Moisture Clouds and Precipitation 18.2 Cloud Formation Recall that condensation occurs when water vapor changes to a liquid. Condensation may form dew, fog, or clouds. Although these three forms are different, all require saturated air to develop. Saturation occurs either when enough water vapor is added to air or, more commonly, when air is cooled to its dew point. Near Earth’s surface, heat is quickly exchanged between the ground and the air above. During evening hours, the surface radiates heat away, causing the surface and adjacent air to cool rapidly. This radiational cooling causes the formation of dew and some types of fog. In contrast, clouds, like those shown in Figure 6, often form during the warmest part of the day. Clearly, some other process must cool air enough to generate clouds. Air Compression and Expansion If you have pumped up a bicycle tire, you might have noticed that the pump barrel became warm. The increase in temperature you felt resulted from the work you did on the air to compress it. When air is compressed, the motion of gas molecules increases and the air temperature rises. The opposite happens when air is allowed to escape from a bicycle tire. The air expands and cools. The expanding air pushes on the surrounding air and cools by an amount equal to the energy used up. Adiabatic Temperature Changes Temperature changes that happen even though heat isn’t added or subtracted are called adiabatic temperature changes. They result when air is compressed or allowed to expand. When air is allowed to expand, it cools, and when it is compressed, it warms. Figure 6 Clouds form when air is cooled to its dew point Expansion and Cooling As you travel from Earth’s surface upward through the atmosphere, the atmospheric pressure decreases. This happens because there are fewer and fewer gas molecules. Any time a volume of air moves upward, it passes through regions of successively lower pressure. As a result, the ascending air expands and cools. Unsaturated air cools at the constant rate of 10°C for every 1000 meters of ascent. In contrast, descending air encounters higher pressures, compresses, and is heated 10°C for every 1000 meters it moves downward. This rate of cooling or heating applies only to unsaturated air and is called the dry adiabatic rate. If a parcel of air rises high enough, it will eventually cool to its dew point. Here the process of condensation begins. From this point on as the air rises, latent heat of condensation stored in the water vapor will be released. Although the air will continue to cool after condensation begins, the released latent heat works against the adiabatic cooling process. This slower rate of cooling caused by the addition of latent heat is called the wet adiabatic rate. Because the amount of latent heat released depends on the quantity of moisture present in the air, the wet adiabatic rate varies from 5–9°C per 1000 meters. Figure 7 shows the role of adiabatic cooling in the formation of clouds. Note that from the surface up to the condensation level the air cools at the dry adiabatic rate. The wet adiabatic rate begins at the condensation level. Processes That Lift Air In general, air resists vertical movement. Air located near the surface tends to stay near the surface. Air far above the surface tends to remain far above the surface. Some exceptions to this happen when conditions in the atmosphere make air buoyant enough to rise without the aid of outside forces. In other situations, clouds form because there is some mechanical process that forces air to rise. Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting. Orographic Lifting When elevated terrains, such as mountains, act as barriers to air flow, orographic lifting of air occurs. Look at Figure 8A. As air goes up a mountain slope, adiabatic cooling often generates clouds and precipitation. Many of the rainiest places on Earth are located on these windward mountain slopes. By the time air reaches the leeward side of a mountain, much of its moisture has been lost. If the air descends, it warms adiabatically. This makes condensation and precipitation even less likely. A rain shadow desert can occur on the leeward side of the mountain. For example, the Great Basin Desert of the western United States lies only a few hundred kilometers from the Pacific Ocean, cut off from the ocean’s moisture by the Sierra Nevada Mountains. Frontal Wedging If orographic lifting was the only mechanism that lifted air, the relatively flat central portion of North America would be an expansive desert instead of the nation’s breadbasket. Fortunately, this is not the case. In central North America, masses of warm air and cold air collide, producing a front. Here the cooler, denser air acts as a barrier over which the warmer, less dense air rises. This process, called frontal wedging, is shown in Figure 8B. Weather-producing fronts are associated with specific storm systems called middle-latitude cyclones. You will study these in Chapter 20. Convergence Recall that the collision of contrasting air masses forces air to rise. In a more general sense, whenever air in the lower atmosphere flows together, lifting results. This is called convergence. When air flows in from more than one direction, it must go somewhere. Because it cannot go down, it goes up, as shown in Figure 8C. This leads to adiabatic cooling and possibly cloud formation. The Florida peninsula provides an example of how convergence can cause cloud development and precipitation. On warm days, the airflow is from the ocean to the land along both coasts of Florida. This leads to a pileup of air along the coasts and general convergence over the peninsula. This pattern of air movement and the uplift that results is helped along by intense solar heating of the land. The result is that the peninsula of Florida experiences the greatest number of mid-afternoon thunderstorms in the United States. Localized Convective Lifting On warm summer days, unequal heating of Earth’s surface may cause pockets of air to be warmed more than the surrounding air. For example, air above a paved parking lot will be warmed more than the air above an adjacent wooded park. Consequently, the parcel of air above the parking lot, which is warmer and less dense than