Radiational Cooling

Technically speaking there is radiational cooling every night it’s just that during our cooler season it often takes front and center stage when we give the forecast. A classic radiational cooling night is one where a very dry air mass moves in with high pressure nearby causing winds to become light or calm. Dry air cools very quickly after sunset and because cold air is heavier than warm air the cold drains off of hill sides settles into low lying areas. Hilltops can have temperatures 10-15 degrees warmer than low areas. Normally we think of hilltops as having cooler temperatures than low lying areas and this is true when the wind is blowing but not so on a radiational cooling night. When we give the forecast low temperature we post the expected low for San Antonio Airport because that is where the official records are kept but you really should know how your particular area runs compared to the airport. I know for example that at my house my low temperature will run very close to the official airport temperature on a radiational cooling night even though I live 17 miles north of the airport. Nearby though, in Leon Springs the morning temperature will run 7 to 8 degrees lower. They typically will see a frost 2 to 3 weeks earlier than I will. That’s because I live up on a hillside and Leon Springs is down in a valley.

In the winter when we get a stong norther with over running moisture my house will run about 3 degrees colder than the airport or Leon Springs. I have seen on a number of occasions where that 3 degrees made all the difference between freezing rain and just plain rain. On those occasions you could see ice coating the hilltops of the Dominion and no ice at all in downtown Leon Springs.

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One Comment on “Radiational Cooling”

  1. Bob M near Boerne Says:

    Of course there was the craziness when Austin started breaking lots of low temp records when the main airport was relocated from n of downtown to old Bergstrom down in the Colorado river basin.

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