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 2018-11-30.

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%
17318
16%
51724
11%
17279
44%
17279
19%
36362

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
19%
778
12%
2522
6%
776
26%
776
10%
1950
January16th to endNight
16%
832
13%
2704
6%
832
25%
832
11%
2055
February1st to 15thNight
18%
762
10%
2482
5%
761
25%
761
12%
1850
February16th to endNight
15%
683
12%
2039
5%
683
22%
683
10%
1593
March1st to 15thNight
23%
780
15%
2145
8%
780
28%
780
15%
1599
March16th to endNight
24%
832
13%
2288
8%
832
29%
832
15%
1741
April1st to 15thNight
30%
778
15%
2053
7%
778
35%
778
20%
1469
April16th to endNight
38%
633
17%
1789
9%
633
43%
633
21%
1245
May1st to 15thNight
40%
585
14%
1755
8%
585
45%
585
19%
1225
May16th to endNight
49%
609
13%
1828
10%
609
52%
609
21%
1294
June1st to 15thNight
63%
585
14%
1755
12%
585
66%
585
27%
1139
June16th to endNight
62%
585
15%
1755
13%
585
65%
585
28%
1108
July1st to 15thNight
59%
555
11%
1665
11%
555
59%
555
23%
1132
July16th to endNight
71%
612
14%
1739
12%
579
72%
579
29%
1090
August1st to 15thNight
64%
575
19%
1715
16%
572
65%
572
26%
1148
August16th to endNight
69%
693
19%
1935
16%
693
70%
693
32%
1239
September1st to 15thNight
61%
780
21%
2083
16%
780
62%
780
29%
1375
September16th to endNight
54%
777
20%
2145
17%
777
56%
777
28%
1428
October1st to 15thNight
49%
780
22%
2304
20%
780
54%
780
25%
1502
October16th to endNight
37%
832
22%
2496
16%
832
46%
832
22%
1638
November1st to 15thNight
41%
820
22%
2559
17%
820
48%
820
22%
1674
November16th to endNight
32%
840
19%
2729
13%
840
40%
840
17%
1833
December1st to 15thNight
20%
780
11%
2535
7%
780
25%
780
10%
1970
December16th to endNight
18%
832
14%
2704
6%
832
28%
832
11%
2065

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%45%6%31%
52568
Night
14%9%47%5%3445%6%34%
51917
Cloud CoverDay
16%9%47%6%35%5%5%6%31%
52578
Night
13%9%47%6%3445%6%34%
51921
TransparencyDay
51%7%11%17%11%1
52279
Night
48%6%11%18%12%1
51724
SeeingDay
23%11%21%21%16%6%
7145
Night
21%12%21%27%16%1
17324

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
15%11%6%13%9%35%335%22%
1746
16th to endDay
17%15%7%7%5%2333428%
1872
February1st to 15thDay
17%10%7%11%5%33446%24%
1710
16th to endDay
19%13%6%10%5%25%33425%
1720
March1st to 15thDay
18%10%5%8%323445%32%
2134
16th to endDay
19%11%46%5%34445%31%
2288
April1st to 15thDay
17%9%44324346%38%
2223
16th to endDay
17%8%35%5%24446%36%
2457
May1st to 15thDay
21%7%35%6%35%5%5%6%28%
2526
16th to endDay
16%7%36%6%46%5%6%7%28%
2639
June1st to 15thDay
11%6%26%6%46%5%7%9%32%
2535
16th to endDay
12%5%37%6%46%6%6%8%33%
2535
July1st to 15thDay
14%7%37%6%35%6%6%8%29%
2535
16th to endDay
12%6%37%7%5%7%6%6%9%26%
2673
August1st to 15thDay
16%7%36%8%36%6%8%8%23%
2487
16th to endDay
11%6%26%8%46%7%7%8%28%
2604
September1st to 15thDay
14%7%27%6%35%5%5%6%35%
2204
16th to endDay
17%7%27%5%45%5%45%33%
2145
October1st to 15thDay
18%7%35%42345%7%37%
1986
16th to endDay
18%11%24323335%40%
2080
November1st to 15thDay
15%11%34333346%39%
1956
16th to endDay
16%11%46%5%24345%35%
1886
December1st to 15thDay
21%12%5%7%5%24436%26%
1755
16th to endDay
20%14%6%8%5%23336%24%
1872
January1st to 15thNight
17%10%6%13%8%4444422%
2522
16th to endNight
17%15%7%9%6%2433425%
2704
February1st to 15thNight
15%12%6%9%7%33345%26%
2480
16th to endNight
16%12%6%10%7%35%45%5%22%
2039
March1st to 15thNight
17%11%47%6%3344430%
2133
16th to endNight
16%13%46%6%25%445%30%
2288
April1st to 15thNight
15%9%35%6%345%48%33%
2053
16th to endNight
14%6%36%7%44447%36%
1789
May1st to 15thNight
17%7%35%5%34447%35%
1755
16th to endNight
12%8%36%6%35%5%5%7%36%
1826
June1st to 15thNight
10%415%5%246%7%10%42%
1755
16th to endNight
9%5%244246%6%9%42%
1755
July1st to 15thNight
10%6%36%6%4446%7%38%
1755
16th to endNight
6%4245%35%7%9%10%40%
1847
August1st to 15thNight
10%5%25%5%35%5%6%8%39%
1726
16th to endNight
8%6%24425%5%7%9%44%
1935
September1st to 15thNight
11%5%24435%5%5%6%44%
2083
16th to endNight
13%6%25%42445%7%41%
2145
October1st to 15thNight
14%6%244345%5%6%42%
2304
16th to endNight
15%10%25%423335%41%
2496
November1st to 15thNight
16%8%35%424335%40%
2559
16th to endNight
14%10%5%7%5%24335%36%
2729
December1st to 15thNight
18%13%5%10%7%35%345%21%
2535
16th to endNight
18%14%7%8%6%34335%24%
2704

