In the quadrant of the earth's surface occupied by North America the climatic conditions are controlled in a large measure by the planetary winds. In the equatorial belt of calms the barometric pressure is lower than on
either side, the temperature is uniformly high, the air is heavily charged with moisture, and torrential rains are frequent. In the belt of the northeast trades the weight of the air for a given area is greater than in the doldrums, the wind blows with remarkable uniformity both of direction and force, the sky is normally clear, and rain infrequent except when the warm moist air is forced upward either by local storms or on coming in contact with high land. The trade-winds blow across the West Indies, Mexico, and much of Central America. To the north of the trade-wind belt is a belt of prevailingly high barometrical pressure, light variable winds, narrower and less well defined than the doldrums, which encircles the earth in the region of the Tropic of Cancer. This belt of calms, although familiar to sailors, to whom it is known as the "horse latitudes," is ill-defined on the land, where its presence is masked by changes due to local conditions. To the north of the tropical calm belt the prevailing surface winds are from the westward, and owe their direction to the constant flow of the upper air-currents in their poleward journey, under the influence of the earth's rotation. This great belt of winds from the westward crosses the portion of North America including the United States and southern Canada, but it is subject to many disturbances. The northern portion of the continent extends into the little known polar region of prevailingly low barometrical pressure, where midsummer and midwinter calms normally prevail.
The great world-encircling currents of the atmosphere, namely, the trade-winds, blowing towards the southwest or west across the Caribbean and Mexican region, and the prevailing westerlies, or winds blowing in an easterly direction, over the broad temperate portion of North America, exert the main control on the climate of the continent.
The Seasons.—Of primary importance to the inhabitants of North America is the fact that the climatic belts determined by the inclination of the earth's axis to the plane of the ecliptic are subject to annual migration towards the north and south. In the torrid zone the equatorial belt of calms, with its humid and oppressively hot atmosphere, prevailing
cloudiness, and heavy rains, and the belt of the northeast trades, with its prevailingly clear skies and refreshing breezes, do not occupy the same positions throughout the year, but migrate with the sun. The migration of these two strongly contrasted climatic belts brings to the otherwise remarkably uniform conditions of the atmosphere over the West Indies, Central America, and Mexico, two, in general well-defined, periods each year, namely, a wet and a dry season, the former occurring in the summer and the latter in the winter. It is to be borne in mind that between the tropics there are, with certain local exceptions, but two seasons each year, the leading contrasts of which are determined by differences in rainfall.
To the north of the Tropic of Cancer the seasonal changes are more varied than in the torrid zone, and contrasts in temperature become the most marked climatic feature; while precipitation, although in general somewhat evenly distributed throughout the year, is more abundant in winter than in summer. On account, however, of the greater diversity in the climatic changes experienced each year within the temperate zone, four seasons are recognised, the most distinctive features of which depend on changes in both temperature and humidity.
In the northern portion of the temperate zone, and extending over the arctic zone, the seasons are again reduced to two, summer and winter, the contrasted conditions pertaining mainly to temperature and light.
A marked variation, which has an important bearing not only on climate, but on the distribution of life encountered in passing from equatorial to polar regions, is found in the distribution of light. Between the tropics the number of hours of light and darkness each day is approximately equal; in the temperate zone there is considerable diversity from season to season, which increases with increase in latitude; and uniformity, of a different character than at the far south, again becomes prominent in the frigid zone, where the number of hours of light each day is greatly prolonged during the summer and correspondingly decreased during
the winter. The extreme contrast occurs in the neighbourhood of the pole, where during the summer season the sun is continuously above, and in winter continuously below the horizon, or in familiar language, there is a six-months day (light) and a six-months night (darkness).
In going from the equatorial to north polar regions there is a general decrease in mean annual temperature, and in general a decrease also in precipitation, but great variations in these gradual changes, with increase in latitude, occur which are both continental and local in character. In winter the interior portions of the continent, and especially the plateaus and mountains, are colder than the lands in corresponding latitudes near the oceans; while in summer the reverse is true, the margin of the continent being cooler than the broad interior.