The new air world

by Willis L. Moore

Follow the currents down the coast of Spain and of northeast Africa; then note on [Chart 14] the southward trend of the lines of equal temperature, as the currents bring colder water southward to cool the air. Next examine the currents of the Pacific and the isothermals. The currents moving northward towards the equator along the west coast of South America, and those moving southward, also toward the equator, along the west coast of the United States and Mexico cause a bulging of the isothermal lines from the positions that they would occupy if there were no currents coming from colder regions.

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.

Figure 27.

Summer day temperature in a narrow valley.

Summer night temperature in the same valley.

Effect of Mountains on Climate. The rarity of the atmosphere of mountains readily allows the rays of the sun to pass through it and thus the surface of mountains is quickly warmed, but the same conditions permit a rapid radiation at night, so that there are considerable extremes of temperature. Air cooled by contact with a mountain may flow down its sides at night and collect in depressions below, often causing frost on still nights where the temperature higher up is much above freezing. Mountains may be more cloudy and rainy than plains, for the currents of air that cross them must rise, and in rising they cool by expansion and often reach the dew point of the air, moisture being precipitated in the form of clouds, rain, or snow. Often a peak is constantly capped with a crown of clouds. Mountains may intercept vapor-bearing winds from oceans, force them to such an elevation that their vapor is largely precipitated on the windward side of the mountain, and receive them on the leeward side as dry, rainless winds. Vast desert areas are often the result. A good example is presented in the case of the Pacific coast mountains and the desert plateau to the east.

Mountain peaks may be covered with snow, even though they be located in the tropics, if their elevation be sufficient. This is because the absorption of both incoming and outgoing radiation is so much greater in the lower reaches of the atmosphere, where the water vapor is densest. Wherever observations have been made they have shown that the temperature of the air on high mountain peaks and crests and for a distance of one to three hundred feet above them is cooler than adjacent free air of the same height, due to upward deflection of air currents and their cooling by expansion, and to radiation from the peak.

The Himalayan Mountains exercise a profound effect on the climate of Asia. The monsoon (any wind that alternates annually in direction or force) of summer brings the moist air from the Bay of Bengal and precipitates torrential rains from it as it ascends to higher and higher elevations in passing over the great heights of the mountains. At a place four thousand feet above the sea and not distant from Calcutta, the annual rainfall is 466 inches, while the average for most of the region east of the Mississippi River is only forty inches. More than forty inches have been known to fall in one day in the Himalayan Mountains. As in the case of all very high mountains, the rainfall increases in these mountains up to a certain elevation and then decreases. North of the mountains the monsoon passes into the interior of Asia with withering dryness and vast deserts are the result.