The rise and fall of the tides play an important role in the natural world and can have a marked effect on maritime-related activities. Tides are one of the most reliable phenomena in the world. As the sun rises in the east and the stars come out at night, we are confident that the ocean waters will regularly rise and fall along our shores. [Source: NOAA]
When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide. When the lowest point, or the trough, reaches a coast, the coast experiences a low tide. Tidal dates and times are made on the basis of observation over time rather than calculations based of the movement of the sun and the moon. Exceptionally large tidal surges can be caused by large storms or extreme changes in atmospheric pressure. NOS scientists advanced tidal recording systems as well as satellite imagery to monitor tides and water levels. These data are used to predict ocean behavior in order to protect our coasts and coastal communities.
Tides are very long-period waves that move through the ocean in response to the forces exerted by the moon and sun. Tides originate in the ocean and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest, of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range. Incoming tides are called flood tides and outgoing ones are called ebb tides.
Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org
Why Tides are Important
Tidal bulge According to the “Introduction to Physical Oceanography”: Tides have been so important for commerce and science for so many thousands of years that tides have entered our everyday language: time and tide wait for no one, the ebb and flow of events, a high-water mark, and turn the tide of battle.” They are important for many reasons, including: 1) Tides produce strong currents in many parts of the ocean. Tidal currents can have speeds of up to 5 meters per second in coastal waters, impeding navigation and mixing coastal waters. 2) Tidal currents generate internal waves over seamounts, continental slopes, and mid-ocean ridges. The waves dissipate tidal energy. Breaking internal waves and tidal currents are a major force driving oceanic mixing. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
3) Tidal mixing helps drive the deep circulation, and it influences climate and abrupt climate change. 4) Tidal currents can suspend bottom sediments, even in the deep ocean. 5) Tides influence the orbits of satellites. Accurate knowledge of tides is needed for computing the orbit of altimetric satellites and for correcting altimeter measurements of oceanic topography. 6) Tidal forces on other planets and stars are important for understanding many aspects of solar-system dynamics and even galactic dynamics. For example, the rotation rate of Mercury, Venus, and Io result from tidal forces.
Study results show that “tides are perhaps responsible for a large portion of the vertical mixing in the ocean” and mixing helps drive the abyssal circulation in the ocean. Among other things this that an understanding of the influence of the ocean on climate requires accurate knowledge of tides?
Forces That Create Tides
Ocean tides are caused by gravitational pulls of the moon and the sun. The pull of the sun is about half that of the moon because even though it larger it is much further from the earth than the moon.
Scientist don't completely understand all the mechanics of tides. "The attractive forces of the moon and the sun operate to pile up ocean waters in a wave directly beneath them, forming at the same time a similar wave on the opposite side of the world. These tidal waves have a wavelength of one half the circumference of the earth and as the earth rotates they try to follow."
Imagine the ocean is shaped like a football pointing at the moon. The football’s pointed ends represent the parts of the Earth experiencing high tide and the football’s flat sides are the parts of the earth experiencing low tide. The point facing the moon is formed because the gravitational pull of the moon is strongest on whichever side of the Earth faces it. Gravity pulls the ocean towards the moon and high tide occurs.
The bulge on the far side of the Earth is caused by inertia. The water moving away from the moon resists the gravitational forces that attempt to pull it in the opposite direction. Because the gravitational pull of the moon is weaker on the far side of the Earth, inertia wins, the ocean bulges out and high tide occurs. As the Earth spins, different areas of the planet face the moon, and this rotation causes the tides to cycle around the planet.
Frequency of Tides
While some places have one high tide and one low tide per day, most coastal locations have two high tides and two low tides a day. These highs and lows typically aren't equal. This is why, in most places, using the phrase "high tide" might be unclear. There's actually high tide and higher high tide (and low and lower low tide). The difference between these tides vary around the world.
Coastal areas experience two low tides and two high tides every lunar day, or 24 hours and 50 minutes. A lunar day is how long it takes for one point on the Earth to make one complete rotation and end up at the same point in relation to the moon. The reason that a lunar day is longer than a normal 24-hour day is because the moon rotates around the Earth in the same direction that the Earth is spinning. It takes the Earth an extra 50 minutes to “catch up” to the moon. If there were no geological features high tides would repeat about every 12 hours and 25 minutes but due to variation in ocean depth, coast lines topography and other features the tides vary a great deal.
