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-27 °F (-31 °C) was the low temperature in Alpena, Michigan this morning, February
28, 2014. That was the official measurement at the airport, which is typically a few degrees warmer than here on
Long Lake, about 15 miles north of there. I knew from the star-filled sky last night that it was going to be frigid.
Without a cloud cover to insulate the Earth, radiative cooling can be quite pronounced, especially in low humidity
conditions as found in the desert and frozen-solid, ice-covered ground. Our daytime highs and nighttime lows have
consistently been 15 to 20 degrees below the long-term averages since we arrived nearly three months ago - a brutal
introduction to northern Michigan. The summer had better better be nice and cool.
Stefan-Boltzmann Law: P = Aεσ T4
| Where: |
P (watts) is the radiated power from a body of area A (m2), at temperature T (K). ε is
emissivity, a dimensionless number between 0 and 1 that determines the efficiency of a body
to radiate and absorb energy. A black body has an emissivity of 1. Soil, asphalt and human skin
have emissivity of about 0.95. The emissivity of the clear night sky is approximately 0.74 at
0 °C. σ is the Stefan-Boltzmann constant, 5.67x10-8 Wm-2T-4 |
Here is a short tutorial of
nighttime radiative
cooling, with examples.
Posted February 28, 2014
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