It’s snowing again.
That seems to happen a lot here—after all nyc is mired in one of the snowiest winters on record. But the weather here, much like the citizenry, seems to have a flair for the dramatic and the indecisive.
It rarely just snows. Often it snows, then rains, then snows again. Maybe there’s some sleet mixed in for good measure. Perhaps some freezing rain if it’s really a party. Then back to snow. All of which leads to a weather (and pavement) phenomenon I’ve taken to calling “aggressive slush.”
Between dodging epic icy puddles and navigating around mountains of frozen garbage (winter wonderland, nyc style), I started to wonder why it sometimes snows, sometimes sleets, sometimes rains, and sometimes slesnorains (working title). Here’s what I found out:
The type of precipitation that falls during any winter storm depends on temperature.
Ok duh, you might say to yourself.
But wait! I say. There’s more!
The temperature that you and I feel while cowering in a bus shelter during a sleet storm is not the same temperature that all those little ice pellets experience as they come racing out of the clouds. Those pellets, like the rest of Earth’s weather for the most part, originate in the lowest level of the Earth’s atmosphere, called the troposphere. The troposphere starts at the surface of our planet and continues up for several miles.
If you were to superhero jump to the top of the troposphere and/or the cloud level, you would notice that the temperature was actually rising as you got higher and higher. But as you approached the clouds, the temperature would drop once more.
Those in the know (which is about to be you!) call those temperature swings ‘vertical temperature distribution.’ There are several layers involved in vertical temperature distribution: the air right below the clouds, a middle layer of air below that, and the air directly above the surface.
Like we learned during our superhero leap, those layers are different temperatures. The width and temperature variation within those layers are essentially what determines whether you need rain boots, snow boots, ski goggles and a sturdy umbrella, or another day curled up on the couch.
Up towards the top of the clouds, all of the precipitation is snow. If all of the layers that the precip passes through are below freezing, then the precipitation remains snow.
Likewise, if the temperature between the clouds and the ground is entirely above freezing, then the precipitation will just be boring old rain and you will totally have to go to school tomorrow.
If the middle layer of air is above freezing, but is not very wide, the precip will fall as sleet. Those tiny ice pellets that hit and bounce off the ground are actually snowflakes that partially melted in the warmer layer, then refroze in the bottom layer of air.
The process of thawing and refreezing demolishes the crystal structure that makes snowflakes what they are, then the below freezing temperatures closer to the ground turn them into the tiny, violent little ice pellets we were hiding from before.
If, on the other hand, there is a big, fat layer of warmer (above freezing) air between the clouds and the ground, then the precip fully melts during its fall. But if the temperature near, and most importantly, on the ground is well below freezing, the newly melted raindrops will freeze upon contact with a surface, such as my bangs, your car, and the sidewalks, rather than in the air… presto! Freezing rain!
It’s important to remember scale here. It isn’t as if precipitation is doing some wild and crazy dance. See the amazing precipitation shape shift from snow to rain to snow in fractions of a second! That isn’t really it. A trip of several miles is quite a journey for a tiny ball of moisture and dust less than a few millimeters across.
One final point: sleet and hail are not the same thing. Sleet, if you recall, is made of tiny ice pellets—the kind that bounce off the ground when they hit. Hail is also made of ice, but much, much larger bits of ice. The National Weather Service, one of the great loves of my life and the place you should absolutely be getting your weather information, defines hail as a “ball of ice more than 5mm in diameter” that falls from a cumulonimbus cloud. Warm updrafts of air keep the hail in the clouds, thus ensuring that it has enough time to grow big and dangerous.
Now, go and find out what makes a cumulonimbus cloud so cululonumbus-y and please make sure to work ‘troposphere’ into your next cocktail party conversation. I, on the other hand, will be researching what kind of heathen god I can bribe into bringing spring a little faster.