Altona Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altona 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
25%
15149
14%
48353
7%
15118
34%
15118
16%
33394

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
14%
765
11%
2662
3%
761
23%
761
8%
2015
January16th to endNight
14%
768
13%
2688
4%
768
25%
768
11%
1881
February1st to 15thNight
13%
704
9%
2287
2%
701
21%
701
10%
1746
February16th to endNight
18%
631
14%
1893
4%
631
30%
631
12%
1368
March1st to 15thNight
15%
720
13%
2160
6%
720
24%
720
13%
1606
March16th to endNight
23%
768
20%
1995
6%
768
33%
768
20%
1391
April1st to 15thNight
17%
718
16%
1796
3%
718
25%
718
17%
1348
April16th to endNight
21%
630
16%
1710
5%
630
29%
630
19%
1184
May1st to 15thNight
27%
411
17%
1491
10%
411
40%
411
16%
987
May16th to endNight
30%
374
13%
1497
8%
374
41%
374
17%
935
June1st to 15thNight
29%
360
10%
1440
8%
360
40%
360
14%
958
June16th to endNight
37%
360
11%
1440
10%
360
47%
360
22%
847
July1st to 15thNight
46%
340
13%
1360
14%
340
57%
340
26%
770
July16th to endNight
48%
376
13%
1418
13%
354
55%
354
24%
825
August1st to 15thNight
48%
383
16%
1454
14%
381
54%
381
26%
821
August16th to endNight
48%
764
22%
1910
10%
764
53%
764
38%
1178
September1st to 15thNight
43%
720
16%
1800
7%
720
47%
720
34%
1181
September16th to endNight
43%
717
24%
1973
16%
717
48%
717
33%
1205
October1st to 15thNight
29%
720
18%
2115
9%
720
40%
720
21%
1416
October16th to endNight
20%
768
12%
2456
5%
768
29%
768
13%
1771
November1st to 15thNight
22%
760
15%
2472
9%
760
32%
760
15%
1720
November16th to endNight
19%
780
14%
2694
7%
780
29%
780
11%
1897
December1st to 15thNight
13%
780
8%
2730
2%
780
21%
780
8%
2109
December16th to endNight
11%
832
10%
2912
2%
832
21%
832
7%
2235

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
15%9%37%6%35%5%5%7%30%
48985
Night
15%8%46%6%35%45%6%32%
48522
Cloud CoverDay
15%8%37%6%35%5%5%6%31%
48995
Night
15%8%36%6%35%45%6%33%
48528
TransparencyDay
51%411%20%9%1
48672
Night
50%412%21%9%1
48353
SeeingDay
21%15%31%19%9%2
5938
Night
21%15%31%22%8%
15150

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
13%10%5%11%9%45%6%5%7%20%
1520
16th to endDay
8%11%5%11%7%45%6%5%5%26%
1536
February1st to 15thDay
10%14%5%12%9%5%6%5%5%5%20%
1572
16th to endDay
11%13%5%10%9%45%435%26%
1580
March1st to 15thDay
14%13%5%8%7%46%5%5%425%
1790
16th to endDay
15%11%5%5%5%33446%34%
2225
April1st to 15thDay
17%11%47%6%34345%30%
2152
16th to endDay
18%8%36%5%33446%35%
2248
May1st to 15thDay
18%8%36%6%35%446%32%
2469
16th to endDay
17%7%36%6%35%5%5%8%31%
2618
June1st to 15thDay
23%7%25%5%345%5%7%27%
2520
16th to endDay
18%6%26%6%45%45%8%30%
2520
July1st to 15thDay
12%5%36%6%36%6%6%8%34%
2520
16th to endDay
12%5%26%5%36%6%7%10%32%
2654
August1st to 15thDay
13%6%36%7%46%6%6%8%30%
2482
16th to endDay
10%5%25%5%45%5%6%9%39%
2292
September1st to 15thDay
15%5%25%5%34437%41%
2160
16th to endDay
10%4245%3445%8%44%
1986
October1st to 15thDay
17%8%35%5%36%5%5%6%32%
1842
16th to endDay
19%12%38%7%3444427%
1767
November1st to 15thDay
14%11%47%5%35%5%5%5%31%
1716
16th to endDay
16%10%5%9%5%46%446%27%
1592
December1st to 15thDay
14%16%8%10%6%5%435%6%18%
1560
16th to endDay
19%15%7%8%5%35%5%5%6%17%
1664
January1st to 15thNight
12%10%5%10%7%48%6%5%6%21%
2662
16th to endNight
9%12%49%8%36%5%5%7%27%
2688
February1st to 15thNight
10%10%5%10%9%48%5%5%6%21%
2285
16th to endNight
12%10%48%8%5%6%45%6%27%
1893
March1st to 15thNight
16%10%5%7%7%45%445%28%
2147
16th to endNight
14%8%36%5%3445%437%
1995
April1st to 15thNight
18%9%47%6%35%35%5%29%
1796
16th to endNight
18%7%35%5%34447%36%
1710
May1st to 15thNight
19%7%26%5%24447%34%
1491
16th to endNight
15%6%35%5%26%45%8%35%
1495
June1st to 15thNight
22%6%35%4235%6%7%31%
1440
16th to endNight
14%425%326%5%7%8%39%
1440
July1st to 15thNight
8%4245%35%47%9%42%
1440
16th to endNight
12%4235%25%5%6%9%42%
1514
August1st to 15thNight
11%4245%2447%9%43%
1464
16th to endNight
10%5%243235%5%7%49%
1910
September1st to 15thNight
13%5%235%4445%6%45%
1800
16th to endNight
15%42332435%7%47%
1973
October1st to 15thNight
16%7%36%5%3345%7%37%
2115
16th to endNight
21%8%47%5%35%446%28%
2456
November1st to 15thNight
17%7%46%6%36%46%5%31%
2472
16th to endNight
16%10%37%7%35%45%6%28%
2694
December1st to 15thNight
18%13%6%8%8%45%5%5%5%19%
2730
16th to endNight
16%13%5%9%7%45%45%6%20%
2912

