Weather In Depth: What’s with the cool nights?

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Another morning this week with a chill in the air. This Friday morning’s low of 45 has been the coolest our morning temperatures have been since May 17, when we had a morning low of 43.

What is the cause of the cool mornings lately? Firstly, this is thanks in part to a fall airmass that has rolled in across our region, but our overnight low dropping under the clear skies can be directly attributed to a process known as “radiational cooling.” This phenomenon occurs when there are clear skies and calm winds at night. The clear skies and calm winds allow the heat at the surface of the Earth to escape freely into the upper atmosphere without any restrictions to keep it at the surface like when there is cloud cover or windy conditions. This happened Thursday night and early Friday morning across our region as temperatures around midnight were around the 50 degree mark and then dropped off into the mid and low 40s. This process is delicate, as a sudden gust of wind and the thinnest layer of cloud cover can stop temperature from cooling down like it did last night for some locations.

This process will once again be evident in Friday night’s lows as the conditions will be clear and calm with temperatures dropping into the mid and low 40s once again. Not only that, but as we progress through the next month or two, it will be even more pronounced as the trees begin to lose their leaves.