Sunday, January 07, 2007

OAT

Today I was thinking about the temperature inversion that was forecast for yesterday. Each flying day as I'm preparing, I look at (among other things), the atmospheric temperature forecast from NOAA's NAM site. The closest forecast to Hemet is for San Diego (I think Miramar NAS), so I don't quite believe it, but it's the best I can find. If an inversion is forecast, then I don't hope for too much out of the day.

Sometimes I can see evidence of an inversion, a haze layer with a defined top. Sometimes it seems to correspond to the forecast inversion top, but I've never paid too much attention. I've sometimes wondered whether the inversion develops locally as forecast. The local commercial operator could, I suppose, take temperature soundings as they tow or go up for a morning checkout flight, but I don't think they do.

Today it hit me. Duh! The PW-5 can display Outside Air Temperature (OAT) on its flight computer LCD. It displays in degrees Celsius, just like the DUATS and NOAA sounding forecasts that I use for my daily thermal forecast. I've never used it, I leave the computer set to display average lift. But I could use OAT to confirm or refute the sounding forecasts during my climb! So... I should pay attention to the forecast temperature gradient, and then keep an eye on the OAT during the tow. That should tell me whether there is or is not an inversion... whether the air is or is not stable.

Anyone else do this?

2 comments:

Anonymous said...

Worth a try. Though my experience suggests most OATs probably won't be accurate/fast enough to detect weak inversions. Too bad you can't record them electronically for later analysis.

Roger said...

Actually, I might be able to. The PW5 has a Borgelt B50 flight computer, talking to a Volkslogger, and I'm sending the output to my IPAQ running SeeYou Mobile. (I've tested it but have not flown with it yet.) The B50 manual says "later models" send the OAT in the NMEA data stream. So if it works, and CU can display the OAT, maybe I can record a temperature profile. It'll be some weeks before I try it.