Great Wind Systems

In olden days, when ships were powered by sails, mariners relied a lot on the weather. They learned that at certain latitudes great winds blew constantly and reliably and would move them long distances over the world’s oceans. These winds are shown on the diagram. However, there were other areas where they had problems. Around the equator, for example, were the doldrums, where they could be becalmed for weeks waiting for a favorable wind. About 2,000 miles (3,200 km) away from the equator was a region that they called the “horse latitudes.” According to one source, this was so named because sailors were at times becalmed there for so long that they started to throw the horses overboard to conserve water!

Undoubtedly, sailors moving with the great winds often blessed the weather, while those becalmed in the uncertain regions said unfriendly things about it. Really, though, both regions are vital to our weather system, and both are caused by the sun.

Our atmosphere is really a gigantic convection system, powered by the heating of the air near the ground in the tropics. This hot air rises to the cooler upper altitudes, and is replaced by air from the North and the South. The great winds you see on the diagram are the result of a complicated circulation system induced by this basic drive, and modified by the rotation of the earth. Those winds are a blessing, in that they blow the rain-bearing clouds from above the seas to the land areas.

But look at the region where the trade winds converge. Yes, it is near the equator, in the doldrums. When all those millions of tons of air rush together in the same latitude, they can only go upward. As they rise from the warmth of sea level to the cool high altitudes, they have to give up some of their load of water vapor. The result is an area of uncertain winds, clouds and rain.

When a body of air ascends, the rotation of the earth makes it revolve like an upside-down whirlpool. Hence, all along this region, bodies of air are being established, in some cases thousands of kilometers across, which are circulating upward, causing low pressure at sea level, clouds and sometimes strong winds. They are called cyclones, and play an important part in moving rain-bearing clouds over the land. Hence, even if the old-time sailors would complain, we can be grateful for this weather system. Similar low-pressure areas are formed where the polar easterlies meet with the prevailing westerlies, where two bodies of air of different temperatures react, or even over locally heated areas.

Two thousand miles (3,200 km) north of the equator exactly the opposite occurs. Here, great bodies of air are moving away from each other​—the trade winds and the prevailing westerlies. The result is that air comes spiraling down from above. The pressure of this air increases, it warms up and an area of fine, cloudless weather is created. This is an anticyclone. The well-known good weather of Hawaii and the Azores comes from stable high-pressure areas that usually hold sway near there. Anticyclones can also form in the polar regions. Coming from there, they will also bring fine, clear weather, but they will be c-o-l-d!

These huge bodies of air, because they are revolving, tend to stay distinct from the surrounding atmosphere. They can move from their place of origin and influence the weather in other locations. Other circulating systems sometimes appear too. A cyclone over the tropical seas may intensify into a typhoon (“hurricane” in the West). This may be hundreds of kilometers across with center winds moving at high speeds. Smaller circulating systems are thunderstorms. Still smaller are the tornadoes, which rip through the central United States each year.

No one fully understands typhoons, thunderstorms or tornadoes. They seem to be systems for relieving unbalance, or perhaps syphoning off excess heat from sea level. But despite their frightening aspect, they undoubtedly play an important role in our atmosphere.

These wind systems are responsible for much of our weather. As high-pressure areas meet up with low-pressure areas, and as both are influenced by the prevailing winds, by the parts of the earth’s surface that they pass over, and by other things, they bring a lot of the variability that we experience in our day-to-day weather.

[Diagrams on page 27]

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POLAR EASTERLIES

SUBPOLAR LOWS

PREVAILING WESTERLIES

HORSE LATITUDES

TRADE WINDS

DOLDRUMS

TRADE WINDS

HORSE LATITUDES

PREVAILING WESTERLIES

60°

30°

0°

30°

[Diagram]

COLD AIR

LOW PRESSURE

WARM AIR

RISING WARM AIR

COLD AIR

WARM AIR