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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 2005-11-05 to 2020-06-03.

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
69%
19522
35%
57691
28%
19474
76%
19474
50%
28458

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 15thNight
54%
897
29%
3103
23%
896
66%
896
38%
1538
January16th to endNight
53%
960
35%
2922
24%
960
67%
960
39%
1566
February1st to 15thNight
59%
879
35%
2645
29%
877
69%
877
44%
1368
February16th to endNight
52%
792
31%
2375
24%
792
63%
792
36%
1340
March1st to 15thNight
55%
900
26%
2700
22%
900
64%
900
37%
1507
March16th to endNight
62%
960
29%
2507
20%
960
68%
960
42%
1436
April1st to 15thNight
60%
899
29%
2249
16%
899
65%
899
42%
1334
April16th to endNight
65%
892
32%
2230
15%
892
71%
892
53%
1199
May1st to 15thNight
66%
764
28%
2114
18%
764
69%
764
47%
1137
May16th to endNight
69%
705
32%
2117
24%
705
72%
705
51%
1017
June1st to 15thNight
84%
557
35%
1831
33%
557
86%
557
61%
771
June16th to endNight
94%
420
38%
1680
47%
420
94%
420
69%
553
July1st to 15thNight
94%
400
38%
1600
45%
400
94%
400
64%
544
July16th to endNight
90%
728
32%
1941
26%
687
91%
687
63%
900
August1st to 15thNight
92%
825
39%
2060
25%
821
92%
821
80%
934
August16th to endNight
93%
892
43%
2230
30%
892
94%
892
79%
1010
September1st to 15thNight
92%
840
43%
2134
34%
840
92%
840
76%
975
September16th to endNight
85%
836
44%
2310
40%
836
89%
836
74%
994
October1st to 15thNight
73%
840
38%
2324
33%
840
80%
840
58%
1146
October16th to endNight
74%
896
42%
2722
40%
896
81%
896
58%
1216
November1st to 15thNight
71%
880
37%
2860
35%
880
80%
880
55%
1270
November16th to endNight
64%
900
35%
2925
32%
900
72%
900
44%
1443
December1st to 15thNight
57%
900
31%
2925
26%
900
66%
900
34%
1594
December16th to endNight
48%
960
34%
3187
24%
960
63%
960
34%
1666

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%312212235%68%
58686
Night
5%312212235%69%
57891
TransparencyDay
18%46%17%27%25%
58378
Night
19%46%16%26%27%
57699
SeeingDay
7%10%32%34%11%3
9556
Night
6%315%35%31%7%
19533

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 15thDay
8%323313246%60%
1822
16th to endDay
12%413312235%58%
2353
February1st to 15thDay
11%312112236%62%
2201
16th to endDay
13%6%13312245%55%
1979
March1st to 15thDay
10%5%13423447%52%
2250
16th to endDay
8%32332346%7%53%
2767
April1st to 15thDay
8%5%23312347%57%
2689
16th to endDay
6%212212336%67%
2665
May1st to 15thDay
5%413212346%64%
2822
16th to endDay
6%312212246%65%
3053
June1st to 15thDay
421112235%74%
2834
16th to endDay
11112385%
2940
July1st to 15thDay
21111223477%
2940
16th to endDay
311212236%75%
2808
August1st to 15thDay
111111286%
2473
16th to endDay
111122483%
2676
September1st to 15thDay
1111112383%
2484
16th to endDay
22111222281%
2310
October1st to 15thDay
6%21121222472%
2293
16th to endDay
32221123375%
2203
November1st to 15thDay
42232323470%
1989
16th to endDay
7%213223346%61%
2025
December1st to 15thDay
8%41231345%8%56%
2025
16th to endDay
12%412322346%56%
2085
January1st to 15thNight
8%312313345%60%
3103
16th to endNight
10%413212345%60%
2922
February1st to 15thNight
8%422212236%63%
2641
16th to endNight
12%42232334457%
2375
March1st to 15thNight
9%5%1232335%7%56%
2700
16th to endNight
6%41322345%7%58%
2507
April1st to 15thNight
8%413222336%61%
2249
16th to endNight
3311112235%72%
2230
May1st to 15thNight
43221225%6%67%
2114
16th to endNight
222212226%74%
2116
June1st to 15thNight
111111236%78%
1831
16th to endNight
11111288%
1680
July1st to 15thNight
11111212382%
1680
16th to endNight
31112236%74%
2056
August1st to 15thNight
111112384%
2070
16th to endNight
111122286%
2230
September1st to 15thNight
111112384%
2134
16th to endNight
111112285%
2310
October1st to 15thNight
421221122474%
2324
16th to endNight
32121122377%
2722
November1st to 15thNight
321221223373%
2860
16th to endNight
7%313313346%62%
2925
December1st to 15thNight
9%324423346%56%
2925
16th to endNight
10%413212335%60%
3187

