This page made possible by the sponsorship of: Joel E. Guthals.

Billings
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Billings clear sky chart from 2005-11-06 to 2024-01-16.

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%
24436
19%
74019
12%
24392
48%
24392
27%
46501

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
31%
947
16%
2647
8%
946
37%
946
24%
1771
January11th to 20thNight
28%
926
20%
2591
10%
926
37%
926
24%
1751
January21st to endNight
23%
814
19%
2596
8%
814
37%
814
22%
1704
February1st to 10thNight
25%
720
14%
2340
8%
720
33%
720
16%
1642
February11th to 20thNight
26%
700
12%
2273
6%
700
33%
700
16%
1603
February21st to endNight
32%
592
15%
1888
9%
592
40%
592
19%
1303
March1st to 10thNight
35%
720
16%
2039
9%
720
41%
720
25%
1379
March11th to 20thNight
41%
720
18%
1980
10%
720
48%
720
28%
1320
March21st to endNight
38%
774
13%
2160
9%
774
42%
774
20%
1567
April1st to 10thNight
38%
540
13%
1800
9%
540
44%
540
19%
1221
April11th to 20thNight
39%
533
15%
1735
12%
533
43%
533
19%
1208
April21st to endNight
43%
540
14%
1620
12%
540
45%
540
20%
1124
May1st to 10thNight
45%
540
14%
1508
11%
540
47%
540
23%
1054
May11th to 20thNight
51%
531
14%
1418
12%
531
52%
531
25%
985
May21st to endNight
48%
584
10%
1552
7%
582
51%
582
22%
1122
June1st to 10thNight
52%
540
11%
1440
6%
540
54%
540
28%
968
June11th to 20thNight
48%
396
13%
1296
9%
396
53%
396
24%
809
June21st to endNight
62%
360
18%
1260
14%
360
67%
360
36%
669
July1st to 10thNight
75%
350
19%
1225
22%
350
76%
350
43%
587
July11th to 20thNight
59%
517
21%
1387
14%
517
64%
517
47%
758
July21st to endNight
66%
591
25%
1492
20%
558
69%
558
51%
795
August1st to 10thNight
64%
537
21%
1602
18%
534
68%
534
39%
879
August11th to 20thNight
68%
534
27%
1602
22%
534
71%
534
44%
833
August21st to endNight
64%
594
27%
1870
24%
594
66%
594
36%
1031
September1st to 10thNight
65%
540
26%
1800
25%
540
71%
540
41%
916
September11th to 20thNight
61%
576
27%
1836
26%
576
65%
576
36%
1012
September21st to endNight
57%
720
27%
2042
21%
720
61%
720
38%
1205
October1st to 10thNight
48%
720
19%
2160
14%
720
53%
720
26%
1413
October11th to 20thNight
44%
720
27%
2160
15%
720
53%
720
30%
1314
October21st to endNight
42%
805
25%
2389
13%
805
52%
805
32%
1432
November1st to 10thNight
39%
925
19%
2552
12%
925
44%
925
27%
1696
November11th to 20thNight
34%
950
24%
2660
12%
950
43%
950
29%
1691
November21st to endNight
35%
950
22%
2660
11%
950
44%
950
31%
1666
December1st to 10thNight
28%
950
16%
2660
7%
950
35%
950
23%
1838
December11th to 20thNight
30%
949
22%
3142
8%
949
41%
949
25%
1848
December21st to endNight
27%
1031
12%
4637
5%
1026
37%
1026
18%
2387

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%7%35%5%35%5%6%8%45%
75257
Night
9%8%36%5%345%6%8%43%
74561
TransparencyDay
36%6%13%24%16%5%
74955
Night
39%6%13%24%14%4
74385
SeeingDay
12%8%27%31%18%4
14757
Night
13%7%30%36%12%1
24458

