Climates of Asia: Seasonal Patterns, Rainfall Patterns and Other Factors

The weather (day-to-day changes in atmos­pheric conditions) and climate (long-run atmospheric conditions) have perhaps had a more lasting impact on the course of hu­man history than other environmental influences. Though not entirely determi­nistic, climate can generally provide us a good deal of understanding of the pat­terns—economic and cultural—over the earth. For example, climate has a direct and obvious relationship with agricul­ture, transport, and human life in general.

Drought in Somalia, unpre­dictable storm in India, frost in Florida—all have direct consequences on our lives. In a more direct relationship, climatic conditions profoundly affect the development of the physical landscape—soils, vegetation, animal life, drainage pat­terns, and topography. Before attempting to describe the Asian climates, it may be useful to briefly review the essential factors that shape them. The most basic of these is the lati­tude of a given place, measured in degrees in distance from north and south of equa­tor.

The earth being almost spherical in shape, the noon sun is always at a high an­gle all the year near the equator than at the poles where the sun never rises far above the horizon. Broadly speaking, these latitu­dinal differences result in the formation of more or less consistent atmospheric pressure belts based on a nearly constant heating and cooling but migrating seasonally in re­sponse to seasonal changes in the receipt of solar energy on the earth’s surface. Thus, the distribution of heat and cold is the first and foremost function of latitude.

The for­mation of zones, belts of atmospheric pressure over the earth is attributed to the unequal heating and cooling in differ­ent latitudes. Thus, a low pressure zone extends roughly 5° north and south of the equator, and a polar high (pressure zone) from 80° north and south. Such pressure belts give rise to wind systems (winds blow from high to low pressure) which are either dry or moist depending on several other factors related to the latitudinal, altitudinal and other differences, and in turn are responsible for conditions producing rainfall or dryness.

If the earth were a stationary body in space, atmospheric circulation (or wind systems) would be simple: heated air would rise from the equator, flow evenly toward the poles cool and sink and return to the earth in a vast, convectional pattern, but such is not the case. Other factors are also at work.

The earth’s rotation round its inclined axis of 23½° and its yearly revolution in space around the sun, in a more or less fixed orbit, is the next most decisive factor in patterning earth’s climates. These move­ments complicate the location, and size of the pressure belts and the resulting wind systems and other climatic phenomena, like the amount of precipitation, as also their seasonal migrations.

The rotational motion of the earth causes air to rise at the equator, pile up above 30 to 40 degrees lati­tude in each hemisphere, and descend due to its own weight at these latitudes, and be­gin to blow near the surface to areas of lower pressure (e.g., the equatorial low pressure zone and the areas between 60 and 70 degrees north and south which have become low pressure areas as com­pared to the high pressure areas between 30 and 40 degrees latitude). Winds blow from high to low pressure zones.

Another important factor is the direc­tion and carrying capacity of winds. Some wind systems bring moisture and some are dry. That depends on several conditions: whether these are ascending or descending air masses (winds), or whether these are moisture-laden winds blowing from over the water bodies (onshore) or are winds blowing from land (offshore).

There are several other factors which influence the climates, like the distribution of land and water over the earth, the role of mountains (as barriers or otherwise), the local wind direction (onshore and off­shore winds mentioned above), the impact of ocean currents on the adjoining land and the shape and nature of coastlines that are all important in understanding climatic patterns. Their impacts on Asia will be briefly reviewed in the discussion that fol­lows.

Asia’s enormous size spanning the en­tire length of the northern hemisphere, its compact shape enclosing a highland core of complex series of mountains acting as ef­fective barriers to air masses and several deserts tucked up in its deep interior, all in­teract to give most of Asia a markedly “continental” climate.

Within it exist large differences in conditions of solar radiation and hence temperature distribution, at­mospheric circulation, and precipitation. The outstanding characteristics of Asian climates can be described in such expres­sions as continentality, variety, and complexity. A continental climate is char­acterized by extremes of annual range of temperatures and is generally associated with large landmasses.

Seasonal Patterns:

Temperature, Pressure and Wind Systems:

The most dominant feature that controls the Asian climates is its enormous size. The conti­nent extends over some 6,000 miles (9,654 km) from east to west along the 40°N and well over 5,300 miles (8,527 km) from north to south along longitude of 105°E (both 40°N, and 105°E being central in the landmass). Another climatic control is the distribution of land and water.

Since land and water behave very differently in their attitude toward retention of solar heat dur­ing the year, the location of places with reference to the oceans is very important. Vast interiors in the continent absorb more solar energy during summer than do water bodies, and conversely lose more en­ergy during winter.

