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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 2017-08-12.

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%
16063
33%
46600
29%
16022
76%
16022
50%
23735

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
62%
716
29%
2327
28%
716
68%
716
40%
1211
January16th to endNight
59%
768
32%
2313
27%
768
66%
768
36%
1321
February1st to 15thNight
65%
698
32%
2112
30%
698
70%
698
40%
1149
February16th to endNight
55%
632
28%
1895
29%
632
58%
632
29%
1182
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%
701
36%
1750
23%
697
93%
697
75%
818
August16th to endNight
92%
700
40%
1750
28%
700
92%
700
73%
836
September1st to 15thNight
92%
660
40%
1719
33%
660
93%
660
74%
788
September16th to endNight
85%
656
46%
1815
42%
656
88%
656
75%
778
October1st to 15thNight
74%
660
38%
1815
34%
660
77%
660
55%
918
October16th to endNight
75%
704
38%
2112
39%
704
78%
704
52%
1010
November1st to 15thNight
75%
700
34%
2275
36%
700
79%
700
49%
1075
November16th to endNight
70%
720
34%
2340
36%
720
74%
720
44%
1174
December1st to 15thNight
64%
720
31%
2340
31%
720
68%
720
36%
1276
December16th to endNight
56%
768
32%
2496
29%
768
63%
768
34%
1395

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%
47363
Night
4413313335%65%
46800
TransparencyDay
20%25%14%26%29%
47063
Night
22%25%14%24%30%
46601
SeeingDay
7%9%31%35%11%4
5751
Night
6%213%31%35%10%
16076

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%5%23323346%58%
1611
16th to endDay
10%6%1232345%5%53%
1908
February1st to 15thDay
8%423224336%57%
1751
16th to endDay
11%7%26%323447%45%
1579
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
11121223478%
2114
16th to endDay
11223345%75%
2100
September1st to 15thDay
1122222477%
1910
16th to endDay
1111222381%
1815
October1st to 15thDay
4313322235%66%
1815
16th to endDay
341231232470%
1756
November1st to 15thDay
4312312235%66%
1584
16th to endDay
6%413213347%60%
1620
December1st to 15thDay
8%5%1442335%8%52%
1620
16th to endDay
12%6%24323335%53%
1728
January1st to 15thNight
7%414323435%58%
2327
16th to endNight
9%6%24423335%54%
2313
February1st to 15thNight
7%6%13323345%56%
2101
16th to endNight
10%8%25%5%3433448%
1895
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%78%
1760
16th to endNight
1111223380%
1750
September1st to 15thNight
111222481%
1719
16th to endNight
1111111384%
1815
October1st to 15thNight
3213212336%67%
1815
16th to endNight
2413412245%67%
2112
November1st to 15thNight
3323212335%68%
2275
16th to endNight
45%23323336%60%
2340
December1st to 15thNight
8%5%2342345%7%52%
2340
16th to endNight
10%6%2432324455%
2496

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%12%26%24%
1612
16th to endDay
32%26%11%19%27%
1908
February1st to 15thDay
28%25%13%23%24%
1751
16th to endDay
37%36%14%23%13%
1579
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%40%
2100
16th to endDay
12%4410%32%35%
2100
September1st to 15thDay
12%339%28%42%
1910
16th to endDay
10%128%32%44%
1815
October1st to 15thDay
21%2415%27%29%
1815
16th to endDay
18%25%11%21%41%
1756
November1st to 15thDay
20%16%13%24%32%
1584
16th to endDay
24%3410%25%31%
1620
December1st to 15thDay
30%27%16%17%24%
1620
16th to endDay
34%26%14%24%17%
1728
January1st to 15thNight
27%25%12%23%27%
2327
16th to endNight
32%36%10%19%28%
2313
February1st to 15thNight
29%25%13%22%26%
2112
16th to endNight
40%2313%22%17%
1895
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
8%3315%31%39%
1750
16th to endNight
10%2310%32%40%
1750
September1st to 15thNight
9%25%11%27%44%
1719
16th to endNight
9%118%32%47%
1815
October1st to 15thNight
20%1416%29%27%
1815
16th to endNight
20%2310%21%42%
2112
November1st to 15thNight
21%2412%25%34%
2275
16th to endNight
25%2413%20%33%
2340
December1st to 15thNight
31%36%14%20%24%
2340
16th to endNight
33%27%13%23%19%
2496

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
24%5%27%35%6%
90
16th to endDay
18%15%27%31%5%1
276
February1st to 15thDay
10%13%31%33%9%1
266
16th to endDay
18%17%33%23%7%
238
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
1126%47%14%9%
300
16th to endDay
2118%55%10%11%
268
September1st to 15thDay
3134%45%10%4
255
16th to endDay
38%35%39%10%3
255
October1st to 15thDay
6%12%40%31%8%
255
16th to endDay
7%7%32%31%19%2
205
November1st to 15thDay
6%7%15%33%28%7%
90
16th to endDay
14%8%22%36%14%3
90
December1st to 15thDay
6%6%21%44%18%2
90
16th to endDay
20%8%26%35%9%
96
January1st to 15thNight
8%217%37%24%9%
720
16th to endNight
13%314%34%26%7%
768
February1st to 15thNight
10%315%36%26%7%
704
16th to endNight
14%5%18%34%20%6%
632
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
1221%55%18%
701
16th to endNight
1114%58%24%
700
September1st to 15thNight
221%57%16%
660
16th to endNight
28%31%40%16%
656
October1st to 15thNight
5%213%33%36%9%
660
16th to endNight
4111%31%34%15%
704
November1st to 15thNight
4211%33%35%12%
700
16th to endNight
7%213%29%31%15%
720
December1st to 15thNight
11%412%35%27%8%
720
16th to endNight
15%5%14%34%23%6%
768
Page generated at 2017-08-13 11:16:17. Version ac04

Copyright 2017 A. Danko


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