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Tides - Why They Happen

Animation of spring and neap tides

Why do we get some very High tides and some very Low tides ?  

Together, the gravitational pull of the moon and the sun affect the Earth’s tides on a monthly basis. When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tides — both commonly called spring tides. One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides. During each lunar month, two sets of spring and two sets of neap tides occur.  (courtesy Sumich, J.L., 1996, and the NOOA OSE).

 

But don't forget about the impact of Air Pressure  

The effect of air pressure on tidal height is substantial and all too easy to forget. The invisible medium in which we live (the atmosphere) has a not inconsiderable weight, causing an effect which can be embarrassing at times !
The weight of the atmosphere creates a force pushing down on the sea. Perhaps surprisingly, one cubic metre of air at sea level weighs about one kilogram. A rough guide is that a change in pressure of one millibar will change the sea level by one centimetre. Tide tables assume a standard pressure of 1013 millibars. This means that a pressure of 1040 mb, pretty high but not abnormally so, could give a sea level lower by nearly 30 cms than expected. That could make the difference between safe passage through the Swashway and ignominiously grounding.

The lowest pressure recorded around the British Isles is about 925 mb which would give sea levels nearly a metre above tide table predictions. Unless air clearance is critical (sailing under a bridge ?), a skipper is unlikely to worry overmuch about too much rise of tide. The highest pressure around the UK is about 1050 mb which would give sea levels about 40 cm lower.

In a nutshell, worry about the pressure effect if the pressure is higher than 1020 mb and your depth is getting critical. (courtesy Mail-A-Sail / Franks-Weather)