billcummings wrote:Ninety nine times out of one hundred the thermal was still where I left it and it had not fizzled out.
billcummings wrote:Terry, and Hawks,
Having a good deal of flying from boat towing and later flying over the wheat fields of Washington State this is what I have noticed that many other pilots may not yet have made this same discovery.
When a thermal, dust devil, leaves a bare field of dirt and enters a wheat field many times the thermal will not pick up enough dust to continue to mark itself.
The foot print of the thermal can be tracked, or seen, by following the lighter shade of wheat as the thermal swirls it’s way downwind. It looks like a bright spotlight moving and swirling across the wheat field.
Just the opposite happens when a thermal moves out over a body of water. The foot print of the thermal looks darker than the surrounding water. This dark wind print over water can be seen from many thousands of feet above whereas leaves fluttering as the thermal moves across the top of a grove of trees cannot be seen from much more than about a thousand feet above.
For new pilots trying to enter these thermals, that can be seen leaving a footprint on the ground, I tell them to lead the thermal to the downwind side. If they ask me, by how much they should lead the thermal to intersect it I tell them this:
Stand up and move one of your hands about a foot away from your leg. That is the angle to be downwind of the thermals foot print on the ground. Strong wind days it will of course need a greater angle. This is just a starting reference point for them.
Just like putting some “English” (spin) on a ping pong ball, tennis ball, or baseball a person can make the path of the ball curve or tack off of the direction the ball was thrown. (Oh yes, you can be seated and finish reading now.)
The same thing happens as a spinning thermal is being blown downwind.
The thermal will not blow straight downwind but tack off to the left or the right of the winds course line depending on the thermals rotation (spin/“English”).
This will also make the thermal lean to the left or right as it tacks its way slightly off of the downwind course line.
So If you are flying downwind to the next thermal and the foot print or dust near the ground gives you an indication of which way the thermal is rotating, clockwise or counter clockwise, this will tell you which way left or right of the footprint you need to fly to intersect the leaning invisible thermal at your altitude.
Darn it! ---- I’ve got to get out more! I used to know which way to search for the thermal above its footprint as it moved downwind depending on its rotation and now I can’t remember!
I guess you will have to sort this out yourselves.
Let’s see now ---if a ping pong ball has right “English” on it and it curves left---then a thermal blowing down wind would go------????
Oh well! It was a lot of fun back when I figured it out for myself so I wouldn’t want to spoil the fun for everyone else.
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