DeLan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DeLan Observatory clear sky chart from 2005-11-05 to 2018-11-30.

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
27%
16193
12%
51686
7%
16162
33%
16162
14%
38170

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
16%
782
10%
2726
4%
778
23%
778
8%
2162
January16th to endNight
17%
832
12%
2912
3%
832
29%
832
10%
2168
February1st to 15thNight
19%
763
10%
2562
4%
761
27%
761
11%
1951
February16th to endNight
17%
683
12%
2049
5%
683
26%
683
12%
1593
March1st to 15thNight
21%
780
13%
2323
8%
780
27%
780
14%
1781
March16th to endNight
32%
832
17%
2217
8%
832
38%
832
20%
1617
April1st to 15thNight
26%
755
16%
1923
7%
755
31%
755
18%
1477
April16th to endNight
28%
578
15%
1734
8%
578
33%
578
17%
1241
May1st to 15thNight
32%
517
13%
1687
10%
517
38%
517
16%
1212
May16th to endNight
31%
407
11%
1544
8%
406
39%
406
15%
1081
June1st to 15thNight
36%
390
7%
1365
5%
390
41%
390
13%
1002
June16th to endNight
39%
390
7%
1365
7%
390
41%
390
16%
955
July1st to 15thNight
41%
370
8%
1389
9%
370
45%
370
19%
919
July16th to endNight
43%
408
9%
1546
12%
386
46%
386
16%
1013
August1st to 15thNight
47%
460
12%
1601
11%
458
50%
458
19%
1085
August16th to endNight
41%
665
15%
1907
10%
665
45%
665
24%
1292
September1st to 15thNight
38%
780
13%
2018
7%
780
41%
780
25%
1428
September16th to endNight
36%
777
17%
2145
10%
777
41%
777
26%
1422
October1st to 15thNight
28%
780
15%
2251
6%
780
38%
780
21%
1606
October16th to endNight
22%
832
9%
2704
5%
832
27%
832
12%
2048
November1st to 15thNight
25%
820
15%
2667
10%
820
31%
820
12%
1983
November16th to endNight
20%
887
11%
3006
5%
887
28%
887
11%
2285
December1st to 15thNight
13%
825
6%
2925
1%
825
18%
825
6%
2397
December16th to endNight
14%
880
11%
3120
4%
880
23%
880
8%
2452

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
17%9%36%5%3445%6%31%
52618
Night
18%10%47%5%34445%29%
51853
TransparencyDay
52%6%12%19%8%
52304
Night
56%5%11%17%8%
51686
SeeingDay
24%11%28%22%9%2
7712
Night
25%10%29%25%8%
16194

By Month

Percentage Cloud Cover NE (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
16%15%5%8%6%3445%6%21%
1539
16th to endDay
10%12%6%9%6%46%46%6%27%
1664
February1st to 15thDay
9%13%6%10%6%46%445%26%
1626
16th to endDay
15%13%6%9%6%35%446%26%
1710
March1st to 15thDay
16%13%5%7%5%34436%28%
1956
16th to endDay
14%8%47%7%35%446%34%
2356
April1st to 15thDay
21%9%36%5%24436%32%
2354
16th to endDay
23%8%35%423345%35%
2501
May1st to 15thDay
21%7%26%433446%34%
2604
16th to endDay
20%6%36%6%35%5%5%6%30%
2920
June1st to 15thDay
20%6%26%5%25%6%6%8%29%
2925
16th to endDay
18%7%35%6%5%6%5%6%7%27%
2925
July1st to 15thDay
12%6%35%5%35%5%6%9%35%
2821
16th to endDay
16%7%25%5%35%6%6%7%33%
2878
August1st to 15thDay
14%6%36%6%47%6%6%9%29%
2589
16th to endDay
11%5%25%4245%5%8%41%
2646
September1st to 15thDay
18%6%25%434336%41%
2272
16th to endDay
17%6%24323347%44%
2145
October1st to 15thDay
19%8%46%5%2345%5%33%
2035
16th to endDay
26%10%47%6%2445%5%23%
1872
November1st to 15thDay
20%10%37%7%35%45%5%27%
1851
16th to endDay
20%15%6%7%5%22345%24%
1608
December1st to 15thDay
23%16%48%5%34336%19%
1365
16th to endDay
19%16%5%8%5%3434422%
1456
January1st to 15thNight
17%14%5%9%8%35%44421%
2726
16th to endNight
12%12%6%8%7%46%5%45%25%
2912
February1st to 15thNight
8%14%5%11%8%5%5%5%5%5%24%
2561
16th to endNight
15%15%6%11%6%3433424%
2049
March1st to 15thNight
18%12%48%5%245%45%28%
2310
16th to endNight
13%12%46%6%34446%33%
2217
April1st to 15thNight
23%11%47%423445%28%
1923
16th to endNight
25%8%25%424345%34%
1734
May1st to 15thNight
24%9%34424445%31%
1687
16th to endNight
23%6%25%5%4434433%
1542
June1st to 15thNight
22%7%246%46%45%5%28%
1365
16th to endNight
17%7%36%5%34447%33%
1365
July1st to 15thNight
15%6%26%5%35%5%6%8%34%
1466
16th to endNight
14%6%45%6%345%6%8%35%
1642
August1st to 15thNight
15%6%36%5%335%6%8%34%
1611
16th to endNight
14%5%25%5%345%6%7%38%
1907
September1st to 15thNight
18%6%26%32445%6%38%
2018
16th to endNight
20%6%14323346%42%
2145
October1st to 15thNight
21%8%35%423446%34%
2251
16th to endNight
25%10%37%423345%26%
2704
November1st to 15thNight
19%11%47%6%25%445%28%
2667
16th to endNight
19%14%5%8%5%23345%25%
3006
December1st to 15thNight
21%15%6%10%7%4443417%
2925
16th to endNight
20%15%49%7%3433422%
3120

