5.3: Cloud Droplets

Aerosols

When air becomes saturated, excess water vapor in the atmosphere can condense to form liquid water. However, water vapor requires a surface on which to condense. The surface is provided by tiny atmospheric particles known as aerosols, or, more generally, cloud condensation nuclei (CCN). Aerosols are everywhere in the atmosphere. They can be composed of earthy matter, such as dust, clay, or soot. They can also be composed of sea salt from the ocean, black carbon from fires, or sulfate from volcanic activity. They can be composed of material from plant or organism life, such as sulfates and nitrates, volatile organic compounds, or even phytoplankton. Anything small enough to be lofted and suspended in the atmosphere can become an aerosol. If there were no aerosols in the atmosphere, relative humidities well over 100% would be needed for clouds to form. In laboratory studies under a clean atmosphere (no aerosols), relative humidities up to 400% have been measured. In Earth’s atmosphere, aerosols are abundant, so water vapor does not struggle to find CCN to condense onto.

Many CCNs are hygroscopic or “water-loving”, meaning they tend to absorb moisture, so condensation may start before the relative humidity reaches 100%. For example, when condensation occurs on extremely hygroscopic salt particles, it can begin at 80% relative humidity or lower. Imagine there are many CCNs of differing sizes in a body of humid, but unsaturated air. If the air were lifted by a mountain or a rising thermal, it would cool, and the relative humidity would increase. As air nears saturation, condensation will begin on the largest and most hygroscopic CCNs. Soon, a cloud of many small cloud droplets, far too small to fall at any significant speed, will form.

Raindrops

So how can cloud droplets grow to form raindrops? The condensation process is insufficient and far too slow to produce raindrops. From a volumetric view, it takes 1 million cloud droplets (10 μm radius) to combine together to make one single raindrop (1000 μm = 1 mm radius). There has to be another faster process by which cloud droplets can grow or combine together to become large and heavy enough to fall.

We will discuss two primary theories of rain formation.

1. Collision-Coalescence Process
2. Ice Phase Processes (Bergeron)

Collision-Coalescence Process

In warm clouds, where all cloud droplets are liquid, the collision-coalescence process is the primary mechanism for producing precipitation. This is thought to be especially true over tropical oceans. The collision-coalescence process is exactly as it sounds: cloud droplets collide and coalesce or stick together. Larger cloud droplets have slightly higher terminal velocities because they have a smaller surface-area-to-weight ratio. This advantage allows them to fall faster and collide with smaller cloud droplets. Sometimes cloud droplets stick together and coalesce into a larger droplet. This begins a positive feedback loop in which these larger droplets fall even faster, collide with even more smaller droplets in their path, and aggregate more and more cloud droplets. However, note that a collision between cloud droplets does not always mean that coalescence will occur. Sometimes droplets will bounce apart during collisions if their surface tension is too high. For collision-coalescence to begin, a cloud needs to have a wide distribution of cloud droplet sizes. This can occur from variations in CCN type—for example, sea-salt aerosols are particularly large—or from random collisions between droplets.

The total amount of liquid water in a cloud, as well as the time a cloud droplet spends in a cloud, influences how large it can grow through collision-coalescence. The cloud height is, of course, a factor here, but it's a little more complicated than that. Rising motion in a cloud will slow the downward speed of a falling droplet. This can increase the time a cloud droplet spends in a cloud and the size it grows to. Let’s cover a few examples.

Deep cumulus clouds with strong convective updrafts tend to produce larger raindrops because the updrafts are strong and droplets have more time in the cloud to grow. In fact, the droplets need to become sufficiently large in order for their fall velocity to overcome the updraft velocity. On the other hand, stratus clouds are typically not very thick and have weak updrafts, so droplets in these clouds don’t spend a long time in the cloud itself, and therefore are not able to grow very large. If moist air lies below the stratus cloud, the drops may reach the ground as a light drizzle. However, if there is dry air below the stratus cloud, the drops may evaporate before reaching the ground.

To summarize, in warm clouds, cloud droplets grow to precipitation-sized drops through collision-coalescence. The most important factor in raindrop production is the liquid water content of a cloud. Assuming the cloud has sufficient water, other factors that affect raindrop production are: thickness of the cloud; strength of updrafts within the cloud; cloud drop distribution of sizes; and difference in electric charge of the droplets and the cloud itself. Thin stratus clouds with weak vertical motion may produce weak drizzle, if any, while tall cumulus clouds with strong updrafts can produce heavy rain showers. Figure \(\PageIndex{1}\) illustrates the collision-coalescence process of raindrop production.

Figure \(\PageIndex{1}\): The collision-coalescence process occurring in warm clouds. The left image shows a slightly larger spherical cloud droplet falling in an environment of smaller cloud droplets and demonstrates the importance of a range of cloud droplet sizes to initiate the collision-coalescence process. The image at right shows a fully formed oval-shaped raindrop rapidly collecting additional droplets as it falls, and highlights the acceleration of the collision-coalescence process once a raindrop forms (CC-BY-SA 4.0; Britt Seifert via Atmospheric Processes and Phenomenon).

