The language of wildfires

The Quick Study “Fluid dynamics of wildfires ” by Rod Linn (Physics Today, November 2019, page 70) applies physics principles to the increasingly urgent global issue of wildfires. It is both thought provoking and relevant.

Included in the piece are the phrases “rising air draws in air below it … in much the same way as air is drawn into a fireplace and rises up a chimney,” “a rising plume can draw cool air,” and “ambient wind drawn into the gaps.” In those contexts, the dictionary meanings of the word “draw” seem to invite the common misapprehension that the heated air of a wildfire is somehow pulled or drawn upward from above.

Heated gases from wildfires are not drawn upward, nor do they rise spontaneously. Wildfire gases are better characterized as being pushed, pressed, or buoyed upward from beneath, ultimately by ambient pressure. The heated gases of a wildfire, much like air bubbles rising in water, are driven upward by a hydrostatic pressure gradient. The greater pressure is at the lower elevation, as established by gravity. Absent gravity, the gradient would disappear and the heated gases would not rise but would instead smother the wildfire.

Precise language is important when talking about science, particularly to accurately represent wildfires as being pushed from behind or below rather than to suggest, as is common in popular parlance, that they are drawn or pulled upward from above.