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In the eighth instalment of our Sea Signs series William reveals the wealth of knowledge you can gain by simply observing air pressure.




We have always had a barometer by the front door at home, but I never paid any attention to it while growing up. Dad would occasionally tap the glass, presumably to check it was still working - but apart from that it simply ticked the box as a piece of décor every coastal home has to have. But when I bought a snazzy new Suunto watch and noticed it had a barometer, I started to pay attention to what it was showing me; the air pressure. I also began to notice that certain things happened in the sea and sky depending on whether the air pressure was high or low, rising or falling. Before I share what I have learnt, let’s start with the basics; what exactly is air pressure? Air pressure is essentially the weight of air around us. When there’s high pressure, it means cold air is sinking, increasing the weight of air at sea level. In contrast, when there’s low pressure, it means warm air is rising, reducing the weight of air at sea level.


These two natural processes create very different weather conditions.  When I wake up in the morning and my barometer is showing steady high pressure I know the winds will be light, the skies clear and the seas calm. These days are perfect for making the boat shipshape; airing the wetsuits, sweeping sand from the floor and wiping salt spray from the windows. These jobs can then be rewarded with adventures perfectly adapted to the conditions; swimming, snorkelling or paddleboarding in particular are ideally suited to high-pressure days because the waters are calm and clear. However, the price is paid when the sun goes down because the day’s heat escapes through the cloudless skies, creating ice-cold nights. This means you’ll need to remember extra blankets for your coastal camping when there’s high pressure; especially because the clear skies will be perfect for stargazing.


If you notice air pressure starting to fall, batten down the hatches because it means bad weather is coming. On a trip to Devon in the van a few years back I noticed barometer dropping to 988mb [low]. The weather was still glorious, but sure enough that night a huge storm struck. The van shook so much I started to feel seasick. Luckily the weather soon cleared up; when air pressure drops rapidly the storms are often short-lived. In contrast, when the pressure drops steadily over the course of a few days, prolonged bad weather is in store. 




One of the first people to notice the effects of air pressure was Robert Fitzroy, a Vice-Admiral in the Royal Navy and founder of what is now the Met Office. Fitzroy’s life reads like an adventure book; in 1831, aged 26, he was a naval protégé and captain of the Beagle. His mission was to circumnavigate the world on a voyage of discovery and for this he invited a young gentleman, Charles Darwin, to be the ship’s naturalist. Although this was Darwin’s official role, his unofficial job was to dine with the Captain every single night of the four-year voyage; Fitzroy suffered from severe depression and he hoped having someone to talk to would keep his ‘black dog’ at bay. Darwin performed both his tasks admirably and the voyage was a success, with both men going on to live incredible lives.

“if you notice air pressure starting to fall, batten down the hatches because it means bad weather is coming”

In the end, however, Fitzroy’s depression overwhelmed him and he took his own life - but not before he spent another three decades developing barometers to help predict the weather and save lives at sea. On once occasion he was sailing back to Britain from New Zealand as a passenger when they anchored in the treacherous Straits of Magellan, at the southern tip of South America. The weather was calm with no indication of bad weather, but Fitzroy’s personal barometer started to fall rapidly so he warned the captain to prepare for an impending storm. The captain ignored him and went to bed. Fitzroy then persuaded the officer of the watch to override the captain’s orders and lay out another anchor with extra thick rope; a huge breach of naval discipline with the consequence of severe punishment. But luckily for the officer, the storm struck at 2am [as predicted by Fitzroy] and the rope of the original anchor snapped. If it wasn’t for the extra anchor that Fitzroy suggested, the ship would certainly have been wrecked upon the rocks, with all lives lost. 


Air pressure doesn’t just affect the weather; it also has a huge effect on tides. When there’s high pressure, the weight of air is literally pushing down on the surface of the sea and it makes tides lower than predicted. This is often quite difficult to spot with the naked eye, but I got definitive proof when travelling to Mersea Island in Essex for a Tide Talk with Stitches + Steel, who make exceptional conversions for VW campers [that’s one of their vans opposite]. I had read that when the tide is over 5 metres The Strood causeway gets flooded, so I timed my arrival for when my Imray Tides Planner app said the tide would be 5.2 metres and falling. 


This way I could experience Mersea as an island, but not have to wait too long before safely crossing The Strood. However, on my punctual arrival the tide was already well below the causeway and it didn’t look as though it had got wet at all. I checked the barometer on my watch and the air pressure was a remarkably high 1030mb; fascinating proof of the effect of air pressure on tides. In contrast, when warm air rises and creates low pressure, it literally pulls up the sea and makes higher tides. For every millibar drop in pressure, the sea rises 1cm. 


This is dramatically illustrated through the phenomena of ‘meteo-tsunamis’, where the extreme pressure differences along squall lines creates a wave that actually travels directly beneath the weather front. While meteo-tsunamis are exceptionally rare, a much more common effect of low pressure is the storm surge. These happen when a low-pressure weather system rises up the sea along an entire coast – in extreme cases by 3 metres – and if this happens at the same time as a high spring tide, flooding occurs. North Norfolk is particularly vulnerable, with plaques on Blakeney quayside showing mind-bogglingly high water levels of past – all because of a combination in the invisible forces of gravity and air pressure. 

In the next Sea Signs feature William will decipher buoys, the signposts of the sea.

About this series

Every day the sea is different, a  result of the ever-changing interaction between the moon, sun, tides, wind, waves,  buoys, boats and the beach. While the picture may appear complex, like a piece of classical music, it is essentially just an arrangement of simple elements, or notes. By dismantling the machine into its component parts and exploring each one in detail we can create order out of chaos; clarity out of confusion. That is the  purpose of this series 'Sea Signs'.

When we are young we learn to navigate urban environments; what traffic lights  mean or how to cross a road. What many of us aren’t taught is how to read nature’s  signposts; how to judge wind by watching birds or what weather different clouds  bring. Then there’s a whole technical world of buoys and boats with different light  sequences, colours and signals communicating messages. My goal in this series is to  share with you all the signs – both natural and man-made - that you may encounter in  your adventures along the coast so you can read them as well as any yachtmaster  skipper or crusty old seadog. 

With this knowledge, nothing is going to slip under your radar.

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