Stellamar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellamar Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
27%
24066
12%
74935
7%
24010
33%
24010
15%
55420

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
12%
950
6%
2648
2%
946
15%
946
7%
2277
January11th to 20thNight
14%
910
7%
2547
3%
910
18%
910
7%
2189
January21st to endNight
17%
990
7%
2772
3%
990
20%
990
8%
2339
February1st to 10thNight
12%
746
8%
2366
2%
746
18%
746
7%
1950
February11th to 20thNight
16%
701
10%
2158
3%
699
23%
699
10%
1696
February21st to endNight
14%
592
11%
1776
1%
592
23%
592
9%
1404
March1st to 10thNight
23%
720
18%
2160
9%
720
31%
720
18%
1485
March11th to 20thNight
20%
720
18%
1992
7%
720
30%
720
18%
1432
March21st to endNight
26%
790
17%
2176
9%
790
33%
790
20%
1562
April1st to 10thNight
21%
540
13%
1800
6%
540
30%
540
14%
1304
April11th to 20thNight
28%
533
20%
1633
10%
533
38%
533
21%
1133
April21st to endNight
30%
540
15%
1620
5%
540
38%
540
20%
1122
May1st to 10thNight
29%
540
14%
1620
7%
540
36%
540
19%
1128
May11th to 20thNight
30%
531
16%
1567
9%
531
39%
531
22%
1065
May21st to endNight
43%
511
17%
1480
9%
510
52%
510
31%
907
June1st to 10thNight
36%
360
16%
1260
13%
360
45%
360
24%
769
June11th to 20thNight
45%
360
15%
1260
12%
360
50%
360
25%
756
June21st to endNight
44%
360
16%
1260
13%
360
50%
360
27%
736
July1st to 10thNight
54%
455
18%
1330
12%
455
60%
455
41%
736
July11th to 20thNight
53%
522
17%
1392
11%
522
57%
522
35%
860
July21st to endNight
54%
591
17%
1487
12%
558
57%
558
35%
922
August1st to 10thNight
51%
537
19%
1567
13%
534
57%
534
36%
921
August11th to 20thNight
57%
534
20%
1598
14%
534
63%
534
35%
916
August21st to endNight
47%
594
18%
1810
12%
594
52%
594
28%
1122
September1st to 10thNight
46%
584
18%
1844
13%
584
52%
584
29%
1107
September11th to 20thNight
45%
720
22%
1980
14%
720
50%
720
31%
1255
September21st to endNight
41%
720
17%
1980
13%
720
45%
720
25%
1355
October1st to 10thNight
34%
720
17%
2011
11%
720
40%
720
23%
1380
October11th to 20thNight
20%
720
12%
2212
6%
720
27%
720
14%
1663
October21st to endNight
20%
792
12%
2574
6%
792
27%
792
11%
1981
November1st to 10thNight
21%
740
15%
2406
9%
740
30%
740
14%
1755
November11th to 20thNight
17%
760
12%
2486
6%
760
25%
760
10%
1897
November21st to endNight
11%
760
8%
2660
3%
760
17%
760
8%
2146
December1st to 10thNight
14%
935
6%
2835
2%
935
18%
935
7%
2407
December11th to 20thNight
11%
950
4%
3508
2%
949
15%
949
5%
3027
December21st to endNight
9%
1038
3%
5160
1%
1026
13%
1026
3%
4716

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%11%5%8%6%35%5%5%7%31%
74564
Night
14%12%5%9%7%45%5%5%6%29%
75141
TransparencyDay
53%6%10%19%10%1
74269
Night
55%6%11%18%9%1
74971
SeeingDay
21%18%31%19%8%3
14391
Night
21%15%31%24%9%1
24076

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
14%17%8%16%10%4443414%
1517
11th to 20thDay
14%18%9%14%11%5%5%445%11%
1453
21st to endDay
13%17%10%15%10%5%7%43313%
