January 22, 2010 - The Big Deal About Sleet and Freezing Rain
Freezing rain and sleet - what's the difference and why? Read all about it!
I have to admit, some of my biggest pet peeves lie within the two terms of sleet and freezing rain. The first pet peeve – sleet is not hail, hail is not sleet. Yes, they are both ice that fall from the sky, but not for the same reason. The ice balls are formed in two completely different processes in the atmosphere. Generally, hail is associated with thunderstorms and sleet is associated with winter weather. That’s the simplest way to remember it.
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One down, one to go. The difference between sleet and freezing rain comes down to the depth of the cold layer at the surface. Let’s break it down a little further. For the most part, precipitation starts in the form of snow. We spoke about CAD in the Jan 21st blog and how when another system interacts with the Valley while we’re “dammed”, warm air overruns the cool air at the surface. Therefore, at some point in the atmosphere (usually between 5,000-10,000 feet), there’s a warm layer of air above 32 degrees F. The snow would melt in that layer and we would have raindrops.
Here is where the difference between sleet and freezing rain comes in. Sleet is a solid ball of ice – therefore we need a deep enough cold layer for that raindrop to completely refreeze before hitting the surface. Sleet can fall when the temperature is above freezing at the surface. With freezing rain, the depth of the cold layer at the surface is more shallow, so the raindrop can’t completely refreeze. Instead, it hits the surface and freezes on contact. The key with freezing rain is that the surface layer has to be at or below freezing. Otherwise, you have plain rain.
We’ve had quite a few winter weather lessons over the past few days as another round of winter weather is now in the books for the extraordinarily active 2009-2010 winter season.
Taking a quick peak into next week – after some rain on Sunday/Monday morning, we’ll start our next cool down. It’ll be quite noticeable by Thursday. Some models even bring in another storm by the weekend. We’ll discuss that much more next week!