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Introduction
How many times when at work, Uni, wherever,
you've asked yourself whether it's worth
taking the day off, chucking a sicky or
something and just bailing down the coast for
a few waves. A fair few I bet. "But what's it
going to be like?" "Is it worth going?" "Is it
worth missing that 3 hour prac. in the
afternoon?" "It could be good, but if I don't
go now before the sea-breeze hits it won't
be worth it." Does all this sound familiar?
When and where to catch the best waves
requires an understanding of "wave
prediction".
Much of the info. here will be familiar if
you've been surfing for a while, but if you
are one of the "uninitiated" to the game then
read on.
Wave prediction is "the art" of making
educated guesses as to where to surf, or
whether it's worth going for a surf. These
"guesses" are based upon a number of
factors that are combined together, then
interpreted and finally, a conclusion, or
decision reached. To predict waves
successfully we need to have an
understanding of the earth's weather,
understand how swell is generated, possess a
tide table and have a good local knowledge of
the area you surf (i.e. the breaks in your area
and the best conditions they work on).
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Elements of Wave Prediction
Wave prediction can be divided into two
facets. These are:


dynamic elements
static elements
Dynamic elements are those that change. The
change can occur over a period of few days
to a matter of 30 minutes (sometimes less).
Wind, swell and tides are examples of
dynamic elements. It can be argued that
shifting sand banks caused by wave action
and long shore currents are dynamic.
Sometimes changes can occur to sand banks
over a couple days, but rarely do significant
changes occur within this time at most sandy
beaches.
Static elements include the breaks
themselves, both beach and reef breaks.
These elements change over large time scales
in comparison to dynamic elements.
In wave prediction the most important
factors that determine what the surf is
going to be like are the dynamic elements.
Secondary to this are the static elements
(the breaks themselves).
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Dynamic Elements - Weather, Swell and
Tides
The processes involved in the earth's
weather are quite complex, so I'll try and
keep things simple. Bear in mind that the
information here is for the southern
hemisphere.
The earth's weather occurs in the
Troposphere. As solar radiation hits the
earth it is absorbed and the ground surface
is heated causing warming of the air. The
greatest heating occurs around the equator
producing an oversupply of heat. This heat
surplus produces a global-scale circulation
effect whereby the heated air moves toward
the earth's poles.
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Low Pressure Cells
Rising of warm air causes a decrease in
pressure (i.e. lowering of pressure) on the air
mass resulting in expansion of the air and
therefore cooling. This process forms a low
pressure cell. Due to the earth's rotational
forces (the Coriolis Force) the rising warm
air rotates. In the southern hemisphere,
rising warm air rotates in a clockwise
direction. Warm air absorbs water through
evaporation and can hold greater volumes of
water vapour than cold air. As the air rises in
a low pressure system it is cooled through
expansion. Cooling of the "wet" air causes the
water vapour to condense into droplets like a
fog, forming clouds. When the air becomes
over saturated with condensed water,
precipitation occurs. Precipitation is manifest
by rain, hail or snow.
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High Pressure Cells
As cool air descends the pressure exerted on
the air mass increases (i.e. increasing of
pressure) and the air undergoes compression
resulting in warming of the air at the earth's
surface. This process forms a high pressure
cell. The descending air rotates in an anticlockwise direction due to the Coriolis Force
and then moves toward an area of low
pressure. The moving air mass moves nearly
parallel to the pressure gradient (i.e.
isobars). Due to the warming of the air, the
air is able to hold more moisture producing
cloud free skies and settled weather.
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Wind
High and low pressure cells form circulating
belts around the earth. This coupled with
variations in earth surface temperatures
causes the formation of pressure variations,
or gradients. These pressure gradients
generate wind. The higher the gradient the
stronger the winds produced and vice-versa.
Pressure is measured these days in the
metric unit known as hectopascals. The
contour lines drawn on weather maps
represent pressure variations. These lines
are called isobars (i.e. lines of equal
pressure). The closer the isobars are
together the higher the pressure gradient
and therefore the stronger the winds.
The speed or velocity of wind is measured in
knots. 1 knot is equal to about 2 kilometres
per hour (kph). There is also another
measureable parameter, known as the "wind
chill factor". This is a measurement of the
chilling, or cooling potential of the wind.
Typically strong cold winds have high wind
chill factors.
Winds in high pressure systems circulate in
an anti-clockwise motion. Whereas, winds
associated with low pressure systems rotate
in a clockwise motion. Remember this is true
for the southern hemisphere. In the
northern hemisphere, the opposite occurs.
