Southwest Montana Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southwest Montana Astronomical Society clear sky chart from 2005-11-06 to 2024-03-28.

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%
24219
16%
80796
12%
24136
46%
24136
21%
52736

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
32%
936
12%
4667
6%
926
36%
926
11%
3116
January11th to 20thNight
31%
819
14%
4493
11%
805
38%
805
10%
3210
January21st to endNight
33%
830
10%
4450
10%
814
40%
814
11%
2851
February1st to 10thNight
23%
760
8%
2470
6%
760
26%
760
11%
1919
February11th to 20thNight
29%
742
8%
2403
6%
740
33%
740
14%
1753
February21st to endNight
27%
628
12%
1964
8%
628
34%
628
16%
1453
March1st to 10thNight
33%
760
15%
2158
9%
760
39%
760
21%
1525
March11th to 20thNight
39%
760
15%
2090
10%
760
43%
760
23%
1441
March21st to endNight
36%
820
9%
2255
6%
820
38%
820
17%
1682
April1st to 10thNight
36%
620
10%
1876
6%
620
40%
620
18%
1350
April11th to 20thNight
37%
533
16%
1601
11%
533
42%
533
20%
1116
April21st to endNight
41%
540
12%
1620
8%
540
43%
540
19%
1147
May1st to 10thNight
43%
540
11%
1620
8%
540
45%
540
22%
1105
May11th to 20thNight
48%
531
12%
1445
11%
531
50%
531
23%
1013
May21st to endNight
45%
561
8%
1530
6%
560
47%
560
20%
1125
June1st to 10thNight
49%
360
11%
1260
12%
360
52%
360
18%
865
June11th to 20thNight
42%
360
12%
1260
7%
360
48%
360
19%
843
June21st to endNight
64%
360
19%
1260
19%
360
70%
360
38%
649
July1st to 10thNight
72%
350
18%
1225
20%
350
75%
350
43%
586
July11th to 20thNight
69%
348
27%
1218
27%
348
78%
348
46%
569
July21st to endNight
68%
542
25%
1606
25%
512
73%
512
48%
768
August1st to 10thNight
67%
537
21%
1602
20%
534
69%
534
41%
840
August11th to 20thNight
70%
534
28%
1602
23%
534
73%
534
46%
826
August21st to endNight
63%
594
29%
1782
23%
594
66%
594
40%
949
September1st to 10thNight
62%
544
26%
1793
23%
544
67%
544
38%
950
September11th to 20thNight
64%
720
26%
1980
21%
720
67%
720
40%
1159
September21st to endNight
55%
720
28%
1980
21%
720
59%
720
36%
1197
October1st to 10thNight
51%
720
19%
2127
13%
720
55%
720
29%
1355
October11th to 20thNight
46%
720
23%
2160
17%
720
52%
720
29%
1346
October21st to endNight
42%
792
19%
2465
13%
792
48%
792
24%
1609
November1st to 10thNight
38%
806
19%
2471
14%
806
44%
806
22%
1675
November11th to 20thNight
31%
950
16%
2660
9%
950
38%
950
20%
1905
November21st to endNight
39%
950
17%
2660
11%
950
44%
950
27%
1723
December1st to 10thNight
31%
950
13%
2660
7%
950
35%
950
19%
1925
December11th to 20thNight
29%
950
14%
3369
9%
949
34%
949
16%
2260
December21st to endNight
25%
1032
9%
5014
6%
1026
31%
1026
11%
2931

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
8%10%47%6%35%5%6%8%39%
78540
Night
8%12%5%8%6%35%5%5%7%38%
81480
TransparencyDay
42%5%13%22%13%4
78239
Night
44%6%14%23%11%3
81303
SeeingDay
16%10%27%28%16%4
14555
Night
16%9%24%37%13%1
24261

By Month

Percentage Cloud Cover SW (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
