UM-Dearborn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UM-Dearborn Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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
35%
24317
14%
80751
10%
24212
41%
24212
18%
57103

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
17%
943
11%
4385
4%
926
25%
926
8%
3280
January11th to 20thNight
17%
941
6%
4414
3%
924
21%
924
6%
3625
January21st to endNight
21%
832
5%
4515
4%
814
27%
814
6%
3721
February1st to 10thNight
19%
760
8%
2470
3%
760
24%
760
10%
1922
February11th to 20thNight
20%
740
13%
2274
6%
739
27%
739
13%
1696
February21st to endNight
18%
611
13%
1831
4%
611
28%
611
15%
1344
March1st to 10thNight
27%
720
16%
2111
9%
720
33%
720
18%
1508
March11th to 20thNight
27%
720
19%
1980
9%
720
36%
720
22%
1362
March21st to endNight
29%
792
17%
2178
10%
792
36%
792
19%
1587
April1st to 10thNight
27%
636
16%
1896
8%
636
35%
636
20%
1296
April11th to 20thNight
30%
533
21%
1625
9%
533
42%
533
22%
1095
April21st to endNight
33%
540
18%
1620
7%
540
42%
540
23%
1093
May1st to 10thNight
34%
540
12%
1620
7%
540
38%
540
19%
1150
May11th to 20thNight
41%
531
17%
1592
11%
531
48%
531
23%
1078
May21st to endNight
52%
583
15%
1745
11%
582
58%
582
30%
1079
June1st to 10thNight
50%
491
17%
1430
12%
491
56%
491
33%
877
June11th to 20thNight
52%
360
20%
1260
21%
360
58%
360
27%
749
June21st to endNight
54%
528
20%
1428
14%
528
60%
528
34%
903
July1st to 10thNight
63%
525
17%
1400
14%
525
66%
525
38%
855
July11th to 20thNight
61%
522
21%
1392
15%
522
67%
522
39%
843
July21st to endNight
65%
591
19%
1654
14%
558
69%
558
37%
958
August1st to 10thNight
61%
537
21%
1600
16%
534
65%
534
36%
947
August11th to 20thNight
62%
534
23%
1598
18%
534
67%
534
34%
949
August21st to endNight
61%
594
23%
1795
17%
594
66%
594
35%
1038
September1st to 10thNight
56%
656
20%
1916
15%
656
60%
656
30%
1189
September11th to 20thNight
60%
720
28%
1980
21%
720
63%
720
37%
1182
September21st to endNight
50%
720
22%
1980
16%
720
55%
720
29%
1290
October1st to 10thNight
46%
720
21%
2034
17%
720
51%
720
29%
1297
October11th to 20thNight
30%
720
16%
2160
9%
720
39%
720
19%
1511
October21st to endNight
26%
792
16%
2505
11%
792
33%
792
16%
1771
November1st to 10thNight
35%
740
20%
2406
15%
740
43%
740
23%
1520
November11th to 20thNight
24%
760
17%
2470
10%
760
34%
760
17%
1714
November21st to endNight
22%
760
14%
2565
8%
760
29%
760
14%
1848
December1st to 10thNight
18%
760
8%
2660
5%
760
22%
760
9%
2093
December11th to 20thNight
17%
826
8%
3356
4%
824
23%
824
9%
2508
December21st to endNight
14%
1039
4%
4906
2%
1026
19%
1026
5%
4225

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%9%47%6%45%5%6%7%32%
78236
Night
16%10%47%6%35%5%5%6%33%
81064
TransparencyDay
49%7%12%19%11%1
77948
Night
50%7%12%19%11%1
80874
SeeingDay
18%12%29%23%13%5%
14361
Night
19%10%29%29%12%1
24341

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
16%16%7%10%7%45%5%5%5%21%
2527
11th to 20thDay
