Ever notice the ocean suddenly creep up the shore, and then pull back hours later? About twice a day, the coast experiences two high tides and two low tides. But what makes these waves?
Well, you can look up at the night sky and find your answer.
What causes high and low tides?
Gravity, from both the moon and Earth, is constantly pulling on everything around them. But when you take into account that Earth is significantly bigger than the moon, it’s safe to say that Earth’s gravitational pull is stronger than the moon’s.
But since the moon is relatively close to Earth, its– albeit weaker– gravitational pull still has an impact.
The moon’s gravity pulls Earth’s water closer, essentially squeezing the Earth, creating what’s known as a tidal force. This “squeeze” creates two water bulges– these are the high tides.
As Earth spins (making its full rotation in just under 24 hours), wherever you are on Earth, you’ll pass through both of these bulges, which is why there are two high tides in a day.
When your region of Earth is not in either of the “high tide” regions, you can expect to see low tides.
But certain times of the year, there are what’s commonly known as “King Tides,” that can turn the usual rise and fall of the ocean into water levels spanning more than 10 feet high.
King tides prompt safety warnings at Cannon Beach: 20-foot-waves ‘spectacular’ but deadly – Cannon Beach Fire photo
What causes King Tides?
King Tides, formally known as Perigean-Spring Tides, are exceptionally high tides. These usually occur about three times a year, typically in the winter months, and are caused by two main factors.
1. The Lunar Phase
About once a month, the moon shifts through eight different phases, but for now, we’re just going to focus on the new moon and the full moon.
The new moon occurs when the moon is aligned between the Earth and the sun, and appears invisible or very dark, as the illuminated side of the moon is facing away from Earth.
Skip ahead about two weeks, and the moon will be on the other side of the Earth, further away from the sun. This is the full moon, and unlike the new moon, the entire face of the moon is illuminated by the sun.
But still, the full moon is in line with both Earth and the sun.
Remember how I said the moon’s gravitational force pulls Earth’s water closer, creating high tides? The sun actually has a similar effect, just weaker.
The moon’s gravitational pull is about twice as strong as the sun’s, since it’s much closer to Earth.
When the Earth, sun, and moon are aligned (during the new moon and full moon), the sun and moon work together to make the high tides just a bit higher and the low tides just a bit lower. These are called spring tides.
Full winters moon in Sealrock Oregon – Caitlynn Driskell.jpg
2. Perigee
The second factor for a King Tide is Perigee, which is when the moon is closest to the Earth (think “p” for “proximity”). Since the moon is closer, its pull grows stronger. This occurs about every 28 days, and also boosts the higher high tides and lower low tides.
When both of these criteria are met, meaning either a full or new moon at the same time as perigee, we have Perigean-Spring Tides, or King Tides.
These very high tides can cause flooding, debris flow, and lingering sneaker waves.
The next bout of King Tides is expected between Dec. 4-6, 2025.
Tap here to learn about tips for safely exploring King Tides.