Wednesday, December 05, 2012

Weather Whys Wednesday: Mixed Precip

Continuing our discussion of winter type weather issues. Today we focus on "mixed precipitation!"

One of the biggest pet peeves of meteorologists and weather enthusiasts is the ritual of butchering precipitation type that occurs in winter. While it's normally obvious that when someone in mid-January says, "It's hailing!" that they actually mean it's sleeting, it can still just be one of those things. And there are differences between the various forms of winter precip that don't fall under the category of "snow." So then, I present the user's guide to wintry and "unknown" forms precipitation.

Sleet and Ice Pellets

They are essentially the same thing. Sleet is the more meteorologically accepted term.

How does Sleet form?

Sleet, credit Wikipedia
In most cases, precipitation falling out of clouds is doing so as snowflakes. If the air is below 32F from cloud to ground, you get snow. But, if for some reason, there's an elevated warm layer, say between 5,000 and 10,000 feet, the snowflake will melt. Then, if the partially melted snowflake refreezes below 5,000 feet, you get an ice pellet...or sleet. So to get sleet, you need a warm layer aloft, that causes partial or total melting of snowflakes, but you will also need a subfreezing layer below that, to allow the liquid time to refreeze into ice. If that frozen layer is a little too close to the ground....

Freezing Rain

Freezing rain occurs by the same process as described above with sleet...but this time, the layer of temperatures below freezing is limited too close to the ground. So the partially or fully melted snowflakes will refreeze on the surface. Thus: It's raining, but freezing on contact...or freezing rain.

So both sleet and freezing rain form by melting snowflakes that refreeze: If they refreeze aloft, it's sleet...if they refreeze on the surface, it's freezing rain.

So then, what the Hail?

Hail forms in thunderstorms. Water droplets rise in cumulonimbus clouds, picked up by the wind within the cloud, known as an updraft. As it rises, it will encounter subfreezing air and, naturally become frozen. What goes up must come down, so the frozen droplet will get pushed back down by a "downdraft" and may even begin to melt. It then can get picked up by another updraft, and so on. This process repeats and the droplet will develop many layers of ice before often falling to the ground...known as hail. But the biggest difference here, is that while hail can form in the cold season, it's predominantly a warm season process, as opposed to a cold season one.

What is this Graupel stuff?

Graupel Through a Microscope. Credit: Wikipedia
When I worked in TV, I once used the term "graupel" on air and received a phone call from a viewer who looked through his dictionary and found nothing. The viewer accused me of "making up words." So naturally, I have learned that not a lot of people actually know what it is. Within clouds, you can have what we call "supercooled" droplets, which are droplet of liquid water that can stay in liquid form, even below as much as -40F no less! If one of those droplets encounters a snowflake, it will bind to the snowflake in a process called "accretion." When the snowflake encounters so much accretion that you can no longer tell it was a snowflake, we call this "graupel." Graupel differs from hail in that it is a cold season process and isn't usually the result of thunderstorms. Graupel also usually falls apart if you try and touch it.

Rime Time 

And then there's also Rime. Supercooled water droplets in a cloud (or fog) freeze to surfaces like trees or mountains. When this occurs (usually on the windward side of these surfaces), we call this rime. It can also stick to aircraft flying through clouds as well, and often you will read about "rime" in pilot reports and other aviation meteorology. It's also frequently reported on Mt. Washington in New Hampshire.

So let's recap....

Sleet: Falling snowflakes aloft melt, and then they refreeze before reaching the ground, falling as ice pellets.
Graupel: Supercooled water droplets freeze to snowflakes, falling to the ground as something similar to ice pellets, but different in formation.
Freezing Rain: Falling snowflakes aloft melt and do not refreeze again until reaching the surface.
Hail: Forms in thunderstorms due to updrafts and downdrafts.
Rime: Supercooled droplets in clouds (fog) that freeze to surfaces on the ground.