Visibility, cloud and weather
1The lesson in plain words
Estimate visibility against known ranges, give cloud amount in oktas with the cloud types, and describe present and past weather.
Visibility
Visibility is the greatest distance at which you can see and identify a suitable dark object against the horizon sky in daylight. At night, estimate equivalent daylight visibility; a light’s range also depends on brightness. Use radar ranges of suitable objects just visible, checking that Earth’s curvature does not hide them. Assess all directions and report the shortest distance. The software codes it in steps from under 50 m to 50 km or more. Fog is in Fog and visibility.
Cloud amount and type
Give the total cloud amount in oktas, eighths of the sky covered: 0 is clear and 8 is overcast. The code uses 9 when fog or snow hides the sky. Then name the low, middle and high cloud types you see and the height of the lowest cloud base. Cloud names and what they tell you are in Clouds and humidity; here you pick the code that fits best, using the cloud atlas pictures in the software.
Present and past weather
Present weather describes now or the previous hour. Use table priorities, usually the highest applicable code; code 17, thunderstorm without precipitation now, takes precedence over codes 20–49. Let the software apply these priorities, rather than choosing by number alone. Past weather covers 6 hours at main synoptic hours and 3 hours at intermediate hours: rain, snow, thunderstorm, fog or haze. Select all applicable conditions; the software fills the two past-weather fields.
Ice and spray
Report ice accretion on the ship: its cause, such as spray, fog or freezing rain, its thickness and how fast it builds. Severe icing with gale-force wind and sub-freezing air is also a danger message, covered at the end of this unit. Iceberg reporting in the North Atlantic is in Ocean weather.
2Key data
- Cloud amount
- Oktas: 0 = clear, 8 = overcast, 9 = sky obscured
- Visibility
- Coded steps, from under 50 m to 50 km or more
- Present weather
- Use table priorities; usually the highest applicable code
- Past weather
- 6 h at main hours, 3 h at intermediate hours
3Key points
- 1
Assess all directions; report the shortest visibility using suitable targets and allowing for Earth’s curvature.
- 2
Cloud amount in oktas; 9 means the sky cannot be seen.
- 3
Present weather covers now and the last hour; past weather the 6 or 3 hours before.
- 4
Report ice accretion with its cause, thickness and rate.
4Frequent mistakes
Wrong: Visibility is good because the radar shows targets at 12 nm.
Right: Use a suitable object you can just see and identify, checking its radar range and horizon limit. Assess all directions; report the shortest visibility.
Wrong: With fog all round and no sky visible, report 8 oktas.
Right: When fog hides the sky, the cloud amount is coded 9: sky obscured.
5Practice case & exam keys
At 0600 UTC, a ship just visible at 4.5 nautical miles (nm) gives the shortest visibility around the horizon; radar confirms its range and the horizon does not hide it. A headland at 7 nm is invisible. Drizzle began 20 minutes ago; rain fell from 0100 to 0300. Low stratus covers 7/8 of the sky.
Show the answer
Visibility 4.5 nm, or 8.3 km: the step for 4 km to under 10 km. Present weather: drizzle, since it is falling now. Past weather for the 6 hours since 0000: rain and drizzle; select both in the software. Cloud: 7 oktas, low cloud stratus, with its estimated base height.
Exam keys
- Visibility: identify suitable targets, allow for the horizon, assess all directions and report the shortest distance.
- Cloud: total amount in oktas, then low, middle and high types and lowest base.
- Past weather: 6 h at main hours, 3 h at intermediate hours.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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