ASU Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASU Dark Sky Observatory clear sky chart from 2005-11-06 to 2024-01-10.

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
41%
24153
20%
74373
11%
24099
51%
24099
23%
47111

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
20%
758
24%
2458
6%
757
42%
757
18%
1475
January11th to 20thNight
24%
720
21%
2340
8%
720
40%
720
16%
1525
January21st to endNight
21%
792
26%
2574
7%
792
43%
792
19%
1527
February1st to 10thNight
27%
720
20%
2340
11%
720
40%
720
20%
1441
February11th to 20thNight
14%
702
21%
2116
5%
699
32%
699
14%
1373
February21st to endNight
20%
592
22%
1768
7%
592
35%
592
19%
1162
March1st to 10thNight
24%
720
26%
1980
9%
720
39%
720
22%
1262
March11th to 20thNight
26%
720
18%
1980
6%
720
37%
720
19%
1335
March21st to endNight
27%
792
16%
2178
7%
792
35%
792
21%
1467
April1st to 10thNight
31%
720
21%
1980
7%
720
43%
720
25%
1235
April11th to 20thNight
34%
702
23%
1781
7%
702
45%
702
27%
1146
April21st to endNight
42%
540
24%
1620
12%
540
51%
540
27%
1003
May1st to 10thNight
39%
540
15%
1620
6%
540
45%
540
24%
1046
May11th to 20thNight
47%
531
14%
1593
8%
531
50%
531
21%
1090
May21st to endNight
54%
582
14%
1745
9%
582
57%
582
23%
1190
June1st to 10thNight
61%
540
14%
1620
11%
540
63%
540
26%
1080
June11th to 20thNight
60%
540
14%
1620
11%
540
63%
540
28%
1028
June21st to endNight
65%
540
16%
1620
12%
540
69%
540
29%
1018
July1st to 10thNight
67%
525
12%
1575
10%
525
69%
525
26%
1050
July11th to 20thNight
69%
522
11%
1566
8%
522
71%
522
26%
1047
July21st to endNight
70%
591
14%
1672
10%
558
72%
558
26%
1106
August1st to 10thNight
59%
537
12%
1600
9%
534
60%
534
22%
1124
August11th to 20thNight
61%
534
18%
1598
15%
534
63%
534
23%
1112
August21st to endNight
65%
692
17%
1880
12%
692
66%
692
28%
1275
September1st to 10thNight
61%
720
19%
1970
14%
720
64%
720
28%
1310
September11th to 20thNight
54%
720
19%
1980
15%
720
56%
720
25%
1353
September21st to endNight
46%
720
19%
1980
13%
720
52%
720
24%
1354
October1st to 10thNight
46%
720
23%
2074
19%
720
53%
720
28%
1309
October11th to 20thNight
41%
720
28%
2160
19%
720
55%
720
29%
1286
October21st to endNight
43%
792
31%
2376
20%
792
58%
792
30%
1366
November1st to 10thNight
44%
740
27%
2241
17%
740
57%
740
29%
1318
November11th to 20thNight
40%
760
28%
2470
17%
760
54%
760
28%
1350
November21st to endNight
37%
760
27%
2470
15%
760
52%
760
26%
1419
December1st to 10thNight
27%
760
23%
2470
9%
760
44%
760
17%
1590
December11th to 20thNight
28%
759
26%
2923
11%
758
46%
758
18%
1690
December21st to endNight
27%
830
14%
4435
8%
817
42%
817
10%
2649

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
15%5%25%5%35%5%6%8%41%
74893
Night
15%5%2443445%7%45%
74796
TransparencyDay
40%9%12%20%15%4
74616
Night
37%8%13%21%17%4
74592
SeeingDay
19%15%24%22%14%6%
14027
Night
17%14%22%27%17%3
24165