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
6%10%48%8%345%7%8%37%
1518
11th to 20thDay
7%10%48%6%345%6%8%38%
1480
21st to endDay
6%12%46%6%35%6%6%10%36%
1774
February1st to 10thDay
8%13%6%8%6%46%5%48%32%
1620
11th to 20thDay
6%12%49%7%5%6%5%6%9%31%
1575
21st to endDay
6%12%5%8%7%35%5%7%7%35%
1368
March1st to 10thDay
7%9%5%7%7%35%5%6%8%39%
1919
11th to 20thDay
11%7%47%5%346%7%10%38%
1980
21st to endDay
11%11%48%6%35%5%7%9%32%
2196
April1st to 10thDay
12%11%45%6%245%6%8%36%
2160
11th to 20thDay
13%10%37%6%45%6%6%8%32%
2172
21st to endDay
16%10%45%5%345%5%6%36%
2340
May1st to 10thDay
14%7%36%5%36%6%5%7%38%
2452
11th to 20thDay
14%7%25%6%35%5%7%9%37%
2478
21st to endDay
20%6%35%5%36%6%7%9%31%
2724
June1st to 10thDay
11%5%25%6%35%6%7%9%42%
2520
11th to 20thDay
8%5%25%6%36%7%6%9%43%
2659
21st to endDay
4313435%5%7%9%56%
2700
July1st to 10thDay
4213335%5%7%10%58%
2700
11th to 20thDay
22133245%7%9%62%
2521
21st to endDay
321332446%9%64%
2736
August1st to 10thDay
5%3133245%6%10%57%
2328
11th to 20thDay
6%22343446%7%60%
2299
21st to endDay
6%31332446%8%60%
2485
September1st to 10thDay
8%314324447%61%
2160
11th to 20thDay
10%424424446%56%
2123
21st to endDay
13%5%2331335%7%57%
1918
October1st to 10thDay
12%9%36%4345%5%7%43%
1800
11th to 20thDay
14%8%25%5%235%46%48%
1800
21st to endDay
9%7%36%5%345%5%8%44%
1948
November1st to 10thDay
12%10%35%43445%7%43%
1524
11th to 20thDay
8%10%35%5%35%5%6%8%43%
1520
21st to endDay
6%9%36%6%345%6%7%44%
1520
December1st to 10thDay
6%9%5%8%6%36%5%5%8%39%
1520
11th to 20thDay
48%37%6%35%6%7%10%41%
1926
21st to endDay
6%9%46%5%35%46%7%46%
2794
January1st to 10thNight
7%9%46%6%36%6%5%7%39%
2647
11th to 20thNight
8%10%5%6%6%3445%8%40%
2591
21st to endNight
8%10%48%5%45%5%6%9%37%
2596
February1st to 10thNight
9%14%6%9%6%45%5%5%7%31%
2340
11th to 20thNight
8%14%5%9%6%45%5%5%7%33%
2272
21st to endNight
7%13%6%9%7%346%6%6%34%
1888
March1st to 10thNight
7%9%5%8%6%45%5%6%8%37%
2039
11th to 20thNight
8%7%27%6%35%6%6%9%39%
1980
21st to endNight
13%12%47%6%35%5%6%7%33%
2160
April1st to 10thNight
13%8%47%6%36%5%47%37%
1800
11th to 20thNight
13%11%5%6%7%445%5%7%32%
1735
21st to endNight
17%8%27%7%3446%6%36%
1620
May1st to 10thNight
16%8%46%6%3446%7%36%
1508
11th to 20thNight
14%7%35%5%35%7%5%8%38%
1417
21st to endNight
18%7%25%6%45%5%7%10%31%
1552
June1st to 10thNight
12%5%25%5%45%7%7%10%39%
1440
11th to 20thNight
11%426%5%36%6%7%9%41%
1296
21st to endNight
6%5%2443445%8%54%
1260
July1st to 10thNight
5%3134245%7%10%56%
1260
11th to 20thNight
4313435%5%6%9%57%
1434
21st to endNight
33234245%7%9%57%
1580
August1st to 10thNight
5%315%5%45%5%6%10%51%
1611
11th to 20thNight
4313435%5%6%8%57%
1602
21st to endNight
8%4145%35%46%8%51%
1870
September1st to 10thNight
7%5%24333446%59%
1800
11th to 20thNight
11%41433345%9%53%
1836
21st to endNight
13%5%23334347%53%
2042
October1st to 10thNight
16%7%245%3446%6%42%
2160
11th to 20thNight
14%7%24323446%50%
2160
21st to endNight
10%6%35%5%2446%9%47%
2389
November1st to 10thNight
11%10%36%4345%6%7%41%
2552
11th to 20thNight
9%9%45%5%345%6%7%44%
2660
21st to endNight
7%8%35%6%345%6%8%45%
2660
December1st to 10thNight
5%11%5%8%7%45%5%5%6%38%
2660
11th to 20thNight
5%7%38%6%345%6%7%47%
3271
21st to endNight
6%9%35%5%345%5%7%50%
4873

