Wildfires that raged across California in October could cost the insurance industry more than $1.5bn. These blazes can no longer be considered rare, warns Tomas Girnius.

On 21 October 2007, Santa Ana winds from the California desert fanned the first in a series of infernos that could cost the insurance industry more than $1.5bn. Southern California was on fire again. Aggravated by extreme conditions that included low humidity, high temperatures, and Santa Ana winds, 23 separate fires devastated more than 500,000 acres, torching modest vacation cottages and luxury homes alike.

The siege prompted Governor Arnold Schwarzenegger to declare a state of emergency in seven counties stretching from Santa Barbara to the Mexican Border. California experienced its largest evacuation in history as residents of San Diego, Los Angeles and surrounding areas fled in advance of the spreading flames.

Devilish Santa Anas

Santa Ana winds, also nicknamed “Devil’s Breath”, play a significant role in shaping California’s wildfire risk. These winds tend to occur between October and February, peaking in December.

Typically, Santa Ana wind events last for 24 to 48 hours with sustained winds of 30mph-40mph. The Santa Anas that fuelled October’s wildfires, however, were an exception. They persisted for nearly five days and were unusually strong. The highest recorded gust was 111mph at Laguna Peak and gusts of more than 80mph were common throughout the area.

These uncharacteristically fierce winds were spawned early on 20 October. Almost simultaneously, dew points dropped nearly 50°F, resulting in 10% humidity. One weather station at Fontana, CA recorded a dew point of -22°F, resulting in a relative humidity of just 2%. This is a level of moisture normally observed only in places like Siberia during winter when the air is so cold it cannot hold moisture.

Temperatures in Southern California were well above normal, rising to 32°C to 35°C during the day. This high heat occurred across locations that had had little or no rain for several months. The last measurable precipitation in Riverside, for example, had occurred on 23 April. The combination of dry fuels, high temperatures, low humidity, and sustained winds created the ideal wildfire setting.

Assessing the damage

Little more than a week after the first fire ignited, AIR sent a team to survey the damage on the ground. Areas covered included the Witch (by most estimates the costliest), the Rice, the Slide and the Grass Valley Fires.

Witch Fire – One of the key findings from the survey was the spottiness of the damage. In the Witch Fire, as with many of the other fires, chaparral was the primary fuel source. Chaparral is a highly flammable mix of low, scrubby bushes and stunted trees, and is found across wide expanses of Southern California.

Throughout areas affected by the Witch Fire damage to properties was as spotty as damage to the landscape. Next to one hill on which structures and vegetation had been reduced to charcoal, was another that was untouched. Much of this area is ranch country where homes are separated by considerable distances.

However, the Witch Fire also penetrated more densely populated outskirts of San Diego. Some 360 homes were destroyed in the Rancho Bernardo community. Yet even here, spottiness was evident. On some streets, for example, one side of the street had been devastated by fire, while houses on the other side survived.

What was more difficult to discern was to what extent the spottiness was due to the selectivity of the fire, to natural firebreaks in the form of roads, or to the effectiveness of fire suppression activities. The worst damage could be found on streets adjacent to wildland without setbacks (setbacks are the distance that a building is set back from a road, river, floodplain or land that it needs protection from).

The dominant construction type throughout the Witch Fire was stucco exterior walls, or cladding, over wood frames. Evidence from other fires shows that stucco exterior walls have proven to be resistant to ignition from flying embers. Once the fire penetrates the door, however, the underlying timber frame has little chance of survival. The AIR survey team saw numerous examples of stucco clinging intact to the wire mesh on which it is applied, but the wood frame of the home had been reduced to a pile of ashes.

While tile roofs are generally thought to be fire resistant, the wooden soffits on which they rest are not. In many of the areas surveyed, barrel-style roof tiles were open ended, presenting an entry point for embers that ignited underlying sheathing.

Another key finding from the survey was the fact that there were very few partially damaged homes. In large part, houses were either a total loss or they survived unscathed. This pattern, which is very different from damage patterns seen in the aftermath of hurricanes and earthquakes, is reflected in the most recent official figures from all fires combined (see table opposite). The pattern is less apparent in commercial structures, which are typically better engineered and constructed of more fire resistant materials.

Slide and Grass Valley Fires – The fuels for the Slide and Grass Valley Fires were different from those that fed the Witch Fire. Rather than chaparral and grassland, these fires occurred in the pine forests of the San Bernadino Mountains. In forests, fire spread rates can be quite complex.

Homes in these mountains are predominantly wood frame with clapboard siding. Roofs were either wood or asphalt shingle. The steepness of the terrain exacerbated the damage. Since heat rises, fuels immediately upslope from a fire are robbed of moisture and preheated prior to actual ignition, allowing the fire to spread more rapidly. Extreme slopes also hinder firefighter access and impede fire suppression efforts.

Lessons learned

With each devastating wildfire, it seems we have to learn the same lessons all over again. According to an analysis by USA Today, since 2000, more than 55,000 people moved into some 16,000 new homes in the neighbourhoods impacted by October’s fires.

These fires highlight the upward trend in property loss due to wildfires that we’ve seen over the last few decades. While some suggest increased wildfire activity during this period has been influenced by climate change, no direct causal link has yet been established, and this continues to be an area of active research by scientists. There is, however, a direct causal link between increased wildfire-related losses and the rapid growth in population and property in the Wildland Urban Interface, a region where undeveloped forests, grasslands and chaparral meets urban expansion.

California has the highest number of homes in these areas as crowded coasts push people wanting larger plots to the mountains for space and views. According to the California Department of Forestry and Fire Protection, close to five million homes in California are at high to extreme risk from wildfires, and 84% of them are in areas adjoining designated wildlands.

Insurers underwriting properties exposed to the wildfire hazard must re-evaluate; they can no longer rely on historical data alone to estimate future wildfire-driven losses.

Tomas Girnius is a research scientist at AIR Worldwide.