Thank god for air friction. Without it, falling rain would smack into our heads at hundreds of miles per hour. But friction works both ways—falling raindrops also slow down the movement of air molecules in the atmosphere. A new paper in Science calculated that raindrops dissipate as much kinetic energy from the atmosphere as air turbulence. Granted, at 1.8 watts per square meter and 0.75% of the atmosphere’s total kinetic energy, that’s not very much. What’s surprising is that rain drops are pulling more than their weight, as they make up only 0.01% of the atmosphere’s mass.
Researchers calculated the kinetic energy dissipated by a single raindrop and put it together with precipitation rates around the world. Since satellite precipitation data also show the height from which rain started falling, the researchers could plug how far raindrops fell into their energy calculations. It all adds up across the whole globe: the researchers estimate the total rate of energy dissipation from rainfall to be 1015 Watts. That’s a lot of energy, but still unlikely to affect major weather phenomena like hurricanes or tornados.
[via Nature News]
Image via Shutterstock
February 24th, 2012 at 5:50 pm
Hey, your link to the article doesn’t work!
February 24th, 2012 at 6:20 pm
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?
February 24th, 2012 at 6:20 pm
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.
February 25th, 2012 at 12:33 am
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.
February 25th, 2012 at 12:08 pm
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…
February 26th, 2012 at 7:15 am
The first (two) links do not work still. The satellite link does work.
February 26th, 2012 at 9:28 pm
@ 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.
February 27th, 2012 at 11:41 am
Links fixed!
February 27th, 2012 at 9:44 pm
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…
March 1st, 2012 at 4:10 pm
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.