When air rises, it cools and usually produces clouds. Why do clouds vary so much in size, and why does the resulting precipitation vary so much? The answers are closely related to the stability of the air.

Recall that a parcel of air can be thought of as having a thin, flexible cover that allows it to expand but prevents it from mixing with the surrounding air (picture a hot-air balloon). Imagine that you find such a parcel, with some given starting temperature, and forcibly lift it to a higher location. Its temperature will decrease because of expansion; its final temperature will depend on how warm it was to begin with and how high you lift it up.

When you let go of the parcel, what happens? If it is cooler (and hence denser) than the surrounding air, it will sink back down to its original location. Air of this type, called stable air, resists upward movement. But if it is warmer than the surrounding air (and hence less dense), it will continue to rise. Specifically, it will rise until it reaches an altitude where its temperature equals that of its surroundings. This is exactly how a hot-air balloon works, rising as long as the air in the balloon is warmer and less dense than the surrounding air (▼). This type of air is termed unstable air.

Types of Stability

Stability is a property of air that describes whether it resists rising (is stable) or may rise spontaneously (is unstable). To determine the stability of a given parcel of air, we first need to know how the temperature of the atmosphere above the parcel changes with height. This measure, determined from observations made by radiosondes and aircraft, is called the environmental lapse rate. It is important not to confuse this with adiabatic temperature changes, which are changes in the temperature of a rising or sinking parcel of air caused by expansion or compression.

To illustrate, we examine a situation in which the environmental lapse rate is 5°C per 1000 meters (▼). Under this condition, when air at the surface has a temperature of 25°C, the air at 1000 meters will be 5° cooler, or 20°C, the air at 2000 meters will have a temperature of 15°C, and so forth. At first glance, it appears that the air at the surface is less dense than the air at 1000 meters because it is 5° warmer. However, if the air near the surface were unsaturated and were to rise to 1000 meters, it would expand and cool at the dry adiabatic rate of 10°C per 1000 meters. Therefore, upon reaching 1000 meters, its temperature would have dropped 10°C. Being 5° cooler than its environment, it would be denser and tend to sink to its original position. Hence, we say that the air near the surface is potentially cooler than the air aloft and therefore will not rise on its own. The air just described is stable and resists vertical movement.

Absolute Stability

Stated quantitatively, absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. The figure below (▼) depicts this situation using an environmental lapse rate of 5°C per 1000 meters and a wet adiabatic rate of 6°C per 1000 meters. Note that at 1000 meters, the temperature of the surrounding air is 15°C, while the rising parcel of air has cooled to 10°C and is therefore the denser air. Even if this stable air were to be forced above the condensation level, it would remain cooler and denser than its environment, and thus it would tend to return to the surface.

The most stable conditions occur when the temperature in a layer of air actually increases with altitude rather than decreases. When such a reversal occurs, a temperature inversion is said to exist. Temperature inversions frequently occur on clear nights as a result of radiation cooling of Earth’s surface. Under these conditions, an inversion is created because the ground and the air immediately above will cool more rapidly than the air aloft. When warm air overlies cooler air, it acts as a lid and prevents appreciable vertical mixing. Because of this, temperature inversions are responsible for trapping pollutants in a narrow zone near Earth’s surface.

Absolute Instability

At the other extreme from absolute stability, air is said to exhibit absolute instability when the environmental lapse rate is greater than the dry adiabatic rate. As shown in ▼, the ascending parcel of air is always warmer than its environment and will continue to rise because of its own buoyancy. However, the conditions needed to render the air absolutely unstable mainly occur near Earth’s surface. On hot, sunny days the air above some surfaces, such as shopping center parking lots, is heated more than the air over adjacent surfaces. These invisible pockets of more intensely heated air, being less dense than the air aloft, will rise like a hot-air balloon. This phenomenon produces the small, fluffy clouds we associate with fair weather. Occasionally, when the surface air is considerably warmer than the air aloft, clouds with considerable vertical development can form.

Conditional Instability

A more common type of atmospheric instability is called conditional instability. This occurs when moist air has an environmental lapse rate between the dry and wet adiabatic rates (between 5°C and 10°C per 1000 meters). Simply, the atmosphere is said to be conditionally unstable when it is stable for an unsaturated parcel of air but unstable for a saturated parcel of air. Notice in the figure below (▼) that the rising parcel of air is cooler than the surrounding air for nearly 3000 meters. With the addition of latent heat above the lifting condensation level, the parcel becomes warmer than the surrounding air. From this point along its ascent, the parcel will continue to rise because of its own buoyancy, without an outside lifting force. Thus, conditional instability depends on whether the rising air is saturated. The word conditional is used because the air must be forced upward, such as over mountainous terrain, before it becomes unstable and rises because of its own buoyancy.

Atmospheric conditions that result in conditional instability

Conditional instability may result when warm air is forced to rise along a frontal boundary. Note that the environmental lapse rate of 9°C per 1000 meters lies between the dry and wet adiabatic rates. A. The parcel of air is cooler than the surrounding air up to nearly 3000 meters, so in this range its tendency is to sink toward the surface (it is stable). Above this level, however, condensation causes the parcel to be warmer than its environment, so it rises because of its own buoyancy (it is unstable). Thus, when conditionally unstable air is forced to rise, the result can be towering cumulus clouds. B. Graphical representation of the conditions shown in part A.

In summary, the stability of air is determined by measuring the temperature of the atmosphere at various heights. In simple terms, a column of air is deemed unstable when the air near the bottom of the column is significantly warmer (less dense) than the air aloft, indicating a steep environmental lapse rate. Under these conditions, the air actually turns over, as the warm air below rises and displaces the colder air aloft. Conversely, the air column is considered to be stable when the temperature drops relatively slowly with increasing altitude. The most stable conditions occur during a temperature inversion, when the temperature actually increases with height. Under these conditions, there is very little vertical air movement.

Stability and Daily Weather

From the previous discussion, we can conclude that stable air resists upward movement, whereas unstable air ascends freely because of its own buoyancy. But how do these facts manifest themselves in our daily weather?

Because stable air resists upward movement, we might conclude that clouds will not form when stable conditions prevail in the atmosphere. Although this seems reasonable, recall that processes exist that force air aloft. These processes include orographic lifting, frontal wedging, and convergence. When stable air is forced aloft, the clouds that form are widespread and have little vertical thickness when compared to their horizontal dimension, and precipitation, if any, is light to moderate.

By contrast, clouds associated with the lifting of unstable air are towering and often generate thunderstorms and occasionally even tornadoes. For this reason, we can conclude that on a dreary, overcast day with light drizzle, stable air has been forced aloft. On the other hand, during a day when cauliflower-shaped clouds appear to be growing as if bubbles of hot air are surging upward, we can be fairly certain that the ascending air is unstable.

In summary, stability plays an important role in determining our daily weather. To a large degree, stability determines the type of clouds that develop and whether precipitation will come as a gentle shower or a heavy downpour.

• Stable air resists vertical movement, whereas unstable air rises because of its buoyancy. The stability of a parcel of air is determined by the local environmental lapse rate (the temperature of the atmosphere at various heights). The three fundamental conditions of the atmosphere are (1) absolute stability, when the environmental lapse rate is less than the wet adiabatic rate; (2) absolute instability, when the environmental lapse rate is greater than the dry adiabatic rate; and (3) conditional instability, when moist air has an environmental lapse rate between the dry and wet adiabatic rates.

• In general, when stable air is forced aloft, the associated clouds have little vertical thickness, and precipitation, if any, is light. In contrast, clouds associated with unstable air are towering and can produce heavy precipitation.

• XXXXX: