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 2005-11-05 to 2018-03-11.

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
43%
16616
18%
48833
12%
16577
48%
16577
27%
31527

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 15thNight
28%
971
14%
2716
8%
970
33%
970
22%
1894
January16th to endNight
24%
913
20%
2785
9%
913
36%
913
23%
1846
February1st to 15thNight
29%
757
13%
2483
8%
757
35%
757
17%
1727
February16th to endNight
33%
684
12%
2195
9%
684
38%
684
18%
1548
March1st to 15thNight
38%
764
16%
2143
8%
764
45%
764
27%
1429
March16th to endNight
38%
756
12%
2100
6%
756
43%
756
22%
1498
April1st to 15thNight
40%
539
12%
1798
10%
539
43%
539
19%
1223
April16th to endNight
43%
540
14%
1650
12%
540
45%
540
20%
1147
May1st to 15thNight
48%
539
16%
1484
12%
539
50%
539
26%
1004
May16th to endNight
50%
563
11%
1498
9%
561
52%
561
23%
1070
June1st to 15thNight
47%
504
10%
1404
6%
504
49%
504
25%
939
June16th to endNight
59%
360
17%
1260
13%
360
64%
360
35%
664
July1st to 15thNight
67%
393
17%
1243
15%
393
70%
393
43%
635
July16th to endNight
62%
564
22%
1419
16%
531
66%
531
50%
762
August1st to 15thNight
66%
539
20%
1611
16%
536
70%
536
41%
856
August16th to endNight
69%
573
30%
1778
27%
573
71%
573
42%
937
September1st to 15thNight
64%
540
24%
1800
24%
540
67%
540
37%
959
September16th to endNight
61%
680
28%
1986
23%
680
65%
680
39%
1138
October1st to 15thNight
47%
720
21%
2160
14%
720
53%
720
25%
1421
October16th to endNight
47%
777
28%
2313
16%
777
57%
777
33%
1363
November1st to 15thNight
41%
950
21%
2635
14%
950
46%
950
29%
1709
November16th to endNight
37%
975
22%
2730
12%
975
45%
975
28%
1781
December1st to 15thNight
28%
975
14%
2730
5%
975
35%
975
20%
1949
December16th to endNight
29%
1040
18%
2912
6%
1040
38%
1040
22%
2028

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%6%25%4245%5%8%44%
49819
Night
9%7%35%5%3445%7%41%
48998
Cloud CoverDay
9%6%25%42445%8%44%
49819
Night
10%8%35%5%3445%7%41%
49000
TransparencyDay
37%5%13%23%15%4
49488
Night
40%5%13%23%13%3
48834
SeeingDay
13%7%26%31%17%3
6607
Night
13%6%29%36%13%
16628

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 15thDay
6%11%48%7%3345%8%34%
1553
16th to endDay
6%11%36%5%345%5%8%38%
1801
February1st to 15thDay
8%10%5%8%5%36%5%49%31%
1710
16th to endDay
5%10%5%8%6%346%7%7%33%
1565
March1st to 15thDay
7%8%36%6%3445%9%39%
2051
16th to endDay
12%9%37%5%3446%10%31%
2123
April1st to 15thDay
13%9%35%5%345%6%9%34%
2150
16th to endDay
14%9%35%5%35%5%6%6%33%
2310
May1st to 15thDay
13%8%25%425%447%40%
2466
16th to endDay
19%5%25%5%35%5%6%8%32%
2616
June1st to 15thDay
10%5%245%35%5%7%7%41%
2553
16th to endDay
3212335%5%6%9%55%
2700
July1st to 15thDay
311332447%10%57%
2640
16th to endDay
32222445%9%61%
2651
August1st to 15thDay
5%2123246%5%9%56%
2340
16th to endDay
421331345%7%61%
2423
September1st to 15thDay
9%413312336%59%
2160
16th to endDay
11%313223446%56%
1972
October1st to 15thDay
12%8%35%43345%6%42%
1800
16th to endDay
9%6%26%425%445%47%
1899
November1st to 15thDay
10%7%244235%6%6%45%
1552
16th to endDay
7%8%345%25%46%6%44%
1560
December1st to 15thDay
6%10%6%8%6%445%5%7%33%
1560
16th to endDay
6%12%5%7%5%3447%9%33%
1664
January1st to 15thNight
9%10%46%5%35%45%7%35%
2716
16th to endNight
9%9%47%5%2445%8%38%
2785
February1st to 15thNight
9%12%5%7%5%35%45%7%32%
2472
16th to endNight
6%13%6%8%6%345%5%7%31%
2195
March1st to 15thNight
7%7%46%5%45%6%6%9%38%
2143
16th to endNight
13%9%36%6%35%6%5%8%32%
2097
April1st to 15thNight
13%8%36%6%25%5%47%35%
1798
16th to endNight
17%8%26%6%245%5%6%34%
1650
May1st to 15thNight
14%6%35%4345%47%40%
1484
16th to endNight
16%6%25%5%45%5%6%8%33%
1498
June1st to 15thNight
11%425%5%35%6%8%8%38%
1404
16th to endNight
8%31342446%7%52%
1260
July1st to 15thNight
42143245%7%9%54%
1317
16th to endNight
321232447%8%57%
1515
August1st to 15thNight
3235%25%6%6%11%50%
1620
16th to endNight
5%31342445%8%56%
1778
September1st to 15thNight
10%423313445%56%
1800
16th to endNight
12%313313348%52%
1986
October1st to 15thNight
16%6%245%3445%5%42%
2160
16th to endNight
9%5%2442345%8%49%
2313
November1st to 15thNight
10%8%25%5%2445%7%43%
2635
16th to endNight
8%9%45%5%3445%7%42%
2730
December1st to 15thNight
5%11%5%10%7%35%45%6%36%
2730
16th to endNight
7%12%47%5%35%5%47%36%
2912

