Understanding Rain: How It Forms, Falls and Shapes Our World

Rain is such an ordinary part of life that it's easy to forget how much is happening every time it rains.

Water that may have evaporated from an ocean, lake, river or even the soil eventually returns to Earth as drops falling from a cloud. Some of that rain soaks into the ground. Some runs into streams and rivers. Some is taken up by plants, some becomes groundwater, and eventually much of it will make its way back into the atmosphere to begin the journey again.

But there are plenty of interesting things happening along the way. How does a raindrop form? How far does it fall? Why aren't raindrops actually shaped like the familiar teardrop? Why can one side of town get an inch of rain while the other side stays dry? Why do we sometimes see a rainbow when a storm passes? And where does all that water go once it reaches the ground?

Here's a closer look at rain, from the formation of a tiny drop in a cloud to what happens after it reaches Earth.

What Is Rain?

Rain is liquid precipitation: water that falls from clouds and reaches the ground as liquid drops. It's one part of the much larger water cycle that continuously moves water among the atmosphere, land, oceans, rivers, lakes, soil, groundwater, plants, snow and ice.

That definition is simple. Getting water from invisible water vapor in the atmosphere to a raindrop large enough to fall out of a cloud is considerably more interesting.

How Does Rain Form?

Clouds contain enormous numbers of tiny water droplets and, depending on temperature, ice crystals. The individual droplets are much too small to simply fall out of the sky as rain.

Rain begins with moisture in the atmosphere. As moist air rises and cools, water vapor can condense onto tiny particles in the air, forming cloud droplets. Those droplets can collide and combine with other droplets, gradually becoming larger. In cold clouds, ice crystals can also play an important role in the development of precipitation.

Eventually, particles become large and heavy enough to fall. Ice particles falling through warmer air can melt on their way down and reach us as rain.

There's quite a bit more happening inside a cloud than that brief explanation suggests. Learn more about how rain forms and why some clouds produce more rain than others.

And even then, their journey isn't necessarily straightforward.

What Does a Raindrop Really Look Like?

If you were asked to draw a raindrop, there's a good chance you'd draw the familiar shape with a rounded bottom and a point at the top.

Real raindrops don't look like that.

Very small raindrops are approximately spherical. As a drop gets larger and falls faster, air resistance pushes against its underside. The bottom becomes flatter while the top remains rounded. Larger drops can become increasingly distorted, and very large drops can become unstable and break apart.

So the classic teardrop is a useful symbol for rain, but it isn't a particularly accurate picture of a raindrop falling through the sky.

See what raindrops really look like and how their size affects their shape and speed.

How Far Does a Raindrop Fall?

There isn't one answer because raindrops don't all begin at the same altitude, and clouds themselves form at very different heights.

What happens during the trip down is just as interesting as the distance. A falling drop encounters air resistance and eventually approaches a terminal velocity determined partly by its size. Larger drops generally fall faster than smaller ones. Drops can collide with other drops, grow, break apart or evaporate as they pass through different layers of air.

Some precipitation doesn't make it all the way down. It can begin falling from a cloud and evaporate before it ever reaches the ground.

Learn how far raindrops can fall—and why some never reach the ground.

Why Isn't All Rain the Same?

Anyone who has watched rain knows that it doesn't always fall the same way.

There is fine drizzle that seems to hang in the air, a gentle rain that continues for hours, brief showers that come and go, and torrential downpours that can soak everything in minutes. Drizzle, showers, downpours and cloudbursts all describe different kinds of rainfall.

The differences have to do with the clouds and weather systems producing the precipitation, the size and number of drops, atmospheric conditions and how quickly the rain is falling.

A slow, steady rain spread over many hours is very different from the same total amount of water arriving in a short cloudburst. Rainfall intensity matters once that water reaches the ground, too. If rain falls faster than the ground can absorb it, more water can run off instead of soaking in. Here's how much rain can fall in an hour during particularly intense storms.

