Grand Rapids
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Rapids clear sky chart from 2005-11-06 to 2024-01-13.

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
33%
24972
14%
74662
7%
24914
39%
24914
21%
52168

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
28%
950
13%
2837
6%
946
36%
946
18%
2061
January11th to 20thNight
22%
915
9%
2652
4%
915
27%
915
15%
2028
January21st to endNight
22%
990
13%
2636
4%
990
28%
990
15%
2055
February1st to 10thNight
28%
900
15%
2340
3%
900
35%
900
22%
1663
February11th to 20thNight
28%
878
18%
2274
5%
874
35%
874
23%
1605
February21st to endNight
30%
723
16%
1907
6%
723
37%
723
20%
1403
March1st to 10thNight
27%
611
16%
2053
11%
611
34%
611
18%
1424
March11th to 20thNight
28%
540
19%
1980
11%
540
40%
540
20%
1279
March21st to endNight
32%
594
15%
2153
12%
594
40%
594
16%
1478
April1st to 10thNight
28%
540
14%
1800
10%
540
36%
540
18%
1229
April11th to 20thNight
31%
533
18%
1700
13%
533
40%
533
20%
1181
April21st to endNight
34%
540
15%
1620
10%
540
40%
540
20%
1140
May1st to 10thNight
33%
540
16%
1467
7%
540
37%
540
24%
1024
May11th to 20thNight
38%
531
16%
1416
6%
531
43%
531
27%
961
May21st to endNight
43%
584
12%
1551
5%
582
46%
582
28%
1044
June1st to 10thNight
42%
540
10%
1321
3%
540
45%
540
32%
874
June11th to 20thNight
41%
540
13%
1260
5%
540
46%
540
37%
820
June21st to endNight
52%
540
14%
1260
8%
540
53%
540
37%
836
July1st to 10thNight
60%
525
15%
1225
7%
525
62%
525
50%
711
July11th to 20thNight
54%
522
18%
1254
7%
522
57%
522
45%
762
July21st to endNight
62%
591
19%
1487
8%
558
64%
558
48%
820
August1st to 10thNight
59%
537
19%
1543
11%
534
64%
534
40%
883
August11th to 20thNight
55%
534
20%
1600
15%
534
61%
534
35%
935
August21st to endNight
53%
594
20%
1844
19%
594
59%
594
34%
1066
September1st to 10thNight
52%
540
18%
1800
17%
540
58%
540
31%
1038
September11th to 20thNight
48%
540
18%
1831
15%
540
55%
540
32%
1041
September21st to endNight
34%
602
18%
2042
13%
602
42%
602
23%
1284
October1st to 10thNight
33%
720
16%
2160
11%
720
37%
720
23%
1436
October11th to 20thNight
27%
720
17%
2160
7%
720
36%
720
20%
1515
October21st to endNight
24%
796
12%
2442
5%
796
31%
796
14%
1822
November1st to 10thNight
28%
925
17%
2591
8%
925
36%
925
21%
1823
November11th to 20thNight
22%
950
11%
2660
4%
950
28%
950
15%
2057
November21st to endNight
23%
950
12%
2660
5%
950
30%
950
17%
1959
December1st to 10thNight
24%
950
8%
2660
3%
950
28%
950
12%
2103
December11th to 20thNight
17%
949
10%
3300
2%
949
25%
949
10%
2573
December21st to endNight
18%
1038
6%
5176
3%
1026
25%
1026
6%
4235

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%47%6%35%5%5%7%34%
74801
Night
13%10%48%6%35%5%5%7%34%
74947
TransparencyDay
49%5%13%23%9%1
74495
Night
50%413%22%10%1
74787
SeeingDay
17%11%31%25%13%3
14945
Night
16%9%33%30%11%1
24984

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 10thDay
9%11%7%12%9%46%5%6%8%23%
1328
11th to 20thDay
11%14%7%14%7%5%7%5%5%7%18%
