The conditions necessary for catastrophic fire include certain fuel types, weather, topographic conditions.
Fuel Types
In this study, fuels consist of vegetation in the wildlands and both vegetation and built structures in the
urban-wildland nexus. Major characteristics that determine how fuel types will burn are in two data
dictionaries, one for wildlands and one for residential areas. In the wildlands, we observed characteristics including: the size of common vegetation patch, amount of fuel, presence of volatile products, moisture level (based on vegetation type), proportion of dead material, and distribution or continuity of fuel. In residential areas we observed both vegetation conditions and building structures. These characteristics were defined and analyzed by a committee of experts with first hand experience of fire behavior in the East Bay Hills.
The risk of a catastrophic fire is increased exponentially by particular weather conditions. Temperature, wind velocity, and relative humidity impact fuel moisture, ignition potential, flame length and the rate of spread. From November to June the climate in the East Bay Hills is considered 'fire safe'. In the summer, dense fog adds moisture to the eucalyptus (Eucalyptus globulus) and Monterey pines (Pinus radiata).
Additionally afternoon westerly winds off San Francisco Bay can reach velocities in excess of 20 miles per hour. These two conditions can move a fire quickly in the hills. At least five historic fires have burned a cumulative 1200 acres under these conditions. However, these are not the extreme conditions that fuel a catastrophic firestorm.
The weather conditions during the Oakland firestorm are primarily created by warm, dry, easterly winds, commonly known as Diablo winds. These winds regularly reach velocities in excess of 20 miles per hour, bring temperatures in excess of 80 degrees Fahrenheit and drop the humidity to less than 20 percent. Diablo winds occur on average 4 to 6 days a year usually in late September or early October.
Topographic data allows calculation of slope gradient and slope aspect which are critical parameters in fire models. Furthermore, the behavior of wildland fires responds to the morphology of the landscape. For instance, fire spreads more rapidly moving uphill as the heat generated from below heats and dries the fuel accelerating the ignition and burn mechanisms. Slope and aspect can also influence the micro climate and produce variations in temperature and fuel moisture content which influences the ignition potential of the fuel. In summary, the drier and hotter the conditions, the greater the risk of fire.
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