All planets with atmospheres have moving winds that cause changing weather patterns. Because there are so
many factors that change wind patterns, forecasting weather can be difficult.
Global Heat Budget
The sun causes all of earth’s weather. It rays reach the earth unevenly.
At the earth’s equator the sun hits the earth directly at about a 90 degree angle with a power of about 1 kw/m 2 .
Towards the poles the sun’s energy is spread over a larger area so that it has a power there of about .5 kw/m2 .
The lower angle also causes more light to be reflected and therefore less is absorbed. Snow, oceans and clouds
also reflect sunlight so that less is absorbed.
About 30% of incoming sunlight is reflected back into space.
The air near the surface of the earth is warm and is like a bath keeping us warm. On bodies without atmospheres
like the moon, objects in the sun get as hot as 100 C while in the shade objects may be at temperatures like -170
C.
The way the earth gets heated:
Sunlight radiation strikes the earth and is absorbed, warming the surface of the earth. The earth in turn reradiates much of the energy as longer wavelengths of infrared energy towards space. If clouds or greenhouse
gases (H2O, CO2 carbon dioxide, or CH4 methane) are in the atmosphere, they reflect the longer wavelengths (heat
radiation) back towards the earth’s surface and also absorb it making the atmosphere warmer. The absorbing
and reflecting back of infrared radiation by these gases is called the greenhouse effect.
Without the greenhouse gases, the earth’s atmosphere would have an average temperature of -18 C but having
these gases, the earth’s atmosphere is actually and average +15 C.
The planet, Venus, has an atmosphere high in greenhouse gases (especially CO2 ) and has an average temperature
of 462 C , hot enough to melt lead.
As the earth warms through solar radiation, the air in contact with the earth warms by conduction as earth
molecules bump into and “warm up” air molecules next to the earth. These warmed air molecules then rise
because they are less dense and the rising air creates a current up of warm air (convection current) and a current
down toward the earth of cooler air. In this way the upper atmosphere is warmed by convection currents.
When sun radiation evaporates water to make clouds, the water vapour in the clouds contains the solar energy
that evaporated it. (Every g of water would contain 540 calories of energy, 540 times the amount of energy that 1
g of water gets as it rises 1 celsius degree in temperature) Thus clouds store enormous amounts of previous solar
energy in the kinetic energy of their molecules.
Eventually the heat in clouds and the atmosphere is radiated out into space which keeps the earth from getting
hotter and hotter.
Hot and Cold Air
Hot air rises since it expands and becomes less dense. Cold air falls since it contracts and becomes more dense.
Where air expands and rises, it creates a low pressure region since there are less molecules if the air moves
upward. Falling colder air creates a high pressure region as more molecules press together.
On earth, air flows from high pressure regions (cooler falling air) to low pressure region (warmer rising air) . Air’s
movement from high pressure regions to low pressure regions creates winds.
The Coriolis Effect
The following simplified air flow would be expected if the earth’s spinning motion were not a factor. Warmer air
rising from the equator would move to the poles while cooler polar air would move towards the equator. But the
earth’s spinning changes this picture.
The rotation of the earth causes a body moving across its surface to be deflected to the right in the northern
hemisphere and to the left in the southern hemisphere. This is called the Coriolis Effect.
The Arctic and Antarctica have steady prevailing easterly winds (winds from the east). Antarctica is the windiest
place on our planet. On the other hand, the equator is the calmest place with little breeze.
Warm air from the equator rises and moves towards the poles.
The speed of the earth changes as one moves from the equator to the poles. If air moving faster at the equator
moves northward, it moves forward, faster than the earth at 30 o latitude. This creates a north and westerly wind
flow from the equator to 30 o latitude.
Thus the air from the equator does not reach the poles but circulates in a Hadley cell, rising from the equator and
dropping at 30 degrees from the equator. This creates a low pressure area at the equator and a high pressure
area at 30 degrees latitude. Polar Hadley Cells develop from 90 degrees to 60 degrees with low pressure regions
at 60 degrees and a high pressure region at 90 degrees latitude.
Air moving from the poles is slower than the 60 degree latitude earth, so it falls behind creating a north and
easterly wind (left below).
