SCSU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SCSU Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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
36%
24998
15%
80043
9%
24905
43%
24905
22%
54109

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
29%
940
10%
4600
6%
926
37%
926
11%
3332
January11th to 20thNight
26%
945
8%
4320
6%
930
30%
930
9%
3276
January21st to endNight
24%
1032
9%
4483
4%
1023
31%
1023
10%
3334
February1st to 10thNight
29%
948
15%
2464
3%
948
37%
948
24%
1718
February11th to 20thNight
29%
879
18%
2273
5%
874
36%
874
25%
1543
February21st to endNight
30%
730
17%
1914
4%
730
38%
730
22%
1376
March1st to 10thNight
32%
563
13%
2003
8%
563
38%
563
18%
1332
March11th to 20thNight
31%
540
19%
1980
12%
540
42%
540
20%
1262
March21st to endNight
33%
594
15%
2178
12%
594
41%
594
16%
1500
April1st to 10thNight
28%
540
16%
1810
10%
540
38%
540
21%
1204
April11th to 20thNight
33%
533
19%
1702
15%
533
42%
533
20%
1173
April21st to endNight
33%
540
16%
1620
10%
540
41%
540
20%
1137
May1st to 10thNight
32%
540
15%
1548
8%
540
38%
540
25%
1048
May11th to 20thNight
38%
531
17%
1417
8%
531
43%
531
27%
961
May21st to endNight
46%
584
14%
1552
6%
582
50%
582
31%
998
June1st to 10thNight
51%
540
13%
1424
5%
540
54%
540
36%
901
June11th to 20thNight
47%
540
16%
1260
6%
540
50%
540
39%
825
June21st to endNight
56%
540
17%
1260
6%
540
59%
540
41%
805
July1st to 10thNight
65%
525
18%
1225
8%
525
67%
525
52%
710
July11th to 20thNight
61%
522
21%
1337
9%
522
64%
522
47%
759
July21st to endNight
65%
591
22%
1538
11%
558
67%
558
47%
837
August1st to 10thNight
62%
537
21%
1601
17%
534
65%
534
36%
938
August11th to 20thNight
61%
534
22%
1600
20%
534
67%
534
35%
930
August21st to endNight
57%
594
24%
1845
23%
594
64%
594
38%
1012
September1st to 10thNight
54%
540
21%
1800
21%
540
61%
540
36%
968
September11th to 20thNight
48%
540
24%
1872
19%
540
58%
540
33%
1035
September21st to endNight
39%
576
23%
2016
15%
576
51%
576
29%
1147
October1st to 10thNight
39%
720
21%
2160
13%
720
44%
720
29%
1322
October11th to 20thNight
31%
720
21%
2160
10%
720
39%
720
24%
1406
October21st to endNight
29%
792
18%
2376
9%
792
38%
792
20%
1625
November1st to 10thNight
34%
911
19%
2569
9%
911
42%
911
25%
1696
November11th to 20thNight
27%
950
16%
2660
6%
950
35%
950
18%
1931
November21st to endNight
27%
950
15%
2660
7%
950
34%
950
22%
1820
December1st to 10thNight
28%
950
10%
2660
4%
950
34%
950
18%
1950
December11th to 20thNight
22%
949
11%
3211
3%
949
29%
949
12%
2414
December21st to endNight
20%
1038
6%
4945
4%
1026
26%
1026
7%
3884

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
13%8%37%6%35%5%5%7%37%
78458
Night
12%9%47%6%35%5%5%7%39%
80512
TransparencyDay
45%7%14%23%10%1
78154
Night
45%7%13%23%11%1
80337
SeeingDay
16%10%31%27%14%3
14947
Night
15%9%33%32%11%1
25034

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
11%13%5%12%10%5%6%5%5%6%22%
2290
11th to 20thDay
