|OVERNIGHT RADIATIONAL COOLING FACTORS
METEOROLOGIST JEFF HABY
Weather and surface conditions can help maximize radiation loss and/or surface cooling. These include clear skies, long nights, light wind,
dry air, ground surface cover type (snow, less vegetation), and being in a valley. Each of these will be explained for
how cooling takes place.
Clear Skies: Allows maximum amount of longwave radiation to escape to space. Less radiation is absorbed and reemitted back toward the
surface. Thus, an overall net cooling is maximized. When clouds are present, less longwave
radiation escapes to space.
Long nights: The nights are longest in the late fall and early winter. The longer the night then the longer the amount of time longwave
radiation can result in cooling of the ground surface. Since cooling is cumulative, a longer
night results in greater cooling.
Light wind: When there are weaker winds then there is less mixing between the air at the surface and warmer air higher aloft. This
allows the cooler air at the ground surface to stay in place and to continue cooling.
Dry air: When the air is drier there is less water vapor in the air. This is important since water vapor is a greenhouse gas that
absorbs and emits longwave radiation. When it is drier then more longwave radiation can escape to space. This produces
a net cooling of the ground surface.
Snow cover: Snow is a good insulator, thus warmth within the soil is kept from warming the air above the ground. Snow
is also a strong emitter of longwave radiation. These two factors help snow cover to significantly cool the air at
the ground surface at night.
Less vegetation: With less vegetation, less moisture is transpirated into the air. Drier air helps
the air cool more at night.
Being in a valley: Cold air is denser than warmer air. Thus, under the influence of gravity, colder air will flow toward a lower
elevation in the valley. This is why valleys can be cooler than areas surrounding the valley at night.