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Being able to accurately forecast the conditions at sea, or sea state, has been the goal of explorers, sailors, and fishermen for thousands of years. Now, through the use of ocean observing systems, we can not only predict, but pinpoint, exactly what the sea state will be like before leaving the dock.
First, let's take a look at what causes a wave. While just about any object can cause a wave, the most common water wave generating force is the wind. So what exactly causes wind?
Wind is the effect of the sun's uneven heating of the Earth's surface. As some parts of the Earth's surface are struck by the sun's energy at a sharper angle (up to 90º), the molecules in the air become warmer (creating a low pressure area) and they spread out in the troposphere. This makes the air mass less dense; therefore, it rises into the atmosphere. As the heated air mass rises, it cools (creating a high pressure area) and begins to sink. At the same time, air from cooler surface-level areas (especially from over water) moves in to replace the warmed air. This movement is known as a convection current of air. These convection currents, in combination with the spin of the Earth, create wind.
As wind moves across water, the friction between the two will cause wrinkles to form on the water's surface. A positive feedback loop is created as more wind catches these wrinkles creating an even rougher surface. The result is more surface area for the wind to catch, creating larger and larger waves. The strength of the wind, combined with how long the wind blows, the water depth and the size of the area over which the wind blows in one direction, or the fetch, will determine how large the waves become. Once any one of these factors decreases, the waves will begin to get smaller. Therefore, wave characteristics can be forecasted from wind data.
The Beaufort Scale
In 1805, British Admiral Sir Francis Beaufort (1774-1857) created a wind speed estimation system based on the conditions at sea. Consisting of a 0-12 scale, the Beaufort wind force scale ranged from calm to hurricane force winds (which are then measured by the Saffir-Simpson scale; tornadoes are measured by the Fujita scale). Despite many different forms, Sir Beaufort's scale is still recognized as the standard and is used all over the world for describing wind conditions both at sea and on land.
We will use four buoys from NOAA's National Data Buoy Center to now-cast real time sea state conditions.
Note: Because the NDBC and other observing systems maintain buoys worldwide, educators are free to choose their own buoys to work with, e.g. a more local waterway or favorite vacation destination. Most buoys should measure wind speed, however, not all buoys record wave height data.
First, locate the approximate location of each of the four observing stations on a map of the United States or a globe:
Next, visit each of the following buoy sites and, using the data sheet below record the data for the first four columns - date and time of the latest observation, wind speed, and wave height. *Be sure all units match!
Click here for a printable data sheet
1 knot = 1.15 mph = 1.852 km/h
1 mph = 0.87 knots = 1.61 km/h
1 km/h = 0.62 mph = 0.54 knots
1 meter = 3.28 feet
1 foot = 0.31 meters
Once you have recorded the data, compare the observed wind speed to the Beaufort Wind Force Scale and record the Beaufort Force and expected sea conditions.
If you have questions about this Data Tip or have suggestions for a future data tip, contact Chris Petrone.
|The Bridge is supported by the National Sea Grant Office, the National Oceanographic Partnership Program, and the National Marine Educators Association.|
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