INSTABILITY

You often hear weather forecasters talk about an unstable atmosphere and how it helps spawn showers and thunderstorms. Stability is one of the main factors that determine where showers and thunderstorms will develop and how strong they will become. Due to the principle of density, a parcel of air will rise if it is heated and becomes warmer than the surrounding environment. Unsaturated air cools at a rate of 5.5 F per 1000 feet of altitude. In an unstable atmosphere, the environment cools quicker than the parcel, allowing the parcel to continue to rise. On the other hand, a stable atmosphere cools slower with height than the parcel, causing the parcel to become colder than the surrounding air and sink.

Moisture can make what normally would be a stable atmosphere, unstable because of condensation. A saturated air parcel cools at a rate of only 3.3 F per 1000 feet of altitude. This is much slower than the unsaturated air parcel, and as a result, the rising, saturated parcel remains warmer than if condensation were not taking place. The difference in cooling rates is due to latent heat released by the condensing water.

Showers and thunderstorms develop in areas of instability where air parcels are allowed to rise uninhibited through the atmosphere. Any weather process that causes colder conditions aloft relative to warmer conditions at the surface will lead to an unstable atmosphere. Some of these weather processes include:

• Heating of the surface. Sunshine or low level warm air advection heats the surface and the air near it, making it relatively warmer than aloft.
• Cold advection aloft. Cold air moves into the higher altitudes and causes temperatures to cool faster than at the surface.
• Mixing of the air. Rising air lowers the temperatures toward the top of the layer while sinking air warms temperatures toward the bottom.
• Large scale rising of the air. A layer of air expands and becomes thicker as it rises. The upper part of the layer rises more than the bottom and cools more as a result. This creates colder temperatures aloft relative to the surface.
  

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