This tendency of extreme temperature ranges (known as the “continental effect”) is at a maximum on the vast Asian land surface where large part of the interiors is over a thousand miles from the oceans. As a consequence of the vastness, and remoteness of the inte­riors from oceans, winters tend to be colder and summers hotter in most of Asia (excepting the coastal fringes and islands) as compared to places in other and smaller continents in those latitudes. The tempera­ture ranges (difference between the average maxima in summer and minima in winter) tends to be very large.

An examination of summer and winter isotherms (lines connecting points of equal temperature). Figures 2.2 and 2.3 illustrate Asia’s marked continentality. The average January temperature over most of north­eastern Siberia is below -5°F (-20°C). Over the coastal areas of the Pacific, the ocean’s proximity moderates the temperatures considerably. The January isotherm of 32°F (0°C) passes through Samarkand (in the deep interior), Beijing (China), and the island of Honshu in Japan.

An isotherm of 70°F (20°C) can be traced along the Tropic of Cancer and of 77°F (25°C) along the equator. In summer, the conditions are re­versed. In July, most of the continent experiences hot temperatures, although the highest temperatures are found in western Asia, in Taklamakan desert (Western China) and in Thar Desert in India.

The 70°F (20°C) isotherm has moved as far north as 55° to 60°N, bending a bit along the comparatively cooler Pacific Ocean (the “maritime effect” as opposed to the “continental effect”). In response to the two basic controls, the latitude and the dis­tance from the oceans, the largest ranges of temperature are experienced in the interior and in inland Siberia, particularly in Sibe­ria’s northeastern section.

The annual temperature range is greatest around Verk­hoyansk area in northeast Siberia (aptly named the “cold pole” due to the ex­tremely low winter temperatures) where the annual temperature range (the differ­ence between January and July averages) may exceed 180°F (83°C)—the range equal to that from ice to boiling water.

The seasonal contrasts between the strong heating of the Asian landmass dur­ing the summer (May to September) and chilling in winter (January to March) are of great significance in explaining the de­velopment of Asia’s pressure systems. The contrasts produce sharp seasonal variations in the atmospheric pressure and wind cir­culation.

During winter, the interior regions are very cold and are responsible for the for­mation of a high-pressure belt within East Siberia which creates conditions suitable for the outflow of strong cold winds as po­lar continental air masses from this belt. The center of this belt is normally located to the south of Lake Baikal.

Originating from this center, these land-oriented winds are closely packed, very cold, dry, and out­ flowing. The winds move eastward and southward in the continent, affecting east­ern Asia and parts of southern Asia as these moves over the fringes of the Pacific bringing precipitation to the lands after these have picked up moisture from the oceans over which these traverse during their long journey. These winds (in south­ern and eastern Asia) are known as northeast winter monsoons and in these latitudes may be called temperate monsoons.

In summer, the rapid and continuous heating of Asia’s interior creates low-pres­sure belt centered around the basin of the Indus river and Iran which attracts hot, moist winds from the Indian Ocean. These winds are the southern or southeastern monsoons that advance along the southern edge of the Indus low bringing copious rainfall to the western Ghats and coast of peninsular India, the southern Himalaya, and as far east as mainland Southeast Asia. These monsoons may be termed as tropical and subtropical monsoons.

This phenomenon of the seasonal re­versal of wind systems or the monsoons (derived from the Arabic word “mausim” meaning the “season”) is most noticeable in southern and eastern Asia, but is pre­sent, to a small extent, in all other continents. In other continents, the land- mass is smaller in which cooling and heating takes place, and the monsoonal im­pact is minimized.

The monsoon phenomenon, as noted above, is associated with a very distinctive seasonal precipitation regime, and is char­acterized by heavy summer rains derived from the moist onshore winds, and a pro­nounced dry season when the continental air mass is moving seaward.

The simplistic explanation of the monsoons on the basis of unequal heating of the continent and oceans as outlined above is, however, insuf­ficient to explain the origin, development, and behavior of the monsoons, particu­larly in the tropical and subtropical regions where winter is not severe enough to generate thermal conditions for the monsoonal development.

The develop­ment of “low” and “high” pressure cells south of the Himalaya during the summer and winter seasons respectively and the de­velopment of seasonal wind system can be inadequately explained by the traditional theory. The traditional model, which em­phasizes purely thermal factors for the timing and incidence of summer rainfall, does not explain the monsoonal “bursts,” “pulls” and “breaks” of the North Indian summer monsoons.

Tropical monsoon winds essentially represent large latitudinal migrations of normal trade winds and the westerly flows. Although an explanation for such extensive migrations is not clear, there is increasing evidence that it is associ­ated with the upper-air phenomena, particularly the jet streams.

