At night, the surface and the air near the surface cools radiatively. Air that comes in contact with the cold surface cools by conduction. If the air and the surface cools to the dew point, the temperature at which saturation occurs, dew forms. If the air temperature drops below freezing, the dew will freeze becoming frozen dew. If the dew point is less that 0C, then we refer to it as the frost point, and frost forms by deposition. Dew, frozen dew and frost form in shallow layers near the surface. They are a form of precipitation, a moisture sink of the atmosphere.

Condensing water to form a cloud is not quite simple.


Homogeneous nucleation occurs when the water vapor molecules condense and form a cloud droplet. To do this requires an environmental temperature of -40C and saturated air, or relative humidity of several hundred percent.


It turns out that saturating the air is not always enough to form a cloud. The water vapor molecules need a site to condense on. This site is called a Condensation Nuclei and the process referred to as heterogeneous nucleation. Cloud condensation nuclei (CCN) are about 1 micron is size. There are two types of condensation nuclei:

HYGROSCOPIC - water-seeking

HYDROPHOBIC - water-repelling

Most particles are released into the atmosphere near the ground, this is where largest concentrations of CCN are. There are generally more CCN over land than over oceans. CCNs are one type of aerosol in the atmosphere.


haze contains a large concentration of nuclei - dust or salt particles.

Dry haze

Wet haze - condensation can occur at relative humidity of 80%.

As the relative humidity increases and approaches 100% the haze particles grow larger and condensation beings on the less-active nuclei. When the visibility lowers to less than 1 km (.62 mi.) and the air contains water droplets we have a FOG.


Fog forms in one of three ways:

  1. by cooling the air
  2. by evaporation of water into the air
  3. mixing of two air parcels

The types of fog are named with reference to the method by which the air becomes saturated.

Favorable conditions:

  1. clear nights,
  2. shallow layer of moist air near the ground,
  3. long nights,
  4. light winds

Radiation fogs are common over land in late fall and winter. Radiation fogs also form in low-lying areas VALLEY FOG. Cold air and high moisture content in river valleys make them susceptible to radiation fog. Radiation fogs form at the ground and are deepest around sunrise - sometimes an increase in thickness at sunrise due to the evaporation of dew supplying moisture to the fog.

The above methods of forming a fog all require cooling the air.

Fogs do not "burn off", the sun warms the ground. The air near the ground warms lowering the relative humidity. This warmer drier air mixes upwards destroying the fog. Sometimes the fog does not totally dissipate, but forms a stratus cloud - California.