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Seasonal Changes to the Weather Patterns
During Summer in the southern hemisphere,
the high and low pressure belts encircling the
earth move south. This is due to the sun
moving toward the southern hemisphere.
High pressure systems are typically located
around the 40 degree latitude in Australia
and the western side of the cells typically
produce north-easterly winds. Prevailing west
to south-westerly winds associated with the
low pressure belt to the south, move further
south into the Southern Ocean and by-pass
most of the continent. So does most of the
swell for the southern states resulting in
small inconsistent swells for much of
Summer. In the northern topics, tropical low
pressure systems predominate with north to
north easterly winds bringing monsoonal
weather.
Sometimes tropical storms, or cyclones on
the north-eastern Australian coast move
south bringing torrential rain to southern
Queensland and the southern states (if the
depression is close enough to the coast) and
good east to north-easterly swells for east
facing coastlines (e.g. the renowned
Queensland cyclone swells). As the system
moves further south toward Victoria it is
possible to get offshore humid northeasterly winds with associated storms and
torrential rain. These systems can move as
far south as Tasmania. Sometimes they bring
good easterly to north-easterly swells to the
east coast of Tassie during the Summer
months.
During the Winter months, the high pressure
belt moves northward to approximately the
25 degree latitude to maintain its position
with the northward shifting position of the
sun. The low pressure belt in the tropics
moves further north and the prevailing winds
shift to south to south-easterlys bringing
clear skies and settled weather (i.e. the "dry
season" in the northern tropics). The low
pressure belt in the Southern Ocean also
moves northward, bringing strong west to
south-westerly winds and stormy cold
weather to southern states and the desired
consistent Winter ground swells. As the high
pressure systems pass across Australia, the
western area of the system produces
dominantly north-west winds.
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Cold Fronts
Cold fronts are more common during the
Winter months in Australia due to the
northward movement of the low pressure
belt encircling the earth. They form when a
cold air mass moving in from the south-west,
replaces a warm air mass, typically defining
the boundary between a high pressure and a
low pressure system. Associated with cold
fronts are sharp changes in pressure and
temperature producing strong cold winds, a
change in the wind direction from
northwesterly to west to south-westerly,
storms and rain. The approach of a cold front
is typically manifested by wispy high cloud
called Cirrus.
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Sea-breezes and Morning Offshores
Sea-breezes are perhaps the most annoying
of winds. The surf may be goin' off with
perfect offshore conditions on a hot sunny
day when suddenly a gust of onshore wind
hits you in the face. 10 minutes later and the
wind is blowing onshore. Sea-breezes can
therefore turn perfect conditions into
onshore slop in a matter of tens of minutes,
often spoiling a session.
Typically sea-breezes occur in the afternoon,
especially when the offshore wind is light. As
the land heats up during the day the
corresponding air mass over the land is also
heated. If the temperature of the air mass
over the land rises above that of the air
mass over the ocean, the heated lighter air is
forced to rise. The rising of the land mass
air creates an area of low pressure and the
cooler air from the ocean flows in toward the
land producing a sea-breeze. The circulation
of air continues until such time the
temperature of the air over the land
decreases to less than or equal to the ocean
air temperature. This typically occurs in the
late afternoon. Often the wind swings back
around to offshore so that you can get a late
glassy session in before heading off for the
day.
The direction of the sea-breeze is dependant
upon the direction of the offshore wind such
that sea-breezes associated with north-
easterly offshores usually have some sort of
easterly direction to them (e.g. southeasterly). Likewise, south-westerly sea
breezes are typically associated with
prevailing northwesterly offshore winds.
Sea breezes are more common in the Summer
months when higher land temperatures are
the norm. The lesson here is that if you
decide to take a sicky and drive down for a
wave mid to late morning and you live a
distance from the coast, chances are that by
the time you get down there the wind may
have swung round from offshore to onshore
with the development of a sea-breeze.
You're better off telling the boss the day
before so that you can get up early the next
day and get down early before the seabreeze hits!
Morning offshore winds work in a similar
fashion, but the air circulation is in the
opposite direction to sea-breezes. Morning
offshore winds are produced when the air
temperature over the ocean exceeds that of
the land. A low pressure zone is formed over
the ocean as the air rises and cooler air from
the land flows out over the ocean replacing
the rising warmer air.
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Swell Formation
Swell is produced by wind blowing over the
ocean. The factors that control the size of
the swell are the speed of the wind, the
length of time it blows and the size, or length
of the area it blows over (i.e. the Fetch).
Swell sizes can be mathematically calculated
using the latter parameters, but I'm not
going to get into the gory details here. Have
a read of Lud's Wave Prediction FAQ for
these details.
