permits rapid radiation. The nights, in regions of very dry air, may be bitterly cold although mid-afternoon has been intolerably hot; indeed, at considerable altitudes, freezing temperatures during summer nights are not unknown in desert regions.
Both the moisture and the dust and smoke content of the air modify the absorption of the sun’s heat and its radiation by the earth. The moisture and, to a less extent, the dust and smoke content of the air absorb a considerable and likewise a measureable part of the heat that passes readily through dry clean air. And if they intercept and retard the passage of heat coming to the earth, they also retard radiation from the earth at night. In other words, the amount of insolation—that is, of solar heat—received at the earth’s surface is quite as variable as is the daily range; indeed, it is the highest expression of the daily range. At sea level dry air does not always indicate warm days and cool nights; but at levels of 5000 feet or more this is the rule rather than the exception. At any level, changes in temperature are more rapid in dry than in moist air, and the reason therefor is obvious.
Over areas of moist air a considerable part of the heat of insolation is absorbed in another way. Almost always in such areas there is a considerable water in the form of mist—that is, minute droplets of water. When the sun’s warmth converts these to vapor the absorbed heat becomes latent heat, and no longer appears as sensible heat. This fact furnishes another reason why the air over desert regions, as well as the ground surface itself, becomes heated to a higher degree.[1]
Conditions of temperature exercise a great control, not only over civilization, but over the distribution of life itself. Humanity may overcome its environment so far as temperature is concerned—man can command fire, food, and fuel to be brought to him; but other forms of life cannot rise superior to conditions of temperature. The line beyond which grass will not grow is determined in part by temperature; it marks the limit beyond which grazing animals cannot thrive, and, with a
- ↑ A moist surface, and very moist air as well, does not have a temperture materially higher than the wet-bulb thermometer; a dry surface, or very dry air, acquires a temperature approximating that registered by the black-bulb thermometer.
