Backus Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Backus Mountain Observatory clear sky chart from 2005-11-05 to 2017-09-20.

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
39%
15719
16%
46932
11%
15680
44%
15680
19%
32926

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
18%
718
13%
2327
6%
716
25%
716
9%
1813
January16th to endNight
15%
768
13%
2496
5%
768
24%
768
11%
1899
February1st to 15thNight
18%
702
11%
2287
5%
701
25%
701
12%
1687
February16th to endNight
14%
631
12%
1884
5%
631
22%
631
11%
1463
March1st to 15thNight
24%
720
15%
1980
9%
720
29%
720
16%
1473
March16th to endNight
25%
768
13%
2112
8%
768
29%
768
15%
1606
April1st to 15thNight
30%
718
15%
1895
6%
718
36%
718
21%
1352
April16th to endNight
40%
592
17%
1672
9%
592
45%
592
22%
1157
May1st to 15thNight
37%
540
13%
1620
7%
540
43%
540
17%
1161
May16th to endNight
51%
561
14%
1684
10%
561
54%
561
22%
1173
June1st to 15thNight
63%
540
14%
1620
11%
540
67%
540
28%
1046
June16th to endNight
63%
540
15%
1620
12%
540
66%
540
28%
1020
July1st to 15thNight
56%
510
10%
1530
9%
510
57%
510
21%
1065
July16th to endNight
73%
564
15%
1595
13%
531
75%
531
31%
987
August1st to 15thNight
64%
539
18%
1609
15%
536
64%
536
26%
1079
August16th to endNight
69%
639
20%
1785
17%
639
70%
639
32%
1146
September1st to 15thNight
60%
720
22%
1923
17%
720
62%
720
31%
1250
September16th to endNight
53%
677
20%
1870
17%
677
56%
677
28%
1246
October1st to 15thNight
47%
660
24%
1950
21%
660
53%
660
26%
1265
October16th to endNight
36%
704
22%
2112
15%
704
45%
704
21%
1393
November1st to 15thNight
45%
700
23%
2185
19%
700
52%
700
25%
1371
November16th to endNight
31%
720
19%
2340
12%
720
40%
720
18%
1563
December1st to 15thNight
21%
720
12%
2340
8%
720
26%
720
11%
1814
December16th to endNight
19%
768
15%
2496
7%
768
29%
768
11%
1897

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
16%9%47%6%35%5%5%6%31%
47894
Night
14%9%47%5%3445%6%34%
47125
Cloud CoverDay
16%9%47%6%35%5%5%7%31%
47904
Night
13%9%47%5%3445%6%35%
47129
TransparencyDay
51%7%11%17%11%1
47605
Night
47%6%11%19%12%2
46932
SeeingDay
22%10%20%21%17%7%
5558
Night
21%12%21%27%15%1
15725

By Month

Percentage Cloud Cover NE (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
16%11%6%14%8%25%325%22%
1611
16th to endDay
17%16%7%7%5%2333427%
1728
February1st to 15thDay
16%10%7%10%5%34446%25%
1575
16th to endDay
16%14%6%10%5%25%33425%
1589
March1st to 15thDay
18%9%5%7%323445%33%
1969
16th to endDay
18%11%47%5%34445%31%
2112
April1st to 15thDay
16%9%44424345%39%
2051
16th to endDay
17%7%35%6%24446%36%
2298
May1st to 15thDay
22%7%35%6%35%5%5%6%26%
2331
16th to endDay
15%7%26%6%46%5%6%7%30%
2431
June1st to 15thDay
11%6%26%6%46%5%7%9%32%
2340
16th to endDay
12%5%37%7%36%6%5%8%33%
2340
July1st to 15thDay
15%7%38%7%35%6%6%8%27%
2340
16th to endDay
12%6%37%7%5%7%6%6%9%27%
2465
August1st to 15thDay
15%7%36%8%36%6%8%8%23%
2340
16th to endDay
11%6%26%8%46%7%7%8%29%
2403
September1st to 15thDay
14%7%27%6%35%5%5%6%36%
2034
16th to endDay
17%7%27%5%45%5%45%32%
1859
October1st to 15thDay
18%7%35%3235%5%7%36%
1680
16th to endDay
18%12%24323335%39%
1760
November1st to 15thDay
12%10%24324346%43%
1670
16th to endDay
17%11%45%5%24345%34%
1620
December1st to 15thDay
21%12%5%7%5%24436%25%
1620
16th to endDay
21%14%6%8%5%23336%24%
1728
January1st to 15thNight
17%10%6%14%8%3434422%
2327
16th to endNight
17%15%7%9%6%3433424%
2496
February1st to 15thNight
14%12%6%9%7%33345%27%
2285
16th to endNight
15%12%7%10%7%35%45%5%22%
1884
March1st to 15thNight
18%11%47%6%3344431%
1968
16th to endNight
16%13%46%6%25%435%31%
2112
April1st to 15thNight
15%10%35%6%345%48%34%
1895
16th to endNight
13%6%36%7%445%5%7%36%
1672
May1st to 15thNight
18%8%35%5%35%447%33%
1620
16th to endNight
11%7%36%5%35%5%5%7%37%
1682
June1st to 15thNight
10%4145%246%7%9%42%
1620
16th to endNight
10%5%244247%7%9%43%
1620
July1st to 15thNight
11%6%36%6%4446%7%36%
1620
16th to endNight
7%423435%7%9%10%40%
1703
August1st to 15thNight
10%5%25%5%35%5%6%8%39%
1620
16th to endNight
9%6%243345%7%8%44%
1785
September1st to 15thNight
11%5%244345%5%6%45%
1923
16th to endNight
12%6%25%5%3445%7%41%
1870
October1st to 15thNight
15%6%25%433446%43%
1950
16th to endNight
15%10%26%423345%41%
2112
November1st to 15thNight
14%7%35%424335%44%
2185
16th to endNight
14%9%5%6%5%24435%37%
2340
December1st to 15thNight
19%13%5%10%7%25%335%22%
2340
16th to endNight
18%14%6%8%6%34335%25%
2496