the surrounding air, will move upward, as shown in Figure 8D. These rising parcels of warmer air are called thermals. The process that produces rising thermals is localized convective lifting. Birds such as hawks and eagles use these thermals to carry them to great heights where they can gaze down on unsuspecting prey. People have learned to use these warm parcels effectively for hang gliding. When warm parcels of air rise above the condensation level, clouds form. These clouds may produce mid-afternoon rain showers. Stability If a volume of air was forced to rise, its temperature would drop because of expansion. If this volume of air was cooler than the surrounding environment, it would be denser, and if allowed to do so, it would sink to its original position. Air of this type, called stable air, resists vertical movement. Density Differences If this imaginary volume of rising air was warmer and therefore less dense than the surrounding air, it would continue to rise until it reached an altitude where its temperature equaled that of its surroundings. This is exactly how a hot-air balloon works. The balloon rises as long as it is warmer and less dense than the surrounding air, as shown in Figure 9. This type of air is classified as unstable air. Stable air tends to remain in its original position, while unstable air tends to rise. Figure 9 Hot-air balloons will rise as long as the air inside them is warmer than the air in the atmosphere surrounding them. Stability Measurements Air stability is determined by measuring the temperature of the atmosphere at various heights. The rate of change of air temperature with height is called the environmental lapse rate. This rate is determined from observations made by aircraft and by radiosondes. A radiosonde is an instrument designed to collect weather data high in the atmosphere. Radiosondes are often carried into the air by balloons. It is important not to confuse the environmental lapse rate with adiabatic temperature changes. Degrees of Stability Air is stable when the temperature decreases gradually with increasing altitude. The most stable conditions happen when air temperature actually increases with height, called a temperature inversion. Temperature inversions frequently happen on clear nights as a result of radiation cooling off Earth’s surface. The inversion is created because the ground and the air immediately above the ground will cool more rapidly than air higher above the ground. Under these conditions, there is very little vertical air movement. In contrast, air is considered unstable when the air close to the surface of Earth is significantly warmer than the air higher above the surface, indicating a large environmental lapse rate. Under these conditions, the air actually turns over, as the warm air below rises and is displaced by the colder air higher above the ground. Figure 10 These clouds provide evidence of unstable conditions in the atmosphere. Stability and Daily Weather Recall that stable air resists vertical movement and that unstable air rises freely. But how do these facts apply to the daily weather? Because stable air resists upward movement, you might conclude that clouds won’t form when stable conditions are present in the atmosphere. Although this seems reasonable, remember that there are processes that force air above Earth’s surface. These include orographic lifting, frontal wedging, and convergence. When stable air is forced above Earth’s surface, the clouds that form are widespread and have little vertical thickness when compared to their horizontal dimension. Precipitation, if any, is light to moderate. In contrast, clouds associated with the lifting of unstable air are towering and often generate thunderstorms and occasionally even a tornado. For this reason, on a dreary, overcast day with light drizzle, stable air has been forced above Earth’s surface. During a day when cauliflowershaped clouds appear to be growing as if bubbles of hot air are surging upward, the air moving up is unstable. Figure 10 shows cauliflower-shaped clouds caused by the rising of unstable air. Condensation Recall that condensation happens when water vapor in the air changes to a liquid. This may be in the form of dew, fog, or clouds. For any of these forms of condensation to occur, the air must be saturated. Saturation occurs most commonly when air is cooled to its dew point, or less often when water vapor is added to the air. Types of Surfaces Generally, there must be a surface for water vapor to condense on.When dew forms, objects at or near the ground, such as grass and car windows, serve this purpose. But when condensation occurs in the air above the ground, tiny bits of particulate matter, called condensation nuclei, serve as surfaces for water-vapor condensation. These nuclei are important because if they are absent, a relative humidity much above 100 percent is needed to produce clouds. Condensation nuclei such as microscopic dust, smoke, and salt particles from the ocean are abundant in the lower atmosphere. Because of these plentiful particles, relative humidity rarely exceeds 100 percent. Some particles, such as ocean salt, are especially good nuclei because they absorb water. When condensation takes place, the initial growth rate of cloud droplets is rapid. It diminishes quickly because the excess water vapor is quickly absorbed by the numerous competing particles. This results in the formation of a cloud consisting of millions upon millions of tiny water droplets. These droplets are all so fine that they remain suspended in air. In the next section, you will examine types of clouds and the precipitation that forms from them. Reviewing Concepts 1. Describe what happens to air temperature when work is done on the air to compress it. 2. What does stability mean in terms of air movement? 3. List four mechanisms that cause air to rise. 4. Describe conditions that cause condensation of liquid water in air. 5. What is a temperature inversion? 6. Which types of condensation nuclei are especially good for condensation to form? Critical Thinking 7. Hypothesizing Study a world map. Hypothesize about other regions on Earth, other than the Florida peninsula, where convergence might cause cloud development and precipitation.