S290 Unit 6 part 2

6-1-S290-EPUnit 6 Atmospheric Stability

Four Lifting Processes That Can Cause Thunderstorms

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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.

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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 andupvalley 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 cloudsare 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 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.

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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

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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 necessaryto 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

• Leading edge of these strong “outflow” winds is called a gust front


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 Indicatorsof Stable Air

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EXERCISE 7

Visual Indicators of Stable and Unstable Air

6-24-S290-EPUnit 6 Atmospheric StabilityExercise 7

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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 ft2) Middle 10,000 to 20,000 ft3) 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

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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

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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

Education

lifting of air

air density

lowering of air pressure

advancing air mass

warmer air mass

passing jet stream

upward deflection of

atmospheric stabilityas