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Watching the weather
Watching the weather

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2.3 Comparing with the view from the ground

So, how does the view from space correlate with the charts of the weather that meteorologists can construct from surface observations? The answer is 'remarkably well'. A geostationary satellite sees higher latitudes increasingly foreshortened owing to the curvature of the Earth's surface. However, image processing experts can tilt and distort the image so that it appears to be taken from overhead. Figure 12 shows both the full globe in the IR, as seen from the MTSAT geostationary satellite, and a reprojection of the same image in the region of Australia and New Zealand, suitable for weather reports in that region. This image was taken at 0630 UTC on 19 February 2008.

Figure 12 MTSAT geostationary satellite infrared images taken at 0630 UTC on 19 February 2008 showing (a) the whole globe, centred on longitude 140° E, and (b) a reprojection of (a) for the area around Australia and New Zealand.
  • There is a bright white, circular region of cloud lying just off the northwestern coast of Australia in Figure 12b, in the centre of a spiral pattern. Why does this cloud appear whiter in the image than the band of cloud over the ocean to the south of Australia?

  • This cloud surrounds the centre of tropical cyclone Nicholas, a region of strong upward motion. The cloud tops are high, and consequently cold, and so the clouds emit little IR radiation and appear white in the IR image. The clouds over the Southern Ocean are associated with a front. The cloud tops here are lower in the atmosphere, warmer and so appear greyer in the image.

The more complex features are not always easy to interpret. However, you should be able to imagine the potential of a sequence of images like this to inform meteorologists of likely developments.

Satellite measurements are described as 'remote', since the instrument is not placed within the atmosphere that it is observing, but instead it measures an electromagnetic spectrum at a distance from its target. From this spectrum, scientists are able to infer the most likely temperatures or water amounts which fit the observed radiation, but they are not directly measuring the physical quantities in which they are interested. Remote measurements are thus different in nature from the type of surface station measurements already discussed in Section 1, and a check against 'ground truth' is always of particular importance for satellite measurement. For example, a check might be made by comparing a surface temperature inferred from a satellite measurement with a direct measurement made using a thermometer on the surface at the same location and time. Satellite instruments must also be monitored in case they degrade with time.