A storm has passed through, and the ground is covered in frozen precipitation. But depending on its journey through the atmosphere, it could be snow, hail, or even sleet or freezing rain.
Three of those four listed need it to be a winter storm, but one can happen year-round – even in the summer.
What makes Snow?
Snow is the most common, of course. Snowflakes come when water vapor freezes directly to ice while skipping the part about being a raindrop first.
"Snowflakes that most of us are used to seeing are not individual snow crystals but are actually aggregates, or collections, of snow crystals that stick or otherwise attach to each other," according to NOAA. "Aggregates can grow to very large sizes compared to individual snow crystals."
Precipitation remains snow as long as the air mass stays below freezing throughout the snowflake's journey from cloud to ground.
"Up in the upper atmosphere -- we're talking about 18,000, 20,000, or upwards of 25,000 feet, precipitation usually starts out as snow," says FOX Weather Meteorologist Jason Frazer. "When it starts to fall, it is going to change based on the air that it is going to encounter. If it continues to see air below that 18,000 feet that is below 32 degrees, it's going to fall to the ground as snow."
But if warmer air starts to intrude into the region, then things get dicey.
How does freezing rain and sleet form?
Both sleet and freezing rain start with a snowflake that melts into a raindrop as it encounters a slice of warmer air on its free-fall through the atmosphere. But if there is a renewed area of freezing temperatures near the ground, the drop will refreeze.
If the wedge of warm air is relatively narrow and the area of freezing air is fairly thick, the raindrop will have time to freeze back into an ice pellet -- this is known as sleet (or on weather observations as "ice pellets" or "P.")
Sleet can accumulate on the ground but is like stepping on zillions of tiny frozen raindrops.
But if the wedge of warm air is relatively thick and the area of freezing air hugging the ground is shallow, the raindrop doesn't have quite enough time to freeze into a solid, but instead becomes "supercooled" to the cusp of freezing.
Once it hits the surface, it will instantly freeze to wherever it landed.
This process will result in freezing rain, or part of an "ice storm," and can be the most dangerous frozen precipitations. If you ever see a weather observation with the code "ZR", it's short for both "freezing rain" and "uh oh".
Unlike sleet, which is more of an accumulation of pellets, freezing rain creates an icy glaze and turns streets and sidewalks into sheets of ice, making driving and walking quite treacherous.
What's worse, the ice is transparent, and many unsuspecting drivers can find themselves hurtling forward with no traction and no way to control their vehicle if they suddenly reach a road that was just hit by a freezing rain event. Some of America's worst accidents involving dozens to hundreds of cars have happened in freezing rain events.
Freezing rain will also coat tree branches and power lines, and if there is enough weight accumulation, it can topple them both, leading to widespread power outages and tree damage.
Simply put: Freezing rain and ice storms are a mess and require extreme caution during these events.
What makes hail?
Hail is the one frozen precipitation that can fall no matter the date on the calendar or the temperature outside.
They are balls of ice that can range from the size of a pea to a volleyball, and fall from a cumulonimbus (thunderstorm) cloud.
A raindrop freezes into an ice pellet and then falls, picking up raindrops through warmer sections of the cloud until it hits the updraft and gets blown skyward where that extra water freezes, making the hail larger and heavier before it falls again.
The process repeats until the hailstone is heavier than the updraft can support, and then it falls to the ground.
But updrafts of 50-plus mph can create huge hailstones such as golf ball- or baseball-sized or, yes, even "gargantuan"-sized.