Comments on: Study: Raindrops Take Energy Out of Air

By: Phil

Phil — Thu, 01 Mar 2012 21:10:24 +0000

I agree with Iain the energy must go somewhere and cedric the energy system must be in balance “rates of vaporization are equalled by condensation”. None of this matters however if the amount of energy being added to the system by solar radiation causes a catastrophic increase in atmospheric temperature. Although I guess that it could be argued that this would lead to a massive increase in evaporation and therefore cloud and that this would act as a counter to the increased temperature.

By: cedric

cedric — Tue, 28 Feb 2012 02:44:13 +0000

The reason the kinetic energy of the air decreases is because the heat from friction from the air is what is supplying the energy of condensation of the raindrop. So the energy from the air goes into evaporating the raindrop. And once the relative humidity is 100% then the saturation pressure is reacched so no more water can evaporate. At that point the raindrop will sensibly warm up from the heat supplied by friction before hitting the ground, but rates of vaporization are equalled by condensation at the saturation pressure found at 100% RH…

By: Sarah Zhang

Sarah Zhang — Mon, 27 Feb 2012 16:41:25 +0000

Links fixed!

By: Iain

Iain — Mon, 27 Feb 2012 02:28:20 +0000

@ cedric
But then again

conservation of energy insists that condensation will release heat, because it took heat to evaporate the water in the first place.
Also at first the rain drops may evaporate at lower altitudes, but once humidity is 100 % well, does it go to 101 or no?
It seems to me that once the rain hits the ground any heat energy would have to go to the air.

Anecdotal – it warms up to snow and rain.

By: Axel

Axel — Sun, 26 Feb 2012 12:15:22 +0000

The first (two) links do not work still. The satellite link does work.

By: cedric

cedric — Sat, 25 Feb 2012 17:08:52 +0000

The heat or energy supplied to the atmosphere is not enough to raise the ambient temperature because most of the heat from friction goes into heating up the raindrop due to the higher capacitance of water over air…this means the raindrop begins to evaporate, lowering the ambient temperature, which is actually the definition of how a wet-bulb temperature is calculated…

By: Kevin

Kevin — Sat, 25 Feb 2012 05:33:38 +0000

Could the loss of heat to the air be related the large amount of energy necessary to heat up a small amount of water? Also, don’t raindrops start as ice in the clouds, so by the time they reach us they have gained lots of energy.

By: tim

tim — Fri, 24 Feb 2012 23:20:35 +0000

I think this is best available on smallish bodies of water, such as lakes, where wind is usually the cause for wave action. During heavy rainfall, I notice the water surface calms right down, in part because of the mediating nature of raindrops on the lake, but also because of less wind.

By: Chris

Chris — Fri, 24 Feb 2012 23:20:24 +0000

I was able to determine what the link was, although I still can’t read the article since I’m not subscribed.

But something doesn’t make sense. The potential energy of a rain drop in the clouds is mgh and on the ground is zero. Kinetic energy in the clouds is zero and when it hits is (1/2)mv^2

If there was no atmosphere since energy is conserved mgh = (1/2)mv^2

Because of friction that velocity will be lower than the ideal velocity so

mgh = (1/2)mv^2 + Frictional energy

Since energy is conserved, shouldn’t a falling raindrop heat up the air?

By: Planktic

Planktic — Fri, 24 Feb 2012 22:50:23 +0000

Hey, your link to the article doesn’t work!