The Top Of The Sky Mini Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Top Of The Sky Mini Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
29%
23372
13%
75088
9%
23317
35%
23317
15%
54895

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
8%
758
9%
2459
2%
757
15%
757
6%
2057
January11th to 20thNight
11%
760
6%
2470
3%
760
14%
760
5%
2109
January21st to endNight
12%
820
8%
2664
3%
820
18%
820
6%
2250
February1st to 10thNight
12%
720
7%
2340
3%
720
17%
720
7%
1941
February11th to 20thNight
12%
702
9%
2243
3%
700
19%
700
8%
1819
February21st to endNight
10%
592
11%
1776
2%
592
20%
592
9%
1424
March1st to 10thNight
18%
720
15%
2016
7%
720
26%
720
13%
1515
March11th to 20thNight
22%
720
17%
1980
8%
720
30%
720
16%
1453
March21st to endNight
22%
792
15%
2178
8%
792
28%
792
16%
1641
April1st to 10thNight
22%
720
12%
1929
5%
720
26%
720
18%
1431
April11th to 20thNight
24%
597
18%
1661
6%
597
33%
597
20%
1171
April21st to endNight
26%
540
17%
1620
6%
540
35%
540
20%
1129
May1st to 10thNight
30%
540
12%
1620
6%
540
35%
540
17%
1164
May11th to 20thNight
38%
531
17%
1593
12%
531
42%
531
21%
1103
May21st to endNight
49%
496
15%
1659
13%
496
54%
496
29%
1000
June1st to 10thNight
43%
360
15%
1440
15%
360
52%
360
20%
870
June11th to 20thNight
45%
360
17%
1440
17%
360
55%
360
21%
857
June21st to endNight
46%
360
17%
1440
17%
360
55%
360
22%
845
July1st to 10thNight
62%
470
16%
1520
16%
470
65%
470
31%
905
July11th to 20thNight
57%
522
17%
1566
15%
522
61%
522
33%
950
July21st to endNight
59%
591
17%
1672
15%
558
61%
558
29%
1059
August1st to 10thNight
61%
537
18%
1600
15%
534
62%
534
32%
967
August11th to 20thNight
59%
534
21%
1598
15%
534
62%
534
29%
1006
August21st to endNight
58%
626
22%
1814
15%
626
61%
626
32%
1117
September1st to 10thNight
53%
720
21%
1867
12%
720
56%
720
30%
1228
September11th to 20thNight
57%
720
26%
1980
18%
720
61%
720
35%
1204
September21st to endNight
42%
720
20%
1980
15%
720
47%
720
25%
1356
October1st to 10thNight
40%
720
19%
2114
15%
720
46%
720
27%
1358
October11th to 20thNight
26%
720
15%
2160
9%
720
35%
720
16%
1540
October21st to endNight
19%
792
16%
2376
8%
792
29%
792
14%
1782
November1st to 10thNight
30%
740
17%
2392
13%
740
38%
740
19%
1604
November11th to 20thNight
17%
760
14%
2535
7%
760
26%
760
11%
1897
November21st to endNight
15%
760
10%
2660
6%
760
21%
760
9%
2105
December1st to 10thNight
12%
760
5%
2660
4%
760
15%
760
6%
2197
December11th to 20thNight
11%
760
7%
3269
2%
759
18%
759
5%
2636
December21st to endNight
8%
832
4%
4797
1%
817
15%
817
3%
4205

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
16%11%5%8%6%35%5%5%7%30%
74723
Night
16%12%5%8%6%3445%6%30%
75324
TransparencyDay
54%6%11%18%10%1
74444
Night
54%6%10%18%10%1
75122
SeeingDay
24%17%29%19%9%3
13500
Night
23%15%27%23%10%1
23381

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 10thDay
17%17%11%12%9%45%43416%
1706
11th to 20thDay
17%20%9%14%7%44435%13%
1710
