ARGUS Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ARGUS Dark Sky Site 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
37%
24404
17%
74606
11%
24348
43%
24348
21%
51364

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
17%
950
12%
2647
5%
946
25%
946
13%
2056
January11th to 20thNight
15%
900
11%
2520
4%
900
21%
900
13%
1993
January21st to endNight
18%
936
10%
2718
4%
936
25%
936
13%
2113
February1st to 10thNight
17%
720
10%
2200
4%
720
23%
720
13%
1691
February11th to 20thNight
21%
699
13%
2099
6%
699
28%
699
15%
1535
February21st to endNight
16%
592
14%
1776
3%
592
25%
592
14%
1322
March1st to 10thNight
25%
720
14%
2160
8%
720
30%
720
17%
1545
March11th to 20thNight
28%
720
19%
2039
9%
720
36%
720
23%
1382
March21st to endNight
27%
729
16%
2115
7%
729
35%
729
18%
1516
April1st to 10thNight
30%
540
16%
1800
9%
540
40%
540
20%
1204
April11th to 20thNight
32%
533
20%
1771
13%
533
41%
533
21%
1160
April21st to endNight
29%
540
18%
1620
7%
540
39%
540
22%
1115
May1st to 10thNight
31%
540
13%
1620
7%
540
36%
540
19%
1172
May11th to 20thNight
42%
531
16%
1592
12%
531
46%
531
23%
1073
May21st to endNight
55%
583
16%
1658
11%
582
59%
582
29%
1061
June1st to 10thNight
57%
540
17%
1440
11%
540
62%
540
36%
892
June11th to 20thNight
56%
540
21%
1440
14%
540
62%
540
36%
900
June21st to endNight
56%
540
19%
1440
14%
540
60%
540
33%
928
July1st to 10thNight
64%
525
16%
1400
15%
525
66%
525
33%
895
July11th to 20thNight
70%
522
22%
1392
18%
522
71%
522
42%
814
July21st to endNight
66%
591
22%
1538
17%
558
69%
558
37%
933
August1st to 10thNight
70%
537
21%
1600
18%
534
72%
534
36%
935
August11th to 20thNight
67%
534
22%
1642
19%
534
70%
534
33%
960
August21st to endNight
65%
594
21%
1980
21%
594
68%
594
34%
1108
September1st to 10thNight
60%
557
21%
1817
20%
557
64%
557
30%
1087
September11th to 20thNight
65%
720
31%
1980
25%
720
69%
720
42%
1128
September21st to endNight
55%
720
27%
1980
19%
720
60%
720
34%
1230
October1st to 10thNight
51%
720
25%
1980
19%
720
56%
720
33%
1222
October11th to 20thNight
33%
720
21%
2140
12%
720
43%
720
24%
1392
October21st to endNight
28%
792
18%
2574
13%
792
37%
792
19%
1742
November1st to 10thNight
34%
740
23%
2406
16%
740
44%
740
26%
1461
November11th to 20thNight
27%
760
21%
2470
12%
760
39%
760
19%
1636
November21st to endNight
26%
760
17%
2470
11%
760
34%
760
15%
1793
December1st to 10thNight
20%
769
12%
2479
5%
769
27%
769
14%
1843
December11th to 20thNight
22%
950
10%
3242
4%
949
28%
949
12%
2469
December21st to endNight
14%
1040
5%
4861
3%
1026
20%
1026
5%
4058

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%9%47%6%35%5%5%7%35%
74837
Night
15%9%47%5%3445%7%37%
74854
TransparencyDay
48%6%12%20%12%2
74543
Night
48%6%11%20%13%2
74669
SeeingDay
17%10%28%23%16%6%
14467
Night
19%9%29%29%13%1
24414

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
18%14%6%11%7%445%5%5%21%
1509
11th to 20thDay
18%18%8%10%6%45%346%19%
1440
