Grand Haven
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Haven clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 10% 4 9% 5% 4 4 3 5% 7% 35%	126322
	Night	15% 13% 5% 11% 6% 4 4 3 4 6% 30%	123290
Transparency	Day	47% 6% 12% 22% 12% 1	129835
	Night	55% 5% 10% 18% 11% 1	127838
Seeing	Day	17% 10% 25% 29% 15% 4	17129
	Night	20% 9% 26% 29% 14% 2	26925

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% 23% 12% 24% 7% 5% 3 2 2 2 9%	3617
	11th to 20th	Day	11% 16% 8% 23% 17% 9% 3 2 2 2 5%	3620
	21st to end	Day	14% 20% 8% 23% 6% 4 6% 2 2 2 12%	3982
February	1st to 10th	Day	12% 17% 6% 17% 10% 7% 4 3 4 4 16%	1820
	11th to 20th	Day	12% 16% 6% 16% 10% 6% 5% 3 4 5% 18%	1948
	21st to end	Day	14% 16% 5% 11% 6% 5% 6% 4 6% 5% 21%	1650
March	1st to 10th	Day	12% 10% 5% 12% 6% 5% 5% 3 5% 6% 33%	4480
	11th to 20th	Day	11% 8% 4 9% 5% 4 4 4 5% 7% 38%	4786
	21st to end	Day	20% 12% 7% 8% 4 4 4 3 4 5% 28%	5771
April	1st to 10th	Day	18% 11% 4 9% 5% 4 3 3 5% 6% 32%	2400
	11th to 20th	Day	16% 10% 3 7% 4 4 4 4 5% 6% 37%	2407
	21st to end	Day	16% 8% 3 7% 5% 5% 4 4 6% 7% 36%	2600
May	1st to 10th	Day	16% 7% 3 6% 4 4 4 3 6% 7% 41%	5862
	11th to 20th	Day	13% 7% 3 6% 4 4 5% 4 6% 9% 42%	5989
	21st to end	Day	13% 6% 2 5% 4 3 4 4 5% 7% 47%	6935
June	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 6% 9% 47%	2800
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 5% 10% 47%	2800
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 9% 47%	2800
July	1st to 10th	Day	11% 7% 3 6% 4 4 4 4 6% 9% 44%	6360
	11th to 20th	Day	7% 5% 2 5% 4 4 3 3 6% 9% 52%	6360
	21st to end	Day	7% 6% 2 6% 4 4 5% 4 7% 9% 47%	6924
August	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 45%	4342
	11th to 20th	Day	11% 4 2 6% 3 3 4 4 6% 7% 51%	4158
	21st to end	Day	7% 5% 3 6% 4 4 4 3 6% 8% 48%	4312
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 6% 8% 47%	2395
	11th to 20th	Day	11% 4 2 4 5% 4 4 4 6% 7% 49%	2200
	21st to end	Day	12% 6% 3 7% 4 4 4 4 6% 8% 42%	2200
October	1st to 10th	Day	16% 6% 3 7% 4 4 4 2 5% 5% 43%	2123
	11th to 20th	Day	22% 11% 3 8% 5% 5% 5% 4 4 7% 25%	1837
	21st to end	Day	23% 12% 4 9% 5% 4 5% 4 5% 7% 23%	1881
November	1st to 10th	Day	15% 14% 4 8% 4 4 4 4 5% 5% 33%	1762
	11th to 20th	Day	17% 24% 5% 9% 5% 4 3 3 4 5% 20%	1800
	21st to end	Day	22% 21% 7% 9% 5% 4 4 2 3 5% 19%	1800
December	1st to 10th	Day	11% 28% 13% 15% 5% 4 3 2 3 4 12%	2611
	11th to 20th	Day	17% 26% 12% 13% 7% 4 4 2 3 3 9%	3008
	21st to end	Day	24% 24% 7% 12% 5% 4 3 2 3 4 11%	3982
January	1st to 10th	Night	11% 24% 11% 22% 8% 4 3 2 3 3 10%	6348
	11th to 20th	Night	13% 21% 11% 24% 12% 7% 2 2 2 1 5%	6360
	21st to end	Night	15% 22% 9% 17% 7% 7% 4 2 3 3 12%	6996
February	1st to 10th	Night	13% 20% 5% 16% 10% 7% 3 3 3 3 18%	2574
	11th to 20th	Night	14% 16% 6% 15% 9% 5% 4 3 3 4 20%	2338
	21st to end	Night	16% 16% 5% 11% 6% 5% 4 3 4 5% 23%	1980
March	1st to 10th	Night	16% 8% 3 10% 5% 4 4 3 5% 6% 35%	5373
	11th to 20th	Night	10% 7% 3 9% 5% 4 4 3 6% 8% 41%	5174
	21st to end	Night	18% 11% 4 11% 5% 4 4 3 4 6% 30%	5202
