remiconnesson/oceanservice-noaa-facts huggingface dataset:
# How frequent are tides?

## Coastal areas experience **two low tides and two high tides** every lunar day, or 24 hours and 50 minutes.

![description of image](tidefrequency.jpg "short caption")

The two tidal bulges caused by inertia and gravity will rotate around the Earth as the moon's position changes. These bulges represent high tides while the flat sides indicate low tides.

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.

[Tides](/facts/tides.html) are very long waves that move across the oceans. They are caused by the gravitational forces exerted on the earth by the moon, and to a lesser extent, the sun. 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.

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.

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.
# What is a current?

You know about ocean tides, but how much do you know about ocean currents? Watch our three-minute video podcast to learn what puts the motion in the ocean.

Ocean currents are driven by wind, water density differences, and tides.

Oceanic currents describe the movement of water from one location to another. Currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). Oceanic currents are driven by three main factors:

**1\. The rise and fall of the tides.** Tides create a current in the oceans, which are strongest near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents change in a very regular pattern and can be predicted for future dates. In some locations, strong tidal currents can travel at speeds of eight knots or more.

**2\. Wind.** Winds drive currents that are at or near the ocean's surface. Near coastal areas winds tend to drive currents on a localized scale and can result in phenomena like coastal [upwelling](/facts/upwelling.html). On a more global scale, in the open ocean, winds drive currents that circulate water for thousands of miles throughout the ocean basins.

**3\. Thermohaline circulation.** This is a process driven by density differences in water due to temperature (thermo) and salinity (haline) variations in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents.

Currents affect the Earth's climate by driving warm water from the Equator and cold water from the poles around the Earth. The warm [Gulf Stream](gulfstreamspeed.html), for instance, brings milder winter weather to Bergen, Norway, than to New York, much further south.
# How fast is the Gulf Stream?

## The Gulf Stream has an **average speed of 6.4 kilometers per hour** (four miles per hour).

![Gulf Stream Current](gulfstreamspeed.jpg "Gulf Stream Current")

The Gulf Stream Current — the main conveyor of heat from south to north in the Atlantic — swirls surface waters in this infrared image from the Suomi NPP satellite on April 16, 2013, centered around 290 kilometers (180 miles) due east of Atlantic City, New Jersey.

The Gulf Stream is an intense, warm ocean current in the western North Atlantic Ocean. It moves north along the coast of Florida and then turns eastward off of North Carolina, flowing northeast across the Atlantic.

The velocity of the Gulf Stream current is fastest near the surface, with the maximum speed typically about nine kilometers per hour (5.6 miles per hour). The average speed of the Gulf Stream, however, is four miles per hour (6.4 kilometers per hour). The current slows to a speed of about 1.6 kilometers per hour (one mile per hour) as it widens to the north. The Gulf Stream transports an amount of water greater than that carried by all of the world's rivers combined.
# Where can I get tide predictions?

## View **tidal predictions** on the Center for Operational Oceanographic Products and Services Website.

![Tide predictions for Santa Monica, CA, as generated through the NOAA CO-OPS Tides and Currents website.](tidepredictions.jpg "The tide predictions for Santa Monica, CA, as generated through the NOAA CO-OPS Tides and Currents website.")

Tide predictions are available for locations on the U.S. coastline, most of the islands in the Caribbean, and many islands in the Pacific Ocean. The image above shows tide predictions for Santa Monica, CA, as generated through the NOAA CO-OPS Tides and Currents website.

The Center for Operational Oceanographic Products and Services (CO-OPS) is primarily responsible for predicting and measuring water levels and currents and disseminating this information. CO-OPS collects, analyzes, and distributes such data to maintain safe maritime navigation and waterborne commerce.

Tide predictions may be generated through the [NOAA Tide Predictions](https://tidesandcurrents.noaa.gov/tide_predictions.html) service of the CO-OPS Tides & Currents website. These predictions are available for locations on the U.S. coastline, most of the islands in the Caribbean, and many islands in the Pacific Ocean. Tide predictions for these locations can be generated for up to two years from the current year – two years in the past to two years into the future. This service offers a wide variety of capabilities and options, allowing customers to generate tide predictions specific to their needs.

