O-Level Geography Chapter 2 Climate Rainfall

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Rainfall

Sources of rainfall in Pakistan :

I. Monsoon Winds

II. The Western Depression

III. Convectional Currents

IV. Relief rainfall

V. Tropical Cyclones

Monsoon Rainfall

Monsoons are seasonal winds that blow from sea towards land from July to August (summers). After it they reverse their direction and blow from land towards the sea (winters). . There are two monsoon systems, one originating from the Bay of Bengal and other from the Arabian Sea. Moist laden winds from high pressure areas of sea blow towards land. From the east the monsoon clouds are deflected along the Himalayas from Nepal to Pakistan. Along the way these clouds rise, cool and thus condense eventually bringing rainfall to Pakistan. They affect northern Punjab, Khyber-Pakhtunkhwa, Gilgit-Baltistan and Azad Kashmir

On the other hand another monsoon system that originates in Arabian Sea also travels inland and delivers little rainfall to Sindh.

The Western Depression

These are wind systems that originate from the Mediterranean Sea and travel eastwards towards Afghanistan and Pakistan. Since they make a long journey they lose most of their water when they reach Pakistan. During the winters (Late November to March) they bring rainfall because during winters the Arabian Sea retains its warmth and thus cold air from coastal areas flows towards sea. These western depressions move from high pressure area (Mediterranean Sea) towards the low pressure area (Indus Plains).

Convection

Convection rainfall occurs when the sun heats up the land. The air near the land absorbs heat by conduction, and thus gets less dense and rises. The moisture in air condenses to form clouds, and when this air is cooled to a certain minimum level rainfall occurs.

Relief Rainfall

Relief or orographic rain is formed when air is forced to cool when it rises over relief features in the landscape such as hills or mountains. As it rises the chilling causes condensation and precipitation on the windward side and forms rain. The leeward side becomes rain shadow.

Thunderstorm

Thunderstorms occur in the same fashion as convectional rainfall with the main difference being that the air here rises very quickly forming Cumulonimbus clouds, which are very tall and big. This is followed by strong winds, lightning and heavy rainfall. Hailstones may also be formed, when the water vapours are cooled multiple times (to form water first and then ice).

Cyclone

Cyclones are formed when the average temperature of large water body exceeds 27° C, thus forming an area of very low pressure. This area then draws further air from high pressure areas quickly. A cone is formed when two oppositely travelling cold and warm winds meet. Intense lightning followed by very strong winds and heavy rainfall occurs. Their effects are similar to that of floods.