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
45%6%14%25%10%
1519
11th to 20thDay
43%415%24%12%3
1480
21st to endDay
43%6%16%23%10%2
1774
February1st to 10thDay
51%5%16%19%9%
1620
11th to 20thDay
50%7%13%21%8%2
1584
21st to endDay
45%6%13%21%12%2
1368
March1st to 10thDay
44%5%14%24%12%1
1919
11th to 20thDay
40%6%13%27%12%2
1980
21st to endDay
48%8%14%22%7%1
2196
April1st to 10thDay
45%7%13%21%11%2
2160
11th to 20thDay
49%7%13%21%9%1
2175
21st to endDay
48%6%11%22%12%1
2340
May1st to 10thDay
44%7%11%25%11%1
2452
11th to 20thDay
42%9%16%23%9%2
2482
21st to endDay
48%9%18%20%6%
2724
June1st to 10thDay
37%8%18%24%12%1
2520
11th to 20thDay
36%8%17%22%14%4
2659
21st to endDay
22%7%15%26%22%8%
2700
July1st to 10thDay
21%6%11%32%23%7%
2626
11th to 20thDay
18%5%7%24%30%15%
2445
21st to endDay
17%48%23%32%17%
2584
August1st to 10thDay
22%5%10%25%26%12%
2313
11th to 20thDay
23%5%7%26%29%10%
2299
21st to endDay
21%5%8%24%28%14%
2485
September1st to 10thDay
23%47%23%30%14%
2160
11th to 20thDay
28%6%10%24%24%8%
2123
21st to endDay
28%49%30%22%7%
1918
October1st to 10thDay
40%5%12%22%16%5%
1800
11th to 20thDay
38%5%11%28%13%6%
1800
21st to endDay
39%5%14%25%15%2
1948
November1st to 10thDay
41%6%12%22%16%4
1524
11th to 20thDay
40%415%24%14%2
1520
21st to endDay
38%5%14%24%16%3
1520
December1st to 10thDay
44%6%12%23%12%2
1520
11th to 20thDay
36%7%24%23%9%1
1924
21st to endDay
38%5%9%27%15%6%
2794
January1st to 10thNight
43%5%16%24%10%2
2647
11th to 20thNight
44%5%14%23%11%3
2591
21st to endNight
44%6%14%24%10%1
2596
February1st to 10thNight
52%5%13%20%9%1
2340
11th to 20thNight
52%6%11%21%9%1
2273
21st to endNight
49%6%13%18%11%3
1888
March1st to 10thNight
44%6%15%22%11%2
2039
11th to 20thNight
39%8%14%24%13%2
1980
21st to endNight
50%7%14%22%7%
2160
April1st to 10thNight
46%8%15%18%11%2
1800
11th to 20thNight
49%7%12%24%8%
1735
21st to endNight
48%7%12%21%12%1
1620
May1st to 10thNight
47%6%10%26%10%1
1508
11th to 20thNight
43%8%15%21%11%1
1418
21st to endNight
46%9%19%20%5%
1552
June1st to 10thNight
39%9%19%23%9%1
1440
11th to 20thNight
38%9%16%21%13%3
1296
21st to endNight
27%7%15%25%21%5%
1260
July1st to 10thNight
22%5%12%36%21%4
1225
11th to 20thNight
23%5%11%21%25%14%
1387
21st to endNight
20%6%11%20%30%13%
1492
August1st to 10thNight
27%6%13%26%20%8%
1602
11th to 20thNight
23%6%10%26%27%9%
1602
21st to endNight
29%6%8%21%25%11%
1870
September1st to 10thNight
26%49%20%25%15%
1800
11th to 20thNight
29%5%11%24%23%7%
1836
21st to endNight
31%5%9%27%22%6%
2042
October1st to 10thNight
42%6%11%22%16%4
2160
11th to 20thNight
36%5%10%26%18%4
2160
21st to endNight
34%6%15%27%15%4
2389
November1st to 10thNight
41%6%12%23%14%3
2552
11th to 20thNight
38%5%13%28%13%2
2660
21st to endNight
37%5%14%27%15%3
2660
December1st to 10thNight
46%6%12%21%12%3
2660
11th to 20thNight
35%6%17%32%10%1
3272
21st to endNight
34%6%11%29%16%5%
4873

