Page:Handbook of Meteorology.djvu/68

56, southwest, northeast, and southeast and that winds from the north, south, east or west are the exception. Winds over the land, however, are apt to be modified by local topography; and it frequently happens that a surface wind differs materially in direction from the wind at low cloud heights.

Throughout the eastern half of the United States, about half the recorded mileage of the wind is from points between north and west. Along the Gulf Coast to a distance of about 300 miles inland, winds with a southerly element of movement prevail. As a rule, the winds are strongest during the winter months and mildest in summer. For the greater part, the prevailing winds of the Pacific Coast region are northwesterly. At San Diego about two-thirds of the mileage is recorded by winds blowing between west and north.

The strongest winds are apt to occur along the coasts of the sea and the Great Lakes. The mean hourly velocity at Sandy Hook, Block Island, Delaware Breakwater and Cape Mendocino exceeds 14 miles. Throughout the plains west of the Missouri River, high winds prevail. The long downward slope over a smooth surface adds to the velocity of westerly winds. During winter months the cold-wave winds from Canada contribute a mass of air which, moving eastward, gives added velocity to the winds of this region.

The latitude of strongest winds in the United States is approximately along the forty-fifth parallel in the summer and a few degrees lower in winter. Winter months are the season of the strongest winds. The winds of greatest strength, however, are storm winds—winter cold-wave blasts, or the recurved portion of West Indian hurricanes which sweep northward along the Atlantic Coast.

Other Features of General Circulation.—The foregoing paragraphs present a very elementary view of the greater circulation of the air. As a matter of fact, not much is known, even of the surface winds over a very large part of the earth.