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%11%5%13%10%3443423%
1746
16th to endDay
16%14%8%7%6%2333429%
1872
February1st to 15thDay
16%10%7%10%6%2435%6%25%
1710
16th to endDay
18%13%6%10%5%2433426%
1720
March1st to 15thDay
17%9%6%8%424445%33%
2145
16th to endDay
18%12%46%5%24445%31%
2287
April1st to 15thDay
16%9%45%324346%39%
2223
16th to endDay
17%7%36%5%24446%36%
2457
May1st to 15thDay
20%7%35%6%35%5%5%7%27%
2526
16th to endDay
15%8%27%6%46%5%6%7%29%
2639
June1st to 15thDay
11%5%27%6%45%5%7%9%32%
2535
16th to endDay
11%6%37%7%46%5%5%8%32%
2535
July1st to 15thDay
14%7%37%7%35%5%6%8%29%
2535
16th to endDay
11%7%37%7%47%7%6%8%26%
2673
August1st to 15thDay
16%7%36%7%46%7%7%8%23%
2487
16th to endDay
11%5%36%8%5%6%7%6%9%28%
2604
September1st to 15thDay
14%7%26%7%345%46%35%
2204
16th to endDay
17%8%36%6%35%5%45%32%
2145
October1st to 15thDay
17%7%26%5%2345%7%37%
1986
16th to endDay
18%11%24323335%41%
2080
November1st to 15thDay
15%11%34433347%39%
1956
16th to endDay
16%10%45%5%24345%35%
1886
December1st to 15thDay
20%12%47%5%24445%26%
1755
16th to endDay
20%13%6%9%5%23446%24%
1872
January1st to 15thNight
16%9%6%14%8%3444423%
2522
16th to endNight
16%13%7%9%6%3433425%
2704
February1st to 15thNight
15%12%6%9%7%34345%27%
2473
16th to endNight
15%12%6%10%7%35%45%5%22%
2039
March1st to 15thNight
17%10%47%7%3444431%
2145
16th to endNight
15%12%46%6%35%5%35%31%
2288
April1st to 15thNight
14%9%45%6%3445%9%33%
2053
16th to endNight
13%6%37%5%45%45%6%37%
1789
May1st to 15thNight
17%7%36%5%34446%36%
1754
16th to endNight
11%8%37%6%35%5%5%8%35%
1826
June1st to 15thNight
9%5%145%245%7%10%41%
1755
16th to endNight
9%5%25%4246%6%10%42%
1755
July1st to 15thNight
9%6%36%6%4446%7%39%
1755
16th to endNight
5%424435%7%8%11%40%
1847
August1st to 15thNight
10%6%25%5%35%6%6%8%39%
1726
16th to endNight
8%5%244345%7%9%44%
1935
September1st to 15thNight
11%6%145%345%5%7%44%
2083
16th to endNight
13%6%35%5%2445%7%41%
2145
October1st to 15thNight
14%6%25%435%45%6%42%
2304
16th to endNight
13%10%26%423345%42%
2496
November1st to 15thNight
15%7%35%5%24345%41%
2559
16th to endNight
13%9%5%7%5%24445%37%
2729
December1st to 15thNight
18%12%5%10%8%35%335%22%
2535
16th to endNight
17%14%7%8%6%34335%24%
2704

