Rain, a precious resource

Rain, rain go away Come again another day

Do you remember these two lines? When it's raining, children recite them in the hope of chasing the rain away, so that they can go out and play. The rain may have troubled you during the avurudu season, preventing you from playing outside and enjoying yourselves. So, do you like to know what actually rain is? Then here we go.

Rain is a source of rainwater which forms when separate drops of water fall to the Earth's surface from clouds. But, did you know that all rain does not fall to the ground?

Some evaporate while falling through dry air. When nothing of the water reaches the ground, it is called virga, a fact often observed in hot, dry desert regions.

The scientific explanation of how rain forms and falls is called the Bergeron. Rain plays a major role in the water cycle, in which moisture from the oceans evaporates, condenses into clouds, falls back to Earth, and eventually returns to the ocean via streams and rivers, to repeat the cycle.

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Types of rain

Rain is classified based on two specifications: the amount of rainfall, and the reason for rainfall.

When classified according to the amount of precipitation (vapour), rain can be divided into:

* Very light rain - when the amount of precipitation is less than 250 mm (250 litre/m2)

* Light rain - when the amount of precipitation is between 250 mm and 750 mm (250 litre/m2 - 750 litre/m2)

* Moderate rain - when the amount of precipitation is between 750 mm and 1500 mm (750 litre/m2 - 1500 litre/m2)

* Heavy rain - when the amount of precipitation is between 1500 mm and 2000 mm (1500 litre/m2 - 2000 litre/m2)

* Very heavy rain - when the amount of precipitation is more than 2000 mm (2000 litre/m2)

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Based on the reason for precipitation, rain is classified into:

Convective rain

This rain mainly occurs in the equatorial climatic regions and tropical climatic regions where it is very hot during the day.

The rate of evaporation of moisture from the water bodies and respiration (breathing) from the dense vegetation is very high. The evaporated moisture along with its hot surrounding air begins to rise.

With gains in altitude, the air expands dynamically due to a decrease in air pressure. Due to this, the wind experiences a decrease in temperature, which results in the increase of the relative humidity.

This causes condensation of water vapour into water droplets to form unstable towering cumulonimbus clouds (See our article on clouds). When the clouds become too heavy to be suspended, rain falls.

Frontal rain

This is also known as Cyclonic rain, and is caused by cyclonic activity and occurs along the fronts of a cyclone. It is formed when two masses of air of different temperature, humidity and density meet.

For example, we can take the meeting of moisture-laden warm tropical wind with a polar air mass. A layer separating them is called the front. This front has two parts - the warm front and the cold front. At the warm front, the warm lighter wind rises gently over the heavier cold air, which, being heavy, stays close to the ground.

As the warm air rises, it cools, and the moisture present in it condenses to form clouds known as altostratus clouds.

This rain falls steadily for a few hours to a few days. The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and is routinely measured with accuracy up to 0.1mm or 0.01in. It is sometimes expressed in litres per square metre (1 litre/m2 = 1 mm).

Orographic rain

This is also known as Relief rain. It is caused when the wind, loaded with warm moisture and blowing into the land from the sea, encounters natural barriers like mountains. This forces the wind to rise.

The same process as in convective rain takes place until the condensation of water, turning vapour into water droplets to form clouds.

The relative humidity continues to increase until the dew point reaches the level of condensation, causing air to be saturated (heavy). This height where the condensation occurs is called the level of condensation. When the clouds become too heavy to be kept suspended (hanging), rain falls.

Shape and size

Most of us think that falling rain drops are the shape of a tear drop, round at the bottom and narrowing towards the top. But, it is incorrect. Only drops of water dripping from sources like taps are tear-shaped at the moment of their formation.

Small raindrops are nearly circular. Larger ones become increasingly flattened on the bottom, very large ones are shaped like parachutes. As they get larger (to about 5 mm diameter), they become more doughnut-shaped. Beyond about 5 mm, they become unstable and break into fragments. On average, raindrops are 1 to 2 mm in diameter.

The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004; some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with a particularly high content of liquid water.

Now do you know enough about the rain? These rain drops help to keep all living beings including plants alive. So, never scold the rain. Just say the nursery rhyme and the rain might just consider coming on another day!

Compiled by Janani Amarasekara