Section 5.4:
How Precipitation Forms

Learning Objective

Describe the Bergeron process and explain how it differs from the collision–coalescence process.

Section Content

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

Mini-Lecture Video - How Precipitation Forms (Click to watch the video)

Typical cloud droplets are minuscule—20 micrometers (0.02 millimeter) in diameter (Figure 5.14). In comparison, a human hair is about 75 micrometers in diameter. Because of their small size, cloud droplets fall through still air incredibly slowly. An average cloud droplet falling from a cloud base at 1000 meters (3280 feet) would require several hours to reach the ground. However, it would never complete its journey. Instead, the cloud droplet would evaporate before it fell a few meters from the cloud base into the unsaturated air below.

Figure 5.14
Diameters of particles involved in condensation and precipitation processes

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 (Figure 5.14). 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. You might suspect that additional condensation creates drops large enough to survive the descent to the surface. However, clouds consist of many billions of tiny cloud droplets that all compete for the available water. Thus, condensation provides an inefficient means of raindrop formation.

Two processes are responsible for the formation of precipitation: the Bergeron process and the collision–coalescence process.

Precipitation from Cold Clouds:
The Bergeron Process

The Bergeron process, which generates much of the precipitation in the middle and high latitudes, is named for its discoverer, the highly respected Swedish meteorologist Tor Bergeron. To understand how this mechanism operates, we must first examine two important properties of water.

Supercooled Water

The Bergeron process operates in cold clouds at temperatures below 0°C (32°F), where liquid cloud droplets and ice crystals coexist. Contrary to what you might expect, cloud droplets do not usually freeze at 0°C (32°F). In order for water to freeze, it must first cool to its freezing temperature, which causes the water molecules to slow down. Next, the molecules need to bond together to form ice crystals, which requires the loss of even more energy. In fact, pure water will not freeze until it reaches a temperature of about −40°C (−40°F). Water in the liquid state below 0°C (32°F) is referred to as supercooled water. However, supercooled water readily freezes if it collides with an object, which explains why airplanes collect ice when they pass through a cold cloud of supercooled droplets. Supercooled water droplets also cause freezing rain, which falls as a liquid but then turns to a sheet of ice when it strikes the pavement, tree branches, and car windshields.

In the atmosphere, supercooled droplets freeze on contact with solid particles that have a shape closely resembling that of ice (silver iodide, for example). These materials, called freezing nuclei, are sparse in the atmosphere and do not generally become effective until the air temperature is about −15°C (5°F) or colder. Thus, at temperatures between 0 and −15°C, most clouds consist only of supercooled water droplets (Figure 5.15). Between −15 and −40°C, most clouds consist of supercooled droplets that coexist with ice crystals, and at temperatures colder than −40°C (−40°F), clouds are composed entirely of ice crystals. For example, the tops of towering cumulonimbus clouds and wispy high-altitude cirrus clouds are usually composed entirely of ice crystals.

Figure 5.15
Distribution of cloud particles found in towering cumulonimbus clouds

Temperature at different heights within these clouds determine where liquid droplets or ice crystals are found.

Saturation Vapor Pressure over Water Versus over Ice

Another important property of water is that the saturation vapor pressure above ice crystals is lower than above water droplets. Stated another way, when the air surrounding a water droplet is saturated (100 percent relative humidity), it is supersaturated relative to a nearby ice crystal. For example, Table 5.3 shows that at −10°C (14°F), when the relative humidity is 100 percent with respect to water, the relative humidity with respect to ice is about 110 percent. This is because ice crystals are solid, so the individual ice molecules are held together more tightly than those of a liquid droplet. For the same reason, water vapor molecules escape (evaporate) from water droplets at a faster rate than from ice crystals at the same temperature.

Table 5.3
Relative Humidity with Respect to Ice When Relative Humidity with Respect to Water Is 100 Percent

How the Bergeron Process Generates Precipitation

When ice crystals and supercooled water droplets coexist, the conditions are ideal for creating precipitation. Because freezing nuclei are sparse, cold clouds consist of relatively few ice crystals (snow crystals) surrounded by numerous liquid droplets (Figure 5.16A). Since the air is supersaturated with respect to the comparatively few ice crystals, the water molecules will begin to collect on the ice crystals by the process of deposition. This in turn lowers the overall relative humidity of the air as the water vapor becomes solid. In response, the surrounding water droplets will begin to evaporate to replenish the lost water vapor (Figure 5.16B). Thus the growth of ice crystals is fed by the continued evaporation and shrinkage of the liquid droplets (Figure 5.16C).

Figure 5.16
The Bergeron process

Ice crystals grow at the expense of cloud droplets until they are large enough to fall. A. As water vapor is deposited on ice crystals, liquid water evaporates from the cloud droplets to maintain air saturation. The ice crystals grow, while the cloud droplets become smaller (B, C).

When ice crystals become sufficiently large, they begin to fall because of gravity. Air movement will sometimes break up these delicate crystals, and the fragments will serve as new freezing nuclei that draw water vapor from other liquid droplets. A chain reaction ensues and produces many ice crystals, which grow into snowflakes.

The Bergeron process can produce precipitation throughout the year in the middle latitudes, provided that at least a portion of a cloud is cold enough, about −15°C (5°F), to generate ice crystals. The type of precipitation (snow, sleet, rain, or freezing rain) that reaches the ground depends on the temperature profile in the lower few kilometers of the atmosphere. When the surface temperature is above 4°C (39°F), snowflakes usually melt before they reach the ground and continue their descent as rain. Even on a hot summer day, a heavy rainfall may have begun as a snowstorm high in the clouds overhead. During a middle-latitude winter, even low clouds are cold enough to trigger precipitation via the Bergeron process.

