TIDES - Tides are another kind of wave motion in the ocean. Tides are a change in the ocean water level, typically reaching a high and low level twice a day usually occurring about six hours apart. The term for the change from low to high tide is called the "flood tide". The change from high tide to low tide is called the "ebb tide". Tides result from the pull of gravity; on the earth alone, between the earth and moon and between the earth and the sun. The gravitational pull of the sun on the earth is about 178 times stronger than the gravitational pull on the earth from the moon. However, because of the close proximity of the moon, when compared to the sun, the tidal pull by the moon is over twice that of the sun. The result of this tidal pull is a bulge in the ocean water almost in line with the position of the moon; one bulge toward the moon and one on the opposite side of the earth, away from the moon. When we observe the tides what we are actually seeing is the result of the earth rotating under this bulge. It is easy to understand why there should be a bulge of water, producing a high tide, on the side of the earth facing of the moon. But why is there a bulge on the opposite side as well? It is obviously not gravity that is doing it but rather, it is the difference in gravitational force across the earth that causes the bulge. This difference in gravitational force comes from the moon's pull at various points on the earth.

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Tides

JetStream, the National Weather Service Online Weather School

 

 

Tides are another kind of wave motion in the ocean.

Tides are a change in the ocean water level, typically reaching a high and low level twice a day usually occurring about six hours apart.

The term for the change from low to high tide is called the "flood tide".

The change from high tide to low tide is called the "ebb tide".

Tides result from the pull of gravity; on the earth alone, between the earth and moon and between the earth and the sun.

The gravitational pull of the sun on the earth is about 178 times stronger than the gravitational pull on the earth from the moon.

However, because of the close proximity of the moon, when compared to the sun, the tidal pull by the moon is over twice that of the sun.

The result of this tidal pull is a bulge in the ocean water almost in line with the position of the moon; one bulge toward the moon and one on the opposite side of the earth, away from the moon.

When we observe the tides what we are actually seeing is the result of the earth rotating under this bulge.

It is easy to understand why there should be a bulge of water, producing a high tide, on the side of the earth facing of the moon. But why is there a bulge on the opposite side as well?

It is obviously not gravity that is doing it but rather, it is the difference in gravitational force across the earth that causes the bulge. This difference in gravitational force comes from the moon's pull at various points on the earth.

Because the pull of gravity becomes stronger as the distance decreases between to object, the moon pulls a little harder at point "C" (closest point to the moon) than it does at point "O" (in the center of the earth), and the pull is weaker still at point "F" (farthest point from the moon).

Figure 2

If it were not for the earth's gravity, the planet would be pulled apart (figure 2).

Yet also because of the earth's gravity which pulls us toward the center of the planet we can, mathematically subtract the moon's pull at the center of the earth from the moon's pull at both point "C" and "F".

Figure 3

When this vector-based subtraction occurs, we are left with two smaller forces; one toward the moon and one on the opposite side point away from the moon (figure 3) producing two bulges.

As the earth makes one rotation in 24 hours, we pass under these areas where the tidal force pulls water away from the earth's surface and experience high tides.

Also, since the difference in gravitation force is constant across the earth, the bulge on both side of the earth is essentially the same.

Which explains why consecutive high tides are nearly the same height each time regardless whether the moon is overhead or on the opposite side of the earth.

The change in the water level with the daily tides from location to location results from many factors.

The oceans and shorelines have complex shapes and the depth, and configuration, of the sea floor varies considerably.

As a result, some locations only experience one high and low tide each day, called a diurnal tide.

Other locations experience two high and low tides daily, called a semi-diurnal tide.

Still, other sites have mixed tides, where the difference in successive high-water and low-water marks differ appreciably.

Another factor in the variation of tides is based on the orbit of the moon around the earth and the earth around the sun. Both orbits are not circles but ellipses.

The distance between the earth and moon can vary by up to 13,000 miles (31,000 km).

Since the tidal force increase with decreasing distance then tides will be higher than normal when the moon is at its closest point (called perigee) to the earth, approximately every 28 days.

Likewise, the earth's elliptical orbit also causes variations in the sun's pull on the tides as we move from the closest point to the farthest point (called apogee) over the course of a year.

And just to complicate things even more, the moon's orbit is inclined 5° to the earth's rotation.

So, the north/south orientations of the bulge also varies between the northern and southern hemisphere over this same 28-day orbital period.

Earth-Moon-Sun configuration for Spring tide

As the moon completes one orbit around the earth (about every 28 days), there are two times in each orbit when the earth, moon and sun are in line with each other and two times when the earth, moon and sun are at right angles.

