Rain

Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall by gravity. It is an important part of the water cycle and helps provide most of the fresh water on Earth. Rain gives water to hydroelectric power plants, helps crops grow through irrigation, and supports many different ecosystems.

Rain happens mainly when moisture moves along areas with differences in temperature and moisture, called weather fronts. When there is enough moisture and upward movement, precipitation can fall from tall clouds known as cumulonimbus, which are often thunder clouds. In mountains, heavy rain can occur on the windward sides where moist air is forced upward, while the leeward sides can become dry and desert-like.

Cities can affect rain patterns through the urban heat island effect, leading to more intense rainfall nearby. Global warming is also changing where and how much rain falls, making some areas wetter and others drier. Scientists measure rainfall using rain gauges and can estimate it with weather radar.

Formation

Air contains tiny drops of water. How much water is in the air depends on the temperature. Warm air can hold more water than cool air. When air cools down, the water in it can turn into clouds.

There are a few ways air can cool. One way is when air rises and spreads out. Another is when air touches a colder surface. Air can also cool by sending out heat or by adding more water through evaporation.

Water in the air usually gathers around tiny particles like dust to form clouds. When water droplets in a cloud bump into each other, they can grow bigger. These bigger droplets can become heavy enough to fall as rain. Sometimes ice crystals in cold clouds fall and melt into rain on the way down.

Raindrops come in different sizes. Small drops are round, but bigger drops look more like flat circles. The biggest raindrops ever found were about the size of a small button.

Causes

Frontal activity

Main article: Weather fronts

Rain can happen because of weather changes near places where warm and cold air meet, called fronts. When air slowly rises in certain weather patterns, it can cause rain. This kind of rain often looks like a big, steady blanket covering the sky. Near cold fronts, where colder air pushes under warmer air, or near warm fronts, where warm air slides over cold air, rain can appear. Rain also happens around big storms called tropical cyclones, which are powerful systems with low pressure in the middle.

Convection

Sometimes rain happens because of strong upward movements in the air, called convection. This kind of rain often comes in short, heavy bursts and can change quickly. It usually falls from tall clouds like cumulonimbus clouds. In places near the equator, most rain happens this way, but steady rain can also occur.

Orographic effects

Main articles: Orographic lift, Precipitation types (meteorology), and United States rainfall climatology

Rain can also happen when moist air is forced to rise over mountains. As the air goes up, it cools and the water in it turns into rain. This is why one side of a mountain often gets a lot of rain, while the other side stays dry. For example, in Hawaii, Mount Waiʻaleʻale gets huge amounts of rain because of this effect.

Within the tropics

See also: Monsoon and Tropical cyclone

Main article: Wet season

In places near the equator, there are times of year when it rains a lot, called the wet season. During this time, most of the rain for the year falls. Some areas, like those with savannas or monsoons, have wet summers and dry winters. Tropical rainforests rain almost all the time. Big storms called tropical cyclones can bring huge amounts of rain to places that usually don’t get much.

Human influence

See also: Effects of climate change, Effects of climate change on the water cycle, and Urban heat island

Pollution from cars and factories can actually help make rain. When pollution builds up during the week, the chance of rain goes up, often peaking on weekends. Cities that are warmer than surrounding areas can also create more rain nearby. Changes in temperature and weather patterns have led to more heavy rain in some places and less in others. Scientists sometimes try to make rain happen on purpose by using techniques called cloud seeding.

Characteristics

Rainbands are long, narrow areas of clouds and rain. They can be created by differences in temperature and are seen on weather maps as stretched-out areas of rain. Rainbands near cold fronts can sometimes produce strong winds and even tornadoes.

Acid rain is rain that has become more acidic because of pollutants in the air. It was first described in 1852. Rain can pick up acids from natural sources like volcanoes and lightning, as well as from human activities like burning fossil fuels. These acids can harm plants, animals, and buildings.

The Köppen climate classification helps us understand different climate types based on temperature and rainfall patterns. Some climates, like tropical rainforests, get a lot of rain all year. Others, like deserts, get very little rain. Different climates affect what kinds of plants and animals can live in an area.

Rain can also carry pollutants from factories, cars, and farms into rivers and lakes. These pollutants can harm wildlife and people. Rain’s composition changes depending on where it falls and what human activities are happening nearby.