As the air from the equator moves northward, it squeezes into a smaller area. With increasing density, the air
pressure builds up, creating a band of high pressure at 30 degrees latitude. At the surface the air is diverted north
and south (left below). As air from 30 latitude moves towards the poles, it is diverted forward again, creating
westerly winds, both north of 30 degrees and south of 30 degrees.
Explorers used the westerlies to sail west from the new world or to sail to the East Indies around the Cape of Good
Hope Africa. The Southern hemisphere westerlies are also known as the roaring 40s.
At 60 degrees, the place where the cooler, easterly sinking polar air meets the warmer westerly air rising from
below is called the polar front. The polar front is where many storms develop as the warmer air meets the cooler
air, causing a belt of rain and storms.
At 30 degrees, the air is sinking (high pressure) and compressing which warms the air and this warm air can
evaporate water more easily at the earth’s surface at this latitude. The world’s great hot deserts are found at this
latitude. In the oceans at 30 degrees latitude, with high pressure the air is very still and quiet with little wind.
These regions were called the horse latitudes because sailors with cargoes of horses often threw their horses
overboard to keep from using up their food since their ships would move very little for days on end.
From 30 degrees latitude to the equator, the winds blow from the east and are referred to as the N.E. Trades and
S.E. Trades since they were used by sailors to return back to Europe from the East Indies or to make a speedy trip
to the New World.
The low pressure zone at the equator where the trade winds converge is called the equatorial trough or the Inter
Tropical Convergence Zone.
The General Circulation of the Atmosphere consists of 7 bands of alternating high and low pressure with wind
movements from high to low in alternating opposing east-west directions.
Seasons
The earth has seasons because its axis is tilted at 23.5 degrees. During the northern hemisphere’s winter
(beginning in December in the diagram below), the earth’s northern axis points away from the sun and the sun’s
rays strike the earth at a lower angle so per square metre there is much less solar radiation in the northern
hemisphere. At the same time, the southern hemisphere receives more direct sunlight energy and thus more
energy per square metre, warming this region and providing Summer temperatures. Day length is shorter and
night length is longer during this time in the northern hemisphere.
During the March equinox (beginning of Spring) when the days and nights are of equal length, solar radiation
increases in the northern hemisphere while it decreases in the southern hemisphere. By June, the northern
hemisphere is receiving its maximum radiation and Summer begins while the southern hemisphere receives its
lowest amount of radiation (the start of its winter).
As the seasons change, the pressure bands also shift. At an equinox the pressure bands are lined up evenly
around the equator. As winter for the northern hemisphere approaches, the bands shift southward. As northern
Spring approaches and then Summer, the band shift northward bring warmer temperatures.
Monsoons
Monsoons are seasonally-reversing tropical winds which bring wet and dry seasons to the tropics. The Inter
Tropical Convergence Zone moves north and south with the seasons. Where the ITCZ moves most it generates
monsoons.
In December/January, the NE Trade winds must cross over the equator to reach the ITCZ low pressure. When they
pass over the equator, the Coriolis effect changes their direction and they bring the monsoon rains to Australia.
In June/July, the ITCZ has moved north and the SE Trades must cross over the equator to reach the ITCZ low
pressure. As they pass over the equator into the northern hemisphere, the Coriolis effect changes their direction
and they bring the monsoon rains to South East Asia. The monsoons happen in Africa, in India and are strongest in
South East Asia and Australia. They are weakest in South America.
The Himalaya Mountains cause the ITCZ to move farther north than anywhere else on the earth. The SE Trades
pick up moisture over the Indian Ocean and become SW Monsoons carrying torrential rains to South West Asia
and India during June/July.
Synoptic Charts
Wind movement is predicted on the basis of pressure difference. Pressure is usually measured in hectopascals
and is displayed on synoptic charts. Isobar lines link regions of identical pressure. Isobar lines are like contour
lines – high pressure lines are like hills and low pressure lines are like the valleys. The numbers indicate higher and
lower pressures.
In the northern hemisphere, air spirals counterclockwise into the lowest pressure region and there is an updraft in
this region. The spiraling air mass around a low pressure region is called a cyclone. Air masses spiral clockwise
around high pressure regions where air is falling or compressing and are called anticyclones. In the southern
hemisphere, directions reverse so that air spirals clockwise around low pressure regions and counterclockwise
around high pressure regions.
Wind flows from high pressure regions to low pressure regions in a spiraling pattern due to Coriolis.