S290 Unit 6 part 2

6-1-S290-EPUnit 6 Atmospheric Stability
Four Lifting Processes That Can Cause Thunderstorms
6-1-S290-EP

6-2-S290-EPUnit 6 Atmospheric Stability 06-2-S290-EP
ThermalsThermals
UpdraftsUpdrafts
Strong Surface Heating
Unstable
Atmosphere
Thermal or Convective LiftThermal or Convective Lift
Strong heating of air near the ground produces
thermal updrafts and convective currents. It is
most common in the summer.
Strong heating of air near the ground produces
thermal updrafts and convective currents. It is
most common in the summer.
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Orographic
Lift
Lifting air over
mountains.
Caused by air being forced
up a slope or valley by
daytime heating, or by the
upward deflection of air
after colliding with a
mountain barrier.
heating
of slopes
upslope and
upvalley flow
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Frontal Lift
Converging Air Masses
When two air masses
converge, the lighter
and warmer air mass
is forced up and over
the other air mass.
The amount of lift
depends on the depth
and speed of the
advancing air mass,
and how unstable the
air mass is.

Convergence Over Mountain Ridges
Occurs during the
day over mountain
ridges as
thermally or heat
driven winds on
opposing slopes
come together.
Convective clouds
are an indication
that it is occurring.
Clouds produced by
this lift are normally
tall and vertically
developed cumulus
type.

Jet Stream Lift - First, What is the Jet Stream?
It is simply a river of
swiftly moving air
most often at high
altitudes.
It is the change in wind speed and direction, or wind
shear that is largely responsible for producing the lifting
of air associated with the jet stream.
Varies in speed and
direction as it rounds
the top of high
pressure ridges and
low pressure troughs.

As the jet stream passes
overhead, wind shear
produces divergence (the
horizontal spreading out of
winds) and lowers air
density and air pressure
aloft.
Jet Stream
Low PressureLow Pressure
Jet Stream Lift
This pressure reduction
causes air from lower levels
to rise and fill in the “void”
left behind by
divergence.
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If this occurs near a
wildfire or thunderstorm,
explosive growth is
possible.
Jet Stream
Divergence
and Low Pressure
Divergence
and Low Pressure
Jet Stream Lift
This chimney-effect caused
by divergence and a
lowering of air pressure aloft
can be enhanced by strong
heating and convergence
(the coming together of air)
at lower levels.
ConvergenceConvergence
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Jet Stream Winds
A wildland fire can quickly
intensify and become
plume dominated with the
strong lift produced by a
passing jet stream.
Wispy cirrus clouds high in
the sky may be your only
indication that jet stream
winds are passing
overhead.
Cirrus
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Fire fighters
should watch for
any visual signs
of a jet stream
nearby
such as the tops
of thunderstorm
clouds being
sheared off by
strong winds
aloft.
Jet Stream Lift and Thunderstorm Growth
Rapidly
Growing
Thunderstorm
Jet Stream Wind Shear
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EXERCISE 5
Lifting Processes

The Thunderstorm
Local storm nearly always
produced by a cumulonimbus cloud,
and always accompanied by
lightning and thunder.

Lifting, instability and moisture are all necessary
to produce a thunderstorm.
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Three Stages of a Thunderstorm

The Cumulus Stage
• Early stage of a thunderstorm.
• The cloud grows vertically, often with only a
single updraft.
• Precipitation is not produced during this stage,
but gusty winds are relatively common.

The Mature Stage
• Most active stage of the thunderstorm cycle
• First lightning and thunder are observed
• Strong updraft and downdraft
• Cloud is called a cumulonimbus
• Strong rain-cooled downdrafts reach the ground,
can fan out producing strong, sometimes
damaging winds

The Dissipating Stage
• During this final stage, downdrafts exist
throughout the cumulonimbus cloud.
• Without an updraft to supply the thunderstorm
with a source of moisture and energy, the core of
the thunderstorm collapses.
• The collapsing cloud can still produce downdraft
winds, gust fronts, rain and lightning.

All three stages of the thunderstorm pose
a hazard to the wildland firefighter.
It is the mature stage, however, that
poses the greatest risk to
personal safety, and potentially
the greatest impact on
wildland fire behavior.

EXERCISE 6
The Thunderstorm and Its Stages

Visual Indicators
of Stable Air

EXERCISE 7
Visual Indicators of
Stable and Unstable Air

Exercise 7 6-24-S290-EP

Exercise 8
From the following photographs of smoke columns
you may be asked to provide short answer to one
or more of the following questions:
What Can Smoke Tell Us About the
Stability of the Atmosphere?
1. Is the atmosphere stable or unstable? Where?
2. If there is wind, where and how strong?
3. If there is an inversion, what type?

A B
C D
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E F
G H
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I J
K L
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What Is a Cloud?
A visible collection of billions of water
droplets and/or ice particles suspended in
the atmosphere.
A cloud that touches the ground is called fog.
The formation and appearance of a cloud is
strongly influenced by the stability of the
atmosphere.

Clouds form either by:
condensing water vapor in a column of rising
air, or by cooling air to the point of saturation.
Clouds form under:
stable and unstable atmospheric conditions,
and not all clouds produce precipitation.

Clouds Can Be Classified:
• by their appearance, and
• by the height of their bases above
the ground.
There are basically three cloud groups:
1) High > 20,000 ft
2) Middle 10,000 to 20,000 ft
3) Low < 6,500 ft
Vertically developed 1,500-10,000 ft

The Cloud Family
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Clouds Preceding a Significant
Weather Change Such as a Cold Front
• High clouds will first fill the sky, sometimes
up to 3 days in advance of a cold front.
• Next to appear will be the middle clouds,
usually one to two days in advance of a cold front.
• Last to appear in the sky are the low and
vertically developed clouds, typically the
day of the cold front passage.

Cloud Progression With An Advancing Cold Front
6-34-S290-EP

Impact of Cloud Cover On Fire Behavior
Less Than 50% Sky Cover Greater Than 50% Sky Cover
Active Fire and
High Flame Heights
Inactive Fire and
Low Flame Heights
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Critical Clouds to Firefighters
• Cumulonimbus or thunderstorm cloud
• Cirrostratus
• Altocumulus castellanus
• Altocumulus floccus
• Altocumulus standing lenticularus
• Stratus

Cumulonimbus (Cb)
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Cirrostratus (Cs)
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Altocumulus Castellanus
(Accas)
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Altocumulus Floccus (Ac)
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Altocumulus Standing
Lenticularus (Acsl)
Wave Cloud
Wave Cloud
Wave Cloud
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Stratus (St)
(Fog – Stratus in contact with the ground)
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EXERCISE 9
Cloud Types and Descriptions

Unit 6 Objectives
2. Describe temperature lapse rate and stability, and
the different temperature lapse rates used to
determine the stability of the atmosphere.
4. Name four types of temperature inversions and
describe their influence on wildland fire behavior,
including the thermal belt.
3. Describe the effects of atmospheric stability on
wildland fire behavior.
1. Describe the relationship among atmospheric
pressure, temperature, density and volume.

5. Name and describe the four lifting processes that
can produce thunderstorms.
6. Describe the elements of a thunderstorm and its
three stages of development.
7. Use visual indicators to describe the stability
of the atmosphere.
8. Describe the four principle cloud groups, and
identify the six clouds most often associated with
critical wildland fire behavior.
Unit 6 Objectives

S290 Unit 6 part 2

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S290 Unit 6 part 2