6%8%5%14%11%6%8%7%7%6%22%
2594
21st to endDay
6%16%9%10%7%46%5%5%7%24%
3135
February1st to 10thDay
9%11%5%8%7%45%45%7%34%
1701
11th to 20thDay
7%9%5%10%9%46%36%8%33%
1574
21st to endDay
10%11%410%8%46%45%6%31%
1341
March1st to 10thDay
13%11%5%7%7%45%5%6%6%33%
1973
11th to 20thDay
14%9%38%435%5%5%7%37%
1980
21st to endDay
19%10%47%6%3445%7%30%
2178
April1st to 10thDay
17%9%47%6%2445%7%34%
2150
11th to 20thDay
18%12%37%6%35%446%32%
2205
21st to endDay
22%10%36%5%245%5%6%32%
2340
May1st to 10thDay
18%8%37%6%45%5%45%35%
2410
11th to 20thDay
16%7%36%5%35%5%5%8%37%
2479
21st to endDay
15%5%36%5%45%5%6%8%38%
2725
June1st to 10thDay
13%5%25%4345%6%8%46%
2530
11th to 20thDay
13%6%26%6%35%5%6%9%39%
2700
21st to endDay
9%426%5%46%5%7%11%41%
2700
July1st to 10thDay
6%3245%35%5%7%11%50%
2700
11th to 20thDay
9%4245%35%46%8%51%
2574
21st to endDay
6%3145%35%6%7%10%49%
2685
August1st to 10thDay
8%5%36%6%45%5%7%9%43%
2328
11th to 20thDay
13%5%245%45%45%9%45%
2301
21st to endDay
10%5%25%4245%5%8%52%
2510
September1st to 10thDay
10%425%5%3445%9%49%
2160
11th to 20thDay
12%5%245%35%6%5%8%45%
2084
21st to endDay
15%6%25%42345%8%44%
1926
October1st to 10thDay
19%7%25%5%2436%7%40%
1800
11th to 20thDay
14%9%36%5%4445%7%38%
1800
21st to endDay
17%9%47%5%24346%38%
1980
November1st to 10thDay
13%11%47%5%345%6%5%37%
1503
11th to 20thDay
10%12%7%10%8%35%45%6%30%
1520
21st to endDay
13%10%47%6%345%5%8%35%
1520
December1st to 10thDay
9%12%6%8%9%47%6%6%7%27%
1520
11th to 20thDay
10%13%7%10%8%4446%6%28%
1918
21st to endDay
23%14%5%14%6%3343420%
2624
January1st to 10thNight
10%12%6%13%8%3445%6%28%
4643
11th to 20thNight
6%11%5%10%10%5%10%8%6%7%22%
4374
21st to endNight
8%19%8%10%6%345%5%5%26%
4532
February1st to 10thNight
7%11%5%7%7%36%5%6%7%37%
2464
11th to 20thNight
6%8%5%8%6%35%46%8%41%
2273
21st to endNight
12%11%37%7%4446%7%34%
1914
March1st to 10thNight
12%10%47%5%445%48%36%
2003
11th to 20thNight
13%9%37%5%35%446%42%
1980
21st to endNight
17%9%47%6%3445%6%35%
2178
April1st to 10thNight
15%9%46%5%2445%7%39%
1810
11th to 20thNight
18%11%46%5%34445%37%
1702
21st to endNight
20%9%45%424446%37%
1620
May1st to 10thNight
16%7%36%5%2436%7%40%
1548
11th to 20thNight
14%6%35%5%3445%7%43%
1416
21st to endNight
13%6%2443446%9%45%
1551
June1st to 10thNight
10%5%245%25%6%5%10%47%
1424
11th to 20thNight
9%5%26%4346%6%9%47%
1260
21st to endNight
8%4245%3446%8%53%
1260
July1st to 10thNight
2313425%5%6%10%59%
1260
11th to 20thNight
7%41232436%8%59%
1381
21st to endNight
432333346%8%60%
1629
August1st to 10thNight
7%5%25%425%5%5%8%52%
1611
11th to 20thNight
12%424424446%55%
1600
21st to endNight
8%42432345%7%57%
1845
September1st to 10thNight
9%4135%3345%9%54%
1800
11th to 20thNight
10%5%25%42345%7%54%
1872
21st to endNight
13%5%2443446%7%50%
2016
October1st to 10thNight
15%6%2442345%7%47%
2160
11th to 20thNight
15%9%35%43445%7%41%
2160
21st to endNight
18%9%47%434447%38%
2376
November1st to 10thNight
12%8%36%5%35%5%6%6%41%
2569