21st to endDay
15%19%11%16%10%4344411%
1836
February1st to 10thDay
17%19%9%10%10%3433417%
1620
11th to 20thDay
15%14%8%15%9%5%4445%17%
1600
21st to endDay
18%16%5%13%7%35%44422%
1480
March1st to 10thDay
18%12%8%8%5%34447%26%
1944
11th to 20thDay
17%15%5%9%7%34336%28%
1980
21st to endDay
21%12%6%8%5%34345%27%
2178
April1st to 10thDay
14%13%6%8%5%3445%6%32%
2030
11th to 20thDay
19%11%46%7%3445%5%32%
2264
21st to endDay
21%11%37%5%33445%34%
2340
May1st to 10thDay
17%9%46%7%5%5%5%5%7%29%
2340
11th to 20thDay
20%8%36%5%35%5%5%7%33%
2301
21st to endDay
10%7%36%7%46%5%7%8%38%
2615
June1st to 10thDay
15%5%36%6%445%6%8%36%
2520
11th to 20thDay
14%7%37%5%345%6%9%38%
2520
21st to endDay
12%5%445%346%6%10%40%
2520
July1st to 10thDay
11%6%36%6%45%5%5%9%41%
2390
11th to 20thDay
11%5%35%5%46%6%6%10%38%
2340
21st to endDay
11%5%36%6%46%5%7%10%39%
2547
August1st to 10thDay
9%6%35%6%46%6%7%9%39%
2327
11th to 20thDay
11%6%37%7%46%8%8%8%34%
2303
21st to endDay
12%6%25%7%46%6%7%9%38%
2522
September1st to 10thDay
16%7%35%5%45%46%8%39%
2090
11th to 20thDay
12%6%36%6%35%5%7%8%41%
1980
21st to endDay
19%9%25%5%3445%7%37%
1980
October1st to 10thDay
16%7%46%5%35%5%5%7%36%
1846
11th to 20thDay
22%11%37%7%34345%31%
1800
21st to endDay
24%12%47%6%3445%6%24%
1980
November1st to 10thDay
12%11%5%6%7%3445%7%35%
1682
11th to 20thDay
19%16%5%7%5%34447%25%
1642
21st to endDay
18%18%8%9%7%3445%6%18%
1520
December1st to 10thDay
17%21%12%11%7%4434314%
1520
11th to 20thDay
17%18%8%16%6%345%45%14%
1910
21st to endDay
24%25%9%11%7%333349%
2840
January1st to 10thNight
17%19%8%13%7%345%35%16%
2459
11th to 20thNight
19%17%9%13%8%4434416%
2470
21st to endNight
16%17%9%14%9%4433416%
2664
February1st to 10thNight
20%15%6%13%9%3444417%
2340
11th to 20thNight
16%15%7%13%8%45%446%19%
2241
21st to endNight
21%13%6%11%7%4443423%
1776
March1st to 10thNight
16%12%7%9%6%345%5%430%
2016
11th to 20thNight
17%10%5%7%5%3445%6%32%
1980
21st to endNight
19%11%5%8%6%24445%30%
2178
April1st to 10thNight
17%12%6%8%6%3445%6%31%
1929
11th to 20thNight
16%10%37%5%45%5%6%6%34%
1661
21st to endNight
15%9%46%5%445%5%7%35%
1620
May1st to 10thNight
18%10%27%7%35%5%6%7%32%
1620
11th to 20thNight
15%9%45%5%45%46%8%35%
1592
21st to endNight
11%5%25%5%35%5%7%11%41%
1658
June1st to 10thNight
10%6%35%5%36%6%6%7%42%
1440
11th to 20thNight
10%5%37%5%45%5%5%8%42%
1440
21st to endNight
11%425%5%35%6%7%10%42%
1440
July1st to 10thNight
8%5%345%35%46%8%49%
1565
11th to 20thNight
8%435%5%35%5%6%9%47%
1620
21st to endNight
8%5%26%5%45%5%6%8%44%
1771
August1st to 10thNight
7%5%25%5%35%5%7%9%47%
1611
11th to 20thNight
9%7%26%5%35%5%5%9%44%
1598
21st to endNight
9%6%25%5%35%5%5%9%46%
1814
September1st to 10thNight
13%6%25%5%35%5%6%7%44%
1867
11th to 20thNight
10%435%43445%8%50%
1980
21st to endNight
15%6%37%6%3445%6%41%
1980
October1st to 10thNight
13%7%35%5%3446%7%42%
2114
11th to 20thNight
18%9%47%5%35%5%5%6%32%
2160
21st to endNight
26%10%36%5%345%5%7%28%
