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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 2016-05-11.

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%
14103
34%
41648
27%
14055
76%
14055
51%
20391

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
54%
1361
32%
4415
24%
1360
68%
1360
41%
2211
FebruaryNight
58%
1219
32%
3664
27%
1217
68%
1217
41%
1947
MarchNight
61%
1364
29%
3819
22%
1364
69%
1364
44%
2061
AprilNight
62%
1319
30%
3299
15%
1319
67%
1319
48%
1852
MayNight
68%
1013
30%
2911
20%
1013
72%
1013
51%
1461
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%
42286
Night
5%212212235%70%
41848
TransparencyDay
18%47%16%27%25%
41978
Night
18%47%16%26%27%
41656
SeeingDay
6%13%38%33%5%1
4058
Night
5%315%34%32%7%
14114

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%60%
3059
Night
8%313212245%63%
4415
FebruaryDay
11%422212236%59%
3050
Night
10%422212235%61%
3660
MarchDay
8%31322345%8%56%
3678
Night
6%31222335%7%61%
3819
AprilDay
8%422212336%63%
3949
Night
6%312212235%69%
3299
MayDay
5%312112235%69%
4028
Night
33221136%73%
2910
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%
3059
Night
24%36%14%21%29%
4415
FebruaryDay
25%46%17%27%18%
3050
Night
25%46%16%25%21%
3671
MarchDay
24%6%10%21%21%15%
3678
Night
21%6%9%20%23%17%
3819
AprilDay
21%5%10%21%26%13%
3949
Night
22%48%22%24%16%
3299
MayDay
18%5%9%22%30%14%
4031
Night
16%6%10%21%29%15%
2912
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
13%16%39%24%5%
273
Night
9%5%24%38%19%3
1364
FebruaryDay
10%18%36%26%6%1
392
Night
10%5%19%36%23%4
1224
MarchDay
10%19%42%22%4
434
Night
5%423%37%23%4
1364
AprilDay
10%27%39%17%31
419
Night
6%6%20%37%24%4
1319
MayDay
9%17%40%27%4
414
Night
4418%37%30%4
1016
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 2016-05-12 10:52:10. Version ac04

Copyright 2016 A. Danko


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