Global Wind Patterns
Learning Objectives
As temperature changes so does pressure. Air will flow from areas of high pressure to areas of lower pressure. Wind is the result.
Pressure gradient force (PGF): the force that results from pressure differences that will cause the wind to blow. The larger the pressure difference between air masses, the stronger the resulting wind.
Coriolis Effect: because the Earth is rotating fluids on the surface (remember, the atmosphere acts like a fluid) are deflected. In the Northern Hemisphere they are deflected to the right.
Geostrophic winds: wind movement that does not encounter friction. Pressure Gradient Forces causes the air to move from an area of high pressure to an area of low pressure. Coriolis Effect deflects that movement 90 degrees to the right. The net result is that the wind moves parallel to the isobars with low pressure to the left and high pressure t the right.
Gradient Wind: wind movement parallel to curved isobars. Again there is a balance between he PGF and the Coriolis Effect. Therefore wind moves around a high pressure or low pressure system in a circular pattern. Wind moves counterclockwise around a low pressure center and clockwise around a high pressure center.
Upper level winds follow the isobars.
Zonal winds: isobars and wind parallel to lines of latitude.
Meridonal winds: isobars and wind cut across lines of latitude.
Atmospheric Boundary Layer: within approximately 1000 ft of the surface friction affects air movement. The PGF causes air to move from areas of high pressure to areas of low pressure and the Coriolis Effect deflects this movement to the right BUT friction acts as a drag to lessen the Coriolis Effect so that the air does not move at 90 degrees to the right of the PGF but rather at some degree less than 90. This can be integrated with altitude but as an average we can use 30 degrees.
Near the surface air movement is deflected 30 degrees to the right of the Pressure Gradient Force.
Buys-Ballot's Law states the relationship between pressure and surface wind movement.
Stand with the wind to your back. Turn clockwise 30 degrees. The lower pressure is to your left and the higher pressure is to your right.
Therefore wind moves counterclockwise
into a low pressure center and clockwise
out of a high pressure center.
As air moves into the low pressure center it moves upwards. As it rises it cools, condenses and clouds form.
As air moves out of the high pressure center it sinks from above. Sinking air warms and dries and the sky is free of clouds.
Global Wind Patterns:
Large scale wind movements are the result of pressure differences that occur because the Poles are cold, the Equator is hot and the Coriolis Effect has deflected the winds.
Graphic showing global wind patterns.
InterTropical Convergence Zone: In the region near the Equator air rises and moves poleward. This zone shifts with the seasons. In this area rising air causes clouds and rain. Also called the Doldrums, the area lacks surface wind.
30 degrees N & S Latitudes: air sinks causing a drying effect. Many deserts in this region. Also called the Horse Latitudes because lack of wind caused sailors to become becalmed.
Trade Winds: blow equatorward between approximately 30 degrees N&S latitudes and the InterTropical Convergence Zone.
60 degrees N & S Latitudes: another area of rising air, not as strong as ITCZ.
Prevailing Westerlies (in N Hemisphere): between ~60 degrees N&S latitudes and ~30 degrees N&S latitudes air blows towards the poles.
90 degrees N & S Latitudes: air sinks over the poles and moves equatorward.
Polar Easterlies (in N Hemisphere): air between pole and ~60 degrees N&S latitudes moves equatorward.
Global wind patterns vary in intensity.
The InterTropical Convergence Zone controls weather patterns in the tropics. Trade winds dominate.
The Prevailing westerlies are weaker and the Polar easterlies are the weakest wind patterns.
