CMS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CMS 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
51%
24574
22%
74269
15%
24526
58%
24526
36%
41767

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
48%
949
21%
2647
12%
946
55%
946
38%
1502
January11th to 20thNight
42%
860
23%
2511
13%
860
53%
860
35%
1438
January21st to endNight
43%
792
24%
2574
15%
792
55%
792
34%
1418
February1st to 10thNight
35%
720
19%
2340
12%
720
46%
720
28%
1394
February11th to 20thNight
35%
700
15%
2273
10%
700
43%
700
27%
1375
February21st to endNight
42%
592
22%
1896
13%
592
53%
592
31%
1095
March1st to 10thNight
45%
720
23%
1994
12%
720
55%
720
37%
1158
March11th to 20thNight
49%
720
22%
1980
11%
720
58%
720
41%
1109
March21st to endNight
42%
792
17%
2178
7%
792
50%
792
29%
1405
April1st to 10thNight
41%
550
17%
1810
9%
550
51%
550
26%
1098
April11th to 20thNight
38%
533
16%
1732
11%
533
45%
533
22%
1127
April21st to endNight
45%
540
19%
1620
14%
540
50%
540
27%
1034
May1st to 10thNight
51%
540
15%
1580
10%
540
54%
540
29%
986
May11th to 20thNight
57%
531
17%
1418
13%
531
59%
531
31%
912
May21st to endNight
51%
584
12%
1552
8%
582
54%
582
26%
1058
June1st to 10thNight
56%
540
15%
1440
10%
540
60%
540
34%
906
June11th to 20thNight
51%
515
16%
1415
10%
515
56%
515
40%
828
June21st to endNight
74%
360
25%
1260
25%
360
78%
360
46%
574
July1st to 10thNight
78%
443
22%
1318
20%
443
82%
443
55%
619
July11th to 20thNight
71%
522
28%
1392
22%
522
77%
522
65%
662
July21st to endNight
72%
591
28%
1581
24%
558
76%
558
58%
742
August1st to 10thNight
72%
537
27%
1602
23%
534
75%
534
47%
810
August11th to 20thNight
75%
534
31%
1602
27%
534
76%
534
53%
756
August21st to endNight
69%
594
32%
1867
28%
594
73%
594
48%
902
September1st to 10thNight
67%
540
31%
1800
29%
540
76%
540
48%
849
September11th to 20thNight
64%
612
32%
1872
27%
612
72%
612
46%
946
September21st to endNight
60%
720
30%
2019
22%
720
65%
720
44%
1116
October1st to 10thNight
54%
720
23%
2160
16%
720
60%
720
34%
1296
October11th to 20thNight
50%
720
29%
2160
17%
720
61%
720
36%
1240
October21st to endNight
48%
792
26%
2376
15%
792
59%
792
37%
1369
November1st to 10thNight
47%
879
24%
2498
15%
879
53%
879
36%
1457
November11th to 20thNight
42%
950
25%
2660
14%
950
52%
950
37%
1527
November21st to endNight
49%
950
24%
2660
15%
950
56%
950
37%
1555
December1st to 10thNight
44%
950
20%
2660
10%
950
52%
950
37%
1531
December11th to 20thNight
46%
950
23%
3174
14%
949
55%
949
36%
1703
December21st to endNight
43%
1032
15%
4648
11%
1026
51%
1026
24%
2270

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
5%5%25%5%35%5%6%9%48%
74989
Night
5%6%35%5%345%6%9%50%
74779
TransparencyDay
31%5%14%26%17%6%
74698
Night
30%5%15%28%17%5%
74592
SeeingDay
9%10%32%33%13%3
14650
Night
7%6%27%44%14%2
24592

