In the Atlantic these south-westerly winds obviously blow in a direction opposite to the north-east trades, whence the name of anti-trades often given to them. As they blow across the broad Atlantic they arrive off Europe saturated with moisture. As they come from lower latitudes they are warmth bringing. In winter these winds reach the Mediterranean area owing to the southern shift of the trades, and bring moisture with them; while in summer they lie more to the north, and though their moisture affects the coast of Portugal it does not reach the greater part of the Mediterranean area. Within that area the northern limit of the rainless summer may be said, in a rough sense, to correspond with about the 40th parallel of latitude, though it varies according to local conditions in the different peninsulas. To the north of this line, therefore, the climate is more or less affected even in summer by the anti-trades. It must not be supposed that the region of the trade winds and of the anti-trades lie side by side. Between the two there is a zone of variable winds, but in general terms we can explain the peculiarities of the Mediterranean rainfall by saying that the region lies within or just at the edge of the dry trades in summer, and within the zone of the moist anti-trades in winter. Let us next consider how the area is demarcated from the surrounding regions. There is of course no hard and fast line, but we can indicate in broad outline the meteorological limits. To take the absolutely rainless summer as the limit would cut out, as we have suggested above, the greater part of the northern shore of the Mediterranean, except the southern halves of all the great peninsulas. Quite generally, however, we may say that the northern limit of the Mediterranean region, in its western half, is defined by the occurrence of considerable summer rain. That is, it is bounded to the north by a region which is within reach of the rain-bringing anti-trades in summer as well as in winter, and which has a lower temperature than the Mediterranean region proper. To the east the region is limited by deserts, for the westerlies of winter can only carry their moisture a certain distance inwards, and though they are greatly assisted by the long, eastward-stretching, inland sea, yet there comes a time when all their load of moisture is lost, and desert conditions supervene. To the south the desert again forms the boundary, though here for a different cause. North Africa behind the Atlas is permanently within the trade-wind belt, that is, it is permanently subjected to the action of drying winds, and its rainfall is therefore small or nil. Similarly in California the southern limit of the Mediterranean zone of climate is the desert region of Arizona, Mexico, and the north of Lower California. A similar band of desert separates the Mediterranean zone from the tropical region of summer rain in the other places where the Mediterranean type occurs.