Water is the source of all life on earth. The distribution of water is not the same in all locations, some places have plenty of water while others have very little. Water exists on earth as a solid, liquid or gas. Oceans, rivers, clouds, and rain, all of which contain water, are in a frequent state of change. However, the total amount of the earth's water does not change. The circulation and conservation of earth's water is called the "hydrologic cycle". .
Water covers 70% of the earth's surface the oceans contain 97.5% of the earth's water, land 2.4%, and the atmosphere holds less than .001%. The annual precipitation for the earth is more than 30 times the atmosphere's total capacity to hold water. This fact indicates the rapid recycling of water that occurs between the earth's surface and the atmosphere. Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. .
Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world. Most evaporated water exists as a gas outside of clouds and evaporation is more intense in the presence of warmer temperatures. .
Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold water vapor. The excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced by convection in unstable air. .
In meteorology, convection refers primarily to atmospheric motions in the vertical direction. As the earth is heated by the sun, different surfaces absorb different amounts of energy and convection may occur where the surface heats up very rapidly. As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise.
