PNRO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PNRO clear sky chart from 2005-11-06 to 2024-01-12.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
49%
22477
11%
74729
9%
22446
51%
22446
12%
57638

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
27%
756
3%
2646
2%
756
29%
756
4%
2348
January11th to 20thNight
35%
728
4%
2548
3%
728
37%
728
6%
2162
January21st to endNight
38%
792
5%
2772
4%
792
40%
792
7%
2310
February1st to 10thNight
38%
720
6%
2398
5%
720
41%
720
10%
1906
February11th to 20thNight
41%
701
8%
2193
7%
700
43%
700
10%
1754
February21st to endNight
35%
592
10%
1776
8%
592
38%
592
9%
1450
March1st to 10thNight
42%
720
8%
2160
7%
720
45%
720
12%
1735
March11th to 20thNight
46%
720
10%
2102
7%
720
48%
720
12%
1696
March21st to endNight
37%
792
7%
1985
3%
792
39%
792
10%
1683
April1st to 10thNight
43%
720
8%
1800
3%
720
45%
720
13%
1469
April11th to 20thNight
43%
711
15%
1779
7%
711
46%
711
17%
1380
April21st to endNight
45%
570
10%
1650
6%
570
48%
570
15%
1277
May1st to 10thNight
53%
384
13%
1464
13%
384
57%
384
14%
1025
May11th to 20thNight
58%
354
15%
1416
19%
354
62%
354
12%
992
May21st to endNight
57%
388
10%
1552
13%
388
61%
388
11%
1138
June1st to 10thNight
48%
360
6%
1440
7%
360
53%
360
10%
1071
June11th to 20thNight
45%
360
10%
1440
11%
360
51%
360
12%
1014
June21st to endNight
60%
360
14%
1440
20%
360
67%
360
18%
912
July1st to 10thNight
67%
350
15%
1400
24%
350
71%
350
23%
818
July11th to 20thNight
64%
348
20%
1392
27%
348
73%
348
23%
815
July21st to endNight
72%
394
23%
1488
26%
372
84%
372
25%
838
August1st to 10thNight
74%
358
22%
1424
25%
356
79%
356
22%
858
August11th to 20thNight
78%
514
25%
1582
20%
514
81%
514
28%
985
August21st to endNight
73%
792
22%
1980
12%
792
76%
792
30%
1350
September1st to 10thNight
73%
720
23%
1800
13%
720
75%
720
32%
1205
September11th to 20thNight
70%
720
24%
1852
17%
720
72%
720
27%
1292
September21st to endNight
66%
720
19%
1980
17%
720
67%
720
22%
1415
October1st to 10thNight
61%
720
16%
2114
15%
720
64%
720
21%
1501
October11th to 20thNight
56%
720
14%
2182
13%
720
60%
720
16%
1617
October21st to endNight
48%
792
12%
2574
11%
792
51%
792
15%
1904
November1st to 10thNight
42%
740
10%
2405
8%
740
45%
740
9%
1961
November11th to 20thNight
38%
760
6%
2574
6%
760
39%
760
7%
2189
November21st to endNight
45%
760
4%
2660
4%
760
46%
760
8%
2217
December1st to 10thNight
40%
760
5%
2660
4%
760
42%
760
6%
2269
December11th to 20thNight
30%
758
2%
3276
2%
758
31%
758
3%
3005
December21st to endNight
29%
823
2%
4825
1%
817
31%
817
2%
4077

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
8%7%36%5%35%45%7%47%
74789
Night
10%8%47%6%35%5%5%7%41%
74948
TransparencyDay
25%38%8%14%11%5%
74492
Night
29%38%8%12%8%4
74766
SeeingDay
10%9%21%26%23%10%
13281
Night
13%7%21%34%21%3
22500

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
17%16%6%12%9%45%5%6%5%15%
1519
11th to 20thDay
16%15%7%10%8%46%46%6%18%
