The range of a light depends first on its elevation above sea-level and secondly on its intensity. Most important lights are of sufficient power to render them visible at their full geographical range in clear weather. On the other hand there are many harbour and other lights which do not meet this condition.
The distances from which lights are visible, given in lists of lights—except in the cases of lights of low power for the reason given above—are usually calculated in nautical miles as seen from a height of 15 ft. above sea-level, the elevation of the lights being taken as above high water. Under certain atmospheric conditions, and especially Aith the more powerful lights, the glare of the light (by reflection from the clouds) may be visible considerably beyond the calculated range.
The elevation of the light above sea level need not, in the case of landfall lights, exceed 200 ft., which is sufficient to give a range of over 20 nautical miles. One hundred and fifty feet is usually sufficient for coast lights. Lights placed
on high headlands are liable to be enveloped in banks of fog at times when at a lower level the atmosphere is comparatively clear (e.g., the old Beachy Head light). No definite rule can, however, be laid down, and local circumstances, such as configuration of the coast line, must be taken into consideration in every case.
Landfall stations should receive first consid eration and the choice of location for such a light ought never to be made subservient to the lighting of the approaches to a port. Subsidiary lights are available for the latter purpose. Lights installed to guard shoals, reefs, or other dangers should, when practicable, be placed seaward of the danger itself, as it is desir able that seamen should be able to "make" the light with confi dence. Sectors marking dangers seaward of the light should not be employed except when the danger is in the near vicinity of the light. Outlying dangers require marking by a light placed on the danger or by a floating light in its vicinity.