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 2017-05-24.

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%
15520
18%
45598
12%
15481
48%
15481
26%
29530

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
27%
896
14%
2506
8%
895
32%
895
21%
1762
January16th to endNight
24%
843
19%
2571
8%
843
36%
843
22%
1723
February1st to 15thNight
30%
697
12%
2288
8%
697
37%
697
17%
1571
February16th to endNight
31%
632
11%
2028
8%
632
37%
632
17%
1443
March1st to 15thNight
36%
720
15%
2019
7%
720
44%
720
27%
1347
March16th to endNight
38%
756
12%
2100
6%
756
43%
756
22%
1498
April1st to 15thNight
40%
539
12%
1798
10%
539
44%
539
19%
1223
April16th to endNight
43%
540
14%
1650
12%
540
46%
540
20%
1147
May1st to 15thNight
48%
539
16%
1484
12%
539
50%
539
26%
1004
May16th to endNight
50%
542
10%
1442
8%
540
51%
540
23%
1037
June1st to 15thNight
46%
462
11%
1287
6%
462
49%
462
24%
868
June16th to endNight
59%
330
16%
1155
12%
330
64%
330
36%
608
July1st to 15thNight
66%
358
17%
1133
16%
358
69%
358
43%
581
July16th to endNight
63%
522
21%
1307
16%
489
66%
489
51%
697
August1st to 15thNight
64%
494
21%
1476
17%
491
69%
491
41%
776
August16th to endNight
67%
525
28%
1629
25%
525
70%
525
42%
859
September1st to 15thNight
64%
495
25%
1650
25%
495
67%
495
37%
888
September16th to endNight
62%
623
28%
1820
23%
623
66%
623
41%
1030
October1st to 15thNight
47%
660
21%
1980
15%
660
53%
660
25%
1312
October16th to endNight
48%
712
26%
2120
16%
712
55%
712
32%
1272
November1st to 15thNight
41%
875
21%
2427
14%
875
46%
875
29%
1563
November16th to endNight
37%
900
21%
2520
12%
900
45%
900
27%
1655
December1st to 15thNight
28%
900
13%
2520
5%
900
34%
900
19%
1817
December16th to endNight
30%
960
18%
2688
6%
960
39%
960
23%
1849

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%6%25%4345%6%8%44%
46700
Night
9%7%35%5%3445%7%41%
45763
Cloud CoverDay
9%6%25%42445%8%44%
46700
Night
10%8%35%5%3445%7%41%
45765
TransparencyDay
37%5%13%23%14%4
46369
Night
40%5%14%23%13%3
45599
SeeingDay
12%7%27%31%17%3
5545
Night
13%6%29%36%13%
15532

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%35%
1433
16th to endDay
6%11%36%4345%5%8%39%
1662
February1st to 15thDay
6%10%5%8%5%36%5%5%9%32%
1575
16th to endDay
5%10%5%8%6%346%7%8%32%
1446
March1st to 15thDay
7%8%36%6%3446%8%39%
1939
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
20%5%25%5%35%5%6%8%31%
2515
June1st to 15thDay
11%5%25%5%35%6%7%7%40%
2340
16th to endDay
3112335%5%6%9%56%
2475
July1st to 15thDay
31133245%6%10%57%
2420
16th to endDay
32222445%8%62%
2456
August1st to 15thDay
5%2133246%6%8%56%
2145
16th to endDay
5%21332346%6%61%
2220
September1st to 15thDay
9%413312346%59%
1980
16th to endDay
10%313213446%58%
1808
October1st to 15thDay
13%8%3443445%6%41%
1650
16th to endDay
9%7%25%425%446%46%
1741
November1st to 15thDay
10%6%244235%6%6%47%
1430
16th to endDay
7%9%345%25%46%6%43%
1440
December1st to 15thDay
7%10%6%9%7%446%5%7%31%
1440
16th to endDay
5%11%5%7%6%45%46%9%34%
1536
January1st to 15thNight
9%10%5%6%6%35%45%7%35%
2506
16th to endNight
9%10%47%5%2435%7%38%
2571
February1st to 15thNight
8%11%5%7%5%35%45%7%33%
2277
16th to endNight
7%13%6%8%6%345%5%7%31%
2028
March1st to 15thNight
6%7%46%6%45%6%6%9%38%
2019
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%9%33%
1442
June1st to 15thNight
12%425%5%35%5%7%8%38%
1287
16th to endNight
8%32342446%7%52%
1155
July1st to 15thNight
42133245%7%8%56%
1207
16th to endNight
321232447%8%57%
1403
August1st to 15thNight
33135%25%6%6%11%51%
1485
16th to endNight
6%31342445%7%56%
1629
September1st to 15thNight
10%423313445%55%
1650
16th to endNight
11%313312349%54%
1820
October1st to 15thNight
17%5%245%3435%5%41%
1980
16th to endNight
9%5%2442445%8%47%
2120
November1st to 15thNight
9%7%25%5%3445%7%43%
2427
16th to endNight
8%9%45%5%3445%7%41%
2520
December1st to 15thNight
5%10%5%10%7%45%45%6%34%
2520
16th to endNight
7%11%46%5%35%5%47%37%
2688

