weather and water wisdom 267
Diffraction of a Swell
Stay well clear of the area just behind the pointed ends of islands or breakwaters.
Wave energy carries beyond the point into a shadow zone. Th is results in an
abrupt change in wave height, called diff raction. A cone of turbulent water forms
just behind each point. Swell height and strength quickly diminish beyond this
area. Move to the center of the lee side before you lower your anchor.
How to Use Terrain Eff ect in Cruise Planning
A landmass changes the prevailing wind direction and strength; it also decreases
or increases wave heights. Use these elements in cruise and voyage forecast plan-
ning. Annotate your charts (see Chapter 2) to indicate landmass areas that might
cause any of these eff ects.
Sea breeze eff ect. Steady, prevailing winds blowing parallel along the coastline
warn of rough conditions by aft ernoon. Aft er the sea breeze fi lls in, the wind
shift s to a direction between the two winds. Wind speed may increase by 50%.
For example, a steady, 20-knot northerly wind blows along a western coastline.
In the aft ernoon, a westerly sea breeze blows toward shore. By late aft ernoon, the
prevailing wind and sea breeze combine into a 30-knot northwesterly gale.
Cliff eff ect. On your chart, cliff s appear as hachures, which resemble teeth-
like serrations on a saw blade (see Chapter 1). Use caution anytime your charted
position brings you close to these symbols. Cliff eff ects can change wind speed
and direction in three ways:
- Winds blowing onshore at narrow angles oft en change direction and
increase in speed near a cliff base. - If a wind rolls down the face of the cliff wall, it sends high, gusty winds
far off shore. - High winds that blow onto a cliff wall create a ricochet eff ect, and can
send gale force winds off shore to a distance equal to ten times the cliff
height. For example, if you are passing a 600-foot cliff in a breeze, stay at
least 1 mile off shore (10 × 600 feet = 6,000 feet or 1 nautical mile).
Island eff ect. Study the cloud layers over islands with high terrain, such
as mountains or cliff s (and remember that mountainous regions appear on the
chart as non-concentric circles; sometimes the elevation is written somewhere
along the circles, but not always). Low clouds near the peaks, covering all sides
of the island, signify rough conditions on the lee side. Clear skies to leeward
indicate calm waters (a wind shadow).
Mountain eff ect. Fall winds form in Pacifi c and northern Mediterranean
regions with coastal mountain ranges. Th ese winds rise over the snowcapped