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Night Warmth
Name: Nick Pryor
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2000-2001
Question:
Why does it stay warm at night?
If it is sunny during the day the earth absorbs a substantial amount of
the incident sunlight, unless the groun is snow covered. That energy
re-radiates during the night, tending to keep the air warm. The effect is
greatest if it gets cloudy at night because the heat radiation [infrared]
tends to be trapped by the clouds.
Vince Calder
Nick,
Your question is very general -- the answer might depend upon where you
live. It doesn't stay warm everywhere at night. Do you think it stays
warm at night in the Antarctica?
I shall attempt a general answer: During the day, the earth and objects
on it may be warmed by sunlight. The heat that is absorbed enables the
earth and those objects to serve as a heat sources at night when there
is no sunlight. Gradually, things cool off toward morning at which time
the warming cycle begins again.
It is a little like a car engine that is cold when it is first started,
warms while it is running, and cools again when it is turned off.
Objects that are warmed during the day, gradually cool during the night.
This is very apparent when one considers the difference in temperature
between cities that are large sources of heat and the open countryside
that cools much more quickly during the night.
Regards,
ProfHoff
Nick,
If you have ever removed a pot from the stove, you
know it does not cool instantly but instead takes a
bit of time to return to a lower temperature, safe for
handling.
The heating caused by light from the sun occurs as the
light strikes the earth and warms it. The warmth is
highest closest to the earth and dissipates the
further from the earth you move. Note that even
though the sun is not "out" at night, the temperature,
like the pan taken from the stove, remains elevated.
Additional continued warming could be provided by
movement of air currents from warmer areas of the
globe into cooler areas. In this case, the heating
again originated from the sun, but the heat itself is
carried in by adjacent warm air masses.
Both these mechanisms prevent the air temperature from
dropping to the cold temperatures found in the expanse
of space between the planets. Good thing that the sun
rises each day to again create and continue the
warming cycle.
Thanks for using NEWTON!
Ric Rupnik
Nick,
There are at least four reasons that it may
stay warm at night.
If it is cloudy, the clouds present a barrier to
the loss of energy from the air below them. Much
of the energy (long wave radiation) lost from the
surface is absorbed by the clouds and then
re-radiated back into the air by the
clouds, thereby reducing the decrease of
temperature at night.
Secondly, warmer air may be coming into the area
from the south (where it is usually warmer than in
the north); this is called advection.
Third, a strong wind creates mechanical turbulence
that mixes air from greater heights (where it is
usually warmer at night when there are no or thin
clouds) down to the surface, which, in the process,
keeps temperatures at the surface warmer.
Fourth, it may be very humid, and I mean this in
the absolute sense, such as on a very muggy day in
the summer. Long wave radiation is absorbed by
water vapor and thereby keeps the air warmer.
Sometimes, most or all of these conditions exist
at the same time. In that case, nighttime
temperatures are not much cooler than daytime
temperatures.
David R. Cook
Atmospheric Section
Environmental Research Division
Argonne National Laboratory
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