1584
February1st to 10thDay
10%15%10%12%15%6%5%445%14%
1593
11th to 20thDay
12%15%7%15%8%45%5%5%420%
1687
21st to endDay
11%19%8%12%8%45%45%5%19%
1480
March1st to 10thDay
13%11%7%10%8%45%446%29%
1798
11th to 20thDay
12%12%6%7%7%35%35%6%34%
1966
21st to endDay
18%13%47%5%34446%31%
2196
April1st to 10thDay
18%15%5%8%6%245%5%5%27%
2160
11th to 20thDay
19%14%35%434446%32%
2274
21st to endDay
17%12%47%6%3445%6%31%
2340
May1st to 10thDay
18%9%36%5%345%5%7%35%
2340
11th to 20thDay
17%7%46%6%34446%38%
2323
21st to endDay
12%6%36%5%45%5%6%7%42%
2792
June1st to 10thDay
14%5%35%6%3446%8%42%
2700
11th to 20thDay
13%6%26%6%45%6%6%8%38%
2700
21st to endDay
14%6%36%5%345%5%8%41%
2700
July1st to 10thDay
9%5%24435%5%6%10%47%
2585
11th to 20thDay
13%5%36%5%35%5%6%8%41%
2520
21st to endDay
10%6%36%5%35%5%6%9%42%
2743
August1st to 10thDay
10%6%27%6%46%7%6%9%37%
2360
11th to 20thDay
11%6%36%5%35%5%5%7%43%
2303
21st to endDay
14%6%35%5%35%46%7%42%
2543
September1st to 10thDay
13%7%36%5%45%45%8%39%
2098
11th to 20thDay
13%6%25%5%45%5%5%8%43%
1980
21st to endDay
17%8%36%6%345%6%8%35%
1980
October1st to 10thDay
20%10%37%5%34347%33%
1948
11th to 20thDay
24%14%47%7%35%5%5%5%21%
1746
21st to endDay
21%18%5%8%7%36%446%18%
1782
November1st to 10thDay
15%17%5%7%6%345%5%8%24%
1672
11th to 20thDay
14%18%8%10%7%45%6%45%18%
1691
21st to endDay
17%19%8%11%6%35%447%15%
1520
December1st to 10thDay
13%20%11%15%8%45%44412%
1344
11th to 20thDay
15%17%9%14%10%5%5%44413%
1671
21st to endDay
22%24%10%17%7%332337%
2475
January1st to 10thNight
13%15%7%16%9%5%6%5%5%5%13%
2648
11th to 20thNight
15%17%8%14%10%46%445%14%
2547
21st to endNight
11%14%7%13%13%6%7%5%5%5%14%
2772
February1st to 10thNight
10%16%7%14%11%5%6%5%5%5%16%
2366
11th to 20thNight
11%13%8%12%8%46%5%5%6%22%
2158
21st to endNight
15%16%6%11%7%445%4423%
1776
March1st to 10thNight
11%12%48%5%4445%6%36%
2149
11th to 20thNight
12%12%5%8%7%35%45%5%35%
1992
21st to endNight
15%10%47%5%35%5%5%6%34%
2176
April1st to 10thNight
19%12%47%5%35%5%5%6%30%
1800
11th to 20thNight
18%10%35%6%345%6%7%35%
1633
21st to endNight
16%10%35%5%4445%6%36%
1620
May1st to 10thNight
13%9%37%8%45%6%5%7%34%
1620
11th to 20thNight
14%8%47%6%345%5%7%37%
1567
21st to endNight
11%5%25%435%47%7%47%
1479
June1st to 10thNight
13%6%35%5%3445%8%43%
1260
11th to 20thNight
9%426%6%37%5%7%9%43%
1260
21st to endNight
12%6%244235%49%47%
1260
July1st to 10thNight
7%4244245%6%9%54%
1370
11th to 20thNight
9%5%245%35%5%6%7%49%
1440
21st to endNight
8%425%4445%5%8%50%
1575
August1st to 10thNight
8%5%245%345%5%9%50%
1576
11th to 20thNight
12%3134335%6%7%51%
1598
21st to endNight
13%5%146%345%5%7%46%
1810
September1st to 10thNight
12%5%25%425%45%7%49%
1844
11th to 20thNight
15%425%5%335%47%47%
1980
21st to endNight
18%6%25%5%35%45%8%38%
1980
October1st to 10thNight
19%7%26%5%3445%6%39%
2011
11th to 20thNight
22%13%36%5%345%5%6%28%
2212
21st to endNight
24%13%48%6%3445%5%25%
2574
November1st to 10thNight
