Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. Precipitation is the main way atmospheric water returns to the surface of the Earth. Most precipitation falls as rain.
• Water Science School HOME • Water Basics topics • The Water Cycle •
Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud, you can see some parts disappearing (evaporating) while other parts are growing (condensing). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds.
Credit: Howard Perlman, USGS
For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus, binding them together. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop.
A more efficient mechanism for producing a precipitation-sized drop, is a process which leads to the rapid growth of ice crystals at the expense of the water vapor present in a cloud. These crystals may fall as snow, or melt then fall as rain. This process is known as the Bergeron-Findeisen process.
How much water falls during a storm?
You might be surprised at the number of gallons of water that fall from the sky in even a small but intense storm. One inch of rain falling on just a single acre results in 27,154 gallons of water on the landscape. If you'd like to know how much water falls during a storm, use our Interactive Rainfall Calculator (English units or Metric units). Just enter an area size and rainfall amount and see how many gallons of water reach the ground.
What do raindrops look like?
Small raindrops, those with a radius of less than 1 millimeter (mm), are spherical, like a round ball. As droplets collide and grow in size, the bottom of the drop begins to be affected by the resistance of the air it is falling through. The bottom of the drop starts to flatten out until at about 2-3 mm in diameter the bottom is quite flat with an indention in the middle - much like a hamburger bun. Raindrops don't stop growing at 3 millimeters, though, and when they reach about 4-5 mm, things really fall apart. At this size, the indentation in the bottom greatly expands forming something like a parachute. The parachute doesn't last long, though, and the large drop breaks up into smaller drops.
Precipitation rates vary geographically and over time
In 2017, Colombia received the greatest amount of precipitation at 3,240 mm for the year. Contrast that with Egypt for the same year, during which they received only 18 mm of precipitation for the year. Explore more annual precipitation data on this interactive site from Our World in Data or at Precipitation Climatology | NASA Global Precipitation Measurement Mission.
Precipitation size and speed
Have you ever watched a raindrop hit the ground during a large rainstorm and wondered how big the drop is and how fast it is falling? Or maybe you've wondered how small fog particles are and how they manage to float in the air. The table below shows the size, velocity of fall, and the density of particles (number of drops per square foot/square meter of air) for various types of precipitation, from fog to a cloudburst.
| Intensity inches/hour (cm/hour) |
Median diameter (millimeters) |
Velocity of fall feet/second (meters/second) |
Drops per second per square foot (square meter) |
|
|---|---|---|---|---|
| Fog | 0.005 (0.013) |
0.01 | 0.01 (0.003) |
6,264,000 (67,425,000) |
| Mist | .002 (0.005) |
.1 | .7 (.21) |
2,510 (27,000) |
| Drizzle | .01 (0.025) |
.96 | 13.5 (4.1) |
14 (151) |
| Light rain | .04 (0.10) |
1.24 | 15.7 (4.8) |
26 (280) |
| Moderate rain | .15 (0.38) |
1.60 | 18.7 (5.7) |
46 (495) |
| Heavy rain | .60 (1.52) |
2.05 | 22.0 (6.7) |
46 (495) |
| Excessive rain | 1.60 (4.06) |
2.40 | 24.0 (7.3) |
76 (818) |
| Cloudburst | 4.00 (10.2) |
2.85 | 25.9 (7.9) |
113 (1,220) |
Source: Lull, H.W., 1959, Soil Compaction on Forest and Range Lands, U.S. Dept. of Agriculture, Forestry Service, Misc. Publication No.768
Sources and more information
- Rain: A Water Resource, USGS General Interest Publication
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. Precipitation is the main way atmospheric water returns to the surface of the Earth. Most precipitation falls as rain.
• Water Science School HOME • Water Basics topics • The Water Cycle •
Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud, you can see some parts disappearing (evaporating) while other parts are growing (condensing). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds.
Credit: Howard Perlman, USGS
For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus, binding them together. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop.
A more efficient mechanism for producing a precipitation-sized drop, is a process which leads to the rapid growth of ice crystals at the expense of the water vapor present in a cloud. These crystals may fall as snow, or melt then fall as rain. This process is known as the Bergeron-Findeisen process.
How much water falls during a storm?
You might be surprised at the number of gallons of water that fall from the sky in even a small but intense storm. One inch of rain falling on just a single acre results in 27,154 gallons of water on the landscape. If you'd like to know how much water falls during a storm, use our Interactive Rainfall Calculator (English units or Metric units). Just enter an area size and rainfall amount and see how many gallons of water reach the ground.
What do raindrops look like?
Small raindrops, those with a radius of less than 1 millimeter (mm), are spherical, like a round ball. As droplets collide and grow in size, the bottom of the drop begins to be affected by the resistance of the air it is falling through. The bottom of the drop starts to flatten out until at about 2-3 mm in diameter the bottom is quite flat with an indention in the middle - much like a hamburger bun. Raindrops don't stop growing at 3 millimeters, though, and when they reach about 4-5 mm, things really fall apart. At this size, the indentation in the bottom greatly expands forming something like a parachute. The parachute doesn't last long, though, and the large drop breaks up into smaller drops.
Precipitation rates vary geographically and over time
In 2017, Colombia received the greatest amount of precipitation at 3,240 mm for the year. Contrast that with Egypt for the same year, during which they received only 18 mm of precipitation for the year. Explore more annual precipitation data on this interactive site from Our World in Data or at Precipitation Climatology | NASA Global Precipitation Measurement Mission.
Precipitation size and speed
Have you ever watched a raindrop hit the ground during a large rainstorm and wondered how big the drop is and how fast it is falling? Or maybe you've wondered how small fog particles are and how they manage to float in the air. The table below shows the size, velocity of fall, and the density of particles (number of drops per square foot/square meter of air) for various types of precipitation, from fog to a cloudburst.
| Intensity inches/hour (cm/hour) |
Median diameter (millimeters) |
Velocity of fall feet/second (meters/second) |
Drops per second per square foot (square meter) |
|
|---|---|---|---|---|
| Fog | 0.005 (0.013) |
0.01 | 0.01 (0.003) |
6,264,000 (67,425,000) |
| Mist | .002 (0.005) |
.1 | .7 (.21) |
2,510 (27,000) |
| Drizzle | .01 (0.025) |
.96 | 13.5 (4.1) |
14 (151) |
| Light rain | .04 (0.10) |
1.24 | 15.7 (4.8) |
26 (280) |
| Moderate rain | .15 (0.38) |
1.60 | 18.7 (5.7) |
46 (495) |
| Heavy rain | .60 (1.52) |
2.05 | 22.0 (6.7) |
46 (495) |
| Excessive rain | 1.60 (4.06) |
2.40 | 24.0 (7.3) |
76 (818) |
| Cloudburst | 4.00 (10.2) |
2.85 | 25.9 (7.9) |
113 (1,220) |
Source: Lull, H.W., 1959, Soil Compaction on Forest and Range Lands, U.S. Dept. of Agriculture, Forestry Service, Misc. Publication No.768
Sources and more information
- Rain: A Water Resource, USGS General Interest Publication