Parts of northern California have been ravaged by intense and fast-burning wildfires that broke out on October 8, 2017. Blazes that started on a few hundred acres around Napa Valley were fanned by strong northeasterly winds, and by October 10, the 14 fires had consumed as much as 100,000 acres (150 square miles) of land. States of emergency have been declared in Napa, Sonoma, Yuba, and Mendocino counties, and thousands of people were asked to evacuate. The densely populated “wine country” is famous for its vineyards and wine-making operations and the tourists they attract.

In the late morning of October 9, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired a natural-color image (top) of the smoke billowing from the fires. About two hours later, the MODIS instrument on NASA’s Aqua satellite captured the second view.

CalFire and local officials reported that at least 1,500 homes and businesses have been destroyed, and thousands more are being threatened. In some places, entire neighborhoods burned to the ground. Cellular and land-line phone communications have been lost in several areas. Authorities are still accounting for deaths and people reported missing. As of the morning of October 10, none of the fires were even partially contained, according to CalFire bulletins.

While the causes of the fires are still under investigation, we do know what helped them spread quickly: abundant dried vegetation and seasonal wind patterns.

“After more than a decade of drought, the fuel levels—dry brush and grasses—across California are exceptionally high,” said William Patzert, a climatologist at NASA’s Jet Propulsion Laboratory. “Last winter’s welcome rains created more vegetation that, over the past six months, created more fuel.”

The fall season also typically brings hot, dry, and gusty winds. These Diablo winds are driven by atmospheric high-pressure systems over the Great Basin (mostly in Nevada). Winds blow from northeast to southwest over California’s mountain ranges and down through the valleys and coastal regions. These downslope winds can quickly whip up a fire and carry burning embers to the next neighborhood or patch of woodland.

“The simple formula is fuel-plus-meteorology-plus-ignition equals fire. The catalyst is people,” Patzert added. “The fires erupted in areas where wildlands meet urban and suburban development. Californians have built in what are historical fire corridors, and these high-density developments are particularly vulnerable to fast-moving, destructive fires.”

Though it is not visible in this imagery, wildfires also broke out on October 9 in Southern California’s Orange County. That fire was fanned by strong gusts of Santa Ana winds.

NASA Earth Observatory images by Joshua Stevens, using MODIS data from LANCE/EOSDIS Rapid Response. Story by Mike Carlowicz.