Global Air Circulation Flashcards
ITCZ
Inter tropical convergence zone
The zone between the tropics (23 N and South of the equator), where warm, moist air from the subtropical high pressure belts converges
Convergence
The inward flow of air to a low pressure
Divergence
The outward flow of air from a high pressure
Dew point temperature
The temperature at which air becomes saturated by cooling
Jet stream
Strong winds blowing from west to east in the upper atmosphere 10km above the surface at speeds of 100-300km/hr
Geostrophic wind
Blows parallel to isobars under the combined effects of the pressure gradient force and the coriolus force
Föhn
Dry, hot wind that originates in a mountainous area
Adiabatic
A change in temperature through a change in pressure only (or significant changes in altitude
Relative humidity
A measure of the water vapor constant of air at a given temperature (it is expressed as a percentage)
How are winds caused?
Winds are caused by the unequal heating of the earth’s atmosphere which results in the pressure belts
Dynamic
Not directly because of the sun
Thermal
Direct result of sun and temperature
How do high and low pressure cells form?
During the day, the land heats up more quickly than the sea.
The air becomes less dense, expands and rises which results in a low pressure over the land.
Over the cooler sea the air is cooler so more dense, contracts and descends.
In this way high and low pressure cells form.
What is wind?
Movement of air from places of higher pressure to places of lower pressure.
Describe atmospheric pressure and wind
Hot air rises and cooler air flies in to replace it.
The convergence of two air masses will cause air to rise.
Air diverges and moves in opposite directions.
Describe Jet streams
Strong winds blowing from west to east in the upper atmosphere about 10km above the surface.
They blow at speeds of about 100-300km/hr
The sub-tropical jet stream forms the boundary between the Hadley and Ferrel cells while between the polar and Ferrel cells we find the polar front jet stream.
What is the pressure gradient force?
The rate at which pressure changes over a horizontal distance measured from high to low.
The pressure gradient is always at right angles to the isobars.
Isobars…
Closer-steeper PGF, stronger wind speed
Further-weaker PGF, weaker wind speed
Describe primary circulation
If the earth did not rotate on its axis then there would be simple form of pressure belts and wind circulation.
Low pressure at equator-hot/moist
High pressure at poles-cold/dry
Describe secondary circulation
Because earth rotates on its axis, pressure belts aren’t simple.
Air mass characteristics
Temperature and humidity characteristics of air masses are picked up from the surfaces over which they develop, so an air mass developing over the sea produces wet/moist, maritime air.
An air mass developing over land results in dry, continental air.
Describe the global pressure belts with the seasons (situation of ITCZ)
South in December/January when southern hemisphere has summer
North in June/July when northern hemisphere has summer
In January ITCZ mostly situated South of equator
In July will shift North of equator
Discuss the sub-tropical HP belt in the southern hemisphere
It is not a band but separate cells because of the large open ocean surfaces
HIGH pressure cells in southern hemisphere
Sinking or descending
Anti-clockwise
Outward
Descending air is heated by compression
Fine weather (no rain, hot, dry)
