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Latitude

Reading Assignment
Gillespie, Netoff and Tiller, eWeather & Climate, as applicable (also note the search function on the CD).
 
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01. In this unit, we will be discussing Latitude as a climatic control. By latitude we are not talking about geographic location. Rather our interest will center on the effect of latitudinal location on the receipt of solar energy at the Earth's surface. To a degree the Greeks were on to the idea when they proposed their Torrid, Temperate and Frigid temperature zones over 2000 years ago.
 
 

02. Specifically in this section on Latitude, we are going to cover four topics:
 
Earth-Sun Relationships. This section will deal with the actual receipt of solar energy at the Earth's surface. The receipt of solar energy is directly related to the relationships which exist between the Earth and the Sun across the year. All of you would recognize the basic relationships. Each day we can see that the Sun "rises" in the east and sets in the west. Most would also recognize that over a 12 month period the Sun "moves" from north to south in our sky. In other words, the Sun not only "moves" from east to west, but also north to south. And how about the fact that days tend to get longer in the northern hemisphere from December 21 to June 21, and then grow shorter from June 21 to December 21? Earth-Sun relationships cause changes in the amount of insolation received day to day and seasonally. And, depending upon the relationship, the amount of insolation changes locationally and through time. The resultant temperatures created due to these changing relationships create pressure differences which are largely responsible for winds. The winds in turn drive the ocean currents and our weather.
 
Temperature. We will also consider the measurement and transfer of heat -- giving special attention to conduction, convection and radiation as these processes relate to the atmosphere.
 
Atmospheric Gases. While we will touch upon the major gases comprising the atmosphere, we will devote most of our attention to the most important gas related to weather -- that being water vapor -- the gaseous state of water.
 
The Layered Atmosphere. Finally, we will take a look at the various atmospheric layers. While there are in fact four atmospheric layers based on temperature, our interest will rest primarily with the first layer, the troposphere. The troposphere, which extends upward from the Earth's surface an average of maybe 5-10 miles, is where most of our weather takes place. While we think of weather as being all-encompassing, it is actually relatively shallow. Think about the last time you took a trip by air. You most likely flew at about 30,000 to 35,000 feet. As you looked out of the window, most of the clouds were far below you. Or maybe you have stood outside after a front passes and felt a strong north wind in your face, and then upon looking up in the sky are surprised to see clouds moving the opposite direction (heading north). Remember our earlier comments about mountain barriers? Weather is shallow.
 
 

03. Before we begin, let me take a minute to define the terms low, middle and high latitudes as I will be using them in this class. Low latitudes are those locations found between the Equator (0 degrees N/S) and 30 degrees N/S. The middle latitudes are found between 30 degrees N/S and 60 degrees N/S. And the high latitudes are found between 60 degrees N/S and the poles (90 degrees N/S). As we look at these locations in the lesson ahead, we will be especially interested in the annual and seasonal temperature differences found in each of these latitudinal belts, and what causes these differences to occur.
 
 

04.
Earth-Sun Relationships. Our first topic is Earth-Sun relationships. This is an extremely important idea because it is these varying relationships that cause differences in the amount of solar energy that comes to the Earth's surface. For instance, you get a lot better Sun tan in June than you will in December -- because the Sun's rays are more intense. It is the Sun's energy that heats the Earth and drives the ocean currents, winds and our weather.
 
I have found that one of the best analogies to use in discussing Earth-Sun relationships is that of baking a cake. Because that is really what we are talking about -- the Sun baking the Earth. If I take the ingredients for a cake, put them in a pan, stir them up, and then turn on the oven, to say 20,000 degrees F, and throw them in the oven and then immediately jerk the pan right out -- what I end up with is a raw cake. Or what if I stir up those same ingredients, throw them in the same pan, slide the pan into an oven that is heated to only 80 degrees F and leave it in there for 6 weeks. What do I have? Well, most likely a raw, moldy cake! At 80 degrees F, the cake never cooks, no matter how long I leave it in the oven. In other words it takes TWO things to bake the cake (or the Earth) -- heat and time working together -- not just one or the other.
 
 
 

05. Let's begin by taking a look at insolation (
incoming solar radiation). If we were to take a flashlight and project a beam of light onto a surface (and let us assume that 1X of insolation is actually reaching the surface), note that the 1X of insolation actually covers 1X of surface (for example, 3 square inches). Now, if we tilt the flashlight a bit and send the light beam to the surface at an angle (again, the same 1X of insolation is being put to the surface), you will observe that the surface area covered by the beam is increased. So, the same amount of insolation, but spread over a larger area -- the result is less heat energy per square inch -- thus (if insolation were all that were involved), a cooler temperature at the surface.
 
Look at the graphic below. Let us assume that it is March 21 and the Sun (see the high Sun position) is directly overhead the Equator. Note: 1X of heat energy onto 1X of land. All of the insolation available (1X) is absorbed by the Earth at the Equator. Contrast this with what one might see on that same March 21 date from a position of 45 degrees N latitude. At 45 degrees N latitude, the same 1X of insolation is being received at the surface, but, since we have almost 50 percent more area covered, you should expect the temperatures (again, if this were all that were involved in Earth temperatues) at this location to be less than those experienced at the Equator. In other words, the closer one is to the point at which the Sun is vertically overhead, the greater amount of insolation received at the surface.
 
 
    
 

06. The graphic below depicts this Earth-Sun relationship at three different points on the Earth's surface. The Sun is off the graphic to the right. If the Sun is directly overhead the Equator (Point C), then 1X of insolation is being put to 1X of surface at the Equator. It's just like the flashlight being directly over a spot. In short, the Equator is receiving the maximum insolation available (1X of solar energy is being put on 1X of area).
 
If we move poleward, to say 45 degrees N (Point B), here we find the solar energy coming in at a greater angle than was the case at point C. Just like the flashlight when we tilted it. The graphic indicates that at 45 degrees N, 1X of insolation would be spread across 1.4X of surface. It would stand to reason that on a per square mile basis, Point B would receive less heat energy than Point C. And at Point A (60 degrees N), 1X of insolation would cover 2X of surface (approximately half the heat energy per square mile as is being put onto the surface at Point A). And, should we carry the example to 90 degrees N (the pole), the Sun angle would be coming in on the horizon and the heat energy received would be minimal per square mile. So, that even though light is available 24 hours a day for several months on end at or near the poles, the heat energy actually received is minimal.
 
 
 

07. Notice in the graphic below, that all things being equal, solar radiation moving to the Earth's surface from a point directly overhead will pass through ONE atmosphere at the Equator on March 21. But, on this day as you move toward a pole, the Sun angle is less (remember, you are tilting the "flashlight") and that the angle will in fact be 0 at the pole). Because of increasing amounts of atmosphere that the solar radiation must pass through in order to reach the Earth's surface, we can see from the graphic below that while the number of atmospheres is one (1.00) over the Equator and increases gradually and only slightly as you move toward the poles, there is a sharp and very significant increase in atmospheres as the Sun angle approaches the horizon at the poles.
 
The point then -- the closer you are to where the Sun is directly overhead, the more insolation per square mile. The farther away you are from the Sun's direct ray, the less the angle and the less heat energy per square mile.
 