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
63%48%13%8%
1747
16th to endDay
58%39%18%8%1
1872
February1st to 15thDay
58%310%18%7%
1712
16th to endDay
61%49%15%8%
1720
March1st to 15thDay
52%5%10%19%9%2
2145
16th to endDay
53%7%8%19%10%1
2288
April1st to 15thDay
45%411%21%12%3
2223
16th to endDay
47%6%9%18%13%3
2457
May1st to 15thDay
53%8%12%15%8%1
2526
16th to endDay
51%9%13%16%8%1
2643
June1st to 15thDay
44%10%12%19%12%
2535
16th to endDay
46%10%12%16%13%1
2535
July1st to 15thDay
49%9%12%14%12%2
2396
16th to endDay
49%10%13%15%10%1
2518
August1st to 15thDay
53%10%13%14%7%
2474
16th to endDay
47%10%14%17%9%1
2604
September1st to 15thDay
47%7%11%16%15%1
2204
16th to endDay
50%6%10%16%14%
2145
October1st to 15thDay
45%5%11%21%13%2
1986
16th to endDay
45%48%24%14%2
2080
November1st to 15thDay
45%49%18%18%3
1956
16th to endDay
49%49%20%14%
1886
December1st to 15thDay
58%49%15%9%2
1755
16th to endDay
60%5%10%16%7%
1872
January1st to 15thNight
62%48%12%9%1
2522
16th to endNight
61%48%16%8%1
2704
February1st to 15thNight
58%410%16%9%1
2482
16th to endNight
60%5%10%14%7%1
2039
March1st to 15thNight
54%49%20%9%1
2145
16th to endNight
54%7%8%17%11%
2288
April1st to 15thNight
47%6%11%20%11%2
2053
16th to endNight
45%6%12%16%14%4
1789
May1st to 15thNight
48%7%13%18%11%1
1755
16th to endNight
45%9%14%22%6%1
1828
June1st to 15thNight
35%11%18%21%13%
1755
16th to endNight
34%10%16%23%14%1
1755
July1st to 15thNight
42%9%14%15%13%4
1665
16th to endNight
30%13%18%22%13%1
1739
August1st to 15thNight
38%10%14%23%12%
1715
16th to endNight
34%9%15%25%13%1
1935
September1st to 15thNight
37%8%15%19%15%3
2083
16th to endNight
39%7%11%23%16%
2145
October1st to 15thNight
41%5%12%22%14%3
2304
16th to endNight
44%48%23%15%2
2496
November1st to 15thNight
44%49%17%18%5%
2559
16th to endNight
48%410%18%16%2
2729
December1st to 15thNight
61%5%9%13%8%1
2535
16th to endNight
60%410%15%7%
2704

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
30%25%27%11%5%
135
16th to endDay
43%16%23%14%1
144
February1st to 15thDay
40%30%21%6%
135
16th to endDay
37%15%32%13%1
133
March1st to 15thDay
27%21%29%15%5%
330
16th to endDay
28%21%25%16%6%1
352
April1st to 15thDay
28%14%24%23%8%
330
16th to endDay
25%12%24%19%13%5%
319
May1st to 15thDay
28%14%22%17%11%5%
348
16th to endDay
22%8%22%17%20%10%
395
June1st to 15thDay
13%6%20%26%22%10%
375
16th to endDay
14%8%23%25%17%9%
375
July1st to 15thDay
19%320%24%19%12%
365
16th to endDay
12%315%28%24%15%
390
August1st to 15thDay
19%317%21%20%16%
358
16th to endDay
12%215%26%28%15%
396
September1st to 15thDay
17%6%13%24%27%12%
375
16th to endDay
21%7%15%24%23%6%
374
October1st to 15thDay
19%7%18%26%22%5%
375
16th to endDay
26%13%20%21%15%2
400
November1st to 15thDay
31%11%21%20%13%1
284
16th to endDay
27%16%21%26%6%1
178
December1st to 15thDay
33%22%25%11%6%1
135
16th to endDay
42%21%24%11%
144
January1st to 15thNight
29%22%24%19%3
780
16th to endNight
31%19%28%18%1
832
February1st to 15thNight
30%19%26%22%1
764
16th to endNight
25%24%31%17%1
683
March1st to 15thNight
27%18%27%22%4
780
16th to endNight
25%16%27%21%7%
832
April1st to 15thNight
23%16%26%27%6%
778
16th to endNight
17%13%23%31%11%1
633
May1st to 15thNight
22%10%20%29%16%1
585
16th to endNight
14%419%31%25%3
611
June1st to 15thNight
10%315%35%31%3
585
16th to endNight
11%215%38%30%2
585
July1st to 15thNight
13%213%34%32%3
555
16th to endNight
7%210%30%41%7%
612
August1st to 15thNight
14%10%34%37%2
575
16th to endNight
11%18%36%37%5%
693
September1st to 15thNight
13%414%28%33%5%
780
16th to endNight
17%6%14%29%28%2
777
October1st to 15thNight
17%8%19%29%21%3
780
16th to endNight
23%12%22%25%13%1
832
November1st to 15thNight
21%11%20%31%13%1
820
16th to endNight
23%16%23%26%9%
840
December1st to 15thNight
30%17%24%20%6%
780
16th to endNight
29%18%27%21%2
832
Page generated at 2018-12-01 11:35:31. Version ac04

Copyright 2018 A. Danko


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