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Griffith Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Griffith Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
75%
15821
32%
46729
29%
15772
79%
15772
50%
23465

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
69%
716
28%
2464
28%
716
75%
716
42%
1214
January16th to endNight
64%
768
34%
2361
31%
768
72%
768
43%
1237
February1st to 15thNight
71%
698
34%
2101
33%
696
77%
696
46%
1086
February16th to endNight
62%
632
26%
1895
26%
632
67%
632
34%
1099
March1st to 15thNight
67%
720
29%
2160
27%
720
74%
720
46%
1101
March16th to endNight
70%
768
28%
1999
21%
768
74%
768
44%
1145
April1st to 15thNight
66%
719
29%
1799
18%
719
70%
719
49%
1024
April16th to endNight
70%
720
31%
1800
15%
720
75%
720
54%
973
May1st to 15thNight
71%
629
27%
1709
18%
629
73%
629
46%
951
May16th to endNight
73%
561
28%
1685
21%
561
76%
561
53%
833
June1st to 15thNight
80%
453
26%
1533
25%
453
82%
453
48%
732
June16th to endNight
90%
360
33%
1440
40%
360
90%
360
52%
564
July1st to 15thNight
90%
340
32%
1360
39%
340
90%
340
51%
542
July16th to endNight
88%
628
28%
1647
23%
585
88%
585
53%
841
August1st to 15thNight
86%
717
33%
1790
22%
713
86%
713
63%
909
August16th to endNight
90%
764
39%
1910
26%
764
90%
764
67%
951
September1st to 15thNight
87%
700
37%
1770
30%
700
88%
700
64%
891
September16th to endNight
89%
656
40%
1815
39%
656
90%
656
66%
826
October1st to 15thNight
80%
660
35%
1837
35%
660
82%
660
56%
921
October16th to endNight
78%
704
38%
2128
40%
704
81%
704
53%
1011
November1st to 15thNight
80%
700
35%
2275
37%
700
84%
700
56%
985
November16th to endNight
77%
720
37%
2340
38%
720
82%
720
51%
1085
December1st to 15thNight
68%
720
34%
2340
32%
720
74%
720
40%
1218
December16th to endNight
61%
768
32%
2571
30%
768
68%
768
38%
1326

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
4213212236%68%
47867
Night
4213212235%67%
46935
TransparencyDay
18%37%17%27%24%
47561
Night
20%38%16%25%25%
46741
SeeingDay
5%8%27%39%15%4
5918
Night
6%210%32%38%9%
15834

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%41442323463%
1471
16th to endDay
10%314212346%58%
1859
February1st to 15thDay
6%313212348%60%
1751
16th to endDay
11%31332335%7%54%
1579
March1st to 15thDay
422232335%7%62%
1800
16th to endDay
5%31342435%9%57%
2221
April1st to 15thDay
5%312313348%61%
2149
16th to endDay
3312212337%69%
2160
May1st to 15thDay
3223322345%66%
2241
16th to endDay
3323212235%67%
2429
June1st to 15thDay
4414212237%64%
2422
16th to endDay
1121122480%
2520
July1st to 15thDay
2213212235%71%
2520
16th to endDay
1112212247%71%
2398
August1st to 15thDay
12111113480%
2160
16th to endDay
121123480%
2292
September1st to 15thDay
11111123380%
2075
16th to endDay
11121123379%
1815
October1st to 15thDay
421322133468%
1790
16th to endDay
311221234471%
1744
November1st to 15thDay
332312335%70%
1584
16th to endDay
5%213223336%65%
1620
December1st to 15thDay
7%21322445%8%57%
1620
16th to endDay
11%323213236%59%
1647
January1st to 15thNight
6%413212336%63%
2464
16th to endNight
9%423212235%61%
2361
February1st to 15thNight
5%413213235%64%
2101
16th to endNight
12%314323345%54%
1895
March1st to 15thNight
4214312348%62%
2160
16th to endNight
421222446%7%59%
1999
April1st to 15thNight
6%31321233465%
1799
16th to endNight
3213222246%68%
1800
May1st to 15thNight
4323212338%63%
1709
16th to endNight
2314212236%69%
1684
June1st to 15thNight
2213213239%66%
1533
16th to endNight
11311222477%
1440
July1st to 15thNight
1212212235%73%
1440
16th to endNight
2112313346%67%
1764
August1st to 15thNight
2112212237%71%
1800
16th to endNight
112212235%76%
1910
September1st to 15thNight
221221222477%
1770
16th to endNight
122123479%
1815
October1st to 15thNight
31331333471%
1837
16th to endNight
42331223471%
2128
November1st to 15thNight
2212212235%72%
2275
16th to endNight
3212213346%66%
2340
December1st to 15thNight
6%313323236%60%
2340
16th to endNight
11%412212235%62%
2571

