Understanding Sea Fog

Anyone who has spent time on the British coast in late spring or early summer will know the experience. The morning is bright and warm inland, yet at the seaside a thick, cold grey cloud hangs over the beach, hiding the water and muffling every sound. Along parts of the east coast this sea fog is known locally as haar, and it can persist for days while towns only a few kilometres inland enjoy sunshine. For holidaymakers it is a disappointment; for scientists, it is a fascinating example of how air and water interact.

Sea fog forms when warm, moist air moves across a cooler sea surface. As the lowest layer of air is chilled by the water beneath it, it can no longer hold all of its moisture as invisible vapour, and tiny droplets of water condense, forming a cloud at ground level. This is why sea fog is most common in spring and early summer, when the air has begun to warm up but the sea, which heats much more slowly than land, is still cold after the winter. Once formed, the fog can be carried onshore by a light breeze.

Sea fog differs from the fog that forms over land on clear, still nights. Land fog develops when the ground loses heat rapidly after sunset, and it usually clears within a few hours of sunrise as the sun warms the ground. Sea fog, by contrast, is not easily burnt off by the sun, because the cold sea keeps feeding it from below. It may retreat a short distance during the warmest part of the afternoon, only to roll back in as evening approaches. Strong winds can break it up, but light winds tend to make it more persistent.

For sailors, sea fog has always been one of the greatest dangers. Before modern navigation equipment, ships could be driven onto rocks within sight of the shore because the crew could see nothing. Foghorns, which sounded a distinctive pattern of blasts at lighthouses and harbour entrances, were introduced to help vessels judge their position, and some coastal communities still remember the low, mournful sound echoing through the night. Today, satellite positioning and radar have greatly reduced the risk, although small boats and swimmers can still be caught out when visibility drops suddenly.

The fog also shapes the character of coastal towns. In some seaside resorts, the sudden arrival of the haar can empty a crowded beach within minutes, as families pack up and head for cafés and amusement arcades. Local people often learn to read the signs of its approach, such as a bank of grey cloud building on the horizon or a sudden cool breeze from the sea. Artists and photographers, on the other hand, value the soft light and muted colours that the fog creates.

Fog also has more surprising effects. Meteorologist Dr Elinor Pascoe, who has studied coastal weather for many years, points out that sea fog can lower air temperatures on the coast by several degrees, sometimes turning a warm day into a chilly one in less than an hour. It can affect wildlife too: some seabirds struggle to find food when they cannot see fish near the surface, while certain coastal plants depend on the moisture fog deposits on their leaves during dry periods. In some desert regions of the world, people have even built nets to collect drinking water from fog.

Forecasting sea fog remains surprisingly difficult. Small changes in wind direction or sea temperature can determine whether fog forms at all, and whether it stays offshore or spreads inland. According to Pascoe, computer models are improving, but they still find it hard to predict exactly where the edge of a fog bank will lie. This uncertainty matters to airports near the coast, which may have to divert flights, and to organisers of outdoor events, who must decide whether to go ahead.

Climate change may alter the pattern of sea fog in the future, although scientists are not yet sure how. Warmer seas could reduce the temperature difference that allows fog to form, making it less common in some places, while changes in wind patterns could increase it elsewhere. Pascoe hopes that people will come to appreciate the fog for what it is. 'It is easy to see it as a nuisance,' she says, 'but it is also a reminder of how closely the land and the sea are connected.'