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 2017-09-21.

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%
16221
14%
46734
7%
16174
40%
16174
22%
32387

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
27%
900
10%
2684
6%
895
32%
895
16%
1987
January16th to endNight
21%
960
13%
2598
4%
960
29%
960
16%
1974
February1st to 15thNight
32%
879
15%
2285
4%
875
38%
875
23%
1622
February16th to endNight
32%
778
12%
2040
4%
778
38%
778
20%
1498
March1st to 15thNight
30%
590
14%
2030
10%
590
35%
590
17%
1400
March16th to endNight
32%
576
15%
2094
11%
576
40%
576
17%
1415
April1st to 15thNight
31%
539
15%
1797
12%
539
39%
539
20%
1213
April16th to endNight
32%
540
15%
1627
10%
540
39%
540
18%
1167
May1st to 15thNight
32%
540
14%
1457
6%
540
36%
540
23%
1035
May16th to endNight
41%
563
12%
1497
4%
561
45%
561
26%
1027
June1st to 15thNight
44%
540
9%
1300
4%
540
46%
540
33%
868
June16th to endNight
51%
540
11%
1260
6%
540
53%
540
38%
827
July1st to 15thNight
56%
510
13%
1190
6%
510
58%
510
43%
737
July16th to endNight
57%
564
17%
1382
7%
531
59%
531
45%
797
August1st to 15thNight
63%
539
19%
1572
13%
536
66%
536
40%
895
August16th to endNight
53%
573
20%
1761
17%
573
60%
573
36%
1000
September1st to 15thNight
58%
540
18%
1800
17%
540
65%
540
35%
983
September16th to endNight
41%
548
16%
1880
13%
548
47%
548
26%
1145
October1st to 15thNight
35%
660
18%
1980
11%
660
41%
660
25%
1274
October16th to endNight
28%
707
12%
2153
5%
707
34%
707
17%
1571
November1st to 15thNight
27%
875
15%
2451
7%
875
34%
875
21%
1721
November16th to endNight
28%
900
13%
2520
8%
900
33%
900
17%
1908
December1st to 15thNight
22%
900
7%
2520
3%
900
26%
900
12%
2003
December16th to endNight
18%
960
9%
2856
3%
960
26%
960
10%
2320

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%45%7%33%
48139
Night
12%9%47%6%3445%6%35%
46895
Cloud CoverDay
14%8%36%6%35%45%7%33%
48150
Night
12%9%47%6%3445%7%35%
46899
Cloud CoverDay
14%8%37%6%35%5%5%7%32%
48148
Night
12%9%47%6%35%45%7%34%
46899
TransparencyDay
49%5%13%22%8%
47814
Night
47%414%22%9%
46738
SeeingDay
18%7%28%26%14%4
6219
Night
17%7%31%30%11%1
16222

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
11%11%5%11%7%47%5%5%8%21%
1253
16th to endDay
11%12%7%12%7%5%6%5%5%8%17%
1626
February1st to 15thDay
9%11%5%8%6%37%5%5%8%29%
1575
16th to endDay
12%11%48%9%5%5%46%5%26%
1432
March1st to 15thDay
14%9%5%7%7%3445%7%32%
1930
16th to endDay
18%10%46%6%24346%31%
2129
April1st to 15thDay
13%8%46%6%45%445%35%
2151
16th to endDay
22%9%36%4345%45%30%
2331
May1st to 15thDay
21%9%36%6%24345%31%
2493
16th to endDay
17%7%25%5%35%45%8%34%
2617
June1st to 15thDay
18%5%25%5%36%5%6%8%32%
2658
16th to endDay
14%5%26%5%36%5%6%9%34%
2700
July1st to 15thDay
11%414435%5%6%8%44%
2700
16th to endDay
8%425%5%35%47%9%43%
2688
August1st to 15thDay
9%426%5%345%6%7%42%
2376
16th to endDay
12%5%2343445%7%45%
2441
September1st to 15thDay
11%6%245%34449%43%
2160
16th to endDay
17%6%25%424446%40%
1859
October1st to 15thDay
15%11%25%5%245%46%35%
1650
16th to endDay
23%12%46%5%23335%28%
1718
November1st to 15thDay
14%12%6%7%5%24345%31%
1407
16th to endDay
16%13%411%8%45%45%5%20%
1440
December1st to 15thDay
13%13%6%12%10%5%5%5%5%5%17%
1440
16th to endDay
15%17%9%11%9%3434414%
1365
January1st to 15thNight
9%9%6%9%8%45%5%6%7%24%
2684
16th to endNight
11%13%5%9%6%35%46%8%23%
2598
February1st to 15thNight
7%9%48%6%37%5%5%7%33%
2285
16th to endNight
9%10%48%7%47%6%6%7%28%
2040
March1st to 15thNight
12%10%48%6%3445%5%34%
2018
16th to endNight
14%9%5%7%5%335%45%35%
2094
April1st to 15thNight
15%9%46%6%2445%5%36%
1797
16th to endNight
19%9%36%5%3345%432%
1627
May1st to 15thNight
19%8%36%434346%35%
1457
16th to endNight
14%5%35%5%35%45%7%40%
1496
June1st to 15thNight
13%5%26%5%25%437%43%
1300
16th to endNight
11%4134145%7%9%44%
1260
July1st to 15thNight
6%4143235%5%9%54%
1260
16th to endNight
4313325%5%6%9%54%
1477
August1st to 15thNight
8%41332435%7%54%
1581
16th to endNight
10%32432335%8%52%
1761
September1st to 15thNight
9%41443335%7%54%
1800
16th to endNight
13%6%35%42335%6%45%
1880
October1st to 15thNight
14%9%236%23346%42%
1980
16th to endNight
21%10%36%43445%5%30%
2153
November1st to 15thNight
14%11%47%5%24346%35%
2451
16th to endNight
15%10%49%7%35%5%5%6%25%
2520
December1st to 15thNight
12%13%6%10%8%47%5%5%6%18%
2520
16th to endNight
15%15%8%12%8%44346%17%
2856

