How Is Rain Formed? — Definition, Types, And More

Have you ever wondered how does rain form? This phenomenon is one stage of the water cycle, condensation. Water vapor condensation forms a cloud. The wind carries the cloud around in the sky until the water drops become too heavy for suspension. Water drops fall to the ground as soon as it gets too heavy for suspension. 

Here, we’ll discuss the various processes involved in rain formation. You will read about the three forms of rainfall and its effect on the ecosystem. 

Related Read: For more meteorological tidbits, head over to our articles about Mammatus clouds and yellow skies.

How is rain formed? 

Water Cycle 

There are several processes involved in the formation of rainfall. The atmosphere contains water droplets at various levels. The clouds in the atmosphere hold different gas and water molecules. We can attribute the constant presence of water droplets in the atmosphere to the water cycle. 

The water cycle refers to the constant recycling of water through land, water bodies, and the atmosphere. The water cycle processes are evaporation, condensation, and precipitation. Precipitation is the last stage where rain falls, while the two earlier methods are how rain forms². 

Evaporation is the process of liquid on the earth's surface turning into gas. During this process, liquid water in oceans, lakes, and rivers evaporates and turns into water vapor. However, there is a certain percentage of water vapor the air can hold before it becomes saturated. 

The occurrence of the saturation also depends on the temperature of the atmosphere. Warmer air can hold more water than cooler air. 

Rain forms when the air is cool. The atmosphere becomes cooler when it rises and expands. Air moves in upward motion because of large-scale movements in the atmosphere or a physical barrier, i.e., a mountain. Atmospheric cooling can also occur when it comes in contact with a colder surface. 

Clouds form when water vapor condenses on condensation nuclei. Condensation nuclei are particles in the atmosphere that mix with water to create clouds. The condensation nuclei include dust, ice, and sea salt. It could also be other particles like the oxidation of sulfur oxide, sulfur from volcanic eruptions, and black carbon from burned fossil fuels.

Rain Shadows

Another interesting aspect of rain formation is the concept of a 'rain shadow.' This occurs when a mountain range blocks the passage of rain-producing weather systems. Here, one side of the mountains receives abundant rainfall while the other side remains arid.

Such conditions are evident in places like the Cascade Range in the western United States, demonstrating how geographical features play a critical role in determining regional climate patterns.

For a detailed look at the water cycle, check out this diagram from the United States Geological Survey.

Coalescence  

Coalescence is the merging of two or more droplets, bubbles, or particles to form a larger but singular size. This process is an important stage in precipitation. Water vapor is stationary in the clouds because of air resistance. 

However, wind or any other form of air turbulence causes tiny water droplets to merge and form larger drops. The coalescence continues until the water falls to the earth's surface. Coalescence usually occurs in warmer cloud areas, but clouds that are below the freezing temperature also experience it. 

Ice crystals in super cool clouds fall when they gain enough mass to fight against air resistance. The temperature difference between the cool cloud and the ground causes the ice crystals to melt, and rain falls to the earth's surface. 

Because of the joining and fragmentation process, raindrops have various sizes. The sizes range between 0.1 to 9mm. Smaller raindrops have a spherical shape, while larger drops have a flattened bottom caused by air resistance. Larger drops look like parachutes with drops of water around the base.

Types of rainfall 

There are three forms of rainfall on earth¹. They are: 

• Convective rain
• Frontal rain
• Orographic rain  

Convective rain 

Convective rainfalls form when the sun heats the group, and water evaporates, turning into water vapor. The water vapor condenses into liquid water, and a small shower of water droplets falls.

This type of rain falls from convective clouds like cumulonimbus and cumulus congestus clouds. The precipitation from these clouds falls for a short period because convective clouds have a short horizontal reach. 

Convective rain, also known as showers, doesn’t last long. It produces small showers of rain over small areas. This is the reason rain doesn’t fall in a neighboring area while it's pouring in your area. Convective rain is on and off throughout the day, with some sunshine in between shower sessions.  

Frontal rain 

Frontal rain occurs when warm clouds meet with cold clouds. Warm air rises above cool air because it is lighter. The point at which they meet causes saturation, causing the warm air to cool down, and there’s heavy rainfall. The characteristics of frontal rainfall are a gray cloud and heavy, persistent rain all day.

Orographic rain  

Orographic rainfalls are heavy rains that mostly occur around hilly and mountain areas. The wind around hilly areas pushes air upwards because the clouds can’t enter the hills. The air cools down because the higher the atmospheric area, the colder it becomes. With enough moisture, water vapor solidifies, and precipitation occurs.

Related read: Types of Precipitation.

What is acid rain? 

Acid rain describes the acidic components in the rain. It is a broad term that also refers to other types of precipitation, like snow, fog, and hail. The acidic components could be sulfuric or nitric acid. 

Human activities (mainly the burning of fossil fuels) cause air pollution, releasing sulfur dioxide (SO2) and nitrogen oxides (NOX) into the cloud. As the wind transports the pollutants, they react to water, oxygen, and other chemicals. The reaction forms sulfuric acid and nitric acid.

Its pH is less than 5. According to the U.S. Environmental Protection Agency, it is between 4.2 and 4.5. Acid rain is harmful to human health and the environment. It washes the protective layer on leaves away and stunts the growth of trees³. 

Acidic rain changes the biological composition of soil, water, and plants, making it dangerous for animals to ingest. Most fish species will die in water bodies with less than 5pH. It can also damage buildings and other architectural structures. 

Effects of water droplets on Earth  

Rain benefits all of the Earth's ecosystem. Some of these ways are; 

• Effects of rain on soil and plants
• Effects of rain on humans

Effects of rain on soil and plants 

Rain is essential for the success of agriculture in various regions. Rain falling on plants and soil is like humans drinking water to quench their thirst. Soil and plants need rainwater to survive and grow exponentially. 

It helps plant seeds germinate and grow. Water dissolves nutrients in the soil, and mature plants use water to absorb the nutrients. Rainwater contains minerals that aid the growth of plants. These minerals include calcium and magnesium. 

Plants also use water for their photosynthesis process. Their transpiration process releases water that helps keep them cool. Without water, plants will dry up and wither away. 

Effects of rain on humans 

Rain is highly beneficial to humans. It acts as a natural air purifier by removing dust, pollen, and other allergens from the atmosphere. It eases hot flashes and hot weather by making the air cool. 

Rainfall also boosts hydration. It increases the levels of groundwater, providing water for drinking, cooking, and other activities. We use rainwater for our agricultural activities, ensuring a bountiful harvest. It prevents drought and starvation.

Related Read: On an emotional level, experience the benefits of this type of precipitation by reading our curated rain quotes. 

Conclusion: How is rain formed?  

Water vapors evaporate into gas form and enter the sky. They form clouds, which hold a lot of water particles. These water particles become solid and fall onto the earth. These water droplets fall in different ways and with various intensities. 

The next time you witness a small shower of rain over a small area, know it's convective rain. Persistent heavy rain is frontal rain, while freezing rain is orographic rain.