In article ,
Ron Garret wrote:

In article ,
"Martin X. Moleski, SJ" wrote:

Two masses in contact with each other (airplane skin and the air
that contacts it) have got to reach thermal equilibrium, all things
being equal and given sufficent time.

Not quite. They have to reach thermal equilibrium if there is no heat
flowing in or out of the system. But, as you correctly note, heat can
(and doe) flow in and out via radiation. Surfaces can "soak up" the
cold of the night sky (actually, they radiate their heat into the night
sky) and become colder than the surrounding air, just as they can "soak
up" the heat of the sun and become warmer than the surrounding air.
Eventually some of the cold/heat does get transferred to the air. This
is why clear nights tend to be colder than cloudy ones (and why clear
days tend to be warmer, all else being equal).


It is worth noting also that dark surfaces absorb and radiate more
readily than light ones, and so they get hotter during the day and
colder at night. Cirri are all painted white in order to take advantage
of this phenomenon and keep the skin from getting too hot in the sun.
(You'll never see a non-white Cirrus. It's part of the certification
conditions to paint the white.) Accordingly, Cirri are less prone to
radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.

FWIW,
rg