Palomar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Palomar Observatory clear sky chart from 2005-11-06 to 2024-03-28.

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
71%
25198
33%
79867
29%
25107
75%
25107
45%
40783

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 10thNight
55%
752
36%
3992
25%
736
63%
736
28%
1695
January11th to 20thNight
57%
750
35%
3869
30%
740
66%
740
26%
1864
January21st to endNight
53%
827
23%
3969
26%
814
64%
814
21%
2341
February1st to 10thNight
59%
760
32%
2280
30%
760
66%
760
38%
1260
February11th to 20thNight
56%
740
31%
2219
27%
740
64%
740
36%
1277
February21st to endNight
51%
628
28%
1884
27%
628
57%
628
31%
1158
March1st to 10thNight
52%
760
24%
2280
24%
760
57%
760
30%
1398
March11th to 20thNight
63%
760
27%
2180
25%
760
66%
760
38%
1259
March21st to endNight
60%
820
26%
2151
20%
820
64%
820
40%
1308
April1st to 10thNight
60%
720
29%
1800
20%
720
63%
720
42%
1093
April11th to 20thNight
63%
711
33%
1779
20%
711
66%
711
49%
1006
April21st to endNight
68%
720
31%
1800
18%
720
70%
720
49%
1026
May1st to 10thNight
68%
587
32%
1667
23%
587
70%
587
48%
879
May11th to 20thNight
71%
531
33%
1595
26%
531
72%
531
52%
782
May21st to endNight
68%
582
31%
1746
25%
582
70%
582
52%
851
June1st to 10thNight
80%
540
35%
1620
29%
540
82%
540
64%
709
June11th to 20thNight
84%
540
37%
1620
31%
540
86%
540
74%
654
June21st to endNight
87%
540
36%
1620
32%
540
88%
540
72%
659
July1st to 10thNight
94%
525
36%
1575
31%
525
94%
525
78%
612
July11th to 20thNight
94%
522
35%
1566
29%
522
94%
522
69%
654
July21st to endNight
90%
664
33%
1743
25%
627
90%
627
59%
850
August1st to 10thNight
92%
716
35%
1780
23%
712
92%
712
75%
840
August11th to 20thNight
95%
712
41%
1780
26%
712
95%
712
77%
822
August21st to endNight
92%
792
44%
1980
31%
792
92%
792
75%
930
September1st to 10thNight
90%
720
42%
1822
33%
720
91%
720
73%
854
September11th to 20thNight
89%
720
43%
1980
40%
720
91%
720
74%
855
September21st to endNight
84%
720
44%
1980
40%
720
87%
720
69%
886
October1st to 10thNight
76%
720
38%
1980
34%
720
80%
720
55%
997
October11th to 20thNight
76%
720
42%
1980
37%
720
81%
720
62%
931
October21st to endNight
71%
792
40%
2466
38%
792
78%
792
55%
1124
November1st to 10thNight
75%
740
35%
2405
37%
740
79%
740
49%
1130
November11th to 20thNight
71%
760
37%
2470
36%
760
78%
760
50%
1146
November21st to endNight
61%
760
32%
2470
31%
760
67%
760
36%
1332
December1st to 10thNight
60%
760
30%
2470
29%
760
65%
760
35%
1354
December11th to 20thNight
55%
760
29%
2953
29%
759
63%
759
26%
1720
December21st to endNight
55%
827
19%
4396
29%
817
62%
817
20%
2527

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
4413323346%66%
79366
Night
46%24323346%62%
80719
TransparencyDay
20%36%15%26%29%
79098
Night
24%45%14%24%29%
80509
SeeingDay
9%8%28%36%14%5%
14827
Night
8%313%35%33%8%
25240

