You are Required to Check Your SHSU Email Account Every Day


Pressure

Reading Assignment
Gillespie, Netoff and Tiller, eWeather & Climate, as applicable (also note the search function on the CD).
 
Note:  This unit contains a great many graphics and photographs. The load time may vary depending upon your computer and web connection.
 
Click the radio button located on the left page margin opposite selected graphics for additional information. Be SURE and close the message box when you are done.

01. We now turn out attention to pressure, the control that for the most part deals with wind. In particular we will be concerned with the semi-permanent pressure cells -- the cells that generate the world's major wind belts. The Northeast and Southeast Trades, the Prevailing Westerlies and the Polar Easterlies are all caused by the world's semi-permanent pressure belts. In the NOAA satellite photo below, we can see the Prevailing Westerlies in action as they sweep smoke eastward from a series of fires in New Mexico. These are the same winds that tend to move weather in the United States from west to east.

 

02. The second exam will include Pressure as it relates to weather and the world's major pressure belts with their related winds. In addition, we will examine Elevation, Mountain Barriers and Ocean Currents as climatic controls.
 
 

03. To begin with we need to keep in mind that most of the wind any of us are likely to experience in our lifetime is the result of pressure differences. As we shall see, winds tend to move from areas of high pressure to areas of low pressure. These high to low pressure differences are caused by density differences which, to a large degree, are the result of surface temperature differences. In addition, we will see that some pressure differences are caused by the movement of air of different densities from one place to another.
 
This is an extremely important weather-related topic. If you are hot and seek relief, you could wait for the Sun to go down or the season to change, but most likely relief will come on a cooling wind. If you need rain, it is the wind that will most likely bring the moisture. Important -- but tricky. If you can get the big idea as presented, the balance of the course should come much more easily to you.
 
 

04. Let's begin our look at pressure with a few comments about atmospheric density. When we talk about air density, we need to keep in mind that unlike so many pressurized products you deal with on a day-to-day basis, atmospheric gases are not contained. They are perfectly free to move about in response to factors that change their density relative to the air density of other locations.
 
Atmospheric density is really just a term that relates to the number of molecules per given volume of air. Density does not vary significantly horizontally, but the vertical variations may be considerable. As you can see in the graphic below, most of the atmosphere's gas molecules are clustered near the Earth's surface. Here we find the greatest density. It's somewhat similar to a pile of football players -- the weight (read that air density) or force being exerted on the Earth's surface is greatest at the bottom (the surface) and decreases as you go up -- always.
 
We will primarily be discussing air pressure in terms of weight. In reality, what you are dealing with in the atmosphere is the fact that air molecules are in random motion thus pressure is exerted in all directions. As these molecules move about they are exerting a force on you and the Earth's surface. This weight or force decreases as you move upward from the surface -- or in other words the molecules spread out or are less dense. Think about going up on a high mountain and the difficulties associated with breathing. In order to draw in enough oxygen molecules to satisfy your needs, you have to take in a lot more air. Breathing is said to be "difficult" and the air "thin."
 
In the graphics that follow, we will take a look at how air density is measured. Before we begin, let's keep in mind a couple of things. First, air pressure is normally measured at (or adjusted to) sea level. It is important when measuring air pressure that we be able to communicate our findings with others in different locations. Sometimes these other places will be at a greater elevation (above sea level) than we are; other times places will be lower. If all of our pressure readings were not adjusted to some standard (in this case sea level which is more or less the same the world over), the information would be worthless to others. For instance, if you are taking a reading in Denver and wanting to communicate with the weather service in Galveston, you would have to adjust the reading to sea level lest you not take into account your location some 5000 feet (and with a great deal of the denser air below you) above Galveston.
 
Secondly, adjustments must be made for the fact that the Earth is not perfectly round. It tends to bulge at the Equator (a little more atmosphere here), and is somewhat flattened at the poles (a little less atmosphere here). Therefore in taking pressure readings it is important to adjust for this difference. Usually the adjustment is made to 45 degrees of latitude -- to split the difference between the poles and Equator.
 
We can think about measuring the air pressure (think force or weight) in several ways. Though useless as related to weather, one way to think about is would be in terms of pounds per square inch (psi). If on an "average day" you were to go to the beach (at 45 degrees of latitude) and take a square inch of air on your arm and run it up to the edge of space; take it off and weigh it, you would find that it would weigh 14.7 psi (that's about a gallon and a half of milk). This same exercise would result in a reading of 1013.25 millibars (mb, a reading of force) or a reading of 29.92 inches of mercury. They are all saying the same thing -- 14.7 psi, 1013.25 mb, or 29.92 inches of mercury. These readings represent the average weight/force/density of the air pressing down upon you at sea level, at 45 degrees of latitude.
 
In this class, we will use the 29.92 inches of mercury and related readings. For the most part this is what your local weatherman uses on the nightly forecast. You will make use of the millibar set of readings in lab if you happen to be taking the lab this semester.
 
 
 

05. The next three graphics are designed to give you some idea of just what is involved in a pressure reading and what exactly is meant by the terms high and low pressure.
 
The Italian, Torricelli (a student of Galileo's), developed a forerunner of the mercurial barometer (bar/force, ometer/to measure) for measuring air pressure in 1643. As your textbook points out, he could have used water in his experiment, but the denser mercury was preferred. The mercurial barometer works somewhat like the mercurial thermometer we are all familiar with. With the thermometer, the mercury is sealed in a glass tube and as the mercury is heated, it expands up the tube to register warmer temperatures, and as it is cooled it contracts in the sealed tube to register lower temperatures. With the mercurial barometer, the density/force/weight of the air bearing down on the mercury in the open dish or bowl causes the mercury in the bowl to offer more resistance (cold air) or less resistance (warm air) to the mercury in the tube
 
In this first graphic, we have a mercurial barometer on an average day. The experiment ran something like the following (assuming Torricelli was at the coast and 45 degrees): on this day he took the glass tube that was open at one end. He filled it with mercury, put his finger over the open end and set it down in an open bowl of mercury (we can surmise that he did not live to a ripe old age handling all that mercury). The weight/density of the air on this average day pressing down on the mercury in the open bowl, offering some average resistance, only allowed the mercury in the tube to fall to a point 29.92 inches above sea level. Average air temperature -- average pressure (29.92 inches).
 
 
 

06. That evening he goes to bed. He gets up the next morning and discovers that during the night a cold front has passed. Consider the density/weight of cold air compared to average air. If you open the refrigerator at your house, does the cold air hit you in the face or fall to your feet? It falls to your feet. Cold air is heavier/denser than "average" or warm air.
 
He again performs the experiment. Putting the open end of the tube down into the bowl of mercury, he finds that the cold air (offering greater resistance since it is heavier) did not permit the mercury in the tube to fall as far in the tube as it had the day before. Let us say that it only fell to a point 30.02 inches above sea level. Again, the weight of the cold air was bearing down on the mercury in the bowl with greater force/weight, thus the mercury in the tube did not fall as far. Since the reading was above 29.92 inches, we can think of the 30.02 reading as a high pressure reading -- it is above 29.92 inches. Average air temperature -- average pressure (29.92 inches), cold air temperature -- high pressure (30.02 inches).
 
 
 

07. That evening he goes to bed. In getting up the next morning he finds that a warm front has passed in the night. Again, keep in mind that warm air is lighter and less dense that either "average" or cold air.
 
He performs the same experiment for a third time. As he puts the open-ended tube down into the bowl of mercury, he finds that the warm air weighs less, thus bears down less forcefully on the mercury in the bowl. As a result, the mercury in the tube can fall further, and does, to say 29.81 inches above sea level. Since the reading was below 29.92 inches, we can think of the 29.81 reading as a low pressure reading -- it is below 29.92. Average air temperature -- average pressure (29.92 inches), cold air temperature -- high pressure (30.02 inches), warm air temperature -- low pressure (29.81).
 
