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Elevation
Reading Assignment
- Gillespie, Netoff and Tiller, eWeather & Climate, as applicable (also note the search function on the CD).
01. The fourth climatic control we will explore is that of Elevation. All of us who have been into high mountains can attest to the influence of this control. In fact, you don't even have to have been in mountains to have seen this control at work. A drive from Texas to Los Angeles will take you through the deserts of the southwestern United States. Here it is possible at certain times of the year to be driving in a dry, desolate desert and to see in the distance mountains covered with trees and often snow. And in many areas of the world (including the Equator), it is possible to ascend to such heights as to have snow cover the year round.
02. Central to the idea of Elevation as a climatic control is the notion of air density. Air pressure and density decrease with increased height above sea level. As the air gets "thinner" it tends to get cooler. Too, keep in mind that as you move higher into the atmosphere, you are getting further away from the general land/water surface which is responsible for much of the heating and cooling of the lower-most levels of the atmosphere.
03. Mountainous areas are particularly sensitive to increases or decreases in solar radiation. In higher elevations, the Sun rapidly heats the slopes via radiation. Out over the valley at the same elevation, the air tends to be considerably cooler -- there being no land to heat the air via conduction save for the general land surface which may be many thousands of feet below.
04. And at night when the Sun goes down, heat rapidly leaves the mountain slopes -- the "thinner" air at these higher elevations offering little in the way of insulation. As a result, temperatures typically drop rapidly due to infrared heat loss.
05. There is a general tendency for the temperature to drop an average of 3.5 degrees F per 1000 feet as we ascend into the atmosphere, and to rise an average of 3.5 degrees F per 1000 feet as we move back toward the Earth's surface. This temperature change is known as the Environmental (sometimes referred to as the Average or Normal) Lapse Rate. Thus we can drive through a desert surrounded by mountains covered with trees and sometimes even snow and ice (depending upon the height of the mountains). By the same token, while much of the western United States in dry and relatively warm, places like Death Valley, California (some 280 feet below sea level) routinely reach temperatures over 120 degrees F in summer duein part to the lack of elevation.
06. Nocturnal Inversions. In the six graphics that follow, we will elaborate on those factors that tend to cause inversions. Let's begin with a graphic of a typical nocturnal inversion. As you can readily see, instead of the air temperature falling with increases in elevation, the temperature is cold at the surface, it increases, sometimes rather abruptly, a few feet off the ground, and then resumes the "normal" pattern of gradual decreases as one moves higher into the atmosphere. A number of factors can contribute to the formation of inversions.
07. One of the most prominent factors causing inversions is the presence of clear skies. Where we find clear skies at night, there is little to impede the loss of heat from the surface. Lack of cloud cover will cause the ground to cool quickly by permitting infrared radiation to "escape" more rapidly into the atmosphere and to in turn chill (by conduction) the air immediately above the surface. And remember throughout this discussion that air is a poor conductor of heat, so that the heat loss experienced at the surface, while chilling the air immediately above the surface via conduction, will have relatively little effect on the air temperatures at higher altitudes.
08. The cooling of the surface is accelerated where the air is not only clear, but also relatively dry. As the amount of water vapor is decreased in the atmosphere, less terrestrial radiation is absorbed by the atmosphere. Once the Sun goes down at night, the temperature drops rapidly as the heat escapes into the higher levels of the atmosphere -- there being relatively little water vapor to absorb the escaping terrestrial radiation.
09. Inversions are more common during the late fall, winter and early spring months. Here we find our shortest days and longest nights. The short days and low Sun angles mean relatively little opportunity for the surface to soak up a great deal of heat energy during the day. The long nights mean there is ample time for the heat to leave the surface and cool the air above.
10. The lack of wind is especially important in the formation of nocturnal inversions. With calm conditions, there is little opportunity for the warmer overlying air to be mixed to the surface. As the air is chilled, this relatively dense air tends to hug the surface.
11. While not always present, snow-covered surfaces will also encourage the formation of inversions in two ways. First, the snow cover tends to reflect a great deal of the incoming solar radiation during the short daylight period, thus robbing the surface of an opportunity to warm; and secondly the snow cover acts as an insulating blanket preventing heat from the land from reaching the surface where it could warm the air. Keep in mind when looking at a six inch snow cover that much of that six inches is actually air (a poor conductor of heat).
12. Upper Air Inversions. The second type of inversion, the upper air inversion, is responsible for many of the world's great desert areas.
13. For our example, assume a location of 30 degrees N. The air over the Sahara Desert cools at the Environmental Lapse Rate average of 3.5 degrees F per 1000 feet. At the same time, keep in mind that air that was ascending over the Equator (the ITCZ) is now descending (subsiding) at about 30 degrees N/S. As this air descends it warms. The graphic below indicates what happens when these two columns of air meet. While warming as it is descending, the upper air (though very cold) remains warmer that the air beneath it which is cooling at the Environmental Lapse Rate. The result is an inversion -- warmer air overlying cold air. In this case, not at the surface, but aloft many thousands of feet above the desert floor. The descending air acts as a cap, preventing any air located beneath it from rising.
You have now completed Unit 5: Elevation. 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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