Big M Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big M Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
36%
24697
17%
74701
10%
24642
43%
24642
23%
50149

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
24%
950
14%
2648
5%
946
32%
946
19%
1891
January11th to 20thNight
22%
918
11%
2569
5%
918
27%
918
14%
1978
January21st to endNight
22%
990
10%
2628
2%
990
28%
990
16%
2012
February1st to 10thNight
22%
900
12%
2340
3%
900
27%
900
18%
1756
February11th to 20thNight
25%
804
17%
2202
6%
803
32%
803
22%
1525
February21st to endNight
21%
592
18%
1776
6%
592
33%
592
18%
1250
March1st to 10thNight
23%
720
14%
2160
6%
720
30%
720
19%
1505
March11th to 20thNight
34%
563
20%
2003
13%
563
44%
563
22%
1227
March21st to endNight
25%
594
14%
2052
9%
594
34%
594
14%
1489
April1st to 10thNight
26%
540
16%
1800
9%
540
36%
540
20%
1187
April11th to 20thNight
33%
533
20%
1775
16%
533
43%
533
19%
1195
April21st to endNight
34%
540
15%
1799
9%
540
43%
540
20%
1202
May1st to 10thNight
32%
540
16%
1486
9%
540
38%
540
24%
1049
May11th to 20thNight
37%
531
18%
1417
10%
531
44%
531
25%
988
May21st to endNight
47%
584
17%
1551
9%
582
52%
582
33%
996
June1st to 10thNight
55%
540
16%
1440
8%
540
59%
540
35%
909
June11th to 20thNight
49%
540
19%
1440
11%
540
53%
540
35%
901
June21st to endNight
54%
540
19%
1440
10%
540
58%
540
34%
907
July1st to 10thNight
71%
525
18%
1400
12%
525
72%
525
44%
808
July11th to 20thNight
63%
522
22%
1392
14%
522
67%
522
45%
804
July21st to endNight
66%
591
22%
1487
17%
558
68%
558
43%
869
August1st to 10thNight
68%
537
23%
1533
21%
534
71%
534
40%
893
August11th to 20thNight
67%
534
22%
1679
24%
534
70%
534
37%
945
August21st to endNight
66%
594
22%
1980
25%
594
69%
594
34%
1093
September1st to 10thNight
61%
540
23%
1800
24%
540
66%
540
33%
1003
September11th to 20thNight
59%
540
30%
1800
29%
540
65%
540
34%
1013
September21st to endNight
49%
720
25%
2002
15%
720
56%
720
34%
1204
October1st to 10thNight
48%
720
22%
2160
15%
720
53%
720
32%
1284
October11th to 20thNight
34%
720
21%
2160
12%
720
44%
720
26%
1374
October21st to endNight
29%
792
18%
2488
11%
792
37%
792
21%
1674
November1st to 10thNight
32%
740
25%
2406
16%
740
44%
740
28%
1431
November11th to 20thNight
23%
817
17%
2527
8%
817
32%
817
18%
1758
November21st to endNight
25%
950
18%
2660
8%
950
34%
950
21%
1837
December1st to 10thNight
22%
950
12%
2660
4%
950
28%
950
17%
1970
December11th to 20thNight
22%
949
13%
3198
4%
948
29%
948
13%
2314
December21st to endNight
16%
1037
7%
4843
3%
1026
25%
1026
7%
3908

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
13%8%46%6%35%5%5%7%37%
74776
Night
13%8%47%5%3445%7%39%
75006
TransparencyDay
45%6%12%22%12%1
74476
Night
45%5%12%22%14%2
74826
SeeingDay
16%10%29%26%14%5%
14716
Night
17%9%31%31%11%1
24709

By Month

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 10thDay
10%11%7%9%8%46%45%7%30%
1517
11th to 20thDay
11%15%6%12%7%3445%8%24%
1464
