Ancient Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ancient Sky Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
50%
15953
27%
48367
18%
15906
58%
15906
36%
27341

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
36%
955
27%
2472
11%
951
46%
951
34%
1534
January16th to endNight
34%
915
31%
2451
11%
915
47%
915
35%
1491
February1st to 15thNight
33%
704
29%
2106
11%
698
47%
698
31%
1199
February16th to endNight
25%
632
30%
1894
9%
632
41%
632
30%
1082
March1st to 15thNight
33%
549
28%
1989
15%
549
48%
549
25%
1151
March16th to endNight
32%
576
25%
2112
15%
576
46%
576
28%
1171
April1st to 15thNight
30%
539
23%
1977
13%
539
45%
539
27%
1110
April16th to endNight
30%
540
25%
1752
14%
540
48%
540
29%
1050
May1st to 15thNight
35%
539
23%
1620
16%
539
47%
539
32%
964
May16th to endNight
48%
562
25%
1684
19%
561
59%
561
36%
979
June1st to 15thNight
67%
540
23%
1620
22%
540
73%
540
50%
799
June16th to endNight
69%
540
23%
1620
25%
540
73%
540
40%
899
July1st to 15thNight
77%
510
23%
1530
26%
510
79%
510
47%
797
July16th to endNight
83%
564
28%
1594
30%
531
86%
531
48%
815
August1st to 15thNight
89%
539
32%
1628
38%
536
89%
536
58%
748
August16th to endNight
75%
573
21%
1979
26%
573
76%
573
33%
1100
September1st to 15thNight
75%
540
20%
1980
25%
540
77%
540
32%
1095
September16th to endNight
68%
537
29%
1980
30%
537
72%
537
32%
1075
October1st to 15thNight
69%
564
34%
2004
31%
564
73%
564
41%
981
October16th to endNight
58%
768
37%
2304
23%
768
66%
768
48%
1145
November1st to 15thNight
51%
777
28%
2297
18%
777
58%
777
37%
1310
November16th to endNight
45%
975
33%
2535
16%
975
54%
975
40%
1485
December1st to 15thNight
40%
975
28%
2535
15%
975
47%
975
33%
1609
December16th to endNight
35%
1040
25%
2704
10%
1040
44%
1040
31%
1752

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
11%424424447%48%
48961
Night
10%423323347%54%
48569
Cloud CoverDay
11%42442445%7%48%
48959
Night
10%423323347%54%
48569
TransparencyDay
35%410%20%19%9%
48659
Night
30%411%21%20%12%
48374
SeeingDay
11%418%32%26%7%
5779
Night
12%6%26%35%17%1
15956

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
16%5%1321233452%
1710
16th to endDay
13%41331335%452%
1782
February1st to 15thDay
14%5%24312336%54%
1750
16th to endDay
15%6%23222335%52%
1580
March1st to 15thDay
19%5%24313335%46%
1969
16th to endDay
13%5%34413345%50%
2112
April1st to 15thDay
12%5%25%5%23447%46%
1971
16th to endDay
14%6%34334345%46%
2205
May1st to 15thDay
14%6%44425%45%6%40%
2331
16th to endDay
14%6%25%5%35%5%7%7%36%
2430
June1st to 15thDay
5%313435%6%6%9%50%
2340
16th to endDay
9%5%245%35%5%7%9%41%
2340
July1st to 15thDay
7%2246%46%5%5%9%44%
2340
16th to endDay
7%323436%5%5%10%47%
2466
August1st to 15thDay
211232445%10%61%
2319
16th to endDay
7%4245%37%5%5%9%42%
2223
September1st to 15thDay
9%425%6%35%45%7%44%
1980
16th to endDay
9%3145%35%5%5%8%46%
1980
October1st to 15thDay
11%314423335%56%
1943
16th to endDay
7%212211235%67%
1920
November1st to 15thDay
12%4245%2333450%
1888
16th to endDay
12%413313235%57%
1755
December1st to 15thDay
16%6%24323345%47%
1755
16th to endDay
15%7%34413435%44%
1872
January1st to 15thNight
15%414313236%52%
2472
16th to endNight
14%423223236%53%
2451
February1st to 15thNight
13%5%2421223456%
2102
16th to endNight
12%423423336%53%
1894
March1st to 15thNight
14%5%24413346%48%
1989
16th to endNight
11%423323446%52%
2112
April1st to 15thNight
11%5%23424446%51%
1977
16th to endNight
13%5%2432445%6%47%
1752
May1st to 15thNight
10%42431346%8%48%
1619
16th to endNight
8%4133245%7%8%48%
1683
June1st to 15thNight
321232335%9%62%
1620
16th to endNight
5%2133245%5%10%53%
1620
July1st to 15thNight
421332346%11%58%
1620
16th to endNight
32123135%6%11%58%
1702
August1st to 15thNight
212212449%68%
1638
16th to endNight
6%425%5%2346%10%49%
1979
September1st to 15thNight
8%3134345%7%8%49%
1980
16th to endNight
8%32442345%7%51%
1980
October1st to 15thNight
8%212223336%62%
2004
16th to endNight
8%212212235%66%
2304
November1st to 15thNight
9%423313336%55%
2297
16th to endNight
11%5%1332223460%
2535
December1st to 15thNight
14%6%24322336%49%
2535
16th to endNight
15%6%34323235%49%
2704

