But what’s the difference between these wintry precipitation types? It all comes down to processes happening high up in the sky.
The simplest form of wintry precipitation is snow, and it’s the coolest. Snowflakes form when water vapor crystalizes on something like dust or pollen that makes its way into the atmosphere. From there, a six-sided ice crystal develops.
You can probably guess what the temperature profile is like — cold all the way down! No melting, no refreezing. Instead, a snowflake just has to delicately waft its way from the clouds to the ground.
It may surprise you, however, to know that it doesn’t have to be freezing at ground level for it to snow. Flurries have been observed at temperatures as warm as 49 degrees. It can snow when it’s above freezing when the warm layer near the ground is shallow and the air aloft is cold, and the flakes don’t have a chance to melt.
Sleet is a bit more of a wild card for forecasters.
Starting off as snow, sleet forms when this snow passes through a relatively shallow warm layer and partially melts. It then passes back into a deep layer of frozen air. In contrast, freezing rain, discussed below, doesn’t have enough time to refreeze before hitting the surface, as sleet does.
Because of its melting and refreezing, sleet doesn’t boast nearly as much of a “fluff factor” as snow. An inch’s worth of rain might only stack up to three inches or so of sleet. For comparison, the same liquid equivalent might fall as eight to 12 inches of snow on average, with higher or lower ratios largely depending on the temperature.
Most of the time when sleet falls, the surface temperature is very close to freezing. Sometimes the ground is just above that crucial threshold. Even then, thanks to the density of sleet, it might cause headaches as it takes time to melt. Sleet pellets and crushed-up granules can also deposit an icy film over the road, making it very easy to skid.
Before moving on, we must briefly address how sleet is not hail, since they are often confused.
Hail is formed in convective updrafts. That’s when the rising air in a thunderstorm lofts water droplets high enough into the atmosphere to reach freezing temperatures — even in the steamy dog-days of summer. It’s this vigorous upward motion that sustains the hailstone and allows it to grow bigger and bigger until it outweighs the force of the updraft. Although they are both balls of ice, sleet and hail are formed through considerably different processes.
Freezing rain is perhaps the species of wintry precipitation most hated by forecasters and the public. It’s challenging to predict, it is extremely dangerous, and it is quite finicky.
Freezing rain is exactly what it sounds like — rain that freezes on contact. It typically begins as snow, then falls through a large enough warm layer to fully melt the snow. Unlike the air that creates sleet, the cold air nearest the ground is not thick enough to fully refreeze it. The rain instead becomes supercooled so it can more easily freeze on contact.
Freezing rain is especially hazardous following cold-air outbreaks, when days of repeated freezing conditions permeate the soil and lower the ground temperature below 32 degrees. It then takes only small amounts to turn sidewalks and streets into treacherous sheets of ice.
Ice storms caused by freezing rain can be far more destructive than snowstorms because of the full weight of ice accumulating on all surfaces. Although it takes only a trace of ice to be a big problem, the worst freezing rain events can bring two to three inches or more of ice accretion. They can shut down entire communities for days and take down virtually every power line.
Graupel is an oddball.
It looks a bit like sleet, but it’s generally softer, larger and cloudier. Sometimes called snow grains or snow pellets, graupel starts as snow. In a process somewhat similar to hail formation, graupel develops as the snowflakes get “rimed” and grow larger as additional ice builds on them as they are picked up by updrafts in a shower. Riming transforms your typical snowflake into something of a ball.
Like sleet, graupel is relatively common in above-freezing conditions at the surface. It occurs when there is significant instability for the cool season, and it does tend to come in showery-type precipitation, or squalls along a front.
Keep an eye to the sky
Now you’re better equipped to know what’s falling from the sky and why. If you want to take the next step and become a citizen scientist, one thing you can do is grab the mPing app and start reporting when and where you see these wintry precipitation types falling.
Capital Weather Gang’s Wes Junker contributed to this article.