Newsletter

Meteorological Terminilogy

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	poor	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