Where the trade-winds heap up the surface water against the east coasts of the continents the currents turn poleward.
The prevailing winds respond to the stronger poleward temperature gradients of winter by rising to a higher velocity and a more frequent and severer cyclonic storminess; and to the weaker gradients of summer by relaxing to a lower velocity with fewer and weaker cyclonic storms; but furthermore the northern zone occupied by the prevailing westerlies expands as the winds strengthen in winter, and shrinks as they weaken in summer; thus the stormy westerlies, which impinge upon the north-western coast and give it plentiful rainfall all through the year, in winter reach southern California and sweep across part of the Gulf of Mexico and Florida; it is for this reason that southern California has a rainy winter season, and that the states bordering on the Gulf of Mexico are visited in winter by occasional intensified cold winds, inappropriate to their latitude.
In each of the three oceans there are two maxima of salinity-one in the north, the other in the south tropical belt, separated by a zone of minimum salinity in the equatorial region, and giving place poleward to regions of still lower salinity.
As a result, the isotherms of July are strongly convex poleward as they cross the United States, the isotherm of 70 Sweeping up to the northern boundary in the north-west, and the heat equator leaping to the overheated deserts of the south-west, where the July mean is over 90.