This page made possible by the sponsorship of: Michael Rudy.

Mount Wilson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Wilson Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
65%
24383
34%
74372
26%
24321
74%
24321
49%
36422

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
47%
757
28%
2646
20%
756
61%
756
34%
1342
January11th to 20thNight
49%
740
36%
2418
24%
740
68%
740
42%
1190
January21st to endNight
44%
792
37%
2376
20%
792
64%
792
38%
1270
February1st to 10thNight
51%
720
36%
2160
26%
720
65%
720
42%
1119
February11th to 20thNight
50%
700
34%
2098
24%
700
65%
700
41%
1098
February21st to endNight
47%
592
28%
1776
20%
592
59%
592
36%
990
March1st to 10thNight
48%
720
25%
2160
19%
720
59%
720
34%
1226
March11th to 20thNight
57%
720
26%
2111
21%
720
65%
720
39%
1149
March21st to endNight
53%
792
31%
1980
17%
792
62%
792
42%
1150
April1st to 10thNight
59%
720
30%
1800
17%
720
65%
720
45%
1051
April11th to 20thNight
57%
711
33%
1779
15%
711
65%
711
51%
955
April21st to endNight
61%
720
31%
1800
13%
720
68%
720
53%
963
May1st to 10thNight
61%
650
28%
1730
15%
650
65%
650
49%
942
May11th to 20thNight
68%
531
31%
1595
22%
531
71%
531
49%
790
May21st to endNight
69%
582
32%
1746
24%
582
72%
582
57%
792
June1st to 10thNight
78%
527
31%
1607
26%
527
81%
527
60%
732
June11th to 20thNight
87%
360
38%
1440
41%
360
92%
360
63%
496
June21st to endNight
86%
360
39%
1440
47%
360
89%
360
67%
480
July1st to 10thNight
93%
350
39%
1400
46%
350
94%
350
70%
449
July11th to 20thNight
93%
375
37%
1419
42%
375
93%
375
63%
520
July21st to endNight
90%
740
34%
1815
25%
699
91%
699
66%
877
August1st to 10thNight
89%
716
36%
1780
23%
712
89%
712
76%
833
August11th to 20thNight
93%
712
41%
1780
26%
712
94%
712
84%
788
August21st to endNight
91%
792
43%
1980
30%
792
92%
792
78%
906
September1st to 10thNight
88%
720
43%
1800
32%
720
90%
720
74%
839
September11th to 20thNight
87%
720
45%
1934
39%
720
90%
720
79%
822
September21st to endNight
79%
720
44%
1980
36%
720
87%
720
73%
858
October1st to 10thNight
71%
720
38%
1980
30%
720
81%
720
58%
981
October11th to 20thNight
71%
720
42%
2087
36%
720
79%
720
61%
930
October21st to endNight
66%
792
41%
2420
35%
792
77%
792
56%
1092
November1st to 10thNight
66%
740
36%
2405
33%
740
77%
740
54%
1066
November11th to 20thNight
58%
760
38%
2470
27%
760
75%
760
52%
1112
November21st to endNight
54%
760
34%
2470
29%
760
65%
760
42%
1230
December1st to 10thNight
51%
760
33%
2470
24%
760
64%
760
37%
1287
December11th to 20thNight
45%
760
28%
2976
22%
759
59%
759
24%
1778
December21st to endNight
42%
832
20%
4544
22%
817
58%
817
20%
2319

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
6%312323345%69%
75038
Night
6%312323345%68%
74751
TransparencyDay
18%47%17%27%27%
74762
Night
20%48%16%25%28%
74548
SeeingDay
8%12%32%34%12%3
14454
Night
6%5%19%36%27%6%
24396

