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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 - PowerPoint PPT Presentation
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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 massesPortion 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