Dear Science: How do millions of gallons of water defy gravity before they fall as rain?
Here's what science has to say:
This is the kind of salt-of-the-Earth science question that might make a reader or two roll their eyes, but I bet that plenty of us have failed to consider how weird and awesome the rain cycle is. Here's a mind-blowing fact: The average puffy, white rain cloud weighs as much as 100 elephants. One hundred! Elephants!
With that in mind, this question becomes a lot more interesting. How do all those elephants stay up in the sky? And if they're so good at defying gravity, what makes them finally come tumbling down?
The rain cycle is pretty simple: Bodies of water evaporate under the warmth of the sun, turning from liquid to gas. That warm gas rises up into the sky. But as it rises, it hits cooler air — and as it cools, it turns liquid again.
At first, these water droplets are itty-bitty. The average drop inside a cloud is about one-fifth of the thickness of a piece of paper. So while an entire cloud may be as massive as an elephant, the water inside isn't grouped into elephant-sized clumps, and the individual droplets are much lighter than the air around them. On average, the space a cloud sits in will weigh about a thousand times more than the cloud itself. The complex wind patterns that swirl below and around these clouds also help the micro-droplets stay afloat.
"They stay up there because the mechanism that forms the water in the first place is rising air," explained Mark Wysocki, an earth and atmospheric science expert at Cornell University.
Just like tiny dust particles that float on the air, seemingly defying gravity as they dance in the light, these droplets hang suspended in the sky — for a time. As molecules cool down, they slow, and that makes them more likely to clump together. For rain droplets, that means the cooler air causes bigger and bigger drops to cluster together. You can compare the water drops to pieces of paper: If you took two sheets of the same size, then tore one up into tiny pieces and crumpled the other into a ball, the wind would be able to lift those tiny pieces up into the air. The crumpled ball, which packs the same mass into a single high-density lump, would need some seriously gusty weather to keep from shooting straight to the ground.
Eventually, the droplets in the clouds are heavy enough to be pulled down by gravity — and that's rain!
Well, it's sometimes rain. If the air is cold near the surface, we might get sleet or snow instead. And if the air is too dry, the water might evaporate before it even hits the ground. Scientists call these streaks of would-be precipitation "virga," and they're the reason weather forecasts seem so wishy-washy.
"It's difficult for us to forecast how much rain will actually fall," Wysocki said. Meteorologists look at humidity, cloud density (how many water droplets are in a cubic centimeter of cloud) and how tall a cloud is to try to estimate rainfall. The distance of the cloud base from the ground is also a factor: In some regions, he explained, cloud bases can sit over 10,000 feet above the ground (on the East Coast, they're usually half as high). If the air is dry and the cloud is high, raindrops are unlikely to survive the trip down. In these cases, the rainfall above a lake — which injects the surrounding air with water vapor, making it a friendlier vessel for falling raindrops — might be significantly more than the rainfall over a piece of dry land just a mile away.
"That's why in some places in the West, if there's a wildfire, they don't want thunderstorms happening," Wysocki explained. The combination of high cloud bases and dry air mean that storms rarely send much rain to the ground. "They get the lightning striking and starting fires, but the rain doesn't actually fall to help put them out."
On the other end of the spectrum sits a phenomenon called the cloudburst. Scientists aren't totally sure what causes them, and they're rare. But when they happen, it's as if an entire cloud full of water condenses all at once and drops from the sky. Wysocki noted that this phenomenon was not responsible for the recent catastrophic rainfall in Louisiana.
The biggest cloudburst ever occurred almost exactly 60 years ago in Unionville, Md. On July 4, 1.2 inches of rain fell in just one minute. Nearly three inches of rain fell in an hour. That's close to the average rainfall for the entire month of July.
If you want to watch us make it rain, check out the video at the top of the post. Spoiler alert: We throw stuff off buildings. You should watch it.