Mt. SAC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. SAC Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
72%
24443
32%
74232
29%
24391
76%
24391
46%
38546

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
64%
756
27%
2638
28%
756
69%
756
37%
1351
January11th to 20thNight
67%
728
35%
2303
31%
728
75%
728
42%
1191
January21st to endNight
60%
792
37%
2376
30%
792
70%
792
41%
1281
February1st to 10thNight
65%
720
36%
2160
32%
720
72%
720
46%
1120
February11th to 20thNight
62%
700
33%
2098
29%
700
71%
700
45%
1089
February21st to endNight
61%
592
29%
1776
28%
592
67%
592
37%
999
March1st to 10thNight
63%
720
25%
2160
25%
720
68%
720
37%
1229
March11th to 20thNight
65%
720
26%
2120
26%
720
69%
720
42%
1156
March21st to endNight
64%
792
29%
1980
20%
792
69%
792
45%
1176
April1st to 10thNight
65%
720
27%
1800
18%
720
68%
720
43%
1082
April11th to 20thNight
63%
711
31%
1779
18%
711
67%
711
50%
997
April21st to endNight
65%
720
29%
1800
17%
720
68%
720
49%
1022
May1st to 10thNight
64%
640
25%
1720
17%
640
66%
640
44%
995
May11th to 20thNight
67%
531
26%
1595
20%
531
68%
531
43%
893
May21st to endNight
64%
582
28%
1746
23%
582
66%
582
48%
918
June1st to 10thNight
70%
540
25%
1620
22%
540
71%
540
44%
904
June11th to 20thNight
82%
401
34%
1481
38%
401
82%
401
48%
638
June21st to endNight
86%
360
35%
1440
43%
360
86%
360
48%
604
July1st to 10thNight
88%
350
35%
1400
42%
350
88%
350
53%
538
July11th to 20thNight
89%
424
34%
1468
35%
424
89%
424
54%
647
July21st to endNight
84%
728
30%
1804
22%
688
84%
688
55%
954
August1st to 10thNight
85%
716
33%
1780
21%
712
85%
712
65%
896
August11th to 20thNight
89%
712
38%
1780
24%
712
89%
712
68%
874
August21st to endNight
87%
792
40%
1980
28%
792
88%
792
66%
984
September1st to 10thNight
81%
720
38%
1800
29%
720
82%
720
61%
934
September11th to 20thNight
86%
720
42%
1948
38%
720
88%
720
69%
881
September21st to endNight
83%
720
41%
1980
38%
720
87%
720
66%
909
October1st to 10thNight
78%
720
35%
1980
33%
720
82%
720
55%
1001
October11th to 20thNight
78%
720
40%
2064
39%
720
82%
720
55%
1007
October21st to endNight
75%
792
39%
2432
40%
792
79%
792
52%
1160
November1st to 10thNight
78%
740
35%
2405
37%
740
83%
740
52%
1096
November11th to 20thNight
78%
760
38%
2470
38%
760
83%
760
52%
1126
November21st to endNight
70%
760
33%
2470
35%
760
74%
760
43%
1239
December1st to 10thNight
65%
760
32%
2470
30%
760
71%
760
39%
1297
December11th to 20thNight
60%
760
28%
2968
30%
759
66%
759
27%
1756
December21st to endNight
59%
824
20%
4441
29%
817
68%
817
21%
2602

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%313323346%67%
75141
Night
6%423323346%64%
74588
TransparencyDay
20%46%17%26%27%
74864
Night
23%47%17%23%25%
74386
SeeingDay
6%7%25%37%19%6%
14502
Night
7%312%37%35%7%
24465

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
9%41342335%7%57%
1519