When all three are in line (around full and new moons), the combined effect of the moon's and sun's pull on the earth's water is at its greatest resulting in the greatest ranges between high and low tide.

This called a "spring" tide (from the water springing or rising up).

Seven days after either full or new moon, the earth, moon and sun are at right angles to each other.

Earth-Moon-Sun configuration for Neap tide

At this time the pull of the moon and the pull of the sun partially cancel each other out. The resulting tide, called a "neap" tide, has the smallest range between high and low tide.

This graph indicates how the ocean level changes in height daily for May 2019 in Santa Barbara, CA. For this location there are two high and low tides daily with one ebb and flow greater than the other.

Example of the daily tide and differences between Spring and Neap tides at Santa Barbara, CA from May 2019.

The difference in sea-level height between each high and low tide changes daily depending upon the position of the Moon.

The greatest difference in height occurs around new and full moons; 6.27 ft. (1.91 m) and 7.18 ft. (2.19 m) respectively.

The least difference in height occurs at both first- and last-quarter moon phases; 4.72 ft. (1.44 m) and 3.16 ft. (0.96 m) respectively.

Local influences effect the actual timing of spring and neap tides and therefore they may not necessarily align precisely with the phases of the moon.

Also, this graph represents this location for this month and year only. High and low tides, and their timing, for every location world-wide are different day-by-day, month-by-month, and year-by-year.

Welcome to JetStream, the National Weather Service Online Weather School. This site is designed to help educators, emergency managers, or anyone interested in learning about weather and weather safety.

https://www.weather.gov/jetstream/tides

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STORM SURGE - Storm surge is the rise in seawater level caused solely by a storm. Storm tide is the total observed seawater level during a storm, which is the combination of storm surge and normal high tide. Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore. The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the local bathymetry. Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. Astronomical tides are caused by the gravitational pull of the sun and the moon and have their greatest effects on seawater level during new and full moons—when the sun, the moon, and the Earth are in alignment. As a result, the highest storm tides are often observed during storms that coincide with a new or full moon. Powerful winds aren’t the only deadly force during a hurricane. The greatest threat to life actually comes from the water – in the form of storm surge. Storm surge combined with waves can cause extensive damage. It can severely erode beaches and coastal highways. The pounding waves can take out boats and buildings.

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This example illustrates water level differences for storm surge,
 storm tide, and a normal (predicted) high tide as compared to sea level. 
Storm surge is the rise in seawater level caused solely by a storm.
 Storm tide is the total observed seawater level during a storm,
 which is the combination of storm surge and normal high tide

Storm Surge

What is storm surge?

Storm surge is the rise in seawater level caused solely by a storm.

The Ocean Today

Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. 

The surge is caused primarily by a storm’s winds pushing water onshore. 

The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the local bathymetry.

Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. 

Astronomical tides are caused by the gravitational pull of the sun and the moon and have their greatest effects on seawater level during new and full moons—when the sun, the moon, and the Earth are in alignment. 

As a result, the highest storm tides are often observed during storms that coincide with a new or full moon.

Powerful winds aren’t the only deadly force during a hurricane.

The greatest threat to life actually comes from the water – in the form of storm surge.

Storm surge is water from the ocean that is pushed toward the shore by the force of the winds swirling around the hurricane.

This advancing surge combines with the normal tides and can increase the water level by 30 feet or more.

Storm surge combined with waves can cause extensive damage. It can severely erode beaches and coastal highways. 

The pounding waves can take out boats and buildings.

As the waters move inland, rivers and lakes may be affected, and add to the rising flood levels.

While we can’t prevent storm surge, we do have a system that can warn us of the incoming threat.

As a hurricane develops over the open ocean, forecasters at the National Hurricane Center closely monitor its path to evaluate the risk of a coastal strike.

They use a computer model called SLOSH to predict storm surge heights. The model depends critically on the hurricane’s track, intensity, and size.

SLOSH uses water depths, land elevations, and barriers to the flow of water to compute surges as they move inland.

This data helps determine which areas may need to be evacuated.

When a hurricane slams our coast, it’s important to be aware of all the dangers. 

As a reminder, emergency managers want us to run from the water and hide from the wind. 

Don’t take unnecessary risks during a storm.  Conditions can change in the blink of an eye.

Storm surge is a dangerous event during a hurricane, where furious winds are driving deadly flows of water from our seas to our shores.

Fast Facts

Storm surge is the most deadly part of a hurricane or tropical storm.