5%17%9%10%7%45%46%7%27%
2219
11th to 20thDay
5%14%6%11%10%45%6%7%6%26%
2453
21st to endDay
411%5%8%7%3446%7%40%
3110
February1st to 10thDay
8%19%6%11%8%446%5%7%23%
1710
11th to 20thDay
416%8%12%9%45%46%6%25%
1665
21st to endDay
414%8%13%10%36%7%5%5%25%
1488
March1st to 10thDay
7%12%5%11%7%46%6%6%8%29%
2021
11th to 20thDay
6%13%6%9%7%46%5%6%7%31%
2090
21st to endDay
8%16%6%11%8%46%5%6%7%23%
2256
April1st to 10thDay
8%16%5%8%7%45%5%5%7%29%
2081
11th to 20thDay
8%13%6%9%8%46%5%6%7%28%
2307
21st to endDay
12%15%47%6%445%6%7%29%
2340
May1st to 10thDay
10%11%46%6%45%6%6%8%32%
2340
11th to 20thDay
11%7%36%6%45%6%8%11%31%
2448
21st to endDay
17%10%47%6%46%6%7%9%24%
2743
June1st to 10thDay
14%7%36%5%36%7%7%9%34%
2700
11th to 20thDay
13%7%36%7%46%6%6%8%34%
2700
21st to endDay
4325%5%35%6%6%8%54%
2700
July1st to 10thDay
3213435%7%8%10%55%
2700
11th to 20thDay
42134345%6%7%61%
2700
21st to endDay
43124345%6%6%61%
2613
August1st to 10thDay
4424435%5%5%9%54%
2328
11th to 20thDay
5%3134345%6%7%59%
2299
21st to endDay
5%414435%5%7%7%55%
2574
September1st to 10thDay
6%41332235%9%60%
2163
11th to 20thDay
9%5%24435%5%6%7%51%
1980
21st to endDay
10%6%25%42346%7%51%
1980
October1st to 10thDay
13%10%36%5%3446%7%40%
1832
11th to 20thDay
13%11%36%433447%42%
1800
21st to endDay
9%13%47%5%36%5%6%6%36%
1890
November1st to 10thDay
11%13%5%8%5%3445%7%36%
1589
11th to 20thDay
8%14%6%6%7%5%445%8%33%
1520
21st to endDay
6%14%5%8%6%35%446%39%
1520
December1st to 10thDay
5%13%6%11%7%46%46%7%32%
1520
11th to 20thDay
414%7%10%7%45%7%8%8%27%
1719
21st to endDay
5%12%6%10%6%35%5%46%38%
2442
January1st to 10thNight
5%14%8%11%6%36%5%5%7%30%
4726
11th to 20thNight
5%15%6%12%10%45%45%7%26%
4537
21st to endNight
6%15%5%8%6%45%5%5%7%33%
4553
February1st to 10thNight
7%20%9%12%7%34445%24%
2470
11th to 20thNight
6%19%8%11%6%345%5%6%28%
2402
21st to endNight
5%13%9%15%8%35%45%6%28%
1964
March1st to 10thNight
6%12%5%10%8%45%46%8%32%
2158
11th to 20thNight
6%13%5%7%6%45%6%5%7%36%
2090
21st to endNight
8%18%6%9%6%345%6%8%26%
2255
April1st to 10thNight
9%15%5%9%6%35%5%5%8%30%
1876
11th to 20thNight
9%12%5%7%7%45%6%6%8%30%
1601
21st to endNight
15%14%45%5%345%5%7%33%
1620
May1st to 10thNight
11%11%46%5%35%6%6%8%34%
1620
11th to 20thNight
13%8%35%6%46%5%8%9%34%
1445
21st to endNight
15%11%5%7%5%35%7%8%9%26%
1530
June1st to 10thNight
14%7%35%8%46%6%5%7%35%
1260
11th to 20thNight
12%9%5%7%6%35%5%6%6%35%
1260
21st to endNight
6%31343445%9%56%
1260
July1st to 10thNight
5%2134345%6%10%58%
1260
11th to 20thNight
3213325%5%6%8%60%
1260
21st to endNight
421443446%8%61%
1700
August1st to 10thNight
5%413435%6%8%8%53%
1611
11th to 20thNight
43243245%6%9%57%
1602
21st to endNight
6%424435%47%8%54%
1782
September1st to 10thNight
6%4146%35%45%8%55%
1793
11th to 20thNight
9%4344335%6%7%52%
1980
21st to endNight
10%7%25%433445%53%
1980
October1st to 10thNight
13%9%35%5%3445%7%44%
2127
11th to 20thNight
12%10%35%423447%46%
2160
21st to endNight
10%13%36%5%3446%6%40%
2465