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
10%423324347%57%
1519
11th to 20thDay
9%31332335%6%61%
1643
21st to endDay
12%6%14312335%59%
1980
February1st to 10thDay
11%323213336%64%
1800
11th to 20thDay
11%41232225%6%61%
1750
21st to endDay
11%8%24322346%57%
1480
March1st to 10thDay
12%6%25%5%245%5%8%48%
1800
11th to 20thDay
10%5%2333336%8%55%
1842
21st to endDay
8%4233345%5%7%57%
2376
April1st to 10thDay
8%423323348%60%
2160
11th to 20thDay
4422323346%66%
2129
21st to endDay
7%313222346%68%
2160
May1st to 10thDay
6%5%23323446%63%
2227
11th to 20thDay
431321335%5%68%
2301
21st to endDay
5%313323347%66%
2530
June1st to 10thDay
4312213337%71%
2347
11th to 20thDay
2211112235%81%
2520
21st to endDay
111211222383%
2520
July1st to 10thDay
221111223381%
2520
11th to 20thDay
321221223481%
2488
21st to endDay
312212347%73%
2392
August1st to 10thDay
111121222385%
2149
11th to 20thDay
11111122486%
2122
21st to endDay
11211232482%
2376
September1st to 10thDay
211221223481%
2160
11th to 20thDay
11111222386%
2025
21st to endDay
221111233480%
1980
October1st to 10thDay
5%312213335%72%
1980
11th to 20thDay
421121213478%
1870
21st to endDay
431221233475%
1933
November1st to 10thDay
5%31232333472%
1674
11th to 20thDay
5%413223335%69%
1710
21st to endDay
6%32433435%6%63%
1710
December1st to 10thDay
8%5%2442335%6%59%
1710
11th to 20thDay
19%5%25%42345%8%44%
2169
21st to endDay
13%7%25%6%34446%47%
2986
January1st to 10thNight
11%41332345%6%59%
2646
11th to 20thNight
7%413313446%63%
2418
21st to endNight
10%5%23312235%62%
2376
February1st to 10thNight
8%423312236%65%
2160
11th to 20thNight
9%423323345%63%
2098
21st to endNight
11%6%2332335%5%57%
1776
March1st to 10thNight
9%6%1343436%8%53%
2160
11th to 20thNight
8%5%1332345%7%59%
2111
21st to endNight
7%5%1432345%6%60%
1980
April1st to 10thNight
7%313323347%63%
1800
11th to 20thNight
45%12214435%69%
1779
21st to endNight
5%422213336%71%
1800
May1st to 10thNight
5%5%1331245%6%65%
1730
11th to 20thNight
3412313335%72%
1594
21st to endNight
2312213446%73%
1746
June1st to 10thNight
1312212347%75%
1607
11th to 20thNight
11111133485%
1440
21st to endNight
111121122386%
1440
July1st to 10thNight
11111212387%
1440
11th to 20thNight
11222223480%
1467
21st to endNight
4111212247%75%
1922
August1st to 10thNight
121121123482%
1790
11th to 20thNight
1111122387%
1780
21st to endNight
21111222385%
1980
September1st to 10thNight
311111123483%
1800
11th to 20thNight
11111212287%
1934
21st to endNight
121111122385%
1980
October1st to 10thNight
431232232474%
1980
11th to 20thNight
421221122380%
2087
21st to endNight
331222322376%
2420
November1st to 10thNight
5%31332223472%
2405
11th to 20thNight
5%313223345%71%
2470
21st to endNight
6%323424446%62%
2470
December1st to 10thNight
8%414424346%59%
2470
11th to 20thNight
17%6%244235%46%46%
3041
21st to endNight
11%6%25%436%5%5%7%48%
4654

