Radiation Fog

CHAPTER FIVE

AVIATION WEATHER

513.

TYPES OF FOG

The two main types of fog are radiation and advection.

Radiation Fog

Radiation fog (Figure 5-18) occurs due to nocturnal cooling, usually on clear nights, when the

Earth releases relatively large amounts of radiation into the atmosphere, cooling the surface.

Cloudy nights, on the other hand, reflect most terrestrial radiation back to the Earth, reducing the

amount of cooling through a "blanket" effect. Radiation cooling actually begins after the

maximum daily temperature is reached, usually between 1530 and 1600 local time. Cooling

continues until sunrise or shortly after sunrise and it affects only the lower limits of the

atmosphere. If nocturnal cooling reduces the air temperature to the dew point temperature, fog

or low ceiling clouds will develop in the area. Winds play an important factor in fog formation.

Winds less than five knots usually results in shallow fog. Winds of five to ten knots will usually

cause dense fog. Winds of greater than ten knots will usually dissipate the fog and cause low

stratus or stratocumulus clouds to form. The other way radiation fog can dissipate is through

Figure 5-18 Radiation Fog

The rate at which the ground temperature can increase after sunrise affects the dissipation of fog

and low clouds. Vertically thick fog or multiple cloud layers in the area will slow down the

morning heating of the ground. Only the heating of the ground can increase the temperature of the

air overlying the ground. Once the surface air temperature rises, the ability of the air to hold more

water vapor increases and the fog particles tend to evaporate (Figure 5-19).

5-26

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds