Comparisons with other RGB images


HRV Fog and Natural Colour RGBs

The HRV Fog RGB and the Natural Colours RGB images are composed from 'similar' channels. Both RGB types present the NIR1.6 channel in red, and visible channels are presented in green and blue. That is why the snow and the clouds will have similar colours (Fig. 13). However, the appearance of the snow-free land is very different. The Natural Colour RGB uses different channels in the blue and green components. The 0.8 and 0.6 micrometer channels (VIS0.8 and VIS0.6) together provide vegetation information which is totally missing from the HRV Fog RGB image. The reason is demonstrated in Fig. 14; the reflectivity of green vegetation differs considerably in the VIS0.8 and VIS0.6 channels. More green in the Natural Colours RGB means that the pixel is covered by vegetation containing more chlorophyll.

Figure 13: HRV Fog RGB (left) and Natural Colour RGB (right) for 13 February 2014 at 11:40 UTC
Figure 14: Typical reflectivity functions of green leaf and bare soil (green and brown curves) in the shortwave spectral region plotted with the top of the atmosphere irradiance (red curve) and the response functions of the shortwave SEVIRI channels (gray curves) (Source: MSG Interpretation Guide, EUMETSAT)

Suggestions:

  • For information on vegetation, one has to use the Natural Colour RGB.
  • If we are interested in small scale details like valley fog, the HRV Fog RGB provides more details (Fig. 15).

Figure 15: Valley fog in HRV Fog RGB (left) and Natural Colours RGB (right) for 8 January 2014 at 11:10 UTC.

HRV Fog and HRV Cloud RGBs

Both RGB types use HRV in two colour beams. The HRV Fog RGB combines the HRV information (high resolution cloud structure and cloud thickness) with NIR1.6 channel information (cloud phase and snow on the surface). The HRV Cloud RGB combines the HRV information with IR10.8 channel information (cloud top temperature). HRV Fog RGB is mainly useful in winter while the HRV Cloud RGB is mainly applicable during summer, as its main application is monitoring convection.

Figs. 16 and 17 compare HRV Fog and HRV Cloud RGB images. One can see that in HRV Cloud RGB there is no colour contrast between snow and water clouds. In winter, HRV Fog RGBs are more useful than HRV Cloud RGB images.

Figure 16: HRV Fog RGB (left) and HRV Cloud RGB (right) for 07 March 2014 at 08:40 UTC
Figure 17: HRV Fog RGB (left) and HRV Cloud RGB (right) for 29 February 2012 at 07:40 UTC