One of the best things about winter is playing in the snow. (Maybe you’re doing that today.) But as the weather gets colder, you may wonder why you often see icy rain — or just plain ice — instead of snow.
Here’s a quick reminder of how rain works: Water from the ocean evaporates (which means it turns from liquid into gas) as the sun warms it up. It rises high into the sky, but the air around it gets colder the higher up it goes. Eventually, the water is cold enough to turn back into liquid, and it clumps together with other drops of water. Once the drops are too big and heavy for the air to hold them up, they fall back down as rain.
So does rain turn to snow when it’s cold on Earth, too? Not exactly. Although getting some rain is as simple as having enough water in the air to form into fat, heavy droplets, snow is more complicated.
“Snow is probably the trickiest type of precipitation to forecast,” says Jaclyn Whittal, a stormhunter and meteorologist for the Weather Network. “It’s all about the column of air that it is falling through on its way down to the ground.” The air surrounding our planet isn’t just colder at the top and warmer close to the ground; currents of warm and cold air can circulate through the atmosphere, creating different weather patterns. Sometimes, even when it feels very cold, the air above us is actually forming something like a sandwich made of frozen bread and warm butter.
“All precipitation starts as snow way up in the tops of the clouds,” she says, “and if it encounters a warm layer on the way down, it can melt to rain, or partially melt to sleet or ice pellets. If the column is completely cold all the way down, then we get snow and we all have to go out and shovel it away — or better yet, go out and build a snowman!”
That explains why we sometimes end up with annoying balls of ice even when it’s very chilly — snow needs consistent cold all the way down! Otherwise, it melts and refreezes, losing the delicate crystal structure that makes a snowflake and forming icy sleet instead — sort of like when ice cream gets soupy and turns rock-solid once it’s back in the freezer.
We can get rain even on a below-freezing day if the warm part of that air sandwich is really thick, which is very dangerous. In that situation, raindrops freeze on the cold ground instead of in the air, which is how we get icy roads instead of snowdrifts.
But it gets even trickier: If it’s too cold all the way down that column of air, it becomes less likely to snow. Cold air can’t hold as much water as warm air can, so tiny, fragile ice crystals fall on their own instead of clumping together to make snowflakes.
And not all snow is made equal: The temperature on the day of a blizzard determines just what kind of wonderland we’re in for.
“The warmer the temperature is, the heavier the snow becomes,” she explains, because there’s more moisture to clump together into big, fat flakes. “In colder temperatures, we get light and fluffy snow.”