ASKC Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASKC Dark Site clear sky chart from 2005-11-05 to 2017-09-10.

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
44%
15828
24%
46938
16%
15781
52%
15781
33%
27804

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
31%
900
24%
2327
9%
895
41%
895
34%
1445
January16th to endNight
33%
960
28%
2496
10%
960
44%
960
35%
1540
February1st to 15thNight
27%
880
19%
2288
7%
876
33%
876
27%
1541
February16th to endNight
29%
652
26%
1914
10%
652
41%
652
29%
1171
March1st to 15thNight
34%
540
23%
1980
14%
540
46%
540
24%
1178
March16th to endNight
31%
576
17%
2112
11%
576
41%
576
20%
1343
April1st to 15thNight
35%
539
22%
1851
15%
539
47%
539
28%
1059
April16th to endNight
34%
540
21%
1734
16%
540
45%
540
24%
1094
May1st to 15thNight
41%
539
19%
1620
15%
539
48%
539
28%
1034
May16th to endNight
48%
563
21%
1685
17%
561
54%
561
31%
1010
June1st to 15thNight
65%
540
21%
1443
14%
540
69%
540
45%
828
June16th to endNight
66%
540
25%
1494
17%
540
70%
540
50%
800
July1st to 15thNight
70%
510
18%
1530
18%
510
71%
510
41%
834
July16th to endNight
76%
564
25%
1594
26%
531
79%
531
46%
823
August1st to 15thNight
75%
539
26%
1610
31%
536
76%
536
43%
857
August16th to endNight
73%
573
28%
1829
33%
573
76%
573
43%
940
September1st to 15thNight
64%
525
26%
1791
29%
525
67%
525
33%
990
September16th to endNight
64%
492
30%
1815
27%
492
70%
492
36%
919
October1st to 15thNight
50%
657
29%
1977
23%
657
55%
657
37%
1120
October16th to endNight
40%
704
30%
2112
16%
704
51%
704
38%
1161
November1st to 15thNight
42%
735
32%
2135
16%
735
55%
735
38%
1197
November16th to endNight
32%
900
25%
2480
11%
900
43%
900
32%
1517
December1st to 15thNight
30%
900
20%
2520
8%
900
40%
900
28%
1626
December16th to endNight
31%
960
23%
2601
10%
960
41%
960
25%
1777

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
12%6%2442445%7%44%
47670
Night
11%5%24423346%49%
47133
TransparencyDay
39%312%26%14%2
47370
Night
35%311%27%18%3
46938
SeeingDay
13%5%22%30%22%5%
5854
Night
14%6%28%34%15%1
15829

By Month

Percentage Cloud Cover SE (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
12%5%26%5%2343449%
1611
16th to endDay
13%7%36%422347%44%
1728
February1st to 15thDay
12%10%45%434236%42%
1575
16th to endDay
14%8%36%414346%42%
1566
March1st to 15thDay
15%7%37%5%35%446%36%
1980
16th to endDay
23%9%25%42343434%
2110
April1st to 15thDay
15%8%34424446%42%
2097
16th to endDay
20%7%36%5%2435%6%34%
2223
May1st to 15thDay
19%7%35%5%34346%36%
2331
16th to endDay
15%6%26%5%35%447%37%
2430
June1st to 15thDay
8%415%6%25%6%6%8%44%
2511
16th to endDay
6%2124245%6%10%52%
2461
July1st to 15thDay
8%4244346%6%8%44%
2340
16th to endDay
6%424436%6%6%10%45%
2466
August1st to 15thDay
6%415%5%35%46%10%45%
2340
16th to endDay
8%3134245%6%9%48%
2372
September1st to 15thDay
12%5%2442345%6%48%
2053
16th to endDay
9%31343445%7%52%
1806
October1st to 15thDay
14%5%2342244451%
1650
16th to endDay
12%5%2441343453%
1760
November1st to 15thDay
10%5%23313447%52%
1722
16th to endDay
17%5%23312337%48%
1478
December1st to 15thDay
16%7%35%423336%42%
1440
16th to endDay
16%7%46%414236%41%
1620
January1st to 15thNight
11%6%14423335%51%
2327
16th to endNight
9%6%24424446%50%
2496
February1st to 15thNight
11%9%35%424346%42%
2278
16th to endNight
11%7%35%423346%47%
1914
March1st to 15thNight
12%6%26%5%23246%45%
1980
16th to endNight
18%7%2442445%5%39%
2112
April1st to 15thNight
11%6%2442335%6%48%
1851
16th to endNight
16%6%25%5%23336%43%
1734
May1st to 15thNight
14%5%35%424446%44%
1619
16th to endNight
13%42432436%10%43%
1683
June1st to 15thNight
5%3234236%6%9%53%
1443
16th to endNight
421322447%8%58%
1494
July1st to 15thNight
6%2144335%6%9%52%
1620
16th to endNight
42133245%6%9%54%
1702
August1st to 15thNight
5%21432445%8%56%
1620
16th to endNight
5%31442445%6%57%
1829
September1st to 15thNight
11%41432445%7%50%
1791
16th to endNight
7%31343445%7%54%
1815
October1st to 15thNight
14%413223235%55%
1977
16th to endNight
10%42342233458%
2112
November1st to 15thNight
10%413313335%57%
2135
16th to endNight
15%6%23213235%52%
2480
December1st to 15thNight
17%8%34312347%43%
2520
16th to endNight
15%8%35%423345%41%
2601

