When surrounding air cools relatively slowly with altitude, the air is stable (left). Stable air stops rising after it's no longer being pushed. That initial push can come from being at the center of an area of low pressure at ground level, when wind pushes air over a mountain, or when warm air rides over colder air (right).
Illustration by Charles Floyd
With spring comes a transition from predominantly stable atmospheric conditions to unstable conditions. An unstable atmosphere can set the stage for thunderstorms and turbulence as the air moves up and down. A stable atmosphere gives you smoother rides but can cause poorer visibility than an unstable atmosphere, in which the up-and-down movements carry air pollution and natural haze to higher altitudes.
The atmosphere at a particular time and place is unstable when air that’s pushed up continues rising after it’s no longer being pushed. In a stable atmosphere, air no longer continues rising after the initial shove ends. The upward shove can consist of air rising as it warms or an arriving cold front pushing the air up.
As air rises, it cools at a constant rate. Air rises as long as it is warmer—thus lighter—than the surrounding air. In an unstable atmosphere, rising air remains warmer than the surrounding atmosphere to a high altitude. A stable atmosphere is one in which the rising air grows cooler than the surrounding air and thus stops rising at a relatively low altitude.
To understand atmospheric stability, you need to understand the term “lapse rate,” which is sometimes confusing because it’s used in two ways. The environmental lapse rate is the change in air temperature when the air is neither rising nor sinking. Weather balloons measure this. The adiabatic lapse rate is the decrease in temperature of air that’s rising or the increase in temperature of air that’s sinking. The surrounding air temperatures don’t affect this change.
Jack Williams is an instrument-rated pilot and author of the National Geographic Pocket Guide to the Weather of North America.
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