21st to endDay
12%15%5%13%8%5%5%45%6%24%
1727
February1st to 10thDay
11%11%6%8%6%35%6%6%6%30%
1620
11th to 20thDay
14%9%5%6%6%35%5%5%7%35%
1646
21st to endDay
13%11%5%8%6%4446%7%33%
1480
March1st to 10thDay
14%9%5%7%6%45%45%7%34%
1800
11th to 20thDay
13%8%47%5%345%47%40%
1971
21st to endDay
20%10%47%5%45%45%7%29%
2304
April1st to 10thDay
18%11%47%6%345%5%7%30%
2160
11th to 20thDay
17%10%47%5%4445%5%34%
2134
21st to endDay
21%8%37%6%3446%6%33%
2160
May1st to 10thDay
16%8%36%8%5%5%6%6%7%30%
2470
11th to 20thDay
17%8%36%6%445%6%8%33%
2479
21st to endDay
15%6%36%6%45%6%6%7%36%
2725
June1st to 10thDay
11%425%5%345%6%9%45%
2520
11th to 20thDay
12%5%35%5%345%6%9%43%
2520
21st to endDay
11%6%36%6%35%5%7%9%39%
2520
July1st to 10thDay
8%5%24435%6%6%10%48%
2520
11th to 20thDay
8%425%5%36%6%6%8%46%
2520
21st to endDay
8%4345%45%5%6%10%45%
2743
August1st to 10thDay
8%425%6%46%6%8%9%43%
2394
11th to 20thDay
11%6%35%5%35%5%5%7%45%
2220
21st to endDay
11%5%25%5%345%6%8%47%
2376
September1st to 10thDay
14%4245%35%46%8%44%
2160
11th to 20thDay
12%5%244345%5%8%50%
2142
21st to endDay
14%5%35%5%35%5%5%8%44%
1954
October1st to 10thDay
13%7%35%5%2445%7%45%
1800
11th to 20thDay
20%9%35%5%24446%37%
1800
21st to endDay
18%9%36%5%23346%41%
1868
November1st to 10thDay
11%8%35%5%44447%44%
1672
11th to 20thDay
15%11%38%6%4445%7%33%
1649
21st to endDay
15%10%5%7%5%34436%39%
1520
December1st to 10thDay
13%13%5%9%6%45%45%6%31%
1520
11th to 20thDay
13%12%5%9%7%5%45%5%7%28%
1918
21st to endDay
19%19%10%11%6%345%4416%
2783
January1st to 10thNight
10%10%6%9%6%45%45%6%35%
2648
11th to 20thNight
13%13%6%11%8%4445%5%27%
2569
21st to endNight
13%13%6%10%6%45%45%6%27%
2628
February1st to 10thNight
10%13%5%9%6%5%5%5%5%5%31%
2340
11th to 20thNight
11%9%47%7%35%446%39%
2201
21st to endNight
16%11%47%5%34446%36%
1776
March1st to 10thNight
16%10%5%6%5%34446%37%
2160
11th to 20thNight
13%7%35%5%45%45%7%42%
2003
21st to endNight
21%9%47%6%35%5%46%30%
2052
April1st to 10thNight
17%9%46%6%3345%6%38%
1800
11th to 20thNight
19%8%35%5%345%46%37%
1775
21st to endNight
17%8%35%6%445%5%7%36%
1799
May1st to 10thNight
16%8%36%5%35%5%6%8%35%
1486
11th to 20thNight
15%7%47%5%35%45%7%38%
1416
21st to endNight
13%6%35%32445%10%46%
1551
June1st to 10thNight
9%426%5%35%5%7%7%47%
1440
11th to 20thNight
11%5%35%4235%7%9%47%
1440
21st to endNight
10%5%245%25%5%7%8%49%
1440
July1st to 10thNight
5%325%42347%10%55%
1440
11th to 20thNight
6%3244345%5%8%57%
1440
21st to endNight
6%3245%35%5%6%9%53%
1575
August1st to 10thNight
5%325%435%5%6%9%52%
1542
11th to 20thNight
9%5%25%43346%8%53%
1679
21st to endNight
9%424434447%55%
1980
September1st to 10thNight
10%5%2343435%7%54%
1800
11th to 20thNight
11%41443445%8%52%
1800
21st to endNight
10%5%25%5%345%5%6%52%
2002
October1st to 10thNight
12%5%2342445%5%52%
2160
11th to 20thNight
17%6%25%43445%7%43%
2160
21st to endNight
18%8%36%5%3445%7%38%
2488
November1st to 10thNight
12%7%36%423446%50%
2406
11th to 20thNight
