stormglass

The storm seen through a window slit in the shaft of the lighthouse.

A cold gradient from the bruised cloud at the head of the aperture down to the spray-lit air at its foot, rain drawn as leaning streaks that travel down across the hole, a soft bloom at the middle so the slit reads as the brightest thing on the granite, and the white a stroke of lightning puts through every window at once. slit_base and slit_height place the gradient in world metres, so one material serves every aperture in the tower; flash is the value game.Lighthouse drives while a stroke is running.

It shades itself rather than handing the engine a PbrSurface: an aperture is a hole onto a sky, so nothing about it should darken when the shaft does.