3. 3
Introduction
Tides are the periodic motion of the waters of the sea due to changes in the attractive forces of the Moon and Sun
upon the rotating Earth. Tides can either help or hinder a mariner. A high tide may provide enough depth to clear
a bar, while a low tide may prevent entering or leaving a harbor. Tidal current may help progress or hinder it, may
set the ship toward dangers or away from them. By understanding tides and making intelligent use of predictions
published in tide and tidal current tables and descriptions in sailing directions, the navigator can plan an
expeditious and safe passage through tidal waters.
TIDE
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the
Moon and the Sun and the rotation of the Earth. Most places in the ocean usually experience two high tides and
two low tides each day but some locations experience only one high and one low tide each day. The times and
amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in
the deep ocean and by the shape of the coastline and near-shore bathymetry.
Most coastal areas experience two high and two low tides per day. The gravitational effect of the Moon on the
surface of the Earth is the same when it is directly overhead as when it is directly underfoot. The Moon orbits the
Earth in the same direction the Earth rotates on its axis, so it takes slightly more than a day about 24 hours and
50 minutes or the Moon to return to the same location in the sky. During this time, it has passed overhead once
and underfoot once, so in many places the period of strongest tidal forcing is 12 hours and 25 minutes. The high
tides do not necessarily occur when the Moon is overhead or underfoot, but the period of the forcing still
determines the time between high tides. The Sun also exerts on the Earth a gravitational attraction which results
in a secondary tidal effect. When the Earth, Moon and Sun are approximately aligned, these two tidal effects
reinforce one another, resulting in higher highs and lower lows. This alignment occurs approximately twice a
month at the full moon and new moon. These recurring extreme tides are termed spring tides. Tides with the
smallest range are termed neap tides. Tides vary on timescales ranging from hours to years due to numerous
influences. To make accurate records, tide gauges at fixed stations measure the water level over time. Gauges
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ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference
level usually called mean sea level. While tides are usually the largest source of short-term sea-level fluctuations,
sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges,
especially in shallow seas and near coasts. Tidal phenomena are not limited to the oceans, but can occur in other
systems whenever a gravitational field that varies in time and space is present. For example, the solid part of the
Earth is affected by tides.
WHAT CAUSES TIDES?
Tides are caused by the gravitational pull of the sun and the moon on the earth. Along the Pacific coast, there are
usually two high tides of unequal height and two low tides of unequal height approximately every 24 hours.
Because of the speed of the moon as it travels around the earth, the tides come a little later each day. High tides
occur every 12 hours and 25 minutes. Low tides are halfway, or six hours, 12 minutes and 30 seconds after each
high tide. The tides along most of the Atlantic coast are the same height each day. When the sun and the moon
are in line with the earth, as they are during a new moon and a full moon, the gravitational pull on the earth is
combined. As a result, the highest tides are higher and the lowest tides are lower. These tides are called spring
tides
During a half moon, when the sun and moon are at right angles to the earth, their forces work against each other.
As a result, the tides, called neap tides, are not very high or very low. Spring and neap tides follow each other every
week.
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Although the sun affects the tides, the pull of the moon is stronger due to its proximity to the earth, therefore, for
simplicity’s sake, only the effects of the moon are depicted in the illustrations below. The gravitational pull of
the moon causes the water on two quarters of the surface of the earth to pool, the quarter nearest to the moon, and
the quarter furthest from the moon. The water pools on the face of the earth closest to the moon because the
gravitational force of the moon pulls the water towards it. The water pools on the opposite side of the earth
because the gravitational force of the moon pulls the solid body of the earth away from the water.
Feature of tide
General Features
At most places the tidal change occurs twice daily. The tide
rises until it reaches a maximum height, called high tide or
high water, and then falls to a minimum level called low
tide or low water. The rate of rise and fall is not uniform.
From low water, the tide begins to rise slowly at first, but at
an increasing grate until it is about halfway to high water.
The rate of rise then decreases until high water is reached,
and the rise ceases. The falling tide behaves in a similar
manner. The period at high or low water during which there
is no apparent change of level is called stand. The difference
in height between consecutive high and low waters is the
range. Figure 904 is a graphical representation of the rise
and fall of the tide at New York during a 24-hour period.
The curve has the general form of a variable sine curve.
Tidal Cycles
In the first chapter we learned that according to one definition a tide is a distortion in the shape of one body
induced by the gravitational pull of another nearby object. We also learned that on Earth the term is also used to
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refer to the rise and fall of the sea level. This change in sea level is of course a visible effect of the shape
distortion affecting the solid Earth as well as the oceans. In this chapter we will have a closer look at the sea
level change on a global scale.The observable rise and fall of the sea level is influenced strongly by shoreline
topography, ocean currents and the distribution of the continents on earth. As a result different tidal cycles can
be experienced in different regions of the world. They are described as semi-diurnal, diurnal or mixed tidal
cyles.
1. Diurnal Tides
Ocean tides are caused by the pull of gravity of
the moon and the sun on the ocean's surface. As
the moon is much closer than the sun to earth, its
influence is far greater. The moon's gravitational
force causes a bulge in the oceans surface on the
side of the earth facing the current position of the
moon. Due to the law of inertia, a bulge also forms
on the opposite side of the earth. At the peaks of
each of these bulges is high tide, at the troughs,
low tide. We experience high and low tides at the
beach when these peaks and troughs reach our
shores.
