What Causes Tides?

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What Causes Tides?
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If you’ve ever spent a day at the beach,  
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you’ve probably noticed that the  ocean water is constantly moving.
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Waves cause the ocean to move all day long.  
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And tides cause the ocean to  rise and fall twice each day.
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The water’s highest point is called high tide.
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The water’s lowest point is called low tide.
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Plants, animals, and humans living near the ocean  
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rely on this predictable rising  and falling of the ocean each day.
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But what exactly causes high tides and low tides?
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The moon! That’s right, the moon is the  main cause of tides in Earth’s oceans.
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The moon and Earth both have gravity — the force  
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by which a planet or other body  draws objects toward its center.
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The force of gravity keeps all of  the planets in orbit around the sun.  
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And Earth’s gravity keeps the  moon in orbit around Earth.
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The moon is a smaller object than Earth,  so its force of gravity is weaker.  
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But since the moon is so close, the pull  of its gravity is still significant.
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As the moon’s gravity pulls on  Earth’s water, the water bulges  
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in the direction of the moon. This is  due to something called the tidal force.
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Because of this tidal force, Earth’s  water on the side of the moon always  
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wants to bulge out toward the moon.  This bulge is what we call a high tide.
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As your part of the Earth rotates into this  bulge of water, you might experience a high tide.
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That explains the first high tide each day.  But what explains the second high tide?
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This is where things get a little  tricky. As we mentioned earlier,  
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when the moon’s gravity pulls on one side of  Earth, it creates a tidal bulge toward the moon.
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But the tidal force actually affects the  whole planet, not just one side of Earth.
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The total effect of the tidal force  is that Earth experiences a squeeze,  
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which causes the water to bulge on two sides.
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These two bulges explain why in one day  
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there are two high tides. Earth's surface  rotates through each of the bulges once a day.
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As Earth’s climate warms and oceans rise,  
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high tides are becoming higher. Sometimes, this  leads to flooding in communities near the coasts.
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High-tide flooding happens when tides are  approximately 2 feet above the average high tide.  
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This is when ocean water starts spilling onto  streets or bubbling up from storm drains.
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Scientists use information from many sources,  
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including NOAA’s GOES-R series weather  satellites, to study these high-tide floods.
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Coastal communities can use this information  
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to better plan for the effects  of rising tides in the future.
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Find out more about Earth’s  weather at NOAA SciJinks!