Lightning
Lightning is not created by thunderstorms, instead clouds merely help form the path of least resistance to the Earth from electricity that originates in space. Without clouds lightning is still evident such as on other planets like Venus, Venus will experience large single bolts known as "Bolts from the Blue".
Nearly every lightning discharge exhibits beading due to cooling immediately after a return stroke, and this is visible in the hieroglyph for Lightning.
For this reason the this hieroglyph is Lightning.