In the following description we shall restrict ourselves, for the sake of clearness, to the Mediterranean region itself, the region where the Mediterranean type of climate is developed over the largest area, and where, for many reasons, it is most important. But it must be noted that the conditions which give rise to the Mediterranean type of climate are the same wherever it occurs, though in the Mediterranean area they are greatly modified by the great inland sea of that name, which carries oceanic conditions far into the land. We must note, first, that at all seasons those regions of the earth which are directly beneath the vertical rays of the sun are heated most intensely. Therefore the air over these regions, being rendered light by heating, rises, and a belt of low pressure is thus formed. Only at the equinoxes does this belt of high temperature, low pressure, and light winds or calms, coincide with the equator. In the northern summer it moves north with the sun; in the northern winter it travels south with the sun, being always over what is called the _heat equator_. Into this belt of low pressure air from north and south, where the pressure is greater, tends to rush in, and we have thus formed the constant or "trade" winds, which, owing to the deflection produced by the earth's rotation, appear as the north-east trades in the northern hemisphere and the south-east in the south. These winds are dry winds, because they blow from colder to warmer latitudes, and they accompany the equatorial low-pressure belt in its north and south movements. In the northern summer the trade winds may extend northward to lat. 35° or even 40°, while in winter their northern limit is 10° to 15° further south. A glance at the map, then, will show that in summer the Mediterranean area is within or near the sphere of action of the dry trade winds, which are continental, sweeping into the region after having blown over land surfaces. We must next consider the atmospheric movements in the region to the north of the trade wind belt. An area of more or less permanent low pressure, best marked in winter, exists in the North Atlantic, in about 60° N. lat., and draws the air into it in the direction known as counterclockwise, that is, in the direction opposite to that of the hands of the clock. The result is the production of the winds which appear off the coast of western Europe as the warm south-westerly winds of winter, while they appear off the coast of North America as cold northerly winds. In the southern hemisphere, where, as we have seen, there is less land to interfere with the development of the atmospheric circulation, these winds form the prevailing westerlies.