Winter Storms & Precipitation

04
Dec
2013
Posted by: AK Tutoring  /   Category: Oklahoma Education / Weather   /   1 Comment

As a northerner by birth and meteorologist by profession, it’s time for a subject close to my heart – winter weather!  Rain in summer is fun, but nothing rivals the variety of precipitation that falls from clouds in winter.  I’ve got winter weather on the brain today because Oklahoma is set to get a winter storm, with a Winter Storm Warning going into effect at 6 a.m. tomorrow.  Here is a guide to the different types you might see this week and later on in the winter:

Snow – When you think of winter weather, you probably think of snow!  Snow is able to reach the ground when the air all the way from the cloud in which they form to the ground is at or below freezing.  Look carefully at the snowflakes that fall into your mittens as you stand outside.  The shapes of the flakes are dependent on the temperature in the cloud in which the flakes formed.  The prettiest snowflakes are called “dendrites,” and form when the air is just below freezing or right around 0°F.  These snowflakes are “sticky” because they have a relatively high liquid water content.  As temperatures get colder, you’ll see less and less of the dendrites, and more of the columnar and plate-like flakes, which are also drier and don’t stick together as well.

Sleet – Sleet is an ice pellet that forms when snowflakes fall through an above-freezing layer on their way to the ground and partially melt or fully melt into raindrops.  The rain drops then fall through a deep below-freezing layer next to the surface and re-freeze – but this time into ice pellets – before reaching the ground.  Many people ask me, how can you have a warm, above-freezing layer sandwiched between two cold, below-freezing layers?  In Oklahoma, our cold air is usually Arctic in origin, and our warm air usually originates from the Gulf of Mexico.  You might have heard that cold air is “heavier” than warm air.  What that is actually referring to is the density of the air, which is related to how close together or far apart the molecules are located.  Imagine that a layer of cold Arctic air is located next to the surface.  If the winds are from the south, warm Gulf air will be carried northward into Oklahoma, but will rise up and over the layer of denser cold air, thus creating the cold/warm sandwich.

Freezing rain – It might seem counter-intuitive, but water can remain in liquid form in below-freezing temperatures!  Liquid water below its freezing point is called “supercooled,” and can exist in this form in large part due to the barrier to phase change that the surface tension of a round drop creates.  The first step in the formation of freezing rain is the same as for sleet, as snow fully melts into rain as it falls through a layer of warm air.  In the case of freezing rain, however, the layer of below-freezing air next to the ground is very shallow; the drops become supercooled as they fall through this layer, but do not have enough time spent in freezing air to become sleet.  Instead, they remain in liquid form until they come into contact with a cold surface (roadway, windshield, tree branch, etc.).  The contact with the cold object overcomes the surface tension barrier, and the drops freeze into ice upon contact.  Oklahoman natives and newbies alike quickly realize that ice storms are a common occurrence in Oklahoma.  In fact, the Oklahoma City metro is located in a maximum of ice storms over the past decade!

Snow, sleet, and freezing rain are the most common types of winter weather that people think of, but there are a few other types of winter weather that you might have experienced:

Rime – Rime isn’t precipitation, and doesn’t fall from clouds like freezing rain, but it still creates a layer of ice on exposed objects.  We most often experience riming when “freezing fog” forms in below-freezing temperatures.  The small water droplets that make up the fog have extremely high surface tension and remain supercooled until they come into contact with a cold surface onto which they can freeze.  Riming creates a bumpy, whitish layer of ice, as opposed to the clearer, smoother glaze produced by freezing rain.

Graupel – Small ice pellets may be graupel rather than sleet.  Graupel forms when small supercooled water droplets rime a snowflake as it falls through a cloud and toward the ground.  Graupel is more opaque and softer than sleet.

Frost – Frost is similar to rime in the sense that it isn’t precipitation and creates a layer of ice on exposed objects, but is formed when ice crystals form directly on cold objects, rather than through the freezing of supercooled water droplets.

Winter storm forecasting is notoriously difficult – the transition from rain to freezing rain to sleet to snow may happen over a few tens of miles, and predicting exactly where that rain-snow line will occur is tough.  Forecasters at the National Weather Service office in Norman, OK are working hard to keep Oklahomans as updated as possible about the impending winter storm.  The Norman office is constantly updating their Facebook page and website with new information about the winter storm.  I suggest liking their Facebook page to keep current, both during this winter season and onward into warmer months!

I’m sure many of you have your fingers crossed for a snow day – enjoy the winter weather but stay safe and be wise about travel.  If you don’t have to be on the roads, please stay home and enjoy a relaxing day of hot chocolate, Christmas music, and studying!  Maybe we will have some winter storm and precipitation pictures to share in the next few days.

Kovacik,C., J. Hocker, and M. Shafer, 2010:  An analysis of southern U.S. ice storm frequency from 2000-2009.  Research Experiences for Undergraduates, University of Oklahoma, 17 pp. [Available at https://www.facebook.com/US.NationalWeatherService.Norman.gov]

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