The two tidal bulges caused by inertia and gravity will rotate around the Earth as the moons position changes. These bulges represent high tides while the flat sides indicate low tides. If the Earth were a perfect sphere without large continents, and if the earth-moon-sun system were in perfect alignment, every place would get two equal high and low tides every day. However, the alignment of the moon and sun relative to Earth, the presence of the continents, regional geography and features on the seafloor, among other factors, make tidal patterns more complex.
Around the world, there are three basic tidal patterns: semidiurnal, mixed, and diurnal. When both high tides are about equal to each other, and the low tides are also roughly equal, the pattern is called a semidiurnal tide. If the two highs and lows differ substantially, the pattern is called a mixed tide. Where there's only one high and one low tide a day, it's called a diurnal tide. One location can experience different tide patterns throughout the month.
Mean Higher High Water (MHHW) is the average of the higher of the two daily high tides. The term MHHW is more precise than “high tide,” and it helps specialists in the field communicate clearly about the tides they track. Problematic flooding can happen when water levels climb even a bit higher than normal variations in MHHW at a particular location. This is commonly called high tide flooding, and it can inundate roads, compromise stormwater systems, and damage coastal property.
The highest tides are caused in funnel-like bays like the Bay of Fundy, which separates New Brunswick from Nova Scotia in Canada. The highest tides in the United States can be found near Anchorage, Alaska, with tidal ranges up to 40 feet. [Source: NOAA]
Tidal highs and lows depend on a lot of different factors. The shape and geometry of a coastline play a major role, as do the locations of the Sun and Moon. Storm systems at sea and on land also shift large quantities of water around and affect the tides. Detailed forecasts are available for high and low tides in all sea ports, but are specific to local conditions.
That many of the areas of the world with high ranges of tides are in the areas of Alaska, Canada, and northern Europe has created a misconception that the range of tide increases with increasing latitude (as one moves farther from the equator and closer to the poles). This is incorrect.
Increased tidal ranges in these areas are created by the positions and configurations of the continents in the northern hemisphere. In the higher latitudes of the northern hemisphere, the continents of North America, Europe, and Asia are pressed closer together. This “constriction” of the oceans creates the effect of a higher range of tides.
In the higher latitudes of the southern hemisphere, in the southern tips of South America, southern Africa, Australia, and Antarctica, tidal ranges are not increased. In these areas the continents are not pressed closely together, there is not a “constriction” of the oceans, and the tidal ranges are not increased.
Higher-than-normal tides occur during the new moon spring tide when the sun and moon are on the same side of the earth, pulling together, or are lined up at opposites of the earth so that their pushing waves line up with the pulling waves. Lower-than-normal tides occur when the moon and sun are at right angles to one another when the moon is in its first quarter and third quarter. These are called neap tides.
There are many factors that cause the tides to be higher than what is "normally" seen from day to day. NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun. The NOAA seasonal bulletin tells you when you may experience higher than normal high tides where you live. NOAA also publish annual high tide flooding reports that present a broad outlook of what to expect for a given year in terms of high tide flooding, as well as a summary of high tide flooding events for the previous calendar year.
Tides are long-period waves that roll around the planet as the ocean is "pulled" back and forth by the gravitational pull of the moon and the sun as these bodies interact with the Earth in their monthly and yearly orbits. Higher than normal tides typically occur during a new or full moon and when the Moon is at its perigee, or during specific seasons around the country. During the new and full moons when the moon and earth or more or less in line and thus the gravitational pull of the moon and sun work together, producing large spring tides.
During full or new moons — which occur when the Earth, sun, and moon are nearly in alignment — average tidal ranges are slightly larger. This occurs twice each lunar month (about 29.5 days on average). The moon appears new (dark) when it is between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is ‘added’ to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are higher and low tides are lower than average. These are called 'spring tides.'
It's also important to note that other factors influence the height of the tide as well. Seasonal effects on mean water level and the tide, like higher water level due to the thermal expansion of warmer water, can sometimes mean that some of the highest tides of the year are not perigean spring tides.