21st to endDay
15%19%7%11%6%45%445%20%
1636
February1st to 10thDay
19%15%6%9%9%45%345%22%
1758
11th to 20thDay
17%11%411%7%45%45%5%26%
1748
21st to endDay
17%15%6%10%6%2445%5%26%
1480
March1st to 10thDay
18%12%6%8%7%34446%28%
1800
11th to 20thDay
16%10%5%7%6%345%5%9%29%
1919
21st to endDay
22%12%5%8%7%34446%27%
2257
April1st to 10thDay
16%11%47%6%345%5%6%33%
2160
11th to 20thDay
18%10%36%5%35%447%35%
2138
21st to endDay
20%8%36%6%4445%8%32%
2340
May1st to 10thDay
17%9%38%6%35%45%8%32%
2340
11th to 20thDay
19%8%36%6%35%45%7%34%
2301
21st to endDay
11%5%36%6%46%7%6%8%37%
2614
June1st to 10thDay
11%5%37%6%35%6%6%7%42%
2520
11th to 20thDay
9%5%25%6%35%6%7%9%44%
2520
21st to endDay
10%5%25%6%47%6%7%9%38%
2520
July1st to 10thDay
9%5%35%5%35%6%6%10%43%
2520
11th to 20thDay
8%425%445%6%6%10%46%
2520
21st to endDay
8%5%25%6%36%7%8%8%42%
2687
August1st to 10thDay
6%425%6%45%6%8%10%44%
2327
11th to 20thDay
9%425%6%45%6%7%8%43%
2254
21st to endDay
9%325%5%45%5%7%8%47%
2376
September1st to 10thDay
11%6%25%5%35%5%6%7%45%
2124
11th to 20thDay
11%4245%35%6%6%8%46%
1980
21st to endDay
12%6%25%5%2446%8%46%
1980
October1st to 10thDay
13%6%26%4345%6%6%44%
1980
11th to 20thDay
16%6%36%5%345%6%6%40%
1818
21st to endDay
20%10%37%6%3445%7%30%
1782
November1st to 10thDay
11%6%2443445%7%49%
1672
11th to 20thDay
18%11%47%6%3445%6%33%
1710
21st to endDay
19%11%48%5%34445%32%
1710
December1st to 10thDay
18%16%6%9%8%44445%24%
1696
11th to 20thDay
15%16%7%10%7%35%336%24%
1918
21st to endDay
25%21%9%10%6%2433312%
2783
January1st to 10thNight
17%14%7%10%7%44345%26%
2647
11th to 20thNight
18%16%6%9%7%44335%26%
2520
21st to endNight
16%14%8%10%6%44446%25%
2718
February1st to 10thNight
20%15%6%9%6%33445%25%
2200
11th to 20thNight
13%14%48%8%45%35%6%30%
2098
21st to endNight
20%12%5%9%5%34346%30%
1776
March1st to 10thNight
15%11%47%7%45%445%33%
2160
11th to 20thNight
15%8%46%5%35%5%5%7%37%
2039
21st to endNight
21%10%46%5%35%346%33%
2115
April1st to 10thNight
13%8%36%6%35%5%5%8%37%
1800
11th to 20thNight
18%8%25%5%35%45%7%38%
1771
21st to endNight
19%8%35%4345%5%7%35%
1620
May1st to 10thNight
19%7%47%7%345%6%8%31%
1620
11th to 20thNight
19%7%35%6%3445%7%39%
1592
21st to endNight
8%425%6%36%6%6%9%43%
1657
June1st to 10thNight
7%5%35%5%35%6%6%7%49%
1440
11th to 20thNight
8%5%35%5%245%5%9%48%
1440
21st to endNight
9%426%6%35%5%6%10%45%
1440
July1st to 10thNight
7%6%15%5%35%7%5%9%46%
1440
11th to 20thNight
6%4344245%5%10%53%
1440
21st to endNight
7%5%35%435%5%6%9%49%
1629
August1st to 10thNight
6%424435%7%6%10%49%
1611
11th to 20thNight
7%5%344445%6%8%50%
1642
21st to endNight
8%435%5%345%5%8%52%
1980
September1st to 10thNight
11%435%6%2445%8%48%
1817
11th to 20thNight
9%32432445%7%57%
1980
21st to endNight
10%5%245%345%5%7%50%
1980
October1st to 10thNight
10%5%25%43435%8%49%
1980
11th to 20thNight
18%7%26%5%23445%43%
2140
21st to endNight
21%8%25%5%345%46%37%
2574
November1st to 10thNight
12%8%35%4335%5%6%46%
