1. Fire Weather: Clouds & T-Storms

2. Physical structure of a cloud Minute water droplets Ice crystals Combination of both Why are clouds important for fire weather?

3. Clouds are indicators of… Atmospheric moisture Atmospheric motion Instability Warning of weather change –Precipitation –Winds

4. Cirrus Clouds thin, wispy, feathery Fire weather: high-altitude moisture and wind direction and speed warning of warm-front activity

5. 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.

6. 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…

7. 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.

8. 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

9. 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

10. 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

11. 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.

12. 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

13. 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)

14. T-Storm stages… 3. DISSIPATING STAGE No new condensation to support cloud growth Cell changes to all downdrafts Downdrafts dissipate and surface signs disappear

15. 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

16. 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

17. Cloud-to-cloud discharges –Between the negative charge in the lower cloud and positive charge in core of cloud.

18. 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