Before asking why, let us understand what rain is. Rain is basically a liquid precipitate (water) coming down from the sky. Let us also get a rough idea on some commonly used terms to describe the process that brings about rain.
Vapour is water in gaseous state suspended in the atmosphere. Therefore, evaporation is the process of changing water from its liquid state to a gaseous state.
Condensation is converting gaseous water into liquid water. Remember the word dense = heavy, therefore condense simply means to make something heavy.
Saturation is maximum quantity of water vapour in air. It is affected by temperature. This means that warm air can saturate more water vapour than cold air.
Coalescence is the fusion of tiny water droplets into large ones which are easier to override resistance between clouds and earth surface. The force of collision between individual water droplets is what attracts and pulls these water droplets together.
Now, let’s engage ourselves in a small discussion to understand step by step why it really rains. We know that rain is liquid, but’¦
Where does this liquid come from?
Well, the surrounding air, the one you breathe, is moist. Moisture in the air is invisible to the naked eye just like the oxygen in the atmosphere. Moist air is simply another way of saying water vapour. To get what this means, when you go to take a bath in summer, turn on your hot shower and watch the steam vapourise the water. Watch closely and vigilantly, you will notice tinny tiny water droplets hanging in the air. This is exactly what happens when clouds form in preparation for rain.
How does it form rain?
As the water vapour is formed in the air, it rises up to the sky forming clouds. The higher the sky the colder it gets, therefore less vapour accumulates to form clouds when it is really cold. However, when temperatures are above melting point 00C, enough water vapour is collected. Once it reaches the saturation point, once the warm air cannot accommodate anymore water vapour, the temperatures begin to cool down the extra air. On cooling, condensation begins turning the vaporized water into small water droplets. These droplets are each bound by forces. These forces cause them to collide with each other, therefore forming large water droplets. The other reason for the formation of large droplets is air turbulence which enhances the coalescing of water droplets. The water droplets have to be many, large and heavy for them to fall to the ground. The heaviness in water droplets allows them to escape resistance as they fall to the earth ground.
There are three in which the clouds produce rain after cooling of air in the clouds:
1. Convectional Rain
This one normally happens in summer. When temperatures are really hot, the ground becomes hot too. This warms up the air around it causing it to rise. The moist air rises at about 100 metres above altitude per 10C. And as we said, the higher you go, the cooler it becomes, so the moist air or water vapour continues to rise up as temperatures get colder until a point called condensation point. Once it reaches this point, cumulus clouds begin to form. These are sometimes called cumulonimbus clouds and we see them as grey clouds from below. We use these clouds to predict rain. Cumulus clouds cause heavy rains during summer even thunder and lightning.
2. Frontal Rain
As the warm air rises, it meets the cold air. However, because warm air is lighter than cold air, so it forces its way up above the cold air creating the ‘front’. As warm air continues to rise, so does the temperature. Once it has reached its saturation point, cooling begins and water droplets are formed. The water droplets coalesce forming larger water droplets and we see rainfall. This is typical in UK.
3. Relief Rain
Also known as orographic rain is typically found in mountainous areas. Warm air is forced out of a mountain or a hill or any physical matter up into the air above it. This moist air rises up to the clouds, gets cooled and forms water droplets. However, the water droplets are very few and scanty and thus the rainfall is typically little. Some people call it the ‘rain shadow’. Since the height of the mountains is closer to the clouds, it quickly warms and easily evaporates the water droplets causing the clouds to clear out. That’s why relief rainfall is typically little and lasts a short while.
Now that you know the processes involved in rainfall, let us try this experiment to prove it.
What you need – a Kettle, water, a tray / plate of ice, potholders
Boil the water in your Kettle until you see steam rising. Then hold your plate of ice about 5 -7 inches above the steaming kettle. Use potholders here to avoid getting burnt by the heat. Wait and watch the water drops forming on the side of your ice plate facing the kettle. Wait a little longer for the drops to multiply and get bigger and then watch the fall like rainfall.