Zzz wrote:This is interesting. I'm an open minded guy so I'm trying to reconcile what Mau Mau is saying with my experience and knowledge. I've concluded that he is fundamentally correct.

Thermals are rising columns of lower density air; that reduced density resulting from surface heating. But it's not a perfect "column." Like a lava lamp, the air is more like a bubble that remains adhered to the ground until it's buoyant enough to "release," and then it begins ascending through the cooler air mass.

So every time you fly over a hot parking lot or catch a thermal in your paraglider, it's a hotter, less dense air mass that is rising through a greater, cooler air mass. But the fact that you feel it rising up means it's "released" from the surface.

So what if this warmer, less dense air mass hasn't released from the surface yet? My thinking is that it would behave exactly as Mau Mau describes.

Sinkers in cold air are the inverse: a heavier denser air mass moving downward through a greater, less dense air mass. You feel the sink because the air is on the move.

But all thing being equal, if the air masses are in contact with the surface and there's no movement from buoyancy occurring, what Mau Mau wrote will hold true. Sudden high DA. Hot air only acts lifty when it's on the move, "bubbling" upward.

That said, in the world of light GA aircraft, encountering a stable hot air mass that isn't bubbling upward is the rare condition.

Battson wrote:Interesting idea.
Air flows freely and the flow patterns would be pretty complex in real life, a dynamic model. It's not really like a "bubble" of warm air that sits there and suddenly starts rising, more like a free stream or flow. The air would be moving continuously along the sea, warming over the land and rising immediately, in columns over the hottest areas. I don't think the lava lamp model is a fair comparison, because the viscosity is so different compared to the amount of motive force, and it's not a free stream.

a dude on the internet wrote:...note that convection currents form very readily in a fluid due to density differences caused by heating. These currents serve to distribute the heat more uniformly throughout the fluid. They are more or less steady state and continuous. However, when the heat is applied at the bottom of a fluid at such a rate that that convection currents cannot keep up with the process, then more chaotic penetrative convection occurs. In this case the warmer air “comes away from the heated (surface) in lumps, which we call thermals.” The quote is from R.S. Scorer’s book Natural Aerodynamics.

GumpAir wrote:Bullshit to all of this. In a light airplane that is.

Same as wind shear.... You sink, you climb, you lose a few knots airspeed, whatever, you get instant correction with throttle. That Cherokee driver just didn't react fast enough when the end of the runway started climbing up the windshield.

Gump

Zzz wrote:

a dude on the internet wrote:...note that convection currents form very readily in a fluid due to density differences caused by heating. These currents serve to distribute the heat more uniformly throughout the fluid. They are more or less steady state and continuous. However, when the heat is applied at the bottom of a fluid at such a rate that that convection currents cannot keep up with the process, then more chaotic penetrative convection occurs. In this case the warmer air “comes away from the heated (surface) in lumps, which we call thermals.” The quote is from R.S. Scorer’s book Natural Aerodynamics.

8GCBC wrote:The pilot was practicing to be on the RedBull flying team. Could not afford an Extra 300 yet.

sbmaule wrote:....and to think, some ware out there, at this very moment, in some sunbathing/beachgoer forum, there's an equally passionate and heated debate over how to get black tire marks off your ass...