By Month

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 10thDay
15%35%5%36%6%9%9%47%
1518
11th to 20thDay
15%36%6%346%7%9%50%
1539
21st to endDay
27%36%6%35%5%6%9%47%
1782
February1st to 10thDay
27%49%7%35%6%5%11%41%
1620
11th to 20thDay
28%48%5%45%5%6%9%44%
1575
21st to endDay
28%48%6%36%6%8%8%43%
1358
March1st to 10thDay
25%35%5%35%5%7%12%47%
1963
11th to 20thDay
37%45%7%35%5%7%10%45%
1980
21st to endDay
7%7%47%6%46%6%7%10%37%
2178
April1st to 10thDay
6%8%37%6%46%5%6%9%41%
2150
11th to 20thDay
7%9%47%7%46%6%6%8%36%
2173
21st to endDay
11%10%37%6%36%5%6%7%35%
2340
May1st to 10thDay
10%8%37%6%36%6%7%7%37%
2380
11th to 20thDay
13%7%25%5%46%7%6%9%38%
2478
21st to endDay
15%7%37%5%37%8%6%9%29%
2724
June1st to 10thDay
8%6%35%5%36%7%8%9%40%
2520
11th to 20thDay
7%5%25%5%46%6%7%9%44%
2536
21st to endDay
3313435%5%8%8%58%
2700
July1st to 10thDay
3213437%6%8%8%55%
2592
11th to 20thDay
22134246%7%8%62%
2520
21st to endDay
3314426%5%6%8%57%
2641
August1st to 10thDay
6%3245%35%6%7%8%51%
2328
11th to 20thDay
33245%35%6%7%8%56%
2299
21st to endDay
3313435%5%6%8%59%
2489
September1st to 10thDay
5%31432445%7%63%
2160
11th to 20thDay
8%3143235%45%62%
2084
21st to endDay
8%5%1432445%6%57%
1940
October1st to 10thDay
10%8%35%42445%7%49%
1800
11th to 20thDay
7%6%235%3445%7%55%
1800
21st to endDay
5%7%245%4345%7%52%
1980
November1st to 10thDay
37%36%6%445%6%9%47%
1562
11th to 20thDay
36%35%6%346%7%9%49%
1520
21st to endDay
38%45%5%335%5%9%49%
1520
December1st to 10thDay
25%36%7%35%5%6%8%49%
1520
11th to 20thDay
25%25%5%25%5%8%12%50%
1926
21st to endDay
36%25%5%3446%9%53%
2794
January1st to 10thNight
15%36%5%45%5%8%9%50%
2647
11th to 20thNight
26%37%5%35%5%6%8%50%
2511
21st to endNight
38%38%43445%9%48%
2574
February1st to 10thNight
26%48%7%45%47%9%43%
2340
11th to 20thNight
38%5%8%7%35%5%6%8%42%
2272
21st to endNight
16%47%7%46%5%5%9%46%
1896
March1st to 10thNight
27%36%5%345%6%10%48%
1994
11th to 20thNight
45%25%4345%7%13%49%
1980
21st to endNight
6%8%47%6%45%5%6%10%39%
2178
April1st to 10thNight
7%8%46%5%346%6%7%43%
1810
11th to 20thNight
8%10%48%6%35%6%5%7%38%
1732
21st to endNight
9%11%37%5%2445%7%42%
1620
May1st to 10thNight
12%8%25%426%45%8%44%
1580
11th to 20thNight
10%7%25%5%35%5%6%9%42%
1417
21st to endNight
14%6%26%6%346%7%11%34%
1552
June1st to 10thNight
8%435%6%45%5%7%9%45%
1440
11th to 20thNight
7%424445%46%9%52%
1415
21st to endNight
331343346%8%62%
1260
July1st to 10thNight
21123246%6%10%63%
1358
11th to 20thNight
21232335%10%68%
1440
21st to endNight
321332346%8%66%
1674
August1st to 10thNight
4224425%6%7%8%57%
1611
11th to 20thNight
42133235%5%9%62%
1602
21st to endNight
5%32332447%7%61%
1867
September1st to 10thNight
6%312323447%64%
1800
11th to 20thNight
6%32432345%8%61%
1872
21st to endNight
11%423223446%59%
2019
October1st to 10thNight
11%7%25%4245%6%6%49%
2160
11th to 20thNight
8%5%35%434345%55%
2160
21st to endNight
5%7%35%4345%5%8%51%
2376
November1st to 10thNight
5%6%36%5%3446%9%49%
2498
11th to 20thNight
37%36%5%35%5%6%8%49%
2660
21st to endNight
48%35%5%35%5%5%8%50%
2660
December1st to 10thNight
25%37%6%35%5%7%8%48%
2660
11th to 20thNight
26%25%4346%7%11%49%
3271
21st to endNight
26%36%5%35%5%7%9%50%
4873