2376
November1st to 10thNight
15%9%47%6%3445%7%37%
2392
11th to 20thNight
16%17%6%9%5%3445%6%26%
2535
21st to endNight
18%17%6%8%7%36%45%5%21%
2660
December1st to 10thNight
20%19%9%10%6%34445%17%
2660
11th to 20thNight
17%19%9%11%7%3444419%
3285
21st to endNight
23%23%10%10%7%3433410%
4812

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
73%5%9%9%3
1707
11th to 20thDay
76%5%10%8%2
1710
21st to endDay
78%5%5%9%3
1836
February1st to 10thDay
72%48%11%5%
1620
11th to 20thDay
70%5%7%12%5%
1609
21st to endDay
65%6%9%12%6%1
1480
March1st to 10thDay
58%6%11%19%5%
1944
11th to 20thDay
59%49%17%9%2
1980
21st to endDay
60%5%7%18%9%1
2178
April1st to 10thDay
52%6%11%17%13%1
2030
11th to 20thDay
53%6%10%16%13%2
2264
21st to endDay
54%410%20%10%3
2340
May1st to 10thDay
54%6%10%19%9%2
2340
11th to 20thDay
50%6%11%18%14%2
2305
21st to endDay
42%8%13%23%13%1
2615
June1st to 10thDay
45%7%11%25%12%1
2520
11th to 20thDay
42%8%14%21%14%1
2520
21st to endDay
38%7%13%25%15%2
2520
July1st to 10thDay
40%6%12%24%16%2
2325
11th to 20thDay
40%7%12%25%15%1
2271
21st to endDay
38%8%14%23%15%2
2405
August1st to 10thDay
39%8%14%26%13%1
2314
11th to 20thDay
42%10%14%21%11%2
2303
21st to endDay
40%8%13%22%14%3
2522
September1st to 10thDay
42%7%14%20%14%3
2090
11th to 20thDay
39%7%14%24%15%1
1980
21st to endDay
47%5%10%24%14%1
1980
October1st to 10thDay
45%7%13%21%13%1
1846
11th to 20thDay
55%411%20%8%1
1800
21st to endDay
61%5%12%13%8%1
1980
November1st to 10thDay
49%6%11%19%14%1
1682
11th to 20thDay
60%411%15%7%3
1642
21st to endDay
67%7%8%13%6%
1520
December1st to 10thDay
75%5%8%10%3
1520
11th to 20thDay
72%6%7%12%3
1906
21st to endDay
82%47%6%1
2840
January1st to 10thNight
72%5%7%12%4
2459
11th to 20thNight
75%48%10%21
2470
21st to endNight
74%47%11%31
2664
February1st to 10thNight
70%5%9%12%4
2340
11th to 20thNight
68%48%14%5%
2243
21st to endNight
66%5%7%14%7%1
1776
March1st to 10thNight
57%5%9%21%8%
2016
11th to 20thNight
53%5%10%19%12%2
1980
21st to endNight
56%5%7%18%11%2
2178
April1st to 10thNight
53%6%10%17%12%2
1929
11th to 20thNight
50%5%10%19%14%3
1661
21st to endNight
48%6%10%20%14%3
1620
May1st to 10thNight
52%6%9%17%12%3
1620
11th to 20thNight
46%7%9%19%14%4
1593
21st to endNight
35%8%17%25%14%2
1659
June1st to 10thNight
40%8%11%25%15%2
1440
11th to 20thNight
39%6%13%21%18%2
1440
21st to endNight
36%9%12%25%17%1
1440
July1st to 10thNight
34%7%11%25%19%4
1520
11th to 20thNight
31%9%12%26%19%3
1566
21st to endNight
37%7%13%23%17%3
1672
August1st to 10thNight
32%9%13%29%16%1
1600
11th to 20thNight
36%8%14%26%13%2
1598
21st to endNight
34%7%11%26%17%3
1814
September1st to 10thNight
38%7%12%25%16%3
1867
11th to 20thNight
32%6%13%27%20%1
1980
21st to endNight
44%6%10%22%17%1
1980
October1st to 10thNight
40%6%13%22%18%1
2114
11th to 20thNight
51%6%9%22%11%1
2160
21st to endNight
56%6%10%17%9%1
2376
November1st to 10thNight
46%6%11%22%12%3
2392
11th to 20thNight
60%410%16%7%3
2535
21st to endNight
