Mark Buxton of Palmerston North asks :-

Tides are caused by the moon and, to a lesser extent, the sun, so why does the height of the tides vary from place to place?

Craig Sloss, a physical geographer at Massey University, responded.

The moon has a gravitational pull which causes the water in the oceans to be pulled towards the moon. If the Earth were entirely water-covered two tidal bulges would travel around the globe each day, as the earth rotates relative to the moon. One bulge on the side facing the moon, the other on the opposite side (due to centrifugal force counteracting the gravitational pull). A complete tidal rotation of the tidal bulges around the globe takes approximately 24 hrs 50 min because each day the moon moves on a little in its monthly orbit around the Earth. In reality, it is only in the Southern Ocean that the two tidal bulges can pass relatively freely around the globe (they are restricted to some degree by southern South America and the Antarctic Peninsula). In most places the tidal bulges are restricted by the land masses.

In the open ocean the tidal range rarely exceeds 2.5 m and is usually less than 0.5 m but as water depth decreases near land the range often increases significantly. The variation in tidal range is determined by the size, depth and shape of the basins, continental shelves and nearshore zone. For example, in large gulfs or embayments the tide can resonate resulting in an amplification of tidal frequently and/or tidal amplitude. The tidal range may be relatively high around the margins of shallow semi-enclosed basins where the geometry of a large funnel-shaped coastal inlet causes the incoming tidal wave to converge and peak up towards the head of the inlet. This is because tides are essentially large waves which interact with the ocean floor, increasing in height when entering shallow water.

The largest tidal ranges are 15.4 m in the Bay of Fundy and 11-15 m in places in north-western Australia.