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.

UNDERGROUND WATER- CONTINUED

THE WATER TABLE

Water table is the upper level of water saturated zone in the permeable rock layer, making up the ground water.

Water infiltrates into the ground, then percolates under gravity to reach an impermeable rock.

The permeable rock above the impermeable rock is filled with water; and is said to be saturated.

The upper limit of the saturated permeable rock is the water table.

The water table fluctuates according to seasons leading to three main zones of saturation

Zones of Saturation

 There are three zones of saturation:

•  Zone of permanent saturation
• Zone of intermittent saturation
•  Zone of non-saturation

Zone of Permanent Saturation: is the zone where all the pores and air spaces in the rock are permanently filled with water. 

Also called the phreatic zone

Zone of Intermittent Saturation: is the zone which is only filled with water during rainy season and is unsaturated during the dry season.

Water content fluctuates according to the season.

Zone of Non-Saturation: is a zone of rock layers found immediately below the surface, allows water to pass through but does not stay long to make it saturated

The top of this zone has the soil water belt.

The zone of intermittent and non-saturation are referred to as the vadose zone/zone of aeration

Diagram: zones of saturation

 

Note:

-An Aquifer is a permanently saturated permeable rock layer above an impermeable rock

Existence of underground water leads to the occurrence of:

•  Springs
•   Wells
•  Artesian basins and artesian wells

 

SPRINGS

A spring is a point where underground water flows out naturally as the water table is exposed to the surface along a slope

Conditions/ ways in which a spring is formed:

i.        When a permeable rock overlies an impermeable rock layer

Rain water percolates and saturates the permeable rock

A spring forms where the permeable rock and the impermeable rock meet on the surface

i.        When water enters and saturates a well-jointed rock on a hill

Rain water percolates through the joints ant saturates the rock

Water comes out where the joints open up to the surface forming a spring

May form a line of springs called vauclusian springs

 

i.        Where a dyke cuts across an aquifer/ a layer of permeable rock

The water table is raised to the surface

Where the water table meets the surface, water flows out forming a spring

Where a limestone/chalk  escarpment lies on top of an impermeable rock

Water percolates and saturates the limestone rock

Water flows out to form a line of spring ate the foot of the escarpment where the water table meets the surface.

 

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