On February 18th, 2016 we learned about

Forming snowflakes when it’s not freezing cold

My first grader recently asked if snow angels were real. Not real in the sense of divine flying creatures, but if people ever flopped down in the snow, waving their arms and legs as she’d seen in books and movies. While I’ll attribute this to her being a native of California, it may be a question on more kids’ minds in the future as the United States sees more rain and less snow during the winter. As average temperatures rise, getting to the perfect conditions for a beautiful snowfall may be less common. You might assume that “perfect” means “freezing,” but it’s not actually that simple.

How critical is the cold?

There are actually a couple of temperatures that need to be right for a good snow. The first issue is the temperature up around the clouds, which does indeed need to somewhere at or below freezing. It really can’t be too cold for snow, although colder air isn’t as good at holding moisture, which would leave too little water to freeze into snowflakes. This is why some parts of Antarctica can be very cold but with very little snowfall, since they’re just too dry for any real precipitation.

Cold air close to the ground is also a sure win, but snow can survive slightly warmer conditions as it falls. As they warm slightly, the outer layer of melting water on the ice crystal will trigger evaporation cooling in the air around it, as the newly warmed water had to draw its heat from somewhere. That cooled air then adds less heat to the remaining snowflake, slowing the melting process. This allows a wetter flake to make it to the ground when ambient temperatures are above freezing, even as high as 41° Fahrenheit.

Warmer under the snow

The snow that does accumulate on the ground will often melt slightly, then refreeze if temperatures drop again to form an outer crust. Subsequent snowfalls add more and more layers of snow, which can help keep itself cold while sitting on the not-quite-frozen earth. With sufficient coverage, the snow is actually insulating for creatures living in the ground below, particularly against any sudden drops in the surface temperatures. Rather than see all their warmth get wicked away in a big blizzard as the ground freezes, a blanket of snow will keep temperatures constant for subterranean animals like porcupines or willow ptarmigans. Animals living above ground also benefit, because a layer of snow will keep the ground from hardening, allowing them to forage for food more easily.

It’s also quite pretty, and of course, more conducive to sledding and snow angels.

Source: How Snow Forms, National Snow and Ice Data Center

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