Atmospheric Stability

13.1 Introduction

Convective clouds and precipitation pose a distinctly different flying environment than stratiform clouds and precipitation. These sharply contrasting conditions result from the atmosphere either resisting or accelerating the vertical motion of air parcels. Atmospheric stability is the property of the ambient air that either enhances or suppresses vertical motion of air parcels and determines which type of clouds and precipitation a pilot will encounter.

13.2 Using a Parcel as a Tool to Evaluate Stability

An air parcel can be used as a tool to evaluate atmospheric stability within a specified vertical column of air in the atmosphere. A parcel is selected from a specified altitude (usually the surface) and hypothetically lifted upward to a specified test altitude. As the parcel is lifted, its temperature decreases due to the expansion and latent heat effects discussed in Chapter 12, Vertical Motion and Clouds. The parcel and the surrounding environmental air temperatures are then compared. If the lifted parcel is colder than the surrounding air, it will be denser (heavier) and sink back to its original level. In this case, the parcel is stable because it resists upward displacement. If the lifted parcel is the same temperature as the surrounding air, it will be the same density and remain at the same level. In this case, the parcel is neutrally stable. If the lifted parcel is warmer and, therefore, less dense (lighter) than the surrounding air, it will continue to rise on its own until it reaches the same temperature as its environment. This final case is an example of an unstable parcel. Greater temperature differences result in greater rates of vertical motion.

13.3 Stability Types

The stability of a column of air in the atmosphere is classified by the distribution of parcel stabilities within the column. The Earth’s surface is typically selected as the base while the top determines the column’s depth. Four unique types of atmospheric stability can be identified and are discussed in the following sections.

13.3.1 Absolute Stability

Absolute stability (see Figure 13-1) is the state of a column of air in the atmosphere when its lapse rate of temperature is less than the moist adiabatic lapse rate. This includes both isothermal and inversion temperature profiles. An air parcel lifted upward would be colder (denser) than the surrounding environmental air and would tend to sink back to its level of origin.

13.3.2 Neutral Stability

Neutral stability (see Figure 13-2) is the state of a column of air in the atmosphere in which an ascending (or descending) air parcel always has the same temperature (density) as the surrounding environmental air. If the column of air is unsaturated, then neutral stability exists when its lapse rate of temperature equals the dry adiabatic lapse rate. If the column of air is saturated, then neutral stability exists when its lapse rate of temperature equals the moist adiabatic lapse rate.

13.3.3 Absolute Instability

Absolute instability (see Figure 13-3) is the state of a column of air in the atmosphere when it has a superadiabatic lapse rate of temperature (i.e., greater than the dry adiabatic lapse rate). An air parcel displaced vertically would be accelerated in the direction of the displacement. The kinetic energy of the parcel would consequently increase with increasing distance from its level of origin.

13.3.4 Conditional Instability

Conditional instability (see Figure 13-4) is the state of a column of unsaturated air in the atmosphere when its lapse rate of temperature is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate. An air parcel lifted upward would be initially stable, but at some point, above its LCL, it would become unstable. The term “conditional” means the parcel must be lifted to a particular level (altitude) before it becomes unstable and rises because of its own buoyancy. The Level of Free Convection (LFC) is the level at which a parcel of air lifted dry adiabatically until saturated and moist adiabatically thereafter would first become warmer than the surrounding environmental air (i.e., unstable) in a conditionally unstable column of air in the atmosphere. The LFC is a defining feature of a conditionally unstable column of air.

13.3.5 Summary of Stability Types

Figure 13-5 summarizes the possible atmospheric stability types.

13.4 Processes That Change Atmospheric Stability

Changes in atmospheric stability are inversely related to temperature (density) changes with height (see Figure 13-6). If temperature lapse rates increase, then stability decreases. Conversely, if temperature lapse rates decrease, then stability increases. Most of these changes occur as a result of the movement of air, but diurnal (day/night) temperature variations can play a significant role. In Figure 13-6, the column of air on the right is less stable because its temperature lapse rate is higher.

13.4.1 Wind Effects on Stability

Wind can act to change the stability of a column of air in the atmosphere by changing the temperature lapse rate. Stability increases when wind blows colder air into the bottom of the air column (cold air advection) and/or warmer air at the top (warm air advection). Conversely, stability decreases when wind blows warmer air into the bottom of the air column and/or colder air at the top.

