Finding the Visibility at Sea

In meteorology, visibility is a measure of the transparency of the atmosphere. It is defined as being the greatest distance at which an object of specified characteristics can be seen by a person of normal vision under conditions of average daylight illumination.

The visibility may be impaired by fog, haze, dust-storms, smoke, snow or rain, but for reports from sea the code figures 90 to 99 are used regardless of the cause of any impairment. On land it is easy to obtain accurate measurements of the visibility as the distance to many landmarks around the observing station are known and a much finer scale may be used.

At sea, on a large vessel it is easy to decide from known distances on deck if 90 (<50m) or 91 (50m) or even 92 (200m) is the appropriate code figure. Code figures 93 and 94 are more difficult as they are for distances greater than the length of the vessel, but the visibility will require a reduction in speed.

When coasting and the land can be seen, the distance at which landmarks are first sighted, or last seen, can be measured from the chart or by fixing. In the open sea, radar can be used to measure the range at which ships are first sighted. Note that if the visibility is not the same in all directions, the shortest distance should be reported. An observation of the horizon does not always produce a reliable value and the result should be treated with caution. Abnormal refraction can be very misleading and some allowance must be made for ‘height of eye’.

At night the estimate of visibility is even more difficult. The distance to be reported is the distance that could be seen if normal daylight conditions were assumed to exist. The distance that can be seen depends upon the illumination. If there is no obvious change in meteorological conditions, the visibility after dark will be the same as that recorded just before dark irrespective of the fact that one may not be able to see so far. It is important not to confuse a change in illumination such as the moon rising or setting with a change in visibility.

The presence of a ‘Loom’ around a vessel’s navigation lights is frequently a guide to deteriorating visibility.

The lights of light-houses can be used to help measure the visibility, but the distance at which a light can be seen depends on its power. A luminous range diagram must be used, as a light may be seen at night in conditions when the light-house would not be visible during the day.

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