Ever Wonder What a Thermal Really Looks Like?

[tyrolean_marcus]

Hello All,

These pictures were taken from the flight deck while taxing out out from a Gate at Vancouver international Airport.

The airport fire fighters were sparying fire retardant foam during an equipment test. The foam was light enough that a passing thermal picked up some of the foam, and sent it skyward. When I first saw this happening I didn't realize that the "white stuff" floating around was actually foam.

It is intersting to note that the themal in the pictures seems to be very narrow close to the ground, then significantly wider about 100' up, and then it narrows down again about 200' up.

The wind was blowing about 5 - 7 knots, and the entire mass of airborne foam was traveling downwind.

I've seen lots of dust devils eminating from dusty fields, but never anything like this. Has anybody every seen anything like this before?

Cheers,

Marcus.

[rogerflies]

I would have thought that the fire suppression foam would be heavy enough to stay on the ground in an inferno. If a thermal will pick it up, how can it cover something that's burning like crazy? :confused:

Cool pics, and thanks for sharing them.

Roger

[Thermalin]

good question.... though I think they shoot it at the base of the flames... maybe this is picking up the particles that were airborne prior to landing on the runway as the first pic shows spraying out and not down.

[fprintf]

Very cool pictures!

[Daemon]

That's pretty cool. Now I'm sure someone else has probably
tried this, but I've got a battery operated bubble machine
(50 little bubble blowers on a wheel with a fan behind it) that'll
crank out a few thousand bubbles every minute. Get the
right bubble solution (long lasting bubbles) and set it up on the upwind
side of a decent thermalling field, and one should be able to see how the
thermals pull the air around for quite a while. I've used it a little
on the slope and it's pretty cool to see the cloud of bubbles racing up
the slope and planes flying through them.

ian

[mtnmnstr]

Good Pics.

While flying sailplanes in the Elsinore valley, ca. I've been at 6000agl and seen columns of dust and a few plastic bags.

gene

[TomO]

There is a 160 acre or so field of alfalfa directly behind my own horse pasture. The owner of that field had just cut the hay and it was drying in the sun on a hot day.

I saw a HUGE thermal develop right there that was strong enough to lift several patches of this hay straight up about 25' or so into the air. Kind of like a dust devil I suppose, maybe a hay devil would be a better term. It sure was cool to watch.

[slopemeno]

I lived in Great Falls Montana for several years. The dust devils were massive, and due to the flat terrain around town you could see them coming across the wheat fields for miles, like brown tornadoes spinning downwind across the plowed fields.

[carrinsr]

I've commented about thermals in another thread, but I'll say it again here. Thermals are mini-tornados, smallest at ground level but they get bigger and stronger with altitude. They can even end up as large as the base of the cloud they're swirling under at their highest point. Most thermals lead upward to the base of a cloud as it is developing. If you are at the base of a thermal then put your back to the direction the wind is generally blowing, and you'll likely be facing the direction the thermal is moving. In the Northern hemisphere they rotate counter clockwise...nice to remember when you're thermaling a sailplane. If you want to gain altitude faster then circle clockwise once you zero in on the heart of the thermal...that is to say, banking right instead of left. Works for me here in Florida! (o: David

[tw126a]

I wonder what the actual speed of rotation speed is in a moderate thermal? Apparently not all tornado's rotate counter clockwise in the northern hemisphere, about 10% rotate the other way. I read this somewhere but unfortunately can't provide a reference.
Tom

[Thermalin]

I wonder what the actual speed of rotation speed is in a moderate thermal? Apparently not all tornado's rotate counter clockwise in the northern hemisphere, about 10% rotate the other way. I read this somewhere but unfortunately can't provide a reference.
Tom

Wonder how that can be as its based on earths rotataion and being in norther or southern hemisphere... similar to water going down drain... anomolies in nature to I guess... wonder what happens at equator?

[dgliderguy]

Here's a pic I took in Eastern Washington last spring, on the way home from an aerotow event. It's a dust devil, kicked up by a powerful thermal (they were absolutely fearsome all day long!). What you are looking at is a series of smaller 'satellite' dust devils all spinning around the center of the column of rising air. It was the most amazing natural ballet I have ever had the opportunity to watch, in open-mouthed fascination.

It lasted a good six or seven minutes, and didn't move more than a hundred feet from the spot where it started. All the way up the column there were hawks and other soaring birds working the booming lift. An awesome thing to see.

Don

[tw126a]

Wonder how that can be as its based on earths rotataion and being in norther or southern hemisphere... similar to water going down drain... anomolies in nature to I guess... wonder what happens at equator?
Actually, its easy to prove, try changing the direction in your tub with your hand and see what happens. If the air was not dynamic then I'm sure the counter clockwise direction would occur 100% of the time, but it just takes a little anomoly to throw everything off. I guess at the equator the roatation could be either way. And in the southern hemisphere it will mostly be in the other direction.

Nice picture Don, that would be a fascinating sight. Classic example of differential heating because of the differing fields, that plowed field must have acted like a furnace just feeding itself.

Tom

[Fred_L]

The clockwise/counter clockwise rotation of fluids in the Northern/Southern hemisphere is due to Coriolis forces. Coriolis forces are extremely weak and arise from the rotation of the planet about its axis. The initiation of a preferred rotational direction relies on one side of the structure having the earth rotate under it as it develops and this progressively generates a net force that rotates the whole structure.

