A spinning imager and three channels

The first-generation Meteosat Visible Infra-Red Imager, MVIRI, used spacecraft spin for east–west scanning and stepped its viewing direction south to north. Its normal full-disc cycle was 30 minutes. The image was assembled through a scan; it was not a simultaneous camera exposure of every point on the disc.

In an original sampling exercise, a storm changes during the scan interval. Different parts of the displayed disc were observed at different moments. Before comparing a fast-moving feature across images, retain the scan timing and acquisition geometry. A neatly presented image can conceal the fact that its measurements took time to collect.

MVIRI observed in visible, thermal-infrared and water-vapour bands. Visible observations depend on reflected sunlight; infrared measurements convey information about emitted radiation. The water-vapour band is particularly sensitive to upper-tropospheric moisture emission. The channels are complementary measurements, not simply three alternative display colors.

The Meteosat 4 image near Angola is explicitly a visible-channel observation from April 13, 1991. Bright clouds in that image should not be interpreted as a direct thermometer reading. Ask which channel was used before explaining a bright or dark feature, and retain the source’s processing description.

A 30-minute interval gives two observation cycles per hour. In an original idealized schedule with one timestamp per cycle, a day contains 48 cycle slots. A missing slot is missing evidence; duplicating the preceding image does not create a new observation. Real archives may contain gaps or different operating modes.

Likewise, a satellite observes radiation that can support derived weather products. A cloud image does not directly measure the rainfall accumulated in every street below it. Interpretation needs the appropriate retrieval method and supporting evidence. Keep the distinction between measured signal, derived quantity and a decision based on that quantity.