Rain

We explain what rain is, how it originates and how it is measured. Also, what are its characteristics, environmental risks and benefits.

What is rain?

Rain is a very common atmospheric and hydrometeorological phenomenon on our planet. It consists of the fall of liquid particles water in the form of dispersed dropsa product of the condensation and cooling of water vapor in the atmosphere.

Rain occurs daily in different regions of the planet, in some more frequently than in others, both over the sea and on land. The generation of rain depends on three specific factors: temperature, atmospheric pressure and atmospheric humidity.

Rain is a component of hydrological cycle or water cycle. In this cycle, water deposits in oceans, lakes, rivers and on the earth’s surface evaporate due to the action of the sun and are transformed into water vapor in the atmosphere. When this water vapor cools, it condenses, returns to the liquid state and precipitates to the surface in the form of rain.

How can rain occur?

There are three ways in which rain can occur:

• Convective rains. They happen when the sun heats bodies of water and evaporates them. Water vapor rises into the atmosphere, cools and condenses. This produces water droplets that, due to the force of gravity, precipitate from the atmosphere to the surface in the form of rain. They occur mainly in equatorial areas where the sun hits with greater intensity and has greater evaporation power.
• Orographic rains. They happen when humid air rises along the walls of a mountain. As its altitude rises, the air cools, condenses and precipitates in the form of rain. For this reason, the side of the mountain that receives humid air from the front usually has a rainy climate, while on the other side the air arrives without humidity and produces a much drier climate. These rains occur only in mountainous areas.
• Cyclonic or frontal rains. They are generated by the collision of two air masses with different temperatures. They can be cold or warm front rains and occur when cold air from one of the air masses condenses the humidity in the atmosphere and produces precipitation. Cold front rains produce short-duration storms, a drop in temperature and a lot of water fall in a short time. Those with a warm front produce drizzles that can last several days and an increase in temperature.

How is rain measured?

Rain is measured in millimeters of water fallen. The device used to measure rain is the rain gauge, which is a container that stores the water that falls during a rain and has a measuring ruler that allows you to know how many millimeters it rained.

Rain gauges are installed in open places to avoid obstruction by objects such as signs, trees or buildings. Meteorological services in charge of measuring precipitation usually install them in airports, racetracks, stadiums and other outdoor spaces.

Once the rain is over, the data is collected from the rain gauges and you can find out how much it rained that day in that location. With this information, monthly and annual rainfall data for each location is constructed.

What does rainwater contain?

The content of rainwater may vary depending on geographic location, climatic conditions and other environmental factors, but in general, it may contain the following components:

• pure water. Rainwater is primarily pure water (H₂O), although it may contain small amounts of other chemical elements and dissolved compounds.
• atmospheric gases. Rainwater can contain dissolved atmospheric gases, such as oxygen, nitrogen, carbon dioxide, and nitrogen oxides.
• suspended particles. Rainwater can also contain suspended particles such as dust, volcanic ash, pollen and microorganisms.
• Pollutants. Rainwater in urban and industrial areas can contain contaminants such as heavy metals, volatile organic compounds, and toxic chemicals.

Distribution of rain in the world

At a planetary level, the region with the highest annual rainfall is the Intertropical Convergence Zone, close to the equator. It is a warm and rainy area located between the Tropics of Cancer in the northern hemisphere and Capricorn in the southern hemisphere.

In temperate climate regions, rainfall is highly variable. There can be very dry regions, such as the Sahara Desert, and very rainy areas, such as the Madagascar jungle. The distribution of rainfall in these latitudes is conditioned by the availability of humidity, the circulation of the winds and the layout of the mountainous relief..

In the polar or subpolar regions, rain is infrequent due to the low temperatures of the dry air at the poles.

Types of rain

According to their intensity, rains can be classified as follows:

• Drizzle or drizzling rain. It is a very weak rain, of fine drops, whose accumulation is almost negligible.
• Shower or downpour. It is a rain of medium to heavy intensity, generally of short duration and accompanied by wind.
• Downpour. It is a torrential rain, of medium duration, that accumulates large amounts of water on the surface.
• thunderstorm. It is a rain of strong intensity, accompanied by intense winds and electrical activity. It can be accompanied by hail, lightning and thunder that announce them even before they occur.

What environmental services does rain provide?

Rain provides important environmental services for the planet. Some of them are:

• It provides fresh water that feeds rivers, aquifers, lakes and lagoons, and provides drinking water to human populations.
• It maintains the biodiversity of terrestrial and aquatic ecosystems, which helps sustain the life cycles of plants and animals. In addition, it is essential for food production.
• It allows the balance of atmospheric temperatures as well as salinity and temperature levels in ocean water.
• It transports nutrients and sediment that help maintain water quality and control erosion.

Problems linked to water

Water is one of the most important natural resources on the planet, and its availability and quality can be affected by various environmental problems, such as:

• Water pollution. Water pollution can be caused by discharges of industrial waste, agricultural pesticides and fertilizers, human and animal waste. This can affect its quality and put human health and biodiversity at risk.
• Water shortage. Water scarcity is a serious problem that affects many regions of the world, especially in arid and semi-arid areas. The overexploitation of water resources, urbanization and climate change are some of the factors that contribute to deepening this problem.
• The floods. Floods are a problem that affects an increasing number of people. The increase in rainfall as a consequence of climate change, the waterproofing of soils as a consequence of human activities and the expansion of cities and the lack of urban planning are some of the causes that contribute to deepening this problem.

Other forms of precipitation

In addition to falling liquid water, there are other forms of precipitation:

• snowfall. If the temperature of the atmosphere is low enough, water falls into a semisolid state. This type of rain is called “snow.”
• The hailstorms. If raindrops form at a high altitude, their temperature is very low and they freeze. If they do not get hot enough to melt as they fall to the surface, they reach the ground in the form of small pieces of ice known as “hail.”

Origin of rain

There are two theories regarding how water appeared on planet Earth:

• The theory of volcanic origin. According to this theory, water originated inside the Earth and was later expelled into the atmosphere by volcanic activity.
• The theory of extraterrestrial origin. This theory proposes that water came to Earth in ice-rich meteorites and icy comets. Upon reaching Earth, these ices could have melted and given rise to liquid water on the planet.

Part of the scientific community specializing in these topics maintains that it is possible that the origin of water on Earth has been mixed, and that it has appeared on the planet as a combination of the two theories.

References

• NASA Climate Change (sf). The water cycle. Pot
• Goudie, A. (1993). The nature of the environment. Chap. 2. Operation of global climatology. Progress in Physical Geography.
• González, A (sf). Types of rain. MagicNature