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Mon, Jul 17, 2006

ANN's Daily Aero-Tips (07.17.06): The Lifted Index In Action

Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network.

Aero-Tips 07.17.06

It's hot and humid -- will thunderstorms pop up?

I've scheduled to fly from Wichita to Fayetteville, Arkansas this evening. It was a warm and muggy morning, perfect for air mass thunderstorms in the afternoon. It's now 3 pm and the sky's still untainted blue... so what will it be like around 7 pm?

The Lifted Index (LI) is a measurement of atmospheric stability. Positive numbers indicate positive stability, i.e., air tends to remain stable and lifting action dampens itself out. A "0" LI means a neutrally stable atmosphere, so localized sources of lifting action (mountains, rising heat in very hot areas) may continue to build, but others may dampen themselves out. If the LI is negative the atmosphere is unstable, so any localized lifting action is enhanced. If unstable air is also moist, towering cumulus and thunderstorms are likely. Areas of a LI of -4 or lower often generate severe thunderstorms.

This morning's (7 am local) Lifted Analysis chart is shown above. At each reporting point, the LI is the upper of a pair of numbers separated by a line-looking like the numerator of a fraction. Notice in eastern Kansas and extreme northwestern Arkansas, where I'll be flying, the LI is positive-chances are I won't see any thunderstorm build-ups this evening.

While we're at it, look at the areas where negative LIs appear-central Kansas, Texas through extreme southern Arizona and New Mexico, and in a large but fairly distinct area defined by about the Mississippi River on the west through a Detroit to Washington DC line and just about everything south and east of there.

Now look at the radar picture above, valid at 2:15 pm local on the same day. There's that pocket of storms in west Kansas, mountainous areas of Arizona and New Mexico, large areas in Texas (including one cell with radar tops to 73,000 feet), and almost everywhere inside the defined area in the east.

Notably, see how my east Kansas-NW Arkansas route remains clear, and how distinctly the storm area ends north and east of the Detroit-DC line. The 2:37 pm local Convective SIGMETs and forecasts (below) closely follow the LI lines as well.

With this morning's Lifted Index information being accurately reflected in this afternoon's weather, there's a very good chance I'll make tonight's trip without having to deal with thunderstorms.

Aero-tip of the day: A quick look at the Lifted Index helps you make a strategic plan for dealing with thunderstorms later in the day.

FMI: Aero-Tips

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