There are three major types of rainfall.

1) Convectional rainfall: This type of rainfall is most common in regions that are intensely heated, either during the day, as in the tropics, or in the summer, as in temperate interiors. When the earth’s surface is heated by conduction, moisture-laden vapour rises because heated air always expands and becomes lighter. Air rises in a convection current after a prolonged period of intense heating (Fig. 14.18).

In ascending, its water vapour condenses into cumulonimbus clouds with a great vertical extent. This probably reaches its maximum in the afternoon when the convectional system is well developed. Hot, rising air has great capacity for holding moisture, which is abundant in regions of high relative humidity. As the air rises, it cools and when saturation point is reached torrential downpours occur, often accompanied by thunder and lightning. The summer showers in temperate regions are equally heavy with occasional thunderstoms. These downpours may not be entirely useful for agriculture because the rain is so intense that it does not sink into the soil but is drained off almost immediately.

Thunderstorms often develop when intense overheating of ground surfaces and moist air reaches its maximum. They are very common in the tropics where they may occur every day, but also develop over land in summer in other latitudes.

The characteristic cloud of the thunderstorm is the cumulonimbus. Its summit in temperate latitudes may be 6000 metres (20 000 feet); in the tropics, it can reach 12 000 metres (40 000 feet).

Lightning is the result of a build-up of electrical energy within the cloud. This electrical energy results when the rain droplets are broken up by ascending air currents. The rain droplets combine, divide and recombine so many times that lightning occurs. This lightning is a discharge of electricity either between two clouds or between the cloud and the ground. As the flash travels earthwards, it heats up the air it passes through. This air at first expands, then cools and contracts. This sets up vibrations or thunder, which is the noise following a lightning flash (Fig. 14.19).


2) Orographic or relief rain: Unlike convectional rain which is caused by convection currents, orographic rain is formed wherever moist air is forced to ascend a mountain barrier. It is best developed on the windward slopes of mountains where the prevailing moisture-laden winds come from the sea. The air is compelled to rise as shown in Fig. 14.20 and is thereby cooled by expansion inthe higher altitudes and the subsequent decrease in atmospheric pressure. Further ascent cools the air until the air is completely saturated (relative humidity is 100 percent).

Condensation takes place forming clouds and eventually rain. Since it is caused by the relief of the land, it is also known as relief rain. Much of the precipitation experienced on the windward slopes of the north-east of West Malaysia, western New Zealand,western Scotland and Wales and the Assam Hills of Indo-Pakistan is relief rain.

In Nigeria, rainfall on the Jos Plateau, which lies across the path of the south-westerly rain-bearing winds, is higher than the average for its latitude. For instance, Kafanchan (750 metres, 10°N) has a mean annual rainfall of 1524 mm, while Ibadan (268 metres, 7½°N) to the south and nearer the sea, has 1219 mm. Other areas experiencing relief rain in West Africa include the Cameroon Highlands, the Mampong Scarp (Ghana), the Sierra Leone Peninsula, the Guinea highlands of Liberia and Sierra Leone, the Fouta Djallon of Guinea, and the high relief islands of Fernando Po, Annobon, Sao Tome and Principe.

On descending the leeward slope, a decrease in altitude increases both the pressure and the temperature, the air is compressed and warmed. Consequently, the relative humidity will drop. There is evaporation and little or no precipitation. The area in the lee of the hills is termed the rain-shadow area.

The effects of rain-shadow are felt on the Canterbury Plains of South Island, New Zealand and the western slopes of the Northern and Central Andes and in many other areas.


3) Cyclonic or frontal rain: This type of rainfall is independent of relief or convection. It is purely associated with cyclonic activity whether in temperate regions (depressions) or tropical regions (cyclones). Basically, it is due to the convergence (meeting) of two different air masses with different temperatures and other physical properties. As cold air is denser, it tends to remain close to the ground. The warm air is lighter and tends to rise over the cold air as shown in Fig. 14.22.

In ascent, pressure decreases, the air expands and cools, condensation takes place and light showers called cyclonic or frontal rain occurs. The heavier and colder air masses eventually push up the warmer and lighter air and the sky is clear again.

In West Africa, the weather is controlled by the interaction of two air masses, the tropical continental air mass (Tc) which flows from the Sahara and the Tropical maritime air mass (Tm) from the Atlantic. The continental air mass 15 cool and dry, while the maritime air mass is warm and moist. Where the two air masses meet is called the Intertropical Discontinuity (ITD), which is a moisture discontinuity between the dry and the moist air masses. The position of the (ITD) is not stable, as it moves north and south in accord with the movements of the sun and the subtropical high pressure belts (Fig. 14.23 (a)).

The figure shows that the ITD moves northwards, during the northern summer and it reaches its most northern position (Lat. 22 to 25°N) in July or August but it retreats equatorward to 4-6°N in West Africa in January. These movements control the rainfall distribution (amount of annual rainfall, seasonality and drought occurrences) in West Africa. The ITD is dominant in the north between June and September when more than three quarters of the annual rainfall is recorded.

Drought is a recurrent feature in the northern parts of West Africa. Drought is the periodic deficiency of rain relative to the previous averages. The deficiency is significant enough to affect the raising of crops and the survival of livestock. It is recorded at varying intensities and intervals. The intensity at any given place depends on the extent of the northward advance

of the ITD. Drought occurs in those years when the northward movement of the ITD is limited such that the influence of the moisture-laden Tm air mass is restricted to the southern parts of the subcontinent.

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