There are two types of swell:


ground swells
wind swells
Ground swells are waves that have been
generated many thousands of kilometres
away in the deep ocean where strong winds
have been blowing over a large fetch for
many days. The direction of the ground swell
is independent of the current prevailing
winds at the coast. Typically the wave length
of the waves in the open ocean are large, in
the order of many tens of metres to
hundreds of metres. Ground swells produce
the best waves for surfing because of their
chunky powerful character, and can last from
a couple of days to about a week, forming
perfect lines as they reach the coastline like
"corduroy to the horizon" (using a well known
metaphor). It is during the Winter months
when the low pressure systems have moved
north that consistent ground swells hit the
southern coast of Australia as well as
Tasmania. Speak to most surfers during
Summer and they'll tell you that they can't
wait until the Winter swells start pumping in!
The direction in which the ground swell
travels is important. If it's travelling the
wrong way it may by-pass a coastline. On a
smaller scale, some breaks work best on
particular swell directions. In Victoria, the
prevailing swell comes from the south-west
and there is really no need to worry about
swell directions when it comes to wave
prediction; just how big the swell is. In
Tasmania around Hobart and the east coast
of Tassie, swell direction and size is
extremely important as some breaks for
example, work only on north-easterly swells
and others on southerly swells. For breaks
that only work on these swells having a
south-westerly swell suggests the spots may
not be worth checking. Having to take swell
direction into account adds an extra
complexity when it comes wave prediction in
comparison to Victoria and the east coast of
Australia in general.
Wind swells on the other hand, are produced
by the prevailing wind and travel in the same
direction as the wind. They are produced
over small fetches where the wind blows for
a short period of time. The wave length of
wind swells is much shorter than ground
swells; in the order of metres. Consequently,
they are much less powerful than ground
swells and only last for short periods of time.
Wind swells typically form peaky, messy
waves that are frustratingly gutless. Most of
the time they are just take off waves with
little or no face to surf along. On days where
strong south-westerly winds are blowing, it is
possible to surf the wind swell on some
suburban beaches around Port Phillip Bay,
Victoria.
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Tides
Tides are very important in wave prediction,
because of their effect on swell size and
surf conditions. Tides are changes in sea
level produced by the gravitational forces of
the moon and the sun. Because the moon is
closer to the earth than the sun, it exerts a
greater influence on sea level changes. When
the moon and the sun align together, the
combined gravitational forces are at their
maximum, producing very high tides known as
Spring tides and very low tides. Spring tides
occur about twice a month when there is a
New, or Full moon.
The greatest gravitational force from the
moon at single point on the earth is attained
every 24 hours and 50 minutes, which
essentially advances the tide times at that
point by 50 minutes each day. In southern
Australia, there are two high tides and two
low tides every day. In northern parts of
Australia there can be only one tidal change
per day because of the stronger influence of
the sun's gravitational pull. Tidal variations
(i.e. the height difference between the high
and low tide) also differs depending on your
location in Australia. Up in the north-west
region of Western Australia the tidal
variation can be as much as 12 metres. On
the other hand, in Hobart the tidal variation
may be as small as 30-40cm. The variety of
tidal variations is due to a number of
independent variables such as coastline
geography. Also in some places, the tidal
forces cancel each other out, and in other
places they combine. In Australia, the
greatest tidal variations typically occur
around Summer time, as the sun moves more
overhead. Due to this, the suns influence, or
gravitational pull is stronger than during
Winter. Couple this with full and new moons,
and the tidal variations peak (i.e. high high
tides and low low tides).
On the incoming tide, waves tend to be
bigger and more consistent. On the outgoing
tide waves reduce in size and become less
consistent. This is true for the majority of
surfing breaks including beach breaks. As the
tide recedes, reef breaks become more
shallow and sometimes the waves become
quite "sucky" barrelling over quite shallow
reef. Probably about 80% of reef breaks are
high tide breaks. Beach breaks vary quite a
bit such that some banks work best on low
tide as the water becomes shallower over the
bank. The neighbouring bank (perhaps a 100m
up the beach) may work best on the high
tide. In some cases beach breaks and point
breaks can work both on the high and low
tides, but usually the swell is smaller on the
low tide. Few reef breaks work best on low
tide in comparison to high tide breaks.
Typically on the high tide these breaks are
"too full", or the depth of water over the
reef is too great for the waves to break
properly. It is important to know whether a
break works best on the high or low tide.
Such information comes from local knowledge
of a particular break.
