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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 2014-04-17.

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%
11325
34%
33464
28%
11277
76%
11277
50%
16483

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%
1113
33%
3609
24%
1112
68%
1112
44%
1755
FebruaryNight
54%
991
30%
2980
25%
989
63%
989
37%
1659
MarchNight
59%
1116
29%
3125
22%
1116
67%
1116
43%
1700
AprilNight
62%
1026
29%
2568
15%
1026
66%
1026
46%
1474
MayNight
69%
776
33%
2236
23%
776
73%
776
53%
1105
JuneNight
86%
554
38%
1994
40%
554
88%
554
61%
775
JulyNight
91%
636
35%
1945
33%
595
92%
595
62%
807
AugustNight
92%
989
41%
2470
28%
985
92%
985
78%
1128
SeptemberNight
90%
956
42%
2540
38%
956
91%
956
73%
1136
OctoberNight
72%
992
39%
2884
36%
992
79%
992
55%
1387
NovemberNight
67%
1060
34%
3445
32%
1060
76%
1060
49%
1617
DecemberNight
55%
1116
33%
3668
26%
1116
66%
1116
35%
1940

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%5%70%
33879
Night
5%5%70%
33664
TransparencyDay
8%17%28%27%
32164
Night
8%16%26%28%
32138
SeeingDay
14%42%35%
2360
Night
16%34%31%7%
11270

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%5%63%
2501
Night
8%5%65%
3609
FebruaryDay
13%6%55%
2480
Night
11%5%5%57%
2976
MarchDay
8%5%8%55%
3009
Night
6%5%7%61%
3125
AprilDay
8%6%63%
3063
Night
7%5%68%
2568
MayDay
5%75%
3098
Night
5%78%
2235
JuneDay
81%
3284
Night
85%
1994
JulyDay
76%
3299
Night
77%
2140
AugustDay
84%
2976
Night
83%
2480
SeptemberDay
84%
2740
Night
85%
2540
OctoberDay
6%72%
2569
Night
5%75%
2884
NovemberDay
7%64%
2394
Night
6%5%67%
3445
DecemberDay
12%7%57%
2466
Night
9%6%59%
3668

Transparency (when cloud cover <= 30%)
Color Meanings:
Poor Below Average Average Above average Transparent
MonthOccurrence of each ForecastData
Points
JanuaryDay
6%13%24%30%
2391
Night
7%14%23%31%
3462
FebruaryDay
5%8%19%28%13%
2301
Night
5%8%18%27%17%
2791
MarchDay
6%13%23%22%15%
2762
Night
7%11%22%23%17%
2915
AprilDay
6%11%24%29%13%
2812
Night
5%9%23%26%16%
2367
MayDay
5%9%22%34%14%
3014
Night
7%10%21%32%16%
2174
JuneDay
6%12%32%33%
3196
Night
10%15%27%32%
1957
JulyDay
6%15%27%37%
2914
Night
7%17%23%38%
1880
AugustDay
5%13%35%36%
2941
Night
6%13%34%35%
2451
SeptemberDay
5%10%32%41%
2721
Night
6%10%28%43%
2521
OctoberDay
5%14%24%35%
2495
Night
5%12%26%37%
2821
NovemberDay
7%15%26%28%
2275
Night
8%15%24%31%
3329
DecemberDay
6%8%19%20%21%
2342
Night
5%7%17%23%22%
3470

Seeing (when cloud cover <= 80%)
Color Meanings:
Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthOccurrence of each ForecastData
Points
JanuaryDay
17%43%21%
160
Night
5%25%39%18%
1105
FebruaryDay
24%41%19%
245
Night
6%21%37%21%
982
MarchDay
24%47%19%
270
Night
5%25%38%21%
1106
AprilDay
29%44%19%
251
Night
6%22%39%23%
1010
MayDay
19%44%31%
242
Night
20%38%29%5%
776
JuneDay
8%48%39%
240
Night
9%38%46%
554
JulyDay
28%64%
237
Night
17%62%15%
633
AugustDay
34%60%
248
Night
17%56%21%
989
SeptemberDay
48%45%
240
Night
5%26%48%17%
956
OctoberDay
13%44%34%
227
Night
14%33%33%9%
992
NovemberNight
17%38%27%7%
1059
DecemberNight
8%19%43%16%
1108
Page generated at 2014-04-17 21:20:00. Version ac01


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