Radiation (Land) Fog
Radiation fog occurs at night over land, and is caused by the rapid radiation of land heat into the lower atmosphere. This causes the land temperature to drop sharply and so cool the layer of air immediately above it until the dew-point is reached. Although it does not actually form over the sea, it can drift out from land for two or three miles and persist for several hours.
Advection (Sea) Fog
Advection fog may occur on land or sea at any time, and is caused by the horizontal movement of warm air over a cold surface, with the air temperature falling to dew-point. It often occurs in spring, when the land mass has begun to warm up but the sea remains cold. Advection fog is invariably accompanied by wind, which sometimes can be quite strong, but which will not necessarily disperse the fog; this will occur only when the wind changes and a warmer drier air stream appears, or when the sun becomes strong enough to raise the air temperature above dew-point. Radiation fog however, will not persist in anything more that the lightest of breezes.
Sailing in Poor Visibility
When there is any possibility of sailing into conditions of reduced visibility, it is of vital importance to obtain an accurate position fix before visibility deteriorates so much that nothing can be seen. The time, log readings an speed should be noted, and the course to steer adjusted as necessary according to the circumstances.
Unless the wind is strong which is unlikely though not impossible in foggy conditions, there is little point in trying to reduce speed of a yacht under sail, on the principle that the slower the speed the less the ability to take avoiding action, although, it is good practice to clear the foredeck by handing the head sail and sailing under main sail alone. Under these conditions a forward watch will be easier to maintain, and preparations for anchoring, if necessary, can be made without much trouble.
The principle danger to a yacht in fog is the risk of being run down by a larger vessel, but this can be avoided by sailing out of the shipping lanes into shallow water. There remains the danger of collision with other small craft, but in this respect a vessel under sail has one great advantage over other vessels, and that is silence, thus for early warning of approaching danger a really good listen out is probably more important than a good look out. The prescribed fog signal should be sounded if necessary, but otherwise the crew should maintain silence and look and listen.
The steps to be taken when sailing into reduced visibility may be summed up as follows:
a. Obtain an accurate fix, note time, log readings and speed. Adjust course to steer according to circumstances.
b. Reduce speed, if necessary by shortening sail or handing the head sail.
c. Hoist radar reflector if not permanently hoisted.
d. Have foghorn ready and prepare to sound prescribed signal.
e. Record depths at regular intervals.
f. Post a good look and listen-out in the bow, and tell him to point in the direction of anything he sees or hears.
g. If conditions warrant it, life jackets should be worn.
h. Check life raft and/or dinghy is ready for launching.
i. Prepare the anchor for letting go in the prescribed manner.
j. At night have signal flares and a powerful torch ready for use.
k. Check that the engine is ready for instant starting. The best advice that can be given in respect of fog is to try to avoid it. If there is a forecast of poor visibility or it is already poor it may be advisable to stay in harbour, particularly if the planned course involves crossing shipping lanes.