What is the difference between advection and radiation fog? Meteorologist and weather router Chris Tibbs on everything you need to know about fog at sea
Radar and AIS help to make navigating in fog safer, but it is still an unnerving experience to sail in visibility of less than 1,000m. Foghorns are hard to pinpoint and the rumble of large engines seem closer than they actually are. It usually feels as if you are sailing around in circles.
At some stage all of us will encounter fog and it will have been generated by one of three processes.
What is advection fog?
Advection fog – the widespread fog that covers large sea areas – is caused when a warm moist air mass moves over a cold sea. The cold sea cools the air above to below its dew-point, causing moisture in the air to condense. Fog is formed from the numerous water droplets.
This happens over a large area and will persist until there is a change in the air mass. This can take anything from hours to days. In one recent Round Britain Race, we started from Calais in fog and returned down the North Sea in even thicker fog; it did lift a little for a while in between. It was a fairly memorable race, but not for the right reasons.
Various locations are prone to fog with a given weather pattern. Indeed, it can be so common that the fog receives a local name – the Haar or sea fret along the east coast of England and Scotland, for example. In an east wind, warm continental air moves over the cooler North Sea. By the time it reaches the UK dense fog has formed. Sea fret doesn’t penetrate far inland, but it means the east coast can struggle with low temperatures and poor visibility while the rest of the UK basks in sunshine.
Grand Banks fog
The best known fog in the Atlantic is that found over the Grand Banks. The prevailing wind direction is south-westerly, which brings warm moist air from the Tropics and the Gulf Stream. Cold water moving south with the Labrador Current provides a steep temperature gradient between the currents, causing thick fog banks to form quickly as the warm air moves over the colder waters.
When considering a passage along the northern route from the US to Europe, staying north or south of the Gulf Stream will usually make all the difference in temperature and visibility.
This passage can be quite testing. We found ourselves surrounded by radar echoes on one occasion and although most of these were fishing boats, some may have been ice. We were pushing hard, so we all slept feet forward just in case.
Fog and ice are not great sailing companions and in the days of the Whitbread Round the World Race (see some great images of EF Education from the 1997-98 race here), before the use of ice gates in the Southern Ocean, poor visibility was a source of concern owing to the increased chance of ice.
The fog indicated cold water and bergs or ‘bergy bits’ (small ice that doesn’t show on radar, but is big enough to do damage) could be around; spectacular sailing, but also somewhat on the scary side.
What is radiation fog or coastal fog?
What we often see around coasts of the UK is radiation fog, generated from the radiative cooling of the land at night, which in turn cools the air above. Any moisture condenses and forms fog. This is most common in the early morning after a cold, clear, still night.
Although this is a land fog, it does drift down valleys and estuaries to the sea, and morning fog can spoil those early departures. We used to keep our boat up the Tamar river, between Devon and Cornwall, and it was always a great feeling to slip away at first light. But this plan could be ruined by radiation fog which reduced visibility to a few metres. Radiation fog will burn off quickly as the sun intensifies and is limited to coastal waters as the warmer sea will disperse it.
The final main process that generates fog is known as frontal fog. This forms when rain falls into cooler, drier air and some of the drops evaporate into water vapour. As the air becomes saturated some of the vapour condenses into fog.
How to navigate a yacht in fog
Knowing the different processes of the development of fog helps you to decide your tactics. With advection fog you either need a change in the wind direction to shift the air mass to a drier, cooler source region, or you need to sail to warmer water.
This may be feasible if you’re close to the Gulf Stream, but generally it is impossible. The tide may introduce a change in the water temperature, but it’s not usually enough to clear fog.
So, to get better visibility you will have to wait for a change in wind direction or speed – in most cases once the wind increases to over around 15 knots, fog lifts to low stratus clouds and visibility improves. That said, it will linger even with a strong wind in locations where there is a large difference between the water and air temperatures.
Frontal fog will quickly pass and radiation fog will burn off – a bit of patience really does go a long way.
What is Dewpoint?
Unless you have a means to measure humidity on board it will be hard to estimate the dew point of the air, although some weather reports on the internet will provide this.
However, marine forecasts do give an indication of expected visibility. While GRIB files increase in popularity on board, they should be viewed as adding to the marine forecast rather than replacing it.
Chris Tibbs is a meteorologist and weather router, as well as a professional sailor and navigator, forecasting for Olympic teams and the ARC rally