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 2005-11-05 to 2017-08-12.

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
36%
15556
15%
46547
10%
15516
42%
15516
20%
32332

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 15thNight
16%
898
9%
2506
4%
895
21%
895
13%
1986
January16th to endNight
19%
825
8%
2553
3%
825
25%
825
10%
2024
February1st to 15thNight
19%
701
8%
2266
2%
701
26%
701
11%
1758
February16th to endNight
24%
631
13%
1893
6%
631
31%
631
15%
1396
March1st to 15thNight
27%
720
15%
2067
9%
720
33%
720
17%
1513
March16th to endNight
32%
768
18%
2112
10%
768
39%
768
21%
1485
April1st to 15thNight
29%
602
14%
1818
7%
602
36%
602
19%
1267
April16th to endNight
35%
540
20%
1620
8%
540
45%
540
24%
1069
May1st to 15thNight
37%
540
14%
1620
8%
540
43%
540
21%
1117
May16th to endNight
48%
562
16%
1684
11%
561
54%
561
28%
1074
June1st to 15thNight
51%
447
16%
1372
14%
447
57%
447
30%
852
June16th to endNight
55%
472
18%
1372
14%
472
61%
472
33%
860
July1st to 15thNight
62%
510
17%
1360
13%
510
65%
510
39%
828
July16th to endNight
67%
564
19%
1525
15%
531
71%
531
39%
880
August1st to 15thNight
62%
527
20%
1575
15%
524
66%
524
35%
950
August16th to endNight
65%
525
22%
1583
17%
525
69%
525
36%
915
September1st to 15thNight
59%
621
25%
1776
19%
621
62%
621
34%
1063
September16th to endNight
57%
657
23%
1815
17%
657
61%
657
33%
1127
October1st to 15thNight
47%
660
19%
1903
17%
660
51%
660
28%
1220
October16th to endNight
25%
704
15%
2192
9%
704
34%
704
17%
1523
November1st to 15thNight
35%
700
21%
2276
16%
700
43%
700
23%
1465
November16th to endNight
25%
720
14%
2400
9%
720
31%
720
14%
1726
December1st to 15thNight
18%
720
8%
2520
4%
720
23%
720
10%
1951
December16th to endNight
14%
942
7%
2739
2%
942
19%
942
9%
2283

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
14%8%46%6%35%5%5%7%32%
47397
Night
15%8%36%5%3445%6%35%
46728
Cloud CoverDay
14%8%37%6%35%5%5%7%32%
47407
Night
15%8%36%5%3445%6%35%
46732
TransparencyDay
49%6%12%19%10%1
47096
Night
47%6%12%20%11%1
46547
SeeingDay
17%8%27%24%14%7%
5653
Night
19%9%28%29%12%1
15562

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 15thDay
19%13%49%6%5%6%5%45%17%
1432
16th to endDay
15%17%8%9%8%45%5%4415%
1670
February1st to 15thDay
15%14%5%10%8%46%445%19%
1593
16th to endDay
14%12%6%12%6%35%45%7%21%
1580
March1st to 15thDay
18%10%5%8%6%3445%6%26%
1881
16th to endDay
18%10%5%8%6%33446%28%
2112
April1st to 15thDay
19%10%45%4345%46%30%
2139
16th to endDay
17%8%35%5%24347%37%
2340
May1st to 15thDay
19%8%5%5%5%3445%6%31%
2331
16th to endDay
13%5%25%5%345%5%8%40%
2431
June1st to 15thDay
10%6%25%6%45%5%5%7%40%
2576
16th to endDay
10%6%25%6%45%6%7%11%33%
2579
July1st to 15thDay
7%5%346%35%5%6%7%44%
2520
16th to endDay
8%425%5%36%6%7%10%38%
2537
August1st to 15thDay
8%5%26%6%35%7%7%8%37%
2288
16th to endDay
10%425%5%36%6%7%9%38%
2265
September1st to 15thDay
12%6%344345%5%8%40%
1843
16th to endDay
12%435%5%25%46%9%39%
1815
October1st to 15thDay
14%7%25%5%335%47%40%
1727
16th to endDay
21%8%25%6%33346%32%
1680
November1st to 15thDay
14%7%346%245%47%38%
1582
16th to endDay
17%11%48%5%35%45%6%27%
1557
December1st to 15thDay
17%11%8%10%6%45%45%5%21%
1440
16th to endDay
21%16%7%11%7%3335%6%13%
1479
January1st to 15thNight
21%14%5%7%6%34445%23%
2506
16th to endNight
14%13%8%10%7%35%445%20%
2553
February1st to 15thNight
16%13%49%7%35%44425%
2264
16th to endNight
17%10%5%7%6%25%445%28%
1893
March1st to 15thNight
18%9%47%5%3445%5%31%
2055
16th to endNight
15%7%36%425%45%7%35%
2112
April1st to 15thNight
18%9%35%5%3345%6%34%
1818
16th to endNight
15%7%24425%5%6%7%38%
1620
May1st to 15thNight
17%6%26%5%3446%6%35%
1620
16th to endNight
12%5%35%433448%44%
1682
June1st to 15thNight
9%5%24425%448%46%
1372
16th to endNight
8%4245%25%5%7%8%45%
1372
July1st to 15thNight
7%31442445%7%53%
1440
16th to endNight
4314415%5%6%9%53%
1631
August1st to 15thNight
4425%5%35%46%8%48%
1586
16th to endNight
10%423324447%52%
1583
September1st to 15thNight
10%423324447%51%
1776
16th to endNight
12%425%423446%48%
1815
October1st to 15thNight
15%6%25%41445%7%42%
1903
16th to endNight
19%8%25%5%3435%5%34%
2192
November1st to 15thNight
15%6%344345%46%40%
2276
16th to endNight
22%10%37%5%23447%28%
2400
December1st to 15thNight
20%13%5%9%5%35%445%23%
2520
16th to endNight
23%13%7%10%6%34445%16%
2739

