World Climate:- Climate and seasons, distribution of rain fall and climatic regions
Climates and seasons
Different parts of the world have different climates. Some parts of the world are hot and rainy nearly every day. They have a tropical wet climate. Others are cold and snow-covered most of the year. They have a polar climate. Between the icy poles and the steamy tropics are many other climates that contribute to Earth’s biodiversity and geologic heritage.
Climate is determined by a region’s climate system. A climate system has five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere.
In 1948, American climatologist Charles Thornthwaite developed a climate classification system that scientists still use today. Thornthwaite’s system relies on a region’s water budget and potential evapotranspiration. Potential evapotranspiration describes the amount of water evaporated from a vegetated piece of land. Indices such as humidity and precipitation help determine a region’s moisture index. The lower its moisture index value, the more arid a region’s climate.
The major classifications in Thornthwaite’s climate classification are microthermal, mesothermal, and megathermal. Microthermal climates are characterized by cold winters and low potential evapotranspiration. Most geographers apply the term exclusively to the northern latitudes of North America, Europe, and Asia. A microthermal climate may include the temperate climate of Boston, Massachusetts; the coniferous forests of southern Scandinavia; and the boreal ecosystem of northern Siberia. Mesothermal regions have moderate climates. They are not cold enough to sustain a layer of winter snow, but are also not remain warm enough to support flowering plants (and, thus, evapotranspiration) all year.
Mesothermal climates include the Mediterranean Basin, most of coastal Australia, and the Pampas region of South America. 
Megathermal climates are hot and humid. These regions have a high moisture index and support rich vegetation all year. Megathermal climates include the Amazon Basin; many islands in Southeast Asia, such as New Guinea and the Philippines; and the Congo Basin in Africa.
Köppen Classification System
Although many climatologists think the Thornthwaite system is an efficient, rigorous way of classifying climate, it is complex and mapping it is difficult. The system is rarely used outside scientific publishing.
The most popular system of classifying climates was proposed in 1900 by Russian-German scientist Wladimir Köppen. Köppen observed that the type of vegetation in a region depended largely on climate. Studying vegetation, temperature, and precipitation data, he and other scientists developed a system for naming climate regions.
According to the Köppen climate classification system, there are five climate groups: tropical, dry, mild, continental, and polar. These climate groups are further divided into climate types. The following list shows the climate groups and their types:
Tropical
- Wet (rain forest)
- Monsoon
- Wet and dry (savanna)
Dry
- Arid
- Semiarid
Mild
- Mediterranean
- Humid subtropical
- Marine
Continental
- Warm summer
- Cool summer
Subarctic (boreal)
Polar
- Tundra
- Ice cap
Tropical Climates
There are three climate types in the tropical group: tropical wet; tropical monsoon; and tropical wet and dry.
Tropical Wet: Rain Forests
Places with a tropical wet climate are also known as rain forests. These equatorial regions have the most predictable weather on Earth, with warm temperatures and regular rainfall. Annual rainfall exceeds 150 centimeters (59 inches), and the temperature varies more during a day than it does over a year. The coolest temperature, about 20° to 23° Celsius (68°-73° Fahrenheit), occurs just before dawn. Afternoon temperatures usually reach 30° to 33° Celsius (86°-91° Fahrenheit). Rain forests experience very little seasonal change, meaning average monthly temperatures remain fairly constant throughout the year.
Tropical wet climates exist in a band extending about 10° of latitude on either side of the Equator. This part of the globe is always under the influence of the intertropical convergence zone. The ITCZ follows a pendulum-like path during the course of a year, moving back and forth across the Equator with the seasons. It moves north during summer in the Northern Hemisphere, and south during the northern winter.
Tropical Monsoon
Tropical monsoon climates are most found in southern Asia and West Africa. A monsoon is a wind system that reverses its direction every six months. Monsoons usually flow from sea to land in the summer, and from land to sea in the winter.
Summer monsoons bring large amounts of rainfall to tropical monsoon regions. People living in these regions depend on the seasonal rains to bring water to their crops. India and Bangladesh are famous for their monsoon climate patterns.
Tropical Wet and Dry: Savanna
Tropical wet and dry climates are sometimes called “savanna” climates after the grassland ecosystem defined by wet and dry periods.
Tropical wet and dry climates sit just outside the ITCZ, near the Equator. They have three seasons. One season is cool and dry—when the warm, moist ITCZ is in the opposite hemisphere. Another season is hot and dry as the ITCZ approaches. The last season is hot and wet as the ITCZ arrives and the region experiences months as a tropical wet climate.
