What Causes Rain to Form?


Rain is a very common atmospheric and hydro meteorological phenomenon on our planet. It consists of the fall of liquid water particles in the form of scattered drops. Rain is the product of the condensation and cooling of water vapor in the Earth’s upper atmosphere. There has been an increase in the number of heavy rainfalls over many areas during the past century. Since the 1970s the prevalence of droughts, especially in the tropics and subtropics also increased.

It is sometimes referred to as precipitation, but this is really a broader category. Rain occurs daily in different regions of the planet, in some more frequently than in others, both on land and sea.

The occurrence of rain depends on three specific factors:

  1. Temperature
  2. Atmospheric pressure
  3. Humidity

What we refer to as rain is nothing more than an instant in the hydrological cycle (water cycle). It is the normal water recycling system on Earth. Due to solar radiation, water evaporates from water bodies such as the sea, lakes and rivers. Water vapor enters the atmosphere where it cools and condenses into clouds. Once the cloud becomes saturated with water vapor that it cannot hold any more water, liquid water is released. Then the cycle repeats itself.

Origin of Water on Earth

Rainstorms are as old as the massive presence of water on the Earth. In other words, it is estimated that it originated following the formation of our planet, about 4.5 billion years ago. There are two main theories about the origin of water on Earth:

  1. Volcanic formation: according to this theory, water originated in the center of the Earth. It is a product of intense reactions between oxygen and hydrogen. Consequently, water was expelled in the form of vapor into the atmosphere by intense volcanic activity that occurred on the planet in its beginnings.
  2. Extraterrestrial causes: This theory proposes that water reached the Earth in numerous meteorites or icy comets abundant with ice. Thus, upon impacting our then volcanic planet, the high temperatures melted it and oceans were formed.

How Do Raindrops Form?

Liquid drops are formed in different types of clouds, such as cumulonimbus and nimbostratus clouds, which receive their moisture from the atmosphere. When they rise to cooler regions, their vapor condenses and breaks down, due to the increase in mass.

Rainfall occurs when either one of the following takes place:

Convectional rainfall: The hot air in contact with the earth’s surface is heated by the Sun and rises, cooling down and producing rain.

Orographic rainfall: This occurs when a mass of humid air collides with a mountainous relief. It ascends its slope and discharges its humidity, until it descends dry on the other side of the elevation.

Frontal rainfall: The collision of two masses of humid air with different temperatures (hot and cold) produces this type of rainfall. Storms and hurricanes are considered frontal rainfalls.

How Is Rain Measured?

Rain is measured in millimeters (mm) of water. The value in mm is referring to the amount of rain per square meter in one hour.

The device used to measure rain is the rain gauge. It measures the amount of water that accumulates on a flat and impermeable surface during the duration of the rainfall.

There are various types of rain gauges, widely used by climatologists or geographers in the study of climatic phenomena.

What Does Rainwater Contain?

Raindrops have a diameter of approximately 0.5 mm.

They are spherical (no, they do not have the classic “teardrop” shape) or hemispherical particles of water. Ideally, there is no significant presence of other chemical elements that are present in the atmosphere.

If the opposite occurs, dusty or muddy rains are produced. These raindrops contain ash or dust. Drops of acidic rains contain molecules of some oxide reacting with the water to make its pH more acidic. If there are unstable chemical elements in it, the rain may even be radioactive.

Rain Distribution

Every year about 505,000 km3 (121,000 mi3) of water falls in the form of rain around the world. About 398,000 km3 of rainfall water falls into the oceans, and 107,000 on the land surface.

The region with the highest annual rainfall is the Intertropical Convergence Zone, close to the Equator. It is where the most intense rainfalls take place.

As you move over the temperate zone, the rainfall reaches more stable averages, depending on the seasons or taking into account specific weather conditions.

In the polar regions, also called the frigid zones of the Earth, rains are practically non-existent. The reason for this is the low temperature of the region’s dry air.

Types of Rain

According to their intensity, rainfalls can be classified into 5 types:

  1. Drizzle – A very weak rain of fine droplets often sprayed in the air, the accumulation of which is almost negligible.
  2. Shower – Rains of medium to heavy intensity, usually of short duration and accompanied by wind. They usually fall from individual clouds. It is possible to observe them in the sky. Between showers there are gaps of drier and brighter weather in between.
  3. Downpour – It is a heavy rainfall. A Torrential rain, of medium duration, that leads to a large accumulation of water on the surface.
  4. Thunderstorm – Heavy rains, accompanied by intense winds and electrical activity in the atmosphere, due to the static electricity generated by charged clouds. It can be accompanied by hail, snow, lightning and commonly thunder that announce them even before they occur.
  5. Hurricane or waterspout – This is an extreme storm. During such storms two air masses of different temperatures (hot and cold) meet and rotate around each other. This phenomenon produces a zone of low pressure that feeds the cycle. Its enormous precipitation margins are accompanied by very intense winds.

How Does Rain Affect the Planet?

Rain has important and far-reaching consequences for our planet, among which are:

  • The irrigation of water in the different regions of the earth’s surface, allowing the maintenance of plant life and rivers, lakes and lagoons.
  • Balance of environmental temperatures throughout the world, as well as salinity and temperature levels in ocean water.
  • Surface runoff on the earth’s surface is able to displace solid matter.
  • In cases of excess rainfall, it causes floods when the soil is saturated and no longer able to absorb water.
  • Torrential rains often cause damage to urban infrastructure, floods, river flooding and loss of life.

Why Is Rain Important?

Rain can be a beneficial as well as a destructive factor, but it is without a doubt crucial in preserving the planet’s climate balance.

Due to rainfall, water circulates through various places, keeping them humid enough to houseplant life. Rainfalls also lead to accumulations of water that become rivers, lakes, etc. Without rainfall, life on Earth would be impossible.

The rain also allows the cooling and humidification of the air in different regions of the globe, keeping the temperature relatively stable. This effect helps the planet to combat global warming.

For human beings, it is a source of free, regular and relatively clean water, usable for various purposes, especially agricultural ones. Finally, rainfalls clean the air. They bring freshness and clarity to the atmosphere, and maintain a stable level of salinity in the oceans.

Rainy Season

The rainy season, or the wet season is the time of year when the heaviest rainfall in a region usually occurs.

They generally last one or several months, and are distributed throughout the tropics and subtropics, where they produce recurrent rain belts.

Other Forms of Precipitation

There are other forms of precipitation that do not involve the fall of liquid water, such as:

  • Snowfall – If pressure and temperature conditions are correct, the water that falls from the atmosphere will be in a semi-solid state known as snow.
  • Hailstorms – In case the atmospheric pressure is particularly low, the water in the atmosphere can freeze instead of condensing. This results in the fall of small pieces of solid ice known as hail.

Sleet – Known as granulated ice, sleet is rain that freezes before it reaches the ground.


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