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Over the equatorial trough of low pressure, lying roughly between 5° S and 5° N lat., is the equatorial belt of variable winds and calms, or the doldrums. There is no prevailing surface winds here, but a fair distribution of directions around the compass. Calms prevail as much as a third of the time. Centrally located on a belt of low pressure, this zone has no strong pressure gradients to induce a persistent flow of wind.
2. Main article
North and south of the doldrums are the trade wind belts, covering roughly the zones lying between 5° and 30° N arid S. The trades are a result of a pressure gradient from the subtropical belt of high pressure to the equatorial trough of low pressure. In the northern hemisphere, air moving equator ward is deflected by the earth's rotation to turn westward. Thus, the prevailing wind is from the northeast and the winds are termed the northeast trades. In the southern hemisphere, deflection of the moving air to the left causes the southeast trades. Trade winds are noted for their steadiness and directional persistence.
3. References
The system of doldrums and trades shifts seasonally north and south, through several degrees of latitude, as do the pressure belts that cause them. Because of the large land areas of the northern hemisphere, there is a tendency for these belts to be shifted farther north in the northern hemisphere in summer (July) than they are shifted south in winter (January). The trades are best developed over the Pacific and Atlantic oceans, but are upset in the Indian Ocean region by the proximity of the great Asiatic landmass.
The trade winds provided a splendid avenue for westward travel in the days of sailing vessels. Steadiness of wind and generally clear weather made this a favorite zone of mariners. Crossing of the doldrums was hazardous because of the possibility of being becalmed for long periods and because of the uncertainty of wind direction. The trade wind belts are not altogether favorable for navigation and flying, however, because over certain oceanic portions, at certain seasons of the year, terrible tropical storms known as hurricanes or typhoons occur.
Between latitudes 25° and 35° are what have long been called the subtropical belts of variable winds and calms, or horse latitudes, coinciding with the subtropical high-pressure belt. Instead of being continuous even belts, however, the high-pressure areas are concentrated into distinct anticyclones or cells, located over the oceans. The cells of high pressure are most strongly developed in the summer (January in the southern hemisphere, July in the northern). There is also a latitudinal shifting following the sun's declination. This amounts to less than 5° in the southern hemisphere, but it is about 8° for the strong Hawaiian high located in the northeastern Pacific.
Winds in the high-pressure cells are distributed around a considerable range of compass directions. Calms prevail as much as a quarter of the time. The cells have generally fair, clear weather, with a strong tendency to dryness. Most of the world's great deserts lie in this zone and in the adjacent trade wind belt. An explanation of the dry, clear weather lies in the fact that the anticyclonic cells are centres of descending air, settling from higher levels of the atmosphere and spreading out near the earth's surface. Descending air becomes increasingly dry.
Between latitudes 35° and 60°, both N and S, is the belt of the westerlies, or prevailing westerly winds. Moving from the subtropical anticyclones toward the sub arctic lows, these surface winds blow from a southwesterly quarter in the northern hemisphere, from a north-westerly quarter in the southern hemisphere. It is more accurate to say that within the westerly wind belt, winds blow from any direction of the compass' but that the westerly components are definitely predominant. Storm winds are common in this belt, as are frequent cloudy days with continued precipitation. Weather is highly changeable.
In the northern hemisphere, landmasses cause considerable disruption of the westerly wind belt, but in the southern hemisphere, between the latitudes 40° and 60°, there is an almost unbroken belt of ocean. Here the westerlies gain great strength and persistence, giving rise to the mariner's expressions, 'the roaring forties,' 'the furious fifties,' and 'the screaming sixties.' This belt was extensively used for sailing vessels traveling eastward from the South Atlantic Ocean to Australia, Tasmania, New Zealand, and the southern Pacific islands. From these places it was then easier to continue eastward around the world to return to European ports. Rounding Cape Horn was relatively easy on an eastward voyage, but in the opposite direction, in the face of prevailing stormy westerly winds, was fraught with great danger.
Although the westerly wind belts no longer exert a strong influence over the routes of modern ocean vessels, they are important in long-distance flying. Transoceanic and transcontinental flights in the easterly direction require less fuel and a shorter time. On westward flights, strong head winds may eat dangerously into the fuel supply on the plane and in any event necessitate reduced payloads.
A wind system often termed the polar easterlies has been described as characteristic of the arctic and polar zones. The concept is greatly oversimplified, if not actually erroneous, for winds in these regions take a variety of directions, as dictated by local weather disturbances. Perhaps in Antarctica, where an ice-capped landmass rests squarely upon the pole and is surrounded by a vast oceanic expanse, the outward spiraling flow of polar easterlies is a valid concept. Deflected to the left in the southern hemisphere, the radial winds would spiral counterclockwise, producing a system of southeasterly winds.
