Boot Key Harbor
"Marathon- the Heart of the Florida Keys"

An On-Line Cruising Guide for the Florida Keys & Cuba
Best viewed at 1024x768 or higher

USE YOUR BROWSER BACK BUTTON TO RETURN TO THE PREVIOUS PAGE

MARINE WEATHER DEFINITIONS, AND SIGNIFICANT WAVE HEIGHT

(See the related discussion of FULLY DEVELOPED SEAS)

Significant Wave Height (commonly referred to as Seas in the Marine Forecast)

This is the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. This is measured because the larger waves are usually more significant than the smaller waves. For instance, the larger waves in a storm cause the most beach erosion, or the larger waves can cause navigation problems for mariners. Since the Significant Wave Height (Seas) is an average of the largest waves, you should be aware that many individual waves will probably be higher.

If we take a sample forecast of Seas Beyond the Reef of 2 to 4 feet, this implies that the average of the highest one-third waves will have a Significant Wave Height of 2 to 4 feet. But mariners need to keep in mind that roughly one of every ten waves will be greater than 4 feet; one in every one hundred waves will be greater than 5 feet; and one in every 1000 waves will be greater than 6 feet. 10% of the waves will be 1.29 times the Significant Wave Heights (SWH) listed here. The average seas will be 0.64 times this SWH, and the tallest waves will be twice the SWH.

As a general rule, the largest individual wave one may encounter is approximately twice as high as the Significant Wave Height (or Seas).

Note: Seas can refers to the combination or interaction of wind waves and swells (combined seas) in which the separate components are not distinguished. This includes the case when swells are negligible or are not
considered in describing sea state.

DEFINITIONS

Gale Warning: A warning of sustained winds in the range 34 to 47 knots (KT) inclusive either predicted or
occurring not associated with tropical cyclones.

Gust: A rapid fluctuation of wind speed with variations of 10 KT or more between peaks and lulls.

Hurricane Warning: A warning issued by the National Hurricane Center (also known as the Tropical Prediction Center) for sustained winds of 64 KT or greater either predicted or occurring associated with a tropical cyclone.

Knot: Unit of speed used in navigation, equal to 1 NM per hour or about 1.15 statute miles per hour.

Marine Weather Statement: A product to provide mariners with details on significant or potentially hazardous
conditions not otherwise covered in existing marine warnings and forecasts.

Nautical Mile: Unit of distance used in marine navigation, equal to approximately 1.15 statute miles (length of 1 minute of latitude).

Small Craft Advisory: An advisory in coastal waters for winds from 20 to 33 KT inclusive and/or for seas, either predicted or occurring, that are considered potentially hazardous to small boats. At the Key West office they use 7 feet or greater as the threshold. However, there is no legal definition for "small craft."

Special Marine Warning: A severe local storm warning affecting coastal water areas, or a warning of potentially hazardous weather conditions usually of short duration (2 hours or less) and producing wind speeds of 34 KT or more that is not adequately covered by existing marine warnings. Also, this product is issued for waterspouts, either predicted or occurring.

Storm Warning: A warning of sustained winds of 48 KT or more, either predicted or occurring, not associated with tropical cyclones.

Sustained Wind: The wind speed obtained by averaging observed values over a period of at least 1 minute.

Swell: Wind-generated waves that have traveled out of their generating area. Swells characteristically exhibit
smoother, more regular and uniform crests and a longer period than wind waves.

Tropical Storm Warning: A warning of sustained winds from 34 to 63 KT inclusive either predicted or
occurring, associated with tropical cyclones.

Waterspout: A rotating column of air, usually pendant from a cumulus or cumulonimbus cloud, that forms over water and whose circulation extends to the surface.

USE YOUR BROWSER BACK BUTTON TO RETURN TO THE PREVIOUS PAGE