On the job, the weather journeyman/craftsman attempts to forecast if the skies will be clear or if there will be clouds. This decision can ultimately affect whether an aircraft can complete its mission. Therefore, knowledge of how clouds form and dissipate is of the utmost importance to the forecaster. The science of physics plays a significant part in this process. In the previous sections, we discussed some of the aspects of atmospheric physics. In this next section we discuss physics as it applies to cloud formation and dissipation. Initially, we must understand the microphysics that ultimately lead to cloud development.

Clouds form when the atmosphere is saturated. Saturation occurs when the air holds the maximum amount of water vapor possible. At saturation, the temperature is equal to the dew-point temperature (T = T_d); the vapor pressure is equal to the saturation vapor pressure (e = e_s); and the relative humidity is 100 percent. Saturation is a state of equilibrium. Water is constantly evaporating and condensing. The amount of water vapor molecules in the air is constant, but the actual molecules are constantly changing their state (or phase).

The CCN are necessary for water vapor to convert to liquid. These nuclei are composed of sea salt, volcanic sulfates, forest fire smoke, clays and other fine terrestrial dust particles. These are particles on which water vapor condenses to form water droplets. The most effective nuclei are hygroscopic (water attracting) and water soluble (can be dissolved by water). If the CCN are water soluble, the resulting water droplet is not pure water but a solution. The concentration of the solution (mass of CCN to volume of water in the droplet) depends upon the size of the CCN and the amount of vapor which has condensed onto it (i.e., the volume of the droplet). Cloud droplets are initiated by the condensation of water vapor on CCN. These droplets continue to grow as water vapor condenses onto the droplet.

The air immediately surrounding all the droplets is said to be saturated (e = e_s). Initially, whether a droplet grows or not depends on saturation vapor pressure (e_s) of the droplet and its surrounding environment. If e_{s env} > e_{s droplet} , the droplet grows due to condensation. If e_env < e_{s droplet,}, the droplet shrinks through evaporation.

Differences in saturation vapor pressure

After their initial growth, droplets have different saturation vapor pressures based on their concentration and size. These differences in saturation vapor pressures determine how the solute effect and curvature effect will contribute to droplet growth. Before we disscuss these effects, however, it is important to note that the solute and curvature effects occur between other droplets and not the environment.

The the curvature effect is based on the fact that small droplets, which are tightly curved, have a larger saturation vapor pressure than larger droplets which are less curved (e_{s small} > e_{s big}). For example, if we have two droplets, one small and one large, the larger droplet grows at the expense of the smaller droplet. As we stated earlier, flow between the two factors is from higher to lower pressure.

In a cloud composed of pure water droplets of varying sizes, it is possible that water droplets below a certain critical size evaporate while larger droplets can grow by condensation. However, this possibility is strongly modified by the fact that cloud droplets are not pure water. Rather, they were formed originally by condensation on "condensation nuclei"--some of which are hygroscopic. The contamination of the droplet by the dissolved or solid hygroscopic substance changes the saturation vapor pressure. The result is called the solute effect and counteracts the curvature effect. When the solute effect occurs, droplets consisting of higher concentrations of solution have a lower saturation vapor pressure than more pure droplets. Suppose droplet "b" is less pure than droplet "a." In that case, e_sa > e_sb. As a result, droplet "a" shrinks due to evaporation at the expense of droplet "b" growing due to condensation. Flow between the two factors is from higher to lower pressure.

Once droplets grow large enough to fall, they can collide with other droplets and merge to form even larger droplets. Coalescence means the droplets which collide with each other stick together. However, drops can only become so big before they split apart into two or more drops. These "new" drops, created by the splitting process, can then grow by collision-coalesence. If this process is multiplied many, many times (as occurs in cumulonimbus clouds), the quantity of droplets increases dramatically. Indeed, collision-coalescence is actually the most efficient method of droplet growth (much more efficient than condensation). This process is represented below, in Figure 1-14.

Figure 1-14: collision-coalescence:

Factors that affect efficiency

Different conditions affect the efficiency of the collision-coalescence process. Now let's discuss the factors that affect the efficiency of this process.

The longer a droplet remains in the cloud, the greater chance it has to interact or coalesce with other droplets. Thick clouds, such as nimbostratus and cumulonimbus, provide a greater distance for the droplet to fall, thereby increasing coalescence time. Updrafts associated with cumulus and cumulonimbus clouds account for transporting droplets to upper portions of the cloud. These updrafts also increase the distance and coalescence time of the droplet. The different precipitation states in a cumulonimbus are shown below, in Figure 1-15.

