In keeping with maritime conventions wind speed is reported in knots.

Forecast or observed winds refer to the mean wind. In observations, this is the average wind speed and direction recorded over the previous ten minutes and is recorded at the international standard height of ten metres above the sea surface. It is known that wind gusts will cause temporary fluctuations about this mean and that maximum gusts are typically up to 40% above the reported or forecast mean wind may be observed.

Forecasts of waves in coastal waters are given in metres and describe the significant wave height which is the average height of the highest one third of the waves. Forecasts for the high seas describe seas and swells using terms like slight, moderate, rough etc. The attached table relates these descriptive terms to the heights they represent. It is important to note that waves higher and lower than the significant wave height occur. Generally, in open water, a wave of 1.86 times the significant wave height can be expected in every thousand waves. For example, maximum wave heights corresponding to significant wave heights of 7, 9 and 11 metres are about 13, 17 and 20 metres respectively.

In the description and forecasting of waves, height refers to the vertical distance between the top of a crest and the bottom of a trough. This should not be confused with the length of the face of the wave which will be considerably longer than the vertical height.

In keeping with maritime conventions distances are quoted in nautical miles.

Surface Analysis Charts

Isobars (lines which join points of equal pressure) on the surface pressure analysis charts shown in Figs E2, E3, E6 and E8 have been drawn with a 2 hectoPascal (hPa) spacing. The values have been abbreviated according to the following convention:

1. Pressures above 1000 hPa have had the first 2 digits removed, eg the ‘1012’ hPa isobar is written as ‘12’.
2. Pressures below 1000 hPa have had only the first digit removed, eg the ‘994’ hPa isobar is written as ‘94’.