Palomar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Palomar Observatory clear sky chart from 2005-11-05 to 2018-02-22.

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
72%
16843
34%
48923
29%
16802
76%
16802
50%
24861

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
61%
776
29%
2522
28%
776
67%
776
38%
1333
January16th to endNight
60%
832
33%
2506
29%
832
67%
832
37%
1410
February1st to 15thNight
67%
758
32%
2292
31%
758
72%
758
40%
1238
February16th to endNight
56%
660
30%
1979
30%
660
60%
660
30%
1222
March1st to 15thNight
63%
720
30%
2157
31%
720
68%
720
41%
1166
March16th to endNight
66%
768
30%
2043
24%
768
70%
768
44%
1175
April1st to 15thNight
58%
719
28%
1799
17%
719
62%
719
43%
1071
April16th to endNight
60%
720
31%
1800
16%
720
65%
720
51%
1004
May1st to 15thNight
68%
569
32%
1649
23%
569
71%
569
52%
836
May16th to endNight
67%
561
31%
1685
22%
561
71%
561
53%
818
June1st to 15thNight
79%
540
33%
1620
27%
540
81%
540
67%
691
June16th to endNight
87%
540
38%
1620
32%
540
89%
540
81%
615
July1st to 15thNight
93%
510
30%
1530
26%
510
93%
510
70%
640
July16th to endNight
89%
613
32%
1638
25%
576
89%
576
59%
788
August1st to 15thNight
93%
717
36%
1790
23%
713
93%
713
76%
834
August16th to endNight
92%
764
43%
1910
30%
764
92%
764
75%
900
September1st to 15thNight
91%
720
38%
1875
31%
720
92%
720
73%
868
September16th to endNight
85%
716
47%
1980
43%
716
88%
716
73%
856
October1st to 15thNight
75%
720
37%
1980
34%
720
79%
720
56%
988
October16th to endNight
76%
768
41%
2304
41%
768
78%
768
53%
1090
November1st to 15thNight
75%
760
32%
2470
34%
760
79%
760
46%
1202
November16th to endNight
70%
780
35%
2535
37%
780
74%
780
44%
1274
December1st to 15thNight
65%
780
31%
2535
32%
780
69%
780
37%
1354
December16th to endNight
58%
832
33%
2704
31%
832
65%
832
35%
1488

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
4312213336%66%
49319
Night
4413313345%65%
49123
TransparencyDay
20%25%14%26%29%
49019
Night
22%25%13%24%30%
48924
SeeingDay
7%9%30%34%13%5%
6445
Night
6%212%32%35%10%
16856

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%6%23313346%57%
1746
16th to endDay
9%6%1332335%6%54%
2067
February1st to 15thDay
7%423224336%58%
1901
16th to endDay
11%7%26%323447%45%
1641
March1st to 15thDay
6%41221345%9%58%
1803
16th to endDay
5%314323346%59%
2178
April1st to 15thDay
7%433313346%57%
2149
16th to endDay
4312212346%67%
2160
May1st to 15thDay
3413212237%67%
2298
16th to endDay
232212236%72%
2429
June1st to 15thDay
22121223476%
2340
16th to endDay
1111287%
2340
July1st to 15thDay
2112212346%72%
2340
16th to endDay
11222445%7%65%
2415
August1st to 15thDay
11121223479%
2160
16th to endDay
1122334476%
2292
September1st to 15thDay
112212225%76%
2083
16th to endDay
1111222381%
1980
October1st to 15thDay
431332223469%
1980
16th to endDay
33231232471%
1916
November1st to 15thDay
4312423346%64%
1719
16th to endDay
5%413313347%60%
1755
December1st to 15thDay
7%5%1332335%8%54%
1755
16th to endDay
11%6%24313335%53%
1872
January1st to 15thNight
7%424423436%56%
2522
16th to endNight
9%6%24423345%55%
2506
February1st to 15thNight
7%6%1332445%5%56%
2281
16th to endNight
10%8%25%43433448%
1979
March1st to 15thNight
7%6%13323247%56%
2157
16th to endNight
5%5%23324449%55%
2043
April1st to 15thNight
7%5%24413447%55%
1799
16th to endNight
4322223345%65%
1800
May1st to 15thNight
2312313348%64%
1649
16th to endNight
1312313346%67%
1684
June1st to 15thNight
21212145%76%
1620
16th to endNight
112287%
1620
July1st to 15thNight
11112246%76%
1620
16th to endNight
21222435%6%68%
1750
August1st to 15thNight
111112235%79%
1800
16th to endNight
1111123382%
1910
September1st to 15thNight
11111222480%
1875
16th to endNight
11111111383%
1980
October1st to 15thNight
3212212336%69%
1980
16th to endNight
241331223468%
2304
November1st to 15thNight
3323223345%65%
2470
16th to endNight
4423323337%59%
2535
December1st to 15thNight
7%5%1342345%7%53%
2535
16th to endNight
9%6%2432324455%
2704

