Santa Monica Mountains Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Monica Mountains Observatory clear sky chart from 2005-11-05 to 2017-09-18.

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
65%
15847
33%
46828
26%
15796
72%
15796
44%
23241

By Month

MonthDays of 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
January1st to 15thNight
58%
716
29%
2452
24%
716
68%
716
35%
1198
January16th to endNight
54%
768
34%
2383
26%
768
66%
768
35%
1238
February1st to 15thNight
60%
700
35%
2101
33%
696
68%
696
40%
1072
February16th to endNight
54%
632
27%
1895
22%
632
63%
632
30%
1092
March1st to 15thNight
58%
720
28%
2160
25%
720
67%
720
40%
1082
March16th to endNight
61%
768
28%
1993
19%
768
67%
768
39%
1132
April1st to 15thNight
58%
719
29%
1799
17%
719
64%
719
44%
1020
April16th to endNight
60%
720
31%
1800
14%
720
67%
720
50%
948
May1st to 15thNight
63%
640
27%
1720
16%
640
66%
640
41%
956
May16th to endNight
65%
561
28%
1685
19%
561
70%
561
46%
836
June1st to 15thNight
69%
435
28%
1515
25%
435
74%
435
41%
718
June16th to endNight
75%
360
34%
1440
34%
360
82%
360
49%
524
July1st to 15thNight
77%
340
33%
1360
37%
340
80%
340
46%
524
July16th to endNight
73%
628
29%
1647
21%
585
80%
585
51%
825
August1st to 15thNight
79%
717
34%
1790
21%
713
80%
713
58%
909
August16th to endNight
81%
764
39%
1910
24%
764
84%
764
65%
917
September1st to 15thNight
81%
720
38%
1815
28%
720
83%
720
60%
911
September16th to endNight
80%
667
41%
1847
35%
667
86%
667
65%
811
October1st to 15thNight
73%
660
35%
1848
34%
660
76%
660
52%
918
October16th to endNight
70%
704
39%
2118
35%
704
79%
704
49%
997
November1st to 15thNight
68%
700
35%
2275
36%
700
74%
700
49%
993
November16th to endNight
63%
720
37%
2340
32%
720
73%
720
41%
1075
December1st to 15thNight
56%
720
35%
2340
29%
720
67%
720
33%
1204
December16th to endNight
50%
768
32%
2595
25%
768
63%
768
31%
1341

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
4312212236%68%
47941
Night
5%312212336%66%
47034
TransparencyDay
18%38%17%27%24%
47635
Night
20%38%16%25%25%
46840
SeeingDay
412%19%36%21%5%
5961
Night
5%15%7%26%34%10%
15858

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
6%41332333463%
1483
16th to endDay
10%323213246%59%
1838
February1st to 15thDay
6%312313247%62%
1751
16th to endDay
10%41321335%6%56%
1579
March1st to 15thDay
5%313213447%62%
1800
16th to endDay
5%31232445%8%58%
2226
April1st to 15thDay
5%312213348%62%
2149
16th to endDay
3312112335%71%
2160
May1st to 15thDay
3321212337%67%
2229
16th to endDay
3312212236%69%
2429
June1st to 15thDay
5%5%1222126%66%
2442
16th to endDay
13121212479%
2520
July1st to 15thDay
332212235%72%
2520
16th to endDay
1212212235%73%
2401
August1st to 15thDay
221211211479%
2160
16th to endDay
111111235%79%
2292
September1st to 15thDay
21111112479%
2143
16th to endDay
22111212479%
1838
October1st to 15thDay
4311212235%71%
1779
16th to endDay
3211212335%71%
1752
November1st to 15thDay
321322233470%
1584
16th to endDay
4212412346%65%
1620
December1st to 15thDay
7%32322335%7%58%
1620
16th to endDay
10%313313336%59%
1626
January1st to 15thNight
6%412313345%62%
2452
16th to endNight
10%312212336%60%
2383
February1st to 15thNight
6%324212336%64%
2101
16th to endNight
12%323323435%55%
1895
March1st to 15thNight
5%313313347%62%
2160
16th to endNight
5%41222345%7%59%
1993
April1st to 15thNight
7%5%22222226%64%
1799
16th to endNight
3213113346%68%
1800
May1st to 15thNight
45%13212247%62%
1720
16th to endNight
3412212336%68%
1684
June1st to 15thNight
2422213248%64%
1515
16th to endNight
132122326%73%
1440
July1st to 15thNight
1312312226%70%
1440
16th to endNight
3313213447%65%
1764
August1st to 15thNight
341213236%70%
1800
16th to endNight
312111246%73%
1910
September1st to 15thNight
22121223474%
1815
16th to endNight
12111223477%
1847
October1st to 15thNight
321231323470%
1848
16th to endNight
43221223471%
2118
November1st to 15thNight
221221222473%
2275
16th to endNight
3312223236%67%
2340
December1st to 15thNight
6%414322246%61%
2340
16th to endNight
10%313212235%61%
2595

