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.
The tropical maxima of salinity on the poleward side of the trade-winds coincide with the regions of minimum rainfall, high temperature, strong winds and consequently of maximum evaporation.
In the region of the: westerly winds on the poleward side of 40° N.
The west-wind drifts on the poleward side carry back part of the water southward to reunite with the equatorial current, and thus there is set up an anticyclonic circulation of water between io and 40° in each hemisphere, the movement of the water corresponding very closely with that of the wind.
The effects of the continent are already visible in the mean annual temperatures, in which the poleward temperature gradient is about twice as strong as it is on the neighboring oceans; this being a natural effect of the immobility of the land surface, in contrast to the circulatory movement of the ocean currents, which thus lessen the temperature differences due to latitude: on the continent such differences are developed in full force.
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.