Weather is a concept we live with everyday,
but how much attention do we really pay to it? Weather systems are (in my
opinion) one of the coolest workings of Mother Nature. The interactions all fit
so perfectly! Good on you Mother Nature!
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But where does weather even come from? Solar
radiation from the sun comes into the atmosphere in short wavelengths. The
wavelengths are short because the sun is so hot – about 6000°C! (for further
see Weins Law). After this short wave energy has entered our atmosphere, it has
to make its way past many obstacles (atmospheric gasses) down to the surface of
the Earth where it is absorbed. The Earth then re-emits the radiation, but this
time as long wave, infrared radiation. Because the Earth is nowhere near as hot as the sun it emits long wave radiation. (Did you know that even you emit radiation?
and so does the computer your reading this off! Everything that is above
absolute zero, or -273°C, will emit radiation!) It is a bit harder for the long
wave radiation to get back out of the atmosphere and into space as the long
waves get caught up by the obstacles more easily than the short ones did on their
way down to Earth. These obstacles consist of pollution, carbon dioxide, methane
and other atmospheric gasses and means that the heat energy is trapped. This is
known as the greenhouse effect. So essentially the pollution, carbon dioxide
and methane etc are like a blanket over our Earth heating it up. So the more
CO2 and methane we add anthropogenicly (through coal fired power stations,
cars, plains, etc) the thicker that blanket gets.
The incoming solar rays are not evenly
distributed over the globe, so there is an energy imbalance. Because the Earth
is round the solar rays hit the earth at parallel to the sun but this means the
solar energy is not evenly distributed. The equator receives direct solar
radiation, whereas higher latitudes (towards the poles) receive less direct
radiation and the radiation is spread over a larger area. Thus there is an
energy deficit at the poles and an energy surplus at the equator. This gives
rise to an energy imbalance and atmospheric circulation, or what we know as
weather, is the workings of Mother Nature trying to even out this imbalance!
The Inter-Topical-Convergent-Zone (ITCZ) it
at the equator (it actually moves with the tilt of the Earth in the seasons but
that’s for another post) where there’s an influx of solar radiation. This means
the Earth’s surface gets really hot at the ITCZ and causes atmospheric
instability, or a Low Pressure System. Rain usually occurs in association with
low pressure systems. In these systems the atmosphere is unstable and air is
warmer on the ground than further up in the atmosphere. Warm air is less dense
than cool air meaning it will rise, like a hot air balloon. As the air rises it
cools (at its lapse rate) and this causes its density to rise and its water
holding capacity (saturation mixing ratio) to go down. (This is why we get fog
in winter instead of summer, because the air temperature is cooler and its
water holding capacity is decreased, thus it cant hold the water vapour so it
hangs around like a cloud) The air continues to rise and decrease in
temperature until it reaches its dew point, this is when clouds begin to form
and precipitation results. So low pressure systems are characterized by warm
rising air and usually rain.
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High pressure systems, on the other hand,
are characterized by cool sinking air. The air from the ITCZ rises and then
travels along to sink at about 30° latitude (about the middle/southern part of
Australia) this is known as the high pressure belt. This air has already
squeezed out its moisture so it already relatively dry, and it is also sinking
and warming up as it descends. This means that is water holding capacity is
rising and so cloud formation and rain is very unlikely. This is why central
Australia is very dry all year round, and the red center is the red center!
This is a stable atmosphere as air is not hotter near the ground than the air
above it, it is thus the air is not rising like a hot air balloon which would
cause precipitation. After it has sank this air travels back towards the ITCZ
because of the Pressure Gradient Force, this is what causes our wind. The air
travels from the high pressure system towards the low pressure system due to
the gradient between the two and the speed of that wind depends on the
intensity of the gradient. This general movement of air (rising in the ITCZ,
sinking at 30°, travelling back towards the ITCZ) is called the Hadley Cell of
circulation. And is one of three cells of circulation in the atmosphere, Hadley
cell, the Ferrel cell and the Polar cell.
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This is a broad overview but I hope you now
appreciate just how awesome the weather is that little bit more!
Now you'll see why I think that when someone brings up the weather in a conversation, its not over, its just beginning! :P
Thanks for reading,
Rach
