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 2017-12-20.

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%
16060
15%
48100
10%
16020
42%
16020
21%
33326

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
20%
825
8%
2553
3%
825
25%
825
10%
2024
February1st to 15thNight
19%
701
9%
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
38%
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
56%
447
30%
852
June16th to endNight
55%
472
18%
1372
14%
472
60%
472
33%
860
July1st to 15thNight
62%
510
17%
1360
13%
510
65%
510
39%
828
July16th to endNight
66%
564
19%
1525
15%
531
70%
531
39%
880
August1st to 15thNight
63%
539
20%
1609
15%
536
67%
536
35%
969
August16th to endNight
64%
573
23%
1728
18%
573
69%
573
36%
998
September1st to 15thNight
59%
678
24%
1938
18%
678
62%
678
34%
1162
September16th to endNight
58%
717
25%
1980
18%
717
62%
717
34%
1218
October1st to 15thNight
46%
720
18%
2076
16%
720
50%
720
27%
1346
October16th to endNight
26%
768
17%
2391
11%
768
35%
768
17%
1646
November1st to 15thNight
36%
760
20%
2471
15%
760
43%
760
22%
1609
November16th to endNight
24%
780
15%
2600
9%
780
31%
780
15%
1840
December1st to 15thNight
19%
780
8%
2730
4%
780
24%
780
10%
2093
December16th to endNight
14%
965
7%
2809
2%
965
19%
965
9%
2336

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%36%6%35%5%5%7%32%
48711
Night
15%8%36%5%3445%6%35%
48281
Cloud CoverDay
14%8%37%6%35%5%5%7%32%
48721
Night
15%8%36%5%3445%6%35%
48285
TransparencyDay
48%6%12%19%10%1
48410
Night
47%5%12%20%11%1
48100
SeeingDay
17%8%28%24%14%7%
6092
Night
18%8%28%29%12%1
16066

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%
2340
16th to endDay
10%425%5%36%6%7%9%38%
2472
September1st to 15thDay
11%6%25%5%45%5%5%8%39%
2010
16th to endDay
12%435%43446%9%41%
1980
October1st to 15thDay
14%7%25%5%33447%39%
1884
16th to endDay
20%8%35%6%33346%32%
1832
November1st to 15thDay
14%8%346%245%47%37%
1717
16th to endDay
17%11%48%5%35%446%28%
1687
December1st to 15thDay
16%11%8%10%7%44345%22%
1560
16th to endDay
22%16%7%11%6%3335%6%13%
1508
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
4415%5%36%46%8%48%
1620
16th to endNight
10%413424447%52%
1728
September1st to 15thNight
9%4233245%46%50%
1938
16th to endNight
11%41442345%6%49%
1980
October1st to 15thNight
15%6%25%42445%7%41%
2076
16th to endNight
19%7%25%5%3435%5%35%
2391
November1st to 15thNight
15%7%344345%47%39%
2471
16th to endNight
21%10%37%5%2345%7%29%
2600
December1st to 15thNight
19%13%5%8%5%34445%23%
2730
16th to endNight
22%13%7%9%6%34445%16%
2809

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%
2340
16th to endDay
10%425%6%35%6%7%8%39%
2472
September1st to 15thDay
11%6%25%5%45%5%5%8%39%
2010
16th to endDay
12%5%25%4245%6%8%41%
1980
October1st to 15thDay
14%7%25%5%3345%7%39%
1884
16th to endDay
19%8%36%6%3435%6%33%
1832
November1st to 15thDay
14%7%35%6%34447%37%
1717
16th to endDay
16%10%48%5%45%45%7%28%
1687
December1st to 15thDay
16%12%7%10%7%45%345%22%
1560
16th to endDay
21%17%6%11%7%2345%6%13%
1508
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%3245%35%46%8%49%
1620
16th to endNight
10%324324447%52%
1728
September1st to 15thNight
9%4234245%5%6%50%
1938
16th to endNight
11%42443345%6%49%
1980
October1st to 15thNight
14%6%25%42445%7%41%
2076
16th to endNight
19%7%35%5%3445%5%35%
2391
November1st to 15thNight
15%7%344345%5%6%39%
2471
16th to endNight
21%10%36%5%2335%7%29%
2600
December1st to 15thNight
19%12%5%8%6%35%445%24%
2730
16th to endNight
22%14%7%9%6%35%5%45%16%
2809

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%14%25%11%1
2327
16th to endDay
37%8%12%23%15%1
2472
September1st to 15thDay
39%7%14%25%12%
2010
16th to endDay
36%7%12%29%13%
1980
October1st to 15thDay
43%5%12%24%12%1
1884
16th to endDay
51%5%14%19%8%
1832
November1st to 15thDay
44%6%14%22%11%1
1717
16th to endDay
55%6%13%18%6%
1687
December1st to 15thDay
63%5%12%13%5%
1560
16th to endDay
71%6%11%7%2
1508
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%29%16%2
1609
16th to endNight
30%5%13%27%18%3
1728
September1st to 15thNight
31%7%11%28%18%1
1938
16th to endNight
34%5%11%30%16%1
1980
October1st to 15thNight
40%5%13%22%16%1
2076
16th to endNight
48%5%13%22%8%
2391
November1st to 15thNight
43%6%12%23%12%1
2471
16th to endNight
53%5%13%20%6%
2600
December1st to 15thNight
61%6%13%12%6%
2730
16th to endNight
68%5%11%10%3
2809

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%20%30%18%18%
313
16th to endDay
13%5%15%25%24%16%
332
September1st to 15thDay
11%419%23%26%14%
315
16th to endDay
14%5%16%21%26%15%
314
October1st to 15thDay
16%8%23%24%19%7%
315
16th to endDay
22%11%36%23%32
248
November1st to 15thDay
17%10%32%24%11%2
135
16th to endDay
22%21%31%18%5%
119
December1st to 15thDay
21%21%35%14%7%
90
16th to endDay
34%28%27%9%
92
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%118%40%27%4
539
16th to endNight
11%214%35%33%3
573
September1st to 15thNight
12%5%17%32%26%5%
678
16th to endNight
12%417%31%28%5%
717
October1st to 15thNight
19%7%21%29%18%3
720
16th to endNight
23%11%32%25%6%
768
November1st to 15thNight
19%8%30%27%12%1
760
16th to endNight
26%15%29%21%7%
780
December1st to 15thNight
27%13%33%19%5%
780
16th to endNight
28%12%36%18%3
965
Page generated at 2017-12-21 07:04:50. Version ac04

Copyright 2017 A. Danko


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