2. Semidiurnal Tides
A semidiurnal tidal cycle is a cycle with two nearly equal
high tides and low tides every lunar day. In the world map
shown above regions experiencing a semidiurnal tidal
cyle are marked in red. They have a period of 12 hours
and 25 min, and a wavelength of more than half the
circumference of Earth [5]. It is also the type of tidal
cycle one could expect from a planet covered entirely
with water and without any continents obstructing the
free motion of water. By looking at the oceans on Earth
we can see that most places experience a semidiurnal tidal
cycle. The following diagram shows the sea level change
over time for a typical semidiurnal tidal cycle
3. Mixed Tidal cycle
A mixed tidal cycle is a cycle with two high and low tides with
different sizes each lunar day. The difference in height between
successive high (or low) tides is called the diurnal inequality.
Areas with a mixed tidal cycle can be found alongside the West
cost of the USA, in parts of Australia and in South East asia.
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Types of tide
Here are all kinds of “tides” or at least the word tide is used to describe a range of phenomena. Check out these
different tides.
Bore Tide: A tidal bore (or simply bore in context, or also aegir, eagre, or eygre) is a tidal phenomenon
in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river
or narrow bay against the direction of the river or bay's current.
Neap Tide: When the Sun and Moon form a right angle, as when we see a half moon, their gravitational
pulls fight each other and we notice a smaller difference between high and low tides. These are called
neap tides.
Spring Tide: When the Moon, Earth, and Sun fall in a straight line, which we call syzygy (siz-eh-gee),
we notice the greatest difference between high and low tide water levels. These spring tides occur twice
each month, during the full and new Moon. If the Moon is at perigee, the closest it approaches Earth in
its orbit, the tides are especially high and low.
Rip Tide: A rip current, commonly referred to simply as a rip, or by the misnomer rip tide, is a strong
channel of water flowing seaward from near the shore, typically through the surf line. Typical flow is at
0.5 meter-per-second (1–2 feet-per-second), and can be as fast as 2.5 meters-per-second (8 feet-per-
second), which is faster than any human swimmer. They can occur at any beach with breaking waves,
including oceans, seas and even large lakes.
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Low Tide: In both senses also called low water.
1. The lowest level of the tide.
2. The time at which the tide is lowest.
High Tide:
a. The tide at its fullest, when the water reaches its highest level.
b. The time at which this tide occurs. Also called high water.
Brown Tide: Brown Tide is a bloom (excessive growth) of small marine algae (Aureococcus
anophagefferens). Although algae of many types are found in all natural freshwater and marine
ecosystems, blooms of the Brown Tide organism literally turn the water deep brown, making it
unappealing to swimmers and fishermen alike. While not harmful to humans, the presence of the Brown
Tide is a problem for bay scallops and eelgrass, and to a lesser degree other finfish and shellfish. Brown
Tide is unlike most other algal blooms because of its unusually high concentrations, the extent of area it
covers and the length of time it persists.
Red Tide: Harmful algal blooms, (HAB) occur when colonies of algae grow out of control while
producing toxic or harmful effects on people, fish, shellfish, marine mammals and birds. The human
illnesses caused by HABs, though rare, can be debilitating or even fatal. Many people call HABs 'red
tides,' scientists prefer the term harmful algal bloom. One of the best known HABs in the nation occurs
nearly every summer along Florida’s Gulf Coast.
Crimson Tide: Trademarked name for the University of Alabama Athletics.
Semidiurnal Tide: These are tides occurring twice a day. This means a body of water with semi-diurnal
tides, like the Atlantic Ocean, will have two high tides and two low tides in one day, much like the
eastern seaboard of North America.
Diurnal Tide: These tides occur once a day. A body of water with diurnal tides, like the Gulf of
Mexico, has only one high tide and one low tide in a 25-hour period.
Mixed Tide: Some bodies of water, including most of North America that’s in contact with the Pacific
Basin, have mixed tides, where a single low tide follows two high tides.
Time of Tide
Since the lunar tide-producing force has the greatest effect in producing tides at most places, the tides “follow the
Moon.” Because the Earth rotates, high water lags behind both upper and lower meridian passage of the Moon.
The tidal day, which is also the lunar day, is the time between consecutive transits of the Moon, or 24 hours and
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50 minutes on the average. Where the tide is largely semidiurnal in type, the lunitidal interval (the interval between
the Moon’s meridian transit and a particular phase of tide) is fairly constant throughout the month, varying
somewhat with the tidal cycles. There are many places, however, where solar or diurnal oscillations are effective
in upsetting this relationship. The interval generally given is the average elapsed time from the meridian transit
(upper or lower) of the Moon until the next high tide. This may be called mean high water lunitidal interval or
corrected (or mean) establishment. The common establishment is the average interval on days of full or new
Moon, and approximates the mean high water lunitidal interval. In the ocean, the tide may be in the nature of a
progressive wave with the crest moving forward, a stationary or standing wave which oscillates in a seesaw
fashion, or a combination of the two. Consequently, caution should be used in inferring the time of tide at a place
from tidal data for nearby places. In a river or estuary, the tide enters from the sea and is usually sent upstream as
a progressive wave so that the tide occurs progressively later at various places upstream
Tide Formation
Gravitational Pull of the Moon
Gravity is also the cause of tides. The earth’s gravity keeps water on the planet’s surface. However, the moon is
large enough and close enough that its gravitational force has a noticeable effect on large bodies of water on
Earth. Water on Earth in the region directly beneath the moon is pulled by gravitational force toward the moon
reating a bulge on the surface of the ocean. There is also a bulge on the opposite side of the earth, caused by the
difference in the moon’s gravitational force across the earth. The ocean bulges on both the side of the earth facing
the moon and the side opposite the moon are called tidal bulges. Earth’s land surface also bulges, as does the
moon, although not to the same extent as the ocean.