2406
11th to 20thNight
15%9%37%6%25%45%7%37%
2470
21st to endNight
19%10%47%7%44437%30%
2470
December1st to 10thNight
18%13%5%8%6%2445%6%28%
2479
11th to 20thNight
18%14%7%10%5%345%45%25%
3279
21st to endNight
25%19%7%9%6%3444415%
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
65%49%15%6%
1510
11th to 20thDay
68%5%11%11%5%
1440
21st to endDay
67%5%10%12%6%1
1636
February1st to 10thDay
67%49%12%7%1
1758
11th to 20thDay
60%410%19%6%1
1758
21st to endDay
59%5%11%16%7%1
1480
March1st to 10thDay
58%410%20%8%
1800
11th to 20thDay
52%6%13%18%10%1
1919
21st to endDay
60%412%19%41
2257
April1st to 10thDay
51%6%12%19%12%
2160
11th to 20thDay
49%6%9%21%11%3
2138
21st to endDay
51%5%11%19%10%4
2340
May1st to 10thDay
52%6%12%19%10%
2340
11th to 20thDay
50%7%10%19%13%1
2305
21st to endDay
41%9%12%22%14%2
2614
June1st to 10thDay
39%8%11%24%16%3
2520
11th to 20thDay
35%7%11%24%20%3
2520
21st to endDay
39%9%15%18%15%3
2520
July1st to 10thDay
36%8%13%23%16%4
2451
11th to 20thDay
31%8%15%27%18%2
2445
21st to endDay
35%8%15%23%15%3
2538
August1st to 10thDay
31%9%16%28%14%2
2314
11th to 20thDay
35%9%13%24%16%2
2254
21st to endDay
33%8%13%22%19%6%
2376
September1st to 10thDay
38%7%12%22%17%4
2124
11th to 20thDay
33%8%11%26%20%2
1980
21st to endDay
36%6%11%24%21%2
1980
October1st to 10thDay
38%7%13%23%18%2
1980
11th to 20thDay
42%6%10%23%18%1
1818
21st to endDay
53%5%11%20%11%1
1782
November1st to 10thDay
35%6%14%26%17%2
1672
11th to 20thDay
53%411%19%11%2
1710
21st to endDay
55%411%21%8%1
1710
December1st to 10thDay
64%411%15%6%
1696
11th to 20thDay
63%49%21%3
1915
21st to endDay
79%47%7%4
2783
January1st to 10thNight
62%39%16%10%
2647
11th to 20thNight
63%411%15%7%
2520
21st to endNight
62%412%14%7%
2718
February1st to 10thNight
62%412%14%7%1
2200
11th to 20thNight
56%411%18%10%1
2099
21st to endNight
57%410%17%9%2
1776
March1st to 10thNight
53%5%11%20%11%1
2160
11th to 20thNight
45%6%11%23%12%3
2039
21st to endNight
53%5%11%22%8%1
2115
April1st to 10thNight
45%7%11%23%13%2
1800
11th to 20thNight
45%5%10%21%15%4
1771
21st to endNight
46%7%10%22%12%4
1620
May1st to 10thNight
50%8%11%19%10%2
1620
11th to 20thNight
45%7%11%22%14%2
1593
21st to endNight
36%10%14%22%14%4
1658
June1st to 10thNight
33%7%11%26%20%3
1440
11th to 20thNight
33%7%11%24%21%4
1440
21st to endNight
34%9%16%22%15%4
1440
July1st to 10thNight
33%9%13%23%17%6%
1400
11th to 20thNight
26%7%15%28%21%3
1392
21st to endNight
30%8%15%24%17%6%
1538
August1st to 10thNight
29%8%15%27%20%2
1600
11th to 20thNight
30%9%14%24%21%2
1642
21st to endNight
30%7%12%24%22%6%
1980
September1st to 10thNight
35%7%13%22%17%6%
1817
11th to 20thNight
27%6%11%28%26%2
1980
21st to endNight
33%7%10%24%23%3
1980
October1st to 10thNight
35%5%10%27%21%2
1980
11th to 20thNight
44%48%25%18%2
2140
21st to endNight
48%5%9%22%14%2
2574
November1st to 10thNight
38%5%11%24%20%2
2406
11th to 20thNight
47%5%11%21%14%2
2470
21st to endNight
56%5%10%21%8%
2470