16%17%7%11%8%6%8%5%5%5%12%
2533
21st to endDay
27%18%8%10%6%3444313%
3124
February1st to 10thDay
15%13%5%10%9%5%5%6%5%5%21%
1710
11th to 20thDay
14%12%411%9%46%5%5%7%24%
1788
21st to endDay
17%13%6%10%6%3445%7%26%
1520
March1st to 10thDay
15%10%5%9%8%35%5%5%7%28%
1848
11th to 20thDay
15%11%5%8%6%3435%7%33%
1980
21st to endDay
19%12%5%8%7%45%45%6%26%
2178
April1st to 10thDay
15%13%5%6%5%35%6%5%7%30%
2078
11th to 20thDay
18%8%37%6%345%5%7%33%
2284
21st to endDay
19%9%36%6%345%5%6%34%
2340
May1st to 10thDay
18%8%47%6%45%5%5%7%32%
2340
11th to 20thDay
18%8%46%5%445%5%7%33%
2301
21st to endDay
13%5%345%346%7%8%41%
2531
June1st to 10thDay
11%6%26%7%45%5%6%8%41%
2512
11th to 20thDay
12%5%26%5%45%5%7%9%40%
2699
21st to endDay
9%6%36%6%36%6%7%11%38%
2520
July1st to 10thDay
7%5%25%6%46%6%6%9%45%
2520
11th to 20thDay
9%436%5%35%6%8%10%41%
2520
21st to endDay
8%5%35%6%46%6%8%9%41%
2565
August1st to 10thDay
8%5%25%6%46%7%7%10%39%
2327
11th to 20thDay
10%436%7%36%7%7%9%36%
2303
21st to endDay
10%5%25%6%36%6%7%8%42%
2558
September1st to 10thDay
11%7%36%6%46%5%6%9%38%
2026
11th to 20thDay
9%5%245%45%6%6%9%45%
1980
21st to endDay
14%6%36%435%5%7%9%38%
1980
October1st to 10thDay
12%7%26%5%445%6%7%40%
1926
11th to 20thDay
17%8%46%7%45%5%5%7%32%
1800
21st to endDay
20%10%37%6%4445%7%31%
1850
November1st to 10thDay
11%7%47%6%445%5%8%39%
1672
11th to 20thDay
14%11%47%6%45%46%7%32%
1710
21st to endDay
19%11%49%5%35%45%7%28%
1611
December1st to 10thDay
15%13%7%10%8%46%5%46%21%
1520
11th to 20thDay
17%15%6%10%6%35%46%6%22%
1804
21st to endDay
27%19%7%11%7%33445%10%
2751
January1st to 10thNight
17%16%6%9%7%34445%25%
4428
11th to 20thNight
15%18%8%12%7%35%7%5%5%15%
4434
21st to endNight
25%17%8%10%6%3433417%
4532
February1st to 10thNight
17%14%5%9%8%45%5%45%23%
2470
11th to 20thNight
15%12%6%9%8%35%5%5%5%27%
2274
21st to endNight
20%11%48%6%35%446%29%
1831
March1st to 10thNight
15%10%48%6%45%45%6%33%
2111
11th to 20thNight
14%9%36%5%35%5%7%7%37%
1980
21st to endNight
20%8%46%5%35%446%34%
2178
April1st to 10thNight
15%10%45%5%345%6%6%36%
1896
11th to 20thNight
18%8%35%5%345%6%7%37%
1625
21st to endNight
18%8%35%435%5%5%6%38%
1620
May1st to 10thNight
17%9%36%6%45%45%6%34%
1620
11th to 20thNight
18%5%36%5%35%45%6%40%
1592
21st to endNight
9%6%25%5%45%46%10%45%
1744
June1st to 10thNight
11%5%24425%5%6%9%47%
1430
11th to 20thNight
9%5%26%6%45%5%5%8%45%
1260
21st to endNight
9%5%26%435%46%8%48%
1428
July1st to 10thNight
7%424425%5%6%8%53%
1440
11th to 20thNight
8%3245%345%6%8%52%
1440
21st to endNight
5%325%435%6%6%9%51%
1751
August1st to 10thNight
7%435%5%3446%9%50%
1611
11th to 20thNight
7%5%245%36%5%5%8%51%
1598
21st to endNight
9%4244345%6%8%51%
1795
September1st to 10thNight
10%5%245%35%5%5%7%46%
1916
11th to 20thNight
9%4244245%5%6%53%
1980
21st to endNight
14%6%245%345%5%7%45%
1980
October1st to 10thNight
14%7%35%5%2445%7%44%
2034
11th to 20thNight
18%9%36%5%35%45%6%35%
2160
21st to endNight
21%8%36%6%35%45%6%32%
2505
November1st to 10thNight