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
62%5%10%13%7%
1540
16th to endDay
54%5%11%21%6%
1664
February1st to 15thDay
58%5%12%15%7%
1629
16th to endDay
59%5%7%18%8%1
1710
March1st to 15thDay
58%410%14%11%
1966
16th to endDay
50%5%9%19%11%2
2356
April1st to 15thDay
53%49%18%10%3
2354
16th to endDay
51%410%17%13%2
2501
May1st to 15thDay
50%5%11%19%11%1
2604
16th to endDay
54%7%10%19%8%
2924
June1st to 15thDay
50%8%13%17%8%1
2925
16th to endDay
53%7%11%18%9%
2925
July1st to 15thDay
42%9%14%24%7%
2670
16th to endDay
48%9%13%21%7%
2711
August1st to 15thDay
47%10%18%18%4
2575
16th to endDay
38%8%17%27%8%
2646
September1st to 15thDay
44%5%12%23%12%
2272
16th to endDay
40%414%27%11%
2145
October1st to 15thDay
51%5%12%23%7%
2035
16th to endDay
63%412%15%4
1872
November1st to 15thDay
58%5%11%18%5%
1851
16th to endDay
62%5%11%15%4
1608
December1st to 15thDay
65%6%9%13%4
1365
16th to endDay
65%47%14%8%
1456
January1st to 15thNight
66%49%13%6%
2726
16th to endNight
58%6%9%16%9%
2912
February1st to 15thNight
60%5%10%15%7%
2562
16th to endNight
62%48%13%10%
2049
March1st to 15thNight
58%6%8%14%10%1
2323
16th to endNight
52%49%19%10%2
2217
April1st to 15thNight
57%36%16%13%2
1923
16th to endNight
52%5%9%17%14%2
1734
May1st to 15thNight
54%410%16%12%1
1687
16th to endNight
54%410%16%12%
1544
June1st to 15thNight
55%7%10%17%6%1
1365
16th to endNight
50%7%12%18%10%
1365
July1st to 15thNight
44%8%16%21%7%
1389
16th to endNight
46%8%15%20%9%
1546
August1st to 15thNight
45%8%18%23%3
1601
16th to endNight
42%8%16%25%7%
1907
September1st to 15thNight
46%5%13%22%11%1
2018
16th to endNight
42%5%16%25%9%
2145
October1st to 15thNight
49%5%13%22%8%
2251
16th to endNight
59%5%11%17%6%
2704
November1st to 15thNight
56%6%12%15%8%
2667
16th to endNight
61%411%16%5%
3006
December1st to 15thNight
70%5%10%9%3
2925
16th to endNight
66%39%11%8%
3120

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
26%24%35%11%2
90
16th to endDay
19%28%37%8%42
96
February1st to 15thDay
20%18%37%16%7%
178
16th to endDay
26%15%32%20%4
291
March1st to 15thDay
29%13%27%22%5%
330
16th to endDay
18%10%29%23%13%4
352
April1st to 15thDay
28%15%27%20%7%1
330
16th to endDay
29%9%26%21%7%4
319
May1st to 15thDay
31%10%27%20%8%2
348
16th to endDay
24%13%30%22%8%
473
June1st to 15thDay
24%9%28%26%11%
570
16th to endDay
23%6%29%27%12%1
570
July1st to 15thDay
18%9%27%28%12%3
454
16th to endDay
18%5%30%30%13%1
390
August1st to 15thDay
16%10%28%24%14%4
358
16th to endDay
17%9%25%24%16%6%
396
September1st to 15thDay
21%8%26%24%14%2
375
16th to endDay
19%12%29%23%11%3
374
October1st to 15thDay
27%12%31%17%9%1
354
16th to endDay
37%15%25%12%8%
336
November1st to 15thDay
31%16%25%19%5%
325
16th to endDay
32%13%26%21%41
217
December1st to 15thDay
35%15%27%14%42
90
16th to endDay
36%23%23%12%3
96
January1st to 15thNight
26%17%31%20%3
782
16th to endNight
19%15%35%22%5%
832
February1st to 15thNight
15%15%39%23%6%
764
16th to endNight
27%12%32%21%5%
683
March1st to 15thNight
25%10%35%23%4
780
16th to endNight
20%9%30%29%9%2
832
April1st to 15thNight
30%10%26%24%7%
755
16th to endNight
30%8%26%24%9%
578
May1st to 15thNight
28%7%26%26%10%
517
16th to endNight
28%428%29%7%
407
June1st to 15thNight
25%324%35%10%
390
16th to endNight
18%5%28%34%12%
390
July1st to 15thNight
19%230%31%14%1
370
16th to endNight
16%327%38%12%
408
August1st to 15thNight
17%321%36%18%1
460
16th to endNight
16%5%28%34%12%2
665
September1st to 15thNight
23%426%31%11%1
780
16th to endNight
22%8%28%28%11%
777
October1st to 15thNight
25%10%31%24%7%
780
16th to endNight
31%11%29%18%7%
832
November1st to 15thNight
28%15%24%23%6%
820
16th to endNight
29%13%29%20%5%1
887
December1st to 15thNight
30%17%30%16%3
825
16th to endNight
31%15%28%19%41
880
Page generated at 2018-12-01 11:35:37. Version ac04

Copyright 2018 A. Danko


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