The darkness of fog rolled across the parking lot as I planned to launch a small sailboat
for a day of sailing on the bay. As I uncovered the boat, still on it’s trailer, moisture ran
from the canvas cover. It took only a glance over my shoulder to see that thick fog had
blown across the bay. I could no longer see the harbor entrance. Boats docked in their
slips became only shadows and sailboat masts disappeared upward into a thick cloud of
moisture. Without asking my sailing partner, I knew enough to stay ashore.

As the morning wore on, I asked myself, “what caused the otherwise overcast day to
turn into this? Did I miss something in the weather forecast? How could I have planned
better? Without radar, a good skipper seldom casts off in a fog unless he or she is ready
to deal with the “soup.”

Boating and sailing enjoyment continues for those skippers who understand fog, when it
is likely to occur, and how to safely cope with it. It is possible for fog to form anywhere,
and at any time, if the conditions are right. An alert weather observer can spot the
conditions and make preparations for the arrival of fog.

There are four classifications of fog based on the conditions that cause its formation.

Radiation fog is also called ground fog. It occurs in late summer or early autumn, when
the temperature of the ground is less than the temperature of the air.

Advection fog can occur during any season when warm winds push moist air over
cooler land or water. An example is when fog seems to be blowing ashore from a large
lake or from the ocean.

Steam fog forms when very cold air blows off land over a body of warm water. This
happens in autumn when the ground cools quickly overnight but lakes and rivers remain
warm from summer weather. The surface of the water appears to be steaming or
smoking.

Precipitation fog occurs when falling raindrops descend through a layer of warm air
aloft. The warm air absorbs moisture in the form of tiny droplets creating an opaque
cloud called fog.

Knowing that fog is moisture suspended in the air helps us understand how it forms.
Humidity suspended in the air is invisible, but when the air is cooled, humidity changes
to tiny droplets. All four types of fog share the common elements of warm moist air, cool
air, and a breeze to cause movement. Warm moist air when cooled can no longer hold
the invisible humidity and squeezes it out in the form of tiny droplets, or visible fog.

Sailors who listen carefully to weather forecasts should note the dew point temperature.
At this temperature, the invisible humidity condenses into fog.

Fog occurring late in the day will often dissipate the next morning when the air is
warmed by the sun. As the temperature rises, the droplets return to humidity and
become invisible. we often say, “the sun is burning off the fog.”

In addition to the dew point, weather reports also mention the humidity. This is
expressed as a percentage. High percentage of humidity indicates much moisture in the
air, while low humidity indicates dryer air. The capacity of air to hold moisture increases
when the air temperature increases, and vice-versa. When the weatherman says, “the
humidity will reach 90% today,” he means the air is at 90% of its capacity. When the
humidity reaches 100% it condenses into droplets called fog.

Federal laws are quite specific regarding what each skipper is required to do in times of
reduced visibility, including fog. Coping begins with slowing down. As visibility
decreases, so should your speed until you are able to stop your vessel in one half the
distance that you can see.

Navigation lights will aid other boaters in learning your location, direction, and speed of
travel so they are better able to avoid a collision. Hoist a radar reflector to create a
strong image to other radar observers. Fiberglass boat hulls are difficult to identify
without metal or foil reflectors.

Adjust your own radar to give you the best image display at close range. Assign a crew
member to monitor your radar screen for other vessels, and to watch for aids to
navigation and the shoreline while the skipper remains at the helm.

Assign other members of your crew to lookout stations even if you have radar. Position
a person at the bow, at the stern, high in the rigging, and low near the water. Although
their vision is hindered by fog, they should listen, smell. and visually scan the horizon
with peripheral vision. A barely visible shadow, the faint odor of cigarette or engine
exhaust, or the sound of voices may be the clew needed to avoid danger.

All boats are required by law to sound fog signals. During times of reduced visibility
including fog, rain, snow, dust, or smoke, sound signals tell other boaters of your
presence, status, ability to maneuver, and whether you are fishing, towing, or anchored.

Sounds produced during fog are often distorted. Their exact location may be difficult to
determine as sound bounces off thick fog banks. Several observers may hear the same
sound and each may point to a different direction as the source of the sound.

While under power, stop frequently. Shut down your engine and listen. Without your own
engine noise you may hear engines of other boats, conversations, or shore noises.

Hopefully, boaters will know and sound the proper signals. Power driven vessels
including sailboats under power, must sound a prolonged blast of 3 to 5 seconds at
intervals of every two minutes of less. A power driven vessel with restricted
maneuverability shall sound one prolonged blast followed by two short blasts. Sailing
vessels, while under sail, sound the same signal as those with restricted
maneuverability.

Becoming lost or disoriented in the fog is common and every skipper experiences this
on occasion. When fog is first detected, mark your position on a paper chart, chart
plotter, or GPS. Mark the time of your plot. Keep a record of your progress at frequent
intervals and compare your electronic position to the paper chart noting shallow water,
aids to navigation, and hazards. Plot your DR (dead reckoning position) on your chart if
you do not have electronic navigation instruments and in the event of a power failure.
Paper charts remain unaffected by electrical failures.

Steer a compass course to your intended destination and check it with electronic aids to
verify, if possible. In the event that fog becomes too dense for safe navigation, anchor
and wait for it to dissipate. Remember while at anchor to produce a sound signal once
each minute. Hopefully, it will not be necessary to anchor in a busy channel.

Although going out in the fog may not be the best choice, fog can occur whenever the
conditions are right. If you’re “out in it,” follow the safety rules and proceed slowly to
your destination. If you arrive today in the fog, use tomorrow as a layover day to enjoy a
sunny day ashore. You will deserve it.