Be prepared -- during a storm surge event stay tuned to NOAA Weather Radio or television station and listen carefully for any advisories or specific instructions from local officials.

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https://oceanservice.noaa.gov/facts/stormsurge-stormtide.html

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THE MOON'S EFFECT ON OCEAN TIDES - The gravitational pull of the Moon and the Sun makes the water in the oceans bulge, causing a continuous change between high and low tide. While both the Moon and the Sun influence the ocean tides, the Moon plays the biggest role because it is so much closer to our planet than the Sun. In fact, the tidal effect of the Moon on Earth is more than twice as strong as that of the Sun, even though the Sun's gravitational pull on Earth is around 178 times stronger than that of the Moon. The gravitational force of the Moon and the Sun pulls the water in the oceans upwards making the oceans bulge, which creates high tide in the areas of Earth facing the Moon and on the opposite side. At the same time, in other parts of the planet, the ocean water drains away to fill these bulges, creating low tides. However, the oceans' water is also constrained by the continents and varying ocean depths.

Moon phases visualized in real time, the past or the future. 

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Boats stranded at low tide in Devon, UK.

The Moon's Effect on Ocean Tides

Time and Date AS

The water level changes gradually.

The gravitational pull of the Moon and the Sun makes the water in the oceans bulge, causing a continuous change between high and low tide.

Illustration showing how the gravitational force of the Sun and Moon act together and create spring tides at New Moon and Full Moon.

While both the Moon and the Sun influence the ocean tides, the Moon plays the biggest role because it is so much closer to our planet than the Sun.

In fact, the tidal effect of the Moon on Earth is more than twice as strong as that of the Sun, even though the Sun's gravitational pull on Earth is around 178 times stronger than that of the Moon.

Oceans Are Pulled Up

The gravitational force of the Moon and the Sun pulls the water in the oceans upwards making the oceans bulge, which creates high tide in the areas of Earth facing the Moon and on the opposite side. (See illustration.)

At the same time, in other parts of the planet, the ocean water drains away to fill these bulges, creating low tides.

However, the oceans' water is also constrained by the continents and varying ocean depths.

As a result, the tides behave more like water sloshing around in an oddly shaped bathtub than in a smooth and even basin.

High and Low Nearly Twice a Day

Tides are one of the most reliable phenomena in the world, and we know that they move in and out around twice a day, but not exactly. So, why is that?

A day on Earth is the time it takes our planet to spin once around its own axis and return to the same point under the Sun. This is known as a solar day, and it lasts around 24 hours.

However, the time it takes Earth to reach the same position in relation to the Moon, takes, on average, 24 hours and 50 minutes, known as a lunar day.

The reason the lunar day is longer than a solar day is that the Moon revolves around Earth in the same direction as Earth rotates around its axis, so it takes Earth, on average, an additional 50 minutes to “catch up” to the Moon.

Because the tidal force of the Moon is more than twice as strong as the Sun's, the tides follow the lunar day, not the solar day.

It takes half a lunar day, on average 12 hours and 25 minutes, from one high tide to the next, so we have high and low tides nearly twice a day.

According to the National Ocean Service, there are some exceptions to the main rule of 2 tides every lunar day: there are a few places, for instance along the coastline of the Gulf of Mexico, where there is only 1 tide per day.

This is caused by the local shoreline topography, among other things. This tidal cycle is called a diurnal cycle, as opposed to the normal semidiurnal cycle, where diurnal means daily and semi means half.

Gradual Ebb and Flow

The change from low to high tide is known as flood tide, while the change from high to low tide is called ebb tide.

The technical term for the difference in water level between high tide and low tide is tidal range.

The flow and ebb are gradual, so it is not accurate to say that a high or low tide lasts around 6 hours and 12 minutes, i.e. a quarter of a lunar day.

The speed of the water flow varies during this period, and it also varies from place to place.

The Rule of 12ths

People who have to take the tides into account in their daily life, like sailors, fishermen, and surfers, often use what is called the rule of 12ths to calculate the expected water level.

This rule states that in the 1st hour after low tide the water level will rise by 1/12 of the predicted tidal range in any given area.

In the 2nd hour, it will rise 2/12, and in the 3rd hour, it will rise 3/12.

In the 4th hour, it will also rise 3/12, in the 5th, it will rise 2/12, and in the 6th hour, it will rise 1/12.

The sequence to remember is 1-2-3-3-2-1.

