TYPES OF RAINFALL                                                                                           

1. CONVECTIONAL RAINFALL   Convectional rain is caused by convection where the surface layer of the atmosphere is heated causing the moisture laden air to rise. As it rises it cools to form clouds. The unequal heating of the earth’s surface causes convection. The resulting condition is that more widespread areas of colder air separate rising currents of warm air. The colder air slowly sinks to take the place of rising warm air.     

The condition of rising currents of warm air separated by more wide-spread areas of slowly sinking air is referred to as CONVECTION. This is typical of thunder storms during a hot summer. Convectional rain can also be found year round in regions near the equator.

2. CYCLONIC RAINFALL This rain is caused by depressions or Lows. Cyclonic rain originates where warm tropical air meets cold polar air. The warm air overrides the cold air.

When contrasting air masses make contact, an abrupt zone or boundary is formed. This boundary is called a FRONT and is accompanied by rather abrupt changes in temperature, pressure and humidity.      

When a mass of warm air moves into a region of cold air, the warm air overrides the cold air mass, forcing the cold air to retreat. This situation is called a WARM FRONT, which are characterized by several days of rain.       

A cold air mass moving into a warm air mass produces a frontal surface, which is more vertical than that of a warm front. This situation produces a COLD FRONT. Cold air masses advance rapidly and force the warm air mass upward where it becomes cooled. The movement of the air mass is rapid enough to produce cumulonimbus clouds. Rainfall is heavy but brief in duration.

3. OROGRAPHIC RAINFALL Canada’s west coast. Rain which mountains standing in the path of moisture-laden air cause; the air is forced to rise and is therefore cooled. If enough water vapour is present it is deposited on the windward side of the mountains.

1) Convective 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. The United Kingdom does experience some convectional rainfall during the summer, particularly in the South East of the country.

Convective rainfall occurs when:

The surface of the earth is heated by the sun.

The warm surface heats the air above it. Hot air always rises so this newly heated air does so.

As it rises the air-cools and begins to condensate.

Further rising and cooling causes a large amount of condensation to occur and rain is formed.

Convection tends to produce towering cumulo-nimbus clouds, which produce heavy rain and possible thunder and lightning.

2) CYCLONIC RAINFALL

This rain is caused by depressions or Lows. Cyclonic rain originates where warm tropical air meets cold polar air. The warm air overrides the cold air.

When contrasting air masses make contact, an abrupt zone or boundary is formed. This boundary is called a FRONT and is accompanied by rather abrupt changes in temperature, pressure and humidity.

When a mass of warm air moves into a region of cold air, the warm air overrides the cold air mass, forcing the cold air to retreat. This situation is called a WARM FRONT, which are characterized by several days of rain.

A cold air mass moving into a warm Cold air masses advance rapidly and force the warm air mass upward where it becomes cooled. The movement of the air mass is rapid enough air mass produces a frontal surface, which is more vertical than that of a warm front. This situation produces a COLD FRONT. to produce cumulonimbus clouds. Rainfall is heavy but brief in duration.

3) Frontal Rainfall

 When two air masses meet, they do not mix readily due to differences in temperature and density. A front is an imaginary line separating two contrasting air masses. Fronts are areas where rainfall takes place.

A warm front is found when warm air is advancing and rises over cold air, which is denser and heavier. As the warm air rises it is cooled and condensation takes place.

Continued condensation leads to the growth of water droplets and when heavy enough these fall as rain.

A cold front occurs when heavier cold air is advancing and pushes beneath a body of lighter warm air. The name of the front is specified by the temperature of the moving air.

4) Orographic rainfall is when a parcel of air containing water vapor gets to a mountain, it is then forced upwards (the mountain blocking its way). 

As the parcel of air is forced upwards, it expands (adiabatic expansion) and also cools. As it cools, it becomes saturated because the amount of water it can hold decreases, so the water vapor begins to condense and finally falls as rain.

Orographic Rainfall occurs when: The prevailing winds pick up moisture from the sea as they travel across it, making the air

moist. The moist air reaches the coast and is forced to rise over mountains and hills. This forces the air to cool and condense, forming clouds. The air continues to be forced over the mountains and so it drops its moisture as relief rain. Once over the top of the mountain the air will usually drop down the other side, warming as it

does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow.

5) Artificial rainfall (Clound seeding)

A method of making a cloud give up its moisture as rain, especially by releasing particles of dry ice or silver iodide into cold clouds. Dry ice freezes water droplets in the cloud, turning them into nuclei for the formation of raindrops. Silver iodide particles are used because they have a crystal structure similar to ice and can also serve as nuclei for raindrop formation.

References

^ National Weather Service (2009). "NOAA's National Weather Service Main Page". Retrieved 2009-01-01.

^ a b Dr. Michael Pidwirny (2008). "CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes". Physical Geography. Retrieved 2009-01-01.

^ "Falling raindrops hit 5 to 20 mph (8.0 to 32 km/h) speeds". Weather Quest. Retrieved 2008-04-08.

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