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Oct 30, 2002, 06:13 PM
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Botterii's Avatar

thermal question


today while walking the dog at a local lake i observed a red tail hawk flapping upwind toward the leeward edge of the lake. there was a light wind (<5mph), heavy low clouds (no sun all day), 8" of snow on the ground, and it was near 20 degrees F. i proceeded to watch the hawk begin 'thermaling' about 100' above the downwind edge of the lake. it was turning fairly tight flat circles all the while rising and drifting downwind as we've all seen birds of prey do on those great warm sunny days. i watched the hawk speck out all the while wondering how in the h#!# this bird could be doing this on such a cold dreary day. then it occured to me that maybe the difference in temperature between the lake water (still relatively warm from summer) and the air tempurature created a bubble of rising warm air that the redtail was rising on. any thoughts on this?

Botterii
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Oct 30, 2002, 08:38 PM
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Daemon's Avatar
You got it in one.
All that's needed to create a thermal is warm air with colder air above it. The warm air will rise until it finds air above it that is warmer where it stops. The lake will probably be a pretty reliable thermal generator until it ices over. People have even thermalled on a frozen lake, over holes in the ice. A plowed parking lot in the sun, surrounded by a snowy field should make a good thermal generator, better yet if there's a road with some traffic on it to kick em loose. In my experience, winter thermals tend to be smoother and more consistent, because the warm air collection points are more defined, the trigger points obvious and the wind speed and direction more stable in general. Some of the thermals can be pretty amazing too, because the temperature differential between the air over a dry spot with sun on it, and the winter air above it can be huge.

Example.. Crop from a nearby sounding for 5pm today.

Temperature at ground level.. about 27F. Temperature above that jumps to the left and by 7200 feet is 15 degrees cooler which is way faster than the dry or saturated lapse rate cools the air. If you saw something like that on a sunny day, the conditions would be phenomenal. The shaded area represents the temperatures and altitudes where any air parcel released from the ground would rise. The only real problem with flying on a day like today is seeing through the snow, and very low cloudbase.. oh, and the cold .

ian
Last edited by Daemon; Oct 30, 2002 at 08:42 PM.
Oct 30, 2002, 09:58 PM
Registered User
Even in the summer time thermals can be found over lakes but not during the day. On a clear night radiation cooling of the land occurs faster than the water in the lake and when the land becomes cooler than the lake the lake can become a thermal source.
Oct 31, 2002, 03:58 AM
Whadda YOU lookin' at?
CactusJackSlade's Avatar

Birds WILL find lift...


Yo Bro,

Did you check out my post below? DS-ing Turkey Vultures!


https://www.rcgroups.com/forums/show...threadid=66615

L8R,

CJS
Nov 24, 2002, 05:28 AM
Registered User
fledge's Avatar

water spouts


a classic example of thermals in winter on lakes are water spouts,water can be lifted into the air the same as a dust devil.

lake huron can kick off water spouts that can ruin small boats and its usually the boat that triggers them

we have a very small ridge here that we soar in the fall that has all the right components,,,,,,, its a coastal bluff with no obstrutions out front so the ridge lift starts to build right from the base of the slope,,,,it faces into a prevailing wind [nw]
we fly on the southern shore and cold fronts usually follow down the lake,,its like magic if you hit it right!
you would normally think that you would have to launch into the ridge lift then wait for thermals but it doesnt seem like that!its just as if you were flying a 500 ft ridge!

i fly r/c and hanggliders there and i always launch the r/c first as a wind dummie
we can completely leave the ridge and go out over the water and some pilots have gained a 1000 ft on a 100 ft ridge

like all soaring sometimes you just gotta be there at the right time
fledge


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