Apsley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Apsley clear sky chart from 2005-11-06 to 2024-02-05.

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
28%
22739
13%
80443
8%
22653
36%
22653
15%
57550

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 10thNight
13%
750
8%
4409
3%
737
22%
737
4%
3481
January11th to 20thNight
16%
758
6%
4419
4%
740
23%
740
4%
3532
January21st to endNight
19%
829
8%
4525
7%
814
26%
814
6%
3592
February1st to 10thNight
13%
739
9%
2399
3%
739
21%
739
10%
1866
February11th to 20thNight
14%
702
14%
2275
4%
700
27%
700
12%
1668
February21st to endNight
19%
592
14%
1861
4%
592
30%
592
13%
1377
March1st to 10thNight
18%
720
17%
2030
7%
720
26%
720
16%
1463
March11th to 20thNight
21%
720
16%
1980
6%
720
29%
720
17%
1431
March21st to endNight
27%
792
20%
2152
8%
792
37%
792
22%
1492
April1st to 10thNight
22%
720
16%
1800
5%
720
28%
720
19%
1343
April11th to 20thNight
26%
711
18%
1775
7%
711
33%
711
22%
1269
April21st to endNight
29%
544
17%
1624
7%
544
36%
544
18%
1161
May1st to 10thNight
33%
442
17%
1522
11%
442
41%
442
19%
984
May11th to 20thNight
33%
354
22%
1416
15%
354
52%
354
21%
838
May21st to endNight
42%
388
16%
1551
13%
388
55%
388
26%
859
June1st to 10thNight
34%
360
11%
1440
8%
360
47%
360
19%
884
June11th to 20thNight
43%
360
14%
1440
15%
360
52%
360
22%
826
June21st to endNight
43%
360
11%
1440
13%
360
49%
360
20%
887
July1st to 10thNight
50%
350
15%
1400
19%
350
57%
350
28%
751
July11th to 20thNight
49%
348
17%
1392
21%
348
56%
348
26%
771
July21st to endNight
51%
416
17%
1507
14%
393
59%
393
30%
826
August1st to 10thNight
56%
537
16%
1600
11%
534
61%
534
33%
978
August11th to 20thNight
56%
534
19%
1598
11%
534
61%
534
32%
979
August21st to endNight
46%
713
19%
1901
8%
713
51%
713
33%
1230
September1st to 10thNight
43%
720
17%
1800
7%
720
47%
720
32%
1188
September11th to 20thNight
50%
720
25%
1812
13%
720
55%
720
36%
1139
September21st to endNight
42%
720
21%
2072
16%
720
47%
720
25%
1368
October1st to 10thNight
35%
720
16%
2160
12%
720
42%
720
20%
1495
October11th to 20thNight
22%
720
12%
2160
6%
720
31%
720
15%
1616
October21st to endNight
20%
792
13%
2399
6%
792
29%
792
12%
1855
November1st to 10thNight
30%
740
16%
2502
13%
740
37%
740
17%
1693
November11th to 20thNight
20%
760
14%
2660
7%
760
30%
760
13%
1910
November21st to endNight
19%
760
10%
2660
5%
760
25%
760
9%
2050
December1st to 10thNight
13%
760
7%
2660
3%
760
18%
760
7%
2160
December11th to 20thNight
14%
759
9%
3252
3%
759
22%
759
7%
2489
December21st to endNight
10%
829
4%
4850
1%
817
16%
817
3%
4099

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
17%11%47%6%35%5%5%6%31%
78132
Night
17%11%48%6%35%45%6%31%
80720
TransparencyDay
53%6%11%19%10%1
77839
Night
53%6%11%19%10%1
80547
SeeingDay
23%15%31%19%8%3
13350
Night
22%13%31%24%10%1
22768

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 10thDay
15%20%6%13%6%3433424%
2517
11th to 20thDay
13%17%10%14%9%5%6%43316%
