Reckoning / Weather / Visibility
Weather · marine forecasts
Visibility
Visibility is the greatest distance at which a dark object can be seen and recognised against the sky. At sea it decides how vessels must navigate and which sound signals they must make.
Forecast terms
| Term | Visibility |
|---|---|
| Very poor | less than 1000 metres |
| Poor | 1000 metres to 2 nautical miles |
| Moderate | 2 to 5 nautical miles |
| Good | more than 5 nautical miles |
As used in UK shipping forecasts: Coast and sea glossary: visibility terms used in shipping forecasts.
Fog, mist and haze
| Fog | Visibility below 1000 m because of water droplets in the air. |
| Mist | Visibility reduced by droplets but still 1000 m or more, with high humidity. |
| Haze | Visibility reduced by dry particles such as dust or smoke. |
Then and now
Older seamanship tables graded fog by the distance at which objects disappeared, from dense fog at about 50 yards to excellent visibility beyond 30 miles. Forecasts now use four plain terms with fixed limits.
In restricted visibility ships must sound the fog signals on the sound signals page.