A straightforward shore-based radar has been shown to be a reliable and effective aid to pilots in bringing ships in from sea in fog through the port channel area into the berthing area, with or without the assistance of the ships' own radar. Once there, however, further movement now depends upon the visibility in relation to such factors as radar assistance, shipping density, the presence or absence of emergency anchorage, the nature of the berth, that is, floating stage, river wharf, or dock, and so on. Below a certain visibility, shipping hold-ups and increased risk of stranding for ships on the move may be expected, As an ultimate ideal, we may have in mind the integration of all available facilities and inventions so as to allow shipping to carry out all its normal activities in the worst of fogs, with no less speed and safety than is customary in clear weather. But this is rather idealistic, and I would like to introduce a criterion, suggested by Mr. Eric Parker, which he terms the ‘Critical Visibility’, and which is the visibility at which 50 per cent of ships will proceed no further than the berthing area. This statistical measure of the effect of fog is one which will give a real indication of the value of any improvements. It will of course be different in different ports, but if say it is half-a-mile with no shore radar assistance, the introduction of shore radar might reduce it to quarter-of-a-mile, and in a few years time it might be reduced by improved aids to say a hundred yards. Eventually it would be at such a value that annual delays to shipping would be of negligible importance.