Atmospheric Stability and Instability

Stability (or atmospheric stability) refers to airs tendency to either rise and create storms (instability), or to resist vertical movement (stability). The simplest way to understand how stability works is to imagine a parcel of air having a thin, flexible cover that allows it to expand but prevents the air inside from mixing with the surrounding air—as is true of a party balloon. Next, imagine that we take the balloon and force it up into the atmosphere. Since air pressure decreases with altitude, the balloon will relax and expand, and its temperature will therefore decrease. If the parcel were cooler than the surrounding air, it would be heavier (since cool air is denser than warm air); and if allowed to do so, it would sink back down to the ground. Air of this type is said to be stable. On the other hand, if we lifted our imaginary balloon and the air within it was warmer, and hence, less dense than its surrounding air, it would continue to rise until it reached a point where its temperature and that of its surroundings were equal. This type of air is classified as unstable. Lapse Rates: A Measure of Stability But meteorologists dont have to watch a balloons behavior every time they want to know atmospheric stability. They can arrive at the same answer simply by measuring the actual air temperature at various heights; this measure is called the environmental lapse rate (the term lapse having to do with temperatures decline). If the environmental lapse rate is steep—as is true when the air near the ground is significantly warmer than the air aloft—then one knows the atmosphere is unstable. But if the lapse rate is small, meaning theres relatively little change in temperature, its a good indication of a stable atmosphere. The most stable conditions occur during a temperature inversion when temperature increases (rather than decreases) with height. The easiest way to determine atmospheric stability at a glance is by using an atmospheric sounding. Edited by Tiffany Means.