Percentage Cloud Cover SE (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
15%12%5%13%9%2443423%
1611
16th to endDay
17%14%8%7%5%23335%28%
1728
February1st to 15thDay
15%10%7%10%6%3435%6%25%
1575
16th to endDay
16%13%6%11%5%25%34426%
1589
March1st to 15thDay
17%9%5%7%424445%34%
1980
16th to endDay
17%11%5%7%5%24445%31%
2111
April1st to 15thDay
15%10%45%324346%39%
2051
16th to endDay
18%6%36%5%35%446%36%
2298
May1st to 15thDay
21%7%35%7%35%5%5%6%26%
2331
16th to endDay
14%7%26%6%46%5%6%8%30%
2431
June1st to 15thDay
12%5%27%6%46%5%7%9%32%
2340
16th to endDay
11%6%37%7%46%5%5%9%33%
2340
July1st to 15thDay
15%7%37%7%35%6%6%9%26%
2340
16th to endDay
11%7%37%7%47%7%6%8%27%
2465
August1st to 15thDay
16%7%36%7%46%7%7%8%23%
2340
16th to endDay
11%5%36%8%5%6%7%7%9%28%
2403
September1st to 15thDay
14%7%26%7%345%46%36%
2034
16th to endDay
16%8%26%6%35%5%45%32%
1859
October1st to 15thDay
18%7%26%5%1345%7%37%
1680
16th to endDay
18%12%24323435%39%
1760
November1st to 15thDay
12%10%34323347%43%
1670
16th to endDay
17%11%45%5%25%345%34%
1620
December1st to 15thDay
21%11%5%7%5%24445%26%
1620
16th to endDay
21%13%6%9%5%23445%24%
1728
January1st to 15thNight
17%9%6%14%8%3443423%
2327
16th to endNight
16%14%7%9%6%3433425%
2496
February1st to 15thNight
14%12%6%9%7%34346%28%
2278
16th to endNight
14%12%6%10%7%36%445%23%
1884
March1st to 15thNight
17%10%46%7%3435%431%
1980
16th to endNight
15%12%46%6%35%5%35%31%
2112
April1st to 15thNight
14%9%45%6%3445%9%34%
1895
16th to endNight
13%6%27%5%45%45%7%37%
1672
May1st to 15thNight
18%8%36%5%34446%34%
1619
16th to endNight
10%7%36%6%36%5%48%37%
1682
June1st to 15thNight
9%5%145%245%7%10%42%
1620
16th to endNight
9%5%25%4246%7%10%42%
1620
July1st to 15thNight
10%6%46%6%5%5%47%7%36%
1620
16th to endNight
6%424435%7%8%11%41%
1703
August1st to 15thNight
10%6%25%5%35%6%6%8%39%
1620
16th to endNight
9%6%244245%6%8%44%
1785
September1st to 15thNight
10%6%15%5%345%5%6%46%
1923
16th to endNight
12%6%35%5%3445%7%41%
1870
October1st to 15thNight
14%6%25%434346%43%
1950
16th to endNight
13%11%26%423345%41%
2112
November1st to 15thNight
13%7%34424435%44%
2185
16th to endNight
14%9%5%6%5%24445%37%
2340
December1st to 15thNight
18%12%5%9%7%35%335%23%
2340
16th to endNight
17%13%7%8%6%34335%25%
2496

