SWAOG Dark Site
Clear Sky Chart
History

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

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
37%
24582
17%
74647
11%
24529
44%
24529
23%
50195

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
21%
950
15%
2648
3%
946
30%
946
18%
1921
January11th to 20thNight
20%
905
12%
2533
4%
905
27%
905
15%
1937
January21st to endNight
21%
990
11%
2691
3%
990
27%
990
16%
2074
February1st to 10thNight
19%
881
10%
2321
2%
881
25%
881
17%
1752
February11th to 20thNight
25%
700
18%
2099
8%
700
34%
700
19%
1451
February21st to endNight
22%
592
18%
1776
5%
592
33%
592
18%
1246
March1st to 10thNight
24%
720
13%
2160
6%
720
31%
720
18%
1511
March11th to 20thNight
31%
676
20%
2112
10%
676
41%
676
23%
1362
March21st to endNight
25%
594
15%
1980
9%
594
34%
594
16%
1444
April1st to 10thNight
27%
540
16%
1800
9%
540
38%
540
21%
1183
April11th to 20thNight
31%
533
20%
1775
13%
533
41%
533
21%
1168
April21st to endNight
30%
540
16%
1729
7%
540
40%
540
20%
1186
May1st to 10thNight
33%
540
15%
1620
7%
540
40%
540
23%
1099
May11th to 20thNight
41%
531
18%
1453
9%
531
49%
531
27%
969
May21st to endNight
50%
583
16%
1551
9%
582
55%
582
28%
1040
June1st to 10thNight
55%
540
17%
1440
11%
540
59%
540
37%
885
June11th to 20thNight
53%
540
21%
1440
13%
540
58%
540
36%
897
June21st to endNight
51%
540
17%
1440
11%
540
54%
540
30%
955
July1st to 10thNight
67%
525
17%
1400
12%
525
69%
525
38%
867
July11th to 20thNight
63%
522
22%
1392
15%
522
68%
522
41%
821
July21st to endNight
69%
591
23%
1487
17%
558
72%
558
43%
858
August1st to 10thNight
71%
537
20%
1474
17%
534
73%
534
38%
891
August11th to 20thNight
71%
534
23%
1776
24%
534
74%
534
37%
958
August21st to endNight
66%
594
24%
1980
24%
594
71%
594
37%
1057
September1st to 10thNight
62%
540
24%
1800
23%
540
66%
540
34%
999
September11th to 20thNight
64%
618
34%
1878
32%
618
68%
618
39%
1050
September21st to endNight
51%
720
23%
1980
16%
720
56%
720
33%
1235
October1st to 10thNight
50%
720
24%
2051
19%
720
54%
720
33%
1247
October11th to 20thNight
35%
720
22%
2160
12%
720
45%
720
26%
1358
October21st to endNight
26%
792
17%
2560
11%
792
36%
792
20%
1718
November1st to 10thNight
31%
740
26%
2406
17%
740
44%
740
27%
1422
November11th to 20thNight
28%
760
19%
2470
10%
760
39%
760
19%
1626
November21st to endNight
26%
838
19%
2548
9%
838
36%
838
21%
1735
December1st to 10thNight
23%
950
13%
2660
5%
950
30%
950
18%
1944
December11th to 20thNight
26%
950
14%
3201
5%
948
32%
948
15%
2311
December21st to endNight
16%
1036
6%
4856
3%
1026
23%
1026
6%
4018

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
14%8%46%6%35%5%5%7%37%
74783
Night
14%8%46%5%3445%7%39%
74936
TransparencyDay
45%7%12%22%12%2
74484
Night
45%6%12%22%14%2
74756
SeeingDay
16%10%28%26%15%6%
14621
Night
18%9%30%30%12%1
24595

