While it may be tempting to say that rain comes from clouds, you can also say that rain is clouds, giving up on their dreams of being water vapor and falling back down to Earth, where they start their journey through the precipitation cycle again. If you want a better understanding of why rain comes down from clouds, start with that precipitation cycle, the mechanism through which water moves from the Earth to the atmosphere and back again.
Understanding the Precipitation Cycle
The amount of water available on Earth never changes. But its state (liquid or gas/vapor) does, and that's all thanks to thermal energy from the sun. As liquid water is heated by the sun, it receives enough energy to break its molecules apart and transform into water vapor.
The warmer the air, the more water vapor it can hold. That warm, moisture-saturated air rises, along with the water vapor it contains, and as it rises it cools. Once the air has cooled past the "dew point," it condenses around "condensation nuclei," which are usually teeny-tiny particles of dust, smoke or even salt that are suspended in the air. (If you've ever looked through a shaft of sunlight and seen dust particles dancing in the air, that's a great visual.)
The tiny water droplets that initially form are what you see as clouds – and if you pay close attention to clouds in the sky, you'll see that they're constantly shrinking and growing in response to the warring forces of evaporation and condensation.
Dew point is the temperature at which there's more condensation than evaporation going on in the air, and so water vapor begins to condense and coalesce into water droplets that can fall as rain. Dew point can vary anywhere from the 30s (Fahrenheit) to, on rare occasions, the 80s. See Resources for a longer discussion of dew point versus average humidity.
How Clouds Become Rain
Water vapor that has condensed into tiny droplets and formed clouds is well on its way to becoming rain – but it's not there yet. For now, the water droplets are so tiny that the air currents keep them aloft, just as swirling particles of dust can stay in the air. But as those droplets continue to rise, buoyed by rising bodies of warm air, they have two routes for making it back to Earth.
The first is when water droplets collide and coalesce with other droplets, eventually becoming heavier than the uplift of the air around them, at which point they fall down through the cloud. Or, through something called the Bergeron-Findeisen-Wegener process, the ice process of precipitation or simply the Bergeron process, the droplets rise high enough to freeze into ice crystals, attracting more water vapor to themselves and growing quickly until they're heavy enough to fall as snow or melt and fall as rain.
Did you know? The water droplets falling out of clouds – in other words, rain – are shaped less like the drip from a faucet and more like a little ball. As they get bigger, they're affected by the resistance of the air and start to look more like a hamburger bun or a bean; and if they get big enough, they'll actually break apart into smaller droplets.
How Does Rain Come From Clouds?
Once a water droplet makes the leap from cloud toward Earth, it arrives with the unceremonious splat of a raindrop. Usually. But depending on atmospheric conditions it may also arrive as freezing rain, sleet (ice pellets mixed with rain or snow), hail or of course snow.
You can also see many different types of rain, as anyone who's experience the persistent mists of Ireland or the thundering downpour of the tropics can tell you. The form rain takes is affected not just by atmospheric conditions like air temperature but also landforms. For example, hilly coastal areas are often wetter than flat coastal areas because as the wet air from the ocean rises to go over the hills, it condenses enough for rain to fall.
Some of the most spectacular rain can happen when weather fronts, or masses of warm and cold air, collide. When that happens the mass of warm air – and the water it's carrying – lifts up and over the air of the cool front. As all that warm air rises it cools enough for water vapor to condense and fall in what can become heavy, intense rain. When conditions are correct, this can also be the mechanism that starts a summer thunderstorm rolling.
Thunderstorms are caused by masses of warm air that rise, whether as the result of colliding weather fronts, mountainous topography or updrafts of warm air caused by the sun. If there's enough warm, rising air to keep feeding energy into the cloud, the combination of upward-rising warm, moist air and downward-falling dry, cool air creates the up-and-down cycle of air that forms a thunderstorm cell.
What Type of "Rain" Is That?
As you already know, precipitation can come down to Earth in many ways – and words like "fog," "mist," "drizzle" or "cloudburst" aren't just descriptive, they also have scientific definitions for the size of the water droplets, the speed of their fall, the inches of precipitation per hour, and their density or how many droplets there are in a square foot. From lightest precipitation to heaviest, those terms are:
- Light rain
- Moderate rain
- Heavy rain
- Excessive rain
So when your friendly TV weatherperson says that "it's raining cats and dogs out there," they're embellish a bit – but if they say you can look forward to "excessive rain," they're actually making a scientific statement.
How Much Rain Is There, Anyway?
That's a complicated question. Here's one impressive fact: According to the U.S. Geological survey, enough rain falls on the continental United States to cover the land in 30 inches of water.
With that said, rainfall patterns vary enormously from year to year and between geographic areas. For example, according to the U.S. Geological Survey, the record for most rain in a year is held by the town of Cherrapunji, India, which received a whopping 905 inches (more than 75 feet) of rain in 1861. The record for highest average annual rainfall belongs to Mt. Waialeale, Hawaii, which averages about 450 inches of rainfall every year.
The opposite extremes exist, too: Again according to the U.S. Geological survey, a rainless period in Arica, Chile, lasted 14 years. That's more than 5,000 dry days, which makes a 767-day drought in Bagdad, California, in the early 1910s, seem almost mild.
With that in mind, you might not be surprised to know that parts of South America (especially in Chile) and parts of California are officially deserts. But did you know that large stretches of land above the Arctic Circle are dubbed deserts as well because of their low precipitation? These include large swathes of Greenland, Canada and Siberia. Much of Antarctica is considered a desert, too.
How do your local rain patterns measure up? See Resources for a map of average rainfall in the United states.
About the Author
Lisa studied mathematics at the University of Alaska, Anchorage, and spent several years tutoring high school and university students through scary -- but fun! -- math subjects like algebra and calculus.