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 2025-09-04.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	14% 10% 4 9% 5% 5% 5% 3 5% 7% 33%	125929
	Night	15% 12% 4 10% 5% 5% 4 3 5% 6% 32%	123024
Transparency	Day	48% 7% 12% 20% 12% 1	129455
	Night	51% 7% 11% 19% 11% 1	127559
Seeing	Day	18% 15% 30% 22% 11% 4	16982
	Night	19% 10% 30% 29% 11% 1	26780

By Month

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 19% 8% 20% 6% 5% 5% 3 4 4 13%	3617
	11th to 20th	Day	11% 15% 7% 20% 15% 11% 5% 3 3 3 6%	3620
	21st to end	Day	17% 18% 9% 19% 7% 5% 6% 2 2 3 12%	3982
February	1st to 10th	Day	12% 16% 6% 15% 11% 6% 6% 3 4 4 18%	1834
	11th to 20th	Day	13% 15% 6% 16% 9% 4 4 3 4 5% 21%	1948
	21st to end	Day	15% 16% 6% 12% 6% 4 5% 3 6% 6% 22%	1650
March	1st to 10th	Day	14% 11% 5% 10% 5% 5% 5% 3 5% 6% 31%	4480
	11th to 20th	Day	11% 8% 4 8% 5% 5% 5% 4 6% 7% 36%	4837
	21st to end	Day	20% 14% 6% 8% 5% 4 3 3 4 5% 27%	5680
April	1st to 10th	Day	18% 14% 4 7% 5% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	17% 11% 3 6% 4 4 4 3 5% 7% 36%	2432
	21st to end	Day	17% 10% 3 7% 5% 4 5% 3 5% 6% 35%	2600
May	1st to 10th	Day	19% 8% 3 5% 4 4 4 3 5% 7% 38%	5862
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 6% 8% 38%	5875
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 7% 42%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 4 5% 5% 6% 8% 44%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 9% 45%	2800
	21st to end	Day	11% 6% 2 7% 4 4 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	11% 6% 3 7% 4 5% 6% 4 7% 10% 39%	6360
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	6360
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 10% 46%	6924
August	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 8% 42%	4276
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 3 6% 8% 45%	4174
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 6% 9% 45%	4360
September	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 6% 7% 44%	2287
	11th to 20th	Day	12% 4 2 5% 4 4 5% 4 6% 7% 48%	2090
	21st to end	Day	13% 8% 2 7% 5% 4 4 3 5% 8% 41%	2090
October	1st to 10th	Day	15% 7% 3 7% 5% 4 4 3 4 5% 43%	2090
	11th to 20th	Day	22% 9% 3 7% 6% 6% 5% 4 5% 7% 26%	1849
	21st to end	Day	23% 11% 3 9% 6% 5% 5% 3 4 7% 24%	1881
November	1st to 10th	Day	14% 11% 3 7% 5% 4 5% 3 6% 7% 36%	1762
	11th to 20th	Day	16% 18% 4 11% 6% 4 5% 3 5% 6% 22%	1800
	21st to end	Day	20% 17% 6% 9% 4 5% 4 3 4 6% 23%	1800
December	1st to 10th	Day	11% 23% 8% 14% 7% 5% 5% 3 4 5% 14%	2684
	11th to 20th	Day	17% 23% 12% 13% 7% 4 3 3 4 3 11%	3008
	21st to end	Day	23% 24% 6% 12% 6% 4 3 2 3 4 12%	3982
January	1st to 10th	Night	12% 19% 9% 19% 6% 5% 4 3 3 4 16%	6348
	11th to 20th	Night	12% 17% 9% 22% 11% 9% 4 3 3 3 8%	6360
	21st to end	Night	17% 20% 8% 14% 6% 7% 5% 3 4 4 13%	6996
February	1st to 10th	Night	13% 18% 5% 13% 9% 5% 4 3 4 5% 20%	2560
