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-27 °F in Alpena, Michigan, February 28, 2014

-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.

 

-27 °F in Alpena, Michigan, February 28, 2014 - RF Cafe

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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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

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