As one travels from the equator to the poles, climatic changes are perceptible. In the same way, as one move up into a highland area, changes in climate at different altitudes are similar to those encountered in latitudinal movement from the equator to the pole.
The temperature under normal conditions decreases with altitude, the normal lapse rate being 6.5°C/km, so that the summit area of a mountain is always cooler than its base.
As we have A, C, D, E and F climatic zones on the earth's surface, so there are similar climatic zones such as A, C, D, E and F according to elevations in the mountainous areas. A high mountain located in the tropical area will rise through all the temperature zones as discussed earlier.
If there is a very high mountain, its peak will be in a zone of perpetual snow or ice. Thus, a simple mountain will have many climates depending on its elevation.
Following are the major climatic controls which determine the climates of a highland: elevation, shape of the highland, exposure to winds, and location.
According to Trewartha, 'next to the distribution of land and water, elevation above sea level is probably the most important cause of differences in climate in similar latitudes'.
As the altitude increases, various climatic elements such as pressure, temperature and precipitation undergo well-defined changes in their values.
Climatic changes with altitude are rapid. It is held that climbing 300 meters up a mountain is equivalent to travelling 500 to 1000 km towards the poles.
Shape of the mountains like that of the continents exerts influence on the highland climates. Minor relief features exert profound influence on the minor details of climate, for example, the direction of winds is determined by the alignment of the relief features.
Exposure is another important factor in mountain climates. Windward slopes present atmospheric conditions different from those of the leeward slopes. Windward slopes force the incoming air to rise up with the resultant condensation, cloud formation and precipitation.
The leeward slopes are characterized by descending air which is warmed up adiabatically and produces little precipitation. The exposure affects the daily march of temperature. East-facing slopes have more sunshine in the mornings which are relatively warmer and cool afternoons.
On the contrary, west-facing slopes have the opposite case of cool mornings and warm afternoons. Similarly, exposure with respect to the mid-day sun profoundly influences surface temperature.
Mountain slopes with ample sunshine are preferred for human habitation, especially in higher latitudes. Besides, crops are preferably grown on slopes that face the equator.
This factor is known as 'slope aspect'. At higher elevations in the highland areas direct sunlight in the more important source of warmth and energy for plant as well as animal life.
Location of a mountain affects its climatic zones considerably. Various characteristics of the wind such as, its atmosphere and moisture content are largely determined by the location of a mountain.
A mountain may have a coastal location, or it may be located in deep continental interior. It may be located at high or low latitude. A mountain may be in the path of cyclonic storms or monsoon winds or beyond them.
Highland climates are characterized by complexity. Within a mountain range, there is a mosaic of climates. In other words, various types of local climates exist in every significant mountain range.
According to altitudes and exposure, one finds varying atmospheric conditions. According to Trewartha, there is no such thing as a highland type of climate. At different latitudes and altitudes there are different types of climates.
There are no typical temperature and rainfall regimes in the highland climates. Only the most flexible generalizations can be made about highland climates.
The highland climatic areas on the world map may be considered as the cool, moist islands in the midst of zonal climates that occur in the area around them. Highland areas are also said to be biotic islands with a natural vegetation and animal life adapted to cooler and moister conditions than those of the surrounding low lands.
Mountains or plateaus receive fairly large amount of precipitation by forcing incoming air to rise upward beyond the condensation level. Mountain slopes, if they are rocky and barren, get heated during to day, and cause convection so that afternoon thunderstorms are produced.
Because of abundant precipitation, mountains are the source areas for a large number of streams which feed the rivers of all the continents. Summer melting of the snow fields on the mountains gives birth to perennial rivers.
The distribution of these climates is associated with the distribution of mountains and high plateaus. The most important area of the highland climate is found on the Tibet Plateau and its adjoining lofty mountain ranges.
The highest mountain peak of the word, Mt. Everest, is located in the Himalayan Range. It is on the Tibet Plateau that all the mighty rivers of Asia rise. Since this high plateau lies in the rain shadow of the Himalayas, the climate here is dry, receiving less than 25 cm of annual precipitation.
On the contrary, the southern slopes of the Himalayas receive more than 500 cm of precipitation annually. The climate of this highland is very severe. Temperatures are extremely low, and there are high-velocity winds blowing much of the time. During summer, daily range of temperature is great because of very chilly nights and extremely hot days.
In Europe, the most important highland climate is found in the Alps running from southern France through Switzerland and north Italy to western Austria. Some of the great rivers like the Rhine, Rhone, Danube and Po have their source regions in the Alps.
Carpathian Ranges also have highland climate. There are also some small areas with highland climates in the mountains of Scandinavia, the British Isles and Central Europe.
