Water Cycle Changes: Precipitation and Drought

The Water Cycle

evapotranspiration The water or hydrologic cycle is the constant exchange of water in its various forms of liquid, solid (ice and snow), and gas (water vapor) between Earth's surface, the oceans, and the atmosphere. The hydrologic cycle constantly renews Earth's supply of freshwater.

The sun provides energy to drive the water cycle. As the air and Earth’s surface warm, liquid water evaporates from all the bodies of water on Earth. Water also evaporates from plants’ leaves. Water is transported from plants’ roots to the leaves and into the atmosphere. This is called transpiration.

Water vapor then rises with the less dense warmer air. As a rising mass of air containing water vapor moves farther away from Earth's surface, it is cooled. Cool air cannot hold as much water vapor as warm air. The temperature at which air cannot hold anymore water vapor or moisture is called the dew point. At this temperature, water vapor condenses into droplets of water. Clouds form as water vapor condenses on small particles (cloud condensation nuclei) in the atmosphere.

Precipitation falls toward Earth when the water droplets become too numerous and heavy to stay in the air. Depending on the temperature of the air near Earth's surface, the precipitation can be in the form of rain, snow, ice, or a combination.

When precipitation reaches Earth, it either evaporates or flows over the surface (known as runoff) where it may accumulate in ponds or lakes or eventually reach streams, rivers, and the ocean. Plants require water and absorb it. Water may also be absorbed into the ground and become groundwater or remain frozen on the surface as snow or ice.

Water Cycle
Image Source: Wikipedia