6.9 usable nights
3–10 in the middle half of years · 1.9 typically near a dark moon.
346 astronomical-dark hours in an average month
20-year trip-planning baseline
Best time to stargaze at San Mateo
October: 12.9 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.18.9–19.7 mag/arcsec²
The short answer
October has been strongest.
It averages 12.9 usable nights, versus 5.5 in August. A typical October lands between 10 and 15 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
7.2 usable nights
4–10 in the middle half of years · 2.2 typically near a dark moon.
289 astronomical-dark hours in an average month
7.0 usable nights
6–9 in the middle half of years · 1.7 typically near a dark moon.
281 astronomical-dark hours in an average month
9.0 usable nights
7–11 in the middle half of years · 2.9 typically near a dark moon.
239 astronomical-dark hours in an average month
9.2 usable nights
7–13 in the middle half of years · 3.0 typically near a dark moon.
187 astronomical-dark hours in an average month
11.1 usable nights
9–14 in the middle half of years · 3.4 typically near a dark moon.
180 astronomical-dark hours in an average month
6.0 usable nights
4–8 in the middle half of years · 1.9 typically near a dark moon.
186 astronomical-dark hours in an average month
5.5 usable nights
4–8 in the middle half of years · 1.4 typically near a dark moon.
227 astronomical-dark hours in an average month
9.3 usable nights
7–11 in the middle half of years · 3.1 typically near a dark moon.
251 astronomical-dark hours in an average month
12.9 usable nights
10–15 in the middle half of years · 5.1 typically near a dark moon.
309 astronomical-dark hours in an average month
10.8 usable nights
9–13 in the middle half of years · 3.5 typically near a dark moon.
329 astronomical-dark hours in an average month
7.2 usable nights
5–9 in the middle half of years · 2.1 typically near a dark moon.
364 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.