Only large scale structures that persist for a long period of time will develop a rotation in the defined direction. Hence hurricanes/typhoons/cyclones do all rotate in a fixed direction dependent on hemisphere.

It is an urban myth that water down the plug hole or even thermals have a defined rotational direction determined by what hemisphere they are in. These events are simply not large enough nor do they live long enough for Coriolis forces to affect the rotational direction. They do rotate, but the direction should be random.

Fred

PS This would be a good one for Mythbusters to blow apart

[BMatthews]

By the time a thermal gets to the dust devil stage where it has a tornado like formation it's a REALLY strong thermal.

Free fligthers actually avoid anything that turbulent but actively look for the smaller and lighter types. You may have seen references to thermal poles with mylar tape and such or bubble makers mounted on poles. These help a lot but have the disadvantage that they are stuck in one spot.

Locally a lot of us have been using cattail seeds to find thermals when the rules allow (SAM specifically prohibits thermal finding aids that are not naturally on the field). The cattails are harvested from local ditches and the heads wrapped tightly in newspaper and left to dry over a winter. TO use we peel back the paper a bit, tease out a fluff ball with a thumb and blow the fluff up into the air then watch. A nice thermal will lift the seeds up and when they reach the core they start churning around like in a washing machine. Not only do they spin around in the circular current but there's also a lot of up and down rotor movement at the core. The model is then launched either directly into the thermal or into the wind such that the first turn will bring it back around into the lift. With the flufflys it's pretty much like shooting fish in a barrel.

Fire foam and fluffys are not the only way. Back in the 50's free flighters that worked at Jim Walker's plant would bring a couple of bags of balsa dust with them to meets. A handful or two of dust pulled from a bag and tossed to the wind would suddenly make any thermal look "solid" over a wide area.

[bz1mcr]

The clockwise/counter clockwise rotation of fluids in the Northern/Southern hemisphere is due to Coriolis forces. Coriolis forces are extremely weak and arise from the rotation of the planet about its axis. The initiation of a preferred rotational direction relies on one side of the structure having the earth rotate under it as it develops and this progressively generates a net force that rotates the whole structure.

Only large scale structures that persist for a long period of time will develop a rotation in the defined direction. Hence hurricanes/typhoons/cyclones do all rotate in a fixed direction dependent on hemisphere.

It is an urban myth that water down the plug hole or even thermals have a defined rotational direction determined by what hemisphere they are in. These events are simply not large enough nor do they live long enough for Coriolis forces to affect the rotational direction. They do rotate, but the direction should be random.

Fred

PS This would be a good one for Mythbusters to blow apart

It is not a myth, and it does not have to be large or exist for a long time.
It does take an environment with few random disturbances to establish the preferred direction. The initial forces are low and can be upset by very small turblulance or other counter motions. If the water in your sink is allowed to settle for a time until it is very still (30 min is usually enough), it will always start to spin in the same direction when the drain is opened. That assumes you are not within a few degrees of the equater, then all bets are off.

Do you have a referance to the connection to Coriolis forces? As I remember Coriolis forces are the result of a body spinning about one axis being forced to rotate about another axis. The classic high school physics demonstration is to have a student hold the axels of a spinning bike wheel while standing on a turntable. No problem, the student holds the wheel axles horizontal (in the line of his thumbs pointing to each other) and the teacher spins the wheel. Then the student is asked to move the axles to vertical. Immediatly the turntable rotates. The student usually quickly learns to stop or reverse the rotation, the axle needs to be moved back toward horizontal.
Important if you had a heavy prop spinning and you want to turn an aircraft, but I see no connection to the dust devils or thermal rotations.

They are the result of conservation of angular monentum. Using the same turn table the student is asked to hold is arms out straight to each side with a dumb bell in each hand. The teacher starts them rotating very very slowly and then asks them to bring the weights in closer to their body-- they speed up. And if brought in very close rotation is so fast it is scarey. You have seen many figure skaters do this to generate fast spins.

That is the tornado builder. If the atmosphere is very still it will be rotating ever so slowly due to the earths rotation. Then a thermal breaks loose and the inward flow of air at the bottom is like the skater pulling in there arms. The rotation begins to speed up and is limited only by viscosity and bernouli forces, both of which are pretty small until speeds are pretty high.
Don

[Paul Naton]

I would be guessing that with all of the thermal 'mythology' be thrown around here that you guys have not studied my Secrets of Thermal Soaring DVD.

You can see some of the variety of themal structures that occur on video.

Dust devils are usually the tail end of a stronger thermal coumn aloft which still has a ground/boundary layer connection which is still pulling in ground air. The thermal generating this 'tail' is often far downwind and high up.

Thermals are not influenced by Coriallis, too small a mass and too short lived. They rotate either direction by: wind shear direction with altitude & local eddy and vortex shedding by boundary layer objects and obstructions.

I have seen dust devils rotaing in different directions on the same field and even change directions in a few seconds.

Its false to assume you can catch 'that thermal under a cloud'. Usually a cloud is formed when enough thermal strings/columns come together from the ground level boundary layer to the condensation altitude. Your ground level HLG thermal is not usually going to form a cloud at 4000' ASL. Most of the thermals we ride loose the ground level air input at some point and will not just climb out forever, a collection of joining themal strings can generate a big cloud and eventually a convective heat engine that produces rain/hail/etc.

However, the more you learn about this stuff, the more you see the 'rules' get broken. The convective boundary layer is truely a bizzare and complex system. i learn new stuff and flying techniques everytime I fly.

PN