Wind

a current of air. Air in motion. The origin of wind is easily understood. Were all parts of the earth equally heated by the sun's rays, the atmosphere would be uniformly dense and would rotate with the earth. The atmosphere would be in a state of perpetual calm. Under present circumstances, however, the sun heats certain areas of the atmosphere more than it does others. The heated portions of the air expand and swell up, pouring out over the cooler areas. The heavy air of the cool areas is held back no longer by the lighter air of the warm areas and rushes in to restore the equilibrium. Thus winds are formed. A similar illustration may be noted on any warm seacoast. During the daytime, the air on land is heated more than the air over the sea. A cool breeze from the sea sets in early in the day and continues until night. During the night time the air on land cools off more rapidly than the air on the sea and becomes more dense. Soon after nightfall, therefore, a cool breeze from the land sets out toward the sea. Thus, on tropical coasts, the sea breeze of the daytime is followed by a land breeze at night with unfailing regularity. This alternation of cool breezes renders the climate of many coasts delightful.

The great amount of heat received within the tropics creates a system of atmospheric circulation on a grand scale. The hot air of the torrid zone expands, flows outward, and is unable to balance the cold air of the poles and the temperate zones. This causes two lower currents from the poles to the equator and two upper currents from the torrid zone back to the poles. Owing to the turning of the earth on its axis, the air at the equator has a much greater rotary motion than the air nearer the poles, that is to say, equatorial air is turning eastward with the earth rapidly. As this air travels northward and southward toward the poles it outruns the meridians of longitude, creating upper currents that travel in a northeasterly and southeasterly direction respectively. The lower currents originating in latitudes that have a less rotary, that is to say, a slower easterly, motion than the equator, are outrun by the torrid zone. They fall backward accordingly as they travel northward and southward toward the equator, producing a northeast wind in the northern hemisphere and a southeast wind in the southern hemisphere. These diagonal winds are called trade winds from their influence on commerce. The two belts of trade winds shift northward and southward somewhat according to season, but they blow westerly during the entire year.

Winds are named from the direction from which they come. A north wind is a wind traveling southward. A wind from land is called a land breeze; one from the sea, a sea breeze; a wind coming from the northeast is called a northeast wind, and so on. In the reports of the United States Signal Service, a fresh wind is a wind blowing twelve miles an hour; a high wind has a rate of thirty-five miles an hour; a gale, forty-five miles; a hurricane, ninety miles.

The highest velocity winds have attained since the United States signal stations have been established are as follows: Albany, New York, seventy miles per hour; Alpena, Michigan, seventy-two miles; Bismarck, North Dakota, seventy-four miles; Buffalo, New York, ninety miles; Chicago, Illinois, eighty-four miles; Custer, Montana, seventy-three miles; Denver, Colorado, seventy-five miles; Duluth, Minnesota, seventy-eight miles; El Paso, Texas, seventy-eight miles; Galveston, Texas, eighty-four miles; Memphis, Tennessee, seventy-five miles;

North Platte, Nebraska, ninety-six miles; St. Louis, Missouri, eighty miles; Washington, District of Columbia, sixty-eight miles

From the tables compiled by the United States Weather Bureau the wind pressure per square foot is as follows, according to the velocity of the wind per hour:

See Whirlwind; Cyclone; Hurricane; Monsoon; Chinook; Sirocco; Foehn; Sail; Boreas