Introduction
One cannot learn about the weather we experience
without considering the ocean and its effect on our
weather...and the weather's effect on it. We must
consider the ocean because nearly 71% of the earth's
surface is covered by it and more than 97% of all our
water is contained in it.
We must consider the ocean and its impact as more than
one-half of the world's population lives within 60 miles
(100 km) of the ocean.
We must consider the ocean as its ability absorb, store,
and release heat into the atmosphere is huge and often
directly affects us. In fact, just the top 10 feet of the
Ocean
Surface Area
miles2
Surface Area
kilometers2
Pacific
64,000,000
166,000,000
45.0%
Atlantic
31,600,000
82,000,000
22.2%
Indian
28,400,000
73,600,000
20.0%
Southern
13,523,000
35,000,000
9.5%
Arctic
4,700,000
12,173,000
3.3%
ocean surface contains more heat than our entire
atmosphere.
Major climate events, such as El Niño, result from ocean temperature changes. These temperature changes then
have impacts on weather events such as hurricanes, typhoons, floods and droughts which, in turn, affect the prices of
fruits, vegetables and grains.
It is essential that we consider "the ocean".
Next:
Of all
oceans
Layers of the Ocean
Just as the atmosphere is divided into layers the ocean
consists of several layers itself.
Epipelagic Zone
This surface layer is also called the sunlight
zone and extends from the surface to 660 feet (200
meters). It is in this zone that most of the visible light
exists. With the light comes heating from sun. This
heating is responsible for wide change in
temperature that occurs in this zone, both in the latitude and each season. The sea surface temperatures
range from as high as 97°F (36°C) in the Persian Gulf to 28°F (-2°C) near the north pole.
The sea surface temperature also "follows the sun". From the earth's perspective, the sun's position in the
sky moves higher each day from winter to summer and lower each day from summer to winter. This change
in the sun's position from winter to summer means that more energy is reaching the ocean and therefore
warms the water.
Interaction with the wind keeps this layer mixed and thus allows the heating from the sun to be distributed
vertically. At the base of this mixing layer is the beginning of the thermocline. The thermocline is a region
where water temperature decreases rapidly with increasing depth and transition layer between the mixed
layer at the surface and deeper water.
The depth and strength of the thermocline varies
from season to season and year to year. It is
strongest in the tropics and decrease to nonexistent in the polar winter season.
Mesopelagic Zone
Below the epipelagic zone is the mesopelagic
zone, extending from 660 feet (200 meters) to
3,300 feet (1,000 meters). The mesopelagic
zone is sometimes referred to as the twilight
zone or the midwater zone as sunlight this deep
is very faint. Temperature changes the greatest
in this zone as this is the zone with contains the
thermocline.
Because of the lack of light, it is within this zone
that bioluminescence begins to appear on life. The eyes on the fishes are larger and generally upward
directed, most likely to see silhouettes of other animals (for food) against the dim light.
Bathypelagic Zone
The depths from 3,300 - 13,100 feet (1,000-4,000 meters) comprise the bathypelagic zone. Due to its
constant darkness, this zone is also called the midnight zone. The only light at this depth (and lower)
comes from the bioluminescence of the animals themselves.
The temperature in the bathypelagic zone, unlike that of the mesopelagic zone, is constant. The temperature
never fluctuates far from a chilling 39°F (4°C). The pressure in the bathypelagic zone is extreme and at
depths of 13,100 feet (4,000 meters), reaches over 5850 pounds per square inch! Yet, sperm whales can
dive down to this level in search of food.
Abyssopelagic Zone
The Abyssopelagic Zone (or abyssal zone) extends from 13,100 feet (4,000 meters) to 19,700 feet (6,000
meters). It is the pitch-black bottom layer of the ocean. The name (abyss) comes from a Greek word
meaning "no bottom" because they thought the ocean was bottomless. Three-quarters of the area of the
deep-ocean floor lies in this zone. The water temperature is constantly near freezing and only a few
creatures can be found at these crushing depths. The deepest a fish have ever been found was in the
Puerto Rico Trench at 27,460 feet (8,372 meters).