1451
21st to endDay
13%14%6%12%9%5%7%5%5%6%19%
1584
February1st to 10thDay
15%13%49%8%45%47%6%24%
1557
11th to 20thDay
13%12%5%8%6%46%45%6%32%
1654
21st to endDay
11%9%5%7%8%46%5%7%7%30%
1480
March1st to 10thDay
10%11%47%7%46%6%6%9%31%
1800
11th to 20thDay
9%8%49%8%46%5%6%7%34%
1856
21st to endDay
11%7%47%8%5%7%6%7%7%30%
2369
April1st to 10thDay
9%7%36%7%37%7%7%9%37%
2160
11th to 20thDay
7%7%36%5%35%5%5%8%46%
2129
21st to endDay
8%6%25%4345%6%7%49%
2308
May1st to 10thDay
5%5%245%345%7%8%52%
2492
11th to 20thDay
8%5%25%6%46%6%7%7%44%
2477
21st to endDay
7%6%35%6%45%5%7%8%46%
2723
June1st to 10thDay
10%425%5%35%5%5%7%48%
2520
11th to 20thDay
7%42443345%7%55%
2520
21st to endDay
421332336%7%67%
2520
July1st to 10thDay
1213222335%76%
2520
11th to 20thDay
1112223335%76%
2520
21st to endDay
111111222484%
2741
August1st to 10thDay
2212222336%75%
2507
11th to 20thDay
2112222335%78%
2315
21st to endDay
3413323345%70%
2376
September1st to 10thDay
2211223436%75%
2160
11th to 20thDay
5%31443446%7%59%
2107
21st to endDay
6%5%25%5%3445%8%54%
1980
October1st to 10thDay
8%5%345%33447%54%
1844
11th to 20thDay
6%6%35%5%445%7%8%49%
1777
21st to endDay
13%8%37%4445%5%7%39%
1782
November1st to 10thDay
14%8%37%7%46%6%5%9%33%
1674
11th to 20thDay
13%13%5%8%8%5%6%6%6%6%25%
1605
21st to endDay
12%13%6%11%8%46%6%7%7%21%
1520
December1st to 10thDay
11%11%7%11%11%5%6%6%6%6%21%
1520
11th to 20thDay
14%15%12%16%9%45%445%12%
1928
21st to endDay
15%11%6%14%12%5%6%47%5%15%
2794
January1st to 10thNight
18%17%5%9%9%5%5%5%5%6%17%
2646
11th to 20thNight
15%12%6%11%8%45%6%6%6%21%
2548
21st to endNight
15%11%6%9%8%45%6%6%7%23%
2772
February1st to 10thNight
14%11%5%8%7%46%5%5%6%29%
2398
11th to 20thNight
11%11%48%7%45%5%6%7%33%
2191
21st to endNight
11%10%5%8%8%35%5%5%7%31%
1776
March1st to 10thNight
12%9%48%7%46%45%8%34%
2160
11th to 20thNight
9%9%47%6%45%5%7%9%33%
2102
21st to endNight
11%8%46%7%47%6%8%7%30%
1985
April1st to 10thNight
13%9%37%6%35%45%8%36%
1800
11th to 20thNight
10%6%35%6%5%5%446%45%
1779
21st to endNight
9%5%46%6%35%5%6%7%42%
1650
May1st to 10thNight
6%425%5%35%5%5%8%52%
1464
11th to 20thNight
10%3245%46%6%8%9%43%
1416
21st to endNight
7%5%36%6%5%46%7%9%41%
1552
June1st to 10thNight
10%5%25%5%35%5%7%8%45%
1440
11th to 20thNight
9%7%25%5%35%5%5%6%50%
1440
21st to endNight
431443445%6%63%
1440
July1st to 10thNight
321221345%7%72%
1440
11th to 20thNight
2111322436%75%
1440
21st to endNight
1111212335%80%
1576
August1st to 10thNight
4212222246%72%
1432
11th to 20thNight
2212323345%74%
1582
21st to endNight
2212323346%71%
1980
September1st to 10thNight
321221333475%
1800
11th to 20thNight
441442435%6%63%
1852
21st to endNight
6%3135%245%5%7%58%
1980
October1st to 10thNight
9%435%434446%54%
2114
11th to 20thNight
8%7%345%3445%6%51%
2182
21st to endNight
10%7%46%6%35%45%6%43%
2574
November1st to 10thNight
16%8%37%6%4445%7%35%
2405
11th to 20thNight
17%10%38%7%45%6%5%6%30%
2574
21st to endNight
