Cloud forecasts describe a modelled area and time interval. They are useful for planning, but they are not a live measurement of the patch of sky above your telescope. A nearby hill, coast or valley can create conditions that a regional grid does not resolve.
Read the time information
Check when the forecast was issued or checked and which observing hours it reaches. Later forecast intervals can be coarser than the first part of the forecast. NightSky does not turn a six-hour sample into six independent hourly predictions. A detailed session score needs current hourly coverage for at least three quarters of its eligible darkness.
An unavailable score does not mean the sky will be cloudy. It means the data cannot support that assessment. Astronomy calculations can still show twilight, Moon position and target directions beyond the reliable weather horizon.
Look at the spread, not one number
A low average can coexist with a cloudy period during the hour you planned to observe. Choose your actual start and duration to assess that session. Keep a backup target and recheck conditions before travelling.
Missing precipitation probability or gust information is unknown, not zero. Cloud percentage also does not describe every kind of haze, smoke, transparency or atmospheric steadiness. Use local weather warnings and on-site observations for decisions that depend on those conditions. The free providers use different models and fields; a tidy comparison table cannot make their underlying uncertainty identical. Record what happened locally if you want to learn how well a particular forecast fits your observing spot.
Sources and review
Reviewed October 22, 2026 by PiSpace Sky editorial. Date-sensitive details can change.