Dr Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dr Sky Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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
77%
24218
37%
79467
33%
24144
82%
24144
53%
36283

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
74%
747
38%
3987
34%
736
79%
736
33%
1591
January11th to 20thNight
72%
748
47%
3887
39%
740
78%
740
44%
1262
January21st to endNight
67%
827
23%
4329
35%
814
75%
814
25%
2276
February1st to 10thNight
69%
760
37%
2382
36%
760
76%
760
52%
1113
February11th to 20thNight
65%
740
35%
2219
32%
740
74%
740
48%
1120
February21st to endNight
66%
610
36%
1810
33%
610
72%
610
49%
909
March1st to 10thNight
68%
720
39%
1980
31%
720
76%
720
58%
970
March11th to 20thNight
73%
720
39%
1980
32%
720
78%
720
60%
963
March21st to endNight
71%
792
39%
2178
29%
792
79%
792
60%
1052
April1st to 10thNight
75%
720
37%
1980
26%
720
82%
720
65%
919
April11th to 20thNight
69%
614
40%
1858
31%
614
79%
614
67%
776
April21st to endNight
73%
540
48%
1653
38%
540
83%
540
74%
649
May1st to 10thNight
75%
540
43%
1620
35%
540
81%
540
72%
656
May11th to 20thNight
79%
531
42%
1594
34%
531
83%
531
70%
657
May21st to endNight
82%
582
42%
1746
34%
582
85%
582
78%
680
June1st to 10thNight
86%
540
37%
1620
32%
540
87%
540
73%
651
June11th to 20thNight
89%
540
41%
1620
35%
540
92%
540
88%
586
June21st to endNight
87%
540
33%
1620
29%
540
88%
540
67%
693
July1st to 10thNight
86%
525
26%
1575
22%
525
86%
525
53%
767
July11th to 20thNight
86%
522
27%
1566
22%
522
87%
522
48%
795
July21st to endNight
81%
591
25%
1674
21%
558
81%
558
35%
989
August1st to 10thNight
88%
537
29%
1602
25%
534
88%
534
49%
802
August11th to 20thNight
84%
534
33%
1602
31%
534
84%
534
44%
857
August21st to endNight
83%
705
31%
2014
29%
705
83%
705
46%
1078
September1st to 10thNight
82%
720
33%
1980
30%
720
83%
720
52%
1036
September11th to 20thNight
85%
720
43%
1980
36%
720
87%
720
67%
915
September21st to endNight
83%
720
39%
1980
31%
720
87%
720
64%
928
October1st to 10thNight
81%
720
40%
2011
33%
720
85%
720
64%
930
October11th to 20thNight
85%
720
44%
2160
40%
720
88%
720
76%
855
October21st to endNight
82%
792
47%
2376
39%
792
89%
792
75%
944
November1st to 10thNight
85%
740
43%
2220
42%
740
87%
740
65%
961
November11th to 20thNight
83%
760
41%
2417
41%
760
87%
760
63%
1017
November21st to endNight
73%
760
39%
2470
37%
760
79%
760
53%
1116
December1st to 10thNight
75%
760
39%
2470
40%
760
79%
760
50%
1149
December11th to 20thNight
69%
758
34%
2943
37%
758
74%
758
35%
1538
December21st to endNight
69%
823
23%
4364
36%
817
73%
817
28%
2083

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
3213323346%69%
78918
Night
4313323446%68%
80453
TransparencyDay
17%35%12%24%38%
78645
Night
18%35%12%23%39%
80248
SeeingDay
4313%28%34%18%
14008
Night
4111%44%35%3
24273

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
5%435%423246%63%
2858
11th to 20thDay
4414424346%65%
2860
21st to endDay
8%6%3443446%7%52%
3146
February1st to 10thDay
4313323446%67%
1795
11th to 20thDay
7%323423346%63%
1850
21st to endDay
5%422423346%66%
1539
March1st to 10thDay
432343346%6%64%
1980
11th to 20thDay
3313323347%66%
1980
21st to endDay
3313323336%70%
2178
April1st to 10thDay
2112212448%73%
1980
11th to 20thDay
1211223235%79%
2049
21st to endDay
111212236%80%
2303
May1st to 10thDay
111121223482%
2340
11th to 20thDay
1112112235%80%
2301
21st to endDay
21111122387%
2530
June1st to 10thDay
1112212235%81%
2340
11th to 20thDay
111111112289%
2340
21st to endDay
1112222235%79%
2340
July1st to 10thDay
22234235%6%7%65%
2340
11th to 20thDay
3213435%7%8%12%50%
2340
21st to endDay
44245%46%6%8%12%47%
2545
August1st to 10thDay
3223425%5%5%8%60%
2328
11th to 20thDay
32134345%8%9%57%
2299
21st to endDay
