Fog redirects light.
When light encounters fog droplets, some light leaves the original beam through scattering; absorption can also contribute. A detector looking along the original path receives less of the transmitted beam. A photograph combines this atmospheric behavior with exposure, contrast, illumination, lens effects and image processing. It can document an experiment, but it is not sufficient on its own to quantify extinction.
A controlled optical measurement compares the received signal with a reference, after accounting for detector background and the instrument’s clear-path response. The useful ratio is transmission, T. A longer path through the same homogeneous fog produces a lower T.
β = −ln(T) / L
R5% = −ln(0.05) / β ≈ 2.996 / βI and I₀ are corrected received and reference signals. L is path length in metres; β is extinction in m⁻¹. The natural logarithm is used. The model assumes representative, homogeneous attenuation and qualified optics.
Optical range needs a definition.
The 5% range is the distance at which this model predicts 5% transmission. It is not automatically the distance at which a particular driver or pilot can recognize a particular object. The WMO definition of meteorological optical range specifies a source spectrum and luminous-flux weighting. A monochromatic bench instrument gives a wavelength-specific optical range unless the relevant spectral conditions are met. Sources S3–S4 →
At T = 1, the ideal inferred extinction is zero and the inferred range is unbounded. Real instruments have finite resolution, background and drift, so the appropriate experimental report includes a detection or uncertainty limit. At T ≤ 0, the logarithm is not a valid estimate; the instrument’s low-signal behavior must be handled explicitly.
Why water and visibility can move differently.
Droplet mass grows with diameter cubed. Optical extinction instead depends on each droplet’s optical cross-section and its size- and wavelength-dependent extinction efficiency. Counting droplets, weighing liquid and measuring light therefore weight the population differently.
Extinction = ∫ N(d) Qext(d, λ) πd²/4 ddN(d) is the number distribution per volume and diameter; Qext is the extinction efficiency; λ is wavelength. This identifies the different size weightings. It is not a substitute for a measured spectrum or a suitable scattering calculation.
Removing a subset of larger drops may recover substantial mass while leaving a population that still scatters strongly. Coalescence can reduce number without exporting liquid. Evaporation changes droplet size and transfers liquid into vapor. The optical outcome depends on which process occurs and on the starting distribution. Field microphysics studies help establish these relationships within their observed regimes. Source S5 →
Read a comparison correctly.
Weatherhill’s proposed test compares downstream extinction with a transit-aligned upstream reference. It then compares the powered and passive normalized results. That helps distinguish a treatment effect from fog that changed before entering the apparatus. The protocol governs timing, valid measurements, exclusions and statistical analysis.
Wet or dirty windows, stray light, changing detector alignment and changes in airflow can all affect the measured signal. A clearer beam near a cleaned sensor does not establish a clearer volume around it. Multiple positions and heights become essential as the experiment moves toward an open environment.
FAA runway visual range systems measure conditions at defined runway locations. This educational calculation and a FogLab experiment do not certify runway visibility or authorize an operation. Source S12 →