April	1st to 10th	Night	17% 10% 3 7% 5% 4 4 4 6% 6% 34%	2000
	11th to 20th	Night	20% 9% 3 6% 4 5% 4 4 5% 5% 36%	1941
	21st to end	Night	16% 8% 3 7% 4 4 5% 3 5% 7% 38%	1800
May	1st to 10th	Night	16% 7% 3 8% 5% 5% 5% 4 5% 7% 35%	4033
	11th to 20th	Night	14% 6% 4 7% 4 5% 4 4 6% 8% 39%	3906
	21st to end	Night	17% 7% 2 6% 4 4 4 3 4 6% 42%	3942
June	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 5% 7% 8% 43%	1600
	11th to 20th	Night	10% 6% 2 7% 5% 4 3 4 6% 10% 44%	1600
	21st to end	Night	9% 5% 3 6% 5% 4 4 5% 6% 8% 44%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 8% 52%	3617
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 48%	3947
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	2808
	11th to 20th	Night	11% 5% 1 6% 4 4 3 4 5% 7% 50%	2875
	21st to end	Night	8% 6% 2 7% 5% 4 4 4 6% 8% 47%	3586
September	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 5% 7% 49%	2000
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 6% 56%	2185
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 3 5% 8% 41%	2200
October	1st to 10th	Night	18% 6% 2 6% 3 3 3 3 4 6% 46%	2124
	11th to 20th	Night	24% 11% 3 7% 3 3 4 2 4 6% 33%	2340
	21st to end	Night	26% 10% 3 7% 4 4 4 3 4 6% 29%	2717
November	1st to 10th	Night	18% 13% 3 7% 5% 4 4 3 4 6% 35%	2536
	11th to 20th	Night	19% 21% 5% 11% 5% 3 3 4 5% 6% 20%	2600
	21st to end	Night	20% 21% 6% 10% 5% 4 3 2 4 5% 20%	2600
December	1st to 10th	Night	14% 24% 10% 16% 6% 4 4 3 4 4 12%	4575
	11th to 20th	Night	18% 27% 10% 14% 7% 3 3 2 3 3 10%	5199
	21st to end	Night	24% 21% 6% 11% 5% 4 4 3 4 5% 12%	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	81% 5% 5% 7% 1	3618
	11th to 20th	Day	86% 5% 5% 3 1	3620
	21st to end	Day	79% 4 8% 7% 1	3982
February	1st to 10th	Day	72% 5% 7% 13% 3	1820
	11th to 20th	Day	68% 5% 8% 13% 5%	1958
	21st to end	Day	62% 5% 13% 14% 6%	1650
March	1st to 10th	Day	51% 4 15% 20% 8% 1	4480
	11th to 20th	Day	44% 4 18% 22% 10% 1	4786
	21st to end	Day	58% 4 10% 17% 10% 1	5773
April	1st to 10th	Day	53% 5% 11% 20% 11% 1	2400
	11th to 20th	Day	46% 5% 11% 22% 14% 2	2407
	21st to end	Day	46% 5% 14% 20% 14% 2	2600
May	1st to 10th	Day	41% 6% 11% 24% 17% 1	5897
	11th to 20th	Day	39% 8% 13% 22% 16% 2	5993
	21st to end	Day	36% 5% 11% 28% 19% 1	6949
June	1st to 10th	Day	33% 6% 13% 29% 17% 2	2800
	11th to 20th	Day	32% 7% 15% 24% 21% 1	2800
	21st to end	Day	33% 7% 16% 25% 18% 2	2800
July	1st to 10th	Day	37% 6% 12% 28% 15% 2	6291
	11th to 20th	Day	28% 6% 14% 32% 19% 1	6285
	21st to end	Day	30% 7% 17% 26% 18% 2	6728
August	1st to 10th	Day	34% 6% 13% 32% 13% 1	4327
	11th to 20th	Day	30% 8% 13% 32% 16% 1	4197
	21st to end	Day	31% 6% 15% 29% 17% 2	4311
September	1st to 10th	Day	34% 6% 15% 29% 14% 2	2395
	11th to 20th	Day	33% 5% 13% 32% 16% 2	2200
	21st to end	Day	39% 6% 12% 27% 16%	2200
October	1st to 10th	Day	29% 4 9% 39% 18% 1	3513
	11th to 20th	Day	44% 6% 13% 25% 13%	3014
	21st to end	Day	52% 5% 15% 20% 8%	3081
November	1st to 10th	Day	52% 6% 11% 21% 10%	1762
	11th to 20th	Day	67% 5% 10% 13% 5%	1800
	21st to end	Day	69% 5% 12% 11% 3	1800
