It’s frigid out­side, so we’ll get snow, right? That’s hazy.

The Washington Post - - KIDSPOST - Felt­man is an ed­i­tor at Pop­u­lar Sci­ence mag­a­zine. RACHEL FELT­MAN

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 be­lowfreez­ing 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.”

HISOKA

All pre­cip­i­ta­tion starts as snow in the clouds, but for snowflakes to fall, cer­tain con­di­tions must be in place.

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