Rainforests are called so because they receive a very high amount of rainfall, usually more than 200 centimetres (78.7 inches) per year. For example, the rainforests of the Amazon basin receive an average of 2,000 millimetres (78.7 inches) of rainfall per year. Some rainforests, like those in Hawaii and the Congo, can receive even more rain, with annual rainfall amounts of over 10,000 millimetres (393.7 inches).

The heavy rainfall in rainforests is due to the combination of several factors, including:

- The location of rainforests: Many rainforests are located near the equator, which is a region of the Earth that receives a high amount of solar radiation. This radiation heats the air and water in the equatorial regions, causing it to rise and form clouds. These clouds then release the moisture they contain in the form of rain.

- The presence of mountains: Some rainforests are also located near mountain ranges, which force air to rise. As the air rises, it cools and releases the moisture it contains in the form of rain.

- The presence of large bodies of water: Some rainforests are located near large bodies of water, such as oceans or rivers. These bodies of water release water vapour into the atmosphere through evaporation, which then forms clouds and eventually releases moisture as rain.

The high rainfall in rainforests is essential for the growth of the lush vegetation that is characteristic of these ecosystems. Rainforests are home to a wide variety of plants and animals, many of which are found only in these habitats. The high rainfall also helps to regulate the Earth's temperature and climate.