River valley fog frequently develops in the night-time and quickly dissolves in the morning hours.
04 May 2023
14 August 2013
By Jochen Kerkmann (EUMETSAT)
This Meteosat-10 Day Natural Colours RGB product shows a typical early morning situation during the dry season over Central Africa. First of all, a band of cirrus fibres from the subtropical jet can be seen over the border area Namibia/Angola.
South of this cirrus band, the air is clear, whereas north of this band the air is very hazy/smoky — which can be seen by the blue taint and the unsharp surface features. The smoke is due to the numerous fires which are very common over Angola, Namibia, Zambia and the Democratic Republic of the Congo (DR Congo) during the dry season.
A few thunderstorm clouds (intense cyan colour) appear in the most northern (upper) part of the image — low-level clouds (light pink colour) are present in the western (left) part of the image, e.g over north-west Angola.
While most of Angola and the DRC is cloud-free, there are several stretches of river valley fog along the Kasai and the Sankuru rivers and their tributaries, in the area of Kasai Occidental, one of the 10 provinces of the DRC.
These areas of river valley fog are not visible in the IR-based Night Microphysics RGB image taken two hours earlier at 04:00 UTC (note that the low level clouds over north-west Angola are well visible in the IR-based RGB image). This RGB product, which exploits the difference of the IR10.8 and the IR3.9 channels, is often used to detect low-level clouds and fog in Africa (and Europe).
One explanation could be that the river valley fog only formed after 04:00 UTC, but the more likely explanation is that the fog was too thin to be detected in the IR channels and corresponding RGB products.
According to Prof. Daniel Rosenfeld from the Hebrew University of Jerusalem, a fog cloud with a cloud water content of 1g/m3 requires a geometrical thickness of 20–80m (depending on the cloud droplet effective radius) to appear clearly in the Night Microphysics RGB product. For lower water contents even higher thicknesses is needed. Thus, it appears likely that the river valley fog over the DRC was too thin to be detected in the RGB product.