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>> Global climate patterns are driven
by energy input from the sun.
Because the Earth is a sphere, photons of light energy
from the sun strike the equator at around a 90 degree angle.
At higher latitudes this same amount
of energy strikes Earth at an oblique angle.
So it's spread over a larger area.
The result is that there's a global energy imbalance,
with the equator receiving 2.5 times more insolation per unit
land area in a year than the poles.
The increased solar energy input toward the equator translates
to a global temperature gradient.
We all know the tropics enjoy warmer average temperatures.
The lack of a distinct cold season is perhaps the most
significant difference in climate
between the tropics and higher latitudes.
And it's one reason so many pale-skinned,
light-deprived people flock to tropical destinations
on their winter vacations.
Equatorial regions also experience greater rainfall.
Why is this?
Well, the thermal imbalance I just talked
about drives the circulation of air
that redistributes energy around the globe.
One of the key patterns of global air movement
which dominates weather in the tropics is the Hadley Cell.
Where the sun's energy input is greatest,
air at the Earth's surface is warmed.
Where does warm air go?
Up. It rises by convection near the equator,
initiating circulation of the Hadley cell.
This belt of rising air around the equator is known
as the equatorial low pressure belt.
So what happens to air
as it rises higher into the atmosphere?
Well, it cools.
Cooling air holds less moisture than warm air.
So water vapor condenses out, and clouds form.
The production of clouds through convection around the equator
and the rainfall that results is the single most important
phenomenon explaining why the great tropical rainforests
of the world are always found hugging the equator.
So back to the Hadley cell.
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Air can't continue rising forever.
Instead, as it approaches the stratosphere,
it deflects to the north and south, pushed outward
by the air rising from below.
By this time virtually all
of its moisture has been rained out.
The cool, dry air gets even cooler
as it reaches higher latitudes.
And begins to descend, as cold air does,
falling to the surface just outside the tropics of Cancer
and Capricorn, or at about 30 degrees latitude.
The band of falling air
around 30 degrees latitude literally weighs
down on the surface of the Earth,
creating the subtropical high pressure zone.
It is this band of cool, dry air falling to Earth
at these so-called horse latitudes that is responsible
for the formation of the world's great deserts.
Weather outside the tropics is influenced
by different air circulation patterns.
But the most important aspect
of temperate climate are the seasons, characterized
by strong fluctuations in day length and temperature.
On the equator, daylight or day length is
around 12 hours long all year.
In contrast, at over 40 degrees latitude in Madison, Wisconsin,
day length changes from a minimum of about eight hours
at the winter solstice to a maximum of about 16 hours
at the summer solstice.
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Correspondingly, in the tropics,
average daily temperature is stable.
Temperature change within a day is greater
than temperature change from day-to-day across the year.
In contrast, here in Madison, Wisconsin,
we may experience temperatures of 90 degrees Fahrenheit
in July, but 10 below Fahrenheit in January.
It would be unheard of for temperature
to change this much within a single day.
Here annual temperature change exceeds daily
temperature change.
At higher latitudes, major variations in day length
and temperature over the year give rise to distinct seasons.
The winter, spring, summer and fall that residents
of temperate places know and love.
Well, some don't like winter so much.
Near the tropics, higher insolation,
warm year-round temperatures
and constant day length creates more stable abiotic conditions
year round.
The seasonality of solar energy input throughout the year has
profound ramifications on the productivity, diversity,
form and function of Earth's ecosystems.
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