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Fires > Module 2: Fire Weather > Flashcards

Module 2: Fire Weather Flashcards

(58 cards)

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1
Q

What is the atmosphere?

A

a thin envelope of invisible gases, which surrounds the earth

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2
Q

What is the composition of the atmosphere?

A

21% oxygen 78% nitrogen 1% other gases

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3
Q

What else does the atmosphere contain? (3 things)

A
  1. suspended solids 2. liquid particles 3. water vapour
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4
Q

______________ are important as condensation nuclei.

A

suspended solids

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5
Q

Water vapour concentrations vary throughout the atmosphere from nearly __% over desert and arctic areas to as much as ___% by volume over tropical latitudes

A

0 & 4

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6
Q

________ is an important atmospheric gas as it can change state under certain conditions of temperature & pressure

A

water vapour

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7
Q

What happens when water vapour is cooled?

A

it condenses into water droplets forming clouds, fog, dew or precipitation

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8
Q

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?

A

latent heat of condesation

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9
Q

The atmosphere is divided into a number of significant layers on the basis of temperature and density. T or F

A

True

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10
Q

What is the bottom layer, adjacent to the earth’s surface called and what happens here?

A

troposphere is heated from below and temp generally decreases with height *mostly interested in the troposphere as this is where surface weather & wildfire interact*

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11
Q

The upper limit of the troposphere layer is defined by ____ which is an ______ of -56C

A

tropopause & isotherm

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12
Q

Approximately __% of the total weight of the atmosphere occurs below __km; most weather occurs here

A

50% & 5.5km

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13
Q

What are the general rules in the troposphere? (2 things)

A
  1. 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
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14
Q

What has a direct relationship with temperature?

A

relative humidity

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15
Q

As temperature increases: RH _______ As temperature_______ : RH increases

A

decreases

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16
Q

What is relative humidity?

A

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.

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17
Q

What is the most important element of fire weather?

A

Relative Humidity

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18
Q

Describe the diurnal relationship: Temp, RH & moisture content of cured fine fuels.

A
  • 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
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19
Q

What is dew point?

A

the temperature to which air must be cooled in order to reach saturation (RH=100%)

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20
Q

what is atmospheric pressure?

A

the force per unit area exerted by the weight of the atmosphere at a specific level

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21
Q

the most common measure of atmospheric pressure is?

A

millibars

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22
Q

what is an isobar and what intervals are they drawn at on surface weather maps?

A

a line of equal surface pressure shown in increments of 4 hecto- Pascal

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23
Q

what do isobars on a surface weather map indicate?

A

relative high and low pressure areas

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24
Q

high and low pressure areas can be distinguished on the map by?

Study These Flashcards
A

the pressure values contained within them

25
what exists between high and low pressure areas as the atmosphere works to balance diffences in pressure?
gradient force
26
\_\_\_ 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
27
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
28
what are low pressure areas characterized by?
cyclone vary in size counter-clockwise rotation ascending air increases in velocity divergent aloft & convergent at surface
29
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
30
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
31
what is a jet stream?
a river of strong upper atmospheric wind (tropopause) situated between the cold polar easterlies & the mild westerlies
32
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
33
pressure patterns at the surface are _________ and occure on \_\_\_\_\_\_\_\_scales
asymmetrical very large
34
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.
35
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
36
indicators of stable atmosphere?
stratiform clouds limited vertical motion of air cooler temps light winds low ground fog reduced visibility
37
Frontal Passage Effect on Fire Behaviour (6)
1. Strong gusty winds, atmosphere instability 2. Variable Wind Direction 3. Strong dy cold front worst scenerio 4. Increase in fir activity 5. Rapid cold front may lead to thunder storms 6. Cold front passage most common in fatalities
38
what is lapse rate?
the rate of temperature change with an increase in elevation
39
what is adiabatic lapse rate?
the lapse rate of rising or sinking air
40
Air Mass descriptors and Temperature Characteristics
Dry continental - (c) Moist Maritime - (m) Arctic (cold) - (A) Polar (temperate) - (P) Tropical (warm) - (T)
41
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
42
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
43
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
44
lifting occurs when air is lifted up and over a prominent topographic feature such as a range of mountains.
orographic
45
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
46
lifing occurs when warmer air is forced upward by advaning cooler air
frontal
47
where air at the surface is forced into a narrow pass, excess air is forced to rise
convergence lifing
48
is a function of pressure difference in the horizontal from high surface pressure areas to low surface pressure areas
wind speed
49
initiates the movement of air from areaas of high pressure to areas of low pressure
gradient force
50
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
51
upper atmospheric winds occure a few kilometres above the surface and flow parallel to the isobars
geostrophic winds
52
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
53
winds are always described in terms of
the direction the are coming from
54
only lasts a few seconds
wind gust
55
often associated with erratic shifts in wind direction, which may create difficulties in fire control
gustiness
56
a gust may be caused by | (2 things)
mechanical turbulence, convergence or eddying effects fairly intense unequal heating of the earth's surface
57
what is a squall
a sudden increase in wind speed which is sustained for several minutes
58
what is a squall caused by | (2 things)
sustained down drafts from thunderstorms rapidly moving cold front

Fires (8 decks)

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