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 15thDay
20%13%49%7%5%6%45%5%17%
1432
16th to endDay
15%17%7%9%8%45%5%5%416%
1670
February1st to 15thDay
15%14%5%9%8%45%6%46%19%
1593
16th to endDay
14%12%6%11%7%25%447%21%
1580
March1st to 15thDay
18%11%5%8%7%34345%26%
1892
16th to endDay
18%9%5%8%6%34346%28%
2111
April1st to 15thDay
19%10%45%5%35%5%47%30%
2139
16th to endDay
17%8%36%424446%37%
2340
May1st to 15thDay
19%8%46%5%35%45%6%31%
2331
16th to endDay
13%5%35%5%3445%7%40%
2431
June1st to 15thDay
11%5%25%6%35%5%5%7%40%
2576
16th to endDay
10%6%36%6%35%6%7%11%33%
2579
July1st to 15thDay
7%5%26%5%36%5%6%7%44%
2520
16th to endDay
8%425%6%36%5%8%10%37%
2537
August1st to 15thDay
8%5%27%6%36%6%6%9%36%
2288
16th to endDay
10%425%5%35%6%7%8%39%
2265
September1st to 15thDay
12%6%25%435%5%5%7%40%
1843
16th to endDay
12%5%25%4245%6%8%40%
1815
October1st to 15thDay
13%7%25%5%3345%7%39%
1727
16th to endDay
20%8%36%6%3435%5%32%
1680
November1st to 15thDay
13%6%35%5%34447%39%
1582
16th to endDay
17%10%48%5%45%45%7%27%
1557
December1st to 15thDay
17%12%7%10%7%45%445%20%
1440
16th to endDay
21%17%6%11%7%24446%13%
1479
January1st to 15thNight
21%14%47%7%34445%23%
2506
16th to endNight
14%13%7%10%7%35%445%21%
2553
February1st to 15thNight
15%13%5%9%7%35%44425%
2257
16th to endNight
16%11%5%8%6%35%445%28%
1893
March1st to 15thNight
18%9%47%5%3435%5%31%
2067
16th to endNight
15%7%35%5%35%5%5%6%36%
2112
April1st to 15thNight
18%9%445%245%5%7%33%
1818
16th to endNight
15%6%35%4245%5%7%38%
1620
May1st to 15thNight
17%6%26%5%3446%7%36%
1619
16th to endNight
12%5%35%434348%43%
1682
June1st to 15thNight
9%5%24435%46%8%46%
1372
16th to endNight
8%5%14435%5%6%8%46%
1372
July1st to 15thNight
7%32442446%7%53%
1440
16th to endNight
43144245%5%9%52%
1631
August1st to 15thNight
5%4245%35%46%8%49%
1586
16th to endNight
11%324323347%52%
1583
September1st to 15thNight
10%42342445%6%51%
1776
16th to endNight
11%42443345%6%48%
1815
October1st to 15thNight
14%6%25%41445%7%42%
1903
16th to endNight
20%7%26%5%3445%6%34%
2192
November1st to 15thNight
14%6%344345%5%6%40%
2276
16th to endNight
22%10%46%42335%7%28%
2400
December1st to 15thNight
20%12%5%8%6%35%445%23%
2520
16th to endNight
22%14%7%9%6%35%5%45%16%
2739