	11th to 20th	Night	13% 16% 6% 12% 6% 5% 6% 4 4 4 24%	2338
	21st to end	Night	16% 15% 4 10% 6% 5% 5% 4 5% 6% 25%	1980
March	1st to 10th	Night	15% 9% 4 8% 5% 4 4 3 5% 6% 36%	5373
	11th to 20th	Night	11% 8% 3 6% 4 3 4 3 6% 9% 43%	5122
	21st to end	Night	18% 13% 4 7% 4 4 4 3 5% 6% 32%	5293
April	1st to 10th	Night	17% 11% 4 7% 5% 4 5% 3 4 6% 36%	2000
	11th to 20th	Night	20% 8% 2 7% 4 3 4 4 5% 5% 38%	1917
	21st to end	Night	18% 8% 2 6% 4 5% 4 4 5% 7% 39%	1800
May	1st to 10th	Night	17% 9% 4 7% 4 5% 4 4 4 6% 35%	4033
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 4 6% 41%	4015
	21st to end	Night	18% 7% 2 6% 4 4 4 3 4 5% 43%	3942
June	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 4 6% 9% 45%	1600
	11th to 20th	Night	9% 6% 3 6% 5% 5% 4 4 6% 10% 43%	1600
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 5% 7% 9% 45%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 44%	3618
	11th to 20th	Night	8% 5% 1 5% 4 4 4 3 5% 7% 52%	3617
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 7% 49%	3947
August	1st to 10th	Night	10% 5% 2 6% 3 4 4 3 6% 7% 50%	2874
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 49%	2875
	21st to end	Night	9% 5% 3 7% 5% 5% 4 4 7% 7% 45%	3534
September	1st to 10th	Night	9% 4 2 6% 3 4 4 3 4 7% 52%	1939
	11th to 20th	Night	11% 5% 1 5% 4 3 3 3 4 6% 55%	2090
	21st to end	Night	13% 7% 3 6% 5% 5% 4 2 5% 8% 43%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 3 4 2 5% 6% 47%	2090
	11th to 20th	Night	21% 10% 3 7% 4 4 4 2 5% 6% 35%	2326
	21st to end	Night	24% 9% 3 6% 4 4 4 3 5% 7% 32%	2717
November	1st to 10th	Night	15% 11% 3 7% 4 4 4 4 5% 6% 38%	2536
	11th to 20th	Night	16% 15% 5% 10% 5% 5% 4 4 7% 6% 24%	2600
	21st to end	Night	18% 17% 4 10% 5% 4 4 3 5% 6% 25%	2600
December	1st to 10th	Night	12% 22% 8% 14% 5% 4 4 3 4 6% 18%	4499
	11th to 20th	Night	18% 22% 9% 15% 7% 4 3 3 4 4 12%	5199
	21st to end	Night	26% 21% 6% 10% 5% 4 4 3 4 5% 13%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	71% 9% 10% 9% 1	3618
	11th to 20th	Day	81% 7% 7% 4 1	3620
	21st to end	Day	78% 6% 8% 7% 2	3982
February	1st to 10th	Day	69% 7% 7% 12% 4	1834
	11th to 20th	Day	65% 6% 9% 13% 7%	1958
	21st to end	Day	62% 7% 12% 13% 6%	1650
March	1st to 10th	Day	52% 7% 14% 18% 8% 1	4480
	11th to 20th	Day	44% 8% 16% 22% 9% 1	4837
	21st to end	Day	59% 5% 10% 15% 10% 1	5682
April	1st to 10th	Day	55% 6% 10% 18% 10% 1	2400
	11th to 20th	Day	47% 6% 11% 21% 13% 2	2432
	21st to end	Day	47% 7% 12% 18% 14% 2	2600
May	1st to 10th	Day	43% 7% 12% 20% 16% 2	5897
	11th to 20th	Day	40% 9% 13% 21% 15% 2	5879
	21st to end	Day	39% 8% 10% 23% 18% 2	6949
June	1st to 10th	Day	36% 7% 12% 27% 16% 2	2800
	11th to 20th	Day	36% 7% 12% 24% 19% 1	2800
	21st to end	Day	36% 8% 14% 22% 17% 2	2800
July	1st to 10th	Day	38% 7% 12% 25% 16% 2	6291
	11th to 20th	Day	32% 7% 12% 28% 19% 1	6285
	21st to end	Day	32% 7% 14% 25% 20% 2	6728
