On Tuesday, November 4, 2014 5:39:22 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 8:10:44 PM UTC-5, jfitch wrote:
On Tuesday, November 4, 2014 4:24:00 PM UTC-8, Andy Blackburn wrote:
On Tuesday, November 4, 2014 3:04:41 PM UTC-8, Evan Ludeman wrote:
On Tuesday, November 4, 2014 4:38:33 PM UTC-5, Andy Blackburn wrote:
I did the simple physics on this. A volume of air rising at 600 fpm with no other forces acting on it other than gravity has enough momentum to rise an additional 1.6 feet before its upward velocity reaches zero.

I am now officially at a loss as to why thermals go up if there is no temperature difference versus the surrounding air.

9B

That's for an object in a vacuum. The thermal isn't surrounded by a vacuum, it's surrounded by air at almost precisely the same density.

T8

Wouldn't that make it go up even less due to the frictional resistance?

Just doesn't weigh as much as you assumed

Buoyancy.

T8

Added that in. Buoyancy works opposite to gravity but momentum still based on total mass of the air.

The math seems to indicate that the momentum of a bubble of air is actually small compared to the buoyancy effects of even slight changes in temperature. I also looked at air density versus temperature and humidity. The humidity effects are real but generally small as well. Even a giant swing in humidity from 80 percent to 30 percent generates an increase in density equivalent to just 2 degrees F. I can see why it pays to look for thermals downwind of cattle tanks in the desert, and why you might get a kick in the climb rate at several thousand feet, but it doesn't explain for me why thermals go up in the first place.

I suppose this confirms why a Skew-T plot generally predicts thermal heights based on temperature differences - the height where e a parcel of air at the surface, when adiabatically lifted, reaches the ambient temperature in the sounding. There's no adjustment for mixing of the thermal down low and no adjustment for momentum of the rising air or the humidity of the air - the thermal goes to the height where the core of the thermal adiabatically cools to the surrounding air temperature and it stops.

9B