Mountain Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Skies Observatory clear sky chart from 2005-11-06 to 2024-03-08.

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
64%
24434
21%
80552
20%
24357
67%
24357
30%
47827

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
56%
745
13%
4044
13%
736
58%
736
11%
2623
January11th to 20thNight
53%
755
9%
4097
12%
740
56%
740
9%
3117
January21st to endNight
56%
826
9%
4466
14%
814
58%
814
11%
3047
February1st to 10thNight
55%
760
12%
2470
12%
760
58%
760
20%
1678
February11th to 20thNight
56%
740
17%
2317
16%
740
59%
740
26%
1476
February21st to endNight
54%
628
19%
1884
17%
628
59%
628
27%
1218
March1st to 10thNight
54%
750
17%
2126
15%
750
57%
750
25%
1434
March11th to 20thNight
62%
720
22%
1980
20%
720
65%
720
37%
1191
March21st to endNight
55%
792
19%
2178
14%
792
58%
792
31%
1391
April1st to 10thNight
53%
714
19%
1974
15%
714
56%
714
33%
1219
April11th to 20thNight
57%
533
26%
1629
18%
533
63%
533
35%
951
April21st to endNight
60%
540
25%
1620
18%
540
64%
540
36%
934
May1st to 10thNight
58%
540
22%
1620
17%
540
61%
540
34%
939
May11th to 20thNight
68%
531
23%
1595
19%
531
70%
531
40%
857
May21st to endNight
63%
582
19%
1746
17%
582
66%
582
35%
1011
June1st to 10thNight
67%
540
23%
1620
21%
540
71%
540
43%
831
June11th to 20thNight
67%
540
26%
1620
24%
540
72%
540
52%
773
June21st to endNight
74%
540
27%
1620
26%
540
77%
540
59%
732
July1st to 10thNight
74%
525
20%
1575
20%
525
76%
525
49%
783
July11th to 20thNight
76%
522
22%
1566
21%
522
77%
522
43%
826
July21st to endNight
78%
591
25%
1674
22%
558
79%
558
43%
891
August1st to 10thNight
76%
537
27%
1602
21%
534
78%
534
43%
831
August11th to 20thNight
79%
534
32%
1602
26%
534
81%
534
49%
803
August21st to endNight
79%
594
35%
1795
32%
594
80%
594
49%
887
September1st to 10thNight
78%
682
32%
1942
27%
682
82%
682
58%
938
September11th to 20thNight
75%
720
34%
1980
28%
720
79%
720
54%
1016
September21st to endNight
75%
720
37%
1980
30%
720
78%
720
55%
996
October1st to 10thNight
69%
720
29%
2056
22%
720
73%
720
46%
1103
October11th to 20thNight
72%
720
34%
2160
28%
720
76%
720
47%
1093
October21st to endNight
69%
792
33%
2382
28%
792
73%
792
45%
1220
November1st to 10thNight
62%
740
27%
2405
25%
740
67%
740
35%
1329
November11th to 20thNight
68%
760
26%
2470
25%
760
70%
760
35%
1361
November21st to endNight
62%
809
22%
2519
20%
809
65%
809
30%
1526
December1st to 10thNight
60%
950
15%
2660
13%
950
61%
950
30%
1729
December11th to 20thNight
53%
921
13%
3136
11%
920
54%
920
17%
2197
December21st to endNight
47%
821
8%
4442
12%
817
49%
817
9%
2876

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%6%36%6%35%6%6%8%48%
78574
Night
5%6%36%6%35%6%6%8%47%
81051
TransparencyDay
32%7%12%18%19%12%
78299
Night
33%8%13%20%17%9%
80847
SeeingDay
8%6%19%32%26%8%
13825
Night
8%416%41%27%3
24486

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
49%5%12%12%6%7%7%8%7%24%
