Climate Change and Nighttime Fire Behavior ^†

Abstract

It is well-documented that global nighttime temperatures have been increasing during the past few decades. For example, the average California nighttime temperature has increased at a rate of 0.7 °C per decade over the past 20 years. Temperature and atmospheric moisture (typically indicated by relative humidity in fire danger indices) are closely related, and dead fuel moisture (DFM) is a function of temperature and moisture via the equilibrium moisture content. Typically, as night temperature decreases, relative humidity increases, as does the DFM. Higher values of DFM is a factor in reducing fire behavior as the increased moisture reduces flammability. However, warmer nighttime temperatures and lower humidity allow fuel to stay drier, thus enabling fires to be more active throughout the night. Historically, fire management would often count on fires “laying down” at night as part of their tactical planning. However, an increasing number of incident reports across the western U.S have been highlighting active nocturnal fire behavior. This has consequences for firefighter safety and suppression success, impacting managed fire activities during the night, as well as the carryover into the next day. In this presentation, we examine the western U.S. trend in nighttime temperature in the context of nighttime fire behavior, discuss the potential fire management impact, and provide a global perspective.