Rainfall Patterns:

Rainfall conditions in an extensive continent as Asia are wide ranging. Precipitation would vary markedly from the very rainy areas of the equatorial belt to practically no rainfall in the deserts of Central Asia and Saudi Ara­bia. The amount and seasonality would depend upon a number of factors. In the equatorial belt (Malaysia, most of Indone­sia and Sri Lanka) annual rainfall is approximately 80 inches (2,000 millime­ters); between 80 to 120 inches and more on the windward maritime slopes; and less than 40 inches on the lee slopes (“rain- shadow” side) of the subequatorial areas.

In the subtropical and temperate re­gions which come under the monsoonal tracks, it varies between 25 and 40 inches annually according to the distance from the ocean. It is less than 10 inches in east­ern Siberia, and averages less than six inches in the Central Asian and Arabian deserts. Mountains can also act as barriers, and control the incidence of rainfall consider­ably. Much of the interior Asia remains in the “rain-shadow” and dry because the rain-bearing winds of maritime origin (originating mostly during the summer) cannot penetrate the mountain barriers.

The “onshore” side of the mountain slopes, on the other hand, receives most of the moisture. For example, exceptionally heavy precipitation in summer charac­terizes the west coast of India which lies on the onshore side of the western Ghats (lit­erally, “the mountains”) but the plateau east of the Ghats receives scanty rainfall as it is located in a “rain-shadow” zone.

Simi­larly, the North Indian plain lying on the “leeward” side of the Himalaya receives rainfall by the summer monsoon, the amount of rainfall depends on the distance the winds have traveled inland in their journey from the Bay of Bengal and the Arabian Sea; whereas Tibet which lies in the rain-shadow of the Himalaya remains dry.

Other Factors:

Climatic characteristics are also determined considerably by topog­raphy. Different altitudinal climatic zones are clearly represented on the southern slopes of the Himalaya, where they vary from the subequatorial and tropical cli­mates of the foothills at the lowest levels to the snowy climate of the peaks at the high­est altitudes.

The degree of exposure to the sun also plays an important role. The ori­entation of the opposite slopes of the ridges shows contrasts in temperature con­ditions, and exposure to the winds. The southern slopes are sunny, windward, and receive moisture, whereas the leeward slopes in the rain-shadow are necessarily drier.

In addition, the leeward slopes expe­rience what is known as the foehn effect which occurs when strong wind traverses a mountain range and is deflected downhill on the lee-side, it becomes warm, dry and dusty. Such climatic contrasts and changes are observed in several mountains, like the Himalaya, the Elburz, the Tienshan, and mountains in Japan and Taiwan. The bar­rier and thus isolating effect of the mountains on climates has already been noted. The plateaus of Tibet, Anatolia (Turkey), Iranian plateaus, and Central Asia remain dry, enclosed, as these are, by the flanking mountains.

Cyclones and Typhoons:

Migrating cyclones or typhoons affect the climates of East Asia, and of Southwest Asia. Tropical cyclones (or typhoons) originate in East China Sea and South China Sea, and move swiftly westward toward the mainland, as well as curl northeast, picking up more speed, bring very heavy rainfall, particu­larly where these meet the mountains, or in Taiwan and the Philippines. These ty­phoons are like the hurricanes of the Gulf of Mexico, and bring very heavy rainfall and devastation every year to the Philip­pines, Taiwan, North Vietnam and Bangladesh.

Urban Climates:

Distinctive changes in climates also result from the economic and cultural activities of human society. The Asian cities are becoming increasingly crowded and polluted, perhaps more so than elsewhere, because these were largely unplanned. The older sections of the cities are particularly prone to environmental degradation.

Recent investigations of the air quality of several Asian cities suggest se­venty of air pollution, due to emission of sulphur and nitrogen oxides by vehicles and acidification as many of the Asian cit­ies have acquired a large manufacturing base. Such conditions are characteristic of northern Kyushu in Japan, and West Ben- gal-southern Bihar industrial region in India. Air pollution in these areas has af­fected locally the temperatures and wind patterns.

In the preceding paragraphs, we have discussed, at some length, factors that have shaped Asia’s climates. These may now be summarized before we attempt to look at the continent’s broad climatic regions.

In sum, Asia’s climates may be explained to a large extent by:

(1) Its enormous size ena­bling the establishment of a wide range of climatic types;

(2) The location of gigantic mountain system that encloses plateaus, deserts and depressions in the heart of the continent which act as climatic barriers be­tween the various sections of the continent, and which contain distinctive climatic patterns within them;

(3) The de­velopment of monsoonal winds affecting large sections of southern and eastern Asia;

(4) Its compact shape with a comparatively shorter coast enabling large territory to ex­perience “continental” climate; and

(5) The oceanic influences of the Pacific and the Atlantic Oceans affect Asia’s climates mar­ginally, the coastal fringes, leaving a large territory in the interiors in Central Asia and Southwest Asia as deserts.