7.4 usable nights
5–10 in the middle half of years · 2.3 typically near a dark moon.
348 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to Great Basin National Park
September: 18.6 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Natural-dark skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.21.9–21.9 mag/arcsec²
The short answer
September has been strongest.
It averages 18.6 usable nights, versus 5.8 in December. A typical September lands between 15 and 21 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
6.7 usable nights
4–9 in the middle half of years · 1.9 typically near a dark moon.
290 astronomical-dark hours in an average month
6.9 usable nights
5–8 in the middle half of years · 1.4 typically near a dark moon.
279 astronomical-dark hours in an average month
8.3 usable nights
6–10 in the middle half of years · 2.3 typically near a dark moon.
232 astronomical-dark hours in an average month
9.5 usable nights
5–13 in the middle half of years · 3.0 typically near a dark moon.
186 astronomical-dark hours in an average month
15.3 usable nights
12–18 in the middle half of years · 4.8 typically near a dark moon.
180 astronomical-dark hours in an average month
12.6 usable nights
9–17 in the middle half of years · 3.9 typically near a dark moon.
186 astronomical-dark hours in an average month
15.8 usable nights
13–19 in the middle half of years · 4.7 typically near a dark moon.
219 astronomical-dark hours in an average month
18.6 usable nights
15–21 in the middle half of years · 6.3 typically near a dark moon.
250 astronomical-dark hours in an average month
16.4 usable nights
13–20 in the middle half of years · 5.3 typically near a dark moon.
307 astronomical-dark hours in an average month
10.8 usable nights
9–13 in the middle half of years · 3.3 typically near a dark moon.
329 astronomical-dark hours in an average month
5.8 usable nights
3–9 in the middle half of years · 1.9 typically near a dark moon.
366 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.