1368
21st to endDay
9%12%7%14%12%46%6%6%8%16%
1719
February1st to 10thDay
8%12%5%10%7%48%5%6%7%27%
1620
11th to 20thDay
8%11%47%8%5%6%5%5%7%33%
1574
21st to endDay
11%9%6%10%9%5%6%46%6%28%
1348
March1st to 10thDay
12%10%5%7%8%35%46%8%32%
1919
11th to 20thDay
12%10%49%7%35%45%7%34%
1980
21st to endDay
18%11%5%7%6%2445%6%32%
2202
April1st to 10thDay
15%12%5%7%7%46%445%32%
2160
11th to 20thDay
17%11%47%5%45%6%5%6%30%
2208
21st to endDay
22%9%47%5%345%45%32%
2340
May1st to 10thDay
19%9%46%5%35%45%6%35%
2492
11th to 20thDay
16%6%35%6%45%5%47%38%
2479
21st to endDay
16%8%25%5%445%6%9%35%
2725
June1st to 10thDay
14%5%35%5%36%5%7%10%39%
2635
11th to 20thDay
16%6%26%6%35%5%6%7%37%
2700
21st to endDay
13%5%26%6%36%6%7%10%36%
2700
July1st to 10thDay
9%4145%35%6%6%9%49%
2700
11th to 20thDay
11%5%25%5%35%5%7%8%43%
2660
21st to endDay
8%425%5%35%5%7%9%48%
2743
August1st to 10thDay
9%426%5%35%5%7%8%45%
2383
11th to 20thDay
13%6%35%435%5%6%8%43%
2301
21st to endDay
12%6%2443446%7%47%
2511
September1st to 10thDay
12%7%245%345%5%9%43%
2160
11th to 20thDay
14%7%26%5%445%5%7%40%
2126
21st to endDay
19%7%35%435%5%5%7%38%
1900
October1st to 10thDay
18%11%26%5%35%5%5%6%34%
1800
11th to 20thDay
17%15%47%6%34446%31%
1800
21st to endDay
19%14%6%8%5%34445%27%
1912
November1st to 10thDay
12%14%6%9%8%45%45%5%29%
1487
11th to 20thDay
13%15%7%13%10%35%345%22%
1520
21st to endDay
14%13%6%11%9%45%5%5%6%22%
1520
December1st to 10thDay
11%13%5%13%11%6%6%6%46%19%
1520
11th to 20thDay
11%15%7%14%8%47%6%5%419%
1819
21st to endDay
22%18%7%11%10%33335%15%
2442
January1st to 10thNight
8%9%6%10%9%5%6%6%7%8%26%
2837
11th to 20thNight
10%13%6%11%9%5%5%5%6%8%23%
2652
21st to endNight
11%14%7%11%8%46%5%7%7%21%
2636
February1st to 10thNight
6%10%5%9%6%48%6%7%8%32%
2340
11th to 20thNight
7%10%48%7%47%6%6%7%35%
2273
21st to endNight
10%11%5%9%7%46%6%6%7%29%
1907
March1st to 10thNight
11%12%48%7%35%45%6%35%
2043
11th to 20thNight
11%9%48%6%445%5%6%39%
1979
21st to endNight
15%11%5%7%7%345%45%34%
2153
April1st to 10thNight
16%9%48%7%335%6%7%33%
1800
11th to 20thNight
16%10%5%7%5%4445%5%36%
1700
21st to endNight
20%9%47%5%4446%434%
1620
May1st to 10thNight
16%9%46%5%35%35%6%38%
1467
11th to 20thNight
15%7%35%6%345%5%7%40%
1416
21st to endNight
15%5%36%5%35%45%8%41%
1551
June1st to 10thNight
12%6%26%6%26%46%9%41%
1321
11th to 20thNight
13%6%24425%46%8%47%
1260
21st to endNight
8%6%245%25%6%7%9%45%
1260
July1st to 10thNight
44143245%6%10%57%
1260
11th to 20thNight
6%4245%345%49%54%
1296
21st to endNight
5%3233245%7%9%58%
1575
August1st to 10thNight
7%5%24435%5%6%8%54%
1552
11th to 20thNight
12%5%2432346%8%51%
1600
21st to endNight
10%5%2443445%7%51%
1844
September1st to 10thNight
10%5%25%4345%5%6%50%
1800
11th to 20thNight
10%5%345%3346%7%50%
1831
21st to endNight
16%7%35%5%2435%6%42%
2042
October1st to 10thNight
15%9%35%6%3445%6%39%
2160
11th to 20thNight
16%12%46%6%34446%35%
2160
21st to endNight
19%11%5%8%6%45%5%46%28%
2442
November1st to 10thNight
