The Journey of a Raindrop

The volume of water on the Earth is estimated to be 1386 million cubic kilometers. This is a huge amount, but at the same time, very little. There is more water on our planet than anywhere else in the Universe – as far as we can tell – but it represents only about 0.025 percent of the Earth’s mass. Fresh water accounts for only 2.7 percent of the total volume of this precious liquid. It not only serves to quench the thirst of living beings, but also, more importantly, to transmit the heat of the sun across the globe. In the same way that a wheel transmits a force, the water cycle transports solar energy from the tropical regions to the poles, from the sea to the land, and back again.

1. Evaporation

In the heat of the Sun, the surface of the oceans evaporates. Every day, 4.30,000 cubic kilometers of water thus pass from a liquid to a vapourised state and join the lower layers of the atmosphere. In the course of this transformation, a considerable part of the Sun’s energy, stored in the water molecules, is used up, which causes a slight local cooling of the atmosphere. The atmosphere in fact draws almost 50% of the energy that animates it (the winds) from the cycle of evaporation and condensation. Evaporation is greatest in the tropical regions where the Sun is strongest, on the continents, which are always hotter than the seas and at the poles, which are very dry. Added to this is evaporation from the lakes and rivers, and the transpiration and respiration of living organisms, making in all about 70,000 cubic kilometers of water. Once evaporated water is less dense than the surrounding air and is carried upwards by the winds.


As it rises, the pressure of the hot air loaded with water vapour decreases. This is accompanied by cooling, which may bring about condensation. The water vapour then reappears in the form of spherical droplets of between 0.008 and 0.8 millimetres in diameter. As they gather together, they form clouds, which do not contain a high proportion of condensed water – never more than 10 grams per cubic metre of air. Condensation frees all the energy from the Sun accumulated by the water vapour when it was formed. The slight warming of the local atmosphere that results causes very strong upward currents to form. As the atmosphere is also colder where the water vapour evaporates, convection currents appear between the evaporation zones (subtropical and polar regions) and condensation zones (the equator and medium latitudes).


The droplets in the clouds grow larger, join together and sometimes reach a size of 0.1 millimetre. This gives them a falling speed of about one metre a second. The rising currents then cannot support them any longer. Depending on the air temperature, they fall as rain, hail or snow. In a year, 1,10,000 cubic kilometers of water precipitates on continents, essentially on the regions round the equator or at medium latitudes (between approximately 35º and 55º). Thus, while China receives seven percent of the total annual precipitation that waters the planet, the Amazon gets twice as much. If the air temperature is too high just above the land, the precipitated water will not reach the ground.


Water precipitated above land first falls on vegetation – from which it might evaporate again. The rest pours onto the ground. Part of it filters down to the hydrographic network underground – rivers, phreatic water and springs. Some gathers in depressions and forms lakes and ponds. Yet more wets the soil, and the remainder returns to the sea by swelling watercourses. It may also accumulate in the form of ice (icebergs and permanent glaciers), which forms a volume of 2,90,00,000 cubic kilometers, making the largest freshwater reserve in the world, far more than underground water (82,00,000 cubic kilometers), soil humidity (70,000 cubic kilometers) and water courses (1,700 cubic kilometers).


The last and smallest reserve of water on Earth is found inside living things: 1,100 cubic kilometer, almost as much as the total volume of the water-courses. Water accounts for more than 95 percent of the total mass of a jellyfish and for almost 70 percent of that of a human being. Cells store 70 percent of this water, the rest is spread between the cells and in the various body fluids. A person must absorb at least 2.5 litres of liquid to compensate for the 2.5 litres of water he or she loses every day. It is little compared with cotton, which has to drink 10,000 litres of water to grow one kilogram.


Flowing over agricultural land, water collects fertilizers (based on nitrates and phosphates) and also very toxic pesticides, herbicides and fungicides. Watercourses are contaminated by wastes from industries and towns, which includes heavy metals, solvents and various organic substances like dioxin. All these molecules eventually affect after accumulating for decades in the food chain. More seriously still, they are concentrated in the phreatic water from which most of our drinking water comes. Underground water is only renewed very slowly, every 1,400 years on average, whereas it takes only a few weeks for the volume of water in a river to be replaced. The water ends up in the sea, where all sorts of pollution are concentrated. In spite of everything, evaporation plays the part of a gigantic purifying station, by ridding water of almost all its dirt. It is only above towns that water vapour becomes polluted once more.

Acknowledgement: Extract from ‘The Amazing Secrets of Nature” published by The Reader’s Digest Association, Pages 149,156-157.