Altair Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altair Observatory clear sky chart from 2005-11-06 to 2024-03-08.

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
54%
24256
32%
79727
21%
24161
66%
24161
43%
38987

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
51%
753
34%
4003
23%
736
68%
736
30%
1757
January11th to 20thNight
46%
756
42%
3917
24%
740
61%
740
38%
1362
January21st to endNight
49%
827
24%
4292
24%
814
65%
814
27%
2119
February1st to 10thNight
46%
760
36%
2423
22%
760
65%
760
50%
1103
February11th to 20thNight
40%
742
33%
2218
17%
740
56%
740
44%
1129
February21st to endNight
32%
628
37%
1884
14%
628
55%
628
47%
902
March1st to 10thNight
40%
750
37%
2123
16%
750
59%
750
48%
1057
March11th to 20thNight
47%
720
35%
1980
18%
720
59%
720
48%
1013
March21st to endNight
38%
734
37%
2120
15%
734
58%
734
47%
1049
April1st to 10thNight
35%
540
37%
1800
16%
540
61%
540
49%
770
April11th to 20thNight
37%
533
35%
1777
17%
533
62%
533
43%
818
April21st to endNight
36%
540
38%
1800
18%
540
64%
540
46%
795
May1st to 10thNight
42%
540
31%
1800
19%
540
61%
540
47%
813
May11th to 20thNight
52%
531
26%
1749
20%
531
62%
531
39%
899
May21st to endNight
51%
584
35%
1618
19%
582
66%
582
56%
782
June1st to 10thNight
61%
540
28%
1440
19%
540
67%
540
46%
787
June11th to 20thNight
60%
540
34%
1440
22%
540
69%
540
59%
720
June21st to endNight
66%
540
23%
1440
18%
540
69%
540
40%
853
July1st to 10thNight
67%
525
17%
1549
17%
525
68%
525
29%
975
July11th to 20thNight
78%
522
18%
1566
18%
522
78%
522
35%
906
July21st to endNight
76%
591
17%
1674
18%
558
76%
558
30%
1035
August1st to 10thNight
78%
537
16%
1684
17%
534
78%
534
33%
978
August11th to 20thNight
75%
534
21%
1778
24%
534
76%
534
30%
1060
August21st to endNight
80%
594
23%
1980
28%
594
81%
594
36%
1083
September1st to 10thNight
73%
540
27%
1800
29%
540
76%
540
34%
1012
September11th to 20thNight
72%
540
36%
1800
34%
540
77%
540
40%
923
September21st to endNight
69%
591
34%
1999
28%
591
77%
591
49%
912
October1st to 10thNight
59%
720
31%
2160
20%
720
66%
720
44%
1132
October11th to 20thNight
65%
720
40%
2160
25%
720
76%
720
59%
977
October21st to endNight
57%
792
40%
2376
23%
792
69%
792
57%
1068
November1st to 10thNight
65%
740
44%
2220
27%
740
76%
740
66%
920
November11th to 20thNight
59%
916
39%
2436
22%
916
69%
916
62%
1160
November21st to endNight
53%
950
38%
2470
21%
950
63%
950
50%
1325
December1st to 10thNight
52%
950
38%
2569
20%
950
65%
950
52%
1310
December11th to 20thNight
45%
948
33%
3119
18%
948
58%
948
41%
1573
December21st to endNight
43%
988
23%
4563
19%
979
58%
979
33%
1910

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
342342445%7%61%
78774
Night
332342445%7%61%
80839
TransparencyDay
21%48%19%29%18%
78500
Night
21%5%9%19%29%18%
80634
SeeingDay
7%11%25%29%21%7%
13948
Night
5%8%28%37%20%2
24290

By Month

Percentage Cloud Cover SW (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
16%243245%5%8%61%
2858
11th to 20thDay
231221235%7%72%
2860
21st to endDay
5%7%25%5%345%7%6%53%
3146
February1st to 10thDay
25%1341345%7%64%
1755
11th to 20thDay
45%24423346%64%
1849
21st to endDay
2413323336%69%
1570
March1st to 10thDay
2415%4245%6%8%58%
1994
11th to 20thDay
35%243235%45%63%
1980
21st to endDay
331333446%7%63%
2236
April1st to 10thDay
221232445%7%68%
2160
11th to 20thDay
4313323346%68%
2131
21st to endDay
2312323345%72%
2160
May1st to 10thDay
2213323447%69%
2160
11th to 20thDay
3323435%5%5%8%60%
2142
21st to endDay
211332345%6%71%
2654
June1st to 10thDay
22124245%6%10%62%
2520
11th to 20thDay
2212323347%72%
2520
21st to endDay
5%423435%5%6%7%55%
2520
July1st to 10thDay
5%3245%36%6%9%11%44%
2358
11th to 20thDay
4325%5%47%8%9%11%44%
2340
21st to endDay
5%425%6%5%7%7%9%12%37%
2545
August1st to 10thDay
43246%46%9%9%11%43%
2244
11th to 20thDay
44246%47%8%8%10%42%
2123
21st to endDay
5%425%5%36%6%7%10%49%
2376
September1st to 10thDay
6%5%15%6%35%6%5%8%49%
2160
11th to 20thDay
7%4133345%6%6%58%
2160
21st to endDay
3213313346%70%
1954
October1st to 10thDay
6%424424345%64%
1800
11th to 20thDay
421222234376%
1800
21st to endDay
331221233476%
1980
November1st to 10thDay
221222335%7%72%
1859
11th to 20thDay
1212213345%74%
1724
21st to endDay
35%23323346%66%
1710
December1st to 10thDay
3423312346%68%
1606
11th to 20thDay
34244345%6%8%57%
1926
21st to endDay
3413323347%67%
2894
January1st to 10thNight
25%25%5%3445%6%59%
4097
11th to 20thNight
2312323346%72%
4110
21st to endNight
47%24425%5%6%7%53%
4521
February1st to 10thNight
15%13213348%67%
2423
11th to 20thNight
34234245%5%7%61%
2218
21st to endNight
2414324347%66%
1884
March1st to 10thNight
232342345%7%64%
2123
11th to 20thNight
5%323323346%66%
1980
21st to endNight
432332446%5%65%
2120
April1st to 10thNight
321332335%6%69%
1800
11th to 20thNight
431332335%6%66%
1777
21st to endNight
3313324336%69%
1800
May1st to 10thNight
2313423447%66%
1800
11th to 20thNight
5%324435%45%7%58%
1749
21st to endNight
2213323436%70%
1618
June1st to 10thNight
23244446%6%10%56%
1440
11th to 20thNight
331332435%7%67%
1440
21st to endNight
6%4245%26%47%10%49%
1440
July1st to 10thNight
7%425%8%46%6%8%9%41%
1594
11th to 20thNight
4425%5%46%7%7%13%43%
1620
21st to endNight
4425%6%5%7%7%9%14%39%
1773
August1st to 10thNight
2225%7%36%8%10%14%39%
1693
11th to 20thNight
5%426%5%46%7%9%12%41%
1778
21st to endNight
33246%36%6%9%11%47%
1980
September1st to 10thNight
6%3246%35%6%7%9%48%
1800
11th to 20thNight
6%41443446%8%57%
1800
21st to endNight
4213333346%67%
1999
October1st to 10thNight
5%42443345%6%60%
2160
11th to 20thNight
311222334475%
2160
21st to endNight
3212222335%74%
2376
November1st to 10thNight
2111213447%75%
2220
11th to 20thNight
1212223345%74%
2436
21st to endNight
5%423223346%66%
2470
December1st to 10thNight
4313323446%67%
2569
11th to 20thNight
33244345%5%9%59%
3271
21st to endNight
3423423346%66%
4800

