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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 2017-03-26.

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
70%
15268
34%
45168
28%
15220
76%
15220
51%
22170

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
65%
748
28%
1956
20%
748
70%
748
45%
1100
April1st to 15thNight
59%
659
30%
1649
17%
659
64%
659
44%
957
April16th to endNight
65%
660
29%
1650
13%
660
70%
660
52%
895
May1st to 15thNight
66%
560
30%
1550
19%
560
69%
560
48%
828
May16th to endNight
70%
513
28%
1541
21%
513
74%
513
53%
729
June1st to 15thNight
81%
432
33%
1422
31%
432
84%
432
59%
614
June16th to endNight
94%
330
36%
1320
45%
330
94%
330
70%
430
July1st to 15thNight
94%
310
37%
1240
45%
310
94%
310
64%
423
July16th to endNight
89%
575
30%
1512
25%
534
90%
534
61%
712
August1st to 15thNight
92%
657
40%
1640
27%
653
92%
653
81%
739
August16th to endNight
93%
700
41%
1750
28%
700
93%
700
79%
796
September1st to 15thNight
92%
660
42%
1677
34%
660
93%
660
74%
775
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%
45777
Night
5%212212235%70%
45368
TransparencyDay
18%46%16%27%26%
45469
Night
18%47%15%26%27%
45176
SeeingDay
7%11%36%34%7%2
5233
Night
5%315%34%32%7%
15279

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
8%22322347%7%55%
2149
April1st to 15thDay
9%422312336%59%
1969
16th to endDay
7%312111235%68%
1980
May1st to 15thDay
4412112245%67%
2067
16th to endDay
6%212212236%69%
2221
June1st to 15thDay
5%21112235%73%
2205
16th to endDay
111112286%
2310
July1st to 15thDay
321111223376%
2310
16th to endDay
311212235%75%
2227
August1st to 15thDay
1111387%
1980
16th to endDay
1111122383%
2100
September1st to 15thDay
1111112384%
1952
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%41321345%7%60%
1956
April1st to 15thNight
8%313212235%65%
1649
16th to endNight
4211111235%73%
1650
May1st to 15thNight
332212246%69%
1550
16th to endNight
22112126%76%
1540
June1st to 15thNight
11111136%79%
1422
16th to endNight
11290%
1320
July1st to 15thNight
11121212382%
1320
16th to endNight
41212236%73%
1627
August1st to 15thNight
11112484%
1650
16th to endNight
211121285%
1750
September1st to 15thNight
1111112384%
1677
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
24%7%11%23%19%13%
2149
April1st to 15thDay
25%6%11%23%22%11%
1969
16th to endDay
18%49%20%30%16%
1980
May1st to 15thDay
19%6%8%19%27%17%
2067
16th to endDay
18%49%27%30%9%
2224
June1st to 15thDay
17%5%6%16%31%22%
2205
16th to endDay
9%149%33%42%
2310
July1st to 15thDay
13%3311%26%41%
2161
16th to endDay
11%37%20%31%24%
2079
August1st to 15thDay
5%1413%35%38%
1966
16th to endDay
8%1413%38%33%
2100
September1st to 15thDay
8%1410%33%40%
1952
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
21%8%9%22%22%15%
1956
April1st to 15thNight
25%5%8%24%20%15%
1649
16th to endNight
19%49%20%28%17%
1650
May1st to 15thNight
19%7%9%19%26%17%
1551
16th to endNight
14%6%10%24%32%11%
1541
June1st to 15thNight
11%5%9%16%30%25%
1422
16th to endNight
8%16%11%27%44%
1320
July1st to 15thNight
9%2412%23%47%
1240
16th to endNight
13%47%19%28%26%
1512
August1st to 15thNight
6%15%10%36%38%
1640
16th to endNight
7%25%13%36%35%
1750
September1st to 15thNight
10%15%9%29%43%
1677
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%16%42%24%6%1
267
April1st to 15thDay
13%22%42%14%41
209
16th to endDay
6%31%37%21%11
210
May1st to 15thDay
9%18%39%27%32
228
16th to endDay
8%15%42%27%6%
266
June1st to 15thDay
6%10%40%37%32
255
16th to endDay
2341%40%5%6%
255
July1st to 15thDay
2328%51%10%2
245
16th to endDay
223%56%10%6%
271
August1st to 15thDay
1334%49%7%3
255
16th to endDay
128%51%12%5%
268
September1st to 15thDay
2239%44%7%3
255
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%419%37%27%4
748
April1st to 15thNight
8%6%19%39%22%3
659
16th to endNight
36%22%34%26%5%
660
May1st to 15thNight
4321%37%27%4
561
16th to endNight
35%17%37%32%4
515
June1st to 15thNight
212%40%40%3
432
16th to endNight
1236%53%6%
330
July1st to 15thNight
111%72%13%
310
16th to endNight
11221%57%14%
575
August1st to 15thNight
324%55%15%
657
16th to endNight
2217%57%21%
700
September1st to 15thNight
1226%50%18%
660
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 2017-03-27 08:15:15. Version ac04

Copyright 2017 A. Danko


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