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%47%17%26%19%
1519
11th to 20thDay
25%47%11%27%26%
1643
21st to endDay
29%47%15%19%26%
1980
February1st to 10thDay
24%46%14%24%28%
1800
11th to 20thDay
23%45%15%28%24%
1760
21st to endDay
28%5%7%17%23%19%
1480
March1st to 10thDay
33%5%9%16%18%20%
1800
11th to 20thDay
25%6%9%19%24%16%
1842
21st to endDay
24%7%11%21%23%14%
2376
April1st to 10thDay
23%6%12%25%22%12%
2160
11th to 20thDay
19%5%8%22%28%17%
2132
21st to endDay
18%410%24%33%12%
2160
May1st to 10thDay
22%6%11%20%28%14%
2227
11th to 20thDay
19%49%25%30%12%
2304
21st to endDay
19%411%28%29%9%
2530
June1st to 10thDay
16%5%9%22%31%18%
2347
11th to 20thDay
11%36%16%30%35%
2520
21st to endDay
11%2410%30%44%
2520
July1st to 10thDay
10%3313%24%48%
2451
11th to 20thDay
11%36%18%30%32%
2413
21st to endDay
11%49%22%33%21%
2259
August1st to 10thDay
8%2413%30%43%
2135
11th to 20thDay
6%1414%35%39%
2122
21st to endDay
8%2412%35%39%
2376
September1st to 10thDay
10%26%9%33%41%
2160
11th to 20thDay
7%1414%34%40%
2025
21st to endDay
9%3416%34%33%
1980
October1st to 10thDay
16%36%17%24%33%
1980
11th to 20thDay
13%25%14%30%36%
1870
21st to endDay
14%3413%22%44%
1933
November1st to 10thDay
16%36%16%29%30%
1674
11th to 20thDay
18%45%13%22%37%
1710
21st to endDay
22%48%11%24%31%
1710
December1st to 10thDay
25%6%6%16%21%25%
1710
11th to 20thDay
37%6%9%13%19%16%
2168
21st to endDay
36%6%9%18%18%13%
2986
January1st to 10thNight
27%47%14%26%22%
2646
11th to 20thNight
22%5%7%15%23%28%
2418
21st to endNight
27%48%13%20%29%
2376
February1st to 10thNight
25%37%13%23%29%
2160
11th to 20thNight
24%5%416%25%26%
2099
21st to endNight
29%5%8%14%22%23%
1776
March1st to 10thNight
30%5%9%19%18%19%
2160
11th to 20thNight
24%8%10%15%24%18%
2111
21st to endNight
25%6%9%22%24%14%
1980
April1st to 10thNight
25%5%9%24%20%16%
1800
11th to 20thNight
19%5%9%22%25%20%
1779
21st to endNight
19%412%22%31%12%
1800
May1st to 10thNight
20%7%11%21%24%16%
1730
11th to 20thNight
19%5%9%24%29%13%
1595
21st to endNight
14%413%27%31%11%
1746
June1st to 10thNight
12%6%12%24%26%21%
1607
11th to 20thNight
9%39%16%27%35%
1440
21st to endNight
9%25%11%27%47%
1440
July1st to 10thNight
8%2314%22%51%
1400
11th to 20thNight
10%38%17%27%34%
1419
21st to endNight
12%48%23%31%22%
1815
August1st to 10thNight
9%26%10%30%43%
1780
11th to 20thNight
6%16%15%33%39%
1780
21st to endNight
7%35%14%32%39%
1980
September1st to 10thNight
10%25%13%26%46%
1800
11th to 20thNight
7%15%13%32%42%
1934
21st to endNight
9%35%16%31%37%
1980
October1st to 10thNight
18%35%16%25%34%
1980
11th to 20thNight
13%37%14%27%36%
2087
21st to endNight
15%35%9%23%45%
2420
November1st to 10thNight
17%36%14%27%33%
2405
11th to 20thNight
17%46%12%22%39%
2470
21st to endNight
23%47%15%19%31%
2470
December1st to 10thNight
26%47%14%20%29%
2470
11th to 20thNight
37%5%7%12%22%16%
3041
21st to endNight
32%6%13%18%19%12%
4654

