11.4 usable nights
9–14 in the middle half of years · 4.0 typically near a dark moon.
341 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to San Dimas
September: 25.0 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Bright urban skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.18.0–18.8 mag/arcsec²
The short answer
September has been strongest.
It averages 25.0 usable nights, versus 11.4 in January. A typical September lands between 23 and 27 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
12.3 usable nights
9–18 in the middle half of years · 4.1 typically near a dark moon.
287 astronomical-dark hours in an average month
14.6 usable nights
11–18 in the middle half of years · 4.5 typically near a dark moon.
287 astronomical-dark hours in an average month
17.4 usable nights
15–20 in the middle half of years · 5.5 typically near a dark moon.
240 astronomical-dark hours in an average month
21.8 usable nights
20–24 in the middle half of years · 7.5 typically near a dark moon.
206 astronomical-dark hours in an average month
24.2 usable nights
22–27 in the middle half of years · 8.1 typically near a dark moon.
180 astronomical-dark hours in an average month
22.0 usable nights
20–26 in the middle half of years · 7.3 typically near a dark moon.
191 astronomical-dark hours in an average month
23.9 usable nights
22–25 in the middle half of years · 7.8 typically near a dark moon.
242 astronomical-dark hours in an average month
25.0 usable nights
23–27 in the middle half of years · 8.9 typically near a dark moon.
257 astronomical-dark hours in an average month
22.6 usable nights
20–25 in the middle half of years · 7.8 typically near a dark moon.
310 astronomical-dark hours in an average month
16.1 usable nights
13–19 in the middle half of years · 5.3 typically near a dark moon.
326 astronomical-dark hours in an average month
11.9 usable nights
9–15 in the middle half of years · 4.2 typically near a dark moon.
355 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.