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
29%5%21%30%13%2
1519
11th to 20thDay
29%421%29%13%3
1539
21st to endDay
32%5%18%28%15%3
1782
February1st to 10thDay
36%5%19%27%11%1
1620
11th to 20thDay
36%6%17%24%14%2
1584
21st to endDay
35%7%15%22%16%5%
1358
March1st to 10thDay
27%5%20%32%15%1
1963
11th to 20thDay
32%6%19%27%13%2
1980
21st to endDay
40%8%19%25%6%1
2178
April1st to 10thDay
39%6%16%27%10%2
2150
11th to 20thDay
44%6%15%26%8%1
2176
21st to endDay
46%5%13%26%9%
2340
May1st to 10thDay
43%6%12%27%10%2
2380
11th to 20thDay
41%7%17%26%9%
2482
21st to endDay
47%8%20%19%7%
2724
June1st to 10thDay
35%7%20%24%12%2
2520
11th to 20thDay
32%7%15%21%19%5%
2536
21st to endDay
21%414%26%22%13%
2700
July1st to 10thDay
22%7%11%27%22%10%
2523
11th to 20thDay
17%47%24%26%21%
2445
21st to endDay
22%5%7%21%27%18%
2494
August1st to 10thDay
28%6%8%26%21%12%
2313
11th to 20thDay
23%5%8%28%25%10%
2299
21st to endDay
22%5%7%22%29%15%
2489
September1st to 10thDay
21%38%22%30%16%
2160
11th to 20thDay
24%39%27%28%8%
2084
21st to endDay
28%38%30%26%5%
1940
October1st to 10thDay
35%5%13%23%18%5%
1800
11th to 20thDay
29%5%14%31%15%6%
1800
21st to endDay
31%414%30%18%3
1980
November1st to 10thDay
34%5%13%26%18%3
1562
11th to 20thDay
30%5%18%26%18%3
1520
21st to endDay
32%415%29%18%2
1520
December1st to 10thDay
30%5%18%27%15%4
1520
11th to 20thDay
25%5%24%30%14%2
1924
21st to endDay
28%410%30%19%9%
2794
January1st to 10thNight
27%6%21%30%14%1
2647
11th to 20thNight
30%5%21%27%14%4
2511
21st to endNight
31%419%29%14%3
2574
February1st to 10thNight
36%5%19%29%9%2
2340
11th to 20thNight
37%5%17%25%13%2
2273
21st to endNight
34%416%25%17%4
1896
March1st to 10thNight
30%5%17%31%16%2
1994
11th to 20thNight
26%6%21%32%13%2
1980
21st to endNight
39%5%17%29%8%2
2178
April1st to 10thNight
36%7%19%25%10%3
1810
11th to 20thNight
43%6%15%26%10%1
1732
21st to endNight
41%5%13%32%9%
1620
May1st to 10thNight
38%5%15%28%12%2
1580
11th to 20thNight
37%6%20%26%9%1
1418
21st to endNight
42%7%22%20%7%1
1552
June1st to 10thNight
33%8%19%23%15%2
1440
11th to 20thNight
29%5%18%25%15%7%
1415
21st to endNight
20%413%31%22%10%
1260
July1st to 10thNight
14%6%12%31%27%10%
1318
11th to 20thNight
12%48%28%29%19%
1392
21st to endNight
15%49%21%32%19%
1581
August1st to 10thNight
22%5%11%28%23%12%
1602
11th to 20thNight
17%5%10%28%27%13%
1602
21st to endNight
21%49%25%25%16%
1867
September1st to 10thNight
20%48%23%30%16%
1800
11th to 20thNight
22%5%10%28%26%10%
1872
21st to endNight
27%47%30%28%5%
2019
October1st to 10thNight
34%5%12%29%15%5%
2160
11th to 20thNight
31%410%32%18%4
2160
21st to endNight
30%6%15%28%18%3
2376
November1st to 10thNight
30%6%15%29%17%3
2498
11th to 20thNight
30%418%30%16%3
2660
21st to endNight
31%415%29%18%3
2660
December1st to 10thNight
29%5%21%26%14%4
2660
11th to 20thNight
27%5%21%34%13%1
3272
21st to endNight
29%5%11%30%16%9%
4873

