Explain the relationship between environmental lapse rate and stability.

Why do clouds vary so much in size and shape, and why does the resulting precipitation vary so widely? The answer is closely tied to the stability of the air. Recall that when a parcel of air is forced to rise, its temperature will decrease because of expansion (adiabatic cooling). By comparing the parcel’s temperature to that of the surrounding air, we can determine its stability. If the parcel is cooler than the surrounding environment, it will be more dense and, if allowed to do so, it will sink to its original position. Air of this type, called stable air, resists vertical movement.

If, however, our imaginary rising parcel is warmer and hence less dense than the surrounding air, it will continue to rise until it reaches an altitude where its temperature equals that of its surroundings. This type of air is classified as unstable air. Unstable air is like a hot-air balloon: It will rise as long as the air in the balloon is sufficiently warmer and less dense than the surrounding air (Figure 4.22).

Types of Stability

The stability of the atmosphere is determined by regularly measuring the air temperature at various heights. This measure, called the environmental lapse rate (ELR), should not be confused with adiabatic temperature changes. The environmental lapse rate is rate of change in the actual temperature of the atmosphere with height, as determined from observations made by radiosondes and aircraft. (Recall that a radiosonde is an instrument package that is attached to a balloon and transmits data by radio as it ascends though the atmosphere.) Adiabatic temperature changes, by contrast, are the changes in temperature due to pressure changes that a parcel of air experiences as it moves vertically through the atmosphere.

Figure 4.23 illustrates how the stability of the atmosphere is determined. Notice that the temperature of the environment at 1000 meters above the surface is 5°C cooler than the air at the surface, whereas the environment at 2000 meters is 10°C cooler, and so forth. Thus, the prevailing environmental lapse rate is 5°C per 1000 meters. The air at the surface appears to be less dense than the air at 1000 meters because it is 5°C warmer. However, if the surface air were forced to rise to 1000 meters, it would expand and cool at the dry adiabatic rate of 10°C per 1000 meters. Therefore, on reaching 1000 meters, the rising parcel would have experienced a drop in temperature from 25°C to 15°C. The temperature of the environment at 1000 meters, on the other hand, is 20°C. Because the rising air would be 5°C cooler than its environment, it would be denser and, if allowed to do so, would sink to its original position. Thus, the air near the surface, depicted in Figure 4.23, will not rise unless forced to do so, and is referred to as stable.

We will now look at three fundamental conditions of the atmosphere: absolute stability, absolute instability, and conditional instability.

Absolute Stability

Stated quantitatively, absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. Figure 4.24 depicts this situation by 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 rising parcel is 5°C cooler than its environment, which makes it denser. Even if this stable air were forced above the lifting condensation level, it would remain cooler and denser than its environment and would have a tendency to return to the surface.

Despite its tendency to remain near Earth’s surface, stable air can be forced aloft, most commonly by frontal lifting. If stable air is forced above the lifting condensation level, flat widespread clouds will be generated. Precipitation, if any, will be light to moderate, depending on the moisture content of the air. Dreary, overcast days with periodic light rain throughout the day are likely when stable air is forced to rise.

Absolute Instability

At the other extreme, a layer of air is said to exhibit absolute instability when the environmental lapse rate is greater than the dry adiabatic rate. As shown in Figure 4.25, the ascending parcel of air is always warmer and less dense than its environment and will continue to rise because of its own buoyancy. Absolute instability occurs most often during the warmest months and on clear days, when solar heating is intense. Under these conditions, the lowermost layer of the atmosphere is heated to a much higher temperature than the air aloft. This results in a steep environmental lapse rate—in other words, environment temperature rapidly decreases with height—and an unstable atmosphere. Convective lifting of the air near Earth’s surface generates towering clouds and the potential for midafternoon thunderstorms that tend to dissipate after sunset.

Conditional Instability

A common type of atmospheric instability is called conditional instability. This situation prevails when moist air has an environmental lapse rate between the dry and wet adiabatic rates (between about 5° and 10°C per 1000 meters). Simply stated, the atmosphere is said to be conditionally unstable when it is stable with respect to an unsaturated parcel of air but unstable with respect to a saturated parcel of air.

Notice in Figure 4.26 that the rising parcel of air is cooler than the surrounding air for about 2500 meters. However, because of the release of latent heat that occurs above the lifting condensation level, the parcel eventually becomes warmer than the surrounding air. From this point along its ascent, the parcel will continue to rise without an external force. Keep in mind that conditionally unstable air must be forced upward until it reaches the level where it becomes unstable and rises on its own. The altitude at which air rises because of its own buoyancy is called the level of free convection (LFC).

Conditional instability is usually a summertime phenomenon associated with warm, humid air. When conditionally unstable air is lifted above the lifting condensation level, the result is usually towering thunderstorms. However, clouds do not continue to grow indefinitely (Figure 4.27). The massive clouds associated with thunderstorms, for example, can rise for several thousand meters, but eventually the rising parcels of air within them reach the base of stratosphere (tropopause). Recall from Chapter 1 that the stratosphere is a temperature inversion—a layer of air in which the temperature increases with altitude. When these rising parcels reach the stratosphere, they are cooler than the surrounding environment and lose their buoyancy. Thus, this temperature inversion inhibits further vertical movement and causes the cloud tops to flatten (Figure 4.27).

The level where the rising parcel becomes colder than the environment is called the equilibrium level (EL) and is illustrated on a simplified Stuve diagram in Box 4.3. In strong thunderstorms, part of the cloud may overshoot the equilibrium level because of the strength of the updraft. The result is a dome-shaped structure that protrudes above an otherwise flat cloud top (see Figure 4.27).

• absolute instability: The condition of air that has an environmental lapse rate that is greater than the dry adiabatic rate (1°C per 100 meters).

• absolute stability: The condition of air that has an environmental lapse rate that is less than the wet adiabatic rate.

• conditional instability: The condition of moist air with an environmental lapse rate between the dry and wet adiabatic rates.

• environmental lapse rate (ELR): The rate of temperature decrease with height in the troposphere.

• equilibrium level (EL): The height of a rising air parcel at which the parcel becomes colder than the surrounding environment after the level of free convection.

• stable air: Air that resists vertical displacement. If it is lifted, adiabatic cooling will cause its temperature to be lower than the surrounding environment; if it is allowed, it will sink to its original position.

• temperature inversion: A layer in the atmosphere of limited depth where the temperature increases rather than decreases with height.

• unstable air: Air that does not resist vertical displacement. If it is lifted, its temperature will not cool as rapidly as the surrounding environment, and so it will continue to rise on its own.

• Stable air resists vertical movement, whereas unstable air rises because of its buoyancy.

• The three fundamental conditions of the atmosphere are (1) absolute stability, (2) absolute instability, and (3) conditional instability.

• In general, when stable air is forced aloft, the associated clouds cover most of the sky and produce light to moderate precipitation that may continue all day. In contrast, clouds associated with unstable air are towering and frequently accompanied by heavy rain.