TYPES OF RAINFALL

RAINFALL

There are three main types of rainfall. All have the common theme of air being forced to rise.

As air rises it cools it cannot hold as much moisture as it could when it was warmer. Eventually the rising air reaches a point where it is 100% saturated, in other words it cannot hold any more water. This is called dew point, and it is above this point that condensation occurs.

Condensation is the process by which the water vapour (a gas) held in the air is turned back into water droplets (a liquid), which fall as rain.

Types of Rainfall

a)      Relief/ orographic rainfall

b)     Convectional rainfall

c)      Frontal/cyclonic rainfall

Relief rainfall

Prevailing winds pick up moisture from the sea as they travel across it, making the wind moist and warm. The warm moist reaches the slope of a mountain or high hills and is forced to rise over along the mountain slope. As the warm moist wind rises, it is cooled due to a decrease in temperature until temperatures falls below due point.

The air condenses, forming clouds and raindrops starts to form.

More water vapour condense forming heavy raindrops, that later falls heavily on the windward side as relief rainfall.

The wind continues to be forced over the mountains and begin to descend on the leeward side/ rain shadow. The wind is warmed and causes little rain. The wind descends down the slope as cool dry wind.

Relief rainfall falls in light showers, rains for a long period of time and occurs in highland/mountainous areas.

Convectional Rainfall   

Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons.

Convectional rainfall occurs when:

Water in a lake/sea is heated by intense solar radiation. Water evaporates and the moist air over the water rises in convectional currents.

As the air rises up, it expands leading to cooling. The moist air condenses at a higher altitude/ level of the atmosphere. The condensed moist air form cumulo-nimbus clouds. These clouds cause heavy or torrential rainfall accompanied by lightning and thunder. At times it also contains hailstones.

Convectional rainfall mainly falls in the afternoon due to maximum heating of air and rains for a short period of time (20-30minutes).

Frontal rainfall

Frontal rainfall is associated with the movement of depressions over the country, which are described in more detail elsewhere in this topic.

Frontal rainfall occurs when:

Two air masses meet, one a warm air mass and one a cold air mass. The lighter, less dense, warm air is forced to rise over the denser, cold air. This causes the warm air to cool and begin to condense. As the warm air is forced to rise further condensation occurs and rain is formed. Frontal rain produces a variety of clouds, which bring moderate to heavy rainfall.

Frontal rainfall is characterized by thunder and lightning, and moderate to heavy rainfall.

ATMOSPHERIC PRESSURE AS AN ELEMENT OF WEATHER

ATMOSPHERIC PRESSURE

What is Air Pressure?

It is the force exerted on a point on the Earth surface by the weight of air column above that point.

In figure1 above, there is high atmospheric pressure at point because, the air column above the point A (low altitude area) is longer/larger and therefore, it exerts more force.

At point B (high altitude area), there is a shorter/smaller air column above the point, therefore there is lower atmospheric pressure.

Factors Influencing Atmospheric Pressure

1.      Altitude

2.      Temperature Variation

3.      Earth’s Rotation

Altitude

Pressure decreases with increase in altitude

High altitude areas have a thin/shorter air column, which causes less force to the area. This leads to a lower atmospheric pressure

Low altitude areas have a thick/long air column, which causes more force to the area. This leads to a higher pressure in the area.

Temperature Variation

Atmospheric pressure reduces with increase in temperature.

When temperature increases, air is heated, expands, becomes light and rises creating a low pressure.

When temperature decreases, air is cooled, contracts, becomes dense and descends creating a high pressure.

Earth’s Rotation

Equatorial areas rotate faster than polar areas which pushes air masses towards the equator. The air is expands due to increased space at the equator and rises creating a low pressure.

Air from the polar areas towards the equator crosses longer latitudes and therefore spreads out and rises up reducing pressure at 60◦N and 60◦S of the equator.

As the air moves away from the poles, more air descends from the atmosphere. The air is forced into a small area, leading to a higher pressure at poles.

Air rising at the equator descends at latitude 30ᵒN and 30ᵒS. The air is thus forced into areas of reducing/shorter latitudes, causing a higher pressure at latitude 30ᵒN and 30ᵒS.