=Influence of the Gulf Stream on Climate.= From either side of the equator the surface winds (Charts 15 and 16) blow the water westward, causing what are known as the “Equatorial Currents” (Chart 13). The eastward projection of the coast of South America divides the Atlantic equatorial current into two parts; one goes south along the coast of South America and sets up the circulation in the South Atlantic, which sweeps north along the southwest coast of Africa. The other passes to the northwest, a part setting up the North Atlantic circulation and the remainder sweeping through the Windward Islands and storing itself in the Gulf of Mexico, whence it is driven out at a velocity of some five miles per hour through the narrow channel between Key West and Cuba. Here it has a depth of half a mile and a width of forty miles. Its velocity is accelerated because it enters the Gulf in a broad sweep and passes out through a constricted channel. It retains its individuality as a warm river passing through the ocean because of its greater velocity and higher temperature than the waters in which it finds itself soon after it leaves the Gulf; but it gradually merges with the great Atlantic circulation as it passes to the middle of the ocean. It is the opinion of the writer that its influence on climate has been exaggerated, that the warming of Europe that is credited to the Gulf Stream is accomplished by the mere presence of the ocean to the westward and to the general circulation of that ocean without regard to the wonderful phenomenon known as the Gulf Stream. =Effect of Valleys on Day and Night Temperatures.= Valleys affect temperatures in proportion to their depth and width. A deep, narrow valley might have the effect illustrated by Figure 27, if the time were summer and the sky clear. During the daytime radiation would warm the interior so that the bottom of the valley would have a much higher temperature than the free air at the top of the valley, and the movement of the air would be sluggishly down the center and up the sides of the depression. During nighttime all the conditions would be reversed. Vegetation, losing heat by radiation much faster than the air, would cool the latter as it came in contact with the sides of the valley. The air would slowly descend along the sides through gain in specific gravity and collect at the bottom with a temperature much lower than it had when it started its descent. [Illustration: FIGURE 27. Summer day temperature in a narrow valley. Summer night temperature in the same valley.]