15%15%5%7%7%35%447%28%
2406
11th to 20thNight
14%14%7%9%7%45%5%5%7%22%
2486
21st to endNight
17%18%8%10%6%35%5%46%19%
2660
December1st to 10thNight
13%16%10%14%7%46%5%46%14%
2835
11th to 20thNight
13%18%8%15%10%46%5%46%13%
3526
21st to endNight
22%24%10%13%6%344337%
5175

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
74%5%8%10%3
1518
11th to 20thDay
76%6%6%9%3
1453
21st to endDay
77%46%8%41
1584
February1st to 10thDay
73%49%12%3
1593
11th to 20thDay
66%5%8%12%8%1
1696
21st to endDay
67%5%8%14%6%
1480
March1st to 10thDay
57%5%10%18%9%2
1800
11th to 20thDay
51%411%22%8%3
1966
21st to endDay
54%5%10%19%11%2
2196
April1st to 10thDay
59%5%9%18%9%1
2160
11th to 20thDay
52%49%18%14%1
2274
21st to endDay
53%5%9%16%14%3
2340
May1st to 10thDay
48%5%8%23%13%2
2340
11th to 20thDay
47%5%8%22%15%3
2327
21st to endDay
40%6%12%24%15%2
2792
June1st to 10thDay
41%410%27%15%3
2700
11th to 20thDay
42%7%11%22%17%2
2700
21st to endDay
42%5%12%25%13%2
2700
July1st to 10thDay
33%6%12%27%20%2
2516
11th to 20thDay
39%7%11%26%16%2
2445
21st to endDay
38%7%13%24%17%1
2591
August1st to 10thDay
41%8%14%23%12%1
2345
11th to 20thDay
39%7%12%27%14%1
2303
21st to endDay
41%6%12%26%14%2
2543
September1st to 10thDay
44%5%11%23%15%1
2098
11th to 20thDay
39%6%13%30%11%1
1980
21st to endDay
47%6%14%25%9%
1980
October1st to 10thDay
52%5%10%23%10%1
1948
11th to 20thDay
63%7%9%16%5%
1746
21st to endDay
68%6%10%12%5%
1782
November1st to 10thDay
57%7%10%19%7%
1672
11th to 20thDay
66%7%11%11%5%
1691
21st to endDay
70%6%12%10%3
1520
December1st to 10thDay
76%6%7%10%21
1344
11th to 20thDay
74%6%9%9%2
1671
21st to endDay
86%35%42
2475
January1st to 10thNight
72%7%9%9%3
2648
11th to 20thNight
74%5%8%9%5%
2547
21st to endNight
71%6%9%10%4
2772
February1st to 10thNight
69%5%8%12%5%1
2366
11th to 20thNight
63%6%10%15%6%1
2160
21st to endNight
64%5%11%14%6%
1776
March1st to 10thNight
48%5%11%24%11%1
2160
11th to 20thNight
52%410%20%11%2
1993
21st to endNight
50%5%10%19%13%2
2176
April1st to 10thNight
53%7%8%19%11%2
1800
11th to 20thNight
48%5%8%23%13%2
1633
21st to endNight
49%412%18%14%3
1620
May1st to 10thNight
49%7%10%19%13%3
1620
11th to 20thNight
46%6%9%21%16%2
1568
21st to endNight
35%6%14%26%16%4
1480
June1st to 10thNight
39%5%12%26%17%1
1260
11th to 20thNight
36%7%14%26%14%3
1260
21st to endNight
35%6%14%27%16%3
1260
July1st to 10thNight
27%5%14%31%20%2
1330
11th to 20thNight
32%7%14%29%18%1
1392
21st to endNight
32%8%16%26%17%1
1487
August1st to 10thNight
31%6%18%32%12%1
1567
11th to 20thNight
30%6%17%32%14%1
1598
21st to endNight
38%6%15%26%14%1
1810
September1st to 10thNight
35%6%13%27%17%2
1844
11th to 20thNight
37%6%15%32%10%1
1980
21st to endNight
45%6%16%25%8%
1980
October1st to 10thNight
46%5%12%25%12%1
2011
11th to 20thNight
57%6%12%18%6%1
2212
21st to endNight
61%6%10%17%6%
2574
November1st to 10thNight
55%6%12%20%7%1
2406
11th to 20thNight
61%6%12%15%6%
2486
21st to endNight
66%7%13%11%3