2858
11th to 20thDay
5%13%7%12%8%5%6%7%6%7%26%
2860
21st to endDay
5%12%5%9%8%45%7%6%7%31%
3146
February1st to 10thDay
6%9%48%6%5%7%7%7%6%35%
1710
11th to 20thDay
5%9%48%8%5%5%7%6%7%36%
1749
21st to endDay
5%10%5%8%7%46%6%6%6%36%
1570
March1st to 10thDay
49%46%5%46%6%5%8%41%
1993
11th to 20thDay
5%7%35%6%46%6%7%10%41%
1980
21st to endDay
8%8%38%8%5%6%6%6%8%34%
2178
April1st to 10thDay
6%8%36%7%46%6%5%7%43%
1985
11th to 20thDay
6%7%37%6%37%6%6%6%45%
2276
21st to endDay
5%6%35%6%45%5%5%8%50%
2340
May1st to 10thDay
6%6%25%7%35%6%6%9%45%
2340
11th to 20thDay
5%4245%45%6%7%11%49%
2301
21st to endDay
5%5%25%6%46%7%8%9%43%
2530
June1st to 10thDay
33244245%6%10%57%
2340
11th to 20thDay
4413333446%64%
2340
21st to endDay
2112323446%72%
2340
July1st to 10thDay
22124245%6%8%65%
2340
11th to 20thDay
43234345%5%8%59%
2340
21st to endDay
3214436%6%6%9%55%
2545
August1st to 10thDay
3314435%5%7%8%56%
2328
11th to 20thDay
32134345%5%7%63%
2299
21st to endDay
33244235%5%8%62%
2561
September1st to 10thDay
321222345%6%70%
2015
11th to 20thDay
421333446%7%65%
1980
21st to endDay
5%213223346%68%
1980
October1st to 10thDay
7%5%24435%46%7%55%
1903
11th to 20thDay
5%424433446%61%
1800
21st to endDay
4414425%5%6%8%55%
1972
November1st to 10thDay
8%5%25%5%25%6%6%8%49%
1674
11th to 20thDay
6%7%35%5%35%5%6%8%48%
1710
21st to endDay
36%48%7%37%6%6%8%40%
1635
December1st to 10thDay
6%7%38%7%5%6%5%6%9%38%
1520
11th to 20thDay
6%9%47%7%5%7%7%8%8%31%
1990
21st to endDay
5%10%49%7%47%7%8%9%30%
3146
January1st to 10thNight
5%9%49%10%5%7%7%6%8%31%
4097
11th to 20thNight
5%15%5%10%8%5%6%7%6%8%24%
4110
21st to endNight
5%10%48%9%5%7%9%7%8%27%
4521
February1st to 10thNight
48%47%7%47%8%7%10%35%
2470
11th to 20thNight
49%37%6%47%6%5%8%42%
2316
21st to endNight
49%37%6%46%6%5%9%40%
1884
March1st to 10thNight
48%47%8%5%7%6%5%7%38%
2126
11th to 20thNight
46%35%6%35%7%7%8%48%
1980
21st to endNight
6%6%36%6%46%6%7%9%42%
2178
April1st to 10thNight
7%8%35%5%35%35%7%49%
1974
11th to 20thNight
45%345%36%7%6%8%49%
1629
21st to endNight
7%5%25%5%25%5%6%7%52%
1620
May1st to 10thNight
6%5%35%436%6%7%8%48%
1620
11th to 20thNight
5%5%14435%5%5%8%56%
1594
21st to endNight
7%5%25%5%35%5%5%10%49%
1746
June1st to 10thNight
442243446%7%59%
1620
11th to 20thNight
3312224446%67%
1620
21st to endNight
12123235%5%8%67%
1620
July1st to 10thNight
22245%345%7%7%58%
1620
11th to 20thNight
34245%36%5%5%8%55%
1620
21st to endNight
34245%25%5%7%9%55%
1773
August1st to 10thNight
32145%3447%8%57%
1611
11th to 20thNight
23144346%6%8%59%
1602
21st to endNight
2324325%6%5%8%61%
1795
September1st to 10thNight
3313322346%69%
1942
11th to 20thNight
3313423447%66%
1980
21st to endNight
5%223313335%71%
1980
October1st to 10thNight
5%425%325%446%60%
2056
11th to 20thNight
4314334346%65%
2160
21st to endNight
44143235%47%62%
2382
November1st to 10thNight
7%6%35%5%2445%8%52%
2405
11th to 20thNight
5%424435%5%6%8%53%
2470
21st to endNight
5%6%35%5%45%7%6%8%46%