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 15thDay
67%6%12%9%4
1433
16th to endDay
69%6%10%8%4
1670
February1st to 15thDay
64%5%10%13%5%
1595
16th to endDay
61%6%10%15%4
1580
March1st to 15thDay
59%5%12%16%5%1
1892
16th to endDay
55%5%10%21%6%
2112
April1st to 15thDay
53%5%10%18%11%
2139
16th to endDay
46%5%13%16%14%2
2340
May1st to 15thDay
52%6%9%18%10%2
2331
16th to endDay
41%6%12%24%12%2
2435
June1st to 15thDay
40%7%12%24%13%1
2576
16th to endDay
41%9%13%18%15%1
2579
July1st to 15thDay
36%6%11%24%18%2
2371
16th to endDay
36%8%16%22%14%2
2380
August1st to 15thDay
39%8%13%24%12%1
2275
16th to endDay
37%8%12%23%14%2
2265
September1st to 15thDay
40%7%13%24%13%
1843
16th to endDay
37%7%13%29%12%
1815
October1st to 15thDay
42%6%13%24%12%1
1727
16th to endDay
52%5%14%19%7%
1680
November1st to 15thDay
43%5%14%22%12%1
1582
16th to endDay
56%6%13%18%5%
1557
December1st to 15thDay
64%5%12%12%4
1440
16th to endDay
71%6%11%8%2
1479
January1st to 15thNight
62%5%12%14%6%
2506
16th to endNight
64%5%13%12%4
2553
February1st to 15thNight
61%5%10%15%6%
2266
16th to endNight
55%5%12%18%6%
1893
March1st to 15thNight
53%5%10%19%10%1
2067
16th to endNight
47%6%12%23%10%1
2112
April1st to 15thNight
50%410%20%13%
1818
16th to endNight
43%5%11%18%16%4
1620
May1st to 15thNight
46%6%12%19%12%2
1620
16th to endNight
38%6%11%24%15%3
1684
June1st to 15thNight
35%7%12%27%15%1
1372
16th to endNight
34%7%13%25%18%1
1372
July1st to 15thNight
30%6%9%29%21%2
1360
16th to endNight
26%8%15%25%20%3
1525
August1st to 15thNight
30%8%13%28%16%2
1575
16th to endNight
30%5%14%27%18%3
1583
September1st to 15thNight
31%7%12%28%19%1
1776
16th to endNight
35%5%11%30%15%1
1815
October1st to 15thNight
39%5%13%22%17%1
1903
16th to endNight
49%5%14%22%7%
2192
November1st to 15thNight
42%6%12%23%13%1
2276
16th to endNight
54%5%13%20%6%
2400
December1st to 15thNight
61%6%13%12%5%
2520
16th to endNight
69%5%11%9%3
2739

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 15thDay
21%21%32%18%5%1
90
16th to endDay
34%9%38%14%3
96
February1st to 15thDay
22%22%38%14%2
90
16th to endDay
17%18%40%19%3
81
March1st to 15thDay
31%14%25%20%7%
182
16th to endDay
21%10%32%22%10%2
288
April1st to 15thDay
22%12%28%25%8%1
270
16th to endDay
25%7%32%15%10%8%
270
May1st to 15thDay
25%7%32%19%9%5%
288
16th to endDay
16%8%27%23%15%7%
331
June1st to 15thDay
12%6%32%31%14%3
466
16th to endDay
14%8%33%26%12%5%
495
July1st to 15thDay
9%430%29%18%7%
475
16th to endDay
9%423%27%24%11%
397
August1st to 15thDay
10%119%30%18%19%
297
16th to endDay
13%5%13%24%25%18%
268
September1st to 15thDay
13%416%22%27%14%
255
16th to endDay
14%5%17%23%27%11%
254
October1st to 15thDay
15%7%23%26%20%7%
255
16th to endDay
24%9%38%21%41
192
November1st to 15thDay
11%13%30%30%12%3
90
16th to endDay
24%22%26%18%7%
79
December1st to 15thDay
23%20%35%16%5%
60
16th to endDay
32%28%28%10%
84
January1st to 15thNight
26%14%35%18%4
900
16th to endNight
20%13%34%26%4
825
February1st to 15thNight
23%12%36%24%2
704
16th to endNight
22%11%32%29%3
631
March1st to 15thNight
22%12%31%27%6%
720
16th to endNight
18%11%31%28%9%1
768
April1st to 15thNight
22%12%30%28%5%
602
16th to endNight
18%6%35%28%9%1
540
May1st to 15thNight
19%7%28%32%10%1
540
16th to endNight
14%326%35%18%1
563
June1st to 15thNight
10%227%47%11%
447
16th to endNight
9%225%42%18%1
472
July1st to 15thNight
9%118%42%24%3
510
16th to endNight
5%218%45%24%3
564
August1st to 15thNight
7%119%39%28%4
527
16th to endNight
11%213%35%33%3
525
September1st to 15thNight
13%5%17%32%26%5%
621
16th to endNight
13%418%32%26%4
657
October1st to 15thNight
18%7%22%29%18%3
660
16th to endNight
23%11%33%25%5%
704
November1st to 15thNight
18%9%31%26%13%1
700
16th to endNight
27%15%26%21%7%
720
December1st to 15thNight
28%13%33%18%5%
720
16th to endNight
29%12%35%18%4
942
Page generated at 2017-08-13 14:16:22. Version ac04

Copyright 2017 A. Danko


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