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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.
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