November1st to 10thNight
11%14%47%6%3446%7%36%
2471
11th to 20thNight
9%15%6%9%5%34446%34%
2660
21st to endNight
6%11%47%5%3445%8%41%
2660
December1st to 10thNight
6%15%7%9%7%34447%33%
2660
11th to 20thNight
6%14%6%9%6%35%5%6%8%33%
3468
21st to endNight
5%12%6%9%6%3345%7%40%
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
53%7%15%22%4
2219
11th to 20thDay
54%7%14%13%11%1
2453
21st to endDay
43%7%21%22%7%1
3110
February1st to 10thDay
60%6%13%17%5%
1710
11th to 20thDay
59%6%13%16%6%1
1674
21st to endDay
56%7%10%16%9%2
1488
March1st to 10thDay
50%7%13%23%7%
2021
11th to 20thDay
50%6%14%21%8%
2090
21st to endDay
57%7%15%17%4
2256
April1st to 10thDay
52%8%13%19%8%
2081
11th to 20thDay
53%5%13%20%7%
2310
21st to endDay
52%415%21%7%1
2340
May1st to 10thDay
47%6%12%28%8%1
2340
11th to 20thDay
43%8%18%23%7%1
2452
21st to endDay
53%6%20%15%41
2743
June1st to 10thDay
45%7%19%22%7%1
2700
11th to 20thDay
46%6%13%20%12%4
2700
21st to endDay
26%5%13%28%22%6%
2700
July1st to 10thDay
21%6%12%31%23%7%
2626
11th to 20thDay
21%47%23%26%20%
2619
21st to endDay
21%46%21%30%18%
2467
August1st to 10thDay
27%39%26%24%11%
2313
11th to 20thDay
23%39%26%28%11%
2299
21st to endDay
25%48%25%27%11%
2574
September1st to 10thDay
23%39%25%28%13%
2163
11th to 20thDay
31%410%23%25%7%
1980
21st to endDay
32%410%31%18%5%
1980
October1st to 10thDay
44%412%24%14%2
1832
11th to 20thDay
42%415%25%10%3
1800
21st to endDay
46%6%15%24%8%2
1890
November1st to 10thDay
47%5%12%23%10%2
1589
11th to 20thDay
49%417%20%9%1
1520
21st to endDay
46%413%23%13%1
1520
December1st to 10thDay
49%7%15%21%7%2
1520
11th to 20thDay
50%8%19%19%31
1718
21st to endDay
46%5%10%22%12%5%
2442
January1st to 10thNight
49%9%13%24%4
4726
11th to 20thNight
58%5%11%18%7%1
4537
21st to endNight
48%7%18%20%6%1
4553
February1st to 10thNight
62%6%11%15%6%
2470
11th to 20thNight
56%5%15%16%7%1
2403
21st to endNight
55%6%9%18%10%1
1964
March1st to 10thNight
47%7%13%22%10%
2158
11th to 20thNight
45%7%16%24%8%1
2090
21st to endNight
54%7%16%20%4
2255
April1st to 10thNight
50%8%15%20%7%
1876
11th to 20thNight
48%7%13%25%7%
1601
21st to endNight
50%6%15%22%7%
1620
May1st to 10thNight
44%8%17%25%5%1
1620
11th to 20thNight
44%9%17%21%9%1
1446
21st to endNight
51%8%22%15%5%
1530
June1st to 10thNight
47%8%18%20%7%
1260
11th to 20thNight
48%6%13%21%10%3
1260
21st to endNight
25%6%15%28%20%5%
1260
July1st to 10thNight
22%5%16%31%20%6%
1225
11th to 20thNight
18%6%10%26%24%15%
1218
21st to endNight
20%49%24%29%13%
1606
August1st to 10thNight
25%6%15%29%18%7%
1602
11th to 20thNight
22%5%11%29%21%12%
1602
21st to endNight
27%412%26%22%9%
1782
September1st to 10thNight
29%49%22%23%14%
1793
11th to 20thNight
29%5%12%27%21%6%
1980
21st to endNight
34%49%28%21%5%
1980
October1st to 10thNight
40%5%13%25%15%3
2127
11th to 20thNight
39%413%27%14%3
2160
21st to endNight
43%5%14%23%13%1
2465
November1st to 10thNight
46%6%11%24%11%2
2471
11th to 20thNight
50%6%14%22%7%1
2660
21st to endNight
41%5%16%25%13%2
