Atmospheric Moisture and Smoke

Because water vapor in the atmosphere reduces visibility, if smoke is added to an already humid environment, visibility can be severely degraded. Also, if the air is saturated with water vapor, particles from smoke may act as condensation nuclei causing water droplets to form. This promotes the formation of clouds or fog, which further degrades visibility. Often a deadly combination occurs during the darkness of night as smoldering smoke drains downvalley to encounter high humidities from condensing cold air under a valley inversion. The effect can be fatal, especially along transportation corridors (Achtemeier and others 1998).

Favorable conditions for fog occur when the dew point temperature is within a few degrees of the dry bulb temperature, wind is less than a few meters per second, and there is a high content of moisture in the soil. Fog is most common at night when temperatures often drop to near the dew point value and winds are most likely to be weak. Common places for fog to form are over lakes and streams and in the vicinity of bogs and marshes.

There are times when atmospheric moisture can improve visibility, however. Smoke particles can adhere to rain droplets, causing them to be carried with the rain as it falls. This “scavenging” effect removes smoke particles out of the atmosphere, reducing smoke concentrations and improving visibility.

See: Atmospheric Moisture in the Fire Weather section for more information.