 
 

So it would seem that colder, denser air results in high pressure (readings above 29.92 inches of mercury) and warmer, lighter air results in low pressure (readings below 29.92 inches of mercury).
 
But what about places like the always-cold Antarctica or the ever-warm Equatorial regions? Because there are no opposite (below or above 29.92 inches of mercury) readings, does this mean that there is no wind? Of course not! We need to keep in mind that while pressure differences cause wind, all pressures are relative. This is why Antarctica has winds. With such extremely low temperatures one might think that all pressure readings in Antarctica would be above 29.92 -- and they would most likely be correct as the cold air in the region is far denser/heavier than the average for the Earth's atmosphere. But some places are no doubt less cold than others. This being the case, can we see that the less cold location (reading of 30.99) would, relative to the colder location (reading of 31.52), in fact represent an area of relative low pressure? Because of these pressure differences, we could expect winds.
 
How about a buzzard story to illustrate the air density idea? When I was a little fellow growing up in deep East Texas (we lived so far back in the woods we used to have to drive toward town to go hunting), we were dirt poor -- no money, no toys, playing with sticks, rocks and cow chips and anything else that came along. In other words, we took our fun where we could get it. As you can well imagine, things were always dying on the farm and my father would drag them off into the pasture for the buzzards. As a little boy, my brothers and I always viewed these as a sign of good days to come. After letting the cow lay out for a few days in the warm Sun, the buzzards came in great flocks.
 
On the appointed day, all of us little boys would grab our sticks and head off into the pasture. Now this was in the days just after cotton had disappeared (early 1950s) and most of the old cotton fields were growing up in persimmon bushes. Sneaking through the persimmon bushes, we quickly came upon the buzzards at work.
 
The basic idea was that on signal all of us were to burst out of the bushes and begin hitting the buzzards with the sticks. Now I am sure some of you have seen a buzzard up close -- it's a really big bird -- especially when you are about 8 or 9 years old. What I could never understand then (but I do now after having gotten familiar with weather) was this -- sometimes we would jump out of those bushes and before we could even raise our sticks, the buzzards took two steps and were in flight. But at other times we would come out of those bushes and with sticks in hand begin whapping those buzzards. The buzzards would run, hop and try to get off the ground, but just couldn't seem to get airborne. What fun! When you are an 8 or 10 year old barefooted boy you can run 6 miles (10 with a new pair of high top tennis shoes) and not even be tired. On a good day, we could run buzzards until we were exhausted.
 
Why the difference? Well, if I had just listened to the weatherman and known what I know now, I could have saved myself a lot of early morning outings. Why could we get to those buzzards on some days, while on others they left us in the dust? Air pressure. The fun days were the days of low pressure. The air was less dense and the big birds had a more difficult time getting aloft when they put their wings down on the warmer, lighter, less dense air. On cold mornings the air was denser and heavier and offered more resistance to the wings, thus it was easier for the birds to get airborne.
 
How many of you have ever noticed that birds and insects light (stop flying) just before fronts come in (as we will see later in the course, this is the period of lowest atmospheric pressure)?
 

08. In the three graphics that follow, we will try to get a little more comprehensive view of atmospheric pressure. Let's say that you wish to "capture" 5000 pounds of air in a column.
 
At Point A (see graphic below) you take out your barometer and get a reading of 29.92 inches of mercury. In this "average" air you will have to run a box/column up 10,000 feet to capture enough molecules to weigh 5000 pounds. Now, keep in mind that the 5000 pounds you have captured only occupies the first 10,000 feet of the atmosphere -- the densest to be sure, but many miles of atmosphere, molecules and "weight" or force remain above your column. For purposes of example, let us assume on this day and in this place that should you run your column on up to the edge of outer space your column of air would in fact weigh 25,000 pounds. In other words, there are 20,000 pounds of air above your column that you are not weighing IN YOUR COLUMN -- however, this weight is taken into account when measuring the SURFACE air pressure.
 
 
 

09. Consider now Point B below. Here we wish to capture the same 5000 pounds of air in a column. However, because at Point B we have a cold air mass present in the area -- thus the colder air is denser (it weighs more) and is exerting a greater force at the surface than was the situation at Point A, the barometric pressure reading is 30.07. Here, because of the greater density of the molecules, we only have to go up only 5000 feet in order to capture 5000 pounds of air in a column. But if we were to calculate the "weight" of all the air above us, we would have to include not only the 5000 pounds in the first 5000 feet, but also the 2500 pounds found between 5000 feet and 10,000 feet PLUS the 20,000 pounds that is found between 10,000 feet and the edge of space. Why do we get a higher reading at the surface at Point B than at Point A? Because we have more molecules (in fact we have 2500 pounds more molecules [a total of 27,500 pounds of molecules] than we had at Point A). Remember, in arriving at the force being exerted at the surface, one has to take into account ALL of the molecules above a given point.
 
 

10. Now let's look at Point C in the graphic below. The idea is the same, however because of a warm air mass in the area, the air is "lighter," it is less dense and therefore exerting less force at the surface. So in order for us to capture the 5000 pounds of air in a column, here we will have to go up higher than was the case at either Point A or Point B. For purposes of discussion let's say we will have to go up 13,000 feet to contain a 5000 pound column of molecules. Why the lower pressure reading? Well if you think about it for a moment, how much "weight" do we have above the 13,000 foot level at Column C? Hard to say for sure -- but can we agree that it is something less than 20,000 pounds (since we had to go up an additional 3000 feet above Point A and thereby reducing the 20,000 pounds of weight that we indicated was to be found above 10,000 feet at Point A)? Let's say for purposes of discussion that we have at Point C only 19,000 pounds of air density above 13,000 feet. Total weight at Point C would then be 24,000 pounds -- less than that found at either Point A or Point B -- thus a lower pressure reading than that found at either of those two points.
 
 

11. We know that air rising at the surface does not continue upward forever (into outer space). A number of factors working together cause at some point the air to spread out and fall back toward the surface. Consider the implications of this by looking at the graphic below. Once the air is "thrown" off the top of the column of rising air (see Columns C and E), where will the air go? How about downhill? So that what we will have is air moving up in Columns C and E, spreading out (diverging) at the top of each column then falling (subsiding) toward areas of similar pressure (the 5000 pound level in Column B) converging and falling toward the surface. And since we are now adding molecules/weight to Column B and D, will not the weight/pressure/density increase in these two columns? And, as we will see shortly, this will cause the air to move from more dense locations (Columns B and D) to less dense locations (Column C and E).
 
    
 

12. The graphics below will give us another view of this very important idea. In the first graphic, we have a situation where there is no pressure difference between the land and water -- thus there is no low or high pressure area and as a result, no wind.
 
 
In the graphic below, the Sun has come up and the land has begun heating up. Thus the air above the land surface is warming and becoming less dense. In other words the 980 mb reading over the land is at a greater height above the surface than the 980 mb reading over water. We now see the full development of the circular air flow between highs and lows. As the Sun rises higher in the sky, the heating of the land surface has intensified resulting in a greater spreading of the isobaric surfaces over the land (you have to go higher in the atmosphere above a given point over land to find say the 980 mb isobar than you would have to go over the cooler/more dense air overlying the cooler water). The warm rising air (a low at the surface), once aloft, now has a ready slope to move "downhill" from the "high" position of the 980 mb isobar over the land to the lower position of the 980 mb isobar over the water. Once here, the air subsides only to diverge and move toward the lower pressures around the low over the land.
 
