Sunday, June 04, 2006

Trigger temperature

Yesterday's lift was much better than the NWS forecast and my forecast spreadsheet indicated. Based on a max temp forecast of 100F, my spreadsheet had indicated that the thermal temperature would be very close to the lapse rate temp all the way up, with a soaring index of only -1. The NWS forecast only 126 fpm lift @ 5000' MSL and a trigger temp of 132 F.

But the actual high at Hemet as measured by our thermometer was 106F when I took off and 109F when I landed. So I went back to my spreadsheet and plugged in 106: that changes the thermal index to -4 @ 5000. 109 takes it to -6. Much better! With just a few degrees additional heating, the thermal line pulls away from the ambient line quite a bit. And my trigger temp calculation was 98F, vs. 132 by the NWS. So... I need to look into trigger temp calculations and see how the NWS and I differ. 109-98=11 degrees, clearly over the trigger. And thermals were definitely being triggered... all over the place!

So what did I learn? If my forecast indicates the trigger temp is near the max forecast temp, try a few different temperatures and see how the lift forecast changes. Then I'll be prepared to evaluate the lift possibility as the day's actual temperature progresses.

No comments: