Tamke-Allan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tamke-Allan Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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%
23739
20%
74538
15%
23683
52%
23683
25%
47892

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
24%
760
19%
2459
10%
757
36%
757
19%
1635
January11th to 20thNight
24%
732
17%
2378
9%
732
34%
732
16%
1659
January21st to endNight
24%
792
21%
2538
10%
792
35%
792
20%
1681
February1st to 10thNight
26%
720
19%
2160
10%
720
35%
720
21%
1466
February11th to 20thNight
20%
703
17%
2098
8%
699
29%
699
18%
1480
February21st to endNight
21%
592
19%
1776
10%
592
29%
592
16%
1296
March1st to 10thNight
31%
720
21%
2138
12%
720
40%
720
24%
1396
March11th to 20thNight
30%
720
18%
1980
10%
720
36%
720
22%
1377
March21st to endNight
34%
792
17%
2178
10%
792
39%
792
23%
1485
April1st to 10thNight
40%
622
25%
1882
15%
622
51%
622
31%
1110
April11th to 20thNight
41%
533
26%
1775
17%
533
53%
533
29%
1039
April21st to endNight
42%
540
23%
1638
14%
540
50%
540
28%
1025
May1st to 10thNight
47%
540
19%
1620
11%
540
54%
540
27%
1023
May11th to 20thNight
53%
531
17%
1593
12%
531
57%
531
26%
1032
May21st to endNight
65%
582
17%
1744
13%
582
67%
582
26%
1147
June1st to 10thNight
63%
540
17%
1620
13%
540
65%
540
28%
1037
June11th to 20thNight
74%
540
16%
1620
13%
540
76%
540
38%
919
June21st to endNight
71%
540
17%
1620
15%
540
72%
540
29%
1017
July1st to 10thNight
68%
525
11%
1575
10%
525
69%
525
23%
1076
July11th to 20thNight
72%
522
15%
1566
13%
522
73%
522
29%
995
July21st to endNight
71%
591
16%
1672
15%
558
73%
558
27%
1086
August1st to 10thNight
69%
537
16%
1600
14%
534
70%
534
28%
1022
August11th to 20thNight
66%
534
16%
1698
16%
534
67%
534
24%
1118
August21st to endNight
68%
594
18%
1980
20%
594
69%
594
25%
1246
September1st to 10thNight
65%
656
19%
1916
18%
656
66%
656
29%
1223
September11th to 20thNight
62%
720
20%
1980
17%
720
63%
720
29%
1314
September21st to endNight
58%
720
26%
1980
19%
720
63%
720
34%
1223
October1st to 10thNight
59%
720
30%
1980
25%
720
62%
720
40%
1148
October11th to 20thNight
48%
720
27%
2082
20%
720
56%
720
34%
1246
October21st to endNight
50%
792
29%
2434
25%
792
58%
792
31%
1445
November1st to 10thNight
52%
740
27%
2406
24%
740
60%
740
32%
1371
November11th to 20thNight
40%
760
26%
2470
19%
760
51%
760
29%
1440
November21st to endNight
40%
760
23%
2470
17%
760
50%
760
25%
1550
December1st to 10thNight
32%
760
19%
2470
12%
760
41%
760
18%
1708
December11th to 20thNight
34%
759
22%
2971
16%
759
43%
759
19%
1865
December21st to endNight
30%
830
11%
4471
10%
817
39%
817
9%
2992

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
14%6%35%5%35%5%6%8%40%
74872
Night
14%6%35%5%35%5%5%7%42%
74878
TransparencyDay
41%9%12%20%15%3
74595
Night
40%8%12%21%17%4
74674
SeeingDay
17%11%22%23%19%8%
13646
Night
16%9%23%30%19%3
23748

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
18%9%36%5%346%5%6%36%
1706
11th to 20thDay