Spring and Neap Tides
A spring tide — popularly known as a "King Tide" — refers to the 'springing forth' of the tide during new and full moon. A neap tide — seven days after a spring tide — refers to a period of moderate tides when the sun and moon are at right angles to each other. [Source: NOAA]
A spring tide is a common historical term that has nothing to do with the season of spring. Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long without regard to the season. Neap tides, which also occur twice a month, happen when the sun and moon are at right angles to each other.
During full or new moons — which occur when the Earth, sun, and moon are nearly in alignment — average tidal ranges are slightly larger. This occurs twice each month. The moon appears new (dark) when it is directly between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is "added" to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are a little higher and low tides are a little lower than average.
These are called spring tides, a common historical term that has nothing to do with the season of spring. Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long, without regard to the season.
Seven days after a spring tide, the sun and moon are at right angles to each other. When this happens, the bulge of the ocean caused by the sun partially cancels out the bulge of the ocean caused by the moon. This produces moderate tides known as neap tides, meaning that high tides are a little lower and low tides are a little higher than average. Neap tides occur during the first and third quarter moon, when the moon appears "half full."
Perigean Spring Tide
Typically between 6-8 times each year, the new or full moon coincides closely in time with the perigee of the moon — the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' High tides during perigean spring tides can be significantly higher than during other times of the year. [Source: NOAA]
In order to understand the phenomenon called a 'perigean spring tide,' you first have to know that the gravitational pull of the moon and the sun cause tides. Tides are actually long-period waves that roll around the planet as the ocean is 'pulled' back and forth as the moon and the sun interact with the Earth in their monthly and yearly orbits.
The next thing you need to know is that the moon follows an elliptical path around the Earth in its monthly orbit, and the Earth follows an elliptical path in its yearly orbit around the sun. This means that, at times, the moon and the sun are closer to Earth. At other times, they are farther away. What happens when the moon and the sun are close to the Earth? You guessed it: the gravitational pull they exert is stronger, resulting in slightly higher tides.
While both the moon and the sun influence tides, the moon plays a much larger role because it is so close to the Earth. Its gravitational pull is about twice as strong as that of the sun. Now consider these two cases: Once about every 28 days, the moon reaches a 'perigee,' its closest point of approach to the Earth. This is the point at which the gravitational pull of the moon is strongest. During these periods there will be an increase in the average range of tides. Conversely, about 14 days following the perigee, the moon reaches an ‘apogee’, its furthest point of approach to the Earth. This is the point at which the gravitation pull of the moon is weakest. During these periods there will be a decrease in the average range of tides.
The difference between perigean spring tides and spring tides that occur closer to the moon’s apogee are location dependent and significantly influenced by tidal range, but can be quite large. It is not uncommon for high tides during a perigean spring tide to be more than a foot higher than high tides during ‘apogean spring tides’. In places like Anchorage, Alaska, which has a tidal range over 30 feet, the difference between spring tides can be 3 feet or more at high tide!
Perigean Spring Tides and Coastal Flooding: Coastal flooding doesn't always occur whenever there is a perigean spring tide. However, perigean spring tides combined with seasonal changes in the tide and mean sea level may cause minor coastal flooding in some low-lying areas, often referred to as “high tide flooding” or “nuisance flooding”. Major coastal flooding typically occurs in response to strong onshore winds and barometric pressure changes from a coastal storm. If a storm strikes during a perigean spring tide, flooding could be significantly worse than it otherwise would have been. In some instances, perigean spring tides have coincided with a shift in offshore ocean circulation patterns and large scale shifts in wind that have resulted in unexpected coastal flooding. It is expected that occurrences of minor high tide flooding at the times of perigean spring tides will increase even more as sea level rises relative to the land. NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun.
