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If you have been watching the water level data from our *monitoring equipment in the river, you may have noticed that the water is not at the same height every day, or even every hour. The moon is primarily responsible for the continually changing water level in the Hudson River, known as the tides. (The sun also plays an important role, but to a lesser degree because of its greater distance from the earth). As the moon revolves around the spinning earth, the gravitational force of the moon pulls the water in the oceans into a bulge at the point closest to the moon. A second bulge is created on the side of the earth opposite the moon. As the bulge of water follows the moon, a predictable rise and fall of the water is created (high and low tide). Because high tide is following the moon’s track around earth, the tide cycle corresponds with our moon’s day (“lunar day”) which is slightly longer than our 24 hour day. That is why high and low tides do not happen at the same time every day. There are two high tides and two low tides in the Hudson River every lunar day.
At high tide, rising water from the Atlantic Ocean flows north into the Hudson River. Then, as the water level in the ocean begins to fall, the current in the Hudson switches direction and begins to flow south, back into the ocean until low tide is reached. This switching of the current between north and south is why the Hudson is known as “the river that flows both ways.”
You can see these tides in action using the online data from our monitoring equipment. Looking at the water level graph for any 24-hour period, you will see two peaks and two troughs corresponding to high tide and low tide, respectively.
You might also notice that the levels of the water at high tide and low tide are different from day to day. The height of the high tide on one day, February 12, 2006, for example, is higher than the high tide of another day in the same month, February 23, 2006. This pattern is caused by the sun. When the moon and the sun are aligned, their gravitational forces are combined to create more extreme tides (higher highs and lower lows). These tides, called spring tides, occur every month during the new and full moons.
During the first and last quarter moons, the sun and moon are at right angles to each other and their forces work in opposition, thereby reducing some of the moon’s pull. This creates less of a height change between high tide and low tide; these are called neap tides. The magnitude of the tides can also be affected by the weather. Falling atmospheric pressure (also known as barometric pressure or “BP” in our online chart) creates higher high tides or lower low tides. Strong winds and/or low barometric pressure which correspond with neap tides can create a blow-out tide, a low tide that exposes areas of the river bottom that are normally under water.
Want to see the tides for yourself? Visit Beczak Center to view our new time-lapse video of the tides filmed in our marsh by film maker Philip DeRise. |
