Water Cycle

Precipitation

• After condensation occurs (forming clouds), water droplets fall in various forms of precipitation – rain, snow, freezing rain, sleet, or hail, depending upon weather conditions.

• Temperature variations within clouds and/or within the region between the cloud and Earth allows for the various forms of precipitation.

• Gravity is the driving force for precipitation. 

Evaporation/Transpiration

• Water enters the atmosphere as water vapor through the processes of evaporation and transpiration (i.e. plants releasing water vapor).

• Thermal energy from the sun causes water to evaporate and/or transpire. 

Condensation/Crystallization

• As water vapor rises in the atmosphere it can turn into water droplets or ice crystals which results in cloud formation.

• Condensation of water vapor into water droplets or ice crystals must occur on a surface.

* In the atmosphere, dust particles (or any other particulates) serve as a surface for water to condense on.

• These processes can form on Earth’s surface in the following forms:

* Dew forms when water vapor condenses directly onto a surface;

* Frost forms when water vapor changes from gas directly to ice crystals on a surface when the temperature at which condensing would take place are at the freezing point or below.

• Increases in thermal temperature causes water vapor to rise (evaporation) while a decrease in thermal temperature triggers condensation (clouds, dew, and frost).

Downhill Flow of Water on Land

• If precipitation falls on land surfaces, it always attempts to move back toward sea level as surface-water flow or groundwater flow.

• The surface that receives the precipitation determines its flow back towards sea level.

Examples are:

* Water will remain on the surface when the surface is not porous or the precipitation is falling too fast for the water to sink into the ground.

* Water will sink into the ground when the surface is porous and there is space in the soil to hold the water.

• Gravity is the driving force for downhill flow of water on land.