14%9%5%8%6%34446%36%
2527
21st to endNight
17%8%46%6%3445%6%38%
2660
December1st to 10thNight
13%11%5%8%7%35%5%5%7%30%
2660
11th to 20thNight
14%12%6%8%6%36%5%5%5%30%
3279
21st to endNight
20%16%5%10%8%45%5%45%18%
4884

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
54%414%16%11%
1518
11th to 20thDay
59%6%13%15%6%1
1464
21st to endDay
61%5%12%13%8%1
1727
February1st to 10thDay
53%5%12%20%8%1
1620
11th to 20thDay
48%6%12%22%10%2
1655
21st to endDay
51%6%10%19%10%2
1480
March1st to 10thDay
50%5%14%21%8%1
1800
11th to 20thDay
45%5%12%24%12%2
1971
21st to endDay
55%6%10%20%8%1
2304
April1st to 10thDay
53%6%10%21%10%
2160
11th to 20thDay
51%410%21%12%2
2134
21st to endDay
51%5%10%20%12%2
2160
May1st to 10thDay
52%7%13%21%8%
2470
11th to 20thDay
49%7%11%21%12%1
2483
21st to endDay
46%8%11%19%14%2
2725
June1st to 10thDay
35%8%13%27%16%1
2520
11th to 20thDay
37%7%14%22%19%2
2520
21st to endDay
40%9%12%25%13%1
2520
July1st to 10thDay
32%8%15%29%13%2
2451
11th to 20thDay
33%9%15%27%14%2
2445
21st to endDay
34%7%15%26%15%3
2591
August1st to 10thDay
34%11%14%29%12%1
2379
11th to 20thDay
37%7%13%27%15%2
2220
21st to endDay
34%7%12%27%17%3
2376
September1st to 10thDay
38%6%11%24%18%2
2160
11th to 20thDay
33%6%10%33%16%2
2142
21st to endDay
39%6%11%27%16%1
1954
October1st to 10thDay
39%6%11%29%15%
1800
11th to 20thDay
47%5%11%22%14%1
1800
21st to endDay
46%5%14%22%13%1
1868
November1st to 10thDay
41%5%13%26%13%2
1672
11th to 20thDay
51%5%13%20%10%1
1649
21st to endDay
48%417%22%8%1
1520
December1st to 10thDay
54%6%12%18%8%2
1520
11th to 20thDay
55%5%16%14%8%1
1915
21st to endDay
72%5%9%9%5%
2783
January1st to 10thNight
50%5%11%20%13%1
2648
11th to 20thNight
59%5%13%14%8%
2569
21st to endNight
58%5%12%15%9%1
2628
February1st to 10thNight
55%411%18%11%2
2340
11th to 20thNight
47%5%12%21%13%1
2202
21st to endNight
51%411%20%10%3
1776
March1st to 10thNight
50%413%21%10%2
2160
11th to 20thNight
41%5%14%22%17%2
2003
21st to endNight
56%49%20%9%2
2052
April1st to 10thNight
47%412%22%14%2
1800
11th to 20thNight
48%5%8%19%17%4
1775
21st to endNight
46%6%12%19%13%3
1799
May1st to 10thNight
45%7%13%21%13%
1486
11th to 20thNight
45%6%11%23%14%1
1417
21st to endNight
36%7%14%24%18%2
1552
June1st to 10thNight
34%7%14%25%19%2
1440
11th to 20thNight
34%6%11%24%22%3
1440
21st to endNight
32%9%14%25%16%3
1440
July1st to 10thNight
25%7%14%33%18%3
1400
11th to 20thNight
25%6%13%30%21%4
1392
21st to endNight
26%7%18%27%18%3
1487
August1st to 10thNight
27%8%14%33%16%2
1533
11th to 20thNight
30%5%15%29%19%2
1679
21st to endNight
29%7%12%28%19%6%
1980
September1st to 10thNight
30%7%11%26%25%2
1800
11th to 20thNight
31%6%10%33%18%3
1800
21st to endNight
33%6%10%29%21%1
2002
October1st to 10thNight
33%6%10%27%22%2
2160
11th to 20thNight
41%5%13%23%18%1
2160
21st to endNight
46%6%10%24%13%1
2488
November1st to 10thNight
37%412%26%19%2
2406
11th to 20thNight
50%5%12%20%11%1
2527
21st to endNight
47%5%13%24%10%1
