This combination of factors comes together to make what would be a normal fire season much worse.
The recent wave of deadly and destructive California wildfires – the worst in that state since 1991 – are the direct result of several converging environmental factors that have created the perfect conditions for a major conflagration, say experts familiar with the situation.
Years of drought, for California and for neighboring states, are the main reason for the fires that have already destroyed more acres that the annual average over the past five years, said Miguel Miller, a National Weather Service meteorologist in San Diego. (This fire season surpassed that average – 113,418 acres – on Oct. 1, by more than 20,000 acres, according to the California Department of Forestry and Fire Management. The current wave of fires has added more than 400,000 additional acres to that total.)
California has been chronically dry for four of the last five years. While the period from July 2002 through June 2003 proved exceptional – southern California actually received close to the annual average of 10.77 inches – that one year was not enough to offset the lows of the previous four years, Miller said.
One of those years (2001-2002), saw the lowest recorded rainfall in San Diego in 150 years. Three inches of rain were measured at the San Diego International Airport that year, he said.
And conditions since May 2003 have been terrible as well, Miller said. Not one drop of rain has been recorded in San Diego since then. That's 177 days of no precipitation whatsoever.
Dry conditions in California itself aren't the whole story, however.
"California doesn't get very much rainfall and we import most of our water," Miller said. "In order for southern California to have a drought, that means that other states are having a drought."
And that's exactly the case with neighboring Arizona and Nevada, said climatologist Mark Svoboda, at the National Drought Mitigation Center at the University of Nebraska-Lincoln.
This regional drought has led not only to dry vegetation and low water tables, but also to low reservoir levels.
As if these conditions weren't bad enough, there is also an epidemic of tree-eating bark beetles that have turned tens of thousands of acres of forest into piles of dead firewood, Svoboda said.
Properly watered trees are able to fend off the beetles to a certain extent, he said. But in drought conditions, trees become unable to fight off these pests and can quickly succumb.
"During healthy years with adequate moisture there's adequate sap to correct that problem," Svoboda said. But during prolonged dry spells, "the trees will eventually die, because they're already in shut-down mode."
The beetle problem has thus provided tons of dried-out firewood, just waiting for a spark.
"What you've got is a much greater fuel base," he said.
While Gov. Gray Davis relaxed state regulations for allowing the removal of dead timber in forested areas, there is just too much land and too much timber.
"The bottom line is there are too many acres to cover," he said.
Then there are the so-called Santa Ana winds, which act like a giant bellows bringing in gusts of fresh oxygen to keep the fires raging.
"That's all been sitting there waiting for the ideal conditions of the Santa Ana winds," Svoboda said. "This combination of factors comes together to make what would be a normal fire season much worse. So that's all led up to what you're seeing now in the news."
These winds originate in the high country of Utah, Idaho and Nevada, an area known as the Great Basin, according to Miller. The formation of these winds begins when a cold air mass settles in the Great Basin, creating a high-pressure system close to the ground.
"This pressure is pushing the air downward over the great basin, and it's got to go somewhere," Miller said. "There is lower pressure at the coast at this time, so that's where it goes."
So from the freezing plateaus of the Rockies, these winds descend to the coastal regions of California. On the way there, these winds can be further compressed in canyons.
"As you compress it, you heat it up," Miller said. "Just like the bicycle pump principle."
The result is a blast of hot, dry air for California, and this only serves to fan the flames, as well as dehydrate more vegetation that will in turn become fuel for the spreading fires.
But in addition to all these environmental causes, there are also human factors to take into consideration, Svoboda said.
Major development has occurred in the last few years in the very areas of California that are highly susceptible to wildfires. Svoboda thinks that recent property destruction is an inevitable outcome of this development.
"It's no different than continued development in flood plains," he said.
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