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
7%11%30%34%15%3
270
11th to 20thDay
27%37%37%10%6%
249
21st to endDay
5%13%36%35%8%2
264
February1st to 10thDay
5%20%49%24%2
240
11th to 20thDay
6%18%43%26%7%
240
21st to endDay
8%11%39%30%12%1
198
March1st to 10thDay
6%18%42%30%4
403
11th to 20thDay
7%18%38%27%8%1
420
21st to endDay
13%17%32%29%8%1
462
April1st to 10thDay
13%17%39%25%5%
420
11th to 20thDay
16%19%37%24%5%
412
21st to endDay
20%13%30%25%10%1
420
May1st to 10thDay
14%13%29%27%15%2
463
11th to 20thDay
19%8%23%30%15%4
625
21st to endDay
18%8%22%29%19%4
679
June1st to 10thDay
9%7%26%33%19%5%
630
11th to 20thDay
10%13%30%33%13%2
630
21st to endDay
5%7%29%37%20%3
630
July1st to 10thDay
36%27%36%23%5%
621
11th to 20thDay
35%28%45%17%2
618
21st to endDay
27%32%36%20%3
583
August1st to 10thDay
10%431%39%13%4
447
11th to 20thDay
5%428%40%19%5%
437
21st to endDay
47%31%37%17%3
495
September1st to 10thDay
6%6%36%35%13%4
450
11th to 20thDay
10%8%32%30%16%5%
450
21st to endDay
11%10%30%31%14%3
410
October1st to 10thDay
13%7%26%36%16%3
270
11th to 20thDay
11%14%27%29%14%5%
270
21st to endDay
10%13%35%30%9%2
297
November1st to 10thDay
9%13%37%27%10%4
270
11th to 20thDay
7%14%32%34%10%2
270
21st to endDay
12%10%36%36%6%
270
December1st to 10thDay
410%39%34%9%3
270
11th to 20thDay
9%9%34%40%7%
270
21st to endDay
6%7%37%37%10%1
297
January1st to 10thNight
39%30%45%12%1
950
11th to 20thNight
49%35%36%12%4
860
21st to endNight
6%7%33%44%9%2
792
February1st to 10thNight
48%39%39%8%1
720
11th to 20thNight
7%9%35%38%11%2
704
21st to endNight
47%34%42%11%1
592
March1st to 10thNight
48%33%44%10%1
720
11th to 20thNight
5%7%31%45%11%1
720
21st to endNight
9%8%29%44%10%1
792
April1st to 10thNight
10%7%31%41%9%
550
11th to 20thNight
14%7%33%38%7%1
533
21st to endNight
14%5%23%41%15%1
540
May1st to 10thNight
16%5%20%39%19%1
540
11th to 20thNight
14%416%43%22%2
531
21st to endNight
16%5%17%39%20%2
584
June1st to 10thNight
8%5%20%41%23%2
540
11th to 20thNight
9%6%29%41%15%
515
21st to endNight
3314%53%27%
360
July1st to 10thNight
2213%52%29%2
443
11th to 20thNight
2220%54%22%
522
21st to endNight
3216%50%26%2
591
August1st to 10thNight
4212%54%26%1
537
11th to 20thNight
5%111%55%26%1
534
21st to endNight
5%317%46%26%2
594
September1st to 10thNight
7%220%50%19%2
540
11th to 20thNight
7%418%48%19%4
612
21st to endNight
11%320%43%18%3
720
October1st to 10thNight
14%221%44%16%3
720
11th to 20thNight
9%5%26%42%15%3
720
21st to endNight
8%432%44%10%2
792
November1st to 10thNight
7%5%31%45%10%2
879
11th to 20thNight
6%9%35%44%5%1
950
21st to endNight
9%6%27%47%10%
950
December1st to 10thNight
48%32%44%10%1
950
11th to 20thNight
5%6%33%46%9%1
950
21st to endNight
6%5%33%45%8%2
1045
Page generated at 2024-01-11 02:22:36. Version ac07

Copyright 2024 Attilla Danko


CMS Observatory Clear Sky Chart |Charts Home