Percentage Cloud Cover SW (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
11%11%5%11%7%46%5%5%8%21%
1253
16th to endDay
11%12%7%12%8%5%6%6%5%8%16%
1626
February1st to 15thDay
9%11%5%8%6%37%5%5%8%29%
1575
16th to endDay
12%11%48%9%5%5%46%5%26%
1432
March1st to 15thDay
14%9%5%6%7%2445%7%31%
1941
16th to endDay
18%10%46%7%2435%6%31%
2129
April1st to 15thDay
13%8%46%6%44446%35%
2151
16th to endDay
22%9%36%4335%35%30%
2331
May1st to 15thDay
21%9%46%6%34335%31%
2493
16th to endDay
17%6%25%5%35%5%5%8%34%
2617
June1st to 15thDay
17%5%345%36%5%6%8%32%
2658
16th to endDay
14%5%26%6%36%5%6%9%33%
2700
July1st to 15thDay
11%414435%5%6%7%44%
2700
16th to endDay
7%425%5%35%47%9%43%
2688
August1st to 15thDay
9%426%5%35%5%6%7%41%
2376
16th to endDay
12%5%2343445%7%44%
2441
September1st to 15thDay
11%6%24444449%43%
2160
16th to endDay
16%6%25%424446%40%
1859
October1st to 15thDay
15%11%26%5%25%5%45%35%
1650
16th to endDay
22%12%46%5%24335%28%
1718
November1st to 15thDay
14%12%6%7%5%24345%31%
1407
16th to endDay
16%12%5%11%8%45%45%5%20%
1440
December1st to 15thDay
12%13%6%12%10%45%5%5%5%16%
1440
16th to endDay
15%17%9%12%9%3434414%
1365
January1st to 15thNight
9%9%6%9%8%45%6%6%7%24%
2684
16th to endNight
11%13%6%9%6%35%46%8%23%
2598
February1st to 15thNight
7%9%48%6%37%5%5%7%33%
2278
16th to endNight
9%10%48%7%47%6%6%7%28%
2040
March1st to 15thNight
12%10%48%6%3445%5%34%
2030
16th to endNight
14%9%5%7%5%235%45%34%
2094
April1st to 15thNight
15%9%46%6%2445%5%36%
1797
16th to endNight
19%9%46%6%3445%432%
1627
May1st to 15thNight
19%8%35%425%246%35%
1456
16th to endNight
14%5%35%5%35%447%40%
1496
June1st to 15thNight
13%5%35%425%447%43%
1300
16th to endNight
11%413425%5%7%9%44%
1260
July1st to 15thNight
6%3143235%5%8%54%
1260
16th to endNight
43134245%6%9%54%
1477
August1st to 15thNight
8%413325%35%7%54%
1581
16th to endNight
10%42432345%8%52%
1761
September1st to 15thNight
8%41443335%7%53%
1800
16th to endNight
13%6%35%42335%6%45%
1880
October1st to 15thNight
14%9%246%24346%42%
1980
16th to endNight
20%10%36%5%335%5%5%29%
2153
November1st to 15thNight
14%10%47%5%24346%34%
2451
16th to endNight
15%10%48%7%45%45%6%25%
2520
December1st to 15thNight
12%13%6%10%8%46%6%5%6%18%
2520
16th to endNight
14%15%8%12%8%34346%17%
2856