By Month

Percentage Cloud Cover SE (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
16%6%2442445%5%48%
1697
11th to 20thDay
17%7%25%5%2446%5%43%
1620
21st to endDay
14%6%2442445%6%50%
1782
February1st to 10thDay
13%7%25%5%35%5%5%7%42%
1620
11th to 20thDay
16%7%35%435%446%42%
1727
21st to endDay
19%6%34435%45%7%42%
1488
March1st to 10thDay
13%5%24434447%50%
1980
11th to 20thDay
19%6%35%43445%8%39%
1980
21st to endDay
19%6%25%5%35%45%5%40%
2178
April1st to 10thDay
11%4245%3446%7%50%
1980
11th to 20thDay
17%5%2443446%8%44%
2142
21st to endDay
15%5%25%5%35%5%5%6%44%
2340
May1st to 10thDay
15%5%25%436%6%6%8%39%
2340
11th to 20thDay
18%7%25%6%46%5%6%8%34%
2301
21st to endDay
16%6%25%6%35%6%7%9%36%
2531
June1st to 10thDay
15%5%25%6%46%6%7%9%36%
2340
11th to 20thDay
11%426%6%46%7%8%10%36%
2340
21st to endDay
9%435%7%5%6%7%8%11%35%
2340
July1st to 10thDay
11%6%27%7%47%7%8%10%31%
2340
11th to 20thDay
11%6%36%7%5%8%7%8%11%29%
2340
21st to endDay
11%6%38%7%48%8%8%8%28%
2547
August1st to 10thDay
16%5%26%7%5%7%7%8%9%29%
2327
11th to 20thDay
13%6%36%8%5%9%7%7%9%27%
2303
21st to endDay
14%7%25%5%35%7%7%9%35%
2458
September1st to 10thDay
14%6%35%5%345%6%8%40%
1985
11th to 20thDay
19%6%26%435%45%8%37%
1980
21st to endDay
22%5%245%45%5%47%38%
1980
October1st to 10thDay
19%5%23423245%49%
1886
11th to 20thDay
15%5%13424346%53%
1800
21st to endDay
17%325%312346%54%
1980
November1st to 10thDay
12%324423346%57%
1834
11th to 20thDay
13%4134245%47%55%
1710
21st to endDay
15%424424347%51%
1710
December1st to 10thDay
16%8%37%5%345%47%39%
1710
11th to 20thDay
14%42342345%7%53%
2153
21st to endDay
20%6%233345%6%7%41%
3124
January1st to 10thNight
14%6%234245%5%6%49%
2458
11th to 20thNight
15%7%345%35%45%6%44%
2340
21st to endNight
14%6%2442446%6%49%
2574
February1st to 10thNight
13%5%25%5%35%5%6%7%44%
2340
11th to 20thNight
18%6%24434446%45%
2116
21st to endNight
14%7%35%435%5%6%6%42%
1768
March1st to 10thNight
12%5%24425%5%46%50%
1980
11th to 20thNight
17%5%25%5%35%6%6%7%41%
1980
21st to endNight
16%6%25%4345%5%7%43%
2178
April1st to 10thNight
12%5%235%3346%8%49%
1980
11th to 20thNight
16%5%24424348%49%
1781
21st to endNight
15%5%233245%46%50%
1620
May1st to 10thNight
14%6%245%35%45%8%43%
1620
11th to 20thNight
16%5%25%5%35%5%7%7%40%
1592
21st to endNight
17%5%245%35%46%10%39%
1744
June1st to 10thNight
11%325%5%36%7%6%12%40%
1620
11th to 20thNight
12%4244345%8%10%45%
1620
21st to endNight
7%4246%36%6%7%13%42%
1620
July1st to 10thNight
10%5%246%35%5%9%10%41%
1620
11th to 20thNight
8%425%5%36%7%8%13%39%
1620
21st to endNight
6%426%7%47%6%8%11%39%
1771
August1st to 10thNight
16%6%35%5%346%6%10%37%
1611
11th to 20thNight
14%6%26%5%45%46%9%38%
1598
21st to endNight
12%6%345%35%47%10%40%
1880
September1st to 10thNight
15%4345%35%5%6%7%45%
1970
11th to 20thNight
19%6%35%424447%41%
1980
21st to endNight
21%5%25%5%34436%43%
1980
October1st to 10thNight
19%5%24423347%48%
2074
11th to 20thNight
17%423324345%53%
2160
21st to endNight
16%41332334458%
2376
November1st to 10thNight
15%5%14323345%55%
2241
11th to 20thNight
15%41332335%6%55%
2470
21st to endNight
13%42442445%7%51%
2470
December1st to 10thNight
17%6%25%424446%44%
2470
11th to 20thNight
14%5%25%434446%49%
3042
21st to endNight
23%7%2443446%7%37%
4532