 

08. In San Antonio on June 21, the Sun is not directly overhead; but it is only 6 degrees off the vertical. On this same day in Winnipeg, the Sun is some 26.5 degrees off the vertical. Note the difference in the amount of surface covered by each ray and consider the path through the atmosphere each is taking to the surface. And contrast June 21 at both locations with the situation on December 21.
 
    
 

09. Let's apply this idea to the house you live in. Many of you no doubt live in a house that has a roof overhang (an eave). In the southern United States, the eaves of houses tend to overhang the wall maybe three to four feet. In the South in the summer, when the Sun is high in the sky and the temperatures are very high, every effort is made to reduce the heating of one's house. Houses with substantial eaves are able to intercept the Sun's rays and thus reduce sunlight entering the windows. In winter, when the Sun is lower in the sky, heat energy is admitted to the home through the windows unhindered by the presence of the eave.
 
If you are in the northern part of the United States, it is relatively cool the year round and here one is more likely to find much smaller, narrower eaves. Such construction permits sunlight to come into the house year round.
 
You can see this idea reflected in the location of ski resorts. Think about Colorado (or wherever) the next time you are on one of those slopes. What direction do the slopes face? North in the Northern Hemisphere (south in the Southern Hemisphere). Here temperatures are somewhat cooler (less direct Sun and less evaporation) than those found on say the south-facing slopes in the Northern Hemisphere. Thus the snow will tend to melt on the north-facing slopes last.
 
 
 

10. Heat then is one of the items needed to bake the cake. But it takes two things to bake the cake. Let us now consider time.
 
From the graphic below you can see that on March 21 and September 21 (the equinoxes -- equal day, equal night), the Earth is oriented to the Sun in such a way as to place the vertical ray directly overhead the Equator. Now, shinning a light on a ball such as the Earth will cause only half (180 degrees) of the ball to be lighted. From the point where the light is directly overhead, light will extend in all directions 90 degrees. So that in this example, if the Sun is directly overhead the Equator, light will be visible 90 degrees to each pole. If you are at the Equator, the Sun will be directly over your head -- at the poles the Sun will be on your horizon. But regardless where you are (pole to Equator) all locations will experience 12 hours of day and 12 hours of night since all parallels are bisected.
 
 
 

11. Things look considerably different on June 21. For openers, the Earth-Sun orientation is not the same. On this date the Northern Hemisphere is pitched toward the Sun and, as a result, you can readily see that the Sun's vertical ray is not striking the Earth at the Equator but rather is overhead at 23.5 degrees N. This is the location of the Tropic of Cancer. The tropic marks the northern-most point at which the Sun will be overhead during the year.
 
When you see the Tropic of Cancer on maps and globes, there is usually some distinctive marking (dots, dashes, or the like) to set this parallel off from others. Poleward of 23.5 degrees N there is never a day when the Sun is directly overhead. Poleward of the Tropic of Cancer you will always have to look south to see the Sun.
 
On June 21, when the Sun is directly overhead 23.5 degrees N, note that in shining 90 degrees, the Sun will extend not only to the North Pole (66.5 degrees away from the Tropic of Cancer), but in fact an additional 23.5 degrees (to make a total of 90 degrees) beyond the pole to the back side of the Arctic Circle (66.5 degrees N). Light shines 90 degrees in all directions from the point where the Sun's ray is vertically overhead.
 
Light also shines 90 degrees to the south from the Tropic of Cancer. This means that light will extend to the Equator (23.5 degrees) plus an additional 66.5 degrees (to total 90 degrees) to the front side of the Antarctic Circle (66.5 degrees S).
 
If we could spin the Earth one rotation/one day, we would find that if you are anywhere between 66.5 degrees N and the North Pole, you would always be in light (24 hours). No darkness would occur because at no time would you rotate into the dark area. So with 24 hours of light, why is it not warm to hot in these polar areas? Keep in mind that you are a long way from the vertical ray of the Sun (it is at 23.5 degrees N). You have long days, but the low Sun angle means little heat energy is actually being received at the Earth's surface. Also keep in mind the earlier graphic depicting the number of atmospheres sunlight must pass through on its way to the surface.
 
Now, if you are located between 66.5 degrees S and the South Pole, you will on this date experience 24 hours of darkness because such locations are more than 90 degrees away from the Sun's vertical ray.
 
Finally, if you are on the Equator you will have 12 hours of day and 12 of night on June 21. On the Equator we will see that days and nights are equal 365 days a year. The Circle of Illumination always bisects the Equator. So why then is it not hot on the Equator? After all, the Sun is overhead or near overhead 365 days a year and sunlight has minimal atmosphere to pass through. Think about it. Just about the time you begin building up heat energy on the Earth's surface, the Sun begins to fall and night is upon you. They don't have those 13 to 14 hour days we have here in the mid-latitudes in the summer. They have the solar radiation (heat energy), but they just don't have the time in the oven. And it takes both to really bake the cake.
 
 
 

12. Finally, let's take a look at December 21. On this date you will notice that the Southern Hemisphere is pitched toward the Sun and, as a result, you can readily see that the Sun's vertical ray is not striking the Earth at either the Tropic of Cancer or the Equator, but rather it is overhead at 23.5 degrees S. This is the location of the Tropic of Capricorn. The Tropic of Capricorn marks the southern-most point at which the Sun will be overhead during the year.
 
And like the Tropic of Cancer, when you see the Tropic of Capricorn on maps and globes, there is usually some distinctive marking (dots, dashes, and the like) to set this parallel off from others. Beyond 23.5 degrees S there is never a day when the Sun is directly overhead. Poleward of the Tropic of Capricorn you will always have to look north to see the Sun.
 
On December 21, when the Sun is directly overhead 23.5 degrees S, note that in shining 90 degrees, the Sun will extend not only to the South Pole (66.5 degrees away from the Tropic of Capricorn), but in fact an additional 23.5 degrees (to make a total of 90 degrees) beyond the pole to the back side of the Antarctic Circle (66.5 degrees S). Light shines 90 degrees in all directions from the point where the Sun's ray is vertically overhead.
 
Light also shines 90 degrees to the north from the Tropic of Capricorn. This means that light will extend to the Equator (23.5 degrees) plus an additional 66.5 degrees (to total 90 degrees) to the front side of the Arctic Circle (66.5 degrees N).
 
And again, if we could spin the Earth one rotation/one day, we would find that if you are anywhere between 66.5 degrees S and the South Pole, you would always be in light (24 hours). No darkness would occur because at no time would you rotate into the dark area.
 
Now, if you are located between 66.5 degrees N and the North pole, you will on this date experience 24 hours of darkness because such locations are more than 90 degrees away from the Sun's vertical ray.
 
Finally, if you are on the Equator you will have 12 hours of day and 12 of night on December 21. On the Equator we will see that days and nights are equal 365 days a year. Light always bisects the Equator.
 
 
 

13. Take a look at the graphic below and note that, depending upon where you are latitudinally on a specific date, the hours of light/dark change. Always 12 and 12 on March 21 and September 21 all over the Earth. Always 12 and 12 on the Equator -- 365 days a year, but note the length of day and night at various latitudes on June 21 and December 21.
 