 

13. We often hear the weatherman discuss pressure cells in terms of high, lows, troughs, domes and ridges. Let's take a look at one last set of cross-sections to try and see if we can determine where these terms might come from.
 
Consider the graphic below. We have a man at the surface with a barometer in his hand. The pressure reading on this average day is 29.92 inches of mercury. For the purposes of this example, let's think about him measuring a weight of air represented by the dashed line. This line represents the average pressure. We might liken this dashed line in the atmosphere (average pressure) to sea level in the ocean. This is what one might expect as "normal."
 
 
 

14. At Point B we have another individual with barometer in hand who is registering a reading of 30.08 inches of mercury. In other words he is not only measuring to the dashed line (average density), but he is registering an additional density/weight/force -- high pressure. So what he is measuring might be portrayed as not only the atmospheric weight/force represented by the dashed line, but some additional weight greater than the average. Sounds like cold air to me.
 
If you listen to the weatherman for very long, you will no doubt hear him refer to "domes" of high pressure, or high pressure "ridges." Think about this graphic when you hear these words. If the dashed line were the flat land surface, then does not Point B look like a mountain or ridge towering above the flat landscape? Can you see why the terms "dome" or "ridge" might be appropriate?
 
 
 

15. At Point C we have another individual whose barometer registers a reading of 29.79 inches of mercury. In this instance we are not even registering a reading to the dashed line. We can indicate his pressure reading by a line that must pass below the dashed line. Because his reading is below the average, we can say that he is getting a low pressure reading. Have you ever heard the weatherman refer to a "trough" of low pressure or maybe a "depression?" What is a hurricane before it becomes a hurricane -- ever heard of a tropical depression? We can see from the graphic where such a term might originate. Our man at Point C is measuring pressure that is below the average -- the line representing this pressure appears as a depression below the general (average) surface -- or a trough, something to feed animals from.
 
 

16. Let's return to our ocean analogy for a moment. If in fact the dashed line on the graphic below represents sea level (or average pressure), then it is surely normal to find instances where the water is higher than average (a wave), as well as lower than average. Yet, while this would be normal, such a situation would not be static. Wouldn't nature make an effort to return to "normal" -- sea level? And wouldn't this be accomplished by water moving from places higher than normal (trying to level these "waves" off) to fill in places that were lower than normal?
 
The atmosphere is no different. The atmosphere also seeks normalcy or "sea level" or the average. It does so by air moving from the higher places in our analogy (the highs/more dense air) to the lower places (the lows/less dense air). In other words, nature it is trying to flatten the domes and fill in the depressions/troughs -- to bring everything back to the average. If you were to experience this movement in the ocean you would call it a current. Such movement in the atmosphere is called wind.
 
And let's take this one step further. Can you see if there is no difference with regard to pressure (no high points, no low points -- everything in the area is the same pressure-wise), thus there will be no wind. And that if there is a small difference between the high and low, there will be a small (low speed) wind. But if there is a big difference in pressure, there will be a strong wind.
 
To sum up then, air at the surface flows "downhill," from highs (dense/heavy air) to lows (less dense/lighter air). Water doesn't flow uphill -- neither does air. Always "downhill" from high to low. Our task as related to weather and wind is to determine where is the high and where is the low. At this point, remember that the high is always associated with the cooler location. The low is associated with the warmer location.
 
 
 

17. As we touched on several graphics ago, air tends to move upward in a low. This is because the air is lighter/less dense and is being forced to take the path of least resistance by the denser/heavier air associated with surrounding high pressure areas. The graphics below illustrate this point. In a low pressure cell (for now, caused by warm temperatures and indicated by red arrows), air converges at the surface then rises. As it rises, it expands and cools -- the blue arrows.
 
In a high pressure cell the conditions are reversed. Here, air tends to move downward from aloft. Again, this is because the air over a high is heavier/denser and it will tend to move down and outward toward areas of least resistance (areas of the less dense/light air associated with low pressure cells).
 
A seeming problem here. Aren't high pressure cells supposed to be associated (for now) with areas of cold temperature? Then how can we be looking at descending (and warming) air and these pressure cells be associated with cold temperatures?
 
Well, let's first make sure we all agree that rising air cools and falling air heats. Remember that the air is under greater pressure at the surface than aloft. So if we cause the air to rise, will it not be entering areas of less pressure? To do so will permit the more dense air from the surface to expand as it rises. With the molecules increasing distances apart, the temperature will fall. Think about a can of hair spray. The air inside is compressed (mashed/under great pressure). If you spray the substance into the air, you are releasing the contents into an area of less pressure. Next time you do this, feel the nozzle of the hair spray can. It's cold. As air comes under less pressure, it expands and cools.
 
That being the case, what would we have in the reverse? If you compress air (as in say return air that is aloft to the surface), you are jamming the molecules closer together and the temperature will go up.
 
Now back to the high pressure, descending and warming air situation. No problem. If we have a high pressure and a 20 degree F temperature on the ground, we can still have air descending and warming in the high -- it's just that the temperature aloft is going to be much colder (say maybe -20 degrees F) than the temperature at the surface.
 
 
    
 

18. Let's take a look at how temperature is related to pressure cells.
 
In the graphic below, the date is August 15th. In our example, the noon Sun is shining down on, with equal vigor, both the blacktop parking lot and the adjacent grassy area. I am in the parking lot and you are in the grass. Is the wind blowing from parking lot to grass, or from grass to parking lot?
 
You should be saying to yourself: well, it's August and hot. The air over the parking lot is a lot warmer than the air over the grass. Therefore …
 
 
 

The warmer parking lot is going to have the low pressure, the cooler grass is going to have higher pressure. Air always moves from high to low, therefore the wind will be moving from the grass to the parking lot.
 
    
 

19. And another: It is February 5th. Suppose you have a free-swinging door on the front door of your completely sealed house (the door swings with equal ease from the inside, out, or from the outside, in). On February 5th, will it be easier to open the door from the inside out, or from the outside, in? Again, it's colder outside than it is inside the house. Therefore the high pressure must be outside, the warmer interior of the house will have the lower pressure. It will be easier to go from the outside of the house to the inside.
 
For now, you need to determine which is cooler (that is the high pressure) and which is warmer (that is the low pressure). Air (wind) always moves from areas of high pressure to areas of low pressure.

Let's take a look at a couple of quick examples to illustrate the previous point.
 
This is probably a good place to again make a point we made earlier regarding condensation and precipitation. If we are to have condensation/rain, the temperature of the air containing the water vapor must be falling. If the temperature of this air is rising -- it is never going to rain.
 
Question. Which of the pressure cells, highs or lows, tend to be associated with clouds and precipitation? Lows -- right? Because in a low, isn't the air converging, rising and cooling?
 
Or how about this one? I am the oldest of four boys. In East Texas in a time past, the senior male could do no wrong. We lived in an un-air conditioned house (and no central heat -- we've already talked about the heat problem and the kitchen heater). In addition, we boys lived four to a room and slept in two sets of bunk beds. As summer began to approach and the days got longer and warmer, I began to feel an urge to force my younger (less senior) brother from his bed. The question, in summer, did I sleep in the upper or lower bunk? The lower of course -- doesn't heat rise? And come winter, wasn't it time for the younger boys to move from their upper bunks to the lower?

20. We know that air moves from high to low as a result of pressure differences. We know pressure differences are caused by temperature and density differences. But what factors are at work that give the wind its speed and direction? Why are winds sometimes blowing two miles per hour and sometimes 50 miles per hour? And why do winds sometimes blow from the east and at other times from the west? And why do some places (coasts, roofs of tall buildings) always seem to be windier than other locations?
 
