Darmouth Haar

The flooded valley that the River Dart runs through does some interesting things with clouds, rain, mist and fog. None of them more strange than recently when on a walk around the boat float, we noticed wafts of grey smoke floating up through the town behind where we live. We assumed it was a fire of some sort or a barbecue that had gotten out of control but it was weird that there were no emergency services hanging around.

Naturally curious (or just plain nosey) we wandered along the South Embankment to locate the source of the fire. There clearly wasn't a source on this side of the river and looking out towards the estuary we were confronted with the site of a narrow blanket of smoke originating on the Kingswear bank and hugging the river as it crossed over to our side.

It didn't smell like smoke and it didn't behave like smoke. That was probably because in fact it wasn't smoke at all.

A search of the internet showed no clear signs of localised fires although there was a boat or caravan fire about 7 miles away. This event was not it. It took a while to find but eventually we discovered that the thing was a sea fog or Haar.

Thinking about it, it would have taken quite a fire to produce the amount of 'smoke' that this fog clearly was. That is especially so if you take a look at the lack of visibility that occured at its best or worst . The estuary mouth was completely obscured and Dartmouth castle was nowhere to be seen as you can see from the following images.

So what is a haar?

A sea haar (sometimes called a sea fret) is a dense coastal fog that forms when warm, moist air passes over colder seawater. The air cools rapidly, causing the moisture within it to condense into a blanket of mist or fog that can drift ashore and dramatically reduce visibility.

Although the term is most commonly associated with the east coast of Scotland, the same phenomenon can occur anywhere around the UK coast where the sea is significantly colder than the air above it.

In Dartmouth, the recent event that many people noticed was likely caused by warm, humid air moving in from the south-west over the still-cool waters of the English Channel. As the air cooled over the sea, fog developed and was then carried into the Dart Estuary by light onshore winds. Once established, the haar can be remarkably localised, with thick fog covering the waterfront while higher ground a short distance away remains clear.

A sea haar often appears suddenly, giving the impression that it is "rolling in" from the sea. Temperatures can fall by several degrees within minutes, and visibility may drop to only a few hundred metres. Around Dartmouth, this can create dramatic scenes where Kingswear, the estuary and vessels on the river seem to disappear into a grey-white curtain, before re-emerging as the fog slowly lifts later in the day.

In short, a sea haar is not smoke or pollution but a natural weather phenomenon caused by the meeting of warm moist air and colder sea water.