Tidal bulges are very small—seemingly insignificantly small—compared to the radius of the earth. The tidal
bulges in figures in this unit are greatly exaggerated. The height of the tidal bulge in the open-ocean is less than
a meter in most areas. However, because the ocean is so vast, tidal bulges can raise a huge amount of water. The
tide resulting from the moon’s gravitational pull is called the lunar tide.
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The moon moves very little around the earth each day. During one day, the earth makes a complete rotation on
its axis, while it takes the moon almost a month to orbit around the earth. In Fig. 6.6, we can see that this means
the earth rotates through the tidal bulges. At midnight, a person standing on the shore of the ocean near the equator
would see a high tide, caused by the gravitational force of the moon. If that person remained in the same location
over the course of a full day as the earth rotated, the person would move into a region of low tide at 6:13 a.m.
This is because the place on Earth where she is standing would have rotated into the trough in between the two
tidal bulges . At 12:25 p.m. there would be another high tide, caused by the outward force of the Earth’s rotation.
At 6:38 p.m. there would be another low tide and at 12:50 a.m. another high tide
Because the moon progresses about 12 degrees (˚) in its orbit around the earth during each 24-hour period, and
because it rotates in the same direction as Earth rotates, the moon will not be directly overhead of the observer
again until it completes one rotation and is again full. This explains why, for any given observer on the surface
of the earth, the moon appears to rise about 50 minutes later each day and why the time of the high and low tides
is about 50 minutes later each day. This also explains why the fictional person described in the previous paragraph
in Fig. 6.6 observed high and low tides about six hours and 12.6 minutes apart.
Gravitational Pull of the Sun
The sun also exerts a gravitational force on the earth, producing a solar tide. Just like with the earth and the moon,
water on Earth directly in line with the sun is pulled by gravitational force toward the sun, creating a bulge of
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water. There is also a bulge on the side of the earth opposite the sun. Similar to the lunar tide, as the earth rotates
through the bulge of water produced by the sun, the tide level changes from high to low and back again. Because
the earth rotates every 24 hours, solar tidal changes occur on a 24-hour schedule.
Interactions of the Gravitational Pulls of the Moon and Sun
Even though the mass of the sun is much greater than the mass of the moon, the moon has a greater influence on
the tides than the sun. This is because the sun is much farther away from the earth, so its tidal force is only about
half that of the moon. Gravitational force depends on both the
mass of the objects and the distance between them. Because
the moon moves a little farther each day in its orbital journey
around the earth, the tides caused by the moon’s gravity occur
50 minutes later than the tides caused by the sun’s gravity. It
takes the moon about 29.5 days to complete its orbit around
the earth. This period is called a lunar month. The moon and
the sun cause predictable, periodic changes in tidal range
during a lunar month. Therefore a lunar month is also called a
tidal month.
When the earth, moon, and the sun are lined up, lunar and solar
tides occur at nearly the same time and produce the largest
tidal ranges over the lunar month. They occur during the new
moon, when the moon is between the earth and the sun, or full
moon, when the earth is between the moon and the sun. Extra-
high and extra-low tides occur at this time. They are called spring tides because they “jump” or “spring” up.
When the sun and moon are at a right angle (90˚) to each other, the moon is either in its first quarter or its third
quarter. In this position the solar and lunar tides tend to cancel each other out, and a reduced tide, called a neap
tide, occurs There are two spring tides and two neap tides in a tidal month.
Characteristics
Tide changes proceed via the following stages:
• Sea level rises over several hours, covering the intertidal zone; flood tide.
• The water rises to its highest level, reaching high tide.
• Sea level falls over several hours, revealing the intertidal zone; ebb tide.
• The water stops falling, reaching low tide.
Tides produce oscillating currents known as tidal streams. The moment that the tidal current ceases is called slack
water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high
water and low water. But there are locations where the moments of slack tide differ significantly from those of
high and low water.[5] Tides are most commonly semidiurnal (two high waters and two low waters each day), or
diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height (the daily
inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters
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each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally
small when the Moon is over the equator.
Why do we study tides?
We study tides for safe navigation, recreation, and coastal development. Scientists measure the times, heights,
and extents of both the rise and fall of the tidal waters that support a number of different aspects of our daily lives.
Navigating ships safely through shallow water ports, intracoastal waterways, and estuaries requires knowledge of
the time and height of the tides as well as the speed and direction of the tidal currents. Mariners need accurate
data because the depths and widths of the channels along with increased marine traffic leaves very little room for
error.Engineers need data to monitor fluctuating tide levels for harbor engineering projects such as the
construction of bridges and docks. Projects involving the construction, demolition, or movement of large
structures must be scheduled far in advance if an area experiences wide fluctuations in water levels during its
tidal cycle. Habitat restoration projects also require accurate knowledge of tide and current conditions.
Tidal data is also critical to fishing, recreational boating, and surfing. Commercial and recreational fishermen use
their knowledge of the tides and tidal currents to help them improve their catches. Depending on the species and
water depth in a particular area, fish may concentrate during ebb or flood tidal currents.
