Rain happens when water vapor condenses into droplets in clouds, and the droplets grow heavy enough to fall.
The process
- The Sun heats water, which evaporates into vapor.
- Vapor rises, cools and condenses on tiny particles into cloud droplets.
- Droplets collide and merge until they are heavy enough to fall.
- They fall as rain if the air below is warm. Otherwise they may fall as snow or sleet.
Types of rain
Convectional, frontal and orographic (over mountains).
Facts
A raindrop is about 1 to 5 mm across, and falls at roughly 9 mph (4 m/s) for a large drop.
The water cycle
Rain is part of the water cycle. The Sun heats oceans, lakes and plants, so water evaporates or transpires into the air as vapor. Rising air cools, vapor condenses on tiny particles such as dust and salt, called condensation nuclei, and forms cloud droplets.
How raindrops grow
Cloud droplets are only about 0.01 millimeters wide. Through collisions and collection of smaller droplets, or through ice crystals that grow and melt as they fall, they become raindrops about 1 to 5 millimeters wide. It takes about a million cloud droplets to make one raindrop.
Ways that air rises
- Fronts: warm air rises over cold air.
- Orographic lift: air moves up over mountains.
- Convection: warm surface air rises in thunderstorms.
- Convergence: air flows together, as in tropical systems.
Shape of raindrops
Raindrops are not teardrop shaped. Small ones are spherical, and larger ones flatten like a hamburger bun because of air pressure.
Speed
Raindrops fall at about 9 meters per second (20 miles per hour) for the largest drops, and slower for small ones.
Rain intensity
Light rain is less than 2.5 millimeters per hour. Moderate is 2.5 to 7.6 millimeters. Heavy rain is more than 7.6 millimeters per hour.
Smell of rain
Petrichor is the earthy smell after rain. It comes from oils from plants and compounds called geosmin produced by soil bacteria.
Global numbers
The Earth gets on average around 990 millimeters of precipitation per year. The wettest place with reliable records is Mawsynram, India, with about 11,872 millimeters per year.
Rain and climate
A warmer atmosphere holds about 7% more water vapor for each 1 degree C of warming, which can lead to heavier rainfall events.
Cloud droplet to raindrop
Rain forms when water vapor condenses on tiny particles called condensation nuclei, such as dust or salt, creating cloud droplets about 0.02 mm across. These droplets are so light that rising air holds them up. They grow by colliding and merging, and it takes about one million cloud droplets to make one raindrop. When drops become heavy enough, they fall. Raindrops range from about 0.5 to 5 mm in diameter and are shaped like small hamburger buns, not teardrops, because air pressure flattens the bottom. Large drops fall at up to about 9 meters per second (20 mph).
Rainfall numbers and smell
Air rises and cools through fronts, mountains and convection, which leads to rain. Global average annual precipitation is about 990 mm (39 inches), the US average is about 30 inches, and Mawsynram in India receives about 11,872 mm (467 inches) a year, one of the wettest places on Earth. The smell of rain, called petrichor, a term coined by Australian scientists in 1964, comes from oils released by plants and geosmin, a compound produced by soil bacteria that humans can detect at extremely low concentrations. Rain is vital to the water cycle, replenishing rivers, aquifers and crops.
Based on data from NOAA and the US Geological Survey
