What causes tides

Tides are the periodic rise and fall of sea level, once or twice a day, caused mainly by the sun and moon. Rises caused by winds and air pressure are called surges; unlike tides, they are not regular.

What causes tides:

• Mainly the moon's gravitational pull, and to a lesser extent the sun's.
• The centrifugal force, which acts to balance gravity.
• Together they create two tidal bulges. On the side facing the moon, the moon's pull is greater than the centrifugal force, so water bulges towards the moon. On the opposite side, the moon's pull is weaker (farther away) and the centrifugal force dominates, so water bulges away from the moon.
• The tide-generating force is the difference between these two forces; on the surface, its horizontal parts matter more than the vertical ones.

Shape of the coast matters:

• Tidal bulges are higher over wide continental shelves and lower at mid-ocean islands.
• Funnel-shaped bays and estuaries magnify tides greatly.
• Tides channelled between islands or into bays and estuaries are tidal currents.
• The highest tides in the world are in the Bay of Fundy, Canada, where the tidal bulge is 15-16 m. With two highs and two lows a day, the tide comes in over about six hours: roughly 2.4 m an hour. Someone walking along a cliff-backed beach there must watch the tide closely.

Example (new): in the Gulf of Khambhat, which narrows like a funnel, the tide rises far more than on the open coast nearby, because the incoming water is squeezed into a shrinking space.