If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#41
|
|||
|
|||
Most people can't see a minute hand moving, but it moves nonetheless.
It's a matter of patience. The problem with observing cloud rotation is that the cloud is constantly changing shape, therefore you cannot time lapse the same way you can when observing vertical development (or determining that a minute hand moves). As noted in my impolite post, you need to spend a half hour on your back. Then we can move on to discussing whether there's any real advantage to be had. |
#42
|
|||
|
|||
|
#43
|
|||
|
|||
"Andrew Sarangan" wrote in message om... (Kirk Stant) wrote in message . com... "K.P. Termaat" wrote in message ... My experience is that it works, especially on days with very low humidity, but no boomers and only low. "Mike Rapoport" schreef in bericht ink.net... You will find less lift over water of any kind, even if it is contained in vegetation. The best lift is always over the highest, dryest, darkest surface around. The water vapor idea is...well...it is hard to find a place to start...but it won't work Mike MU-2 Have to disagree with you, Mike - out here in Arizona, in the desert areas that are not irrigated, we often find good lift directly over small cattle "tanks" - small shallow ponds that are scattered around. A lot of us have noticed this and compared notes, and it works; if too low to get to high, dark ground, I'll head for the nearest pond and it will usually turn up a nice thermal. We think it may be due to the fact that the ponds are in a natural low spot, and coupled with the little bit of moisture, could be the necessary trigger for a thermal. Now obviously, large irrigated farm fields or river basins are death to thermals - but a local lake (reservoir) seems to have little effect on thermal activity - could it be all the drunk boaters? What's the old saying about never saying never? Kirk LS6-b The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it is 3C/1000ft. Therefore, assuming that the water temperature is the same as the surrounding ground (which would be true if the water is shallow), I could see how one will experience greater lift above water. Not for unsaturated air it isn't. The moist rate only applies to saturated air (ie in clouds) Mike MU-2 |
#44
|
|||
|
|||
Chris OCallaghan wrote:
Granted, our attention spans are short, but spending a half hour on your back watching the development of a cumulus cloud can be very instructive. We've been talking about dust devils, which rotate as fast as 20 times minute, but you seem to be talking about rotations slower than 10 times per hour. No wonder I don't notice rotations like that while flying. But you needn't even do this if you are observant during your climbs. You will note that around the edges of the cloud you'll occasionally see vortices forming when the light hits them right. I do see these generally small clumps of rotations in the vertical plane, near cloud edges. This isn't what I would call "clouds rotating" in this context (which direction to circle), by which I mean a substantial amount of the cloud going around horizontally like a dust devil does. I have seen roll clouds rotating rapidly, but that isn't relevant to dust devils. Or if there is clag below cloud base, it will often have a discernable rotation. When observed, they often herald localized cores that can be exploited. I have never observed this, though I always head for "tendrils" or clumps of cloud forming below the nominal cloud base, as the lift is usually much better. The movement, if present, has been invariably upwards. What speed are the cores rotating at? I'm sure I'd notice if it was anything like dust devil speed. More than once I've noted vorticity on the edege of a large cloud, shifted my circle to it, and been rewarded with much improved lift. Me too, but it's not been in the horizontal plane. Spending some time on your back, you will observe that not only are there localized vortices, but that the entire system slowly rotates. Sometimes it's easier to see this than at others. However, patience is required since the rotation is very slow, but nontheless discernable. I haven't looked for rotation this slow, and while it's interesting, it doesn't seem relevant to the choice of circling direction. Best to start with a wisp and watch its full development. Choose one as close to directly overhead as possible. The closer your view to directly beneath, the more obvious the cloud rotation becomes. I'll try this next time I'm stuck on the ground under cumulus clouds. |
#45
|
|||
|
|||
