The Water Cycle

Water on Earth is always in circulation and is recycled over and over again. This total process is what is known as the “water cycle”. The cycle begins with the Sun’s heat that provides energy to evaporate water from Earth’s surface. Thereafter the winds lift the water vapor (mostly from the ocean) into the atmosphere. The water vapor then condenses to form clouds and under the right conditions clouds release water as rain or snow. Most of the rain falls into oceans and about 20 percent falls onto land. Rivers and streams collect water from the ground and return it to the ocean so the whole cycle can start all over again.

However, the water cycle is not completely closed. When a volume of water evaporates, a considerable part of the water vapour drops back as precipitation while the rest rises into the upper troposphere. Almost all water vapor reaching this region is condensed into ice particles (cirrus clouds) due to the very low temperatures of the tropopause region. Only 1 water molecule over 10.000 reaches the stratosphere. The exchange of water vapour across the tropopause is a hot topic for current research.

The abundance of water vapor is twice larger at the top of the stratosphere (stratopause) than at the bottom of the stratosphere (tropopause). The additional water vapor implies a local (stratospheric) source. This source is the oxidation of methane, i.e. a chain of photochemical reactions transforms each methane molecule (CH₄) into two water molecules (2H₂O) as air rises into the stratosphere.

The water vapour molecules in the stratosphere are sometimes broken apart by ultraviolet sunrays hitting them (at wavelengths lower than 200 nm). The term “photodissociation” is used for such events. In the mesosphere the UV radiation is much more intense (there is less air that absorbs it), hence all water vapor is lost by photodissociation in this part of the atmposphere.

The above process releases hydrogen atoms and because of their lower density they are able to ascend beyond the thermosphere. Given the very high temperatures at these altitudes, the hydrogen atoms gain enough energy to escape the terrestrial atmosphere forever: for every pair of hydrogen atoms that disappears in this way means the loss of a water molecule.

Earth loses water at a rate of 1.700 tons per day in this way! However, there is no reason for us to worry: the Earth system contains 1.4 billion cubic kilometers of water (1.4x1018 tons). So at this rate, it would take more than 2000 billion years before our planet gets totally dehydrated. However, this is irrelevant as the Solar System will have disappeared by then.

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