If all clouds contain water, why do some produce precipitation while others drift placidly overhead? This seemingly simple question perplexed meteorologists for many years.

Typical cloud droplets are minuscule—0.02 millimeter (20 micrometers) in diameter (▼), or about a quarter the width of a human hair (about 0.075 millimeter [75 micrometers]). Because of their small size, cloud droplets in still air fall incredibly slowly. An average cloud droplet falling from a cloud base would require several hours to reach the ground—except that it would actually evaporate within a few meters in the unsaturated air below the cloud.

How large must a cloud droplet grow in order to fall as precipitation? A typical raindrop has a diameter of about 2 millimeters, or 100 times that of the average cloud droplet (▲). However, the volume of a typical raindrop is 1 million times that of a cloud droplet. Thus, for precipitation to form, cloud droplets must grow in volume by roughly 1 million times. Two processes are responsible for the formation of precipitation: the Bergeron process and the collision–coalescence process.

Precipitation from Cold Clouds:
The Bergeron Process

You have probably watched a documentary in which mountain climbers braved intense cold and ferocious snow storms to scale ice-covered peaks. Although it is hard to imagine, similar conditions exist in the upper portions of towering cumulonimbus clouds, even on sweltering summer days. It is within these cold clouds that a mechanism called the Bergeron process generates much of the precipitation that occurs in the middle and high latitudes.

The Bergeron process is based on the fact that cloud droplets remain liquid at temperatures as low as -40°C (-40°F). Liquid water at temperatures below freezing is termed supercooled, and it becomes solid, or freezes, upon impact with a surface. This explains why airplanes collect ice when they pass through a cloud composed of subzero droplets, a condition called icing. Supercooled water droplets also freeze upon contact with particles in the atmosphere known as ice nuclei, or freezing nuclei. Because ice nuclei are relatively sparse, cold clouds primarily consist of supercooled droplets intermixed with a lesser amount of ice crystals.

When ice crystals and supercooled water droplets coexist in a cloud, the conditions are ideal for generating precipitation. Because ice crystals have a greater affinity for water vapor than does liquid water, they collect the available water vapor at a much faster rate. In turn, the water droplets evaporate to maintain saturation and replenish the diminishing water vapor, thereby providing a continual source of moisture for the growth of ice crystals. As shown in the figure below▼ , the result is that the ice crystals grow larger—at the expense of the water droplets, which shrink in size.

Eventually, this process generates ice crystals large enough to fall as snowflakes. During their descent, these ice crystals become larger as they intercept supercooled cloud droplets that freeze on them. When Earth’s surface temperature is about 4°C (39°F) or higher, snowflakes usually melt before they reach the ground and continue their descent as rain.

Precipitation from Warm Clouds:
The Collision–Coalescence Process

A few decades ago, meteorologists believed that the Bergeron process was responsible for the formation of most precipitation. However, it was discovered that copious rainfall may be produced within clouds located well below the freezing level (warm clouds), particularly in the tropics. This led to the proposal of a second mechanism thought to produce precipitation—the collision–coalescence process.

Research has shown that clouds composed entirely of liquid droplets must contain some droplets larger than 20 micrometers (0.02 millimeter) for precipitation to form. These large droplets usually form when hygroscopic particles (particles that attract water), such as sea salt, are abundant in the atmosphere. Hygroscopic particles begin to remove water vapor from the air when the relative humidity is under 100 percent, and the cloud droplets that form on them can grow quite large. Because the rate at which drops fall is size dependent, these “giant” droplets fall most rapidly. As they plummet, they collide with smaller, slower droplets (▼) becoming larger through accretion of those droplets. After many such collisions, these droplets may grow large enough to fall to the surface without evaporating. Updrafts also aid this process because the airflow propels the droplets so they remain in the cloud longer, allowing for additional collisions.

Raindrops can grow to a maximum size of  millimeters, at which point they fall at a rate of 33 kilometers (20 miles) per hour. Beyond this size and speed, the water’s surface tension, which holds the drop together, is overcome by the drag imposed by the air, causing the drops to break apart (▲). The resulting breakup of a large raindrop produces numerous smaller drops that begin anew the task of sweeping up cloud droplets.

• For precipitation to form, millions of cloud droplets must join together into drops that are large enough to reach the ground before evaporating.

• The two mechanisms that generate precipitation are the Bergeron process, which produces precipitation from cold clouds primarily in the middle and high latitudes, and the collision–coalescence process, which occurs in warm clouds and primarily in the tropics.

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