ACT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ACT Observatory clear sky chart from 2005-11-05 to 2017-04-30.

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
45%
15326
27%
45615
17%
15280
55%
15280
36%
25731

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
32%
899
30%
2327
11%
895
44%
895
37%
1363
January16th to endNight
35%
960
32%
2496
11%
960
47%
960
39%
1448
February1st to 15thNight
32%
852
23%
2260
9%
848
41%
848
34%
1386
February16th to endNight
29%
613
28%
1875
11%
613
44%
613
31%
1085
March1st to 15thNight
33%
540
23%
1980
13%
540
45%
540
24%
1139
March16th to endNight
31%
576
22%
2112
15%
576
42%
576
26%
1215
April1st to 15thNight
31%
539
23%
1920
13%
539
47%
539
29%
1054
April16th to endNight
33%
537
24%
1686
14%
537
49%
537
32%
987
May1st to 15thNight
35%
494
24%
1485
15%
494
49%
494
28%
927
May16th to endNight
46%
515
22%
1540
16%
513
57%
513
34%
907
June1st to 15thNight
62%
495
24%
1485
20%
495
68%
495
44%
779
June16th to endNight
67%
495
23%
1485
19%
495
73%
495
48%
752
July1st to 15thNight
71%
465
21%
1395
21%
465
74%
465
39%
788
July16th to endNight
82%
522
29%
1468
35%
489
84%
489
53%
715
August1st to 15thNight
83%
494
30%
1475
35%
491
85%
491
49%
738
August16th to endNight
77%
525
26%
1649
29%
525
78%
525
40%
873
September1st to 15thNight
63%
495
25%
1800
28%
495
67%
495
33%
972
September16th to endNight
65%
492
33%
1815
31%
492
72%
492
38%
884
October1st to 15thNight
47%
603
31%
1923
22%
603
55%
603
36%
1054
October16th to endNight
48%
704
35%
2112
18%
704
59%
704
46%
1069
November1st to 15thNight
43%
751
32%
2151
16%
751
55%
751
43%
1147
November16th to endNight
36%
900
30%
2340
14%
900
46%
900
38%
1411
December1st to 15thNight
34%
900
26%
2340
11%
900
44%
900
34%
1454
December16th to endNight
33%
960
27%
2496
11%
960
44%
960
32%
1584

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%5%24424447%48%
46107
Night
10%423323346%55%
45811
TransparencyDay
34%5%11%24%19%4
45807
Night
29%410%25%22%6%
45616
SeeingDay
11%6%24%30%22%5%
5103
Night
12%7%30%33%15%1
15328

