Atmospheric Stability, Winds, and Clouds Summary

Atmospheric Stability, Winds, and Clouds

Objectives

  • Describe atmospheric stability and its effects on fire behavior.

  • Describe wind and its effects on fire behavior.

  • Explain cloud classifications and their impact on fire behavior.

  • Explain the similarities between smoke layers and clouds and their impact on fire behavior.

Atmospheric Stability

  • Atmospheric stability is a fundamental factor in understanding weather.

  • Wildfires are greatly affected by atmospheric motion.

  • Changes in surface winds, temperature, and relative humidity are commonly considered by firefighters.

  • Atmospheric stability is the degree to which vertical motion in the atmosphere is enhanced or suppressed.

  • Stability is related to the temperature distribution of the atmosphere and can be determined by on-site measurements or estimated by atmospheric models.

  • Atmospheric temperature structure and stability are constantly changing.

    • Variations occur over time (day to day, season to season).

    • Variations occur by location.

    • Variations occur from one layer of the atmosphere to the next.

  • Atmospheric stability determines whether conditions will encourage or discourage upward motion.

Unstable Atmosphere

  • Encourages upward motion.

  • Allows convective columns to reach greater heights.

  • Provides the fire with oxygen-rich air by increasing in-drafts at the surface.

  • Can result in high-intensity burning conditions.

Stable Atmosphere

  • Suppresses upward motion.

  • Limits convective columns.

  • Reduces oxygen supply to the fire.

Environmental Temperature and Vertical Motion

  • The environmental temperature structure of the atmosphere determines whether the atmosphere will enhance or suppress vertical motion.

  • Horizontal motion can be caused by surface winds or winds aloft.

  • Stability determines vertical motion, influencing whether air stays capped or rises into the upper atmosphere.

Example: Hot Air Balloon

  • Hot air inside a balloon is less dense than the environmental air outside, causing the balloon to rise.

  • As the environmental temperature warms, its density rises, and the air in the balloon becomes less dense in comparison, causing the balloon to descend.

  • Relate the balloon example to a fire: as fire creates heat and a convective current, cooler air outside will cause it to rise.

Determining Atmospheric Stability

  • Consider the environmental temperature and the air parcel temperature (hot air balloon or fire).

  • Stable Environment: Discourages vertical rise.

  • Unstable Environment: Encourages vertical rise.

  • Atmospheric stability is determined by comparing the temperature of an air parcel of smoke to the environmental temperature.

Stable Conditions

  • If the parcel temperature is equal to or cooler than the environmental temperature, it will stay at its current level or sink.

Unstable Conditions

  • If the air parcel temperature is warmer than the environment, it will rise.

Haines Index

  • The Haines Index (HI) indicates the potential for rapid fire growth due to dry and unstable atmospheric conditions over a fire area.

  • It measures the atmospheric contribution to a fire's growth potential.

  • A high Haines Index correlates with large fire growth where winds do not dominate fire behavior.

  • Haines Index values:

    • 3-4: Common during normal fire operations.

    • 5-6: Indicate moderate or high potential for a dry and very unstable atmosphere, typical during red flag days.

Visual Indicators of Stable Atmospheric Conditions

  • Smoke layer: The top layer of smoke levels off like a table.

  • Stratus clouds or fog.

  • Low-intensity fire as a result of suppressed vertical motion and weak inflow wind.

  • Mountain wave clouds (lenticular clouds): Indicate strong winds aloft that could surface on the lee side of a mountain range.

Effects of Stable Atmospheric Conditions on Fire Behavior

  • Limits the rise of the smoke column, resulting in poor smoke dispersion and visibility.

  • Reduces the inflow of fresh air, limiting fire growth and intensity.

  • Lowers the surface wind speeds and fire spread rates, except in mountainous and hilly terrain (e.g., fern winds).

Visual Indicators of Unstable Atmosphere

  • Towering smoke plume.

    • Plume description: light, puffy, and much larger than the surrounding terrain.

  • Cumulus clouds: Formed from rising air parcels.

  • Cumulonimbus clouds: Larger than cumulus, may produce rain, lightning, and virga (rain that evaporates before hitting the ground).

  • High-intensity fire due to increased vertical motion and inflow winds.

  • Gusty winds as a result of mixing of the lower atmosphere.

  • Good visibility.