Tide predictions are also published annually in a series of books; these Tide Tables are are produced by CO-OPS and printed/distributed through several commercial publishers.  A list of these publishers, with contact information, are available from the [Frequently Asked Questions](https://tidesandcurrents.noaa.gov/faq.html) pages of the CO-OPS website.
# What is a rogue wave?

## A 'rogue wave' is **large**, **unexpected**, and **dangerous**.

![rogue wave](roguewave.jpg "the blue ocean")

A rogue wave estimated at 18.3 meters (60 feet) in the Gulf Stream off of Charleston, South Carolina. At the time, surface winds were light at 15 knots. The wave was moving away from the ship after crashing into it moments before this photo was captured.

Rogue, freak, or killer waves have been part of marine folklore for centuries, but have only been accepted as real by scientists over the past few decades.

Rogues, called 'extreme storm waves' by scientists, are those waves which are greater than twice the size of surrounding waves, are very unpredictable, and often come unexpectedly from directions other than prevailing wind and waves.

Most reports of extreme storm waves say they look like "walls of water." They are often steep-sided with unusually deep troughs.

Since these waves are uncommon, measurements and analysis of this phenomenon is extremely rare. Exactly how and when rogue waves form is still under investigation, but there are several known causes:

**Constructive interference**. Extreme waves often form because swells, while traveling across the ocean, do so at different speeds and directions. As these swells pass through one another, their crests, troughs, and lengths sometimes coincide and reinforce each other. This process can form unusually large, towering waves that quickly disappear. If the swells are travelling in the same direction, these mountainous waves may last for several minutes before subsiding.

**Focusing of wave energy**. When waves formed by a storm develop in a water current against the normal wave direction, an interaction can take place which results in a shortening of the wave frequency. This can cause the waves to dynamically join together, forming very big 'rogue' waves. The currents where these are sometimes seen are the [Gulf Stream](https://ocean.weather.gov/gulf_stream.php) and [Agulhas current](http://www.aoml.noaa.gov/phod/trinanes/AGULHAS/agulhas.html). Extreme waves developed in this fashion tend to be longer lived.
# Where is the highest tide?

![Hopewell Rocks, NB, in the Bay of Fundy](/facts/highesttide.jpg "Where is the highest tide?")

_The highest tides in the world can be found in Canada's Bay of Fundy at Burntcoat Head in Nova Scotia. Image credit: Shawn M. Kent_.

The highest tides in the world can be found in [Canada at the Bay of Fundy,](https://tidesandcurrents.noaa.gov/faq.html#08) which separates New Brunswick from Nova Scotia.

The highest tides in the United States can be found near Anchorage, Alaska, with tidal ranges that average around 30 feet.

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.

Did you know? - *Of the top [50 locations in the world](https://tidesandcurrents.noaa.gov/faq.html#08) with the largest tidal range (average difference between high and low tide), only two locations are within the U.S.:*

- *On the U.S. East Coast, the largest tidal range occurs in Maine. The areas near Eastport and Cobscook Bay, Maine, experience tidal ranges of about [20 feet](https://oceanservice.noaa.gov/). Many areas north of Cape Cod have tidal ranges of [8-15 feet](https://oceanservice.noaa.gov/). South of Cape Cod to the Florida Keys, the normal tidal range is between [5-8 feet](https://oceanservice.noaa.gov/)*.

- [Center for Operational Oceanographic Products and Services](https://tidesandcurrents.noaa.gov/)

- [Tides and Water Levels Education](https://oceanservice.noaa.gov/education/tutorial_tides/welcome.html)

-
# What's the difference between a tide and a current?

## Tides go **up and down**; currents move **left and right**.

![Pictured: Lowtide in Islesboro, Maine.](tides-currents.jpg "Pictured: Lowtide in Islesboro, Maine.")

Tides create a current in the oceans, near the shore, and in bays and estuaries along the coast. Pictured: Lowtide in Islesboro, Maine.

Tides are driven by the gravitational force of the moon and sun. Tides are characterized by water moving up and down over a long period of time.

When used in association with water, the term "current" describes the motion of the water. Oceanic currents are driven by several factors. One is the rise and fall of the tides. Tides create a current in the oceans, near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents are the only type of currents that change in a very regular pattern and can be predicted for future dates.

A second factor that drives ocean currents is wind. Winds drive currents that are at or near the ocean's surface. These currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). Winds drive currents near coastal areas on a localized scale and in the open ocean on a global scale.