 

The previous graphics have demonstrated that Earth-Sun relations change across the year. We well understand that the Sun is not doing anything. It is just there -- immobile, relative to the Earth. It is the Earth that is presenting itself differently to the Sun at various times of the year. Some times during the year the Northern Hemisphere is pitched toward the Sun (June 21). Here proximity to the ray, combined with long days and short nights, creates a period of considerable heat. At other times of the year the Southern Hemisphere is pitched toward the Sun and the Southern Hemisphere experiences summer conditions. During this period (December 21) the resulting distance from the Sun's vertical ray, short days and long nights makes for cold winter conditions in the Northern Hemisphere. And finally, at other times of the year, neither hemisphere is pitched toward the Sun (March 21 and September 21 periods), and at these times the insolation received from the Sun tends to be more moderate in both hemispheres resulting in the more temperate spring and fall seasons. Now there is a lot going on here -- remember, the Sun is not doing anything other than just sitting there.
 

14. What are these things that are causing the Earth to be oriented to the Sun differently at different times of the year? There are four factors at work in Earth-Sun relationships. They are: Rotation, Revolution, Inclination, and Parallelism. Let's take a look at each one of these in the graphics that follow.
 
 

15.
Rotation. Again, keep in mind the Sun, relative to the Earth, is NOT moving. The Earth rotates on its axis in a west to east direction. Think of the axis as an imaginary line extending from pole to pole and passing through the center of the Earth. That said, how do you know that the Earth is in fact rotating from west to east? Think about it for a minute. Doesn't New York City come into the light (experience dawn) while it is still dark in Huntsville? If it's 8:00AM here in Huntsville, isn't it 9:00AM in New York City? They are an hour ahead (earlier light) of us. If that is the case, then won't the Earth have to rotate from Huntsville (west of New York City) toward New York City in order for Huntsville to experience the light -- which it generally receives about one hour after New York? And when Huntsville comes into the light, isn't Los Angeles still in the dark? So for Los Angeles to receive light (usually some two hours after Huntsville) won't it be necessary for Los Angeles (to the west of Huntsville) to rotate from the west TO the east in order to reach the light? And finally, as dawn breaks over Los Angeles, hasn't New York City been in the light for several hours? One rotation takes approximately 24 hours. Again, all this assumes that you keep in mind that the Sun is immobile relative to the Earth.
 
 
 
The photograph below was taken by the crew on board the Columbia during its last mission February1, 2003. The photograph, taken on a cloudless day, catches the sun setting over Europe and Africa. The Circle of Illumination is striking as is cuts across western Africa, Spain and France. The more urbanized areas may be clearly seen in the light pattern -- the lights are on in Paris and Barcelona, but it is still daylight in London and Gibraltar. In the middle of the Atlantic Ocean are the Azores and to their southeast are the Canary Islands. On to the south, off the west coast of Africa are the Cape Verde Islands. Across the top of the photo, from right to left is snow-covered Norway, Iceland and Greenland.
 

 


16. Consider for a moment the speed of the Earth's rotation. You might think it is the same everywhere, but such is not the case. Look at the graphic below. We know that the distance around the Earth at the Equator is approximately 25,000 miles. One turn (rotation) in 24 hours. Thus the speed of the Earth at the Equator is a little more than 1000 mph. If you are standing on your big toe on the North Pole, what is the speed of rotation? Well, since you will turn once in 24 hours, but in so doing you will actually go nowhere, the speed at the pole must be 0 miles per hour -- the Earth is just turning in place for you.
 
Therefore, intermediate locations between the Equator and poles will have speeds between 1000 and 0 miles per hour with the speed declining toward the pole. And while we will come back to it later, consider for a moment the speed of the Earth's atmosphere. On average, do you see why the speed of the atmosphere above any given point on the Earth's surface is the same as the Earth's surface? If such were not the case -- for instance say the Earth were turning 800 miles per hour, but the atmosphere above it were only turning 700 miles per hour -- individuals on the Earth would be experiencing a 100 mph wind!
 
 

17. Kind of as an aside, yet related to rotation (which causes day and night), is the idea of the Circle of Illumination. As the Earth rotates, some of the time we face the Sun and are in light; other times we are away from the Sun and in darkness. Keeping in mind that only half of the globe can be lighted at one time, what are we to make of the transition area between light and dark? You know these times as dawn, and dusk or twilight. This area is known as the Circle of Illumination. In the Circle of Illumination it is not really dark nor is it light. Any of you who have ever flown in a plane and crossed the Circle will well remember it. You can look up toward the front of the plane at 35,000 feet and see the light, and look to the rear and see darkness.
 
 

18.
Revolution. The second factor influencing Earth-Sun relations is Revolution. As the Earth is rotating on its axis, it is also revolving around the Sun in a counterclockwise direction. 365 days to make one circuit. In revolving about the Sun, the Earth moves along on what is called the Plane of the Ecliptic (its orbital plane). As you can see in the graphic below, at some times in the orbit the Earth is closer to the Sun than at others. Two dates come to mind: January 3 and July 4. Living in the Northern Hemisphere you might think that the Earth and Sun would be closest on July 4 because this date is warmer than January 3, but to do so would be in error. Do you think you would have made the same assumption if you were living in Australia? In fact, the Earth is closest to the Sun on January 3 and most distant on July 4. January 3 is called perihelion; July 4 is called aphelion (think Aphelion -- Away).
 
What does all this mean? This basically means that distance from the Sun is not what is causing the seasons.
 
 
   
 

19.
Inclination. Inclination refers to the tilt of the Earth on its axis. Note that when you see models of the Earth, the North Pole is not at a 90 degree angle with reference to the Plane of the Ecliptic. Rather it is tilted or inclined 23.5 degrees off the perpendicular. Now, where have you heard this 23.5 degree business? Yes, the Tropics of Cancer and Capricorn. Anywhere else? How about the two circles (Arctic and Antarctic)? They are at 66.5 degrees N/S, but where are these points relative to the adjacent poles? Right, they are 23.5 degrees Equatorward of the poles. This is why we give special significance to these two special parallels -- the two tropics and the two circles. They are related to the tilt of the Earth off the perpendicular.
 
Let's try you out on something here. What if, instead of the Earth being tilted 23.5 degrees, it were tilted 10 degrees off the perpendicular? If such were the case, would summers in the Northern Hemisphere be: warmer than they are now, cooler than they are now, about the same, or can't tell?
 
Think about this for a moment. What is it that makes the Earth's surface warm or cold? Isn't it distance from the vertical Sun ray and length of day (the cake analogy)? Well, that being the case, take it to the extreme. Don't tilt the Earth 10 degrees -- just don't tilt the Earth any degrees. In other words imagine there is no tilt. The Earth's poles are at a 90 degree angle to the Plane of the Ecliptic. Now, if this were the case, would the Sun's vertical ray be striking the Earth at the Equator every day, 365 days a year? Yes. Well, if the Sun is on the Equator all year, doesn't that mean that the Sun's vertical ray does NOT travel into either hemisphere, but instead remains on the Equator all year? Too, that will mean that every day (and night) is 12 hours long. If that is the case, then wouldn't summers be cooler than they are now? True, the question didn't ask for no tilt, but it did ask for LESS tilt (10 degrees) than we now have (23.5 degrees), so doesn't it follow with the example above that, if such were the case, that summers would be cooler than they are now?
 