Hadalpelagic Zone
The deepest zone of the ocean, the hadalpelagic zone extends from 19,700 feet (6,000 meters) to the very
bottom at 35,797 feet (10,911 meters) in the Mariana Trench off the coast of Japan. The temperature is
constant at just above freezing. The weight of all the water over head in the Mariana Trench is over 8 tons
per square inch (the weight of 48 Boeing 747 jets).
Even at the very bottom life exists. In 2005, tiny single-celled organisms, called foraminifera, a type of
plankton, were discovered in the Challenger Deep trench southwest of Guam in the Pacific Ocean.
Sea Water
If there is one thing that just about everyone knows about the ocean is that it is
salty. The two most common elements in sea water, after oxygen and hydrogen,
are sodium and chloride. Sodium and chloride combine to form what we know as
table salt.
Sea water salinity is expressed as a ratio of salt (in grams) to liter of water. In sea
water there is typically close to 35 grams of dissolved salts in each liter. It is
written as 35‰ The normal range of ocean salinity ranges between 33-37 grams
per liter (33‰ - 37‰).
But as in weather, where there are areas of high and low pressure, there are
areas of high and low salinity. Of the five ocean basins, the Atlantic Ocean is the
saltiest. On average, there is a distinct decrease of salinity near the equator and
at both poles, although for different reasons.
Near the equator, the tropics receive the most rain on a consistent basis. As a result, the fresh water falling into the
ocean helps decrease the salinity of the surface water in that region. As one move toward the poles, the region of rain
decreases and with less rain and more sunshine, evaporation increases.
Fresh water, in the form of water vapor, moves from the ocean to the atmosphere through evaporation causing the
higher salinity. Toward the poles, fresh water from melting ice decreases the surface salinity once again.
The saltiest locations in the ocean are the regions where evaporation is highest or in large bodies of water where
there is no outlet into the ocean. The saltiest ocean water is in the Red Sea and in the Persian Gulf region (around
40‰) due to very high evaporation and little fresh water inflow.
Water has a unique property. As the temperature decreases to 40°F (4°C) the molecules slow, water contracts and
the density increases. Below 40°F (4°C) the molecules begin to bond to each other and as they do, the water begins
to expand again, decreasing the density. At 32°F (0°C) all molecule are locked into a crystalline structure with a
resulting nine percent expansion in size. This expansion, and corresponding decrease in density, is the reason ice
floats.
The amount of salt in sea water also determines the temperature at which sea water freezes. Adding salt to water
lowers the freezing temperature. Water with a salinity of 17‰ freezes at about 30°F (-1°C) and 35‰ water freezes at
about 28.5°F (-2C). Yet, despite the saltiness of the ocean, sea ice contains very little salt, about a tenth of the
amount of salt that sea water has. This is because ice will not incorporate sea salt into its crystal structure. Therefore,
sea ice is actually drinkable.
The temperature and salinity of the sea water also help determine its density. As
the temperature of sea water decreases the density also increases. Also, as the salt content of sea water increases,
so does its density. This makes the density of sea water, unlike fresh water, below the freezing point. So in situations
of sea ice formation, the salinity, and therefore the density of the underlying water continues to increase well after an
area is iced over.
In the "Average Salinity" map (right), it shows the lowest salinity in the polar regions. Bear in mind, this image depicts
surface salinity only. The surface salinity is lower in the polar regions than in the tropical regions due to melting each
summer. However, each winter below the ocean surface, the increased salinity in the water due to ice formation,
causes the water below the ice to sink and that sinking motion governs the motion of the ocean's deep water
currents.
Ocean Circulations
In January 1992, a container ship near the International Date Line, headed to Tacoma, Washington from Hong Kong,
lost 12 containers during severe storm conditions. One of these containers held a shipment of 29,000 bathtub toys.
Ten months later, the first of these plastic toys began to wash up onto the coast of Alaska. Driven by the wind and
ocean currents, these toys continue to wash ashore during the next several years and some even drifted into the
Atlantic Ocean.
