Impact of the viewing (scanning) geometry

Not all of the storm are scanned directly from above, being at nadir direction with respect to the satellite (at its sub-satellite point). Actually, most of the storms are scanned at slanted angle, which results in certain degradation of the real storm appearance. The more oblique the view, the greater the distortion of the original feature shape. Ideally, what would appear as a perfect circle when observed exactly from above (at nadir), appears as a more and more elongated ellipse with growing distance from the sub-satellite point (when displayed in the original satellite projection). As most of the storms are far from being circular, the final distorted shape can be quite complex. This shape degradation affects both types of satellites: the images from polar-orbiting satellites (where the key factor is the distance from the sub-satellite track, typically the swath center line), as well as the geostationary satellite imagery (where the distance of the storm from the satellite's nominal position plays the key role). An example below (Fig. 1) shows this effect.

For a full size view, click on the image.

Figure 1a. A larger storm above Russia, as viewed by NOAA-15 directly from above (being almost at nadir  direction with respect to the satellite). 04 August 2010, 13:10 UTC, RGB composite image of bands 1, 2 and 4.

Figure 1b. The same storm as above (center of the image), captured at almost the same time by the Meteosat-8 satellite from its geostationary orbit. As the storm is located far from the nadir location of the satellite, very close to the edge of the Earth (which can be seen at top right of the image), the storm appears strongly distorted, elongated in the NW-SE direction as compared to the NOAA-15 image above. See also the zoom of this image below. 04 August 2010, 13:10 UTC, SEVIRI HRV band, Meteosat-8.

Figure 1c. 2x zoom of the image above (1b), centered at the storm, showing the area at a scale very close to that one of the NOAA-15 image above (Fig. 1a). The storm in this original satellite projection is strongly "flattened" in a direction pointing to the center of the globe, giving the storm an unrealistic shape. To avoid distortion similar to this one, caused by the viewing (scanning) geometry, the satellite images are usually not displayed in the original (satellite) projection like this one, but rather are remapped to one of the standard geographical projections - discussed briefly in the next part.