rodbailey

Lifted Index
by Dixon White ( http://eagleparagliding.com )

This is the 4th article in a series on weather and flying pointers. Contact me or the USHGA for the previous articles which are important for following the thread of these discussions. Spend time with your flying group reviewing your experiences, knowledge and lessons. A recent local club discussion reminded members to be supportive in making wise flying decisions as opposed to challenging each to daredevil activities. A serious accident had recently occurred where there was some possibility that the pilot felt pressured into flying demanding conditions. Do your own homework on the weather and make your own decisions on whether to fly - you're the one who gets to either enjoy the flight or suffer through it.

The "lifted index" is the "thermal index" at the 18,000 ft msl level, which is generally higher than the realm we fly within - so we'll simply look at the thermal index, which is the surface puddles of heat compared to any level above the ground. When you gather temperatures at different AGL's you can chart how strong the thermals may develop. Air basically cools at 5.5f/1000ft. i.e. if it's 100f at sea level you'd expect air at 10,000ft above sea level to be 45f. If the "sounding" (see previous articles) shows that the air at 10,000ft was 35f then you have a -10 thermal index. The stronger the negative number, the stronger the thermals, usually.

Different ground surfaces heat differently - a golf course is pretty "cool" compared to an exposed dirt field. Take a thermometer and place it on different surfaces to get an idea of how much heat can develop on the ground - you'll be shocked. You'll find temperatures of 130f within 10 minutes midday on some "hot" surfaces. We take the forecasted high for the day and the temps we get from actually measuring the surface heat and split the difference to derive a "puddle" temperature basis, i.e. forecasted high for Denver is 85f and the dirt slope below Lookout Launch showed 115f after 10 minutes, so the puddle temperature would be around 100f. We'll use 6,000ft as our ground level and compare this 100f to a reported 32f at 12,000msl. This gives us a thermal index to 12,000msl of -35 (VERY STRONG!!). You will also want to note the barometric pressure level and keep some notes as to how the flying went for you, or your buddies. Didn't like the air because it was too bumpy? Learn to anticipate it by knowing the models of thermal index, pressure, and the other factors we discussed in previous articles. We'll talk about the "K" index in the next article.