2.6 usable nights
1–4 in the middle half of years · 0.5 typically near a dark moon.
357 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to Granby
September: 8.3 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Bright skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.19.5–20.2 mag/arcsec²
The short answer
September has been strongest.
It averages 8.3 usable nights, versus 1.5 in December. A typical September lands between 6 and 10 usable nights.
Record2005–2024 · 20 complete years
Usable nightAt least 3 astronomical-dark hours and NASA POWER nighttime cloud ≤30%
Dark-moon nightUsable night with ≤25% illumination at local solar midnight
ResolutionCERES SYN1deg 1° cloud grid; regional climate, not a campsite guarantee
Seasonal fingerprint
Usable nights by month
Month by month
2.7 usable nights
2–4 in the middle half of years · 0.7 typically near a dark moon.
292 astronomical-dark hours in an average month
5.3 usable nights
4–7 in the middle half of years · 1.8 typically near a dark moon.
268 astronomical-dark hours in an average month
4.2 usable nights
2–6 in the middle half of years · 1.2 typically near a dark moon.
205 astronomical-dark hours in an average month
4.8 usable nights
3–6 in the middle half of years · 1.6 typically near a dark moon.
146 astronomical-dark hours in an average month
4.2 usable nights
3–5 in the middle half of years · 1.5 typically near a dark moon.
118 astronomical-dark hours in an average month
5.8 usable nights
4–8 in the middle half of years · 2.0 typically near a dark moon.
127 astronomical-dark hours in an average month
6.8 usable nights
5–9 in the middle half of years · 2.3 typically near a dark moon.
188 astronomical-dark hours in an average month
8.3 usable nights
6–10 in the middle half of years · 2.5 typically near a dark moon.
245 astronomical-dark hours in an average month
5.0 usable nights
4–6 in the middle half of years · 1.7 typically near a dark moon.
303 astronomical-dark hours in an average month
3.0 usable nights
2–4 in the middle half of years · 1.3 typically near a dark moon.
333 astronomical-dark hours in an average month
1.5 usable nights
1–2 in the middle half of years · 0.5 typically near a dark moon.
372 astronomical-dark hours in an average month
Use the baseline honestly
Climate chooses the month. Forecasts choose the night.
The cloud product is a roughly 1° regional grid. It can reveal seasonality, but it cannot know a mountain ridge, valley fog pocket, locked gate, smoke event or the exact sky you will get.
Source: NASA POWER daily data, parameter CLOUD_AMT_NIGHT from the CERES SYN1deg cloud product. GoodSeeing calculates astronomical darkness and lunar illumination independently for each place.