2519
December1st to 10thNight
5%7%37%7%35%5%6%8%45%
2660
11th to 20thNight
8%8%47%6%46%7%7%9%35%
3230
21st to endNight
5%9%48%7%5%9%7%8%9%29%
4521

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
50%15%16%15%5%
2859
11th to 20thDay
56%11%16%12%41
2860
21st to endDay
48%14%18%13%6%1
3146
February1st to 10thDay
49%9%14%16%10%1
1710
11th to 20thDay
44%9%14%21%11%1
1758
21st to endDay
44%8%16%20%11%1
1570
March1st to 10thDay
37%9%15%26%11%2
1993
11th to 20thDay
36%7%15%25%13%3
1980
21st to endDay
43%9%16%20%10%3
2178
April1st to 10thDay
37%8%14%18%17%7%
1985
11th to 20thDay
35%6%14%22%19%5%
2279
21st to endDay
30%7%12%24%23%3
2340
May1st to 10thDay
31%7%13%21%23%6%
2340
11th to 20thDay
26%7%13%22%24%7%
2304
21st to endDay
30%8%14%21%21%6%
2530
June1st to 10thDay
21%5%9%19%32%14%
2340
11th to 20thDay
20%48%15%28%25%
2340
21st to endDay
14%26%10%27%41%
2340
July1st to 10thDay
15%49%11%24%37%
2276
11th to 20thDay
19%5%10%12%22%32%
2271
21st to endDay
21%6%12%13%21%28%
2404
August1st to 10thDay
22%6%9%16%20%28%
2313
11th to 20thDay
18%48%10%28%32%
2299
21st to endDay
20%47%16%29%24%
2561
September1st to 10thDay
14%38%14%27%33%
2015
11th to 20thDay
16%5%9%18%26%25%
1980
21st to endDay
17%48%18%32%21%
1980
October1st to 10thDay
27%7%10%19%27%10%
1903
11th to 20thDay
24%49%28%28%7%
1800
21st to endDay
25%8%15%28%22%3
1972
November1st to 10thDay
34%7%14%23%19%4
1674
11th to 20thDay
35%8%13%28%16%1
1710
21st to endDay
39%9%16%21%13%2
1635
December1st to 10thDay
44%9%18%20%8%1
1520
11th to 20thDay
48%13%13%19%6%
1988
21st to endDay
45%11%18%14%11%
3146
January1st to 10thNight
44%13%17%21%6%
4097
11th to 20thNight
55%13%14%12%41
4110
21st to endNight
46%14%19%15%6%1
4521
February1st to 10thNight
42%12%18%19%8%1
2470
11th to 20thNight
40%6%14%24%14%1
2317
21st to endNight
40%7%14%25%12%2
1884
March1st to 10thNight
42%8%13%25%11%1
2126
11th to 20thNight
30%7%14%28%17%3
1980
21st to endNight
35%8%16%26%12%4
2178
April1st to 10thNight
36%5%12%23%18%7%
1974
11th to 20thNight
30%7%10%24%25%4
1629
21st to endNight
30%7%13%24%24%3
1620
May1st to 10thNight
30%7%13%21%25%5%
1620
11th to 20thNight
25%7%11%25%24%9%
1595
21st to endNight
28%8%14%26%20%4
1746
June1st to 10thNight
23%5%10%20%29%14%
1620
11th to 20thNight
17%48%17%29%25%
1620
21st to endNight
15%36%9%27%40%
1620
July1st to 10thNight
21%49%12%16%37%
1575
11th to 20thNight
24%5%10%12%18%31%
1566
21st to endNight
25%5%11%15%18%26%
1674
August1st to 10thNight
22%7%11%14%19%27%
1602
11th to 20thNight
20%5%7%13%24%31%
1602
21st to endNight
20%5%9%14%25%28%
1795
September1st to 10thNight
17%37%13%24%35%
1942
11th to 20thNight
18%5%8%16%26%27%
1980
21st to endNight
19%35%16%33%25%
1980
October1st to 10thNight
23%5%9%21%31%11%
2056
11th to 20thNight
21%5%8%26%31%8%
2160
21st to endNight
21%6%13%29%26%5%
2382
November1st to 10thNight
29%6%11%27%22%5%
2405
11th to 20thNight
28%8%15%30%16%3
2470
21st to endNight
33%9%14%27%14%3
2519