21%10%48%5%24435%34%
1638
21st to endDay
18%6%25%433448%42%
1814
February1st to 10thDay
19%11%36%5%35%35%436%
1800
11th to 20thDay
23%8%45%5%24445%35%
1748
21st to endDay
22%8%46%5%2445%5%37%
1480
March1st to 10thDay
15%7%35%5%346%5%7%40%
1822
11th to 20thDay
21%9%46%43445%7%34%
1980
21st to endDay
17%7%35%6%35%45%7%37%
2178
April1st to 10thDay
12%5%25%5%35%46%7%46%
2092
11th to 20thDay
14%7%35%4335%48%45%
2134
21st to endDay
14%6%25%435%5%6%7%43%
2320
May1st to 10thDay
13%6%25%5%35%45%7%45%
2340
11th to 20thDay
11%7%37%7%45%5%5%8%38%
2301
21st to endDay
11%5%25%5%45%5%7%10%41%
2531
June1st to 10thDay
10%6%25%5%46%6%7%10%40%
2340
11th to 20thDay
5%325%6%47%8%9%10%43%
2340
21st to endDay
7%435%6%46%7%7%11%40%
2340
July1st to 10thDay
8%5%37%7%6%6%7%7%11%33%
2340
11th to 20thDay
11%5%35%6%5%6%7%8%11%33%
2340
21st to endDay
10%5%36%7%47%7%8%10%32%
2547
August1st to 10thDay
9%6%26%7%5%7%8%9%10%31%
2327
11th to 20thDay
9%5%37%7%5%7%7%7%9%35%
2198
21st to endDay
10%5%35%6%46%6%7%10%37%
2376
September1st to 10thDay
13%5%35%5%45%5%7%8%41%
2026
11th to 20thDay
15%425%5%446%6%8%41%
1980
21st to endDay
13%5%25%434446%50%
1980
October1st to 10thDay
11%5%24433446%55%
1980
11th to 20thDay
13%5%23423446%53%
1877
21st to endDay
15%5%2442445%7%49%
1918
November1st to 10thDay
11%6%23423446%55%
1672
11th to 20thDay
14%6%35%32345%6%49%
1710
21st to endDay
17%6%345%3445%5%46%
1710
December1st to 10thDay
22%8%35%6%35%5%45%34%
1710
11th to 20thDay
15%9%35%5%35%46%6%37%
2153
21st to endDay
20%12%26%6%3446%7%31%
3124
January1st to 10thNight
19%7%36%5%34446%40%
2459
11th to 20thNight
20%8%37%5%3445%7%33%
2378
21st to endNight
16%8%36%5%35%45%6%39%
2538
February1st to 10thNight
18%7%35%5%45%45%6%37%
2160
11th to 20thNight
23%7%45%5%3445%5%36%
2098
21st to endNight
24%8%346%25%445%33%
1776
March1st to 10thNight
15%6%25%5%3445%7%43%
2138
11th to 20thNight
15%8%46%6%34446%39%
1980
21st to endNight
15%6%36%6%3445%7%40%
2178
April1st to 10thNight
12%414435%45%8%50%
1882
11th to 20thNight
13%5%245%2445%6%50%
1775
21st to endNight
11%5%25%5%345%5%9%45%
1638
May1st to 10thNight
10%5%35%5%35%47%8%44%
1620
11th to 20thNight
11%6%25%5%35%5%6%8%44%
1592
21st to endNight
8%436%5%46%6%7%10%41%
1744
June1st to 10thNight
7%5%36%5%45%6%8%9%43%
1620
11th to 20thNight
5%313425%5%8%12%51%
1620
21st to endNight
6%5%35%6%46%6%6%11%43%
1620
July1st to 10thNight
8%6%36%6%46%6%7%11%36%
1620
11th to 20thNight
8%4246%35%6%8%11%43%
1620
21st to endNight
7%5%246%46%8%9%11%39%
1771
August1st to 10thNight
8%425%5%35%7%8%10%43%
1611
11th to 20thNight
9%6%26%6%35%6%7%7%43%
1698
21st to endNight
10%6%25%6%45%6%6%9%42%
1980
September1st to 10thNight
10%6%25%5%35%47%7%45%
1916
11th to 20thNight
12%6%37%5%3346%9%43%
1980
21st to endNight
12%5%2443435%6%52%
1980
October1st to 10thNight
11%424323446%57%
1980
11th to 20thNight
14%5%24434445%52%
2082
21st to endNight
14%6%34334435%52%
2434
November1st to 10thNight
12%6%24424346%53%
2406
11th to 20thNight
12%6%35%424446%50%
