Ann Arbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ann Arbor clear sky chart from 2005-11-05 to 2017-09-10.

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
37%
15840
15%
46884
10%
15798
43%
15798
21%
32595

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%
900
10%
2506
4%
895
22%
895
13%
1973
January16th to endNight
19%
855
9%
2583
3%
855
24%
855
10%
2063
February1st to 15thNight
18%
701
9%
2250
3%
701
25%
701
10%
1729
February16th to endNight
25%
631
14%
1893
6%
631
33%
631
15%
1399
March1st to 15thNight
26%
720
14%
2082
8%
720
31%
720
17%
1517
March16th to endNight
32%
768
18%
2112
10%
768
39%
768
21%
1495
April1st to 15thNight
30%
581
14%
1812
7%
581
38%
581
19%
1272
April16th to endNight
36%
540
19%
1620
8%
540
45%
540
24%
1085
May1st to 15thNight
37%
540
14%
1620
7%
540
43%
540
21%
1125
May16th to endNight
49%
562
15%
1681
10%
561
55%
561
28%
1068
June1st to 15thNight
57%
540
15%
1440
11%
540
61%
540
33%
931
June16th to endNight
59%
540
17%
1440
13%
540
64%
540
34%
922
July1st to 15thNight
65%
510
17%
1360
14%
510
68%
510
40%
803
July16th to endNight
69%
564
19%
1503
16%
531
72%
531
39%
878
August1st to 15thNight
68%
539
20%
1609
16%
536
71%
536
34%
971
August16th to endNight
66%
573
22%
1751
19%
573
69%
573
34%
1031
September1st to 15thNight
60%
643
26%
1868
20%
643
64%
643
36%
1089
September16th to endNight
58%
657
24%
1815
18%
657
62%
657
33%
1126
October1st to 15thNight
46%
660
21%
1889
17%
660
50%
660
28%
1232
October16th to endNight
27%
704
15%
2208
9%
704
36%
704
18%
1537
November1st to 15thNight
36%
700
21%
2276
16%
700
44%
700
24%
1449
November16th to endNight
24%
720
14%
2340
9%
720
30%
720
14%
1706
December1st to 15thNight
18%
732
8%
2530
4%
732
24%
732
10%
1940
December16th to endNight
15%
960
8%
2696
1%
960
21%
960
9%
2254

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%33%
47725
Night
15%8%36%5%34446%36%
47062
Cloud CoverDay
14%8%36%6%35%5%5%7%33%
47735
Night
15%8%36%5%3445%6%36%
47066
TransparencyDay
48%7%11%19%11%1
47421
Night
47%6%11%20%12%1
46884
SeeingDay
16%7%26%24%17%7%
5812
Night
18%7%28%29%13%1
15844

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
18%14%5%9%7%45%436%18%
1432
16th to endDay
17%15%8%9%8%4445%5%15%
1638
February1st to 15thDay
14%14%6%11%7%45%45%5%20%
1611
16th to endDay
14%13%6%9%7%35%5%5%6%21%
1580
March1st to 15thDay
17%11%5%7%6%44447%27%
1867
16th to endDay
18%9%5%7%6%24446%30%
2112
April1st to 15thDay
18%10%35%5%35%45%6%30%
2145
16th to endDay
17%7%36%423447%38%
2340
May1st to 15thDay
18%7%35%6%35%5%47%31%
2331
16th to endDay
13%6%25%5%3445%7%40%
2431
June1st to 15thDay
10%5%25%6%35%5%7%9%38%
2520
16th to endDay
9%5%25%6%35%6%8%9%35%
2520
July1st to 15thDay
7%5%15%5%35%5%6%8%45%
2520
16th to endDay
8%435%5%37%5%6%10%39%
2560
August1st to 15thDay
9%436%6%45%5%5%8%40%
2340
16th to endDay
10%425%5%45%6%7%9%37%
2450
September1st to 15thDay
13%5%25%5%35%449%40%
1966
16th to endDay
12%5%25%5%3446%9%40%
1815
October1st to 15thDay
14%5%26%5%3445%7%39%
1739
16th to endDay
20%8%36%5%34446%31%
1662
November1st to 15thDay
14%7%25%6%24446%38%
1582
16th to endDay
18%10%5%6%6%35%46%5%27%
1617
December1st to 15thDay
16%12%6%11%7%345%5%5%21%
1425
16th to endDay
21%15%7%11%7%33346%14%
1522
January1st to 15thNight
22%13%5%7%6%33445%23%
2506
16th to endNight
14%12%8%10%9%345%45%21%
2583
February1st to 15thNight
15%13%6%8%7%34345%26%
2248
16th to endNight
16%9%5%9%6%35%345%30%
1893
March1st to 15thNight
18%8%46%5%34446%31%
2070
16th to endNight
15%8%45%5%3445%7%35%
2112
April1st to 15thNight
18%8%345%35%35%7%34%
1812
16th to endNight
16%6%25%425%45%6%39%
1620
May1st to 15thNight
18%6%35%5%35%447%34%
1620
16th to endNight
13%424425%348%44%
1679
June1st to 15thNight
9%425%436%5%48%45%
1440
16th to endNight
8%415%4345%7%9%44%
1440
July1st to 15thNight
7%31442345%8%53%
1440
16th to endNight
4415%4245%5%10%52%
1606
August1st to 15thNight
5%3235%35%6%6%8%48%
1620
16th to endNight
9%5%14434346%51%
1751
September1st to 15thNight
11%41242345%7%51%
1868
16th to endNight
10%5%2442445%7%46%
1815
October1st to 15thNight
13%7%35%414446%43%
1889
16th to endNight
20%7%35%5%34445%34%
2208
November1st to 15thNight
14%7%25%425%45%6%42%
2276
16th to endNight
21%11%45%5%24446%29%
2340
December1st to 15thNight
21%12%5%7%6%35%346%23%
2530
16th to endNight
23%14%6%9%6%35%5%45%16%
2696

