When a tropical storm or hurricane forces large amounts of ocean water on shore, causing inland flooding for coastal communities, this is known as a storm surge. But did you know that the moon can affect how strong this storm surge can become?

Storm surge happens when strong winds push water onto the land. The higher the ocean is to begin with, the higher and farther inland this storm surge can go. And sea level is also affected by the gravitational pull of the moon, in what we know as tides.

(MORE: Hurricane terms and definitions you need to know)

So, here’s an explainer on what conditions of the moon are good and bad omens for coastal communities bracing for storm surge in the midst of a hurricane.

Tides and the daily cycle

To start, here’s how tides work.

On the sides of the Earth closest to and farthest from the moon, the ocean’s waters bulge out, resulting in higher water levels and therefore high tides. 

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Twice a day, as the Earth rotates, different parts of the ocean pass through these high tide spots — and then the low tide areas in between — every 12 hours. 

This is the basic cycle our oceans constantly go through every day, but there are certain states of the moon that can also change these tides, sea levels and, accordingly, the height of storm surge.

Lunar phases and the monthly cycle

About once a month — specifically 29.5 days — the moon goes through a cycle as it orbits around the Earth, which changes how lit up by the sun it appears in our sky.

Twice a month, when the moon is either in its full or new phases, it is in alignment with the sun. The sun also exerts gravity on Earth and affects sea levels — at a little less than half of the moon’s tide-generating force.

(MORE: How the moon could increase Florence's storm surge damage)

But when the two are aligned, that force combines, resulting in especially high tides called spring tides. During the first and last quarter of the cycle, the moon is perpendicular to the sun, and the especially low tides are called neap tides.

Perigee and supermoons

The moon’s orbit around the Earth isn’t a perfect circle; it’s elliptical. This means that at times it’s closer and farther from Earth during another “about monthly” cycle.

When the moon is closest, known as perigee, its gravitational pull is stronger and tides are higher. At its farthest, apogee, they’re lower.

Sometimes, the moon’s full or new phases coincide with perigee, combining those gravitational and tide-rising forces. 

We call this a supermoon because this can make it appear especially big in the sky — which is more noticeable to us when it’s full and lit, but equally true in its new phase.

The moon’s wobble

While the rest of these cycles are daily or monthly, this next cycle is quite different, happening over an 18.6-year period, which is called the lunar nodal precession. 

This has to do with the moon’s tilt. The moon doesn't travel around Earth on the same plane that Earth travels around the sun — its orbit is tipped by about 5 degrees.

And that tilt doesn't stay pointed in one direction. It slowly rotates, taking 18.6 years to come all the way back around.

As it turns, it changes how far north and south of the equator the moon swings each month — and that swing is what stretches or flattens the range between high tide and low tide.

During half of the moon’s 18.6-year cycle, Earth’s regular daily tides are amplified — high tides are higher than normal, and low tides are lower than normal. And in the other half, it’s reversed.

Why the 2030s could be a problem

This 18.6-year wobble cycle is completely normal. But what isn’t completely normal and is a rather newer development is climate change’s effects on rising sea levels.

During the half of the cycle when tides are lower than normal, this counteracts the effect of sea level rise on high tides. But when tides are higher, this only increases the effect.

The next time this cycle hits the tide amplification period will be in the mid-2030s. And between now and then, sea levels will almost certainly have risen even more.

(MORE: What does a 100-year flood actually mean? It's not what you think.)

Scientists warn this will cause coastal cities all around the U.S. to begin a decade of dramatic increases in flood numbers. High tides will exceed known flooding thresholds around the country more often, according to NASA’s Sea Level Change Science Team.

This period of a rise in sea level and tides will reflect in storm surge heights as well.

Adding it all up: When does the moon impact storm surge the most?

So, what does this all mean? What are the moon conditions that could affect you most — producing the highest sea level, tides and storm surge? 

That’s a new or full moon at perigee — a supermoon. And timing-wise, during the spring tide part of the month at the peak of the 18.6-year wobble cycle, this would be further amplified.

If a storm is coming through and the moon is in its first or last quarter phase at apogee, that’ll be the best conditions to put coastal communities in a better spot for a lower and less damaging storm surge.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.