-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