Topography Factors

In the coastal plains areas, features such as swamps, rivers, roads, etc. are a major factor in fire control, but slope is not.  In managing smoke, however, even slight slopes become a very important factor.  The majority of highway accidents have occurred because of down-drainage smoke at night or smoke spreading into open areas.

The sun striking the earth’s surface heats it during the day.  The warmer air close to the surface warms and expands as it rises. The air along the surface of a slope, being warmer and lighter, tends to rise up-slope, resulting in up-slope winds.  At night, the reverse is true.  Such movement of air, which is very noticeable in mountainous areas, is so slow in flat (coastal plains) areas, it is not noticeable.  Also, winds during the day will generally overcome this phenomena in coastal and piedmont areas.  At night however, the general winds usually die down and the local down-slope and down-drainage winds occur, even though very slowly.  This local wind flowing down-slope at the surface will carry residual smoke down-drainage into lower areas.  The smoke is not diffused as it is during the day.  Instead, as drainages come together, it concentrates and becomes thicker.

Smoke can be smelled at very low concentrations (a few parts per million).  Somewhat higher concentrations can be seen, and as it increases, can reduce visibility.  At night, this reduction is also abrupt--the same as with fog.  It does not “feather out” at the edge like it does during the day.  Consequently, the visibility can change from clear to almost zero in a few feet. Smoke will also flow from forested areas into open areas because there is no obstruction to its= movement.

Even a slight reduction of visibility on interstates is a real problem.  Although most people will slow down when smoke is encountered, some people (knowing the interstate is wide and straight) will either continue through the smoke at the same rate of speed or not slow down as much as other drivers in front of them.

This problem is compounded by the fact that the visibility of red taillights is drastically reduced in both smoke and fog.  Taillights on older vehicles, being smaller and generally weaker, may not be visible past a few feet.  Drivers rarely turn on their flashers--even though they have slowed to less than 30 mph on an interstate.  Some people don’t remember to turn them on even when they stop.

Smoke on interstates and major highways can result in drastic consequences including fatalities and damages in the millions of dollars.

Down-drainage Smoke is a Killer!

 Why will smoke flow down-slope and down drainage at night?

  1. Unstable conditions become stable.
  2. Relative humidity increases.
  3. Temperature drops.
  4. Surface cools rapidly when sun goes down.
  5. Fire will also cool down--thus, less heat to lift smoke.

Be prepared for a lawsuit if an accident occurs close to your burn Document!

What to do after an incident occurs.