Plume-Dominated

A plume-dominated fire exhibits the increased role of the convective force generated by the heating of the fire. The fire itself begins to influence the wind field around it. This added vertical development has also been described as "fire in the third dimension" along with the length, width and atmosphere above and around the fire. Consequently, fire spread rate and direction become less predictable because the role of the general wind in fire spread becomes less pronounced. Spotting is normally short-ranged but occurs in all directions.

Large convective plume can influence the winds around a fire

All convective columns have updrafts and downdrafts. Associated with these are indrafts, which is the flow of air into the column, and downdrafts or downbursts of air out of the convective column.

Indrafts

Indrafts develop in response to the low pressure created as air is lifted upward into the convective column in response to heating from the fire. This inflow of air provides oxygen to the flame front, thus increasing the amount of combustion and heat released, which increases convective heat transfer to help preheat fuels. In addition, spotting can occur as firebrands are carried up into the convective column. As the column increases in size and strength, firebrands can be carried even greater distances by the column. This is the classic cycle of reinforcement. The increased inflow increases fire intensity, which in turn, increases convective activity, which increases the strength of the indrafts.

Downbursts

Downbursts or downdrafts develop within the convective column and force air downward to the surface, where it fans out as it hits the ground. These downbursts are similar to thunderstorm outflow winds. As the convective column builds, the rising air cools and clouds and precipitation form. As raindrops fall toward the ground, they begin to evaporate, and the surrounding air is cooled and begins to accelerate toward the ground. Once the downburst hits the ground, the air spreads out in all directions from the column's base.

:Virga is a visual indicator of downburst development

The visual indicators of the possible development or occurrence of downbursts include:

  • The appearance of virga and/or rain indicate that evaporation is occurring and thus a downdraft has developed.
  • The development of a strong convective column is an initial sign that strong downburst winds may soon develop.
  • Winds calm as the indrafts turn to downbursts and signals that outflow winds may soon hit the ground.

Remember, these indicators have occurred prior to fatalities on several fires and should alert you to take immediate action. A three-dimensional fire will first be influenced more strongly by the wind, and then transition to a more plume-dominated situation as the convective column builds and strengthens. The wind field can still influence fire spread, but the fire itself will begin to modify the general winds as the indrafts and downdrafts strengthen with the convective column.

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