Unit 7: Atmospheric Stability and Instability
A Summary: Atmospheric Lapse Rates and Stability
We all know that air in the atmosphere is constantly circulating. Indeed, the troposphere’s convection currents are much like those in a kettle of boiling water. The earth, which absorbs radiant energy from the sun and then reradiates this as heat, is like the heating elements under the kettle. It warms the air, with the greatest effect on that air near the surface.
When gases are heated they expand, becoming less dense and lighter in weight. They are buoyed upward by surrounding denser gases —the reason that warm air rises.
When air expands without the addition of new heat, which is adiabatic expansion, it cools. This can be demonstrated by allowing air to rapidly escape from a balloon. The balloon will feel slightly cooler. Air loses temperature through expansion at a lapse rate determined by its moisture content.
Consider a day in desert country that has a surface air temperature (50° Fahrenheit, while the air at 1000 feet above the surface is 44° Fahrenheit. If a parcel of dry air over barren soils is heated to 51 Fahrenheit—making it warmer and lighter than surrounding air—it will begin to rise. Since the lapse rate for dry air is 5 1/2 Fahrenheit per 1000 feet, the parcel of air will drop in temperature to 45 1/2 Fahrenheit when it rises 1000 feet. The prevailing air temperature at this altitude is 44 degrees, thus the warmer parcel will continue to rise. Under such conditions, the atmosphere is said to be unstable. In converse, a stable atmosphere exists when a rising parcel of air reaches a height, where through expansion, it becomes cooler than surrounding air at that altitude and sinks back to its former position.
Updrafts that are sustained to high altitudes are called thermals. These rising columns of air are sought out by sailplane pilots and by eagles, hawks, and other soaring birds.
When rising moist air cools through expansion, it will eventually reach its dew point. At this point condensation of vapor into drops of water occurs, and latent heat is released. The release of heat through condensation warms the air, thus, temperature drops less rapidly in rising moist air than in rising dry air.
The moist adiabatic lapse rate is about 3 degrees Fahrenheit per 1000 feet as compared to the dry adiabatic lapse rate of 5 1/2° Fahrenheit per 1000 feet. Hot saturated air has a lapse rate even lower than 3° because this air holds much more water vapor, and more latent heat is released during condensation. Conversely, cold saturated air, holding little vapor, approximates the lapse rate of dry air.
Moisture condensing from a rising convection current or thermal usually becomes visible as cumulus clouds. These clouds have flat bases and cauliflower-like tops. Such cumulus clouds often mark the tops of thermals, serving as guideposts to pilots. A thermal rising to great heights will often result in a towering cumulonimbus, or thunderhead, if enough moisture is present in the atmosphere.
When saturated air descends, it compresses, warms and is no longer saturated. This descending moist air behaves thereafter as dry air—gaining temperature at the rate of 5 1/2 Fahrenheit per 1000 feet of fall.
Lapse rates and temperature conditions determine whether the atmosphere will be stable or unstable. Sometimes a warm layer of air may stagnate above cooler air, thus producing a temperature inversion. This creates an unusually stable state which stops the rise of cooler air from below. Such temperature inversions cause smog disasters when industrial smoke, soot and fumes are trapped below the warm air layer.
If you desire further information on the effects of stability/instability on fire behavior, you may wish to refer to U.S.D.A. Handbook 360 "Fire Weather", by M. J. Schroeder and C. C. Buck, 1970.