=Why Does Air Cool with Ascent and Heat with Descent?= If a mass of air be elevated 183 feet it will be found to have lost one degree in temperature, because there is less air above to exert pressure upon it and it therefore expands to greater volume, and in the process of expansion work is performed which employs heat and renders it latent. One minute, one hour, or a thousand years thereafter, if this same air be lowered to its former elevation, it will be compressed into its previous dimensions and the heat energy that formerly was employed to expand it will be restored to the sensible condition. This ratio of 183 feet to one degree does not hold for any extended movement, because, as soon as the dew point of the air is reached, condensation in the form of cloud or rain occurs and the heat of condensation is released; that is to say, the same quantity of heat employed to create the water vapor at some previous time and thereby rendered latent is now become sensible and partly makes up for the loss by expansion as the air ascends. The average is therefore about three hundred feet for one degree. =Height of Freezing Cold in the Free Air.= The frost level remains constant, winter and summer, over the equator at about eighteen thousand feet. Elsewhere this level rises and falls with the seasons, the amplitude of the movement increasing with latitude and being greater over land than over water on the same parallel. =Daily Range of Temperature in the Free Air.= The difference between the temperature of day and that of night decreases with altitude in the free air and ceases at about eight thousand feet. It is greatest during clear weather and least in cloudy weather. Narrow valleys may show a greater daily range than hilltops. When the sky is clear, radiation from the hillsides may heat the air in a valley to almost furnace heat at midday, while at night the air, coming in contact with cool vegetation higher up, chills and, gaining in weight by contraction, flows down and collects in the valley, making the bottom of the valley warmer during day and colder during night than the air above. Often moisture-laden winds precipitate much of their water vapor as the air cools by expansion in passing over a mountain range. These winds carry a comparatively dry air over to the leeward side of the mountain, where the daily range of temperature will be much greater than on the windward side at the same elevation. San Francisco, where the prevailing winds come from the ocean, has a less range than New York, where the predominating winds are from the land; but New York is influenced by its proximity to the ocean, for its range is much less than at Denver, in the interior of the continent. The range is less on the east side of Lake Michigan than on the west side, as it is with relation to all similar bodies of water.