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%11%33%30%7%3
180
11th to 20thDay
14%16%37%23%9%
315
21st to endDay
13%20%31%27%8%1
462
February1st to 10thDay
14%15%32%28%8%2
420
11th to 20thDay
15%20%35%23%5%1
415
21st to endDay
16%20%37%22%5%
346
March1st to 10thDay
18%22%37%17%6%
420
11th to 20thDay
17%18%35%22%7%1
420
21st to endDay
16%21%39%20%5%
462
April1st to 10thDay
12%17%37%24%8%2
420
11th to 20thDay
8%25%40%22%41
412
21st to endDay
6%20%32%29%10%2
420
May1st to 10thDay
13%16%37%26%7%2
423
11th to 20thDay
9%12%36%32%9%3
448
21st to endDay
9%14%38%27%10%2
485
June1st to 10thDay
412%29%38%14%4
450
11th to 20thDay
38%31%41%12%6%
450
21st to endDay
35%33%41%12%6%
450
July1st to 10thDay
2222%50%20%5%
446
11th to 20thDay
215%53%20%9%
444
21st to endDay
1117%54%18%9%
487
August1st to 10thDay
2326%50%15%5%
447
11th to 20thDay
2221%50%20%5%
437
21st to endDay
1124%51%18%4
495
September1st to 10thDay
3334%43%14%3
450
11th to 20thDay
26%38%40%13%1
450
21st to endDay
49%31%41%12%3
450
October1st to 10thDay
5%11%34%37%11%2
450
11th to 20thDay
410%39%36%11%1
450
21st to endDay
6%12%32%33%14%3
448
November1st to 10thDay
6%13%26%40%14%2
270
11th to 20thDay
9%9%37%29%14%3
270
21st to endDay
6%14%35%31%12%2
270
December1st to 10thDay
11%13%31%31%13%1
270
11th to 20thDay
6%16%33%28%14%2
270
21st to endDay
19%19%31%20%8%2
252
January1st to 10thNight
13%8%27%35%14%3
760
11th to 20thNight
9%6%31%37%15%2
740
21st to endNight
14%9%28%35%13%2
792
February1st to 10thNight
10%7%26%36%18%3
720
11th to 20thNight
11%8%26%33%19%4
704
21st to endNight
12%9%27%33%17%2
592
March1st to 10thNight
10%8%29%35%16%2
720
11th to 20thNight
10%6%22%34%23%5%
720
21st to endNight
9%10%24%37%18%3
792
April1st to 10thNight
9%5%21%43%19%3
720
11th to 20thNight
6%8%25%37%22%2
711
21st to endNight
5%7%24%37%22%6%
720
May1st to 10thNight
7%424%39%21%4
650
11th to 20thNight
5%5%17%38%32%4
531
21st to endNight
3420%41%28%4
584
June1st to 10thNight
2214%41%38%3
527
11th to 20thNight
1110%39%43%6%
360
21st to endNight
29%38%45%6%
360
July1st to 10thNight
1426%58%12%
350
11th to 20thNight
2228%57%12%
375
21st to endNight
21232%51%12%
740
August1st to 10thNight
26%34%47%12%
716
11th to 20thNight
1327%53%15%
712
21st to endNight
2430%49%15%
792
September1st to 10thNight
316%33%43%14%
720
11th to 20thNight
110%40%38%11%
720
21st to endNight
2114%37%38%9%
720
October1st to 10thNight
5%217%46%28%3
720
11th to 20thNight
5%418%38%30%6%
720
21st to endNight
46%19%36%24%10%
792
November1st to 10thNight
6%419%40%24%6%
740
11th to 20thNight
6%427%36%21%6%
760
21st to endNight
8%6%28%39%16%4
760
December1st to 10thNight
8%8%26%39%17%2
760
11th to 20thNight
13%9%27%37%13%1
760
21st to endNight
11%12%30%35%11%3
836
Page generated at 2024-01-12 22:38:08. Version ac07

Copyright 2024 Attilla Danko


Mount Wilson Observatory Clear Sky Chart |Charts Home