Light is scattered when it encounters cloud, fog, smoke or dust in the atmosphere. Minute droplets or solid particles scatter rays in every direction, and cause diffusion and polarization. The light is consequently softer and easier to use, particularly for portraits, but colours become less saturated and a noticeable haze can develop.
A similar effect becomes apparent when dust and dirt accumulate on the surface of a lens. When working in dusty environments it is very important to carry lens-cleaning equipment, and to ensure that the front element of a lens is kept largely dust free.