Fire Weather and Fire Behavior Impacts from Clouds
Introduction
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| H1: Cirrus Streaks Single or multiple bands across sky indicating Jet Stream aloft – Possible instability or surface winds increasing fire behavior |
H2: Cirrus
Dense, in patches or sheaves, not increasing, or with tufts – No significant fire weather or fire behavior expected |
H3: Cirrus
Anvil shaped remains of a cumulonimbus – Watch for gusty winds from nearby decaying thunderstorms
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H4: Cirrus
In hooks or filaments, increasing, becoming denser – Often indicates cold air aloft, watch for possible instability leading to more active fire |
H5: Cirrostratus
Cirrus bands, relatively near the horizon – Indicates weather fronts are typically 2 to 3 days away
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H6: Cirrostratus
Cirrus bands, covering a significant portion of the sky with sun mostly obscured – Typically indicates weather fronts are one day or less away
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H7: Cirrostratus Translucent, completely covering the sky – No significant impact on fire weather or fire behavior
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H8: Cirrostratus Not increasing, not covering the whole sky – No significant impact on fire weather or fire behavior
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H9: Cirrocumulus Alone or with some cirrus or cirrostratus – Very high instability, usually little impact on surface conditions |
M1: Altostratus Semi-transparent, sun or moon may be dimly visible – No fire weather impacts, expect reduced fire activity from added shading
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M2: Altostratus or Nimbostratus
Dense enough to hide the or moon – Often contains steady rain, watch for slippery footing, expect minimal fire activity
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M3: Altocumulus Semi-transparent, one level – indicates weak mid-level instability and increasing moisture, possible monsoon thunderstorm development within 24 hours
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M4: Altocumulus Lens-shaped, or continually changing shape and size – Results from strong mid-level winds that may surface under the right conditions resulting in running fire
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M5: Altocumulus
One or more bands or layers, expanding, thickening – indicates instability present, monitor for additional development
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M6: Altocumulus From the spreading of cumulus or cumulonimbus – Could indicate presence of a nearby thunderstorm, caution for gusty winds generally from the direction of the clouds
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M7: Altocumulus Generally opaque layers, possibly containing rain shafts – Weak instability, shafts may result in gusty winds, cloud density will usually inhibit further convection
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M8: Altocumulus Castellanus/
Altocumulus Floccus
With cumulus-like tufts or turrets – When observed in morning hours, often a precursor to afternoon/ evening thunderstorms |
M9: Altocumulus Chaotic sky, cloud bases at several levels – Lower cloud base may block view of higher based storms, hides potential for lightning and gusty winds
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L1: Fair Weather Cumulus
Flattened Appearance – Weak instability present, but rarely results in further development, active fire behavior still possible, especially under low humidity
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L2: Cumulus Moderate/strong vertical development or towering cumulus – Atmosphere is unstable, pay close attention for further storm development and increased fire activity
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L3: Cumulonimbus Tops not fibrous, outline not completely sharp, no anvil – Significant vertical motion present, gusty downdrafts likely, thunderstorms are possible, erratic fire activity
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L4: Stratocumulus Moderate instability – Showers and downdrafts likely, thunderstorms possibly hidden, variable fire behavior |
L5: Stratocumulus Flat appearance, usually associated with cool moist weather – Light showers possible, fire generally inactive |
L6: Stratus
Smooth appearance, continuous low layer – Often inhibits aircraft use, minimal fire activity
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| L7: Stratus Fractus/ Cumulus Fractus Occurs with rain or snow – Usually associated with cold front, winds may push fire |
L8: Cumulus & Stratocumulus
Usually associated with cool coastal influence – Fire activity may increase with afternoon heating and instability |
L9: Mature Thunderstorm
Strong downdraft winds, lightning, heavy rain, and hail possible underneath – Strong terrain channeled winds and lightning possible at some distance |
Pyrocumulus
Unstable conditions where smoke moisture condenses to form cumulus – Downdrafts, rain, lightning possible under extreme conditions
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Tornado
Rapidly rotating column under a cumulonimbus cloud that touches the ground
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Wall Cloud
Lowering of the rain free base of a thunderstorm, often prior to tornado formation |
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| Shelf Cloud Represents the leading edge of strong winds in advance of a thunderstorm |
Wave Cloud
Formed by strong winds over uneven terrain, often repeating downstream – These strong winds may surface resulting in running fire
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