How might fires move?
- Smoldering
- Creeping
- Running
- Torching
- Spotting
- Crowning
What’s Fire Behaviour?
The manner in which fuels ignite, flames develop and fires spread as determined by fuels weather and topography
What might we want to know about the condition of fuels ?
- Are fuels live or dead?
- If living are they healthy?
- If dead are they decayed?
- Are fuels green and lush?
- Dry and brittle?
- Mixture?
What factors determine fuels susceptibility to fire?
- Conifers, deciduous or herbaceous
- Annual or perrenial
- Fire prone or resistant
- Homog or heterog. (all the same size and distribution or different?)
- Fuel size (diameter)
- Arrangement (Does fuel create a ladder?)
- fuel continuity (thickness, spatial arrangement)
What effect do slopes have on fire behavior?
- Fire spreads more rapidly upslope than on level ground, the steeper the more rapid.
- For every 10% slope increase there is a doubling of the ISI
- Slope causes exposed fuel to be closer to flames
- Slope can be affected by local winds
- Reduces amount of available surface moisture due to runoff
How does aspect affect fire behaviour?
What direction is more likely to be at high risk?
- Aspect determines the degree that the sun will effect the land surface. Sun increases temperature and drying effect on fuels.
- South and west receive more solar radiation and are generally at a higher risk
- SW will have greater growth rate and volume of fuel available.
-Effects the time of day that the slope is more fire prone ex (NE morning, SW afternoon)
How do gullies and chimneys affect fire behaviour ?
- Gullies cause wind to funnel and concentrate.
- Gullies usually contain high concentrations of fuel due to constant water supply that promotes vegetation.
Difference between a gully and a chimney?
- Gullies are well defined and related to watercourses
- Chimneys are less obvious, often bowlshaped
What is orographic lift?
-When hills and mountains have an effect on surface winds. Air is forced to rise.
How does topography affect precip?
Topography causes orographic precip. ( when prevailing winds are forced to rise over mountains and release accumulated water on the windward side. The lea side is subject to “rain shadow” and will be drier. )
Why is the east coast of the island drier?
It lies in the rain shadow of the insular mountains.
Which direction does air flow on a slope during the day? Night?
During the day we get upslope winds, During the evening we get downslope winds.
What are some continental topography features that affect Fire Behaviour in North America
- Has huge mountain ranges
- Surrounded by ocean
- Prevailing winds from the west
- Goes from near the equator to the north pole.
What are wind eddies? What kind of movement do they produce in fires?
When air flows over ridge tops and creates turbulence on the leeward side.
-Commonly associated with spotting, when the wind picks up sparks and embers from the windward side and drop them on to the leeward side.
What are some continental effects (large land mass effects) on fire behaviour?
-South
-Major influence of cooling and warming
-Very affected by tropical patterns
- North
mountain ranges (rainforest and lush green veg. on windward side, Dry grasslands forest and desert on leeward side)
How does air pressure effect fire behaviour?
High pressure systems push down on the earth, referred to as highs (stable weather in summer, nasty cold weather in winter)
Lows form depressions of low pressure.
Wind rushes from low to high constantly trying to account for imbalance in pressure. Wind action between pressure gradients can significantly impact surface winds. The greater the difference, the higher the wind velocity.
Low pressure areas are associated with lightning.
What are inflow winds?
- Wind patterns in western North America that predominate
- Weather and moisture originates from the pacific ocean
Briefly describe the adiabatic lapse rate
- As air rises it cools and as it falls it warms.
- Warming is caused by compression
- Rising air cools at 2degrees C per 300m of ascent
- Falling air warms at 3 degrees C per 300m of descent
What are outflow winds?
How are they problematic?
Winds coming from the interior and flow towards the coast. Because outflowing winds are very dry and descend further than they ascend, increasing temperature and decreasing relative humidity. Can result in very dry weather.
What is the 30/30 crossover?
Rule of thumb: when RH drops below 30% and temp exceeds 30 degrees C.
- Expect risk to be serious
- Referred to as Crossover of “The Black Cross”
How do inlets effect air?
Act like chimneys or gullies in that they funnel air and increase velocity, thus increasing ISI