And do you see that if the tilt were increased to say 40 degrees, we would all be in big trouble from a temperature standpoint? In such an instance, wouldn't the Sun's vertical ray (and related surface heat) extend to 40 degrees N in the Northern Hemisphere's summer (thus reaching far beyond the present 23.5 degrees) into the hemisphere, and in addition providing much, much longer days (and shorter nights) than we now experience? Summers of 130 degrees F or more might be common. Too, do you see that winters in such a situation (with a 40 degree tilt) would be much, much colder than we now experience. The cold being related to the fact that the Sun's vertical ray (the heat energy) would not be at its maximum distance as is now the case just 23.5 degrees south of the Equator, but instead would be 40 degrees south of the Equator. Northern Hemisphere locations would be much more distant from the vertical ray than is now the case. Too, Northern Hemisphere days would be much shorter (not as much time to absorb any insolation headed their way) and nights would be much longer than is now the case -- thus more time for the meager heat energy to leave the Earth's surface.
 
If you understand all this, then you've got it!
 
 
 

20.
Parallelism. Parallelism is the idea that as the Earth revolves around the Sun, the axis is always pointed to the same place in space. Where is this place that it is pointed? The North Star. The North Pole is always pointed to the North Star no matter where it is in its orbit about the Sun. Look closely at the graphic below and you can get an inkling of the importance of this idea. Note that in all the views of the Earth, the axis is pointed the same direction (to your right). Consider for a moment the implications of this across a year. Can you see that at some points in the orbit, the North Pole is pitched toward the Sun while at other points the South Pole is pitched toward the Sun?
 
 

Now, let's put all this together by taking a look at the Earth-Sun relationships that are to be found on the four critical dates as the Earth orbits the Sun.

21.
June 21. We will begin with the position of June 21. Note that you are viewing the Earth graphic on the left from the position indicated by a red "X" on the graphic to the right.
 
On June 21 you will note that the Northern Hemisphere is pitched toward the Sun. This means that the Sun's vertical ray is striking the Earth at the Tropic of Cancer (23.5 degrees N). Light extends 90 degrees in all directions from this point. On this date, you will note that light extends from 23.5 degrees N toward and then beyond the North Pole to the back side of the Arctic Circle, and that light also extends toward the South Pole to the front side of the Antarctic Circle (66.5 degrees S).
 
Note that at the Equator we have equal day and night, but as we move toward the North Pole from the Equator days get longer finally reaching 24 hours at the Arctic Circle (66.5 degrees N). It is a combination of the long days and the fact that the Sun's vertical ray is at 23.5 degrees N that makes this the day of maximum solar radiation receipt in the Northern Hemisphere.
 
And we have the reverse situation in the Southern Hemisphere. Here, as you go toward the South Pole, days get shorter until we get to the Antarctic Circle beyond which there is no light. Too, keep in mind that the Sun's vertical ray (the heat energy) is centered 23.5 degrees north of the Equator -- a long way from points in the Southern Hemisphere. Thus solar energy is at its minimum in the Southern Hemisphere on this date (the first day of the Southern Hemisphere winter).
 
Let's see now. The date is June 21. It is the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere. I get the June 21 date in the Northern Hemisphere -- but June 21 in the Southern Hemisphere? Are we sure about this? What are the chances, since it is winter in the Southern Hemisphere, that it is December 21 in the Southern Hemisphere? The chances are ZERO. The season may change as you move from one hemisphere, but the date stays the same. Just checking!
 
What's the best day for a sun tan? June 21. You may think the best day is sometime in August when the temperatures are the highest, but keep in mind you are not baking yourself to get the tan, you are absorbing the Sun's rays. These rays are most intense when the Sun is closest to you. So while June 21 may be the absolute best day for a tan (assuming clear skies), keep in mind that you can move off that date an equal number of days in either direction and get a similar tan.
 
 
    
 

Now let's revolve the Earth counterclockwise 90 degrees (3 months) from the June 21 date. The date is now September 21/22. Note that the North Pole is still pointed toward the North Star. Keep in mind that what we just did (the counterclockwise revolution) happens gradually and that what we will now discuss has evolved over three months from conditions we previously discussed for June 21.

22.
September 21/22. Note where you are -- you are in front of the Earth in its orbital path. You are looking back on the Earth from a point in FRONT of it. The Sun is to your right, thus the right side of the Earth as you look at is lighted. The Earth is moving in its orbit toward you. The Circle of Illumination is bisecting all parallels. This is the fall or autumnal equinox in the Northern Hemisphere, and the spring or vernal equinox in the Southern Hemisphere. Equinox -- equal night (and equal days at all locations). For us in the Northern Hemisphere, the days are getting shorter as the Sun falls/moves lower in the southern sky. Winter is approaching. If you are in the Southern Hemisphere, the Sun is rising in the northern sky and the days are getting longer. Summer is approaching.
 
    
 

23.
December 21. We will now revolve the Earth 90 degrees along its orbit in a counterclockwise direction. Note that you are viewing the Earth graphic on the left from the position indicated by a red "X" on the graphic to the right.
 
On December 21 you will observe that the Southern Hemisphere is pitched toward the Sun. This means that the Sun's vertical ray is striking the Earth at the Tropic of Capricorn (23.5 degrees S). Light extends 90 degrees in all directions from this point. On this date, you will note that light extends from 23.5 degrees S toward and then beyond the South Pole to the back side of the Antarctic Circle, and that light also extends toward the North Pole to the front side of the Arctic Circle (66.5 degrees N).
 
At the Equator we have equal day and night, but as we move toward the South Pole from the Equator days get longer finally reaching 24 hours at the Antarctic Circle (66.5 degrees S). It is a combination of the long days and the fact that the Sun's vertical ray is at 23.5 degrees S that makes this the day of maximum solar radiation receipt in the Southern Hemisphere.
 
And we have the reverse situation in the Northern Hemisphere. Here, as you go toward the North Pole, days get shorter until we get to the Arctic Circle -- beyond which there is no light. Too, keep in mind that the intense energy associated with Sun's vertical ray is centered 23.5 degrees south of the Equator -- a long way from points in the Northern Hemisphere. Thus solar energy is at its minimum in the Northern Hemisphere on this date (the first day of the Northern Hemisphere winter).
 
 
    
 

24.
March 21. We will now revolve the Earth 90 degrees in a counterclockwise direction. Look at where you are -- you are behind the Earth in its orbital path. You are looking at the Earth from a point in BACK of it as it orbits the Sun. The Sun is to your left, thus the left side of the Earth as you look at is lighted. The Earth is moving in its orbit away from you. The Circle of Illumination is bisecting all parallels. This is the spring or vernal equinox in the Northern Hemisphere, and the fall or autumnal equinox in the Southern Hemisphere. Equinox -- equal night (and equal days at all locations). For us in the Northern Hemisphere, the days are getting longer as the Sun rises/moves higher in the southern sky. Summer is approaching. If you are in the Southern Hemisphere, the Sun is falling in the northern sky and the days are getting shorter. Winter is approaching.
 
