- Using the Web Site
- Review of Available Wave Data Sources
- The NMIMET Method of Analysis
- Wind Statistics
- Scatter Tables (Wave Height and Period)
- Extreme Wave Height Estimation
- Persistence of Storms and Calms
- Reliability and Validation
- Validation of the NMIMET Analysis
- Wave Height Data
- Wave Period Data
- Data Quality Control
- Comparison with 'Ocean Wave Statistics'
- Extreme Wave Height Validation
- Persistence Analysis Validation
- European Database
Wave Height Data
The results relating to wave height are shown in Figure 6 to Figure 14. In the case of the instrumental data, the heights are 'significant wave heights' usually denoted by the symbol Hs. This is defined as the mean height (from crest to trough) of the highest third of the waves in a sample, and is widely regarded as approximately equivalent to the visually observed height. It can be computed in various ways  and the formula Hs = sqrt(4m0) (where m0 is the area under the energy spectrum) is commonly used. The NMIMET data were derived using the 'smoothing mode' analysis described in Appendix_A3.
The raw visual data were derived from the so-called '2 Group' wind and wave observations, used as input to the NMIMET analysis. These were restricted to cases in which estimates of both sea height H1 and swell height H2 have been reported, and a resultant height Hr=sqrt(H1+H2) has generally been used (see Appendix A2). It is important that this should be appreciated when assessing the comparison of wave height data in 'Ocean Wave Statistics' with the data in Global Wave Statistics Online. The earlier data were derived from all the available wave observations but taking only the higher of the two heights in cases where both sea and swell were reported; they may thus be seriously misleading in areas where swell is important.
In evaluating the comparisons shown in Figure 6 to Figure 14, it should be noted that although instrumental data are generally considered to be the most reliable, their quality can vary. It should also be recognized that they cover much shorter periods of years than the visual observations. In the case of the Sevenstones Light vessel, comparisons are included using instrumental data available for both one-year and seven-year durations. For the wave height data the results are not very different (Figure 11 and Figure 12). For the wave period comparisons however (Figure 19 and Figure 20) the increased duration leads to considerably closer agreement. The results shown are fairly typical of all the comparisons so far made, and it is considered reasonable to assume that the wave height data in this book should be interpreted as equivalent to statistics of 'significant wave height'. It should thus not be necessary to apply any of the correction factors which are sometimes used  for estimating significant heights from visual observations of height.