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
16%5%1321334452%
1710
16th to endDay
13%5%1432334452%
1782
February1st to 15thDay
14%5%24222335%54%
1750
16th to endDay
15%6%23222335%52%
1580
March1st to 15thDay
19%5%24313335%45%
1969
16th to endDay
13%5%34323345%50%
2110
April1st to 15thDay
11%6%25%42435%7%46%
1971
16th to endDay
13%6%34424346%46%
2205
May1st to 15thDay
14%6%35%5%26%45%6%40%
2331
16th to endDay
13%6%25%5%35%5%7%7%36%
2430
June1st to 15thDay
5%214435%6%6%9%50%
2340
16th to endDay
9%424436%5%7%9%41%
2340
July1st to 15thDay
7%2245%46%5%5%9%44%
2340
16th to endDay
7%323436%5%6%10%46%
2466
August1st to 15thDay
211232445%10%60%
2319
16th to endDay
7%425%5%37%5%6%9%42%
2223
September1st to 15thDay
8%425%6%35%45%7%44%
1980
16th to endDay
8%3145%35%5%6%8%46%
1980
October1st to 15thDay
10%314323335%56%
1943
16th to endDay
7%212222245%66%
1920
November1st to 15thDay
12%5%25%5%2333450%
1888
16th to endDay
12%423313335%57%
1755
December1st to 15thDay
16%6%25%323435%47%
1755
16th to endDay
14%7%34413435%45%
1872
January1st to 15thNight
15%5%14313335%52%
2472
16th to endNight
14%423223236%53%
2451
February1st to 15thNight
13%5%2421223456%
2102
16th to endNight
12%423423336%53%
1894
March1st to 15thNight
14%5%24413346%48%
1989
16th to endNight
11%423423447%52%
2112
April1st to 15thNight
10%5%235%24346%51%
1977
16th to endNight
13%5%2431445%6%47%
1752
May1st to 15thNight
10%425%3235%6%8%48%
1619
16th to endNight
8%4143245%7%8%48%
1683
June1st to 15thNight
321232336%8%62%
1620
16th to endNight
5%213325%46%9%53%
1620
July1st to 15thNight
421332345%11%58%
1620
16th to endNight
32123235%5%11%58%
1702
August1st to 15thNight
212212449%68%
1638
16th to endNight
6%425%5%2346%10%49%
1979
September1st to 15thNight
8%4134345%7%8%49%
1980
16th to endNight
8%4245%2346%7%51%
1980
October1st to 15thNight
8%313223336%62%
2004
16th to endNight
7%212212335%66%
2304
November1st to 15thNight
9%424323336%55%
2297
16th to endNight
10%5%1332323460%
2535
December1st to 15thNight
14%6%24322346%49%
2535
16th to endNight
15%7%34322335%48%
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
35%37%19%22%12%
1711
16th to endDay
35%25%14%26%16%
1782
February1st to 15thDay
34%26%15%23%17%
1751
16th to endDay
36%25%18%22%14%
1580
March1st to 15thDay
42%27%13%22%12%
1969
16th to endDay
36%38%17%23%9%
2112
April1st to 15thDay
38%38%18%17%13%
1971
16th to endDay
40%36%18%19%10%
2205
May1st to 15thDay
44%5%9%14%17%8%
2331
16th to endDay
43%6%11%18%16%3
2434
June1st to 15thDay
28%5%11%26%22%5%
2340
16th to endDay
37%5%14%22%16%3
2340
July1st to 15thDay
34%7%15%26%13%1
2201
16th to endDay
29%6%18%29%14%1
2311
August1st to 15thDay
18%6%16%37%19%2
2305
16th to endDay
35%9%15%24%11%3
2223
September1st to 15thDay
38%5%11%22%15%5%
1980
16th to endDay
33%7%10%20%19%8%
1980
October1st to 15thDay
30%410%22%17%14%
1943
16th to endDay
21%25%16%32%21%
1920
November1st to 15thDay
38%39%20%18%10%
1888
16th to endDay
30%37%16%22%20%
1755
December1st to 15thDay