August	1st to 10th	Day	36% 7% 13% 28% 15% 1	4261
	11th to 20th	Day	35% 8% 12% 27% 17% 1	4213
	21st to end	Day	34% 8% 13% 27% 16% 2	4359
September	1st to 10th	Day	37% 8% 14% 25% 14% 2	2287
	11th to 20th	Day	33% 8% 12% 30% 17% 2	2090
	21st to end	Day	41% 6% 13% 23% 16%	2090
October	1st to 10th	Day	29% 5% 8% 36% 20% 2	3480
	11th to 20th	Day	42% 7% 12% 24% 15%	3039
	21st to end	Day	49% 6% 12% 24% 8% 1	3081
November	1st to 10th	Day	47% 7% 14% 20% 12% 1	1762
	11th to 20th	Day	62% 7% 11% 14% 5% 1	1800
	21st to end	Day	63% 6% 13% 13% 4	1800
December	1st to 10th	Day	70% 10% 11% 7% 1	2684
	11th to 20th	Day	78% 6% 7% 8% 1	3005
	21st to end	Day	76% 6% 6% 10% 2	3982
January	1st to 10th	Night	71% 7% 8% 10% 4 1	6348
	11th to 20th	Night	81% 6% 6% 5% 2	6360
	21st to end	Night	74% 7% 10% 7% 2	6996
February	1st to 10th	Night	67% 7% 8% 13% 5%	2560
	11th to 20th	Night	62% 7% 9% 16% 7%	2339
	21st to end	Night	58% 7% 13% 14% 6% 1	1980
March	1st to 10th	Night	46% 7% 14% 24% 8% 1	5373
	11th to 20th	Night	36% 7% 16% 27% 12% 2	5122
	21st to end	Night	52% 7% 11% 17% 12% 1	5293
April	1st to 10th	Night	48% 7% 11% 20% 14% 1	2000
	11th to 20th	Night	47% 6% 9% 18% 17% 3	1917
	21st to end	Night	44% 7% 13% 19% 15% 3	1800
May	1st to 10th	Night	46% 9% 12% 15% 15% 3	4060
	11th to 20th	Night	43% 7% 9% 21% 17% 3	4016
	21st to end	Night	42% 6% 8% 22% 19% 2	3950
June	1st to 10th	Night	35% 7% 11% 29% 16% 3	1600
	11th to 20th	Night	35% 9% 14% 23% 18% 1	1600
	21st to end	Night	33% 8% 16% 25% 16% 2	1600
July	1st to 10th	Night	34% 7% 13% 28% 14% 5%	3580
	11th to 20th	Night	29% 7% 13% 26% 22% 2	3572
	21st to end	Night	34% 7% 12% 22% 23% 2	3832
August	1st to 10th	Night	32% 7% 14% 29% 17% 1	2865
	11th to 20th	Night	34% 7% 10% 32% 16% 1	2902
	21st to end	Night	35% 7% 11% 28% 17% 2	3534
September	1st to 10th	Night	31% 7% 14% 28% 18% 2	1939
	11th to 20th	Night	29% 6% 12% 32% 19% 2	2090
	21st to end	Night	40% 6% 14% 25% 14% 1	2090
October	1st to 10th	Night	27% 5% 10% 36% 21% 2	3480
	11th to 20th	Night	40% 6% 12% 23% 18%	3862
	21st to end	Night	42% 5% 10% 28% 14%	4467
November	1st to 10th	Night	46% 7% 11% 22% 12% 1	2536
	11th to 20th	Night	58% 8% 13% 14% 6% 1	2600
	21st to end	Night	60% 7% 15% 15% 2	2600
December	1st to 10th	Night	68% 7% 13% 10% 2	4499
	11th to 20th	Night	76% 6% 8% 8% 2	5201
	21st to end	Night	73% 6% 9% 9% 3	6996

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 17% 40% 19% 4 1	300
	11th to 20th	Day	22% 21% 42% 11% 3	300
	21st to end	Day	26% 19% 34% 18% 3	330
February	1st to 10th	Day	22% 30% 28% 16% 2 2	300
	11th to 20th	Day	22% 30% 33% 14% 1	300
	21st to end	Day	25% 33% 33% 7% 2	249
March	1st to 10th	Day	23% 27% 38% 9% 3	300
	11th to 20th	Day	24% 19% 31% 18% 8%	435
	21st to end	Day	29% 18% 29% 18% 5% 2	550
April	1st to 10th	Day	26% 21% 29% 18% 5% 1	500
	11th to 20th	Day	27% 22% 27% 15% 5% 3	492
