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.

Orographic lifting of the air will cause adiabatic cooling at the dry adiabatic rate of 5.5F per 1,000 feet. If an adequate amount of lifting occurs, fog or low clouds will form. This process can create challenges for the forecaster.

The procedures for determining the probability of fog or low clouds during nighttime hours at stations having upslope winds are as follows:

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 cooling.

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 = 13.75F 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 the expected cooling is greater than the late afternoon spread, either fog or low clouds should be expected. Wind velocity will determine which of the two conditions will form.

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