The Equilibrium Theory of Tides
This is sometimes called the theory of “static” tides, a theory that emerged for the first time in Isaac Newton’s
famed Principia. Having identified the tide-producing forces, Newton, and others who followed him, conceived
of a hypothetical global ocean in static equilibrium with these forces – an equilibrium calling for a prolate spheroid
of water covering the earth. “Prolate” means that the sphere in question has been stretched along a line joining
two poles; not the geographic poles in this case but the poles in line with the celestial body (moon or sun) causing
the hypothetical ocean sphere to deform. Taking another look at the graphic illustrating the tractive forces in the
previous module, one can easily imagine water converging on these poles to produce twin “tidal bulges” – terms
that are still very popular in modern day textbooks.
The reason for the popularity of the equilibrium theory - the tidal bulges concept at least – is that it’s easy to
explain certain well-known tidal phenomena with pictures. In the one on the left below, an imaginary observer
named Joe rotates with the earth and encounters the static bulges in the form of high tides. Whenever the moon
crosses Joe’s local meridian, he witnesses high tide. It’s high tide again twelve lunar hours later when the moon
crosses the opposite meridian on the other side of the earth. Two highs and two lows occur in one lunar day lasting
24 hours and 50 minutes in watch (solar) time. In the figure on the right, the moon has progressed in its orbit
around the earth to a position north of the equator (north declination). The static bulges move to remain in line
with the moon and now Joe encounters a diurnal inequality in the high tides (successive high tides of unequal
height). Maximum lunar declination, north or south of the equator, produces tropic tides; tides occurring when
the moon is on the equator are called equatorial tides. Tropic-equatorial tides recur twice in an interval of 27 1/3
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days – the tropic month covering one complete cycle in lunar declination. All these observations are consistent
with equilibrium theory.
Another phenomenon that’s easy to demonstrate in this way is the well-known spring-neap cycle. Solar gravity
also produces a pair of tidal bulges in the hypothetical ocean. When the tractive forces of the sun and moon are
in line, spring tides of greater range (higher highs and lower lows) result as shown in the figure on the left below.
As in the previous figures, when the moon completes another half-cycle in its orbit – this time from full to new
moon - spring tides will occur again.
The figure below on the right illustrates the neap portion of the spring-neap cycle; i.e., when the moon is in the
first quarter (or the third quarter) of that cycle, lunar and solar tractive forces are completely out-of-line, tending
to counteract one another, and neap tides of lesser range (lower highs and higher lows) result. Two spring-neap
cycles (two springs and two neaps) are completed in 29 ½ days, the same period of time required for the moon to
complete one full orbit of earth with respect to the sun.
Other aspects of the observed tide in accordance with equilibrium theory include the perigean-apogean cycle.
This one stems from the fact that the moon’s orbit around the earth describes an ellipse rather than a circle.
Perigean tides of greater range occur at lunar perigee, when the moon is closest to the earth, and apogean tides of
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lesser range occur at lunar apogee, when the moon is farthest from the earth in its elliptical orbit. The perigean-
apogean cycle takes about 27 ½ days to complete.
Although the equilibrium theory does an excellent job of explaining cyclical tidal phenomena and the recurrence
periods associated with many of them, it’s an example of a model of ideal behavior – something that works for
the purpose intended although it may not adhere to the truth in all instances. We don’t have far to look for those
instances. The earth is only partially covered by its waters, land masses prevent anything resembling a bulge from
traveling completely around it, and observations of real tides show that they do not respond instantly to the tide-
producing forces of the moon and sun as the theory requires.
The Dynamic Theory of Tides
Tides in the actual Earth’s oceans behave a bit differently than in our hypothetical ocean-covered Earth due to the
placement of landmasses, the shallow depth of water relative to wavelength of tides, the latitudinal variation of
the rotational velocity of Earth, and the Coriolis Effect. When we take these factors into effect we discover the
dynamic theory of tides.
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Ocean Depth and Rotational Velocity. Because tides are such long wavelength waves, they behave as shallow
water waves. This means that all of the water in the oceans are effected by tides – from the water at the surface
to the water at the deepest depths. Recall that the speed of a shallow water wave is directly proportional to the
water depth – because the seafloor acts to slow down waves.
Based on our equilibrium theory of tides, ocean water always stayed in direct line with the sun and moon, meaning
that in theory the waves traveled at the speed of rotation of the Earth. However, if we calculate the maximum
speed tides can reach (being shallow water waves) we find that they travel slightly slower. This means that the
bulge created by the gravitational pulls and centripetal force actually lags somewhat behind the moon as the moon
orbits the earth. So when the moon is directly overhead a certain location, that location is not experiencing its
high tide at that moment, it comes later.
However, at higher latitudes we find that the tides do not lag behind the sun and the moon. This is because the
rotational velocity of the Earth decreases with latitude and even though the tides still interact with the seafloor,
they are able to “keep up”. This lag time is shown in figure 9.14 shows a map of these systems using co-tidal
lines – lines showing the delay in time between when the moon is directly overhead and the actual high tide
occurs.
Continents and the Coriolis Effect. Landmasses on Earths surface prevent the Earth from simply rotating into
and out of tidal bulges. When the tidal bulge “hits” the side of a continent some of its energy is dissipated, and
some of the energy is reflected back into the ocean basin. This reflection, coupled with the Coriolis Effect causes
water to be rotated around an ocean basin, much in the way water would rotate around a cup if you move the cup
back and forth.