"Brian Case" wrote in message om... On the other hand I can't tell you how many times(numerous) I intercepted a dust devil at 1000-1500 AGL and climbed out at less then 1kt or even did not climb. Most times however I get 3-6 kts out of them. I have see dust devils go to 7-8000 feet up. Hate to tell the one gentleman this that wouldn't fly into a dust devil, but if he flys using thermals he is just flying into dustless dust devils, As far as I can tell the only difference is if it is lifting air over an area were it can pick dust up or not. We are apparently sharing different definitions of "dust devils." The ones that rage across the part of the country I fly mostly do not fit the structure of a thermal. Thermals, at least where I am, are rising volumes of air created by differential temperatures on the surface. One of our best local thermal engines is the black paved surface of Runway 10/28 at IYK. The equipment parking lot for the highway maintenance yard is another. The location of these is pretty constant and reasonably predictable and reasonably benign. Dust devils on the other hand, while they may begin at convective sources, are cyclonic whirlwinds that travel laterally across the ground, sometimes for miles. DDs in our area are typically less than five meters in diameter. One monster dust devil that went across a portable weather station at the Naval Air Warfare Center spun the anemometer over 80 knots before it ripped the mast apart. We watched that one travel about ten miles. On another occasion, a monster went across a mobile home park and took the roof off a home and dissassembled tool sheds like card houses. |
#46
|
|||
|
|||
Casey Wilson wrote:
Dust devils on the other hand, while they may begin at convective sources, are cyclonic whirlwinds that travel laterally across the ground, sometimes for miles. DDs in our area are typically less than five meters in diameter. One monster dust devil that went across a portable weather station at the Naval Air Warfare Center spun the anemometer over 80 knots before it ripped the mast apart. We watched that one travel about ten miles. On another occasion, a monster went across a mobile home park and took the roof off a home and dissassembled tool sheds like card houses. Yeah, those are the kind of dust devils I look for 8^) I remember one I saw in a valley north of Tonopah, when I was down low looking for lift. It had one huge central column and six smaller ones twisting around it. I could see huge pieces of sagebrush literally getting blown out of its path. I pulled into it at roughly 1500 feet AGL, and centered a 14 knot climb with dust and twigs flying all around me. I was at 18000 feet in what seemed like a moment. Even at that altitude, there was plenty of dust, and looking down into was like looking into the maw of a huge snake that stretched all the way back down to the ground. You don't know what you're missing 8^) Marc |
#47
|
|||
|
|||
I've seen this effect many times in Australia. Kept finding
lift over small irrigation ponds (altitude several hundred feet). Decided to do an experiment whilst awaiting a buddy who was interminable slow to get airborne. There was an irrigation pond about 1km from the airfield. After losing sufficient altitude doing aero, spoilered down to several hundred feet over this pond, then climbed out and repeated the procedure. Four or five times (like I said, he's slow). The interesting features here (and in Arizona) a - very dry air, and - shallow irrigation tank/pond subject to good heating (warm water) Don't know that I understand the physics, but extremely consistent. Beaver Pond Lift is however a different phenom... Best Wishes for 2004 to all, Dave "YO" Peter Creswick wrote in message ... Mike Rapoport wrote: The moisture doesn't really help lift until the air is fuly saturated and starts condensing releasing heat. Also, the air above the water is cooled by evaporation and is cooler than the surrounding air. I will never say never and I don't dispute your or others experience, but the explanation doesn't make sense to me. Mike MU-2 "Kirk Stant" wrote in message om... "K.P. Termaat" wrote in message ... My experience is that it works, especially on days with very low humidity, but no boomers and only low. "Mike Rapoport" schreef in bericht ink.net... You will find less lift over water of any kind, even if it is contained in vegetation. The best lift is always over the highest, dryest, darkest surface around. The water vapor idea is...well...it is hard to find a place to start...but it won't work Mike MU-2 Have to disagree with you, Mike - out here in Arizona, in the desert areas that are not irrigated, we often find good lift directly over small cattle "tanks" - small shallow ponds that are scattered around. A lot of us have noticed this and compared notes, and it works; if too low to get to high, dark ground, I'll head for the nearest pond and it will usually turn up a nice thermal. We think it may be due to the