Fig. 28.—Average Monthly Temperature and Rainfall of Typical Places in North America. (Huntington and Cushing.)
[Figure 28] graphically presents the average monthly temperature and rainfall of typical places in North America, and [Figure 29] of places in the Old World. Here may be seen every phase of climate from tropical to temperate and to cold, and from marine to continental. By studying the winds on [Charts 15] and [16] and the ocean currents on [Chart 13], the reader should be able to find an explanation for the different conditions shown. For example: Mazatlan and Vera Cruz are both on the coast of Mexico, the first on the west and the latter on the east. Each has a rainy period in the summer, but at Vera Cruz the rain begins earlier and lasts later and is much heavier. The reason is that they both have north winds in winter ([Charts 15] and [16]), but in summer Vera Cruz receives winds direct from the Gulf of Mexico and at Mazatlan the winds continue to blow from the north, with but a slight inclination landward. Again, the explanation for the fact that Mazatlan has a monthly range of temperature from 60° in winter to 80° in summer, while Vera Cruz has a range of only 70° to 80° is found in the wind direction.
Fig. 29.—Average Monthly Temperatures and Rainfall of Typical Places in the Old World. (From “Principles of Human Geography.” John Wiley & Sons.)
The City of Mexico is wonderfully favored by climate. Here a moderate rainfall occurs from May to September. The oceans are not far distant on either side, as distances are measured continentally, but its great elevation on a table-land relieves it of the torrential rains usual to the tropics; and yet it is close enough to marine influence so that its air has not the nerve-irritating dryness of the plateau of the Rocky Mountains, and it has a remarkable evenness of temperature between winter and summer, with a monthly range between 50° and 60°. Its range between day and night is sufficient to be stimulating.
Still looking at [Figure 28], note the remarkable similarity between the climate of Pittsburgh and Toronto. Each has about the same rainfall and it is almost equally distributed throughout the months of the year. The only difference is that Toronto is a little colder. St. Paul and Kansas City, typical of the climate in the interior cities, have a small amount of precipitation in winter, considerable in summer, and a wide range of temperature; while the Pacific coast cities have dry summers, and winters that vary from three inches of rain at Los Angeles to fourteen inches at Astoria, with no excesses in temperature.
Temperatures Aloft in the Atmosphere. Kite and balloon observations have not been continued long enough, nor have they been made at a sufficient number of places, to give one the data from which the climate of any considerable altitude in the free air may be determined, but from a large number of free balloon observations made with self-recording instruments, in the middle latitudes of this and foreign countries, [Figure 1] (page 12) has been constructed, which shows the manner in which the temperature decreases with elevation up to eighteen kilometers (eleven miles). Note how rapidly it falls with elevation up to eleven and a half kilometers (about seven miles). This depth of air measures the thickness of the turbulent stratum in which cyclones and anti-cyclones operate. At its top the temperature always is about 64° below zero in winter and 70° below in summer. And right here occurs a most wonderful phenomenon,—one of which scientists were entirely ignorant less than two decades ago. At first it was thought that there was something wrong with the recording thermometers, for they failed to register falling temperature with gaining altitude after the storm stratum was passed at seven miles. Then it was noted that all instruments displayed the same peculiarity, and the “Isothermal Stratum” (equally heated region) was discovered, in which the temperature maintains the same degree of intense cold so far as exploration had been made. From Mount Weather, under the direction of the writer, a balloon was flown to nineteen and one tenth miles before it exploded and sent a parachute gently down to earth with its precious record. This flight showed practically no change in temperature after the isothermal stratum was reached. (See [Chapter III].) One is reasonably safe in assuming that there is no oxygen beyond an altitude of thirty miles and that at fifty miles the nitrogen becomes inappreciable, and that, therefore, the temperature must shade away to practically nothing when the void of outer space is reached, notwithstanding the presence of the newly-discovered isothermal stratum nearer the earth.