Module 2: Fire Weather Flashcards
What is the atmosphere?
a thin envelope of invisible gases, which surrounds the earth
What is the composition of the atmosphere?
21% oxygen 78% nitrogen 1% other gases
What else does the atmosphere contain? (3 things)
- suspended solids 2. liquid particles 3. water vapour
______________ are important as condensation nuclei.
suspended solids
Water vapour concentrations vary throughout the atmosphere from nearly __% over desert and arctic areas to as much as ___% by volume over tropical latitudes
0 & 4
________ is an important atmospheric gas as it can change state under certain conditions of temperature & pressure
water vapour
What happens when water vapour is cooled?
it condenses into water droplets forming clouds, fog, dew or precipitation
This process releases heat and its energy contribution is important in weather building processes such as storm clouds & winds leading to atmospheric instability. What is this called?
latent heat of condesation
The atmosphere is divided into a number of significant layers on the basis of temperature and density. T or F
True
What is the bottom layer, adjacent to the earth’s surface called and what happens here?
troposphere is heated from below and temp generally decreases with height *mostly interested in the troposphere as this is where surface weather & wildfire interact*
The upper limit of the troposphere layer is defined by ____ which is an ______ of -56C
tropopause & isotherm
Approximately __% of the total weight of the atmosphere occurs below __km; most weather occurs here
50% & 5.5km
What are the general rules in the troposphere? (2 things)
- both temperature and pressure decrease with increasing elevation 2. subsiding air warms as it descends. as it warms it will expand & take up more space. this air is subject to increasing pressure as it descends & will be compressed against the earth’s surface. this heating, expansions & compression may result in strong surface winds- much like Chinooks
What has a direct relationship with temperature?
relative humidity
As temperature increases: RH _______ As temperature_______ : RH increases
decreases
What is relative humidity?
the ratio between the amount of water vapour present in a given parcel of air compared to the maximum amount of water vapour that parcel can hold at a given temperature & pressure.
What is the most important element of fire weather?
Relative Humidity
Describe the diurnal relationship: Temp, RH & moisture content of cured fine fuels.
- RH < 20% results in aggressive burning conditions
- Temp (r) : RH (d), RH goes down and the moisture content of cured fine fuels also decreases
- peak buring time is between 2 pm & 4 pm
- as RH (d) fire behaviour (i)
- watch for temp : RH crossover
What is dew point?
the temperature to which air must be cooled in order to reach saturation (RH=100%)
what is atmospheric pressure?
the force per unit area exerted by the weight of the atmosphere at a specific level
the most common measure of atmospheric pressure is?
millibars
what is an isobar and what intervals are they drawn at on surface weather maps?
a line of equal surface pressure shown in increments of 4 hecto- Pascal
what do isobars on a surface weather map indicate?
relative high and low pressure areas
high and low pressure areas can be distinguished on the map by?
the pressure values contained within them
what exists between high and low pressure areas as the atmosphere works to balance diffences in pressure?
gradient force
___ connect like valleys btween low-pressure areas while a ___ of high pressure occurs where the isobars connect or come close to connecting between high-pressure centrers
troughs
ridge
a low pressure area is often referred to as?
What are some examples of cyclones?
as air ascends in a low pressure area, what happens?
if sufficient moisture is present, what occurs?
a cyclone or depression
hurricanes, typhoons & tornados
it is cooled
clouds and precipitation
what are low pressure areas characterized by?
cyclone
vary in size
counter-clockwise rotation
ascending air increases in velocity
divergent aloft & convergent at surface
a high pressure system is referred to as?
as air descends, what happens?
anti-cyclone
it is warmed & capable of holding more water vapour resulting in fewer clouds and fair weather
what are high pressure areas characterized by?
anti-cyclone
generally much larger than L pressure areas
clockwise rotation
descending air slows in velocity with surface friction
convergent aloft & divergent at surface
what is a jet stream?
a river of strong upper atmospheric wind (tropopause) situated between the cold polar easterlies & the mild westerlies
how fast can jet streams blow at and what do they do to high and low pressure systems?
100 to 200 kph
steer direction from NW to SE across Canada
pressure patterns at the surface are _________ and occure on ________scales
asymmetrical
very large
Frontal Passage
Extremely important to understanding fire behaviour.
As cold front passed over a fire the winds become very gusty and erratic.
Caused by the dneser cold air undercutting the warmer air causing lift and turbulence.
a parcel of air is considered to be stable if?
unstable if?
air is resistanct to vertical motion
air is not resistanct to vertical motion
indicators of stable atmosphere?
stratiform clouds
limited vertical motion of air
cooler temps
light winds
low ground fog
reduced visibility
Frontal Passage Effect on Fire Behaviour (6)
- Strong gusty winds, atmosphere instability
- Variable Wind Direction
- Strong dy cold front worst scenerio
- Increase in fir activity
- Rapid cold front may lead to thunder storms
- Cold front passage most common in fatalities
what is lapse rate?
the rate of temperature change with an increase in elevation
what is adiabatic lapse rate?
the lapse rate of rising or sinking air
Air Mass descriptors
and
Temperature Characteristics
Dry continental - (c)
Moist Maritime - (m)
Arctic (cold) - (A)
Polar (temperate) - (P)
Tropical (warm) - (T)
what is moist adiabtaic lapse rate (MALR)?
shows a considerable variation because the amount of heating is dependant upon how much water vapour condeses out
more condensations means more heat released
1.5C/ 1000 ft
Common air mass descriptors
cA - only forms over frozen arctic, does not exist in summer slides south bringing cold temps
mA - this is a cA air mass that moves over open oceans
cP - forms over dry central North America
mP - a cA air mass moved over a warmer ocean
cT - forms over arid south-western US regions, summer time only and is rare
mT - forms over tropical ocean latitudes near equator and moves north
is a type of lift, which results from differential heating of the earth’s surface. Air, which has been heated, becomes less dense than the surrounding air and rises.
Convection
lifting occurs when air is lifted up and over a prominent topographic feature such as a range of mountains.
orographic
lifing of the air may be caused by turbulence as airflows over a mountain or hill causing eddies on the lee side which may disrupt the flow of air near the surface
mechanical lifing
lifing occurs when warmer air is forced upward by advaning cooler air
frontal
where air at the surface is forced into a narrow pass, excess air is forced to rise
convergence lifing
is a function of pressure difference in the horizontal from high surface pressure areas to low surface pressure areas
wind speed
initiates the movement of air from areaas of high pressure to areas of low pressure
gradient force
affects the direction of wind by causing it to flow parallel with the isobars at heights that are above the influence of surface friction
coriolis force
upper atmospheric winds occure a few kilometres above the surface and flow parallel to the isobars
geostrophic winds
the force of friction partially counteracts the coriolis force which causes the air to slow and to flow out of high pressure areas and into low pressure area, crossing the isobars at an angle
surface winds
winds are always described in terms of
the direction the are coming from
only lasts a few seconds
wind gust
often associated with erratic shifts in wind direction, which may create difficulties in fire control
gustiness
a gust may be caused by
(2 things)
mechanical turbulence, convergence or eddying effects
fairly intense unequal heating of the earth’s surface
what is a squall
a sudden increase in wind speed which is sustained for several minutes
what is a squall caused by
(2 things)
sustained down drafts from thunderstorms
rapidly moving cold front
