Why do some places have 2 tides a day and others 4?

10 February 2008


Why do certain parts of the world such as the gulf of Mexico have only two tides a day: one high and one low when other bits of the world have four tides a day?


First of all 99% of the world just has 2 tides a day and the reason for that is basically the moon and the sun pull on the Earth and on the water around it. If you're close to something massive it's got a stronger attraction due to gravity than something farther away. Water on the side of the Earth closest to the moon is going to get pulled the hardest and the Earth which is in the middle is doing to get pulled slightly less hard and the water on the far side is going to get pulled even less hard. So you tend to get two bulges of water: one is the bulge of water close to the moon and the other bulge of water on the other side which is getting left behind. That's the reason why most places get 2 tides a day.

Some places get 4 - the only place I know about it is Southampton, Portsmouth in the UK by the Isle of Wight. If you look very closely at the map of the Isle of Wight it has funnels on each side of the channel just north of it. As the water rushes up the channel it sort of piles into these funnels and then as it gets narrower the wave gets higher. You actually get a high tide as the water rushes up. You get another one on the other funnel as the water rushes back down the channel so you get twice as many tides as you should have.

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