Global Winds: Trade Winds, Westerlies and Polar Easterlies

When air moves in a definite direction, it is called wind. If the winds move from west to east, they are called westerlies. If they move from east to west, they are called easterlies.

There are winds because there are differences in pressures. The direction of wind is also affected by coriolis affect. Due to Coriolis Force, the wind flowing from equator towards the North Pole and from North Pole towards the equator are deflected to their right while the winds flowing north-south and south-north in the southern hemisphere are deflected towards their left. The magnitude of the deflection, or “Coriolis effect,” varies significantly with latitude. The Coriolis Effect is zero at the equator and increases to a maximum at the poles. The effect is proportional to wind speed; that is, deflection increases as wind strengthens. The resultant balance between the pressure force and the Coriolis force is such that, in the absence of surface friction, air moves parallel to isobars (lines of equal pressure). This is called the geotropic wind. The Coriolis force explains why winds circulate around high and low pressure systems as opposed to blowing in the direction of the pressure gradient. Central idea behind the Coriolis force is that when the earth rotates from west to east, it produces the centrifugal force and due to this force, there is a change in the direction of the wind. There is Ferrel’s law derived from Coriolis Effect, which says that in northern hemispheres, wind deflects towards the right and in southern hemisphere wind deflects towards left. This means that in northern hemisphere, wind deflects clockwise, while in southern hemisphere, wind deflects anti-clockwise.

Trade winds

Trades wind blow out from the Subtropical High Pressure belts. In the northern hemisphere, they blow towards the equatorial low and called North East Trade Winds. In the Southern hemisphere they blow towards the equatorial low and become the South East Trade winds. This implies that Trade winds blow from North east towards equator in Northern hemisphere and South East Towards equator in southern hemisphere.  It has been shown in the following graphics.

The trade winds are most regular winds of all kinds on earth. They blow with great force and in constant direction that is why they are preferred by the sailors. The trade winds bring heavy rain falls and sometimes contain intense depressions.

Trade winds and Hadley cells

There are three primary circulation cells on earth known as the Hadley cell, Ferrel cell, and Polar cell.  The Hadley cell mechanism provides an explanation for the trade winds. Hadley cell is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas (the Intertropical Convergence Zone, ITCZ) to the tropopause and carried pole ward. At about 30°N/S latitude, it descends in a high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds. Hadley Cells is described to be lying on equator but it follows sun’s zenith point, or what is termed the “thermal equator”.

Origin of Trade Winds

Trade winds are part of the Hadley cell circulation. At the equator, a low-pressure area of calm, light variable winds is known Intertropical Convergence Zone as we discussed above. The air lifts from here and at around 30° North and South, the air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. At the surface, the air flows from these subtropical high-pressure belts toward the Equator but is deflected toward the west in both hemispheres by the Coriolis Effect. Thus, these winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Because winds are named for the direction from which the wind is blowing, these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The trade winds meet at the doldrums.

You can visualize more about trade winds & Hadley Cells in this animation: http://bit.ly/UfHnbt

Implications of Trade winds

• Trade winds are the surface winds in low latitudes, representing the low-level airflow. Back in history, two large belts of winds were discovered blowing toward the equator called North East and South East trade winds. The word trade in those days referred to advance steadily and was synonymous with efficient sailing. The trade winds allowed the sailing vessels to advance steadily—and, of course, to set up patterns of international trade. However, you must note that trade winds are not totally steady in force or direction, but they do trend in the general direction of southwest and northwest.
• Hawaii is located south of Tropic of Cancer, yet, the temperatures are pleasant, temperatures and humidity tends to be a bit less extreme. This makes it one of the most famous tourist destinations of the world. What make such a climate are Trade Winds.

Westerlies

The directions of the Westerlies are opposite to trade winds and that is why they are also called antitrade winds. Westerlies blow in the middle latitudes between 30 and 60 degrees latitude, and originate from the high pressure area in the horse latitudes towards the poles. Under the effect of the Coriolis force, they become the south westerlies in the northern hemisphere and Northern westerlies in the southern hemisphere. Please note that in the southern hemisphere, there is more of ocean and less of land in comparison to the northern hemisphere. Due to this reason, the westerlies blow with much greater force in southern hemisphere in comparison to northern hemisphere.

This also has implications in the Ocean currents. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the Westerlies of each hemisphere.

Generally, Westerlies are strongest in the winter hemisphere and at times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles. Please note the westerlies are also associated with the “extra tropical” cyclones which refer to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet, where the Westerlies steer the system generally from west to east. Whenever there is a convergence of the cold and denser polar winds and warm and light westerlies, there are much variation in the weather. The velocity of the westerlies increases southward and they become stormy. When we move towards poles, the velocity of the westerlies is given different terms as follows:

• Roaring Forties between the 40-50°S
• Furious Fifties at the 50°S and Shrieking Sixties at 60°S.

Polar Easterlies

Polar easterlies blow from the polar high pressure belts towards the temperate low pressure belts. These are extremely cold winds that come from the Tundra and Icecap regions of the poles. The Polar Easterlies are more regular in the southern hemisphere in comparison to the northern hemisphere. These polar cold winds converge with the warm easterlies near 60° latitudes and form the Polar front or Mid Latitude front. This mid-latitude front becomes the centre of the origin of the Temperate Cyclones.