2660
December1st to 10thNight
55%5%11%20%8%1
2660
11th to 20thNight
55%5%14%18%8%1
3281
21st to endNight
69%5%8%13%5%
4884

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
17%13%29%29%10%1
270
11th to 20thDay
21%10%42%19%6%1
249
21st to endDay
21%12%39%22%42
264
February1st to 10thDay
16%19%43%18%4
240
11th to 20thDay
22%21%32%18%6%1
240
21st to endDay
20%23%39%15%2
198
March1st to 10thDay
16%18%43%17%7%
240
11th to 20thDay
17%21%35%18%7%3
240
21st to endDay
26%13%35%16%8%2
390
April1st to 10thDay
25%13%34%21%6%
420
11th to 20thDay
24%17%27%21%10%1
412
21st to endDay
24%12%30%22%10%2
420
May1st to 10thDay
20%11%32%26%9%1
553
11th to 20thDay
21%14%25%25%12%3
625
21st to endDay
17%9%27%25%15%7%
679
June1st to 10thDay
10%6%27%31%21%5%
630
11th to 20thDay
14%5%28%34%15%4
630
21st to endDay
13%8%28%30%15%6%
630
July1st to 10thDay
9%317%33%25%14%
621
11th to 20thDay
9%427%29%24%8%
618
21st to endDay
9%325%35%22%7%
684
August1st to 10thDay
7%219%36%26%10%
626
11th to 20thDay
11%6%19%27%22%14%
534
21st to endDay
11%6%20%33%21%10%
495
September1st to 10thDay
16%6%23%30%18%6%
450
11th to 20thDay
11%420%35%22%9%
450
21st to endDay
17%9%27%25%19%4
424
October1st to 10thDay
19%9%28%27%15%2
270
11th to 20thDay
24%19%32%17%8%
270
21st to endDay
24%16%29%23%6%2
297
November1st to 10thDay
14%16%37%21%9%2
270
11th to 20thDay
17%18%36%20%9%1
270
21st to endDay
23%11%36%24%5%
270
December1st to 10thDay
16%16%40%20%7%1
270
11th to 20thDay
16%11%40%24%7%1
270
21st to endDay
26%17%37%16%3
297
January1st to 10thNight
14%15%37%25%7%
950
11th to 20thNight
19%11%36%28%5%
918
21st to endNight
22%10%35%27%6%
990
February1st to 10thNight
18%12%39%25%6%
900
11th to 20thNight
17%13%39%26%5%
807
21st to endNight
23%13%36%22%41
592
March1st to 10thNight
22%11%36%24%6%1
720
11th to 20thNight
18%10%33%31%7%1
563
21st to endNight
24%10%34%26%6%1
594
April1st to 10thNight
19%14%36%26%4
540
11th to 20thNight
23%13%27%30%7%1
533
21st to endNight
20%9%31%29%10%1
540
May1st to 10thNight
19%7%35%30%7%1
540
11th to 20thNight
19%7%31%33%11%
531
21st to endNight
15%428%35%16%2
584
June1st to 10thNight
11%425%46%14%1
540
11th to 20thNight
14%329%40%14%
540
21st to endNight
11%426%42%16%2
540
July1st to 10thNight
5%115%42%33%4
525
11th to 20thNight
7%221%42%26%2
522
21st to endNight
8%117%45%27%2
591
August1st to 10thNight
6%215%46%28%3
537
11th to 20thNight
9%115%40%30%4
534
21st to endNight
11%116%44%24%3
594
September1st to 10thNight
11%419%39%21%6%
540
11th to 20thNight
12%121%36%26%4
540
21st to endNight
12%8%26%35%18%1
720
October1st to 10thNight
15%6%27%32%19%1
720
11th to 20thNight
21%9%32%29%9%
720
21st to endNight
21%10%33%27%7%1
792
November1st to 10thNight
16%10%38%25%10%1
740
11th to 20thNight
18%13%39%24%5%
817
21st to endNight
21%12%37%24%7%
950
December1st to 10thNight
19%11%39%24%5%1
950
11th to 20thNight
23%10%35%25%6%
950
21st to endNight
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