July 9, 2016 Water No Comments
How Is Rain Formed

Rain is one of the most important natural

processes for human survival. Without it, our lifestyles would be completely different. All of the water on earth would eventually funnel into the ocean, while the continents would become dry and desolate, much like the Sahara Desert or central Australia. A huge part of our existence is governed by rain. For farmers, it determines when crops are planted and harvested, when holidays are taken, and what sort of produce is grown where. For other people, rain controls different factors in your life, for example: whether you cycle or drive to work, what you wear, where you choose to live, and what you choose to do during your free time. This is all ok, but fairly superficial. Read on to discover how rain is formed and why it is really necessary for human existence.

How is rain formed? Rain is formed during a process which is usually known as the water cycle. The water cycle involves a number of steps, including evaporation, cloud formation, precipitation (rain), relocation, and then evaporation again¹.

Evaporation occurs when liquid water is vaporised into water vapour, allowing it to become a part of the atmosphere. Heat from the sun acts on the water molecules in liquid water, increasing their energy. These particles then ‘break away’ from the liquid, becoming airborne. When there is high humidity it simply means that there is a high concentration of water molecules in the air: a lot of evaporation has occurred².

When the amount of water vapour in the air reaches a level where no more water can evaporate, we say that the air is fully saturated, or that the humidity is at 100%. If the conditions then change so that the air can hold less water vapour (for example, if it gets colder or the barometric pressure decreases), then some of it will begin to condense and form liquid water again. To begin with, only tiny particles of water will be formed which are too light for gravity to pull them back to the ground. If this happens near ground level, fog or mist will be formed. However, if it happens in the upper atmosphere, we will see collections of these water particles as clouds³.

Inside a cloud, the tiny water particles move around, bumping into each other. When they hit each other, they can combine to form a larger droplet. Once this has happened enough times, a droplet of water which is heavy enough to fall to the ground is formed. The form of this fall – known as precipitation – depends on atmospheric conditions. It may be snow, rain, sleet, hail, etc. Once precipitation, or rainfall, has occurred, the liquid water will move around the earth’s surface, usually towards the ocean, until it is evaporated again and the cycle restarts³.

Why is rain needed for the planet?

Rain is essential for life as we know it. Without rain, all of the water on earth would eventually end up in static pools, such as oceans, lakes, and seas. The soil would dry out completely, making any form of agriculture extremely difficult. Forests and grasslands would disappear and would instead become desolate wastelands. Most animals would die out, as water and food sources would disappear.

Rain is the process which allows the transport of water from the ocean to land. The water cycle purifies water and delivers fresh water to land dwelling organisms, most of which rely on it to live. Human and animal cells can be up to 90% water – it is obvious how important fresh water is to the existence of life.

Without rain, it is possible that humans couldn’t exist – we certainly couldn’t live with the same luxury and ease that we do now. The face of the earth would become an unrecognizable wasteland, and a huge percentage of life would become extinct.



¹ http://www.mpimet.mpg.de/en/communication/climate-faq/how-do-clouds-and-rain-form/
² http://nationalgeographic.org/encyclopedia/evaporation/
³ http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi

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