Measurement

See also: Precipitation § Rate

See also: Rain gauge, Disdrometer, and Snow gauge

Rain is measured in how much falls over time, usually in millimeters or inches per hour. This tells us how deep the water would be on a flat surface after that time. We use special tools called gauges to collect and measure the rain. Simple gauges look like small cans, while scientists use more advanced ones for better accuracy.

See also: Weather radar

Weather radar helps us measure rain over large areas. This is important for planning things like flood control and building dams. Radar and satellites can estimate how much rain has fallen by looking at signals they receive.

See also: 100-year flood

We describe rain events by how often they happen. A "10-year storm" is a rare rain event that might occur once every ten years. A "100-year storm" is even rarer, happening about once in a hundred years. These events usually bring more rain and worse flooding than less rare storms.

Forecasting

Main article: Quantitative precipitation forecast

A Quantitative Precipitation Forecast (QPF) tells us how much rain or other liquid precipitation is expected to fall over a certain time and area. These forecasts are made for specific times during the day, and they take into account the shape of the land and past weather patterns.

Weather models are very affected by how much moisture is in the lower part of the atmosphere. QPFs can predict either the exact amount of rain or the chance that a certain amount will fall. Radar images are often better at forecasting rain within about 6 to 7 hours compared to computer models. We can check how good these forecasts are by comparing them to actual rain measurements from gauges or radar.

Impact

Rain is very important for farming. Plants need water to grow, and rain is the best way to give them that water. But rain can also be a problem. Too much rain can hurt plants by making mold grow, and too little rain can dry out the land and kill crops. Different plants need different amounts of rain — some need just a little, while others need a lot.

People feel differently about rain depending on where they live. In some places, rain makes people happy, especially after a long dry spell. In other places, rain can be stressful if the weather is not stable. Many cultures have special ways to deal with rain, like using umbrellas or raincoats. Rain also has deep meaning in many religions and traditions around the world.

Main article: List of rain deities

Global climatology

See also: Earth rainfall climatology

Every year, about 505,000 cubic kilometers of water falls as rain around the world, with most of it landing over the oceans. This rain is important for plants, animals, and people because it helps fill rivers and lakes. Deserts are dry places where very little rain falls, usually less than 250 millimeters each year.

Deserts

Main article: Desert

The northern part of Africa has the world's largest hot, dry area called the Sahara Desert. Other deserts are found in southern Africa, such as the Namib and the Kalahari. In Asia, deserts stretch from the Gobi Desert in Mongolia through parts of Pakistan and Iran to the Arabian Desert in Saudi Arabia. Most of Australia is dry or desert-like, making it the driest place where people live. In South America, mountains block rain from reaching parts of western Argentina. In the United States, dry areas include the Sonoran Desert in the southwest and parts of the Great Basin and central Wyoming.

Polar deserts

Main articles: Polar desert and Polar climate

Rain falls as liquid water, so it doesn't often happen when temperatures are very cold. Places that are always cold, like polar deserts, get very little rain. These areas have short summers and long, frozen winters. Antarctica is one of the driest places on Earth because it is so cold.

Rainforests

See also: Rainforest

Rainforests are places with lots of rain and very green forests. Tropical rainforests are near the equator and have rain almost all the time. Temperate rainforests are farther from the equator but still get a lot of rain. Some rainforests can have dry periods.

Monsoons

See also: Monsoon and Monsoon trough

Near the equator, some places get a lot of rain during certain times of the year. This is called a monsoon. In Asia, monsoons bring rain from the Indian Ocean to places like India, the Philippines, China, and Japan. Similar patterns happen in North America and Australia.

During summer, moisture from the Gulf of California and Gulf of Mexico can bring thunderstorms to the southern United States and the Great Plains. The eastern part of the United States, the mountains of the Pacific Northwest, and the Sierra Nevada get more rain than other areas. Tropical cyclones can also bring extra rain to places like Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa.

Impact of the Westerlies

See also: Westerlies

Winds from the Atlantic Ocean bring rain to western Europe, especially to places like Ireland and the United Kingdom. Bergen, Norway gets a lot of rain each year. In the United States, storms from the Pacific bring rain to Hawaii and the western states. The El Niño-Southern Oscillation can change where rain falls, and global warming might be making some areas rainier.

Wettest known locations

Cherrapunji, in India, gets more rain than any other place on Earth, with about 11,430 millimeters each year. Lloró in Colombia might get the most rain, with about 13,300 millimeters each year. Other very wet places include Mawsynram in India, Mount Bellenden Ker in Australia, the Big Bog on Maui, and Mount Waiʻaleʻale on Kauaʻi.