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16 October 2014
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Sea Fog @ Freshwater West - Adam Pitters

Sea Fog

Fret, Haar and Fog - what's the difference?
BBC Meteorologist, Gail Pirie explains...

Quite simply, all of the above are one and the same. They are examples of a weather phenomenon more commonly known as sea fog. And no matter what you call it, it's the damp, drizzly stuff that can ruin what would otherwise be a fine day.

The variation in name simply arises from the locale in which you happen to find yourself when the fog rolls in off the sea. On the east coast of Scotland sea fog is known locally as haar or North Sea Haar, and it is often said to plague local residents during the Summer. Likewise, it's English counterpart - Fret or Sea Fret can make Summer days on the East coast of England miserable.

So why does this weather phenomenon spoil a day that could otherwise be spent on the beach? Where does sea fog come from and how does it form?
Sea fog usually occurs between April and September. But is most likely on the east coast of the UK, or over the Northern Isles during early summer, before the sea has started to warm up. It forms when a parcel of warm air passes over the notoriously cold North Sea.

The warm air at the bottom of the parcel is cooled by the cold air below, until it can no longer hold the moisture that was previously contained within. Therefore, it releases some of the moisture in the form of liquid water through condensation. Add an onshore component like a wind of 5-20 mph, and the cooling in the bottom of the warm parcel of air is spread upwards and generates a fog; sea fog. Moreover, if the wind is coming from a direction between North and South East, the sea fog will make its journey from the sea over the land.

A number of factors determine the locational extent of sea fog and whether it will disperse quickly or linger throughout the day. For instance, if the land is warm when the fog rolls in from the sea, it will readily disperse. This is due to a smaller temperature difference between the surface of the earth and the bottom of the parcel of air.

During the night, when temperatures on land drop, the sea fog can penetrate a long way inland and linger till the next morning. Should there be a blanket of fog greeting you in the morning, then sunshine is needed to burn it back to the coast. But if the sea fog is very thick, sunshine is unlikely to burn through it. And if the wind is blowing steadily in from the sea, any 'haar' that is dispersed will be quickly replaced with a fresh batch.

So all in all, sea fog is usually an unwelcome guest. The length of its stay is determined by wind speed and direction, sea temperatures and humidity levels in the bottom few thousand feet of the atmosphere. All of which makes forecasting its dispersal complex.

Footnote: Be very careful if you're planning on surfing in foggy conditions. It's inadvisable to do so as it's very easy to become dis-orientated. Rip currents can carry you to hazardous areas of the beach without you even realising it.

I remember surfing in similar conditions a few years ago at Freshwater West. Some kids on a day trip were swept off their feet by a freak wave in ankle deep water and quickly pulled out to sea in the rip. Visibility was so poor due to sea fog that they literally vanished.

Were it not for local surfers, the outcome could have been a lot worse. Luckily all of them were found quickly and picked up by a longboarder paddling in.

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