Big Sky Observatory Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Sky Observatory 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
42%
23485
16%
74586
10%
23453
47%
23453
23%
47708

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
33%
948
13%
2834
7%
946
38%
946
18%
1997
January11th to 20thNight
36%
878
17%
2678
10%
878
43%
878
21%
1838
January21st to endNight
35%
792
21%
2772
14%
792
47%
792
20%
1779
February1st to 10thNight
36%
720
14%
2483
11%
720
42%
720
17%
1682
February11th to 20thNight
35%
701
13%
2273
10%
700
42%
700
17%
1618
February21st to endNight
40%
592
16%
1821
12%
592
46%
592
19%
1285
March1st to 10thNight
35%
720
14%
2160
10%
720
41%
720
20%
1500
March11th to 20thNight
33%
720
19%
1981
9%
720
42%
720
24%
1352
March21st to endNight
36%
792
11%
2178
5%
792
41%
792
21%
1563
April1st to 10thNight
33%
661
16%
1741
5%
661
39%
661
21%
1260
April11th to 20thNight
36%
533
15%
1601
9%
533
41%
533
19%
1126
April21st to endNight
40%
540
13%
1620
10%
540
44%
540
20%
1126
May1st to 10thNight
42%
514
11%
1563
9%
514
46%
514
22%
1065
May11th to 20thNight
52%
354
16%
1239
18%
354
55%
354
25%
755
May21st to endNight
46%
388
7%
1358
6%
388
48%
388
17%
933
June1st to 10thNight
47%
360
4%
1260
2%
360
48%
360
20%
807
June11th to 20thNight
40%
360
3%
1260
1%
360
41%
360
19%
840
June21st to endNight
58%
360
4%
1260
1%
360
58%
360
30%
694
July1st to 10thNight
66%
350
5%
1225
2%
350
67%
350
32%
676
July11th to 20thNight
60%
348
10%
1218
10%
348
63%
348
34%
652
July21st to endNight
64%
394
18%
1372
21%
372
73%
372
43%
637
August1st to 10thNight
64%
375
17%
1441
21%
373
70%
373
29%
768
August11th to 20thNight
65%
534
22%
1602
19%
534
68%
534
42%
866
August21st to endNight
58%
594
27%
1782
20%
594
63%
594
40%
959
September1st to 10thNight
54%
616
21%
1735
13%
616
58%
616
37%
1033
September11th to 20thNight
54%
720
21%
1980
14%
720
58%
720
38%
1180
September21st to endNight
46%
720
22%
1980
14%
720
52%
720
33%
1226
October1st to 10thNight
47%
720
21%
2070
12%
720
55%
720
29%
1330
October11th to 20thNight
40%
720
21%
2254
12%
720
52%
720
28%
1387
October21st to endNight
40%
792
20%
2574
13%
792
50%
792
24%
1635
November1st to 10thNight
30%
839
17%
2504
9%
839
39%
839
21%
1740
November11th to 20thNight
32%
950
21%
2664
11%
950
41%
950
26%
1749
November21st to endNight
37%
950
17%
2850
9%
950
43%
950
26%
1814
December1st to 10thNight
41%
950
16%
2850
11%
950
46%
950
21%
1929
December11th to 20thNight
35%
949
14%
3467
9%
949
40%
949
19%
2219
December21st to endNight
33%
1031
10%
4936
8%
1026
38%
1026
13%
2688

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
9%8%36%6%35%5%6%8%40%
74608
Night
7%8%37%6%45%5%6%8%40%
75072
TransparencyDay
40%7%15%24%12%2
74294
Night
40%7%15%23%12%2
74904
SeeingDay
12%9%27%29%18%6%
14205
Night
11%7%30%35%15%2
23506

By Month

Percentage Cloud Cover NW (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
410%5%8%6%45%6%7%8%37%
1322
11th to 20thDay
5%9%48%7%36%5%7%9%35%