Effects of Unstable Atmospheric Conditions on Fire Behavior

  • Increased likelihood of fire whirls and dust devils.

  • Increased likelihood of gusty and erratic surface winds.

  • The height and strength of convection and smoke columns often increase significantly.

  • Increased likelihood of firebrands being lifted to great heights, leading to spot fires.

Temperature Inversions

  • Warm layers exist at different levels of the atmosphere.

  • Meteorologists use weather data and atmospheric models to determine the strength and level of inversions.

  • An inversion is a layer of very stable air.

  • Inversions act like a cap or lid to severely limit the upward movement of air.

  • The level and strength of the inversion are measured and analyzed to determine impacts on the fire environment.

Recognizing a Developing Inversion

  • Surface temperatures decrease.

  • Relative humidity increases.

  • Winds may become light.

  • Smoke flattens after limited rise.

  • Fire behavior typically decreases.

  • Likely occurs during evening and night hours.

Inversion Lifting (Dissipating Inversion)

  • Wind may increase and change direction.

  • Temperature increases and relative humidity decreases.

  • The air mass becomes unstable.

  • Smoke begins to loft and develop a column.

  • Fire behavior increases.

  • Often occurs as daytime hours come, the temperature rises

Look Up, Look Down, Look Around Indicators (IRPG)

  • Fuel characteristics and topography change very little, whereas weather can change rapidly.

  • Winds with speeds over 10 miles per hour.

  • Lenticular clouds indicate high winds aloft.

  • Cold front passage.

  • Cumulonimbus clouds.

  • Dust clouds approaching (potential front).

  • Sudden calm (cold front passage).

  • Battling or shifting wind.

  • Good visibility, battling or shifting winds, dust devils, cumulus clouds, Castellanus clouds (unstable atmosphere).

  • Smoke rising straight up.

  • Inversion lifting.

  • Unusually high Haines values for the local area.

Wind

  • Wind is the most critical factor affecting fire behavior.

  • It is very difficult to predict, especially in complex terrain.

  • Wind is the most variable factor in time and space.

  • It poses safety and control problems.

  • Wind can support tactics if accurately measured and forecasted.

Definition

  • Wind is a horizontal movement of air relative to the surface of the earth.

Scales of Wind

  • General winds: Large scale upper-level winds caused by high and low-pressure systems.

  • Local winds: Found at lower levels of the atmosphere, induced by small-scale local differences in air temperature and pressure.

Wind Direction

  • Wind direction is the compass direction from which the wind is blowing.

General Winds

  • Large-scale winds caused by high and low-pressure systems.

  • Influenced and modified in the lower atmosphere by terrain.

  • Typically found at mid and upper levels of the troposphere.

  • Responsible for transporting weather systems.

Local Winds

  • Generated over a comparatively small area by local terrain and weather.

  • Differ from those appropriate to the general pressure pattern.

  • Found at the lower levels of the troposphere.

  • Induced by small-scale differences in air temperature and pressure.

  • Examples: Up valley/upslope winds, downslope/down valley winds.

  • Influenced by the terrain.

  • May be the only wind impacting a fire or a combination of general and local winds.

Downslope and Down Valley Winds

  • Downslope winds: 2-5 miles per hour.

  • Down valley winds: 5-10 miles per hour.

  • Small-scale convective winds due to local heating and cooling of natural inclines.

  • Result of slopes or valleys cooling at night; cool air sinks.

Land and Sea Breezes

  • Common in coastal regions.

  • Firefighters should anticipate wind shifts near large bodies of water.

Land Breeze

  • During evening and overnight hours, the land mass becomes cooler than the air over water.

  • Cooler air settles.

  • Air moves from land towards the water.

Sea Breeze

  • During late morning and early afternoon, the land mass becomes warmer than the air over water.

  • Air over the land becomes unstable and rises, causing lower pressure.

  • Rising air is replaced by air over the water.

Mid-Flame Wind

Beaufort Scale

  • Tool to estimate 20-foot wind speeds using visual cues such as tree movement.

Mid-Flame Description

  • Wind that acts directly on the flaming fire front at a level of half the flame height.

  • The eye-level wind is an excellent approximation.

Wind Effects on Fire Behavior

  • Carries away moisture-laden air, hastening the drying of fuels.

  • Aids combustion by increasing the supply of oxygen.

  • Increases fire spread by carrying heat and burning embers.

