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Water cycling on Earth

Introduction to water

Water is the only substance that exist naturally on Earth in all three physical states of the matter - gas (vapor), liquid, and solid (ice). Water in all these three states makes a large contribution to the planet's climate. Clouds in the atmosphere and ice sheets on the surface reflect a good deal of the solar radiation preventing it from warming the planet. On the other hand, where the earth's surface has been heated, clouds help trap energy radiated from the heated surface and thus have a warming effect as well. In fact, water vapor (a powerful greenhouse gas) is responsible for more than half the Earth's greenhouse gas warming.

Most plants and animals contain more than 60% water by volume. Water has a number of unique chemical and physical properties that make it essential for life. Without water, life would not have evolved on Earth.

Water molecule consists of two hydrogen atoms bonded to one oxygen atom – H2O (H-O-H). The bond between each hydrogen atom and the oxygen atom results from a pair of electrons shared between the two atoms. Because the oxygen atom has a greater affinity for electrons than does the hydrogen atom the electrons in the O–H bond are more attracted to oxygen. The unequal sharing of negatively charged electrons results in oxygen acquiring a partial negative charge (−) and hydrogen a partial positive charge (+). The H–O–H bond angle in water is 104.5°, which means that the molecule has a bent shape. This geometry and the accumulation of electrons on the oxygen side of the molecule cause the water molecule to have a negative charge on one side, the oxygen side, and a positive charge on the other side, the hydrogen side. Such molecules are called polar molecules.

Polar molecules are attracted to each other. The attraction results from the negative region of one molecule, the oxygen atom, being drawn to the positive region of another molecule, the hydrogen atom. Oxygen atoms have a great affinity for electrons, and so the tiny hydrogen atoms bonded to an oxygen atom acquire a significant concentrated positive charge. Because of the significant polarization of water molecules the attraction between them gets particularly strong. It is so strong that it has been given a particular name: hydrogen bonding. The energy associated with hydrogen bonds in water is about 20 kJ·mol–1, which is about 1/10 the strength of a typical shared-electron bond within a molecule. Namely hydrogen bonding between water molecules gives water its unique properties.

View of the Earth as seen by the Apollo 17 crew traveling toward the moon.

View of the Earth as seen by the Apollo 17 crew traveling toward the moon.

Hydrological cycle

Hydrological cycle is the cycle in which water evaporates from the oceans and the land surface, is carried over the Earth in atmospheric circulation as water vapor, condenses to form clouds, precipitates over ocean and land as rain or snow, which on land can be intercepted by trees and vegetation, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams, and ultimately, flows out into the oceans, from which it will eventually evaporate again. The various systems involved in the hydrological cycle are usually referred to as hydrological systems.

Distribution of water around the globe

The hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet.

Sometimes, definition of hydrosphere includes only total mass of liquid water.

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Hydrologic cycle

"The water cycle is dynamic; it describes the continuous movement of water on, above and below the surface of the Earth and the transitions from one state to another. Sea surface temperature, surface winds, and air temperature influence the rate of evaporation at the ocean surface. In the tropics, warm ocean surface temperatures support high rates of evaporation. Wind also increases evaporation. When the air's temperature is warmer, it can hold more water. While the atmosphere does not store a large quantity of water compared to the ocean, rivers and lakes, it can transport water quickly from one place to another. Low-lying regions of the atmosphere with high moisture and strong winds, can form "atmospheric rivers" to transport water horizontally.
Clouds are formed as water vapor cools and condenses into droplets and ice crystals. Clouds and water vapor act as insulators in the atmosphere. Clouds help shield the Earth from the Sun and trap heat from below. When cloud particles grow large enough, they may fall out as rain or snow. Under the right conditions, areas of precipitation can grow into large storms. As storms grow, they transfer heat vertically into the upper atmosphere. The migration of storms helps to distribute heat between the equator and poles - shaping wind patterns globally. How storms grow and intensify depends upon atmospheric moisture, surface temperatures and wind patterns. Precipitation is concentrated in some parts of the world and scarce in others. It can vary substantially from season-to-season and from year-to-year. Water that falls on the land surface as precipitation is stored within snow packs, lakes, reservoirs, soils and underground aquifers. "

(NASA)

Water cycle

Water cycle on Earth

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References

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Climate change

Variations of this design on various useful items including T-shirts are vailable at at GeoChemBio shop

Climate change mug climate change shirt

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