Night-time fog/low stratus in Brazil, Paraguay & Bolivia

The field of view of Meteosat-8, being positioned over the Equator at 3.4°W, covers Europe, Africa, large parts of the Atlantic Ocean, parts of the Indian Ocean and also large parts of South America (see Meteosat-8 field of view). It may not be well known that for instance Paraguay has the same satellite viewing angle than Southern Sweden and that the most eastern regions of Brazil appear with the same satellite viewing angle than Spain.

Therefore, high-frequency Meteosat-8 images provide essential support to forecasters in countries like Brazil, Argentina, Bolivia, Paraguay, Uruguay, Guyana, French Guyana, Surinam and Venezuela. One example that shows the detection of night-time fog/low stratus is given below. The upper left image shows the recommended night-time RGB composite for cloud classification and monitoring of fog. The low-level fog/stratus areas in northern Bolivia, south-western Brazil and Paraguay appear in a light green to yellow colour, while high-level ice clouds appear in dark red (thick) or dark blue colours (thin clouds). For the colour interpretation, please click on the interpretation link below.

The essential input for the night-time detection of low-level fog/stratus comes from the IR10.8–IR3.9 brightness temperature difference (BTD) images. The technique is based on the principle that the emissivity of water clouds at IR3.9 is less than at IR10.8. This is not the case for cloud-free surfaces (except sandy desert surfaces). The result is that, at night, low-level water clouds have a higher IR10.8–IR3.9 BTD than cloud-free surfaces. For the case shown below, the IR10.8–IR3.9 BTD for fog/low stratus is around of +9K, while the cloud free areas in Argentina and Paraguay appear with BTDs of +6K. In Central Europe, with slightly smaller satellite viewing angles (and, thus, less cooling of the IR3.9 channel due to CO2 absorption), cloud free areas have typical BTDs of +4/+5K (see example from 9 November 2003). In Central Africa, with very small satellite zenith angles, the IR10.8 - IR3.9 BTD for cloud-free vegetated surfaces shows typical values of +2/+3K (see figure source: Météo-France/Nowcasting SAF). It should be noted that, as shown below, fog/low stratus at night can also be detected in the IR10.8–IR8.7 BTD image. However, the contrast between cloud-free surfaces and fog/low stratus is somewhat less pronounced than in the IR10.8–IR3.9 BTD images.

Besides the detection of fog at night, the IR10.8–IR3.9 BTD is also essential for the detection of fires (hot spots), which appear as black spots in this BTD image. Looking at the animation below (in satellite projection), several fires (mostly single pixel hot spots) are well visible in Brazil. One of the fires in south-western Brazil gets covered by fog/low stratus so that it becomes invisible to the satellite. In the reprojected animation, the hot spots appear somewhat distorted in the E-W direction.

Please, note that, if not stated differently, all images below are shown in Mercator projection with a centre latitude of 16°S and a centre longitude of 57°W.