Climate: Mountain

Latitude

The distance poleward from the equator governs the angle at which the sun's rays strike the Earth, the length of the day, and thus the amount of solar radiation arriving at the surface, which dictates temperatures. In the tropics, the sun is always high overhead at midday, and the days and nights are of nearly equal length throughout the year. With increasing latitude, however, the height of the sun changes during the course of the year, and days and nights become longer or shorter depending on the season.

The distribution of mountains in the global circulation system, which has a strong latitude distribution, dictates their basic climate. Mountains near the equator are under the influence of the equatorial lows and receive daily precipitation on their east-facing windward slopes. In contrast, mountains located around 30° latitude experience considerable aridity. Further poleward, mountains receive heavy winter precipitation on westward slopes facing the prevailing westerlies. Polar mountains are cold and dry year-round.

Continentality

The relationship between land and water has a strong influence on the climate of a region. Water heats and cools more slowly than land, so the temperature ranges between day and night and between winter and summer are smaller in marine areas than in continental areas. Generally, the more water dominated an area is, the more moderate its climate. An extreme example is a small oceanic island, on which the climate is essentially that of the surrounding sea: a marine climate. The other extreme is a central location on a large land mass such as Eurasia, far removed from the sea: a continental climate with hot summers and cold winters.

Altitude

Essential to mountain climatology are the changes that occur in the atmosphere with increasing altitude, principally the decrease in temperature, air density, water vapor, carbon dioxide, and aerosols and the increase in cloudiness, precipitation, and solar energy received during clear-sky conditions. The sun is the ultimate source of energy, but little heating of the atmosphere takes place directly. Rather, solar radiation passes through the atmosphere and is absorbed by the Earth's surface, which then reradiates infrared radiation, which heats the atmosphere from the bottom up (i.e., the greenhouse effect). Mountains are part of the Earth, too, but they present a smaller land area at higher altitudes in the atmosphere, so they contribute less to heating the atmosphere above them. A mountain peak is analogous to an oceanic island in the continentality effect. The smaller the island and the farther it is from large land masses, the more its climate will be like that of the surrounding sea. In contrast, the larger the island or mountain area, the more it modifies its own climate. This mountain mass effect can be a major factor in the regional climate.

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