Chapter 13, Atmospheric Stability 13-7
Temperature
Sounding
Unsaturated
Parcel
Saturated
Parcel Stability Type
A Unstable Unstable Absolute Instability
B Neutral Unstable
C Stable Unstable Conditional Instability
D Stable Neutral
E Stable Stable Lapse Absolute
Stability F Stable Stable Isothermal
G Stable Stable Inversion
Figure 13-5. 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.
Figure 13-6. Temperature Lapse Rate Effects on Stability
Chapter 13, Atmospheric Stability 13-8
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 proce ss 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 cool s 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.
Figure 13-7. Vertical Motion Effects on 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
Chapter 13, Atmospheric Stability 13-9
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 lo wer 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.
Figure 13-8. Diurnal Temperature Variation Effects on Stability
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).
Chapter 13, Atmospheric Stability 13-10
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.
Figure 13-9. Lifted Index Example
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 strong er updrafts. Observed values in thunderstorm environments often
exceed 1,000 J/kg and in extreme cases may exceed 5,000 J/kg.
Chapter 13, Atmospheric Stability 13-11
13.6 Convection
Convection is g enerally described a s 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 a ffecting the vertical motion of air. Stable air suppresses
vertical motion while unstable air enhances it.