Grand Rapids
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Rapids 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
35%
15866
13%
46452
9%
15824
40%
15824
19%
33459

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
6%
2506
3%
895
18%
895
8%
2144
January16th to endNight
19%
960
5%
2688
3%
960
21%
960
7%
2341
February1st to 15thNight
18%
719
8%
2208
4%
719
22%
719
9%
1785
February16th to endNight
22%
631
10%
1893
4%
631
28%
631
12%
1482
March1st to 15thNight
30%
720
14%
2139
10%
720
34%
720
19%
1521
March16th to endNight
32%
726
18%
2070
11%
726
39%
726
22%
1428
April1st to 15thNight
29%
539
12%
1797
9%
539
35%
539
18%
1269
April16th to endNight
35%
540
20%
1644
11%
540
43%
540
24%
1065
May1st to 15thNight
36%
540
14%
1620
8%
540
41%
540
20%
1127
May16th to endNight
44%
562
14%
1551
9%
561
49%
561
26%
1049
June1st to 15thNight
49%
540
14%
1440
9%
540
53%
540
32%
926
June16th to endNight
52%
540
14%
1440
8%
540
55%
540
31%
921
July1st to 15thNight
63%
510
16%
1360
12%
510
65%
510
41%
800
July16th to endNight
67%
564
20%
1417
16%
531
69%
531
39%
850
August1st to 15thNight
64%
527
22%
1559
17%
524
68%
524
37%
882
August16th to endNight
64%
525
20%
1674
19%
525
67%
525
32%
984
September1st to 15thNight
61%
555
25%
1710
22%
555
65%
555
34%
997
September16th to endNight
56%
657
21%
1815
16%
657
59%
657
33%
1120
October1st to 15thNight
45%
660
21%
1842
16%
660
49%
660
28%
1207
October16th to endNight
27%
704
13%
2288
8%
704
34%
704
15%
1642
November1st to 15thNight
34%
700
17%
2276
15%
700
39%
700
20%
1558
November16th to endNight
25%
720
10%
2340
7%
720
30%
720
11%
1830
December1st to 15thNight
15%
867
4%
2487
1%
867
17%
867
6%
2190
December16th to endNight
13%
960
6%
2688
1%
960
17%
960
7%
2341

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%9%46%5%3445%7%33%
47530
Night
15%10%47%5%34446%33%
46620
Cloud CoverDay
14%9%46%6%35%45%7%33%
47539
Night
15%9%47%5%34446%33%
46625
TransparencyDay
49%6%12%19%10%1
47216
Night
51%6%11%18%10%1
46452
SeeingDay
17%10%27%24%13%6%
5833
Night
19%8%28%29%11%1
15870

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
17%14%8%11%9%5%6%445%12%
1432
16th to endDay
14%20%11%11%10%4433311%
1536
February1st to 15thDay
12%14%8%14%10%45%43416%
1650
16th to endDay
16%12%7%10%7%35%5%5%5%19%
1580
March1st to 15thDay
15%11%37%6%36%45%7%28%
1811
16th to endDay
17%9%46%5%24346%34%
2166
April1st to 15thDay
21%12%36%42235%6%31%
2151
16th to endDay
17%7%245%245%5%7%37%
2316
May1st to 15thDay
20%6%35%43445%7%35%
2331
16th to endDay
10%6%35%5%3445%8%42%
2561
June1st to 15thDay
11%5%36%436%447%41%
2520
16th to endDay
11%425%5%25%5%5%9%41%
2520
July1st to 15thDay
7%324435%5%7%9%46%
2520
16th to endDay
9%324425%5%6%9%45%
2655
August1st to 15thDay
9%5%25%435%5%6%8%42%
2306
16th to endDay
13%425%43446%9%42%
2173
September1st to 15thDay
12%5%25%5%35%46%7%42%
1909
16th to endDay
12%5%26%5%3446%9%40%
1815
October1st to 15thDay
18%6%27%5%34345%38%
1786
16th to endDay
25%8%47%6%44346%25%
1584
November1st to 15thDay
15%11%25%5%25%445%36%
1582
16th to endDay
20%16%48%5%34446%20%
1620
December1st to 15thDay
16%19%10%12%7%3442413%
1470
16th to endDay
21%21%8%11%5%3433412%
1536
January1st to 15thNight
20%15%7%11%9%3432416%
2506
16th to endNight
13%15%10%12%9%44445%14%
2688
February1st to 15thNight
13%17%8%9%8%35%345%21%
2206
16th to endNight
15%14%410%6%35%5%45%23%
1893
March1st to 15thNight
15%11%36%5%34446%34%
2126
16th to endNight
15%7%36%5%24447%36%
2070
April1st to 15thNight
19%8%35%5%25%436%34%
1797
16th to endNight
17%6%3432345%6%41%
1644
May1st to 15thNight
17%7%25%5%445%5%5%36%
1620
16th to endNight
11%6%45%435%45%7%41%
1549
June1st to 15thNight
10%425%5%335%6%8%45%
1440
16th to endNight
8%4245%36%5%7%10%42%
1440
July1st to 15thNight
6%32442446%8%53%
1440
16th to endNight
6%325%42346%8%52%
1513
August1st to 15thNight
6%325%33346%7%52%
1568
16th to endNight
10%42442345%6%50%
1674
September1st to 15thNight
12%313323446%53%
1710
16th to endNight
10%5%24424347%48%
1815
October1st to 15thNight
18%5%2442335%6%43%
1842
16th to endNight
24%8%25%5%23345%33%
2288
November1st to 15thNight
17%10%35%42346%5%35%
2276
16th to endNight
20%13%5%8%6%34346%22%
2340
December1st to 15thNight
21%19%8%10%7%3433413%
2487
16th to endNight
22%18%8%8%7%3433413%
2688