The ultimate reason for the world's surface ocean currents is the sun. The heating of the earth by the sun has
produced semi-permanent pressure centers near the surface. When wind blows over the ocean around these
pressure centers, surface waves are generated by transferring some of the wind's energy, in the form of momentum,
from the air to the water. This constant push on the surface of the ocean is the force that forms the surface currents.
Around the world, there are some similarities in the currents.
For example, along the west coasts of the continents, the currents flow toward the equator in both hemispheres.
These are called cold currents as they bring cool water from the polar regions into the tropical regions. The cold
current off the west coast of the United States is called the California Current.
Likewise, the opposite is true as well. Along the east coasts of the continents, the currents flow from the equator
toward the poles. There are called warm current as they bring the warm tropical water north. The Gulf Stream, off the
southeast United States coast, is one of the strongest currents known anywhere in the world, with water speeds up to
3 mph (5 kph).
These currents have a huge impact on the long-term weather a location experiences. The overall climate of Norway
and the British Isle is about 18°F (10°C) warmer in the winter than other cities located at the same latitude due to the
Gulf Stream.
While ocean currents are shallow level circulations, there is global circulation which extends to the depths of the sea
called the Great Ocean Conveyor. Also called the thermohaline circulation, it is driven by differences in the density
of the sea water which is controlled by temperature (thermal) and salinity (haline).
In the northern Atlantic Ocean, as water flows north it cools considerably increasing its density. As it cools to the
freezing point, sea ice forms with the "salts" extracted from the frozen water making the water below more dense.
The very salty water sinks to the ocean floor.
It is not static, but a slowly southward flowing current. The route of the deep water flow is through the Atlantic Basin
around South Africa and into the Indian Ocean and on past Australia into the Pacific Ocean Basin.
If the water is sinking in the North Atlantic Ocean then it must raise somewhere else. This upwelling is relatively
widespread. However, water samples taken around the world indicate that most of the upwelling takes place in the
North Pacific Ocean.
It is estimated that once the water sinks in the North Atlantic Ocean that it takes 1,000-1,200 years before that deep,
salty bottom water rises to the upper levels of the ocean.
The Great Ocean Conveyor
Wind, Swell and Rogue Waves
The wind not only produces currents, it creates waves. As wind blows across the smooth water surface, the friction or
drag between the air and the water tends to stretch the surface. As waves form, the surface becomes rougher and it
is easier for the wind to grip the water surface and intensify the waves.
"Stolt Surf", North Pacific 1977
The following images are from the chemical tanker ship "Stolt Surf", built in 1970, caught in a large storm in the North Pacific Ocean.
The largest waves of the storm broke over the Bridge, more than 72 feet (22 meters) high. Images © Karsten Petersen. Used by
permission.
Going into the trough of a wave
Riding up to the next peak
Nearly at the crest
In the book Oceanography and
Seamanship, William G. Van
Dorn provided an example of
what the wave heights would be
if a steady 33 mph (30 knots)
wind blew for 24 hours over a
fetch of 340 miles.
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




Topping the crest...just to have another huge wave behind


it.
10% of all waves will be
less than 3.6 ft (1 m).
The most frequent wave
height will be 8½ ft (2½ m).
The average wave height
will be 11 ft (3 m).
The significant wave height
will be 17 ft (5 m).
10% of all waves will be
higher than 18 ft (5 m).
The average wave height
of the highest 10% of all
waves will be 22 ft (7 m).
A 5% chance of
encountering a single wave
higher than 35 ft (11 m)
among every 200 waves
that pass in about 30
minutes.
A 5% chance of
encountering a single wave
higher than 40 ft (12 m)
among every 2,600 waves
that pass in about five
hours.
Take it to the MAX! Anatomy of a Wave
Storms of equal size can generate much larger waves in the open Pacific Ocean as compared to the other oceans
due to the long open distance of water. How big wind waves get depends on three things:
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Wind strength. The wind must be moving faster than the wave crests for energy to be transferred.
Wind duration. Strong wind that does not blow for a long period will not generate large waves.
Fetch. This is the uninterrupted distance over which the wind blows without significant change in direction.
Storms of equal size can generate much larger waves in the open Pacific Ocean as compared to the other oceans
due to the long open distance of water.