11th to 20thNight
13%10%5%9%6%35%45%6%34%
2660
21st to endNight
12%8%47%6%345%5%7%40%
2660
December1st to 10thNight
10%10%5%9%8%5%5%5%5%7%31%
2660
11th to 20thNight
10%12%5%10%8%45%5%5%6%28%
3279
21st to endNight
21%15%5%11%7%34345%21%
5022

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
60%8%12%16%4
2291
11th to 20thDay
58%7%12%18%5%
2594
21st to endDay
59%8%13%15%5%
3135
February1st to 10thDay
50%5%13%22%8%1
1701
11th to 20thDay
50%6%13%22%7%1
1583
21st to endDay
55%5%10%18%11%1
1341
March1st to 10thDay
52%7%14%19%8%1
1973
11th to 20thDay
46%7%15%23%8%1
1980
21st to endDay
53%7%12%21%7%
2178
April1st to 10thDay
49%6%11%20%12%1
2150
11th to 20thDay
54%6%9%18%11%1
2205
21st to endDay
52%7%12%19%9%1
2340
May1st to 10thDay
50%7%13%20%9%1
2410
11th to 20thDay
45%7%13%23%12%1
2483
21st to endDay
43%8%14%23%11%2
2725
June1st to 10thDay
36%8%15%29%11%1
2530
11th to 20thDay
41%9%15%20%14%1
2700
21st to endDay
34%9%19%27%9%2
2700
July1st to 10thDay
25%9%19%30%16%1
2626
11th to 20thDay
30%8%16%27%17%3
2496
21st to endDay
27%10%18%28%16%2
2536
August1st to 10thDay
35%9%16%26%10%3
2314
11th to 20thDay
36%8%16%29%11%
2301
21st to endDay
30%6%16%31%16%2
2510
September1st to 10thDay
31%7%16%34%12%
2160
11th to 20thDay
36%9%16%30%8%
2084
21st to endDay
38%7%13%33%8%1
1926
October1st to 10thDay
42%7%13%26%12%
1800
11th to 20thDay
45%6%14%23%10%2
1800
21st to endDay
48%5%12%25%9%
1980
November1st to 10thDay
47%6%13%19%12%2
1503
11th to 20thDay
54%7%12%17%10%
1520
21st to endDay
47%7%15%22%6%2
1520
December1st to 10thDay
56%7%15%17%5%
1520
11th to 20thDay
55%7%16%17%4
1915
21st to endDay
68%5%9%12%6%
2624
January1st to 10thNight
55%7%12%18%6%
4643
11th to 20thNight
58%9%13%14%5%
4374
21st to endNight
59%6%10%19%5%1
4532
February1st to 10thNight
45%6%14%23%10%1
2464
11th to 20thNight
41%7%16%24%11%2
2274
21st to endNight
48%6%11%20%13%1
1914
March1st to 10thNight
46%7%17%22%8%
2003
11th to 20thNight
43%6%13%23%14%1
1980
21st to endNight
50%7%11%23%8%2
2178
April1st to 10thNight
45%6%11%23%14%1
1810
11th to 20thNight
49%5%7%21%15%2
1702
21st to endNight
48%7%10%20%13%2
1620
May1st to 10thNight
43%7%13%22%15%
1548
11th to 20thNight
41%7%13%23%15%2
1417
21st to endNight
36%7%15%23%16%2
1552
June1st to 10thNight
31%9%17%27%13%2
1424
11th to 20thNight
31%8%16%26%16%3
1260
21st to endNight
28%8%16%27%17%3
1260
July1st to 10thNight
19%8%19%34%18%1
1225
11th to 20thNight
23%7%15%34%17%4
1337
21st to endNight
21%7%16%34%17%5%
1538
August1st to 10thNight
28%9%15%31%13%5%
1601
11th to 20thNight
30%7%17%30%15%1
1600
21st to endNight
25%8%14%31%18%3
1845
September1st to 10thNight
26%8%17%33%16%1
1800
11th to 20thNight
30%7%17%30%17%
1872
21st to endNight
33%6%14%32%14%
2016
October1st to 10thNight
36%6%12%30%15%2
2160
11th to 20thNight
42%6%13%26%12%1
2160
21st to endNight
47%6%13%23%10%1
2376
November1st to 10thNight
41%7%15%24%12%2
2569
11th to 20thNight
51%7%12%19%11%
2660
21st to endNight
44%7%18%23%7%1
2660
