90 Draft: Chap. 4 Mt. Climate by A. Bach for Mountains and People
Chapter 4: Mountain Climate
Climate is the fundamental factor in establishing a natural environment, it sets the stage upon which all physical, chemical, and biological processes operate. This becomes especially evident at the climatic margins of the earth, i.e., desert and tundra. Under temperate conditions, the effects of climate are often muted and intermingled so that the relationships between stimuli and reaction are difficult to isolate, but under extreme conditions the relationship becomes more evident. Extremes constitute the norm in many areas within high mountains; for this reason, a basic knowledge of climatic processes and characteristics is a prerequisite to an understanding of the mountain milieu.
The climate of mountains is kaleidoscopic, composed of myriad individual segments continually changing through space and time. Great environmental contrasts occur within short distances as a result of the diverse topography and highly variable nature of the energy and moisture fluxes within the system. While in the mountains, have you ever sought refuge from the wind in the lee of a rock? If so, you have experienced the kind of difference that can occur within a small area. Near the margin of a species' distribution, such differences may decide between life and death; thus, plants and animals reach their highest elevations by taking advantage of microhabitats. Great variations also occur within short time‑spans. When the sun is shining it may be quite warm, even in winter, but if a passing cloud blocks the sun, the temperature drops rapidly. Therefore, areas exposed to the sun undergo much greater and more frequent temperature contrasts than those in shade. This is true for all environments, of course, but the difference is much greater in mountains because the thin alpine air does not hold heat well and allows a larger magnitude of solar radiation to reach the surface.
In more general terms, the climate of a slope may be very different from that of a ridge or valley. When these basic differences are compounded by the infinite variety of combinations created by the orientation, spacing, and steepness of slopes, along with the presence of snow patches, shade, vegetation, and soil, the complexity of climatic patterns in mountains becomes truly overwhelming. Nevertheless, predictable patterns and characteristics are found within this heterogeneous system; for example, temperatures normally decrease with elevation while cloudiness and precipitation increases, it is usually windier in mountains, the air is thinner and clearer, and the sun’s rays are more intense.
The dynamic effects of mountains also have a major impact on regional and local airflow patterns that impact the climates of adjacent regions. Their influence may be felt for hundreds or thousands of kilometers, making surrounding areas warmer or colder, wetter or drier than they would be if the mountains were not there. The exact effect of the mountains depends upon their location, size, and orientation with respect to the moisture source and the direction of the prevailing winds. The 2,400‑kilometer‑long (1,500 mi.) natural barrier of the Himalayas permits tropical climates to extend farther north in India and southeast Asia than they do anywhere else in the world (Tang and Reiter 1984). One of the heaviest rainfall records in the world was measured at Cherrapunji, near the base of the Himalayas in Assam. This famous weather station has an annual rainfall of 10,871 mm (428 in.). Its record for a single day is 1,041 mm (41 in.) as much as Chicago or London receives in an entire year (Kendrew 1961)! On the north side of the Himalayas, however, there are extensive deserts and the temperatures are abnormally low for the latitude. This contrast in environment between north and south is due almost entirely to the presence of the mountains, whose east‑west orientation and great height prevent the invasion of warm air into central Asia just as surely as they prevent major invasions of cold air into India. It is no wonder that the Hindus pay homage to Siva, the great god of the Himalayas.
EXTERNAL CLIMATIC CONTROLS
Mountain climates occur within the framework of the surrounding regional climate and are controlled by the same factors, including latitude, altitude, continentality, and regional circumstances such as ocean currents, prevailing wind direction, and the location of semi-permanent high and low‑pressure cells. Mountains themselves, by acting as a barrier, affect regional climate and modifying passing storms. Our primary concern is in the significance of all these more or less independent controls to the weather and climate of mountains.
Latitude
The distance north or south of the equator governs the angle at which the sun's rays strike the earth, the length of the day, thus the amount of solar radiation arriving at the surface. In the tropics, the sun is always high overhead at midday and the days and nights are of nearly equal length throughout the year. As a result, there is no winter or summer; one day differs from another only in the amount of cloud cover. There is an old adage, "Night is the winter of the tropics." 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 (Fig. 4.1). Thus, during summer solstice in the northern hemisphere (June 21) the day is 12 hours, 7 minutes long at Mount Kenya on the equator; 13 hours, 53 minutes long at Mount Everest in the Himalayas (28?N lat.); 15 hours, 45 minutes long at the Matterhorn in the Swiss Alps (41?N lat.); and 20 hours, 19 minutes long at Mount McKinley in Alaska (63?N lat.) (List 1958). During the winter, of course, the length of day and night at any given location are reversed. Consequently, the distribution of solar energy is greatly variable in space and time. In the polar regions, the extreme situation, up to six months of continuous sunlight follow six months of continuous night.
Although the highest latitudes receive the lowest amounts of heat energy, middle latitudes frequently experience higher temperatures during the summer than do the tropics. This is due to moderate sun heights and longer days. Furthermore, mountains in middle latitudes may experience even greater solar intensity than lowlands, both because the atmosphere is thinner and because the sun's rays strike slopes oriented toward the sun at a higher angle than level surfaces. A surface inclined 20? toward the sun in middle latitudes receives about twice as much radiation during the winter as a level surface. It can be seen that slope angle and orientation with respect to the sun are vastly important and may partially compensate for latitude.
