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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 2015-03-27.

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%
12585
34%
37229
27%
12537
76%
12537
51%
18257

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%
1237
33%
4012
24%
1236
69%
1236
43%
1960
FebruaryNight
56%
1103
31%
3316
26%
1101
65%
1101
39%
1808
MarchNight
60%
1224
29%
3433
22%
1224
68%
1224
43%
1873
AprilNight
61%
1079
30%
2699
15%
1079
66%
1079
47%
1544
MayNight
70%
875
31%
2521
21%
875
74%
875
55%
1217
JuneNight
87%
623
34%
2243
36%
623
89%
623
64%
850
JulyNight
90%
719
32%
2214
31%
678
91%
678
60%
933
AugustNight
92%
1113
40%
2780
27%
1109
92%
1109
78%
1273
SeptemberNight
91%
1076
43%
2857
38%
1076
92%
1076
75%
1256
OctoberNight
74%
1116
39%
3244
36%
1116
80%
1116
56%
1546
NovemberNight
67%
1180
34%
3835
32%
1180
76%
1180
49%
1798
DecemberNight
54%
1240
32%
4075
25%
1240
65%
1240
34%
2199

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%70%
37689
Night
5%212212235%70%
37429
TransparencyDay
17%47%16%27%26%
37381
Night
18%47%16%25%27%
37237
SeeingDay
5%14%43%34%1
2646
Night
5%315%34%32%8%
12596

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%312312225%63%
2780
Night
8%312212235%65%
4012
FebruaryDay
12%423213236%56%
2760
Night
11%422212235%59%
3312
MarchDay
8%31322345%8%56%
3295
Night
6%31321335%7%60%
3433
AprilDay
7%422212236%64%
3229
Night
7%312212235%68%
2699
MayDay
3211112235%74%
3495
Night
22111126%78%
2520
JuneDay
3111122382%
3694
Night
12486%
2243
JulyDay
31111223475%
3712
Night
31121223476%
2409
AugustDay
1111121384%
3348
Night
111112383%
2790
SeptemberDay
111112285%
3082
Night
111112286%
2857
OctoberDay
6%21121222472%
2890
Night
421211111376%
3244
NovemberDay
7%21332323465%
2664
Night
6%212212235%67%
3835
DecemberDay
12%412312346%55%
2740
Night
10%413313335%58%
4075

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%36%12%23%29%
2780
Night
22%36%14%22%30%
4012
FebruaryDay
28%5%7%18%26%14%
2760
Night
27%46%16%26%17%
3323
MarchDay
24%6%11%20%21%16%
3295
Night
22%7%10%20%21%17%
3433
AprilDay
21%5%11%22%26%12%
3229
Night
23%48%22%24%15%
2699
MayDay
14%5%10%21%32%16%
3498
Night
13%6%10%20%30%17%
2522
JuneDay
12%36%12%32%33%
3694
Night
9%49%14%28%33%
2243
JulyDay
12%46%16%26%34%
3415
Night
12%47%17%22%35%
2214
AugustDay
7%25%13%35%35%
3334
Night
7%26%13%34%35%
2780
SeptemberDay
7%25%11%32%41%
3082
Night
8%15%10%28%43%
2857
OctoberDay
16%35%15%24%34%
2890
Night
17%2412%26%36%
3244
NovemberDay
22%36%14%25%27%
2664
Night
19%37%15%23%31%
3835
DecemberDay
28%6%7%18%19%20%
2740
Night
28%5%7%16%22%20%
4075

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%18%41%22%2
180
Night
9%5%23%38%19%3
1240
FebruaryDay
13%23%42%19%1
276
Night
11%5%20%36%22%3
1108
MarchDay
10%23%46%18%1
306
Night
5%424%38%22%4
1224
AprilDay
7%29%43%19%
269
Night
6%6%22%38%22%3
1079
MayDay
319%45%30%1
273
Night
3419%37%29%5%
878
JuneDay
29%50%36%
270
Night
8%40%44%4
623
JulyDay
229%64%2
268
Night
11217%62%15%
719
AugustDay
1335%58%
279
Night
1318%55%20%
1113
SeptemberDay
1450%42%
270
Night
1427%48%18%
1076
OctoberDay
5%12%45%34%1
255
Night
5%213%32%35%10%
1116
NovemberNight
6%217%37%27%7%
1180
DecemberNight
10%7%19%42%16%2
1240
Page generated at 2015-03-28 10:47:17. Version ac04

Copyright 2015 A. Danko


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