    
 

25.
Heat Transfer. As you well realize, with the Sun's vertical ray appearing to move between the two tropics, there is a lot of insolation being put into the tropical/low latitudes. Your textbook indicates that between 36 degrees N and 36 degrees S there is a annual surplus of heat energy. In other words there is more heat energy being put into this area by the Sun over a 365 day period than is leaving. Poleward from 37 degrees N/S there is a heat energy deficit -- in other words, more heat energy is being lost than is being received. Keep in mind that if you are at the poles, day may be 24 hours long in the summer, but you are not getting a lot of insolation because of the low angle. And in the winter months, the Sun is extremely low or non-existent angle.
 
Well, a situation like this exist can't exist for very long. If it did, then soon all of the world's population would be fighting for the area around 36 degrees N and S. Toward the Equator from this point it would be too hot to live, and toward the poles it would be too cold. Since this is not the case, something must be causing cold air to be transferred out of the cold deficit areas into the heat energy surplus areas, and warm air to be transferred out of the heat energy surplus area into the heat energy deficit areas. For us, the most obvious factor at work is fronts that bring cold air from the higher latitudes into the lower/warmer mid-latitudes. That air is coming in from Canada -- that's cold air from the heat energy deficit region.
 
And think about the typical summer weather forecast for Houston -- such and such a high temperature, such and such a low temperature AND warm, humid air moving in from the Gulf of Mexico -- or some variation of that. This warm air from the Gulf of Mexico is going over us (the typical south wind of summer) as it heads out of the tropics into the middle and upper middle latitudes. In other words we can all see this heat transfer at work right here in southeast Texas the year round.
 
Air is important, and the winds do move a lot of heat/cold about the Earth, but even more important in the distribution of temperature is water (ocean currents). Water moving from the high latitudes into the mid and low latitudes (cold currents), and water moving from the tropical latitudes into the mid and high latitudes (warm currents) are carrying huge quantities of energy with them.
 
Of lesser importance are hurricanes. They contain huge amounts of heat energy (acquired via evaporation over tropical oceans) that are brought into the mid-latitudes and released in the condensation process (rain).
 
 
 

26. When we consider Latitude alone as a control, we know that the low latitudes (say from the Equator to approximately 30 degrees N/S) are the warmest across the year (on an annual basis). Why? Well, let's consider the question in terms of our cake analogy. We have to consider both temperature and time.
 
We know that in these latitudes the Sun's ray is vertical or near vertical at all times, thus a considerable amount of heat energy is available to the region throughout the year. In addition, the length of day is always 12 hours at the Equator, and as we move poleward toward 30 degrees N/S the length of day gradually increases to maybe 13/14 hours in the summer (and running some 10 to 11 hours in the winter). Thus, with a high Sun ray and days of moderate length all year, you would expect the annual temperature to be relatively warm.
 
 
 

27. And as you should expect, on an annual basis the high latitudes (60 to 90 degrees N/S) are the coldest. Consider: during the cooler late fall, winter and early spring months we have little or no Sun angle (insolation) and thus little or no daylight (time) for these roughly six months. During late spring, summer and early fall months, while we do have extremely long days (ranging up to six months at the pole) the Sun is either on the horizon or barely above it, thus little heat energy is available.
 
 

28. So far so good. All is just as you would have suspected. The Equatorial areas are the warmest and the polar areas are the coldest across the year. Now let's take a look at things seasonally. Keep in mind as we talk about seasons that we have two hemispheres, and that the seasons are reversed in these hemispheres. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. Too, keep in mind that while summer is always summer (summer is always the "warm" season), summer is not always July. It depends on what hemisphere you are in.
 
On a seasonal basis, the high latitudes are the coldest -- just as you thought would be the case. After all, when you have a place that during its cold season has both little Sun angle and little daylight, you should expect it to be cold.
 
 
 

29. Let us now consider the summer (high Sun) situation at say 25 to 35 degrees N/S. We can actually use Texas as the example since most of the state falls within these latitudes -- and I suspect that most of us would agree that our summers seem awfully hot. Why is this?
 
In our summer season, the Sun is approximately 82 to 83 degrees above the southern horizon in Houston. Not 90 degrees like we get at the Equator at equinox, but still 80 plus degrees is pretty high. This means that the Sun, while not vertically overhead, is very close to vertical, and as a result we do get a great deal of insolation. In addition, our days run approximately 14 hours in length. It's warmest along 25 to 25 degrees N in the summer season because here you have both the ray (insolation) and time.
 
Then why doesn't it get warmer as you go poleward? Why are 25 to 35 degrees N/S the warmest latitudes and not 40 to 45 degrees N/S? After all, since these latitudes are experiencing longer days than we are here in south Texas, shouldn't they be warmer?
 
Remember to keep in mind that it takes TWO things to bake the cake -- time and temperature. As one continues to go poleward from 23.5 degrees N, it is true that the days get longer (being 24 hours poleward of 66.5 degrees N on June 21), but, while days are getting longer, the Sun angle is dropping in the sky and thus there is less heat energy available.
 
 
 

30. There are, of course, other controls that have an impact on temperature in addition to latitude. While we are discussing the climatic controls one at a time, if we really wanted to explain why a place experiences the weather/climate that it does, we would need to consider all of the controls together. We are not there yet -- but we will be at the end of the course. However, you do need to keep in mind that there are other controls at work. These controls would include:
 
Air Masses. Fronts and winds can bring large bodies of air into a region whose temperatures and humidities are very different from the resident air mass. Think here about the very cold and dry "blue northers" that occasionally visit Texas in winter.
 
Continentality. This is the idea that land and water do not heat up and cool off at the same rate. Think about Galveston and Dallas.
 
Ocean Currents. There are both warm and cold currents. Jacksonville, FL has a warm current off its coast, Los Angeles has a cold current. While both are coastal in their location, their climates are considerably different.
 
Mountain Barriers. Mountains can block warm or cold air and in so doing have a considerable impact on a location's climate.
 
Elevation. Places located between 25 and 35 degrees N/S may well be the warmest seasonally, but such locations can still have snow-covered mountain peaks the year round.
 
 
 

31. Temperature is a measurement of molecular activity. The faster the speed of the molecules, the greater the temperature. The slower the speed of the molecules, the colder the temperature. Temperature is measured with instruments called thermometers (
thermo/heat, meter/to measure). A number of scales have been devised and are currently in use to measure temperature. The most popular (with the melting point of ice and the boiling point of water) are: Fahrenheit (32 and 212 degrees F), Celsius (0 and 100 degrees C), and Kelvin (273 and 373 Kelvins). In this class we will use the Fahrenheit scale because this is the most commonly used scale in this country -- note your evening weather report. If you are taking the lab with this course, the emphasis will be on the Celsius scale.
 
 

32. In your day-to-day life, you will encounter many different expressions of temperature. Our interest in this class will be with air temperature. But there are other types of temperatures you should at least have some acquaintance with. One of these is what is called sensible temperature. Sensible temperature is how it feels on your skin as contrasted with air temperature that is measured with a thermometer. Sensible temperature is affected by not only the air temperature, but also the relative humidity, wind speed and solar radiation.
 
