How we measure visibility
Visibility can be reduced by fog, pollution, snow or even sand whipped up by the wind.
For a great many years meteorological visibility was estimated by the human observer judging the appearance of distant objects against a contrasting background, usually the sky. Various rules have been established for how visibility should be judged:
- An object should not merely be seen but should be identifiable against the background as a specific object
- Visibility should be estimated at ground level where there is an uninterrupted view of the horizon
- If the visibility varies from one direction to another, the lowest value should be reported
- For the purposes of aviation the prevailing visibility should be reported
Modern day observations
Today, most synoptic observing stations have sensors which provide a measurement of visibility where the station operates in fully automatic mode. At the smaller number of stations where there is still a human observer (e.g. at airfields) the sensor acts as an aid in the estimation of visibility. Visibility sensors measure the meteorological optical range which is defined as the length of atmosphere over which a beam of light travels before its luminous flux is reduced to 5% of its original value. In most instances this is approximately equivalent to, but not the same as, visibility measured by the contrast of a distant object against its background.
A typical visibility sensor used at synoptic observing stations is shown below. It applies the forward scattering characteristics of light to measure the extinction coefficient of a beam generated by a high intensity xenon strobe transmitter directed at a volume of air close to the sensor. Reasonably accurate measurements are possible over a range of visibility extending from a few tens of metres to a few tens of kilometres. The use of light within the visible spectrum allows the sensor to most accurately simulate human perception of visibility.