Every year, Santa Ana Winds drive some of the largest wildfires in Southern California during autumn and winter, and a new analysis of 71 years of data suggests that the total amount of land burned is determined more by wind speed and power line ignitions than by temperature and precipitation. The findings suggest that maintaining utility lines and carefully planning urban growth to reduce powerline ignitions may help to reduce future losses from Santa Ana-driven autumn and winter fires, which occur far less frequently than summer fires but account for the largest blazes annually. While California's summer fires are typically driven by an abundance of fuels such as dry twigs and logs, and are often ignited by lightning in remote areas, the state's autumn and winter fires are typically wind-driven and have increased in frequency in recent decades, along with the expansion of the state's power grid. To understand the factors that determine land area burned by Santa Ana Wind-driven fires, Jon Keeley and colleagues analyzed 71 years' worth (1948-2018) of daily Santa Ana windspeed data, as well as daily and monthly temperature and precipitation measurements covering the same time span. The researchers found that more than 3 million hectares burned in the region during this period, with 45.3% of fires ignited during Santa Ana Wind events. Further analyses suggested that large fires were not associated with higher temperatures, and that precipitation in the week leading up to Santa Ana Wind events likewise did not play a role in area burned. However, Keeley et al. found that higher maximum wind speeds led to greater area burned for large fires covering between 1,000 and 5,000 hectares. Ignitions also made substantial contributions to area burned - 100% of fires occurring during Santa Ana Winds events were sparked by humans, with powerline failures playing the greatest role in recent decades.
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