East Bay Hills Fire (Oakland-Berkely, CA - October 1991) » Climatic Conditions

The East Bay Hills have their own micro-climatic conditions, distinct from the areas to the east and west. The “flatlands” have a cool damp coastal climate, influenced by San Francisco Bay and the Pacific Ocean. The prevailing winds push moist air against the Oakland side of the hills and often create gusty winds, while the flatlands atmosphere is calm. The hills block low clouds and moist air coming through the Golden Gate opening from the ocean, keeping the moisture from reaching Contra Costa County. There is often a temperature differential of 50 to 100 between the hills and the flatlands, with the flatlands cooler sometimes and the hills cooler sometimes.

Between 1986 and 1991 most of California experienced drought conditions. This situation was recognized as creating more and more critical fire risk conditions each year. The unprecedented drought was accompanied by an unusual period of freezing weather, in December of 1990, which killed massive quantities of the lighter brush and eucalyptus. Dead fuel accumulated on the ground in many areas and combined with dropped pine needles and other natural debris to create a highly combustible blanket. Due to the fiscal cutbacks, governmental programs to thin these fuels and create fuel breaks were severely curtailed, so the fuel load was much greater than normal by the second half of 1991. In addition, no measurable rainfall was recorded during the summer and early fall of 1991.

The coastal areas of southern California are extremely vulnerable to the infamous Santa Ana wind, officially classified as a foehn wind condition. A similar condition occurs in the Oakland area, where it is known as a Diablo (or “Devil”) wind. These winds are created when a high pressure weather system is located over the great basin of the inland western states, accompanied by an offshore low pressure system. The high pressure system imports chilled air from the far north, with extremely low moisture content. The interaction of the two pressure systems and their counter-rotational forces creates a wind flow from northeast to southwest, while the pressure differential forces the dry air from high altitudes down to ground level. The result is a strong wind of exceptionally dry air, blowing through the mountain passes and spilling over the coastal hills toward the Pacific Ocean. Increased pressure also heats the air mass (adiabatic compression), which often results in air temperatures of 90 to lOOoF at sea level, with less than 10 percent relative humidity and wind velocities of 35 to 70 miles per hour.

Most of the major wildland fires in California have occurred during foehn wind conditions, which occur most frequently between mid-September and late November. The fire protection agencies in California are highly aware of the danger that is created by these wind conditions and use a Red Flag alert system to warn of extreme fire risk conditions. The National Weather Service monitors weather trends in the western states to issue an early warning of impending Red Flag conditions.

The drought conditions prevailed through October 1991 with warmer than normal temperatures. A Diablo wind condition was predicted for Sunday, October 20, and Red Flag warnings were issued to wildland fire agencies.

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