Identify four mechanisms that cause air to rise.

Although air tends to resist vertical movement (air near the surface “wants” to stay near the surface, a condition discussed in section 4.7), there are several mechanisms that cause air to rise and trigger the formation of clouds. These mechanisms include orographic lifting, frontal lifting, convergence, and localized convective lifting.

Orographic Lifting

Orographic lifting occurs when elevated terrain, such as mountains, act as barriers to the horizontal flow of air (Figure 4.17). As air ascends a mountain slope, adiabatic cooling often generates clouds and copious precipitation. In fact, many of the rainiest places in the world are located on windward mountain slopes (Box 4.2).

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, making condensation and precipitation even less likely. As shown in Figure 4.17, the result can be a rain shadow desert (look ahead to Box 4.4. The Great Basin Desert of the western United States lies only a few hundred kilometers from the Pacific Ocean, but it is effectively cut off from the ocean’s moisture by the imposing Sierra Nevada (Figure 4.17). The Gobi Desert of Mongolia, the Takla Makan of China, and the Patagonia Desert of Argentina are other examples of rain shadow deserts located on the leeward sides of large mountain systems.

Frontal Lifting

If orographic lifting were the only mechanism that forced air aloft, the relatively flat central portion of North America would be an expansive desert rather than the area known as “the nation’s breadbasket.” Fortunately, this is not the case.

In central North America, warm and cold air masses often collide, producing boundaries called fronts. Rather than mixing, the cooler, denser air mass acts as a barrier over which the warmer, less dense air mass rises. This process, called frontal lifting, also referred to as frontal wedging, is illustrated in Figure 4.18.

It should be noted that weather-producing fronts are associated with storm systems called midlatitude cyclones. Because these storms are responsible for producing a high percentage of the precipitation in the middle latitudes, we will examine them in detail in Chapter 9.

Convergence

When the wind pattern near Earth’s surface is such that more air is entering an area than is leaving—a phenomenon called convergence—lifting occurs (Figure 4.19). Convergence as a mechanism of lifting is most often associated with large centers of low pressure, mainly midlatitude cyclones and hurricanes. The inward flow of air at the surface of these systems is balanced by rising air, cloud formation, and usually precipitation.

Convergence can also occur when an obstacle slows or restricts horizontal airflow (wind). For example, when air moves from a relatively smooth surface, such as the ocean, onto an irregular landscape, increased friction reduces its speed. The result is a pileup of air (convergence). When air converges, there is an upward flow of air molecules rather than a simple squeezing together of molecules (as happens when people enter a crowded building).

The Florida peninsula provides an excellent example of the influence of convergence in cloud development and precipitation. On warm days, air flows from the ocean toward 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 convergence and uplift is aided by intense solar heating of the land. As a result, Florida’s peninsula experiences the greatest frequency of midafternoon thunderstorms in the United States (Figure 4.20).

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 (Figure 4.21). For instance, air above a plowed field will be warmed more than the air above adjacent fields of crops. Consequently, the parcel of air above the field, which is warmer (less dense) than the surrounding air, will be buoyed upward. These rising parcels of warmer air are called thermals. Birds such as hawks and eagles use thermals to carry them to great heights, where they can identify unsuspecting prey. Humans have learned to employ these rising parcels to use hang gliders as a way to “fly.”

The phenomenon that produces rising thermals is called localized convective lifting, or simply convective lifting. When these warm parcels of air rise above the lifting condensation level, clouds form and on occasion produce midafternoon rain showers. The height of clouds produced in this fashion is somewhat limited, because the buoyancy caused solely by unequal surface heating is confined to, at most, the first few kilometers of the atmosphere. Also, the accompanying rains, although occasionally heavy, are of short duration and widely scattered.

• convergence: The condition that exists when the wind distribution within a given region results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.

• front: A boundary (discontinuity) separating air masses of different densities, one warmer and often higher in moisture content than the other.

• frontal lifting (a.k.a., frontal wedging): The lifting of air resulting when cool air acts as a barrier over which warmer, lighter air will rise.

• localized convective lifting: The process in which a parcel of air warmed at the surface becomes buoyant and rises above the lifting condensation level to form clouds.

• orographic lifting: The process in which mountains or highlands act as barriers to the flow of air and force the air to ascend. The air cools adiabatically, and clouds and precipitation may result.

• rain shadow desert: A dry area on the lee side of a mountain range.

• Four mechanisms that cause air to rise are (1) orographic lifting, (2) frontal lifting, (3) convergence, and (4) localized convective lifting.