Dry Climates
Regions lying within the dry climate group occur where precipitation is low. There are two dry climate types: arid and semiarid. Most arid climates receive 10 to 30 centimeters (4 to 12 inches) of rain each year, and semiarid climates receive enough to support extensive grasslands. Temperatures in both arid and semiarid climates show large daily and seasonal variations. The hottest spots in the world are in arid climates. The temperature in the arid Death Valley National Park, California, U.S.A. , reached 56.7° Celsius (134° Fahrenheit) on July 10, 1913—the highest weather temperature ever recorded.
Although rainfall is limited in all dry climates, there are a few parts of the world where it never rains. One of the driest places on Earth is the Atacama Desert of Chile, on the west coast of South America. Stretches of the Atacama may have never received rain in recorded history.
Semiarid regions, such as the Australian Outback, usually receive between 25 and 50 centimeters (10-20 inches) of rainfall every year. They are often located between arid and tropical climate regions.
Arid and semiarid climates can occur where the movement of warm, moist air is blocked by mountains. Denver, Colorado, just east of the Rocky Mountains in the U.S., has this type of dry climate.
Mild Climates
Regions with mild and continental climates are also called temperate regions. Both climate types have distinct cold seasons. In these parts of the world, climate is influenced mostly by latitude and a region’s position on the continent.
Mediterranean
Mediterranean climates have warm summers and short, mild, rainy winters. Mediterranean climates are found on the west coasts of continents between 30° and 40° latitude, and along the shores of the Mediterranean Sea.
Humid Subtropical
Humid subtropical climates are usually found on the eastern sides of continents. In cities such as Savannah, Georgia, in the U.S.; Shanghai, China; and Sydney, Australia, summers are hot and humid. Winter can be severely cold. Precipitation is spread evenly through the year and totals 76 to 165 centimeters (30-65 inches). Hurricanes and other violent storms are common in these regions.
Marine
The marine west coast climate, a type of mild climate typical of cities such as Seattle, Washington, in the U.S. and Wellington, New Zealand, has a longer, cooler winter than the Mediterranean climate. Drizzle falls about two-thirds of winter days, and temperatures average about 5° Celsius (41° Fahrenheit).
Continental Climates
Areas with continental climates have colder winters, longer-lasting snow, and shorter growing seasons. They are the transition zones between mild and polar climates. Continental climates experience extreme seasonal changes.
The range of weather in continental climate regions makes them among the most spectacular sites for weather phenomena. In autumn, for instance, vast forests put on their annual show of brilliant color before shedding their leaves as winter approaches. Thunderstorms and tornadoes, among the most powerful forces in nature, form mostly in continental climates.
Warm Summer
Warm summer climate regions often have wet summer seasons, similar to monsoon climates. For this reason, this climate type is also called humid continental. Most of Eastern Europe, including Romania and Georgia, has warm summer climates. Cool Summer
Cool summer
climates have winters with low temperatures and snow. Cold winds, sweeping in from the Arctic, dominate winter weather. People living in these climates have grown accustomed to the harsh weather, but those unprepared for such cold may suffer. Many of Napoleon Bonaparte’s soldiers, for example, were used to the mild Mediterranean climates of France. Thousands died in bitter cold as they retreated from Russia’s cool summer climate in the winter of 1812.
Subarctic
North of regions with cool summer climates are regions with subarctic climates. These regions, including northern Scandinavia and Siberia, experience very long, cold winters with little precipitation. Subarctic climates are also called boreal climates or taiga.
Polar Climates
The two polar climate types, tundra and ice cap, lie within the Arctic and Antarctic Circles near the North and South Poles.
Tundra
In tundra climates, summers are short, but plants and animals are plentiful. Temperatures can average as high as 10° Celsius (50° Fahrenheit) in July. Wildflowers dot the landscape, and flocks of migratory birds feed on insects and fish. Whales feed on microscopic creatures in the region’s cold, nutrient-rich waters. People have adapted to life on the tundra for thousands of years.
Ice Cap
Few organisms survive in the ice cap climates of the Arctic and Antarctic. Temperatures rarely rise above freezing, even in summer. The ever-present ice helps keep the weather cold by reflecting most of the Sun’s energy back into the atmosphere. Skies are mostly clear and precipitation is low. In fact, Antarctica, covered by an ice cap a mile thick, is one of the largest, driest deserts on Earth.
World distribution of precipitation
The yearly precipitation averaged over the whole Earth is about 100 cm, but this is distributed very unevenly. The regions of highest rainfall are found in the equatorial zone and the monsoon area of Southeast Asia. Middle latitudes receive moderate amounts of precipitation, but little falls in the desert regions of the subtropics and around the poles.