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
28%25%13%26%22%
1747
16th to endDay
30%26%13%19%27%
2067
February1st to 15thDay
27%25%13%24%25%
1901
16th to endDay
37%36%14%22%14%
1641
March1st to 15thDay
23%36%19%24%22%
1803
16th to endDay
25%37%21%25%15%
2178
April1st to 15thDay
28%26%21%30%10%
2149
16th to endDay
18%28%20%29%19%
2160
May1st to 15thDay
20%26%17%30%22%
2298
16th to endDay
15%38%20%36%16%
2432
June1st to 15thDay
13%2415%28%36%
2340
16th to endDay
6%125%19%64%
2340
July1st to 15thDay
12%35%11%22%44%
2201
16th to endDay
16%6%8%20%25%23%
2264
August1st to 15thDay
10%2311%30%41%
2146
16th to endDay
12%449%32%37%
2292
September1st to 15thDay
12%439%29%39%
2083
16th to endDay
10%128%33%43%
1980
October1st to 15thDay
19%2414%27%32%
1980
16th to endDay
17%2410%21%42%
1916
November1st to 15thDay
22%16%14%24%30%
1719
16th to endDay
24%3410%26%31%
1755
December1st to 15thDay
29%27%16%17%27%
1755
16th to endDay
34%25%13%25%18%
1872
January1st to 15thNight
29%2413%23%26%
2522
16th to endNight
31%36%11%19%28%
2506
February1st to 15thNight
28%25%12%23%27%
2292
16th to endNight
39%2313%23%18%
1979
March1st to 15thNight
28%25%18%22%22%
2157
16th to endNight
27%47%17%25%17%
2043
April1st to 15thNight
29%27%21%24%14%
1799
16th to endNight
23%27%18%25%22%
1800
May1st to 15thNight
21%27%18%25%24%
1650
16th to endNight
19%29%20%28%18%
1685
June1st to 15thNight
11%15%14%27%38%
1620
16th to endNight
5%125%18%66%
1620
July1st to 15thNight
9%25%9%23%49%
1530
16th to endNight
15%48%20%27%23%
1638
August1st to 15thNight
7%3314%31%39%
1790
16th to endNight
9%239%31%43%
1910
September1st to 15thNight
9%25%12%28%41%
1875
16th to endNight
9%128%32%45%
1980
October1st to 15thNight
19%1415%28%30%
1980
16th to endNight
19%1310%20%44%
2304
November1st to 15thNight
22%2412%25%31%
2470
16th to endNight
25%2412%20%33%
2535
December1st to 15thNight
29%36%13%19%27%
2535
16th to endNight
32%27%12%24%20%
2704

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
22%421%34%15%1
135
16th to endDay
15%14%25%30%11%2
339
February1st to 15thDay
8%12%29%32%11%5%
326
16th to endDay
17%16%31%25%8%
264
March1st to 15thDay
12%15%30%25%12%3
270
16th to endDay
7%21%38%19%11%2
288
April1st to 15thDay
13%19%35%15%11%2
269
16th to endDay
8%20%42%19%6%2
270
May1st to 15thDay
12%13%35%29%6%2
288
16th to endDay
6%13%40%28%9%2
330
June1st to 15thDay
26%43%32%9%4
315
16th to endDay
2232%39%15%7%
315
July1st to 15thDay
1120%52%15%8%
305
16th to endDay
2113%53%17%10%
327
August1st to 15thDay
126%46%14%9%
315
16th to endDay
2118%53%12%12%
332
September1st to 15thDay
2134%44%11%5%
315
16th to endDay
28%33%38%13%3
315
October1st to 15thDay
5%9%38%32%12%
315
16th to endDay
6%6%28%29%23%4
263
November1st to 15thDay
8%5%20%37%23%5%
135
16th to endDay
11%7%19%34%17%8%
135
December1st to 15thDay
5%7%21%40%21%2
135
16th to endDay
14%6%20%33%20%4
144
January1st to 15thNight
9%216%36%25%8%
780
16th to endNight
13%314%34%27%6%
832
February1st to 15thNight
9%314%36%27%8%
764
16th to endNight
14%5%17%35%21%6%
660
March1st to 15thNight
10%5%15%36%27%4
720
16th to endNight
6%417%34%27%8%
768
April1st to 15thNight
7%7%21%33%24%5%
719
16th to endNight
6%5%24%28%27%7%
720
May1st to 15thNight
4220%36%29%6%
570
16th to endNight
3419%35%30%6%
563
June1st to 15thNight
1116%41%35%4
540
16th to endNight
110%37%44%5%
540
July1st to 15thNight
228%57%10%
510
16th to endNight
1321%57%16%
613
August1st to 15thNight
1222%55%17%
717
16th to endNight
1113%58%24%
764
September1st to 15thNight
322%56%15%
720
16th to endNight
28%32%41%15%
716
October1st to 15thNight
4212%33%38%8%
720
16th to endNight
4111%30%35%16%
768
November1st to 15thNight
4210%37%33%11%
760
16th to endNight
7%212%30%32%14%
780
December1st to 15thNight
10%312%36%27%8%
780
16th to endNight
14%5%13%34%25%6%
832
Page generated at 2018-02-23 08:21:37. Version ac04

Copyright 2018 A. Danko


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