40%37%17%21%9%
1755
16th to endDay
41%49%15%19%9%
1872
January1st to 15thNight
35%25%18%21%16%
2472
16th to endNight
34%35%13%24%18%
2451
February1st to 15thNight
33%25%14%22%21%
2112
16th to endNight
34%26%17%21%17%
1894
March1st to 15thNight
37%48%13%23%13%
1989
16th to endNight
33%410%16%22%11%
2112
April1st to 15thNight
34%48%17%18%16%
1977
16th to endNight
38%310%17%18%11%
1752
May1st to 15thNight
33%5%11%21%17%9%
1620
16th to endNight
30%5%16%27%15%3
1685
June1st to 15thNight
18%5%15%28%26%5%
1620
16th to endNight
26%6%15%29%17%3
1620
July1st to 15thNight
19%7%22%33%15%1
1530
16th to endNight
18%7%22%36%13%
1594
August1st to 15thNight
13%5%20%38%19%2
1628
16th to endNight
30%6%20%26%11%3
1979
September1st to 15thNight
30%6%16%24%14%6%
1980
16th to endNight
31%5%12%23%18%9%
1980
October1st to 15thNight
23%411%25%19%15%
2004
16th to endNight
21%25%17%27%24%
2304
November1st to 15thNight
31%310%22%19%12%
2297
16th to endNight
29%26%15%24%20%
2535
December1st to 15thNight
37%38%18%20%11%
2535
16th to endNight
40%26%15%22%12%
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
20%7%30%36%4
123
16th to endDay
15%10%41%26%42
96
February1st to 15thDay
6%17%34%34%6%
90
16th to endDay
18%445%29%1
81
March1st to 15thDay
26%6%34%18%13%
90
16th to endDay
8%6%43%27%13%1
96
April1st to 15thDay
13%13%36%32%4
90
16th to endDay
14%11%36%26%11%
315
May1st to 15thDay
16%10%36%26%9%
468
16th to endDay
20%426%29%16%1
517
June1st to 15thDay
5%112%46%26%7%
495
16th to endDay
10%39%38%29%8%
495
July1st to 15thDay
6%8%39%35%9%
475
16th to endDay
4432%46%12%
515
August1st to 15thDay
3228%51%14%
472
16th to endDay
11%234%34%15%
263
September1st to 15thDay
221%51%23%
135
16th to endDay
9%2226%42%17%
135
October1st to 15thDay
35%29%42%17%
135
16th to endDay
14%5%18%29%27%4
144
November1st to 15thDay
29%121%32%11%2
135
16th to endDay
20%7%30%21%16%3
135
December1st to 15thDay
21%5%28%22%14%6%
135
16th to endDay
26%12%30%25%41
144
January1st to 15thNight
18%9%30%32%8%
956
16th to endNight
17%8%36%31%6%
915
February1st to 15thNight
17%12%32%32%4
704
16th to endNight
16%12%43%24%2
632
March1st to 15thNight
17%10%34%32%5%
549
16th to endNight
13%10%41%30%4
576
April1st to 15thNight
14%7%44%30%2
539
16th to endNight
16%7%41%24%9%
540
May1st to 15thNight
12%7%40%31%7%
540
16th to endNight
9%433%34%16%
563
June1st to 15thNight
3120%47%25%1
540
16th to endNight
6%117%47%24%3
540
July1st to 15thNight
410%49%30%3
510
16th to endNight
3449%36%4
564
August1st to 15thNight
2345%42%5%
539
16th to endNight
9%340%42%4
573
September1st to 15thNight
7%15%39%40%5%
540
16th to endNight
10%110%35%38%5%
537
October1st to 15thNight
9%213%43%24%6%
564
16th to endNight
8%5%25%34%24%2
768
November1st to 15thNight
12%326%36%18%2
777
16th to endNight
14%7%28%32%15%1
975
December1st to 15thNight
18%7%29%33%10%
975
16th to endNight
20%8%30%32%8%
1040
Page generated at 2018-01-12 21:01:28. Version ac04

Copyright 2018 A. Danko


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