The essential answer
Visibility responds to how strongly airborne material attenuates contrast, not directly to the mass of water available for collection. Fog droplets scatter light, but droplet size distribution changes the relation between extinction and liquid water content. An optical estimate of fog density therefore needs assumptions. Treat visibility as its own measurement, especially when comparing scenes with different droplet spectra or aerosol backgrounds. A clearer view alone does not establish how much water a device has collected.
How to reason about it
In a simple uniform medium, transmitted direct light follows an exponential attenuation law. If beta is the extinction coefficient and L the path length, transmission is exp of minus beta times L. This model makes path length explicit and is useful for checking dimensions. Meteorological visibility adds a contrast threshold and background assumptions, so it is not identical to a single transmission reading. The cited fog study examines the relationship between extinction and LWC rather than assuming a universal conversion. For a diagnostic plot, keep measured transmission, inferred extinction, and inferred water content in separate layers. Changes in illumination, wet optics, alignment, or aerosols can alter the observation. Comparing a short optical path near a device with a long distant view also samples different volumes of air.
ILLUSTRATIVE · NOT A REPORTED RESULT
An example to work through
Assume a uniform fog with extinction coefficient 0.01 per meter, a 100-meter path, and negligible instrument offsets. The model gives transmission exp(-1), about 0.368, or 36.8%. Over 200 meters in the same fog, transmission becomes exp(-2), about 13.5%. Nothing about this calculation specifies LWC because the scattering-to-mass relation has not been supplied. If an instrument reports a higher transmission after a test, first check that path length, optics, and background remained comparable before interpreting the result as a change in atmospheric conditions.
Where interpretation fails
A photograph is not a calibrated extinction meter. Automatic exposure can brighten a foggy scene, and a clear channel near a fan can coexist with unchanged conditions elsewhere. Inferring removed water from image contrast alone requires assumptions that are usually much stronger than the image supports.
The next decision
Choose extinction when the question is optical attenuation and LWC when the question is suspended liquid inventory. If converting between them, state the size-distribution assumption and its uncertainty. For device evaluation, pair optical observations with mass measurements and a spatially appropriate untreated baseline.
Sources & context
These sources support the concepts noted below. Our synthetic examples and research framing are educational interpretation; they are not findings from these organizations or validation of our ventures.
- Relationship between optical extinction and liquid water content in fogs ↗
Extinction-to-LWC relationship depends on fog microphysics.
Source check: Article landing page and abstract reviewed
Educational research context, not a validated device specification or clinical protocol. Read our editorial and correction policy.