The second great physical region of Asia is the Northern Low land, which everywhere extends from the borders of the mid-world mountain system to the shores of the Arctic Ocean. This lowland, which is practically continuous with that of Northern Europe, has its greatest breadth in the west, and gradually becomes narrower towards the north-east. In the west, also, its altitude is less (the districts around the Caspian are below sea-level), while in the north east it rises to a height of 1,700 feet. This difference in altitude is partly due to differences in geological structure, the western section being of much later date than the eastern. The Caspian and the Sea of Aral are in regions of inland drainage, and most of the land around them is covered with shifting sands. In the basins of the Ob and the Irtysh recent alluvium is the prevailing formation, though in places older rocks come to the surface. Further east, with some Secondary rocks prevail over the greater part of the country.
In the south, the mid-world mountain system is separated by the great alluvial valleys of the Tigris and Euphrates, and the Indus and Ganges, from the ancient plateau lands of Arabia and Southern India. These two plateaus, which seldom exceed 6,000 feet in height, are composed of ancient rocks, and seem to have remained above sea-level for the greater part of the geological period. In places they have been considdrably affected by volcanic action.
CLIMATE.—Only a brief account of the general conditions which determine the climate of Asia need be attempted at present, as it will be necessary later to examine in some detail the climatic characteristics of those regions which are most important from the economic point of view.
Among the factors which have to be considered are the great size of the continent, the situation of the whole of it within the northern hemisphere and of the most of it outside of the tropics, the great system of mountains and plateaus by which it is traversed and the relation of these to the surrounding regions, and the exist ence of the African land mass on the south-west, and of the Indian and Pacific Oceans on the south and east.
During the winter months the greater part of Asia is an area of low temperature, and the coldest known region on earth is in north-east Siberia where, at Verkhoyansk, the mean temperature in January is —60°F. Partly as a result of the great cold, and
partly because of the position of the northern belt of high pressure, the region is one of high pressure, the maximum being established over Mongolia. Hence the winds blow outwards, and being cold, give to the districts over which they blow temperatures that are below the normal. They bring little rain except when they have crossed the sea as in north-west Japan, or where, as in Farther India, they are merely reinforcing the ordinary trade winds. On the west of the continent, Asia Minor, Northern Arabia, and Western Persia fall within the region with a Mediterranean type of climate and receive their rainfall during the winter half of the year.
During the summer months the land surface becomes greatly heated, and an area of low pressure, which is deepest between the Red Sea and Northern India, is developed over the continent. This low pressure region sucks in the trade winds both of the Indian and Pacific Oceans, and so gives rain to the whole of the region backed by the mid-world mountain system from India to Korea. The mountain system itself, on account of its position and configuration, receives but little rain, except on the slopes exposed to the monsoon rains, and the interior plateaus are there fore dry at all seasons of the year. Over the lowlands, precipitation takes place mainly in the summer months. The south-west, around the Caspian Sea and the Sea of Aral, is dry at all seasons of the year, as in the summer it is sheltered by the configuration of the land, while in winter it falls within a region of high pressure. Further north there is a gradually increasing rainfall, and a great part of Siberia has between 10 and 20 inches, the most of which falls in summer, when moisture is sucked into the northern part of the continent from various directions, but mainly from the west. This region is bounded both on the north and on the south by a belt of country in which precipitation is less than 10 inches.