Four factors are involved: gravity, pressure gradient force, Coriolis Effect and friction. Let's take a look at each of these as they pertain to weather.
 
 
 

21. Gravity. Gravity, the force that pulls all things toward the center of the Earth, starts the air down the hill (the high pressure cell). Nature is trying to flatten out the high pressure "dome" and fill in the adjacent low pressure (depression). The tendency in all high pressure cells will be to flatten themselves out by winds moving away from the high pressure toward areas of less dense, lower pressure. Gravity just begins the process.
 
 

22. Pressure Gradient Force. How fast the air/wind will move down the slope toward the low pressure cell will depend on the slope (pressure difference) between the two. This difference, or rate of change between the two pressure cells, is called the pressure gradient force. The greater the pressure difference, the greater the slope/the pressure gradient force, and thus the greater the speed. The graphic below presents the same high pressure cell from two different perspectives: on the right is the cell looking at it from a side view; on the left is the same cell looking down on it from above. And remember, if we have a high (a mountain) what is all around that mountain relative to it? The land is lower. Same in pressure. If we have a high pressure cell, everything around that high pressure cell is, relative to the high pressure cell, lower. Thus low(er) pressure surrounds all high pressure cells and high(er) pressure surrounds all low pressure cells.
 
You can see on both graphics that the center of the high pressure cell is indicated by a capital "H." Notice as well that the slope to the right of the cell is steeper than the slope to the left. In other words the pressure gradient force is greater on the right slope than the left. If this were in fact a mountain, and I poured water down both slopes, the water would flow more rapidly down the right slope. Or in this instance, if a wind were blowing off this high, it may well be blowing 20 miles per hour off the right slope, but only five miles per hour off the left slope. The relative strength of the pressure gradient force is indicated by the width of the arrows in the graphic.
 
The lines you see on the graphic are called isobars (iso/equal, bar/measure of force). You can see that where the isobars are close together (as you look down on the cell from above) the slope/pressure gradient force is steeper and the winds faster.
 
Gravity starts the air down the slope of the high pressure cell. Pressure gradient force gives the air (wind) its speed.
 
 
 

23. The graphic below is intended to show you that pressure gradient force acts at a right angle to the isobars. In other words, as the air moves from the high down the slope toward the low, the movement will be across (at right angles to) the isobar.
 
 

24. As you look at the graphic of the United States below, can you see that there is a "mountain" of high pressure along the Gulf coast and an even stronger/higher high in northwestern Canada? If you have these two pictured in your mind, can you also see the "valley" of low pressure that runs from northeast to southwest across the map between the two highs? In looking at the valley, can you see that while the entire area is lower than the two mountains, that both ends of the valley are in fact depressions (even lower lows) within the valley?
 
 

25. Coriolis Effect. The third factor influencing the speed and direction of wind is Coriolis Effect. This is the same factor that causes hurricanes to spin inward in a counterclockwise direction in the Northern Hemisphere and inward in a clockwise direction in the Southern Hemisphere.
 
From the previous graphic, you might think that wind would always flow at right angles to the isobars. And it would if the Earth were not rotating. But the Earth is rotating, and this does put a little kink in things. As a general statement, all things moving in the Northern Hemisphere tend to be "deflected" somewhat to the right of their apparent path; and in the Southern Hemisphere all things are "deflected" to the left of their apparent path. The effect is only on direction -- there is no impact on the object's speed.
 
I think we can best get this idea if we use a couple of real world examples.
 
For openers, we need to keep in mind that the Earth moves at varying speeds -- approximately 1000 miles per hour at the Equator decreasing to 0 miles per hour at the poles. Whoa here -- that didn't sound right. Think about this for a moment. The distance around the Earth at the Equator is approximately 25,000 miles. For our purposes here, let's call it 24,000 miles. The Earth rotates once each 24 hours. Thus the speed of the Earth at the Equator must be 1000 miles per hour. You stand on your big toe on the North Pole. How far (linear distance) will you go in 24 hours? Nowhere -- you will just spin in place. Thus the speed of the Earth at the Poles -- 0 miles per hour. Faster at the Equator, gradually less as you move toward the poles to 0 miles per hour at the poles.
 
Now, think about a pickup tearing down a country road at 70 miles per hour. You are standing in the back of the truck with a brick in hand. Up ahead is a lone (stationary/not moving -- going slower than you are) mailbox just waiting to be zinged. If you are from the city and unaccustomed to such things, you might wait until you got even with the mailbox to throw the brick. If such were the case, you would see the brick fly past the mailbox and land in the ditch on up the road. If you are from the country (or just have some experience in these sorts of things) you know that in order to hit the mailbox, you are going to have to "lag" the brick (this is a rural art-form) -- in other words you are going to have to throw the brick at the box before you actually get to it. What's going on here? Well, when you throw the brick, aren't you not only sending the brick toward the mailbox with a degree of force (your throw), but isn't there also a force (the forward speed down the road of the pickup) that is sending the brick straight down the road? The result is these two forces (your throwing and the pickup's forward speed) are working at a 90 degree angle to each other. The result is the brick will most likely split the difference and seem to "veer" off from the target about 45 degrees.
 
Here's another childhood example. And maybe it is more realistic in what is happening with Coriolis Effect.
 
When I was a little boy going to school, times were always the best at recess. There seemed to be no rules governing play so long as you didn't make too much noise (which would surely indicate you were doing something you weren't supposed to be doing). When time came to go out for recess, we all lined up in rows (boys in one line and girls in another) and we marched down the hall to the outside door. The teacher opened the door and got out of the way.
 
Depending upon what was the agreed upon activity for the day, the advantage went to either speed or size. In those days, it was not unusual to find an old boy who had been "put back" a couple of times who might weigh 150 pounds or so. Great if you were playing football or baseball. But the all-time favorite activity was the merry-go-round. Size might have been important, but if you were big, you were probably a lot slower than those little 45 pound 4th graders who were your classmates. And when it came to the merry-go-round, speed mattered.
 
Upon clearing the door of the school, all the little boys took off for the merry-go-round. Now, I usually point out that the merry-go-round at our school in the 1940s was a real merry-go-round. It was probably 15 to 18 feet across, big heavy 2 X 12 boards with brass hand bars so large that no 4th grader could ever hope to get his hand around it. In the center of the merry-go-round was a large brass plate. It was on this plate that the first little boy to reach the merry-go-round sat. As the next fastest boys took up their places around the merry-go-round (one boy to a hand bar), we were usually ready to go before the 150 pounders even got up a head of steam running.
 
Now think about what we have here for a minute and let's relate it to Coriolis Effect -- the topic at hand. Imagine looking down on the merry-go-round scene. Does this look something like the Earth as seen from the North Pole, with the brass plate being the North Pole and the brass hand bars representing meridians of longitude? And think about what the little boys are about to do. Which way did we begin to push the merry-go-round? Since right-handed was all that mattered, did we begin pushing in a counterclockwise direction -- the same way the Earth rotates if seen from above the North Pole? And what about a spinning merry-go-round? Isn't the speed greater if you are on the outside of the merry-go-round than if you were at its center?
 
Again, no teachers in sight. Faster and faster we went -- and make no mistake once you got something as big as this merry-go-round going, nothing was going to stop it. At some point, one of the little boys stumbles -- this is the trigger that moves us all to the next stage. We all jump aboard the merry-go-round with left arm encircling the hand bar -- again no little hand could get a grip on that big bar. Our left arm and hand are occupied, but out of our right pocket we all take out that rock or dirt clod we had kept hidden under the merry-go-round, Then with the right hand, one at a time over the next 5 to 10 minutes, we began to throw the rock/dirt clod at the little boy in the center of the merry-go-round. Fun, and Coriolis Effect at its best.
 