11th to 20thDay
8%32332435%7%61%
1684
21st to endDay
10%423322335%61%
1980
February1st to 10thDay
9%423322335%64%
1800
11th to 20thDay
9%323223346%63%
1750
21st to endDay
9%4243235%47%56%
1480
March1st to 10thDay
9%5%25%5%25%5%5%8%51%
1800
11th to 20thDay
8%42342346%9%57%
1840
21st to endDay
6%32343446%7%59%
2371
April1st to 10thDay
7%42342335%8%59%
2160
11th to 20thDay
5%423423345%65%
2129
21st to endDay
5%423313236%68%
2160
May1st to 10thDay
6%5%23323446%61%
2237
11th to 20thDay
5%423423346%63%
2301
21st to endDay
6%5%23323346%63%
2530
June1st to 10thDay
4425%31225%7%64%
2340
11th to 20thDay
4413212235%74%
2471
21st to endDay
2213222335%76%
2520
July1st to 10thDay
1212313346%74%
2520
11th to 20thDay
2212312245%76%
2440
21st to endDay
211221346%7%71%
2403
August1st to 10thDay
221121223481%
2149
11th to 20thDay
211121223481%
2122
21st to endDay
111322335%78%
2376
September1st to 10thDay
3212211236%78%
2160
11th to 20thDay
321221112382%
2012
21st to endDay
221121224479%
1980
October1st to 10thDay
5%223323346%70%
1980
11th to 20thDay
421221223476%
1892
21st to endDay
431222234474%
1922
November1st to 10thDay
4212322346%72%
1674
11th to 20thDay
5%212223345%71%
1710
21st to endDay
5%323433347%64%
1710
December1st to 10thDay
8%323423347%61%
1710
11th to 20thDay
14%5%2442346%9%47%
2169
21st to endDay
12%5%25%6%3445%6%47%
3140
January1st to 10thNight
10%424323446%58%
2638
11th to 20thNight
7%413313337%63%
2303
21st to endNight
10%424322336%61%
2376
February1st to 10thNight
8%423423335%65%
2160
11th to 20thNight
9%423313335%64%
2098
21st to endNight
11%4134235%36%57%
1776
March1st to 10thNight
8%5%35%43445%8%52%
2160
11th to 20thNight
9%42432335%7%58%
2120
21st to endNight
8%5%2333335%7%57%
1980
April1st to 10thNight
9%5%34323437%57%
1800
11th to 20thNight
6%7%23223245%65%
1779
21st to endNight
6%6%24313346%63%
1800
May1st to 10thNight
6%8%24423346%59%
1720
11th to 20thNight
5%7%34324346%59%
1594
21st to endNight
5%8%33312236%63%
1746
June1st to 10thNight
46%25%32345%6%60%
1620
11th to 20thNight
3413312246%69%
1481
21st to endNight
242431325%6%69%
1440
July1st to 10thNight
1413321226%73%
1440
11th to 20thNight
212332335%7%69%
1516
21st to endNight
3313423448%66%
1910
August1st to 10thNight
3312212246%75%
1790
11th to 20thNight
2212211236%77%
1780
21st to endNight
121322325%5%74%
1980
September1st to 10thNight
4313322225%73%
1800
11th to 20thNight
231121123480%
1948
21st to endNight
2212211235%78%
1980
October1st to 10thNight
4313423345%69%
1980
11th to 20thNight
5%412212245%73%
2064
21st to endNight
431332324471%
2432
November1st to 10thNight
5%212322236%70%
2405
11th to 20thNight
4323323246%70%
2470
21st to endNight
6%41432335%7%60%
2470
December1st to 10thNight
8%424413347%58%
2470
11th to 20thNight
14%6%35%424347%48%
3041
21st to endNight
13%7%26%5%245%5%6%45%
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
