What Causes Tides?

Tides, which are rising and falling sea levels, are caused by the effects of gravitational forces of the moon and sun combined with the Earth's rotation. Tidal magnitudes are affected by shoreline shape, the shape of bodies of water and environmental factors.

While the main factors contributing to the size of tides are the relative distances and positions of the sun, moon and Earth, other factors also affect tides. One such factor is the shape of the shoreline. Near shores bounded by wide continental margins, the height of the tides can be magnified. The opposite effect is experienced by mid-oceanic islands not near continental margins where tides are very small.

The shape of bays and estuaries also contribute to tidal magnitudes. For example, funnel shaped bays, such as the Bay of Fundy in Nova Scotia, may cause extremely high tides. Narrow inlets and shallow water may cause barely noticeable tides. In estuaries where there are strong tidal rivers, seasonal river flows in the spring can severely alter or hide incoming tides.

Environmental factors such as local wind and weather patterns affect tides as well. Strong offshore winds may move water away from the shoreline, exaggerating a low tide. In contrast, onshore winds may pile water onshore, virtually eliminating low tides. In addition, high and low pressure weather systems contribute to tidal effect.