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
15%11%5%9%8%46%5%5%6%28%
1517
11th to 20thDay
14%15%7%11%7%35%446%24%
1440
21st to endDay
15%15%7%11%7%435%46%23%
1661
February1st to 10thDay
18%13%5%7%7%45%5%5%5%25%
1638
11th to 20thDay
15%10%48%7%44446%34%
1748
21st to endDay
16%13%5%10%5%3345%6%29%
1480
March1st to 10thDay
16%13%47%7%35%5%46%32%
1800
11th to 20thDay
15%9%46%6%35%45%8%34%
1863
21st to endDay
20%10%5%8%8%45%5%46%25%
2376
April1st to 10thDay
16%11%5%8%5%345%6%7%31%
2160
11th to 20thDay
16%10%36%5%35%5%5%6%36%
2134
21st to endDay
21%8%37%6%345%5%7%33%
2229
May1st to 10thDay
17%8%36%6%35%5%6%9%30%
2340
11th to 20thDay
19%8%36%5%45%45%8%34%
2440
21st to endDay
14%6%36%6%45%5%7%9%36%
2725
June1st to 10thDay
10%5%346%36%46%8%46%
2520
11th to 20thDay
9%6%35%6%35%5%6%7%46%
2520
21st to endDay
11%5%36%6%46%6%7%8%38%
2520
July1st to 10thDay
8%4245%35%5%7%9%47%
2520
11th to 20thDay
9%5%24435%5%7%9%48%
2520
21st to endDay
7%425%6%45%6%7%9%44%
2743
August1st to 10thDay
7%5%35%5%35%7%8%9%44%
2450
11th to 20thDay
9%435%6%35%6%7%9%43%
2125
21st to endDay
9%324435%5%7%10%48%
2376
September1st to 10thDay
11%6%245%4445%8%46%
2160
11th to 20thDay
11%424433447%54%
2062
21st to endDay
9%5%36%5%35%46%8%46%
1980
October1st to 10thDay
12%7%35%5%235%5%7%46%
1907
11th to 20thDay
18%8%36%5%345%5%8%36%
1800
21st to endDay
19%9%47%423446%37%
1796
November1st to 10thDay
12%7%35%4335%5%7%45%
1672
11th to 20thDay
16%11%5%7%5%35%5%47%33%
1710
21st to endDay
19%10%49%434345%35%
1630
December1st to 10thDay
15%12%48%6%45%45%6%32%
1520
11th to 20thDay
14%13%6%7%5%46%6%5%6%29%
1918
21st to endDay
23%20%9%12%7%3432414%
2783
January1st to 10thNight
14%12%5%8%6%34446%34%
2648
11th to 20thNight
15%14%6%9%7%34446%28%
2533
21st to endNight
14%13%7%10%7%4445%6%27%
2691
February1st to 10thNight
17%14%5%8%7%33346%30%
2321
11th to 20thNight
14%10%47%6%45%5%37%37%
2098
21st to endNight
19%10%49%5%33445%35%
1776
March1st to 10thNight
17%11%37%5%45%445%37%
2160
11th to 20thNight
13%8%46%5%3445%6%42%
2112
21st to endNight
21%10%48%5%345%5%5%30%
1980
April1st to 10thNight
16%9%46%5%3445%6%39%
1800
11th to 20thNight
19%9%25%5%3345%5%40%
1775
21st to endNight
19%7%36%6%35%5%5%7%35%
1729
May1st to 10thNight
17%6%36%6%445%6%9%36%
1620
11th to 20thNight
16%6%25%435%5%5%8%41%
1452
21st to endNight
12%5%26%5%46%46%9%40%
1551
June1st to 10thNight
10%425%5%345%5%8%51%
1440
11th to 20thNight
9%5%245%345%7%10%46%
1440
21st to endNight
9%5%36%6%35%6%6%7%44%
1440
July1st to 10thNight
6%425%4245%6%10%50%
1440
11th to 20thNight
6%4235%446%6%8%53%
1440
21st to endNight
43245%35%5%7%10%52%
1575
August1st to 10thNight
5%5%25%5%35%6%6%9%51%
1483
11th to 20thNight
8%3144245%5%9%53%
1776
21st to endNight
9%32433436%8%55%
1980
September1st to 10thNight
10%5%35%32335%8%54%
1800
11th to 20thNight
9%42343345%7%55%
1878
21st to endNight
9%5%25%5%345%6%7%49%
1980
October1st to 10thNight
10%6%25%42445%6%51%
2051
11th to 20thNight
18%7%245%24346%45%
2160
21st to endNight
20%9%36%433446%38%
2560
November1st to 10thNight
12%7%35%42345%6%50%
2406
11th to 20thNight
15%8%47%6%3445%6%38%
2470
21st to endNight
18%8%37%6%34445%39%
2548
December1st to 10thNight
17%9%47%6%45%446%33%
2660
11th to 20thNight
16%12%5%9%5%34446%33%
3279
21st to endNight
24%16%7%9%6%45%445%16%
4884