14%7%35%5%345%5%6%44%
2406
11th to 20thNight
17%10%47%6%35%5%6%7%31%
2470
21st to endNight
21%12%47%5%3345%7%29%
2565
December1st to 10thNight
17%14%7%9%7%45%5%5%5%22%
2660
11th to 20thNight
15%14%5%10%7%46%45%6%24%
3388
21st to endNight
28%21%7%9%5%3344412%
4912

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
63%7%12%11%6%
2528
11th to 20thDay
72%7%11%7%3
2533
21st to endDay
76%5%9%6%31
3124
February1st to 10thDay
62%7%11%13%7%
1710
11th to 20thDay
59%7%9%16%7%1
1798
21st to endDay
58%6%11%16%7%1
1520
March1st to 10thDay
55%6%13%19%6%1
1848
11th to 20thDay
51%5%12%19%11%2
1980
21st to endDay
59%5%8%20%6%1
2178
April1st to 10thDay
51%7%11%18%12%
2078
11th to 20thDay
49%6%12%18%13%2
2284
21st to endDay
49%6%14%16%13%1
2340
May1st to 10thDay
50%8%10%20%11%2
2340
11th to 20thDay
49%7%11%18%13%2
2305
21st to endDay
39%6%14%24%14%2
2531
June1st to 10thDay
39%7%12%26%13%3
2512
11th to 20thDay
38%8%13%22%16%3
2699
21st to endDay
38%9%13%21%17%2
2520
July1st to 10thDay
34%7%11%25%19%4
2451
11th to 20thDay
34%7%15%24%17%2
2445
21st to endDay
34%8%15%23%17%3
2423
August1st to 10thDay
34%10%15%28%12%1
2314
11th to 20thDay
39%10%13%24%12%2
2303
21st to endDay
36%9%11%25%17%2
2558
September1st to 10thDay
41%8%16%20%13%2
2026
11th to 20thDay
32%9%13%32%13%1
1980
21st to endDay
40%8%14%25%13%
1980
October1st to 10thDay
40%8%12%25%13%2
1926
11th to 20thDay
49%6%14%21%10%
1800
21st to endDay
53%6%14%19%9%
1850
November1st to 10thDay
43%7%13%23%13%1
1672
11th to 20thDay
51%6%14%19%9%1
1710
21st to endDay
55%7%12%18%7%
1611
December1st to 10thDay
64%7%12%13%4
1520
11th to 20thDay
60%7%13%15%5%
1800
21st to endDay
78%5%9%5%3
2751
January1st to 10thNight
60%6%12%15%6%1
4428
11th to 20thNight
69%7%11%9%4
4434
21st to endNight
72%5%9%10%3
4532
February1st to 10thNight
62%6%12%13%6%1
2470
11th to 20thNight
57%5%10%18%9%1
2275
21st to endNight
55%5%12%18%8%2
1831
March1st to 10thNight
51%6%11%22%10%1
2111
11th to 20thNight
44%7%12%20%15%2
1980
21st to endNight
52%5%9%22%11%1
2178
April1st to 10thNight
47%6%11%22%14%2
1896
11th to 20thNight
45%7%10%20%17%2
1625
21st to endNight
46%5%12%19%15%3
1620
May1st to 10thNight
49%7%11%18%12%2
1620
11th to 20thNight
44%7%9%20%17%3
1593
21st to endNight
34%8%14%25%16%3
1745
June1st to 10thNight
34%6%13%26%17%3
1430
11th to 20thNight
36%7%13%26%17%2
1260
21st to endNight
33%8%11%27%21%1
1428
July1st to 10thNight
29%8%10%30%20%3
1400
11th to 20thNight
28%8%13%30%19%2
1392
21st to endNight
27%9%16%24%20%4
1654
August1st to 10thNight
30%8%14%29%17%2
1600
11th to 20thNight
30%8%14%31%13%3
1598
21st to endNight
30%7%14%28%19%2
1795
September1st to 10thNight
34%8%13%26%16%2
1916
11th to 20thNight
29%7%11%32%20%1
1980
21st to endNight
39%6%13%27%14%1
1980
October1st to 10thNight
39%5%12%25%18%1
2034
11th to 20thNight
48%7%12%19%14%1
2160
21st to endNight
52%6%12%21%9%1
2505
November1st to 10thNight
40%7%12%25%15%1
2406
11th to 20thNight
50%6%14%20%9%1
2470
21st to endNight