22134345%6%9%61%
2334
September1st to 10thDay
341432345%6%65%
1980
11th to 20thDay
211121234478%
1980
21st to endDay
221232323476%
1980
October1st to 10thDay
3212223346%73%
1948
11th to 20thDay
11211123484%
1800
21st to endDay
1112212235%80%
1980
November1st to 10thDay
2112122337%76%
1859
11th to 20thDay
1212312345%75%
1761
21st to endDay
6%323323446%65%
1710
December1st to 10thDay
6%314434445%62%
1710
11th to 20thDay
6%426%4345%6%8%54%
2169
21st to endDay
8%5%25%43345%6%55%
3146
January1st to 10thNight
4423424446%62%
4097
11th to 20thNight
4212322235%74%
4110
21st to endNight
12%5%25%42445%7%51%
4521
February1st to 10thNight
4423323336%67%
2382
11th to 20thNight
6%424323446%63%
2218
21st to endNight
6%213433345%65%
1810
March1st to 10thNight
42123135%5%7%66%
1980
11th to 20thNight
5%223212246%70%
1980
21st to endNight
421231335%7%69%
2178
April1st to 10thNight
2212223446%72%
1980
11th to 20thNight
231212233478%
1858
21st to endNight
221212135%82%
1653
May1st to 10thNight
1212212246%78%
1620
11th to 20thNight
111222223381%
1594
21st to endNight
211111122287%
1746
June1st to 10thNight
1111213236%78%
1620
11th to 20thNight
11122388%
1620
21st to endNight
2111323335%76%
1620
July1st to 10thNight
23234345%6%8%61%
1620
11th to 20thNight
22134347%7%13%54%
1620
21st to endNight
4425%5%46%6%9%13%41%
1773
August1st to 10thNight
2213425%5%8%11%57%
1611
11th to 20thNight
33245%35%6%6%10%55%
1602
21st to endNight
23145%35%6%6%8%57%
2014
September1st to 10thNight
33244245%6%8%60%
1980
11th to 20thNight
2112322246%76%
1980
21st to endNight
2112423335%74%
1980
October1st to 10thNight
2212323335%74%
2011
11th to 20thNight
1211111235%82%
2160
21st to endNight
121221123482%
2376
November1st to 10thNight
3212212345%75%
2220
11th to 20thNight
121322233476%
2417
21st to endNight
5%313323346%67%
2470
December1st to 10thNight
6%323323447%64%
2470
11th to 20thNight
7%426%5%2345%6%54%
3041
21st to endNight
7%5%244345%5%5%55%
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
23%35%16%36%16%
2859
11th to 20thDay
21%4419%33%19%
2860
21st to endDay
30%5%7%16%26%16%
3146
February1st to 10thDay
19%36%15%25%33%
1795
11th to 20thDay
23%35%13%27%29%
1860
21st to endDay
19%3413%31%29%
1539
March1st to 10thDay
20%35%11%27%35%
1980
11th to 20thDay
18%3412%25%38%
1980
21st to endDay
18%248%34%34%
2178
April1st to 10thDay
12%238%35%41%
1980
11th to 20thDay
11%136%26%53%
2052
21st to endDay
8%137%24%56%
2303
May1st to 10thDay
9%138%22%57%
2340
11th to 20thDay
9%238%22%57%
2304
21st to endDay
6%12420%66%
2530
June1st to 10thDay
8%12419%66%
2340
11th to 20thDay
6%1129%81%
2340
21st to endDay
10%245%13%67%
2340
July1st to 10thDay
16%5%7%10%17%45%
2276
11th to 20thDay
22%8%14%21%20%15%
2271
21st to endDay
26%9%14%21%17%12%
2404
August1st to 10thDay
20%6%9%16%17%31%
2313
11th to 20thDay
19%7%11%19%21%23%
2299
21st to endDay
18%6%7%16%27%26%
2334
September1st to 10thDay
19%47%12%24%33%
1980
11th to 20thDay
10%247%29%48%
1980
21st to endDay
12%446%22%52%
1980
October1st to 10thDay
13%3411%25%43%
1948
11th to 20thDay
6%237%24%59%
1800
21st to endDay
10%249%26%50%
1980
November1st to 10thDay
10%2313%32%41%
1859
11th to 20thDay
14%2410%27%43%
1761
21st to endDay
20%25%11%25%36%
1710
December1st to 10thDay
25%4415%24%29%
1710
11th to 20thDay
27%48%18%24%19%
2168
21st to endDay
27%5%8%18%27%14%
3146
January1st to 10thNight
20%5%5%15%34%20%
4097
11th to 20thNight
15%2320%39%21%
4110
21st to endNight
31%7%7%12%26%17%
4521
February1st to 10thNight
19%35%12%23%39%
2382
11th to 20thNight
23%3414%25%31%
2219
21st to endNight
22%3311%26%34%
1810
March1st to 10thNight
17%3411%25%40%
1980
11th to 20thNight
16%2310%24%44%
1980
21st to endNight
16%249%33%37%
2178
April1st to 10thNight
13%238%26%48%
1980
11th to 20thNight