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
64%48%13%8%
1612
16th to endDay
59%39%18%7%1
1728
February1st to 15thDay
57%311%18%8%
1577
16th to endDay
61%49%15%8%
1589
March1st to 15thDay
51%5%11%20%9%2
1980
16th to endDay
53%7%8%20%9%1
2112
April1st to 15thDay
45%411%21%13%3
2051
16th to endDay
47%6%10%19%13%3
2298
May1st to 15thDay
55%8%11%14%8%1
2331
16th to endDay
50%8%13%17%8%1
2435
June1st to 15thDay
45%11%12%19%12%
2340
16th to endDay
46%10%12%16%13%1
2340
July1st to 15thDay
51%9%12%12%11%2
2201
16th to endDay
48%10%13%15%10%1
2310
August1st to 15thDay
53%9%13%14%7%
2327
16th to endDay
47%9%14%17%9%1
2403
September1st to 15thDay
46%7%10%17%16%1
2034
16th to endDay
50%6%10%16%14%
1859
October1st to 15thDay
45%5%11%21%13%2
1680
16th to endDay
47%5%8%23%13%2
1760
November1st to 15thDay
41%39%19%21%3
1670
16th to endDay
50%49%19%15%
1620
December1st to 15thDay
58%5%9%15%9%2
1620
16th to endDay
60%5%9%16%7%
1728
January1st to 15thNight
63%48%12%9%2
2327
16th to endNight
61%38%16%8%1
2496
February1st to 15thNight
57%410%16%10%1
2287
16th to endNight
59%6%10%14%7%1
1884
March1st to 15thNight
54%49%21%8%1
1980
16th to endNight
54%7%8%16%11%
2112
April1st to 15thNight
47%6%10%20%12%2
1895
16th to endNight
44%6%12%17%13%4
1672
May1st to 15thNight
50%7%12%17%11%
1620
16th to endNight
43%9%14%23%6%1
1684
June1st to 15thNight
35%10%18%21%13%
1620
16th to endNight
33%11%16%23%13%1
1620
July1st to 15thNight
44%9%14%15%12%3
1530
16th to endNight
29%13%18%22%14%1
1595
August1st to 15thNight
38%10%14%22%13%
1609
16th to endNight
34%9%15%25%14%1
1785
September1st to 15thNight
36%8%14%20%17%3
1923
16th to endNight
39%8%11%21%18%
1870
October1st to 15thNight
41%5%11%23%15%3
1950
16th to endNight
44%49%23%14%2
2112
November1st to 15thNight
41%310%17%21%5%
2185
16th to endNight
47%49%20%15%2
2340
December1st to 15thNight
61%5%9%13%8%1
2340
16th to endNight
60%410%16%7%
2496

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
34%24%26%6%7%
90
16th to endDay
50%15%19%13%1
96
February1st to 15thDay
33%36%23%5%1
90
16th to endDay
31%18%34%13%2
93
March1st to 15thDay
26%19%29%17%6%
270
16th to endDay
23%21%27%18%7%1
288
April1st to 15thDay
26%14%21%26%10%
270
16th to endDay
23%11%24%18%16%5%
270
May1st to 15thDay
32%15%22%14%10%5%
288
16th to endDay
20%8%20%17%19%12%
331
June1st to 15thDay
13%6%19%25%24%11%
315
16th to endDay
13%9%24%25%17%9%
315
July1st to 15thDay
22%422%23%15%12%
305
16th to endDay
11%312%27%28%17%
326
August1st to 15thDay
19%317%21%22%15%
313
16th to endDay
12%215%25%28%15%
332
September1st to 15thDay
17%6%12%25%27%10%
315
16th to endDay
20%8%16%21%25%8%
270
October1st to 15thDay
21%7%19%28%18%5%
255
16th to endDay
27%11%18%22%18%2
272
November1st to 15thDay
24%7%20%25%19%2
178
16th to endDay
30%21%20%20%7%1
90
December1st to 15thDay
33%21%25%11%6%2
90
16th to endDay
47%16%20%14%
96
January1st to 15thNight
30%22%24%18%3
720
16th to endNight
31%20%29%17%1
768
February1st to 15thNight
29%19%27%22%1
704
16th to endNight
24%25%31%17%
631
March1st to 15thNight
28%16%26%23%4
720
16th to endNight
25%16%27%21%7%
768
April1st to 15thNight
23%17%25%26%6%
718
16th to endNight
16%13%23%32%11%1
592
May1st to 15thNight
23%11%21%28%13%1
540
16th to endNight
13%420%31%25%4
563
June1st to 15thNight
10%315%35%31%4
540
16th to endNight
11%115%40%28%2
540
July1st to 15thNight
13%214%34%30%2
510
16th to endNight
8%29%28%42%8%
564
August1st to 15thNight
14%10%35%36%2
539
16th to endNight
12%18%37%35%5%
639
September1st to 15thNight
13%415%30%33%4
720
16th to endNight
15%7%16%30%27%2
677
October1st to 15thNight
17%9%20%30%19%3
660
16th to endNight
23%13%23%26%12%1
704
November1st to 15thNight
19%11%20%32%14%1
700
16th to endNight
24%16%23%25%9%
720
December1st to 15thNight
30%17%24%20%6%
720
16th to endNight
28%18%27%22%2
768
Page generated at 2017-09-21 07:18:24. Version ac04

Copyright 2017 A. Danko


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