2470
21st to endNight
16%7%34425%5%48%43%
2470
December1st to 10thNight
21%8%36%435%45%6%35%
2470
11th to 20thNight
17%8%36%5%34446%41%
3042
21st to endNight
20%8%37%6%46%6%7%7%27%
4532

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
48%7%9%19%15%3
1707
11th to 20thDay
55%48%18%13%2
1638
21st to endDay
42%6%11%23%17%1
1814
February1st to 10thDay
52%5%9%17%15%2
1800
11th to 20thDay
52%6%8%19%13%3
1758
21st to endDay
50%7%8%14%15%6%
1480
March1st to 10thDay
43%6%10%18%18%5%
1822
11th to 20thDay
49%7%10%17%13%3
1980
21st to endDay
46%9%10%21%12%1
2178
April1st to 10thDay
37%8%12%23%17%3
2092
11th to 20thDay
39%5%10%17%24%5%
2134
21st to endDay
38%8%13%25%14%2
2320
May1st to 10thDay
38%9%12%26%13%2
2340
11th to 20thDay
43%11%14%19%12%1
2305
21st to endDay
37%11%16%22%12%2
2531
June1st to 10thDay
36%12%17%19%13%2
2340
11th to 20thDay
30%14%19%21%13%2
2340
21st to endDay
35%14%17%18%12%4
2340
July1st to 10thDay
41%15%14%16%11%2
2275
11th to 20thDay
39%15%18%14%10%4
2271
21st to endDay
40%15%16%20%9%1
2405
August1st to 10thDay
41%15%19%17%8%1
2314
11th to 20thDay
41%14%16%15%12%2
2198
21st to endDay
40%11%17%17%12%3
2376
September1st to 10thDay
37%10%14%19%15%4
2026
11th to 20thDay
39%9%13%20%16%3
1980
21st to endDay
34%6%13%22%19%6%
1980
October1st to 10thDay
31%6%10%23%23%6%
1980
11th to 20thDay
32%6%10%21%24%7%
1877
21st to endDay
34%5%9%24%23%4
1918
November1st to 10thDay
31%5%8%24%26%6%
1672
11th to 20thDay
36%48%24%24%5%
1710
21st to endDay
40%5%10%23%20%3
1710
December1st to 10thDay
51%6%7%21%12%3
1710
11th to 20thDay
46%8%10%19%15%2
2150
21st to endDay
49%7%13%21%9%1
3124
January1st to 10thNight
46%6%9%20%16%3
2459
11th to 20thNight
51%5%11%15%14%2
2378
21st to endNight
46%5%10%22%15%2
2538
February1st to 10thNight
47%5%11%20%15%2
2160
11th to 20thNight
51%5%8%19%13%4
2099
21st to endNight
53%6%8%15%14%4
1776
March1st to 10thNight
41%6%11%16%20%6%
2138
11th to 20thNight
45%7%10%20%15%3
1980
21st to endNight
44%6%12%22%14%2
2178
April1st to 10thNight
32%7%12%24%22%3
1882
11th to 20thNight
35%5%9%21%22%7%
1775
21st to endNight
36%8%11%28%15%1
1638
May1st to 10thNight
36%9%12%25%15%2
1620
11th to 20thNight
37%10%16%23%13%1
1593
21st to endNight
35%13%15%23%13%2
1745
June1st to 10thNight
36%10%15%24%14%2
1620
11th to 20thNight
24%11%23%28%12%2
1620
21st to endNight
34%10%16%24%12%5%
1620
July1st to 10thNight
40%13%17%17%13%2
1575
11th to 20thNight
31%14%21%18%11%5%
1566
21st to endNight
32%15%19%22%10%2
1672
August1st to 10thNight
33%12%21%23%10%1
1600
11th to 20thNight
37%13%19%16%14%2
1698
21st to endNight
36%11%18%21%12%2
1980
September1st to 10thNight
35%10%17%18%15%5%
1916
11th to 20thNight
38%9%14%17%21%1
1980
21st to endNight
33%7%11%22%22%6%
1980
October1st to 10thNight
29%410%21%27%8%
1980
11th to 20thNight
33%6%10%21%22%8%
2082
21st to endNight
35%45%23%25%7%
2434
November1st to 10thNight
33%5%8%22%24%8%
2406
11th to 20thNight
35%5%8%21%24%7%
2470
21st to endNight
41%5%7%23%21%3