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%45%13%23%28%
1471
16th to endDay
27%47%15%17%27%
1859
February1st to 15thDay
22%36%15%25%24%
1751
16th to endDay
29%48%19%27%10%
1579
March1st to 15thDay
21%311%20%22%21%
1800
16th to endDay
23%412%24%20%14%
2221
April1st to 15thDay
22%412%24%24%11%
2149
16th to endDay
16%310%23%28%17%
2160
May1st to 15thDay
20%49%21%27%16%
2241
16th to endDay
19%310%26%29%9%
2432
June1st to 15thDay
22%38%16%31%17%
2422
16th to endDay
12%148%34%37%
2520
July1st to 15thDay
17%16%11%26%36%
2371
16th to endDay
14%48%21%31%20%
2252
August1st to 15thDay
11%1314%35%32%
2146
16th to endDay
9%25%12%40%30%
2292
September1st to 15thDay
11%15%12%32%36%
2075
16th to endDay
11%2414%34%33%
1815
October1st to 15thDay
18%25%20%26%25%
1790
16th to endDay
15%45%14%22%38%
1744
November1st to 15thDay
17%26%15%23%33%
1584
16th to endDay
20%37%13%22%31%
1620
December1st to 15thDay
24%49%16%20%23%
1620
16th to endDay
27%39%16%21%20%
1647
January1st to 15thNight
23%47%13%22%28%
2464
16th to endNight
26%38%15%16%30%
2361
February1st to 15thNight
23%47%15%22%27%
2112
16th to endNight
32%48%15%24%14%
1895
March1st to 15thNight
22%49%19%23%20%
2160
16th to endNight
23%7%12%20%21%14%
1999
April1st to 15thNight
24%39%26%22%13%
1799
16th to endNight
19%311%22%24%17%
1800
May1st to 15thNight
23%6%10%19%25%14%
1710
16th to endNight
18%413%26%25%11%
1685
June1st to 15thNight
20%413%16%29%14%
1533
16th to endNight
15%28%11%30%30%
1440
July1st to 15thNight
16%36%13%26%34%
1360
16th to endNight
19%410%22%23%19%
1647
August1st to 15thNight
16%26%12%35%27%
1790
16th to endNight
13%26%12%37%27%
1910
September1st to 15thNight
14%36%9%31%34%
1770
16th to endNight
12%25%13%31%34%
1815
October1st to 15thNight
17%35%17%28%25%
1837
16th to endNight
18%35%12%23%37%
2128
November1st to 15thNight
15%27%15%21%37%
2275
16th to endNight
19%26%17%20%32%
2340
December1st to 15thNight
26%47%15%20%25%
2340
16th to endNight
27%38%14%24%21%
2571

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
14%7%40%30%8%
99
16th to endDay
11%10%35%33%8%
278
February1st to 15thDay
10%10%29%31%13%3
266
16th to endDay
14%19%35%20%9%
238
March1st to 15thDay
7%15%35%23%15%2
270
16th to endDay
7%16%36%30%7%2
288
April1st to 15thDay
9%14%42%22%7%3
269
16th to endDay
319%44%23%43
270
May1st to 15thDay
5%14%35%31%9%2
288
16th to endDay
49%39%35%8%2
330
June1st to 15thDay
8%5%29%41%12%2
315
16th to endDay
118%50%19%9%
315
July1st to 15thDay
112%53%24%6%
305
16th to endDay
210%51%26%9%
327
August1st to 15thDay
4114%56%18%5%
315
16th to endDay
212%53%23%8%
332
September1st to 15thDay
4124%51%14%4
294
16th to endDay
1227%52%11%4
255
October1st to 15thDay
5%8%32%38%13%1
255
16th to endDay
5%6%28%39%16%3
256
November1st to 15thDay
45%10%27%35%16%
90
16th to endDay
5%6%22%36%26%2
90
December1st to 15thDay
6%6%15%40%30%1
90
16th to endDay
15%14%18%32%18%1
83
January1st to 15thNight
8%113%42%26%5%
720
16th to endNight
12%214%37%28%5%
768
February1st to 15thNight
7%311%39%30%8%
704
16th to endNight
12%416%36%25%5%
632
March1st to 15thNight
6%217%40%26%6%
720
16th to endNight
6%314%39%30%5%
768
April1st to 15thNight
8%316%40%27%3
719
16th to endNight
4416%37%31%5%
720
May1st to 15thNight
6%314%35%35%4
630
16th to endNight
3117%37%33%4
563
June1st to 15thNight
310%39%42%3
453
16th to endNight
2225%61%7%
360
July1st to 15thNight
219%68%17%
340
16th to endNight
3117%57%19%
628
August1st to 15thNight
2222%55%15%
717
16th to endNight
3215%55%22%
764
September1st to 15thNight
4320%51%20%
700
16th to endNight
21326%52%14%
656
October1st to 15thNight
419%35%39%10%
660
16th to endNight
6%19%29%39%14%
704
November1st to 15thNight
419%35%36%13%
700
16th to endNight
5%110%35%34%11%
720
December1st to 15thNight
7%312%38%31%6%
720
16th to endNight
13%5%15%37%22%4
768
Page generated at 2017-09-11 16:35:32. Version ac04

Copyright 2017 A. Danko


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