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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 2016-01-22.

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%
13668
34%
40441
27%
13620
76%
13620
51%
19799

By Month

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
JanuaryNight
55%
1325
32%
4307
24%
1324
68%
1324
42%
2151
FebruaryNight
56%
1103
31%
3316
26%
1101
65%
1101
39%
1808
MarchNight
60%
1240
28%
3473
22%
1240
68%
1240
43%
1889
AprilNight
62%
1199
30%
2999
15%
1199
68%
1199
48%
1690
MayNight
69%
974
29%
2806
20%
974
72%
974
51%
1402
JuneNight
86%
693
34%
2493
37%
693
88%
693
63%
949
JulyNight
90%
801
32%
2482
31%
760
90%
760
60%
1048
AugustNight
92%
1233
40%
3080
27%
1229
92%
1229
78%
1409
SeptemberNight
90%
1196
42%
3174
37%
1196
91%
1196
74%
1412
OctoberNight
73%
1240
38%
3604
35%
1240
79%
1240
56%
1726
NovemberNight
68%
1300
35%
4225
33%
1300
77%
1300
50%
1949
DecemberNight
54%
1364
33%
4482
26%
1364
65%
1364
35%
2366

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%
41068
Night
5%212212235%70%
40641
TransparencyDay
18%47%16%27%25%
40760
Night
18%47%15%26%27%
40449
SeeingDay
6%13%39%34%41
3616
Night
5%315%34%32%7%
13679

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
MonthOccurrence of each ForecastData
Points
JanuaryDay
10%412312235%61%
2963
Night
8%313212245%63%
4307
FebruaryDay
12%423213236%56%
2760
Night
11%422212235%59%
3312
MarchDay
8%31322345%8%56%
3343
Night
6%31221335%7%61%
3473
AprilDay
8%322212236%64%
3589
Night
6%312212235%69%
2999
MayDay
4312112235%70%
3891
Night
322211136%74%
2805
JuneDay
3111122381%
4104
Night
11112485%
2493
JulyDay
311212235%74%
4125
Night
311212235%76%
2677
AugustDay
1111121384%
3708
Night
111112383%
3090
SeptemberDay
11111122283%
3425
Night
111112285%
3174
OctoberDay
6%31121222471%
3212
Night
431211111375%
3604
NovemberDay
6%21322323467%
2934
Night
5%21221223469%
4225
DecemberDay
11%412313347%57%
3014
Night
10%413313336%59%
4482

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthOccurrence of each ForecastData
Points
JanuaryDay
26%36%13%22%27%
2963
Night
24%37%14%21%29%
4307
FebruaryDay
28%5%7%18%26%14%
2760
Night
27%46%16%26%17%
3323
MarchDay
24%6%11%20%20%16%
3343
Night
22%7%10%20%21%18%
3473
AprilDay
21%5%10%21%27%13%
3589
Night
22%49%22%24%16%
2999
MayDay
17%5%9%21%31%14%
3894
Night
16%6%9%20%30%16%
2807
JuneDay
13%35%12%31%33%
4104
Night
9%38%14%28%34%
2493
JulyDay
13%35%16%26%32%
3828
Night
13%36%17%24%34%
2482
AugustDay
7%2414%37%33%
3694
Night
7%25%13%36%34%
3080
SeptemberDay
8%2411%33%38%
3425
Night
9%25%11%31%40%
3174
OctoberDay
17%35%15%24%33%
3212
Night
17%25%12%26%34%
3604
NovemberDay
20%36%14%25%29%
2934
Night
18%36%14%23%33%
4225
DecemberDay
26%6%7%17%21%20%
3014
Night
26%5%7%15%23%21%
4482

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
MonthOccurrence of each ForecastData
Points
JanuaryDay
14%15%41%23%4
235
Night
9%5%23%38%19%3
1328
FebruaryDay
13%23%42%19%1
276
Night
11%5%20%36%22%3
1108
MarchDay
10%23%46%18%1
310
Night
5%423%37%22%4
1240
AprilDay
8%27%45%18%
299
Night
6%6%21%37%24%4
1199
MayDay
8%16%41%28%4
370
Night
4418%37%30%4
977
JuneDay
48%43%37%22
390
Night
18%39%45%4
693
JulyDay
3229%56%6%1
392
Night
2217%62%13%
801
AugustDay
1332%53%6%3
399
Night
1319%56%19%
1233
SeptemberDay
2444%41%5%2
390
Night
1426%49%17%
1196
OctoberDay
7%15%40%30%5%
377
Night
5%313%34%34%9%
1240
NovemberDay
116%32%33%13%3
90
Night
6%317%37%27%7%
1300
DecemberDay
417%28%32%15%1
88
Night
10%8%19%41%17%2
1364
Page generated at 2016-01-23 03:47:43. Version ac04

Copyright 2016 A. Danko


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