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 2005-11-05 to 2017-09-10.

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
43%
15680
21%
46830
12%
15641
53%
15641
24%
29725

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 15thNight
25%
717
24%
2327
8%
716
45%
716
18%
1416
January16th to endNight
23%
768
24%
2496
7%
768
43%
768
18%
1529
February1st to 15thNight
24%
703
23%
2244
8%
701
42%
701
20%
1322
February16th to endNight
20%
631
24%
1887
8%
631
37%
631
20%
1168
March1st to 15thNight
29%
720
24%
1980
10%
720
41%
720
22%
1289
March16th to endNight
29%
768
17%
2112
8%
768
36%
768
22%
1409
April1st to 15thNight
36%
718
21%
1925
8%
718
46%
718
27%
1196
April16th to endNight
42%
593
20%
1673
9%
593
51%
593
25%
1091
May1st to 15thNight
42%
540
14%
1620
7%
540
47%
540
22%
1090
May16th to endNight
56%
561
14%
1684
8%
561
59%
561
24%
1120
June1st to 15thNight
65%
540
13%
1620
11%
540
66%
540
27%
1076
June16th to endNight
67%
540
15%
1620
12%
540
69%
540
31%
1009
July1st to 15thNight
68%
510
11%
1530
9%
510
69%
510
26%
1016
July16th to endNight
73%
564
13%
1595
10%
531
75%
531
29%
1018
August1st to 15thNight
61%
539
14%
1609
12%
536
62%
536
23%
1113
August16th to endNight
68%
639
18%
1785
15%
639
69%
639
27%
1227
September1st to 15thNight
61%
700
19%
1919
15%
700
63%
700
27%
1280
September16th to endNight
50%
657
17%
1815
13%
657
53%
657
23%
1270
October1st to 15thNight
46%
660
27%
1928
22%
660
55%
660
29%
1187
October16th to endNight
44%
704
32%
2112
19%
704
60%
704
31%
1201
November1st to 15thNight
48%
700
27%
2173
19%
700
60%
700
31%
1220
November16th to endNight
38%
720
26%
2340
16%
720
51%
720
26%
1346
December1st to 15thNight
32%
720
24%
2340
11%
720
46%
720
19%
1457
December16th to endNight
26%
768
21%
2496
9%
768
41%
768
15%
1675

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%5%7%39%
47795
Night
14%42442445%7%45%
47027
TransparencyDay
41%9%12%18%14%3
47496
Night
36%7%12%20%16%4
46830
SeeingDay
18%11%20%21%17%11%
5527
Night
16%12%21%26%19%3
15686

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 15thDay
15%5%24425%46%5%44%
1611
16th to endDay
13%5%1442345%6%48%
1728
February1st to 15thDay
12%6%2442445%7%45%
1615
16th to endDay
13%434334347%45%
1586
March1st to 15thDay
14%42433435%7%46%
1980
16th to endDay
18%6%245%24445%39%
2111
April1st to 15thDay
13%21442445%8%47%
2021
16th to endDay
16%6%25%425%45%6%40%
2298
May1st to 15thDay
17%5%15%436%5%5%8%35%
2331
16th to endDay
14%5%246%35%5%7%8%35%
2431
June1st to 15thDay
13%426%6%46%6%8%9%31%
2340
16th to endDay
9%3246%5%7%6%7%10%35%
2340
July1st to 15thDay
13%6%26%6%47%6%7%9%28%
2340
16th to endDay
10%5%36%8%48%7%8%10%26%
2465
August1st to 15thDay
16%5%25%7%47%7%7%9%25%
2340
16th to endDay
14%7%25%5%46%7%7%9%29%
2405
September1st to 15thDay
15%7%26%5%345%5%7%36%
1926
16th to endDay
23%6%24425%447%34%
1815
October1st to 15thDay
18%5%1332333451%
1702
16th to endDay
16%31441223454%
1760
November1st to 15thDay
10%214413446%57%
1682
16th to endDay
16%414323346%48%
1620
December1st to 15thDay
14%5%25%4235%47%44%
1620
16th to endDay
19%5%2442435%6%42%
1728
January1st to 15thNight
14%5%1232445%6%48%
2327
16th to endNight
14%5%2332445%5%46%
2496
February1st to 15thNight
12%41332345%7%49%
2235
16th to endNight
11%424325%5%6%7%46%
1887
March1st to 15thNight
12%424425%45%6%46%
1980
16th to endNight
16%5%2432445%7%43%
2112
April1st to 15thNight
11%41342445%7%50%
1925
16th to endNight
17%5%23324346%46%
1673
May1st to 15thNight
16%6%2343446%7%40%
1619
16th to endNight
14%414425%47%10%39%
1682
June1st to 15thNight
9%425%436%7%6%11%40%
1620
16th to endNight
7%424425%5%7%12%44%
1620
July1st to 15thNight
10%3235%35%5%8%11%40%
1620
16th to endNight
5%3145%36%6%7%10%41%
1703
August1st to 15thNight
17%42443445%9%39%
1620
16th to endNight
11%5%245%45%47%10%38%
1785
September1st to 15thNight
15%4234345%5%8%42%
1919
16th to endNight
22%5%25%323436%40%
1815
October1st to 15thNight
17%5%13323235%50%
1928
16th to endNight
13%31222333458%
2112
November1st to 15thNight
13%413213235%57%
2173
16th to endNight
15%312313446%51%
2340
December1st to 15thNight
14%5%24424345%47%
2340
16th to endNight
20%6%15%5%3445%5%38%
2496

