New light on “fog”?

„The term ‘fog’ is used when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than 1 km” (https://cloudatlas.wmo.int/en/fog-compared-with-mist.html). This simple, traditional definition has proven very useful, it has been applied for decades in meteorological observations by the naked eye and – more recently – by instrument recordings. The term ‘mist’ is used “when the droplets do not reduce horizontal visibility to less than 1 km” (cloudatlas.wmo.int, see above). Both fog and mist are often associated with air pollution, specifically high aerosol load within the boundary layer, which leads to large number concentrations of small droplets. Such small hydrometeors are often non-activated. The increase of air quality (decrease of aerosol particle concentrations) worldwide leads to a decrease of fog. Yet, what are the microphysical conditions of fog in a clean atmospheric environment? Will fog form only when droplets are activated? Recently employed fog droplet size spectrometers of various manufacturers allow a deeper insight into the microphysical conditions of fog and will shed light on the role of activation of fog condensation nuclei (FCN) on the formation of fog, its interaction with air pollution, and its trends before the backdrop of climate change.