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Continentality

Reading Assignment
The textbook for this course is entitled: eWeather and Climate by Netoff, Gillespie, Fujimoto-Strait and Tiller. Upon purchase of the Lab Manual for the Course GEO 1401/Lab, you will be provided a link to a downloadable pdf which will serve as the textbook for this course GEOG 1401/Lecture. The Lab Manual/cd text is only available for purchase online at: bearkatsonline.com.
 
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. For the first exam we are taking a look at the two primary climatic controls responsible for heat: Latitude and Continentality. In the previous section on Latitude, we discussed Sun angle and time as factors in the receipt of solar energy by the Earth. In addition, we took a look at the concepts of radiation, conduction and convection.
 
In this unit we will take up the second climatic control related to heating of the Earth's surface -- Continentality.

 

02. As you will remember from the previous unit, the Earth's surface significantly influences the air temperature. Thus, if the Earth's land surfaces do not heat up and cool off at the same rate as water surfaces, one could expect such a difference to have a very substantial impact on air temperature. And in fact land and water do not heat up or cool off at the same rate. This notion of differential heating and cooling is called Continentality and this concept will be the topic for discussion in this section. A couple of examples to illustrate the point follow.
 
Suppose it is August 15 and you and I are driving across a large shopping center parking lot. I suddenly open the door and throw you out in your bare feet. The high Sun (radiation), long days and minimal albedo have made the parking lot surface an inferno. You begin jumping and hopping around (conduction works fast!). Spying a puddle of water, you head for it to soak your scortching feet. I don't get it! The Sun is beating down on both the parking lot and water puddle with equal intensity, yet you find the water to be cooler (I do understand the fact that the water is soaking up less heat than the parking lot and that there is some cooling related to evaporation, but still …). What's at work here?
 
Or how about a short trip to Lake Livingston east of Huntsville. The date is June 1. The temperature as we leave Huntsville is 90 degrees F (why is it warm?). As we cross the bridge over the lake, you decide to go for a swim. You dive off the bridge (illegal, of course), hit the water and are immediately yelling for rescue and a blanket. The land may be warm, but not the water. Again, what's at work here?
 
Now, while the parking lot situation might not have much effect on the weather, such could not be said of something as large as Lake Livingston. If in June the lake water is substantially cooler than 90 degrees F, then the air over the lake will be cooler as well. And since many people do not like heat, we find that lakes are especially popular places for summer homes. The homeowner may be interested in water-related activities, but more likely than not they just want to get away from the heat. What's at work here? Why do water and land, both receiving the same amount of heat energy, not respond in the same way with respect to heat?
 
Continentality is what is going on here. Let's take a look at the five factors the cause this differential heating and cooling of land and water.
 
 
 

03. Specific Heat. One reason land and water do not heat up and cool off at the same rate has to do with the nature of the substances themselves. They have different specific heat values. Specific heat has to do with how a substance responds to the input/output of heat. We could define specific heat as the heat required to raise the temperature of 1 gram of a substance 1 degree Celsius (C). From the table below we can see that the specific heat benchmark (pure water) has a specific heat value of 1.0. What this means is that 1 calorie of heat applied to 1 gram of pure water will raise the temperature of that water 1 degree C.
 
Now take a look at the substance concrete. The specific heat of concrete is .20. That means that if you apply .20 calories of heat to 1 gram of concrete you will raise the temperature of that gram of concrete 1 degree C. Or put another way, if you were to apply 1 calorie of heat to the 1 gram of concrete, you would raise the temperature of the gram of concrete 5 degrees C! Or put still another way, concrete will heat up (and cool off) five times faster than pure water.
 
You can see from the chart that various substances have different specific heat values. For purposes of example, let us say that dirt (just general dirt) has a specific heat of .33. Now of course different kinds of dirt in reality have different specific heat values. But we just want to establish a value we can make some generalizations with. Assuming general dirt to have a specific heat value of .33, what we are saying is that dirt heats up and cools off three times faster than water.
 
One reason then that Lake Livingston is cooler in June than the surrounding land is that the specific heat value of water (dirty water in this case, not pure) is greater than that of the adjacent land, thus the land (and as a result the air above the land) is going to respond more rapidly than the lake water to the rising Sun angle and longer periods of daylight associated with late spring and early summer days.
 
