Record-setting heat helped keep California wildfires active at night

July in the state had the highest minimum temperature on record

Above: Credit Los Angeles Times

There are many ways that a warmer climate can influence wildfires, causing them to burn more intensely. Higher temperatures can lower the relative humidity, lower the amount of moisture in the vegetation (fuel), raise the temperature of the fuel itself, and cause more powerful thunderstorms with  lightning. But one factor that we don’t think about very often is that the heat can persist through the night, influencing fire behavior.

When today’s senior firefighters began their careers, they could usually count on fires “laying down” at night. The intensity and rate of spread would decline to the point where night shift personnel could more easily and safely “go direct”, constructing fireline very close to the edge of the fire.

During the month of July in California, many of the large fires continued to grow rapidly at night, which often required firefighters to drop back to a safety zone and simply watch, since there is little that they could do without putting themselves in harms way.

Of course it is too early to say that this will be a permanent change, but in the last month a new record was set for California’s average minimum temperature; it was the highest since records have been kept. And this was not just a one-month event. The trend has been increasing since the 1980s.

Many of those senior firefighters have been known to to lament the trend in the last couple of decades of incident management teams declining to have a night shift. The justification of the teams was that it was not safe to have firefighters working at night because of snags falling, steep terrain, and other issues. After observing the nighttime fire behavior in recent years, the senior firefighters might now be less inclined to argue strongly in favor of night shifts, at least in certain geographical areas and weather conditions.