Friday, February 4, 2011

Weather 102: Basics on the wet-bulb process and why you should care

Warning: this post is a little technical and lengthy! I've tried to simplify it for the average reader.  Let me know how I've done!

In developing winter weather scenarios, meteorologists (often not in front of you, lest they forever be known as weather geeks!) will often refer to the "wet-bulb temperature," or the wet-bulb process.  In it's simplest form, the wet-bulb temperature is a combination of the air temperature (or what is actually called the dry-bulb temperature) and the amount of water vapor in the air, typically measured by the dewpoint temperature.  Temperature and dewpoint are terms you have probably heard before.  Dewpoint is a measure of the amount of moisture in the air, which, if the air were cooled to that temperature, there would be 100% relative humidity.  So, if the temperature is 40 degrees and the humidity were 55%, the dewpoint would be 25.  In other words, cooling the air down to 25 degrees would produce 100% humidity.

The wet-bulb, on the other hand, falls between the air temperature and the dewpoint temperature.  It is defined as the lowest temperature that can be reached by evaporation only.  As rain/snow falls from clouds, evaporation takes place as the precipitation falls into drier air.  As the evaporation occurs, the amount of moisture in the air goes up, the dewpoint increases, and the temperature decreases, bringing them closer together.

"Wet-bulbbing" is important in winter weather scenarios (and actually all precipitation onset situations) because as the initial precipitation falls from clouds, it first saturates the atmosphere it is falling into through evaporation.  This process (called "evaporative cooling") lowers the temperature and increases the dewpoint.  If the temperature lowers to a point below freezing (when the wet-bulb temperature is below freezing), a temperature in the upper 30s could end up falling into the lower 30s and producing winter weather!

An example of the wet-bulb process can be seen as precipitation began on January 9, 2011 in the graph of temperature and dewpoint in Bartlett below.  Notice the temperature (white) fall and dewpoint (yellow) quickly rise between 6-8pm.  The 30 degree temperature ended up in the mid 20s and the dewpoint rose from the single digits to the lower 20s.  Precipitation falling shortly after 6pm did not reach the ground, instead evaporating and causing the wet-bulb process to occur. By 8pm, moderate snow was reaching the ground.  The difference between the temperature and dewpoint until 6pm also delayed the initial onset of the precipitation, likely resulting in virga (precipitation seen falling from the clouds but evaporating before reaching the surface).

The wet-bulb process occurred between 6-8pm on January 9, 2011.
Why bring this up now? Well, the process is occurring right now in the Memphis area!  Very light precipitation is falling from the clouds and evaporating. The difference between temperature and dewpoint is about 10 degrees (38 and 28). Through the evening, those two numbers will approach each other (nearing the wet-bulb temperature) and light rain will change to light snow due to the surface temperature falling!  Watch for it on the WXLIVE! graphical page, noting the temp/dewpoint graph.

You can get an idea of what the wet-bulb temperature is by moving air over wet skin. Are you cold when you first get out of the shower? In essence, you're experiencing the wet-bulb temperature of your bathroom!  The air feels cooler when exposed to a wet surface - you!

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