2532
21st to endDay
19%20%7%13%6%3333319%
3124
February1st to 10thDay
14%16%7%13%9%35%5%4420%
1656
11th to 20thDay
14%14%7%12%7%3444427%
1573
21st to endDay
17%16%6%9%7%45%44424%
1394
March1st to 10thDay
15%12%6%9%8%3445%6%29%
1923
11th to 20thDay
13%16%5%9%7%35%446%29%
1980
21st to endDay
19%11%46%433445%37%
2204
April1st to 10thDay
21%13%46%6%3445%5%30%
2160
11th to 20thDay
22%9%35%444345%37%
2145
21st to endDay
23%10%36%6%34446%32%
2340
May1st to 10thDay
19%7%46%6%35%46%6%34%
2438
11th to 20thDay
16%8%36%5%3445%7%38%
2478
21st to endDay
15%5%26%5%45%5%6%7%40%
2725
June1st to 10thDay
20%6%46%7%345%5%7%32%
2520
11th to 20thDay
16%6%25%5%46%5%6%9%36%
2520
21st to endDay
17%7%25%6%46%5%6%8%35%
2520
July1st to 10thDay
12%5%35%5%45%5%6%9%42%
2520
11th to 20thDay
12%5%35%6%46%6%6%9%38%
2520
21st to endDay
12%5%26%6%46%7%7%10%34%
2718
August1st to 10thDay
12%6%35%6%36%8%6%9%36%
2327
11th to 20thDay
13%6%26%7%45%5%6%7%37%
2303
21st to endDay
15%6%25%6%35%5%7%8%38%
2435
September1st to 10thDay
14%7%36%6%35%5%6%9%39%
2160
11th to 20thDay
13%5%26%5%345%5%7%45%
2145
21st to endDay
17%6%26%6%5%45%6%6%38%
1887
October1st to 10thDay
22%8%47%6%245%5%7%33%
1800
11th to 20thDay
26%10%47%6%35%46%6%25%
1800
21st to endDay
27%13%47%6%3435%5%23%
1955
November1st to 10thDay
17%11%36%6%35%45%8%32%
1575
11th to 20thDay
17%13%6%8%6%345%6%6%28%
1520
21st to endDay
22%17%6%8%5%5%5%5%5%419%
1520
December1st to 10thDay
19%21%8%10%7%33445%16%
1520
11th to 20thDay
18%18%6%9%5%2445%426%
1906
21st to endDay
26%20%5%11%5%2323418%
2772
January1st to 10thNight
18%19%8%11%6%44435%18%
4429
11th to 20thNight
14%14%6%13%12%5%6%5%5%5%16%
4426
21st to endNight
19%15%8%10%7%5%4445%18%
4532
February1st to 10thNight
13%16%5%11%7%46%6%5%6%21%
2399
11th to 20thNight
13%14%6%9%7%4445%5%29%
2273
21st to endNight
15%14%5%8%6%45%46%6%27%
1861
March1st to 10thNight
15%11%5%7%6%4445%5%33%
2020
11th to 20thNight
15%12%6%9%5%34446%33%
1979
21st to endNight
16%8%37%5%4445%6%38%
2152
April1st to 10thNight
20%10%5%6%6%3445%6%32%
1800
11th to 20thNight
17%9%37%5%34446%38%
1775
21st to endNight
19%8%36%6%445%6%6%33%
1624
May1st to 10thNight
19%7%25%6%345%5%6%38%
1522
11th to 20thNight
15%4345%35%46%8%43%
1415
21st to endNight
11%5%15%435%5%5%7%49%
1550
June1st to 10thNight
16%6%36%445%5%6%9%38%
1440
11th to 20thNight
13%5%25%5%46%5%48%44%
1440
21st to endNight
13%7%36%5%35%5%5%8%40%
1440
July1st to 10thNight
8%426%5%445%5%6%51%
1440
11th to 20thNight
11%5%245%3446%9%45%
1440
21st to endNight
10%5%24335%6%7%9%47%
1596
August1st to 10thNight
10%5%15%5%35%46%9%48%
1611
11th to 20thNight
9%5%25%5%35%5%5%9%46%
1598
21st to endNight
10%5%26%5%45%5%6%7%46%
1901
September1st to 10thNight
11%5%36%5%3447%9%44%
1800
11th to 20thNight
13%424435%45%6%50%
1812
21st to endNight
18%6%15%5%3445%5%43%
2072
October1st to 10thNight
18%7%37%5%35%5%47%36%
2160
11th to 20thNight
23%9%47%5%345%5%6%29%
2160
21st to endNight
23%13%46%6%35%5%5%5%26%
2399
November1st to 10thNight