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This oscillation of water around an ocean basin is called an amphidromic system and causes the high tide wave
crests and low tide wave troughs to move around ocean basins in a clockwise (S. Hemisphere) or counterclockwise
(N. Hemisphere) pattern. In the center of this rotating wave is a node where the tidal range is zero. These systems
have been found to occur in all the ocean basins except the Southern Ocean, where tide crests and troughs simply
move
Why tide necessary?
Fishing
Fish may concentrate during ebb tides. Commercial fishermen follow the tides and learn to fish during levels of
highest concentration to improve their economic investment and to make more efficient use of their time.
Recreational fishermen may also fish during ebb tides because the concentrations of smaller fish attract the larger
trophy fish.
Tides affect other aspects of oceanic life, including the reproductive activities of fish and ocean plants. Floating
plants and animals ride the tidal currents between the breeding areas and deeper waters. The tides help remove
pollutants and circulate nutrients ocean plants and animals need to survive.
Tidal Zone Foods
Crabs, mussels, snails, seaweed and other edible sea life inhabit the tidal zone. Small tide pools may also contain
small fish and sea vegetables. The sea life found in these regions are often harvested for food. Without the regular
washing of the tides, these complex and abundant creatures would die and food resources would diminish.
Navigation
Tides affect the depth and currents in and around coastal areas. Ships may need to navigate the waters during high
tide in some areas or risk running aground. Pilots take into consideration the water level, width of channels and
direction of the water flow to determine the best time to travel. Pilots may choose to travel when tides are at ebb
in order to get tall loads under bridges.
Tidal flows can also help or impede the progress of a ship in the water. Pilots can take advantage of the current
to get the craft where it needs to go. A thorough understanding of how tides affect navigation and how to use the
tides in navigation can improve the productivity of marine and inland shipping.
Weather
Tides and tidal currents affect the weather by stirring the ocean waters. The tides and tidal currents mix arctic
water that can’t absorb lots of sunlight with warmer topic water that does. The stirring produces more predictable
and habitable climate conditions and balances temperatures on the planet.
Tidal Energy
Two high tides and two low tides occur during every 24-hour period. The predictability of the tides, fast movement
of water during the inflow and outflow can provide a source of renewable energy to communities living along the
coast. Hydroelectric plants can exploit the water flow in ways similar to those used on rivers.
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Atmospheric circulation
On Earth, an atmospheric circulation takes place (see picture) which is triggered by the temperature difference on
the ground at the equator and poles. During the year, the sun is shining perpendicular at the equator whereby there
is no sun in winter. In the summer, the sun only shines from a shallow angle. Thus, different pressure areas which
trigger a large circulation between the equator and the poles are formed. Because of the earth´s rotation, a direct
flow between anticyclone (equator) and depression (poles) is prevented. In the northern hemisphere, the air
masses are defelcted to the right and in the southern hemisphere to the left. For that reason, three large circulation
cells are generated (Hadley cell, Ferrel cell and the Polar cell).
The main effects of the atmospheric circulation:
Continuous transport of humidity from the equator to the north and to the south tropics.
Transport of hot air and humidity from the tropics to the temperate zones.
Transport of warmer air and humidity from the temperate to the colder zones.
Hadley cell
At the equator, the air rises up, because of strong heating by the sun. At the tropopause (temperature inversion in
about 18km above ground), the air masses will deflect to the North and South. Through area correction, the air
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masses slide down to the poles. Furthermore, through the earth´s rotation, the winds fall until the 30th
latitude and
flow back to the equator as trade winds. At the equator, these winds meet in the Intertropical Convergence Zone
(ITCZ). This circulation is called Hadley cell.
During the ascend process, the air cools down, the steam inside condenses, clouds are build and it starts to rain
very strongly. In the descending process, the exact opposite is happening. The air gets warm and the water in it
starts to evaporate. Desert areas (such as Sahara or Namib Desert) around the 30th
latitude are consequences of
this procedure. There are several anticyclones in this region, which is caused by the warm air on the ground.
These are assigned to the subtropical ridge (horse latitudes).
Polar cell
Near the ground level, air currents flow from the poles toward the equator. These are called polar easterlies,
because they are distracted from eastside by the rotation of the earth. Near the 60th
latitude, the winds are
heated and rise up on the way to the South. This second circulation is called Polar cell.
Ferrel Cell
Based on air ascent (60th
latitude) and air cooling (30th
latitude), a third circulation is formed in the area between
the 60th
and 30th
latitude. This circulation is called Ferrel cell. Near ground level, there is an air transport towards
the poles wherey the air flows towards the equator at higher levels. In the northern hemisphere, the air on the
ground is distracted to the right and in the southern hemisphere to the left. The winds from the West are called
westerlies. The polar front is located on the border between the polar easterlies (cold) and the easterlies (warm).
This border is usually between the 60th
and the 70th
latitude. In this area, depressions often occure.
