Tens of thousands of people have been evacuated from the Gironde region surrounding Bordeaux, as France continues to battle wildfires fueled by unprecedented “pyrocumulonimbus,” or fire clouds.

Scientists sometimes describe these clouds as a fire-generated storm because the fire itself provides the energy needed to build the cloud.

The enormous amount of heat generated by forest fires makes the The air above the fire rises rapidly, carrying smoke, ash and water vapor into the atmosphere.

As the air rises, it cools and the water vapor condenses, forming a cloud.

If the fire is powerful enough and the atmosphere is unstable, that cloud can become a towering cumulonimbus, the same type of cloud associated with thunderstorms.

The turbulent winds created by fire clouds can cause embers to fly in all directions, making wildfires unpredictable and more difficult to fight. Lightning can also cause more fires.