Brr!!! It's not quite time for the Four P's (plants, pipes, pets and people), but we're in for a couple of cold mornings across Southeast Texas, starting with tonight. The National Weather Service issued its first freeze warning of the fall for the highlighted counties north of Houston. Its here where temperatures will likely fall below freezing for a few hours. If that's where you are and you have tender plants, bring 'em in for the next few nights.
A dry, cold air mass that moved in behind Sunday night's rain has been gradually settling in across the area. Tuesday night, we missed out on bitter cold thanks to a blanket of clouds that drifted in late that afternoon. However, Tuesday morning quite a few places woke up to temperatures in the 30s. Technically, no one had a freeze, but Conroe came pretty close and so did Huntsville.
Clear skies, dry air and light winds all contribute to what is called radiational cooling. Basically, in these conditions, the earth releases all the heat it obtained during the day back up into the atmosphere. The coldest part of the day is just prior to sunrise; after that, the big light bulb in the sky heats us back up.
One of the key numbers I look at on a night like this is the dew point, a measure of the amount of moisture in the air. Typically, on a night when radiative cooling is expected, the overnight low approaches the dew point measured around 10 or 11 pm. The air doesn't add moisture overnight so this is a good rule of thumb.
The freeze warning doesn't include Harris county or the city of Houston. The "typical" date of the first freeze in H-town is usually around the first week of December.