How Does Rain Form—and Why Do Some Clouds Produce More Rain Than Others?

We see clouds all the time without giving much thought to what’s happening inside them. Some drift overhead without producing a drop. Others bring a light, steady drizzle. And sometimes a dark summer cloud seems to open up all at once, dumping an inch of rain in a remarkably short time.

So what makes the difference?

Rain begins with water vapor, but turning invisible moisture in the air into raindrops large enough to reach the ground takes several steps. The type of cloud, the amount of moisture available and the movement of air inside the cloud all help determine whether we get no rain at all, a gentle shower or a soaking downpour.

How Does Rain Form?

The process starts with the water cycle.

Water evaporates from oceans, lakes, rivers, soil and plants and enters the atmosphere as invisible water vapor. When warm, moist air rises, it expands and cools. Eventually it can cool enough to reach its dew point—the temperature at which the air becomes saturated. At that point, some of the water vapor begins to condense.

But water vapor generally needs something to condense onto. Tiny particles floating in the atmosphere—such as dust, sea salt, pollen and smoke—act as cloud condensation nuclei. Water collects around these microscopic particles, forming tiny cloud droplets.

A cloud is essentially an enormous collection of these suspended water droplets or, when temperatures are cold enough, ice crystals. But forming a cloud doesn't automatically mean rain is coming.

Why Don't All Clouds Produce Rain?

Cloud droplets are extremely small. A typical cloud droplet is only a tiny fraction of the size of a raindrop and is light enough to remain suspended in moving air. For rain to fall, those droplets need to grow.

In warmer clouds, droplets collide and merge with other droplets in a process known as collision and coalescence. As a droplet becomes larger, it falls faster, allowing it to collide with even more droplets.

Eventually it becomes heavy enough to overcome the rising air currents inside the cloud and falls toward the ground.

In colder clouds, another process becomes important. Clouds can contain a mixture of ice crystals and tiny droplets of liquid water that remain unfrozen even below 32°F. Water vapor tends to collect on the ice crystals, causing them to grow. The crystals may eventually fall as snowflakes or melt on their way down and reach the ground as rain.

If cloud droplets or ice crystals never become large enough, however, the cloud may pass overhead without producing measurable precipitation.

Sometimes Rain Falls From a Cloud but Never Reaches the Ground

You may occasionally see wispy streaks hanging beneath a cloud that seem to disappear before reaching the surface. This is called virga.

Virga occurs when precipitation falls into a layer of dry air below the cloud and evaporates—or, in the case of ice crystals, sublimates—before reaching the ground.

So a cloud can actually be producing precipitation overhead while your rain gauge remains completely dry.

Why Do Some Clouds Produce Light, Steady Rain?

The character of rainfall depends partly on the type and structure of the cloud producing it.

Broad, layered clouds such as nimbostratus often produce widespread, relatively steady precipitation. These clouds can cover enormous areas, and the vertical air movement inside them is generally less vigorous than it is inside a thunderstorm.

The result may be hours of light or moderate rain rather than a sudden downpour. This kind of rainfall can add up substantially even when it never seems particularly heavy.

Why Do Thunderstorms Produce Such Heavy Rain?

Thunderstorms are a very different kind of cloud system.

A mature cumulonimbus cloud can extend from relatively low in the atmosphere to tens of thousands of feet above the ground. Strong updrafts carry warm, moisture-rich air high into the cloud, where water droplets and ice particles can continue to grow.

Those powerful upward air currents can temporarily keep large amounts of water suspended inside the storm. Eventually, precipitation becomes too heavy for the updrafts to support. That's when the rain can really come down.

A thunderstorm may therefore produce an intense burst of rainfall in a relatively small area, while a layered storm system produces lighter rain over a much larger area and for a longer period of time.

Why Can One Thunderstorm Drop More Rain Than Another?

Not every thunderstorm has the same amount of water available to it. Several factors affect how much rain a storm can produce, including:

• Atmospheric moisture. The more water vapor available, the more potential precipitation a storm can produce.
• Strength of the updrafts. Strong rising air can support the growth of larger droplets and ice particles.
• Storm speed. A slow-moving storm can dump far more rain in one location than a fast-moving storm with a similar rainfall rate.
• Storm development. Some storms repeatedly form new cells over the same area, a phenomenon sometimes called training.
• Temperature and atmospheric conditions. These influence how efficiently cloud droplets and ice crystals grow into precipitation.

This is one reason rainfall totals from thunderstorms can vary so dramatically over surprisingly short distances. Your neighborhood might receive an inch of rain while a location only a few miles away gets barely enough to wet the pavement.

Sometimes the difference can even be noticeable from one part of a large property to another, particularly when a small, intense storm cell passes directly overhead.

Drizzle, Showers and Downpours: What's the Difference?

Not all liquid precipitation is the same.

Drizzle consists of very small water droplets and typically falls from low, relatively uniform clouds. It may seem insignificant, although prolonged drizzle can eventually produce measurable rainfall.

Rain consists of larger droplets and can range from light to heavy.

Showers tend to begin and end more abruptly and are often associated with clouds created by rising, unstable air.

A downpour isn't a separate type of precipitation so much as a description of very heavy rainfall over a short period.

That distinction matters when measuring rainfall. Ten minutes of an intense summer downpour can sometimes put more water in a rain gauge than several hours of light rain.

What About Hail?

Hail forms differently from ordinary rain.

Inside strong thunderstorms, powerful updrafts can carry ice particles repeatedly through regions containing supercooled water droplets. Those droplets freeze onto the particles, allowing hailstones to grow in layers.

Eventually the hailstones become too heavy for the storm's updrafts to keep aloft and fall to the ground.

That is why hail is most closely associated with strong thunderstorms rather than ordinary rain clouds.

From Water Vapor to Your Rain Gauge

What eventually lands in your rain gauge began as invisible water vapor. That vapor cooled and condensed into microscopic droplets or formed ice crystals. Those particles grew through collisions and other processes until gravity finally won the battle against the air currents holding them aloft.

And even then, whether that water reached your yard depended on where the cloud traveled, how quickly the storm moved and what happened to the precipitation on its way to the ground. It's a surprisingly complicated journey for something that looks as simple as rain.

And it's also why measuring rainfall where it actually falls can tell you something a regional forecast or weather station can't: how much water your own garden and landscape actually received.