The first thing we need to understand is what exactly a fog is. It is, simply put, a ground level cloud. All clouds are formed when invisible water molecules (in the form of water vapor or gas) cool and condense onto tiny particles (condensation nuclei such as dust, sea sale and other aerosols) in the air to form water drops (liquid). If the condensation occurs very high up in the atmosphere then the chilled molecules condense into ice crystals (ice). Since fog is a ground level cloud it takes on the liquid state of matter. However only the fog droplets are liquid. In between is air (a gas).