Weather by Malcolm RileySea fog occurred in much of NSW on this day as warm moist air from around Lord Howe Island was brought over the cooler water of the NSW south and central coasts. 



You are sailing in your fibreglass vessel off the coast during late spring when a bank of fog moves over your area and you do not have radar. Then you start to hear it, one long blast every two minutes and you can tell it is from something big and fast.
It is in times like this you start to wonder how well your vessel shows up on radar.
Noise in fog can be confusing; the speed a sound travels through the air is mostly a function of the temperature of the air. However, moist air absorbs less sound than dry air especially at higher frequencies. Low frequency noise is less affected by any absorption, this is why fog signals are often deep low noise.
The mariner relies on their eyes as the number one sense. We all judge a closing vessel by watching its progress. In fog the ears then take over, doing an unfamiliar task; judging distance by change in volume and direction.
The chances are that the big and fast vessel you can hear has more than one expensive high performance radars and professional crew that know how to use it; but you just don’t know. Collisions between large well equipped vessels that are in fog do occur.
One of the most famous collisions in fog was between the SS Andrea Doria and the MS Stockholm off Nantucket Island in the US in the mid 1950s. The Andrea Doria (29,000 tons) sank as a result of the collision. Both had radar and saw each other and both were travelling at around 20 knots; a potential closing speed of 40 knots. If you think that a tale of this vintage is not relevant today have a look at this article from maritime NZ.
For any given temperature a parcel of air can hold a specific amount of water vapour; the higher the air temperature the more water vapour it can hold. When a given parcel of air is holding as much water vapour as it can for a particular temperature it is said to be saturated and the relative humidity in that parcel would be 100%.
Saturation of a parcel of air can occur by either adding more moisture (evaporation) or by cooling the temperature of the parcel. If you cool a parcel of air enough it will reach the dewpoint. The dewpoint is the temperature where the relative humidity reaches 100%.
Once the temperature goes past the dewpoint some of the water vapour will condense out. Often this is seen as dew on the ground on cold nights. The vapour can also condense out onto tiny particles in the air and we know this as either fog or cloud.
Fog that forms in the coastal waters of the southeastern sea board is usually the result of north-easterly winds bringing warm moist air (high dewpoint) over cool water. The air moves over the cooler water, the layers of air having contact with the water cools and reaches its dewpoint. The vapour then commences to condense out of the air and fog forms.
The weather systems that bring these north-easterly winds are high pressure systems in the Tasman Sea. They are often slow moving, so the conditions that make sea fog likely may persist for several days.
In the waters around South Australia a slightly different process occurs. Warm dry air blows off the continent and well out to sea. As this warm air moves across the sea, evaporation occurs and the warm dry air becomes warm moist air. If this air is then blown back onto the coast by the arrival of another weather system and moves over areas of cooler water fog can form. In South Australia’s case there are areas of upwelling (deeper colder water brought up to the surface) along the coast that provides this cooling.
One of the most famous collisions in fog was between SS Andrea Doria (above) and MS Stockholm off Nantucket Island in the 1950s.Sea fog can form and persist in quite strong winds whereas the fogs that most of us experience over land tend to break up once the wind reaches around 5 knots.
Forecasts from the Bureau of Meteorology will only have the briefest mention of fog; South Coastal Waters, Ulladulla to Gabo Island and 60nm seawards: Tuesday until midnight: Wind: NE 10/15 knots. Sea: 1 to 1.5 metres. Swell: NE 1 to 1.5 metres. Patches sea fog
As a refresher the sound signals for restricted visibility are:
Power Driven Vessel underway        One long blast every two minutes.
Sailing vessel Underway         One long and two short blasts every two minutes.
There are a few more restricted visibility sound signals (see the ColRegs for full details).
If you’re are unsure of your vessel’s radar reflectivity, next time when at sea get on the VHF to a passing vessel with radar and ask them, not a warship because their radars may be a bit high performance and give you a false sense of security.
San Francisco harbour is notorious for fog. At the end of World War II the Liberty ship Henry Bergh was heading into San Francisco harbour. The ship’s official capacity was 560 but it was carrying 1,300 returning troops.
A thick fog had hung over the ship for many hours but did little to dampen the enthusiasm of a raucous deck party that continued into the small hours. Wind and current had pushed the ship off course and the noise from the party prevented the fog warning from a nearby lighthouse being heard. The ship ran aground and despite frigid water and treacherous seas all on board were saved.
A video of sea fog formation can be seen at:
Startling images of a collision as the result of sea fog can be seen at: