A very curious illustration of the similarity of climate in the different regions named is to be found in the fact that in parts of the Mediterranean area two introduced American plants, the agave and the prickly pear, are more obvious and abundant than most native plants; while in California, Cape Colony and southern Australia the cultivated plants are chiefly of Mediterranean origin.
The main features of the Mediterranean climate may be briefly summarised. The most important character, next to the mild temperature, is the fact that no rain (or very little) falls in summer, the growing season further north, which is here largely a period of cessation of plant growth. The rain, which tends to be scanty or even absent in the interior of land masses, _e. g._ in Spain and Asia Minor, and also to the south, _e. g._ in the Desert of Sahara, in the Mediterranean region proper falls in the winter months. It is this winter rainfall and the summer drought which define the Mediterranean type of climate.
The reason for this seasonal distribution of rainfall is as interesting as the fact itself, and to understand it we must turn to the circulation of air on the surface of the globe.
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.