Rainstorms and Other Weather Phenomena

Rain often arrives with other interesting weather. Thunderstorms can bring lightning and thunder, sudden changes in wind, hail and dramatic shifts in temperature. Some storms darken the sky in the middle of the day, while others produce heavy rain even though sunshine and blue sky are visible nearby.

Lightning develops because electrical charges become separated within a thunderstorm. When that electrical imbalance becomes great enough, a lightning discharge can occur within a cloud, between clouds or between a cloud and the ground. Thunder is the sound created as air around a lightning channel heats and expands extremely rapidly.

Then there are rainbows. When sunlight enters raindrops, the light is bent, reflected inside the drops and separated into its different colors before reaching our eyes. Under the right conditions, a second reflection inside the drops can produce a double rainbow.

There are stranger rainy-day sights as well. Rain can fall while the sun is shining, creating what we call a sunshower. Sometimes precipitation falls from a cloud but evaporates before reaching the ground, leaving streaks beneath the cloud known as virga. Strong thunderstorms can also produce hail, powerful downdrafts and other dramatic weather.

Rain may be the main event, but there's often a lot happening around it.

Why Can It Rain on One Side of Town but Not the Other?

Rainfall can be surprisingly local.

Large weather systems may produce widespread rain across an entire region, while thunderstorms and scattered showers can affect much smaller areas. A summer storm may produce a downpour in one neighborhood while another nearby gets little or nothing.

Terrain can influence rainfall, too. Mountains can force moist air upward, affecting where clouds and precipitation develop, while areas on the opposite side may be considerably drier. This effect can create a rain shadow. Wind, storm movement and atmospheric conditions all contribute to the uneven distribution of rain.

This is why two people who live only a few miles apart can report very different rainfall totals from what seems like the same storm.

It's also one reason your local rainfall may not match the amount shown by a weather app. We look at that more closely in Why Your Rain Gauge Doesn't Match Your Weather App.

Learn more about why rainfall can vary so much across a surprisingly short distance.

What Does an Inch of Rain Actually Mean?

When a weather report says an inch of rain fell, it means enough water fell to cover a level surface one inch deep, assuming none of it ran off, soaked in or evaporated.

An inch doesn't sound like much until you consider the area over which it's falling.

One inch of rain falling evenly over a single acre amounts to about 27,154 gallons of water. Over one square mile, that same inch represents more than 17 million gallons.

Suddenly, an inch of rain sounds like quite a lot of water.

Rainfall totals also don't tell you everything about a storm. An inch falling gradually over an entire day can behave very differently once it reaches the ground than an inch that arrives in 20 minutes.

Learn more about what an inch of rain actually means.

Where Does Rain Go After It Falls?

The answer depends greatly on where it lands.

Rain falling on soil may soak into the ground, a process called infiltration. Some remains near the surface where plant roots can use it. Some moves deeper and can eventually recharge groundwater. That process can take water deep underground, where groundwater may remain for years, centuries or even longer. Rain that doesn't soak in may travel across the surface as runoff, eventually reaching ditches, streams, rivers, lakes and oceans.

Rain behaves differently when it lands on a forest than when it lands on a parking lot. Vegetation can intercept and slow water, while roads, roofs, sidewalks and other hard surfaces prevent much of it from soaking into the ground and can increase runoff.

Plants take up some of the water in soil and eventually release moisture back into the atmosphere. Water also evaporates directly from soil, puddles, lakes, rivers and oceans. Groundwater may remain below the surface for a long time before eventually emerging elsewhere.

So when a puddle disappears from your yard, the water isn't necessarily "gone." It has simply moved somewhere else.

Take a closer look at where rain goes after it falls.

What Does a 100% Chance of Rain Actually Mean?

Weather forecasts create another set of rain questions, and few numbers are misunderstood more often than the percentage next to the little rain-cloud symbol on a weather app.

Probability of precipitation describes the likelihood that a particular location in the forecast area will receive measurable precipitation during the forecast period.

It doesn't tell you how long it will rain or how much rain will fall. A brief shower and an all-day soaking rain can both occur on days with a very high probability of precipitation.

That little percentage is useful. It just may not mean quite what you thought it did. And if you've ever wondered why two forecasts don't even agree on that percentage, there are several reasons weather apps can give different forecasts for exactly the same place.