2660
December1st to 10thNight
70%7%9%11%2
2835
11th to 20thNight
72%8%9%8%3
3525
21st to endNight
83%5%5%5%3
5175

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%21%40%12%3
270
11th to 20thDay
24%23%38%11%41
245
21st to endDay
31%19%35%9%5%1
264
February1st to 10thDay
24%32%35%6%3
240
11th to 20thDay
25%38%29%8%2
240
21st to endDay
33%29%33%32
198
March1st to 10thDay
27%33%29%10%2
240
11th to 20thDay
24%20%36%14%5%1
406
21st to endDay
32%17%27%18%6%1
462
April1st to 10thDay
30%21%29%15%5%
420
11th to 20thDay
33%17%26%14%8%2
412
21st to endDay
27%17%33%14%6%3
420
May1st to 10thDay
26%21%29%14%6%4
423
11th to 20thDay
18%21%32%19%7%4
469
21st to endDay
15%16%34%24%9%2
679
June1st to 10thDay
16%20%36%21%7%2
630
11th to 20thDay
17%16%36%23%7%2
630
21st to endDay
17%13%38%23%8%2
630
July1st to 10thDay
12%13%35%26%12%4
621
11th to 20thDay
15%13%32%26%10%4
618
21st to endDay
13%12%35%25%10%4
684
August1st to 10thDay
14%8%28%28%14%8%
479
11th to 20thDay
14%8%23%29%15%12%
439
21st to endDay
18%11%27%26%13%4
495
September1st to 10thDay
17%12%28%23%14%6%
450
11th to 20thDay
16%10%28%28%13%6%
450
21st to endDay
22%12%23%25%12%5%
450
October1st to 10thDay
26%13%28%18%13%2
450
11th to 20thDay
29%21%32%14%41
396
21st to endDay
32%24%32%8%3
297
November1st to 10thDay
27%23%32%14%3
270
11th to 20thDay
25%28%31%12%31
261
21st to endDay
25%29%31%13%11
180
December1st to 10thDay
28%23%33%10%6%
180
11th to 20thDay
26%28%36%8%2
180
21st to endDay
31%31%24%10%21
213
January1st to 10thNight
23%22%33%18%5%
950
11th to 20thNight
26%20%29%19%6%1
910
21st to endNight
19%16%35%23%5%1
990
February1st to 10thNight
19%24%35%18%3
746
11th to 20thNight
21%23%32%19%5%
703
21st to endNight
27%18%33%20%2
592
March1st to 10thNight
19%21%32%23%5%1
720
11th to 20thNight
23%20%33%19%6%
720
21st to endNight
22%14%32%26%6%1
790
April1st to 10thNight
25%15%34%21%41
540
11th to 20thNight
26%13%30%24%6%1
533
21st to endNight
23%12%31%27%7%1
540
May1st to 10thNight
16%12%36%27%8%1
540
11th to 20thNight
19%12%32%29%8%
531
21st to endNight
12%9%33%33%13%1
512
June1st to 10thNight
14%11%33%32%10%1
360
11th to 20thNight
10%9%32%38%11%
360
21st to endNight
14%8%28%38%11%1
360
July1st to 10thNight
7%429%43%16%1
455
11th to 20thNight
10%7%26%33%23%2
522
21st to endNight
9%6%25%42%17%1
591
August1st to 10thNight
11%427%37%21%
537
11th to 20thNight
12%5%20%35%24%4
534
21st to endNight
15%7%24%35%17%2
594
September1st to 10thNight
13%8%27%29%19%4
584
11th to 20thNight
17%8%25%31%16%3
720
21st to endNight
21%8%25%28%14%3
720
October1st to 10thNight
22%11%27%27%11%1
720
11th to 20thNight
27%16%32%18%7%
720
21st to endNight
32%14%30%20%41
792
November1st to 10thNight
25%16%33%20%6%1
740
11th to 20thNight
24%19%34%19%5%
760
21st to endNight
28%16%40%14%3
760
December1st to 10thNight
23%20%34%17%5%
935
11th to 20thNight
23%24%32%17%31
950
21st to endNight
29%18%33%16%3
1045
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