Stull Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stull 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
25%
22981
12%
74784
7%
22928
31%
22928
13%
55319

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
8%
2649
4%
757
14%
757
4%
2174
January11th to 20thNight
12%
727
6%
2435
4%
727
16%
727
6%
2007
January21st to endNight
10%
792
9%
2574
4%
792
17%
792
5%
2135
February1st to 10thNight
8%
720
6%
2340
2%
720
14%
720
6%
1909
February11th to 20thNight
7%
703
7%
2275
1%
700
15%
700
6%
1827
February21st to endNight
10%
592
10%
1812
2%
592
18%
592
9%
1433
March1st to 10thNight
12%
720
13%
2017
5%
720
21%
720
10%
1591
March11th to 20thNight
17%
720
13%
1980
5%
720
23%
720
12%
1527
March21st to endNight
17%
792
15%
2152
5%
792
25%
792
15%
1634
April1st to 10thNight
18%
720
12%
1800
3%
720
24%
720
16%
1379
April11th to 20thNight
21%
711
13%
1775
5%
711
25%
711
19%
1314
April21st to endNight
26%
629
14%
1709
6%
629
32%
629
19%
1251
May1st to 10thNight
33%
511
13%
1591
8%
511
38%
511
16%
1167
May11th to 20thNight
35%
354
16%
1416
14%
354
43%
354
16%
947
May21st to endNight
41%
389
15%
1551
16%
388
48%
388
20%
942
June1st to 10thNight
36%
360
15%
1440
13%
360
47%
360
18%
906
June11th to 20thNight
44%
360
13%
1440
15%
360
51%
360
19%
892
June21st to endNight
44%
360
15%
1440
14%
360
52%
360
22%
848
July1st to 10thNight
53%
350
14%
1400
18%
350
57%
350
23%
832
July11th to 20thNight
52%
348
19%
1392
23%
348
59%
348
25%
803
July21st to endNight
50%
520
16%
1605
11%
491
54%
491
26%
1009
August1st to 10thNight
55%
537
15%
1600
11%
534
58%
534
29%
993
August11th to 20thNight
49%
534
16%
1598
11%
534
52%
534
26%
1040
August21st to endNight
50%
770
18%
1958
10%
770
52%
770
29%
1334
September1st to 10thNight
44%
720
17%
1800
8%
720
48%
720
30%
1204
September11th to 20thNight
50%
720
24%
1800
15%
720
54%
720
31%
1201
September21st to endNight
38%
720
18%
2079
13%
720
42%
720
22%
1414
October1st to 10thNight
34%
720
14%
2160
10%
720
40%
720
21%
1491
October11th to 20thNight
23%
720
12%
2160
5%
720
32%
720
16%
1553
October21st to endNight
15%
792
13%
2376
5%
792
25%
792
12%
1840
November1st to 10thNight
24%
740
16%
2379
11%
740
33%
740
17%
1646
November11th to 20thNight
15%
760
12%
2651
7%
760
23%
760
10%
1971
November21st to endNight
15%
760
9%
2660
5%
760
21%
760
8%
2067
December1st to 10thNight
10%
760
5%
2660
3%
760
12%
760
5%
2250
December11th to 20thNight
10%
760
6%
3253
2%
759
14%
759
4%
2642
December21st to endNight
8%
832
3%
4857
2%
817
13%
817
2%
4146

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%12%5%9%7%45%5%5%6%28%
74675
Night
14%12%5%9%7%45%45%6%28%
75025
TransparencyDay
55%7%11%17%9%1
74388
Night
55%7%12%17%8%1
74846
SeeingDay
24%20%29%17%8%3
13503
Night
23%19%28%20%9%1
22987

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
10%18%9%15%9%5%6%445%16%
1517
11th to 20thDay
12%19%9%14%10%45%44416%
1552
21st to endDay
8%16%9%16%12%5%6%5%5%5%13%
1782