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
37%7%9%23%19%5%
1698
11th to 20thDay
41%7%9%19%19%5%
1620
21st to endDay
36%48%25%20%7%
1782
February1st to 10thDay
40%7%12%20%16%5%
1620
11th to 20thDay
43%6%9%19%18%4
1737
21st to endDay
42%7%13%16%16%6%
1488
March1st to 10thDay
34%6%9%23%21%7%
1980
11th to 20thDay
44%8%10%17%16%5%
1980
21st to endDay
45%7%11%21%13%3
2178
April1st to 10thDay
32%8%12%24%20%5%
1980
11th to 20thDay
37%8%12%18%19%6%
2142
21st to endDay
40%10%12%22%14%3
2340
May1st to 10thDay
39%12%16%19%12%3
2340
11th to 20thDay
46%11%14%19%8%2
2305
21st to endDay
41%12%15%23%8%1
2531
June1st to 10thDay
42%12%16%20%9%1
2340
11th to 20thDay
39%14%15%18%13%2
2340
21st to endDay
39%14%15%18%13%1
2340
July1st to 10thDay
44%15%15%15%9%2
2275
11th to 20thDay
44%15%19%12%9%1
2271
21st to endDay
45%15%17%15%6%1
2405
August1st to 10thDay
46%16%15%16%6%1
2314
11th to 20thDay
47%16%16%15%6%1
2303
21st to endDay
42%12%16%20%7%3
2458
September1st to 10thDay
40%11%14%19%14%4
1985
11th to 20thDay
44%9%10%19%16%2
1980
21st to endDay
46%6%12%18%14%3
1980
October1st to 10thDay
37%6%10%20%22%5%
1886
11th to 20thDay
33%6%10%24%20%7%
1800
21st to endDay
33%49%24%21%8%
1980
November1st to 10thDay
30%5%9%22%27%8%
1834
11th to 20thDay
30%5%9%22%26%7%
1710
21st to endDay
35%5%8%21%26%5%
1710
December1st to 10thDay
45%8%7%19%15%5%
1710
11th to 20thDay
31%6%10%20%24%10%
2150
21st to endDay
40%7%10%23%17%3
3124
January1st to 10thNight
35%7%11%19%21%7%
2458
11th to 20thNight
41%8%9%17%20%5%
2340
21st to endNight
36%5%9%22%23%5%
2574
February1st to 10thNight
37%7%12%22%17%4
2340
11th to 20thNight
41%6%9%20%17%7%
2117
21st to endNight
40%7%13%17%16%6%
1768
March1st to 10thNight
35%8%8%20%22%7%
1980
11th to 20thNight
40%9%10%23%15%3
1980
21st to endNight
40%8%14%19%15%3
2178
April1st to 10thNight
33%8%12%23%20%4
1980
11th to 20thNight
34%7%11%22%18%8%
1781
21st to endNight
35%7%15%25%16%3
1620
May1st to 10thNight
38%9%16%24%11%1
1620
11th to 20thNight
42%10%15%21%10%1
1593
21st to endNight
40%11%16%24%8%1
1745
June1st to 10thNight
36%13%18%22%10%1
1620
11th to 20thNight
32%13%21%22%11%1
1620
21st to endNight
31%14%20%22%10%4
1620
July1st to 10thNight
35%13%18%19%12%2
1575
11th to 20thNight
34%15%22%18%8%2
1566
21st to endNight
34%14%22%22%8%1
1672
August1st to 10thNight
42%13%19%18%9%
1600
11th to 20thNight
42%11%17%20%9%1
1598
21st to endNight
38%10%19%22%9%2
1880
September1st to 10thNight
37%10%15%19%15%3
1970
11th to 20thNight
42%9%12%19%18%1
1980
21st to endNight
44%6%12%19%17%3
1980
October1st to 10thNight
37%6%10%22%21%4
2074
11th to 20thNight
34%5%9%26%20%5%
2160
21st to endNight
31%5%7%22%24%10%
2376
November1st to 10thNight
33%5%9%20%24%9%
2241
11th to 20thNight
30%5%9%20%26%10%
2470
21st to endNight
32%6%8%24%25%5%
2470
December1st to 10thNight
42%7%9%19%18%5%
2470
11th to 20thNight
35%6%9%19%24%7%
3044
21st to endNight
42%8%13%20%15%2
4532