You have no doubt heard the weatherman talk about something that is sort of like sensible temperature -- wind chill. Wind chill is a measurement that takes into account air temperature and wind speed. But you no doubt recognize that there are other things that go into determining the "temperature" you feel. Cold, dry air can be very cold, but cold, wet air can make one miserable. And to combine either with a good stiff wind adds but another layer of misery.
 
Think about those times you have seen on the news people snow skiing in shorts and without a shirt. Are they cold? They are probably cold only if they fall down in the snow or if they ski into the shade. The air temperature is certainly cold, but in the dry mountain air, and with the bright Sun above and the Sun being reflected off the snow, the skiers are getting a lot of radiation thus helping to offset the chilling tendencies of the air temperature.
If you have ever been to a high elevation, you can relate to the skier's situation. If you stand out in the Sun, the temperature can be very pleasant. But if you step back into the shade, the temperature drops very quickly. The air temperature in both the Sun and the shaded area is the same, but they feel very different on your skin. This is why when you hang a thermometer outside you should hang it in the shade. With the thermometer you are attempting to measure air temperature. If you hang the thermometer in the Sun, you will in fact measure the air temperature, but, in addition, the reading will include any heat generated as a result of radiation striking the thermometer resulting in a higher reading of the air temperature.
 
 
 

33. We may well make passing reference to sensible temperature from time to time in this course, however, our primary focus will be on air temperature.
 
While temperature and other weather element readings have in the past typically been taken by instruments placed in shelters such as those pictured in your textbook, readings today are much more likely to be taken by stations such as pictured below. There are literally thousands of such measuring stations in this country and around the world taking weather readings on a daily basis and sending them in to central collection points where they are complied and mapped. This information forms the basis for the weather forecasts all of us are familiar with. This same information, over the long term, provides the basis for climatic statistics.

 

34. As we move through the course we will make use of a number of terms related to temperature. For the moment, you should be sure that you understand each of the following:
 
Daily Mean Temperature. The daily mean temperature is arrived at by securing a daily temperature average. This may be calculated in a number of ways. The most common is to average the maximum and minimum temperatures for the day. Another method would be to average the hourly temperature readings. You have to be careful in using this temperature when trying to provide someone with an idea of what kind of temperatures they might could expect at a given location. Galveston, influenced by its proximity to the coast, may have an average daily mean temperature of 80 degrees F during the summer (75 degrees F for a low, 85 degrees F for a high). Las Vegas may have the same average (80 degrees F) but their low may be 50 degrees F and their high 110 degrees F. Same daily mean temperature, two very different sets of temperatures to contend with.
 
Daily Range of Temperature. The daily range of temperature is the difference between the high and low temperatures for a given day. In the example above, Galveston's daily range would be 10 degrees F while Las Vegas would have a daily range of 60 degrees F.
 
Monthly Mean Temperature. The monthly mean temperature is the average temperature for the month. This is usually arrived at by adding the daily means and dividing the result by the number of days in the month.
 
Annual Mean Temperature. The annual mean temperature is simply an average of the 12 monthly temperature means.
 
Annual Temperature Range. The annual temperature range is the difference between the means of the warmest and coldest months.
 
 
 

35. As you can well imagine, trying to make sense out of hundreds or even thousands of temperature readings across the United States would be a daunting task. A weather map exhibiting such data would be no more than a great mass of incomprehensible numbers. How to make some sense out of all these numbers? Meteorologists make use of isotherms (
iso/equal, therms/temperature -- lines of equal temperature) to accomplish the task. Isotherms, usually drawn in 5 to 10 degree intervals, are a convenient method for mapping temperatures. In principle, this idea is very similar to that of the contour line in geology -- contour lines being lines connecting points of equal elevation.
 
The graphic below depicts an uncolored weather map with several weather stations on it and a series of isotherms drawn in at 10 degree F intervals. Such a graphic is a major improvement over a map depicting the simple plotting of data. But such a map can itself be significantly enhanced by the addition of color. Here, at a glance, once can get an overview of the temperature gradations across the country.
 
 

36. We now have some idea of what is involved in getting the heat energy of the Sun to the Earth's surface. Now the question is how is the heat energy absorbed by the Earth and then distributed upward to the atmosphere? Three mechanisms are involved in this process: conduction, convection and radiation. Let's now take a closer look at each of these.
 
 

37.
Radiation. Radiation can be defined as the transfer of heat energy by electromagnetic waves -- no medium of transport is required -- no solid, no liquid no gas, no land, no air is required. It is in this manner that energy from the Sun is transferred to the Earth. Here the term "insolation" is frequently used. Think of the Sun's warmth on your face on a cold winter afternoon. You step into the shadows and it gets cold. Radiation comes to the Earth from the Sun as short wave radiation -- which just means that the distance between the electromagnetic waves is "short." The radiation is absorbed by the surfaces it reaches and is then re-radiated back to the atmosphere as long wave (or infrared) radiation.
 
Consider: a winter morning with a heavy frost. As you drive to work or school, you notice that your roof along with several others in your neighborhood are covered with frost, while on others frost is only visible around the edges of the roof. What's going on here? Well, if you are one who has a frost covered roof, could it be that you have a lot of insulation in your ceiling? Next door you notice that the only frost on your neighbor's roof is around the edges (above the eaves). Could it be that with so little insulation that he has a lot of heat escaping from the inside of the house thus heating the underside of his roof and preventing frost from forming?
 
And just as you turn the corner you see a small airplane from your local power company flying overhead. As you later learn, the plane is snapping infrared photographs of the neighborhood. About a week or so later you get a postcard from them in the mail indicating that, based on their analysis of their photos of the neighborhood, you have one of the "tightest" houses in the area -- very minimal loss of heat. Smiling at your good luck, you later learn that your next door neighbor has decided to make use of the energy company's services to run an audit on his home to find out how he might make his home more energy efficient.
 
 

38.
Conduction. Once radiation has heated the surface, how is heat energy transferred to the air above (to then be moved about by the wind)? By the process of conduction. Conduction might be defined as the transfer of heat from molecule to adjacent molecule with the heat flowing from the warmer object to the colder object.
 
Look at the graphic below. The bar is placed over the flame and heat is transferred to the bar and then, with the bar held in place, heat slowly works its way down the flame to the point that it becomes too warm to be held. While this bar may conduct heat very efficiently, such is not the case with all substances. Overall, solids tend to conduct heat better than liquids, and liquids tend to be better conductors of heat than gases. Since our concern is with the atmosphere, our interest is primarily with gases -- the poorest conductors of heat.
 
In your everyday life you can see plenty of evidence of the poor conducting qualities of air. Think about a winter coat. The purpose of a coat on a cold winter day is NOT to keep the cold air OFF of you, but rather to keep the warm air IN. Isn't this the same idea that is at work when one layers their clothing? Each layer of clothing is surrounded by a thin layer of air, and it is this thin layer of air (a poor conductor of heat) that inhibits the escape of warm air from beneath the clothing.
 
