HiSET Science: Water Cycle and Energy Transfers Involved

Hydrologic Cycle
The hydrologic (water) cycle refers to the circulation of water in the Earth's hydrosphere (below the surface, on the surface, and above the surface of the Earth). This continuous process involves five physical actions. Evaporation entails the change of water molecules from a liquid to gaseous state. Liquid water on the Earth's surface (often contained in a large body of water) becomes water vapor and enters the atmosphere when its component molecules gain enough kinetic (heat) energy to escape the liquid form. As the vapor rises, it cools and therefore loses its ability to maintain the gaseous form. It begins to the process of condensation (the return to a liquid or solid state) and forms clouds. When the clouds become sufficiently dense, the water falls back to Earth as precipitation. Water is then either trapped in vegetation (interception) or absorbed into the surface (infiltration). Runoff, caused by gravity, physically moves water downward into oceans or other water bodies.

Evaporation
Evaporation is the change of state in a substance from a liquid to a gaseous form at a temperature below its boiling point (the temperature at which all of the molecules in a liquid are changed to gas through vaporization). Some of the molecules at the surface of a liquid always maintain enough heat energy to escape the cohesive forces exerted on them by neighboring molecules. At higher temperatures, the molecules in a substance move more rapidly, increasing their number with enough energy to break out of the liquid form. The rate of evaporation is higher when more of the surface area of a liquid is exposed (as in a large water body, such as an ocean). The amount of moisture already in the air also affects the rate of evaporation—if there is a significant amount of water vapor in the air around a liquid, some evaporated molecules will return to the liquid. The speed of the evaporation process is also decreased by increased atmospheric pressure.

Condensation
Condensation is the phase change in a substance from a gaseous to liquid form; it is the opposite of evaporation or vaporization.
When temperatures decrease in a gas, such as water vapor, the material's component molecules move more slowly. The decreased motion of the molecules enables intermolecular cohesive forces to pull the molecules closer together and, in water, establish hydrogen bonds. Condensation can also be caused by an increase in the pressure exerted on a gas, which results in a decrease in the substance's volume (it reduces the distance between particles). In the hydrologic cycle, this process is initiated when warm air containing water vapor rises and then cools. This occurs due to convection in the air, meteorological fronts, or lifting over high land formations.

Precipitation
Precipitation is water that falls back to Earth's surface from the atmosphere. This water may be in the form of rain, which is water in the liquid form. Raindrops are formed in clouds due to the process of condensation. When the drops become too heavy to remain in the cloud (due to a decrease in their kinetic energy), gravity causes them to fall down toward Earth's surface. Extremely small raindrops are called drizzle. If the temperature of a layer of air through which rain passes on its way down is below the freezing point, the rain may take the form of sleet (partially frozen water). Precipitation may also fall in the form of snow, or water molecules sublimated into ice crystals. When clumps of snowflakes melt and refreeze, hail is formed. Hail may also be formed when liquid water accumulates on the surface of a snowflake and subsequently freezes.

Transportation of Water in the Water Cycle
In the hydrologic cycle, the principal movement of water in the atmosphere is its transport from the area above an ocean to an area over land. If this transport did not occur, the hydrologic cycle would be less a cycle than the vertical motion of water from the oceans to the atmosphere and back again. Some evaporated water is transported in the form of clouds consisting of condensed water droplets and small ice crystals.
The clouds are moved by the jet stream (strong winds in the upper levels of the atmosphere that are related to surface temperatures) or by surface winds (land or sea breezes). Most of the water that moves through the atmosphere is water vapor (water in the gaseous form).