2660
December1st to 10thNight
51%6%15%18%9%2
2660
11th to 20thNight
45%9%18%23%5%
3469
21st to endNight
43%6%11%25%10%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
15%8%19%35%19%3
180
11th to 20thDay
21%7%23%32%11%7%
180
21st to endDay
14%8%31%34%11%2
286
February1st to 10thDay
24%15%33%21%6%
270
11th to 20thDay
21%13%34%24%7%2
270
21st to endDay
18%15%34%21%11%1
300
March1st to 10thDay
21%16%32%23%7%1
460
11th to 20thDay
19%14%28%25%11%2
460
21st to endDay
22%15%27%26%9%1
491
April1st to 10thDay
21%20%31%19%8%1
420
11th to 20thDay
20%16%34%22%7%1
412
21st to endDay
24%12%27%23%10%4
420
May1st to 10thDay
19%14%22%26%16%3
423
11th to 20thDay
19%8%25%26%18%4
595
21st to endDay
22%10%19%22%21%6%
679
June1st to 10thDay
17%7%24%26%22%5%
630
11th to 20thDay
19%14%26%26%13%2
630
21st to endDay
7%5%27%35%23%4
630
July1st to 10thDay
5%6%24%36%24%6%
621
11th to 20thDay
6%7%24%37%24%2
618
21st to endDay
6%7%24%38%20%6%
508
August1st to 10thDay
7%6%28%31%22%5%
447
11th to 20thDay
7%5%26%34%20%8%
437
21st to endDay
7%7%28%28%23%7%
495
September1st to 10thDay
8%8%30%30%20%4
450
11th to 20thDay
16%8%27%30%16%4
450
21st to endDay
17%11%23%24%20%4
450
October1st to 10thDay
19%12%21%24%20%5%
302
11th to 20thDay
23%9%28%23%10%7%
270
21st to endDay
22%9%30%25%11%3
297
November1st to 10thDay
23%10%26%27%12%2
270
11th to 20thDay
19%10%31%29%9%1
270
21st to endDay
18%12%22%37%9%1
270
December1st to 10thDay
14%15%27%29%14%1
270
11th to 20thDay
22%15%25%28%9%1
196
21st to endDay
20%14%23%26%15%3
198
January1st to 10thNight
18%12%27%34%8%
950
11th to 20thNight
19%12%27%29%10%2
825
21st to endNight
17%13%27%34%9%1
836
February1st to 10thNight
20%14%30%27%8%1
760
11th to 20thNight
19%12%27%35%6%1
744
21st to endNight
14%14%32%27%10%2
628
March1st to 10thNight
13%12%29%38%8%
760
11th to 20thNight
14%11%26%38%11%1
760
21st to endNight
20%10%23%35%10%2
820
April1st to 10thNight
21%9%25%35%10%1
620
11th to 20thNight
17%11%27%36%9%1
533
21st to endNight
23%9%19%33%15%1
540
May1st to 10thNight
19%8%22%33%18%1
540
11th to 20thNight
18%7%18%34%20%2
531
21st to endNight
23%6%17%31%22%2
562
June1st to 10thNight
19%416%34%25%1
360
11th to 20thNight
19%7%25%38%11%
360
21st to endNight
8%217%51%22%
360
July1st to 10thNight
7%115%47%30%1
350
11th to 20thNight
5%317%51%24%
348
21st to endNight
4318%47%26%1
542
August1st to 10thNight
6%316%54%20%2
537
11th to 20thNight
7%215%49%25%2
534
21st to endNight
9%319%43%25%1
594
September1st to 10thNight
8%320%45%22%2
544
11th to 20thNight
11%314%44%25%2
720
21st to endNight
15%7%16%40%20%3
720
October1st to 10thNight
18%418%43%16%1
720
11th to 20thNight
20%7%21%35%15%2
720
21st to endNight
21%8%22%37%10%2
792
November1st to 10thNight
23%8%25%37%8%
806
11th to 20thNight
20%13%28%33%6%
950
21st to endNight
12%12%30%38%8%
950
December1st to 10thNight
17%12%28%32%9%1
950
11th to 20thNight
18%11%30%33%7%1
950
21st to endNight
17%15%28%31%8%1
1045
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