Mountain Climates

Mountain Climates

 

climatic conditions in mountainous areas. The primary reason to climatic differences between mountains and neighboring plains is the increase in elevation above sea level. In addition, important characteristics of mountain climates are created by the relief of the locality (including the degree of ruggedness. the relative elevation and direction of mountain ranges. the exposure of slopes, and the width and orientation of valleys). as well as by glaciers and névés.

A distinction is made between the mountain climate proper at elevations of less than 3,000–4.000 m and the high mountain climate at greater elevations. The mountain climate differs fundamentally from climatic conditions in the free atmosphere over a plain at the same elevations. Climatic conditions on vast, high plateaus also differ from conditions in valleys, on mountain slopes, or on particular peaks. Because the atmospheric pressure and the temperature. humidity. and other properties of the air change with elevation, there are climatic belts located one above another in the mountains, resulting in a division of the landscape as a whole into elevation zones.

Atmospheric pressure and air density decrease with elevation. The water vapor and dust content of the air decreases even more rapidly, increasing the transparency of the air for solar radiation in mountainous areas. The intensity of direct solar radiation in the mountains is higher than on the plains, and the intensity of scattered radiation is lower. As a result. the intensity of illumination is higher, especially in snow-fields. and the sky has a deeper blue color. The effective radiation of the earth’s surface is also higher in the mountains.

In the troposphere, the temperature of the air drops as the elevation increases. In the mountains the temperature of the air also depends on the elevation of the locality, and it is lower than in the lowlands. Furthermore. the air temperature depends on the exposure of the slopes: on southern slopes, where the influx of radiation is greater, the temperature is higher than on northern slopes. Therefore. mountain ranges, particularly those located latitudinally. are important climatic boundaries (the Himalayas and the Caucasus). At high elevations in the mountains temperature conditions are also influenced by glaciers and neves. In the interior areas of mountainous massifs. chilled air may be stagnant at night and in the winter. resulting in the frequent formation of temperature inversions (increase in temperature as elevation increases). The daily change of air temperature on certain peaks is reduced. approximating conditions in the free atmosphere. However, in valleys and on plateaus the daily temperature change may be very significant (for example in Tibet and the Pamirs). The annual variation in temperature corresponds to conditions on a plain in a given latitudinal zone. The amplitude of the variation is great in the middle and high latitudes but small in the low latitudes.

Precipitation in the mountains increases with elevation, but only to a certain level. which varies in different cases. The increase in precipitation varies. depending on the exposure of the slopes. The greatest precipitation is observed on slopes facing prevailing winds, especially if the air masses carried by the prevailing winds have a high moisture content (for example. in the Western Tien-Shan and the Pamirs). On the leeward slopes, on the other hand. the foehn and bora are observed. Local air circulations—the so-called mountain-valley winds—arise in the mountains. and there are also glacier winds over glaciers.

In many cases. mountain climates have a beneficial physiological effect (mountain health resorts). Especially important are the moderately rarefied condition and purity of the mountain air, the increased solar radiation (including ultraviolet radiation), and the cool weather. At the same time the foehn. increased precipitation, and other characteristics of mountain climates can also have a negative effect on the human organism. Symptoms of altitude sickness usually begin at elevations of more than 3.000 m, where the intensity of solar radiation is too great, the air temperature and pressure are low, and there is little precipitation. Therefore, life under the conditions of a high mountain climate frequently requires prolonged acclimatization. It is interesting to note. however, that many cities in Bolivia and Peru are located at elevations up to 3,800 m. Small settlements and farming are found in the mountains at elevations up to 4.000–5.000 m.

REFERENCE

Berg. L. S. Osnovy klimatolo’gii, 2nd ed. Moscow. 1938.

S. P. KHROMOV

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