27%37%18%23%21%
1519
11th to 20thDay
25%46%14%23%28%
1684
21st to endDay
29%36%16%18%29%
1980
February1st to 10thDay
24%35%14%25%29%
1800
11th to 20thDay
23%45%16%26%26%
1760
21st to endDay
27%6%7%17%24%19%
1480
March1st to 10thDay
29%6%8%18%18%21%
1800
11th to 20thDay
25%5%10%20%23%17%
1840
21st to endDay
23%6%11%23%21%15%
2371
April1st to 10thDay
23%6%11%26%19%14%
2160
11th to 20thDay
21%5%6%22%28%18%
2132
21st to endDay
19%48%25%31%13%
2160
May1st to 10thDay
23%6%9%20%27%15%
2237
11th to 20thDay
22%5%7%23%29%13%
2304
21st to endDay
23%5%9%28%27%9%
2530
June1st to 10thDay
22%5%8%20%30%15%
2340
11th to 20thDay
16%3414%33%30%
2471
21st to endDay
15%3310%29%40%
2520
July1st to 10thDay
13%3411%24%46%
2451
11th to 20thDay
13%36%19%29%30%
2365
21st to endDay
13%5%8%24%32%18%
2269
August1st to 10thDay
11%2214%32%39%
2135
11th to 20thDay
9%2213%36%37%
2122
21st to endDay
10%4313%35%36%
2376
September1st to 10thDay
12%3410%32%39%
2160
11th to 20thDay
11%2212%35%38%
2012
21st to endDay
10%3317%32%35%
1980
October1st to 10thDay
17%5%5%19%23%32%
1980
11th to 20thDay
16%3415%26%36%
1892
21st to endDay
16%3412%20%44%
1922
November1st to 10thDay
15%36%17%25%35%
1674
11th to 20thDay
17%46%16%18%40%
1710
21st to endDay
22%47%12%23%32%
1710
December1st to 10thDay
25%47%18%19%27%
1710
11th to 20thDay
34%6%9%14%18%18%
2168
21st to endDay
38%6%7%21%15%13%
3140
January1st to 10thNight
28%46%16%24%23%
2638
11th to 20thNight
25%5%6%16%20%29%
2303
21st to endNight
27%48%16%15%31%
2376
February1st to 10thNight
25%36%15%22%29%
2160
11th to 20thNight
24%47%18%21%26%
2099
21st to endNight
29%5%7%15%21%23%
1776
March1st to 10thNight
30%6%8%20%18%18%
2160
11th to 20thNight
27%5%10%18%24%17%
2120
21st to endNight
26%6%10%23%21%14%
1980
April1st to 10thNight
28%5%12%23%18%14%
1800
11th to 20thNight
22%5%9%23%22%18%
1779
21st to endNight
23%5%10%25%26%11%
1800
May1st to 10thNight
26%6%11%22%20%14%
1720
11th to 20thNight
25%7%11%26%21%11%
1595
21st to endNight
23%412%30%24%8%
1746
June1st to 10thNight
25%5%10%21%25%12%
1620
11th to 20thNight
17%5%6%19%32%22%
1481
21st to endNight
19%3413%29%33%
1440
July1st to 10thNight
15%3314%29%36%
1400
11th to 20thNight
17%36%22%28%24%
1468
21st to endNight
20%48%24%30%14%
1804
August1st to 10thNight
15%3413%36%29%
1780
11th to 20thNight
13%2415%37%29%
1780
21st to endNight
14%3314%35%31%
1980
September1st to 10thNight
18%3411%32%34%
1800
11th to 20thNight
11%2414%31%37%
1948
21st to endNight
13%3415%33%32%
1980
October1st to 10thNight
18%5%6%19%22%30%
1980
11th to 20thNight
18%3416%26%32%
2064
21st to endNight
18%4310%21%43%
2432
November1st to 10thNight
17%45%16%23%35%
2405
11th to 20thNight
18%45%15%19%39%
2470
21st to endNight
24%47%16%17%31%
2470
December1st to 10thNight
27%5%8%15%17%29%
2470
11th to 20thNight
36%5%8%14%20%16%
3041
21st to endNight
37%8%9%18%15%13%