	21st to end	Day	24% 16% 30% 20% 9% 1	500
May	1st to 10th	Day	23% 18% 29% 21% 7% 2	503
	11th to 20th	Day	19% 14% 29% 22% 11% 5%	528
	21st to end	Day	15% 13% 28% 25% 14% 4	789
June	1st to 10th	Day	12% 12% 34% 28% 10% 4	730
	11th to 20th	Day	13% 12% 31% 28% 11% 4	730
	21st to end	Day	14% 9% 35% 26% 12% 4	730
July	1st to 10th	Day	8% 7% 27% 31% 19% 8%	721
	11th to 20th	Day	10% 10% 27% 29% 18% 7%	718
	21st to end	Day	9% 5% 31% 32% 16% 7%	794
August	1st to 10th	Day	10% 5% 25% 25% 23% 13%	556
	11th to 20th	Day	10% 6% 23% 29% 24% 8%	519
	21st to end	Day	11% 7% 23% 29% 20% 10%	583
September	1st to 10th	Day	13% 8% 22% 28% 19% 10%	502
	11th to 20th	Day	9% 6% 19% 28% 28% 10%	490
	21st to end	Day	16% 12% 22% 27% 16% 7%	490
October	1st to 10th	Day	16% 14% 26% 24% 16% 3	490
	11th to 20th	Day	22% 19% 30% 24% 4	393
	21st to end	Day	30% 17% 38% 10% 3 1	330
November	1st to 10th	Day	24% 18% 36% 16% 6% 1	300
	11th to 20th	Day	24% 26% 34% 12% 5%	300
	21st to end	Day	28% 18% 40% 11% 3	300
December	1st to 10th	Day	25% 28% 27% 13% 5% 1	300
	11th to 20th	Day	23% 28% 33% 13% 3	300
	21st to end	Day	26% 26% 32% 12% 3 1	330
January	1st to 10th	Night	20% 17% 33% 23% 6% 1	1000
	11th to 20th	Night	24% 14% 35% 23% 4 1	1000
	21st to end	Night	23% 11% 34% 28% 4	1100
February	1st to 10th	Night	22% 15% 35% 25% 3	830
	11th to 20th	Night	21% 15% 35% 24% 4	783
	21st to end	Night	23% 16% 38% 19% 3	660
March	1st to 10th	Night	19% 14% 35% 26% 6%	800
	11th to 20th	Night	17% 15% 36% 25% 6% 1	800
	21st to end	Night	22% 12% 32% 27% 7% 1	809
April	1st to 10th	Night	19% 13% 36% 27% 5%	600
	11th to 20th	Night	24% 12% 32% 24% 7% 1	593
	21st to end	Night	23% 11% 32% 27% 7% 1	600
May	1st to 10th	Night	18% 10% 33% 32% 8% 1	600
	11th to 20th	Night	18% 9% 31% 31% 11%	591
	21st to end	Night	14% 7% 27% 34% 17% 1	650
June	1st to 10th	Night	11% 6% 30% 38% 14% 1	600
	11th to 20th	Night	12% 5% 31% 37% 15%	600
	21st to end	Night	11% 5% 28% 38% 18% 2	600
July	1st to 10th	Night	9% 3 20% 41% 25% 2	585
	11th to 20th	Night	7% 2 21% 43% 23% 3	582
	21st to end	Night	8% 1 21% 43% 27% 1	657
August	1st to 10th	Night	8% 2 18% 43% 26% 3	597
	11th to 20th	Night	11% 3 18% 41% 24% 4	594
	21st to end	Night	10% 3 18% 41% 25% 3	660
September	1st to 10th	Night	9% 4 21% 38% 23% 4	588
	11th to 20th	Night	11% 4 19% 38% 24% 3	760
	21st to end	Night	14% 10% 22% 32% 19% 3	760
October	1st to 10th	Night	18% 6% 26% 31% 18% 1	760
	11th to 20th	Night	25% 13% 26% 27% 9% 1	760
	21st to end	Night	28% 12% 32% 22% 6%	836
November	1st to 10th	Night	21% 12% 32% 26% 9%	780
	11th to 20th	Night	24% 15% 34% 21% 6% 1	800
	21st to end	Night	26% 15% 31% 25% 4	800
December	1st to 10th	Night	25% 16% 31% 23% 5%	945
	11th to 20th	Night	26% 15% 35% 19% 4 1	1000
	21st to end	Night	32% 13% 36% 18% 2	1100

Page generated at 2025-09-01 01:51:07. Version ac07

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