What are king tides?

King Tides are one of several threats to Vancouver's coasts during the winter months - what exactly are they?

A few astronomical factors come together for these big tides

In 2014, Highway 101 on B.C.'s Sunshine Coast was flooded due to the king tide blocking the culvert outlet. (Ministry of Transportation/Twitter)

It's that time of the year again for the South Coast, where winter storm surges and king tides can lead to flooding for low-lying coastal areas. 

King tide is a colloquial term for the highest tides. These kinds of tides happen due to extra gravitational forces on Earth's oceans that are produced by specific alignments of the sun and the moon.

Every two weeks, the moon and the sun align on the same side of our planet to produce gravitational forces on Earth that result in tides that are about 20 per cent higher than normal. This is known as a spring tide (although it has nothing to do with the season).

About three or four times a year, the moon will be at its closest approach to Earth on its elliptical orbit. This is called a perigean tide. 

Because the moon's orbit around the Earth is elliptical and lopsided, it's 50,000 kilometres closer to the Earth on one side of its orbit (the perigee) than the other (the apogee). (NASA)

If these two astronomical events line up, then you get a perigean spring tide — otherwise known as a king tide. In this situation, the sun and the moon line up on the same side of Earth, and the moon is also at its closest approach to Earth. 

This usually happens around late December or early January, a time when Vancouver also gets storms. When you factor in a storm surge, which is the rising waters that comes with low-pressure systems, plus the forward push of water thanks to the winds around the low-pressure systems — that's a lot of extra water.

And that's why there is so much concern when a winter storm lines up with King tides. While the king tides can be calculated ahead of time, predicting winter storms much farther out than a week is a different story.  

Global warming will continue to mean rising sea levels and a future for B.C. that will include more extreme weather events.  All the more reason for the South Coast to start preparing for bigger annual floods. 

About the Author

Johanna Wagstaffe

Senior Meteorologist

Johanna Wagstaffe is a senior meteorologist for CBC, covering weather and science stories, with a background in seismology and earth science. Her weekly segment, Science Smart, answers viewers' science-related questions.

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