We're seeing the first freeze this fall across Southeast Texas and it is a little early. Temperatures will slip below freezing in five counties: Houston, Trinity, Polk, San Jacinto and Walker. In fact, Tuesday morning, Huntsville did drop below 32°. It's all courtesy of a cold Canadian air mass that sunk into the state Monday as a high pressure cell pushed in behind a strong cold front.
Sometimes I'm asked how we can forecast the low temperature and the answer is in the dew point and the wind speed. Dew point is the measure of the amount of moisture in the air. The Canadian air mass brought down some very dry and cold air behind Monday's cold front. When skies are clear at night and the winds are calm, a process called radiative cooling releases the heat from the ground back to space. The ground releases the heat until sunrise the next day. That's why the morning low is usually reached around sunrise. The temperature can't fall below the dew point and typically dew point stops changing around 10-11 pm the night before. So at 10 pm, I check the dew points in the area and if the winds are calm, as they should be tonight, I use that to forecast the overnight low.
The process isn't always smooth. Winds tend to mix the air near the ground, preventing all the heat from escaping. Also, the winds might change direction overnight. For instance, if the winds pick up out of the southeast, this will bring moist air from the Gulf of Mexico and dew points will actually go up overnight. Tonight, the high will be parked right over us so the forecast method should work.