February1st to 10thDay
14%17%9%12%12%46%43416%
1620
11th to 20thDay
11%13%9%16%9%46%445%20%
1573
21st to endDay
16%15%5%11%8%4345%5%22%
1444
March1st to 10thDay
13%14%7%14%8%33445%25%
1936
11th to 20thDay
14%15%7%8%7%35%45%427%
1980
21st to endDay
18%14%5%9%7%44446%26%
2202
April1st to 10thDay
13%17%5%8%5%2435%6%29%
2160
11th to 20thDay
18%12%5%5%6%45%45%6%32%
2134
21st to endDay
20%12%47%5%35%445%31%
2266
May1st to 10thDay
17%12%48%6%35%5%5%6%29%
2366
11th to 20thDay
19%9%47%7%345%5%7%31%
2478
21st to endDay
12%8%36%6%446%6%7%37%
2725
June1st to 10thDay
16%7%36%6%36%5%5%7%35%
2520
11th to 20thDay
15%6%37%7%35%5%5%9%34%
2520
21st to endDay
13%7%37%5%46%5%5%9%36%
2520
July1st to 10thDay
10%6%46%7%47%6%7%9%35%
2520
11th to 20thDay
11%7%36%6%46%7%7%9%34%
2520
21st to endDay
12%6%37%8%47%6%7%8%33%
2614
August1st to 10thDay
11%6%36%6%5%6%6%7%9%35%
2327
11th to 20thDay
13%7%38%7%5%6%7%7%9%29%
2303
21st to endDay
14%7%36%8%46%6%6%7%32%
2379
September1st to 10thDay
19%7%26%4445%6%7%36%
2160
11th to 20thDay
16%7%36%5%3445%7%41%
2158
21st to endDay
19%8%37%6%45%46%7%33%
1880
October1st to 10thDay
18%9%5%6%6%3445%8%32%
1800
11th to 20thDay
18%13%48%7%345%46%28%
1800
21st to endDay
22%18%5%7%5%3344424%
1980
November1st to 10thDay
12%15%5%8%6%3445%6%32%
1696
11th to 20thDay
14%19%7%9%5%345%5%6%23%
1525
21st to endDay
15%19%8%10%7%35%445%20%
1520
December1st to 10thDay
14%20%12%14%7%45%43413%
1520
11th to 20thDay
14%18%8%14%8%245%5%5%17%
1906
21st to endDay
18%24%10%13%8%3432412%
2772
January1st to 10thNight
11%15%8%15%11%5%6%446%15%
2649
11th to 20thNight
13%16%8%14%11%46%45%6%16%
2435
21st to endNight
9%14%8%14%14%6%7%5%4415%
2574
February1st to 10thNight
13%14%9%12%12%5%6%445%16%
2340
11th to 20thNight
11%13%8%14%10%46%6%45%18%
2273
21st to endNight
14%14%7%14%9%345%5%421%
1812
March1st to 10thNight
10%15%8%13%7%5%5%45%5%24%
2007
11th to 20thNight
15%14%6%10%8%34345%28%
1979
21st to endNight
14%14%6%9%7%34446%28%
2152
April1st to 10thNight
15%13%6%9%5%45%45%7%28%
1800
11th to 20thNight
17%13%46%6%34437%33%
1775
21st to endNight
16%11%48%6%4445%6%33%
1709
May1st to 10thNight
17%12%36%6%36%5%5%6%31%
1591
11th to 20thNight
13%9%25%6%45%6%6%7%35%
1415
21st to endNight
12%7%35%43445%8%44%
1550
June1st to 10thNight
16%5%25%7%45%5%7%9%36%
1440
11th to 20thNight
12%8%36%425%6%5%8%40%
1440
21st to endNight
10%6%35%5%345%6%7%45%
1440
July1st to 10thNight
7%6%47%6%45%46%9%43%
1440
11th to 20thNight
9%4345%36%5%7%7%47%
1440
21st to endNight
11%5%35%6%35%5%8%9%41%
1700
August1st to 10thNight
9%7%25%5%36%6%7%9%43%
1611
11th to 20thNight
12%8%36%43447%9%40%
1598
21st to endNight
13%6%35%5%346%5%8%41%
1958
September1st to 10thNight
16%6%25%5%3445%8%44%
1800
11th to 20thNight
15%7%34335%447%45%
1800
21st to endNight
16%8%26%6%345%5%7%37%
2079
October1st to 10thNight
16%10%37%5%3445%7%37%
2160
11th to 20thNight
17%13%36%6%445%47%31%
2160
21st to endNight
26%13%46%5%34446%25%
2376
November1st to 10thNight
