SAFE BOATING

Use caution when encountering fog

By Roz Butziger
Posted 3/31/16

You drop anchor in Potter Cove in Prudence Island, swim, have a nice supper and then turn in early. You and your family have plans to meet friends ashore for a Sunday BBQ the next day. No problem. …

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SAFE BOATING

Use caution when encountering fog

Posted

You drop anchor in Potter Cove in Prudence Island, swim, have a nice supper and then turn in early. You and your family have plans to meet friends ashore for a Sunday BBQ the next day. No problem. You can easily get underway by 8 a.m. and be at the dock by 10. You settle into your bunk knowing you are in total control. At 6 the next morning you go up on deck and see – nothing! In every direction a blanket of fog engulfs you. Where did that come from? How long will it stay? Should you call your friends and cancel? You’d sit down, but every surface is wet. Your thoughts stray to that Jaime Lee Curtis horror movie “The Fog.”

Actually, you are in the middle of a low cloud. The land around the cove cooled down at night and when warm damp air sits over the cool surface, the temperature of the air is cooled. What happens when you cool moist air? Well, what happens when you have a glass of iced lemonade on an 80-degree day? You pour the lemonade into a dry glass. Right away the outside of the glass becomes wet. Why? The moisture in the air is cooled down when it touches the cold glass, and water vapor particles condense out, becoming liquid droplets on the outside of the glass. When warm moist air meets cold the result is water droplets. If air is warm and moist, but the sun sets and it cools off, radiation fog results. When the sun comes up, it eventually warms it up again and all the condensed out water droplets can be reabsorbed, and fog dissipates in the morning.

Advection fog, on the other hand, is what happens when a large mass of warm air blows over cooler water. The warm moist air above the water cools and its moisture is squeezed out. A fog bank can form in minutes! A sunny trip can turn into a time for fog signals and great caution. Waking up in a sheltered harbor in a fog bank is one thing, but when you underway, if you see the advection fog bank move in, and about to be engulfed by the fog bank, there are several things you should do.

Mark your position while you can still see your surroundings. Of course reduce your speed. Alert your crew and post lookouts. They should listen for any other boats that also are flying blind. Don’t anchor in a shipping channel, but if you are in a safe area, you could anchor and wait a while, even a day or so, until the advection fog bank moves on. Make sure to have someone assigned to ring your bell or sound a horn periodically. An anchored boat needs to signal its position.  If your boat is under 36 ft long you are required to give a sound signal at least every two minutes. If it is over 65 ft you must ring a bell for five seconds every minute. If you are a powerboat underway in a fog, you must sound one prolonged blast every two minutes. Listen for other signals. If you hear two blasts every two minutes, there is a powerboat adrift near you. It is a good idea to keep aboard a card with the fog signals on it so you know if you are encountering a tug with a tow or a sailboat, which will sound a long blast followed by two short blasts or an anchored power boat etc. 

How can you predict fog? At night, a halo around lights indicates moisture in the air. If there is a gentle wind that is warm and moist, the elements for fog formation are there. Look at the sky. Is it hazy? Check the weather report and note the air temperature and dew point. If they are within 10 degrees of each other, fog is possible.

So what about your plans to leave Potter Cove by 8 a.m.? If it is radiation fog, due to the land cooling down, as soon as the sun heats it up, the fog most likely will dissipate, and you can be on your way. Maybe you will be a little late, but no big deal. However, if it is the advection type of fog, settle in for a stay, heat up some soup and grab a good book. Maybe The Fog.

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