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
7%11%47%7%2346%7%35%
1432
16th to endDay
6%11%36%5%345%5%9%38%
1662
February1st to 15thDay
6%11%5%6%5%47%5%5%8%32%
1575
16th to endDay
6%11%48%6%35%5%6%8%32%
1446
March1st to 15thDay
7%8%46%5%345%5%8%39%
1939
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
20%425%5%26%5%6%9%31%
2515
June1st to 15thDay
11%5%246%35%6%6%7%40%
2340
16th to endDay
32124245%7%9%56%
2475
July1st to 15thDay
321332446%9%58%
2420
16th to endDay
321232435%7%62%
2456
August1st to 15thDay
5%2123245%6%9%56%
2145
16th to endDay
5%21232336%6%61%
2220
September1st to 15thDay
9%313313236%58%
1980
16th to endDay
10%413312347%57%
1808
October1st to 15thDay
14%8%24423447%41%
1650
16th to endDay
10%7%25%5%245%46%45%
1741
November1st to 15thDay
10%7%2442445%7%47%
1430
16th to endDay
7%9%25%5%345%6%6%43%
1440
December1st to 15thDay
7%11%5%9%7%36%5%5%6%31%
1440
16th to endDay
7%11%48%5%25%46%7%35%
1536
January1st to 15thNight
9%10%46%6%35%45%6%35%
2506
16th to endNight
9%10%47%5%3435%8%38%
2571
February1st to 15thNight
9%12%5%7%5%3445%7%33%
2277
16th to endNight
8%13%6%8%6%345%5%6%31%
2028
March1st to 15thNight
7%6%47%5%35%5%6%9%37%
2019
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
16%6%25%6%346%6%8%33%
1441
June1st to 15thNight
12%5%26%5%25%5%7%8%39%
1287
16th to endNight
7%414424448%52%
1155
July1st to 15thNight
5%3123145%6%9%56%
1207
16th to endNight
322232347%7%58%
1403
August1st to 15thNight
3335%345%6%10%52%
1485
16th to endNight
6%31342436%7%56%
1629
September1st to 15thNight
10%423223346%56%
1650
16th to endNight
12%4332325%8%54%
1820
October1st to 15thNight
17%6%2442445%6%41%
1980
16th to endNight
9%5%2442445%8%47%
2120
November1st to 15thNight
10%8%35%42445%7%43%
2427
16th to endNight
8%9%45%5%3445%7%41%
2520
December1st to 15thNight
5%10%5%10%7%35%45%5%34%
2520
16th to endNight
8%11%46%5%35%5%5%7%37%
2688