1300
21st to endDay
49%49%8%37%7%7%10%31%
1584
February1st to 10thDay
5%11%49%8%5%8%6%6%8%29%
1476
11th to 20thDay
5%13%5%7%9%5%5%6%7%9%30%
1575
21st to endDay
414%7%9%8%45%6%5%8%31%
1430
March1st to 10thDay
312%48%9%45%6%6%9%33%
1800
11th to 20thDay
6%8%38%7%445%7%8%39%
1978
21st to endDay
8%12%6%8%7%45%5%6%8%31%
2178
April1st to 10thDay
9%12%5%7%8%46%6%5%8%31%
2216
11th to 20thDay
10%13%47%6%35%5%7%8%32%
2310
21st to endDay
11%9%36%7%45%5%7%9%35%
2340
May1st to 10thDay
13%9%37%6%36%5%6%8%33%
2394
11th to 20thDay
13%6%35%6%45%5%6%9%38%
2654
21st to endDay
20%7%37%5%35%6%5%7%31%
2917
June1st to 10thDay
15%7%25%6%36%7%7%9%33%
2700
11th to 20thDay
18%7%36%7%46%6%6%8%30%
2700
21st to endDay
8%4235%35%7%7%9%47%
2700
July1st to 10thDay
7%424435%6%7%9%50%
2700
11th to 20thDay
7%4144345%6%8%53%
2700
21st to endDay
5%32342346%9%60%
2862
August1st to 10thDay
8%4144345%7%9%52%
2490
11th to 20thDay
7%31433447%9%53%
2299
21st to endDay
9%4244345%6%9%53%
2574
September1st to 10thDay
10%5%24425%45%6%53%
2223
11th to 20thDay
13%6%3443435%7%48%
1980
21st to endDay
10%6%26%43445%8%48%
1980
October1st to 10thDay
13%9%26%5%3445%8%40%
1890
11th to 20thDay
9%9%36%5%445%6%8%41%
1702
21st to endDay
8%9%36%6%36%5%7%10%37%
1782
November1st to 10thDay
10%13%47%5%36%46%7%35%
1556
11th to 20thDay
5%10%37%7%35%5%5%9%41%
1512
21st to endDay
5%9%5%8%5%245%7%10%38%
1330
December1st to 10thDay
49%5%9%7%45%5%410%37%
1330
11th to 20thDay
49%5%10%9%5%7%6%7%6%32%
1682
21st to endDay
38%36%6%35%6%6%9%44%
2442
January1st to 10thNight
410%6%9%8%47%6%7%9%30%
2834
11th to 20thNight
5%9%49%7%46%5%6%9%35%
2678
21st to endNight
5%9%48%6%46%7%7%8%36%
2772
February1st to 10thNight
5%9%411%8%46%5%7%9%32%
2483
11th to 20thNight
6%11%6%9%7%5%6%7%6%8%28%
2272
21st to endNight
410%7%11%9%5%5%5%47%33%
1821
March1st to 10thNight
410%5%10%9%45%5%8%8%33%
2160
11th to 20thNight
5%11%47%7%35%7%6%11%35%
1981
21st to endNight
6%12%5%8%7%45%6%7%9%32%
2178
April1st to 10thNight
9%11%49%6%5%5%5%6%7%34%
1741
11th to 20thNight
10%11%48%7%45%7%5%6%33%
1601
21st to endNight
11%9%47%5%46%5%6%9%34%
1620
May1st to 10thNight
11%9%48%5%36%5%5%7%37%
1563
11th to 20thNight
11%6%246%26%46%7%46%
1239
21st to endNight
19%5%35%5%46%6%6%8%32%
1358
June1st to 10thNight
13%6%35%5%45%6%7%8%39%
1260
11th to 20thNight
18%6%36%7%346%5%8%34%
1260
21st to endNight
7%3245%35%5%8%10%49%
1260
July1st to 10thNight
6%4145%35%6%7%10%49%
1260
11th to 20thNight
7%3244245%8%10%53%
1260
21st to endNight
42124335%6%7%62%
1453
August1st to 10thNight
7%326%4345%5%7%54%
1449
11th to 20thNight
5%3134245%6%10%55%
1602
21st to endNight
8%32442345%7%58%
1782
September1st to 10thNight
10%425%5%3345%7%51%
1735
11th to 20thNight
12%6%2432335%6%53%
1980
21st to endNight
8%7%245%445%6%7%48%
1980
October1st to 10thNight
11%7%25%6%345%5%7%44%
2070
11th to 20thNight
9%7%35%5%35%5%7%9%44%
2254
21st to endNight
8%8%36%6%36%6%6%8%40%
2574
November1st to 10thNight
9%11%46%6%35%6%7%8%35%
2504
11th to 20thNight