A third factor that drives currents is thermohaline circulation - a process driven by density differences in water due to temperature (thermo) and salinity (haline) in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents.
# How does sea ice affect global climate?

![sea ice in the Arctic Ocean](/facts/sea-ice.jpg "Arctic Ocean sea ice")

_Sea ice in the Arctic Ocean. While sea ice exists primarily in the polar regions, it influences the global climate._

##### **Did you know?**

Sea ice also affects the movement of ocean waters. The ocean is [salty](/facts/whysalty.html) and when sea ice forms, much of the salt is pushed into the ocean water below the ice, although some salt may become trapped in small pockets between ice crystals. Water below sea ice has a higher concentration of salt and is denser than the surrounding ocean water, so it sinks and moves from the surface. In this way, sea ice contributes to the circulation of the [global ocean conveyor belt](/facts/conveyor.html). Cold, dense [polar water](/facts/thermocline.html) descends from the surface and circulates along the ocean bottom toward the equator, while warm water from mid-depth to the surface travels from the equator toward the poles.

Sea ice is [frozen water](/facts/cryosphere.html) that forms, expands, and melts in the ocean. It is different from icebergs, glaciers, ice sheets, and ice shelves, which originate on land. For the most part, sea ice expands during winter months and melts during summer months, but in certain regions, some sea ice remains year-round. About 15 percent of the world's oceans are covered by sea ice during part of the year.

While sea ice exists primarily in the polar regions, it influences the global climate. While sea ice exists because the polar regions are cold, the brightness of the sea ice acts to reinforce the coldness.

When warming temperatures gradually melt sea ice over time, fewer bright surfaces are available to reflect sunlight back into the atmosphere. More solar energy is absorbed at the surface and ocean temperatures rise. This begins a cycle of warming and melting. Warmer water temperatures delay ice growth in the fall and winter, and the ice melts faster the following spring, exposing dark ocean waters for a longer period the following summer.

Changes in the amount of sea ice can disrupt normal ocean circulation, thereby leading to changes in global climate. Even a small increase in temperature can lead to greater warming over time, making the polar regions the most sensitive areas to climate change on Earth.

Did you know? - Sea ice also affects the movement of ocean waters. The ocean is [salty](https://oceanservice.noaa.gov/facts/whysalty.html) and when sea ice forms, much of the salt is pushed into the ocean water below the ice, although some salt may become trapped in small pockets between ice crystals. Water below sea ice has a higher concentration of salt and is denser than the surrounding ocean water, so it sinks and moves from the surface. In this way, sea ice contributes to the circulation of the [global ocean conveyor belt](https://oceanservice.noaa.gov/facts/conveyor.html). Cold, dense [polar water](https://oceanservice.noaa.gov/facts/thermocline.html) descends from the surface and circulates along the ocean bottom toward the equator, while warm water from mid-depth to the surface travels from the equator toward the poles.
# What is the Bermuda Triangle?

## The Bermuda Triangle is a region in the western part of the North Atlantic Ocean in which ships, planes, and people are alleged to have mysteriously vanished.

![Bermuda Triangle map](/facts/bermuda-triangle.jpg "Bermuda Triangle")

This map shows the general location of the infamous Bermuda Triangle.

For decades, the Atlantic Ocean’s fabled Bermuda Triangle has captured the human imagination with unexplained disappearances of ships, planes, and people.

Some speculate that unknown and mysterious forces account for the unexplained disappearances, such as extraterrestrials capturing humans for study; the influence of the lost continent of [Atlantis](/facts/atlantis.html); vortices that suck objects into other dimensions; and other whimsical ideas.  Some explanations are more grounded in science, if not in evidence.  These include oceanic flatulence (methane gas erupting from ocean sediments) and disruptions in geomagnetic lines of flux.

Environmental considerations could explain many, if not most, of the disappearances.  The majority of Atlantic tropical storms and hurricanes pass through the Bermuda Triangle, and in the days prior to improved weather forecasting, these dangerous storms claimed many ships.  Also, the [Gulf Stream](/facts/gulfstreamspeed.html) can cause rapid, sometimes violent, changes in weather.  Additionally, the large number of islands in the Caribbean Sea creates many areas of shallow water that can be treacherous to ship navigation. And there is some evidence to suggest that the Bermuda Triangle is a place where a “magnetic” compass sometimes points towards “true” north, as opposed to “magnetic” north. 