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
17%14%8%10%10%5%6%445%12%
1432
16th to endDay
14%19%11%12%10%4443310%
1536
February1st to 15thDay
12%15%7%13%11%37%32416%
1650
16th to endDay
15%13%7%11%7%45%45%6%19%
1580
March1st to 15thDay
15%10%47%7%35%5%48%27%
1821
16th to endDay
17%9%5%6%5%24446%33%
2165
April1st to 15thDay
21%12%35%323346%31%
2151
16th to endDay
16%8%2443445%6%37%
2316
May1st to 15thDay
20%7%25%42445%7%34%
2331
16th to endDay
10%6%35%5%3445%8%41%
2561
June1st to 15thDay
10%6%26%5%36%5%47%40%
2520
16th to endDay
10%5%25%5%35%5%5%9%40%
2520
July1st to 15thDay
6%4244345%6%10%45%
2520
16th to endDay
9%324425%5%7%10%43%
2655
August1st to 15thDay
9%5%25%5%35%5%7%8%40%
2306
16th to endDay
12%6%245%3446%9%41%
2173
September1st to 15thDay
12%5%25%435%5%5%6%42%
1909
16th to endDay
11%6%25%43445%9%40%
1815
October1st to 15thDay
17%7%36%6%23445%38%
1786
16th to endDay
23%9%37%7%44336%26%
1584
November1st to 15thDay
15%11%25%5%24347%36%
1582
16th to endDay
20%16%5%8%435%446%21%
1620
December1st to 15thDay
16%19%8%12%7%45%42413%
1470
16th to endDay
20%22%8%10%6%34345%11%
1536
January1st to 15thNight
20%14%7%12%8%3433416%
2506
16th to endNight
13%15%9%12%9%5%5%445%14%
2688
February1st to 15thNight
13%16%7%9%8%35%43421%
2199
16th to endNight
15%14%49%6%45%5%45%24%
1893
March1st to 15thNight
16%10%45%5%3445%6%33%
2139
16th to endNight
15%7%36%5%25%45%6%37%
2070
April1st to 15thNight
19%8%36%434435%34%
1797
16th to endNight
17%6%3432445%5%42%
1644
May1st to 15thNight
17%6%36%5%345%46%36%
1619
16th to endNight
11%6%36%425%5%5%7%41%
1549
June1st to 15thNight
9%425%6%235%5%9%45%
1440
16th to endNight
7%424436%6%6%10%42%
1440
July1st to 15thNight
6%31442335%8%53%
1440
16th to endNight
6%31442446%7%53%
1513
August1st to 15thNight
6%32442437%7%52%
1568
16th to endNight
10%5%2443345%6%50%
1674
September1st to 15thNight
12%3143245%47%53%
1710
16th to endNight
9%6%2443435%7%49%
1815
October1st to 15thNight
18%5%24423247%43%
1842
16th to endNight
23%8%36%424345%33%
2288
November1st to 15thNight
16%10%36%42345%6%35%
2276
16th to endNight
20%13%5%7%5%4445%5%23%
2340
December1st to 15thNight
21%18%8%10%7%45%33413%
2487
16th to endNight
22%18%9%9%8%24335%13%
2688

