Why Another Freeze Warning Tonight?
By Erica Grow on April 29, 2012, 11:07pm
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The National Weather Service has once again issued a Freeze Warning for our area. Tonight's freeze warning covers the entire state, even the shoreline. Previous Freeze Warnings over the past few days have been smaller in scope, leaving out the shoreline or more. It's not surprising that, with similar weather conditions persisting in our area for several days in a row, that the extreme element of those weather conditions would be more significant with each passing day... or, in this case, each night.
If you've been watching our weather reports over the past few days, you know the 3 main components which are leading to near-record cold nights: clear skies, light winds, and low humidity. Let's go into each of these in detail to see how they contribute to a cold night.
Clear skies: Cloud cover inhibits the loss of daytime heat when it's present in the overnight hours. After the sun goes down, the ground starts cooling off by emitting heat in the form of longwave radiation. Clouds absorb that longwave radiation and emit some of that radiation back down toward the Earth's surface. So, when clouds are in place, we lose a lot less of the day's heat.
Light winds: The wind speeds generally slow down at night because the daytime sun provides atmospheric instability. The atmosphere, therefore, is usually more stable when the wind is lighter. If overnight winds are calm, this allows daytime heat to rapidly escape away from the Earth's surface; on the other hand, breezes will mix the outgoing longwave radiation back down toward the surface.
Low humidity: Water vapor has a much higher capacity than dry air to hold onto heat. You can observe this in your kitchen-- turn on the stove, and hover your hand above the burner. The air is hot right away. But if you put a pot of cool water on that same burner, you could stick your hand in the water, even a minute or two after the burner is turned on. Conversely, when the burner is turned off, the air cools right away, but the water takes much longer to return to room temperature. Apply that principle to the air, and you can see why a humid airmass cools down more slowly than a dry airmass.
Tonight's conditions combine all three principles described above. This is why a Freeze Warning is once again in effect. Moisture and clouds come back into the forecast on Monday night, so the freezing temperatures won't be in the forecast beyond tonight.
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