This page made possible by the sponsorship of: Michael Rudy.

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 2015-08-20.

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%
13048
34%
38562
27%
13000
76%
13000
51%
18901

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%
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%
1189
41%
2970
28%
1185
92%
1185
78%
1356
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%
39542
Night
5%212212235%70%
38762
TransparencyDay
18%47%16%27%26%
39234
Night
18%47%16%25%27%
38570
SeeingDay
6%13%41%34%2
3097
Night
5%315%34%32%8%
13059

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%
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%
3574
Night
111112383%
2980
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%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%25%14%36%34%
3560
Night
7%25%13%35%35%
2970
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
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
1334%55%31
355
Night
1319%56%19%
1189
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 2015-08-21 09:03:18. Version ac04

Copyright 2015 A. Danko


Mount Wilson Observatory Clear Sky Chart | Charts Home