So, let’s say the predicted tidal range is 12 feet. In the 1st hour, the tide would rise 1 foot. In the 2nd hour, it would rise 2 feet. In the 3rd and 4th hours, it would rise 3 feet. In the 5th hour, the tide would rise 2 feet, and in the 6th hour, 1 foot.

Storm Tides and Surge

The astronomical forces which drive the tides can be predicted very accurately, and these predictions are published in local tidal tables.

However, different weather conditions also affect the sea level and may cause both lower and higher tides than expected.

If there is a storm, the seawater level often increases. This is called a storm tide and is caused by a combination of storm surge and normal tidal movement.

Strong offshore winds can move water away from coastlines, exaggerating the low tide, while onshore winds may cause the water to pile up onto the shoreline, making the low tide higher than normal.

High pressure weather systems can lead to days with exceptionally low tides, while low pressure systems may contribute to causing much higher tides than predicted.

Geography and Topography Important

The depth and shape of the ocean and the distance between continents are also important in determining the water level along the shores.

In the northern parts of North America, Europe, and Asia, the continents are close together, which creates a bigger difference between high and low tides than in areas farther south, where the continents are farther apart.

Average and Extreme Tides

The average tidal range in mid-ocean is around 1 meter or 3 feet. However, in some coastal areas, the tidal range can be more than 10 times higher in the most extreme areas.

To give an average for tidal range along the world's coastlines doesn't make much sense, as they vary so much from place to place.

The world's highest tide is in the Bay of Fundy in Canada, where the difference between low and high tide can be up to 16.3 meters (53.5 feet).

The highest tides in the US can reach 12.2 meters (40 feet) near Anchorage, Alaska. Along the coast of the UK, the tidal range varies from as little as 0.5 meters (1.6 feet) to a maximum of 15 meters (50 feet).

Spring Tides

The Moon’s phase also plays a part in the tidal range. The greatest difference between high and low tide is around New Moon and Full Moon.

During these Moon phases, the solar tide coincides with the lunar tide because the Sun and the Moon are aligned with Earth, and their gravitational forces combine to pull the ocean’s water in the same direction.

These tides are known as spring tides or king tides. The name has nothing to do with the season spring, but rather it is a synonym for jump or leap.

If a spring tide coincides with either the March equinox or the September equinox, it is called an equinoctial spring tide.

At these times we can expect the largest tidal range of the year because, at the equinoxes, the Moon and Sun are aligned with the equator.

Several times a year, the Full Moon or New Moon happens as the Moon is around its closest point to Earth, called perigee.

This is popularly known as a Supermoon and leads to even larger variation between high and low tides, known as perigean spring tides.

However, the difference from a normal spring tide is only around 5 cm or 2 inches.

The opposite happens when the Full or New Moon is around its farthest from Earth, apogee, also known as Micromoon.

The apogean spring tides are around 5 cm (2 inches) smaller than regular spring tides.

The tidal range is smallest around the 2 Quarter Moons (Half Moons) because the gravitational force from the Moon and the Sun counteract each other at these 2 points of the lunar month.

These tides are called neap tides or neaps, from Anglo-Saxon, meaning without the power. Neaps always occur about 7 days after spring tides.

Oceans and Some Rivers

There is a difference between having noticeable tides and having true tides. For tides to be noticeable, the body of water has to be huge, like an ocean.

Even though true tides also occur in smaller water basins, like big lakes, the tidal variations here are too small to notice.

For example, in the Great Lakes in the US, the largest tidal range is less than 5 cm or just under 2 inches.

Different weather conditions, such as wind and barometric pressure, creates bigger differences in the water level than tides on these lakes.

This is also the case in the Baltic Sea, the Black Sea, the Caspian Sea, and even the Mediterranean.

Many rivers connecting to the ocean do have high and low tides. In some of these tidal rivers, the water drains away almost entirely at low tide, making it possible to walk across the bottom of the river.

If a part of a larger river is affected by the tides, the section affected is known as tidal reach.

In a few areas, where the tide comes into a narrow bay or river, tidal bores can form. Created by the incoming tide, tidal bores are waves which travel against the direction of the water current.

Tides in the Human Body?

Many people believe that the Moon’s gravitational force also affects humans, as our bodies are made up of approximately 70% fluid.

However, there is no scientific evidence supporting this belief. The amount of liquid in a human body is far from big enough to experience tides.

Time and Date AS is a fast growing company based in Stavanger, Norway. We are always looking for good candidates to join our team.

https://www.timeanddate.com/astronomy/moon/tides.html

The oceans bulge. (Not to scale.)