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August 04, 2012 at 04:34 AM EDT | No Comments
The recent heat wave that seems to have overtaken the country is affecting all of us. I haven't flown anywhere near my normal routine, because of the heat, and the resulting instability of the air. I've had a couple of rough experiences with thermals already this year, and don't particularly want to get caught in another.
Since I didn't fly this morning (probably should have), I was catching up on teaching materials and reading. There's a very interesting article in the August 2012 edition of AOPA Pilot - a part of the Weather Watch series, it always merits a first read when I open the magazine. This month's article talks about stability guidelines, and gives a couple of excellent resources that I'm sharing with my students, and thought it worth passing along to the group.
The article discusses Lifted Index and CAPE (Convective Available Potential Energy) Index in some detail, and provides probably the best description of what those mean on a pretty easy to understand level. For those who don't get the AOPA magazine (and you really should - lots of great stuff every month, particularly for the active instructors), it says:
CAPE index values =
Less than 300: Mostly stable, little or no convection
300-1,000 marginally unstable; weak thunderstorm activity
1,000 – 2,500: Moderately unstable; possible severe thunderstorms
2,500 – 3,500: Very unstable; very severe thunderstorms; possible tornadoes
3,500 – plus: Extremely unstable; severe thunderstorms; tornadoes likely
Lifted Index values:
Greater than 2: No significant activity
2 to 0: Showers/thunderstorms possible with other source of lift
0 to minus 2: Thunderstorms possible
Minus 2 to minus 4: Thunderstorms probable, only a few severe
Less than minus 4: Severe thunderstorms possible
The CAPE index and Lifted Index are both found on the RUC soundings, in the data in the upper right hand corner of the graphic.
There are also a number of other sources, such as http://www.emc.ncep.noaa.gov/mmb/namsvrfcst/lift.animate.html for the Lifted Index, and for the CAPE, have a look at the Twister website hosted by Ohio State University (http://twister.sbs.ohio-state.edu/imageg.php?dispimg=severe/cape&imgname=CAPE)
Why is this important to you? Well, witness the accident in central Georgia, back in March of this year. As an observation, the CAPE index that day was 2719, and the Lifted Index was -7. Definitely something to consider.