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
9%6%24423335%54%
1611
16th to endDay
12%423313327%54%
1728
February1st to 15thDay
11%7%45%323336%47%
1612
16th to endDay
13%6%34414345%49%
1598
March1st to 15thDay
15%7%25%323345%46%
1980
16th to endDay
16%8%34414246%42%
2110
April1st to 15thDay
12%5%2443345%6%46%
2027
16th to endDay
16%6%25%424345%43%
2250
May1st to 15thDay
18%6%345%23445%39%
2136
16th to endDay
13%5%26%5%35%5%6%8%37%
2222
June1st to 15thDay
7%325%5%25%5%5%8%47%
2145
16th to endDay
7%223435%5%6%8%50%
2145
July1st to 15thDay
8%324435%6%7%10%42%
2145
16th to endDay
5%223435%45%9%50%
2284
August1st to 15thDay
43135%25%5%6%9%52%
2145
16th to endDay
7%3144445%6%9%47%
2201
September1st to 15thDay
14%4245%2445%7%43%
1827
16th to endDay
9%31332345%7%54%
1815
October1st to 15thDay
13%42342323454%
1694
16th to endDay
9%322212325%63%
1760
November1st to 15thDay
9%313212347%59%
1704
16th to endDay
13%413312245%56%
1620
December1st to 15thDay
16%5%34422235%49%
1620
16th to endDay
14%7%3432333448%
1728
January1st to 15thNight
12%323312335%57%
2327
16th to endNight
9%423313246%57%
2496
February1st to 15thNight
12%5%24323335%52%
2250
16th to endNight
10%6%24323335%54%
1875
March1st to 15thNight
12%6%24333445%49%
1980
16th to endNight
16%423312346%50%
2112
April1st to 15thNight
12%323314446%53%
1920
16th to endNight
12%424423246%51%
1686
May1st to 15thNight
13%43442445%6%47%
1484
16th to endNight
10%42442445%10%47%
1539
June1st to 15thNight
421232436%7%59%
1485
16th to endNight
42122235%5%9%60%
1485
July1st to 15thNight
4313435%5%5%10%50%
1485
16th to endNight
41132133410%62%
1576
August1st to 15thNight
31123245%7%10%59%
1485
16th to endNight
3223426%46%9%53%
1649
September1st to 15thNight
12%42342335%7%50%
1800
16th to endNight
8%313422346%59%
1815
October1st to 15thNight
11%414423345%55%
1923
16th to endNight
8%31221233465%
2112
November1st to 15thNight
7%312312335%62%
2151
16th to endNight
11%6%23223235%56%
2340
December1st to 15thNight
14%5%33323346%51%
2340
16th to endNight
14%6%34312335%50%
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
33%37%26%25%2
1612
16th to endDay
32%28%29%22%4
1728
February1st to 15thDay
39%310%25%16%4
1613
16th to endDay
37%47%27%18%5%
1598
March1st to 15thDay
39%48%23%18%6%
1980
16th to endDay
43%411%22%14%2
2112
April1st to 15thDay
37%6%12%23%15%5%
2027
16th to endDay
42%49%21%18%3
2250
May1st to 15thDay
46%49%18%17%2
2136
16th to endDay
41%8%13%22%12%
2226
June1st to 15thDay
33%6%15%24%18%2
2145
16th to endDay
28%6%13%25%20%4
2145
July1st to 15thDay
33%7%15%28%15%
2006
16th to endDay
26%7%15%29%19%1
2129
August1st to 15thDay
25%6%15%24%21%6%
2131
16th to endDay
31%6%14%26%14%5%
2201
September1st to 15thDay
38%5%11%19%15%9%
1827
16th to endDay
27%5%8%26%24%6%
1815
October1st to 15thDay
33%310%22%22%7%
1694
16th to endDay
25%38%27%26%8%
1760
November1st to 15thDay
25%210%29%23%8%
1704
16th to endDay
30%29%24%25%6%
1620
December1st to 15thDay
39%29%26%19%1
1620
16th to endDay
40%39%22%19%5%
1728
January1st to 15thNight
30%37%27%26%4
2327
16th to endNight
28%48%27%24%7%
2496
February1st to 15thNight
35%39%27%19%5%
2260
16th to endNight
33%39%26%21%5%
1875
March1st to 15thNight
36%5%10%18%21%7%
1980
16th to endNight
34%5%10%26%18%2
2112
April1st to 15thNight
31%5%10%21%21%8%
1920
16th to endNight
35%39%24%20%6%
1686
May1st to 15thNight
37%5%11%18%20%6%
1485
16th to endNight
32%5%17%27%14%2
1541
June1st to 15thNight
22%5%14%34%19%3
1485
16th to endNight
19%6%13%29%24%6%
1485
July1st to 15thNight
25%8%12%32%18%1
1395
16th to endNight
17%5%11%35%25%4
1468
August1st to 15thNight
17%7%16%27%22%8%
1475
16th to endNight
25%5%12%27%20%7%
1649
September1st to 15thNight
32%312%21%17%12%
1800
16th to endNight
26%38%25%28%7%
1815
October1st to 15thNight
31%310%20%23%10%
1923
16th to endNight
23%26%25%31%11%
2112
November1st to 15thNight
25%38%24%27%11%
2151
16th to endNight
31%37%20%29%7%
2340
December1st to 15thNight
34%311%23%21%4
2340
16th to endNight
37%39%21%21%6%
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
22%14%31%25%6%
90
16th to endDay
9%13%50%20%6%
96
February1st to 15thDay
11%16%50%20%2
90
16th to endDay
12%14%49%19%3
81
March1st to 15thDay
23%17%42%12%4
90
16th to endDay
9%17%39%23%9%
96
April1st to 15thDay
10%15%34%32%7%
146
16th to endDay
20%13%38%18%9%
368
May1st to 15thDay
17%13%35%24%6%2
393
16th to endDay
19%6%27%28%14%3
437
June1st to 15thDay
7%5%24%34%23%5%
420
16th to endDay
7%216%38%28%6%
420
July1st to 15thDay
9%115%42%24%7%
400
16th to endDay
46%30%46%10%
445
August1st to 15thDay
315%37%43%8%
418
16th to endDay
8%29%33%35%10%
369
September1st to 15thDay
417%36%39%9%
102
16th to endDay
5%20%31%23%20%
90
October1st to 15thDay
11%5%24%27%22%8%
90
16th to endDay
7%6%31%37%12%5%
96
November1st to 15thDay
14%126%23%30%4
90
16th to endDay
26%16%30%18%6%1
90
December1st to 15thDay
20%8%44%15%8%2
90
16th to endDay
18%11%44%20%4
96
January1st to 15thNight
14%13%37%29%5%
900
16th to endNight
11%11%39%31%5%
960
February1st to 15thNight
15%11%36%32%4
852
16th to endNight
13%13%41%26%5%
613
March1st to 15thNight
15%11%35%32%4
540
16th to endNight
17%9%39%26%7%
576
April1st to 15thNight
13%11%41%28%4
539
16th to endNight
14%8%40%26%8%
537
May1st to 15thNight
12%8%39%29%9%
495
16th to endNight
10%433%32%17%1
515
June1st to 15thNight
6%321%43%23%1
495
16th to endNight
4221%46%22%2
495
July1st to 15thNight
5%115%48%27%2
465
16th to endNight
5%5%38%47%3
522
August1st to 15thNight
37%44%39%4
494
16th to endNight
49%40%38%5%
525
September1st to 15thNight
14%114%35%31%2
495
16th to endNight
9%117%36%28%5%
492
October1st to 15thNight
13%231%29%20%2
603
16th to endNight
9%7%32%34%14%
704
November1st to 15thNight
9%8%34%31%15%
751
16th to endNight
14%11%36%28%9%
900
December1st to 15thNight
18%11%33%29%7%
900
16th to endNight
18%9%35%28%7%
960
Page generated at 2017-04-30 19:40:01. Version ac04

Copyright 2017 A. Danko


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