High air pressure leads to lower than expected tides. The opposite is also true: low air pressure leads to higher than expected tides.
We can think of high air pressure sitting on the sea and squashing it down and low pressure releasing the sea to rise higher.
The effect is small but noticeable, anything up to about 30cm or 12 inches is possible when an extreme high or low pressure goes through.
In the charts above we can see the extreme low pressure system of Storm Ciaran that went through the English Channel overnight between the 1st and 2nd November 2023. At its centre it was 955 mb/Hpa, which is 58 mb below standard atmospheric pressure (1013.25 mb/Hpa).
A leap in wind speeds is expected but the effect on tidal heights is less well known.
Have a look at the chart below. We can see the green line showing the tidal heights that we would expect if there was normal atmospheric pressure. The green line is what the moon and sun would have the tide do, based on measurements over the years. The red line shows the actual, measured height, minute by minute. There is a difference of half a metre at times. This is the result of the extreme low pressure and a wind effect called ‘storm surge’ when a storm’s winds shepherd more water into an area leading to height increases.
Out in the ocean, the moon and sun account for the majority of changes in tidal height, but there are actually hundreds of factors at play, including atmospheric pressure and temperature. Nearer to land the shape of coastlines lies behind the most dramatic effects – you never get massive tidal height changes far out to sea, only when the sea meets the land.
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