14%11%48%6%3445%7%33%
2591
11th to 20thNight
15%11%6%11%8%35%5%46%25%
2660
21st to endNight
13%9%410%9%45%5%6%7%28%
2660
December1st to 10thNight
9%12%7%13%9%5%7%6%6%7%19%
2660
11th to 20thNight
11%13%6%12%9%5%6%5%6%6%20%
3374
21st to endNight
23%17%6%10%7%44435%16%
5225

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
56%5%11%20%8%
1329
11th to 20thDay
66%49%14%7%
1368
21st to endDay
66%5%7%13%8%
1719
February1st to 10thDay
55%311%19%11%1
1620
11th to 20thDay
50%5%13%21%10%2
1583
21st to endDay
55%311%18%11%1
1348
March1st to 10thDay
51%414%22%8%
1921
11th to 20thDay
51%412%21%10%1
1980
21st to endDay
54%5%12%20%8%1
2202
April1st to 10thDay
55%48%18%13%2
2160
11th to 20thDay
55%5%10%20%10%1
2208
21st to endDay
54%410%21%11%1
2340
May1st to 10thDay
50%5%10%23%10%2
2492
11th to 20thDay
46%5%12%26%10%2
2483
21st to endDay
46%6%12%23%12%1
2725
June1st to 10thDay
41%5%19%26%9%1
2635
11th to 20thDay
45%5%13%24%12%1
2700
21st to endDay
41%7%15%28%8%1
2700
July1st to 10thDay
31%6%15%35%13%1
2626
11th to 20thDay
37%6%16%28%13%1
2579
21st to endDay
32%5%19%30%13%1
2591
August1st to 10thDay
35%5%17%32%9%2
2368
11th to 20thDay
38%6%19%27%9%
2301
21st to endDay
36%5%15%31%12%1
2511
September1st to 10thDay
39%5%15%31%10%
2160
11th to 20thDay
43%419%26%7%
2126
21st to endDay
45%5%14%30%5%
1900
October1st to 10thDay
50%415%24%6%
1800
11th to 20thDay
56%411%21%8%1
1800
21st to endDay
59%411%19%6%
1912
November1st to 10thDay
57%5%11%18%8%1
1487
11th to 20thDay
66%312%12%7%1
1520
21st to endDay
62%5%12%16%5%1
1520
December1st to 10thDay
65%5%11%15%4
1520
11th to 20thDay
67%39%17%3
1819
21st to endDay
74%2413%5%1
2442
January1st to 10thNight
54%411%21%9%1
2837
11th to 20thNight
60%5%12%16%7%1
2652
21st to endNight
61%49%18%8%1
2636
February1st to 10thNight
49%413%21%13%1
2340
11th to 20thNight
47%5%12%21%13%1
2274
21st to endNight
53%5%10%21%11%1
1907
March1st to 10thNight
50%5%12%20%11%1
2054
11th to 20thNight
47%412%23%14%1
1980
21st to endNight
52%5%12%19%10%2
2153
April1st to 10thNight
51%410%19%15%1
1800
11th to 20thNight
51%49%22%12%1
1700
21st to endNight
52%39%22%13%1
1620
May1st to 10thNight
48%411%25%12%1
1467
11th to 20thNight
43%6%11%25%12%3
1417
21st to endNight
42%6%12%24%15%2
1552
June1st to 10thNight
40%421%24%10%1
1321
11th to 20thNight
35%5%16%25%17%2
1260
21st to endNight
34%7%17%30%11%2
1260
July1st to 10thNight
23%6%19%35%16%1
1225
11th to 20thNight
28%5%17%32%16%2
1254
21st to endNight
21%7%20%33%16%2
1487
August1st to 10thNight
30%416%35%11%3
1543
11th to 20thNight
32%5%18%31%13%1
1600
21st to endNight
34%414%30%16%2
1844
September1st to 10thNight
34%5%16%32%12%
1800
11th to 20thNight
33%6%21%31%10%
1831
21st to endNight
43%413%32%8%
2042
October1st to 10thNight
46%414%26%8%1
2160
11th to 20thNight
51%412%26%8%
2160
21st to endNight
58%5%12%19%7%
2442
November1st to 10thNight
51%413%23%9%1
2591
11th to 20thNight
60%413%15%7%1
2660
21st to endNight
53%5%18%17%6%1
2660
