What is a King Tide?

By |2018-09-20T14:24:02+00:00November 9th, 2017|

In the months of September, October and November, look out for the King Tide. A King Tide is a higher than usual high tide (and lower than usual low tide). It is strongest during the Full Moon and the New Moon of the fall months, and it typically brings flooding to coastal areas.

What causes the tides?

Tides are caused by the gravitational pull of the Moon on the Earth’s water. The Sun also exerts a pull, but it’s weaker than the Moon’s pull, because the Sun is much farther away. As the Earth rotates, the Moon pulls the water toward it, creating a “bump” of water in the part of the Earth closest to the Moon. Centrifugal force creates a slightly smaller “counter bump” on the opposite side of the Earth. These bumps appear to us as high tide coming in as our part of the Earth rotates into the gravitational pull. The tide “goes out” as our part of the Earth rotates out of the pull zone.

Why are some tides higher than others?

The lunar cycle, and the juxtaposition of the Earth, Moon, and Sun create a stronger gravitational pull on some days of the month than on others. During waxing and waning moons, the Moon and Sun are pulling at different angles to the Earth, and they cancel each other’s pull out somewhat. But when Sun, Earth and Moon are aligned, as they are at the full and new Moons, the tides are stronger because the Moon and Sun are pulling together. These stronger tides are called spring tides – not because of the season, but because the water “springs” forth. High tide is higher and low tide is lower.

What causes a King Tide?

The Moon orbits the Earth every 29 days or so in an elliptical path. When the Moon is farthest away from the Earth, it is said to be at its apogee, and its pull is weaker. When it comes closest to Earth, it is said to be at its perigee and its pull is stronger.

The Earth orbits the Sun every 365 days or so, also in an elliptical path. When the Earth is farthest from the Sun in July, it is at its aphelion. When it is closest to the Sun in January, it is at its perihelion.

When the Moon reaches its perigee, closest to the Earth reaching its perihelion, the Moon and Sun can pull even more strongly together, especially during the new and full Moons of those months. This typically happens in the autumn months.

The scientific names for these tides is perigean spring tides, but they are colloquially known as King Tides.

When the perigee and full moon fall exactly on the same date closest to the perihelion, you get very strong King Tides. In 2017 this is not the case, so our King Tide effect is somewhat lessened.

Other impacts

It’s not just the tidal effect that causes more or less water to come up, there are other factors:

  • Wind: Wind can either push water toward shore or away from the shore. Heavy wind can create storm surge.
  • Rain: Heavy rain can add to the volume of water in waterways, especially as rain falling over land gets channeled via storm drains into waterways.
  • Water temperature: warm water expands and takes up more space. Cold water contracts.
  • Air pressure: Water is compacted when air pressure is high; it expands when air pressure is low.
  • Ocean and other currents: Currents can bring water toward a shoreline or away from it.
  • Manmade causes: Construction and development can alter the landscape and overland water flows, affecting waterways.

Many factors influence tides. Each on its own can either be diminished or augmented by other factors. The more factors that are working together at the same time, the more coastal regions will experience flooding during strong tide cycles.