Figure 1–15: distribution of water and ice crystals in a thunderstorm

If droplets are falling at different rates (velocities), then the faster falling droplets catch up, collide, and coalesce with the slower falling droplets. Larger droplets fall at a faster rate than smaller ones. A droplet with a diameter of 20 microns has a terminal velocity 0.01 ms^-1. A much larger droplet, with a diameter of 4,000 microns, has a terminal velocity of 6.5 ms^-1.

A wide spectrum of droplet sizes is indicative of variable fall velocities, and enhancement of the collision-coalescence process. Strong thunderstorm updrafts (which may exceed 20 ms^-1), will also enhance this process. A drop can be carried to great heights a number of times in a cloud, leading to repeated growth by sweeping up small droplets and resulting in a large drop.

Droplets exposed to an electric field develop a net charge. Opposite charges attract droplets to each other. The electrical attraction causes more collisions between droplets to occur.

Ice nuclei (IN), which are similar to CCN, are necessary for liquid or vapor to form ice crystals. Water droplets at temperatures below 0°C are called supercooled. These droplets exist in a liquid state. If these droplets encounter an ice crystal, they automatically freeze onto the ice crystal and form graupel (through accretion). Once a crystal is formed, it grows by deposition (conversion of water vapor to ice). This is because the saturation vapor pressure over ice is less than that over water (e_{s ice} < e_{s water}).

Therefore, the ice crystals grow at the expense of water droplets. Since the e_s over ice is less than that of a water droplet, the ice crystal grows by deposition.

Once the crystal grows large enough, it undergoes a process similar to collision-coalescence. The crystal can collide with other cloud droplets. If the cloud is colder than 0° C, the droplets freeze on contact. The crystal can also collide with other ice crystals. This process (aggregation) is responsible for the formation of large snowflakes. These large snowflakes (with larger terminal velocities) may then fall below the freezing level -- melt-- and form larger droplets through collision-coalescence.

Generally, cloud types and amounts are determined by the amount of atmospheric moisture available, temperature, stability, and lifting mechanisms. Let's discuss these general cloud dynamics now.

First and foremost, there must be enough available atmospheric moisture present for clouds to form. No amount of lift or cooling will produce clouds if sufficient moisture is not present.

Cooling processes such as adiabatic cooling and radiational cooling are the principle condensation mechanisms. Adiabatic cooling is the most effective means of cooling water vapor until it condenses.

Lifting mechanisms include orographic lift, frontal lift, and low-level convergence.

Horizontal motion is converted to vertical motion proportional to the slope of the terrain. Even relatively flat terrain may have slopes of 1 mile vertical to 200 miles horizontal. Meteorologists must have a thorough grasp of geographic details over the forecast region to assist in the cloud forecast. This phenomenon occurs quite often on the windward side of mountains.

The amount of frontal lift depends on the frontal slope. The frontal slope is significant because it represents the potential lift of a front. A steep slope suggests the frontal lift could be strong if the wind flow forces air to ascend the frontal surface. Conversely, a shallow slope suggests the frontal lift would be weak if the wind flow forces air to ascend the frontal surface.

Convergence is a measure of the rate of the net addition of mass into a volume at a given point. This convergence can be directional, speed, or a combination of both. The mass of air converging in the low levels just above the surface must go somewhere. Since it cannot go into the ground, it rises, giving lift to the air.

Directional convergence is the coming together of wind flow, which results in mass being added to an area.

Speed convergence is caused by winds rapidly decreasing speed downstream. The higher speed winds push mass into an area faster than it can be removed by the slower speed winds, thus increasing the mass.

Just as there were a number of reasons for cloud formation, cloud dissipation also has more than one reason for occurrence.

Evaporation of water droplets and the sublimation of ice crystals (conversion from ice directly to vapor) are ways that clouds can dissipate. These dissipation dynamics are caused by two mechanisms, decreasing the moisture and increasing the air temperature.

Decreasing the moisture of the cloud can be accomplished in two ways. The first includes the cloud "raining itself out". The precipitation evaporates while it falls or after it reaches the surface.

Moisture can also be decreased by a process known as dry-air entrainment. Dry-air entrainment is the process by which the outer edge of the cloud mixes with dry air outside of the cloud (As shown in Figure 1-16, below). This outer edge of cloud then evaporates or sublimates with the drier air.

Figure 1-16:Dry air entrainment into a cumulonimbus (indicated by the red arrows on the edge of the cloud)

Increasing the air temperature within the cloud can also be accomplished in two ways. The first includes adiabatic warming associated with downward vertical motions (sinking air). These motions occur with either subsidence or as a leeside mountain effect where moist air is compressed and warmed. The cloud either evaporates or sublimates.

An increase in cloud temperature can also be achieved by warming from below. For example, increased air temperature is the primary mechanism for the dissipation of radiational fog. The factor that increases the air temperature of the fog (a cloud on the ground) is solar heating.