waves and swell forecasting
Now days forecasting waves, tides, swell
and surfing conditions is quite easy, there are many websites
offering from daily to hourly surf conditions.
Read on for an overview of how waves are made and how wave
predictions are made for your local surf report.
How are waves made?
Waves are formed by
wind blowing over the water surface. The three things
needed to make waves are the wind speed; it's duration
and fetch (the size of the wind area). If the wind duration
is more than 24 hours, the swell is fully developed and
even if it blows for a longer time at that speed, the
waves will not get any bigger.
If the fetch is more than 200 miles or so, then the wave
becomes fairly well developed. If the wind blows for less
than 24 hours or the fetch is smaller than 200 miles the
waves will be smaller.
Lastly but most importantly
is the wind speed. The stronger the wind the bigger the
wave. Also waves made in a stronger wind will last over
a longer period of time.
How to recognise a swell
On a weather chart
there are high and low pressure systems. They are separated
by isobars (the lines on the chart). If you see on a weather
map an area with the isobar lines close together and straight
for more than 200 miles and the pattern is not moving
very quickly, then this would probably mean it is a good
swell making weather pattern.
How does the swell move?
Waves move away from
the swell making area in 'wave groups'. If you have winds
stronger than 35 knots the waves will have a bigger wave
period and move quickly through the water.
The swell will move in roughly the same direction as the
wind, but will spread out at an angle of 15 degrees. This
spreading out is why swells die out. After a swell travels
about a 1000 miles it will be half its original size.
Over 1000 miles the swell holds it size for longer.
In the above weather chart you can see
that the isobar lines at the bottom are close together and
stay straight for more than 200 miles. This is a perfect swell
making pattern, with strong winds and big waves.
Deep and shallow water
As waves pass from the
oceans to the shallow continental shelf they will change
from being deep water waves to shallow water waves. This
causes a drag on the wave and slows it down. When the
water becomes less than one and a half times the wave
height, it is likely to break, which is what happens on
Shallow water wave
If a wave approaches
a beach at an angle, the part closest to the beach is
in shallower water and therefore travels slower than those
further away so the whole wave front tends to be turned
into the beach. When the wave reaches the beach, it will
be parallel to it.
At a headland, waves
are turned towards the point and break along its side
at a much bigger size than on the beach. Also, on individual
sand bars the surf will break at a bigger size than anywhere
else. This makes ideal surf conditions.
How are rogue waves made?
If there are two swells
coming from different areas they could become in sync
and so the wave height becomes the sum of the two in sync
waves. Therefore two waves which are 3 feet in height
if in sync could produce a wave of 6 feet. Read more about
Where can I find a weather chart?
The best source of charts
is on the Internet. There are heaps of charts to choose from.
You can also get hold of satellite pictures and weather forecasts
in addition to computer generated swell forecasts.
Check out these charts
books and information
mailing/postal addressSaltwater Dreaming
108/3 moo 3
Tel: +66 76 271050
Fax: +66 76 325629
Surf Shop (location map)
Surin Beach, Phuket, Thailand
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