Section 4.8:
Stability & Daily Weather

Learning Objective

List the primary factors that influence the stability of air.

Section Content

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.

Mini-Lecture Video - Atmospheric Stability (Click to watch the video)

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:

Stability is enhanced by the following:

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.

Box 4.4

Orographic Effects: Windward Precipitation and Leeward Rain Shadows

Orographic lifting is a significant factor in the development of windward precipitation and leeward rain shadows. A simplified hypothetical situation, illustrated in Figure 4.D, shows prevailing winds forcing warm moist air over a nearly 3000-meter-high mountain range. As the unsaturated air ascends the windward side of the range, it cools at a rate of 10°C per 1000 meters (dry adiabatic rate) until it reaches the dew-point temperature of 20°C. Because the dew-point temperature is reached at 1000 meters, we can say that this height represents the lifting condensation level and the height of the cloud base. Notice that above the lifting condensation level, latent heat is released, which results in a slower rate of cooling, the wet adiabatic rate.

Figure 4.D

Orographic lifting and the formation of rain shadow deserts.

From the cloud base to the top of the mountain, water vapor within the rising air condenses to form more and more cloud droplets. As a result, the windward side of the mountain range experiences abundant precipitation.

For simplicity, we will assume that the air that was forced to the top of the mountain is cooler than the surrounding air and hence begins to flow down the leeward slope of the mountain. As the air descends, it is compressed and heated at the dry adiabatic rate. As the descending air reaches the base of the mountain range, its temperature has risen to 40°C, or 10°C warmer than the temperature at the base of the mountain on the windward side. The higher temperature on the leeward side is a result of the latent heat that was released during condensation as the air ascended the windward slope of the mountain range.

Two factors account for the rain shadow commonly observed on leeward mountain slopes. First, water is extracted from air in the form of precipitation on the windward side. Second, the air on the leeward side is warmer than the air on the windward side. (Recall that an increase in temperature results in a drop in relative humidity.)

A classic example of windward precipitation and leeward rain shadows is found in western Washington State. As moist Pacific air flows inland over the Olympic and Cascade Mountains, orographic precipitation is abundant (Figure 4.E). By contrast, precipitation data for Yakima indicates the presence of a rain shadow on the leeward side of these highlands.

Figure 4.E

Distribution of precipitation in western Washington State.

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.

Figure 4.28
Temperature profile typical of a low-level temperature inversion

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).

Figure 4.29
Pollution trapped by a temperature inversion

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.

Figure 4.30
Comparison of the three types of atmospheric stability

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

What weather conditions would lead you to believe that air is unstable? Stable?

Towering cauliflower-shaped clouds, heavy precipitation, and isolated afternoon showers in summer are conditions that are associated with unstable air. Clear skies and low humidity are conditions associated with stable air. If the stable air is forced aloft, clouds with little vertical thickness and light precipitation may be present.

List four ways instability can be enhanced.

Instability can be enhanced by the following:

a) intense solar heating warming the lowermost layer of the atmosphere;

b) the heating of an air mass from below as it passes over a warm surface;

c) general upward movement of air caused by processes such as orographic lifting, frontal wedging, and convergence;

d) radiation cooling from cloud tops.

List three ways stability can be enhanced.

Stability can be enhanced by the following:

a) radiation cooling of Earth’s surface after sunset;

b) the cooling of an air mass from below as it traverses a cold surface;

c) general subsidence within an air column.