Rain

We all know what Rain is. It is one of the most exciting season of the year. But have you ever wondered where does rain come from? Where all the pouring water comes from?

We will do a small experiment to understand the concept of Rain. Take an aluminium kettle and cover it with a lid in such a way that there remains a little gap between the lid and the vessel for the exit of the steam. Now boil the water till steam starts coming out. Take some pieces of ice or chilled water in a pan with a handle.

Now see where from the rain water comes in the sky. Hold the pan at a distance from the nozzle of the kettle blowing out the steam and see what happens. The hot steam strikes the outer chilled wall of the pan and gets again converted into water. That is the process of condensation takes place. When many such drops combine together and become too heavy to withstand their own weight, they start falling to the ground. This precisely is the process by which the rain falls.

The heat coming from the sun converts the water bodies into vapour. This vapour gets absorbed in atmosphere. As the air ascends it becomes colder and the vapour present in it takes the shape of tiny drops of water owing to colder climate above. Large collection of such water drops forms the cloud. When the water drops in the cloud become too heavy to withstand their own weight they start falling to the earth and thus we get the delight of rains.

What causes precipitation (rain and snow)?

Precipitation forms when cloud droplets (or ice particles) in clouds grow and combine to become so large that their fall speed exceeds the updraft speed in the cloud, and they then fall out of the cloud. If these large water drops or ice particles do not re-evaporate as they fall farther below the cloud, they reach the ground as precipitation. Precipitation that does re-evaporate before reaching the ground is called "virga", two examples of which are shown below:

The more watervapour there is below the cloud, and the stronger the updrafts that cause this water vapour to condense into cloud water or ice particles, the more likely it is that precipitation will form within the cloud.
So, a cloudy day with no precipitation indicates that there is either (1) not enough water vapour available to the cloud for precipitation to form, or (2) that the rising motion creating the cloud is not strong enough -- or both. At the opposite extreme is a tropical rain shower that has large amounts of water vapor available to it (like the one pictured at the top of this page), and which can rain heavily from even a small cloud with weak updrafts.

In warm air masses, precipitation occurs primarily within localized shower clouds that have strong updrafts. In cooler air masses, precipitation (rain or snow) usually occurs in large cloud systems associated with lowpressure zones. These low pressure zones usually form along the boundary between warm and cold air masses, where the flow of air around the low pressure causes large areas of weak rising motion as air from the warmer air mass flows up and over the colder air mass.

Interesting facts:

PRECIPITATION EQUALS EVAPORATION

All of the precipitation that falls originated as water vapour that was evaporated from the surface of the Earth. It is always raining somewhere on the Earth, just as evaporation is always occurring over most of the Earth's surface. At any given time, precipitation covers only about 2% to 5% of the surface of the Earth, while evaporation is occurring over the remaining 95% to 98% of the Earth. Thus, as water vapour slowly evaporates over most of the Earth, an approximately equal amount gets "concentrated" into relatively small rain systems that turn some of the vapour into precipitation.

So, averaged over the whole Earth over a period of months, the amount of precipitation almost exactly balances the amount of evaporation. If this were not so, the atmosphere would either be filling up with water vapour, or be depleted in water vapour.

BY

KISHORE, NSS VOLUNTEER