11%14%47%6%3445%6%36%
2379
11th to 20thNight
14%17%49%7%345%5%6%25%
2651
21st to endNight
13%17%7%11%8%35%5%46%21%
2660
December1st to 10thNight
15%20%10%13%8%35%5%45%13%
2660
11th to 20thNight
15%18%9%13%8%35%345%16%
3288
21st to endNight
19%18%10%11%8%45%5%5%411%
4884

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
69%5%10%11%5%
1518
11th to 20thDay
71%6%8%8%6%
1552
21st to endDay
72%5%9%8%5%1
1782
February1st to 10thDay
72%6%10%9%3
1620
11th to 20thDay
67%5%7%15%7%
1582
21st to endDay
61%6%10%16%5%2
1444
March1st to 10thDay
60%5%11%16%6%1
1939
11th to 20thDay
58%410%16%9%2
1980
21st to endDay
60%47%16%11%2
2202
April1st to 10thDay
56%5%10%17%11%1
2160
11th to 20thDay
52%6%10%17%13%1
2134
21st to endDay
54%6%10%17%13%2
2266
May1st to 10thDay
55%6%9%17%10%2
2366
11th to 20thDay
52%6%10%18%12%1
2482
21st to endDay
44%9%14%20%12%1
2725
June1st to 10thDay
47%7%12%19%14%1
2520
11th to 20thDay
46%8%13%22%11%1
2520
21st to endDay
45%7%13%23%12%
2520
July1st to 10thDay
43%9%12%24%11%1
2450
11th to 20thDay
42%9%16%21%11%
2445
21st to endDay
45%9%14%19%11%1
2468
August1st to 10thDay
43%9%15%23%10%
2314
11th to 20thDay
48%11%15%19%7%1
2303
21st to endDay
48%8%11%21%10%1
2379
September1st to 10thDay
45%9%10%23%11%2
2160
11th to 20thDay
41%7%11%25%15%1
2158
21st to endDay
50%6%10%21%12%1
1880
October1st to 10thDay
50%7%14%16%13%1
1800
11th to 20thDay
56%6%12%17%8%1
1800
21st to endDay
62%6%10%14%6%2
1980
November1st to 10thDay
51%7%12%16%12%3
1696
11th to 20thDay
59%7%11%16%5%2
1525
21st to endDay
64%8%10%13%5%1
1520
December1st to 10thDay
73%6%9%9%3
1520
11th to 20thDay
67%6%13%9%41
1906
21st to endDay
76%6%7%9%1
2772
January1st to 10thNight
69%6%9%10%6%
2649
11th to 20thNight
68%7%12%9%31
2435
21st to endNight
70%6%9%10%6%1
2574
February1st to 10thNight
68%5%12%10%5%
2340
11th to 20thNight
65%7%11%11%5%1
2275
21st to endNight
63%6%10%13%7%1
1812
March1st to 10thNight
61%6%10%17%5%1
2017
11th to 20thNight
59%6%10%16%9%1
1980
21st to endNight
56%6%9%14%12%2
2152
April1st to 10thNight
55%6%11%17%10%1
1800
11th to 20thNight
52%5%11%17%13%1
1775
21st to endNight
51%6%11%17%13%2
1709
May1st to 10thNight
52%8%8%20%10%2
1591
11th to 20thNight
46%9%11%21%12%1
1416
21st to endNight
38%9%15%22%14%2
1551
June1st to 10thNight
43%7%13%24%13%1
1440
11th to 20thNight
40%9%15%22%12%2
1440
21st to endNight
37%8%15%26%13%1
1440
July1st to 10thNight
36%9%14%24%16%2
1400
11th to 20thNight
33%9%14%27%15%1
1392
21st to endNight
37%9%15%23%14%1
1605
August1st to 10thNight
34%9%19%27%10%1
1600
11th to 20thNight
38%9%18%27%8%
1598
21st to endNight
38%9%14%25%12%2
1958
September1st to 10thNight
38%7%16%25%11%3
1800
11th to 20thNight
38%8%12%25%16%2
1800
21st to endNight
44%6%15%21%13%1
2079
October1st to 10thNight
47%7%14%20%13%1
2160
11th to 20thNight
51%7%15%17%10%1
2160
21st to endNight
59%6%11%15%7%2
2376
November1st to 10thNight
48%7%13%19%12%2
2379
11th to 20thNight
58%6%12%15%7%2
2651
21st to endNight