December1st to 10thNight
57%6%11%17%8%1
2479
11th to 20thNight
61%411%17%6%1
3281
21st to endNight
73%410%8%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
23%18%36%16%6%1
267
11th to 20thDay
26%15%40%14%6%
240
21st to endDay
28%15%34%17%5%
264
February1st to 10thDay
27%20%38%13%3
240
11th to 20thDay
27%19%35%14%41
240
21st to endDay
27%20%41%10%2
198
March1st to 10thDay
21%22%38%16%4
240
11th to 20thDay
25%12%35%20%7%1
359
21st to endDay
32%14%29%18%7%1
462
April1st to 10thDay
21%12%36%19%10%2
420
11th to 20thDay
21%18%28%20%10%3
412
21st to endDay
25%16%30%18%10%2
420
May1st to 10thDay
23%12%33%20%8%3
423
11th to 20thDay
22%7%30%23%13%6%
447
21st to endDay
13%5%29%25%18%10%
568
June1st to 10thDay
11%7%29%26%21%5%
630
11th to 20thDay
12%6%27%27%21%7%
630
21st to endDay
13%6%28%27%18%7%
630
July1st to 10thDay
12%317%27%25%16%
621
11th to 20thDay
7%223%30%24%13%
618
21st to endDay
10%318%30%28%11%
628
August1st to 10thDay
6%213%30%33%15%
446
11th to 20thDay
10%319%29%24%15%
439
21st to endDay
8%416%27%32%13%
495
September1st to 10thDay
14%5%17%27%27%11%
450
11th to 20thDay
11%217%24%30%16%
450
21st to endDay
15%9%20%25%23%8%
450
October1st to 10thDay
13%6%26%30%18%8%
450
11th to 20thDay
17%19%30%24%10%1
386
21st to endDay
27%15%37%19%11
297
November1st to 10thDay
11%15%34%25%13%3
270
11th to 20thDay
25%17%31%17%9%1
270
21st to endDay
28%14%37%18%4
270
December1st to 10thDay
23%21%35%13%8%
270
11th to 20thDay
21%16%40%17%6%
270
21st to endDay
30%19%33%16%2
297
January1st to 10thNight
23%16%34%21%5%
950
11th to 20thNight
26%13%37%21%4
900
21st to endNight
21%15%36%23%4
936
February1st to 10thNight
29%10%35%23%3
720
11th to 20thNight
21%14%37%25%3
703
21st to endNight
27%14%37%18%3
592
March1st to 10thNight
20%14%34%26%6%
720
11th to 20thNight
20%11%36%26%6%1
720
21st to endNight
25%10%33%26%6%
729
April1st to 10thNight
17%11%36%28%7%1
540
11th to 20thNight
21%11%33%26%8%1
533
21st to endNight
23%8%36%24%9%1
540
May1st to 10thNight
22%7%32%30%9%
540
11th to 20thNight
20%5%25%32%16%2
531
21st to endNight
10%223%43%19%1
584
June1st to 10thNight
8%325%44%19%1
540
11th to 20thNight
10%424%40%21%1
540
21st to endNight
10%322%39%26%1
540
July1st to 10thNight
10%216%36%31%5%
525
11th to 20thNight
7%216%40%31%4
522
21st to endNight
8%214%42%32%2
591
August1st to 10thNight
7%213%42%32%4
537
11th to 20thNight
8%116%38%31%4
534
21st to endNight
8%218%33%35%5%
594
September1st to 10thNight
13%314%36%29%4
557
11th to 20thNight
11%315%35%32%4
720
21st to endNight
12%5%23%35%23%2
720
October1st to 10thNight
13%6%24%34%20%4
720
11th to 20thNight
21%12%28%28%10%1
720
21st to endNight
25%11%32%25%7%
792
November1st to 10thNight
16%10%36%26%11%1
740
11th to 20thNight
20%15%33%25%7%
760
21st to endNight
25%13%31%23%8%
760
December1st to 10thNight
25%13%35%21%5%
769
11th to 20thNight
25%13%31%24%6%
950
21st to endNight
31%13%33%20%2
1045
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