Learn more about what a chance of rain really means.

Can Animals Tell When It's Going to Rain?

Stories about animals predicting weather have been around for generations. Cows lie down before rain. Birds fly differently. Frogs get noisy. Insects behave strangely. Dogs seem to know a storm is coming.

Some observations may have plausible explanations. Animals can respond to environmental changes associated with approaching weather, including changes in atmospheric pressure, humidity, wind and temperature. Other familiar weather sayings are much harder to separate from coincidence and folklore.

The interesting question isn't whether animals have a magical ability to predict the weather. It's whether some animals detect changes around them before we notice those changes ourselves.

Learn more about what animals may be able to sense before it rains.

Why Does Rain Smell So Good?

That distinctive earthy smell associated with rain has a name: petrichor. But what we think of as the smell of rain isn't simply the smell of water falling from the sky. Compounds from soil, plants and microorganisms contribute to the scents we notice before, during and after rainfall.

One of the best known is geosmin, a compound produced by microorganisms in soil. Rain can help release these earthy-smelling compounds into the air, where our noses can detect them.

It's one of the reasons you may sometimes smell an approaching rainstorm before you feel the first drop.

Learn more about why rain smells so good and the science behind petrichor.

Can Humans Make It Rain?

People have been trying to influence rainfall for a long time. One modern approach is cloud seeding, a form of weather modification intended to encourage precipitation from suitable clouds.

Cloud seeding involves introducing particles into existing clouds to encourage physical processes that may lead to precipitation. The important part is existing clouds. Cloud seeding can't simply create a rainstorm in a clear blue sky; atmospheric conditions already need to be suitable.

How well it works, under what conditions and how much additional precipitation it can produce are more complicated questions—and a good example of how difficult it is to control something as complex as weather.

Learn more about the history and science of cloud seeding.

How Is Rain Measured?

Rainfall is generally expressed as a depth, most commonly in inches in the United States and millimeters in much of the rest of the world.

The basic principle behind a rain gauge is remarkably simple: collect precipitation through an opening and measure how much has accumulated. Weather stations and weather radar can provide information about precipitation across larger areas, while a rain gauge provides a measurement at a particular location.

That location matters. As we've already seen, rainfall can vary considerably over surprisingly short distances.

If you measure rainfall at home, proper placement matters too. Buildings, trees and other obstructions can interfere with how much rain reaches a gauge. Our guide to where to place a rain gauge for accurate readings explains what to look for.

If you enjoy keeping track over time, you can also learn more about tracking rainfall and keeping a rainfall log.

Rain Is Part of a Much Bigger Cycle

A raindrop reaching the ground isn't really the end of its journey.

Water that soaks into soil may be taken up by a plant or continue downward into groundwater. Runoff may enter a creek, then a river and eventually the ocean. Water on the surface can evaporate, while plants return moisture to the atmosphere through transpiration. In the atmosphere, water vapor can once again condense, become part of a cloud and eventually return as precipitation.

The water cycle doesn't really have a beginning or an end. Rain is simply one part of water's continuous movement around the planet.

And the next time you hear rain hitting the roof, there's a lot more going on than water falling out of the sky.

Explore More About Rain

There's always something else to wonder about when it rains. As we add to our rain library, we'll explore the science of raindrops, rainstorms and weather as well as what happens to all that water once it reaches the ground.

You can also explore:

• Rain, Snow, Sleet and Freezing Rain: What Determines What Falls From the Sky?
• Does 100% Humidity Mean Rain?
• How Did People Predict the Weather Before Modern Forecasts?
• The History of Weather Forecasting
• How Far Ahead Can We Really Predict the Weather?
• Weather Watches, Warnings and Advisories: What's the Difference?
• Where Do Wild Animals Go During Heavy Rain?
• What Do Worms, Frogs, Bees and Bugs Do During Heavy Rain?
• Why Does Rain Sound So Relaxing?

Interested in measuring rain in your yard or garden? We have a complete collection of decorative rain gauges available for purchase here.