Precipitation

Precipitation Formation

Precipitation is any form of water that falls from the sky to the Earth’s surface. It is a key part of the water cycle, which moves water around our planet. Precipitation starts when water vapor in the air cools and condenses into tiny droplets or ice crystals, forming clouds. When these droplets or crystals become heavy enough, gravity pulls them down as precipitation.

The Collision-Coalescence Process

The collision-coalescence process is one way raindrops form in clouds, especially in warm clouds where temperatures are above freezing. It starts with tiny cloud droplets floating in the air. These droplets move at different speeds and sometimes collide with each other. When two droplets collide and stick together, they form a larger droplet. This process repeats over and over, with droplets merging and growing bigger.

Eventually, the droplets become heavy enough that gravity pulls them down from the cloud as rain. The collision-coalescence process is more common in tropical regions, where clouds are thick and full of water droplets.

Essentially, with the collision-coalescence process, the precipitation begins as rain and hits the ground as rain.

The Bergeron Process

The Bergeron process is one way that precipitation forms in cold clouds, where temperatures are below freezing. In these clouds, tiny ice crystals and supercooled water droplets (liquid water below 0°C) exist together. Ice crystals grow because water vapor in the cloud moves from the supercooled droplets to the ice crystals. As the ice crystals get bigger, they eventually become heavy enough to fall from the cloud as snow or ice.

Sometimes, the falling ice melts on its way down if it passes through warmer air, turning into rain. The Bergeron process is especially important in mid- and high-latitude regions, where cold clouds are common. It helps explain why snow and other frozen precipitation form even in clouds that contain liquid water.

Essentially, with the Bergeron process, the precipitation begins with snow and then depending on the air temperature at lower altitudes, could hit the ground as snow, rain, or another type of precipitation.

Types of Precipitation

Rain

Rain forms when water droplets inside a cloud grow large enough to fall to the ground. This usually happens through the collision-coalescence process in warm clouds or when ice crystals in cold clouds formed by the Bergeron process melt as they fall through a thick layer of warm air. If the air near the surface is above freezing, the melted droplets reach the ground as rain.

Fun Fact:

The speed at which raindrops fall depends on their size.

Small drops (0.5 mm) → about 2 mph
Medium drops (1–2 mm) → about 10–15 mph
Large drops (4–5 mm) → about 20–25 mph
Very large drops (6 mm, almost the largest raindrops possible before breaking apart) → about 30 mph

The reason rain doesn’t fall faster is because air resistance slows them down. Larger drops eventually break apart if they get too big, so there’s a limit to how fast a raindrop can fall.

Snow

Snow forms in cold clouds by the Bergeron process, where temperatures are below freezing. Ice crystals grow as water vapor sticks to them, causing them to form snowflakes. If the air all the way to the ground stays below 32°F (0°C), the snowflakes never melt and reach the surface as snow.

Fun Fact:

Have you ever wondered why there are so many different types of snowflakes? Why some snow falls as a light powder and some snow is heavy and wet? There are different types of snow because temperature, humidity, and conditions inside clouds and near the ground all affect how snow crystals form, grow, and change as they fall.

1. Temperature in the cloud

Snowflakes form when water vapor freezes onto tiny ice crystals.

Very cold clouds → crystals grow slowly and stay small and powdery.
Slightly warmer clouds → crystals stick together into bigger, fluffier flakes.

Different temperatures allow different crystal shapes, such as plates, needles, columns, and dendrites (the classic “branching” snowflakes).

2. Temperature as the snow falls

The snowflake may pass through layers of warmer or colder air.

Stays below freezing all the way down → snowflakes remain intact and fluffy.
Near-freezing air → flakes start to melt a little, becoming wetter and heavier.
Falls through a slightly warm layer, then cold air again → can form graupel or sleet instead of classic snow.

3. Humidity

Humidity affects how quickly crystals grow and how complex they become.

High humidity → big, branching, fluffy flakes (dendrites).
Low humidity → small, simple crystals like plates or needles.

4. Collisions inside the cloud

If snowflakes bump into supercooled water droplets, ice can build up around them, creating graupel (soft hail-like pellets).

If snow crystals collide and stick together, they form aggregates, the big flakes that look like “snow clumps.”

Sleet

Sleet starts as snow high in the cloud. As the snow falls, it passes through a layer of warm air and melts into raindrops. Then the drops enter a deep layer of cold air closer to the ground and freeze into tiny ice pellets. These pellets bounce when they hit the surface—this is sleet.

Glaze (Freezing Rain)

Glaze, or freezing rain, also begins as snow that melts into rain on its way down. The difference from sleet is that the cold layer near the ground is shallow and not cold enough to refreeze the drops in the air. Instead, the supercooled raindrops freeze instantly when they hit cold surfaces such as roads, trees, and power lines, forming a coating of smooth, clear ice. When you hear about “black ice” on roads and sidewalks, this is glaze.

Fun Fact:

Supercooled water is liquid water that’s below 0 °C (32 °F)—sometimes as low as about −40 °C—but hasn’t frozen yet.

Freezing isn’t just about temperature; water also needs a starting point to build ice crystals. Scientists call this nucleation.

In normal water, tiny impurities, dust, or rough surfaces give ice crystals something to form on.
In very pure water with no disturbances, there may be nothing to start the freezing process, even though it’s cold enough.

So the water stays liquid… kind of on a knife edge.

Hail

Hail forms inside strong thunderstorms with powerful updrafts. Ice pellets are lifted upward repeatedly by the updraft, gaining new layers of ice each time they rise through supercooled water droplets. The more times they go up and down within the storm, the larger they grow. When the hailstones become too heavy for the updraft to hold, they fall to the ground as chunks of ice—hail.

Fun Fact:

The largest hailstone ever recorded was found in Vivian, South Dakota in 2010. It measured 8 inches across. This would be slightly smaller than the size of a basketball (a basketball is about 9.5 inches across). Its weight measured about 1.94 pounds - about the weight of a large pineapple.

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