The Naked Scientists Forum

Author Topic: Why can avalanche speeds exceed human terminal velocity?  (Read 6614 times)

alkan123

• First timers
• Posts: 2
Why can avalanche speeds exceed human terminal velocity?
« on: 12/07/2012 21:09:12 »
Hello
It's been worked out that a body in freefall reaches a speed of 120 mph(approx). If so, how can an avalanche reach much greater speeds than that? The avalanche is flowing down a mountain at an angle so it shouldn't even reach the freefall speed. What am I not taking into consideration?
« Last Edit: 12/07/2012 22:03:48 by chris »

CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
Re: Avalanches
« Reply #1 on: 13/07/2012 07:14:47 »
Interesting.
I'm seeing skydiving terminal velocity at around 50 to 76 mph.
And the "dry" snow avalanche travelling at about 200 mph.
However a "wet" snow avalanche is much slower, perhaps at 20 mph.

Terminal velocity is a function of mass, gravity, and the coefficient of drag.

Thinking of skydiving, if one holds one's arms out like a sail, one goes much slower than if one dives head first as one is diving into water.  In fact there is a new type of wingsuit flying with skydivers looking like flying squirrels, increasing their surface area and drag, and decreasing the terminal velocity.

Semi-trucks can apparently hit in excess of 70 MPH on a steep downhill, carrying a lot of mass, and relatively low drag.

The terminal velocity of a bomb can be as high as 2,500 mph (4,000 km/h) due to its high mass and streamlined design.

The avalanche has a lot of mass.  The "dry" snow will essentially ride down the hill on an air cushion.  The air gets pushed along with the avalanche, creating hurricane-like winds.  And, perhaps in a sense, it would also push some of the air upwards and slides under the air like a F1 racecar.  Of course, a fast moving wind would create a low pressure, and increase some of the lift to the snow, and thus decrease the density, and friction of the mass of snow moving down the mountain.  You can thinking of the wind moving with the avalanche, but the wind is also being pushed by the avalanche.

Anyway, I think that like the bomb example, the avalanche has a relatively high mass to drag ratio.

alkan123

• First timers
• Posts: 2
Re: Why can avalanche speeds exceed human terminal velocity?
« Reply #2 on: 13/07/2012 20:58:24 »
Thankyou for your succint reply. I didn't realise that other forces were involved. Have a better understanding now.
Talking about sky diving it's funny how some people think that the diver goes up when the chute opens. I admit it looks like that to the video camera.
Cheers

The Naked Scientists Forum

Re: Why can avalanche speeds exceed human terminal velocity?
« Reply #2 on: 13/07/2012 20:58:24 »