If Earth’s surface were perfectly uniform, the long-term average rainfall would be distributed in distinct latitudinal bands, but the situation is complicated by the pattern of the global winds, the distribution of land and sea, and the presence of mountains. Because rainfall results from the ascent and cooling of moist air, the areas of heavy rain indicate regions of rising air, whereas the deserts occur in regions in which the air is warmed and dried during descent. In the subtropics, the trade winds bring plentiful rain to the east coasts of the continents, but the west coasts tend to be dry. On the other hand, in high latitudes the west coasts are generally wetter than the east coasts. Rain tends to be abundant on the windward slopes of mountain ranges but sparse on the lee sides.
In the equatorial belt, the trade winds from both hemispheres converge and give rise to a general upward motion of air, which becomes intensified locally in tropical storms that produce very heavy rains in the Caribbean, the Indian and southwest Pacific oceans, and the China Sea and in thunderstorms that are especially frequent and active over the land areas. During the annual cycle, the doldrums move toward the summer hemisphere, so outside a central region near the Equator, which has abundant rain at all seasons, there is a zone that receives much rain in summer but a good deal less in winter.
The dry areas of the subtropics—such as the desert regions of North Africa, the Arabian Peninsula, South Africa, Australia, and central South America—are due to the presence of semipermanent subtropical anticyclones in which the air subsides and becomes warm and dry. These high-pressure belts tend to migrate with the seasons and cause summer dryness on the poleward side and winter dryness on the equatorward side of their mean positions. The easterly trade winds, having made a long passage over the warm oceans, bring plentiful rains to the east coasts of the subtropical landmasses, but the west coasts and the interiors of the continents, which are often sheltered by mountain ranges, are very dry.
In middle latitudes, weather and rainfall are dominated by traveling depressions and fronts that yield a good deal of rain in all seasons and in most places except the far interiors of the Asian and North American continents. Generally, rainfall is more abundant in summer, except on the western coasts of North America, Europe, and North Africa, where it is higher during the winter.
At high latitudes and especially in the polar regions, the low precipitation is caused partly by subsidence of air in the high-pressure belts and partly by the low temperatures. Snow or rain occur at times, but evaporation from the cold sea and land surfaces is slow, and the cold air has little capacity for moisture.
The influence of oceans and continents on rainfall is particularly striking in the case of the Indian monsoon. During the Northern Hemisphere winter, cool dry air from the interior of the continent flows southward and produces little rain over the land areas. After the air has traveled some distance over the warm tropical ocean, however, it releases heavy shower rains over the East Indies. During the northern summer, when the monsoon blows from the southwest, rainfall is heavy over India and Southeast Asia. These rains are intensified where the air is forced to ascend over the windward slopes of the Western Ghats and the Himalayas.
The combined effects of land, sea, mountains, and prevailing winds show up in South America. There the desert in southern Argentina is sheltered by the Andes from the westerly winds blowing from the Pacific Ocean, and the west-coast desert not only is situated under the South Pacific subtropical anticyclone but is also protected by the Andes against rain-bearing winds from the Atlantic.
The long-term average amounts of precipitation for a season or a year give little information on the regularity with which rain may be expected, particularly for regions where the average amounts are small. For example, at Iquique, a city in northern Chile, four years once passed without rain, whereas the fifth year gave 15 mm; the five-year average was therefore 3 mm.
The variability of the annual rainfall is closely related to the average amounts. For example, over the British Isles, which have a very dependable rainfall, the annual amount varies by less than 10 percent above the long-term average value. A variability of less than 15 percent is typical of the mid-latitude cyclonic belts of the Pacific and Atlantic oceans and of much of the wet equatorial regions. In the interiors of the desert areas of Africa, Arabia, and Central Asia, however, the rainfall in a particular year may deviate from the normal long-term average by more than 40 percent. The variability for individual seasons or months may differ considerably from that for the year as a whole, but again the variability tends to be higher where the average amounts are low.
The heaviest annual rainfall in the world was recorded at the village of Cherrapunji, India, where 26,470 mm fell between August 1860 and July 1861. The heaviest rainfall in a period of 24 hours was 1,870 mm, recorded at the village of Cilaos, Réunion, in the Indian Ocean on March 15–16, 1952. The lowest recorded rainfall in the world occurred at Arica, a port city in northern Chile. An annual average, taken over a 43-year period, was only 0.5 mm.
Although past records give some guide, it is not possible to estimate very precisely the maximum possible precipitation that may fall in a given locality during a specified interval of time. Much will depend on a favourable combination of several factors, including the properties of the storm and the effects of local topography. Thus, it is possible only to make estimates that are based on analyses of past storms or on theoretical calculations that attempt to maximize statistically the various factors or the most effective combination of factors that are known to control the duration and intensity of the precipitation. For many important planning and design problems, however, estimates of the greatest precipitation to be expected at a given location within a specified number of years are required.
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