Visual Data

Visual observations of wave height, period and direction have been reported from ships in normal service all over the world since 1949; wind speeds (Beaufort Scale) and directions and wave heights in a coarse code have been reported since 1854. This vast quantity of observations has been collected under the auspices of the World Meteorological Organization (WMO), and has been archived by meteorological agencies in a number of designated member countries. Since the observations are made in accordance with guidance notes prescribed by the WMO, they represent data which are globally homogeneous in quality. They also cover most sea areas of practical interest, with concentration of density on the main shipping routes.

A considerable number of atlases containing data derived from these observations have been published [1] [2] [19 - 23], in addition to several regular series of 'Monthly Climatological Summaries' published by the data holding agencies.

'Ocean Wave Statistics' [2] was published in 1967 and contains raw statistics of wave heights, periods and directions covering 50 sea areas involving about a million sets of visual wave observations from the archive then available at the UK Meteorological Office.

The book 'Global Wave Statistics' [1] on which this program and database are based, was published in December 1986. The major advantages offered by the new book and program database are the much greater reliability and area coverage made possible by the development of the NMIMET analysis program, and the massive increase in the number of observations available.

The reliability of visual data is more fully discussed in later sections, but the quality achieved by use of the NMIMET analysis has been demonstrated by extensive comparisons with measured data on a world-wide basis and has been well documented [24 - 27]. It should be emphasized that the statistics of wave period found on this site have been derived from the wave height observations using well validated methods. They are thus much more reliable than the visual observations of period given in various atlases, which have been shown to be very misleading.

Area coverage is discussed more fully later, but it should be noted that the Marine Databank at the UK Meteorological Office [28] now contains about 55 million sets of observations, which include all the available archives from the majority of the WMO member countries. It has thus now been possible to achieve much denser coverage of more than twice the sea areas than was the case with 'Ocean Wave Statistics'; it has also been possible to include the Pacific, which was not covered in the earlier book.