List the primary factors that influence the stability of air.

How does air stability manifest itself in our daily weather? When stable air is forced aloft, relatively thin widespread clouds typically form, and any precipitation that results is light to moderate. In contrast, as unstable air rises, towering clouds are generated that are usually accompanied by heavy precipitation.

How Stability Changes

Any factor that causes air near the surface to become warmed relative to the air aloft makes air more unstable (increases instability), whereas any factor that causes the surface air to be chilled increases stability. Stated another way, any factor that increases the environmental lapse rate renders the air less stable, whereas any factor that reduces the environmental lapse rate increases the air’s stability.

Instability is enhanced by the following:

• Warming of the lowermost layer of the atmosphere by solar radiation during daylight hours

• Heating of a cold air mass from below as it passes over a warm surface or bringing in warm air near the surface (warm air advection)

• Upward movement of air caused by processes such as orographic lifting, frontal lifting, or convergence

• Radiation cooling of cloud tops

Stability is enhanced by the following:

• Radiation cooling of Earth’s surface after sunset

• The cooling of an air mass from below as it traverses a cold surface or the arrival of cold surface air (cold air advection)

• Subsidence within an air column

Note that most processes that alter stability result from temperature changes caused by horizontal or vertical air movement, although daily temperature changes are important as well.

Solar Heating and Stability

On clear summer days, when there is abundant surface heating, the lower atmosphere may become warmed sufficiently to cause parcels of air to rise—localized convection. After the Sun sets, surface cooling generally renders the atmosphere stable again.

Horizontal Air Movement and Stability

Changes in stability may occur as air moves horizontally over surfaces that have markedly different temperatures. For example, in the winter, when warm air from the Gulf of Mexico moves northward over the cold, snow-covered Midwest, the air is cooled from below. This increases the stability of the air and often produces widespread fog but no cloud development.

On the other hand, instability can be enhanced in the winter when frigid polar air moves southward over the open waters of the Great Lakes. Although the Great Lakes are cold in the winter, they are as much as 25°C warmer than a subfreezing polar air mass as it pushes southward across the lakes. During its journey, moisture and heat are added to the frigid polar air from the comparatively warm water below, rendering the air humid and unstable. The result can be heavy snowfalls on the downwind shores of the Great Lakes—called “lake-effect snow.”

Vertical Air Movement and Stability

When there is a general downward flow of air, called subsidence, the upper part of the air column is heated by compression, more so than the air below. (Usually, the air just above Earth’s surface is not involved in subsidence because the ground inhibits sinking motion, so its temperature remains unchanged.) Because the air aloft is warmed more than the air near the surface, subsidence tends to stabilize the atmosphere. Subsidence can occur for different reasons, including the downward motion of a convection cell or air descending the leeward side of a mountain range (Box 4.4). The warming effect of a few hundred meters of subsidence is enough to evaporate clouds. Thus, one sign of subsiding air is a deep blue, cloudless sky.

Upward movement of air generally enhances instability and is particularly significant in generating towering clouds and thunderstorms during the warm summer months. Recall that when conditionally unstable air is forcefully lifted along a front, it can become unstable and continue to rise because of its buoyancy (see Figure 4.26). This can also occur when warm moist air is forced up a mountain, leading to dangerous thunderstorms, which may cause flash flooding in the adjacent valleys.

Radiation Cooling from Clouds

The loss of heat by radiation emitted from cloud tops during evening hours adds to their instability and growth. Unlike air, which is a poor radiator of heat, cloud droplets emit considerable energy to space. Towering clouds that owe their growth to surface heating lose that source of energy at sunset. After sunset, however, radiation cooling at their tops steepens the lapse rate near the tops of these clouds and can lead to additional upward flow of warmer air below. This process is responsible for producing nocturnal thunderstorms from clouds whose growth temporarily ceased around sunset.

Temperature Inversions and Stability

The most stable atmospheric conditions are associated with temperature inversions. Recall that a temperature inversion is a layer in which the temperature increases with altitude rather than the more common condition of decreasing with altitude (Figure 4.28). This phenomenon can occur within any layer of the atmosphere. Temperature inversions act like a lid that keeps rising air from penetrating the inversion. There are two main types of inversions—those that occur near the surface and those that develop aloft.

Many processes can generate a temperature inversion, such as radiation cooling of Earth’s surface on a clear night. After sunset, Earth’s surface loses energy quickly and, through conduction, cools the air near the surface. However, because air is a poor conductor of heat, the air aloft remains comparatively warm.

When the air near the surface is cooler and heavier than a layer of air aloft, minimal vertical mixing occurs between the two layers. Because pollutants are generally added to the atmosphere from below, a temperature inversion confines the pollutants to the lowermost layer, where their concentration will continue to increase until the temperature inversion dissipates (Figure 4.29).

Widespread fog can also be enhanced by a temperature inversion. Fog often forms after sunset because of radiation cooling. If an inversion develops, it inhibits mixing between the moist, fog-laden air near the surface and dryer air aloft—thus preventing the fog from dissipating.

Temperature inversions that occur aloft can cause convective clouds to spread out and take on a flattened appearance. One example is the flattened tops of towering storm clouds that reach the top of the troposphere. This temperature inversion is the result of solar heating of the ozone layer that is found in the stratosphere (see Figure 4.27). Subsidence as a mechanism for generating temperature inversions aloft is discussed in Chapter 13.

The role of stability in determining our daily weather is summarized in Figure 4.30. The air’s stability, or lack of it, determines to a large degree whether clouds develop and produce precipitation and whether that precipitation will come as a gentle shower or a violent downpour. When stable air is forced aloft, the associated clouds generally have little vertical thickness, and precipitation, if any, is light. In contrast, unstable air can result in towering clouds frequently accompanied by thunderstorms and heavy precipitation. The most stable conditions occur during a temperature inversion, when the air temperature increases with height and inhibits vertical air movement.

• subsidence: An extensive sinking motion of air, most frequently occurring in anticyclones. The subsiding air is warmed by compression and becomes more stable.

• Any factor that causes air near the surface to become warmed in relation to the air aloft increases the air’s instability. The opposite is also true: Any factor that causes the surface air to be chilled compared to air aloft results in the air becoming more stable.

• The most stable atmospheric conditions are associated with temperature inversions.