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
21%29%24%37%8%
2859
11th to 20thDay
13%310%32%35%7%
2860
21st to endDay
30%48%18%32%8%
3146
February1st to 10thDay
19%39%20%31%18%
1755
11th to 20thDay
22%36%20%30%19%
1859
21st to endDay
19%2415%34%27%
1570
March1st to 10thDay
23%26%17%31%21%
1994
11th to 20thDay
22%46%15%31%23%
1980
21st to endDay
20%35%19%34%19%
2236
April1st to 10thDay
16%3415%38%23%
2160
11th to 20thDay
19%25%15%30%29%
2134
21st to endDay
15%36%17%38%21%
2160
May1st to 10thDay
15%36%14%35%27%
2160
11th to 20thDay
22%6%8%17%25%22%
2145
21st to endDay
14%47%15%28%32%
2654
June1st to 10thDay
16%5%10%18%27%24%
2520
11th to 20thDay
14%3412%23%45%
2520
21st to endDay
25%5%10%15%22%23%
2520
July1st to 10thDay
28%9%14%16%17%17%
2294
11th to 20thDay
28%9%16%22%15%9%
2271
21st to endDay
33%12%20%24%10%1
2404
August1st to 10thDay
28%11%18%23%14%6%
2229
11th to 20thDay
31%10%14%22%17%6%
2123
21st to endDay
28%7%14%23%22%5%
2376
September1st to 10thDay
31%6%10%19%22%12%
2160
11th to 20thDay
23%7%9%16%26%19%
2160
21st to endDay
15%47%22%31%21%
1954
October1st to 10thDay
24%38%17%27%21%
1800
11th to 20thDay
14%3412%32%36%
1800
21st to endDay
13%35%19%41%19%
1980
November1st to 10thDay
13%25%20%39%22%
1859
11th to 20thDay
14%2423%38%18%
1724
21st to endDay
21%2422%33%19%
1710
December1st to 10thDay
19%36%29%29%14%
1606
11th to 20thDay
25%49%20%30%12%
1924
21st to endDay
19%27%18%34%19%
2894
January1st to 10thNight
25%49%19%32%11%
4097
11th to 20thNight
16%47%28%36%9%
4110
21st to endNight
27%6%9%18%30%9%
4521
February1st to 10thNight
17%39%20%31%20%
2423
11th to 20thNight
21%46%20%30%19%
2218
21st to endNight
20%2415%34%25%
1884
March1st to 10thNight
19%37%19%33%20%
2123
11th to 20thNight
20%37%17%35%18%
1980
21st to endNight
20%36%19%34%18%
2120
April1st to 10thNight
16%35%16%37%23%
1800
11th to 20thNight
19%36%14%29%29%
1777
21st to endNight
17%37%17%35%20%
1800
May1st to 10thNight
19%36%11%33%28%
1800
11th to 20thNight
24%6%10%15%25%20%
1750
21st to endNight
16%47%14%27%32%
1618
June1st to 10thNight
22%6%13%16%17%26%
1440
11th to 20thNight
17%37%14%18%42%
1440
21st to endNight
28%7%11%15%16%24%
1440
July1st to 10thNight
35%9%13%16%14%13%
1549
11th to 20thNight
28%10%20%25%13%6%
1566
21st to endNight
31%12%22%24%9%2
1674
August1st to 10thNight
28%10%23%22%10%6%
1684
11th to 20thNight
31%11%18%21%14%6%
1778
21st to endNight
25%10%17%25%17%6%
1980
September1st to 10thNight
29%8%14%19%20%10%
1800
11th to 20thNight
25%6%12%16%22%19%
1800
21st to endNight
18%48%16%31%22%
1999
October1st to 10thNight
24%49%17%24%21%
2160
11th to 20thNight
14%3414%30%35%
2160
21st to endNight
14%36%17%38%22%
2376
November1st to 10thNight
10%35%22%39%22%
2220
11th to 20thNight
13%3420%38%22%
2436
21st to endNight
21%3417%36%19%
2470
December1st to 10thNight
18%47%26%31%14%
2569
11th to 20thNight
22%49%21%31%13%
3272
21st to endNight
21%45%21%32%17%
4800

