The Mountain Shadow is the triangular shadow of a mountain extending up to over hundred miles. The mountain shadow is not just the mere shadow of a mountain but has some other unique features. The peculiar feature about the mountain shadow is that the shadow cast by the mountain always looks triangular to the observer, irrespective of the size and shape of the mountain. Even if a mountain is rectangular in shape, the shadow looks triangular from its peak. This happens due to the perspective effect. Mountain shadow is more of a natural phenomenon than weather illusion .

When the observer stands at the rim of a big tunnel of shadowed air and looks along the length of the mountain he finds that the shape of the mountain is same as the cross section of the tunnel, but the final end of the mountain is located at such a distance that it cannot be distinguished properly. Actually the dispersion of the rays of the sun at an angle of 0.5º makes the view at the distance very blurred.

The definite size of the sun also makes the mountain shadow look triangular. Owing to the sun the completely shaded parts of the shadow converges and narrows down. The mountain shadow cast by a high mountain can extend up to two to three hundred miles or more. The edges of the mountain shadow unite at a point because of the perspective.

However such shadows can be cast only by enormous mountains as the shadows cast by small hills are not long and significant enough.