AVHRR images
The polar orbiting satellites have AVHRR instruments, with channels that differentiate from those of Seviri. Differentiating between water and ice clouds is more difficult with AVHRR channels than with those of Seviri. Having a better resolution than Seviri images, they complete the information received from MSG. This is true especially at high latitudes, where the images also come more often than middle latitudes, so as to be actually useful in nowcasting and short-range forecasting.
In middle latitudes, the Seviri images have a relatively high resolution, and visible channels can be used throughout the winter in daytime, so there the importance of AVHRR images is lower - as is their availability.
The most important AVHRR product that is available day and night and can tell the difference between water and ice clouds is "comb 345". In daytime this combination is (1.6 μm + 10.9 μm + 12,0 μm), and in nighttime (3,7 μm + 10.9 μm + 12,0 μm).
Fig. 3.9. AVHRR combination image 345
During daytime a combination of visual and infrared channels can also be used: "comb124" (0,6 μm + 0.9 μm + 10.9μm). In practice, as most icing cases occur in winter, this image is seldom of use. It can, however, reveal quite nicely different clouds in autumn and spring (October, March).
Fig. 3.10. AVHRR combination image 124