249
What will the weather bring? Is the yacht and her crew fully prepared?
—Erroll Bruce, This Is Rough Weather CruisingA falling barometer and southerly winds warn of a low-pressure system headed
your way. What do you need to do to get out of its path? How far from a cliff
face must you stay to avoid deadly “ricochet” winds? What four things must you
do to prepare for an approaching squall line under power or sail?
In Th is Chapter, You’ll Learn How To:
D Use a barometer and wind direction to avoid storms at sea
D Maneuver during the fury of a line squall
D Modify a sea state forecast for greater accuracy
D Use terrain eff ects to safely forecast cruising weather
D Predict rough weather on the lee side of an island anchorage
A Mariner’s Most Essential Weather Predictor
Th e barometer is to weather forecasting what a knife is to the sailor. Th is simple
instrument measures the weight of a column of air at the sea surface. Modern
barometers convert this weight to millibars or inches. Normal sea level pressure
is 1013 millibars or 29.92 inches. If you observe changes above or below these
average pressures, you can predict wind, waves, and precipitation.
WIND CREATION
Wind occurs when the air masses over two locations contain diff erent barometric
pressures. Small changes in pressure between two locations usually result in
light to moderate breezes. Large pressure changes can brew reefi ng breezes and
a steep chop in protected waters, or heavy seas off shore.
BAROMETRIC READINGS—TRACKING THE TREND
Base your forecasts on the trends you detect in a series of hourly barometric
readings. Note whether the barometric pressure remains steady, rises, or falls.
WEATHER AND
12 WATER WISDOM