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
5%17%38%29%11%
270
11th to 20thDay
10%15%35%24%14%1
270
21st to endDay
5%13%36%30%11%4
297
February1st to 10thDay
6%19%33%27%14%
270
11th to 20thDay
10%20%40%23%6%
270
21st to endDay
13%29%32%18%6%
225
March1st to 10thDay
11%20%37%23%9%1
385
11th to 20thDay
13%20%33%24%9%2
420
21st to endDay
13%24%37%18%7%2
462
April1st to 10thDay
9%20%38%23%8%3
420
11th to 20thDay
12%20%39%20%8%
412
21st to endDay
11%19%39%22%8%1
420
May1st to 10thDay
6%16%40%28%9%2
423
11th to 20thDay
6%14%31%29%15%5%
448
21st to endDay
411%34%33%17%1
609
June1st to 10thDay
3422%39%28%4
630
11th to 20thDay
35%22%37%29%4
630
21st to endDay
7%215%37%29%10%
630
July1st to 10thDay
428%34%35%17%
464
11th to 20thDay
5%421%37%32%
444
21st to endDay
6%1321%39%30%
487
August1st to 10thDay
217%27%43%20%
447
11th to 20thDay
6%29%30%37%16%
437
21st to endDay
7%15%32%38%17%
495
September1st to 10thDay
8%211%33%30%16%
450
11th to 20thDay
5%214%36%35%8%
450
21st to endDay
6%7%18%30%31%8%
299
October1st to 10thDay
7%10%23%29%23%9%
270
11th to 20thDay
10%6%22%32%24%6%
270
21st to endDay
10%12%22%31%19%6%
297
November1st to 10thDay
8%12%35%27%15%3
270
11th to 20thDay
310%28%33%20%6%
270
21st to endDay
10%17%34%26%11%2
270
December1st to 10thDay
5%16%36%27%14%1
270
11th to 20thDay
8%15%37%31%9%
270
21st to endDay
11%18%36%25%9%1
297
January1st to 10thNight
5%8%32%37%17%1
760
11th to 20thNight
5%10%34%34%15%2
760
21st to endNight
6%10%32%36%14%2
836
February1st to 10thNight
411%36%36%12%1
760
11th to 20thNight
6%11%38%36%8%
744
21st to endNight
7%13%46%30%4
628
March1st to 10thNight
5%13%41%31%10%1
750
11th to 20thNight
6%12%32%36%13%1
720
21st to endNight
5%12%42%32%9%
734
April1st to 10thNight
5%11%46%31%7%1
540
11th to 20thNight
6%15%39%32%7%1
533
21st to endNight
5%13%43%31%7%1
540
May1st to 10thNight
49%42%37%9%
540
11th to 20thNight
6%5%30%41%16%1
531
21st to endNight
36%35%43%13%
584
June1st to 10thNight
35%25%46%20%
540
11th to 20thNight
4229%47%17%1
540
21st to endNight
7%114%47%28%3
540
July1st to 10thNight
7%28%41%37%5%
525
11th to 20thNight
5%2430%51%8%
522
21st to endNight
5%2427%53%10%
591
August1st to 10thNight
315%29%50%12%
537
11th to 20thNight
5%17%35%47%4
534
21st to endNight
41435%50%6%
594
September1st to 10thNight
7%110%42%36%4
540
11th to 20thNight
9%111%47%31%1
540
21st to endNight
5%318%41%29%4
591
October1st to 10thNight
6%423%43%21%3
720
11th to 20thNight
45%22%46%20%2
720
21st to endNight
5%8%27%34%22%4
792
November1st to 10thNight
35%26%42%22%3
740
11th to 20thNight
36%29%40%20%2
916
21st to endNight
6%11%26%34%20%2
950
December1st to 10thNight
5%9%31%38%16%1
950
11th to 20thNight
6%10%33%37%13%
950
21st to endNight
6%13%33%37%9%1
998
Page generated at 2024-03-05 05:24:01. Version ac07

Copyright © 2024 Attilla Danko.


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