Although winds can change on a daily basis, there are some consistencies in world wind patterns. Working from the equator to the poles, there are three bands of wind direction in each hemisphere. From the equator to about 30 degrees latitude (about a third of the way to the poles), the winds come from the east and are called the trade winds. Actually, they come from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. In the middle latitudes, from 30 degrees to 60 degrees, they come from the west and are called the prevailing westerlies. And from the poles, they come from the east and are called the prevailing easterlies. The Coriolis force causes them to arrive at slight angles.
What regulates these winds are three distinctive cells or circular wind patterns in both the northern and southern hemispheres - one near the equator, one at the poles and one in between. The trade winds from both hemispheres converge on the surface at the equator, which sends the air aloft, reducing the pressure. This is why it rains so much around the equator. When the air reaches the upper altitudes, some of it heads north and some heads south in an effort to even out the heat difference between the equator and the poles.
At about 30 degrees latitude, the high-altitude winds sink to the surface. Sinking winds create high-pressure zones, with lots of sun, which is why 30 degrees latitude is where the world’s great deserts are. Some of that sinking air heads back to the equator, which keeps this cell or circular wind pattern going.
Some of the high winds descend at the poles. From there they head back toward the equator and converge with the prevailing westerlies at 60 degrees. When winds converge, they rise. This, you get another area of frequent storms at 60 degrees.
In general, there are a number of prevailing wind conditions on the surface and at higher altitudes in both hemispheres that greatly affect the weather. Where winds converge on the surface at the equator, the air moves upward and you will have lots of storms and rain. At 30 degrees winds diverge at the surface, high winds move downward and you get deserts and dry weather. At 60 degrees latitude, you get converging winds and another uplifting with resultant storm activity.
Though surface winds may differ at different latitudes, above 15,000 feet in the troposphere, the wind is generally from the west. This is why big storms move from the west to the east. During the summer the winds can shift in the tropics, which is why hurricanes head east.
So we have this complex system, where air moves in complex patterns, stirring up trouble in the process. Now what are a few extra globally warm degrees going to do? An increase of just eight to ten degrees can cause an El Nino effect, which we know from recent history can have dramatic effects on the climate.
Michael Russell
Your Independent guide to Environment
