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 2018-03-11.

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%
16408
21%
49072
12%
16369
53%
16369
24%
31090

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
26%
777
24%
2522
8%
776
45%
776
18%
1529
January16th to endNight
24%
832
24%
2704
9%
832
43%
832
19%
1637
February1st to 15thNight
25%
763
23%
2435
9%
761
42%
761
19%
1463
February16th to endNight
21%
683
23%
2043
8%
683
37%
683
20%
1283
March1st to 15thNight
29%
764
23%
2101
9%
764
40%
764
21%
1365
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
60%
720
19%
1974
15%
720
62%
720
26%
1322
September16th to endNight
52%
717
19%
1980
16%
717
56%
717
24%
1357
October1st to 15thNight
48%
720
26%
2103
22%
720
56%
720
29%
1301
October16th to endNight
43%
768
34%
2304
21%
768
61%
768
31%
1303
November1st to 15thNight
48%
760
26%
2359
19%
760
59%
760
30%
1348
November16th to endNight
40%
780
28%
2535
17%
780
54%
780
27%
1427
December1st to 15thNight
32%
780
24%
2535
11%
780
47%
780
19%
1576
December16th to endNight
26%
832
21%
2704
8%
832
42%
832
15%
1814

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%
49554
Night
14%42442445%7%45%
49269
TransparencyDay
41%9%12%18%14%3
49255
Night
36%7%12%20%17%4
49072
SeeingDay
18%11%21%21%16%10%
6143
Night
16%12%21%26%19%3
16414

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%45%445%
1746
16th to endDay
13%5%1342345%5%49%
1872
February1st to 15thDay
12%6%25%42435%7%44%
1754
16th to endDay
14%5%34334347%44%
1716
March1st to 15thDay
14%424334346%46%
2096
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
16%7%26%5%345%5%7%35%
1983
16th to endDay
21%5%245%35%45%7%34%
1980
October1st to 15thDay
18%41332323450%
1857
16th to endDay
16%31431223455%
1920
November1st to 15thDay
11%314413446%54%
1826
16th to endDay
15%314323346%50%
1755
December1st to 15thDay
14%5%25%4235%46%44%
1755
16th to endDay
19%6%2442435%6%41%
1872
January1st to 15thNight
14%41232445%6%49%
2522
16th to endNight
14%5%2332445%5%48%
2704
February1st to 15thNight
13%41442345%7%48%
2426
16th to endNight
12%424325%5%6%7%44%
2043
March1st to 15thNight
12%414425%45%6%46%
2101
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%48%41%
1974
16th to endNight
20%415%323436%42%
1980
October1st to 15thNight
17%413323345%49%
2103
16th to endNight
13%31222333459%
2304
November1st to 15thNight
14%413213235%55%
2359
16th to endNight
14%312313446%53%
2535
December1st to 15thNight
14%5%24424345%47%
2535
16th to endNight
20%6%15%5%2445%6%37%
2704

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%5%
1747
16th to endDay
35%5%8%26%18%6%
1872
February1st to 15thDay
37%6%11%21%18%3
1756
16th to endDay
39%6%14%20%16%3
1716
March1st to 15thDay
37%5%9%22%17%6%
2096
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
44%10%10%16%13%3
1983
16th to endDay
48%6%12%14%15%1
1980
October1st to 15thDay
37%5%10%21%19%5%
1857
16th to endDay
33%37%24%21%8%
1920
November1st to 15thDay
30%6%9%18%27%7%
1826
16th to endDay
35%48%20%25%5%
1755
December1st to 15thDay
39%7%8%20%18%5%
1755
16th to endDay
43%6%11%17%15%4
1872
January1st to 15thNight
34%8%10%17%21%7%
2522
16th to endNight
36%6%9%21%21%5%
2704
February1st to 15thNight
35%6%10%23%19%4
2435
16th to endNight
36%7%14%22%15%3
2043
March1st to 15thNight
36%8%8%22%17%5%
2101
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
39%9%14%18%15%3
1974
16th to endNight
44%6%11%15%19%1
1980
October1st to 15thNight
36%5%10%24%19%3
2103
16th to endNight
30%46%25%24%8%
2304
November1st to 15thNight
31%5%9%19%23%10%
2359
16th to endNight
31%5%8%22%26%6%
2535
December1st to 15thNight
39%6%9%19%18%6%
2535
16th to endNight
46%5%10%16%17%3
2704

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
28%27%27%12%2
135
16th to endDay
21%34%30%11%2
144
February1st to 15thDay
26%32%31%8%
135
16th to endDay
28%26%35%7%1
126
March1st to 15thDay
18%25%27%19%7%
312
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
18%212%18%29%19%
315
16th to endDay
20%5%14%23%22%13%
314
October1st to 15thDay
21%7%17%22%21%8%
315
16th to endDay
17%13%21%24%17%5%
336
November1st to 15thDay
21%13%20%23%13%5%
244
16th to endDay
14%20%26%27%9%2
135
December1st to 15thDay
20%25%33%16%3
135
16th to endDay
35%20%34%7%1
144
January1st to 15thNight
17%25%27%23%6%
780
16th to endNight
18%25%28%23%3
832
February1st to 15thNight
17%24%29%26%2
764
16th to endNight
16%25%33%21%3
683
March1st to 15thNight
15%22%30%26%4
764
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
15%212%22%39%7%
720
16th to endNight
21%313%24%32%5%
717
October1st to 15thNight
20%7%18%24%23%6%
720
16th to endNight
15%13%23%26%16%3
768
November1st to 15thNight
17%11%19%31%15%4
760
16th to endNight
15%18%20%29%13%1
780
December1st to 15thNight
17%18%25%27%9%1
780
16th to endNight
24%18%25%25%5%
832
Page generated at 2018-03-12 13:28:02. Version ac04

Copyright 2018 A. Danko


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