How Is Sleet, Why Is Hail, and WTF Is Graupel?
Here's your cheat sheet to all the weird frozen stuff that falls from the sky.
They say the Inuit have 50 words for snow, and that may be true, depending on who you ask, and how you count. In English, too, you might be surprised at how many different ways there are to describe cold stuff that falls from the sky.
What you call that stuff falling from the sky is a totally different matter. As a way to standardize weather reports, meteorologists and pilots use METAR codes to distinguish between precipitation types. Designed to be short and sweet, snow becomes SN, rain becomes RA, and drizzle is DZ. Winter, in particular, brings strange things falling from the clouds. Here’s your guide to getting the names right, so you don’t sound like an idiot.
Sleet
Americans call this sleet, but most everyone else call it ice pellets.
Sleet is probably the most confusing of the precipitations, because its definition changes depending on where you are in the world. In most Commonwealth countries, including England and Canada, sleet refers to a wet snow, or a mix of rain and snow (RASN). America, always daring to be different, defines sleet as ice pellets (PL). Ice pellets are formed when snow that forms high up in the atmosphere melts as it comes through a layer of warmer air below. A third layer of air, this time cold again, then refreezes what had been small water droplets in pellets that fall to the ground, bouncing off hard surfaces on contact.
Graupel
A graupel shower is like a miniature snowball fight.
Graupel (GS), also known as soft hail or snow pellets, forms when snowflakes encounter tiny droplets of supercooled water as they fall. This water immediately freezes and binds to the flake, and if this happens enough times, it stops looking like a snowflake and starts to look like a tiny, squishy snow ball.
Snow Grains
Snow grains (SG) are like baby graupel, and they are sometimes referred to as granular snow. Because these flakes are less built up than ones we’d call graupel, they tend to be flat and elongated rather than fully round. “Descriptions of the physical structure of snow grains vary widely and include very fine, simple ice crystals; tiny, complex snow crystals; small, compact bundles of rime; and particles with a rime core and a fine glaze coating,” according to the American Meteorological Society’s glossary of meteorology. One telltale sign — they don’t bounce or shatter when they hit the ground.
Diamond Dust
Like the dust of real diamonds, diamond dust is rare and sparkly. It’s similar to ice fog, another awesome frozen phenomenon, except in an ice fog the frozen particles are suspended in air, whereas in diamond dust they tumble gently to Earth. Meteorologists refer to this precipitation by the less poetic term, ice crystals (IC).
Freezing Rain
Freezing rain is beautiful and treacherous.
Freezing rain (FZRA) looks like rain, but it’s a lot more dangerous. It occurs when snow melts to rain as it passes through warmer air, then cools off to freezing or even below freezing temperatures before hitting the ground. Water that is still liquid despite below freezing temperature is called supercooled — this happens pretty easily in very cold weather, so long as it is free of contaminants and free of a crystal nucleus to catalyze the transition from liquid to solid.
The trouble comes when the supercooled liquid hits the ground, or a tree branch, or a powerline. The water will instantly freeze, encasing anything it hits in ice. If enough freezing rain falls, tree branches will start collapsing under their weight, and take out the powerlines with them. Roads and sidewalks become like skating rinks. In December 2008 an ice storm in the Northwestern U.S. left 1.25 million homes and businesses without power.
Freezing drizzle (FZDZ) is to freezing rain what normal drizzle is to normal rain.
Hail
Hail (GR) is the weirdo of frozen precipitation, because it happens almost always in the summer time, during thunderstorms. Hail needs warm, wet air and strong updrafts. The updrafts carry water in high levels of the atmosphere, where it’s very cold and freezes instantly. After freezing it will fall, and might get caught in the same updraft again, gaining in size each time through the cycle. The stronger that vertical force, the heavier the hail can get before it tumbles to Earth. That’s why you can get major hail storms in strange places, like the desert in Dubai.
