Chapter 6 Atmospheric Stability The Concept of Stability

Chapter 6 

Atmospheric Stability 

The Concept of Stability 

Why do we care about atmospheric stability 

A non-atmospheric example of stability: 

What happens if we push the ball to the left or right in each figure below

Air parcel - a distinct blob of air that we will imagine we can identify as it 

moves through the atmosphere 

Air parcels and stability: 

Stable: if the parcel is displaced vertically, it will return to its original 

position 

Neutral: if the parcel is displaced vertically, it will remain in its new position 

Unstable: if the parcel is displaced vertically, it will accelerate away from its 

original position in the direction of the initial displacement 

What happens to the size of an air parcel as it moves vertically in the 

atmosphere 

An air parcel will expand as it rises because it encounters lower pressure. 

An air parcel will compress as it sinks because it encounters higher 

pressure.

What happens to the temperature of an air parcel as it expands (rises) or 

compresses (sinks) 

The air parcel cools as it expands (rises). 

The air parcel warms as it compresses (sinks). 

Adiabatic process – a process in which an air parcel does not mix with its 

environment or exchange energy with its environment 

An air parcel will expand and cool as it rises and will compress and 

warm as it sinks. 

Lapse Rates 

Dry adiabatic lapse rate – the rate at which an unsaturated air parcel will 

cool if it rises or warm as it sinks (applies to an air parcel with a relative 

humidity of less than 100%) 

Dry adiabatic lapse rate = 10 deg C / km 

Moist adiabatic lapse rate – the rate at which a saturated air parcel will cool 

if it rises or warm if it sinks (applies to an air parcel with a relative humidity 

of 100%) 

Moist adiabatic lapse rate = 6 deg C / km (on average in the troposphere) 

Why is the moist adiabatic lapse rate different Why is it less than the dry 

adiabatic lapse rate 

What happens to a saturated air parcel as it rises 

1. The air parcel will cool as it rises 

2. Water vapor will condense as the parcel rises (a cloud forms) 

3. As water vapor condenses latent heat is released 

4. The latent heat that is released will offset some of the cooling that 

occurred as the air parcel rose (the amount of latent heat released 

will vary, but on average will offset about 4 deg C / km of cooling) 

Use the dry adiabatic lapse rate or moist adiabatic lapse rate to determine 

the temperature of an air parcel as it rises or sinks in the atmosphere.

Environment – the atmosphere outside of an air parcel 

Environmental lapse rate – the rate at which the environment’s temperature 

decreases with increasing altitude 

Use the environmental lapse rate to determine the temperature of the 

environment as you move up or down in the atmosphere. 

Idealized environmental temperature 

profile 

How do we measure the 

environmental temperature profile 

Is the environmental temperature 

profile always the same 

Inversion layer – a layer of the 

atmosphere where the 

environmental temperature 

increases with increasing altitude 

What does a negative environmental 

lapse rate mean 

Determining Stability 

To determine stability we need to compare the temperature of an air parcel 

to the temperature of its environment. 

If an air parcel is warmer than its environment it will rise. 

If an air parcel is colder than its environment it will sink.

Two sample environmental temperature profiles: 

Examples of determining stability: 

For these examples 

an air parcel starts 

at the surface, and 

has the same 

temperature as the 

environment at the 

surface. 

The air parcel is 

then lifted 1 km. 

How does the 

temperature of the 

air parcel change 

as it is lifted 

What is the stability 

of the air parcel in 

each panel of this 

figure

Conditionally unstable – the condition required for instability is that the 

displaced air parcel is saturated. 

An example of a conditionally unstable atmospheric layer: 

Summary of Categories of Atmospheric Layer Stability 

Environmental Lapse Rate (ELR) Stability 

ELR > 10 deg C / km 

Unstable 

ELR = 10 deg C / km 

Neutral if unsaturated 

Unstable if saturated 

6 deg C / km < ELR < 10 deg C / km Conditionally unstable (see above) 

ELR = 6 deg C / km 

Neutral if saturated 

Stable if unsaturated 

ELR < 6 deg C 

Stable

The environmental 

temperature profile is 

often more complex 

than was shown in the 

examples above. On 

the left are two 

examples of actual 

environmental 

temperature profiles 

and the temperature 

of an air parcel as it 

rises through the 

environment. 

What is the stability of 

the air parcel as it 

rises through the 

environmental 

temperature profile 

Does the stability 

change if we look at 

different levels of the 

atmosphere

How can the stability of an atmospheric layer be changed 

What happens to the stability of the lower part of the atmosphere over the 

course of a day 

Stability and thunderstorm development 

Convection – an air parcel rising buoyantly because it is warmer than its 

environment 

What happens to an 

unsaturated air parcel 

as it is lifted 

Size 

Temperature 

Relative humidity 

What happens when 

the relative humidity of 

the air parcel increases 

to 100% 

Lifting condensation level – the level where condensation first occurs as an 

air parcel is lifted (where the relative humidity of the air parcel becomes 

100%) 

Level of free convection – the level where an air parcel first becomes 

buoyant (warmer than its environment)

Lifting Mechanisms 

How are air parcels lifted in the atmosphere

Clouds 

How do clouds form 

(In class experiment to make a cloud) 

Ingredients for a cloud: 

1. 

2. 

How is the 2 nd ingredient achieved in the atmosphere 

3. 

Do we need a 3 rd ingredient Why 

What serves as the 3 rd ingredient 

Clouds form when an air parcel is lifted to the lifting condensation 

level and condensation occurs 

There are four basic types of clouds based on Latin words: 

Stratus: “spread out” 

layered cloud 

Cumulus: “heap” 

puffy (cotton ball) cloud 

Cirrus: “curl of hair” 

wispy cloud 

Nimbus: “rain cloud” or “rain storm” 

raining cloud

Cloud categories: 

There are also four categories based on: 

Height/altitude, vertical development, and appearance of the cloud 

 

Low (0-2 km): no special prefix for height 

stratus, stratocumulus, nimbostratus 

Middle (2-6 km): alto----- 

altostratus, altocumulus 

High (>6 km): cirrus or cirro----- 

cirrus, cirrostratus, cirrocumulus 

Vertically developed: cumulus or cumulo---- 

cumulus, cumulonimbus 

Cumuliform clouds occur more in unstable environments when the air 

parcels undergo a lot of vertical motion 

Stratiform clouds occur in more stable environments

Chapter 6 Atmospheric Stability The Concept of Stability

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