13.4.2 Vertical Air Motion Effects on Stability

A column of air in the atmosphere will become more stable when it descends (subsides) (see Figure 13-7). As it subsides, it becomes compressed by the weight of the atmosphere and shrinks vertically. The entire layer warms due to adiabatic compression. However, the upper part of the column sinks farther and, thus, warms more than the bottom part. This process acts to decrease the temperature lapse rate and increase stability.

Conversely, a column of air in the atmosphere will become less stable when it ascends (rises). As it rises, the rapid decrease in air density aloft causes the column to stretch out vertically. As long as the layer remains unsaturated, the entire layer cools at the dry adiabatic lapse rate (see Figure 13-7). However, due to the stretching effect, air at the top of the column cools more than the air at the bottom of the column. This process acts to increase the temperature lapse rate and decrease stability.

A rising column of air will become less stable when air at the bottom has a higher relative humidity than air at the top. As the air moves upward, the bottom becomes saturated first and cools at the lesser moist adiabatic lapse rate. The net effect is to increase the lapse rate within the column and decrease stability. This process is called convective instability and is associated with the development of thunderstorms.

13.4.3 Diurnal Temperature Variation Effects on Stability

Diurnal (day/night) temperature variations can have a significant impact on atmospheric stability (see Figure 13-8). Daytime heating of the surface increases temperature lapse rates and decreases stability. Conversely, nighttime cooling of the surface decreases temperature lapse rates and increases stability. Diurnal temperature variations are most pronounced in the lower troposphere because air is a poor conductor of heat (see Table 5-3).

The magnitude of diurnal temperature (and stability) variation is primarily influenced by surface type, latitude, sky cover (e.g., clouds and pollutants), water vapor content of the air, and wind speed. Temperature variation is maximized over land, at low latitudes, with a clear sky, dry air, and light wind. Conversely, temperature variation is minimized over large bodies of water, at high latitudes, with a cloudy sky, moist air, and strong wind.

13.5 Measurements of Stability

Several stability indices and other quantities exist that evaluate atmospheric stability and the potential for convective storms. The most common of these are Lifted Index (LI) and Convective Available Potential Energy (CAPE).

13.5.1 Lifted Index (LI)

The LI (see Figure 13-9) is the temperature difference between an air parcel (usually at the surface) lifted adiabatically (see Chapter 12) and the temperature of the environment at a given pressure (usually 500 mb) in the atmosphere. A positive value indicates a stable column of air (at the respective pressure), a negative value indicates an unstable column of air, and a value of zero indicates a neutrally stable column of air. The larger the positive (negative) LI value, the more stable (unstable) the column of air. LI is generally used in thunderstorm forecasting; however, CAPE is generally considered a superior measurement of instability. However, LI is easier to determine without using a computer.

13.5.2 Convective Available Potential Energy (CAPE)

CAPE is the maximum amount of energy available to an ascending air parcel for convection. CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel over the layer, wherein the latter is warmer than the former. Units are joules per kilogram (J/kg) of air. Any value greater than 0 J/kg indicates instability and the possibility of thunderstorms.

CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate the potential for stronger updrafts. Observed values in thunderstorm environments often exceed 1,000 J/kg and in extreme cases may exceed 5,000 J/kg.

13.6 Convection

Convection is generally described as the transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms “convection” and “thunderstorms” often are used interchangeably, although thunderstorms are only one form of convection. Cumulonimbus clouds, towering cumulus clouds, and altocumulus clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection occurring without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

13.6.1 Surface-Based Convection

Surface-based convection occurs within a surface-based layer (i.e., a layer in which the lowest portion is based at or very near the Earth’s surface) and is primarily generated by the daily heating of the surface of the Earth by the energy from the Sun.

13.6.2 Elevated Convection

Elevated convection occurs within an elevated layer (i.e., a layer in which the lowest portion is based above the Earth’s surface). Elevated convection often occurs when air near the ground is relatively cool and stable (e.g., during periods of isentropic lift, when an unstable layer of air is present aloft, etc.). In cases of elevated convection, stability indices based on near-surface measurements (such as the LI) will typically underestimate the amount of instability present. Severe weather is possible from elevated convection but is less likely than with surface-based convection.

13.6.3 Level of Free Convection (LFC)

The LFC is the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

13.6.4 Popcorn Convection

Popcorn convection is a term often used for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms are sometimes referred to as air mass thunderstorms. They are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

13.7 Summary

Atmospheric stability influences weather by affecting the vertical motion of air. Stable air suppresses vertical motion while unstable air enhances it.