N.B. Make sure you have a copy of the tide
tables for your area. These are available
from Port Authorities, local surf shops,
fishing tackle shops, yacht clubs and on the
web (for Australia).
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Static Elements
Static elements are those that change little
over time such as coastlines and surfing
breaks. Beach breaks however, are prone to
seasonal changes due to long shore current
shifts and changes in swell size. Infact,
changes can occur to the sand banks over a
week.
Types of surfing breaks
Surfing breaks can be broken up into four
types:




reef breaks
beach breaks
point breaks
river mouth breaks
Reef breaks are those where the waves
break over a shelf of rocks or coral. They
typically never change and the waves break in
a consistent manner (i.e. you can return to a
reef break after a number of years and find
that the wave characteristics haven't
changed). A well known example is Bells, in
Victoria.
Beach breaks are those that occur along
sandy beach coastlines such as Surfers
Paradise. Waves break over sand banks that
are sculptured by wave forces, long shore
currents and outgoing rips. Sand banks
change over time and often vary depending on
the season. For example, storm surf during
Winter causes erosion of the beach and
redistribution of the sand further out to sea.
Calm surf during Summer causes the built up
of sand on the shoreline in general.
Therefore, wave characteristics at individual
banks are liable to change over time. The
position of sand banks at Woolamai in
Victoria remain fairly consistent, often
because there is some reef below the sand.
So much so that some are named (e.g.
Anzacs). However, even though they remain
in the same spot along the beach, the wave
characteristics can vary from one month to
another and from Winter to Summer. A
common question that surfers ask one
another is "what are the banks like?" before
making a decision to surf the "beachies".
Point breaks occur where a bluff of land
extends out into the ocean. Approaching
waves break along the side of the point
usually producing good quality waves. Waves
may either break on a sand base that has
built up around the bluff, or on reef. Like
reef breaks, wave characteristics rarely
change. Some examples of point breaks
include National Park at Noosa (Queensland),
The Bluff (Western Australia) and Lennox
Head (New South Wales).
River mouth breaks occur where a river flows
out into the sea. They are similar to beach
breaks, but the main difference is that sand
is built up around a river mouth. River mouth
sand banks can be quite mobile and can vary
significantly from month to month, or day to
day. Therefore, wave characteristics readily
change over time. In some circumstances,
large bays with narrow entrances to the open
ocean have sand banks on their ocean side
that produce consistent breaks. Next to the
sand bank there is usually a deep channel
where water flows in through the narrow
entrance and out as the tide changes.
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Wave Characteristics at a Break
As a wave approaches shallower water it will
break when the water depth is 1.3 times the
height of the wave. For example a 2 metre
wave will break when the water depth is 2.6
metres. Waves come in sets of three to five.
In between the sets there may be no waves,
or just smaller waves. However, the wave
characteristics of various breaks differ.
Some breaks produce fast powerful
barrelling waves, whereas others produce
weak, frustratingly gutless waves. Therefore,
wave characteristics at breaks can be
divided into:


gutless slow waves
powerful fast waves
Wave characteristics are dependant on the
profile of the base that the wave travels
over. If an approaching wave moves across a
base that has a mellow profile or slight
gradient, the wave will break slowly. Such
waves are slow and gutless, difficult to catch
and a strain to get any speed once you've
caught one. A good example of such wave
characteristics occurs at Tidal River, Wilsons
Promontory (Victoria).
If the profile of the bottom is steep, or has
a sharp gradient, then the waves will jack-up,
and have some decent grunt. If the wave
approaches from deep water and then
suddenly hits a shallow reef, it will suck and
jack-up, surging forward producing fast
sucky barrelling waves. These waves can also
be described as "dredgey" waves because
they suck so hard on the take-off (e.g. "a
dredgey take-off", or "the break really
dredges"). Experienced surfers try and surf
these waves as much as possible due to their
power, barrelling nature, and challenge. They
also get well and truly drilled by them as
well!. These types of "heavy" waves are found
exclusively at reef breaks. Good examples
include Bird Rock and Meanos (pronounced
"mean-oh's") in Victoria. Some steep profile
beaches also produce fast sucky waves as
swell moves in from deep water and hits
shallow sand banks (they are also fairly
treacherous for a leisurely swim, because of
strong rips). A good example are some of the
beachies at Kilcunda (Victoria).
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Local Knowledge - The Most Important
Factor Of All!
