Page:Handbook of Meteorology.djvu/249

 Obtaining wind direction from the movement of the clouds is frequently misleading as to results. Sometimes it happens that the surface wind blows from one direction, while clouds move toward another. If the clouds are low, the direction whence they come is most accurately obtained by facing them and then turning at a right angle to check the observation.

Cross-winds, that is wind currents of different directions, are more common than is generally known. Airmen have learned their meaning and watch sharply for them. They are apt to occur before and after a storm. At such times they practically mark the advancing or the retreating edge of a cyclonic movement. Billow clouds are the earmarks of cross-winds and such cross-winds are usually at a considerable height. Cross-winds are very common along the coasts of large bodies of water where the land and the sea breeze alternate. These alternating winds are shallow, however, and the airman usually finds the steady prevailing wind at an altitude of half a mile or more. The alternating mountain valley winds are cross-winds of similar character.

Cross-winds are not always discernible to the airman or to the marine pilot. They become visible as to position only when difference in temperature and humidity of the two layers produces cloudiness at the interface.

Wind Velocity.—The velocity of the wind at any locality varies greatly. The dead calm of tropical seas is frequently followed by hurricane winds having a velocity exceeding 100 miles an hour. The hurricane that wrecked Galveston blew with a velocity estimated at more than 125 miles an hour. At Cape Mendocino, California, a velocity of 144 miles was registered, and at Mount Washington a mean hourly velocity of in miles per hour was registered for a whole day. At Battery Park, New York city, the anemometer has registered a velocity of 96 miles; and storm winds along the coast have reached a velocity of 100 miles a dozen times or more. Some of the strongest winds along the Atlantic Coast of the United States are storm winds of a recurved part of West Indian hurricanes.

For the greater part, the mean hourly velocity of the wind at the various stations provided with anemometers varies from