Counting Usable Observing Nights from Public Data: Cloud, Transparency, and Smoke in Eight Canadian Cities, 2020–2025

An observer in a Canadian city can check the Clear Sky Chart for tonight, but nothing says how many usable nights to expect in a year, or which months to plan around. We present a method that counts them from public data and apply it to eight cities over 2020–2025. A night is good when it holds at least 4 h of clear astronomical darkness (hourly METAR cloud reports) and its air is transparent (ERA5 water vapour) and free of surface smoke (PM2.5 and the METAR smoke code); we also count the moonless good nights and those with a weak jet stream. Human cloud reports overstate clearing after dark by 8–17 percentage points; we correct for this night by night against automated ceilometers, which removes 8–23% of each city’s nights of clear darkness. Toronto has the most such nights, 98 a year, but its air is often humid, and only 58 are good. Calgary’s high, dry air works the other way: seventh on clear dark nights with 76, it keeps 86% of them and has 65 good nights. Winnipeg (70) and Calgary have significantly more good nights than Edmonton (49) and Vancouver (48). The first two have more than the last two in all 20 overlapping six-year windows since 2001, with or without the correction. Good nights come between September and May, with modest peaks in March and October in the east; the summer star-party season falls in months with fewer than three good nights. The method applies to any airport with a human-augmented METAR record and a satellite or automated cloud reference, and the code is released.