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 15thDay
8%11%47%7%2346%7%34%
1552
16th to endDay
6%10%36%5%345%5%8%38%
1801
February1st to 15thDay
8%12%5%7%5%46%5%48%31%
1710
16th to endDay
5%11%48%6%35%5%6%8%33%
1565
March1st to 15thDay
8%8%46%5%345%5%8%39%
2051
16th to endDay
12%9%36%435%5%7%9%32%
2124
April1st to 15thDay
13%9%35%5%245%5%9%35%
2150
16th to endDay
15%10%36%5%35%45%7%33%
2310
May1st to 15thDay
13%7%35%5%2445%6%41%
2466
16th to endDay
19%425%5%25%5%6%9%31%
2616
June1st to 15thDay
10%5%246%25%6%6%7%41%
2553
16th to endDay
3212425%5%7%9%55%
2700
July1st to 15thDay
321232446%10%58%
2640
16th to endDay
321232445%8%62%
2651
August1st to 15thDay
5%2123245%6%9%56%
2340
16th to endDay
5%21232336%7%62%
2423
September1st to 15thDay
9%313313236%59%
2160
16th to endDay
11%413313346%56%
1972
October1st to 15thDay
13%8%25%4235%47%42%
1800
16th to endDay
9%7%25%4245%46%46%
1899
November1st to 15thDay
11%8%2442445%6%45%
1552
16th to endDay
8%8%25%5%345%6%6%44%
1560
December1st to 15thDay
7%11%5%9%6%36%5%5%7%33%
1560
16th to endDay
7%12%5%8%5%25%36%7%34%
1664
January1st to 15thNight
9%10%46%6%25%5%5%7%36%
2716
16th to endNight
9%10%47%43445%8%38%
2785
February1st to 15thNight
10%12%5%7%5%3445%7%32%
2472
16th to endNight
7%13%5%8%6%345%5%6%32%
2195
March1st to 15thNight
7%7%46%5%35%5%6%9%38%
2143
16th to endNight
13%9%36%6%25%5%6%7%32%
2100
April1st to 15thNight
13%8%46%5%35%5%47%35%
1798
16th to endNight
17%8%26%6%3346%5%34%
1650
May1st to 15thNight
14%6%35%4245%5%7%40%
1484
16th to endNight
15%6%25%5%346%6%8%34%
1497
June1st to 15thNight
11%5%26%435%6%7%8%38%
1404
16th to endNight
7%313424449%52%
1260
July1st to 15thNight
42123145%7%10%54%
1317
16th to endNight
322232447%7%58%
1515
August1st to 15thNight
4335%345%6%10%52%
1620
16th to endNight
5%31342436%7%56%
1778
September1st to 15thNight
10%413323345%56%
1800
16th to endNight
12%4332335%8%53%
1986
October1st to 15thNight
16%6%2443445%6%42%
2160
16th to endNight
9%5%2442345%8%49%
2313
November1st to 15thNight
10%8%36%42345%7%43%
2635
16th to endNight
8%9%45%5%3445%7%42%
2730
December1st to 15thNight
6%10%5%9%7%35%45%5%35%
2730
16th to endNight
8%12%47%5%25%5%5%6%35%
2912

