3.6 usable nights
2–5 in the middle half of years · 0.8 typically near a dark moon.
350 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to Boulder
September: 12.3 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.19.2–19.9 mag/arcsec²
The short answer
September has been strongest.
It averages 12.3 usable nights, versus 3.3 in February. A typical September lands between 10 and 14 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
3.3 usable nights
2–4 in the middle half of years · 0.9 typically near a dark moon.
291 astronomical-dark hours in an average month
4.0 usable nights
2–5 in the middle half of years · 1.1 typically near a dark moon.
277 astronomical-dark hours in an average month
4.5 usable nights
2–7 in the middle half of years · 1.0 typically near a dark moon.
228 astronomical-dark hours in an average month
5.0 usable nights
2–8 in the middle half of years · 1.3 typically near a dark moon.
186 astronomical-dark hours in an average month
10.3 usable nights
8–13 in the middle half of years · 3.6 typically near a dark moon.
141 astronomical-dark hours in an average month
10.3 usable nights
9–11 in the middle half of years · 3.5 typically near a dark moon.
176 astronomical-dark hours in an average month
9.4 usable nights
8–11 in the middle half of years · 3.4 typically near a dark moon.
214 astronomical-dark hours in an average month
12.3 usable nights
10–14 in the middle half of years · 4.8 typically near a dark moon.
249 astronomical-dark hours in an average month
10.0 usable nights
8–13 in the middle half of years · 3.0 typically near a dark moon.
306 astronomical-dark hours in an average month
5.7 usable nights
4–8 in the middle half of years · 1.9 typically near a dark moon.
330 astronomical-dark hours in an average month
3.9 usable nights
2–5 in the middle half of years · 1.1 typically near a dark moon.
368 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.