fact that the ponds are in a natural low spot, and coupled with the little bit of moisture, could be the necessary trigger for a thermal. Now obviously, large irrigated farm fields or river basins are death to thermals - but a local lake (reservoir) seems to have little effect on thermal activity - could it be all the drunk boaters? What's the old saying about never saying never? Kirk LS6-b Have seen similar effects over the small dams on farms here too. My idea is that the air over the water cools by evaporating water out of the pond. In so doing it looses more heat and hence contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both colder and denser overall than the surrounding surface air. As the dense pool of air becomes greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond / dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the ground. |
#48
|
|||
|
|||
I can claim no specialist meteorological knowledge, but I believe the
following to be undeniable: 1. When a thermal forms, there is to some extent at least an inflow of air, to prevent a vacuum forming under the rising airmass. 2. It is extremely unlikely that the inflow from every direction is equal and symmetrical. If Coriolis effects are likely to be swamped by local initial conditions for the inflow, the resultant rotation may be in either direction. 3. It follows that there will be an element of rotation imparted to the resulting thermal bubble/plume/call-it-what-you-will. 4. In some cases at least, the resultant rotation may be enough to be noticeable. 5. During the inflow phase, any resultant rotation will speed up, by conservation of angular momentum, like a dancer speeding up a spin by pulling in the arms. 6. This rotation is certainly visible in dust devils, tornados, and other smaller local eddies. Even in the UK's usually mild thermals, one occasionally sees bits of straw, grass cuttings, or dead leaves picked up and whirling round. In the days of stubble fires (farmers buring off fields, after harvest - now banned) bits of burning straw could be seen in the rotating thermals. [I have used "rotation" in the layman's sense that the air and its contents are going round. There is a technical use of the word, if I recall my fluid dynamics correctly, that the water in a whirlpool has "zero rotation" which may also apply in thermals, for the same reason - it is a mechanism where things can go round quite fast without a significant input of angular kinetic energy. It is a feature of a rotating fluid mass where the middle is going round faster than the outside, which happens in whirlpools and when you pull the plug out of a circular basin full of water. I propose to leave it there. ] Chris N. |
#49
|
|||
|
|||
On Mon, 29 Dec 2003 10:24:40 GMT, "K.P. Termaat" wrote:
My experience is that it works, especially on days with very low humidity, but no boomers and only low. I'am talking about small shallow ponds in dry area's especially when the ponds are surrounded by sandy grounds with higher vegetation like trees. The buoyancy impuls from the evaporated water is apparently just good enough to start the thermal which then sucks air from its heated up vicinity. Has saved me many times when I was still flying my Pik20D or more recently my DG800S. Karel, NL V-2cxT Water vapour has a molecular weight of a bit over 18 and dry air a bit more than 28. Water vapour at the same pressure as the air around it is considerably less dense than dry air. More water vapour= more bouyancy. Then again this may have more to do with low spots in the ground. I've always found quarries (holes in the ground)to be excellent lift sources when low. Mike Borgelt |
#50
|
|||
|
|||
I'll buy that one, JJ.
-- Bert Willing ASW20 "TW" "JJ Sinclair" a écrit dans le message de ... I am not disputing the facts, I am disputing the explanation. OK, Its time for my Minden pond theory. We have a small pond about 5 miles east of the airport that consistently produces thermals. JJ's explanation; Heated air that is slowely moving over the ground by the wind, suddenly comes to the cooler pond edge. This cool edge of the pond acts as a trigger that forces the heated air to break loose and start rising and that's why thermals can be found at the edge of small ponds. JJ Sinclair |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Aerobatics | 28 | January 2nd 09 02:26 PM |
Dover short pilots since vaccine order | Roman Bystrianyk | Naval Aviation | 0 | December 29th 04 12:47 AM |
[OT] USA - TSA Obstructing Armed Pilots? | No Spam! | Military Aviation | 120 | January 27th 04 10:19 AM |
[OT] USA - TSA Obstructing Armed Pilots? | No Spam! | General Aviation | 3 | December 23rd 03 08:53 PM |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Piloting | 25 | September 11th 03 01:27 PM |