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 15thDay
38%8%9%20%18%4
1612
16th to endDay
36%5%8%27%16%6%
1728
February1st to 15thDay
36%6%11%21%19%4
1617
16th to endDay
37%6%14%21%16%3
1586
March1st to 15thDay
37%5%9%23%18%5%
1980
16th to endDay
45%7%11%18%14%2
2112
April1st to 15thDay
33%6%14%24%17%4
2021
16th to endDay
42%8%10%17%15%5%
2298
May1st to 15thDay
44%11%15%17%9%1
2331
16th to endDay
43%10%15%19%8%2
2435
June1st to 15thDay
44%12%17%17%8%
2340
16th to endDay
39%13%14%17%13%1
2340
July1st to 15thDay
48%13%14%13%8%1
2201
16th to endDay
44%15%19%12%7%1
2310
August1st to 15thDay
50%14%14%13%6%
2327
16th to endDay
45%13%14%18%6%1
2405
September1st to 15thDay
43%10%10%17%14%3
1926
16th to endDay
49%6%12%14%15%2
1815
October1st to 15thDay
37%410%21%19%5%
1702
16th to endDay
34%38%23%20%9%
1760
November1st to 15thDay
28%5%9%18%29%8%
1682
16th to endDay
36%5%8%21%22%5%
1620
December1st to 15thDay
38%7%8%19%18%5%
1620
16th to endDay
43%6%11%16%17%4
1728
January1st to 15thNight
35%8%10%18%21%6%
2327
16th to endNight
37%6%9%21%20%4
2496
February1st to 15thNight
33%6%10%23%21%5%
2244
16th to endNight
35%7%14%23%16%2
1887
March1st to 15thNight
36%8%8%23%16%5%
1980
16th to endNight
40%7%13%20%15%2
2112
April1st to 15thNight
32%7%12%26%16%4
1925
16th to endNight
38%7%13%18%16%6%
1673
May1st to 15thNight
41%8%15%21%11%
1620
16th to endNight
37%11%18%22%7%1
1684
June1st to 15thNight
35%12%20%21%9%
1620
16th to endNight
30%12%21%21%11%3
1620
July1st to 15thNight
35%12%18%19%11%2
1530
16th to endNight
32%12%23%21%8%1
1595
August1st to 15thNight
42%10%16%16%12%1
1609
16th to endNight
39%10%16%23%8%1
1785
September1st to 15thNight
38%9%14%18%15%3
1919
16th to endNight
46%6%11%14%18%1
1815
October1st to 15thNight
35%5%10%23%19%3
1928
16th to endNight
30%5%6%25%24%8%
2112
November1st to 15thNight
30%5%8%19%25%11%
2173
16th to endNight
33%5%8%22%24%5%
2340
December1st to 15thNight
39%6%9%18%18%7%
2340
16th to endNight
47%5%9%16%17%3
2496

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 15thDay
30%26%26%12%31
90
16th to endDay
18%41%30%8%1
96
February1st to 15thDay
26%34%28%8%1
90
16th to endDay
26%26%36%8%2
87
March1st to 15thDay
17%24%27%22%8%
270
16th to endDay
22%19%27%19%9%1
288
April1st to 15thDay
17%20%21%27%10%3
270
16th to endDay
19%11%22%18%20%7%
270
May1st to 15thDay
19%17%27%17%10%8%
288
16th to endDay
17%6%22%19%17%15%
331
June1st to 15thDay
13%6%16%25%23%14%
315
16th to endDay
11%422%26%20%13%
315
July1st to 15thDay
12%119%24%22%18%
305
16th to endDay
8%111%30%26%20%
326
August1st to 15thDay
17%215%22%20%21%
313
16th to endDay
14%9%21%25%27%
332
September1st to 15thDay
15%211%19%30%20%
294
16th to endDay
25%5%15%22%18%12%
254
October1st to 15thDay
21%9%19%20%18%9%
255
16th to endDay
18%12%19%26%16%5%
272
November1st to 15thDay
17%14%21%24%15%6%
190
16th to endDay
18%21%23%27%7%1
90
December1st to 15thDay
17%27%33%16%31
90
16th to endDay
41%17%30%8%2
96
January1st to 15thNight
17%25%27%22%5%
720
16th to endNight
18%25%29%23%2
768
February1st to 15thNight
17%25%29%25%2
704
16th to endNight
15%27%34%19%2
631
March1st to 15thNight
15%21%30%27%4
720
16th to endNight
18%15%31%24%9%
768
April1st to 15thNight
14%21%25%26%11%
718
16th to endNight
20%11%21%30%15%1
593
May1st to 15thNight
19%8%25%30%15%1
540
16th to endNight
15%414%25%32%6%
563
June1st to 15thNight
10%114%30%35%7%
540
16th to endNight
8%213%29%40%5%
540
July1st to 15thNight
12%18%28%42%6%
510
16th to endNight
6%9%23%46%13%
564
August1st to 15thNight
17%7%30%36%7%
539
16th to endNight
12%6%25%46%8%
639
September1st to 15thNight
14%211%22%40%8%
700
16th to endNight
23%314%25%29%3
657
October1st to 15thNight
19%8%19%24%21%6%
660
16th to endNight
15%13%24%27%15%3
704
November1st to 15thNight
15%12%20%31%15%4
700
16th to endNight
16%19%21%28%12%1
720
December1st to 15thNight
17%18%25%27%9%1
720
16th to endNight
24%18%25%25%5%
768
Page generated at 2017-09-11 19:35:16. Version ac04

Copyright 2017 A. Danko


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