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

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