December1st to 10thNight
53%6%16%19%6%
2660
11th to 20thNight
55%9%12%17%5%2
3281
21st to endNight
65%6%10%15%4
5022

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
13%13%31%29%13%1
270
11th to 20thDay
14%11%37%32%6%
270
21st to endDay
12%8%40%24%13%2
297
February1st to 10thDay
13%17%40%25%4
267
11th to 20thDay
14%20%38%22%5%1
240
21st to endDay
17%12%40%28%3
198
March1st to 10thDay
18%13%38%23%6%2
240
11th to 20thDay
21%9%35%22%11%2
240
21st to endDay
24%13%27%23%11%2
264
April1st to 10thDay
26%14%31%21%8%
410
11th to 20thDay
27%13%27%24%9%1
483
21st to endDay
23%12%32%20%10%2
600
May1st to 10thDay
21%12%32%25%8%2
603
11th to 20thDay
20%10%30%25%13%2
625
21st to endDay
19%7%29%29%14%3
679
June1st to 10thDay
11%7%28%31%20%4
630
11th to 20thDay
14%7%35%30%12%1
630
21st to endDay
12%6%32%30%16%5%
630
July1st to 10thDay
7%423%33%23%9%
621
11th to 20thDay
9%7%28%30%20%7%
618
21st to endDay
7%5%25%35%23%6%
684
August1st to 10thDay
8%424%33%25%5%
626
11th to 20thDay
14%5%23%29%23%6%
615
21st to endDay
11%6%24%32%21%5%
629
September1st to 10thDay
12%6%30%32%16%4
450
11th to 20thDay
11%430%31%20%4
374
21st to endDay
20%12%31%21%13%3
270
October1st to 10thDay
20%10%31%21%11%6%
270
11th to 20thDay
14%20%33%23%8%1
270
21st to endDay
21%17%38%16%7%1
297
November1st to 10thDay
21%14%32%25%7%2
270
11th to 20thDay
14%19%43%21%2
270
21st to endDay
19%14%43%19%6%
270
December1st to 10thDay
14%12%41%23%10%
270
11th to 20thDay
18%15%39%24%3
270
21st to endDay
29%12%37%18%31
297
January1st to 10thNight
10%12%37%30%9%1
950
11th to 20thNight
16%10%36%30%7%1
950
21st to endNight
16%11%36%29%8%
1045
February1st to 10thNight
12%10%40%30%8%1
948
11th to 20thNight
11%12%41%28%7%1
879
21st to endNight
17%11%33%29%9%1
730
March1st to 10thNight
17%10%36%28%9%1
563
11th to 20thNight
18%9%38%28%6%1
540
21st to endNight
21%9%32%26%10%2
594
April1st to 10thNight
19%12%36%27%6%
540
11th to 20thNight
23%12%29%28%8%
533
21st to endNight
22%9%30%29%9%2
540
May1st to 10thNight
17%9%36%29%8%1
540
11th to 20thNight
15%9%31%33%11%
531
21st to endNight
14%8%27%37%14%
584
June1st to 10thNight
11%5%27%40%16%
540
11th to 20thNight
10%5%33%43%9%
540
21st to endNight
9%5%24%44%16%1
540
July1st to 10thNight
3224%44%26%1
525
11th to 20thNight
8%324%49%15%1
522
21st to endNight
5%324%45%21%2
591
August1st to 10thNight
8%220%47%22%1
537
11th to 20thNight
12%319%40%25%2
534
21st to endNight
8%5%25%36%24%2
594
September1st to 10thNight
11%426%39%18%2
540
11th to 20thNight
12%6%30%32%19%1
540
21st to endNight
13%9%35%28%13%1
576
October1st to 10thNight
17%9%31%30%12%1
720
11th to 20thNight
19%10%35%28%7%1
720
21st to endNight
23%11%32%27%7%
792
November1st to 10thNight
16%11%33%32%8%1
911
11th to 20thNight
17%14%35%26%7%1
950
21st to endNight
16%13%38%25%8%
950
December1st to 10thNight
15%12%35%30%8%1
950
11th to 20thNight
19%9%37%29%6%1
950
21st to endNight
25%10%33%26%5%1
1045
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