Hand-held Gold visibility meters have been available for many years to aid the estimation of visibility, but it was not until the 1990s that visiometers have been widely installed at stations in the synoptic network for the automation of the measurement. Visiometers measure the transmissivity of a sample volume of air, expressed as the Meteorological Optical Range (MOR), which is converted for observational use into a visibility and coded directly into the SYNOP message at automatic stations. Where the station is manned the visiometer measurement may be altered by the observer if, in his judgement, it is in error or unrepresentative of the conditions obtaining at the time.