Think about what is happening (no squealing, of course, as that would draw the teachers). You can probably see why the fastest little boy wanted in the center. As each boy took his turn with his rock or dirt clod, he was obligated to throw straight down the hand bar to the center of the merry-go-round. If you did otherwise, you were not allowed to play again. The little boy in the center had the thrill of seeing a rock come zinging down the hand bar at him only to veer off to his left (the thrower's right). Could he sit there and not flinch? The thrower got the thrill of trying to hit the boy on the brass plate. The other little boys strung out around the merry-go-round had the thrill of dodging the rocks coming from the various throwers. Something for everyone!
 
Can you see that the little boys around the merry-go-round are going faster than the little boy in the center. As they throw their rocks, the rock is being influenced not only by the force the thrower put on it, but the speed of the merry-go-round. Working together these two forces (along with friction) cause the rock not to hit the boy in the center of the merry-go-round, or to zip along the edge of the merry-go-round, but rather to actually go about 45 degrees to the right of the intended path as you look from behind the thrower. The effect is the opposite in the Southern Hemisphere. Imagine if you will laying down on the merry-go-round just described and looking at what would be going on if the situation were taking place under the merry-go-round.
 
A final example may be seen in the graphic below. Imagine there is no rotation of the Earth. If you were to fire a cannon toward Paris, the projectile would take a straight path to the city. But, because the Earth rotates from west to east, were you to fire the cannon the Earth (along with the stationary cannon and stationary Paris) would rotate to the east while the projectile, because it was aloft in the air while the Earth was rotating, would land to the west of Paris.
 
 
 
 
 

26. The same effect can be seen on wind being "thrown" from a high pressure (the thrower) toward a low pressure (the little boy in the center of the merry-go-round). The path taken is in fact not at a 90 degree angle across the isobars, but rather across the isobars at about a 45 degree angle. So, gravity starts the wind down the high, pressure gradient force (slope) gives the wind its speed, Coriolis Effect "deflects" the wind about 45 degrees from its intended path -- to the right in the Northern Hemisphere, to the left in the Southern Hemisphere.
 
  

27. Friction. And finally there is friction. As the air moves it encounters trees, buildings, landforms and the like, and in so doing is slowed and diverted from its intended path. Isn't it windier on high buildings than on lower ones? Or how about at the coast? Why are coastal locations so windy? Could it be, in addition to the land/sea temperature (and pressure) differences, there is little out over the water impeding the flow of air?
 
 

28. The graphic below is intended to represent the situation near the Earth's surface regarding wind. Here winds, while blowing from high pressure areas toward low pressure areas, do not blow in a straight line at a 90 degree angle across the isobars. Rather what we find is that the winds tend to move at about a 45 degree angle across the isobars. This is in large measure because of Coriolis Effect which is "pulling" the wind to the right in the Northern Hemisphere. As a result the wind "splits the difference." Think about driving 70 miles an hour down that country road in the back of that pickup with the brick in your hand. You may release the brick when you are even with the mailbox, but the brick doesn't hit the mailbox, nor does it move down the road parallel to the pickup truck -- it splits the difference and will move about 45 degrees between the mailbox and the road.
 
 

29. The two graphics below will serve as a reminder to you that we have both low-level and high/upper-level winds. The winds near the surface tend to move across the isobars at a 45 degree angle; those aloft tend to move parallel to the isobars. Once we move away from the surface, friction becomes less of a factor. Above 3000 to 4000 feet we find that instead of the winds moving across the isobars at 45 degrees (or any degree for that matter), they tend to move parallel to the isobars. Such winds are called geostrophic winds. A jet stream is an example of a geostrophic wind. Let's take a brief look at the forces at work that cause geostrophic winds.
 
As you can see from the graphic on the right, winds aloft, once they begin moving, move rather quickly to a position of balance between the pressure gradient force and Coriolis Effect. The result is a wind that moves (coasts as your textbook describes it) not across the isobars, but parallel to them so long as the pressure gradient force remains balanced against the Coriolis Effect. In reality, this is an idealized situation and is rarely found.
 
    
 

30. To recap, in a low pressure cell winds are converging, rising (cooling) and, at the top, moving outward (diverging) and then down the barometric slope. Aloft, over surface high pressure areas, winds converge, then move down (subside) and at the surface diverge to move toward the less dense areas of low pressure.
 
 

31. The graphic below depicts air circulation around a Northern Hemisphere high pressure cell. Such cells are often called anti-cyclones (high pressure cells). You will observe that as the air moves away from the center of the high, Coriolis Effect acts to deflect the wind to the right of its apparent path. Thus we can say that winds associated with Northern Hemisphere high pressure cells are descending, diverging and moving about the high pressure cell in a clockwise direction.
 
  
 

32. The graphic below depicts air circulation around a Southern Hemisphere high pressure cell. As was the case in the Northern Hemisphere, air moves away from the center of the high, but here in the Southern Hemisphere Coriolis Effect acts to deflect the wind to the left of its apparent path. Thus we can say that winds associated with Southern Hemisphere high pressure cells are descending, diverging and moving about the high pressure cell in a counter-clockwise direction.
 
  

33. Air flow around low pressure cells (often called cyclones) is a little trickier. Let's first get the wind flow established, and then we will take some time to account for the seeming inconsistent air flow associated with low pressure cells.
 
The graphic below depicts the movement of air around a Northern Hemisphere low pressure cell. You can see that the flow is down the pressure gradient slope into the low from the surrounding areas of higher pressure. As the air converges, it then rises. For now, note that the air moves into a Northern Hemisphere low in a counterclockwise direction. Granted the air seems to go against the grain of what we know about Coriolis Effect -- we will discuss this seeming inconsistency shortly.
 
 

34. Around a Southern Hemisphere low pressure cell the air flow is reversed. Here air still moves into the low, but in a clockwise direction.
 
 

35. Now that you have the basic hemispheric air flow around highs and lows, let's take a closer look at the flow around low pressure areas. As you can plainly see, there seems to be some sort of problem here. For instance, in the Northern Hemisphere while we know that wind is deflected to the right, it is very clear that the air is moving into the low cell with a leftward deflection. What's going on here?
 
Let's take a look at the graphic below and get our bearings. The top portion of the graphic is a side view of both a high and low pressure cell. You can see the familiar mountain and bowl analogy. For purposes of discussion, assume that the dashed line separating the red and blue colors represents 29.92 inches of mercury. Note the wind is moving down the (pressure gradient) slope from the high to the low. In the bottom portion of the graphic observe that we have begun the air's descent from the high to the low. As we move off of the high pressure area, Coriolis Effect begins to deflect the air to the right. For the purposes of this graphic, I have only taken the wind to the 29.92 inch line. All looks well.
 
 
 

36. In the graphic below, the wind continues to be deflected to the right by Coriolis Effect. Notice the path taken -- especially as it relates to color. Out of the blue, crossing the 29.92 inch line, into the red color. Continuing to be deflected to the right, the wind eventually re-crosses the 29.92 inch line and moves into the blue color again. What's wrong with this graphic?
 
Well, it looks to me like the air was moving down the pressure gradient slope from the top of the blue high pressure area across the 29.92 inch line and was still moving down the slope for approximately the first half of its journey in the red portion of the graphic. Keep in mind where the slope is. The top of the "hill" is located at the "H" in the center of the blue field, and the bottom of the "valley" is located at the "L" in the center of the red field. Notice what happened to the wind about half way into its journey in the red field -- did it not begin to go "uphill?" And in fact it went so far uphill that it went from some reading below 29.92 inches back to 29.92 inches and then into the blue field with its even higher readings. Water doesn't go uphill -- neither does air. Something is in need of adjustment.
 
