What Is a Tidal Range?
A tide refers to the rise and fall of sea levels due to the gravity from the sun and moon, as well as the rotation of the earth. A tidal range is the difference in the height of a high tide and its corresponding low tide. Tidal ranges are not fixed but vary depending on the location of the sun and the moon. Additionally, the greatest tidal ranges tend to occur during spring tides, when the gravitational forces of the sun and the moon are in alignment. Tidal ranges are also large during the first and last stages of a moon phase. During a new moon, the gravitational forces of the moon and the sun reinforce one another, while the two forces are opposed during a full moon. While coastal regions will receive higher than normal tidal ranges during these times, the highest tidal ranges are experienced when a spring tide coincides with the equinox.
Tidal Range Classifications
The average tidal range in an open ocean is approximately 0.6 m, with a global range between zero and 12 m. A tidal range typically becomes larger closer to a coast. Other factors that can determine a tidal range include the amount of water near the coast, as well the size and shape of the water basin. Tidal ranges are often greater in larger bodies of water, and the geography of a water basin can either funnel or disperse a tide, increasing and decreasing the tidal range, respectively.
One common misconception about the factors affecting tidal ranges is that they increase as one moves north from the equator. This erroneous theory stems from the fact that most areas with high tidal ranges are located in areas north of the equator.
Tidal ranges are typically grouped into three categories. Tidal ranges less than 2 m are classified as micromareal, those between 2 m and 4 m are classified as mesomareal, and tidal ranges greater than 4 m are classified as macromareal.
The World’s Largest Tidal Range
The world's largest tidal range is observed at Canada's Bay of Fundy, which can reach 16.3 m (53 feet). The Bay of Fundy is an example of how geography can increase a tidal range by funneling. The bay sits between the two Canadian provinces of New Brunswick and Nova Scotia, and the bay becomes narrower from the mouth of the bay to the inner shores. The tide at the Bay of Fundy is semidiurnal, meaning that there are two high tides and two low tides each day, spaced about 6 hours and 13 minutes apart. The difference between low tide and high tide can reach an astonishing 16.3 m with over 115 billion tonnes of water flowing in and out of the bay. The highest recorded tidal range was in 1869 where the water level rose 21.6 m (71 ft) during the Saxby Gale.
Large tidal ranges also occur in the United Kingdom, reaching up to 15 m at the Severn Estuary, which is between England and Wales. Parts of the United States that receive high tidal ranges of up to 12 m feet include Anchorage, Alaska.