Perhaps the most important factor in wave
prediction is local knowledge of your surfing
area. If you don't know where the breaks
are, or haven't worked out the best
conditions they work on, you can "wave
predict" till you're blue in the face and still
not get a wave. Local knowledge can be the
most difficult aspect of wave prediction to
learn and usually takes the longest. Once you
have the local knowledge you will be able get
waves under most swell and wind conditions,
even if it's just to get wet. Some of the
things that you need to get a grip on are;



where the surfing spots are
what are the best conditions for these
to work on
indicators of where to go
The first aspect is obvious. You need to know
where to get waves. If you don't know this,
then get out a map of your coastline and have
a look.
The second aspect takes time, experience
and patience to learn. Things that you need
to know are the best tide, the best sized
swell (and swell direction at some places), and
the direction the wind has to be for offshore
conditions (i.e. "what's offshore?") for
breaks within your area. Other things that
you may want to know for personal
preferences is whether the break is a left or
right-hander, what sort of break it is (i.e.
reef, point etc.) and the wave characteristics
(i.e. whether it's a gutless wave, or a full-on
down the line hooter!).
The third aspect also takes time, experience
and patience to learn. To understand the
concept of using indicators, let's take some
simple examples......
Example 1
You get down the coast to find
that the waves where you usually
surf are too small. However, down
the road a bit you know the next
beach cops more swell and you
have a greater chance of getting
bigger waves. What you've done is
used your local knowledge and
used your normal surfing beach as
an indicator for potential better
waves down the road.
Example 2
You may have worked out through
experience that if a particular
local beach (that is never surfed)
has two foot waves, it means that
the swell must be southerly and
that a break you know of will be
about 3-4 foot on the sets. What
you've done to draw this
conclusion (without even going to
the break to check it) is you've
used an indicator.
Indicators are important for wave prediction.
If you don't know them for your area, or
haven't worked them out yet, you will
definitely miss out on good waves (believe
me!). If you are new to place and have met
and now surf with some of the locals, then
you will pick up on important indicators quite
quickly. If not, you will have to experiment.
To do this, make a note of how big the surf is
at your favourite surf break on a particular
given day. Then go and check some other
beaches or breaks (that may be closer to
home) to see how they compare. Make a note
of how they compare and remember it so
that next time you can use them as
indicators to predict how big, or even if it's
worth checking your favourite spot.
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Wave prediction - "Making the Big
Decision"
The majority of surfers have a "plan of
action" in mind when they head-off for a
surf. They hardly ever go down the coast
without a plan. This plan is a prediction of
what the waves are going to like and the
possible best breaks to surf given the
current weather and swell conditions. To
predict what the swell is going to be like, look
at the weather maps and swell charts. For
example, let's assume you've been looking at
the weather maps over a couple days and are
monitoring a low pressure system in the
Southern Ocean that is moving across the
Great Australian Bight toward the east. The
centre of the cell may be around 980
hectopascals and deepening. The isobars are
quite close together indicating sharp
pressure gradients and therefore strong
winds. You also notice that the isobars (that
are close together) cover a large area of the
ocean and therefore strong west to southwesterly winds are blowing over a large area,
or fetch. Under these conditions you can
expect a decent ground swell to be
generated. For the full brunt of the swell to
hit your favourite surfing coastline, the
prevailing winds at the top of the system
should be at least blowing toward you.
Take a look at the few examples of weather
charts and satellite images I've put together
during good swells. Hopefully they will give an
idea of "how" and "what" to look out for.
Finally the swell hits and you have to decide
where you're going to go for a wave. The
things that you need to look at are wind
direction, tides, swell size (and swell
direction in some places). Use these
combined with your local knowledge of
surfing breaks in your area to plan where to
go. You may want to check out a few
indicators before heading off on a decent
drive in search for a wave.
Something to mention in passing are
barometers. Barometers measure air
pressure and are a useful tool in wave
prediction as they possess the potential to
help you forecast weather conditions a couple
of days in advance. Use a barometer to tell
you whether the pressure is increasing, or
decreasing. If the pressure is decreasing it
is likely there is an approaching low pressure
system bringing unsettled weather and
possible increasing swell. Try and gauge how
quickly it is decreasing. The faster it
decreases the sharper the pressure gradient,
the closer the isobars are on a weather map,
and the stronger the winds. If the pressure
is increasing, then settled weather is on the
way as well as warmer temperatures and
offshore winds depending on where you're
going to surf.
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Finale
Hopefully you now know some of the finer points of wave prediction
and how complicated it can be. The majority of keen surfers have a
good understanding of the weather and their local surf spots, and
can forecast when and where the best waves will occur. To finish up,
make a point of monitoring the changing weather patterns and
developing your local knowledge of surfing spots and potential surf
indicators where you live. If you stuff up, don't despair: remember
that it's all a learning process and that you learn from your
mistakes.