This article was written in response to two articles that were printed in the Paragliding Magazine about Thermals in the earlier part of 2002.
Thank-you Peter Grey and Dennis Pagan. You stimulate our sometimes sluggish and complacent minds.
I would like to throw my non-scientific, but experience referenced thoughts on the subject in the ring anyway. Stimulating conversations, like yours, and my observations of thermals through being in them and viewing their shapes have led me to reach the following always changing and useful (to me) conclusions about thermals.
Three generally shaped categories are useful to me as a foot launched, non-motorized, pilot: pancake, cone, and snake. Each category creation has to do with energy reserve and inertia. How much and how quickly has the heating been happening, is it happening now and how long and strong is it continuing to happen? The energy of a thermal of course has to do with more than just heating. Otherwise we would only have to check the temperature differential between degrees and amount of time to figure out if it is going to be a good thermal day. But it is not just lapse rate that we check. We also check the jet stream and pressure and moisture content, top of the lit and lifted index and k index, look at the sky, feel the cycles etc.. There are also many things we don’t understand enough about yet to even understand that we should be checking it, or how to check it or what “it” is. If this were not true we would be 100% accurate on our thermal predictions every day. I have not yet met the pilot who can do that.
- Amoebae Pancake: inertia is weakening rather than strengthening. Relatively weak energy reserve (heating) and inertia. Usually too weak to be useful to us except as a welcomed extension to our sled ride but occasionally useable as with the end of the day glass off lifty conditions we sometimes know and love or cloud base flying at or near “0” sink. etc… Cloud base “0” sink flying is one of the rare times when we can “see” the pancake model. The lift has mixed and weakened enough to become the pancake model similar to Peter’s drawn model we saw several months ago.
- Cone shaped model: Moderate thermals – energy reserve is great enough to create a moderate and comfortable (for us) strengthening of the thermal lift (ice cream cone shaped). These are the thermals we find the most useful. I’m guessing 150 to 1500 or more up would usually, but not always, be somewhat cone shaped. (How is that for ambiguously taking a stab at defining a usually non visual experience, gutsy hey?) The best visual model of this I have ever seen was in central Utah on a flight from Frisco Peak in ’95. Over the flats I could not get more than 3000 feet above the ground with 800 ft/min max and bumpy. After 3 hours I came to an area that had recently burned in a brush fire. The black ash was so light that every thermal in the entire area was blackened in the shape of a cone (one of the many great photos I have failed to get in this sport. The above was drawn by Annie Doryk). Most of the cones were 1000 ft to 1700 ft tall and then the ash dispersed to the point where I could not see it. They were of remarkably similar size, shape and color and were somewhat evenly spaced approximately 100 to 500 or so yards apart. I did not measure any of this and am just guessing from memory so take it for what it is worth. We have all seen this as dust devils when the lift is stronger and/or when the debris is lighter as with this ash or the dirt on the flats near Chelan, WA. This ash thermal view was the only time I have ever been able to see what I think was every thermal, at least 40 and probably over 50, in a relatively large area, 2 by 4 miles.
- Snake shaped model: great energy, reserves and inertia. As thermals get stronger 1500 ft to 2 or 3000 ft/min they usually become more snake shaped and have sharper edges. We get these at Snowbird UT in the summer and we sometimes call then gravel devils. These snakes only happen there when the lift is very strong and comes up from all sides of the 3-sided peak, mixing near the middle of the 200-ft. diameter flat top.
These 3 models are useful to me whether they are “real” or not. I have seen, felt and experienced them being useful to me while flying. There may be other additional or replacement models that I find more useful next week. I certainly hope we all continue to improve our modeling so that we don’t stagnate. We are grateful to you Peter and Dennis for stirring our sometimes-stagnant thermal pond.
I find the triggering model useful also but let’s see what someone else has to say about that.