10.6 usable nights
6–14 in the middle half of years · 3.4 typically near a dark moon.
345 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to Saint George
September: 21.1 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Brightened skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.19.8–20.4 mag/arcsec²
The short answer
September has been strongest.
It averages 21.1 usable nights, versus 9.4 in December. A typical September lands between 19 and 24 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
10.2 usable nights
8–13 in the middle half of years · 3.3 typically near a dark moon.
289 astronomical-dark hours in an average month
10.7 usable nights
8–13 in the middle half of years · 2.9 typically near a dark moon.
282 astronomical-dark hours in an average month
13.4 usable nights
12–15 in the middle half of years · 4.0 typically near a dark moon.
240 astronomical-dark hours in an average month
15.8 usable nights
13–19 in the middle half of years · 5.3 typically near a dark moon.
188 astronomical-dark hours in an average month
19.9 usable nights
17–22 in the middle half of years · 6.6 typically near a dark moon.
180 astronomical-dark hours in an average month
13.7 usable nights
9–17 in the middle half of years · 4.4 typically near a dark moon.
186 astronomical-dark hours in an average month
18.4 usable nights
16–21 in the middle half of years · 5.5 typically near a dark moon.
229 astronomical-dark hours in an average month
21.1 usable nights
19–24 in the middle half of years · 7.5 typically near a dark moon.
252 astronomical-dark hours in an average month
19.8 usable nights
17–22 in the middle half of years · 6.9 typically near a dark moon.
309 astronomical-dark hours in an average month
15.1 usable nights
14–17 in the middle half of years · 5.3 typically near a dark moon.
328 astronomical-dark hours in an average month
9.4 usable nights
7–12 in the middle half of years · 3.2 typically near a dark moon.
363 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.