Should A Fire Weather Warning Worry You?
Fire weather won't concern most city dwellers, but understanding it could be critical to people living in a rural environment.
Every year in the USA there are tens of thousands of wild fires, mostly small and easy to control. In bad years thenumber may climb to around a quarter of a million, and the cost of fighting them runs well over the billion dollar mark.
Better communications, emergency services and fire fightingtechniques have resulted in reductions in deaths from thehundreds who died in major fires in the 19th and early 20th centuries, but damage to housing, forests, and agricultural and grazing land remains high.
Three major factors interact to determine the extent and severity of a wildfire - weather, terrain, and availability of fuel. This interaction makes fire fighting and fire prevention a very complex challenge, and even the effectsof weather can be quite diverse.
Setting The Scene
Fire weather is the combination of temperature, humidity,wind strength and chance of thunderstorms that meansthat the chance of fires is high, or that control ofany existing or new fires will be difficult and livesand property may be in danger. When weatherconditions are threatening, fire weather warnings areissued to areas at risk, and can be monitored on
But the conditions that favour fires may have resulted from
weather patterns months or even years in the past.
For example, good winter or spring rains will encourage
strong grass growth. A hot summer will dry off the grass,
leaving a thick accumulation of fuel available for any fire
that starts. Similarly heavy winter rains in southern
California will encourage rapid growth of highly flammable
scrub, ready and waiting for the hot dry summer.
Strong winds occurring in
thunderstorms can also knock down acres of trees, which
die and dry out to form a thick layer of fuel guaranteed
to cause problems during the next fire.
Not so far in the past are the weather events of the preceding
days or weeks. A stationary ridge of high pressure in
summer can result in days of hot fine weather, often with
gradually increasing temperatures each succeeding day. The
continued heat dries out the vegetation, increasing its
fuel value for any subsequent fire.
Even worse is the longer lasting effect of
particularly on forests, as both the living trees and
debris on the ground lose much of their moisture.
Trees flattened by downburst
Courtesy NOAA Photo Library
Fire Weather Days
There are many combinations of weather conditions that
combine together to create a strong chance of fire weather -
the combination of high temperatures, low humidity and
But there are two considerations here - risk of fires
starting, and weather that aids the spread of fires and
hinders their control.
In accessible or settled areas most wildfires are started
by humans. Most are accidental, but many of the deliberately
lit fires occur, not by chance, on high risk fire weather
days. So in some ways the warnings can serve as wake up
calls to the arsonists.
But in remote areas the chief cause of fire is lightning,
and the greatest number of lightning-caused fires occurs
on days where there is enough instability to cause
continued convection and thunderstorm
development, but not enough moisture in the air to
result in significant rainfall. Dry thunderstorms form,
cloud to ground lightning can ignite fires, but there is
not enough rain in the storm to put them out or hinder
And even dry storms are accompanied by strong winds, so the
chances are high for a lightning started fire to take a
firm hold with the help of gusty winds.
Storms can also occur in advance of cold fronts, so a few
hours of lightning caused fires can be followed by wind
changes, often strong, as the front passes through. These
conditions can increase the severity of the fire, as well as
increasing the dangers for fire fighters.
So overall fire weather is characterized by heat and
strong winds. Short lived strong winds accompany
thunderstorms and cold fronts, but the most dangerous
elements of fire weather are winds which may continue for
days, bringing heat with them but no relieving rain.
The main culprits go by the general name of foehn
winds. They occur on the lee side of high mountain ranges
when a weather system passes through after dumping most
of its moisture as rain or snow on the windward side of
As the winds cross the mountains they begin to descend,
and increasing air pressure causes them to warm up.
Once they hit the plains or basins they have accumulated
significant heat, and are moving at 30 - 50mph (50-80
kph), with stronger gusts particularly when they
channel through valleys or passes.
In North America these winds are known as Chinooks on
the eastern side of the Rockies, as East Winds in
Washington State, Oregon and northern California, and as
the Santa Ana in southern California.
So we have the ingredients of ideal fire weather- hot dry
air with strong winds, and even stronger gusts. In some
cases the overall conditions may also be right for
thunderstorms to form, adding a little lightning to the mix.
However experience would suggest that there is sufficient
human carelessness and malice to provide more than enough
artificially started fires.
Other factors which may complicate the picture include
low level jets. Related to the jet stream flows, these
winds occasionally occur around the tops of ranges.
As turbulence increases during the day, these winds can
descend to ground level, bringing winds with speeds around
30mph (50kph). Not only can these add to the problems of
fire fighters by increasing wind velocity, but they can
also come from a different direction, turning the fire
to a new path.
Forest Fire, Saganaga, Minnesota, 1995
Source; NOAA Photo Library
Fire Generated Weather
As normally used, fire weather is a term for weather that
supports the start or growth of fires. Another form of
fire weather is that created by the fire itself.
Flames heat the air, which rises, carrying with it soot,
water vapour, and the other products of combustion.
This rapidly rising air is replaced by air flowing in
horizontally, bringing with it oxygen to assist the fire.
It also makes the fire harder to control, and may combine
with existing winds to help carry sparks and embers in
front of the fire, starting spot fires.
The turbulence caused by this combination of incoming and
rising air can also generate firewhirls or fire whirlwinds
in larger fires. Fire fighters really do have a lot
to think about.
Convection created by the fire can generate its own private
thunderstorm, as water vapour released by burning vegetation
condenses above the fire. Lightning and rain can result,
although the rain is rarely enough to have much effect on
the fire. The name for the clouds formed in this way is
Photo courtesy of NOAA
Large fires in rugged or remote country are extremely difficult
to control, let alone extinguish. There are really only two
ways to put out a fire - deprive it of fuel by creating
barriers in its path, or wait for a change in the weather
to turn the balance in favour of the fire fighters. Heavy
rain is of course the ideal.
But not all the effect of the fire is confined to its
immediate vicinity. The plume of smoke and gases is picked up
by the prevailing winds at height and transported down wind.
Many of the products of a fire can be dangerous, toxic, or
health hazards - they would never be allowed from a
commercial operation. They include very fine soot and carbon
monoxide, which can cause problems to people with
Consequently another function of the National Weather Service
is to track the distribution of the plume. Health warnings
are issued when necessary.
Fire Weather and Weather Stations
Fire weather is rather obvious when it arrives, and is easily
recognised from the temperature, humidity and wind sensors.
It is often worth looking back to the beginning of the conditions
which led to fire weather warnings being issued, and maybe
even further back to the weather events which may have
provided favorable conditions for fuel accumulation.
Alternatively, as often happens, fire follows or accompanies
drought, which may have a much longer period of development.
But it is also interesting to investigate the cause of any
particular episode of fire weather. Quite often, more so
in some areas than others, it is related to a change in
medium to large scale pressure systems, both at ground
level and at higher elevations.
A common happening is the break down of an upper atmosphere
high pressure ridge. The ridge is likely to have been the
underlying cause of high temperatures and dry air which
dried out the fuel prior to the fire. The ridge can decay
quite rapidly under the attack of a surface trough, which
often brings with it instability, convection, thunderstorms
and gusty winds, all favourable for starting and growing
Clues to these features include pressure charts for the
upper atmosphere (500mb charts), satellite images showing
water vapour, and of course surface charts and radar.
For more information I can thoroughly recommend a multimedia
wildfires and fire weather
the University Corporation for Atmospheric Research.
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Last update 05/24/2011