Transcript Weather and the Earth’s Heat Balance

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
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Temperature & Moisture
Atmospheric Stability
Winds
Masses & Fronts
Clouds & T-Storms
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
DAY
NIGHT
Rising air expands and cools.
Sinking air is compressed & warmed
Air
Rising
Air
Falling
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)
Effect of vegetation on air temperatures…
Openings = warm pockets (chimney effect)
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
4
metal
1
air
5
water
2
soil
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 reflectance
Dark surfaces – high absorption
Water – high conductance
high heat capacity
high 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 conductivity
Metal – high conductivity
Moisture & Weather
• Moisture in the atmosphere is continually
changing its physical state:
Solid
(ice)
Liquid
(water)
Gas
(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…
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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%
How is heat exchanged between the Earth’s surface and the atmosphere?
DAY
DAY
How do the physical properties of
air change when it rises or sinks?
NIGHT
How do these features affect the Earth’s
surface temperature?
How does heating and cooling
differ between these materials?
How can vegetation affect air temperatures?
Conductivity of surface properties
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List the following from good (1) to poor (5) conductors:
wood/litter
metal
air
water
soil
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Which is the better conductor and why?
– Dry vs. wet soils?
– Compact soil vs. porous duff?
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How will good and poor conductors differ in their
temperature between night and day?