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
38%211%27%17%1
1612
16th to endDay
40%214%26%13%1
1728
February1st to 15thDay
45%414%25%9%1
1576
16th to endDay
44%211%25%13%2
1566
March1st to 15thDay
48%311%21%12%2
1980
16th to endDay
53%213%19%9%1
2112
April1st to 15thDay
43%312%25%14%
2097
16th to endDay
50%311%22%11%
2223
May1st to 15thDay
48%411%23%11%
2331
16th to endDay
45%415%22%10%
2434
June1st to 15thDay
35%5%14%25%16%1
2511
16th to endDay
25%5%16%33%16%2
2461
July1st to 15thDay
33%5%15%29%13%2
2201
16th to endDay
30%5%15%33%13%2
2311
August1st to 15thDay
32%417%26%17%1
2326
16th to endDay
31%5%12%27%17%5%
2372
September1st to 15thDay
36%310%26%16%5%
2053
16th to endDay
31%410%30%19%3
1806
October1st to 15thDay
35%210%24%21%4
1650
16th to endDay
35%19%29%18%5%
1760
November1st to 15thDay
31%310%27%21%5%
1722
16th to endDay
38%210%28%20%
1478
December1st to 15thDay
43%312%29%11%
1440
16th to endDay
46%213%24%10%3
1620
January1st to 15thNight
35%212%27%19%1
2327
16th to endNight
34%313%30%16%2
2496
February1st to 15thNight
43%311%25%14%1
2288
16th to endNight
38%211%27%17%2
1914
March1st to 15thNight
40%310%24%16%4
1980
16th to endNight
45%311%23%13%1
2112
April1st to 15thNight
36%39%25%21%3
1851
16th to endNight
43%39%24%17%1
1734
May1st to 15thNight
40%412%24%16%1
1620
16th to endNight
35%416%28%12%1
1685
June1st to 15thNight
26%417%30%18%2
1443
16th to endNight
21%414%32%22%3
1494
July1st to 15thNight
28%313%33%16%3
1530
16th to endNight
23%316%30%23%2
1594
August1st to 15thNight
24%414%34%18%3
1610
16th to endNight
25%311%27%25%6%
1829
September1st to 15thNight
32%410%25%19%7%
1791
16th to endNight
28%39%30%24%3
1815
October1st to 15thNight
32%38%24%24%6%
1977
16th to endNight
29%28%31%22%5%
2112
November1st to 15thNight
29%29%26%23%7%
2135
16th to endNight
36%27%28%21%3
2480
December1st to 15thNight
41%313%27%13%
2520
16th to endNight
46%311%22%12%4
2601

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
25%16%31%22%4
90
16th to endDay
18%13%37%21%7%1
96
February1st to 15thDay
7%23%42%20%6%
90
16th to endDay
14%20%41%14%6%1
81
March1st to 15thDay
26%17%36%15%3
90
16th to endDay
27%12%34%17%6%2
96
April1st to 15thDay
15%12%33%25%11%
216
16th to endDay
24%7%32%24%11%
333
May1st to 15thDay
18%8%29%28%12%2
468
16th to endDay
19%427%29%14%4
517
June1st to 15thDay
7%421%35%25%6%
495
16th to endDay
7%226%34%25%4
495
July1st to 15thDay
11%214%34%29%8%
475
16th to endDay
6%16%32%42%10%
515
August1st to 15thDay
8%17%35%38%8%
493
16th to endDay
9%111%32%35%9%
412
September1st to 15thDay
10%211%29%34%11%
250
16th to endDay
10%120%36%17%14%
90
October1st to 15thDay
11%127%34%12%13%
90
16th to endDay
11%239%34%10%2
96
November1st to 15thDay
11%6%17%28%24%11%
90
16th to endDay
34%15%26%15%6%1
90
December1st to 15thDay
15%13%36%23%6%4
90
16th to endDay
22%11%37%23%4
96
January1st to 15thNight
15%11%38%27%6%1
900
16th to endNight
13%10%38%32%5%
960
February1st to 15thNight
16%11%37%30%3
880
16th to endNight
16%11%38%30%3
652
March1st to 15thNight
16%8%35%32%6%
540
16th to endNight
20%8%35%30%4
576
April1st to 15thNight
14%10%34%32%8%
539
16th to endNight
18%6%34%30%10%
540
May1st to 15thNight
15%6%29%34%12%1
540
16th to endNight
15%325%34%18%2
563
June1st to 15thNight
7%219%45%25%1
540
16th to endNight
4222%48%21%1
540
July1st to 15thNight
9%12%45%30%1
510
16th to endNight
5%7%37%45%3
564
August1st to 15thNight
6%15%43%38%4
539
16th to endNight
7%9%39%39%3
573
September1st to 15thNight
13%113%38%27%4
525
16th to endNight
8%217%37%26%6%
492
October1st to 15thNight
14%323%33%21%2
657
16th to endNight
13%7%34%31%11%
704
November1st to 15thNight
13%7%31%31%15%
735
16th to endNight
18%10%34%26%8%
900
December1st to 15thNight
20%9%33%30%4
900
16th to endNight
19%8%33%32%5%
960
Page generated at 2017-09-11 18:35:18. Version ac04

Copyright 2017 A. Danko


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