2470
December1st to 10thNight
49%6%10%19%13%3
2470
11th to 20thNight
42%7%10%20%19%2
3044
21st to endNight
49%10%13%17%10%1
4532

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
24%26%30%17%3
270
11th to 20thDay
30%24%28%14%41
246
21st to endDay
20%24%41%10%4
264
February1st to 10thDay
29%27%25%15%4
240
11th to 20thDay
38%22%30%10%
240
21st to endDay
38%22%30%9%1
198
March1st to 10thDay
24%21%34%17%3
262
11th to 20thDay
23%21%31%19%5%1
420
21st to endDay
25%15%32%19%8%
462
April1st to 10thDay
19%18%26%24%12%2
420
11th to 20thDay
19%15%28%24%12%1
412
21st to endDay
20%11%28%25%12%4
420
May1st to 10thDay
15%13%33%23%12%4
423
11th to 20thDay
10%8%26%28%18%11%
447
21st to endDay
16%7%16%24%24%14%
485
June1st to 10thDay
12%5%19%25%25%13%
450
11th to 20thDay
6%414%28%30%18%
450
21st to endDay
10%220%28%25%15%
450
July1st to 10thDay
9%411%25%29%22%
445
11th to 20thDay
10%410%25%31%20%
444
21st to endDay
9%39%26%33%20%
488
August1st to 10thDay
12%411%24%32%16%
446
11th to 20thDay
12%315%28%32%11%
439
21st to endDay
11%313%22%32%19%
495
September1st to 10thDay
11%314%23%32%16%
450
11th to 20thDay
15%311%22%33%16%
450
21st to endDay
16%5%14%26%30%9%
450
October1st to 10thDay
14%5%23%24%25%9%
450
11th to 20thDay
11%15%24%29%18%3
450
21st to endDay
19%9%25%28%15%3
433
November1st to 10thDay
19%9%30%26%13%3
270
11th to 20thDay
14%14%35%31%6%
270
21st to endDay
17%12%36%23%10%1
270
December1st to 10thDay
29%18%28%19%6%
270
11th to 20thDay
23%17%29%21%9%
270
21st to endDay
29%18%30%18%5%1
297
January1st to 10thNight
22%16%33%24%4
760
11th to 20thNight
23%16%32%24%5%
732
21st to endNight
20%15%37%26%2
792
February1st to 10thNight
21%13%36%26%4
720
11th to 20thNight
28%15%35%21%2
703
21st to endNight
26%17%34%22%1
592
March1st to 10thNight
19%13%32%33%3
720
11th to 20thNight
20%14%33%25%8%1
720
21st to endNight
20%12%29%30%8%
792
April1st to 10thNight
13%12%32%32%10%
622
11th to 20thNight
16%9%30%31%13%
533
21st to endNight
12%8%31%37%10%1
540
May1st to 10thNight
11%6%27%37%16%2
540
11th to 20thNight
12%5%22%34%23%4
531
21st to endNight
8%414%34%37%5%
584
June1st to 10thNight
8%215%36%33%6%
540
11th to 20thNight
5%214%33%36%9%
540
21st to endNight
7%111%36%38%9%
540
July1st to 10thNight
10%28%26%42%12%
525
11th to 20thNight
7%48%28%43%11%
522
21st to endNight
7%29%28%42%10%
591
August1st to 10thNight
10%18%26%45%10%
537
11th to 20thNight
10%29%31%43%4
534
21st to endNight
11%17%30%41%9%
594
September1st to 10thNight
10%311%29%39%8%
656
11th to 20thNight
13%313%31%35%6%
720
21st to endNight
14%318%32%32%2
720
October1st to 10thNight
11%7%18%26%31%7%
720
11th to 20thNight
17%6%25%33%19%2
720
21st to endNight
16%8%23%36%16%2
792
November1st to 10thNight
14%8%21%36%18%3
740
11th to 20thNight
14%11%32%33%10%1
760
21st to endNight
19%10%27%32%11%
760
December1st to 10thNight
24%13%27%28%8%
760
11th to 20thNight
22%11%30%30%8%
760
21st to endNight
26%14%24%31%5%
836
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