To determine the surface temperature necessary for
the dissipation of fog by using the Skew T Log P
Diagram, trace dry adiabatically from the intersection
of the average mixing ratio line and the temperature
curve to the surface level. The temperature of the dry
adiabat at the surface level is the temperature necessary
This temperature is known as the
CRITICAL TEMPERATURE. This temperature is an
approximation, since it assumes no changes will take
place in the stratum from the time of observation to the
time of dissipation.
This temperature should be
modified on the basis of local conditions. See figure
In considering the dissipation of fog and low clouds,
you should consider the rate at which the surface
temperature will increase after sunrise. Vertically thick
fog, or multiple cloud layers, will slow up the morning
heating at the surface. If advection fog is present, the
fog may be lifted off the ground to a height where it is
classified as stratus. If ground fog is present, the
increase in surface air temperature will cause the fog
particles to evaporate, thus dissipating the fog. Further
heating may evaporate advection fog and low clouds.
FORECASTING ADVECTION FOG
OVER THE OCEANS
In the absence of actual temperature and dewpoint
data and with a stationary high (a southerly flow is
assumed), use the following method to forecast
advection fog over the ocean.
1. Pick out the point on an isobar at which the
highest sea temperature is present (either from the
surface chart or a mean monthly sea temperature chart).
Assume that at this point, the air temperature is equal to
that of the water and has a dewpoint 2 degrees lower.
2. Find the point on the isobar northward where the
water is 2 degrees colder. From this point on, patchy
light fog should occur.
3. From a saturation curve chart (fig. 5-14), find
how much further cooling would have to occur to give
an excess over saturation of 0.4 GM/KG, and also 2.0
GM/KG. The first represents the beginning of moderate
fog and the second represents drizzle.
4. As the air continues around the northern ridge
of the high, it will reach its lowest temperature, and from
then on will be subject to warming. The pattern will then
be drizzle until the excess is reduced to 2.0 GM/KG, and
moderate fog until 0.4 GM/KG is reached.
If actual water and temperature data are available,
use these in preference to climatic mean data. If the high
is moving, trajectories will have to be calculated.
The fog is usually less widespread than calculated,
and drizzle is less extensive. Also, clearing and lifting
on the east side of the high is slightly faster. This method
appears to work well in the summer over the Aleutian
areas where such fog is frequent.
FORECASTING UPSLOPE FOG
Orographic lifting of the air will cause adiabatic
cooling at the dry adiabatic rate of 5.5°F per 1,000 feet.
If an adequate amount of lifting occurs, fog or low
clouds will form. This process can create challenges for
The procedures for determining the probability of
fog or low clouds during nighttime hours at stations
having upslope winds are as follows:
1. Forecast the amount of nocturnal cooling,
2. Determine the expected amount of upslope
cooling by using the following steps:
a. Determine the approximate number of hours
between sunset and sunrise.
b. Estimate the expected wind velocity during
the nighttime hours.
c. Multiply a by b. This will give the distance
the upslope wind will move during the period of the day
when daylight heating cannot counteract upslope
d. Determine the approximate terrain elevation
difference between the station and the distance
computed in c. Elevation difference should be in feet.
(Example, 2.5 thousand feet.)
e. Multiply the elevation difference by the dry
adiabatic rate of cooling. (Example, 2.5 times 5.5 =
13.75°F of upslope cooling.)
3. Add the expected amount of upslope cooling to
the expected nocturnal cooling to arrive at the total
amount of cooling.
4. Determine the late afternoon temperature
dewpoint spread at the station under consideration. If