Saturday, November 04, 2006

Thermal forecast spreadsheet

A few months ago I created an Excel spreadsheet to produce the standard thermal forecast chart. In training you learn how to take the forecast high temperature on the ground and the standard lapse rate, compare it to the forecast "soundings" at 3,000' intervals, and calculate a probable maximum thermal height and thermal index. This gets tedious, and would take a lot of time in the morning when preparing for a day of soaring. There are various resources on the web that present this information in one way or another, but not in an easy-to-take-along format.

So, being a computer programmer, I naturally thought of automating it. And since I thought others might want to use it, I wanted a platform that could be easily shared. I decided that Microsoft Excel would be suitable - especially since it has graphing capabilities. It took me a couple of days to work it out, and a couple of versions to get the bugs out. I offered it to people on my club's mailing list and a few took me up on it.

The NWS does not prepare a sounding forecast for Hemet. The closest ones available are for Miramar NAS near San Diego and for Ontario. They're about equidistant from Hemet, and I think the Miramar forecast would have too much ocean influence, so I built my program to use both and calculate an average. It seems to work pretty well. I still use it, every soaring day.

One thing I learned is that just a few degrees of difference at the ground level can cause a huge difference in thermal height. There's a "trigger temperature" at which thermal production should start. Often the forecast maximum ground level temperature is close to the trigger temp, which would mean little or no lift. Just a few more degrees means that the thermals can go thousands of feet higher. Playing with the spreadsheet really shows this. I've always looked at a lot of weather info on each soaring day, and one thing I've noticed is that the forecast temps for Hemet are almost always lower than the maximum we see at the airport. I look at the newspaper (I don't know their source), and Weather Underground, and the NWS. They're all low a lot of the time. Today the forecast was 76F to 78F, and the actual high on our thermometer (4 ft from ground level, in the shade, out of the wind) was 87F.

So I've learned to be a little more optimistic than my thermal forecast indicates.

I don't think there's any way to upload files to this blog site. If anyone wants a copy of my graphing thermal forecast spreadsheet, please reply to this post with your email address, and I'll send it to you. It is not totally generic, it's specific for our field's 1500' MSL altitude. If you want to work with a different field elevation, you'd have to study and adjust some formulas.


Anonymous said...

Roger, I would like to take a look at the spreadsheet you developed. I live in Florida though.
As far as your actual temperature readings in Hemet. I believe they will only be accurate if the thermometers are housed within an instrument shelter. White wood structure with louvered siding.Elevated above a grass surface. The ground underneath the thermometer could be generating additional heat at the level you are taking the readings. Concrete surfaces would effect it greatly I am sure.Just my thinking.
Thank You

Roger said...

To the person at I tried to send you the spreadsheet but your email bounced. Please comment again and double-check your address.

Anonymous said...

Roger I just stumbled on your blog and would be interested in looking at your excel spreadsheet.

Thanks in Advance
Austin, TX