In Africa, the volcanic highlands of Ethiopia and Ruwenzori Range have highland climate. Mt. Kenya and Mt. Kilimanjaro, even though situated almost on the equator, have perpetual snow and ice on their peaks. The Atlas Mountains in northwest Africa are another small area of the highland climate.
The Andes in South America is high enough to have its crest in the zone of permanent snow. It has tropical to polar type of climate at different elevations. Vertical climatic zonation can be seen in the volcanic mountains of Central America and Mexico.
The highland climates are characterized by their distinct zonation by latitude. The climatic zonation can be seen in an ideal form in the Himalayas or in the tropical mountain of South America. A typical example of climatic zonation by altitude can be given in case of the Andes Mountains in South America.
The tropical rainforest climate is found on the foothills of this mountain in the east, whereas in the west are the dry climates produced by the cool ocean currents. Above 1200 meters are the sub-tropical climatic zones.
At 2400 m elevation, one can see the mesothermal climate with the type of natural vegetation that occurs in the Mediterranean type of climate. But at 3600 m appear the micro-thermal climates that continue up to 4800 m. beyond this altitude there is perpetual snow and ice like those of the polar areas.
It is noteworthy that temperature continues to decrease in these altitudinal zones, but winds tend to increase with elevation. Precipitation, fog and cloud cover also show an increasing trend.
In the highland climates atmospheric pressure becomes an element and is very important. With increasing elevation, the weight of the atmosphere decreases rapidly. At an elevation of 5 km the atmospheric pressure is reduced nearly one- half of its sea level value.
People prefer to settle on mountain slopes below this level. At altitudes above about 3000 meters people feel physiological disturbances resulting in nosebleed, nausea, headache, and sleeplessness.
On exposed slopes and peaks, friction is reduced to a minimum so that winds are much stronger than at low levels. Due to the relief of the mountains, there are certain local winds. Like land and sees breezes, there is a diurnal reversal of wind direction.
During day time, there are upslope winds, whereas at night, winds generally blow down the slopes; such winds are called valley breeze and mountain breeze according to their places of origin.
Up-slope winds are also called anabatic, whereas down-slope winds are called katabatic. Upslope winds often produce cumulus clouds over the mountains which may develop into cumulonimbus clouds giving heavy showers in the afternoon.
In coastal areas the down-slope winds blow with great violence. The mistral in southern France, the bora in the coastal areas of Adriatic and the santa ana of southern California are such local winds which arrive as cold currents.
If valleys are fairly wide, the mountain breeze may displace more moist air lying over the valley bottoms so that night-time clouds are produced. If there is inversion of temperature in the valley, clouds thus produced may be of cumulus or stratocumulus type.
If the air filling the valley happens to be unstable, large cumulus or cumulonimbus clouds may build up to produce heavy night-time precipitation. Magdalena valley and the Llanos of Columbia receive much of their precipitation in this way.
When the mountains obstruct the horizontal flow of air, it is forced to ascend and then descend. During its ascent or descent the air under-goes changes of pressure and temperature resulting in changes of relative humidity.
Thus, the ascending winds cause precipitation, whereas descending winds tend to cause evaporation. In the Alps, the descending winds are known as the foehn which is extremely hot and dry. Its occurrence is associated with the passage of a cyclone.
Similar warm and dry winds occur along the eastern slopes of the Rockey Mountains where they are known as chinook. Similar local winds occur in all the mountains which are visited by cyclonic storms.
The Samun of Persia, the norwesters of New Zealand and many other local winds, dry and warm, occur in all the mountainous areas where the snow cover vanishes from the ground in no time.
In mountainous regions, relief is the dominant controlling factor for the distribution of precipitation. Since relief in high-land areas is highly complex, so the distribution of precipitation is also complex.
In low latitudes, there is an increase of precipitation in mountains up to and altitude of about 1500 meters beyond which it shows a declining trend. But in the middle latitude mountains, the total annual precipitation tends to increase up to the summit areas.
The greater amount of precipitation at higher levels is of greater significance in the arid lands, since here alone is water available. In arid regions, because of the greater amount of precipitation, highlands are the only areas which have luxuriant growth of vegetation.
Besides, rivers originating from the humid slopes of the mountains carry water to the surrounding arid lowlands where it is used for irrigation. The Nile River and the Colorado River have their source regions in the rainier high mountains.
Similarly, many streams originating from the Andes supply water to the Peruvian coastal desert. Besides, heavy precipitation occurring in highlands is used for generating hydroelectric power at the lower levels.
Generally summers are rainier than winters in a mountain region. Similarly, more rains fall during the day-time than at night.
In a mountain region the windward slopes are rainier than the leeward side which is often drier. The rain shadow always lies on the leeward side. However, leeward slopes also receive greater precipitation at higher levels.
This is because of the fact that rising air on the windward slopes does not start descending after crossing the crest of a mountain range, but it continues to rise even after crossing the crest, even though only for a short distance.