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Atmospheric Circulation in tropical & subtropical region :
The Tropical Hadley cells are clearly apparent on the global circulation map. The zone of uplift between the two
cells forms the Intertropical convergence zone (ITCZ). The energy for uplift is supplied by longwave radiation
from the surface and the latent heat released during condensation in rising cumulus clouds. The associated low
pressure is most intense over the tropical continents and the western Pacific archipelago: these are the main sites
of ascending warm, moist air. The ITCZ and associated low pressure zones are pulled polewards in the summer
hemisphere: southward into South America, Africa, and Indonesia/Australia in January, and northward into
northern Africa and the Himalaya/Indian subcontinent in July. The most northward position of the ITCZ is over
India in July, due to the intense heating of the subcontinent by the early summer sun. The northward and
southward motion of the ITCZ causes a seasonal reversal of wind patterns. In India, winds are south- westerly in
22. 22
the summer, bringing moist air on to the land, and northerly in the winter, bringing cooler, dry air from the
Himalayas. Similar patterns exist over East Asia, Africa, Australia, and South America. This seasonal reversal of
weather patterns is known as a monsoonal climate. The torrential rains commonly associated with the word
'monsoon' are a characteristic of the summer monsoon season.
The equatorial low pressure is less intense over the tropical Indian, Atlantic and eastern Pacific Oceans, where
more solar energy is consumed by evaporation. In many parts of these tropical seas, the uplift less intense, with
only weak surface winds in the zone of uplift. These latitudes were known as the Doldrums in the days of sail,Ê
when sailing ships could drift for weeks with limp sails in blistering heat. However, intense storms (known as
tropical cyclones or hurricanes) can form over the Tropical oceans.
The Inter Tropical Convergence Zone is fed by the low-level Trade winds, which are north-easterly in the northern
hemisphere and south-easterly in the southern hemisphere. These are particularly clear in the January map.
Coriolis deflection is weak at these low latitudes, so air flows across the isobars at a relatively low angle.
In the Sub-tropics, the descending limbs of the Hadley cells create areas of clear calm weather: these are the Sub-
Tropical Highs. These form permanent high pressure zones over the subtropical oceans, and tend to be best
developed in the winter hemisphere. Over the oceans, the calm sub-tropical high pressure zones were known as
the Horse latitudes. The origin of the term is obscure, although it has been suggested that it comes from the
frequent death of horses on ships becalmed en route to the colonies, so that the sailors would eat horse meat in
these parts of the world. (In his fascinating book ÔIn PatagoniaÕ, Bruce Chatwin recounted the lurid tale of a
hold-full of penguin meat that turned rotten as a northbound ship traversed these latitudes, with hellish
consequences for all on board.)
General Circulation of the Atmosphere
The circulation of wind in the atmosphere is driven by the rotation of the earth and the incoming energy from the
sun. Wind circulates in each hemisphere in three distinct cells which help transport energy and heat from the
equator to the poles. The winds are driven by the energy from the sun at the surface as warm air rises and colder
air sinks
23. 23
The circulation cell closest to the equator is called the Hadley cell. Winds are light at the equator because of the
weak horizontal pressure gradients located there. The warm surface conditions result in locally low
pressure. The warm air rises at the equator producing clouds and causing instability in the atmosphere. This
instability causes thunderstorms to develop and release large amounts of latent heat. Latent heat is just energy
released by the storms due to changes from water vapor to liquid water droplets as the vapor condenses in the
clouds, causing the surrounding air to become more warm and moist, which essentially provides the energy to
drive the Hadley cell.
The Hadley Cell encompasses latitudes from the equator to about 30°. At this latitude surface high pressure
causes the air near the ground to diverge. This forces air to come down from aloft to "fill in" for the air that is
diverging away from the surface high pressure. The air flowing northward from the equator high up in the
atmosphere is warm and moist compared to the air nearer the poles. This causes a strong temperature gradient
between the two different air masses and a jet stream results. At the 30° latitudes, this jet is known as the
subtropical jet stream which flows from west to east in both the Northern and Southern Hemispheres. Clear
skies generally prevail throughout the surface high pressure, which is where many of the deserts are located in
the world. From 30° latitude, some of the air that sinks to the surface returns to the equator to complete the
Hadley Cell. This produces the northeast trade winds in the Northern Hemisphere and the southeast trades in the
Southern Hemisphere. The Coriolis force impacts the direction of the wind flow. In the Northern Hemisphere,
the Coriolis force turns the winds to the right. In the Southern Hemisphere, the Coriolis force turns the winds to
the left.
From 30° latitude to 60° latitude, a new cell takes over known as the Ferrel Cell. This cell produces prevailing
westerly winds at the surface within these latitudes. This is because some of the air sinking at 30° latitude
continues traveling northward toward the poles and the Coriolis force bends it to the right (in the Northern
Hemisphere). This air is still warm and at roughly 60° latitude approaches cold air moving down from the poles.
With the converging air masses at the surface, the low surface pressure at 60° latitude causes air to rise and
form clouds. Some of the rising warm air returns to 30° latitude to complete the Ferrel Cell.
24. 24
The two air masses at 60° latitude do not mix well and form the polar front which separates the warm air from
the cold air. Thus the polar front is the boundary between warm tropical air masses and the colder polar air
moving from the north. (The use of the word "front" is from military terminology; it is where opposing armies
clash in battle.) The polar jet stream aloft is located above the polar front and flows generally from west to east.
The polar jet is strongest in the winter because of the greater temperature contrasts than during the
summer. Waves along this front can pull the boundary north or south, resulting in local warm and cold fronts
which affect the weather at particular locations.
Above 60° latitude, the polar cell circulates cold, polar air equatorward. The air from the poles rises at 60°
latitude where the polar cell and Ferrel cell meet, and some of this air returns to the poles completing the polar
cell. Because the wind flows from high to low pressure and taking into account the effects of the Coriolis force,
the winds above 60° latitude are prevailing easterlies.
