|
Photo courtesy of Western Fire, Inc.
Folklore tells
how Indian ancestors used fire to rejuvenate the land, create new
vegetation as a food source for game, and control underbrush. However,
in modern times communities are springing up where suburbia meets
the undeveloped countryside and wildfires are an ever-present danger
in the Urban/Rural Interface.
Wet winter
means more fire fuel
The western
United States receives most of its precipitation during the winter
months, with summers being relatively hot and dry. Vegetation receives
ample moisture for spring growth, but by the time summer arrives,
the vegetation has dried and becomes fuel for wildfires. Generally
speaking, the wetter the winter, the more fuel there will be for
the fire season.
The summer monsoons
reach the Desert Southwest and Intermountain region when massive
high-pressure systems stagnate over the Midwest. The general clockwise
flow brings warm moist air from the Gulf of Mexico across the central
Mexican plateau until it meets the Sierra Madre Mountains and the
Intermountain Region of the American Southwest. This warm unstable
air can extend as far north as the Canadian border and as far west
as the Pacific Coast. Moist air is naturally more buoyant than dry
air, thus relatively unstable.
When coupled
with local mountain topography (called orographic lift) and diurnal
heating, this monsoonal air mass triggers thunderstorms. Heat lightning
results when thunderstorms develop but produce little or no rain.
Virga (rain that falls but evaporates before it reaches the ground)
leaves the land parched. And lightning sparks fires in these tinder-dry
areas.
Wind is a
major concern
Wind is caused
by differential heating, which causes differential pressure within
an air mass. For example, mountains are cooler than valleys because
of the elevation. The cooler, denser air sinks and replaces warmer,
less dense air on the valley floor, creating wind.
Wind is normally
funneled through valley passes, which intensifies its effects. During
wildland fires, wind fans the flames and can cause a fire to spread
out of control.
Fire, itself,
is one of the few natural phenomena that can create its own microclimate.
Large fires heat massive amounts of air causing it to rise into
the atmosphere; this is known as convection. Cooler surface air
from outside the fire zone creates in-drafts to replace the rising
air. This causes violent winds, which fan the fire.
Coupled with
low humidity and rough terrain, these extremes seem to take on a
life of their own and create devastating firestorms. Firestorms
can generate fire whirls, which are fire vortexes or small tornadoes,
reaching as much as 500 feet in diameter and extending hundreds
of feet in the air.
Katabatic
winds
Another wind
phenomenon that wreaks havoc with firefighters is the infamous Santa
Ana winds. The Santa Anas are actually katabatic winds, normally
created during the fall and winter months by colder, denser air
pooling and piling up over the higher desert elevations of southern
California. Katabatic winds in other parts of the world are known
as Chinook or Foehn winds.
When pressure
spills through mountain passes, it creates powerful winds funneled
and warmed through atmospheric compression. The atmosphere dries
as it descends, removing any available moisture from the environment.
These conditions are further exacerbated by strong northerly winds
in the upper atmosphere passing perpendicular across the local mountain
ranges causing a natural "chimney effect" to occur. Add fire to
the mix and it's a natural recipe for disaster.
Santa Ana conditions
can last for several days to a week and can blow in excess of 80
mph over the entire region.
Predicting
fire behavior
Fire meteorologists
at the National Interagency Fire Center in Boise, Idaho have the
daunting task of providing fire weather forecasts with sufficient
detail to help predict fire behavior. In most wildland locations,
real-time weather data is sketchy, so meteorologists don't have
the necessary tools to base their predictions on. The National Weather
Service and NOAA work closely with meteorologists to help predict
atmospheric conditions, but this is a monumental task.
Private companies,
such as Columbia Weather Systems, are now
rigging wildland fire trucks with automated weather systems to put
on the fire lines. Data is then sent back to the Incident Command
meteorologists in the field. This information is essential in helping
predict fire behavior in real-time and in strategic locations where
the fire is actually being fought.
In addition,
fire hazard monitoring at ranger stations can be enhanced with WeatherMaster
weather software which incorporates calculated fuel moisture
readings. The calculations are based on environmental parameters
and replace the traditional and cumbersome fuel sticks.
In the realm
of wildland firefighting, knowing the weather conditions helps define
the rules of engagement. Each year millions of acres of timber and
grasslands are burned and the property damage ranges in the billions.
More importantly, lives are lost. Giant strides have been made to
effectively manage our forests and minimize the fire fuels. Our
best efforts will not eliminate wildland fires, but it will help
protect the heritage we pass on to future generations.
|