That is why the higher slopes of the leeward side also receive heavy precipitation. According to E. De Mortonne, "the variability of mountain climates is best illustrated by the conditions of atmospheric humidity.
Sometimes mist covers one slope of a hill, whilst the sun shines on the other; in one place sleet and hail may fall with unusual violence, whilst a hundred yards away the sky may be clear."
The snow line:
The lower limit of permanent snow on a mountain is called the snow line. The elevation of the snow line is not constant, but it undergoes change, migrating up and down the mountain slopes in summer and winter respectively.
However, as the temperature decreases away from the equator, so the snow line becomes increasingly lower with increasing latitude. In the vicinity of the Arctic Circle the snow line reaches the sea level.
The snow line, in fact, represents the line of balance between the amount of snow and the rate of its melting and evaporation. Various factors account for variations in the altitude of snow line, such as latitude, seasonal distribution of precipitation, exposure to the sun, steepness of the slope, and other local influences.
Vertical zonation is the keynote of the distribution of natural vegetation in a highland climate. The lowermost zone comprises the natural vegetation of the surrounding region, but upward the changes in vegetation are more rapid than those found on the surface of the earth.
According to Austin Miller "the variations of temperature and rainfall which mountains bring about are sudden and abrupt, so that the climatic optimum may be displaced within a very short distance from one vegetation type to another."
Therefore within a small space various types of plant associations are to be seen in a mountainous region. The salient feature of the natural vegetation of a mountain is the fact that the zone of maximum rainfall is invariably forest, deciduous below, passing upwards into coniferous.
Between the coniferous forest and the snow line there is an alpine zone or grassland, because this is the region of decreasing rainfall. The climatic conditions, i.e., the low temperature, the high rate of evaporation, low pressure and high- velocity winds favour the growth of xerophytic vegetation. Moreover, at higher elevations the season available for plant growth becomes shorter and shorter.
In India the mountain forests vary according to altitude and the amount of precipitation. They comprise the sub-tropical through temperate to alpine forests.
The Himalayas exhibit distinct vertical zonation of natural vegetation. Different vegetation zones as are found on these mountain ranges are the following -
(a) Wet hill forest:
A high and dense forest occurs between 915 m and 1830 m mostly on the eastern Himalayan ranges. Dominant species are ever-green oaks and chestnuts. These forests have some other species like ash and beech. At lower elevations, sal may also be found in certain suitable locations. Climbers and epiphytes are not uncommon in these forests.
(b) Sub-tropical pine forest:
This forest is found on the mountain slopes generally at elevations ranging from 915 m to 1830 m between 73° and 88° East longitudes. Chir is the most important tree which is useful in many ways. Khasya pine is found in the Khasi Hills and the neighbouring areas at 1220-1525 m.
(c) Sub-tropical dry evergreen forest:
This forest is confined to the western Himalayas which are relatively drier. It occupies the Himalayan foothills at elevations varying from 450 m to 1215 m. Wild olives are the dominant species.
(d) Sub-tropical hill savannah:
This type of vegetation consists of grasslands which are found above 1370 meters. It occupies a long belt from the western Himalayas to the Burmese border in the east. Savannah comprises various species of grasses like arundinella and themed.
(e) Moist temperate forest:
This forest occurs at altitudes varying from 1525 m to 3355 m in mil different sections of the Himalayan Mountains. Oaks, laurels, pines, cedars, chestnuts, rhododendrons are the dominant species.
East of 88° E, the wet temperate forest is found between 1525 m and 3355 m. In the wetter eastern Himalayas coniferous trees are mixed with broad- leaved evergreen forest. There are open forests of pines, silver firs, and spruce. Bamboos are also found as undergrowth.
(f) Dry temperate forest:
This forest covers the inner Himalayan ranges where average annual precipitation is less than 100 cm. Much of the precipitation is in the form of snow. It found in Kashmir and the northern Sikkim. Dominant trees comprise deodars, junipers, oak and ash. Ground in this forest is covered with various kinds of grasses.
(g) Montane temperate grassland: This type of vegetation is found at about 2000 m.
(h) Alpine forest:
This forest consists of birches, pines, silver firs, junipers and rhododendrons. It occupies higher altitudes ranging from 2800 m to 3660 m. It is largely a dense shrubby forest.
(i) Alpine grassland:
At elevations ranging from 2135 to 2440 m the temperate grasslands gradually merge into pastures which are used for transhumant herds and flocks.
In the Himalayas the snowline is to be found at about 4880 m. It may be pointed out that the vegetation zone of alpine tundra lies above the timber line and resembles the tundra of the arctic region.
To quote Spate and Lear-month, "the vegetation mosaic in the topographical conditions of the Himalayas is naturally often very complex, it is impossible in a general review to do justice to its permutations."