Precipitation

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Precipitation

Introduction

Falling rain or snow, known as precipitation, is an essential part of the hydrological cycle. It replenishes water supplies for plants, agriculture, and other human uses. Precipitation levels vary widely from place to place and help to shape local weather and overall climate. There are some very dry places on Earth, such as Antarctica, and also locations such as rain forests where there may be 100 in (250 cm) or more of rain every year. Drought, which is a reduction in the usual level of precipitation, can spell disaster by causing crops to fail.

Precipitation is pure compared to seawater, which contains mineral salts. However, it may react with certain gases (such as sulfur dioxide, or SO₂, which is emitted from power stations). These reactions can form acid precipitation or acid rain, which can fall far from the source of pollution. This kind of acidic pollution can damage buildings, lakes and rivers, and trees.

Historical Background and Scientific Foundations

In the hydrological cycle, also known as the water cycle, liquid water evaporates to form water vapor in the atmosphere. The liquid water comes from bodies of water such as oceans and lakes, as well as from respiration and plant transpiration. Evaporation is the escape of the more energetic water molecules from the surface of water, leaving salts and other materials behind. Therefore, water vapor is purer than the water that is its source.

The amount of water vapor held in the air is known as humidity and it increases with temperature. A useful measure is relative humidity, which is humidity compared to the maximum amount of water vapor the air could hold at that temperature. Saturation point occurs when the air is holding as much water vapor as possible at a given temperature. Beyond this it will undergo condensation, where vapor turns into liquid or solid droplets of water. Condensation occurs around tiny, often invisible, particles of dust, ash, spores, or smoke called nuclei. Then the droplets accumulate, forming clouds. Air currents can keep cloud particles suspended in the atmosphere when they are small. However, they tend to increase in size and at some point gravity forces them to fall to Earth as precipitation. Many clouds do not form precipitation because their droplets are too small and continue to be supported in the atmosphere by air currents.

There are various kinds of precipitation. Rain refers to liquid droplets of a diameter between 0.02 and 0.2 in (0.05 and 0.5 cm). Smaller droplets are known as drizzle. Sleet is transparent pellets of frozen water that start off as rain but freeze as they fall through colder air. Snow forms when water vapor condenses as hexagonal solid particles, while hail is ice pellets more than 0.2 in (0.5 cm) in diameter.

Levels of precipitation vary widely around the world. Global air circulation patterns are very influential in shaping the amount of precipitation occurring in a region. Generally, where there is high pressure, there is low precipitation. This means that there tends to be low precipitation between latitude 20° and 40° North and South, but high precipitation between latitude 40° and 60° North and South, as well as near the equator. There is also more precipitation near the oceans and on the windward side of mountains, where air pressure is lower. Conversely, there tends to be less precipitation on the other, leeward, side of a mountain where air pressure rises.

Impacts and Issues

Precipitation is essential for replenishing water resources on land. But it can be damaging. Excessive rainfall can cause flooding that destroys crops and buildings in addition to putting lives at risk. Persistent rainfall can cause soil erosion and hail can devastate crops.

Sulfur (S) emissions from power stations running on coal can react with water vapor to create a dilute form of sulfuric acid known as acid precipitation or acid rain. This form of pollution was discovered by the Scottish chemist Robert Angus Smith in the mid-nineteenth century. Acid rain caused rapid forest decline at high elevations in many parts of Europe and North America in the 1980s. It also caused many lakes in Sweden to become so acidic that many fish species could no longer live there. Buildings and statues in cities around the world, from India’s Taj Mahal to Michaelangelo’s statue of David in Florence, are also under threat from acid rain. However, legislation against air pollution in recent years has started to limit the damage caused by this form of precipitation.