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 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
46%
16251
27%
48343
17%
16205
55%
16205
37%
27202

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%
955
29%
2472
11%
951
44%
951
37%
1447
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%
540
24%
1695
14%
540
49%
540
32%
996
May1st to 15thNight
36%
539
23%
1620
15%
539
49%
539
29%
997
May16th to endNight
46%
563
24%
1684
17%
561
58%
561
35%
983
June1st to 15thNight
63%
540
22%
1620
18%
540
68%
540
43%
853
June16th to endNight
68%
540
26%
1620
21%
540
73%
540
49%
811
July1st to 15thNight
71%
510
19%
1530
20%
510
74%
510
39%
863
July16th to endNight
83%
564
31%
1594
37%
531
84%
531
53%
776
August1st to 15thNight
83%
539
28%
1610
33%
536
84%
536
47%
823
August16th to endNight
78%
573
28%
1800
32%
573
79%
573
41%
943
September1st to 15thNight
65%
540
25%
1964
28%
540
68%
540
35%
1030
September16th to endNight
64%
537
34%
1980
32%
537
71%
537
37%
980
October1st to 15thNight
48%
658
30%
2098
21%
658
56%
658
37%
1153
October16th to endNight
48%
768
37%
2304
20%
768
60%
768
47%
1152
November1st to 15thNight
43%
815
30%
2335
15%
815
54%
815
41%
1266
November16th to endNight
37%
975
32%
2535
15%
975
47%
975
39%
1512
December1st to 15thNight
35%
975
27%
2535
11%
975
45%
975
36%
1554
December16th to endNight
32%
1040
27%
2704
11%
1040
43%
1040
31%
1736

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%49%
48988
Night
9%423323346%55%
48539
TransparencyDay
34%5%11%24%19%4
48688
Night
29%410%25%22%6%
48344
SeeingDay
11%6%23%30%22%5%
6105
Night
11%7%30%33%15%1
16253

