Smoke-Induced Fog (SIF)

  • A dense fog (water droplets suspended in the atmosphere) that forms away from a fire site when surface smoke comprised of increased fine particulates and water vapor, combines with ambient air that is already prone to natural fog formation.
    • Definition Extension: Under these conditions, smoke serves as a catalyst and induces surface fog formation, reducing visibility to a greater extent than smoke or fog individually. Smoke can induce fog in two ways.
      1. Surface smoke contains additional water which is a byproduct of combustion. This excess water can induce fog or if present increase the density of existing fog.
      2. Smoke contains particulate matter which may serve as condensation nuclei thus increasing the number of smaller fog droplets in smoke.
      In either case, SIF reduces visibility beyond that of the separate individual impacts of smoke or fog. SIF can occur anywhere within surface smoke plume but will usually be found in low-lying areas and drainages where ambient conditions most likely favor fog formation. Fog has formed when observed relative humidity is under 100% and thus, it forms more easily in the presence of smoke. SIF is also most likely to be found late at night or early in the morning when ambient conditions most likely favor fog formation.
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