Length of Daylight and Temperature
How does the length of daylight affect maximum
The longer the day, the longer the time a particular region of the Earth
is exposed. However, the problem is much more complicated for several
1 The angle of the region of the Earth's orientation alters the
amount of light striking that area. The maximum is when the region is
perpendicular the Sun; the minimum is when the area is almost parallel to
the Sun and the light just grazes the surface.
2 Cloud cover is very important since it scatters the sunlight and
prevents most of it from
reaching the Earth's surface. Dust, and various aerosols also scatter or
absorb light, preventing it from reaching the Earth's surface.
3. Not all (or possibly even most) of the light that hits the Earth's
surface goes to
heating the surface. A significant amount is converted to chemical energy
by photosynthesis reactions in plants (from pine trees to plankton). There
are also chemical reactions that use the sunlight that prevents it from
heating the Earth directly. One example is the conversion of oxygen to
4 A portion is reflected back into space and that also does not
contribute directly to increasing the temperature of the Earth's surface
(for example the surface ice at the north and south poles). These various
complicating factors are not independent, that is, they affect one another
in ways that are not always straightforward.
So the question of just how does the time of exposure of a region of the
Earth to the Sun is really very complicated.
The length of daylight can have a tremendous effect
on the maximum temperature.
Consider two days with the same temperature at sunrise.
On the longer day, more energy from the Sun reaches the ground
simply because the day is longer, but also because when the days
are longer the Sun reaches to a greater height in the sky (closer
to the zenith) and therefore passes through less atmosphere most
of the day (meaning that less energy is scattered by the atmosphere
and thus that even more energy can reach the surface).
By adding more energy to the Earth's surface on the longer day,
more can be released from the surface to heat the air, resulting in
a higher maximum temperature on a day with more daylight than on a
day with a shorter period of daylight.
David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012