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
14%5%27%36%17%1
180
11th to 20thDay
12%7%26%32%21%2
331
21st to endDay
10%11%32%32%14%2
462
February1st to 10thDay
11%9%28%32%15%5%
420
11th to 20thDay
12%10%34%33%9%2
415
21st to endDay
12%11%33%33%10%
346
March1st to 10thDay
13%14%31%28%12%2
420
11th to 20thDay
13%12%28%31%14%3
420
21st to endDay
10%14%36%27%11%2
462
April1st to 10thDay
8%11%32%27%16%5%
420
11th to 20thDay
317%37%28%11%3
412
21st to endDay
415%30%30%14%7%
420
May1st to 10thDay
10%13%31%28%15%3
423
11th to 20thDay
8%10%30%32%16%4
448
21st to endDay
7%12%32%29%18%3
485
June1st to 10thDay
38%23%37%22%6%
450
11th to 20thDay
45%23%39%22%8%
450
21st to endDay
3421%44%21%8%
450
July1st to 10thDay
2215%46%26%10%
446
11th to 20thDay
111%44%28%16%
444
21st to endDay
1211%46%25%14%
487
August1st to 10thDay
2115%48%22%12%
447
11th to 20thDay
2211%49%24%13%
437
21st to endDay
1116%45%26%11%
495
September1st to 10thDay
3222%44%19%10%
450
11th to 20thDay
3424%42%20%8%
450
21st to endDay
26%26%33%24%9%
450
October1st to 10thDay
48%26%34%23%5%
450
11th to 20thDay
47%27%35%22%4
450
21st to endDay
5%7%22%37%23%7%
437
November1st to 10thDay
5%416%40%30%5%
270
11th to 20thDay
6%420%40%24%7%
270
21st to endDay
6%426%36%22%6%
270
December1st to 10thDay
10%415%45%23%4
270
11th to 20thDay
7%5%22%41%21%4
270
21st to endDay
16%5%24%38%15%2
295
January1st to 10thNight
11%415%43%23%5%
760
11th to 20thNight
9%315%43%27%3
728
21st to endNight
13%318%43%21%3
792
February1st to 10thNight
10%315%43%24%5%
720
11th to 20thNight
10%318%43%23%5%
704
21st to endNight
13%5%16%43%19%4
592
March1st to 10thNight
8%318%48%19%3
720
11th to 20thNight
11%215%39%26%6%
720
21st to endNight
9%418%44%23%3
792
April1st to 10thNight
10%413%43%26%4
720
11th to 20thNight
7%5%17%41%26%3
711
21st to endNight
6%5%16%38%29%7%
720
May1st to 10thNight
8%5%14%38%31%4
640
11th to 20thNight
7%315%34%37%4
531
21st to endNight
6%6%16%38%30%4
584
June1st to 10thNight
4315%39%36%3
540
11th to 20thNight
325%36%48%5%
401
21st to endNight
31235%55%4
360
July1st to 10thNight
21114%72%11%
350
11th to 20thNight
2221%63%12%
424
21st to endNight
21323%56%14%
728
August1st to 10thNight
315%27%52%13%
716
11th to 20thNight
11222%56%18%
712
21st to endNight
21321%56%17%
792
September1st to 10thNight
5%26%24%47%16%
720
11th to 20thNight
225%31%48%11%
720
21st to endNight
317%33%46%10%
720
October1st to 10thNight
5%19%43%39%5%
720
11th to 20thNight
5%39%37%38%7%
720
21st to endNight
4212%37%33%11%
792
November1st to 10thNight
5%19%42%33%10%
740
11th to 20thNight
419%45%33%8%
760
21st to endNight
7%313%44%29%5%
760
December1st to 10thNight
9%313%42%28%5%
760
11th to 20thNight
13%316%41%22%4
760
21st to endNight
10%320%42%19%5%
836
Page generated at 2024-01-09 22:24:58. Version ac07

Copyright 2024 Attilla Danko


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