On Visibility

Hello There

Often, as I work into the early hours, the sound of whatever the Home Service has become these days interrupts my trains of thought with a most annoying expression. "Channel Light Vessel Automatic: visibility 20 miles", announces the dinner-jacketed meteorologist in a perfunctory monotone.

I become intensely disturbed by this, because it is a short phrase crammed with contradiction, inconsistency and invention. Let me expand.

If this unmanned, yawing hulk anchored somewhere between Plymouth and the Channel Islands is ‘automatic’, how can it measure visibility? Is it sentient? Can it watch? If so, how does it know what to watch? Or is there equipment on it which provides an image which, through various relaying devices, can be watched by someone elsewhere? Are jpegs sent hourly to the Met Office in Exeter? Does an Elder Brethren sit haunched high on a headland somewhere on the South Coast peering at a television screen? And if so, how does he know that as far as he can see is further than 19 miles away, yet closer than 21?

One would assume therefore that the Admiralty has festooned our western approaches with carefully positioned buoys, each displaying individually numbered tags similar to those forensic officers place next to the bullets around the corpse of a machine-gunned mafia victim. However, consider the days that salty sea-dogs must therefore have spent in open boats, diligently placing these calibrated markers at precisely nominated distances from the light ship. We have always been a nation of great maritime ingenuity yet the process of visualising such a laborious scenario tires me immensely.

As the great liners steam west, why do passengers not notice these numbered floats - indeed, why are they not aroused from their bunks as they hear them smash against the hull?

This is a theoretically lame (sea)horse which falls at the first hurdle of comprehension. When the aforesaid agent examines the image from the automatic device, how does he know that the resolution of the image he has been sent has not been compromised in transmission? Has sea spray dried on the apparatus lens? Rogue seaweed may have become lodged in the mechanism. The image may have become distorted. If a Golden Eagle can read a newspaper at three miles, would it be able to read the same newspaper if it had previously wrapped greasy fish and chips? And would it be able to read it as well at night, assuming the newspaper is no longer in the illuminated fish-and-chip shop?

Naturally, being both a great thinker and born communicator, I can see huge possibilities in my exploring this quandary in a visually exciting television programme. Some of you will remember me from the small screen previously and I am sure you would be delighted to watch me once again in my investigation: “How visibility can be determined when there is no-one there to look”.

This is a paradoxical issue which vexed me inordinately until only a few moments ago, when the telephone rang and an ex-colleague from nobel gases at Culham explained. Apparently some bright spark came up with a tube-like device which takes a sample of atmosphere and bounces a laser beam back and forth within it. The intensity of the laser is measured as it subsequently diminishes. When visibility is poor, the laser loses strength faster than when visibility is good.

So the Met Office bought a dozen, bolted them onto converted trawlers and gave a fine body of hawk-eyed matelots the heave-ho. Such is progress.

With warm regards

Vernon Thornycroft