A Chilly September Night

It’s a sure sign that fall is around the corner. The temperature at Norfolk, CT dipped into the 30s early this morning. Across the rest of the state low to mid 40s were widespread which sent many (myself included) scrambling to close windows last night around nightfall!

Even colder weather is expected tonight. Last night the wind was relatively gusty but tonight the winds are expected to go nearly calm. Radiational cooling will take place in many areas, particularly in the valleys, and result in many readings in the 30s Saturday morning.

Calm winds and clear skies promote what we call radiational cooling. The longwave radiation emitted by earth into space is maximized on clear nights. On cloudy nights some of that radiation is absorbed by the clouds. A portion of that absorbed longwave radiation is emitted into space and a portion is re-emitted back to earth. On clear nights there are no clouds around to re-emit longwave energy back to earth. The more radiation escapes into space the cooler we get.

Calm winds prevent the atmosphere from mixing. The coldest readings will be found within an inch of the ground on a radiational cooling night with the temperature warming up the higher from the ground you are (to a thousand or two feet, maybe). We measure temperature at 2 meters but if you were to take a temperature measurement at 6 inches above the ground on a radiational cooling night you may be 5 or even 10 degrees colder in some situations! A gusty wind will promote mixing colder air up and warmer air down to make the lower part of the atmosphere more homogenous (this lower part of the atmosphere is known as the boundary layer). When the wind is calm, however, the cold near the earth’s surface is able to remain there in a very shallow area without being disturbed by the warm air above.

Just how cold temperatures get depends on a number of localized factors including how rural an area is, the topography of a neighborhood (relative low areas will cool better), the type of soil (sandy soil radiates the best), snow cover, and moisture content of soil. It’s not unheard of to have wild swings (at least 20 degrees) in temperature between say downtown Hartford and New Haven and a rural, valley community 20 miles away.

12z NAM 24-hour SLP Forecast

Tonight our computer models are forecasting a high pressure system right overhead. With the high overhead I expect winds to become calm in many areas and the bottom to drop out on the thermometer. Don’t be surprised to see frost in some “favored” areas Saturday morning.

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