Behind the Forecast: Why do clouds float

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Sep. 23, 2022 at 9:49 AM EDT
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LOUISVILLE, Ky. (WAVE) - Clouds look so light and fluffy as they float above our heads, but clouds are exceptionally heavy.

Tiny water droplets comprise clouds. For a cumulus cloud, the estimated water density is nearly half a gram of water per cubic meter. A one cubic kilometer cloud contains a billion cubic meters. Five hundred million grams of water droplets make up a one cubic meter cloud. This means clouds weigh about 500 thousand kilograms or 1.1 million pounds; that’s around five times as heavy as a blue whale, the largest animal in the world.

If a cloud is this heavy, why doesn’t it fall? The key is density.

As clouds form, rising air expands as atmospheric pressure decreases. As the rising air enters the higher altitudes, the air cools while the water vapor within it condenses. It’s this cooling and condenses that contributes to the growth and lifespan of clouds.

The distance from a water droplet’s edge to its center ranges from a few microns (thousandths of a millimeter) to tens of microns.

The speed at which an object descend correlates to its surface area and mass. Since cloud droplets are so tiny, they do not fall quickly; an ice crystal’s shape further reduces its velocity. The average cloud droplet has a terminal velocity of 1.3 cm per second in motionless air, according to the National Weather Service. It would take this droplet more than 10 hours to reach the ground from a cloud base of 1,650 feet. The smallest upward motions in the atmosphere are strong enough to offset a water droplet’s velocity.

All of these tiny water droplets or ice crystals that make up are so small and spread out over such a wide area that the effect of gravity on them is inconsequential. This is why clouds seem to float overhead.

It’s also important to note that a single water molecule weighs 18 g/mol. Dry air is mainly comprised of nitrogen and oxygen. Nitrogen weighs 28 g/mole while oxygen weighs 32 g/mole. So a single water molecule is lighter than both nitrogen and oxygen; this is why moist air is lighter than dry air.

Once droplets aggregate into large enough raindrops, they will eventually fall.