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
57%5%12%16%10%
1518
11th to 20thDay
62%6%12%14%5%1
1440
21st to endDay
63%6%10%13%8%
1661
February1st to 10thDay
60%6%11%15%7%2
1638
11th to 20thDay
52%6%11%21%8%2
1758
21st to endDay
56%6%10%19%7%2
1480
March1st to 10thDay
53%6%13%21%7%1
1800
11th to 20thDay
48%6%14%20%10%1
1863
21st to endDay
60%6%11%17%5%1
2376
April1st to 10thDay
51%7%11%21%10%
2160
11th to 20thDay
46%7%11%21%11%3
2134
21st to endDay
51%6%12%17%12%2
2229
May1st to 10thDay
50%8%12%21%8%
2340
11th to 20thDay
48%7%10%23%11%
2444
21st to endDay
43%7%13%19%15%2
2725
June1st to 10thDay
35%6%12%27%17%2
2520
11th to 20thDay
35%8%11%24%20%3
2520
21st to endDay
41%9%13%22%14%2
2520
July1st to 10thDay
31%8%15%28%15%3
2451
11th to 20thDay
31%8%16%27%16%2
2445
21st to endDay
33%9%17%23%15%2
2591
August1st to 10thDay
33%8%17%29%12%2
2435
11th to 20thDay
35%8%15%25%16%1
2125
21st to endDay
30%8%13%27%19%3
2376
September1st to 10thDay
35%7%14%23%16%5%
2160
11th to 20thDay
30%6%12%28%20%3
2062
21st to endDay
35%6%9%27%20%2
1980
October1st to 10thDay
38%6%11%26%18%2
1907
11th to 20thDay
47%5%12%22%13%1
1800
21st to endDay
48%5%12%22%10%3
1796
November1st to 10thDay
38%6%13%24%16%2
1672
11th to 20thDay
51%411%22%10%1
1710
21st to endDay
53%5%11%23%9%
1630
December1st to 10thDay
54%5%13%20%8%1
1520
11th to 20thDay
54%6%14%18%8%1
1915
21st to endDay
76%48%8%3
2783
January1st to 10thNight
52%5%11%20%12%1
2648
11th to 20thNight
58%6%14%13%9%
2533
21st to endNight
58%412%15%9%1
2691
February1st to 10thNight
57%411%17%9%2
2321
11th to 20thNight
49%6%10%22%12%1
2099
21st to endNight
52%411%18%12%3
1776
March1st to 10thNight
51%5%12%21%10%1
2160
11th to 20thNight
43%5%12%21%16%3
2112
21st to endNight
54%6%11%18%10%1
1980
April1st to 10thNight
45%5%12%23%13%2
1800
11th to 20thNight
46%5%8%22%15%4
1775
21st to endNight
47%7%11%22%11%3
1729
May1st to 10thNight
45%6%13%24%11%
1620
11th to 20thNight
40%8%13%24%14%1
1453
21st to endNight
39%8%14%19%17%2
1552
June1st to 10thNight
31%7%13%25%20%4
1440
11th to 20thNight
32%8%12%24%22%3
1440
21st to endNight
37%9%14%21%16%3
1440
July1st to 10thNight
30%9%14%26%17%5%
1400
11th to 20thNight
27%7%14%27%22%3
1392
21st to endNight
25%7%17%27%19%5%
1487
August1st to 10thNight
27%9%18%27%16%2
1474
11th to 20thNight
26%7%16%28%20%2
1776
21st to endNight
26%7%12%29%20%5%
1980
September1st to 10thNight
30%6%13%25%22%4
1800
11th to 20thNight
28%6%8%32%24%2
1878
21st to endNight
33%7%11%28%17%3
1980
October1st to 10thNight
34%6%10%27%21%3
2051
11th to 20thNight
40%410%24%20%2
2160
21st to endNight
48%49%24%13%1
2560
November1st to 10thNight
36%5%10%26%19%4
2406
11th to 20thNight
46%5%13%22%13%2
2470
21st to endNight
48%411%24%12%1
2548
December1st to 10thNight
53%5%11%22%8%1
2660
11th to 20thNight
53%5%12%18%10%1
3281
21st to endNight
70%6%8%11%4
4884