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 10thDay
6%7%23322346%64%
2858
11th to 20thDay
6%6%2342445%8%56%
2988
21st to endDay
11%8%244335%5%7%48%
3498
February1st to 10thDay
10%7%24324345%57%
1900
11th to 20thDay
8%6%3432445%8%54%
1850
21st to endDay
8%11%36%323446%48%
1570
March1st to 10thDay
8%7%244245%6%9%50%
1900
11th to 20thDay
7%6%2443445%8%55%
2000
21st to endDay
5%425%33335%7%60%
2356
April1st to 10thDay
7%5%24424447%56%
2160
11th to 20thDay
3323324446%67%
2129
21st to endDay
4413213347%67%
2160
May1st to 10thDay
45%2432325%7%64%
2290
11th to 20thDay
2312213336%74%
2301
21st to endDay
3312322347%70%
2530
June1st to 10thDay
2311212235%77%
2340
11th to 20thDay
1221112385%
2340
21st to endDay
111211212484%
2340
July1st to 10thDay
1112212335%79%
2340
11th to 20thDay
221222345%7%71%
2340
21st to endDay
2213325%5%6%8%64%
2468
August1st to 10thDay
211121333479%
2149
11th to 20thDay
1112212336%79%
2122
21st to endDay
11221334478%
2376
September1st to 10thDay
2212313336%76%
2137
11th to 20thDay
111111122387%
1980
21st to endDay
121221333478%
1980
October1st to 10thDay
3423323335%70%
1980
11th to 20thDay
321332223475%
1980
21st to endDay
331221233475%
1885
November1st to 10thDay
4412322346%69%
1674
11th to 20thDay
3313423346%67%
1710
21st to endDay
6%6%2333435%7%57%
1710
December1st to 10thDay
6%7%2442435%8%54%
1710
11th to 20thDay
11%7%35%5%35%5%6%7%44%
2169
21st to endDay
7%10%36%7%45%5%5%6%43%
3146
January1st to 10thNight
5%6%25%423435%61%
4097
11th to 20thNight
47%25%5%3445%9%52%
3982
21st to endNight
12%10%35%42345%5%46%
4180
February1st to 10thNight
8%8%23314446%55%
2280
11th to 20thNight
7%8%25%433335%56%
2218
21st to endNight
9%11%35%43445%5%48%
1884
March1st to 10thNight
7%11%36%5%3435%8%45%
2280
11th to 20thNight
7%8%3442335%8%53%
2180
21st to endNight
5%8%344345%5%8%51%
2151
April1st to 10thNight
5%7%35%5%2345%8%53%
1800
11th to 20thNight
5%6%24423436%62%
1779
21st to endNight
46%23333446%62%
1800
May1st to 10thNight
35%135%3336%9%60%
1667
11th to 20thNight
2324423447%64%
1594
21st to endNight
2424423346%65%
1746
June1st to 10thNight
313222246%74%
1620
11th to 20thNight
121121113482%
1620
21st to endNight
12121222483%
1620
July1st to 10thNight
1111111235%84%
1620
11th to 20thNight
111122335%6%76%
1620
21st to endNight
221332346%7%68%
1846
August1st to 10thNight
111213245%79%
1790
11th to 20thNight
111112335%82%
1780
21st to endNight
111221223482%
1980
September1st to 10thNight
11221223481%
1822
11th to 20thNight
111221222284%
1980
21st to endNight
121221222481%
1980
October1st to 10thNight
4224323337%68%
1980
11th to 20thNight
231331223375%
1980
21st to endNight
2424322235%71%
2466
November1st to 10thNight
3423223336%68%
2405
11th to 20thNight
35%23422345%67%
2470
21st to endNight
5%6%36%435%45%7%54%
2470
December1st to 10thNight
8%7%245%245%6%6%52%
2470
11th to 20thNight
9%10%36%43446%8%44%
3041
21st to endNight
6%12%37%6%3445%5%45%
4521