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
11%11%38%30%8%3
270
11th to 20thDay
13%10%34%30%10%4
255
21st to endDay
14%14%41%24%6%1
264
February1st to 10thDay
19%15%44%20%2
240
11th to 20thDay
12%15%43%24%5%1
240
21st to endDay
11%12%41%25%10%1
198
March1st to 10thDay
12%12%33%32%9%1
359
11th to 20thDay
14%11%34%27%11%3
420
21st to endDay
19%14%25%29%11%2
462
April1st to 10thDay
17%15%34%23%9%2
420
11th to 20thDay
18%15%33%25%8%1
412
21st to endDay
22%10%27%23%14%4
420
May1st to 10thDay
17%7%23%27%21%5%
535
11th to 20thDay
20%5%22%29%19%6%
625
21st to endDay
18%8%18%26%21%8%
679
June1st to 10thDay
10%5%21%35%23%6%
630
11th to 20thDay
10%12%27%30%18%2
630
21st to endDay
6%5%23%36%26%4
630
July1st to 10thDay
5%319%35%32%5%
621
11th to 20thDay
35%25%39%24%3
618
21st to endDay
3319%42%28%6%
678
August1st to 10thDay
6%5%22%39%22%6%
447
11th to 20thDay
8%222%33%25%9%
437
21st to endDay
6%421%32%28%9%
495
September1st to 10thDay
7%7%24%38%20%5%
450
11th to 20thDay
10%6%19%34%23%8%
450
21st to endDay
16%6%21%29%24%4
388
October1st to 10thDay
17%6%17%29%26%5%
270
11th to 20thDay
16%9%26%32%13%5%
270
21st to endDay
15%12%36%21%13%3
297
November1st to 10thDay
21%9%30%27%11%2
270
11th to 20thDay
12%11%38%30%7%1
270
21st to endDay
13%8%41%29%10%
270
December1st to 10thDay
7%10%45%30%7%
270
11th to 20thDay
13%12%36%31%7%
270
21st to endDay
14%9%39%25%11%2
297
January1st to 10thNight
12%9%41%31%7%1
950
11th to 20thNight
14%10%42%26%7%1
926
21st to endNight
15%9%44%24%7%1
814
February1st to 10thNight
14%11%39%28%7%1
720
11th to 20thNight
16%9%38%31%7%1
704
21st to endNight
14%9%35%32%9%1
592
March1st to 10thNight
12%6%36%33%12%
720
11th to 20thNight
12%7%32%36%12%2
720
21st to endNight
18%7%27%34%13%1
774
April1st to 10thNight
17%10%27%37%9%1
540
11th to 20thNight
17%7%30%39%7%
533
21st to endNight
20%6%21%38%14%1
540
May1st to 10thNight
18%7%20%37%16%1
540
11th to 20thNight
17%6%20%33%23%1
531
21st to endNight
20%5%20%35%19%1
584
June1st to 10thNight
12%5%25%37%18%3
540
11th to 20thNight
12%7%27%40%15%
396
21st to endNight
7%221%49%20%
360
July1st to 10thNight
6%212%51%29%
350
11th to 20thNight
4427%50%15%
517
21st to endNight
4322%55%16%1
591
August1st to 10thNight
6%321%51%20%1
537
11th to 20thNight
7%318%52%20%1
534
21st to endNight
8%416%52%20%1
594
September1st to 10thNight
8%419%50%19%1
540
11th to 20thNight
13%218%47%19%1
576
21st to endNight
15%419%40%20%2
720
October1st to 10thNight
19%422%37%16%2
720
11th to 20thNight
17%8%26%32%17%1
720
21st to endNight
13%5%34%36%11%1
805
November1st to 10thNight
15%9%32%35%8%1
925
11th to 20thNight
13%10%38%32%7%
950
21st to endNight
11%8%39%36%6%
950
December1st to 10thNight
12%9%41%31%7%
950
11th to 20thNight
10%8%43%31%7%1
950
21st to endNight
13%11%37%30%9%1
1045
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