The principle of this local change of winds, together with the rotation of the earth, the absorption of moisture by air, and the fact that the latter becomes cooler when it expands on rising and warmer when it is compressed by descending, serves to explain all the major phenomena usually observed in connection with winds. The air of the equatorial belt, heated throughout the year, necessarily rises and creates an indraught from both north and south; but since the air thus flowing in has a lower rotary velocity than the earth’s surface at the Equator, it lags behind and so gives rise to northeast and southeast winds, respectively, between the two tropics and the equatorial belt. These regular winds, from the aid they give to commerce in passing from continent to continent, are known as the _trade winds_. On the other hand, the air that has risen from the hot equatorial belt, cooling by expansion as it rises and flowing northward and southward from the Equator, on descending as it mainly does into the temperate zones, has a higher rotary velocity than the land-surface and so tends to give rise to southwest and northwest winds in the northern and southern hemispheres respectively. At sea, on coasts and in level inland regions to windward of mountain chains, such winds are often quite regular during a portion of the year. _Cyclones._—But local disturbances arising from heated land areas or mountain slopes, as well as wide atmospheric changes whose causes are not fully understood, give rise to waves of alternating high and low barometric pressure, largely converting rectilinear or slightly curved wind-motion into whirls or “cyclones”[102] ranging from a thousand to over two thousand miles in diameter. These in the case of low-pressure waves or centers, _toward_ which the air flows from the _outside_, revolve in the direction contrary to the movement of the hands of a clock, and commonly produce rain in their east portion. A high-pressure wave or center, _from_ which the air naturally flows toward the _outside_, will usually bring about an “anti-cyclone” area with fair, and in winter cold (“blizzard”) weather, the direction of the whirl being, in this case, the reverse, or in the same direction as the hands of a clock. Both cyclones and anti-cyclones move in North America from west to east, mostly entering from the Pacific Ocean off the northwest coast and traversing the continent with a slight southeast (or in the case of cold weather almost south) trend, with a velocity of twenty to thirty miles an hour; until upon reaching the region of the Great Lakes they generally turn northeastward and pass into the Atlantic Ocean from the New England and Canada coasts.—It is upon these general facts, roughly outlined here, that the weather forecasts are in the main based; taking into consideration, of course, the local or regional conditions, topography, etc., which modify the application of the general rules.