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 10thDay
24%2417%33%19%
2859
11th to 20thDay
26%5%9%23%23%14%
2988
21st to endDay
35%6%7%13%24%16%
3498
February1st to 10thDay
31%25%12%22%27%
1900
11th to 20thDay
29%36%14%26%22%
1860
21st to endDay
37%46%14%20%19%
1570
March1st to 10thDay
31%38%17%21%20%
1900
11th to 20thDay
28%5%7%15%26%18%
2000
21st to endDay
25%49%23%26%14%
2356
April1st to 10thDay
28%37%22%28%12%
2160
11th to 20thDay
19%38%19%30%22%
2132
21st to endDay
18%38%20%36%16%
2160
May1st to 10thDay
22%39%18%28%21%
2290
11th to 20thDay
13%37%21%36%20%
2304
21st to endDay
16%39%24%34%14%
2530
June1st to 10thDay
12%36%18%34%28%
2340
11th to 20thDay
7%2411%27%50%
2340
21st to endDay
9%236%18%63%
2340
July1st to 10thDay
10%348%20%56%
2276
11th to 20thDay
12%5%7%19%26%31%
2271
21st to endDay
16%8%9%21%27%21%
2331
August1st to 10thDay
10%3411%27%45%
2135
11th to 20thDay
10%3414%27%42%
2122
21st to endDay
11%4410%32%40%
2376
September1st to 10thDay
13%449%26%44%
2137
11th to 20thDay
7%128%33%49%
1980
21st to endDay
11%3410%30%42%
1980
October1st to 10thDay
18%35%15%25%34%
1980
11th to 20thDay
15%25%10%25%43%
1980
21st to endDay
14%25%10%23%46%
1885
November1st to 10thDay
18%2413%27%36%
1674
11th to 20thDay
19%25%10%27%36%
1710
21st to endDay
26%45%11%24%31%
1710
December1st to 10thDay
29%36%16%19%28%
1710
11th to 20thDay
37%5%8%12%20%17%
2168
21st to endDay
40%46%17%19%13%
3146
January1st to 10thNight
24%5%5%14%30%22%
4097
11th to 20thNight
28%6%8%22%21%15%
3982
21st to endNight
38%6%5%8%26%17%
4180
February1st to 10thNight
30%35%12%21%28%
2280
11th to 20thNight
34%25%11%23%26%
2219
21st to endNight
39%3313%21%21%
1884
March1st to 10thNight
39%45%15%18%20%
2280
11th to 20thNight
29%6%6%14%27%18%
2180
21st to endNight
32%48%18%23%15%
2151
April1st to 10thNight
30%47%19%23%16%
1800
11th to 20thNight
25%36%19%23%23%
1779
21st to endNight
25%37%17%29%19%
1800
May1st to 10thNight
23%39%17%23%25%
1667
11th to 20thNight
21%28%23%26%20%
1595
21st to endNight
19%310%22%30%16%
1746
June1st to 10thNight
13%37%15%31%31%
1620
11th to 20thNight
9%1313%21%52%
1620
21st to endNight
9%236%18%62%
1620
July1st to 10thNight
6%238%20%60%
1575
11th to 20thNight
9%48%19%28%33%
1566
21st to endNight
16%48%23%29%20%
1743
August1st to 10thNight
8%45%12%28%43%
1780
11th to 20thNight
7%3316%28%43%
1780
21st to endNight
9%339%30%46%
1980
September1st to 10thNight
9%35%13%24%46%
1822
11th to 20thNight
9%239%25%52%
1980
21st to endNight
11%249%31%43%
1980
October1st to 10thNight
20%2416%24%34%
1980
11th to 20thNight
16%15%12%25%40%
1980
21st to endNight
18%239%20%47%
2466
November1st to 10thNight
21%3410%26%37%
2405
11th to 20thNight
21%2311%23%39%
2470
21st to endNight
30%35%12%19%32%
2470
December1st to 10thNight
31%45%12%19%29%
2470
11th to 20thNight
37%47%13%21%18%
3041
21st to endNight
39%6%6%17%20%13%
4521

