Embers fly through the Kincade wildfire leaving streaks of light that reveal the strong winds helping drive the fire. This unintentional flow visualization mirrors techniques used by researchers to understand how flows are moving. The shutter of the camera remains open for a fixed time, so the length of each streak tells us about the speed of the flow. Longer streaks occur where embers moved faster.
Here we see the longest streaks in the upper left side of the image, which tells us that the wind was moving faster there than it did at lower heights, like near the photographer in the picture. That’s in keeping with what we would expect. In general, winds move faster above the ground than they do near the surface. That speed difference is one of the reasons wildfires are so difficult to contain; a single ember caught by high winds is easily carried to unburnt areas, allowing the fire to spread more quickly than if it had to burn along the ground. (Image credit: J. Edelson/Getty Images; via Wired)