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
13%11%5%11%8%46%5%5%6%21%
1520
16th to endDay
9%12%5%10%7%35%7%5%5%26%
1536
February1st to 15thDay
11%13%5%12%10%47%5%4420%
1572
16th to endDay
12%12%5%10%8%45%5%45%26%
1580
March1st to 15thDay
14%13%5%8%6%36%46%425%
1800
16th to endDay
15%11%45%5%33445%36%
2225
April1st to 15thDay
17%11%47%5%44345%31%
2152
16th to endDay
18%7%36%5%34436%36%
2248
May1st to 15thDay
18%8%36%5%35%347%33%
2469
16th to endDay
18%6%26%6%35%5%5%7%32%
2618
June1st to 15thDay
22%6%25%6%35%45%8%28%
2520
16th to endDay
18%5%36%5%45%5%5%10%29%
2520
July1st to 15thDay
12%435%7%36%6%6%8%34%
2520
16th to endDay
12%5%25%6%36%7%6%9%32%
2654
August1st to 15thDay
13%6%27%6%45%6%7%7%31%
2482
16th to endDay
10%425%5%35%5%6%9%40%
2292
September1st to 15thDay
14%6%25%5%24446%42%
2160
16th to endDay
10%5%245%35%35%8%45%
1986
October1st to 15thDay
18%8%25%5%36%5%5%6%33%
1842
16th to endDay
19%11%36%6%44445%27%
1767
November1st to 15thDay
14%10%48%5%35%5%5%6%31%
1716
16th to endDay
17%10%46%6%35%45%5%29%
1592
December1st to 15thDay
16%16%6%9%7%5%4345%19%
1560
16th to endDay
20%14%6%9%6%3445%5%18%
1664
January1st to 15thNight
12%11%5%10%8%47%6%6%422%
2662
16th to endNight
11%11%49%7%35%5%47%28%
2688
February1st to 15thNight
11%11%5%10%9%47%6%5%6%21%
2278
16th to endNight
12%10%48%8%45%446%29%
1893
March1st to 15thNight
17%10%5%7%6%46%335%29%
2160
16th to endNight
14%8%25%5%25%346%38%
1995
April1st to 15thNight
18%9%36%7%25%345%30%
1796
16th to endNight
18%7%26%5%24447%36%
1710
May1st to 15thNight
18%7%36%5%3335%7%34%
1491
16th to endNight
16%425%5%35%5%5%8%36%
1495
June1st to 15thNight
21%6%25%5%335%5%8%32%
1440
16th to endNight
13%5%243346%7%9%38%
1440
July1st to 15thNight
8%4345%35%5%7%9%41%
1440
16th to endNight
11%4235%25%5%6%8%42%
1514
August1st to 15thNight
11%42442446%10%43%
1464
16th to endNight
9%4145%2345%7%50%
1910
September1st to 15thNight
14%4145%3445%6%46%
1800
16th to endNight
15%32341435%7%48%
1973
October1st to 15thNight
16%7%25%5%3435%6%38%
2115
16th to endNight
21%8%36%6%25%45%6%28%
2456
November1st to 15thNight
17%7%36%5%35%5%5%6%32%
2472
16th to endNight
17%10%36%7%3445%5%30%
2694
December1st to 15thNight
19%14%5%8%6%35%45%6%20%
2730
16th to endNight
17%13%5%9%7%4445%5%22%
2912