Water Circulation
Water that is normally present in porous rocks, such as sand or gravel, fills all the voids in a continuous manner.
However, in karst rocks, the water forms courses of water which at times become large underground rivers that
flow in enormous galleries whose diameter is many meters wide and are many kilometres long. The water of the
underground streams flows in the same way as those on the surface, and similarly they are subjected to floods
caused by rainfalls on the surface (in caves, floods arrive with a certain delay in time, due to slow seeping in the
catchement zone). Water is able to entrench and erode rock by means of mechanical abrasion processes, to
transport sediments of various granulometries, and to create alluvial deposits inside caves.
25. 25
Causes of water circulation
The movement of air through Earth's -- or any planet's -- atmosphere is called wind, and the main cause of Earth's
winds is uneven heating by the sun. This uneven heating causes changes of atmospheric pressure, and winds blow
from regions with high pressure to those with low pressure. This happens on a local scale to produce storms as
well as gentle breezes, and it happens on a planet-wide scale to produce global wind patterns.
Atmospheric Pressure
Winds are a result of the tendency of warm air to rise and, conversely, cool air to sink. When the sun's energy
heats up a part of the ground or the sea and the heat radiates into the air, the air rises, creating a low pressure area
underneath it. Because the atmosphere constantly flows, like a fluid in a container, air rushes into such low
pressure areas from cooler, high-pressure ones, and the movement of the air creates wind. The strength of the
wind depends on the pressure difference. A small difference produces a gentle breeze, while a large difference
may produce a gale.
The Trade Winds
The sun heats the equator more than it does the poles, so there is a perennial movement of air between these two
regions. Hot air at the equator rises into the troposphere at the equator, creating a low pressure area underneath it
that is filled by cooler air from higher latitudes. The hot air cools as it moves toward the poles, and at about 30
degrees north and south latitude, it cools, sinks and circulates back to the equator as the west-moving trade winds.
This air pattern produces dry conditions at 30 degrees latitude, where many of the world's deserts are located.
The Jet Streams
Because the air at the poles is cooler than the air at lower latitudes, it has a tendency to move toward the equator.
It doesn't move in a straight line, however, because the Earth's rotation exerts a tangential force called the Coriolis
force. The effect of this force isn't straightforward. It bends winds at the poles toward the west, but between 30
and 60 degrees latitude, it bends them in the opposite direction, creating the westerlies. In the upper troposphere,
the westerlies can blow at speeds of 160 kilometers per hour (100 miles per hour). These upper tropospheric winds
are known as the jet streams.
26. 26
Local Winds
Topography and ground composition are two of the factors that can influence local wind patterns. For example,
beach sand radiates more of the sun's heat back into the atmosphere than seawater, so people at the sea shore
usually enjoy a sea breeze on hot days. In mountainous areas where the tops of the mountains receive more
sunlight than the lowlands, there is often an updraft along the mountain slopes. Such drafts are an important factor
in the spread of forest fires because air from cooler regions rushes to replace the air displaced by the heat of the
fire, creating a wind that helps the fire to spread.
Water circulation patterns
As the prevailing winds in earth’s atmosphere blow across the surface of the oceans, the winds push water in the
direction that they’re blowing. As a result, the surface water of the oceans moves in concert with the air above
it.This dual movement creates large circular patterns, or gyres, in each of the planet’s oceans. The ocean gyres
move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Ocean gyre circulation moves cold surface water from the poles to the equator, where the water is warmed before
the gyres send it back toward the poles. The water’s temperature influences the temperature of the air: Cold
currents bring cooler air to the coastline as they move toward the equator, and they bring warmer air to the
continents they pass on their way back toward the poles.
Patterns of ocean circulation: Upwelling
Sometimes the movement of surface currents along a coastline leads to a circulation process called upwelling. As
a result of the Coriolis effect, upwelling commonly occurs on the west coast of continents, where the surface
waters moving toward the equator are replaced by deeper cold water that moves up to the surface. The deep water
brings with it nutrients from the bottom of the ocean. These nutrients support the growth of primary producers,
which support the entire food web in the ocean.Regions of the world where deep ocean upwelling occurs are often
very productive with high numbers of many different types of organisms living in them.
27. 27
Patterns of ocean circulation: Thermohaline circulation
The largest circulation of water on the planet is a direct result of changes in temperature and salinity. Salinity is
the measure of dissolved salt in water. The pattern of ocean currents related to salinity and temperature is called
the thermohaline circulation (thermo = heat; haline = salt). This figure gives you a general idea of what this pattern
looks like.
Sometimes called the thermohaline conveyor belt, this circulation pattern moves cold water around the globe in
deep water currents and warmer water in surface currents. A single molecule of water being transported by
thermohaline circulation may take a thousand years to move completely throughout the Earth’s oceans.
The conveyor is driven by changes in the density of water as a result of changes in both temperature and salinity.
Here’s how this circulation pattern works:
1. Warm water in a shallow current near the surface moves toward the North Pole near Iceland. As this
water reaches the colder polar region, some of it freezes or evaporates, leaving behind the salt that was
dissolved in it. The resulting water is colder and has more salt per volume than it did before (and thus is
more dense).
2. The cold, dense, salty water sinks deeper into the ocean and moves to the south, as far as Antarctica.
After it makes its way near Antarctica, the cold, deep current splits, one branch moving up toward India
into the Indian Ocean and the other continuing along Antarctica into the Pacific Ocean.