19%9%36%6%3445%6%35%
2502
11th to 20thNight
19%12%46%6%35%45%7%29%
2660
21st to endNight
20%12%48%7%36%5%5%7%23%
2660
December1st to 10thNight
21%17%7%9%6%3445%7%17%
2660
11th to 20thNight
17%17%6%11%6%33335%24%
3288
21st to endNight
27%19%6%11%6%3434414%
4884

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 10thDay
65%6%10%11%7%1
2518
11th to 20thDay
70%5%11%8%6%1
2532
21st to endDay
71%48%10%5%1
3124
February1st to 10thDay
67%5%8%13%5%1
1656
11th to 20thDay
61%5%9%14%10%2
1582
21st to endDay
63%48%14%10%1
1394
March1st to 10thDay
56%5%14%15%9%2
1926
11th to 20thDay
57%410%17%10%2
1980
21st to endDay
51%5%7%19%15%4
2204
April1st to 10thDay
55%5%8%19%10%3
2160
11th to 20thDay
49%49%22%15%1
2145
21st to endDay
54%49%19%12%3
2340
May1st to 10thDay
50%5%9%19%14%3
2438
11th to 20thDay
46%5%8%24%15%1
2482
21st to endDay
42%7%14%21%14%3
2725
June1st to 10thDay
49%6%12%19%13%1
2520
11th to 20thDay
44%6%11%24%13%2
2520
21st to endDay
46%7%13%21%12%1
2520
July1st to 10thDay
38%6%12%27%15%2
2450
11th to 20thDay
39%8%16%27%10%
2445
21st to endDay
42%8%14%25%10%1
2566
August1st to 10thDay
41%9%15%24%11%1
2314
11th to 20thDay
43%7%15%24%10%1
2303
21st to endDay
42%7%13%26%11%1
2435
September1st to 10thDay
42%6%13%27%12%1
2160
11th to 20thDay
37%6%14%29%14%1
2145
21st to endDay
45%7%14%25%9%
1887
October1st to 10thDay
52%5%12%19%11%1
1800
11th to 20thDay
59%7%13%14%7%1
1800
21st to endDay
64%5%10%15%6%1
1955
November1st to 10thDay
51%5%15%20%8%1
1575
11th to 20thDay
56%7%13%15%8%2
1520
21st to endDay
65%7%11%11%6%1
1520
December1st to 10thDay
72%5%9%9%5%
1520
11th to 20thDay
63%8%8%12%8%
1906
21st to endDay
72%47%13%4
2772
January1st to 10thNight
70%5%9%12%4
4429
11th to 20thNight
68%7%11%9%5%1
4426
21st to endNight
68%5%10%10%6%
4532
February1st to 10thNight
62%6%10%15%6%1
2399
11th to 20thNight
58%410%19%9%
2275
21st to endNight
56%5%11%18%9%1
1861
March1st to 10thNight
52%5%11%18%12%2
2030
11th to 20thNight
52%5%10%20%11%2
1980
21st to endNight
46%5%9%21%15%3
2152
April1st to 10thNight
52%5%7%20%13%2
1800
11th to 20thNight
47%5%9%21%15%2
1775
21st to endNight
50%7%9%17%15%2
1624
May1st to 10thNight
45%6%10%20%16%4
1522
11th to 20thNight
38%7%9%25%18%3
1416
21st to endNight
34%7%15%23%18%3
1551
June1st to 10thNight
42%6%13%23%14%1
1440
11th to 20thNight
37%7%13%24%19%2
1440
21st to endNight
43%5%15%22%13%2
1440
July1st to 10thNight
31%7%12%28%19%3
1400
11th to 20thNight
34%6%14%32%14%
1392
21st to endNight
32%6%18%31%13%1
1507
August1st to 10thNight
33%7%19%28%13%1
1600
11th to 20thNight
34%7%18%26%14%1
1598
21st to endNight
36%7%14%29%13%1
1901
September1st to 10thNight
36%8%16%25%14%1
1800
11th to 20thNight
34%5%14%33%13%1
1812
21st to endNight
43%5%12%30%10%
2072
October1st to 10thNight
48%6%11%20%14%1
2160
11th to 20thNight
55%6%12%20%6%
2160
21st to endNight
60%6%11%14%10%
2399
November1st to 10thNight
48%6%14%23%9%
2502
11th to 20thNight
54%6%13%17%9%1
2660
21st to endNight
59%8%13%14%4