65%5%10%15%5%
2660
December1st to 10thNight
72%410%10%4
2660
11th to 20thNight
69%5%10%13%2
3288
21st to endNight
78%6%8%7%1
4812

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
34%26%29%9%3
270
11th to 20thDay
33%32%28%5%11
270
21st to endDay
32%33%27%6%2
282
February1st to 10thDay
40%23%35%3
240
11th to 20thDay
36%33%27%3
240
21st to endDay
40%31%25%4
198
March1st to 10thDay
29%25%30%12%3
384
11th to 20thDay
31%23%30%11%41
420
21st to endDay
35%23%24%15%3
462
April1st to 10thDay
25%26%27%15%6%
420
11th to 20thDay
32%18%27%14%8%1
412
21st to endDay
32%19%29%14%5%1
420
May1st to 10thDay
26%21%28%17%43
423
11th to 20thDay
26%11%32%21%7%3
447
21st to endDay
16%10%32%25%13%5%
485
June1st to 10thDay
16%12%38%22%10%2
450
11th to 20thDay
18%12%29%27%12%2
450
21st to endDay
16%13%32%25%11%3
450
July1st to 10thDay
13%8%23%29%20%7%
445
11th to 20thDay
13%9%27%27%18%6%
444
21st to endDay
11%9%31%29%14%6%
488
August1st to 10thDay
10%8%24%30%20%7%
446
11th to 20thDay
15%8%28%26%15%8%
439
21st to endDay
12%7%27%29%17%8%
495
September1st to 10thDay
17%8%24%26%19%6%
450
11th to 20thDay
12%6%23%28%21%9%
450
21st to endDay
22%13%25%22%13%5%
450
October1st to 10thDay
16%14%29%21%17%3
450
11th to 20thDay
27%20%32%15%41
450
21st to endDay
36%19%30%13%3
495
November1st to 10thDay
26%21%29%15%8%1
280
11th to 20thDay
35%20%27%13%5%
237
21st to endDay
33%22%39%6%1
180
December1st to 10thDay
33%27%25%12%12
180
11th to 20thDay
34%29%29%7%1
180
21st to endDay
34%30%28%7%
218
January1st to 10thNight
30%26%30%11%3
760
11th to 20thNight
31%26%28%13%2
760
21st to endNight
27%25%29%16%2
820
February1st to 10thNight
33%22%29%15%1
720
11th to 20thNight
30%22%32%15%1
703
21st to endNight
32%24%29%13%1
592
March1st to 10thNight
26%22%31%18%3
720
11th to 20thNight
26%20%29%18%6%1
720
21st to endNight
27%18%29%20%6%1
792
April1st to 10thNight
26%17%31%22%5%
720
11th to 20thNight
22%17%33%18%8%1
597
21st to endNight
20%15%34%24%6%
540
May1st to 10thNight
24%10%29%27%9%1
540
11th to 20thNight
20%9%29%32%9%2
531
21st to endNight
12%430%34%18%1
498
June1st to 10thNight
12%434%38%12%1
360
11th to 20thNight
12%6%30%39%13%1
360
21st to endNight
12%8%28%37%15%
360
July1st to 10thNight
8%419%38%28%4
470
11th to 20thNight
10%324%35%24%3
522
21st to endNight
10%5%19%41%23%2
591
August1st to 10thNight
8%5%19%40%26%2
537
11th to 20thNight
11%5%16%37%27%4
534
21st to endNight
12%419%32%29%3
626
September1st to 10thNight
15%7%19%31%24%3
720
11th to 20thNight
14%419%33%26%4
720
21st to endNight
18%9%26%27%16%4
720
October1st to 10thNight
19%9%28%26%16%2
720
11th to 20thNight
25%17%28%23%7%1
720
21st to endNight
33%15%30%18%4
792
November1st to 10thNight
21%17%28%24%9%1
740
11th to 20thNight
28%23%28%14%6%1
760
21st to endNight
32%20%30%13%4
760
December1st to 10thNight
33%24%28%12%4
760
11th to 20thNight
29%26%29%14%2
760
21st to endNight
34%21%31%13%1
836
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