After the wind begins to blow for a while, the waves get higher from trough to crest, and both the wave length and
period become longer. As the wind continues or strengthens, the water first forms whitecaps and eventually the
waves start to break. This is referred to as a fully developed sea.
Ta
it to the MAX! Beaufort Wind Scale
The waves in a fully developed sea outrun the storm that creates them, lengthening and reducing in height in the
process. There are called swell waves. Swells organize into groups smooth and regular in appearance. They are able
to travel thousands of miles unchanged in height and period.
The longer the wave, the faster it travels. As waves leave a storm area, they tend to sort themselves out with the long
ones ahead of the short ones, and the energy is simultaneously spread out over an increasingly larger area. As the
waves close in on the coast, they begin to feel the bottom and their direction of travel might change due to the
contour of the land. Eventually, the waves run ashore, increasing in height up to 1.5 times their height in deep water,
finally breaking up as surf.
There are many sailor tales of "rogue waves", "freak waves", "three sisters" and other "killer waves". Properly called
"extreme storm waves" these tales were ridiculed and mariners were accused of using them as an excuse to cover
their own mistakes in wrecks. Rogue waves are simply unusually large waves appearing in a set of smaller waves.
Some of the characteristics of rogue waves are:
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their height is greater than twice the size of surrounding waves,
they often come unexpectedly from directions other than prevailing wind and waves, and
they are unpredictable.
Most reports of extreme storm waves say they look like "walls of water," and are seen as steep-sided with unusually
deep troughs. The USS Ramapo reported one such wave with a height of 112 feet in the Pacific in 1933. Another
report of a freak wave occurred with it struck the Queen Mary amidships, south of Newfoundland, at the end of World
War II, rolling her to within a degree or two of capsizing.
Just recently, a 70 foot wave crashed down on the Norwegian Dawn cruise ship in April 2005. The average waves
that day were 25 to 30 feet high before this monster wave struck. The wave even damaged the hull of the Norwegian
Dawn.
What causes these enormous waves? Generally they form because of 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
happen to coincide and reinforce each other, combining to form unusually large waves that tower then disappear. If
the swells are travelling in the same direction, these mountainous waves may last for several minutes before
subsiding.
It is very seldom that huge waves over 65 feet (20 meters) are developed and normally sailors do not even see them,
because ships nowadays will try to avoid such conditions by altering course before the storm hits. But they do occur.
Next:
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.
Take it to the MAX! Battle of the Bulge
The change in the water level with the
daily tides from location to location
results from a 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 vary between the
northern and southern hemisphere over this same 28-day orbital period.
Take it to the MAX! The Fun Bay of Fundy
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 inline (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. 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.
Learning Lesson: Next:
The Sea Breeze
The ability for the ocean to absorb and store energy from the sun is huge. This is due to...
1.
The transparency of the water that allows the sun's rays to penetrate deep into the ocean. In clear, tropical
water light can reach a depth of 500-650 feet (150-200 meters) meaning that it takes a great amount of heat
to raise the temperature through such a large volume of water, and
2.
The constant turbulence from wind and weather continually mixes the water, distributing surface heating
through the water column.
In contrast to the ocean the sun's rays do not to penetrate deep into the earth but are confined to the top few inches
of soil. Consequently, temperature fluctuations between daytime and nighttime are much greater over land than over
water. For coastal areas, this difference of heating can have a large impact on the weather by the formation of the
sea and land breezes.
The sea breeze
circulation is
comprised of two
opposing flows; one
at the surface (called
the sea breeze) and
one aloft (which is a
return flow). These
two flows are a
result of
the difference in air
density between the
land and sea caused
by the sun's heating.
The sun warms both the ground and ocean at the same rate. However, since the ground's heat remains confined to
the top few inches of soil it radiates back into the atmosphere warming the air. As the air warms, its density
decreases creating a weak low pressure area called a "thermal low" (1).
Over the adjacent water the cooler, more dense air, being pull down by gravity, begins to spread inland (2).
This inland push of air from the ocean undercuts the less dense air over land forcing it to rise (3). A sharp boundary
develops due to the large difference between the air temperature over land and over water. This boundary, called a
sea breeze front, acts in the same manner as the cold front we typically experience.
