Rain sucks energy out of world's wind

Every time it rains, a little bit of energy gets wasted. For the first time we have an estimate of the amount of energy falling raindrops pull from the air.

The team behind the calculations say that the atmosphere's energy balance will drop as climate change increases rainfall, slightly weakening winds around the world.

As a raindrop falls it is slowed by contact with the air. This friction takes energy away from the droplet, dissipating it in the atmosphere. The energy ? originally from the sun ? is not destroyed, but is converted into diffuse heat that cannot be used to generate winds.

This is basic physics, but that doesn't tell us how much energy is lost, says Olivier Pauluis of New York University. With Juliana Dias of the NOAA Earth System Research Laboratory in Boulder, Colorado, he set about finding out.

We know roughly how much energy is dissipated by a single raindrop, so Pauluis just needed an estimate of total global rainfall. He got that from the TRMM (Tropical Rainfall Measuring Mission) satellite, which also told him how far each raindrop fell ? a crucial point, as drops that fall further dissipate more energy. On average, 1.8 watts were dissipated for every square metre of atmosphere.

That's a lot, says Dargan Frierson of the University of Washington in Seattle. "Falling water droplets and ice crystals make up only a tiny fraction of the total mass of the atmosphere," he says, but nevertheless they take out a lot of energy. Pauluis's estimate may even be too low, as TRMM is thought to underestimate rainfall.

Small changes in future

As Earth's climate warms due to greenhouse gases, rainfall is expected to increase. The raindrops will also fall further, because in a warmer atmosphere water vapour has to rise higher before it condenses.

Pauluis expects energy losses from rainfall to increase by a few per cent for every degree of warming ? meaning there will be marginally less energy available for wind.

That's in line with existing predictions, says Frierson. "We do expect large-scale tropical circulations like the Hadley Circulation and the Walker Circulation to decrease in strength with global warming," he says. The Hadley Circulation helps to power the trade winds and jet streams, which control how weather systems move around the world, a weaker Hadley Circulation would mean weather systems move around slightly less.

The change is unlikely to affect our daily lives, however. The energy loss probably won't affect hurricanes, which are expected to become more powerful as the world warms. That's because the strength of hurricanes is determined by sea surface temperatures, not energy in the atmosphere.

Neither will the loss of energy be great enough to affect the operation of wind turbines.

Journal reference: Science, DOI: 10.1126/science.1215869

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