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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 2017-09-10.

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
71%
15818
34%
46714
28%
15770
77%
15770
51%
22815

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
55%
717
29%
2479
23%
716
68%
716
40%
1211
January16th to endNight
52%
768
34%
2337
23%
768
66%
768
39%
1263
February1st to 15thNight
60%
699
34%
2105
28%
697
70%
697
44%
1091
February16th to endNight
54%
632
28%
1895
24%
632
62%
632
34%
1094
March1st to 15thNight
60%
720
29%
2160
24%
720
70%
720
45%
1108
March16th to endNight
64%
768
30%
2006
21%
768
70%
768
45%
1120
April1st to 15thNight
61%
719
29%
1799
17%
719
66%
719
46%
1029
April16th to endNight
65%
720
32%
1800
15%
720
70%
720
53%
963
May1st to 15thNight
66%
611
29%
1691
19%
611
70%
611
49%
897
May16th to endNight
71%
561
31%
1685
23%
561
74%
561
54%
792
June1st to 15thNight
82%
471
32%
1551
30%
471
85%
471
60%
661
June16th to endNight
94%
360
39%
1440
49%
360
95%
360
71%
463
July1st to 15thNight
93%
340
36%
1360
43%
340
93%
340
65%
459
July16th to endNight
90%
623
33%
1644
27%
582
91%
582
63%
763
August1st to 15thNight
92%
717
37%
1790
25%
713
92%
713
81%
808
August16th to endNight
93%
764
44%
1910
30%
764
94%
764
79%
864
September1st to 15thNight
92%
700
40%
1777
32%
700
92%
700
74%
824
September16th to endNight
87%
656
44%
1815
41%
656
90%
656
74%
779
October1st to 15thNight
73%
660
37%
1826
33%
660
79%
660
55%
922
October16th to endNight
74%
704
39%
2139
38%
704
80%
704
54%
993
November1st to 15thNight
70%
700
36%
2275
33%
700
80%
700
55%
1003
November16th to endNight
68%
720
36%
2340
35%
720
75%
720
48%
1107
December1st to 15thNight
60%
720
32%
2340
27%
720
70%
720
35%
1254
December16th to endNight
48%
768
32%
2550
24%
768
62%
768
33%
1347

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%212212235%69%
47884
Night
5%212212235%70%
46914
TransparencyDay
18%46%16%27%26%
47576
Night
18%46%15%26%28%
46722
SeeingDay
7%11%34%34%9%3
5897
Night
5%314%34%33%7%
15829

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%313313235%62%
1456
16th to endDay
14%412312335%57%
1882
February1st to 15thDay
10%322112237%61%
1751
16th to endDay
16%6%23312235%52%
1579
March1st to 15thDay
7%313213448%57%
1800
16th to endDay
7%22322346%7%56%
2213
April1st to 15thDay
8%422312347%60%
2149
16th to endDay
6%211111236%69%
2160
May1st to 15thDay
4413112345%67%
2256
16th to endDay
5%212212236%69%
2429
June1st to 15thDay
4211112235%73%
2406
16th to endDay
11112287%
2520
July1st to 15thDay
22111223377%
2520
16th to endDay
311212235%75%
2403
August1st to 15thDay
11111286%
2160
16th to endDay
1111121383%
2292
September1st to 15thDay
11111112383%
2069
16th to endDay
1211222282%
1815
October1st to 15thDay
7%21121222469%
1801
16th to endDay
43221233372%
1731
November1st to 15thDay
5%3331322370%
1584
16th to endDay
7%213222245%64%
1620
December1st to 15thDay
8%41232335%8%56%
1620
16th to endDay
14%412312335%56%
1668
January1st to 15thNight
8%312312345%62%
2479
16th to endNight
10%413312235%59%
2337
February1st to 15thNight
7%423212236%63%
2101
16th to endNight
15%42222334454%
1895
March1st to 15thNight
6%41222325%7%63%
2160
16th to endNight
5%31321245%7%61%
2006
April1st to 15thNight
8%313212235%65%
1799
16th to endNight
4211111235%74%
1800
May1st to 15thNight
332212246%69%
1691
16th to endNight
221212226%76%
1684
June1st to 15thNight
111111236%78%
1551
16th to endNight
11290%
1440
July1st to 15thNight
11111212282%
1440
16th to endNight
31112246%74%
1759
August1st to 15thNight
111112484%
1800
16th to endNight
111122286%
1910
September1st to 15thNight
1111112383%
1777
16th to endNight
111112286%
1815
October1st to 15thNight
431221122473%
1826
16th to endNight
42121222375%
2139
November1st to 15thNight
421221223473%
2275
16th to endNight
6%312213235%66%
2340
December1st to 15thNight
8%323423346%58%
2340
16th to endNight
11%413212325%60%
2550

