It seems that, in the summer, the Flight Service Station weather briefers always hedge their bets by adding ‘potential for afternoon thunderstorms’ onto every briefing. Is there a way to narrow down this blanket forecast?
Summertime is prime thunderstorm season because all the thunderstorm ingredients are present: heat, moist air, and lifting (unstable) action of the air. No wonder weather briefers add the threat of thunderstorm to almost every summer forecast. But pilots need more specific information to make safe decisions.
The solution: The ‘Lifted Index.’
Next time when getting weather information ask the briefer about the Lifted Index. Also many on-line and computer weather services also provide the LI on a Composite Moisture and Stability Chart. The Lifted Index is a measurement of how much stability is in the atmosphere. Remember the atmosphere must be unstable to produce thunderstorms. Air cools as it gets higher in the atmosphere. The standard cool-off rate is 3 degrees Celsius per thousand feet. If air were lifted from sea level to 18,000 feet the standard cool-off would be 54 degrees (3 x 18). The difference between this standard and what the actual air would do today is the Lifted Index. A positive number means that the air would be cooler than standard. Cool air, being heavy, would sink and the air would be stable. Cool, sinking, stable air resist thunderstorms. But a negative number means the air, if lifted to 18,000 feet, would be warmer than standard. Warm air, being lighter, will rise (like a hot air balloon) and this upward lifting is a clue to unstable air and thunderstorms.
Having a large negative Lifted Index number alone does not guarantee a thunderstorm. There must also be moisture present. The measure for moisture that meteorologists use a ‘K Index.’ K calculates temperature, moisture level, and saturation. The higher the value of K the higher the moisture potential. The Lifted Index and K Index are usually shown together with the Lifted number above a line and the K number below the line.
A Lifted Index of 0 to –2 and a K Index number of 0 to 15 will have a near zero chance of thunderstorms. But if Lifted is –6 or more while K is 40 or more then you have a near 100% chance of Airmass Thunderstorms popping up all afternoon. Use this as an extra tool to predict and avoid thunderstorm activity.