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Weather
Our single largest concern in making any ocean passage is the
prevailing weather. Because sailboats move slowly, most passages
take from several days to several weeks to accomplish. During
that entire time, we are at the mercy of the elements. For us,
the winds and the currents
determine where we can go, when we can go, and how comfortable
(or uncomfortable) we are while we're getting there. Our onboard
electrical systems and electronics
help us with our navigational tools,
and we work hard at our navigational
methods, but in the end, the weather can never be tamed. All
we can do is to try to understand it a little better.
In the middle of the ocean, there is nowhere for us to go should
we encounter a bad storm or big seas. While we always welcome
the rain to help us fill our water tanks, the accompanying high
winds and large waves can make life on the boat very uncomfortable
at times. Also, the direction of the prevailing winds can dictate
our course and force us to steer hundreds of miles out of our
way to reach our desired destination. So for our comfort and safety,
we keep a very close eye on the weather, and try to predict a
favorable "weather window" to make each passage. Because we can't
always just listen to the local weatherman, we need to constantly
watch the weather and make our own predictions.
Fortunately, scientists have been studying the winds and currents
of the world's oceans for so long that we have a very accurate
picture of the major prevailing winds and currents. It is these
major patterns that drive the earth's various climates and weather
changes around the world.
Winds
Three factors are primarily responsible for the earth's winds:
- The earth rotates on its axis from west to east
- Warm air rises and cold air sinks.
- Air moves from areas of higher pressure to areas of lower
pressure.
The world's winds are born at the equator, where the intense
sunlight heats the water. The warm water heats the air, which
rises and moves toward the poles, where it cools, sinks, and moves
back towards the equator.
At the same time, the earth's rotation causes west-to-east winds
at the middle latitudes of both hemispheres, and east-to-west
trade winds in the tropics of both hemispheres. These prevailing
global conditions are what creates the "jet stream" that your
local weatherman describes on television.
Jet Streams
Jet streams are like "rivers of wind" high in the atmosphere
(above about 20,000 feet) that steer storms. The jet stream essentially
controls the majority of our weather in the United States. Strictly
speaking, as defined in the American Meteorological Society's
Glossary of Weather and Climate, a jet stream is "a relatively
narrow river of very strong horizontal winds (usually 50 knots
or greater) embedded in the planetary winds aloft." The "planetary
winds aloft" are the part of the earth's general circulation of
air that's not at the surface.
Jet streams form along the upper air boundaries of large masses
of warm and cold air. During major cold outbreaks over the United
States, the jet stream often dives south, staying above the warm-cold
boundary and sometimes moving as far south as the Gulf of Mexico.
During unusually mild winter weather and during the summer, the
jet stream retreats far to the north all the way into Canada.
Highs and Lows
As the name says, a "high" is an area where the air's pressure
is higher than the pressure of the surrounding air. A "low" is
where it's lower. No particular number divides high from low pressure;
it's the relative differences that count. The pressure is high
at the surface where air is slowly descending (much too slowly
to feel) over a large area. As air descends, it warms, which inhibits
the formation of clouds. This is why high pressure is generally
associated with good weather.
In an area of low pressure the air has a tendency to rise. This
general upward motion means that there is less pressure from the
air pushing down on the earth. As air rises, it cools. If there
is enough water vapor it may condense to form clouds and rain.
This is why a low pressure is generally associated with wet weather.
The greater the difference between a given set of high
and low pressure systems, the harder the wind will blow. The harder
the wind blows over water, the larger the waves on the water's
surface become.
Currents
Ocean currents are created when ocean water is driven by the
global winds, by the earth's rotation, and by differences in temperature
and salinity.
Cold arctic water flows from west to east and from the poles
to the equator, and from the equator back toward the poles, creating
big circulating rivers of water. The Gulf Stream is one of these
ocean currents.
The warm Gulf Stream flows from near the equator northward up
the east coast of North America. As it nears the Arctic, it bends
eastward towards Europe, cooling as it flows. The European landmass
forces the now-cold current to turn southward. Where it does this,
it is renamed the Canary current.
The cold Canary current flows south down the European and African
coasts towards the equator, where it is progressively warmed.
By the time it gets to about 15 degrees North latitude, it is
fully warmed and turns back to the west, flowing towards the Caribbean
and southern US. There, it is forced to turn north, becomes the
Gulf Stream again, and carries its warm water up the coast until
it cools back down in the arctic and repeats the journey.
So this clockwise rotation of currents in the Northern Hemisphere
creates our familiar Gulf Stream, which brings warm air and water
up the East Coast of the United States and makes our Southeastern
beaches so pleasant and breezy. This same type of current activity
is mirrored in the Pacific Ocean. However, while the United States
sits on the western side of the Atlantic Ocean, we sit
on the eastern side of the Pacific Ocean. So on the Pacific
Coast of North America, due to the clockwise motion of the currents,
rather than having warm air and water flowing north up the coast
from the equator, the West Coast beaches experience cold air and
water flowing south from the Arctic. Ever notice how many California
surfers wear wetsuits, even in the summer? Well, that's why!
South of the equator, these currents flow counterclockwise, with
the air and water currents making the circuit between the Antarctic
and the equator.
Mixing it All Up
For the most part, it is the conflict of hot and cold air from
the jet stream and the Gulf Stream that causes our storms on the
east coast of the United States. The hot air rising causes "low
pressure" systems and sinking cold air causes "high pressure"
systems. Since all matter migrates from areas of higher pressure
to areas of lower pressure, when this happens to air in the atmosphere,
the result is wind.
Air and water warm up and cool off at different rates. Because
of this, there are times when the air currents and ocean currents
flow in different directions. This condition will cause the waves
on the water's surface to become steeper. Along the Gulf Stream,
this condition happens so often that some sailors even refer to
these waves as "square elephants". When the forces of
wind and water oppose each other more violently, they can cause
the large circular storms known as hurricanes (in the Atlantic)
or cyclones (in the Pacific).
As water currents pass through the warmer temperatures closer
to the equator, the heat causes some of the water to evaporate
and rise up into the atmosphere. This water vapor is in turn carried
along with the surrounding air. As this air moves up and away
from the equator it begins to cool; the water condenses and eventually
forms clouds and ultimately rain, sleet or snow.
Armed with this basic knowledge about global wind and current
patterns, and the interaction of high pressure and low pressure
systems, we can watch the weather maps on the Internet to form
our own predictions of what the local weather will be over the
next few days. We also carry a barometer and wind meter on board
Landfall to verify what we see on the weather maps.
We can also use our knowledge of the prevailing winds and currents
to plan our routes. We pick routes to allow us to take advantage
of the elements and to get an extra push from the natural flow
of the winds and ocean currents. We can also use this knowledge
to avoid certain areas during the hurricane and cyclone seasons.
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