How the label is built
The parts always come in the same order:
- Rhythm: how the light flashes, for example Fl, Oc, Iso or Q, with any group in brackets.
- Colour: W white, R red, G green, Y yellow. If no colour is given, the light is white.
- Period: how long one complete sequence takes, in seconds, for example 10s.
- Elevation: the height of the light, in metres, for example 12m.
- Range: its nominal range, in nautical miles, for example 9M.
So Fl(3)G.10s is a green light flashing in groups of three, the whole pattern repeating every 10 seconds. Oc.WRG.6s12m9M is an occulting sector light showing white, red or green depending on your bearing, repeating every 6 seconds, 12 metres high, with a range of 9 miles.
The rhythms
| On the chart | Name | What you see |
|---|---|---|
| F | Fixed | A continuous, steady light. |
| Fl | Flashing | Light shorter than dark, flashing fewer than 50 times a minute. |
| LFl | Long flashing | A single flash lasting at least 2 seconds. |
| Fl(2) | Group flashing | Flashes in groups, here of two, with a longer dark gap between groups. |
| Fl(2+1) | Composite group flashing | A group of two flashes, then a single flash. |
| Oc | Occulting | Light longer than dark: a steady light that briefly goes out. |
| Oc(2) | Group occulting | Eclipses in groups, here of two. |
| Iso | Isophase | Equal periods of light and dark. |
| Q | Quick | 50 to 79 flashes a minute, usually 50 or 60. |
| VQ | Very quick | 80 to 159 flashes a minute, usually 100 or 120. |
| UQ | Ultra quick | 160 or more flashes a minute. |
| IQ | Interrupted quick | Quick flashes broken by regular dark gaps. |
| Mo(A) | Morse code | Flashes of different lengths spelling a Morse letter, here A (· —). |
| Al.WR | Alternating | A light that changes colour, here white and red. |
Where you'll see them
- Cardinal marks use white Q or VQ, in groups that follow a clock face: continuous for north, three for east, six plus a long flash for south, nine for west. See cardinal marks explained.
- Preferred channel marks are the ones that use Fl(2+1). See lateral marks explained.
- Isolated danger marks show white Fl(2), and safe water marks white Iso, Oc, LFl 10s or Mo(A).
- Sector lights show different colours over different arcs, usually to lead you down a safe channel in the white sector, with red and green either side.
Identifying a light at night
Count the flashes in a group and time the full period with a stopwatch, from the start of one sequence to the start of the next. Then match it to the chart or almanac. Two lights that look similar at a glance, say Fl(3) 10s and Fl(3) 15s, are easy to tell apart once you time them.