2660
December1st to 10thNight
67%7%10%12%4
2660
11th to 20thNight
64%411%14%6%
3288
21st to endNight
75%5%7%10%4
4884

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 10thDay
23%23%31%18%5%1
180
11th to 20thDay
27%20%34%15%31
186
21st to endDay
26%20%36%14%41
297
February1st to 10thDay
31%23%37%8%2
252
11th to 20thDay
29%23%42%5%1
240
21st to endDay
37%19%34%9%1
260
March1st to 10thDay
25%19%37%16%4
420
11th to 20thDay
26%20%34%17%31
420
21st to endDay
33%16%29%14%7%1
462
April1st to 10thDay
26%22%31%14%7%
420
11th to 20thDay
32%20%21%16%8%2
412
21st to endDay
30%16%34%14%5%1
420
May1st to 10thDay
27%14%30%18%5%5%
423
11th to 20thDay
21%15%36%17%8%3
447
21st to endDay
20%12%36%18%10%4
485
June1st to 10thDay
23%13%34%21%8%1
450
11th to 20thDay
19%12%32%23%10%3
450
21st to endDay
20%11%35%22%10%2
450
July1st to 10thDay
13%8%31%31%13%4
445
11th to 20thDay
15%9%31%26%14%5%
444
21st to endDay
13%10%34%29%10%3
488
August1st to 10thDay
13%7%31%29%14%6%
446
11th to 20thDay
15%5%26%31%16%7%
439
21st to endDay
15%9%32%23%13%8%
495
September1st to 10thDay
15%10%31%24%14%7%
450
11th to 20thDay
12%11%28%22%18%9%
450
21st to endDay
18%14%27%24%11%6%
450
October1st to 10thDay
27%14%23%18%14%4
450
11th to 20thDay
25%23%28%17%5%1
450
21st to endDay
33%19%27%16%4
482
November1st to 10thDay
27%17%30%18%7%1
269
11th to 20thDay
30%23%31%11%42
180
21st to endDay
29%20%37%11%21
180
December1st to 10thDay
33%19%28%18%2
180
11th to 20thDay
34%22%27%13%21
180
21st to endDay
33%22%32%11%2
198
January1st to 10thNight
28%19%31%17%5%
760
11th to 20thNight
28%16%32%19%5%
760
21st to endNight
20%15%35%23%6%1
836
February1st to 10thNight
23%20%36%18%2
739
11th to 20thNight
22%17%40%17%4
703
21st to endNight
24%22%30%20%3
592
March1st to 10thNight
23%19%36%20%3
720
11th to 20thNight
23%18%34%19%5%1
720
21st to endNight
21%12%33%24%9%1
792
April1st to 10thNight
25%15%32%22%5%
720
11th to 20thNight
23%17%31%20%8%1
711
21st to endNight
23%12%31%24%10%1
544
May1st to 10thNight
23%6%31%26%13%1
442
11th to 20thNight
17%10%36%29%9%
354
21st to endNight
13%7%34%33%12%1
389
June1st to 10thNight
16%7%36%29%11%
360
11th to 20thNight
15%9%30%35%11%1
360
21st to endNight
14%8%28%36%13%
360
July1st to 10thNight
7%6%30%40%16%1
350
11th to 20thNight
11%5%29%36%18%1
348
21st to endNight
11%330%39%16%1
416
August1st to 10thNight
14%221%36%25%2
537
11th to 20thNight
11%5%21%35%26%2
534
21st to endNight
13%430%30%20%2
713
September1st to 10thNight
13%7%31%29%18%3
720
11th to 20thNight
14%5%26%30%21%4
720
21st to endNight
21%9%21%27%18%3
720
October1st to 10thNight
22%12%27%23%15%2
720
11th to 20thNight
29%14%30%19%8%1
720
21st to endNight
30%14%31%20%5%
792
November1st to 10thNight
24%13%28%27%8%
740
11th to 20thNight
27%16%32%18%6%2
760
21st to endNight
29%16%30%20%5%1
760
December1st to 10thNight
31%18%32%15%4
760
11th to 20thNight
25%19%35%17%4
760
21st to endNight
34%15%32%17%3
836
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