Page:Handbook of Meteorology.djvu/62

50 Trade Winds.—The broad belt of easterly winds within the tropics is popularly known as the Trade Winds. Their direction is southwesterly in the northern and northwesterly in the southern part of the belt. The heating of the air in equatorial regions causes a convectional updraught; and this is balanced by an inflow of air from higher latitudes. The rotation of the earth deflects the movement of the air, giving a westerly motion to the winds. Their force and direction are best studied from the monthly pilot charts published by the United States Hydrographic Office. Although the pilot charts refer specifically to ocean winds, the general information published, so far as wind direction is concerned, applies to land winds also.

The strength of the Trade Winds varies according to latitude and also according to season. The velocity is highest near the edges of the belt and lowest at its center; it varies from about 8 miles per hour in the fall months to about twice this rate in the spring and summer months. The southeast winds are materially stronger than the northeast winds. The easterly component is the important commercial factor—hence the popular name. For the year their average is from 11 to 14 miles per hour, or from 2 to 3, Beaufort scale. On the Pacific Ocean the Trade Winds are neither so strong nor so regular as in the Atlantic; on the Indian Ocean only the southern part of the belt is observable.

Prevailing Westerlies.—The two broad belts which flank the Trade Winds are variously named “Counter Trades,” “Return Trades,” and “Anti-Trades.” Their direction varies—northeast, east, and southeast as shown on the pilot charts, and their strength is indicated by the arrows. In the southern hemisphere because of their strength, they are known, as the “Roaring Forties.” In the days of sailing vessels, a ship from a port of Europe to Australia could usually make the return trip more expeditiously by way of Cape Horn. The force of the Prevailing Westerlies is from 2 to 4, Beaufort scale.

The Prevailing Westerlies begin as an upper wind in Trade Wind latitudes, descending to sea level at the edges of the Trade Wind belt, approximately Lat. 30° N. and S. Over Cuba the airman may find them at the height of about 11,500 feet; over Jamaica about 19,500 feet; and over Trinidad about