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
27%34%28%9%1
267
11th to 20thDay
28%26%35%11%
240
21st to endDay
28%36%27%8%2
264
February1st to 10thDay
29%29%30%10%2
240
11th to 20thDay
41%29%25%4
240
21st to endDay
36%31%26%7%
206
March1st to 10thDay
21%28%31%15%5%
420
11th to 20thDay
28%28%25%15%31
420
21st to endDay
29%22%27%15%6%
462
April1st to 10thDay
21%24%30%18%6%1
420
11th to 20thDay
22%22%24%19%12%2
412
21st to endDay
21%21%28%16%10%4
420
May1st to 10thDay
18%21%32%16%8%5%
423
11th to 20thDay
19%13%26%23%12%7%
447
21st to endDay
20%6%21%25%17%11%
485
June1st to 10thDay
14%9%23%28%18%8%
450
11th to 20thDay
11%8%21%31%20%10%
450
21st to endDay
10%7%29%30%15%9%
450
July1st to 10thDay
11%316%29%28%13%
445
11th to 20thDay
7%317%31%28%15%
444
21st to endDay
10%216%31%27%13%
488
August1st to 10thDay
16%5%21%24%19%16%
446
11th to 20thDay
11%316%28%26%15%
439
21st to endDay
14%315%29%23%17%
495
September1st to 10thDay
14%316%28%26%13%
450
11th to 20thDay
20%314%27%25%10%
450
21st to endDay
23%9%18%28%17%5%
450
October1st to 10thDay
24%9%19%23%19%7%
450
11th to 20thDay
16%18%25%23%15%3
450
21st to endDay
20%15%29%24%9%3
495
November1st to 10thDay
19%16%26%23%12%3
432
11th to 20thDay
20%25%31%17%41
270
21st to endDay
19%24%33%19%6%1
270
December1st to 10thDay
23%29%35%11%2
270
11th to 20thDay
21%23%35%16%5%
270
21st to endDay
32%21%30%11%42
297
January1st to 10thNight
18%28%30%20%4
760
11th to 20thNight
21%23%28%24%4
720
21st to endNight
17%27%32%22%3
792
February1st to 10thNight
17%22%30%27%4
720
11th to 20thNight
24%29%29%17%1
703
21st to endNight
20%24%33%20%3
592
March1st to 10thNight
16%24%32%24%3
720
11th to 20thNight
22%18%32%21%7%1
720
21st to endNight
19%18%30%25%8%
792
April1st to 10thNight
17%22%27%26%8%
720
11th to 20thNight
20%18%25%24%13%1
702
21st to endNight
18%12%25%32%12%
540
May1st to 10thNight
17%11%28%28%15%1
540
11th to 20thNight
17%9%19%33%19%3
531
21st to endNight
17%415%25%33%6%
584
June1st to 10thNight
12%216%35%30%5%
540
11th to 20thNight
12%118%28%33%7%
540
21st to endNight
9%215%35%35%4
540
July1st to 10thNight
11%38%29%43%6%
525
11th to 20thNight
10%110%27%44%9%
522
21st to endNight
6%210%32%41%9%
591
August1st to 10thNight
16%211%27%37%6%
537
11th to 20thNight
15%29%28%37%8%
534
21st to endNight
14%18%29%40%8%
692
September1st to 10thNight
14%313%25%37%8%
720
11th to 20thNight
18%413%25%33%6%
720
21st to endNight
22%5%17%27%26%2
720
October1st to 10thNight
19%10%19%23%24%5%
720
11th to 20thNight
20%11%25%28%15%3
720
21st to endNight
19%14%20%32%12%3
792
November1st to 10thNight
18%12%22%30%14%3
740
11th to 20thNight
17%16%24%32%10%1
760
21st to endNight
14%21%24%29%10%1
760
December1st to 10thNight
20%21%27%25%6%
760
11th to 20thNight
18%20%29%26%7%1
760
21st to endNight
26%19%24%27%4
836
Page generated at 2024-01-07 10:06:52. Version ac07

Copyright 2024 Attilla Danko


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