Fire Weather:

Clouds & T-Storms

Physical structure of a cloud

• Minute water droplets

• Ice crystals

• Combination of both

Why are clouds important for fire weather?

Clouds are indicators of…

• Atmospheric moisture

• Atmospheric motion

• Instability

• Warning of weather change– Precipitation– Winds

Cirrus Clouds thin, wispy, feathery

Fire weather: high-altitude moisture and wind direction and speed warning of warm-front activity

Fair Weather Cumulus Clouds puffy cotton balls floating in the sky

Flat bases and distinct outlines, irregular shapes (“cauliflower”).Slight vertical growth; cloud tops = limit of the rising air. Fire weather: warning of convection in surface layer.Can later develop into towering cumulonimbus clouds.

                        

             

Altocumulus Clouds parallel bands or rounded masses

Portion of cloud = shaded.Formed by frontal or orographic lifting.Fire weather: May develop into Altocumulus Castellanus clouds…

Altocumulus Castellanus Clouds (“turrets”)

Convection in unstable layer aloft.Often result of gradual lifting of air in advance of a cold front.Fire weather: warning of possible thunderstorms later in the day.

Cumulonimbus Clouds - towering high into the atmosphere-   Moist and unstable air = towering cumulus clouds Fueled by vigorous convective updrafts (> 50 knots, tops > 60,000 ft) Thunderhead (with anvil) Fire weather: gusty and high speed surface winds, dust devils, whirlwinds, turbulence, downdrafts

Lenticular Clouds• Indicate waves of air flow caused by strong winds blowing across a

range (“mountain waves”).• Usually appear over the ridge on the lee side of mountains.• Fire weather: may increase fire activity if air flows descend

Thunderstorms • Major influence on fire behavior

– Wind patterns– Lightning (can cause fires anywhere in the U.S.)

• Three conditions required:– Unstable air – Triggering mechanisms (lifting process)

• Orographic, frontal, convergence, thermal/convection

– Sufficient moisture in air

Group Exercise• Describe the processes that occur in each of the

three stages of thunderstorm development: cumulus, mature, dissipating.

• What is the effect on fire behavior in each stage?

• Note: you will be given a diagram of each stage to facilitate your discussion and formulation of your response.

• Each member of the group should be prepared to be called upon to present the group’s answer to the class.

3 Stages of a Thunderstorm

1. CUMULUS STAGE– Lifting of moist air above

condensation level– Updrafts increase in speed– Droplets increase in size– Light downdrafts (settling of air)– Gentle wind change– Fire behavior: convection

columns may cause fire to

become more active

T-Storm stages…2. MATURE STAGE– Rain falls from cloud base –

drowndrafts– Updrafts and downdrafts in

different portions of cloud– Downdrafts strongest at front edge

(>30 mph)– Convection cell - maximum height– Anvil top – points in

direction of travel– Fire behavior:

turbulent, strong wind

(horizontal flow)

T-Storm stages…

3. DISSIPATING STAGE• No new condensation

to support cloud growth• Cell changes to all

downdrafts• Downdrafts dissipate

and surface signs disappear

Lightning

• Occurs in a T-Storm when electrical potential builds up

• Movement of particles with positive and negative charges

• Atmosphere: positive charge with respect to the Earth

• Cumulonimbus clouds – alters and intensifies electric fields – creates a positive charge on the ground

Lightning Discharge

Cloud-to-ground: - between negative lower portion of

cloud and positive charge on ground– Cold stroke: intense current, short

duration (345,000 amps)– Hot stroke: lesser current, longer

duration (200 amps)– Hot stokes more likely to start fires– 20% lightning bolts in West = hot

strokes

Cloud-to-cloud discharges – Between the negative

charge in the lower cloud and positive charge in core of cloud.

Thunder

• Compression wave from sudden heating and expansion of air along path of lightning discharge

• Reflected from the ground surface = sound• Light moves faster than sound – see

lightning first• Approx. 1 mile to the flash for every 5

seconds elapsed time