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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 2014-04-23.

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%
11350
34%
33525
28%
11302
76%
11302
50%
16521

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%
1051
29%
2629
15%
1051
67%
1051
46%
1512
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%2%1%2%2%1%2%2%3%5%70%
33951
Night
5%3%1%2%2%1%2%2%3%5%70%
33725
TransparencyDay
18%4%7%16%27%25%
33643
Night
19%4%7%16%25%26%
33533
SeeingDay
6%14%42%35%1%
2385
Night
5%3%16%34%31%7%
11361

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%3%1%2%3%1%2%2%2%5%63%
2501
Night
8%3%1%3%2%1%2%2%4%5%65%
3609
FebruaryDay
13%4%2%3%2%1%3%2%3%6%55%
2480
Night
11%5%2%3%2%1%2%2%3%5%57%
2976
MarchDay
8%3%1%3%2%2%3%4%5%8%55%
3009
Night
6%3%1%2%2%1%3%3%5%7%61%
3125
AprilDay
7%4%2%2%2%1%2%2%3%6%63%
3135
Night
7%3%1%2%2%1%2%2%3%5%68%
2629
MayDay
4%2%1%1%1%1%2%1%3%5%75%
3098
Night
2%2%1%1%1%1%2%5%78%
2235
JuneDay
3%1%1%1%2%2%3%81%
3284
Night
1%1%1%2%4%85%
1994
JulyDay
3%1%1%1%1%2%2%3%4%76%
3299
Night
3%1%1%1%1%2%1%3%4%77%
2140
AugustDay
1%1%1%1%2%1%3%84%
2976
Night
1%1%1%2%2%2%3%83%
2480
SeptemberDay
1%1%1%1%1%1%2%2%84%
2740
Night
1%1%1%1%1%2%3%85%
2540
OctoberDay
6%2%1%1%2%1%2%2%2%4%72%
2569
Night
5%3%1%1%1%1%2%2%1%3%75%
2884
NovemberDay
7%3%1%3%3%2%3%2%3%4%64%
2394
Night
6%3%1%2%2%1%2%2%3%5%67%
3445
DecemberDay
12%4%1%2%2%1%2%3%3%7%57%
2466
Night
9%4%1%3%3%1%3%2%3%6%59%
3668

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthOccurrence of each ForecastData
Points
JanuaryDay
25%3%6%12%23%28%
2501
Night
22%3%7%14%22%30%
3609
FebruaryDay
29%5%7%18%26%12%
2480
Night
28%5%7%17%25%15%
2987
MarchDay
24%6%12%21%21%14%
3009
Night
22%6%11%21%22%15%
3125
AprilDay
22%5%10%22%26%12%
3135
Night
24%4%9%22%24%15%
2629
MayDay
14%5%9%22%33%14%
3101
Night
14%7%9%20%31%15%
2237
JuneDay
13%3%6%12%31%32%
3284
Night
9%4%10%15%27%32%
1994
JulyDay
12%3%6%15%26%36%
3002
Night
11%3%7%16%22%37%
1945
AugustDay
6%2%5%13%35%36%
2962
Night
7%1%6%13%34%35%
2470
SeptemberDay
8%2%5%10%32%41%
2740
Night
9%2%5%10%27%43%
2540
OctoberDay
17%3%5%14%24%34%
2569
Night
17%2%5%12%25%36%
2884
NovemberDay
23%3%6%14%24%26%
2394
Night
19%3%7%14%23%30%
3445
DecemberDay
26%6%7%18%19%20%
2466
Night
26%5%7%16%22%21%
3668

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
15%17%43%21%2%
162
Night
9%5%24%39%18%3%
1116
FebruaryDay
14%23%41%19%1%
248
Night
11%6%21%36%20%3%
996
MarchDay
10%23%46%18%1%
279
Night
5%5%25%37%21%4%
1116
AprilDay
7%29%43%19%
262
Night
6%6%21%38%23%3%
1051
MayDay
3%19%44%31%1%
242
Night
3%3%20%37%29%5%
779
JuneDay
3%8%48%39%
240
Night
1%9%38%46%4%
554
JulyDay
2%28%64%2%
237
Night
1%1%2%17%62%15%
636
AugustDay
2%34%60%
248
Night
1%3%17%56%21%
989
SeptemberDay
1%4%48%45%
240
Night
1%5%26%48%17%
956
OctoberDay
6%13%44%34%2%
227
Night
6%3%14%33%33%9%
992
NovemberNight
6%2%17%38%27%7%
1060
DecemberNight
9%8%19%43%16%2%
1116
Page generated at 2014-04-23 21:41:44. Version ac03


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