Variation in Tidal Range


Why are ocean tides so much larger in some places than in others?


tidal vectorsIn our discussion of the origins of tides we previously concluded that tides result from a gravitational effects of the moon and sun. When solar systems are formed there is a tendency for the revolving of planets around a star, the revolving of satellites around planets and the rotation of all these bodies about their axis to be all in the same direction. In the case of the sun, moon earth trio this is approximately true. In fact the earth's axis of rotation is tipped a bit but not enough to change the following argument significantly.

The squeeze put on the earth by the moon and sun is applied near the equator. We already noted that this squeeze tends to pile water up around the equator so that there are high spots toward and away from the moon for example. There is another effect taking place though. If we look at any particular line of longitude, water is going to be pushed toward the poles by the squeeze at the equator. The fact that the water does not all accumulate at the poles is due to the rotation of the earth. The rotation tends to sling the water out to the maximum available radius, which is at the equator. The net effect of these opposing forces is that the tidal bulges tend to be highest near the middle latitudes.

FundyThe world's most spectacular tides occur in the Bay of Fundy between the state of Maine in the US and the province of Nova Scotia in Canada. This is at approximately 45 degrees north latitude so it meets the mid-latitude criteria but it is the shape of that particular bay that makes the most difference.

Try this. Draw eight inches of water into a bathtub and start sloshing it back and forth with your hand. If you time your pushes according to the time it takes a wave to return to your end of the tub you can build up some impressive changes in height of the water. The Bay of Fundy is a tub whose natural period is about 12 hours so every high tide gives it another push in resonance with it own sloshing tendency. A tidal range of over 40 feet is seen in this bay.