13%9%49%7%46%5%7%7%30%
2660
December1st to 10thNight
14%11%7%11%8%5%6%6%6%6%22%
2660
11th to 20thNight
16%17%7%14%9%45%5%45%14%
3276
21st to endNight
14%12%10%11%10%5%6%45%5%18%
4862

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
41%44%10%5%1
1519
11th to 20thDay
37%45%8%9%1
1451
21st to endDay
38%39%13%7%2
1584
February1st to 10thDay
36%39%13%10%2
1557
11th to 20thDay
31%40%11%12%6%
1663
21st to endDay
34%40%10%12%4
1480
March1st to 10thDay
32%39%11%13%5%
1800
11th to 20thDay
32%38%13%14%4
1856
21st to endDay
35%40%11%11%3
2369
April1st to 10thDay
28%39%9%16%8%
2160
11th to 20thDay
23%37%11%15%10%3
2132
21st to endDay
24%38%10%16%10%1
2308
May1st to 10thDay
21%37%8%18%13%2
2492
11th to 20thDay
22%38%7%18%13%3
2480
21st to endDay
26%39%8%15%10%2
2724
June1st to 10thDay
27%38%8%15%9%3
2520
11th to 20thDay
20%36%9%19%12%3
2520
21st to endDay
13%36%6%20%16%9%
2520
July1st to 10thDay
7%36%5%14%23%15%
2451
11th to 20thDay
7%36%49%22%22%
2445
21st to endDay
5%36%39%26%21%
2589
August1st to 10thDay
9%36%6%15%23%11%
2490
11th to 20thDay
7%33%414%24%17%
2315
21st to endDay
11%35%417%24%8%
2376
September1st to 10thDay
8%36%5%17%21%13%
2160
11th to 20thDay
15%37%7%17%17%8%
2107
21st to endDay
19%36%9%20%12%4
1980
October1st to 10thDay
21%36%8%18%14%2
1844
11th to 20thDay
19%39%12%19%9%1
1777
21st to endDay
26%39%11%19%6%1
1782
November1st to 10thDay
33%43%9%11%3
1674
11th to 20thDay
36%43%9%10%1
1605
21st to endDay
38%43%11%7%1
1520
December1st to 10thDay
37%43%8%10%2
1520
11th to 20thDay
56%35%5%4
1928
21st to endDay
54%30%8%7%1
2794
January1st to 10thNight
42%42%8%7%1
2646
11th to 20thNight
36%43%11%9%2
2548
21st to endNight
37%39%12%10%11
2772
February1st to 10thNight
32%39%13%13%31
2398
11th to 20thNight
31%40%11%14%5%
2193
21st to endNight
34%38%11%11%5%1
1776
March1st to 10thNight
31%39%11%14%5%
2160
11th to 20thNight
31%39%11%14%4
2102
21st to endNight
37%38%9%13%3
1985
April1st to 10thNight
31%38%9%13%8%
1800
11th to 20thNight
26%37%8%18%8%3
1779
21st to endNight
27%38%10%14%9%2
1650
May1st to 10thNight
22%38%9%16%13%2
1464
11th to 20thNight
22%39%7%17%12%4
1416
21st to endNight
25%39%9%15%10%2
1552
June1st to 10thNight
27%39%9%14%7%3
1440
11th to 20thNight
24%37%9%18%11%2
1440
21st to endNight
15%36%6%18%16%9%
1440
July1st to 10thNight
8%37%6%13%20%17%
1400
11th to 20thNight
8%36%411%19%22%
1392
21st to endNight
6%36%38%23%25%
1488
August1st to 10thNight
12%35%5%12%24%12%
1424
11th to 20thNight
10%35%311%21%20%
1582
21st to endNight
11%36%5%13%23%12%
1980
September1st to 10thNight
9%35%5%14%19%17%
1800
11th to 20thNight
15%36%5%15%18%11%
1852
21st to endNight
18%37%8%15%15%7%
1980
October1st to 10thNight
20%36%7%17%17%2
2114
11th to 20thNight
23%39%7%17%11%3
2182
21st to endNight
24%39%10%16%10%1
2574
November1st to 10thNight
32%41%7%15%4
2405
11th to 20thNight
38%41%9%11%1
2574
21st to endNight
34%42%11%11%2
2660
December1st to 10thNight
38%42%9%10%2
2660
11th to 20thNight
