| The human observer on dry land measures visibility by determining the greatest distance at which he can distinguish objects, ideally black, against the horizon sky with the unaided eye. At night an equivalent is obtained by using lights of known intensities at various distances. At sea the Officer of the watch has no fixed reference points unless, of course, the visibility is very poor and he is using the various objects on deck. In reasonable visibility he uses his long experience by looking at the horizon and seeing how sharp it is, trying to see other vessels that he has detected on his |
radar and so on. At night, he will use the brightness of the stars, how well he can see the cloud, the clarity of lights on board and so on. The ability to distinguish objects by day or see lights at night is determined by scattering of light by dust, smoke, water and other particulate matter in the air. The greater the scattering of light, the poorer will be the visibility. The extreme case is dense fog at night when light is scattered directly back at your car headlights. |
| At automatic stations, such as those used on light vessels, there are various ways of measuring this scattering effect. First, there are forward scatter instruments. These emit a beam of light. Sensors a little way off the direct, narrow beam, measure only light that has been scattered. Poor visibility leads to more scattering and, therefore, more light at the sensors. The poorer the visibility, the greater the signal.. The next technique, known as back scattering, is in use on the M25 where observant drivers will have seen sensors alongside the carriageway. These measure light scattered directly back to the source and, again, the greater the received light the poorer is the visibility. This technique is particularly useful for fog and the sensors are
used to activate fog warning signs. A third system, using instruments known as transmissometers, measures the light |
loss, due to scattering, by putting the sensors in the direct beam. Here the less light received by the sensor then the poorer is the visibility. Each sensor type has its particular virtues but the forward scatter visibility sensors on the light-vessels were chosen by the Met Office as being reliable, robust and not unduly affected by salt spray contaminating the optics. They are good at measuring the visibility well throughout a large range although, perhaps, slightly inferior to the other techniques at very poor visibilities. This is not too critical at sea where anything below 1000 metres is, effectively, fog and defined as such by meteorologists. The same definition is also used for aviation. To a car driver visibility really only becomes a hazard
at much lower values, say around 200 metres. For this reason, "fog" in forecasts for the general public refers to visibilities of less than 200 metres. |
| The moored buoys that are used to the west of the UK on the edge of the Continental Shelf, although large, are not big enough to carry these instruments. But most automatic stations on land and large platforms at sea are increasingly being fitted with visibility sensors. At manned stations they are also used although here the observer can override the instrument if he believes that the value is not realistic. The rules for human observers define visibility as the least |
value around the horizon. Those who sail close to Sandettie Light Vessel in the Dover Strait, Greenwich Light vessel on a crossing to the baie de Seine or Channel Light Vessels at the western end of the Casquets separation zone, can do their own checks. The measurements usually look good to me. |