Why does air rise on some occasions to produce clouds, but not on others? Generally, the tendency is for air to resist vertical movement; air near the surface tends to stay near the surface, and air aloft tends to remain aloft. However, the following four processes can cause air to rise, thereby generating clouds:

1. Orographic lifting, in which air is forced to rise over a mountainous barrier

2. Frontal lifting, in which warmer, less-dense air is forced over cooler, denser air

3. Convergence, which is a pileup of horizontal airflow that results in upward movement

4. Localized convective lifting, in which unequal surface heating causes localized pockets of air to rise because of their buoyancy

Orographic Lifting

Orographic lifting occurs when elevated terrain, such as a mountain range, act as a barrier to the flow of air (▼). 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.

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 ▲ , the result can be a rainshadow desert like the Great Basin Desert of the western United States, which lies only a few hundred kilometers from the Pacific Ocean but is effectively cut off from the ocean’s moisture by the imposing Sierra Nevada. The Gobi Desert of Mongolia, the Takla Makan of China, and the Patagonia Desert of Argentina are other examples of deserts that exist because they are 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 for rich soils and climates favorable for large-scale grain and wheat production. 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 rises. This process, called frontal lifting, or frontal wedging, is illustrated in ▼. In the middle latitudes, most precipitation is due to weather-producing fronts associated with storm systems called midlatitude cyclones. We will examine fronts in detail later this semester.

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 (▼). When air is compressed by convergence, it escapes by moving upward. One situation in which convergence causes lifting is when air is drawn into a center of low pressure, such as a midlatitude cyclone or hurricane. The inward flow of air at the bottom 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).

The Florida peninsula provides an excellent example of the role that convergence can play in initiating cloud development and precipitation (refer to ▲). 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 convergence and uplift is aided by intense solar heating of the land. As a result, Florida’s peninsula experiences the greatest frequency of mid-afternoon thunderstorms in the United States.

Localized Convective Lifting

On warm summer days, unequal heating of Earth’s surface may cause some pockets of air to be warmed more than the surrounding air (▼). For instance, a plowed field or parking lot will absorb more radiation and hence impart more warmth to the overlying air than will adjacent fields of crops. Consequently, the overlying parcel of air, being warmer (less dense) than the surrounding air, will be buoyed upward. Such rising parcels of warmer air are called thermals. Thermals can carry birds such as hawks and eagles—and also human hang gliders—to great heights.

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 occasionally produce mid-afternoon 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, a phenomenon called sun showers.

• Four mechanisms that cause air to rise are (1) orographic lifting, where air is forced to rise over elevated terrain, such as a mountain barrier; (2) frontal lifting, where warmer, less-dense air is forced over cooler, denser air along a front; (3) convergence, a pileup of horizontal airflow resulting in an upward flow; and (4) localized convective lifting, where unequal surface heating causes localized pockets of air to rise because of their buoyancy.

• convergence: The condition resulting from a net horizontal inflow of air into an area. Convergence at lower levels is associated with an upward movement of air and is favorable to cloud formation and precipitation.

• front: The boundary between two adjoining air masses having contrasting characteristics.

• frontal lifting (a.k.a. frontal wedging): Lifting of air that results when cool air acts as a barrier over which warmer, lighter air rises.

• localized convective lifting (a.k.a. convective lifting): Rising parcels of buoyant air (thermals) produced by unequal surface heating.

• orographic lifting: Mountains acting as barriers to the flow of air, forcing the air to ascend. The air cools adiabatically, and clouds and precipitation may result.

• rainshadow (desert): A dry area on the lee side of a mountain range. Many middle-latitude deserts are of this type.