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
45%6%15%21%8%1
1433
16th to endDay
42%416%24%10%2
1662
February1st to 15thDay
48%5%16%20%7%
1575
16th to endDay
46%6%16%21%6%2
1446
March1st to 15thDay
41%5%16%25%9%
1939
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%18%7%
2519
June1st to 15thDay
39%8%18%21%10%1
2340
16th to endDay
21%6%15%26%21%7%
2475
July1st to 15thDay
21%5%10%29%22%10%
2266
16th to endDay
19%47%22%30%16%
2289
August1st to 15thDay
22%5%9%24%25%12%
2130
16th to endDay
21%47%24%27%14%
2220
September1st to 15thDay
26%47%22%28%11%
1980
16th to endDay
26%411%27%24%6%
1808
October1st to 15thDay
42%6%12%22%12%2
1650
16th to endDay
37%5%12%26%13%2
1741
November1st to 15thDay
36%5%13%24%17%3
1430
16th to endDay
39%5%17%23%12%1
1440
December1st to 15thDay
51%5%16%20%5%
1440
16th to endDay
46%5%16%22%7%1
1536
January1st to 15thNight
48%5%16%20%7%1
2506
16th to endNight
45%5%12%23%11%1
2571
February1st to 15thNight
49%5%15%19%8%
2288
16th to endNight
52%5%14%19%6%2
2028
March1st to 15thNight
41%6%16%23%9%1
2019
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
46%8%19%17%7%
1442
June1st to 15thNight
40%9%17%22%9%
1287
16th to endNight
27%5%15%25%20%4
1155
July1st to 15thNight
23%5%12%31%19%7%
1133
16th to endNight
20%5%11%19%31%12%
1307
August1st to 15thNight
24%6%15%24%20%9%
1476
16th to endNight
25%47%25%26%11%
1629
September1st to 15thNight
29%49%21%24%11%
1650
16th to endNight
27%410%25%24%6%
1820
October1st to 15thNight
42%6%11%22%13%2
1980
16th to endNight
34%5%14%27%14%3
2120
November1st to 15thNight
39%414%24%14%2
2427
16th to endNight
41%414%24%11%2
2520
December1st to 15thNight
50%6%14%21%7%
2520
16th to endNight
45%5%14%24%8%1
2688

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
13%5%24%36%14%5%
90
16th to endDay
16%8%37%25%11%1
96
February1st to 15thDay
12%13%40%28%5%
90
16th to endDay
8%14%40%28%7%
81
March1st to 15thDay
14%13%38%25%8%
229
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%7%19%25%21%6%
481
June1st to 15thDay
11%6%22%35%21%3
420
16th to endDay
46%27%36%22%2
420
July1st to 15thDay
6%324%35%26%3
400
16th to endDay
35%24%39%24%3
441
August1st to 15thDay
7%329%37%18%3
255
16th to endDay
7%425%33%21%6%
268
September1st to 15thDay
11%7%28%35%13%2
255
16th to endDay
11%5%23%31%24%2
217
October1st to 15thDay
20%220%38%15%3
90
16th to endDay
13%418%32%27%4
96
November1st to 15thDay
8%228%24%26%8%
90
16th to endDay
15%8%32%36%6%
90
December1st to 15thDay
14%438%33%8%
90
16th to endDay
18%7%35%26%9%3
96
January1st to 15thNight
15%8%41%26%6%
900
16th to endNight
15%9%43%23%7%
843
February1st to 15thNight
13%9%35%30%9%1
704
16th to endNight
15%8%34%32%8%1
632
March1st to 15thNight
10%6%37%32%12%
720
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%419%35%20%1
542
June1st to 15thNight
12%5%28%37%14%1
462
16th to endNight
8%323%45%19%
330
July1st to 15thNight
5%219%48%23%
358
16th to endNight
4225%48%17%
522
August1st to 15thNight
5%221%49%20%
494
16th to endNight
6%315%52%20%
525
September1st to 15thNight
12%217%46%19%1
495
16th to endNight
13%215%40%25%2
623
October1st to 15thNight
19%5%21%35%15%2
660
16th to endNight
12%6%26%36%16%1
712
November1st to 15thNight
12%8%31%36%10%
875
16th to endNight
13%8%34%38%5%
900
December1st to 15thNight
12%7%39%31%7%1
900
16th to endNight
13%8%36%30%9%
960
Page generated at 2017-05-25 11:27:39. Version ac04

Copyright 2017 A. Danko


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