If you are wearing a coat and the coat gets wet, you are in big trouble quickly as the warmth of the body is conducted to the cooler water and you get cold (take on the temperature of the water). At the extreme, think about the problem of a person forced into the cold ocean after a shipwreck. Conduction transfers the warmth of the body (98.6 degrees F) to the much colder ocean water that is completely surrounding, and in contact with, the body. The body temperature will drop quickly with the result often being death.
 
Or think about the use of insulation in a house. When insulation is placed in the attic, it contains a lot of air. That area of air is very difficult to heat or cool. The air in the insulation helps prevent the escape of warmth (or coolness) from the living space below. In time, as the insulation settles, its insulating qualities diminish.
 
Because air is such a poor conductor of heat, as you might suspect it is very difficult for conduction to heat the air to any great height. A few feet above the ground, the temperature may be much cooler or warmer than the surface. For instance, it is generally recommended that when placing a thermometer outside to measure air temperature that the instrument be placed some five feet off the ground (and in the shade). The five feet gets it away from the very warm layer right at the surface caused by conduction (for instance it may be 90 degrees F at five feet, but 110 degrees F at the ground on a warm summer day). You should place the thermometer in the shade to get it out of the Sun's radiation that will provide a false reading by increasing the temperature of the instrument itself (and, by conduction, the temperature reading).
 
 
 

39.
Convection. And finally there is the process of convection that we might define as the transfer of heat within gases or liquids as these gases/liquids move. We can also think of this as the vertical movement (up or down) of air in the atmosphere. Some of you may have heard someone refer to a summer afternoon thunderstorm as a convectional storm. What is meant here is that the storm is caused by the intense heating (radiation) of the land in the afternoon by the high Sun angle and long summer day. The heat of the warmed surface is then conducted to the air above. This warm air then rises (convection) and cools to the point of condensation. Clouds are formed, often followed by heavy rains. Convection is important for moving air vertically through the atmosphere from the surface upward (and sometimes from aloft downward to the surface). Convection is also at work in moving air from the warmer regions of the Earth (the Tropics) to the higher latitudes and vice versa.
 
Convection is the vertical movement or air. But what about what you and I feel daily -- the horizontal movement or air? Such air is termed "advection."
 
Note on the graphic below that warm rising (lighter) air is being uplifted. As this air leaves the surface, other cooler, more dense air moves in to replace the rising warm air. As the air rises aloft, it does not go on out into space. A number of forces are at work that cause the rising, cooling air to spread out (diverge) and then begin to fall back toward the surface (subside). Upon reaching the surface the air begins to move toward areas that are being heated and uplifted. This is a classic convection cycle. We will discuss this in greater detail when we get to the control: Pressure.
 
We will discuss condensation and precipitation later as well, but for now keep in mind that there are at least two conditions that must be present in order to have condensation and/or precipitation. One: moisture (water vapor) must be present in the atmosphere; and (2) the temperature of the air containing the moisture must be falling. You can see by this graphic why we tend to get a lot of afternoon rain in the summer months. Sun heats the land; the warm (and humid) air rises; it expands and cools to the point of condensation and rains. Think of the rainforest located along the Equator. Is it warm along the Equator year round? Then do you think almost every afternoon we would find warm air rising, cooling, condensing and raining?
 
 
 

40. Let's put it all together with a couple of examples. You drive to school for a class. The outside temperature is 25 degrees F. You lock your car, go in for the class and upon returning find as you open the car door that the temperature inside is almost 80 degrees F! What's going on here?
 
Short wave radiation from the Sun is penetrating the car's windows, it strikes the dash, steering wheel and seats and is absorbed. The warmed surfaces inside the car, by conduction, warm the air above. The warmed air then rises to swirl about inside the car. You return to the car, open the door and are hit with a blast of hot air -- not to your feet, but to your face as the warm (and light) air exits the car and moves upward through the atmosphere.
 
Or say you are going to visit your 105 year old grandfather in deep East Texas. This old guy is not into modern conveniences -- like central heat. It's cold -- 20 degrees F. No heat in the house as you get ready to go to bed. Now you have been through this drill before on previous visits so you know just what to do.
 
Before getting into bed, you carefully lay out the clothes you will be putting on in the morning. Now, ready to get into bed you note that the bed is covered with four or five hand-made quilts -- not the kind with the cheap, light-weight batting in it, but rather the real McCoy -- with plenty of heavy cotton batting. You know that you have to decide right now, as you get into bed, what position you are going to want to take for the night. Because once in bed there will be no turning over -- the quilts will keep you pressed to the lumpy mattress in the same position all night. Heavy, but effective. Isn't there a layer of air between each quilt? The air contained within will keep you warm.
 
Morning arrives at 4:30AM with a forceful bang on the door by Granpa. "Time to get up! It's sinful to sleep past daylight!" Again, you've been here before. You know that the only heat in the house is the single gas heater in the kitchen. The trick is to jump out of bed, grab the clothes you laid out the night before, get them on while making your way to the kitchen. And Granpa will demand that you be fully dressed before you show up in the kitchen. You make it without freezing to death along the way.
 
As you ease up to the fire, you are careful not to let the jeans touch the back of your legs. The raging heater will, within minutes, bring the back of the jeans to a boil. A slight tilt of the knees in the wrong direction will cost you all of the hair on the back of your legs. Ain't radiation wonderful!
 
Got it? 

41. The graphic below is a relatively famous photo taken by the NASA astronauts from the moon looking back to the Earth. Why is it that you can see the Earth? It's because a percentage of the total light being sent from the Sun is being reflected from the Earth back into your eyes. This reflected light is called the planetary albedo -- some might call it "Earthshine." We can define albedo as the reflection of radiant energy expressed as a percentage, and this percentage varies with the surface encountered. Of all of the insolation coming to the Earth, approximately 30 percent is reflected back into space and never gets to the Earth's surface. Again, this is why you can see the Earth in the photo taken from the surface of the moon.
 
In addition to the planetary albedo lost, there is another approximately 20 percent that is lost via scattering, absorption and reflection as the radiant energy passes through the atmosphere on its way to the Earth's surface. Of the solar radiation that reaches the outer layer of the atmosphere, only about 50 percent, on average, actually reaches the Earth's surface. And of the 50 percent that does get through then, depending upon the surface encountered, maybe as much as 95 percent of that may be reflected back and lost to the surface.
 
You will note in the table that accompanies the NASA photo that the albedo rate varies considerable. Let me call your attention to two items in particular.
 
First, in looking at the percentages, notice that they vary widely depending upon the surface encountered. First, in looking at the percentage for fresh snow (75 percent to 95 percent ) think back on our earlier discussion of Sun angle at high latitudes. Does this 75 percent to 95 percent figure help you to better understand why during the winter, the snow-covered surfaces of Canada (or Russia) have such low temperatures? Not only is (1) the Sun angle low, thus little heat energy is received per square mile, and (2) the days short so there is relatively little time for solar radiation to be received, and long nights allowing plenty of time for the meager amount of solar radiation received to be lost, but (3) even of what little radiation may actually get to the surface, a very high percentage is reflected back and thus lost to the surface for the purposes of heating.
 
