Creeping Coastal Fog
by Ann Backus, MS
Ann Backus, MS is an Instructor in Occupational Health at Harvard School of Public Health, 665 Huntington Ave., Boston MA 02115, 617/432-3327, abackus@hohp.harvard.edu
What fog stories do you have? Undoubtedly nearly 100% of you who read Fishermen’s Voice have experienced coastal fog. Whether you are a commercial fisherman, a sailor, a kayaker, a year-round resident or a late summer visitor, there is likely a fog story in your past. Maine boaters are particularly likely to encounter fog.
Many years ago I was clamming on an island in midcoast Maine with my kids, ages 3 and 6, when I looked up to see the fog had engulfed us. Those were the days before the hand-held GPS, and we always carried a chart and compass for just this eventuality.
How can we prepare for fog?
Most fishermen and boaters check the weather forecast before launching. Fog may be included in the forecast, or one can estimate the likelihood of fog from weather data such as water temperature, air temperature and dew point. We are familiar with water and air temperature, but have less familiarity with dew point and how to use it in combination with air and water temperature.
Dew point is the temperature at which (for a given barometric pressure and relative humidity) the air will be saturated with water vapor, i.e. the humidity will be 100%. If the dew point is given as 65oF and the air temperature is falling, when the air temperature reaches 65oF, the water vapor will show up as fog.
There are two types of fog. Radiation or ground fog usually occurs overnight as the land cools (the warmth of the land radiates away from the earth) and the air temperature reaches the dew point. This type of fog generally burns off the next day the temperature rises.
Advection fog is the type of fog that boaters worry about. It moves horizontally. Boaters can see often a fog bank in the distance and can observe whether it is advancing towards them and how fast.
Warm air “holds” more water vapor than cold air. For example, the outside of your cup of hot coffee doesn’t “sweat” because the warm air next to the cup can “hold” the water vapor it has in it. But the outside of your cup of iced coffee will “sweat” because the air next to the cup is cold and can’t hold its water vapor. [You can try this with the kids and if you have a thermometer you might be able to find the dew point of the air at your house, by sequentially filling your cup with cooler and cooler water.]
So, say we have warm air coming in from the south, and water temperature of about 63oF (which is reasonable for coastal Maine in summer). If the dew point is 65oF, then when the water cools that warm incoming air above it to 65oF, the water vapor will condense and be suspended as fog.
You can get dew point forecasts from weather underground (www.wunderground.com) and from weather.com by clicking on current details. Weather Undergound also supports marine forecasts and provides buoy data from various locations. According to Wikipedia, for every 10% that the humidity is below 100% subtract 3oF from the ambient air temperature to find the dew point. Simply speaking, if the dew point temperature is close to the air temperature, there is a greater likelihood of fog, especially if the humidity is high as in the case of an advancing warm front. If you or your teenagers become interested in weather predicting, check out “dew point” on Wikipedia.
After checking out the weather forecast, draw-up a brief “float plan” and leave it with those on shore.
What should we have on board?
According to the Boater’s Guide to Maine Boating Laws and Responsibilities available at http://www.maine.gov/IFW/laws_rules/boatlaws.htm, certain flotation and signaling devices must be on board various watercraft. The following table is excerpted from the Boater’s Guide and the devices chosen from the guide are those that may help ensure safe passage in fog. The usual warnings apply – check the laws where you boat and check the US Coast Guard and commercial fishing regulations as appropriate.
Personal Water Craft (PWC) Vessel less than 16 ft in length Vessel more than 16 ft in length, but less than 40 ft
PaddlecraftPersonal Flotation Device (PFD) (Types I, II, and III are the typical lifejackets; Type IV is a throwable PFD; Type V is for specialized sports such as windsurfing, kayaking, waterskiing.)Yes, all on board must wear PFD.Yes, available for each person; children under 10 years must wear PFD Type I, II, or III.Yes, available for each person; children under 10 years must wear PFD Type I, II, or III; thowable PFD must be available except for kayaks and canoes.Yes, Type V each person; children under 10 years must wear PFD Type I, II, or III; thowable PFD not required.Sound Signal (must last more than 2 seconds)YesYesYesYes, if motorizedVisual Distress Signals/daytimeNoNoYes, in federal watersNoVisual Distress Signals/nightMay not be permitted to operate at nightYes, in federal watersYes, in federal watersYesNavigation LightsMay not be permitted to operate at nightYesYesYes, if motorized; if not motorized, lantern or flashlight with white light required
Sound Signals such as whistles and air horns are available. Tie a whistle to each PFD including those for children and consider having an air horn on board.
Visual Distress Signals for daytime include an orange flag, orange smoke, orange floating smoke and hand signals; for night time (or for fog!) they include pyrotechnic devices such as red meteors and red flares and an electric lantern/flashlight. Even though the requirements specify federal waters, it would be advisable for each boat to have a minimum of 3 of some type of pyrotechnic device on board.
Navigation Lights are required at night for every type of vessel and could be of use during daytime fog.
For way-finding in fog, your GPS is very valuable, especially if you set a few waypoints such as myharbor or mymooring before you set out and/or “trak” your adventure on the hand-held GPS and retrace the trak as you return. Just be sure your batteries are fresh.
Having a compass is great if you know the directional relationships of the coastal landmarks, and a compass combined with a nautical chart could help ensure safe return in fog. It is a good idea for boating novices to practice with the compass and chart on every trip in order to be familiar with chart markings and relationships as they relate to visible landmarks.
I hope you don’t have a fog story to tell at the end of the summer, or if you do, that it ended well because you were prepared, mindful of the conditions that result in fog, and had a healthy respect for how quickly fog can close in on us on the coast.