The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail... all forms of precipitation, the superhighway moving water from the sky to the Earth's surface.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
• 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
Precipitation and the Water Cycle
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
How do raindrops form?
Credit: Howard Perlman, USGS
The 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 (condensation). 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.
For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. 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 (known as the Bergeron-Findeisen process) for producing a precipitation-sized drop is through 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 and fall as rain.
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) to find out - you just enter an area size and rainfall amount and see how many gallons of water reach the ground.
What do raindrops look like?
Let me introduce myself - I'm Drippy, the (un)official USGS water-science icon! It's obvious that I'm a raindrop, right? After all, all of you know that raindrops are shaped, well ... like me. As proof, you've probably seen me on television, in magazines, and in artists' representations. Truth is, I'm actually shaped more like a drip falling from a water faucet than a raindrop. The common raindrop is actually shaped more like a hamburger bun!
As Alistair Frasier explains on his web page, Bad Rain, 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
Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mawsynram in Meghalaya State, India, where it averages about 467.4 inches (1,187.2 cm) per year. Contrast this excessive precipitation to Arica, Chile, where no rain fell for 14 years, and in Bagdad, California, where precipitation was absent for 767 consecutive days from October 1912 to November 1914.
The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?
Credit: Earth Forum, Houston Museum of Natural Science
On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).
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
Below are other topics associated with precipitation and the water cycle.
Rain and Precipitation
Evaporation and the Water Cycle
Infiltration and the Water Cycle
Condensation and the Water Cycle
Ice, Snow, and Glaciers and the Water Cycle
Groundwater Flow and the Water Cycle
Evapotranspiration and the Water Cycle
Freshwater (Lakes and Rivers) and the Water Cycle
Follow a Drip Through the Water Cycle
Precipitation and Streamgage Flood Warning System
Below are multimedia items associated with precipitation and the water cycle.
- Overview
The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail... all forms of precipitation, the superhighway moving water from the sky to the Earth's surface.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
• 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
Precipitation and the Water Cycle
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
How do raindrops form?
A localized heavy summer rainstorm in Colorado, USA.
Credit: Howard Perlman, USGSThe 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 (condensation). 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.
For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. 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 (known as the Bergeron-Findeisen process) for producing a precipitation-sized drop is through 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 and fall as rain.
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) to find out - you just enter an area size and rainfall amount and see how many gallons of water reach the ground.
What do raindrops look like?
Let me introduce myself - I'm Drippy, the (un)official USGS water-science icon! It's obvious that I'm a raindrop, right? After all, all of you know that raindrops are shaped, well ... like me. As proof, you've probably seen me on television, in magazines, and in artists' representations. Truth is, I'm actually shaped more like a drip falling from a water faucet than a raindrop. The common raindrop is actually shaped more like a hamburger bun!
As Alistair Frasier explains on his web page, Bad Rain, 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
Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mawsynram in Meghalaya State, India, where it averages about 467.4 inches (1,187.2 cm) per year. Contrast this excessive precipitation to Arica, Chile, where no rain fell for 14 years, and in Bagdad, California, where precipitation was absent for 767 consecutive days from October 1912 to November 1914.
The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?
Generalized map of global precipitation.
Credit: Earth Forum, Houston Museum of Natural ScienceOn average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).
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
- Science
Below are other topics associated with precipitation and the water cycle.
Rain and Precipitation
Rain and snow are key elements in the Earth's water cycle, which is vital to all life on Earth. Rainfall is the main way that the water in the skies comes down to Earth, where it fills our lakes and rivers, recharges the underground aquifers, and provides drinks to plants and animals.Evaporation and the Water Cycle
Evaporation is the process that changes liquid water to gaseous water (water vapor). Water moves from the Earth’s surface to the atmosphere via evaporation.Infiltration and the Water Cycle
You can't see it, but a large portion of the world's freshwater lies underground. It may all start as precipitation, but through infiltration and seepage, water soaks into the ground in vast amounts. Water in the ground keeps all plant life alive and serves peoples' needs, too.Condensation and the Water Cycle
Condensation is the process of gaseous water (water vapor) turning into liquid water. Have you ever seen water on the outside of a cold glass on a humid day? That’s condensation.Ice, Snow, and Glaciers and the Water Cycle
The water stored in ice and glaciers moves slowly through are part of the water cycle, even though the water in them moves very slowly. Did you know? Ice caps influence the weather, too. The color white reflects sunlight (heat) more than darker colors, and as ice is so white, sunlight is reflected back out to the sky, which helps to create weather patterns.Groundwater Flow and the Water Cycle
Yes, water below your feet is moving all the time, but not like rivers flowing below ground. It's more like water in a sponge. Gravity and pressure move water downward and sideways underground through spaces between rocks. Eventually it emerges back to the land surface, into rivers, and into the oceans to keep the water cycle going.Evapotranspiration and the Water Cycle
Evapotranspiration is the sum of all processes by which water moves from the land surface to the atmosphere via evaporation and transpiration.Freshwater (Lakes and Rivers) and the Water Cycle
Freshwater on the land surface is a vital part of the water cycle for everyday human life. On the landscape, freshwater is stored in rivers, lakes, reservoirs, creeks, and streams. Most of the water people use everyday comes from these sources of water on the land surface.Follow a Drip Through the Water Cycle
You may be familiar with how water is always cycling around, through, and above the Earth, continually changing from liquid water to water vapor to ice. One way to envision the water cycle is to follow a drop of water around as it moves on its way. Read on to learn more about the journey.Precipitation and Streamgage Flood Warning System
Rainfall amounts associated with the September 2013 Colorado Floods exceeded 15 inches in some locations and resulted in significant flooding along the Front Range (Hydrometeorological Design Studies Center, 2013). These events resulted in streamflows that compromised a variety of transportation structures such as bridges and culverts and roadways. By coupling the National Oceanic and Atmospheric... - Multimedia
Below are multimedia items associated with precipitation and the water cycle.