 

04. In addition to specific heat, we need to consider the opacity of land and water.
 
Land is Opaque. You can't see (and light won't shine) through very much dirt. Thus, sunlight shining on land has a difficult time penetrating very far. What heat energy is received at the surface is then transferred slowly downward via the process of conduction. And the process of conduction is not all that efficient. Think about how far you would have to dig a hole into the ground on August 15th before the ground would begin to feel cool to the touch. What -- maybe a foot? What this means is that all of the heat energy being put to the Earth's surface by the Sun is being concentrated in the top foot or so of the Earth. And remember, in order to raise the air temperature to 90 degrees F, we first have to raise the temperature of the surface of that one foot of the land surface to 90 degrees F. If all of the Sun's heat energy is being concentrated in the upper-most foot of the surface, can you see that it will not take very long to raise the temperature of the Earth to 90 degrees F. And once this is accomplished, the surface can then begin to raise the temperature of the air above it to 90 degrees F.
 
Water is Translucent. Water on the other hand is translucent. It's not opaque like land, nor is it totally transparent. And of course some water is more translucent that others. The clear waters of the Texas Lake-country rivers is a far cry from the muddy Mississippi (or a small farm pond). Note on the graphic below, that sunlight can penetrate much deeper (maybe 10 or even 20 or more feet) than was the situation with land. Let's say for this discussion that light penetrates a water body 10 feet. This alone means that compared to the surrounding land, a pond will take 10 times longer to heat to a similar temperature. And remember, in the real world, we also have to include the specific heat values of the two substances. So can we see that to raise the temperature of a 10 foot deep water body might well take 30 times the heat energy that it would take to heat land to the same temperature?
 
Water is Mobile. And one other aside regarding water. Unless you are in Los Angeles or San Francisco, the land surface does not move. Such is not the case with water. Water is highly mobile -- constantly mixing upward and downward. So while heat may penetrate 10 feet into a water body, in fact one might well end up having to heat a much greater depth due to the mobility of the water.
 
 

05.
Evaporation. Finally there is evaporation. Evaporation (a cooling process) is greater over water than over land. Because energy is needed for the evaporation process, the energy used is not available for heating.
 
The graphic below summarizes the five factors influencing the differential heating of land and water
 
 
 

06. Consider the graphics below of the Northern and Southern Hemispheres. The Northern Hemisphere is comprised of approximately 61 percent water vs 39 percent land, whereas the Southern Hemisphere is approximately 81 percent water and 19 percent land.
 
Now, which of these hemispheres is most likely to have the greater annual temperature range? Yes, the Northern Hemisphere -- because it has more land area. As a result, summers will be warmer and winters cooler in the Northern Hemisphere when compared to those of the Southern Hemisphere.
 
    
 

07. Below is a climate map of the world from your textbook. Note that, except for Antarctica, there are no cold Continental or Polar climates in the Southern Hemisphere. This is no doubt due in part to the greater presence of water (and its temperature-moderating influence) in the Southern Hemisphere.
 
But there is another Continentality-related factor at work here as well that contributes to the more moderate temperatures found on Southern Hemisphere landmasses. You will also observe that as you go poleward in the Northern Hemisphere the landmasses become larger, but as you move poleward in the Southern Hemisphere, the landmasses are for the most part actually narrowing. What we have then is not only more water in the Southern Hemisphere moderating the temperatures, but we also find as we move poleward that, instead of encountering what we would expect to be increasingly colder climates, we in fact find that the narrowing landmasses and increase in water is actually moderating the temperatures found at higher latitudes.
 
 
 

08. If you were to ask the average person on the street what time of day is the coldest, most would probably indicate the period just before sunrise. This might seem strange since we know that the Sun is actually most distant from us (180 degrees around on the other side of the Earth) at 12:00AM or midnight. Their answer is, of course, most likely based on their personal experience. And they are almost right, but not quite. From the graphic below and our discussion to this point, I suspect you might be able to provide them a better answer.
 
Let's assume that we have an equinox (12 hours of day, 12 of night with the Sun rising at 6:00AM, rising to its highest point in the sky at 12 noon and setting at 6:00PM) -- see the line labeled insolation on the graphic below. Note the second line on the graphic labeled temperature. As you can see, it continues to fall after the Sun has risen. It doesn't begin to rise until maybe 6:15 or 6:30AM. Why? Keep in mind that before the air temperature can rise, we have to heat the surface. The Sun may come up at 6:00AM, but it is going to take some time for the Sun to warm the land, and the land in turn to warm the air above it. Remember air temperature is measured at approximately five feet off the ground, and air is a poor conductor of heat.
 