The basic pattern of global atmospheric pressure systems reflects the role of latitude in determining climatic patterns (Fig. 4.2). These systems are known as the equatorial low (0?‑ 20? lat.), subtropical high (20?‑ 40? lat.), polar front and subpolar lows (40?‑ 70? lat.), and polar high (70?‑ 90? lat.). The equatorial low and subpolar low are zones of relatively heavy precipitation while the subtropical high and polar high are areas of low precipitation. These pressure zones create the global circulation system (Fig. 4.2). General circulation dictates the prevailing wind direction and types of storms that occur latitudinally. The easterly Trade Winds have warm, very moist convective (tropical) storms, which seasonally follow the direct rays of the sun. The subtropical highs have slack winds and clear skies year round. The subpolar lows and polar front are imbedded in the Westerlies, bringing cool, wet cyclonic storms and large seasonal temperature fluctuations. The cold and dry Polar Easterlies develop seasonally, dissipating in the summer season.
The distribution of mountains in the global circulation system has a major influence on their climate. Mountains near the equator, such as Mount Kilimanjaro in East Africa, Mount Kinabalu in Borneo, or Mount Cotopaxi in Ecuador, are under the influence of the equatorial low and receive precipitation almost daily on their east-facing windward slopes. By contrast, mountains located around 30? latitude may experience considerable aridity; as do the northern Himalayas, Tibetan highlands, the Puna de Atacama in the Andes, the Atlas Mountains of North Africa, the mountains of the southwestern United States, and northern Mexico (Troll 1968). Farther poleward, the Alps, the Rockies, Cascades, the southern Andes, and the Southern Alps of New Zealand again receive heavy precipitation on westward slopes facing prevailing Westerlies. Leeward facing slopes and lands down wind are notably arid. Polar mountains are cold and dry year round.
Altitude
Fundamental to mountain climatology are the changes that occur in the atmosphere with increasing altitude, especially the decrease in temperature, air density, water vapor, carbon dioxide, and impurities. 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. The earth itself becomes the radiating body, emitting long-wave energy that is readily absorbed by CO2, H2O and other greenhouse gases in the atmosphere. The atmosphere, therefore, is heated directly by the earth, not by the sun. This is why the highest temperatures usually occur near the earth’s surface and decrease outward. Mountains are part of the earth, too, but they present a smaller land area at higher altitudes within the atmosphere, so they are less able to modify the temperature of the surrounding air. A mountain peak is analogous to an oceanic island. 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. By contrast, the larger the island or mountain area, the more it modifies its own climate. This mountain mass effect is a major factor in the local climate (see pp. 77 ‑ 81).
The density and composition of the air control its ability to absorb and hold heat. The weight or density of the air at sea level (standard atmospheric pressure) is generally expressed as 1013 mb (millibars, or 760 mm [29.92 in.] of mercury). Near the earth, pressure decreases at a rate of approximately 1 mb per 10 m (30 mm/300 m (1 in./1,000 ft.) of increased altitude. Above 5,000 m (20,000 ft.) atmospheric pressure begins to fall off exponentially. Thus, half the weight of the atmosphere occurs below 5,500 m (18,000 ft.) and pressure is halved again in the next 6,000 m (Fig. 4.3).
The ability of air to hold heat is a function of its molecular structure. At higher altitudes, molecules are spaced farther apart, so there are fewer molecules in a given parcel of air to receive and hold heat. Similarly, the composition of the air changes rapidly with altitude, losing water vapor, carbon dioxide, and suspended particulate matter (Tables 4.1 and 4.2). These constituents, important in determining the ability of the air to absorb heat, are all concentrated in the lower reaches of the atmosphere. Water vapor is the chief heat‑absorbing constituent, and half of the water vapor in the air occurs below an elevation of 1,800 m (6,000 ft.). It diminishes rapidly above this point and is barely detectable at elevations above 12,000 m (40,000 ft.).
The importance of water vapor as a reservoir of heat can be seen by comparing the daily temperature ranges of a desert to that of a humid area. Both areas may heat up equally during the day but, due to the relative absence of water vapor to absorb and hold the heat energy, the desert area cools down much more at night than the humid area. The mountain environment responds in a similar fashion to that of a desert, but is even more accentuated. The thin pure air of high altitudes does not effectively intercept radiation, allowing it to be lost to space. Mountain temperatures respond almost entirely to radiation fluxes, not on the temperature of the surrounding air (although some mountains receive considerable heat from precipitation processes). The sun's rays pass through the high thin air with negligible heating. Consequently, although the temperature at 1,800 m (6,000 ft.) in the free atmosphere changes very little between day and night, next to a mountain peak, the sun's rays are intercepted and absorbed. The soil surface may be quite warm but the envelope of heated air is usually only a few meters thick and displays a steep temperature gradient.
In theory, every point along a given latitude receives the same amount of sunshine; in reality, of course, clouds interfere. The amount of cloudiness is controlled by distance from the ocean, direction of prevailing winds, dominance of pressure systems, and altitude. Precipitation normally increases with elevation, but only up to a certain point. Precipitation is generally heaviest on middle slopes where clouds first form and cloud moisture is greatest, decreasing at higher elevations. Thus, the lower slopes can be wrapped in clouds while the higher slopes are sunny. In the Alps, for example, the outer ranges receive more precipitation and less sunshine than the higher interior ranges. The herders in the Tien Shan and Pamir Mountains of Central Asia traditionally take their flocks higher in the winter than in summer to take advantage of the lower snowfall and sunnier conditions at the higher elevations. High mountains have another advantage with respect to possible sunshine: in effect, they lower the horizon. The sun shines earlier in the morning and later in the evening on mountain peaks than in lowlands. The same peaks, however, can raise the horizon for adjacent land, delaying sunrise or creating early sunsets.