49%37%7%35%6%7%8%42%
2664
21st to endNight
5%9%46%6%445%7%10%40%
2850
December1st to 10thNight
410%48%7%5%7%6%6%8%36%
2850
11th to 20thNight
38%5%9%8%45%6%7%10%35%
3510
21st to endNight
49%37%6%35%6%6%8%44%
5214

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
43%8%15%24%7%3
1322
11th to 20thDay
45%8%14%23%8%3
1300
21st to endDay
44%8%16%18%12%2
1584
February1st to 10thDay
51%8%13%18%9%2
1476
11th to 20thDay
48%8%16%19%8%1
1584
21st to endDay
52%6%11%17%12%2
1430
March1st to 10thDay
46%8%16%19%11%1
1800
11th to 20thDay
41%7%15%22%13%1
1979
21st to endDay
50%9%15%20%6%1
2178
April1st to 10thDay
50%6%15%21%8%1
2216
11th to 20thDay
49%7%11%21%10%2
2310
21st to endDay
46%7%14%21%10%1
2340
May1st to 10thDay
47%8%13%23%8%
2394
11th to 20thDay
41%8%14%25%11%1
2658
21st to endDay
50%8%13%20%9%1
2918
June1st to 10thDay
45%8%16%20%9%1
2700
11th to 20thDay
49%8%14%19%9%1
2700
21st to endDay
31%8%19%28%12%2
2700
July1st to 10thDay
29%9%16%30%13%3
2626
11th to 20thDay
28%9%17%27%16%3
2619
21st to endDay
21%8%14%31%21%5%
2703
August1st to 10thDay
27%8%16%31%14%3
2473
11th to 20thDay
26%8%19%31%12%3
2299
21st to endDay
28%6%16%29%18%4
2574
September1st to 10thDay
31%6%13%26%17%7%
2223
11th to 20thDay
36%5%12%29%14%3
1980
21st to endDay
35%6%13%30%14%3
1980
October1st to 10thDay
43%7%14%23%11%2
1890
11th to 20thDay
40%6%17%24%11%3
1702
21st to endDay
41%8%15%23%12%2
1782
November1st to 10thDay
49%7%12%21%10%1
1556
11th to 20thDay
40%7%18%24%10%1
1512
21st to endDay
40%7%19%22%11%3
1330
December1st to 10thDay
45%8%13%19%11%3
1330
11th to 20thDay
49%7%15%21%6%1
1684
21st to endDay
34%7%14%22%19%3
2442
January1st to 10thNight
47%8%15%18%10%3
2834
11th to 20thNight
45%8%15%21%8%2
2678
21st to endNight
42%7%15%20%14%2
2772
February1st to 10thNight
47%7%14%18%12%2
2483
11th to 20thNight
50%8%12%19%10%1
2273
21st to endNight
50%6%11%20%10%3
1821
March1st to 10thNight
47%8%14%22%9%1
2160
11th to 20thNight
43%7%16%22%11%1
1981
21st to endNight
47%9%17%19%7%1
2178
April1st to 10thNight
49%7%10%22%12%1
1741
11th to 20thNight
49%7%13%20%9%1
1601
21st to endNight
46%8%12%23%9%2
1620
May1st to 10thNight
46%6%13%24%10%1
1563
11th to 20thNight
37%8%13%30%10%2
1239
21st to endNight
48%9%14%20%8%2
1358
June1st to 10thNight
42%9%15%24%11%1
1260
11th to 20thNight
47%7%17%20%8%2
1260
21st to endNight
29%9%19%26%15%2
1260
July1st to 10thNight
27%11%19%28%13%2
1225
11th to 20thNight
26%9%17%25%18%5%
1218
21st to endNight
19%7%15%28%25%6%
1372
August1st to 10thNight
28%9%15%29%15%4
1441
11th to 20thNight
23%8%17%32%16%4
1602
21st to endNight
26%6%11%32%21%5%
1782
September1st to 10thNight
31%6%13%27%18%5%
1735
11th to 20thNight
31%5%13%27%18%5%
1980
21st to endNight
34%6%15%26%15%4
1980
October1st to 10thNight
38%7%14%24%16%1
2070
11th to 20thNight
35%8%15%24%15%3
2254
21st to endNight
39%8%15%25%13%1
2574
November1st to 10thNight
46%8%12%23%11%1
2504
11th to 20thNight
38%8%16%21%16%1
2664
21st to endNight
38%7%18%21%12%3
2850
December1st to 10thNight