  • Bends flames closer to unburned fuels, preheating them.

  • Changes in wind direction and speed can rapidly change fire behavior.

  • The direction of fire spread and smoke transport are determined mostly by the wind direction.

Clouds

  • Clouds are a visible collection of moisture suspended in the atmosphere.

  • They form under stable or unstable atmospheric conditions.

  • Not all clouds produce precipitation.

Cloud Impacts on Fire Environment

  • Clouds can modify the fire environment by changing:

    • Temperature

    • Relative humidity

    • Atmospheric stability

    • Wind

    • Fuel temperature

    • Fuel moisture

    • Fire behavior

  • They reflect incoming solar radiation, creating cooler temperatures, increasing humidity, and changing wind and stability.

  • Changes in fire weather conditions directly impact fuel moisture and decrease fire behavior.

  • Once clouds move away, fire behavior can increase.

Cloud Classification

  • Clouds are classified by height:

    • High clouds: 6,000 to 50,000 feet, usually pure white, made of ice crystals. (Cirrus, Cirrostratus, Cirrocumulus).

      • Cirrostratus: Cirrus bands transparent relatively near the horizon, typically indicates whether fronts are two to three days away. Cirrus bands covering majority of the sky with sun obscured typically indicates whether fronts are one day or less away

    • Middle clouds: 6,500 to 23,000 feet, a combination of white and gray, made of water droplets and ice crystals. (Altostratus, Altocumulus).

      • Altocumulus castellanus or floccus: Flat base with the turrets or cumulus like tufts on the top. When observed in morning hours, often indicates afternoon, evening thunderstorms.

    • Low clouds: 0 to 6,500 feet, usually gray, made of water droplets. (Stratus, Stratocumulus, Nimbostratus).

    • Vertically Developed Clouds: Bases range 1,000 - 10,000, with tops from 5,000-80,000. Cumulus and Cumulonimbus
      *Cumulonimbus: Tops are not fibrous. No anvils developed yet, significant vertical motion present. Dusty downdrafts likely, thunderstorms are possible, and erratic fire activity is definitely present, around those downdrafts.
      *Stratus: Smooth in appearance, continuous low layer of gray often inhibits aircraft use and minimal fire activity

  • Cumulonimbus: Strong anvil shape, strong downdraft winds, lightning, heavy rain, and hail possible underneath these clouds. Distant terrain channeled winds and lightning possible. Around these mature thunderstorms, very erratic winds can be the cause of some significant fire activity and was one of the leading contributors to the Yarnell Hill fire

Special Cases

  • Pyrocumulus clouds: Unstable conditions where smoke moisture condenses to form cumulus or cumulonimbus clouds.

    • Downdrafts, rain, and lightning are possible.

  • Fire whirls: Localized intense wind swirls, like a tornado.

    • Can scatter fire across lines and enter safety zones.

    • Unstable conditions, speeds exceeding 50 mph.

  • Virga: Rain that doesn't touch the ground, evaporates before hitting.

    • Creates downdrafts and cooling, which can cause erratic fire behavior.

Vertically Developed Clouds

  • Cumulus and cumulonimbus clouds are made of water and ice and indicate unstable atmospheric conditions.

  • An unstable atmosphere can result in an increase in fire behavior.

  • Identifying this type of cloud is important when predicting fire weather.

Cloud Formation

  • The sun heats the ground, which in turn heats the air near it.

  • Warm air rises, cools, and condenses, forming clouds.

  • The dew point is the temperature at which air becomes saturated and clouds form.

  • Storms often form over mountains first due to elevated heat sources.

Altocumulus Castellatus

  • Small clusters of puffy clouds.

  • Sign of an unstable day.

Virga

  • Rain falls from a cloud but doesn't reach the ground.

  • Indicates very gusty winds.

  • Cooled air rushes down to the ground in all directions.

Cold Fronts

  • In winter, clouds and weather come toward you.

  • Rapid shift of wind direction.

  • In the summer, it can be so dry that there is no indication of a cold front coming through.

Importance of Observation

  • Observe the sky every day to learn more about weather patterns.

  • Get the latest information and constantly look at the sky.

Objectives Review

  • Describe atmospheric stability and discuss the effects on fire behavior.

  • Describe wind and its effect on fire behavior.

  • Explain cloud classifications.

  • Explain the similarities between smoke layers and clouds.