55%6%13%20%6%
2565
December1st to 10thNight
63%7%11%13%6%
2660
11th to 20thNight
58%7%14%14%6%1
3391
21st to endNight
76%5%10%7%2
4912

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%22%33%15%6%1
270
11th to 20thDay
24%19%37%16%31
270
21st to endDay
29%14%34%18%5%
297
February1st to 10thDay
23%21%37%16%31
270
11th to 20thDay
24%19%36%19%2
270
21st to endDay
25%26%38%8%3
210
March1st to 10thDay
26%20%31%18%4
288
11th to 20thDay
22%13%36%22%6%1
420
21st to endDay
27%16%31%19%7%1
462
April1st to 10thDay
19%18%31%22%8%1
420
11th to 20thDay
27%17%29%16%8%4
412
21st to endDay
28%14%30%18%8%3
420
May1st to 10thDay
25%15%30%19%8%4
423
11th to 20thDay
20%11%31%21%12%5%
447
21st to endDay
14%7%24%30%18%7%
485
June1st to 10thDay
13%9%31%33%12%3
586
11th to 20thDay
15%8%30%31%13%3
630
21st to endDay
12%9%32%29%14%4
630
July1st to 10thDay
8%5%24%31%22%9%
621
11th to 20thDay
11%6%27%31%18%7%
618
21st to endDay
8%427%27%24%8%
506
August1st to 10thDay
7%324%30%22%13%
446
11th to 20thDay
13%323%29%21%11%
439
21st to endDay
12%5%21%29%23%11%
495
September1st to 10thDay
13%6%23%27%20%11%
450
11th to 20thDay
9%5%19%27%28%12%
450
21st to endDay
16%12%24%22%18%8%
450
October1st to 10thDay
14%12%28%22%18%7%
450
11th to 20thDay
19%19%30%24%6%2
450
21st to endDay
27%17%35%16%41
365
November1st to 10thDay
15%20%33%21%9%1
270
11th to 20thDay
22%23%33%17%5%1
270
21st to endDay
27%19%34%14%6%
223
December1st to 10thDay
22%22%34%14%6%1
180
11th to 20thDay
26%24%36%10%31
180
21st to endDay
29%24%31%14%2
288
January1st to 10thNight
23%17%35%20%5%1
950
11th to 20thNight
27%14%32%22%41
944
21st to endNight
20%14%34%27%5%
836
February1st to 10thNight
25%12%37%23%3
760
11th to 20thNight
23%14%33%26%3
743
21st to endNight
25%15%37%21%2
611
March1st to 10thNight
19%13%34%28%6%
720
11th to 20thNight
19%15%36%25%5%1
720
21st to endNight
24%11%29%28%8%1
792
April1st to 10thNight
20%14%32%29%4
636
11th to 20thNight
21%10%35%25%8%1
533
21st to endNight
21%8%34%29%8%
540
May1st to 10thNight
19%9%30%31%10%1
540
11th to 20thNight
21%5%27%33%13%1
531
21st to endNight
11%325%39%20%2
584
June1st to 10thNight
13%327%45%13%
491
11th to 20thNight
10%5%25%48%13%
360
21st to endNight
10%5%23%41%20%1
528
July1st to 10thNight
8%319%39%27%4
525
11th to 20thNight
9%321%40%23%4
522
21st to endNight
6%220%46%24%2
591
August1st to 10thNight
8%318%40%27%3
537
11th to 20thNight
9%220%36%31%3
534
21st to endNight
10%319%36%30%3
594
September1st to 10thNight
13%6%18%33%24%5%
656
11th to 20thNight
10%419%34%29%4
720
21st to endNight
15%7%22%30%22%4
720
October1st to 10thNight
17%8%25%30%17%3
720
11th to 20thNight
23%13%29%24%10%1
720
21st to endNight
25%14%30%26%5%
792
November1st to 10thNight
17%12%31%26%12%1
740
11th to 20thNight
22%16%32%22%7%1
760
21st to endNight
27%13%33%20%6%
760
December1st to 10thNight
26%15%34%19%6%
760
11th to 20thNight
24%15%36%20%4
826
21st to endNight
31%12%33%20%3
1045
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