For example, the air temperature drops significantly after the sea breeze front passes a location, sometimes as much
as 15-20°F (8-11°C). The skies also clear after the sea breeze front pass by. Another change that takes place with
the passage of the sea breeze front is an increase in humidity. Finally, there can be a significant change in wind
direction and/or speed.
Over land, air forced up by the sea breeze front will begin to cool. This cooling means the density increases again
forming a small area of high pressure (4). Typically, this occurs from 3,000 to 5,000 feet (1,000 to 1,500 meters) in
elevation. At this level the air pressure and density, being greater than the same elevation over the water, causes air
to flow back over water (5).
Once over water again, the air cools, increases in density and sinks toward the earth's surface (6). This enhances
high pressure near the ocean's surface (7) and the whole process repeats as land flowing air pushes the sea breeze
front further inland. While the sea breeze is generally associated with the ocean, they can occur along the shore of
any large body of water such as the Great Lakes.
Opposing sea breezes meeting over Cuba forming a line of
cumulus clouds.
These changes occur in a relatively small scale weatherwise. However, if there are larger scale atmospheric
conditions also affecting the weather, then the sea breeze
and sea breeze front can have a much larger impact on the
type and intensity of weather one observes. Just like along
cold fronts, if weather conditions are right, thunderstorms often develop along sea breeze fronts.
The location and amount of thunderstorms will vary depending on the overall weather pattern over the region. For
example, in Florida the amount of sunshine and prevailing surface wind over the state has a large impact on sea
breeze thunderstorms. If the wind is relatively calm then the sea breeze can move well inland but with only scattered
thunderstorms occurring about 1/3rd of the way across the peninsula.
Light west wind (5-10 mph) keeps the sea breeze front confined to the eastern coast but also makes for more
widespread thunderstorms along the boundary. Stronger west winds can prevent the sea breeze front from moving
onshore or forming at all, so no thunderstorms will occur. With prevailing east winds, they actually help push the sea
breeze front and thunderstorms as much as half way across the peninsula.
On smaller peninsulas, such as at the northern tip of New Zealand, sea breezes from opposite coasts may collide. In
these situations, two lines of thunderstorms may collide and combine into a single but intense, short-lived line.
The opposite of a sea breeze is a land breeze. While sea breezes occur during the day, land breezes occur at night.
Despite the difference in times at which the land breezes and sea breezes occur, the reason for the land breeze's
formation is basically the same as the sea breeze, but the role of the ocean and land is reversed.
Land
breezes can occur when the land's nighttime temperature is less than the sea surface temperature. They are most
common during the fall and winter seasons when water temperatures are still fairly warm and nights are cool.
However, unlike the sea breeze, the land breeze is often much weaker.
At night, the land temperature falls to below that of the ocean resulting in an increase in the air's density. Gravity's
downward pull moves air downhill spilling it onto the water (1). This more dense air undercuts the lighter, warmer air
over the water (2) forcing it up into the atmosphere (3). This rising air forms a weak low pressure area (4).
The rising air accumulates aloft forming an area of higher pressure (5). Relative to the land at the same elevation, air
flows back toward land from high pressure to low pressure (6). Once back over land, the air cools, increases in
density and then sinks causing an increase in density and high pressure (7). Gravity pulls the dense air offshore
again completing the circulation.
Land breezes are weaker than sea breezes but not because of the difference in heating. Daytime heating and
nighttime cooling occur at about the same rate so the potential for both land and sea breezes to be the same strength
exists. But at night,
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Ne
The cooling ground inhibits vertical motion which, in turn, weakens the land breeze circulation,
Nighttime cooling also produces a shallower change in temperature so land breeze circulation is shallower,
and
Terrain, vegetation, and buildings inhibit the flow of air from land to water.
xt
The Marine Layer
Somewhat similar to the sea breeze, the marine layer also represents a difference between a cool, moist airmass and
a warmer airmass. Unlike the sea breeze, which reforms almost every day along the east coast in Summer, the
marine layer can persists for days and weeks along the west coast, particularly Southern California.
degrees F | degrees C
The reason is the water along the west coast of the United States, comes from the Gulf of Alaska and is much cooler
than at the same latitude along the east coast, where the Gulf Stream brings tropical water north. The surface
temperature of the water off the California coast can be as much as 30°F (17°C) or more lower than at the same
latitude on the east coast. The colder water means the air in contact with the water is colder and therefore is denser.
