Fire Weather: Temperature & Moisture
Dec 22, 2015
Fire Weather:
Temperature & Moisture
Weather and the Earth’s Heat Balance
• Weather = motion in the atmosphere due to unequal heating• Over time, the amount of energy lost and received by the
atmosphere must be in balance• But, the atmosphere is not in balance• Attempt to regain balance disturbance weather
Components of Weather
• Temperature & Moisture
• Atmospheric Stability
• Winds
• Masses & Fronts
• Clouds & T-Storms
DAY NIGHT
SURFACE HEAT EXCHANGE
• Solar radiation: heats air directly 0.5-1 F/day (depending on water vapor)
• Conduction: Air heated (or cooled) through contact with Earth’s surface
• Convection: warm air rises, cooler air falls (mixing)• Cooling at night: Radiation, conduction, convection
Rising air expands and cools.Sinking air is compressed & warmed
AirRising
AirFalling
Factors affecting the Earth’s surface temperature
• Season – angle of sun, day length
• Topography – slope, aspect
• Shading/scattering – clouds, smoke, haze, vegetation
• Absorption – clouds, water vapor
• Surface properties
Effect of vegetation on air temperatures (moderating influence)
• Intercepts radiation• Green foliage
– greater reflection
• Type of vegetation – shrub, tree, grass
• Density (canopy cover)
* Temperatures in forest = 5-8 F cooler than open areas (warmer during the night)
Openings = warm pockets (chimney effect)
Effect of vegetation on air temperatures…
Effects of surface properties on temperature
• Absorption and reflection: dark vs. light– Dark materials absorb more radiation– Light materials reflect radiation
• Transparent vs. opaque materials– Transparent materials allow radiation
penetrate deeper layers (water)– Opaque materials concentrate radiation at the
surface (soil, litter)
Surface properties continued…• Conductivity
– Good conductors – transmit heat rapidly to the interior wood/litter metal air water soil 4 1 5 2 3
– Dry vs. wet soils?– Compact soil vs. porous duff?
• Poor conductors = hotter during the day and cooler at night (example – dry wood with frost)
• Good conductors = moderating influence, less temperature change (example – lake water)
Surface properties continued…
• Heat capacity - definition– The capacity of a material to contain, hold, or
absorb heat– The amount of heat required to raise the
temperature– Water VS. soil, rocks, wood, litter
• Water has higher HC (and good conductor, high transparency)
• Water requires large amounts of heat to raise its temperature; but loses and gains heat slowly
• Soil, rocks, wood, litter: Low HC
How does heating and cooling differ between these materials?
Vegetation – high reflectanceDark surfaces – high absorption
Water – high conductance high heat capacityhigh transparency (heat transferred
to deep er layers)
Dark dry soil – low conductance, (opaque) high absorption,
low heat capacity opaque (concentrates heat
near surface)
Air – low conductivityMetal – high conductivity
Moisture & Weather
• Moisture in the atmosphere is continually changing its physical state:
Solid Liquid Gas
(ice) (water) (vapor)
Heat Energy Required (Melting, Evaporation) = cooling
Heat Energy Released (Condensation, Freezing) = warming
Atmospheric Humidity• Absolute humidity:
– The amount of water vapor in a given volume of air
• Relative humidity: – The ratio of the amount of water in a volume of air to the total
amount of which that volume can hold at a given temperature and atmospheric pressure.
= actual amount of water x 100 total amount the air can hold
* RH VARIES WITH TEMPERATURE!!! *** Warm air can hold more water
Homework - Select a recently or currently burning fire…
• Name & location• How did it start?• Fuel type?• Topography?• Fire behavior?• Weather conditions (RH/TEMP – see outcome sheet)• …Find the hourly temperature, dew point, and relative
humidity for 6:00 am – midnight on September 2nd, 2008 for that site.
Relationship between temperature and relative humidity
Saturation and Dew Point
• Saturation – when the air contains all the water vapor that it can
hold
• Dew point – The temperature to which air must be cooled to reach
its saturation point – Further cooling will cause condensation to occur
(clouds, fog, dew)– For an air mass with a given amount of water
content, dew point remains the same even if temperature changes
Psychrometer• Instrument used to measure atmospheric
moisture – 2 thermometers:– Dry bulb – measures air temperature– Web bulb – measures temperature of
evaporating water from a muslin wicking clot• Use tables to determine:
– Dew-point temperature– RH
Dew point = Top number (red)RH = Bottom number (black)
Psychrometer: Examples
• Example 1:– Dry Bulb = 80 F; Wet bulb = 70– DP = ?; RH = ?– DP = 65; RH = 61%
• Example 2:– AM: temperature = 70 F & RH = 60%– PM: temperature = 90 F & RH = ?– DP=55, RH = 30-31%
DAY
How is heat exchanged between the Earth’s surface and the atmosphere?
How do the physical properties of
air change when it rises or sinks?
NIGHT
How do these features affect the Earth’s surface temperature?
DAY
How does heating and cooling differ between these materials?
Conductivity of surface properties
• List the following from good (1) to poor (5) conductors:
wood/litter metal air water soil
• Which is the better conductor and why?– Dry vs. wet soils?– Compact soil vs. porous duff?
• How will good and poor conductors differ in their temperature between night and day?
How can vegetation affect air temperatures?