Atmospheric Stability

The great ocean of air that is the atmosphere is far from quiescent. Place-to place heating differences make air flow horizontally (see winds and pressure systems) and also vertically. The vertical rise of air has particular significance because it is the rise of air that is associated with clouds and precipitation. How strong is the rise of air? The answer is that it depends on the stability of the atmosphere and that stability differs by location, season, and time of day. If lift is gentle or air is descending there is not likely to be significant precipitation. It is the strong ascent of unstable air that is associated with all of the world’s great storminess. The basis of understanding of atmospheric stability is rooted in the ready ability of air to compress and decompress. From physics, the Gas Law (Equation of State) holds that temperature, pressure, and density are interrelated. As air is heated and cooled, its density can change dramatically. In this regard, air is usually considered in small, discrete pieces known as air parcels. Air parcels are useful constructs with which to consider the thermodynamic properties of air. A few meters across, an air parcel has consistency of temperature, pressure, and density within and is compared to the air around it (known as the environmental air).

As air parcels rise they decompress; that is, the same mass takes up more volume and the density lowers. This decompression decreases the number of molecular collisions in the air parcel and causes a corresponding drop in temperature. Importantly, the temperature drop is the result of the spatial rearrangement of the molecules and not a loss of energy to the surrounding environmental air. This is known as the adiabatic process and is a profound influence on the temperature the rising and sinking air. Rising air parcels that are not saturated cool at the unsaturated (dry) adiabatic lapse rate of 10°C for every kilometer of ascent. In saturated air, the decrease of temperature averages 6°C for every kilometer (the saturated or wet adiabatic lapse rate). 

This retardation of the saturated adiabatic lapse rate is the result of the conversion of latent heat to sensible heat during the cooling of the air parcel. Descending air parcels warm at the unsaturated adiabatic lapse rate, whether or not they begin the descent saturated: descending air warms so that it soon becomes unsaturated. Air parcel temperatures can change dramatically in a few minutes because of adiabatic processes; similar temperature change because of gain or loss of radiant energy would take hours to occur. Consider an air parcel rising from the surface. The air parcel is lifted to altitude X and its temperature decreases at the appropriate adiabatic lapse rate. The air parcel at altitude X is less dense and warmer than the environmental air, so the air parcel will rise spontaneously, and this air is absolutely unstable. An air parcel already at altitude X has the same density as the environmental air, will remain at altitude X, and it has neutral stability. If an air parcel is denser and cooler than the environmental air at altitude X, the air parcel will sink spontaneously, and this air is termed absolutely stable. On average, the troposphere is absolutely stable because the average environmental lapse rate is less than the saturated and unsaturated adiabatic lapse rates. How, then, does the troposphere sometimes destabilize? 

The answer is straightforward: the environmental lapse rate is quite variable over short amounts of time. The atmosphere destabilizes as the environmental lapse rate increases to be greater than the adiabatic lapse rates. The environmental lapse rate might have been increased by warming near Earth’s surface or by cooling of the air aloft. Warming of the lower air is accomplished by air moving over a warm surface, and daytime heating by the influx of solar energy, or advection of warm air. Air aloft can be cooled by air and clouds radiating energy to space or by cold air advection resulting from disturbances in the jet stream. Moreover, both the warming of the lower air and cooling of air aloft can be simultaneous and result in rapid destabilization. Over the southern Great Plains of the United States maritime tropical air flows at low level and the polar front jet stream flows at the top of the troposphere. The combination makes for thousands of meters of instability and some of the largest thunderstorms on Earth. The presence of plentiful water vapor helps makes the air more unstable. Although air can be unstable with modest amounts of vapor, greater amounts of vapor make the air more unstable to greater altitude. The key is in the saturated adiabatic lapse rate. Moister air parcels become saturated with less lift than dryer parcels. The retardation of the cooling of a saturated parcel via the saturated environmental lapse rate keeps the rising air parcel warmer, less dense, and unstable to heights greater than possible in unsaturated air.