Passive remote sensors have been monitoring the state of the atmosphere from space for several decades. Their measurements have led to reliable climatologies of clouds distribution on a global scale. Unfortunately, these retrievals were penalized by significant uncertainties above certain surface types (ice, snow, deserts, and more generally everything that is not ocean). The ability to detect clouds was moreover strongly dependent on their optical depth, with thinnest clouds often escaping detection. Along the vertical dimension, retrievals were often limited to the cloud top temperature, which required strong assumptions on cloud density that were rarely realistic and led to underestimations by several kilometers \cite{Sherwood_2004}. Considerable effort and ingenuity went into solving these problems and reducing uncertainties as much as possible, but the error bars remain significant. They mean that passive observations cannot answer specific questions about how the cloud cover reacts to changes in its atmospheric environment [Chepfer et al., in review].
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