December1st to 10thNight
37%8%18%24%10%2
2660
11th to 20thNight
45%11%16%20%8%
3231
21st to endNight
48%9%18%16%8%
4521

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
16%9%17%31%18%9%
270
11th to 20thDay
10%7%21%35%21%7%
270
21st to endDay
12%5%17%37%23%7%
297
February1st to 10thDay
13%11%23%34%14%4
270
11th to 20thDay
10%8%24%34%20%3
354
21st to endDay
8%9%27%38%16%1
382
March1st to 10thDay
12%9%24%35%15%4
449
11th to 20thDay
9%8%27%31%21%4
420
21st to endDay
15%11%28%30%14%1
462
April1st to 10thDay
12%9%26%30%19%5%
420
11th to 20thDay
12%12%30%28%15%3
412
21st to endDay
11%13%25%26%20%5%
420
May1st to 10thDay
7%9%27%34%17%6%
423
11th to 20thDay
9%5%23%31%23%8%
448
21st to endDay
7%6%21%26%28%12%
485
June1st to 10thDay
37%18%29%33%9%
450
11th to 20thDay
6%7%21%34%24%8%
450
21st to endDay
3422%34%32%7%
450
July1st to 10thDay
1214%36%35%12%
446
11th to 20thDay
4213%34%39%7%
444
21st to endDay
38%31%44%13%
487
August1st to 10thDay
4213%35%36%10%
447
11th to 20thDay
3313%30%37%15%
437
21st to endDay
4212%30%42%10%
495
September1st to 10thDay
3313%32%33%16%
450
11th to 20thDay
5%5%17%28%32%13%
450
21st to endDay
6%6%15%24%36%14%
450
October1st to 10thDay
6%5%21%27%27%15%
373
11th to 20thDay
6%5%17%30%27%15%
270
21st to endDay
6%6%20%36%26%6%
297
November1st to 10thDay
13%9%14%30%24%11%
270
11th to 20thDay
8%7%14%38%23%10%
270
21st to endDay
10%5%16%31%28%9%
270
December1st to 10thDay
9%8%13%36%24%10%
270
11th to 20thDay
11%7%13%44%21%4
270
21st to endDay
15%8%19%35%21%2
297
January1st to 10thNight
13%5%14%41%22%5%
760
11th to 20thNight
12%5%17%36%25%5%
760
21st to endNight
12%5%14%41%23%4
836
February1st to 10thNight
10%6%19%47%15%4
760
11th to 20thNight
11%418%46%19%2
744
21st to endNight
10%5%21%45%18%1
628
March1st to 10thNight
9%7%20%45%17%2
750
11th to 20thNight
9%417%44%24%2
720
21st to endNight
11%6%21%45%15%2
792
April1st to 10thNight
14%8%19%39%17%2
714
11th to 20thNight
7%8%23%47%15%1
533
21st to endNight
9%5%21%40%24%2
540
May1st to 10thNight
7%6%22%42%21%1
540
11th to 20thNight
6%415%40%31%4
531
21st to endNight
8%6%17%36%29%4
584
June1st to 10thNight
6%6%16%39%31%2
540
11th to 20thNight
46%20%41%27%1
540
21st to endNight
3414%41%35%2
540
July1st to 10thNight
3112%37%42%5%
525
11th to 20thNight
429%44%38%3
522
21st to endNight
5%16%31%54%2
591
August1st to 10thNight
4110%41%41%2
537
11th to 20thNight
319%43%41%4
534
21st to endNight
239%39%45%2
594
September1st to 10thNight
5%29%38%40%6%
682
11th to 20thNight
4214%35%38%7%
720
21st to endNight
7%311%32%38%8%
720
October1st to 10thNight
8%416%40%28%5%
720
11th to 20thNight
6%314%40%32%6%
720
21st to endNight
6%416%41%28%4
792
November1st to 10thNight
11%416%35%29%4
740
11th to 20thNight
7%416%48%20%5%
760
21st to endNight
9%416%41%24%6%
809
December1st to 10thNight
11%415%45%21%3
950
11th to 20thNight
16%316%44%18%2
922
21st to endNight
15%6%18%44%16%2
836
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