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Handbook No. 1
See recent photos from NDBC weather buoy 42039 in the Gulf of Mexico.
Glossary of Terms for Detailed Wave Information
Date/Time Format (YYYY MM DD hh)
The date/time is of the form, 'YYYY MM DD hh', which represents the year, month, day and hour of the measurement in GMT. GMT is Greenwich Mean Time and is five hours ahead of Eastern Standard Time. For Daylight Savings Time, the difference is one hour less. In other words, 12 GMT is 7 AM EST (8 AM EDT). For West Coast mariners, Pacific Standard Time = GMT - 8 hours.
Swell Direction (SwD)
This is the direction that the swells are coming from. Swells are waves not produced by the local wind and come in at a higher period (longer wave length) than waves produced by the local wind. Direction is given on a 16 point compass scale.
MM or -99 denote missing data. Less than half of our buoy stations report directional wave data because of the costs involved with additional instrumentation. NO indicates that no swells could be determined.
Swell Height (SwH)
This is the estimated average height of the highest one-third of the swells. It is estimated from determining how the wave energy is distributed among various periods (frequencies), determining if a separate swell energy peak exists, and then, picking a frequency to separate swell and wind-waves. The swell height is calculated from the wave energies below the separation frequency.
Swell Period (SwP)
This is the peak period in seconds of the swells. If more than one swell is present, this is the period of the swell containing the maximum energy.
Wind-Wave Direction (WWD)
This is the direction that the wind-waves are coming from. Wind-waves are produced (or were recently produced) by the local wind. If a swell is present, these waves arrive at a lower period (more frequently) than do the swells. Direction is given on a 16 point compass scale.
MM or -99 denote missing data. Less than half of our buoy stations report directional wave data because of the costs involved with additional instrumentation.
Wind-Wave Height (WWH)
This is the average height of the highest one-third of the wind-waves. Again, it is estimated by the process mentioned under "Swell Height", except that it is the calculated from the energies above the separation frequency.
Wind-Wave Period (WWP)
This is the peak period in seconds of the wind-waves.
Overall Height and Period (H0 and AVP)
This is the significant wave height and dominant wave period that has been traditionally available. Significant wave height is the average height of the highest third of the waves. If both swell and wind-waves are present, it should equal the square root of the sum of the squares of the swell and wind-wave heights. Dominant period is the period with maximum energy and is always either the swell period or the wind-wave period.
Reported as either "VERY STEEP", "STEEP", "AVERAGE", or "SWELL". For a given wave height, steep waves represent a more serious threat to capsizing vessels or damaging marine structures than broad swell. It is determined by examining the significant wave height and the dominant wave period when compared to climatology.