Ice Phase (Bergeron) Process

Figure \(\PageIndex{2}\): Ice crystals forming on a window at 30,000 feet (CC-BY-SA 3.0; Brocken Inaglory via Wikimedia Commons).

Outside the tropics, the ice-phase process of rain formation is the primary mechanism producing most of the world's precipitation. The ice-phase process occurs in cold clouds, where temperatures are below 0°C. To understand why, we need to know how liquid water droplets freeze.

Supercooled Water and Ice Nuclei

Supercooled water is liquid water below its freezing point (0°C). Similar to how cloud droplets need a surface on which to condense, ice crystals also need a nucleus or ice embryo to freeze. Without an ice nucleus, liquid water drops can remain liquid in temperatures as low as -40°C. Once beyond -40°C, all hydrometeors (water particles) will exist in the solid state. Typically, the distribution of liquid and solid hydrometeors in a cloud is similar to Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): A simple diagram showing the distribution of liquid and solid phase hydrometeors in a mixed phase cloud. Description in the following text. (CC-BY-SA 3.0; BoH via Wikimedia Commons)

At low elevations above 0°C (region 4), the hydrometeors in the cloud exist as liquid droplets. Between 0°C and -10°C (region 3), supercooled liquid droplets exist. In the layer between -10°C and -23°C, some liquid droplets begin to freeze, and both liquid and ice-phase hydrometeors coexist (region 2). Finally, above some level where the temperature is below -40°C, all hydrometeors will exist in their solid state (region 1).

When liquid water droplets freeze without any nucleus, this is known as homogeneous or spontaneous freezing. While this occurs within a large body of freshwater at temperatures slightly below 0°C, cloud droplets will not freeze spontaneously until temperatures are -40°C or lower. For droplets to freeze spontaneously, enough molecules within the droplet must form a rigid pattern, forming a tiny ice structure known as an ice embryo. When this embryo reaches a certain size, it will act as an ice nucleus. The other molecules in the droplet then become attached to the ice structure, and the entire droplet freezes.

Tiny ice embryos form when the in-cloud temperature drops just below freezing, but at these temperatures, thermal agitation is typically enough to weaken their structure and break them apart. At lower temperatures, there is less thermal motion, and ice embryos are more likely to grow large enough to freeze the surrounding water. When you have larger volumes of water, ice embryos have a better chance of growing large enough to freeze the surrounding liquid before being broken up, but this becomes more and more difficult with smaller volumes of water. Only the largest cloud droplets can freeze spontaneously without a nucleus at temperatures below -40°C. In most cases, ice nuclei are required for ice crystals to form in sub-freezing clouds.

Just as CCNs are required for liquid cloud droplets to form, ice crystals form on particles called ice nuclei (IN). Particles serve as effective IN if they have a similar geometry to an ice crystal; for example, ice itself is an effective IN. There are not many IN in the atmosphere, especially at temperatures above -10°C, but certain types of particles become active IN with lower temperatures. For example, dust can be an effective IN. Ice nuclei are rare compared to hygroscopic cloud condensation nuclei. A common cloud-seeding agent is silver iodide (AgI).

Some IN allow water vapor to immediately become solid ice when they come into contact with each other. These are known as deposition nuclei because water vapor changes phase directly into solid ice without first becoming liquid (the phase change from gas to solid is called deposition). INs that are effective at freezing supercooled liquid droplets are called freezing nuclei. Some freezing nuclei must be immersed in a liquid drop to freeze, while others are effective at inducing condensation and then freezing. Many freezing nuclei cause supercooled droplets to freeze upon collision, a process called contact freezing; these nuclei are called contact nuclei. These different freezing methods are outlined in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Four processes of ice formation in the atmosphere through four mechanisms: homogeneous freezing, deposition nucleation, immersion/condensation freezing, and contact freezing. Each process involves interactions among cloud droplets, water vapor, aerosol particles, and ice embryos, leading to the formation of ice crystals at temperatures below freezing. (CC-BY-SA 4.0; Pre2grk, via Wikimedia Commons).

A detailed description of Figure 5.3.4 is provided in Table 5.3.1.

Table 5.3.1. Different mechanisms of frozen precipitation formation.

Mechanism	Temperature range	Initial phase	Trigger/Process	Resulting phase
Homogeneous freezing	≤ ~−38°C	Liquid droplet	Spontaneous freezing without particles	Ice crystal
Deposition nucleation	< 0°C (often well below)	Water vapor	Vapor deposits directly onto aerosol particle	Ice crystal
Immersion freezing	< 0°C	Liquid droplet	Aerosol particle immersed in droplet induces freezing	Ice crystal
Condensation freezing	< 0°C	Water vapor → droplet	Condensation onto particle followed by freezing	Ice crystal
Contact freezing	< 0°C	Liquid droplet	Collision with aerosol particle triggers freezing	Ice crystal

To summarize, cloud droplets may freeze spontaneously, but only at very low temperatures. Ice nuclei can help ice crystals grow, but they are not naturally abundant.