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 15thDay
26%35%14%25%24%
1822
16th to endDay
28%47%14%21%23%
2353
February1st to 15thDay
23%35%15%25%25%
2201
16th to endDay
30%45%18%27%13%
1979
March1st to 15thDay
30%5%8%18%19%17%
2250
16th to endDay
26%6%10%21%20%13%
2767
April1st to 15thDay
26%5%10%23%22%11%
2689
16th to endDay
18%48%21%32%14%
2665
May1st to 15thDay
22%5%8%20%27%14%
2822
16th to endDay
21%48%26%29%9%
3056
June1st to 15thDay
16%46%19%32%21%
2834
16th to endDay
9%1310%31%43%
2940
July1st to 15thDay
12%3312%26%41%
2791
16th to endDay
11%38%21%30%23%
2660
August1st to 15thDay
6%2312%35%40%
2459
16th to endDay
7%1313%39%33%
2676
September1st to 15thDay
8%1412%34%38%
2484
16th to endDay
9%2313%36%34%
2310
October1st to 15thDay
17%3417%26%31%
2293
16th to endDay
14%3312%23%42%
2203
November1st to 15thDay
17%35%14%26%31%
1989
16th to endDay
23%46%13%22%29%
2025
December1st to 15thDay
25%6%7%17%20%22%
2025
16th to endDay
28%5%7%16%22%19%
2085
January1st to 15thNight
24%46%16%23%25%
3103
16th to endNight
26%47%13%20%26%
2922
February1st to 15thNight
23%46%14%24%26%
2652
16th to endNight
30%45%16%25%18%
2375
March1st to 15thNight
28%5%9%17%21%17%
2700
16th to endNight
23%7%8%22%23%14%
2507
April1st to 15thNight
26%5%8%23%21%14%
2249
16th to endNight
18%48%21%30%16%
2230
May1st to 15thNight
21%6%8%21%27%15%
2115
16th to endNight
17%5%9%25%30%11%
2117
June1st to 15thNight
10%5%7%20%30%24%
1831
16th to endNight
8%15%12%27%44%
1680
July1st to 15thNight
9%2412%25%45%
1600
16th to endNight
12%47%20%29%25%
1941
August1st to 15thNight
6%15%10%34%40%
2060
16th to endNight
6%2413%37%35%
2230
September1st to 15thNight
9%1411%30%42%
2134
16th to endNight
8%2313%33%38%
2310
October1st to 15thNight
17%2415%29%30%
2324
16th to endNight
14%2410%22%44%
2722
November1st to 15thNight
16%35%14%26%34%
2860
16th to endNight
22%46%15%20%30%
2925
December1st to 15thNight
28%5%6%14%19%24%
2925
16th to endNight
26%5%6%14%26%20%
3187

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 15thDay
16%11%33%25%11%1
180
16th to endDay
12%14%31%30%9%
465
February1st to 15thDay
14%15%28%29%9%2
446
16th to endDay
14%18%38%21%6%
398
March1st to 15thDay
17%19%33%20%9%
450
16th to endDay
12%17%39%23%6%1
480
April1st to 15thDay
12%18%36%22%7%1
449
16th to endDay
6%23%35%24%7%2
438
May1st to 15thDay
10%13%36%28%7%3
468
16th to endDay
9%14%38%26%9%1
522
June1st to 15thDay
47%31%39%11%5%
444
16th to endDay
2232%44%11%7%
435
July1st to 15thDay
2221%47%18%7%
425
16th to endDay
218%53%15%9%
455
August1st to 15thDay
2228%48%12%5%
419
16th to endDay
122%50%19%6%
460
September1st to 15thDay
2236%43%12%2
435
16th to endDay
38%37%37%10%2
435
October1st to 15thDay
5%12%40%33%6%1
435
16th to endDay
48%34%31%17%3
427
November1st to 15thDay
36%22%43%20%4
225
16th to endDay
8%9%34%29%14%3
225
December1st to 15thDay
8%11%28%34%16%1
225
16th to endDay
10%17%29%27%12%3
215
January1st to 15thNight
9%6%24%40%16%2
900
16th to endNight
11%5%23%39%17%2
960
February1st to 15thNight
9%5%19%38%22%4
884
16th to endNight
12%6%22%34%20%2
792
March1st to 15thNight
9%5%24%37%19%4
900
16th to endNight
7%5%20%38%24%4
960
April1st to 15thNight
9%5%19%39%23%3
899
16th to endNight
46%21%36%25%5%
892
May1st to 15thNight
6%320%36%28%5%
765
16th to endNight
4417%39%29%4
707
June1st to 15thNight
110%38%43%4
557
16th to endNight
1233%54%8%
420
July1st to 15thNight
1111%70%15%
400
16th to endNight
2223%56%14%
728
August1st to 15thNight
327%52%15%
825
16th to endNight
1220%55%19%
892
September1st to 15thNight
1429%47%16%
840
16th to endNight
19%33%42%12%
836
October1st to 15thNight
4214%41%31%5%
840
16th to endNight
4313%33%32%11%
896
November1st to 15thNight
4216%42%26%7%
880
16th to endNight
8%417%38%23%7%
900
December1st to 15thNight
9%7%18%43%19%1
900
16th to endNight
11%10%23%38%13%2
960
Page generated at 2020-06-04 00:18:55. Version ac04

Copyright 2020 A. Danko


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