Stargazing needs two things the weather decides: a clear sky and a discreet moon. A cloud cover of 30 % sounds low, but it is enough to interrupt your viewing as the clouds drift by. Less than twenty per cent is what counts as a clear sky.
Why this indicator matters
The moon is the other variable. At full moon, it lights the sky enough to wipe out the Milky Way and the faintest stars, even under a perfectly clear sky. At new moon, the sky is at its darkest. The illumination is calculated from the 29.53-day cycle, with no outside data.
Clear nights are also the coldest, because the ground radiates freely into space: the same condition that promises stars heralds frost outside the warm season. Knowing when the sky clears lets you dress for it and choose the time.
How to read it
The box looks at the night from 22:00 to 04:00. It speaks up only if the average cloud cover stays below 30 %, and gives the longest slot of hours with less than 20 % cloud, the phase of the moon and its illumination.
When the moon is above 70 %, the sentence warns that it will mask faint objects: it is still a fine night for the moon itself, the planets and the bright constellations, but not for the Milky Way.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Night examined: from 22:00 to 4:00.
- The indicator speaks up if the average cloud cover is below 30 %.
- An hour is “clear” below 20 % cloud; at least 2 clear hours in a row are needed.
- Bright moon: illumination of at least 70 %.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13, this indicator stays silent: none of the conditions above is met at the moment. The average cloud cover for the coming night is 100.0 %.
Its limits
The model's cloud cover is averaged over a grid cell a few kilometres across: a local bank of mist or a trail of high cloud may escape it. The town's light pollution is not taken into account, and the moon phase is calculated to within a day, which is enough to say whether it interferes or not.
Frequently asked questions
Why take the moon into account?
Because a full moon lights the sky so much that it hides the Milky Way and many faint stars, even without a single cloud.
Is the sky clearer in the countryside?
The clouds are the same, but the light pollution of towns cuts the number of visible stars from several thousand to a few hundred.
How long does it take to get used to the dark?
About twenty minutes for your eyes to adapt, and a phone screen resets the clock to zero. A red torch avoids losing that adaptation.