that have spent the season in southern climes
Weather Systems
are once again northbound. To determine a
Air near the earth’s surface tends to move
good “weather window” for departure, you
from areas of high pressure to areas of low
should always try to time your departure
pressure. The winds and waves that affect
based on the most recent forecasts.
mariners are attributable mainly to that
En route: While en route during an ex-
movement. In the absence of powerful local
tended passage, your weather strategy shifts
factors, a large difference in pressure be-
to finding the most efficient and comfort-
tween the highs and lows (a steep gradient)
able track for your vessel. It’s important to
produces strong winds; a small difference
think about “the big picture” and to obtain
(usually indicated by steady or slowly chang-
synoptic analyses and forecasts. You need to
ing atmospheric pressure in your area) pro-
study weather patterns that are hundreds,
duces weak winds.
and even thousands, of miles away to deter-
Pressure differentials are usually created in
mine what type of wind and seas they will
the first instance by the uneven heating of
bring and whether you can plot a course to
air. When air over a warm surface is heated,
avoid rough conditions.
it expands and becomes lighter. It rises and
Pre-arrival: When your destination is a day
the pressure over this warm area decreases.
or two away, your focus should again be on
The process over a cold surface is reversed.
coastal and local conditions—this time, to de-
There, the cooled air near the surface be-
termine the safest way to make port at a desir-
comes heavier, contracts, and sinks. At high-
able time. Consider not only coastal weather,
er levels, air moves horizontally to fill the
but also how winds, waves, and tides will in-
space vacated by the sinking air. This adds
teract with coastal geography at your destina-
to the total air mass over the cold region and
tion. Consider the probable sea state in areas
increases the atmospheric pressure there.
of shoaling (at the Continental Shelf or near
Through this process heat is redistributed
shore) and possible wind anomalies caused
between warmer and colder regions. If this
by prominent landmasses. Consider also the
were a simpler world, a single circulating cell
possibility of fog when there is a significant
r
would be operating in each hemisphere. Ris-
temperature difference between the air and
ing warm air from the low pressure equato-
water. A prudent mariner will often delay his
rial belt would flow at high altitudes toward
nighttime arrival at a new or unfamiliar port
e
athe
the poles, where it would be cooled and sink
until daybreak when visibility is optimal.
W
to the surface in the high pressure polar re-
Weather Routing gions and then flow south near the surface
To make the most of the abundant weather
back toward the equatorial low.
information available today, some com-
In the real world, the process is much more r
mercial mariners and offshore yacht racers
complicated, mainly because the earth is
and cruisers hire professional shore-based
spinning. The spin creates the Coriolis ef-
weather routers, a trend that is growing in
fect, which induces the wind to curve to the
e
athe
popularity for contemporary sailors. A rout-
right in the Northern Hemisphere and to the
er analyzes government and private weather
left in the Southern Hemisphere. The warm,
data using various computer models. A
polar-bound air from equatorial regions
package typically contains weather forecasts
cools enough in comparison to surrounding
for a week or more, including surface and
air that it tends to sink when it reaches the
upper-air weather charts, ocean current
vicinity of about 25 to 35 degrees of latitude,
r
standing W
analysis (if appropriate), custom wind and
where the pressure is semi-permanently high.
sea state forecasts, and a suggested route with
Likewise, the cold, equator-bound air from nde
specific waypoints based on your vessel’s
polar regions warms enough by the time it
U
performance profile. Depending on the path
reaches the vicinity of 50 to 60 degrees of
and duration of your voyage, the router may
latitude that it tends to rise, creating a semi-
also provide updates while you are at sea and
permanent belt of low pressure there. So, in
propose alterations to the suggested route. A
fact, there are three circulating cells in opera-
list of professional routers appears on page
tion in each hemisphere, with separate pre-
811.
vailing wind patterns in each. At the surface,
winds flowing from the east (the trade winds)
781