Note the surface continues to heat the air several hours past the time the Sun reaches it highest point (and the time of peak insolation) at 12:00 noon. This difference between the insolation peak and the time of greatest air temperature (usually between 2 and 4:00PM in the afternoon) is called the daily lag (or march) of temperature. We are all familiar with this fact, and this should serve as a reminder to us that Continentality involves the heating first of the surface, and then of the air above.
 
Finally, we can see from the 2:00 to 4:00PM temperature peak, air temperature falls steadily, past sunset, past midnight and past sunrise, only to begin rising again once the rising Sun has heated the land which in turn heats the overlying air.
 
While the above discussion would describe the typical day, variations can and do occur. Most such variations are caused by general cloud cover, the passage of weather disturbances and/or location along coasts subject to onshore and offshore breezes.
 
 
 

09. The graphic below depicts the seasonal lag of temperature concept. This is essentially the same idea as just discussed with respect to the daily lag of temperature. It would seem like it ought to be coldest in December when the Sun is most distant from us here in Texas (you remember it is at 23.5 degrees S on December 21). Instead, we typically experience our coldest temperatures in January or February. And one might think it would be warmest in June when the Sun is overhead at 23.5 degrees N, yet our warmest month in Texas tends to be either July or August.
 
What we see at work here is Continentality on a seasonal basis. As with the daily temperature lag, we cannot begin to heat the air until the surface below it is heated. Here the lag is measured in weeks rather than hours, but the concept is the same. Use the accompanying table and compare the average temperatures for the months of June and December with the actual high and low average monthly temperatures.
 
 
    
 

10. Earlier we considered a map that depicted the distribution of temperature across the United States using isotherms (lines of equal temperature) and colors to give you a better idea of the nation's temperature pattern. Take a look at the graphic below. Here we have an isotherm -- let's call it the 60 degree F isotherm for the purposes of discussion (all points along the line/isotherm have a temperature of 60 degrees F). Out over the ocean at 40 degrees N, the air temperature is 60 degrees F. As we trace the isotherm in off the water onto the land in summer (July) we can see that we have to go poleward to maintain a 60 degree F temperature reading. Because land heats up faster than water in the summer, if we were to continue straight east along the 40th parallel, we may well find the temperature over the land to be 75 or 80 degrees F or more. In order to maintain a 60 degree F temperature, we have to swing north (poleward) to areas where the Sun's rays are less intense.
 
Note the reverse situation in January. At this time of the year, the water is warmer than the land. Thus, to find warmer temperatures (and to maintain the 60 degree F isotherm), we have to swing south (Equatorward) over the land (which is colder at this time of the year than the water due to a lower Sun ray and shorter days).
 
 
 

11. Finally, let's take a look at several locations on the North American continent and compare their annual ranges of temperatures. You can see that Omaha, Nebraska, with an annual temperature range of 56 degrees F, is located more or less in the center of the lower 48 states. Compare this with the 70 degree F annual temperature range of Winnipeg, Canada, a city located in roughly the center of the North American landmass. While Winnipeg's climate is more extreme than that of Omaha, both can be said to have a continental, or land-dominated climate. As you might suspect, the temperatures of a continental climate would be characterized by warm to hot summers and cool to cold winters -- the widely varying seasonal temperatures the result of intense heating and cooling of the adjacent landmasses.
 
Now contrast the temperatures of these two locations with those of Miami (annual temperature range of 14 degrees F) and San Francisco (annual temperature range of 8 degrees F). Both Miami and San Francisco reflect the influence of the adjacent water by their rather narrow temperature ranges. Both of these locations can be said to have a marine, or water-dominated climate. Because of the proximity to water, such climates will experience cooler summers and warmer winters than locations at similar latitudes further inland.
 
 
 

12. Taken together then, Continentality causes water to warm more slowly than land when exposed to high amounts of solar radiation, to cool more slowly than land and to store more heat than land. Specific heat, opacity of the land, the translucent nature of water, water's mobility and evaporation are all factors to be considered. Too, always keep in mind that the temperature of the air is directly related to the surface beneath it.

 

You have now completed Unit 3: Continentality. You might wish to check your knowledge of the material presented in this section by working through the Short Answer Review Questions, Multiple Choice/True-False Quiz Questions and the Drop-Down Statements available for Exam 1.To return to the top of the page.

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