Met Instruments - Transmissometers
Although several of the met instruments used in forecasting have been mentioned, another that might be recalled is the transmissometer, a device used to assist with the calculation of horizontal visual range. For anyone who hasn't seen this equipment, essentially it consists of two tubes mounted horizontally about two metres above the ground, each facing the other some 10 metres apart. Through one tube a beam of energy, usually a laser, is sent to be received by the opposing tube where the amount of energy that has been lost in transition (eg through mist, fog, precipitation, etc) can be measured and the results used in calculations.
In the mid 1970s I was involved with the Blind Landing Experimental Unit at the Royal Aircraft Establishment at Bedford where we would fly an HS748, specially instrumented, in what was called "Fog Flying" in order to obtain results that could be used to assist with various experiments, one of which was the aim of finding an accurate means of determining slant visual range. This was where the horizontal visual range provided by the transmissometer played an essential part. And obviously our scientists needed accurate readings. Often during the season when fog was likely to form the equipment at Bedford would be left on overnight to obtain a record of visibilities arising on the aerodrome.
However, all too often an otherwise steady reading would precipitously drop to zero, and for no obvious reason. Anyway, one night an enthusiastic and inquisitive met officer stayed up to see if the cause could be determined, and his keen attention paid off. What he observed was the arrival in the middle of the night of a Little Owl that landed on the lower lip of the transmitting tube, inserted himself within it, and snuggled down such that he received a constant supply of warmth from the laser transmitter!
Problem solved!