61%7%13%14%4
2660
December1st to 10thNight
71%8%10%9%3
2660
11th to 20thNight
69%6%10%12%3
3288
21st to endNight
74%7%9%9%1
4884

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
32%31%23%11%4
180
11th to 20thDay
30%36%27%5%1
208
21st to endDay
25%36%30%8%2
264
February1st to 10thDay
39%34%23%4
240
11th to 20thDay
35%37%27%1
240
21st to endDay
39%30%24%6%
310
March1st to 10thDay
33%28%26%10%2
420
11th to 20thDay
28%30%29%9%4
420
21st to endDay
36%23%25%10%41
462
April1st to 10thDay
33%22%27%12%5%1
420
11th to 20thDay
28%26%26%11%7%2
412
21st to endDay
30%22%30%12%42
420
May1st to 10thDay
29%21%28%13%6%3
423
11th to 20thDay
24%20%30%17%6%2
447
21st to endDay
16%16%30%23%9%5%
485
June1st to 10thDay
18%19%31%23%7%2
450
11th to 20thDay
19%14%32%21%13%2
450
21st to endDay
13%16%39%23%6%3
450
July1st to 10thDay
11%10%30%30%13%5%
445
11th to 20thDay
11%10%32%30%12%5%
444
21st to endDay
13%10%38%27%9%3
488
August1st to 10thDay
11%9%29%28%16%7%
446
11th to 20thDay
14%9%32%26%13%7%
439
21st to endDay
14%12%31%22%13%8%
495
September1st to 10thDay
18%12%26%22%14%7%
450
11th to 20thDay
14%11%26%21%19%8%
450
21st to endDay
21%16%29%17%13%4
450
October1st to 10thDay
22%16%28%18%14%2
450
11th to 20thDay
26%25%28%14%6%1
450
21st to endDay
33%24%23%17%2
495
November1st to 10thDay
27%19%29%15%8%2
377
11th to 20thDay
35%25%25%12%2
185
21st to endDay
33%23%37%6%1
180
December1st to 10thDay
38%26%27%441
180
11th to 20thDay
42%23%27%8%1
180
21st to endDay
34%33%28%41
198
January1st to 10thNight
26%29%31%12%2
760
11th to 20thNight
27%28%29%13%2
727
21st to endNight
22%32%30%15%1
792
February1st to 10thNight
28%30%29%12%1
720
11th to 20thNight
30%28%31%10%1
703
21st to endNight
31%28%29%11%2
592
March1st to 10thNight
25%28%31%13%2
720
11th to 20thNight
29%25%26%16%41
720
21st to endNight
24%24%31%15%5%1
792
April1st to 10thNight
26%21%32%19%2
720
11th to 20thNight
27%19%30%17%7%1
711
21st to endNight
22%14%33%26%5%
629
May1st to 10thNight
23%13%25%29%10%1
511
11th to 20thNight
18%12%31%30%9%
354
21st to endNight
14%10%28%31%16%1
389
June1st to 10thNight
16%9%32%29%14%1
360
11th to 20thNight
15%10%24%37%13%
360
21st to endNight
9%9%33%37%11%
360
July1st to 10thNight
8%7%23%42%18%3
350
11th to 20thNight
10%6%26%35%21%1
348
21st to endNight
13%5%26%35%19%2
520
August1st to 10thNight
10%6%24%37%22%2
537
11th to 20thNight
12%7%22%31%26%2
534
21st to endNight
14%7%23%32%22%2
770
September1st to 10thNight
18%8%25%29%17%3
720
11th to 20thNight
17%7%22%25%25%4
720
21st to endNight
18%13%25%24%16%4
720
October1st to 10thNight
21%13%27%22%16%2
720
11th to 20thNight
25%20%29%21%5%1
720
21st to endNight
33%19%29%16%3
792
November1st to 10thNight
20%21%32%20%7%
740
11th to 20thNight
28%23%29%14%42
760
21st to endNight
25%27%29%16%4
760
December1st to 10thNight
31%28%27%10%4
760
11th to 20thNight
28%29%29%12%2
760
21st to endNight
29%29%30%11%2
836
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