· Climatic patterns originate as the
earth absorbs the energy in sunlight.
· The ability to absorb sunlight varies across the earth's
surfaces and this creates differential heating and cooling.
· The earth's climate is cold and dry towards the poles
and hot and wet towards the equator.
· The primary cause of global variation in climate is the
greater intensity of sunlight at the equator.
· The light that does strike the poles, does so at a lower
angle and it is spread over a greater area whereas the light that
strikes the equator does so at a higher angle and a smaller area
therefore the warming effects of the sun clearly diminish from
the tropics to the poles.
· There are daily changes in climate result from the rotation
of the earth around its axis.
· The revolution of the moon around the earth influences
the tides on a monthly basis, and
· The rotation of the earth around the sun brings seasonal
changes to part of the world.
· The earth is tilted 23.5 degrees with respect to the
path it follows on its rotation around the sun.
· The northern hemisphere receives more sun during our
summer than the southern hemisphere and they have summer while
we have winter.
· The seasonal range in temperature in the northern hemisphere
is greater than that for the southern hemisphere because there
is less water in the north (more land masses).
· The solar equator is directly under the suns zenith.
The location of the solar equator varies during the year. It
is at 23.5 deg. N on 21 June and at 23.5 deg S on 21 Dec.
· Heated air in the tropics rises and spreads to the north
and south in upper layers of the atmosphere.
· The air is replaced from below by surface-level air
from the subtropics.
· The rising tropical air mass cools as it radiates heat
back into space,
· When the air has moved to about 30 deg. N and S it has
become dense enough to sink back to the earth's surface and this
is how the Hadley cells are formed
· Around the equator there are two Hadley cells that are
like waistbands encircling the earth.
· The sinking air of tropical Hadley cells drives secondary
cells in temperate regions which circulate in the opposite direction.
· The circulation of temperate Hadley cells (30-60 degrees)
cause air to rise at 60 degrees north and south forming polar
· The intertropical convergence is where the tropical Hadley
cells meet. As moisture laden tropical air rises and cools in
the convergence zone, the moisture condenses to form clouds and
· The regions north and south of the equator are known
as subtropical high pressure belts because the descending mass
of heavy air creates high atmospheric pressure.
· As air sinks and spreads to the north and south, it draws
moisture from land into the air creating deserts at 30 degrees
north and south. All the great deserts of the world are found
within subtropical high pressure belts
· The rotation of the earth deflects surface flow in Hadley
cells and surface flows are shifted. The tradewinds blow from
east to west from 30 degrees N or S towards the equator and westerlies
blow from the west to the east from 30 degrees N or S to 60 degrees
N or S
· The positions of the continental landmasses exert a secondary
effect on the global pattern of rainfall.
· More rain falls in the Southern Hemisphere because oceans
and lakes make up more of the surface areas (81% of the surface
as opposed to 61% in the Northern Hemisphere).
· Interior parts of the continents receive less rain than
· Coastal (maritime) climates vary less than interior (continental
climates) because the heat storage capacity of water reduces temperature
· Global wind patterns interact with other features of
the landscape to create precipitation.
· Mountains force air upward causing it to cool and lose
moisture as precipitation on the windward side of a mountain range.
As air descends on the leeward slope and travels across lowlands,
it picks up moisture creating arid environments in rain shadows
· The physical conditions of the oceans are also complex.
· Winds propel major surface currents and the underlying
topography of ocean basins also affects currents.
· Deep currents are established because of differences
in density caused by variation in temperature and salinity.
· In large ocean basins, cold water circulates towards
the tropics along the western edges of continents and warm water
circulates towards temperate latitudes along the eastern coasts
· This figure shows the location of the major ocean currents.
The cold Peru Current off the eastern Pacific moves northward
from the Antarctic Ocean along the coasts of Chile and Peru.
This creates cool, dry environments along the west coast of South
· Upwelling is the upward movement of ocean water, and
it occurs when surface currents diverge.
· As surface currents move apart during upwelling, they
draw water upward from deeper layers.
· Strong upwelling zones are established on the western
sides of continents where surface currents move towards the equator.
· Within the tropics, the movement north and south of the
solar equator determines rainfall seasonality.
· The intertropical convergence zone follows the solar
equator producing a moving belt of rainfall.
· Seasonality is most pronounced at the higher tropical
· At middle latitudes, climates are influenced by westerly
· Many aspects of climate are unpredictable, and it is
difficult to forecast far into the future. In southern Florida,
we can absolutely understand this.
· The Peru Current flows north along the western boundary
of the continent then veers offshore near Ecuador and heads out
to the Galapagos.
· Each year, a warm countercurrent known as El Niño
moves southward along the coast.
· In some years, the El Niño current forces the
Peru Current offshore and this has a negative effect on fishes.
· During El Niño events, there is an oscillation
of high pressure areas (center moves from Tahiti to Darwin, Australia).
This switch is known as the Southern Oscillation and together
these phenomena are known as the El Niño Southern Oscillation
Events or ENSO events for short.
· The change in the location of the high pressure system
causes winds to shift.
· Warm water piles up off the coast of South America, the
westward flowing equatorial currents stop or reverse and upwelling
weakens or ceases.
· This cartoon shows what happens during El Niños.
There is a high pressure over Australia and a low over Tahiti.
· Weak easterly winds allow warm water to move east.
· The subtropical jet stream carries moisture east and
there are fewer Atlantic hurricanes.
· El Niños are followed by La Niña events.
· During the La Niñas, strong trade winds accentuate
normal currents and the high and low pressure areas switch to
the "normal configuration".
· The cloud bank moves north, the jet stream is weakened,
and Atlantic hurricanes develop at an increased rate.
· ENSO events are felt globally -- some areas are unusually
wet and some are unusually dry.