A tidal wave is a regularly reoccurring shallow water wave caused by effects of the gravitational interactions between the Sun, Moon, and Earth on the ocean. The term "tidal wave" is often used to refer to tsunamis; however, this reference is incorrect as tsunamis have nothing to do with tides. [Source: NOAA]
The term tidal can refer to : 1) A tidal bore, a large movement of water formed by the funnelling of the incoming tide into a river or narrow bay; 2) storm surge, or tidal surge, which can cause waves that breach flood defences; 3) a tsunami, a series of water waves in a body of water caused by the displacement of a large volume of water; 4) a megatsunami, which is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis; and 5) the crest (physics) of a tide as it moves around the Earth. [Source: Wikipedia]
According to American Oceans: Tidal waves, in their most basic form, are waves that follow a tide and are moved by the wind. However, when talking about the more dangerous and more significant tidal waves, we find that they are usually formed as the result of an earthquake or extremely high speed winds. While some people use the phrases “tidal wave” and “tsunami” interchangeably, the two phenomena do have their differences. A tidal wave is formed by high winds or a seismic interruption (usually an earthquake), while a seismic interruption is the sole cause of a tsunami. Tidal waves are understandably very dangerous. When high winds and the seismic force of an earthquake combine, they can create a powerful disturbance on the surface of the ocean that can have devastating effects. The higher the wind speed and the stronger the earthquake, the more dangerous the tidal wave becomes. A wave officially becomes a “tidal wave” when it gathers enough water and momentum to crash into the coastline.
The intertidal zone is the area where the ocean meets the land between high and low tides. Intertidal zones exist anywhere the ocean meets the land, from steep, rocky ledges to long, sloping sandy beaches and mudflats that can extend for hundreds of meters. [Source: NOAA]
Four physical divisions, each with distinct characteristics and ecological differences, divide the intertidal zone. They are the: 1) Spray Zone: dampened by ocean spray and high waves and is submerged only during very high tides or severe storms. 2) High Intertidal Zone: floods during the peaks of daily high tides but remains dry for long stretches between high tides. It is inhabited by hardy sea life that can withstand pounding waves, such as barnacles, marine snails, mussels, limpets, shore crabs, and hermit crabs. 3) Middle Intertidal Zone: over which the tides ebb and flow twice a day, and which is inhabited by a greater variety of both plants and animals, including sea stars and anemones. Low Intertidal Zone: virtually always underwater except during the lowest of spring tides. Life is more abundant there because of the protection provided by the water.
Sea creatures arrange themselves vertically in the intertidal zone depending on their abilities to compete for space, avoid predators from above and below, and resist drying out. Residents of the higher intertidal zones can either close themselves up in their shells to remain moist and ward off predators, or are mobile enough to retreat to a submerged zone when the tide goes out. In the lower parts of the intertidal zone, many plants and animals attach themselves in place and are very sturdy, very flexible, or otherwise well suited to stand up to wave energy. Larger marine life, such as seals, sea lions, and fish, find foraging for food ideal at high tide in the intertidal zone, while a large variety of shorebirds, looking for their meals, stroll hungrily over the intertidal zone at low tide.
High Tide Flooding
“High tide flooding” describes increasingly common flooding conditions along the coast due to rising sea levels, sinking land, and the loss of natural barriers. As relative sea level rises, it no longer takes a strong storm or a hurricane to cause coastal flooding. High tide flooding occurs when sea level rise combines with local factors to push water levels above the normal high tide mark. Changes in prevailing winds, shifts in ocean currents, and strong tidal forces (which occur during full or new moon) can all cause high tide flooding, inundating streets even on sunny days. [Source: NOAA]
High tide flooding falls into three levels of severity: minor, moderate, and major. The classifications measure how much water levels exceed average high tide for that location. 1) Minor high tide flooding is when water levels reach 0.55 meters (1.8 feet) above average high tide. This minor flooding is mostly disruptive, causing stormwater backups and road closures. 2) Moderate high tide flooding is 0.85 meters (2.8 feet) above average high tide. This can cause more disruption and can damage homes and businesses. 3) Major flooding is flooding 1.20 meters (3.9 feet) above average high tide. Floods of this severity are quite destructive, may lead to evacuations, and often require repairs to infrastructure and property.
Because of rising seas, land subsidence, and the loss of natural barriers, high tide flooding is now twice as frequent in U.S. coastal communities as it was 20 years ago. Predictions from the latest interagency Sea Level Rise Technical Report show that high tide flooding will become more common and more severe over the coming decades. As sea levels continue to rise, conditions that cause minor and moderate high tide flooding today will cause moderate and major high tide flooding by 2050.
Image Sources: Wikimedia Commons; YouTube, NOAA
Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated March 2023