Visible satellite imagery takes a snapshot of what the sky (and ground) "look" like at any one time. Therefore, these images are only available during daylight hours. Thicker cloud will show up as brighter white in these images, regardless of its height. Thin cloud shows up as paler grey shades.
Infra-red satellite imagery detects the temperature of the sky (or ground). As it does not rely on visible light for illumination, these images are available 24 hours a day, making them more useful for long-term tracking of weather systems. Colder temperatures are shown in brighter shades of white and warmer temperatures in darker shades of grey. Therefore, low cloud, which is close to the surface and, as such, warm, will show up very faintly, if at all, on infra-red imagery, as it will be hard to distinguish from the nearby land. High cloud, which is colder, will show up as bright white.
Therefore, during daylight hours, it is possible to use visible and infra-red satellite imagery in tandem to get a better snapshot of the state of the atmosphere. Visible imagery can be used to get an idea of the thickness of clouds (and also to detect low cloud that may not have been apparent on infra-red imagery) and infra-red imagery can give an idea of the height of those clouds.
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