Ask Sam: Why Are Tides So Different From Place To Place?

Oct 17, 2019

Credit Photo by Jonathan Combe, https://bit.ly/2BhLrRu

Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown tackles a question from a listener. This week, we're buzzing because...

...Patty from Northampton asks: "I should understand the tides but I really don't. So in North Hampton, we have a very small beach even at low tide. And I was recently at Ogunquit beach with a friend, and the tide goes the way the heck out at low tide. And I don't know why!"

So first off, readers probably know that the tides are cause by the gravitation pull of the moon and the force of the Earth's rotation. These two forces cause our oceans to sort of 'bulge out' on two sides, and thin out on the others. The bulges, as they slosh across the globe, are actually called "tidal waves", which have little to do with the image of a tsunami that you might have previously associated with the phrase. 

These are large forces of course  - so why should the tides differ so much from beach to beach? The answer has to do with the underlying regional geography. While the tide may rise an identical number of feet from one beach to another (if they're a relatively short distance away) the horizontal distance the tide travels can be greater or shorter depending on the shape of the coastline. If the underlying slope of a coastlines is steep (as it may be in North Hampton) it may not look as dramatic as it does in Ogunquit, where the coastline rises more gradually.

Additionally, the underlying topography can make tides slosh differently in different places, as it crashes against underwater obstacles, or enters narrow channels. Consider the Bay of Fundy in Canada, which has made it's astounding tides something of a tourist attraction.

Credit Courtesy of Will Verafong, Flickr CC.

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