Why a clear forecast can still be a bad night for astronomy
The forecast says clear, the sky says otherwise. Cloud is only one of six things between you and a faint galaxy.
Cloud, and which cloud
A forecast’s cloud figure is the whole sky, all layers together. High, thin cloud lets the Moon and planets through and kills everything faint. Low cloud is the one that sends you indoors. That’s why Meanwhile shows the layers separately, and asks four forecasting centres rather than one: when the Met Office, ECMWF, DWD and NOAA disagree, believe the disagreement.
Fog, which the cloud figure doesn’t count
Fog is cloud on the ground, and most cloud forecasts don’t include it. On 2 November 2026 the forecast here said cloudy with dew. Fog arrived at about 20:45 and sat on the roofs all night: by the small hours the sky let through less than a fifth of the light it had the night before. (The diary has the night.) Fog forms when the air cools to its dew point and the wind drops, so Meanwhile warns when the forecast has the air within half a degree of its dew point with hardly any wind, or visibility under a kilometre.
Dew
On a clear night the ground and your telescope lose heat to the sky. When they cool to the dew point, water condenses on them, and a lens or mirror fogs over in minutes. When the forecast has the air within 2 °C of its dew point, cover the optics or heat them.
Transparency
Clear isn’t the same as transparent. Haze, humidity, dust and smoke dim the sky without showing up as cloud. A transparent night shows the Milky Way’s dark lanes; a hazy one turns faint galaxies into nothing. Meanwhile reads 7Timer’s transparency forecast for this.
Seeing
Seeing is how steady the air is. Turbulence, often from the jet stream far overhead, makes stars twinkle and planets boil in the eyepiece. Bad seeing barely matters for a wide view of a cluster, and ruins Jupiter’s cloud belts. 7Timer forecasts this too, which is why Meanwhile judges the Moon and planets separately from faint galaxies.
The Moon, and the Sun
A bright Moon lights the whole sky, so faint things vanish even on a perfect night: the week either side of full Moon is the time for the Moon itself, planets and bright clusters, not galaxies. And the sky only gets properly dark once the Sun is 18° below the horizon. In summer, far from the equator, it never does.
See what tonight looks like where you are
What’s observed and what’s a rule
The fog night is what happened here, from the diary. The thresholds (fog when visibility is under a kilometre, or the air is within 0.5 °C of its dew point with wind at 5 km/h or less; dew when it’s within 2 °C) are rules telescopeyoke, the software that drives my telescope, applies to the forecast. They’re rules of thumb, not physics: fog can form a little outside them, and stay away inside them.
Sources: the Met Office on fog and cloud types; the US National Weather Service on dew point and fog; 7Timer’s documentation for its seeing and transparency forecasts; Krisciunas and Schaefer, A model of the brightness of moonlight (1991), which Meanwhile uses for how much the Moon brightens the sky. The night itself: the diary, 2 November 2026.