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?