CLIMATE.-A comparison and study of the position of the iso thermal lines for January and July throw a considerable amount of light upon the general conditions which determine the tempera ture of North America. In January the isotherms, outside of the tropics, trend from north-west to south-east with a curve to the south ; in July, over a great part of the continent, they run in a some what similar direction, but in the west they bend towards the north and are crowded together along the Pacific coast, to which they run almost parallel for considerable distances, and which they leave in lower latitudes than those in which they enter along the Atlantic. In the winter months the land is much colder than the sea, and there is a decrease in temperature along the parallels of latitude with an increase in distance from the coast. Further, as there is during the winter months a high pressure area over the land, the winds blow outwards towards the sea, especially on the east coast, as far south as the twenty-fifth parallel. As these winds, blowing from the west and north-west, are cold, the temperature of the eastern shores of Canada and the United States is greatly reduced by them. The west coast, on the other hand, is protected from cold land winds by the Western Cordillera, and the prevailing winds in higher latitudes are those blowing from the tropic high pressure belt, which extends across the Pacific ocean, and which at this time of the year lies well to the south. These south-westerly and westerly winds, having come from the south and having blown over the Pacific, are relatively warm, and have a moderating effect upon the climate of the west coast. South of about the twenty-fifth parallel, the country lies within the belt of constant trade winds, the land mass is comparatively narrow, and the isotherms tend to follow the parallels of latitude.
In summer, the heat equator has moved northwards, the land is much warmer than the sea, there is an area of low pressure over the continent, and the winds move inland from the sea. On the east coast, in higher latitudes, they tend to blow from the south and south-west. In lower latitudes the trade winds have been sucked in from their usual course by the continental low pressure area. On the west coast, in higher latitudes, winds from the west and south-west are blowing, and these exercise a cooling influence which prevents the range between summer and winter temperatures from being great. In lower latitudes the trade winds blow off-shore, except in the extreme south where monsoon conditions prevail, the trade winds of the southern hemisphere having been pulled across the equator.
As a result of these various atmospheric conditions, the tempera ture along the coast is much lower than in the interior along the same parallels of latitude. The greatest extension northward of the isotherms is in the western part of the continental interior, where the influence of sea winds is least felt, and where the dryness of the atmosphere and the barrenness of the land lead to very high temperatures.
North America may therefore be divided into several regions as far as temperature alone is concerned. The Arctic lands in the north have very long, cold winters, and short and cool summers. The eastern parts of Canada and the United States have winters which are cold in the north but cool in the south, and warm along the Gulf coast ; and summers which are warm in the north, but become hot in the south and around the Gulf. The great interior region has, on the whole, a more extreme climate, varying, however, according to latitude. The winters in central Canada and in the northern States are very cold, while the summers are warm. Further south the cold of winter is seldom so great, but the heat of summer is more intense. Along the Pacific coast the climate is more equable than in any other part of North America outside of the tropics.
The rainfall of North America occurs chiefly during the summer months. At this period of the year a low pressure area, running north and south, has developed over the interior of the continent, while high pressure areas have spread northward over the Atlantic and the Pacific. The moist air is therefore sucked inwards into the very heart of the continent, and precipitation takes place as a result of cooling caused partly by increasing altitude, but chiefly by the expansion and uprising of the air in the hot continental interior. The Atlantic trade winds, moreover, are, to some extent, drawn out of their usual course by the low-pressure area over the land at this season, and a heavy monsoonal rainfall takes place along the southern part of the Atlantic coastal plain, around the Gulf, and on the eastern slopes of the mountains of Mexico. On the west coast there is a heavy rainfall as far south as Vancouver, brought by the north-westerly winds from the Pacific high pressure area, but further south the winds contain little moisture, and, as temperature increases rapidly inland, there is no precipitation. On the west coast of Mexico, however, there is a heavy monsoonal rainfall due to the trade winds of the south Pacific being pulled across the equator and turned to the right.