Saturation Vapor Pressure

So, we have cold clouds that contain many more liquid cloud droplets than ice crystals, even at sub-freezing temperatures, and these particles are not large/heavy enough to precipitate out of the cloud. How do we get rain and snow out of the ice-crystal process, then?

Imagine a cloud with supercooled liquid water and saturated air. At saturation, the liquid droplets are in equilibrium with the water vapor in the air. The number of water molecules leaving and entering the surface of the liquid droplets is equivalent. Now imagine that an ice crystal forms by one of the processes described above. In the below-freezing portion of a cloud, this ice crystal is surrounded by many liquid supercooled droplets. Because the saturation vapor pressures over liquid and ice differ slightly, the presence of this new ice crystal significantly impacts the cloud.

Figure \(\PageIndex{5}\): The saturation vapor pressure with respect to liquid (blue) and with respect to ice (purple). The difference is shown at the top (CC-BY-NC-SA 4.0; Roland Stull via Practical Meteorology).

Figure \(\PageIndex{5}\) compares water vapor saturation over liquid water and ice at different temperatures. The upper graph illustrates the difference in vapor pressure between water and ice, with regions labeled as saturated or unsaturated relative to each phase. The difference between saturation vapor pressure with respect to liquid and saturation vapor pressure with respect to ice starts out small (around 0.005 kPa) at a temperature below -40°C, but then gradually increases to a peak of aproximately 0.025kPa at a temperature of approximately -10°C before becoming 0 at 0°C. The lower graph depicts how saturation vapor pressure increases with temperature, with separate curves for saturation relative to liquid water and to ice. The difference between the two curves is calculated in the upper graph, but note that between approxmately -40°C and 0°C, saturation vapor pressure with respect to ice is lower than saturation vapor pressure with respect to liquid.

With respect to the liquid, the liquid droplets were at saturation. But now, with respect to ice, the ice crystal is in a supersaturated environment. You can think of this as the following: it is easier for water molecules to escape a liquid surface through evaporation than to escape a solid surface. This means that, at a given temperature, many more molecules will escape from the liquid water surface, and it will require more water vapor around the droplet to keep it in saturation. At the same temperature, the saturation vapor pressure above a surface of water is greater than the saturation vapor pressure above a surface of ice.

This difference in saturation vapor pressure between water and ice causes water vapor molecules to deposit onto the ice crystal from the environment. Because vapor molecules are removed from the surrounding environment, the vapor pressure above the water surface decreases. This throws the water droplets out of equilibrium with their surroundings, causing them to evaporate and replenish the water vapor removed from the environment. This provides additional moisture for the ice crystal, allowing it to grow at the expense of the liquid droplets. This process is called the Bergeron (or, more generally, the ice-phase) process. Ice crystals in a sub-freezing region of a cloud will grow larger at the expense of surrounding water droplets.

Falling Ice Crystals

Imagine this happening throughout a large cloud. The water vapor within the cloud, as well as that from evaporating supercooled liquid droplets, provides a continuous source of moisture for ice crystals, allowing them to grow rapidly. Eventually, these ice crystals become large enough to fall. The same issues with updrafts and rising air apply, but at some point, the crystals will fall faster than the updrafts within the cloud.

Sometimes, ice crystals collide with nearby supercooled droplets in the cloud, causing the droplets to freeze onto the crystals. The ice crystal will grow larger and larger as it collides with more droplets; this is called riming or accretion. This forms an icy clump called graupel (also known as snow pellets), which may break apart into tiny ice particles as it collides with more droplets. These splinters may form graupel of their own as they collide with other droplets, which in turn may also splinter, causing a chain reaction.

In colder clouds, ice crystals may collide and break apart into smaller ice particles, which act as tiny seeds that can freeze supercooled droplets on contact. This could also cause a chain reaction that produces many ice crystals. As these ice crystals fall, they can collide and stick together. This process of collision and sticking is called aggregation. A fully grown ice crystal is what we call a snowflake.

Figure \(\PageIndex{6}\): The growth of ice crystals occurs during their descent through aggregation (CC 0; Pekachu via Wikimedia Commons).

Figure \(\PageIndex{6}\) illustrates the formation and growth of an ice crystal in a supersaturated environment. In the first panel, a small disturbance occurs among water molecules. In the second, an ice embryo begins to form as molecules cluster. In the third stage, the embryo develops into a larger crystal as more water molecules join. There must be many times more water droplets in a cloud than ice crystals for ice crystals to get large enough to fall as snow, on the order of 100,000:1 to 1,000,000:1.