Explain the roles of adiabatic cooling and cloud condensation nuclei in cloud formation.

Clouds consist of billions of minute water droplets and/or ice crystals that are suspended above Earth’s surface. In addition to being prominent and sometimes spectacular features in the sky, clouds are of continual interest to meteorologists because they provide a visual indication of atmospheric conditions. For condensation to generate clouds, the air must reach saturation, and there must be a surface on which the water vapor can condense to form liquid droplets.

How Does Air Reach Saturation?

Saturation occurs in air aloft in one of two ways. First, cooling air to its dew-point temperature causes saturation, which results in condensation and cloud formation. Recall from Chapter 4 that clouds most often form when air rises and cools to its dew-point temperature by the process of adiabatic cooling. When a parcel of air ascends, it passes through regions of successively lower air pressure, causing the parcel to expand and cool adiabatically. At a height called the lifting condensation level, the ascending parcel will have cooled to its dew-point temperature, and saturation is reached.

Saturation also occurs when cool, unsaturated air passes over a warm water body and sufficient water vapor is added from below. This process is mainly responsible for the formation of low clouds, particularly those that form over the subtropical oceans.

The Role of Condensation Nuclei

Another requirement for condensation is that there must be a surface on which water vapor can condense. Objects at or near the ground, such as blades of grass, are such surfaces. When condensation occurs aloft, tiny particles known as cloud condensation nuclei serve this purpose. Without condensation nuclei, a relative humidity well in excess of 100 percent is necessary to produce cloud droplets. (At very low temperatures—low kinetic energies—water molecules will “stick together” in tiny clusters even in the absence of condensation nuclei.)

Dust storms, particulates from volcanic eruptions, and pollen from plants are major sources of cloud condensation nuclei. In addition, condensation nuclei are introduced into the atmosphere as by-products of combustion (burning) from such sources as forest fires, automobiles, and coal-burning furnaces.

The most effective particles for condensation aloft are hygroscopic (water-seeking) nuclei. Common food items such as crackers and cereals are hygroscopic: When exposed to humid air, they absorb moisture and quickly become stale. Over the ocean, salt particles are released into the atmosphere when sea spray evaporates. Because salt is hygroscopic, water droplets begin to form around sea salt particles at relative humidities less than 100 percent. As a result, the cloud droplets that form on hygroscopic particles such as sea salt are generally much larger than those that grow on hydrophobic (water-repelling) nuclei. Although hydrophobic particles are not efficient condensation nuclei, cloud droplets will form on them when the relative humidity reaches 100 percent.

Because cloud condensation nuclei have a wide range of affinities for water, cloud droplets of various sizes often coexist in the same cloud—an important factor for the formation of precipitation.

Growth of Cloud Droplets

Initially, the growth of cloud droplets occurs rapidly. However, the rate of growth slows as the large numbers of competing droplets consume the available water vapor. The result is the formation of a cloud consisting of billions of tiny water droplets—usually having radii of 20 micrometers (μm) or less. These cloud droplets are so minute that they remain suspended in air by the smallest updraft.

Even in very moist air, the growth of cloud droplets by additional condensation is quite slow. Furthermore, the size difference between cloud droplets and raindrops is vast—it takes about 1 million cloud droplets to form a single raindrop. Thus, it is not condensation that is responsible for the formation of raindrops (or ice crystals) large enough to fall to the ground without evaporating. We will investigate the processes that generate precipitation later in this chapter.

• cloud: A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.

• cloud condensation nuclei: Microscopic particles that serve as surfaces on which water vapor condenses.

• hydrophobic (water-repelling) nuclei: Particles that are not efficient condensation nuclei. Small droplets will form on them whenever the relative humidity reaches 100 percent.

• hygroscopic (water-seeking) nuclei: Condensation nuclei having a high affinity for water, such as salt particles.

• Clouds form in the atmosphere when water vapor condenses on cloud condensation nuclei. This produces tiny cloud droplets held aloft by the slightest updrafts.

• Clouds, visible aggregates of minute droplets of water and/or tiny crystals of ice, are one form of condensation.