By: Bill Jennings
Fog sucks. Boating in poor visibility is dangerous. Fog is one weather condition that is guaranteed to reduce visibility, sometimes to zero. Such conditions are not abnormal and being trapped in a fog bank can be a scary experience. Not surprisingly, boaters avoid areas where fog is reported, but all too often, fog occurs in remote places where it is not reported. The best way for you to avoid a ‘boating blackout’ from fog, is to understand what causes it. By learning this, you will know when and where to expect it.
Fog is basically a cloud on the water. Fog results when temperatures cool. To predict when a dropping ‘temperature number’ will cause fog, you need to know another number --- ‘dew point.' Armed with both of these numbers, you simply have to know that when a dropping ‘temperature number’ gets close to the ‘dew point number' (within a couple of degrees), water vapor condenses into tiny suspended droplets that we call “fog." Conversely, these droplets will evaporate as the temperature rises and the fog will “lift." The cause of the lifting being the increasing spread between temperature and dew point.
When meteorologists report a close temperature/dew point spread, there is a good chance you will run into fog on the water. Overnight air temperature drops often bring temperatures down to the above dew point, which explains the morning fog we see on small lakes. Clear nights allow surface heat to dissipate, further increasing the chances of fog. When lake water evaporates overnight into cooler air above it, the dew point can rise close to the air temperature, causing moist air to turn into fog.
Fog will rarely drop on you like a wet blanket while boating. You will usually see it ahead of you or creeping up behind you. If ahead, turn 180 degrees and find shelter. If behind, you may be able to outrun it. Should you ever get caught in a fog, your GPS is no help, because it cannot spot traffic. Radar is the best way for you to see other boats and mounting a radar reflector is the best way for you to help other boats see you. Of course, the best practice is to slow to an idle and follow the signal regulations. Vessels under 39.4 feet (12m) that are moving in restricted visibility such as fog are required to sound an effective sound signal, for 4 to 6 seconds, every two minutes.
If you reside in areas where fog occurs regularly, you will get to know almost by feel when fog is likely. A safer procedure would be to do your own evaluation of risk by sourcing dew point numbers from either detailed weather information or aviation reports, then based upon their closeness to temperatures, make your own decision as to whether fog will be a concern for your travels.