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
61%411%16%5%
1521
16th to endDay
56%413%16%8%
1536
February1st to 15thDay
64%5%7%17%4
1574
16th to endDay
60%212%13%10%1
1580
March1st to 15thDay
60%312%14%8%1
1800
16th to endDay
49%39%18%15%4
2225
April1st to 15thDay
55%27%20%11%2
2152
16th to endDay
50%39%20%13%3
2248
May1st to 15thDay
52%38%19%14%1
2469
16th to endDay
48%5%11%20%11%2
2622
June1st to 15thDay
53%411%19%9%
2520
16th to endDay
49%411%23%9%
2520
July1st to 15thDay
43%5%12%27%10%
2371
16th to endDay
44%5%15%24%9%
2487
August1st to 15thDay
47%5%15%22%7%
2468
16th to endDay
39%414%28%12%
2292
September1st to 15thDay
41%310%28%13%
2160
16th to endDay
37%413%31%12%1
1986
October1st to 15thDay
50%411%22%10%
1842
16th to endDay
58%314%17%6%
1767
November1st to 15thDay
52%314%19%7%2
1716
16th to endDay
56%312%18%7%1
1592
December1st to 15thDay
67%311%14%2
1560
16th to endDay
66%410%11%7%
1664
January1st to 15thNight
61%411%16%5%
2662
16th to endNight
54%312%20%9%
2688
February1st to 15thNight
61%413%14%5%
2287
16th to endNight
55%312%17%10%
1893
March1st to 15thNight
58%210%18%8%1
2160
16th to endNight
47%210%20%15%3
1995
April1st to 15thNight
56%27%19%13%
1796
16th to endNight
48%39%21%13%3
1710
May1st to 15thNight
48%48%18%16%3
1491
16th to endNight
44%5%13%22%11%1
1497
June1st to 15thNight
49%5%12%21%9%
1440
16th to endNight
38%6%17%25%12%
1440
July1st to 15thNight
35%5%15%30%12%
1360
16th to endNight
36%6%16%25%12%1
1418
August1st to 15thNight
36%420%27%10%
1454
16th to endNight
33%315%33%13%
1910
September1st to 15thNight
38%414%30%11%1
1800
16th to endNight
35%312%33%13%1
1973
October1st to 15thNight
46%312%24%12%
2115
16th to endNight
55%314%19%5%
2456
November1st to 15thNight
51%314%17%9%2
2472
16th to endNight
55%313%20%6%
2694
December1st to 15thNight
64%312%15%3
2730
16th to endNight
64%311%14%6%
2912

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%25%32%10%9%
82
16th to endDay
29%28%29%9%3
64
February1st to 15thDay
30%28%34%7%1
100
16th to endDay
22%27%29%15%4
239
March1st to 15thDay
24%24%29%17%3
270
16th to endDay
26%21%28%14%7%1
288
April1st to 15thDay
29%20%30%15%3
270
16th to endDay
24%17%29%18%8%1
270
May1st to 15thDay
31%14%29%13%8%2
288
16th to endDay
18%17%35%20%5%1
331
June1st to 15thDay
27%12%34%16%7%
315
16th to endDay
14%15%36%24%7%1
315
July1st to 15thDay
15%9%34%28%10%2
305
16th to endDay
14%437%30%10%3
326
August1st to 15thDay
14%7%30%28%14%4
313
16th to endDay
12%11%23%29%15%6%
332
September1st to 15thDay
15%9%28%22%19%5%
315
16th to endDay
10%10%28%25%17%7%
314
October1st to 15thDay
22%16%28%13%15%2
315
16th to endDay
27%18%31%16%6%
298
November1st to 15thDay
24%15%31%17%10%1
280
16th to endDay
31%23%27%14%1
122
December1st to 15thDay
21%24%33%15%41
90
16th to endDay
34%28%29%6%2
96
January1st to 15thNight
20%21%37%16%3
765
16th to endNight
20%21%34%18%3
768
February1st to 15thNight
17%19%39%20%2
704
16th to endNight
20%22%32%20%3
631
March1st to 15thNight
26%18%32%19%2
720
16th to endNight
23%19%30%20%5%1
768
April1st to 15thNight
25%18%33%20%2
718
16th to endNight
25%18%30%18%6%
630
May1st to 15thNight
24%13%31%22%8%
411
16th to endNight
16%11%34%26%9%
375
June1st to 15thNight
25%11%26%26%10%
360
16th to endNight
16%7%35%29%10%1
360
July1st to 15thNight
7%7%32%32%18%
340
16th to endNight
11%5%29%36%15%1
376
August1st to 15thNight
13%426%36%16%2
383
16th to endNight
13%6%26%33%18%1
764
September1st to 15thNight
16%6%28%32%14%1
720
16th to endNight
17%8%26%23%20%3
717
October1st to 15thNight
21%14%27%23%12%
720
16th to endNight
26%16%30%19%5%
768
November1st to 15thNight
22%16%31%19%8%1
760
16th to endNight
25%21%27%15%9%
780
December1st to 15thNight
27%21%31%16%3
780
16th to endNight
26%23%30%15%3
832
Page generated at 2018-01-12 22:01:32. Version ac04

Copyright 2018 A. Danko


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