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
18%14%5%9%7%5%5%5%36%18%
1432
16th to endDay
16%16%7%10%8%45%45%5%16%
1638
February1st to 15thDay
14%13%6%11%7%45%45%6%20%
1611
16th to endDay
14%13%5%9%7%35%5%5%6%22%
1580
March1st to 15thDay
17%10%5%7%6%34347%26%
1878
16th to endDay
18%9%47%6%2445%6%29%
2111
April1st to 15thDay
18%10%35%5%3445%6%31%
2145
16th to endDay
16%8%36%433347%37%
2340
May1st to 15thDay
19%7%36%6%35%447%30%
2331
16th to endDay
13%6%25%5%3445%7%40%
2431
June1st to 15thDay
10%5%25%5%345%6%9%38%
2520
16th to endDay
9%5%25%5%36%6%7%10%35%
2520
July1st to 15thDay
7%5%25%5%35%5%6%8%45%
2520
16th to endDay
7%5%25%6%37%6%6%10%39%
2560
August1st to 15thDay
8%5%26%6%46%46%8%40%
2340
16th to endDay
10%5%245%45%6%7%9%38%
2450
September1st to 15thDay
13%5%25%5%35%45%9%40%
1966
16th to endDay
12%5%25%5%345%5%9%40%
1815
October1st to 15thDay
14%5%26%5%35%5%5%7%39%
1739
16th to endDay
20%8%36%5%34446%31%
1662
November1st to 15thDay
14%7%35%6%34446%38%
1582
16th to endDay
18%10%47%7%35%45%5%27%
1617
December1st to 15thDay
16%12%6%11%6%35%5%46%21%
1425
16th to endDay
21%15%7%11%7%34346%14%
1522
January1st to 15thNight
22%13%48%6%33445%23%
2506
16th to endNight
14%12%8%10%8%445%45%21%
2583
February1st to 15thNight
14%13%6%8%6%34345%27%
2241
16th to endNight
16%9%5%8%6%35%445%30%
1893
March1st to 15thNight
19%8%47%5%34446%31%
2082
16th to endNight
15%8%45%5%3445%7%36%
2112
April1st to 15thNight
18%8%245%35%45%6%34%
1812
16th to endNight
15%6%25%5%25%45%6%39%
1620
May1st to 15thNight
18%6%25%5%3445%7%34%
1619
16th to endNight
13%5%25%425%448%44%
1679
June1st to 15thNight
9%425%435%5%5%9%44%
1440
16th to endNight
8%415%43447%9%44%
1440
July1st to 15thNight
7%31442345%8%54%
1440
16th to endNight
43244245%5%9%52%
1606
August1st to 15thNight
5%3235%35%5%7%8%48%
1620
16th to endNight
9%4145%23347%51%
1751
September1st to 15thNight
11%41342345%7%51%
1868
16th to endNight
10%5%25%42445%7%46%
1815
October1st to 15thNight
13%6%35%42345%5%43%
1889
16th to endNight
20%7%36%434446%34%
2208
November1st to 15thNight
14%6%34425%446%42%
2276
16th to endNight
21%11%45%5%2435%6%29%
2340
December1st to 15thNight
21%12%5%7%6%25%336%24%
2530
16th to endNight
23%13%6%9%6%35%45%5%16%
2696

