The Hydrologic Cycle
The Water Cycle is a process which consists of evaporation, transpiration, condensation, and precipitation. The whole process is controlled by the sun, which produces kinetic energy or heat energy. Evaporation occurs when the kinetic energy of the water molecules increases, causing the molecules to move more quickly, and undergo a liquid-to-gas phase change. Evaporation is stimulated by heat or sunlight. Transpiration is evaporation of water through the leaves of plants. The next part of the cycle is condensation and this is when gas molecules slow down, release energy, and turn into water molecules. Precipitation is one form in which the condensed water molecules return to Earth, in the form of rain, snow, sleet, hail, fog, or dew. Most precipitation falls in either coastal areas or in high elevations. Some of the water which falls in high elevations becomes runoff water, the water which runs over the ground to lower elevations to form rivers, lakes, and fertile valleys. Sometimes this water collects nutrients from the soil it goes over making valleys fertile for plant growth.
This image of the hydrologic cycle was taken from: http://www.geocities.com/capecanaveral/lab/8375/cycle.html
Evaporation is the phase change of liquid water into a vapor (gas). Evaporation is an important means of transferring energy between the surface and the air above. The energy used to evaporate water is called "latent energy". Latent energy is "locked up" in the water molecule when water undergoes the phase change from a liquid to a gas. Eighty-eight percent of all water entering the atmosphere originates from the ocean between 60 degrees north and 60 degrees south latitude. Most of the water evaporated from the ocean returns directly back to the ocean. Some water is transported over land before it is precipitated out. The water vapor may condense back into a liquid and in so doing, releases latent heat which is converted into sensible heat that warms the surrounding air. This warming fuels the uplift of air to help promote adiabatic cooling and further condensation. As droplets of water coalesce into larger droplets they attain a size big enough to fall towards the earth's surface as precipitation. Located high in the troposphere they possess a high degree of potential energy that is converted into kinetic energy once they begin to fall toward the surface. Impacting the surface they convert this kinetic energy into work done on the surface (erosion for example). Rather than condensing and precipitating directly back into the ocean, water vapor may be transported to some other location where it condenses and precipitates out (http://www.uwsp.edu/geo/faculty/ritter/geog101/lectures/lecture_atmospheric_moisture.html)