December1st to 10thNight
63%5%14%14%31
2660
11th to 20thNight
63%411%15%5%1
3373
21st to endNight
72%37%12%5%
5225

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
13%14%33%26%13%1
270
11th to 20thDay
17%12%35%26%10%1
248
21st to endDay
15%9%36%24%15%1
264
February1st to 10thDay
15%13%48%17%7%
240
11th to 20thDay
16%20%39%14%9%2
240
21st to endDay
16%10%46%22%41
198
March1st to 10thDay
13%13%42%23%8%3
240
11th to 20thDay
17%10%35%23%12%3
240
21st to endDay
23%14%31%19%11%2
288
April1st to 10thDay
23%14%33%23%7%1
420
11th to 20thDay
23%12%26%28%10%2
486
21st to endDay
23%10%32%21%13%1
600
May1st to 10thDay
21%14%31%23%8%3
603
11th to 20thDay
20%11%29%25%12%2
625
21st to endDay
18%10%28%30%11%3
679
June1st to 10thDay
14%10%29%29%13%5%
630
11th to 20thDay
15%11%34%28%9%3
630
21st to endDay
15%9%29%26%15%6%
630
July1st to 10thDay
12%425%28%22%9%
621
11th to 20thDay
11%8%29%26%17%8%
618
21st to endDay
8%5%27%31%21%8%
684
August1st to 10thDay
11%5%23%33%22%6%
626
11th to 20thDay
14%5%25%28%21%7%
615
21st to endDay
16%8%25%27%20%5%
630
September1st to 10thDay
13%8%30%29%17%2
450
11th to 20thDay
18%7%31%27%14%4
416
21st to endDay
25%14%29%19%10%3
270
October1st to 10thDay
24%11%30%19%12%3
270
11th to 20thDay
25%15%34%22%41
270
21st to endDay
30%16%33%15%41
297
November1st to 10thDay
19%18%33%22%7%1
270
11th to 20thDay
23%19%36%16%5%
270
21st to endDay
22%16%38%20%3
270
December1st to 10thDay
19%13%39%25%5%
270
11th to 20thDay
20%17%31%27%5%
270
21st to endDay
27%11%32%22%6%2
297
January1st to 10thNight
13%9%38%27%11%2
950
11th to 20thNight
16%8%39%27%9%
915
21st to endNight
17%10%38%28%7%
990
February1st to 10thNight
12%9%39%30%10%1
900
11th to 20thNight
13%11%40%25%9%2
879
21st to endNight
14%10%37%27%11%2
723
March1st to 10thNight
16%10%36%27%10%1
614
11th to 20thNight
14%10%39%30%6%1
540
21st to endNight
20%9%29%26%12%4
594
April1st to 10thNight
19%9%35%28%7%1
540
11th to 20thNight
20%9%34%27%9%1
533
21st to endNight
24%7%28%30%11%
540
May1st to 10thNight
20%9%33%29%8%1
540
11th to 20thNight
18%6%32%31%11%1
531
21st to endNight
16%8%29%35%12%
584
June1st to 10thNight
14%6%30%36%13%1
540
11th to 20thNight
15%6%34%36%9%
540
21st to endNight
12%425%39%18%1
540
July1st to 10thNight
6%325%41%23%2
525
11th to 20thNight
8%5%28%40%18%2
522
21st to endNight
6%326%42%21%2
591
August1st to 10thNight
9%324%42%21%1
537
11th to 20thNight
13%323%36%23%2
534
21st to endNight
12%6%24%38%19%2
594
September1st to 10thNight
13%5%26%38%16%2
540
11th to 20thNight
11%5%31%35%17%2
540
21st to endNight
18%10%34%25%11%2
602
October1st to 10thNight
22%9%30%29%10%
720
11th to 20thNight
23%10%34%27%6%1
720
21st to endNight
26%12%31%25%6%
796
November1st to 10thNight
19%10%33%30%8%
925
11th to 20thNight
19%13%35%25%7%1
950
21st to endNight
18%14%38%24%6%1
950
December1st to 10thNight
16%11%35%28%10%1
950
11th to 20thNight
20%10%38%24%8%1
950
21st to endNight
24%9%34%25%6%1
1045
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