A LAPSE FOR JUDGEMENT...

Severe weather forecasting is one of the most important tasks for any meteorologist. The list of severe parameters is seemingly endless, including a wide variety of acronyms such as LCL, LFC, EL, LI, CIN, and CAPE. It would take several blog entries to explain each of these terms in adequate detail. For now, we'll talk about about the most common gauge of severe potential- the "lapse rate".

Simply stated, the "lapse rate" is the rate of temperature change with height. Steep lapse rates indicate unstable air masses. After all, atmospheric instability is fueled by changes in temperature. The uneven heating of the earth's surface leads to variation in air pressure, wind fields, and (eventually) storm development.

When we think of changing temperatures we usually refer to horizontal changes (such as the difference in temperature between Burgaw and Wilmington). However, the "lapse rate" is more concerned with temperature change as you travel from sea-level upward into the atmosphere. Meteorologists often divide the atmosphere into imaginary layers and calculate temperature changes within each "slice".

For example, a meteorologist may calculate the change in temperature from 4,500 feet to 18,000 feet above sea-level. The numerical value of "lapse rate" can then become a measurable gauge of instability. A change of 6 degrees Celsius per 1000 kilometers indicates a fairly stable atmosphere, whereas a change of 8 degrees indicates a more unstable atmosphere, and so on. Such temperature changes may not sound like much, but even a few degrees can make the difference between quiet weather and severe storms.