Re: Winds and temperature

[John Denker <jsd@AV8N.COM>]

Quoting Gary Turner <turner@MORNINGSIDE.EDU>:

> I hear frequently on weather reports comments along the lines of "little
> wind tonight so expect temperatures to drop".  I haven't managed to
> convince myself of the reason for this,

Various people have mentioned some correct parts of the answer;
let me try to pull together something more complete.

First of all, let me change the question slightly:  If we
have a CLEAR SKY and light winds, then we expect a big
temperature drop ... specially in the mountains, the desert,
or extra-especially the high desert.

Factors include:
 -- dry air is transparent to thermal IR.
 -- water vapor (clouds or humid air) is pretty much
    a black body at thermal IR wavelengths.
 -- terrain (including rocks, soil, plants, puddles, etc.) is
    pretty much a black body.
 -- the terrain cools by radiation.  Stefan-Boltzmann and all that.
    It's trying to come into equilibrium with the sky, which is at
    three kelvin!
 -- typical terrain (not including the ocean surface) has rather
    little heat conductivity, so the surface of the terrain cools
    quickly.
 -- the lowest levels of the air cool by conduction, by contact
    with the cold terrain.
 -- if you stir the air (e.g. due to wind) you would have to cool
    the entire air column, which has a considerable heat capacity.
    OTOH if you don't stir the air, only the lowest bits of air
    are subject to cooling, so they cool quicker and get colder.
    Most people (not including pilots) generally only care about
    the temperature of the lowest few feet of air.
 -- Once a cold layer forms, it will tend to stay there since it
    is an inversion.  The normal profile of the air is to be
    warmer below and cooler above, right at the very edge of
    convective instability.

Non-factors include:
 -- Sinking of cold air.   There's usually not any supply of cold
air available for sinking, and if it did sink, it would warm
itself adiabatically anyway.