Meteorology: Understanding the Atmosphere            Ackerman and Knox

Meteorologists use stability indices to quickly assess the susceptibility of the atmosphere to severe weather. Stability indices are a measure of the atmospheric static stability. The table below lists some of the commonly used stability indices. The indices are based on the stability concepts discussed above and in Chapters 3 and 10, and the fact that moist air is less dense than dry air (Box 2.1).

A Total Totals (TT) of greater than 48 indicates favorable conditions for development of severe thunderstorms, a value of 55 indicates favorable conditions for tornadoes. Notice that increasing the dew point at 850 mb or decreasing the temperature at 500 mb (temperatures at 500 mb are usually less than 0°C) makes the atmosphere more favorable for thunderstorm development.  Dry air over warm moist air is unstable.

Lifted Index (LI) has proved useful for indicating the likelihood of severe thunderstorms. The chances of a severe thunderstorm are best when the lifted index is less than or equal to -6. This is because air rising in these situations is much warmer than its surroundings and can accelerate rapidly and create tall, violent thunderstorms. Values less than -9 reflect extreme instability. An LI of between 0 and –2 indicates that there is a small chance of having a severe thunderstorm. Airmass thunderstorms can occur when the LI is slightly positive.

The K-Index has proved useful in indicating the probability of air mass thunderstorms. As the K-Index increases, so does the probability of having an airmass thunderstorm.

The Showalter Index is the simplest measure of the local static stability of the atmosphere. Positive values indicate the lifted parcel is colder than its new environment, and thus the atmosphere is stable. Cumulonimbus generally fail if the Showalter Index is greater than 4.

Remember, thunderstorms are likely to develop in regions where the stability indices are at critical levels AND an initial lifting mechanism exists. The lifting mechanism can be solar heating or air flow patterns.

Thermograph Applet

Use the mouse to drag the temperatures and dewpoints to new values; when you release the mouse, new stability values will be computed. The lifted parcel profile will appear in magenta. Adjust the temperature and dew point profiles to make the atmosphere unstable according to the different indices discussed above.