HRV is able to monitor fog and even fog initiation, with a threshold for air turbidity similar to the human eye.
05 May 2023
07 July 2013
By Fausto Polvorinos Pascual (AEMET)
The probability of marine fog over the Alboran Sea is at a maximum during late spring and summer, when the temperature difference between the air and the sea surface is highest. A strong inversion (tenths of metres up to 350m thick) is found between the warm dry subsiding air from the land and the cool moist marine air below.
Marine fog occurs predominantly after sunset under clear skies and calm winds. A nearly saturated layer at dusk is paramount to the fog formation. The same effect can be observed west of the Strait of Gibraltar.
Hygroscopic particles attract water molecules before the relative humidity reaches 100%. The particles grow and increase light scattering, reducing visibility. As with the human eye, HRV satellite images are an important tool to distinguish dry masses of air from wet masses during day.
A thin layer of air near the surface appears brighter on the HRV channel when it nears saturation. The HRV detects the moist marine layer before fog generates, and is, therefore, a warning of imminent fog. HRV is a powerful tool for the short-range forecast of marine fog.
The two Meteosat-10 images below show humidity conditions connected to easterlies at the Strait of Gibraltar.
The two Meteosat-9 images below show, respectively, the fog initiation around sunset, when HRV gives an indication of high humidity and hygroscopic aerosol, and extensive fog the following morning.