OROGRAPHICAL ASPECTS Orographic Control of Rainfall.—The sun's heat raises water-vapour from the hydrosphere and sets up a system of winds which carry it over the land. When vapour-laden air is raised in the atmosphere by any cause it cools and the vapour condenses to fall as rain, snow or hail. When the uprising is due to storms of the cyclonic, squall or thunder type the place where the rain falls depends on the intensity of the meteorological disturbance and the path it takes, conditions which have no clear relation to the form of the land surface. The dominance of orographic con trol asserts itself, however, in the much more common case of a steady sea wind blowing in horizontally towards the land. Vapour carried by such a wind does not condense over the sea and to a very slight extent on low islands or level plains, but when it en counters rising ground the wedge-like action of the land compels the air to rise and as the slope increases in steepness the rate of rising is accelerated, cooling becomes rapid and rain falls more and more heavily as the height increases until if the range is high enough all available moisture is deposited as rain on the slopes, or as snow on the summits and the air descends on the other side as a drying wind. In these circumstances the windward slope is abundantly watered ; the leeward lands are arid. On lower hills the maximum rainfall occurs just over the summit as the air continues to rise by inertia for a little way after the summit is passed. So close and intimate is the relation between rainfall and land forms that a map of the rainfall of western Europe, to take a typical example, is a close repetition of the orographical map.
Orographic Control of Glaciers and Rivers.—When snow is the material dragged down by gravity from the mountain tops, its plastic nature, even when compressed by its own weight into ice, causes it to flow along the valley lines extremely slowly until the glacier meets the sea or melts in a valley giving rise to a stream or river. Glaciers effect a certain amount of erosion and polishing on their beds and leave very characteristic marks. The clay formed in this process with the boulders which fell on the ice from above heap great moraines on the lower ground which often produce an entirely new configuration of the surface.
When rain falls on a hillside it runs over the surface in trickles which follow the steepest and shortest path downwards, gradually wearing a deeper bed for themselves, meeting where the slopes converge and forming definite streams. In their steep torrential course the streams foam along carrying boulders and rock-rubbish with them which continually deepen the channel in which they flow. Where an abrupt change of level takes place the stream plunges over as a waterfall, where a hollow in the ground is encountered the stream fills it up forming a lake whence the water overflows at the lowest point and proceeds along the steepest path to the sea, or in an arid region to a salt lake with no outlet. As the slope of the land diminishes its control of the flow becomes feebler, the current slackens, the material carried along by the water is dropped by degrees until only fine mud is moved by the slow current which meanders on its plain, shifting its course with every flood, and if the tides at its mouth are not too strong, gradually building a delta out to sea. The course of a river and of the ramified tributaries coming into it is absolutely dictated by the primitive land forms ; but by degrees the river system entrenches itself in the land, always deepening its valleys, until rivers become the strongest and most permanent lines in nature. The deepest drainage line is termed the thalweg, literally "dale-way" of a river. The area sending the rain which falls upon it to one river is termed the basin, the drainage area or the gather ing ground of that river, and the line which divides it from neigh bouring drainage areas is termed a watershed, water-parting or divide. Taking all the continents into account it has been cal culated that rather more than half the land area drains into the Atlantic and Arctic Sea the greater part of America sloping east ward and northward and most of Europe, Asia and Africa sloping westward and northward, leaving little more than one-quarter of the land surface to send its rivers in equal measure to the Indian Ocean and the vast Pacific. The rainfall of the remainder of the land-surface, amounting almost to one-quarter of its area, does not reach the sea at all but is intercepted by lakes in arid regions where the evaporation so far exceeds the rainfall of the drainage areas in question that the rivers cannot fill up the lake basins to overflowing. Regions of internal drainage occur in each of the continents in accordance with climatic conditions the origin of which is still obscure. In the case of Lake Titicaca in South America and of the great Salt Lake of Utah, Lakes Van and Urumia in Armenia, and the many salt lakes of Tibet the position is on a high plateau shut in by mountain ranges which shut off the rain-bearing winds. In the case of the Aral Sea, Lake Balkhash and Lob Nor in Asia and of Lakes Eyre and Torrens in Australia the elevation is moderate, and in the case of the Caspian Sea, the Dead Sea in Asia and the shotts of the Sahara in Africa, is considerably below the level of the sea.