Precipitation is water vapor that has condensed from clouds to fall as liquid (rain) or solids (snow, hail). Precipitation is the part of the water cycle that delivers water from the atmosphere to the Earth’s surface. When and where precipitation falls is determined by the climate system especially by the patterns of atmospheric and ocean circulation, and how much water returns in the atmosphere.
Various human activities and environmental phenomena influence precipitation patterns, including:
- The burning of fossil fuels, agricultural activities, and deforestation, which increase the concentration greenhouses gases in the atmosphere, and thus the Earth’s average temperature. Increases in atmospheric and sea surface temperatures alter atmospheric circulation patterns, and thus how water cycles through the Earth system. These changes can also increase the intensity and duration of extreme weather events, such as hurricanes and droughts.
- Trees return a significant proportion of rainfall to the atmosphere via a process called transpiration. Thus, deforestation decreases the amount of water cycling through the biosphere and atmosphere, while reforestation increase the amount of water cycling.
- Over millions of years mountain building alters local wind and precipitation For example, some of the wettest places on Earth are where warm moist air cools as it is forced to rise over the mountains, causing the water vapor to condense as clouds and be released as rain or snow. Once the air is past the peaks of the mountain range, precipitation deceases. This is because the air has depleted much of its water vapor and the descending air warms and can carry more water vapor. Thus, the downwind side of a mountain range is typically much drier than the upwind side, and is said to be in a rain shadow.
- Over millions of years, changes in the distribution of continents and oceans can alter where and how much sunlight is absorbed or reflected by the surface of the Earth. This differential heating influences the direction and strength of wind and precipitation patterns. The distribution of continents and oceans also directly affects the pattern of and atmospheric and especially oceanic circulation, and thus where, when, and how much precipitation occurs.
Precipitation affects various Earth system processes and phenomena, including:
- Regional temperature and humidity.
- Terrestrial ecosystem productivity and biomass, species ranges, and population sizes because terrestrial organisms require a source of freshwater to grow and survive.
- Life cycles and traits of organisms. Temperature and precipitation patterns are used by some species as environmental cues to signal the transition between different stages in their life cycles.
- The distribution and concentration of pollutants and airborne particles, which influence air quality. Precipitation can remove or dissolve particles in the atmosphere.
- The amount of water run-off and erosion of sediments from land into the ocean, rivers, and lakes.
- The frequency and size of fires. Regions that experience dry seasons are often susceptible to fires. In many of these regions climate change due to global warming is making their dry seasons longer.
Can you think of additional cause and effect relationships between precipitation and other parts of the Earth system?