What the cloud percentage means for astronomy
You can see bright stars through a veil of high cloud and still lose the faint nebula you meant to shoot. On another night, a bank of low cloud leaves a useful patch of sky untouched. The cloud cover percentage helps describe both nights, but the number alone will not tell you whether to start a long run. Look at the layers, then at where the target sits.
A share of the sky, not a measure of thickness
Cloud cover describes how much of the sky is covered. It is not the chance that cloud will arrive, and it does not tell you how much light will get through. Thin cloud counts too. A large percentage can describe a thin veil you can still see stars through, rather than a dark lid of rain cloud.
That is why The Quiet Sky includes a note that cloud cover includes thin cloud and a high percentage can still mean a bright sky. For imaging, read a large number as a reason to inspect the details. A bright star visible through the haze is not much evidence that a faint target will record well. Transparency matters here; the seeing and transparency guide separates loss of faint detail from unsteady air.
Give high cloud a closer look
Low cloud often hides the target completely as it passes. Mid cloud can build into a sheet that closes the night down. High cloud is often thinner, but a pale veil can still reduce contrast and leave halos around bright stars. Its effect may become obvious only when you compare a new sub with one taken through clearer sky.
Treating every layer as equally obstructive misses that difference. The Quiet Sky gives high cloud less weight than low or mid cloud in the score, using an assumed thickness for each layer. The low, mid and high split is available in each hour's details. Check it before writing off a night with high cloud, but do not read the lighter weighting as permission to ignore it. The assumption cannot tell you the actual thickness of the veil in front of the telescope.
Check the direction of the telescope
An overhead forecast describes the cloud at your location. When the scope points low across the sky, the light reaches you through layers farther away. At 20° altitude, The Quiet Sky's sightline crosses its high cloud layer about 29 km away. The forecast there can look quite different from the one above your garden.
With an account, the app follows the target's direction and reads the forecast at the crossings through the low, mid and high cloud layers. The hourly label changes from “Cloud overhead” without a target to “Cloud on target” with one. Hours missing sightline data use overhead cover and are marked. The guide to choosing a clear sky forecast puts that distinction alongside darkness and the time your target stays above the skyline.
Read Broken as an invitation to work the gaps
In The Quiet Sky, Broken means “work the gaps”. It sits above Unsettled because it describes gaps long enough to shoot between, while Unsettled means “short windows at best”. These are forecast verdicts on the imaging night, based on the weather model chosen for your area. They are not another way of writing the cloud percentage.
For a broken night, look for a stretch long enough to justify getting on target. Keep the session simple so that you do not spend each opening changing the setup; use the session planning guide to allow for the pauses between exposures. Watch the incoming subs when cloud is nearby; a guiding run that keeps going does not mean the target stayed clear. The forecast and nowcast guide explains how recent observations can help with the evening decision.
Put the cloud back into the night
A hole in the cloud may arrive while the sky is still too bright, or after the target has gone out of reach. In The Quiet Sky, the usable window is the longest unbroken run of dark hours meeting your go threshold. With a target, only hours above your skyline and your rig's minimum altitude count. Saved sites and rigs need an account.
Compare that window with the time you can actually be outside. If they barely overlap, it may be a night for a modest run rather than a full setup. You can read the forecast without an account, or return to the guides for help with the other weather numbers.