12.7.12 What type of winter precip is THAT? - FOX44 - Burlington / Plattsburgh News, Weather & Sports

Coming up soon in the forecast (Sunday night to Monday), you will hear plenty of "wintry" words. More specifically, "wintry" precipitation! Do you know the difference? You SHOULD! You live in the northeast, for cryin' out loud!

So let's start with the forecast for late Sunday night to early Monday. (Keep in mind, this is as of Friday, so be sure to check back with our full forecast page through the weekend for updates). I anticipate most of Sunday will be dry, but a slug of moisture will move in Sunday night, beginning as snow and sleet. Then, as warmer air begins to flux in, a turn to freezing rain will develop for many areas, especially east of the Green Mountains. Then, once enough warm air fills in, freezing rain will change to rain.

Did you follow all that? Do you really know what kinds of precipitation will fall? Let's talk about each type!

First, *snow*. I think that one is pretty clear around this part of the country. Snow is frozen precipitation, a collection of ice crystals, that falls as a light white flake or collection of snow crystals. The temperatures in the atmosphere from the earth's surface to the higher atmosphere are completely below 32.  See the atmosphere profile for snow on the left, top image (click to enlarge)

Sleet is frozen precipitation that has partially melted, and refrozen as it falls through the atmosphere. Sleet forms when the earth's surface temperature is still at or below freezing, but a warm layer of air exists in the mid atmosphere. Sleet will begin as snowflakes in the clouds, but with a layer of warmer air in the atmosphere, that snowflake will melt. Once that melted mass drops to the Earth's surface, it will refreeze into a small glob of icy precip- often it will refreeze into a solid hard ball of ice. See the atmospheric profile for sleet on the left, middle image (click to enlarge).

Freezing rain is probably the most impactful/dangerous type of winter precipitation. Picture the same set up in teh atmosphere as sleet with snowy clouds, a warm layer lower in the atmosphere, and then a sub-freezing layer of air at the earth's surface. The difference will be the depth of the sub-freezing surface layer. Unlike sleet, that cold layer is much more shallow, so the melted precip doesn't have as much time to totally re-freeze into ice. Instead, that melted particle becomes "supercooled" or cooled to a temperature at or below 32 but hasnt yet frozen into a solild particle. Once that freezing rain droplet makes contact with any surface, it will freeze instantly. This will create a layer of ice on any and all surfaces that gets "rained" on. See the atmospheric profile for freezing rain on the left, bottom image (click to enlarge).

Finally, plain rain is also a familiar precip type. Rain is all liquid, not frozen precipitation. This forms as the atmosphere is all above 32 degrees from surface to cloud.

Finally, you will see how these types of precipitation progress in the atmosphere as a warm front advances into a region. Precip will typically follow the order: snow, sleet, freezing rain, then rain as the atmospheric temperature gradually climbs in the higher atmosphere, gradually down to the surface.