Secondly, note the wide variation in albedo for water (from 5 percent to 80 percent ). Why such a wide variation? Well, note in the NASA photo that the darkest color to be seen is that of the ocean. For sure the clouds would be associated with a high albedo, as would the lighter colored landmasses. But the ocean water? When the Sun is overhead or nearly so, can you see that a huge percentage of radiation would be absorbed by the dark blue oceans relative to the land? However, as one moves poleward, the Sun angle would decrease, reflection would increase as would the albedo. As a result, what we generally find in high latitude locations is very cold land (among other things, the result of high albedo over the snow-covered surfaces) and relatively cold water (among other things, the result of high rates of albedo over the water).

 
 

42. The atmosphere is comprised of a number of gases, and several of them are directly and immediately useful to life on Earth -- oxygen for animals, carbon dioxide for plants being the two that come quickly to mind. In addition, certain of these gases screen out harmful rays (most notably those related to skin cancers), and of course the atmosphere is very important in that it insulates/holds in terrestrial radiation -- without which the Earth would experience extreme swings of temperature.
 
In terms of their presence in the atmosphere, the most prominent gases are nitrogen, oxygen and argon -- none of these are directly important in weather. However, these three gases make up, on average, some 99.9 percent of our atmospheric gases. Combined, the rest of the gases comprising the atmosphere make up just 1/10 of one percent of the gases present -- these include two gases that are meteorologically important: carbon dioxide and water vapor.
 
 

43. The importance of carbon dioxide lies more in the realm of climatology than meteorology. This gas has the ability to absorb out-going long wave radiant energy and thus to heat the Earth. Coming from a variety of sources, most notably volcanic eruptions, decaying vegetation and the burning of fossil fuels, carbon dioxide, as a percentage of the atmosphere, has been rising for several centuries. This gas is most closely associated with those who attribute its increase to global warming. This is a controversial theory and is covered at some length in your text. For now, let us just remember that all notions such as this have two sides -- and besides, a little global warming might be good for a lot of people. Try telling a Russian or a Canadian that global warming is all bad!
 
 

44. Water vapor is the colorless, odorless, gaseous form of water. This gas, largely confined to the lower-most layers of the atmosphere, has the ability to absorb BOTH incoming, short wave solar radiation AND outgoing, long wave terrestrial radiation. In addition, water vapor, unlike most atmospheric gases, changes states at temperatures found at the Earth's surface. As water vapor changes state, heat is either absorbed or released by the water molecule. This heat is the source of energy that drives our weather and storms. Consider a hurricane. The storm forms over the tropical oceans. Here it absorbs huge quantities of heat as water is evaporated into the storm. The hurricane then moves into the middle latitudes where both moisture and heat are released.
 
The amount of water vapor in air varies widely depending on a number of factors. Generally, warm, tropical locations contain considerably more water vapor that do colder polar locations. Isn't this what a clothes dryer is all about? Clothes dryers do not make use of cold air (such air can't hold much water). Instead, liberal use is made of warm air to hot air. Such air, with its increased ability to hold moisture, encourages evaporation off the wet clothes.
 
Many of you have no doubt heard the saying, "It's too cold to snow." This is just a reminder that warm air can hold more moisture than cold air. And if we have really cold temperatures, there may be so little moisture in the air that only very small amounts of snow are to be expected.
 
Consider Antarctica. You might think such a snow and ice covered location would experience considerable annual snowfalls. Such is not the case, and we should all be grateful. The temperatures in Antarctica are so cold that the air holds very little moisture, thus there is little to fall out (maybe a couple of inches across the year). Then where does all the snow/ice come from? It's been there a long time. Think about it. If it did snow a lot in Antarctica, and it continued to be as cold as it now is, we would have very little melting or evaporation. Thus, in short order, the snow pack would increase; the thickness of the ice sheets would increase and we would likely see a return to an Ice Age.
 
 
 

45. One other gas we might take note of is ozone. Most of this gas is concentrated about 12 to 18 miles up. Ozone, whose "hole" is in the news from time to time, has the ability to absorb certain parts of the short wave spectrum (ultraviolet). Acting as a screen, ozone is important in keeping these harmful rays which have been shown to be a major contributor to certain types of skin cancers to a minimum. The graphic below depicts the famous "hole" in the ozone.
 
 

46. Gases are not the only items found in the atmosphere. Solid particles such as dust, pollen, spores and the like are found in abundance, especially in the lower-most levels. Think about the last time you gave your house a real good cleaning. Vacuuming, dusting, Spic and Span -- the whole nine yards. Then you walk by one of the newly cleaned rooms and see Sun streaming in a window and all kinds of "dust" floating in the air. The moral of this story -- atmospheric particles are always present, especially in the lower-most levels of the atmosphere. It is sunlight, being reflected and scattered off these particles that give rise to the beautiful orange and red sunrises and sunsets that are so prominent in the more arid portions of our landmasses and along our coasts (salt particles).
 
Particulate matter in the atmosphere is also important from a weather standpoint. In the condensation process, it is necessary for particulate matter to be present in order for there to be something for the water vapor to condense around. We will discuss this topic at much greater length in a later unit.
 
The graphics which follow depict several of the major sources of particulate matter.

 

47. Volcanoes (Mt. St. Helens, USA).

 

48. Dust Storms. Kansas (Photo 1) and over Ethiopia and the Red Sea (Photo 2).

 
 

49. Fires. Forest fire in California (Photo 1) and large forest fires in Florida (Photo 2, see the red color along the east coast).

 

50. Pollution. Air pollution over Mexico City (Photo 1). Oil fires in Kuwait (Photo 2).

 

51. The atmosphere is usually divided into four layers on the basis of temperature. The lower-most layer, the troposphere (tropo/to turn over or change), will be the focus of our attention. The troposphere extends upward some 10 to 12 miles over the Equator and gradually declines in height to some five to six miles over the poles. As one ascends into the troposphere, the temperature drops on average approximately 3.5 degrees F per 1000 feet. This temperature change is fairly consistent and is often called the environmental lapse rate, or sometimes the normal lapse rate. And it follows that temperatures will rise by 3.5 degrees F per 1000 feet as one descends toward the surface.
 
It is in the troposphere, and especially in the first 3000 to 5000 feet of the troposphere (the so-called Boundary Layer), that we find virtually all of our weather. This is where most of the water vapor is found as well as most of the particulate matter. Here are the clouds, the fronts and storms. You will often hear the troposphere referred to as the "Weather Sphere."
 
Above the troposphere, from 10 miles to about 30 miles above the Earth's surface, lies the stratosphere. Here temperatures begin to rise due to the presence of ozone. Little weather is to be found in the stratosphere as this layer contains very little moisture. While volcanic eruptions may throw particulate matter into this layer, little particulate matter is typically found at these altitudes. Because of the lack of clouds and storms, this is the preferred area for air travel. Think about the last time you flew. Looking out the window, most of the clouds (except for a few powerful thunderstorms) were below you.
 
The layers above the stratosphere (the mesosphere at 30 to 50 miles and the thermosphere at 50 to about 300 miles) are not meteorologically significant.
 
 
 

You have now completed Unit 2: Latitude. 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.

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