Anticrepuscular rays, photographed in Arizona. The rays are called inscrepuscularins because they appear at dawn and dusk when the Sun is a few degrees below the horizon; they are insanticrepuscularins because they appear in the sky opposite to where the Sun is - the direction away from the sunrise or sunset. They are believed to be caused by the shadows cast by cloud tops well below the horizon. The parallel shadows are cast into the upper atomosphere, & appear to converge by the effect of perspective. They can only be seen when the sky is particularly clear, since any significant amount of dust will reduce the contrast between shadows & sky.