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
25%35%12%24%27%
1456
16th to endDay
29%47%13%19%25%
1882
February1st to 15thDay
23%47%16%25%24%
1751
16th to endDay
33%5%5%17%27%10%
1579
March1st to 15thDay
23%5%9%19%22%19%
1800
16th to endDay
23%7%11%23%20%13%
2213
April1st to 15thDay
23%5%10%24%24%10%
2149
16th to endDay
17%49%21%29%16%
2160
May1st to 15thDay
20%6%8%20%28%16%
2256
16th to endDay
19%49%26%30%10%
2432
June1st to 15thDay
17%46%16%31%22%
2406
16th to endDay
8%138%31%46%
2520
July1st to 15thDay
12%3311%26%42%
2371
16th to endDay
12%37%20%31%25%
2255
August1st to 15thDay
6%2312%35%39%
2146
16th to endDay
7%1412%38%34%
2292
September1st to 15thDay
9%1411%33%39%
2069
16th to endDay
8%3312%33%37%
1815
October1st to 15thDay
18%35%17%27%26%
1801
16th to endDay
17%3412%22%38%
1731
November1st to 15thDay
18%35%14%27%29%
1584
16th to endDay
22%36%12%23%30%
1620
December1st to 15thDay
25%6%8%17%20%21%
1620
16th to endDay
28%6%7%16%20%19%
1668
January1st to 15thNight
24%36%14%23%27%
2479
16th to endNight
27%47%13%18%29%
2337
February1st to 15thNight
23%47%14%24%25%
2112
16th to endNight
31%5%6%16%24%15%
1895
March1st to 15thNight
21%5%9%19%23%20%
2160
16th to endNight
20%8%9%22%23%15%
2006
April1st to 15thNight
24%48%24%23%14%
1799
16th to endNight
18%49%21%27%18%
1800
May1st to 15thNight
19%6%8%20%27%16%
1692
16th to endNight
14%6%10%24%31%12%
1685
June1st to 15thNight
11%5%8%18%30%25%
1551
16th to endNight
7%16%10%25%47%
1440
July1st to 15thNight
9%2411%25%47%
1360
16th to endNight
12%47%20%28%26%
1644
August1st to 15thNight
7%15%10%35%39%
1790
16th to endNight
6%2412%36%37%
1910
September1st to 15thNight
10%15%9%29%42%
1777
16th to endNight
8%2313%31%40%
1815
October1st to 15thNight
18%3415%31%26%
1826
16th to endNight
16%35%11%23%39%
2139
November1st to 15thNight
16%25%13%27%33%
2275
16th to endNight
19%36%15%20%33%
2340
December1st to 15thNight
27%5%7%14%21%24%
2340
16th to endNight
26%5%7%15%24%20%
2550

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
21%15%39%19%4
84
16th to endDay
16%15%35%26%5%
282
February1st to 15thDay
11%15%32%30%8%1
266
16th to endDay
18%19%36%18%5%
238
March1st to 15thDay
10%17%35%22%12%
270
16th to endDay
8%17%41%24%6%1
288
April1st to 15thDay
11%21%41%17%5%2
269
16th to endDay
5%29%38%21%31
270
May1st to 15thDay
10%18%37%27%42
288
16th to endDay
7%13%41%28%8%
330
June1st to 15thDay
5%10%35%37%6%3
315
16th to endDay
2237%41%8%7%
315
July1st to 15thDay
2224%49%14%6%
305
16th to endDay
219%56%12%8%
327
August1st to 15thDay
2231%50%8%4
315
16th to endDay
127%50%15%5%
332
September1st to 15thDay
2239%43%8%2
294
16th to endDay
26%44%36%8%2
255
October1st to 15thDay
6%15%39%31%6%1
255
16th to endDay
6%9%36%32%13%1
243
November1st to 15thDay
311%14%35%26%8%
90
16th to endDay
5%12%38%32%10%1
90
December1st to 15thDay
310%24%43%17%1
90
16th to endDay
16%16%32%26%8%
86
January1st to 15thNight
8%425%41%16%3
720
16th to endNight
12%5%23%36%19%3
768
February1st to 15thNight
9%5%19%39%22%4
704
16th to endNight
14%7%18%34%22%3
632
March1st to 15thNight
5%425%37%22%5%
720
16th to endNight
5%5%19%37%27%4
768
April1st to 15thNight
8%5%18%40%23%3
719
16th to endNight
37%22%35%26%5%
720
May1st to 15thNight
4320%36%28%5%
612
16th to endNight
3416%38%31%4
563
June1st to 15thNight
111%40%41%3
471
16th to endNight
1235%53%7%
360
July1st to 15thNight
210%71%15%
340
16th to endNight
11221%58%14%
623
August1st to 15thNight
324%55%15%
717
16th to endNight
1217%56%21%
764
September1st to 15thNight
2226%50%18%
700
16th to endNight
17%30%46%14%
656
October1st to 15thNight
5%213%36%36%6%
660
16th to endNight
5%312%33%34%11%
704
November1st to 15thNight
4217%37%29%8%
700
16th to endNight
7%416%39%25%7%
720
December1st to 15thNight
8%7%15%45%21%2
720
16th to endNight
12%10%22%39%13%2
768
Page generated at 2017-09-11 16:35:25. Version ac04

Copyright 2017 A. Danko


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