12.9 usable nights
10–15 in the middle half of years · 4.5 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 Kitt Peak National Observatory
May: 22.3 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Threatened dark skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.21.6–21.7 mag/arcsec²
The short answer
May has been strongest.
It averages 22.3 usable nights, versus 4.5 in July. A typical May lands between 20 and 25 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
9–16 in the middle half of years · 3.9 typically near a dark moon.
284 astronomical-dark hours in an average month
15.8 usable nights
12–19 in the middle half of years · 5.0 typically near a dark moon.
292 astronomical-dark hours in an average month
18.9 usable nights
17–20 in the middle half of years · 6.3 typically near a dark moon.
240 astronomical-dark hours in an average month
22.3 usable nights
20–25 in the middle half of years · 7.0 typically near a dark moon.
221 astronomical-dark hours in an average month
19.9 usable nights
16–24 in the middle half of years · 6.5 typically near a dark moon.
180 astronomical-dark hours in an average month
4.5 usable nights
2–6 in the middle half of years · 1.8 typically near a dark moon.
200 astronomical-dark hours in an average month
6.0 usable nights
3–9 in the middle half of years · 1.8 typically near a dark moon.
248 astronomical-dark hours in an average month
16.0 usable nights
14–19 in the middle half of years · 5.5 typically near a dark moon.
261 astronomical-dark hours in an average month
22.0 usable nights
17–26 in the middle half of years · 7.4 typically near a dark moon.
310 astronomical-dark hours in an average month
16.8 usable nights
16–18 in the middle half of years · 5.6 typically near a dark moon.
324 astronomical-dark hours in an average month
12.7 usable nights
9–15 in the middle half of years · 5.1 typically near a dark moon.
344 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.