Sea State

Forecast Conditions at Sea

Summary
Cast real time sea state conditions using buoys from NOAA's National Data Buoy Center.

Introduction
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.

Data Activity
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:

1. Gulf of Alaska
2. San Francisco, CA
3. Gulf of Mexico
4. Cape Canaveral, FL

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. Station 46078 - Albatross Banks, Alaska
2. Station 46012 - Half Moon Bay southwest of San Francisco, CA (Pacific Ocean)
3. Station 42001 - Gulf of Mexico south of Southwest Pass, Louisiana
4. Station 41010 - Offshore Cape Canaveral, Florida (Atlantic Ocean)

Conversions:
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.

Discussion Questions

1. How are the ocean and the atmosphere related?
2. Why are the wind and the air-sea interface important to fish and other underwater fauna?
3. Why are land animals, like humans, dependent upon the air-sea interface?

Extensions

1. Using an anemometer, have your students record wind data outside your classroom and determine the Beaufort Force. For a more in-depth project, have students record wind data (at school or from ocean observing buoys) several times a day over the course of the school year and evaluate the data. What month/time of year does the wind blow the most? What time of day does the wind blow the most?
2. Have students make their own anemometer.
3. Using a digital camera, have students take pictures of objects around campus under different wind strengths, and create a custom Beaufort Scale. *Use discretion if it's too windy outside!
4. Examine current and wind regimes in the buoy locations. When are the strongest currents and winds? Do they both flow in the same direction year-round?
5. Explore the global winds and circulation and the Coriolis Effect.
6. Further explore density and convection currents.