53%36%7%5%
3276
21st to endNight
50%31%9%7%3
4862

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
23%11%21%22%18%5%
180
11th to 20thDay
22%7%23%29%13%5%
163
21st to endDay
21%8%24%26%19%2
176
February1st to 10thDay
23%13%23%24%13%4
277
11th to 20thDay
22%13%21%23%17%4
371
21st to endDay
15%15%21%25%20%3
346
March1st to 10thDay
19%15%26%22%15%4
420
11th to 20thDay
12%12%22%28%19%6%
420
21st to endDay
13%13%29%27%15%3
462
April1st to 10thDay
14%16%25%21%15%8%
420
11th to 20thDay
11%16%31%22%14%7%
412
21st to endDay
8%13%27%27%18%7%
420
May1st to 10thDay
8%10%23%27%25%7%
423
11th to 20thDay
12%7%24%20%25%12%
448
21st to endDay
10%6%22%25%25%12%
485
June1st to 10thDay
10%10%19%22%25%14%
450
11th to 20thDay
8%12%26%28%19%6%
450
21st to endDay
26%18%29%30%14%
450
July1st to 10thDay
2215%33%40%8%
446
11th to 20thDay
25%16%37%29%12%
444
21st to endDay
1312%31%34%20%
487
August1st to 10thDay
2411%29%32%22%
447
11th to 20thDay
2112%23%34%27%
437
21st to endDay
5%5%18%27%28%17%
495
September1st to 10thDay
3415%30%33%15%
450
11th to 20thDay
7%6%16%24%28%19%
450
21st to endDay
8%8%16%29%27%12%
450
October1st to 10thDay
12%6%18%27%25%12%
450
11th to 20thDay
11%10%18%23%26%12%
427
21st to endDay
16%11%23%27%17%7%
297
November1st to 10thDay
16%12%25%25%19%3
270
11th to 20thDay
19%11%23%26%17%5%
220
21st to endDay
13%11%22%23%27%6%
180
December1st to 10thDay
13%11%23%28%13%13%
180
11th to 20thDay
20%12%22%33%11%3
180
21st to endDay
19%8%27%30%12%4
198
January1st to 10thNight
27%9%21%25%15%3
760
11th to 20thNight
19%9%18%33%18%3
728
21st to endNight
20%6%18%33%20%3
792
February1st to 10thNight
20%11%20%32%16%2
720
11th to 20thNight
17%9%20%33%19%3
704
21st to endNight
16%13%23%34%12%2
592
March1st to 10thNight
15%10%24%31%17%3
720
11th to 20thNight
13%8%23%34%21%1
720
21st to endNight
14%10%31%31%14%1
792
April1st to 10thNight
16%8%23%34%16%3
720
11th to 20thNight
13%11%28%27%18%4
711
21st to endNight
11%8%29%30%20%2
570
May1st to 10thNight
7%7%28%36%21%2
384
11th to 20thNight
11%6%17%37%27%2
354
21st to endNight
8%7%23%35%23%3
389
June1st to 10thNight
11%7%27%33%18%3
360
11th to 20thNight
11%9%33%34%13%1
360
21st to endNight
4427%38%24%3
360
July1st to 10thNight
4324%40%27%2
350
11th to 20thNight
2328%44%20%2
348
21st to endNight
1321%40%32%2
394
August1st to 10thNight
5%117%43%31%3
358
11th to 20thNight
3115%36%37%9%
514
21st to endNight
3318%39%31%6%
792
September1st to 10thNight
4118%39%32%5%
720
11th to 20thNight
4217%33%36%9%
720
21st to endNight
7%419%32%31%7%
720
October1st to 10thNight
11%318%38%26%4
720
11th to 20thNight
11%8%17%33%25%6%
720
21st to endNight
12%7%24%31%20%5%
792
November1st to 10thNight
19%5%24%34%17%1
740
11th to 20thNight
20%9%22%30%16%3
760
21st to endNight
16%8%16%35%21%4
760
December1st to 10thNight
17%9%17%35%18%4
760
11th to 20thNight
22%8%21%34%13%2
760
21st to endNight
26%9%21%30%13%1
836
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