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
11%11%5%11%8%46%5%6%8%20%
1253
16th to endDay
11%12%7%11%8%6%6%6%5%8%16%
1626
February1st to 15thDay
9%11%5%8%6%47%5%48%29%
1575
16th to endDay
12%11%48%9%5%6%45%5%26%
1432
March1st to 15thDay
14%9%48%6%345%5%7%31%
1941
16th to endDay
17%10%47%6%2435%6%30%
2127
April1st to 15thDay
14%8%46%6%44446%34%
2151
16th to endDay
22%9%46%434445%30%
2331
May1st to 15thDay
21%9%46%5%34345%30%
2493
16th to endDay
17%6%26%4345%6%8%33%
2617
June1st to 15thDay
17%6%25%5%36%5%6%7%32%
2658
16th to endDay
14%5%26%6%36%5%6%9%32%
2700
July1st to 15thDay
11%4244345%5%7%44%
2700
16th to endDay
8%415%5%35%5%7%9%43%
2688
August1st to 15thDay
9%426%5%445%6%7%41%
2376
16th to endDay
12%4335%3435%8%44%
2441
September1st to 15thDay
11%5%25%44445%8%43%
2160
16th to endDay
16%7%26%434447%39%
1859
October1st to 15thDay
16%10%35%5%25%5%4436%
1650
16th to endDay
23%12%46%5%24335%27%
1718
November1st to 15thDay
14%12%6%7%5%3435%431%
1407
16th to endDay
16%13%5%11%8%45%5%46%20%
1440
December1st to 15thDay
12%13%5%12%10%5%5%5%5%5%16%
1440
16th to endDay
15%18%9%12%9%34435%14%
1365
January1st to 15thNight
9%10%6%9%8%45%6%6%7%24%
2684
16th to endNight
11%12%6%10%6%35%5%6%8%22%
2598
February1st to 15thNight
7%9%5%7%7%37%46%7%32%
2278
16th to endNight
8%10%48%7%38%6%6%7%28%
2040
March1st to 15thNight
11%10%47%6%35%446%34%
2030
16th to endNight
14%9%5%7%5%245%46%34%
2094
April1st to 15thNight
15%9%46%5%3445%5%36%
1797
16th to endNight
19%8%46%6%3444432%
1627
May1st to 15thNight
18%9%35%5%3435%5%35%
1456
16th to endNight
13%6%25%5%35%45%7%39%
1496
June1st to 15thNight
13%5%35%435%347%44%
1300
16th to endNight
11%4144245%6%10%44%
1260
July1st to 15thNight
6%41432346%8%54%
1260
16th to endNight
431332446%10%53%
1477
August1st to 15thNight
8%31332435%7%54%
1581
16th to endNight
10%42432335%8%51%
1761
September1st to 15thNight
8%41342345%7%53%
1800
16th to endNight
12%7%245%2335%6%44%
1880
October1st to 15thNight
14%9%25%433346%42%
1980
16th to endNight
20%11%46%5%245%45%29%
2153
November1st to 15thNight
14%11%47%5%24346%34%
2451
16th to endNight
15%10%5%8%7%35%45%7%25%
2520
December1st to 15thNight
12%13%7%10%8%47%6%6%6%17%
2520
16th to endNight
14%16%7%13%8%44446%17%
2856