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 10thDay
19%6%29%34%10%1
270
11th to 20thDay
14%10%25%35%15%2
346
21st to endDay
16%12%31%31%9%1
506
February1st to 10thDay
18%10%30%27%11%3
460
11th to 20thDay
15%15%33%29%7%
455
21st to endDay
20%17%35%21%7%
382
March1st to 10thDay
18%16%36%20%9%1
460
11th to 20thDay
17%13%35%25%10%1
460
21st to endDay
15%17%38%22%8%1
491
April1st to 10thDay
13%13%32%28%11%3
420
11th to 20thDay
9%20%35%25%9%2
412
21st to endDay
9%14%32%28%12%5%
420
May1st to 10thDay
14%12%33%27%11%4
423
11th to 20thDay
9%9%35%32%12%3
448
21st to endDay
8%13%34%29%13%3
485
June1st to 10thDay
46%27%39%17%6%
450
11th to 20thDay
2429%43%15%7%
450
21st to endDay
2428%41%18%8%
450
July1st to 10thDay
2116%52%18%11%
446
11th to 20thDay
2112%51%20%14%
444
21st to endDay
2110%51%23%13%
487
August1st to 10thDay
2121%47%19%11%
447
11th to 20thDay
215%49%17%16%
437
21st to endDay
2115%51%21%11%
495
September1st to 10thDay
2226%45%18%7%
450
11th to 20thDay
3431%41%17%4
450
21st to endDay
49%28%38%17%5%
450
October1st to 10thDay
6%8%32%34%18%2
450
11th to 20thDay
6%8%33%37%16%1
450
21st to endDay
5%6%27%37%20%4
386
November1st to 10thDay
9%6%21%40%20%4
270
11th to 20thDay
6%8%23%41%19%4
270
21st to endDay
15%8%31%30%11%5%
270
December1st to 10thDay
13%5%31%34%14%3
270
11th to 20thDay
8%9%28%37%15%3
270
21st to endDay
23%12%23%29%12%2
297
January1st to 10thNight
12%323%36%21%6%
760
11th to 20thNight
10%420%40%20%6%
760
21st to endNight
16%5%17%40%19%3
836
February1st to 10thNight
14%418%35%24%5%
760
11th to 20thNight
12%5%20%35%22%6%
744
21st to endNight
17%6%19%35%18%4
628
March1st to 10thNight
15%6%18%39%19%2
760
11th to 20thNight
14%314%39%22%8%
760
21st to endNight
11%5%18%38%23%5%
820
April1st to 10thNight
10%6%16%40%24%4
720
11th to 20thNight
9%7%17%37%26%5%
711
21st to endNight
9%415%36%29%7%
720
May1st to 10thNight
7%416%38%30%5%
587
11th to 20thNight
5%315%35%35%7%
531
21st to endNight
6%5%16%37%31%4
584
June1st to 10thNight
1312%39%38%6%
540
11th to 20thNight
129%43%41%4
540
21st to endNight
218%38%44%7%
540
July1st to 10thNight
1230%55%11%
525
11th to 20thNight
1224%63%10%
522
21st to endNight
12121%59%16%
664
August1st to 10thNight
11130%52%16%
716
11th to 20thNight
1116%64%18%
712
21st to endNight
2116%59%21%
792
September1st to 10thNight
21324%56%14%
720
11th to 20thNight
22433%49%11%
720
21st to endNight
328%35%41%12%
720
October1st to 10thNight
6%210%41%34%7%
720
11th to 20thNight
4312%37%32%11%
720
21st to endNight
6%313%33%30%15%
792
November1st to 10thNight
7%211%41%29%10%
740
11th to 20thNight
6%214%40%29%10%
760
21st to endNight
11%417%37%24%7%
760
December1st to 10thNight
11%418%39%21%7%
760
11th to 20thNight
16%5%16%42%19%3
760
21st to endNight
14%5%19%39%18%6%
836
Page generated at 2024-03-24 20:37:04. Version ac07

Copyright © 2024 Attilla Danko.


Palomar Observatory Clear Sky Chart |Charts Home