CONDENSATION: DEW, FOG AND CLOUDS

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

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.

HETEROGENEOUS NUCLEATION

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

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

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.

• RADIATION (GROUND) FOG - produced by the radiational cooling at the surface

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.

• ADVECTION FOG - air is cooled to saturation by having warm moist air moving over a cold surface. Pacific Coast has a lot of advection fog since the surface water near the coast is much colder than the water farther off shore. Warm moist air from the Pacific Ocean is advected over the cold coastal waters, chilling the advected air from below. Advection fog, unlike radiation fog always involves the movement of air. Fog, while a traffic hazard, is very important water source for some plants and animals - particularly along the coast of California.
• UPSLOPE FOG - forms as moist air flows up an elevated plain, hill or mountain. Winter and spring on the east side of Rockies are a common site. Also common in the Appalachian and Adarondak mountains. Upslope fogs may last for several days.

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

• EVAPORATION (or MIXING) FOG - formed by mixing two unsaturated air masses. Moist air coming out of your mouth is an example of mixing fog. Contrails are another example. Saturation vapor pressure depends on the temperature - the higher the temperature the higher the saturation vapor pressure. Warm rain falling into a cool air mass near the surface can form a fog. An example of such a fog is the type that forms in the vicinity of warm fronts.
• STEAM FOG - forms when cold air moves over warm water (heated outside swimming pool) water evaporates from the pool into the air, increasing the dew point and if mixing is sufficient the air above becomes saturated. The colder air directly above the water is heated from below and rises forming what appears to be steam. Observed over unfrozen lakes in autumn or winter. Steam fog also forms above a wet surface on a sunny day - over a road after a summer rain shower.

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.