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In this article we will discuss about:- 1. Location of Equatorial Climate 2. Temperature of Equatorial Climate 3. Precipitation 4. Distribution.
Location of Equatorial Climate:
Equatorial type of climate, also known as tropical rainforest wet climate or simply Af climate, is located up to 5° to 10° latitudes on either side of the equator (fig. 39.1) but at some places it extends up to 15°-25° latitudes mainly along the eastern margins of the continents. This climatic zone is subjected to seasonal shifting due to seasonal shifting of pressure and wind belts consequent upon the northward and southward migration of the sun.
The equatorial climate is characterized by two major properties e.g.:
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(i) Uniformly high temperature throughout the year, and
(ii) Uniformly adequate rainfall throughout the year received through convective mechanism.
The equatorial climate is found in the following localities:
(i) The Amazon Basin in South America,
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(ii) The Congo Basin in Africa,
(iii) Guinea coast in Africa,
(iv) Much of the Indo-Malaysian Region mainly in Java, Sumatra, Borneo, Malaysia, Singapore and New Guinea,
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(v) Philippine Islands,
(vi) Eastern central America (parts of Panama, Costarica, Nicargua, Honduras, Guatemala etc.), some islands in the Caribbean Sea, western Columbia and eastern Madagascar.
Temperature of Equatorial Climate:
Since mid-day sun is almost overhead throughout the year and there is little difference between the lengths of day and night during the year and hence the equatorial region receives maximum amount of insolation which causes uniformly high temperature throughout the year as the average monthly temperature is always more than 18°C.
The mean monthly temperature of most of the places ranges between 24°C and 27°C. Mean annual temperature is around 20°C but the maximum temperature of the year touches 30°C. The mean annual range of temperature of island areas ranges between 0.5°C and 1°C but other areas record annual ranges of temperature between 2°C to 3°C.
The annual range of temperature of Iquitos (located in Peru falling in the Amazon Basin, 4°S of the equator) is 2°C. Similarly, Akassa (located at the mouth of the Niger River, Africa) records annual range of temperature of 2°C but Para records less than 2°C as annual range of temperature.
The annual range of temperature becomes minimum over the oceans. For example, Jaluit, located on Marshall Island in the central Pacific Ocean records annual range of temperature of only 0.4°C. Thus, uniformly high temperatures of the equatorial regions, though lower than the temperatures of the hot desert climate, becomes unpleasant and injurious to human beings because of its uniformity and monotony.
The daily range of temperature varying between 5°C and 10°C is usually far greater than the annual range of temperature. Usually, mid-day temperature rises to 29°C-34°C and comes down to 21°C-24°C during nights. Thus, the relatively low nocturnal temperature becomes uncomfortable to local people.
This is why nights of the equatorial regions are called winters of the tropics. The annual range of temperature of Bolobo of the Belgian Congo is 1°C but the daily range becomes 9°C. Belam city records daily maximum and minimum temperatures as 32.8°C and 20.1°C respectively thus registering diurnal range of 12.7°C.
Similarly, Santaram of the Amazon Basin records 35.5°C and 19.5°C as maximum and minimum daily temperatures and thus diurnal range of temperature becomes 16.0°C. The daytime temperature becomes oppressive and unbearable due to high relative humidity, weak air-circulation, bright sunlight etc.
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The inhabitants of the equatorial regions are so used and habitual to uniformly high temperatures throughout the year that they feel immediately even a plight fall in temperature. They feel cold if temperature falls below 20°C and they burn wood to ward off relative cold (though there is no winter season).
Precipitation of Equatorial Climate:
Equatorial regions receive rainfall throughout the year and thus there is no dry season. Average annual rainfall exceeds 200cm to 250cm. Even the driest month of the year receives rainfall more than 6cm. Iquitos (Peru), Akassa and Ocean Island receive 261 cm, 366cm and 213cm of rain per annum respectively. Though most of the rainfall occurs through convective mechanism but wherever mountain barrier becomes effective the amount of rainfall increases substantially.
For example, annual rainfall reaches 1000cm in the foothill Zone of Cameroon Mountain in Africa. Most of the annual rainfall in the equatorial region is received in the form of convectional rainfall.
The strong daily vertical convective mechanism due to intense heating of ground surface because of high amount of insolation, horizontal convergence of trade winds forming intertropical convergence, a fairly large number of atmospheric disturbances (cyclonic storms) and thunderstorms yield heavy rainfall daily throughout the equatorial regions.
Distribution of Equatorial Climate:
Inspite of high rainfall throughout the year there is no uniform spatial distribution of rainfall in all parts of equatorial climatic region. Though no month goes dry but definitely some months of the year receive more rainfall than the other months. Thus, the months having more rainfall are called wet months while the months receiving less rainfall are known as less wet months.
