Pictographs, Canyonlands National Park, Utah
Air moves upward and downward in the atmosphere when blown over mountains and in response to thermals. It is easiest to explain what happens is if we examine things from the perspective of a small, imaginary, parcel of air. Think of holding the air in your hands and then letting go.
Pressure decreases with elevation in the atmosphere. When a parcel of air rises it expands as the pressure lowers. If no external energy is added or removed (adiabatic expansion) the expansion causes the air parcel to cool. The rate of cooling for dry air is about 0.98°C per 100 meters of rise (5.4°F/1000 ft). The cooling rate is called the dry adiabatic lapse rate. If the air parcel is moved downward it warms at the same rate.
Figure 1. Lifting Condensation Level, a) temperature changes with elevation at the dry adiabatic lapse rate, b) temperature changes with elevation at the wet adiabatic lapse rate. The bottom of the clouds is where the air becomes supersaturated with water vapor and moisture drops out.
The vapor pressure of water is temperature dependent. As the air cools it tends to become water saturated. Note that there are two effects here a) the air is expanding and thus diluting the water vapor and b) the air is cooling meaning it will hold less water vapor per unit volume. It turns out that the second effect is dominant, and water eventually begins to condense out of the air parcel in the form of a cloud.
The elevation where the cloud first begins to form is called the lifting condensation level.
Figure 2. Water content of air at 25, 50 75 and 100% relative humidity.
The lifting condensation level shown in Figure 1 is eventually reached if the parcel keeps rising. The lifting condensation level (LCL) is the point where the water vapor begins to condense and is visible as clouds. The LCL is the cause of the flat bottom on many clouds. As the air rises and falls, the water either condenses (cloud forms) or evaporates (cloud dissipates). The total water content of the air does not change, only the visibility of the water – either as invisible water vapor or visible water droplets (clouds).
A special example of the lifting condensation level is orographic clouds. Orographic clouds form when humid air blows over the top of the mountain. The air must rise to go over the mountain range. When the air rises it reaches its LCL and clouds form. On the downwind side of the mountains the air sinks back into the valley and warms. During warming the water droplets (i.e., clouds) evaporate into invisible water vapor. It is fascinating to watch orographic clouds and understand that a single cloud is not hanging onto the mountain range. Rather the cloud is rapidly forming and dissipating at the speed of the wind as air rises over the mountain range then sinks on the other side. The parcel of air suddenly becomes visible as it passes over the top of the mountains and clouds temporarily form.
Figure 3. Orographic clouds forming above the Franklin Mountains, El Paso, Texas. The wind is blowing toward the camera. The second set of clouds are lee wave clouds as the air rises in a standing wave set up by the mountains.
Figure 3 illustrates several phenomena. Two sets of clouds are present, one over the Franklin Mountains, and a second line of clouds some distance downwind from the mountains and parallel to the mountains. The lifting condensation level is visible at the base of both sets of clouds. The clouds over the mountains are orographic clouds caused when the air rising over the mountains cools below the condensation point on the upwind side of the mountain then re warms (cloud dissipates) on the downwind side of the mountain (wind was blowing toward the camera when picture was taken). The line of clouds downwind of the mountain is evidence of a lee wave. When air passes over the mountains a series of standing waves are created downwind. When atmospheric conditions are just right, as they were for this picture, the air rises above the LCL in the standing wave, marking it with a line of clouds.
When water condenses to form clouds the latent heat of vaporization of water is released. The release of latent heat tends to warm the parcel (actually slow its cooling). The slope of the cooling of the air parcel changes from the dry adiabatic to the wet adiabatic lapse rate at the condensation point. The wet adiabatic lapse rate is about 0.36 to 0.55°C per 100 meters of rise (2-3°F per 1000 feet of rise). The precise value depends upon the temperature. The major point to remember is that the wet adiabatic lapse rate is lower because of the release of energy – the latent heat of vaporization. The latent heat of vaporization is the power that drives the immense energy in thunderstorms. Under some humid, warm conditions a moist air mass rises to the LCL. This releases the latent heat of vaporization. Because the wet adiabatic lapse rate is lower, the moist air is likely to be warmer than the surrounding dry air. This causes the air mass to continue rising and releasing more latent heat and rainfall, viola, a thunderstorm is born!
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Copyright 2014John Walton