°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 =
||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
of a body to radiate and absorb energy. A black body has an
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.
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