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
21%11%34%27%7%
270
11th to 20thDay
21%14%36%23%5%1
240
21st to endDay
22%18%30%23%7%1
264
February1st to 10thDay
25%17%38%17%3
240
11th to 20thDay
26%14%36%19%5%
240
21st to endDay
27%16%39%17%2
198
March1st to 10thDay
26%20%35%18%2
240
11th to 20thDay
22%18%34%18%7%1
244
21st to endDay
27%15%31%19%7%2
462
April1st to 10thDay
23%12%33%25%7%
420
11th to 20thDay
19%20%27%19%12%3
412
21st to endDay
24%13%30%24%8%2
420
May1st to 10thDay
23%11%29%26%9%2
423
11th to 20thDay
21%9%28%25%12%4
586
21st to endDay
15%7%29%22%18%8%
679
June1st to 10thDay
9%7%28%29%20%6%
630
11th to 20thDay
11%7%28%30%19%5%
630
21st to endDay
12%7%30%30%15%5%
630
July1st to 10thDay
10%316%30%24%18%
621
11th to 20thDay
11%5%22%31%21%10%
618
21st to endDay
7%225%35%23%8%
684
August1st to 10thDay
7%216%37%25%13%
595
11th to 20thDay
11%5%15%26%28%15%
439
21st to endDay
7%319%31%27%13%
495
September1st to 10thDay
13%7%20%28%25%7%
450
11th to 20thDay
12%217%30%29%11%
450
21st to endDay
11%11%24%26%21%7%
450
October1st to 10thDay
14%7%27%29%18%5%
377
11th to 20thDay
18%23%31%21%7%
270
21st to endDay
30%14%33%16%6%1
297
November1st to 10thDay
13%16%36%22%8%4
270
11th to 20thDay
24%18%31%19%6%2
270
21st to endDay
26%11%33%23%7%
270
December1st to 10thDay
17%17%38%19%7%1
270
11th to 20thDay
17%15%38%23%7%
270
21st to endDay
27%20%35%15%3
297
January1st to 10thNight
20%13%38%23%5%
950
11th to 20thNight
22%12%37%22%5%1
905
21st to endNight
20%12%35%29%3
990
February1st to 10thNight
25%11%37%23%4
881
11th to 20thNight
21%12%36%26%4
703
21st to endNight
23%12%37%23%41
592
March1st to 10thNight
22%13%35%24%6%
720
11th to 20thNight
17%9%37%29%7%1
676
21st to endNight
25%11%33%24%7%1
594
April1st to 10thNight
19%13%36%26%6%
540
11th to 20thNight
22%11%32%27%8%
533
21st to endNight
21%9%35%27%8%
540
May1st to 10thNight
19%7%34%31%7%1
540
11th to 20thNight
18%7%28%34%12%1
531
21st to endNight
13%5%26%38%19%
584
June1st to 10thNight
11%424%44%16%
540
11th to 20thNight
11%326%46%13%1
540
21st to endNight
11%425%41%18%1
540
July1st to 10thNight
8%216%42%30%3
525
11th to 20thNight
8%121%39%28%3
522
21st to endNight
5%217%47%28%2
591
August1st to 10thNight
5%312%46%30%3
537
11th to 20thNight
6%312%41%35%3
534
21st to endNight
9%217%37%33%3
594
September1st to 10thNight
12%219%37%27%3
540
11th to 20thNight
9%418%36%30%4
618
21st to endNight
10%7%24%36%21%2
720
October1st to 10thNight
13%6%26%34%19%2
720
11th to 20thNight
21%12%28%29%10%
720
21st to endNight
25%11%32%24%8%
792
November1st to 10thNight
16%9%39%25%11%1
740
11th to 20thNight
19%15%32%27%7%
760
21st to endNight
21%11%35%26%7%
838
December1st to 10thNight
22%12%36%25%5%
950
11th to 20thNight
23%11%33%28%5%
950
21st to endNight
28%14%33%22%3
1045
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