Breakdown: Why instability is to blame for thunderstorms

Published: Jan. 31, 2020 at 10:16 AM CST
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MEMPHIS, Tenn. (WMC) - Sometimes you may hear meteorologists talk about unstable air and you may have wondered, what is unstable air and how did it get that way. Let’s examine.

Air is considered unstable, in the lowest layers of an air mass when the air is warmer and or more humid than the surrounding air. When this occurs the air will rise, as that air parcel is warmer than the air surrounding it. In an unstable environment, the weather can change suddenly and can be violent.

When rising air, through expansion, cools to the point where some of the water vapor molecules compact together at a faster rate than they are torn apart, some of the water vapor condenses (change from a gas to liquid) and forms clouds. Condensation releases heat, which warms the air parcel and can cause the parcel to rise even higher. This is the basis for thunderstorm development.

The more unstable the air mass the more clouds you get. The more clouds you get the greater the chance of precipitation. The more unstable the air mass, the more energy there is for thunderstorms to feed off of. Thunderstorms transport the extra heat into the upper troposphere, and at the same time cool the lower troposphere. This combination will gradually reduce the instability of the air mass.

If an air mass is stable in the atmosphere, if air rises, it is usually colder than the surrounding air (due to expansion), causing it to sink down again. When the air is colder and sinks, we call this a stable air mass. Stable air means that the weather is likely calm but it doesn’t mean that the weather will be sunny. It could mean a steady rain or snowfall but no rapid changes.

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