 
 

37. What we've got to account for here is the ultimate effect of gravity on the air. In the end, isn't gravity going to put the wind in the bottom of the "bowl?"
 
Look at the two graphics below and consider. As the air moves off the high and down the gradient crossing the 29.92 inch line, pressure gradient force and gravity continue to exert themselves on the wind. At the same time, Coriolis Effect is pulling the air to the right. If the air continues to move rightward at this 45 degree angle, then it will ultimately begin to move uphill. Not possible. What happens is that gravity is going to ultimately take the air to the bottom of the "bowl" -- but the wind is only going there grudgingly. It will continue to resist these two forces -- but in the end, it will end up in the bottom of the "bowl."
 
Look at the graphic on the right to see what is going on as the wind moves deeper and deeper into the low pressure area. You can see that pressure gradient force continues to move the air toward the center of the low. At the same time, Coriolis Effect continues to exert its rightward "pull." The result is a wind that ultimately gets to the center of the low, but with a rightward pull that in the end causes the air to move grudgingly into the center of the low with a counterclockwise motion.
 
 
    
 

38. We are now ready to consider the world's major pressure and wind system. Because the pressure system is arranged along selected parallels, we can best illustrate our discussion with a graphic that places the two poles at either end of a latitudinal cross-section and the Equator in the center. Think of laying the world on its side.
 
 

39. If you will remember from our previous discussion of Latitude as a climatic control, the Equatorial regions experience, on an annual basis, the world's warmest temperatures. Specifically, while the Sun may appear to move from one hemisphere to the other across the year, the average location of the Sun's vertical ray (and thus the area of maximum solar radiation) is the Equator. With this in mind, what we tend to find along the Equator is an area of relatively warm temperatures the year-round. Associated with these warm temperatures is a belt of low atmospheric pressure -- centered on the Equator and ranging outward, both to the north and south, for maybe an average of 15 degrees. Within this area, and generally within 5 degrees or so of the Equator, we find ascending air, disturbed weather and weak highly unpredictable surface winds -- the Doldrums, or what is often called the Inter-Tropical Convergence Zone (the ITCZ).
 
    
 

40. If it were an average day at 30 degrees N/S (let us use our little stick man with barometer in hand analogy), we would be getting a reading of 29.92 inches of mercury. But things are not "average" here. Thirty degrees away (at the Equator), air is ascending, diverging and moving away toward our location at 30 degrees N/S. And here at 30 degrees N/S, this air, which was rising at the Equator, begins to move back toward the Earth (the air is subsiding).
 
So, at both 30 degrees N/S, our man with the barometer is not only measuring the average atmospheric pressure above him (29.92 inches of mercury); he must also add to his reading the air that is piling in on top of him from the Equator. In other words, he has more than his share of air over him -- he is in an area of high pressure. And this is not a here-today, gone-tomorrow high pressure area. Because air is ascending at the Equator all year, it is descending at 30 degrees N/S all year. The warmth that makes the area along the Equator a low pressure region the year-round, creates the belts of high pressure found along 30 degrees N/S.
 
While the Equatorial regions with their rising air are well known for their warm climates and high rainfalls, the areas adjacent to 30 degrees N/S are associated with warming, descending air and as a result are home to some of the world's driest deserts.
 
 
 
 

41. And what about around the poles? It's extremely cold here the year-round, and as one might suspect the polar regions are closely associated with dense air and high atmospheric pressure. So far, so good. As you can see we are developing a nice little array of alternating pressure cells -- the low at the Equator caused by the warm temperatures and rising air; the high pressure areas centered at 30 degrees N/S caused by subsidence and the high pressure cells at the poles caused by cold temperatures. Now, let's take a look at what we have at 60 degrees N/S.
 
 

42. Think about what we have at 60 degrees N/S. Equatorward of these locations we have high pressure belts centered on 30 degrees N/S -- the result of subsidence. Poleward of these locations we have high pressure areas centered on the poles -- the result of cold temperatures. Can you see that despite the high latitudinal location and what are surely some very cold annual temperature averages, the areas on and either side of 60 degrees N/S are belts low pressure? Low? Yes! Consider: if I have a mountain on one side of me (say a high caused by subsidence at 30 degrees N/S) and a mountain on the other side of me (caused by cold temperatures, the poles at 90 degrees N/S), then where am I? I must be in a valley -- a low. A low, not based on warm temperatures, but a relative low -- lower that the subsidence-caused high at 30 degrees N/S and lower than the temperature-related high at and around the poles caused by cold temperatures.
 
 

43. A quick review of the world pressure belts. Centered at the Equator and extending an average 15 degrees either side we have a belt of low pressure. The air rises here, diverges and descends into high pressure belts located some 30 degrees away in both the Northern and Southern Hemispheres. At the cold poles, we have areas of high pressure the year-round. And at 60 degrees N/S we have, relative to the well developed high pressure regions at 30 and 90 degrees N/S, a belt of relatively lower pressure.
 
 

44. Let us superimpose these belts on the Earth -- not as continuous belts, but as discontinuous cells -- belts of cells if you will. Again, lows along the Equator, highs at 30 degrees N/S, lows at 60 degrees N/S and highs at both poles.
 
 

45. Now, if the Earth did not rotate, our discussion of the world wind patterns would be almost complete. All we would have to do is remind you that wind blows from areas of high pressure to areas of low pressure, and the world's wind pattern would be as indicated below. We would essentially be dealing with a series of what your textbook calls "Hadley Cells." We could also call these giant convectional cells -- with air ascending in lows, moving aloft and descending to highs, then returning to the surrounding lows at ground level.
 
        
 

46. But the Earth does rotate. Coriolis Effect does come into play, and our major wind belts do not blow north to south, or south to north. Rather what we have is a fairly straight forward system of winds found between the world pressure belts just described that for our general purposes blow at approximately a 45 degree angle across the isobars.
 
Let us begin our discussion of the world's wind belts with the Trade Winds. The air that descends into the highs at 30 degrees N/S moves, in part, Equatorward. As this air moves, it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Moving across the isobars at roughly a 45 degree angle, we find that the winds between 30 degrees N and the Equator move toward the Equator from the northeast to the southwest. Because winds are named for the direction from which they come, these winds are called the Northeast Trades (we will discuss the "Trade" portion of the name shortly). Remember, you must always stand behind the high, look to the low and then deflect for Coriolis Effect -- left or right. In the Southern Hemisphere, the wind moves from the high at 30 degrees S, 45 degrees across the isobars to the Equator. Because these winds are moving from southeast to northwest, these winds are referred to as the Southeast Trades.
 
The "Trade" portion of the name was given to this wind belt by early European sea captains headed to the New World. Five hundred years ago when wind was the primary power source for ships, those coming to the New World from Europe sailed south to points off the coast of Africa between the Equator and 30 degrees N. Here they picked up the easterly winds that took them in a direction to the New World. These winds were generally found to be strong (Columbus made some 100 miles a day on his initial voyage), steady and highly reliable.
 
The Doldrums or the Inter-Tropical Convergence Zone is located at the point where the two trade wind belts converge.
 
 
 

47. Poleward from the high pressure belt at 30 degrees N/S, air is descending and moving poleward toward the belts of relatively low pressure at 60 degrees N/S. As these winds move poleward, they too are deflected to the right (Northern Hemisphere) and left (Southern Hemisphere) by Coriolis Effect. As a result, the winds between 30 degrees N and 60 degrees N blow in a generally southwest to northeast direction. Those found between 30 degrees S and 60 degrees S blow in a generally northwest to southeast direction. Winds in both of these regions are known as the Prevailing Westerlies (the winds generally blow from west to east).
 
