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 Rapid Loss of Heat by Radiation on the Earth.

In order better to comprehend what this minimum may be under extreme conditions, it will be useful to take note of the effects it actually produces on the earth in places where the conditions are nearest to those existing on the moon or on Mars, though never quite equalling, or even approaching very near them. It is in our great desert regions, and especially on high plateaux, that extreme aridity prevails, and it is in such districts that the differences between day and night temperatures reach their maximum. It is stated by geographers that in parts of the Great Sahara the surface temperature is sometimes 150°F., while during the night it falls nearly or quite to the freezing point—a difference of 118 degrees in little more than 12 hours. In the high desert plains of Central Asia the extremes are said to be even greater. Again, in his Universal Geography, Reclus states that in the Armenian Highlands the thermometer oscillates between 13°F. and 112°F. We may therefore, without any fear of exaggeration, take it as proved that a fall of 100°F. in twelve or fifteen hours not infrequently occurs where there is a very dry and clear atmosphere permitting continuous insolation by day and rapid radiation by night.