3. Each branch of the cold, deep current is eventually warmed in the Indian Ocean or the northern part of
the Pacific Ocean. Although the water still contains the same amount of salt, it’s a little less dense
because it’s warmer than the cold water surrounding it; as a result, it moves upward, becoming a surface
current.
4. The warm, shallow, less dense surface current moves to the west, across the Pacific Ocean, and into the
Indian Ocean, where it rejoins the Indian Ocean branch. Both branches then continue into the Atlantic
Ocean and head back toward the North Pole.
Environmental scientists who study global climate change are interested in how increased ice melting in the
Arctic and Greenland will affect the thermohaline circulation. The addition of large amounts of fresh water will
reduce the salinity and density and may change the pattern of global ocean circulation
Water circulation pattern of tropical region
Tropical oceans encircle Earth in an equatorial band between the Tropic of Cancer (23.5° North latitude) and
the Tropic of Capricorn (23.5° South latitude). The central portions of the Pacific and Atlantic Oceans and most
of the Indian Ocean lie in the tropics. The warm tropical oceans play a critical role in regulating Earth's climate
and large-scale weather patterns. Much of the planet's biological diversity resides in the tropics, and the global
distribution of species and ecosystems depends on oceanographic and atmospheric processes that occur in the
equatorial oceans.
Heat from the Sun drives global circulation of Earth's oceans and atmosphere. Much of that critical solar
radiation initially falls on the tropics, where the Sun lies almost directly overhead for the entire year. The water
temperature of tropical oceans thus typically exceeds 20°C (68°F) and stays relatively constant throughout the
year. Particularly intense radiation directly over the equator evaporates seawater and forms a mass of very
28. 28
warm, humid tropical air that subsequently rises and cools as it flows north and south. Because cool air holds
less moisture than warm air, the water vapor quickly condenses into clouds and falls as precipitation. Heavy,
warm, yearround rains are a hallmark of Earth's tropical regions. Fragile, biologically diverse ecosystems such
as rainforests and coral reefs thrive in the warm, wet tropics.
Uneven heating of the sea surface between the tropics and the poles creates heat-driven convection currents in
the atmosphere and oceans. * Vertical circulation in the tropical oceans also affects the distribution of heat and
biological nutrients throughout the global ocean. In general, surface water sinks at downwellings where surface
currents flow toward a continental coastline, or where two surface currents converge. Deep ocean water rises to
the surface at upwellings where surface currents flow away from land, or where surface currents diverge.
Tropical downwellings transfer heat and nutrients to the deep-ocean circulation system. At tropical upwellings
White sand, palm trees, and warm, shallow water comprise the classic image of a tropical beach. The brilliant
turquoise hue of clear tropical waters is largely the result of the selective scattering and absorption of visible
light .cool, oxygen-rich and nutrient-rich deep water supports abundant marine life. Because normal tropical
currents flow from east to west, downwellings often occur along the east coasts of tropical continents, and
upwellings are common along their west coasts. In the Pacific Ocean, for example, an upwelling off the west
coast of South America usually feeds extremely productive fisheries of coastal Peru and Ecuador, and a
downwelling in Polynesia forces warm, oxygen-depleted water into the deep ocean.
Tropical upwellings support huge populations of microscopic plants and animals called phytoplankton and
zooplankton. Plankton, in turn, feed many species of fish and other marine life, and humans who depend on fish
for food. Tropical fisheries account for about half of the world's fish catch, even though tropical oceans
represent only 0.01 percent of Earth's ocean volume. Coral reefs are another well-recognized feature of tropical
oceans. The seas surrounding tropical islands and low-latitude continental shelves away from major river deltas
are ideal for coral reef formation. Over millennia, very large reefs have formed in the Caribbean Sea, and
29. 29
especially in the southwest Pacific Ocean. For example, the Great Barrier Reef of northeastern Australia covers
thousands of square kilometers.
Effects of ocean water circulation
Benificial
1. Increased oxygen levels through improved water movement at the surface
2. Allow food to be carried to the tank inhabitants and corals
3. Allow wastes to be carried away from the tank inhabitants and corals
4. Help with the photosynthesis and calcification processes of corals
5. Help prevent the build-up of detritus
6. Helps to create a more natural environment for your tank inhabitants.
7. Can help to prevent some types of marine algae from getting a strong foot hold in your aquarium.
Harmful
1. Excessive nitrates and phosphates from allowing detritus to build up and remain in dead spots.
2. Good conditions for pathogenic bacteria to grow (the bacteria that causes infections in fish and corals).
3. Slow growth in corals
4. Ineffective or reduced biological filtration from your live rock
5. Ineffective or reduced filtration from your skimmer (or other forms of filtration you are using).
Importance of water circulation
Increased oxygen levels through improved water movement at the surface.
Allow food to be carried to the tank inhabitants and corals.
Allow wastes to be carried away from the tank inhabitants and corals .
Help with the photosynthesis and calcification processes of corals.
Help prevent the build-up of detritus.
Helps to create a more natural environment for your tank inhabitants.
Can help to prevent some types of marine algae from getting a strong foot hold in your aquarium.
Some of the problems you could have if you have too low water movement or too low flow :
Excessive nitrates and phosphates from allowing detritus to build up and remain in dead spots.
Good conditions for pathogenic bacteria to grow (the bacteria that causes infections in fish and corals).
Slow growth in corals.
Ineffective or reduced biological filtration from your live rock.
Ineffective or reduced filtration from your skimmer (or other forms of filtration you are using