While discernable wind belts, the Prevailing Westerlies are less steady or persistent than the Trades. This is in large measure due to the greater temperature differences one encounters with changes in latitude, the presence of mid-latitude storms with their varying wind, and the monsoonal circulations around the large landmasses found in these latitudes.
 
While much of the year these latitudes are dominated by sub-tropical and tropical air masses, they are subject to invasion by much colder air from the higher latitudes. As a result, these latitudes are known from their frequent and sudden weather changes.
 
 
 

You are probably more familiar with these two wind belts than you think. All of you have watched hurricanes develop out in the Atlantic and then move toward the Gulf of Mexico. This may not seem strange, but it is if you consider our weather here in the United States. As you watched that hurricane, were you thinking about the direction it was moving? Right! It was moving from the east to the west. Carried along by what we have just described as the Northeast Trades. And think about the path that hurricane took -- let us assume that it came into the Gulf of Mexico and made landfall in Texas. Up to the time it hit Texas, its path was generally from east to west. But once it came inland, how many times have you ever seen a hurricane go to Denver? Almost never. Once on land, don't hurricanes tend to curve and turn back to the east (carried along by the Prevailing Westerlies)?
 
Think about our weather here in the United States. It may be sunny and "warm" in Chicago in mid-December, but as a Chicago resident, you know this can't last. Therefore, you keep a close eye on the weather -- not in nearby Cleveland to your east, but rather in Fargo or far away Seattle to your west, because you know that your weather comes not from the east, but from the west. What is in Seattle on Saturday, will most likely be in Chicago on Monday (again, carried along by the Prevailing Westerlies).
 
Columbus was well aware of these winds patterns as well. As a resident of Europe, he knew that those latitudes experienced winds that generally blow from west to east. And he knew that if he wanted to sail west to go east, he was going to have to find another route other than to go due west. With the sailing ships of the day so dependent on wind power, to set sail due west from Spain would only put him back in the harbor (the Prevailing Westerlies). He wasn't going anywhere by sailing due west.
 
But he also knew that further to the south toward the Equator (below 30 degrees N) the wind belts were different. Here they blew from east to west (the Northeast Trades). So ... let's take the Trades over to the New World, do a little pillaging and return via the Gulf Stream to 30 degrees or so north where we can pick up the Prevailing Westerlies and return home. We will even get some of our countrymen to build St. Augustine (the oldest European city on the mainland United States) on the coast of Florida at about 30 degrees N in a few years to keep a chokehold on this route home to Europe. The biggest danger for many of these earlier voyages, aside from the hostile Indians, was most likely the threat of hurricanes. After a hard season of pillaging, many were ready to return home in late summer with ships laden with treasure. To leave later would be to risk getting caught in the ferocious North Sea storms of winter. Unfortunately for some, they were leaving the New World at the height of the hurricane season -- and more than a few ships went to the bottom off the coast of Florida as they prepared to catch the north-flowing Gulf Stream to the Prevailing Westerlies wind belt.

48. And finally there are the Polar Easterlies wind belts. These winds, generated by air moving from the high pressure cells at the poles to the relative low pressure cells at 60 degrees N/S, blow from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere.
 
 

49. How do these pressure belts and wind belts conform to the real world? Fairly closely as we can see if we take a look at the graphics below. You can see that along the Equator there is a rough belt of low pressure in both January and July. Likewise we find high pressure along 30 degrees N/S and low pressure along 60 degrees N/S. Now, the match is not perfect. Most of the variations can be accounted for by the presence of landmasses and the related effects of Continentality. Look at that big (and strong) high pressure cell in central Asia in January (it's very cold then -- thus a strong high pressure cell). Or how about the strong low pressure cell over south central Asia in July -- a low where we ought to find a high? Again, this one is caused by the intense heating of the land to the point of displacing the resident high typically found at 30 degrees N.
 
 
 

50. Compare the two graphics above as regards the location of the ITCZ. The influence of Continentality and the Sun's shifting vertical ray on the location of the low pressure belt along the Equator largely accounts for the latitudinal swing of the ITCZ. 

51. The pressure cell/wind relationship is all around us. Remember the Pilgrims who came to America in the early 1600s? Here they were from northern Europe. Looking at the map below and taking into consideration latitude, why would they not assume that in leaving their homes and sailing south toward the Equator that they would enjoy a much milder climate than that they knew in Europe? But as we all know, that first winter we almost lost them all. To their great surprise, they found the New World climate to be much, much colder than that of Europe. Why?
 
Keeping in mind that both locations are between 30 and 60 degrees N, is it any wonder that, despite their high middle latitude location in Europe, they were accustomed to fairly warm winter temperatures -- the result of the Prevailing Westerlies blowing over the relatively warm ocean and taking those influences onto the adjacent land? But once in the New World, they were operating under a different set of circumstances. Same Prevailing Westerlies wind belt, but in this case these winds were blowing across the much colder land (Continentality), and it was these cold, land-dominated temperatures that were taken to the Pilgrims.
 
 
 

52. How about a southeast Texas example. Suppose you are in the market for a lot on one of the local lakes in the region. The graphic below depicts four possibilities -- all identical except for the relative location on the lake. Which one would you select if you were most interested in keeping cool in your un-airconditioned lake house?
 
Let's begin with a couple of assumptions. One, that you want the lot to build a summer lake house on. Two, that the prevailing wind belts begin and end precisely on the 0, 30, 60 and 90 degree parallels. You should come quickly to either the A or B choice (I hope). Keep in mind that, since this area of Texas is generally north of 30 degrees N, we are located in the belt of the Prevailing Westerlies. If you chose C or D -- bad choice. Can you see that the prevailing wind will move across the land to you bringing with it the warmer temperatures associated with land during the summer months?
 
 
 

53. If you chose B, you did fine. You were thinking about the area lying within the belt of Prevailing Westerlies and since these winds prevail from southwest to northeast, it would be to your advantage to purchase the B lot since it would receive winds which had traveled over a considerable expanse of the lake, picking up the relatively cooler temperatures, before coming to you.
 
 

54. But you who chose lot A would be correct if we consider what we actually find on the ground. If you live in or near Houston, you know that in reality the wind tends to prevail, not out of the southwest (we can only wish for the low relative humidities associated with southwest winds in the summer), but rather from the south to southeast -- see the graphic on the right. If you chose A or B, you did fine, it you chose C or D, you need to go back over the material presented here and in your textbook.
 
Before we leave this, just a word on why the wind in the real world tends to blow from the southeast along the Texas Gulf Coast. In the graphic to the right, air is moving off the high pressure cell at approximately 30 degrees N, Coriolis is nudging it to the right so that it moves onto the Gulf Coast from roughly the southeast. However, as the air continues to move, see how it swings around to the south, then southwest and finally back to the south as it makes its way into the low pressure area at 60 degrees N. Again, this is how it works in the real world. For our purposes, let us assume that if you are one millimeter poleward of 30 degrees N/S, you are in the belt of Prevailing Westerlies. If you are one millimeter Equatorward of 30 degrees N/S, you are in either the Northeast or Southeast Trade winds depending upon the hemisphere you are in. This same generalization will hold for the other wind belts of the world.
 
        
 

55. Land Sea Breezes. Any of you who have spent any time along a beach will readily recognize the ebb and flow of winds from sea to land and then back out to sea. These local winds, rarely felt more than 20 to 30 miles inland, are well known by coastal residents.
 
