15.1 usable nights
13–18 in the middle half of years · 5.0 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 Green Valley
May: 22.6 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.20.7–21.2 mag/arcsec²
The short answer
May has been strongest.
It averages 22.6 usable nights, versus 4.8 in July. A typical May lands between 19 and 26 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
13.6 usable nights
10–17 in the middle half of years · 4.0 typically near a dark moon.
284 astronomical-dark hours in an average month
15.9 usable nights
13–18 in the middle half of years · 5.1 typically near a dark moon.
292 astronomical-dark hours in an average month
19.4 usable nights
17–21 in the middle half of years · 6.3 typically near a dark moon.
240 astronomical-dark hours in an average month
22.6 usable nights
19–26 in the middle half of years · 7.2 typically near a dark moon.
221 astronomical-dark hours in an average month
19.3 usable nights
15–22 in the middle half of years · 6.4 typically near a dark moon.
180 astronomical-dark hours in an average month
4.8 usable nights
3–7 in the middle half of years · 1.7 typically near a dark moon.
201 astronomical-dark hours in an average month
7.0 usable nights
4–9 in the middle half of years · 2.0 typically near a dark moon.
248 astronomical-dark hours in an average month
17.4 usable nights
14–21 in the middle half of years · 6.0 typically near a dark moon.
261 astronomical-dark hours in an average month
22.3 usable nights
20–26 in the middle half of years · 7.7 typically near a dark moon.
310 astronomical-dark hours in an average month
18.0 usable nights
16–21 in the middle half of years · 5.7 typically near a dark moon.
324 astronomical-dark hours in an average month
14.4 usable nights
11–18 in the middle half of years · 5.4 typically near a dark moon.
343 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.