When the air well above the water is warmer than the water, as it is normally for all seasons except winter but most
common in late spring/early summer, a temperature inversion develops, where instead of the air cooling with
increasing elevation the air actually increases in temperature with height. The cooler air below the inversion is called
the marine layer and is cooled to the point at clouds form. Because of its persistence in early summer, the people in
Southern California it is often refer to it as the "May Gray" or "June Gloom".
However, it is not just a West Coast phenomena, it can occur near any large body of water such as the Great Lakes
region when the water temperature is significantly colder than the air moving over it.
The depth of the marine layer depends upon the large scale weather patterns that pass overhead. High pressure
systems (at 15,000 to 30,000 feet) tend to squish the marine layer down near the surface. When the inversion is very
strong and relatively shallow, the coastal clouds and foggy weather will be confined to the beaches with warm, sunny
conditions beginning just a mile or so inland.
In Summer, it is not uncommon for someone to drive across the San Francisco's Golden Gate bridge in fog with a
temperature in the upper 50s to lower 60s only to go to the top of Mount Tamalpais, just a few miles north of the
bridge (elevation 2,600 feet), and have clear skies with a temperature in the 80s or even the lower 90s because of
moving above the cool marine layer.
As the pressure aloft decreases, the downward forcing on
the marine layer decreases and the marine layer begins to lift. Near the beach, the fog lifts into a low cloud layer. The
leading edge of the marine layer extends farther inland pushing the fog inland.
Further lifting of the marine layer will allow the cooler air to spill over the mountains into the interior valleys.
Sometimes, if the air in the low pressure system aloft is cold enough, the inversion that defines the top of the marine
layer will dissipate and so will the marine layer. Because of this, often the maximum daily temperatures in the San
Joaquin and Sacramento Valleys of California will undergo a roller coaster effect.
Rip Currents
Rip currents are powerful, channeled
currents of water flowing away from shore.
They typically extend from the shoreline,
through the surf zone, and past the line of
breaking waves. Rip currents can occur at
any beach with breaking waves, including
the Great Lakes.
Rip currents most typically form at low spots or breaks in sandbars, and also near structures such as groins, jetties
and piers. Rip currents can be very narrow or extend in widths to hundreds of yards. The seaward pull of rip currents
varies: sometimes the rip current ends just beyond the line of breaking waves, but sometimes rip currents continue to
push hundreds of yards offshore.
Rip currents form as incoming waves create an underwater sandbar close to shore (#1 above), and the waves push
more and more water in between the sandbar and the shore (#2) until a section of this sandbar collapses and the
water rushes back toward the sea (#3) through a narrow gap. Once the flowing water pass through the narrow gap it
begins to spread out (#4). It is here where the velocity and strength of the rip current circulation begins to weaken
considerably.
Rip currents can be killers as they are the leading surf
hazard for all beachgoers. They are particularly
dangerous for weak or non-swimmers. Rip current speeds
are typically 1-2 feet per second. However, speeds as
high as 8 feet per second have been measured which is
faster than an Olympic swimmer can sprint!
The United States Lifesaving Association estimates that the annual number of deaths due to rip currents on our
nation's beaches exceeds 100. Rip currents account for over 80% of rescues performed by surf beach lifeguards.
The drowning deaths occur when people, pulled offshore, are unable to keep themselves afloat and swim to shore.
This may be due to any combination of fear, panic, exhaustion, or lack of swimming skills.
Dispelling the Myth of the Rip
A rip current is a horizontal motion not a vertical motion. Rip currents do not pull people under the water; they pull
people away from shore. The rip current is typically the strongest about a foot off of the bottom, which can cause your
feet to be knocked out from under you making it feel like something under the water was pulling you. This is where
the incorrect term "undertow" comes from.