The U.S. Navy and U.S. Coast Guard contend that there are no supernatural explanations for disasters at sea.  Their experience suggests that the combined forces of nature and human fallibility outdo even the most incredulous science fiction. They add that no official maps exist that delineate the boundaries of the Bermuda Triangle. The U. S. Board of Geographic Names does not recognize the Bermuda Triangle as an official name and does not maintain an official file on the area.

The ocean has always been a mysterious place to humans, and when foul weather or poor navigation is involved, it can be a very deadly place.  This is true all over the world.  There is no evidence that mysterious disappearances occur with any greater frequency in the Bermuda Triangle than in any other large, well-traveled area of the ocean. 

**_This text of this Ocean Fact was posted on January 4, 2010, and has not been altered since then._**
# What is bombogenesis?

[![Environmental Visualization Laboratory visualization of wind speed data](/facts/bombogenesis.jpg "Fourteen of 20 hurricane-force wind events underwent bombogenesis in the North Atlantic during the first two months of 2014.")](https://www.nesdis.noaa.gov/?MediaID=1515&MediaTypeID=1)

_Fourteen of 20 hurricane-force wind events underwent bombogenesis in the North Atlantic during the first two months of 2014. This unusual activity can be seen in wind speed data from the period. In this image, blues indicate areas with wind speeds that are faster than the 30-year historical average (1981-2010)._

Bombogenesis, a term used by meteorologists, occurs when a midlatitude (the latitudes between the tropics and polar regions) [cyclone](/facts/cyclone.html) rapidly intensifies, or strengthens, over a 24 hour period. This intensification is represented by a drop in millibars, a measurement of pressure used in meteorology. The intensification required to classify as "bombogenesis" varies by latitude. At 60 degrees latitude, it is a drop of at least 24 millibars (24 hectopascals) over 24 hours. At the latitude of New York City, the required pressure drop is about 17.8 millibars (17.8 hectopascals) over 24 hours.

Bombogenesis can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. It is popularly referred to as a bomb cyclone.
# What is a waterspout?

## A waterspout is a whirling column of air and water mist.

![waterspout](/facts/waterspout.jpg "short caption")

According to NOAA's National Weather Service, the best way to avoid a waterspout is to move at a 90-degree angle to its apparent movement. Never move closer to investigate a waterspout. Some can be just as dangerous as tornadoes.

Waterspouts fall into two categories: fair weather waterspouts and tornadic waterspouts.

Tornadic waterspouts are tornadoes that form over water, or move from land to water. They have the same characteristics as a land tornado. They are associated with severe thunderstorms, and are often accompanied by high winds and seas, large hail, and frequent dangerous lightning.

Fair weather waterspouts usually form along the dark flat base of a line of developing cumulus clouds. This type of waterspout is generally not associated with thunderstorms. While tornadic waterspouts develop downward in a thunderstorm, a fair weather waterspout develops on the surface of the water and works its way upward. By the time the funnel is visible, a fair weather waterspout is near maturity. Fair weather waterspouts form in light wind conditions so they normally move very little.

If a waterspout moves onshore, the National Weather Service issues a tornado warning, as some of them can cause significant damage and injuries to people. Typically, fair weather waterspouts dissipate rapidly when they make landfall, and rarely penetrate far inland.
# Where is all of the Earth's water?

## The ocean holds about 97 percent of the Earth's water; the remaining three percent is found in glaciers and ice, below the ground, in rivers and lakes.

![The ocean](/facts/oceanwater.jpg "the ocean")

Of the world's total water supply of about 332 million cubic miles of water, about 97 percent is found in the ocean.

The ocean holds about 97 percent of the Earth's water; the remaining three percent is [distributed in many different places](http://water.usgs.gov/edu/earthwherewater.html), including glaciers and ice, below the ground, in rivers and lakes, and in the atmosphere.

According to the [U.S. Geological Survey](https://waterdata.usgs.gov/nwis/rt), there are over 332,519,000 cubic miles of water on the planet. A cubic mile is the volume of a cube measuring one mile on each side. Of this vast volume of water, NOAA's National Geophysical Data Center estimates that 321,003,271 cubic miles is in the ocean.