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 2018-03-11.

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%
16549
33%
48977
29%
16500
79%
16500
50%
24531

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%
776
28%
2670
29%
776
74%
776
41%
1340
January16th to endNight
65%
832
35%
2558
32%
832
73%
832
44%
1317
February1st to 15thNight
72%
758
35%
2281
34%
756
77%
756
47%
1165
February16th to endNight
62%
684
28%
2051
27%
684
68%
684
36%
1160
March1st to 15thNight
67%
764
28%
2292
27%
764
73%
764
45%
1181
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%
720
37%
1822
30%
720
88%
720
63%
921
September16th to endNight
89%
716
42%
1980
41%
716
90%
716
67%
900
October1st to 15thNight
81%
720
35%
2004
34%
720
83%
720
56%
1000
October16th to endNight
78%
768
40%
2322
42%
768
81%
768
54%
1097
November1st to 15thNight
79%
760
34%
2470
36%
760
84%
760
54%
1100
November16th to endNight
77%
780
38%
2535
40%
780
82%
780
50%
1181
December1st to 15thNight
69%
780
34%
2535
33%
780
75%
780
42%
1293
December16th to endNight
62%
832
34%
2785
32%
832
70%
832
39%
1411

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
4213212336%68%
49619
Night
4213212236%67%
49183
TransparencyDay
18%37%17%27%25%
49313
Night
20%38%16%25%25%
48989
SeeingDay
5%7%26%38%15%4
6544
Night
6%210%33%37%9%
16562

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
7%31442323462%
1595
16th to endDay
9%314212346%59%
2014
February1st to 15thDay
6%313212347%62%
1901
16th to endDay
10%313222347%56%
1709
March1st to 15thDay
5%22342235%7%61%
1904
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
11111123479%
2136
16th to endDay
11121123379%
1980
October1st to 15thDay
421322133470%
1953
16th to endDay
3112212345%72%
1902
November1st to 15thDay
332312435%68%
1719
16th to endDay
5%213323337%64%
1755
December1st to 15thDay
7%213213449%59%
1755
16th to endDay
10%323213236%60%
1784
January1st to 15thNight
6%413222336%61%
2670
16th to endNight
8%313212236%62%
2558
February1st to 15thNight
5%313223236%65%
2281
16th to endNight
11%314323335%57%
2051
March1st to 15thNight
5%21332335%8%60%
2292
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
221221222476%
1822
16th to endNight
121123480%
1980
October1st to 15thNight
31331333471%
2004
16th to endNight
42221223471%
2322
November1st to 15thNight
2213213235%69%
2470
16th to endNight
3213213347%65%
2535
December1st to 15thNight
6%213323247%61%
2535
16th to endNight
10%412212235%62%
2785

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
26%45%14%22%26%
1595
16th to endDay
26%47%16%17%27%
2014
February1st to 15thDay
21%36%15%26%26%
1901
16th to endDay
27%38%19%27%12%
1709
March1st to 15thDay
22%311%20%21%21%
1904
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%13%32%34%
2136
16th to endDay
11%2414%34%32%
1980
October1st to 15thDay
18%25%19%26%27%
1953
16th to endDay
15%35%14%22%39%
1902
November1st to 15thDay
18%36%17%23%30%
1719
16th to endDay
20%37%12%22%32%
1755
December1st to 15thDay
23%49%16%19%27%
1755
16th to endDay
26%39%16%22%22%
1784
January1st to 15thNight
25%47%14%21%26%
2670
16th to endNight
24%38%16%17%30%
2558
February1st to 15thNight
22%37%15%23%27%
2292
16th to endNight
31%47%15%25%16%
2051
March1st to 15thNight
23%49%19%23%19%
2292
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%32%33%
1822
16th to endNight
12%25%13%32%34%
1980
October1st to 15thNight
17%35%17%28%27%
2004
16th to endNight
18%25%12%22%38%
2322
November1st to 15thNight
17%28%16%21%34%
2470
16th to endNight
19%26%17%21%32%
2535
December1st to 15thNight
25%37%14%20%28%
2535
16th to endNight
25%37%14%25%22%
2785

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
15%5%33%30%14%1
133
16th to endDay
9%10%31%34%12%2
337
February1st to 15thDay
9%10%27%30%15%7%
326
16th to endDay
13%20%32%22%11%
290
March1st to 15thDay
9%14%34%23%15%2
312
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
4125%49%14%4
315
16th to endDay
2326%48%14%5%
315
October1st to 15thDay
47%30%39%15%2
315
16th to endDay
5%6%26%37%17%6%
318
November1st to 15thDay
5%5%17%30%29%11%
135
16th to endDay
5%5%20%34%26%8%
135
December1st to 15thDay
46%15%37%28%7%
135
16th to endDay
10%12%16%30%25%4
124
January1st to 15thNight
9%113%42%27%5%
780
16th to endNight
11%214%37%29%5%
832
February1st to 15thNight
6%211%38%31%9%
764
16th to endNight
11%416%36%25%4
684
March1st to 15thNight
7%217%40%26%6%
764
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
4322%50%19%
720
16th to endNight
2327%51%13%
716
October1st to 15thNight
48%35%40%9%
720
16th to endNight
6%18%28%38%15%
768
November1st to 15thNight
419%37%35%12%
760
16th to endNight
5%110%35%35%11%
780
December1st to 15thNight
7%312%39%31%6%
780
16th to endNight
12%5%15%37%23%5%
832
Page generated at 2018-03-12 10:28:13. Version ac04

Copyright 2018 A. Danko


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