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
25%36%12%24%27%
1483
16th to endDay
27%39%15%17%27%
1838
February1st to 15thDay
21%47%15%25%25%
1751
16th to endDay
28%5%9%18%28%10%
1579
March1st to 15thDay
21%5%9%21%21%21%
1800
16th to endDay
23%412%24%19%15%
2226
April1st to 15thDay
20%5%13%23%23%12%
2149
16th to endDay
14%5%10%23%27%18%
2160
May1st to 15thDay
17%6%12%20%26%17%
2229
16th to endDay
17%412%28%28%9%
2432
June1st to 15thDay
20%410%17%31%16%
2442
16th to endDay
11%25%10%35%34%
2520
July1st to 15thDay
15%25%12%29%33%
2371
16th to endDay
14%38%22%30%20%
2255
August1st to 15thDay
11%2415%36%29%
2146
16th to endDay
8%25%13%40%30%
2292
September1st to 15thDay
10%26%14%31%34%
2143
16th to endDay
11%1416%33%32%
1838
October1st to 15thDay
17%36%21%25%25%
1779
16th to endDay
15%45%14%22%37%
1752
November1st to 15thDay
19%25%14%24%33%
1584
16th to endDay
19%48%12%22%32%
1620
December1st to 15thDay
24%5%10%15%20%23%
1620
16th to endDay
28%48%16%22%20%
1626
January1st to 15thNight
23%46%14%22%28%
2452
16th to endNight
26%38%15%16%30%
2383
February1st to 15thNight
21%47%16%23%26%
2112
16th to endNight
32%48%15%24%14%
1895
March1st to 15thNight
21%311%18%25%20%
2160
16th to endNight
24%6%12%19%21%14%
1993
April1st to 15thNight
24%39%25%22%13%
1799
16th to endNight
16%412%22%24%19%
1800
May1st to 15thNight
22%6%9%20%25%15%
1721
16th to endNight
17%6%12%26%25%12%
1685
June1st to 15thNight
19%5%13%18%28%14%
1515
16th to endNight
12%210%14%30%30%
1440
July1st to 15thNight
16%16%15%25%35%
1360
16th to endNight
18%49%24%24%19%
1647
August1st to 15thNight
16%16%11%35%27%
1790
16th to endNight
12%17%13%36%29%
1910
September1st to 15thNight
14%36%10%31%34%
1815
16th to endNight
10%15%15%31%35%
1847
October1st to 15thNight
18%36%18%27%26%
1848
16th to endNight
17%35%13%22%37%
2118
November1st to 15thNight
15%27%15%21%37%
2275
16th to endNight
18%36%16%20%33%
2340
December1st to 15thNight
25%47%15%20%26%
2340
16th to endNight
26%47%14%25%21%
2595

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
16%12%31%34%5%
111
16th to endDay
11%18%24%33%12%1
275
February1st to 15thDay
10%15%26%29%15%3
266
16th to endDay
12%19%27%27%11%1
238
March1st to 15thDay
6%19%29%20%21%2
270
16th to endDay
420%27%33%11%2
288
April1st to 15thDay
5%22%30%27%10%4
269
16th to endDay
23%35%29%7%3
270
May1st to 15thDay
5%19%26%34%12%2
288
16th to endDay
313%26%41%12%2
330
June1st to 15thDay
8%12%19%39%17%3
315
16th to endDay
10%8%40%29%11%
315
July1st to 15thDay
36%6%36%38%8%
305
16th to endDay
19%335%37%12%
327
August1st to 15thDay
26%6%40%34%7%
315
16th to endDay
145%35%43%10%
332
September1st to 15thDay
4411%49%24%5%
315
16th to endDay
16%12%50%21%7%
264
October1st to 15thDay
29%25%42%17%2
255
16th to endDay
49%20%42%19%4
264
November1st to 15thDay
447%24%38%20%
90
16th to endDay
36%18%38%30%2
90
December1st to 15thDay
38%13%35%37%1
90
16th to endDay
11%13%20%30%21%2
79
January1st to 15thNight
7%17%10%34%22%6%
720
16th to endNight
10%18%10%30%25%4
768
February1st to 15thNight
5%17%10%32%25%7%
704
16th to endNight
11%17%11%29%25%4
632
March1st to 15thNight
5%17%12%33%24%6%
720
16th to endNight
5%18%10%31%29%5%
768
April1st to 15thNight
7%18%11%31%26%3
719
16th to endNight
419%11%30%27%6%
720
May1st to 15thNight
7%17%6%30%31%5%
641
16th to endNight
317%11%32%31%3
563
June1st to 15thNight
216%6%29%39%4
435
16th to endNight
316%118%51%8%
360
July1st to 15thNight
311%16%53%22%
340
16th to endNight
316%19%49%20%
628
August1st to 15thNight
49%116%50%17%
717
16th to endNight
28%112%50%24%
764
September1st to 15thNight
48%217%47%20%
720
16th to endNight
28%319%49%16%
667
October1st to 15thNight
5%9%6%26%41%11%
660
16th to endNight
5%10%7%23%37%14%
704
November1st to 15thNight
315%7%29%31%13%
700
16th to endNight
6%17%8%27%29%10%
720
December1st to 15thNight
7%18%9%32%25%6%
720
16th to endNight
12%20%12%31%17%5%
768
Page generated at 2017-09-19 04:32:23. Version ac04

Copyright 2017 A. Danko


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