A fog seal is a light application of asphalt, usually emulsion, applied to retain aggregate. It is sometimes used over an asphalt concrete surface or a new seal coat, particularly if a porous aggregate has been used as the cover aggregate.

Sometimes a fog seal is used on an asphalt concrete surface that is exhibiting raveling. In some cases, a fog seal is applied to a seal coat that is exhibiting aggregate shelling. Aggregate shelling can often occur during the first cold spell after a seal coat. A light fog seal immediately applied at this time can minimize further shelling of seal coat aggregate.

Fog seals should be used with extreme caution as they can cause a temporary loss of friction on the roadway surface. Traffic should not be allowed on a fog seal until it has adequately cured.