Low, mid and high cloud, and why we split them

A single "60% cloud" figure hides the thing you need to know. Sixty per cent low cloud is a wall. Sixty per cent high cirrus is a veil you might still image through, and it will quietly ruin your data while looking fine to your eye.

The World Meteorological Organization divides the troposphere into three levels, historically called étages. In its International Cloud Atlas, low cloud has a base up to about 2,000 m, middle cloud a base usually between 2,000 and 7,000 m, and high cloud a base above roughly 5,000 m. The ranges overlap deliberately, and their limits vary with latitude — the same genus sits higher in the tropics than near the poles.

Low cloud is the opaque one. It either blocks your target or it does not. It is also the layer most likely to thin after sunset as the ground cools, which is why an overcast afternoon does not always mean a lost night.

Middle cloud is the awkward one. It breaks into patches and drifts, so it gives you gaps, and whether a gap is usable depends entirely on how long it lasts relative to what you are doing.

High cloud is ice crystal cirrus, and it is the layer that catches people out. You can look up, see stars, and conclude the sky is clear while thin cirrus is quietly scattering light across your whole frame. Visual observers often barely register it. Long exposures integrate it, so it accumulates.

That asymmetry between what your eye notices and what a sensor records is the reason this forecast splits the layers out instead of reporting one total, and why the wide-field and long-exposure modes in the window finder penalise high cloud far harder than the visual modes do.