Atmospheric opacity: A study of visibility observations in the British Isles

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Abstract
A station may be classified as to visibility by using visibility frequencies to evaluate the atmospheric opacity, which is directly proportional to the extinction coefficient. Mean values of the atmospheric opacity in clear conditions and in foggy conditions at various stations in the British Isles are utilised to infer the contribution of the various types of nuclei and particles to the opacity, and in particular to infer the relative efficiency of each type as a fog‐producer.It is concluded that sea‐salt nuclei are much larger than combustion nuclei and play a leading part in the determination of visibility. The effect of combustion nuclei is negligible unless they are extremely numerous, but the concomitant smoke particles contribute largely to impairment of visibility and when plentiful may even produce thick fog.Water fogs are due to sea‐salt nuclei and if supersaturation is attained the fogs may thicken considerably owing to the deposition of water on the sea‐salt nuclei which consequently become very much larger. Such deposition does not occur on the combustion nuclei if sea‐salt nuclei are present.The variation in opacity with relative humidity is calculated theoretically and the calculations are applied to observations of nuclei, particles, and relative humidity at Kew Observatory in the afternoon. Fair agreement is found between the calculated values of opacity and those which correspond to the visibility observed.In one appendix to the paper factors are evaluated by which the visibility of reflecting objects viewed against sky or terrestrial backgrounds may be compared with the visibility of a black object silhouetted against the sky. In a second appendix the increase in opacity during precipitation is calculated.

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