Tuesday, November 30, 2010

Sleet and Snow at 45 Degrees?

A cold front moved through the area yesterday ushering in cooler and drier air to East Texas. Behind the front precipitation developed in the colder air in the form of snow around 12,000 feet in the atmosphere. As this snow fell towards the surface, most of it evaporated not reaching the surface. However, in a few areas where the precipitation was heavy enough, light rain and sleet did make it to the surface mixed at times with a few snowflakes. So how is it possible for sleet and snow to reach the ground when the surface temperature is well above freezing, in the middle 40s?

Well, you have to look at the temperature and moisture content of the entire atmosphere to see if sleet and snow will reach the surface, not just the surface temperature. By looking a forecast sounding from Tyler this morning, or a profile of the atmosphere, precipitation was developing in an area where the temperature was between -10°C and -20°C, the perfect temperature for snow crystal growth. As the snow fell from the clouds it entered a layer of the atmosphere that was very dry allowing most of the precipitation to evaporate. However, in areas where the precipitation was heavy enough, a few flakes remained. Once the snow reached around the 5000’ mark in elevation it entered an area where the temperature was slightly above freezing. So much of the remaining snow partially melted in this layer only to refreeze, in the form of sleet, as it entered below freezing temperatures again around 4000’ in elevation.

Now according to the forecast sounding for Tyler the depth of above freezing temperatures from the surface was about 1900’, usually enough to melt the sleet and snow before reaching the surface. But another temperature that is very important in winter weather forecasting in the wetbulb temperature or the temperature that would be reached when evaporation occurs. The depth of the above freezing wetbulb temperature this morning was only around 800’. So in areas of more intense precipitation this morning, the process of evaporational cooling would bring temperatures to the wetbulb level allowing sleet and snow to reach the surface. I must say I expected a little light sleet this morning, like we saw last Friday. But I was very surprised to see a few snowflakes hitting the wind shield this morning until I analyzed the forecast sounding. There always seems to be something exciting in the world of weather.

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