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%7%23323335%54%
1710
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%
2262
May1st to 15thDay
17%6%245%23445%41%
2331
16th to endDay
12%5%26%5%35%5%6%8%39%
2430
June1st to 15thDay
7%325%5%25%5%5%8%47%
2340
16th to endDay
7%323425%46%8%51%
2340
July1st to 15thDay
8%324436%6%7%10%42%
2340
16th to endDay
5%223435%46%9%51%
2466
August1st to 15thDay
5%3135%25%5%6%9%50%
2340
16th to endDay
6%314435%5%6%9%48%
2402
September1st to 15thDay
13%4245%2445%6%46%
1993
16th to endDay
10%42332345%8%52%
1980
October1st to 15thDay
13%423423235%55%
1848
16th to endDay
8%322222325%64%
1920
November1st to 15thDay
10%423212347%57%
1849
16th to endDay
12%413312245%57%
1755
December1st to 15thDay
15%5%24422235%50%
1755
16th to endDay
14%7%35%323345%47%
1872
January1st to 15thNight
11%423322335%57%
2472
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%50%
1695
May1st to 15thNight
12%42442345%6%48%
1619
16th to endNight
9%42342446%10%47%
1683
June1st to 15thNight
431332446%7%59%
1620
16th to endNight
42122235%5%9%61%
1620
July1st to 15thNight
5%314435%5%5%10%51%
1620
16th to endNight
41132133410%62%
1702
August1st to 15thNight
31123245%7%10%57%
1620
16th to endNight
3223425%45%9%55%
1800
September1st to 15thNight
11%323323347%53%
1964
16th to endNight
8%313422346%58%
1980
October1st to 15thNight
11%414423345%55%
2098
16th to endNight
7%21221233466%
2304
November1st to 15thNight
8%312312335%61%
2335
16th to endNight
11%5%2321323457%
2535
December1st to 15thNight
13%423323336%52%
2535
16th to endNight
15%7%35%312235%49%
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
33%38%27%23%2
1711
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
2262
May1st to 15thDay
45%49%18%19%2
2331
16th to endDay
39%8%13%23%14%
2434
June1st to 15thDay
32%6%15%25%18%2
2340
16th to endDay
28%6%13%25%21%4
2340
July1st to 15thDay
33%7%14%27%15%
2201
16th to endDay
25%7%16%29%20%1
2311
August1st to 15thDay
27%7%15%24%19%5%
2326
16th to endDay
31%6%14%26%16%5%
2402
September1st to 15thDay
36%5%10%21%16%8%
1993
16th to endDay
29%5%8%26%23%6%
1980
October1st to 15thDay
32%310%22%22%7%
1848
16th to endDay
25%38%26%27%9%
1920
November1st to 15thDay
27%210%28%22%8%
1849
16th to endDay
29%29%25%26%6%
1755
December1st to 15thDay
38%29%26%20%2
1755
16th to endDay
40%310%22%18%5%
1872
January1st to 15thNight
30%38%27%25%4
2472
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%
1695
May1st to 15thNight
36%5%10%18%22%6%
1620
16th to endNight
31%6%16%28%15%2
1685
June1st to 15thNight
22%6%14%34%19%3
1620
16th to endNight
19%5%13%30%25%6%
1620
July1st to 15thNight
26%7%12%31%19%1
1530
16th to endNight
17%5%11%35%25%4
1594
August1st to 15thNight
19%7%16%27%21%7%
1610
16th to endNight
24%5%12%28%20%7%
1800
September1st to 15thNight
30%311%22%19%12%
1964
16th to endNight
27%38%26%27%7%
1980
October1st to 15thNight
30%311%21%22%11%
2098
16th to endNight
21%26%24%32%11%
2304
November1st to 15thNight
26%39%24%26%10%
2335
16th to endNight
30%37%20%30%7%
2535
December1st to 15thNight
33%311%23%22%5%
2535
16th to endNight
38%39%21%20%6%
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
21%12%30%28%8%
123
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%37%18%9%
372
May1st to 15thDay
16%13%33%26%8%2
468
16th to endDay
18%7%30%27%12%3
517
June1st to 15thDay
8%423%34%23%5%
495
16th to endDay
7%217%38%27%6%
495
July1st to 15thDay
8%113%39%29%8%
475
16th to endDay
45%28%46%13%
515
August1st to 15thDay
5%15%36%43%7%
493
16th to endDay
7%210%34%36%8%
442
September1st to 15thDay
327%31%37%18%
148
16th to endDay
10%14%40%21%13%
135
October1st to 15thDay
11%8%24%30%20%5%
135
16th to endDay
411%31%34%13%4
144
November1st to 15thDay
19%229%22%22%4
135
16th to endDay
21%13%32%24%7%
135
December1st to 15thDay
17%8%45%18%8%1
135
16th to endDay
18%14%45%16%4
144
January1st to 15thNight
13%13%37%29%5%
956
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%9%40%26%8%
540
May1st to 15thNight
12%8%39%29%11%
540
16th to endNight
10%433%33%17%1
563
June1st to 15thNight
6%321%43%24%1
540
16th to endNight
4221%47%22%2
540
July1st to 15thNight
6%14%47%29%2
510
16th to endNight
5%5%37%48%3
564
August1st to 15thNight
37%43%39%4
539
16th to endNight
419%42%38%4
573
September1st to 15thNight
13%114%35%33%2
540
16th to endNight
9%218%37%27%5%
537
October1st to 15thNight
12%330%30%20%2
658
16th to endNight
8%7%32%35%14%
768
November1st to 15thNight
10%8%35%31%14%
815
16th to endNight
14%10%35%29%9%
975
December1st to 15thNight
16%10%34%30%7%
975
16th to endNight
19%9%34%28%7%
1040
Page generated at 2018-01-12 21:01:32. Version ac04

Copyright 2018 A. Danko


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