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
57%5%11%18%6%
1254
16th to endDay
64%5%9%14%4
1626
February1st to 15thDay
52%414%19%8%
1576
16th to endDay
57%412%19%6%
1432
March1st to 15thDay
51%5%14%19%9%
1941
16th to endDay
54%5%13%20%5%
2129
April1st to 15thDay
49%412%18%13%1
2151
16th to endDay
55%5%9%19%9%
2331
May1st to 15thDay
55%410%21%6%
2493
16th to endDay
46%6%13%22%10%
2621
June1st to 15thDay
47%5%17%22%6%
2658
16th to endDay
44%7%15%23%8%
2700
July1st to 15thDay
35%6%13%30%13%1
2541
16th to endDay
34%6%19%27%11%
2520
August1st to 15thDay
38%5%15%30%8%1
2361
16th to endDay
37%5%17%27%11%
2441
September1st to 15thDay
38%5%16%31%8%
2160
16th to endDay
43%417%29%4
1859
October1st to 15thDay
49%415%22%8%
1650
16th to endDay
58%411%18%6%
1718
November1st to 15thDay
54%412%17%8%
1407
16th to endDay
63%410%13%5%
1440
December1st to 15thDay
66%5%10%13%3
1440
16th to endDay
72%47%10%4
1365
January1st to 15thNight
55%5%12%20%6%
2684
16th to endNight
57%412%18%6%
2598
February1st to 15thNight
48%413%22%10%
2288
16th to endNight
51%5%13%21%8%
2040
March1st to 15thNight
49%5%14%18%10%
2030
16th to endNight
50%413%21%8%1
2094
April1st to 15thNight
48%410%22%12%1
1797
16th to endNight
53%49%20%11%
1627
May1st to 15thNight
50%312%23%9%
1457
16th to endNight
42%7%11%22%13%2
1498
June1st to 15thNight
39%419%24%11%1
1300
16th to endNight
33%6%20%27%10%1
1260
July1st to 15thNight
28%6%16%31%15%1
1190
16th to endNight
23%7%20%31%15%1
1382
August1st to 15thNight
29%416%33%12%2
1572
16th to endNight
30%416%29%16%2
1761
September1st to 15thNight
29%5%17%36%10%
1800
16th to endNight
39%5%16%31%7%
1880
October1st to 15thNight
42%415%28%8%
1980
16th to endNight
53%5%14%20%5%
2153
November1st to 15thNight
50%313%22%8%1
2451
16th to endNight
56%5%15%15%6%
2520
December1st to 15thNight
63%5%13%14%3
2520
16th to endNight
67%310%13%4
2856

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%12%28%22%12%3
90
16th to endDay
25%5%27%28%14%
96
February1st to 15thDay
17%18%38%21%3
90
16th to endDay
27%240%19%6%3
81
March1st to 15thDay
18%8%34%30%7%
90
16th to endDay
24%6%31%21%15%
113
April1st to 15thDay
14%10%34%27%10%2
270
16th to endDay
28%7%26%25%10%1
441
May1st to 15thDay
26%11%28%21%8%2
468
16th to endDay
23%7%29%23%12%4
517
June1st to 15thDay
18%9%31%24%11%4
495
16th to endDay
14%9%30%27%13%4
495
July1st to 15thDay
14%5%22%28%20%9%
475
16th to endDay
9%427%28%21%8%
515
August1st to 15thDay
11%5%20%29%28%4
493
16th to endDay
16%325%28%20%6%
481
September1st to 15thDay
11%327%32%20%4
315
16th to endDay
21%8%32%25%7%4
142
October1st to 15thDay
15%8%33%23%12%6%
90
16th to endDay
30%7%35%14%10%2
96
November1st to 15thDay
14%5%45%23%10%1
90
16th to endDay
43%10%21%21%4
90
December1st to 15thDay
16%345%30%4
90
16th to endDay
27%8%29%25%6%4
96
January1st to 15thNight
14%8%35%27%11%2
900
16th to endNight
17%9%38%27%6%
960
February1st to 15thNight
13%7%35%30%12%1
880
16th to endNight
14%7%34%28%12%2
778
March1st to 15thNight
15%9%36%28%9%
590
16th to endNight
19%7%30%26%11%4
576
April1st to 15thNight
20%6%34%29%7%
539
16th to endNight
23%6%29%29%10%
540
May1st to 15thNight
23%8%30%28%7%
540
16th to endNight
17%5%30%33%11%1
563
June1st to 15thNight
16%7%28%37%9%1
540
16th to endNight
13%425%39%16%
540
July1st to 15thNight
10%325%39%20%1
510
16th to endNight
6%326%40%21%1
564
August1st to 15thNight
11%120%41%23%1
539
16th to endNight
11%325%37%19%2
573
September1st to 15thNight
10%222%37%22%2
540
16th to endNight
14%6%32%29%14%2
548
October1st to 15thNight
21%8%30%29%10%
660
16th to endNight
26%9%27%29%6%
707
November1st to 15thNight
19%10%33%27%7%1
875
16th to endNight
19%12%31%26%9%1
900
December1st to 15thNight
19%8%34%26%10%
900
16th to endNight
22%8%33%25%7%2
960
Page generated at 2017-09-22 06:35:37. Version ac04

Copyright 2017 A. Danko


Grand Rapids Clear Sky Chart | Charts Home