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
73%7%9%7%1
1433
16th to endDay
78%5%7%5%2
1536
February1st to 15thDay
71%5%6%12%2
1652
16th to endDay
64%7%11%12%4
1580
March1st to 15thDay
54%7%14%16%6%
1821
16th to endDay
50%5%13%22%6%1
2166
April1st to 15thDay
53%5%9%19%11%
2151
16th to endDay
45%6%13%16%14%3
2316
May1st to 15thDay
47%6%12%22%9%1
2331
16th to endDay
40%7%13%26%12%
2565
June1st to 15thDay
40%6%11%24%15%
2520
16th to endDay
37%8%13%20%17%1
2520
July1st to 15thDay
31%7%13%26%17%2
2371
16th to endDay
33%8%14%24%16%2
2488
August1st to 15thDay
36%7%13%27%12%1
2291
16th to endDay
37%7%13%24%14%2
2173
September1st to 15thDay
39%6%11%26%14%1
1909
16th to endDay
40%5%13%24%15%
1815
October1st to 15thDay
48%5%10%21%13%1
1786
16th to endDay
59%5%14%14%4
1584
November1st to 15thDay
48%6%12%20%11%1
1582
16th to endDay
64%6%11%13%3
1620
December1st to 15thDay
74%6%8%7%1
1470
16th to endDay
75%5%9%7%1
1536
January1st to 15thNight
72%5%9%9%3
2506
16th to endNight
71%7%10%6%2
2688
February1st to 15thNight
66%5%9%14%4
2208
16th to endNight
60%6%12%14%5%
1893
March1st to 15thNight
50%6%13%19%9%1
2139
16th to endNight
47%5%12%23%9%2
2070
April1st to 15thNight
51%5%9%17%14%1
1797
16th to endNight
42%5%12%19%15%4
1644
May1st to 15thNight
47%6%11%19%11%2
1620
16th to endNight
41%7%11%21%15%2
1551
June1st to 15thNight
35%7%11%27%16%1
1440
16th to endNight
34%8%16%22%15%1
1440
July1st to 15thNight
27%6%13%28%20%2
1360
16th to endNight
29%7%15%25%20%2
1417
August1st to 15thNight
28%7%13%30%17%2
1559
16th to endNight
33%6%9%27%19%3
1674
September1st to 15thNight
30%7%12%29%18%1
1710
16th to endNight
35%413%29%15%
1815
October1st to 15thNight
40%5%11%24%15%1
1842
16th to endNight
53%6%13%18%8%
2288
November1st to 15thNight
48%5%13%19%12%1
2276
16th to endNight
62%6%12%15%2
2340
December1st to 15thNight
75%6%7%6%3
2487
16th to endNight
75%410%7%2
2688

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
20%16%43%18%1
90
16th to endDay
30%9%40%16%21
96
February1st to 15thDay
22%31%34%11%1
90
16th to endDay
22%19%29%24%3
81
March1st to 15thDay
33%17%22%18%9%
111
16th to endDay
27%9%28%22%9%3
288
April1st to 15thDay
29%11%27%23%5%1
270
16th to endDay
23%11%30%18%11%5%
270
May1st to 15thDay
22%14%23%24%9%5%
288
16th to endDay
14%11%29%24%13%5%
461
June1st to 15thDay
12%8%32%29%12%4
495
16th to endDay
13%9%34%25%12%5%
495
July1st to 15thDay
8%6%26%32%16%9%
475
16th to endDay
8%428%30%19%8%
515
August1st to 15thDay
11%220%24%23%17%
315
16th to endDay
16%418%24%22%13%
268
September1st to 15thDay
13%218%31%23%10%
255
16th to endDay
14%417%26%26%10%
254
October1st to 15thDay
18%10%21%27%18%3
254
16th to endDay
22%12%47%16%
96
November1st to 15thDay
13%17%31%20%15%2
90
16th to endDay
31%18%33%12%4
90
December1st to 15thDay
28%25%33%6%5%
90
16th to endDay
29%39%26%41
96
January1st to 15thNight
26%15%31%19%6%
900
16th to endNight
19%10%34%30%3
960
February1st to 15thNight
21%14%35%24%4
722
16th to endNight
21%13%32%26%5%
631
March1st to 15thNight
20%10%31%27%7%
720
16th to endNight
16%10%34%28%8%1
726
April1st to 15thNight
22%9%33%28%6%
539
16th to endNight
21%8%32%27%7%1
540
May1st to 15thNight
19%6%31%32%9%
540
16th to endNight
12%5%30%33%16%
563
June1st to 15thNight
10%429%38%16%
540
16th to endNight
10%129%39%17%1
540
July1st to 15thNight
8%20%43%23%2
510
16th to endNight
7%18%42%29%1
564
August1st to 15thNight
7%18%43%26%3
527
16th to endNight
11%216%38%28%3
525
September1st to 15thNight
12%317%39%21%4
555
16th to endNight
12%6%20%32%24%4
657
October1st to 15thNight
20%6%23%31%17%
660
16th to endNight
28%11%28%21%8%
704
November1st to 15thNight
21%10%29%28%10%
700
16th to endNight
26%12%30%24%6%
720
December1st to 15thNight
29%13%32%21%3
867
16th to endNight
30%11%36%18%3
960
Page generated at 2017-08-13 14:16:53. Version ac04

Copyright 2017 A. Danko


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