You wake up early one summer morning while visiting some relatives who live along the Texas coast. Being a good citizen, you go out and get the morning newspaper just as you do each morning at home in Dallas. Picking up the paper you look to the northern sky and, as you are so accustomed to seeing, you observe the sky is perfectly clear -- not a cloud to be seen. You walk back into the house and onto the back porch that faces south. You are somewhat startled to see a line of clouds off in the distance. Stepping outside to get a better look, you notice a gentle, but steady wind moving from north to south. Remembering your old geography class, you know in a heartbeat what is happening.
 
Let's see now. Early morning, land is cooler than the water -- so there must be a high pressure cell over the land. High pressure means air is descending to the surface, diverging and moving away toward adjacent lows. Any indication of where the low is? Yes! True, it is summer, but keep in mind that night, with its rapidly falling temperatures relative to water, is just coming to a close and in the early morning hours the Sun has yet to heat the land to a temperature greater than the water. The water in the adjacent Gulf is warmer than the land, so it will have the low pressure. True, it is summer, but keep in mind that night with its rapidly falling temperatures relative to water is just coming to a close and in these early morning hours the Sun has yet to heat the land to a temperature greater than the water. And you remember that low pressure is associated with rising and cooling air. And if you are thinking ahead now, it will occur to you that for water vapor to condense requires cooling (think about that "sweating" glass of ice tea). And there you have it. The clouds are there because of the rising air associated with the low pressure out over the warmer Gulf waters.
 
And what about that wind? In weather, that land to sea breeze so common in the early mornings along the coast is known as a land breeze. As you might suspect, the strength of the wind is related to the temperature difference. And while merchant seamen were reluctant to sail out of sight of land 2000 years ago for fear of not being able to find the next port, coastal fishermen have long known of the presence of land breezes and made extensive use of these winds to sail considerable distances beyond the sight of land with no fear of being able to find their way back.
 
And how did these early fishermen find their way back? You can already hear it coming can't you? By making use of the late afternoon sea breezes.
 
 
 

56. The graphic below depicts the factors at work in creating a sea breeze. As the Sun begins to rise in the morning sky, the land begins to heat more rapidly than the adjacent water. By 10:00AM or so, the land is usually warmer than the water and a pressure reversal takes place. Now, with low pressure developing over the warm land and the cooler water assuming a high pressure mantle, the winds of evening reverse and the winds begins to move from the sea to the land. Isn't this one of the reasons we all go to the beach in the summer -- to experience the cooling breezes from the sea? Remember the water is cooler (hence the air temperature) and the breeze moves toward us on the beach by virtue of the land's lower pressure. And if you think about it for a moment, how many times have you been at the beach in the afternoon and observed the clear skies over the water (high pressure) and looked inland and there were the clouds -- with rain often experienced inland between 2 and 4:00PM (the heat of day).
 
And what about those fishermen of 2000 years ago? Well they just had to make sure that they finished up their fishing duties and were headed in to port before dark on the sea breezes which continued to blow until ... that's right! Until the land cooled to a temperature below that of the water and the pressure field reversed again.
 
 
 

57. Below is a NOAA satellite photo of the island of Puerto Rico. Is this an early morning or late afternoon shot of the island? Click the radio button for the correct answer.

 

58. Monsoons. Many of you have no doubt heard the term "monsoon." Most of you probably associate it with heavy rainfall. You are sort of correct. Actually the term is translated from the Arabic and means "seasonal shift of winds." It rains a lot because as these winds shift, they bring moisture inland where mountains are encountered, the air is uplifted and cooled and heavy rains are the result.
 
The cause of the monsoons (and there are two -- a summer and a winter monsoon) can in many ways be likened to land-sea breezes. Except with the monsoon we are dealing with much larger scale phenomena, and seasonal rather than daily patterns. While monsoons occur all over the world where there are large landmasses, the term is most closely associated with Asia -- especially eastern and southern Asia.
 
While the causes are complex and, like many things in weather not fully understood, essentially what we have is a summer situation where the land warms to a greater degree than the sea. A low pressure region is established over the land, with a high pressure pattern prevailing over the sea. As the air begins moving from sea to land, it winds bring to the land much needed rainfall and relief from sweltering summer temperatures.
 
 
 

59. During the winter monsoon, the pattern is reversed. Because the air tends to move from the colder land (high pressure) to the warmer sea (low pressure), this is generally the dry season in monsoonal regions of the world.
 
 

60. A comment about the relationship between large and small scale winds. The prevailing wind pattern of the world is more or less always present. That is not to say there will always be winds. But it is rare to find a time when the influence of the prevailing winds is not to be felt.
 
Let's say it is September and a dying hurricane is directly overhead Huntsville. A hurricane, you will remember, is a large low pressure cell. If you are in Galveston, you will experience a south wind; if you are in Dallas, you will experience a north wind. In other words around any low pressure cell the winds blow into the low from all directions. If you are near the low, you can experience wind from any direction depending upon where you are with respect to the low. And if there is a low pressure cell in your area generating a wind, it will most likely displace the prevailing wind pattern. But this is not to say that the prevailing winds to not exert an influence.
 
As we discussed earlier, hurricanes are moved along toward the west in their earlier stages by the Trade Winds, only to be moved eastward in their later stages by the Prevailing Westerlies.
 
 
 

61. No discussion of winds would be complete without a few comments on the upper level circulation. In the upper levels of the troposphere, we know there is a general pressure gradient slope from the Equator toward the poles. At these high levels, pressure gradient force and Coriolis Effect are in balance, friction is minimal, the winds are geostrophic and they generally flow from west to east around the Earth. Within this west to east flow we find a number (generally two to five or so) of undulations called Rossby Waves. At the outer margins of these waves are the jet streams -- on the poleward margin is the Polar Jet Stream, on the Equatorward margin is the Subtropical Jet Stream.
 
We have known of the presence of jet streams since World War II. Today airline pilots going from San Francisco to New York (west to east) often seek out these rapidly moving "rivers" of air to speed along their flights. As indicated in the graphic, these discontinuous steams of air can obtain speeds in excess of 200 miles per hour. We also know that jet streams have a major effect on the movement of storms at the surface. We will take a little closer look at this in the graphics that follow.
 
 
    
 

62. In the graphics below we see the jet stream that more or less divides North America in half with cold air to the north and warmer air to the south. In the graphic on the right, we began to get an undulation in the jet stream and along with this comes the movement of warmer air into higher latitudes along the west coast and a dipping of colder Canadian air into the mid-section of the United States.
 
    
 
As a rule, high pressure ridges are associated with the up-ticks of jet streams. Think about this for just a moment and picture the air flow around a high pressure cell. Think about this for just a moment and picture the air flow around a high pressure cell. As you can see on the graphic to the right, the high is bringing cold air down into the central United States via the counterclockwise flow on its east side, but this same high is sweeping warm air to the north on its west side.
 
The down-ticks of jets typically are associated with low pressure cells (mid-latitude cyclones). While we will take up mid-latitude cyclones much later in the course, for now note that as a low pressure cell, these storms are bringing warm air up from the south (in this case all the way into New England and adjacent Canada) while at the same time they are drawing air into themselves (the bowl) on the back/west side.
 
Keeping an eye on the movement of jet streams is important in trying to predict the movement of surface storms. 

You have now completed Unit 4: Pressure. You might wish to check your knowledge of the material presented in this section by working through the Multiple Choice, and True-False Quiz Questions as well as the essay-style Review Questions available through The Course dropdown located in the header of this page. To return to the top of the page.

Copyright 2004, The START Group, All Rights Reserved
PO Box 1972
Huntsville, TX 77342-1972