Why can you see beams of light in the air when it is foggy outside?
(Q&A by Tara Nikolischen)

On a dreary morning when it is still dark but foggy outside, you turn your car headlights on. What do you see? Well, you should see beams of light, not just where the headlights are shining because of what is in the air. Fog is a colloid, meaning that it has two phases, the dispersed phase and the continuous phase. The air is the continuous phase. The colloidal particles (water droplets) which make up the dispersed phase, are able to scatter light. The size of these particles vary from 1 to 100 nanometers in size. This scattering of light is known as the Tyndall effect. This is why beams of light are visible in fog.
Colloids are divided into different classes depending on their dispersed and continuous phases. Fog is an example of an aerosol because it is a liquid dissolved in a gas. Mayonnaise is a colloidal emulsion, a liquid dissolved in a liquid. Foams, such as whipped cream, are gases dispersed in liquids or solids. Sols, such as jellies or paints, are dispersions of solids in liquids.

Source: Smoot, Smith, and Price. Chemistry. Glencoe; New York, NY. 1998