Fire Weather: Clouds

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Fire WeatherClouds T-Storms
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Physical structure of a cloud

• Minute water droplets
• Ice crystals
• Combination of both

Why are clouds important for fire weather?
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Clouds are indicators of

• Atmospheric moisture
• Atmospheric motion
• Instability
• Warning of weather change
• Precipitation
• Winds

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Cloud formation

• Saturation ? condensation (or sublimation)
• Addition of moisture
• Cold air over warm water (evaporation)
• Warm rain falls through cold air (beneath a warm
  front)
• Lowering of air temperatures - lifting
• Thermal
• Orographic
• Frontal
• Convergence (low pressure systems)

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Cirrus Clouds thin, wispy, feathery
Fire weather high-altitude
moisture and wind direction and speed
warning of warm-front activity
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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.
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Altocumulus Clouds parallel bands or rounded
masses
Portion of cloud shaded. Formed by frontal or
orographic lifting. May develop into Altocumulus
Castellanus clouds
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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.
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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
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Nimbostratus Clouds dark, low-level clouds with
precipitation
Because of the fog and falling precipitation
commonly found beneath and around nimbostratus
clouds, the cloud base is typically very diffuse
and difficult to accurately determine.
Fire weather increased moisture (fog, rain)
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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)
• Sufficient moisture in air

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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 groups answer to the
  class.

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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

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T-Storm stages

• 3. DISSIPATING STAGE
• No new condensation to support cloud growth
• Cell changes to all downdrafts
• Downdrafts dissipate and surface signs disappear

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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)

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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

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Lightning Discharge 3 types

• Cloud-to-ground
• between negative lower portion of cloud and
  positive charge on ground
• 1/3 of all discharges
• 4 strokes on average (bolt/flash 40-50
  milli-sec)
• 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

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• 2) Hybrid discharge
• After the return stroke, a continuous charge
  transfer takes place (200 mill-sec)
• 15 of all cloud-to-ground discharges
• Usually responsible for starting fires
• 3) Cloud-to-cloud discharges
• Between the negative charge in the lower cloud
  and positive charge in core of cloud

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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

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