Mist & Fog

Book Extract

For a given temperature the atmosphere will hold only a limited amount of water as vapour.

If the air mass is sufficiently moist, once a particular temperature is reached, the vapour will condense out into minute, visible droplets; mist and fog will form.

Types of Fog
There are three types of fog:

• Radiation fog, which forms because of the reduction of temperature, usually inland on a clear, cool night
• Advection fog, caused by warmer or cold temperatures moving in (advecting) from elsewhere
• Hill fog, which is cloud covering the hilltops

Radiation Fog
Usually forming on a clear night with light winds, radiation fog occurs inland. It isn’t a major problem for coastal sailors, but can be blown towards the coasts.

Radiation fog forms on cool, clear nights.

For radiation fog to form, certain circumstances are required:

• Clear skies
• Moist air
• Falling temperatures, close to the dew point
• Light winds (5 to 7 knots), which mix air in the lower levels aiding the fog formation

It is possible to forecast the formation of radiation fog. As with all things “forecasting”, it is necessary to keep a watchful eye on weather conditions and spot the circumstances for its development. The sailor should look at the forecast charts for midnight and 6 A.M. and then ask a series of questions:

• Is there a ridge of high pressure over the area of interest, or a col?
• Will skies be clear?
• What will wind speeds be? (5 to 7 knots?)
• Will dew points fall, rise or stay the same? Dew points do not vary as much through the day as do temperatures. Monitor the afternoon dew points.
• Will wind speeds increase or decrease overnight to fall within the ideal 5- to 7-knot range?
• Will cloud increase or decrease overnight? If cloud increases, temperatures will increase; if cloud decreases, temperatures will fall.
• If you’re wondering how to predict temperature and dew points, several Web sites (such as Weatheronline) now offer forecasts of dew points and temperatures throughout the night.

Advection (Coastal Fog)
This is the fog that affects sailors most often. The classic advection fog occurs on western and southern coasts of the British Isles and Ireland.

As its name suggests, advection fog is fog caused by warmer air being advected in from another location.

Typical locations in the UK and Ireland for advection and coastal fog are southwest England, Wales, southern and western Ireland, northwest England and the west coasts of Scotland.

Advection fog forms in the following way:

• A moist flow of air is present, usually in a tropical maritime air mass
• Moist warm air flows over a colder sea (e.g., warm air from the Azores traveling over cooler Atlantic waters southwest of the UK)
• Warmer air contains more moisture than cold air, and so as the warm air comes into contact with a colder sea, it can no longer hold the water as invisible vapour
• The water vapour condenses and forms fog

The most common fog to sailors is advection fog.

A classic scenario is for advection fog to form in the warm sector, behind a warm front.

Note that unlike radiation fog, wind speed is not a factor in the formation of advection fog.

A similar effect occurs when cold air flows over a warm sea. The air in contact with the sea can no longer hold its water as invisible vapour and so condenses. This phenomena is often seen in the Arctic and is referred to as Arctic sea smoke.

To forecast advection fog, you should ask yourself:

• How moist is the air?
• What are the dew points downwind? Are they higher or lower than the current dew points?
• Is the sea cold or warm when compared with the air mass above it?
• Where did the air mass originate?

Advection fog is well forecast in the Shipping and Inshore Waters Forecasts, so you should be alerted to the possibility of it occurring. However, it’s always useful to be able to spot the circumstances leading to advection fog formation, as a slight change in wind direction could mean the difference between good and poor visibility.

 

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Sailor’s Book Of The Weather is written by Simon Keeling. Simon is proud to be a self-confessed weather anorak, and loves to convey enthusiasm for weather, climate and forecasting to viewers, listeners and readers. From the age of nine he dreamed of being a TV weatherman and in 1995 that dream came true when he became weatherman for GMTV. He has done stints on all the main UK networks, but now spends more time writing about the weather, running weather courses for TV weather presenters, pilots and sailors, as well as running his own weather company. He is a Fellow of the Royal Meteorological Society and has a Master of Science Degree in Applied Meteorology & Climatology from the University of Birmingham.