If the temporal distribution of rainfall in the equatorial regions is considered carefully it appears that there are two periods of maximum rainfall and two periods of minimum rainfall in a year. Normally, April and November receive maximum rainfall but the period of maximum rainfall varies spatially. For example, Lagos receives maximum rainfall in May, June and July amounting to 100cm whereas October records 25cm of rainfall.
Cloudiness:
Equatorial climate is characterized by fairly large amount of cloudiness throughout the year. Generally, cumulus type of clouds dominates daily weather conditions. On an average, there is about 60 per cent cloudiness daily. The maximum amount of cloudiness is found between 3 and 4 P.M. daily because of maximum convective activity during this period but the sky is generally clear in the morning and at night.
Though the daily period of cloudiness is less in comparison to high middle latitude areas dominated by temperate cyclones but there is strong heavy down-pour due to convective mechanism and resultant convectional rainfall.
Rainfall regime:
Equatorial rainfall is convectional in character wherein there is daily heavy downpour from cumulo-nimbus clouds. The sky is usually free from clouds in the early morning. As the sun rises above the horizon, the amount of insolation received at the ground surface increases and hence air temperature also increases accordingly.
Air is heated, becomes light and moves upward and thus becomes unstable which causes convectional system. The ascending air cools at the dry adiabatic lapse rate (10°C per 1000m) and the air soon becomes saturated and condensation level is reached. Clouds are formed.
In the beginning they are cumulus and few in number but as the day advances, humidity increases due to increasing evaporation, the clouds are thickened and darkness increases. By afternoon the whole sky becomes overcast with thick cumulo-nimbus clouds. Thus, heavy rain starts with lightning and cloud thunder.
As the day draws towards evening the rains become slow and weak and they completely stop by evening, clouds are cleared and weather becomes pleasant for some time. The aforesaid mechanism is repeated daily. Sometimes, this rhythemic daily mechanism is interrupted as rains continue uninterruptedly for several days. Continuous rains for 40 hours have been reported in Ivory Coast (Africa). The most characteristic feature of equatorial rainfall is that it is usually associated with strong thunderstorms as about 75 to 150 rainy days are associated with them.
Variability of rainfall:
Inspite of very high mean annual rainfall there is temporal variability in the amount of rainfall and this variability is more than the variability of temperature. The crops grown in this climatic region are such that they require more moisture and thus the year having little less than average annual rainfall is termed as a drought year because the crops are damaged.
Though the word drought is unfamiliar in equatorial climatic region but sometimes brief drought conditions are created. There is also variability of rainfall within a year. For example, Belem (in the Amazon Basin) receives average annual rainfall of 239 cm and March receives more rainfall than other months.
March has 28 rainy days against 10 rainy days in November. It may be concluded that rainfall in equatorial rainforest climate is adequate enough to support field crops and luxurious dense forests. Most of the rains is intercepted by forest canopy and thus reaches ground surface slowly in the form of aerial streamlets through leaves, branches and stems of trees and thus there is maximum infiltration of rainwater.
Recent clearance of rainforests in the equatorial regions in general and in Amazonia in particular has converted once forest-converted surface into a bare ground surface which is subjected to accelerated rate of soil erosion due to daily heavy rains.
Effects of Climate on Natural Vegetation:
The tropical rainforest or equatorial climatic region accounts for the largest number of plant species and luxuriant growth of natural vegetation due to high temperature and high rainfall throughout the year. The climatic region is characterized by broad-leaf evergreen dense forests comprising valuable trees such as mahogany, rosewood, coconut palm, avony, cincona, plaintain, bamboos, wild rubber, sandal wood etc.
The number of tree species is so large and their diversity is so great that one hectare of land in the equatorial region accounts for 40 to 100 species. It may be pointed out that tree species account for 70 per cent of the total plant species of the tropical evergreen forests. Creepers or climbers are the second important members of the rainforests.
The creepers comprising:
(i) Climbers of lower strata,
(ii) Long woody climbers known as ‘lianas’, and
(iii) Epiphytes are so circuitous and highly irregular in form that it becomes difficult to find out their actual length.
“They ramble through the forest, scaling the highest emergent trees and frequently looping down to the ground and then ascending further sections of the forest”. The climbers so greatly bind several trees and plants together that the accessibility in the forest cover becomes almost zero.
The vertical stratification of vegetation community consists of 5 layers or strata viz.:
(i) First or top layer or dominant layer representing the canopy of tallest trees (30 to 60m in height),
(ii) Second layer or co- dominant layer (25m to 30m in height),
(iii) Third layer of smaller trees (12m to 20m in height),
(vi) Fourth layer of shrubs (5m in height), and
(v) Ground layer of herbaceous plants and ferns.