11%235%24%54%
1858
21st to endNight
9%126%21%62%
1653
May1st to 10thNight
9%138%22%56%
1620
11th to 20thNight
10%237%18%61%
1595
21st to endNight
7%11315%72%
1746
June1st to 10thNight
9%13413%70%
1620
11th to 20thNight
312410%80%
1620
21st to endNight
12%2449%69%
1620
July1st to 10thNight
19%49%12%13%44%
1575
11th to 20thNight
18%8%14%21%22%17%
1566
21st to endNight
29%9%12%19%16%15%
1674
August1st to 10thNight
18%7%9%18%16%32%
1602
11th to 20thNight
23%7%11%16%18%25%
1602
21st to endNight
22%6%7%13%24%27%
2014
September1st to 10thNight
21%49%16%18%32%
1980
11th to 20thNight
12%3310%21%51%
1980
21st to endNight
13%447%19%54%
1980
October1st to 10thNight
14%36%11%20%47%
2011
11th to 20thNight
8%137%20%61%
2160
21st to endNight
8%139%21%57%
2376
November1st to 10thNight
13%238%26%47%
2220
11th to 20thNight
14%249%25%47%
2417
21st to endNight
19%2410%27%38%
2470
December1st to 10thNight
20%35%17%22%33%
2470
11th to 20thNight
29%36%14%26%22%
3041
21st to endNight
27%6%8%16%24%19%
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
8%313%40%28%9%
270
11th to 20thDay
10%211%32%34%11%
270
21st to endDay
45%16%38%25%12%
297
February1st to 10thDay
6%420%30%31%9%
355
11th to 20thDay
9%7%22%33%25%4
455
21st to endDay
8%7%23%40%19%3
363
March1st to 10thDay
6%9%23%36%19%7%
420
11th to 20thDay
6%6%21%28%24%15%
420
21st to endDay
5%9%23%34%24%6%
462
April1st to 10thDay
46%17%35%26%12%
420
11th to 20thDay
28%25%30%27%8%
412
21st to endDay
36%22%31%27%12%
420
May1st to 10thDay
1422%28%30%15%
423
11th to 20thDay
1216%30%37%15%
448
21st to endDay
15%17%32%34%11%
485
June1st to 10thDay
228%29%39%19%
450
11th to 20thDay
1210%31%34%22%
450
21st to endDay
117%26%44%21%
450
July1st to 10thDay
21419%47%28%
446
11th to 20thDay
4414%42%36%
444
21st to endDay
4314%40%38%
487
August1st to 10thDay
2217%40%38%
447
11th to 20thDay
31215%45%34%
437
21st to endDay
11115%43%40%
495
September1st to 10thDay
5%16%22%42%24%
450
11th to 20thDay
416%24%45%20%
450
21st to endDay
4213%27%32%23%
450
October1st to 10thDay
3313%33%35%14%
418
11th to 20thDay
1113%26%37%21%
270
21st to endDay
1213%27%38%18%
297
November1st to 10thDay
3212%33%37%12%
270
11th to 20thDay
319%34%39%14%
270
21st to endDay
6%16%34%33%10%
270
December1st to 10thDay
9%17%40%33%11%
270
11th to 20thDay
5%213%42%31%8%
270
21st to endDay
11%418%37%23%7%
297
January1st to 10thNight
6%211%53%26%2
760
11th to 20thNight
7%212%49%28%2
760
21st to endNight
10%115%50%22%2
836
February1st to 10thNight
7%217%46%26%3
760
11th to 20thNight
8%219%52%19%1
744
21st to endNight
8%318%51%20%1
610
March1st to 10thNight
5%319%54%17%2
720
11th to 20thNight
6%215%43%29%6%
720
21st to endNight
5%216%47%28%2
792
April1st to 10thNight
3216%52%25%2
720
11th to 20thNight
4222%48%24%1
614
21st to endNight
2220%49%24%3
540
May1st to 10thNight
2218%50%26%2
540
11th to 20thNight
2115%48%31%2
531
21st to endNight
3113%51%31%2
584
June1st to 10thNight
218%51%36%2
540
11th to 20thNight
18%51%38%2
540
21st to endNight
215%40%50%3
540
July1st to 10thNight
3436%52%4
525
11th to 20thNight
3323%63%7%
522
21st to endNight
5%1317%64%10%
591
August1st to 10thNight
2120%65%10%
537
11th to 20thNight
1425%64%6%
534
21st to endNight
4320%62%11%
705
September1st to 10thNight
5%15%29%54%7%
720
11th to 20thNight
217%35%51%4
720
21st to endNight
219%41%43%5%
720
October1st to 10thNight
310%49%36%2
720
11th to 20thNight
219%47%38%2
720
21st to endNight
2111%43%37%6%
792
November1st to 10thNight
417%49%36%3
740
11th to 20thNight
210%48%37%3
760
21st to endNight
5%213%48%29%3
760
December1st to 10thNight
8%210%48%31%1
760
11th to 20thNight
10%210%51%25%2
760
21st to endNight
8%113%51%23%3
836
Page generated at 2024-02-22 04:05:47. Version ac07

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