I clean the ashes out of my wood stove every 3 days. I've noticed that on some days, the ashes seem to be so airborne it's difficult to get them from the stove into the bucket without covering my whole house with ash dust. But on other days, the ashes seem to be heavier and less apt to fly around. I assume that the difference occurs because of different amounts of humidity in the air. When the air is more humid and contains more water vapor the small ash particles become slightly damp and this makes them stick to each other.
Humidity also acts to cut down on static electricity.
When the ash picks up small electric charges, it repels other like-charged pieces of ash. As you try to sweep up the ashes, they rub against the walls of the stove and pick up charges. I'm guessing the answer may be simple, but I'd certainly like to know it. Because the particles scatter all the wavelengths of light right back at the viewer, the material appears white. If the material didn't scatter the light so strongly, the light could penetrate the material to a deeper level and would travel a longer distance inside the material before it gets back out to the viewer's eye.