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 15thDay
47%6%15%21%8%1
1553
16th to endDay
43%415%23%10%1
1801
February1st to 15thDay
50%5%16%20%7%
1710
16th to endDay
45%6%15%21%8%2
1565
March1st to 15thDay
41%5%16%26%9%
2051
16th to endDay
47%7%14%25%5%
2124
April1st to 15thDay
45%7%15%20%9%1
2150
16th to endDay
49%6%12%20%9%
2310
May1st to 15thDay
42%6%12%27%9%1
2466
16th to endDay
48%7%17%19%6%
2620
June1st to 15thDay
37%7%18%22%11%1
2553
16th to endDay
22%6%15%26%21%6%
2700
July1st to 15thDay
21%5%10%29%24%9%
2486
16th to endDay
18%37%22%31%16%
2484
August1st to 15thDay
22%5%10%24%26%11%
2325
16th to endDay
21%47%25%27%14%
2423
September1st to 15thDay
26%46%21%28%12%
2160
16th to endDay
28%410%27%23%5%
1972
October1st to 15thDay
41%5%12%22%14%2
1800
16th to endDay
37%5%12%26%14%4
1899
November1st to 15thDay
38%413%23%16%3
1552
16th to endDay
38%5%16%24%12%1
1560
December1st to 15thDay
49%5%16%20%6%
1560
16th to endDay
47%5%16%21%7%1
1664
January1st to 15thNight
47%5%16%20%7%1
2716
16th to endNight
44%5%13%23%12%1
2785
February1st to 15thNight
50%5%14%19%8%
2483
16th to endNight
51%5%13%20%7%2
2195
March1st to 15thNight
41%6%16%24%9%1
2143
16th to endNight
48%6%15%22%6%
2100
April1st to 15thNight
46%7%16%19%8%1
1798
16th to endNight
48%6%13%20%10%
1650
May1st to 15thNight
42%6%12%26%11%2
1485
16th to endNight
45%8%18%19%7%
1498
June1st to 15thNight
39%9%17%23%9%
1404
16th to endNight
28%5%15%26%20%4
1260
July1st to 15thNight
23%5%11%32%20%6%
1243
16th to endNight
21%5%10%19%30%12%
1419
August1st to 15thNight
24%6%14%24%21%8%
1611
16th to endNight
25%47%25%26%10%
1778
September1st to 15thNight
28%49%20%24%12%
1800
16th to endNight
29%410%26%22%5%
1986
October1st to 15thNight
42%6%11%22%15%2
2160
16th to endNight
33%414%27%15%4
2313
November1st to 15thNight
39%5%13%24%14%2
2635
16th to endNight
41%414%25%11%2
2730
December1st to 15thNight
48%6%13%22%7%1
2730
16th to endNight
47%5%14%23%7%1
2912

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 15thDay
16%5%26%37%11%3
135
16th to endDay
15%8%42%23%7%2
144
February1st to 15thDay
24%10%34%25%5%
135
16th to endDay
7%10%45%25%8%1
120
March1st to 15thDay
14%11%37%27%7%
268
16th to endDay
22%12%32%23%7%1
288
April1st to 15thDay
15%15%32%26%8%1
269
16th to endDay
26%11%27%22%9%2
270
May1st to 15thDay
18%6%24%26%19%4
423
16th to endDay
19%6%18%26%22%6%
517
June1st to 15thDay
10%7%23%34%21%3
495
16th to endDay
5%6%26%36%22%2
495
July1st to 15thDay
5%322%34%29%4
475
16th to endDay
3425%38%24%3
510
August1st to 15thDay
6%326%38%20%6%
315
16th to endDay
6%321%31%25%9%
332
September1st to 15thDay
11%6%25%34%15%6%
315
16th to endDay
15%5%20%28%25%3
273
October1st to 15thDay
17%425%37%12%2
135
16th to endDay
14%8%24%31%18%2
144
November1st to 15thDay
20%225%27%19%5%
135
16th to endDay
14%9%36%33%6%
135
December1st to 15thDay
12%5%38%35%8%
135
16th to endDay
21%9%34%24%7%2
144
January1st to 15thNight
15%7%41%27%6%
975
16th to endNight
15%8%44%23%7%
913
February1st to 15thNight
15%9%35%29%8%1
764
16th to endNight
15%7%34%32%9%1
684
March1st to 15thNight
11%6%35%33%12%
764
16th to endNight
19%5%28%32%12%
756
April1st to 15thNight
17%8%26%38%9%
539
16th to endNight
22%5%20%35%14%1
540
May1st to 15thNight
15%7%21%35%18%1
540
16th to endNight
19%319%36%19%1
563
June1st to 15thNight
12%5%28%37%14%1
504
16th to endNight
8%323%46%18%
360
July1st to 15thNight
5%219%48%24%
393
16th to endNight
5%226%47%17%
564
August1st to 15thNight
5%220%49%21%
539
16th to endNight
6%315%51%22%1
573
September1st to 15thNight
12%216%45%20%2
540
16th to endNight
14%215%40%25%2
680
October1st to 15thNight
19%5%22%36%14%2
720
16th to endNight
11%7%27%36%15%1
777
November1st to 15thNight
13%7%30%37%10%
950
16th to endNight
13%8%35%37%5%
975
December1st to 15thNight
12%7%40%31%7%
975
16th to endNight
14%8%37%29%9%
1040
Page generated at 2018-03-12 11:28:05. Version ac04

Copyright 2018 A. Danko


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