Trade Winds also move from
east to
west in the
southern hemisphere, i.e, southeast to northwest, as described above.
The trade winds
originate in the vicinity of 30
degrees north and south
latitude, in bands of
elevated
atmospheric pressure known as the
subtropical highs.
The warm air at the equator rises to the
tropopause and heads
poleward and
eastward. By the time the air gets to about 30 degrees from the equator it begins to cool and
descend, and
converges, since there is less room than at the equator, so the air pressure builds. Some of the air near the surface heads toward the equator and the cycle begins again. This large-scale north-south
circulation comprises one
cell of the
three-cell model of general
atmospheric circulation.
The air moves west as it goes south due to
conservation of momentum. The
circumference of the
earth,
measured along lines of latitude, is greater at the equator than at 30 degrees (or
elsewhere). A
parcel of air that was
still at some latitude above the equator, i.e. moving at the same speed as the surface of the earth, would end up over a
longitude west of where it started as it moved south, since to stay over the same latitude it would have to speed up somehow to cover the greater east-west distance. This is also why air moving poleward also travels east relative to the surface - it is going faster relative to the surface at higher latitudes than it was at lower latitudes.