Meteorological Terminology
To simplify the description of weather, meteorologists use very specific terminology.
Wind strength: The Beaufort Scale
| wind speed (kts) | descriptive term | sea state in open water and max. wave height (in m,) | ||
| 0 | <1 | calm | mirror-like | |
| 1 | 1-3 | light air | ripples | 0.1 |
| 2 | 4-6 | light breeze | small wavelets | 0.3 |
| 3 | 7-10 | gentle breeze | some breaking crests | 1 |
| 4 | 11-16 | moderate breeze | fairly frequent white horses | 1.5 |
| 5 | 17-21 | fresh breeze | many white horses and possibly spray | 2.5 |
| 6 | 22-27 | strong breeze | extensive white horses and spray | 4 |
| 7 | 28-33 | near gale | foam begins to form streaks downwind from crests | 5.5 |
| 8 | 34-40 | gale | well – marked foam streaks | 7.5 |
| 9 | 41-47 | severe gale | tumbling crests, spray may effect visibility | 10 |
| 10 | 48-55 | storm | sea surface generally white with foam | 12.5 |
| 11 | 56-63 | violent storm | wave crests blown into froth, forming long white patches on surface | 16 |
| 12 | >64 | hurricane | air filled with foam and spray, surface white, visibility badly affected | >16 |
The wind speed refer to sustained wind speed (at 10 m above the surface), not to transient gusts which may be considerably stronger.
Note that the sea state descriptions are a guide to what may be expected in the open sea: the Beaufort scale should not be used to describe sea states, and you should bear in mind that conditions may be very different in coastal waters.
Sea State
*crest to trough
| code | descriptive term | wave height (m)* |
| 0 | calm-glassy | 0 |
| 1 | calm-rippled | 0 – 0.1 |
| 2 | smooth | 0.1 – 0.5 |
| 3 | slight | 0.5 – 1.25 |
| 4 | moderate | 1.25 – 2.5 |
| 5 | rough | 2.5 – 4.0 |
| 6 | very rough | 4 – 6 |
| 7 | high | 6 – 9 |
| 8 | very high | 9 – 14 |
| 9 | phenomenal | >14 |
Visibility
Different terms are used to describe visibility, depending on the context – land, sea, or air.
| marine | aviation & coastal | land | |
| >5 NM | good | ||
| 2 – 5 NM | moderate | ||
| 1 km- 2 NM | poor | ||
| 1 km- 2 km | mist or haze | ||
| 200m- 1 km | fog | fog | fog |
| 50-200m | thick fog | ||
| <50m | dense fog |
Timing
Timing can be crucial, especially if you are looking for a ‘weather window’ in which to make a dash for home.
| time from issue of forecast | |
| imminent | < 6 hours |
| soon | 6 – 12 hours |
| later | > 12 hours |
Movement of weather systems
| slowly | < 15 knots |
| steadily | 15 – 25 knots |
| rather quickly | 25 – 35 knots |
| rapidly | 35 – 45 knots |
| very rapidly | > 45 knots |
Pressure Changes
| change in past 3 hours | |
| steady | > 0.1 mb |
| slowly | 0.1 – 1.5 mb |
| rising or falling | 1.6 – 3.5 mb |
| quickly | 3.6 – 6.0 mb |
| very rapidly | > 6.0 mb |
| now rising | a previous trend has reversed, and pressure is now rising or now falling |
| now falling |
RYA Courses
Our Sailing school
Impression
