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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
UnstableAtmosphere
Thermal or Convective LiftThermal or Convective LiftStrong 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.
6-2-S290-EP
6-3-S290-EPUnit 6 Atmospheric Stability
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 andupvalley flow
6-3-S290-EP
6-4-S290-EPUnit 6 Atmospheric Stability
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.
6-5-S290-EPUnit 6 Atmospheric Stability
Convergence Over Mountain Ridges
Occurs during the day over mountain
ridges as thermally or heat driven winds on opposing slopes come together.
Convective cloudsare an indication
that it is occurring.
Clouds produced by this lift are normally
tall and vertically developed cumulus
type.
6-6-S290-EPUnit 6 Atmospheric Stability
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.
6-7-S290-EPUnit 6 Atmospheric Stability
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 StreamLow PressureLow Pressure
Jet Stream Lift
This pressure reduction causes air from lower levels to rise and fill in the “void”
left behind by divergence.
6-7-S290-EP
6-8-S290-EPUnit 6 Atmospheric Stability
If this occurs near a wildfire or thunderstorm,
explosive growth is possible.
Jet Stream
Divergenceand Low Pressure
Divergenceand 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
6-8-S290-EP
6-9-S290-EPUnit 6 Atmospheric Stability
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
6-9-S290-EP
6-10-S290-EPUnit 6 Atmospheric Stability
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
6-10-S290-EP
6-11-S290-EPUnit 6 Atmospheric Stability
EXERCISE 5
Lifting Processes
6-12-S290-EPUnit 6 Atmospheric Stability
The Thunderstorm
Local storm nearly always produced by a cumulonimbus cloud,
and always accompanied by lightning and thunder.
6-13-S290-EPUnit 6 Atmospheric Stability
Lifting, instability and moisture are all necessaryto produce a thunderstorm.
6-13-S290-EP
6-14-S290-EPUnit 6 Atmospheric Stability
Three Stages of a Thunderstorm
6-15-S290-EPUnit 6 Atmospheric Stability
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.
6-16-S290-EPUnit 6 Atmospheric Stability
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
• Leading edge of these strong “outflow” winds is called a gust front
6-17-S290-EPUnit 6 Atmospheric Stability
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.
6-18-S290-EPUnit 6 Atmospheric Stability
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.
6-19-S290-EPUnit 6 Atmospheric Stability
EXERCISE 6
The Thunderstorm and Its Stages
6-20-S290-EPUnit 6 Atmospheric Stability
Visual Indicatorsof Stable Air
6-21-S290-EPUnit 6 Atmospheric Stability 6-21-S290-EP
6-22-S290-EPUnit 6 Atmospheric Stability
6-22-S290-EP
6-23-S290-EPUnit 6 Atmospheric Stability
EXERCISE 7
Visual Indicators of Stable and Unstable Air
6-24-S290-EPUnit 6 Atmospheric StabilityExercise 7
6-24-S290-EP
6-25-S290-EPUnit 6 Atmospheric Stability
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?
6-26-S290-EPUnit 6 Atmospheric Stability
A B
C D
6-26-S290-EP
6-27-S290-EPUnit 6 Atmospheric Stability
E F
G H
6-27-S290-EP
6-28-S290-EPUnit 6 Atmospheric Stability
I J
K L
6-28-S290-EP
6-29-S290-EPUnit 6 Atmospheric Stability
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.
6-30-S290-EPUnit 6 Atmospheric Stability
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.
6-31-S290-EPUnit 6 Atmospheric Stability
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 ft2) Middle 10,000 to 20,000 ft3) Low < 6,500 ft
Vertically developed 1,500-10,000 ft
6-32-S290-EPUnit 6 Atmospheric Stability
The Cloud Family
6-32-S290-EP
6-33-S290-EPUnit 6 Atmospheric Stability
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.
6-34-S290-EPUnit 6 Atmospheric Stability
Cloud Progression With An Advancing Cold Front
6-34-S290-EP
6-35-S290-EPUnit 6 Atmospheric Stability
Impact of Cloud Cover On Fire BehaviorLess Than 50% Sky Cover Greater Than 50% Sky Cover
Active Fire and High Flame Heights
Inactive Fire and Low Flame Heights
6-35-S290-EP
6-36-S290-EPUnit 6 Atmospheric Stability
Critical Clouds to Firefighters
• Cumulonimbus or thunderstorm cloud
• Cirrostratus
• Altocumulus castellanus
• Altocumulus floccus
• Altocumulus standing lenticularus
• Stratus
6-37-S290-EPUnit 6 Atmospheric Stability
Cumulonimbus (Cb)
6-37-S290-EP
6-38-S290-EPUnit 6 Atmospheric Stability
Cirrostratus (Cs)
6-38-S290-EP
6-39-S290-EPUnit 6 Atmospheric Stability
Altocumulus Castellanus (Accas)
6-39-S290-EP
6-40-S290-EPUnit 6 Atmospheric Stability
Altocumulus Floccus (Ac)
6-40-S290-EP
6-41-S290-EPUnit 6 Atmospheric Stability
Altocumulus Standing Lenticularus (Acsl)
Wave Cloud
Wave Cloud
Wave Cloud
6-41-S290-EP
6-42-S290-EPUnit 6 Atmospheric Stability
Stratus (St)
(Fog – Stratus in contact with the ground)
6-42-S290-EP
6-43-S290-EPUnit 6 Atmospheric Stability
EXERCISE 9
Cloud Types and Descriptions
6-44-S290-EPUnit 6 Atmospheric Stability
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.
6-45-S290-EPUnit 6 Atmospheric Stability
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