3.1 usable nights
1–5 in the middle half of years · 0.5 typically near a dark moon.
350 astronomical-dark hours in an average month
20-year trip-planning baseline
Best time to stargaze at Longmont
September: 11.7 usable nights on average. Use this seasonal climate baseline to choose a month, then use the live forecast to choose a night.
Site darkness · modelled, not measured
Bright skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.18.9–19.6 mag/arcsec²
The short answer
September has been strongest.
It averages 11.7 usable nights, versus 2.4 in December. A typical September lands between 10 and 13 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
4.2 usable nights
3–6 in the middle half of years · 1.2 typically near a dark moon.
291 astronomical-dark hours in an average month
5.8 usable nights
4–7 in the middle half of years · 1.6 typically near a dark moon.
277 astronomical-dark hours in an average month
5.8 usable nights
4–8 in the middle half of years · 1.9 typically near a dark moon.
227 astronomical-dark hours in an average month
5.8 usable nights
4–8 in the middle half of years · 1.8 typically near a dark moon.
186 astronomical-dark hours in an average month
8.8 usable nights
6–11 in the middle half of years · 3.1 typically near a dark moon.
136 astronomical-dark hours in an average month
9.3 usable nights
7–11 in the middle half of years · 2.9 typically near a dark moon.
175 astronomical-dark hours in an average month
9.3 usable nights
7–11 in the middle half of years · 3.3 typically near a dark moon.
214 astronomical-dark hours in an average month
11.7 usable nights
10–13 in the middle half of years · 4.3 typically near a dark moon.
249 astronomical-dark hours in an average month
9.8 usable nights
8–12 in the middle half of years · 3.4 typically near a dark moon.
306 astronomical-dark hours in an average month
5.8 usable nights
4–7 in the middle half of years · 2.1 typically near a dark moon.
330 astronomical-dark hours in an average month
2.4 usable nights
1–4 in the middle half of years · 0.6 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.