Red Flag Days in California - A Hasty Climatology
Introduction With all the California wildfires in the news, and questions over whether these fires can be attributed to climate change, human activity (e.g. arson), or forestry management, I decided to take a look at the trends in the number of Red Flag days over the past few decades. "Red Flag" conditions are weather conditions that when combined with critical fuels (e.g. dead trees, grass build-up, etc.) and an ignition source (e.g. lightning, an ill-placed cigarette butt, etc.), will foster rapid fire growth. In general, the two main things you need for rapid fire growth are low humidity and high winds. The criteria for the National Weather Service to issue a Red Flag Warning will vary depending where in the country you are, and the criteria may be eased (either direction) a bit depending on fuels. For example, a local weather forecast office (WFO) may issue a warning below criteria if fuels are really primed (such as during a severe, long-term drought). Conversely, a WFO ...