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Landfall-Learning > Science > 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.


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.


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.



If you would like to try your hand at predicting the weather, check out the Websites below to see the conditions where you are and where Landfall is. For a classroom exercise, let a different student predict the weather for both places and give the weather report to the class each morning.