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Increasing elevation brings a great reduction in both pressure and temperature. Thus, climates change greatly within a vertical range of a few thousand feet. In a general way, a rise of altitude is equivalent to an increase in latitude, so that the tundra and icecap climate equivalents can be found among the glaciers of a mountain mass above timber line. In one major respect, however, the analogy is inadequate. Whereas intensity of insolation is progressively less toward the poles, it is increased at higher altitudes. Thus, daily temperature ranges are excessive at high altitudes in middle and low latitudes, but are much less pronounced in equivalent arctic climates. Pressure and temperature - The physiological effects of a pressure decrease are well known from the experiences of flying and mountain climbing. The principal influence is through an insufficient amount of oxygen to supply the blood through the lungs. At altitudes of 10,000 to 15,000 ft (3000 to 4500 m) mountain sickness (altitude sickness) occurs, characterized by weakness, headache, nosebleed, or nausea. Persons who remain at these altitudes for a day or two normally adjust to the conditions, but physical exertion is always accompanies by shortness of breath.

At reduced pressures the boiling point of water or other liquids is reduced so that cooking time of various foods is greatly lengthened.

The table gives some data on pressure and boiling point relationships. From these figures it is evident that the use of pressure cookers will be of great value about 5000 ft (1500 m) wherever the cooking involves boiling of water.

From the standpoint of weather and climate, reduced atmospheric pressure is principally effective in that the thinner atmosphere, with relatively less carbon dioxide, water vapour, and dust, absorbs and deflects less solar energy and thus permits a high intensity of insolation at the ground.

Increasing intensity of insolation at higher elevation has a profound influence upon temperature relations. Surfaces exposed to sunlight heat rapidly and intensely, shaded surfaces are quickly and severely cooled. This results in rapid air heating during the day and rapid cooling at night in high-mountain locations. Thus at mountain location resorts rot only is the air pure and the landscape features sharply outlined as if washed clean, but the cool nights and warm days are stimulating physically.

The contrast between exposed and shaded surfaces is particularly noteworthy at high altitudes. It is said that temperatures of objects in the sun and in the shade differ by as much as 40° to 50°F (22 to 28°C).

Increased intensity of insolation is accompanied by an in-crease in intensity of violet and ultraviolet rays. Sunburn is very much more rapid above 5000 ft (1500 m) than at sea level, as many a person has learned by unfortunate experience. The red and infrared rays of the spectrum, on the other hand, are relatively less intensified by increased elevation because they are better able to pass through a dense atmosphere.

The general decrease in air temperature with elevation follows the environmental lapse rate of 3 1/2°F per 1000 ft (2°C per 300 m). Thus, we might expect a station 10,000 ft (3000 m) in elevation to have a temperature about 35°F (20°C) below that of a nearby sea-level station.

At high altitudes in the equatorial regions, annual range of temperature is very small, much as in the wet equatorial rainforest climates, which surround these mountains at low elevations.

In middle and high latitudes, on the other hand, a wide annual temperature variation is to be expected, following the marked variations of insolation from summer to winter.

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