Precipitation from Warm Clouds:
The Collision–Coalescence Process

The collision–coalescence process is the dominant process for generating precipitation in warm clouds—clouds with tops warmer than −15°C (5°F). Simply, the collision–coalescence process involves multiple collisions of tiny cloud droplets that stick together (coalesce) to form raindrops large enough to reach the ground before evaporating.

One of the requirements for the formation of raindrops by the collision–coalescence process is the presence of larger-than-average cloud droplets. Research has shown that clouds made entirely of liquid droplets usually contain some droplets larger than 20 micrometers (0.02 millimeter). These relatively large droplets may form when “giant” condensation nuclei are present or when hygroscopic particles (such as sea salt) are carried by updrafts into the atmosphere. Hygroscopic particles begin to collect water vapor at relative humidity below 100 percent. When large cloud droplets are intermixed with numerous smaller droplets, the conditions are ideal for the formation of precipitation.

Cloud Droplet Size and Fall Velocities

The maximum speed at which an object falls, called its terminal velocity, occurs when air resistance equals the gravitational pull on the object. Because large droplets have a smaller ratio of surface area as compared to their weight, they fall faster than small droplets. Imagine that you go skydiving while wearing a baseball cap. As you make the jump, your cap comes off. Because your body has a low ratio of surface area compared to your weight, you will have a much higher terminal velocity than your baseball cap. Table 5.4 summarizes how this principle applies to cloud droplets and their fall velocities.

Table 5.4
Fall Velocity of Water Drops

As the larger droplets fall through a cloud, they collide with smaller, slower droplets and coalesce. They become larger in the process and fall even more rapidly (or, in an updraft, they rise more slowly), which increases their chances of more collisions and growth (Figure 5.17A). After a million or so cloud droplets coalesce, they form a raindrop that is large enough to fall to the surface without evaporating.

Figure 5.17
The collision–coalescence process

The collision–coalescence process involves multiple collisions of tiny cloud droplets that stick together (coalesce) to form raindrops large enough to reach the ground before evaporating.

Because of the huge number of collisions required for growth to raindrop size, clouds that have great vertical thickness and contain large cloud droplets have the best chance of producing precipitation. Updrafts associated with unstable air also aid this process because the droplets can traverse the cloud repeatedly, which results in more collisions.

As raindrops grow in size, their fall velocity increases. This in turn increases the frictional resistance of the air, which causes the drop’s “bottom” to flatten out (Figure 5.17B). As a drop approaches 4 millimeters in diameter, it develops a depression, as shown in Figure 5.17C. Raindrops can grow to a maximum of 5 millimeters when they fall at the rate of 33 kilometers (20 miles) per hour. At this size, the water’s surface tension, which holds the drop together, is surpassed by the frictional drag of the air. The depression grows almost explosively, forming a donutlike ring that immediately breaks apart. The resulting breakup of a large raindrop produces numerous smaller drops that begin anew the task of sweeping up cloud droplets (Figure 5.17D).

How Do Cloud Droplets Coalesce?

The collision–coalescence process is not as simple as it may first seem. First, as the larger droplets descend, they produce an airstream around them similar to that produced by an automobile traveling rapidly down the highway. The airstream sweeps aside objects, especially the smallest cloud droplets. Imagine driving on a summer night along a country road. The bugs in the air are like cloud droplets: Most are pushed aside, but larger bugs (cloud droplets) have an increased chance of colliding with the car (giant droplet).

Further, collision does not guarantee coalescence. Experimentation has indicated that the presence of atmospheric electricity may be the key to what holds these droplets together once they collide. If a droplet with a negative charge collides with a positively charged droplet, their electrical attraction may bind them together.

The air over the tropical oceans is an ideal setting for the development of precipitation by the collision–coalescence process because the relatively clean air contains fewer condensation nuclei compared to the air over populated urban regions. With fewer condensation nuclei to compete for available water vapor (which is plentiful), condensation is fast-paced and produces comparatively few large cloud droplets. Within developing cumulus clouds, the largest drops quickly gather smaller droplets to generate the warm afternoon showers associated with tropical climates.

In the middle latitudes, the collision–coalescence process contributes to the precipitation from a large cumulonimbus cloud by working in tandem with the Bergeron process—particularly during the hot, humid summer months. High in these towers, the Bergeron process generates snow that melts as it passes below the freezing level. When snowflakes melt, they generate relatively large drops with fast fall velocities. As these large drops descend, they overtake and coalesce with the slower and smaller cloud droplets that comprise much of the lower regions of the cloud. The result can be a heavy downpour.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Describe the Bergeron process.

The Bergeron process operates in cold clouds, at temperatures below 0°C (32°F), where liquid cloud droplets and ice crystals coexist. The Bergeron process relies on the fact that cloud droplets do not freeze until they reach a temperature below the freezing point and, even then, only in the presence of freezing nuclei (solid particles that have a crystal form like ice). Because freezing nuclei are much less abundant than condensation nuclei, many clouds exist in the liquid state while at temperatures well below 0°C (32°F). The freezing nuclei present promote the formation of a few scattered ice crystals.

Explain how snow that formed high in a towering cloud might produce rain.

High in the towering clouds, the Bergeron process generates snow that melts as it passes below the freezing level. When snowflakes melt, they generate relatively large drops with fast fall velocities. As these large drops descend, they overtake and coalesce with the slower and smaller cloud droplets that comprise much of the lower regions of the cloud. The result can be a heavy downpour.

Briefly summarize the collision–coalescence process.

The collision–coalescence process produces precipitation in warm clouds (above 0°C). In the collision–coalescence process, large cloud droplets fall more rapidly than smaller droplets, sweep up the smaller ones in their path, and grow. Experiments show that atmospheric electricity may play an important role in this process.