Also, another incorrect term used for rip currents is the "rip tide". Rip currents would exist with or without tides.
However, low tide can enhance the intensity of the current.
Rip Currents: Where's the Rip?
Signs that a rip current is present are very subtle and difficult for the average beachgoer to identify. Look for
differences in the water color, water motion, incoming wave shape or breaking point compared to adjacent conditions.
Look for these clues as one, all or none may be visible:
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Channel of churning, choppy water
Line of foam, seaweed, or debris moving steadily seaward
Area having a notable difference in water color
Break in the incoming wave pattern
Channel of churning, choppy water
Photo courtesy of Dr. Wendy Carey, Delaware Sea Grant
Line of foam, seaweed, or debris moving seaward
Photo courtesy of Dr. Wendy Carey, Delaware Sea Grant
Area having a notable difference in water color
Photo courtesy of Dr. Wendy Carey, Delaware Sea Grant
Area having a notable difference in water color
Photo courtesy of Dr. Wendy Carey, Delaware Sea Grant
Break in the incoming wave pattern
Photo courtesy of Dr. Wendy Carey, Delaware Sea Grant
Break in the incoming wave pattern
Photo courtesy of Dr. Tom Herrington, Stevens Institute of
Technology
Next:
Rip Currents: Break The Grip of the Rip
TM
National Weather Service and National Sea Grant Program, in partnership with the United States Lifesaving
Association, are working together to raise awareness about the dangers of rip currents and how to protect yourself.
Before you go to the beach...
Rip Current poster - click to go to the download page.
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KNOW HOW TO SWIM.
Seems simple enough, but those who do not know how to swim can be pull out to sea by a rip current and
will stand little chance of survival. And just because you are in shallow water does not mean you are safe
either. A person standing waist deep in water can be dragged out into deeper waters, where they can drown.
In April 2004, an 19-year-old man was killed by a rip current simply by standing in knee-deep ocean water.
Knocked off his feet, this non-swimmer was pulled out to sea and drowned.
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Check the Surf Zone Forecast for your region.
Surf Zone Forecasts will contain Rip Current Outlooks using the following, three-tiered set of qualifiers:
o Low Risk of rip currents. Wind and/or wave conditions are not expected to support the
development of rip currents; however, rip currents can sometimes occur, especially in the vicinity of
groins, jetties, and piers. Know how to swim and heed the advice of lifeguards.
o Moderate Risk of rip currents. Wind and/or wave conditions support stronger or more frequent rip
currents. Only experienced surf swimmers should enter the water.
o High Risk of rip currents. Wind and/or wave conditions support dangerous rip currents. Rip
currents are life-threatening to anyone entering the surf.
When you get to the beach...
Whenever possible, swim at a lifeguard-protected beach. Ask a lifeguard about the
conditions before entering the water. This is part of their job.
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Never swim alone
Never swim at night. Rip currents can be more dangerous at night simply
because you cannot see them like you can during daylight hours.
 Stay at least 100 feet away from piers and jetties. Permanent rip currents
often exist alongside these structures.
 Use polarized sunglasses. They will help you to spot signatures of rip
currents by cutting down glare and reflected sunlight off the ocean's surface.
 Avoid the "it won't/didn't happen to me" syndrome. Obey all
instructions/orders from lifeguards and posted signs. They are there for your
wellbeing.
If caught in a rip current...
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Remain calm. You will not be pulled under the surface of the water.
Only swim parallel to the shore to escape the current. As soon as you are
out of the current, only then swim toward the beach. You will not make it swimming
directly against the current. It will be too strong for you.
 Another option is to just float. Eventually you will reach the end of the current.
Then either...
o Swim parallel to the shore to get out of the path of the rip current and once you do so, only then
swim toward the beach OR
o Draw attention to yourself by waving your arm and yelling for help (which you can do because you
are not swimming alone...right?)
If you see someone in trouble...
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Throw the rip current victim something that floats--a life vest, a cooler, an inflatable ball.
Get help from a lifeguard. If a lifeguard is not available, call 9-1-1.
Yell instructions on how to escape the rip current. Tell them which way to swim.