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
65%7%10%11%4
1433
16th to endDay
68%7%9%10%4
1638
February1st to 15thDay
63%7%7%14%6%
1613
16th to endDay
60%7%11%15%5%
1580
March1st to 15thDay
56%6%11%17%6%1
1878
16th to endDay
54%5%10%20%7%1
2112
April1st to 15thDay
53%5%10%18%11%
2145
16th to endDay
46%5%13%18%14%2
2340
May1st to 15thDay
51%6%10%18%10%1
2331
16th to endDay
42%6%12%24%12%2
2435
June1st to 15thDay
39%7%12%23%14%1
2520
16th to endDay
39%10%13%19%16%1
2520
July1st to 15thDay
35%7%10%23%20%2
2371
16th to endDay
36%9%13%21%16%2
2400
August1st to 15thDay
40%7%12%24%14%1
2327
16th to endDay
39%8%11%21%16%2
2450
September1st to 15thDay
40%6%13%24%14%1
1966
16th to endDay
38%6%10%30%14%
1815
October1st to 15thDay
42%6%11%24%13%1
1739
16th to endDay
52%5%14%18%8%
1662
November1st to 15thDay
45%5%11%22%13%1
1582
16th to endDay
57%5%11%18%5%
1617
December1st to 15thDay
62%6%11%13%4
1425
16th to endDay
71%6%10%8%3
1522
January1st to 15thNight
62%410%15%6%
2506
16th to endNight
63%6%12%12%4
2583
February1st to 15thNight
59%6%10%15%5%1
2250
16th to endNight
56%410%19%7%
1893
March1st to 15thNight
52%6%10%18%10%1
2082
16th to endNight
47%6%11%22%10%1
2112
April1st to 15thNight
48%6%9%20%14%
1812
16th to endNight
43%5%12%17%17%3
1620
May1st to 15thNight
48%6%10%20%12%2
1620
16th to endNight
38%6%12%24%14%3
1681
June1st to 15thNight
35%8%11%28%15%1
1440
16th to endNight
33%8%14%24%18%1
1440
July1st to 15thNight
28%6%11%28%21%2
1360
16th to endNight
26%8%14%25%21%3
1503
August1st to 15thNight
30%9%12%29%15%2
1609
16th to endNight
32%5%11%26%19%3
1751
September1st to 15thNight
31%5%10%30%21%1
1868
16th to endNight
35%5%10%29%18%
1815
October1st to 15thNight
40%5%11%23%18%1
1889
16th to endNight
49%5%12%22%8%
2208
November1st to 15thNight
41%6%12%23%14%1
2276
16th to endNight
55%5%12%18%7%
2340
December1st to 15thNight
62%412%13%5%
2530
16th to endNight
67%6%9%10%4
2696

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
17%13%40%21%6%1
90
16th to endDay
32%8%41%13%4
96
February1st to 15thDay
21%18%47%10%2
90
16th to endDay
20%14%35%24%3
81
March1st to 15thDay
31%12%29%19%6%
168
16th to endDay
22%9%28%27%10%2
288
April1st to 15thDay
22%10%32%23%10%
270
16th to endDay
20%11%28%20%11%7%
270
May1st to 15thDay
21%7%30%20%13%5%
288
16th to endDay
16%9%24%24%15%9%
331
June1st to 15thDay
11%5%28%32%16%4
495
16th to endDay
12%6%31%28%14%6%
495
July1st to 15thDay
11%322%27%24%9%
475
16th to endDay
8%320%27%26%13%
420
August1st to 15thDay
10%115%30%23%18%
313
16th to endDay
14%115%22%28%16%
332
September1st to 15thDay
11%421%20%24%18%
294
16th to endDay
16%414%21%29%12%
254
October1st to 15thDay
17%423%28%18%8%
255
16th to endDay
24%8%34%24%8%
174
November1st to 15thDay
7%15%25%28%18%3
90
16th to endDay
21%15%38%17%6%
89
December1st to 15thDay
18%23%35%16%5%1
60
16th to endDay
29%21%40%8%
94
January1st to 15thNight
29%11%34%17%5%
900
16th to endNight
18%12%36%26%6%
855
February1st to 15thNight
22%13%35%24%4
704
16th to endNight
20%10%34%30%4
631
March1st to 15thNight
23%11%31%26%7%
720
16th to endNight
17%9%32%29%9%1
768
April1st to 15thNight
21%12%30%29%5%
581
16th to endNight
21%6%32%29%9%1
540
May1st to 15thNight
21%429%32%10%1
540
16th to endNight
15%126%33%20%2
563
June1st to 15thNight
11%123%47%16%
540
16th to endNight
9%222%41%23%1
540
July1st to 15thNight
9%115%41%28%3
510
16th to endNight
5%16%46%27%2
564
August1st to 15thNight
6%17%45%25%5%
539
16th to endNight
11%112%34%35%4
573
September1st to 15thNight
12%418%30%28%5%
643
16th to endNight
13%315%32%28%5%
657
October1st to 15thNight
16%6%22%32%18%3
660
16th to endNight
23%10%34%25%6%
704
November1st to 15thNight
18%9%30%26%14%1
700
16th to endNight
27%12%30%21%7%
720
December1st to 15thNight
26%12%34%21%4
732
16th to endNight
29%10%35%20%3
960
Page generated at 2017-09-11 19:35:22. Version ac04

Copyright 2017 A. Danko


Ann Arbor Clear Sky Chart | Charts Home