Subtle Workings of Mother Nature
You know (or should know) that it is the tilt of Earth's rotation
to its orbital plane (by 23.44 degrees) about the Sun that accounts for
the changes in the
Greater amounts of light energy are deposited within the northern
during its summer because (1) the Sun's rays are then more direct and
more intense, and (2) the number of daylight hours are longer (and so
the duration of heating in our hemisphere is thus
longer). Just the
holds in winter.
But here is a surprising tidbit regarding Earth's seasons...
The Earth's global average temperature is actually higher in
when it is the furthest from the Sun.
Averaged over the globe, sunlight falling on Earth in July (at aphelion
Earth's most distant point from the Sun) is about 6.8% less intense
is in January (at perihelion - Earth's closest point to the Sun).
average temperature of Earth's entire surface at aphelion is
4 degrees F (2.3 degrees C) higher than it is at perihelion!
Why is the Earth's average surface warmer when we're farther from the
All else being equal, the flux of light radiation arriving at Earth
Sun is proportional to the Sun's luminosity divided by the square of
the Sun's distance from Earth (and a geometric constant). The
change in Earth's distance
from the Sun accounts
the 6.8% deficit of sunlight arriving at Earth in July as
January. But the rise in temperature of a substance due to an increase
in the amount of energy deposited depends upon a quantity called specific
heat. It's a measure of the amount of energy required to raise the
of a unit mass of material by 1 degree Kelvin. It's because there's
in the northern hemisphere and more water in the south that accounts
the surprising result. During July the land-crowded northern half of
planet is tilted toward the Sun during the long day time hours. Land
more rapidly than does liquid water1 due to the deposition
of solar energy. Earth's average global surface temperature is slightly
in July because the Sun is shining down on all that land, which "heats
This effect is similarly responsible for the lag in time between the
and winter solstices (June 21 and December 22) and when we in the
hemisphere experience the hottest and coldest temperatures of the
respectively. It simply takes time to deposit sufficient energy in the
hemisphere to bring us our consistently hottest days of summer, usually
in mid-late July. Likewise, it takes time for the northern hemisphere
it's minimum average temperature due to the deficiency of solar energy,
usually occuring between mid January - early February.
1Although, this effect is
somewhat by the fact that a large body of liquid water, e.g., an ocean,
energy than does typical land of equal area, due to water's generally
albedo (reflectivity). Spatial and temporal differences in land and ice
albedos, as well as in hemispheric
cloud cover also affect how much light energy of the Sun
is deposited into Earth.
2One can also consider a further effect that the northern
winter is approximately 3 days shorter than its summers because Earth
traveling more quickly in its orbit about the Sun near perihelion than