Exploring the 3 Types of rainfall

There are three major types of rainfall at different parts of the World and these are:

1. Convectional rainfall
2. Orographic or relief rain
3. Cyclonic or frontal rain

Convectional Types of rainfall.

• These types 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
• When ascending, the water vapor 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 accompanied by thunder and lightning.

2. Orographic or relief rain

• Unlike convectional rain which is caused by convection currents, orographic rain is formed whenever 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 and is thereby cooled by expansion in the higher altitudes and the subsequent decrease in the atmospheric pressure
• Further ascent cools the air until the air is completely saturated (relative humidity is 100 per cent)
• Condensation takes place forming clouds and eventually rain
• Since it is covered by the relief of the land it is also known as relief rain
• The other side of the mountain where there is little or no rainfall is called the leeward slope
• The wind is dry and does not bring rain
• The area is termed the rain shadow e.g. the Luangwa valley in the lee of the Muchinga Escarpment.

Cyclonic or frontal rain

• This type of rainfall 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

In ascent, pressure decreases, the air expands and cools, condensation takes place and

• Light showers called cyclonic or frontal rain occur.

Measurement of rainfall

An instrument called a rain gauge is used to measure rainfall. Rainfall is measured in millimetres or inches.

A rain gauge consists of the following parts;-

a. A metal or plastic container
b. A funnel
c. A graduate jar or measuring cylinder.

• Rain falling in the funnel trickles into the jar or measuring cylinder shown above
• Measurements are taken every 24 hours.
• The reading obtained is the depth of rain that has fallen over area equipment to the top of the funnel.

Position of rain Gauge

• It must be sunk into the ground level. This is to prevent rain from splashing into the funnel from the g round.
• It must be sunk into the ground to prevent excessive evaporation of rain water from the jar
• It must be placed in an open space. This is to prevent run – off water from buildings and trees from entering the funnel.

Recording rainfall

• On weather maps all places having the same quantity of rainfall are joined by a line known as isohyets.

Humidity of the air

• Humidity refers to the amount of water vapour that may be present in the air.
• This amount of watervapour varies from what air can hold at a given temperature and the actual amount of water vapour in the air is very important.
• This is called relative Humidity (R. H.). It is expressed as a percentage (%).

Example:

• If R. H. is 60% a temperature of 30°C, then air is only holding 6/10th of the water vapour it could hold at that temperature.
• There is a limit to the quantity of water vapour that can be held by the air.
• This limit is known as the saturation point.
• This means that the air is saturated when the R. H. is 100%.