Santa Ana winds rapidly spread wildfires through chaparral shrubland, tall grasses, and brush in southern California in December 2017. The Thomas Fire burned 237,500 acres from the time it started on December 4 through December 13.

A pair of images from Landsat 8 shows the area before and during the fire near Ventura, northwest of Los Angeles. Extremely dry, strong winds spread the fire aggressively and increased as they gusted through mountain passes and canyons.

On the south edge of the burn scar, developed and agricultural areas slowed the spread of the fire. At the time this image was acquired, the major concern was with the northwest portion of the burned area, where the wind was carrying the fire toward other populated areas.

Landsat’s 30-meter resolution allows detailed mapping of burn severity. Its shortwave infrared (SWIR) and near-infrared (NIR) bands combine to provide an accurate distinction between burned and unburned vegetation. The diagonal lines in the second image are an airplane contrail and its shadow cast on the ground from a plane that flew past just before the image was acquired.

The image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite complements Landsat imagery. The combination of visible and near-infrared bands from ASTER shows vegetation as red and the burned area as gray. Its 15-meter resolution shows the area in slightly more detail.