44%8%15%20%10%3
2850
11th to 20thNight
41%9%22%21%6%1
3507
21st to endNight
37%6%13%25%13%5%
5214

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
6%9%29%30%13%12%
178
11th to 20thDay
13%5%33%24%16%8%
160
21st to endDay
39%34%35%16%3
176
February1st to 10thDay
10%10%40%30%9%3
176
11th to 20thDay
7%14%33%30%13%3
240
21st to endDay
12%10%31%28%16%3
296
March1st to 10thDay
10%17%33%26%11%3
420
11th to 20thDay
9%12%32%29%14%4
420
21st to endDay
12%16%31%24%12%5%
462
April1st to 10thDay
15%15%31%27%10%2
420
11th to 20thDay
14%14%26%27%15%4
412
21st to endDay
16%14%29%24%13%4
420
May1st to 10thDay
14%11%23%29%15%8%
454
11th to 20thDay
17%7%21%27%22%6%
625
21st to endDay
19%7%18%29%19%8%
679
June1st to 10thDay
14%8%29%28%18%3
630
11th to 20thDay
19%10%26%28%16%2
630
21st to endDay
10%5%22%32%23%7%
630
July1st to 10thDay
8%321%35%27%6%
621
11th to 20thDay
10%5%23%35%21%6%
618
21st to endDay
5%425%34%25%7%
608
August1st to 10thDay
9%5%23%34%21%8%
447
11th to 20thDay
7%6%21%33%24%10%
437
21st to endDay
8%8%26%30%18%10%
495
September1st to 10thDay
11%8%29%26%21%4
450
11th to 20thDay
15%7%25%30%17%6%
450
21st to endDay
16%15%28%22%15%4
450
October1st to 10thDay
18%8%21%23%23%7%
360
11th to 20thDay
13%11%30%23%15%7%
270
21st to endDay
15%14%28%28%12%2
297
November1st to 10thDay
21%9%30%25%12%3
270
11th to 20thDay
11%14%34%28%11%2
266
21st to endDay
5%17%28%28%13%9%
180
December1st to 10thDay
8%12%29%32%14%6%
180
11th to 20thDay
7%11%42%23%13%3
180
21st to endDay
10%8%22%34%19%8%
198
January1st to 10thNight
9%12%34%31%13%2
950
11th to 20thNight
9%8%35%34%12%2
878
21st to endNight
10%8%37%31%13%2
792
February1st to 10thNight
9%12%29%32%16%3
720
11th to 20thNight
11%6%32%37%12%2
704
21st to endNight
9%7%26%33%22%3
592
March1st to 10thNight
8%6%37%34%13%2
720
11th to 20thNight
10%8%38%30%12%2
720
21st to endNight
11%9%30%34%12%3
792
April1st to 10thNight
13%8%34%32%12%2
661
11th to 20thNight
15%7%32%34%10%1
533
21st to endNight
16%9%27%34%13%1
540
May1st to 10thNight
15%7%25%34%18%1
514
11th to 20thNight
13%6%21%37%21%1
354
21st to endNight
20%5%22%32%20%1
389
June1st to 10thNight
15%426%36%17%1
360
11th to 20thNight
21%6%26%32%14%
360
21st to endNight
9%324%41%22%2
360
July1st to 10thNight
6%319%43%27%1
350
11th to 20thNight
7%223%45%21%1
348
21st to endNight
5%224%44%23%3
394
August1st to 10thNight
9%215%42%29%2
375
11th to 20thNight
7%221%46%22%2
534
21st to endNight
9%5%23%42%20%1
594
September1st to 10thNight
12%324%39%20%2
616
11th to 20thNight
16%321%37%21%2
720
21st to endNight
11%6%30%35%15%4
720
October1st to 10thNight
15%6%24%34%19%3
720
11th to 20thNight
13%5%34%30%15%3
720
21st to endNight
12%7%34%33%13%1
792
November1st to 10thNight
15%9%38%30%8%1
839
11th to 20thNight
10%11%41%28%9%2
950
21st to endNight
10%10%35%32%11%2
950
December1st to 10thNight
9%9%30%35%14%3
950
11th to 20thNight
8%9%36%37%9%2
950
21st to endNight
12%8%32%34%12%2
1045
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