JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 effects 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 5 knots usually results in shallow fog. Winds of 5 to 10 knots will usually cause
dense fog. Winds of greater than 10 knots will usually dissipate the fog and cause low stratus or
stratocumulus clouds to form. The other way radiation fog can dissipate is through solar heating.
Figure 4-15 — Dissipation of Radiation Fog
In considering the dissipation of fog and low clouds, consideration should be given to the rate at
which the ground temperature can increase after sunrise. 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 4-15).
Advection Fog
Advection fog occurs when warm, moist air moves over a cold surface and the air is cooled to
below its dew point. Common in coastal areas, it is often referred to as sea fog when observed to
come from the sea. Fog of this type becomes thicker and denser as the wind speed increases, up
to about 15 knots. Winds much stronger than this lift the fog into a layer of low stratus.
However, in some oceanic areas, sea fog has been known to persist with winds as high as 40
knots. Advection fog can stay over the water for weeks, moving over the land late in the day and
moving back over the water the next morning.
Version 3.2/Dec 08 4-23