December	1st to 10th	Day	75% 8% 9% 7% 1	2611
	11th to 20th	Day	81% 5% 8% 6% 1	3005
	21st to end	Day	77% 4 6% 9% 3	3982
January	1st to 10th	Night	80% 5% 5% 7% 2	6348
	11th to 20th	Night	88% 4 4 3 1	6360
	21st to end	Night	80% 3 9% 6% 2	6996
February	1st to 10th	Night	72% 3 8% 11% 5%	2574
	11th to 20th	Night	69% 5% 6% 14% 6%	2339
	21st to end	Night	63% 4 12% 13% 6% 1	1980
March	1st to 10th	Night	49% 4 14% 24% 8%	5373
	11th to 20th	Night	40% 5% 14% 28% 11% 2	5174
	21st to end	Night	56% 4 9% 18% 12% 1	5202
April	1st to 10th	Night	49% 6% 10% 21% 13% 1	2000
	11th to 20th	Night	49% 4 7% 19% 17% 3	1941
	21st to end	Night	45% 5% 11% 20% 15% 4	1800
May	1st to 10th	Night	49% 5% 11% 17% 15% 3	4060
	11th to 20th	Night	43% 6% 10% 22% 15% 4	3907
	21st to end	Night	45% 4 7% 22% 19% 3	3950
June	1st to 10th	Night	35% 7% 9% 29% 17% 3	1600
	11th to 20th	Night	36% 5% 12% 25% 19% 2	1600
	21st to end	Night	36% 7% 14% 25% 16% 3	1600
July	1st to 10th	Night	38% 5% 11% 27% 14% 5%	3580
	11th to 20th	Night	29% 5% 13% 26% 25% 2	3572
	21st to end	Night	35% 6% 12% 23% 23% 2	3832
August	1st to 10th	Night	32% 6% 13% 28% 19% 1	2799
	11th to 20th	Night	33% 6% 10% 32% 17% 1	2902
	21st to end	Night	34% 7% 10% 26% 21% 2	3586
September	1st to 10th	Night	33% 7% 11% 28% 18% 2	2000
	11th to 20th	Night	29% 5% 10% 34% 20% 2	2185
	21st to end	Night	41% 7% 11% 27% 13% 1	2200
October	1st to 10th	Night	31% 4 8% 34% 21% 2	3514
	11th to 20th	Night	41% 5% 13% 25% 16%	3889
	21st to end	Night	46% 5% 9% 26% 13%	4467
November	1st to 10th	Night	51% 7% 9% 21% 11% 1	2536
	11th to 20th	Night	65% 5% 10% 13% 6% 1	2600
	21st to end	Night	67% 4 12% 14% 2	2600
December	1st to 10th	Night	74% 7% 11% 6% 2	4575
	11th to 20th	Night	80% 4 8% 6% 2	5201
	21st to end	Night	74% 5% 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	20% 14% 35% 24% 4 2	300
	11th to 20th	Day	22% 18% 32% 20% 6% 1	300
	21st to end	Day	27% 15% 31% 23% 4 1	330
February	1st to 10th	Day	19% 20% 34% 19% 6% 2	300
	11th to 20th	Day	21% 18% 37% 18% 6%	300
	21st to end	Day	22% 18% 39% 16% 4	249
March	1st to 10th	Day	19% 18% 40% 18% 5%	300
	11th to 20th	Day	18% 15% 32% 23% 10% 1	410
	21st to end	Day	25% 13% 28% 26% 7% 1	550
April	1st to 10th	Day	23% 14% 30% 25% 7%	500
	11th to 20th	Day	21% 17% 28% 23% 7% 3	492
	21st to end	Day	22% 10% 28% 28% 11% 1	500
May	1st to 10th	Day	19% 12% 27% 31% 10% 1	503
	11th to 20th	Day	18% 6% 23% 33% 16% 4	577
	21st to end	Day	15% 5% 23% 30% 21% 5%	789
June	1st to 10th	Day	11% 6% 25% 39% 16% 4	730
	11th to 20th	Day	13% 5% 24% 34% 19% 4	730
	21st to end	Day	10% 7% 26% 38% 15% 4	730
July	1st to 10th	Day	7% 4 21% 36% 25% 8%	721
	11th to 20th	Day	8% 4 22% 37% 22% 6%	718
	21st to end	Day	8% 2 20% 44% 22% 5%	794
August	1st to 10th	Day	10% 3 16% 32% 30% 9%	596
	11th to 20th	Day	10% 4 17% 32% 26% 11%	519
	21st to end	Day	11% 4 16% 32% 27% 9%	583
September	1st to 10th	Day	13% 6% 19% 31% 22% 9%	530
	11th to 20th	Day	8% 4 15% 27% 32% 14%	530
	21st to end	Day	14% 8% 23% 26% 20% 9%	530
October	1st to 10th	Day	18% 8% 24% 27% 17% 6%	502
	11th to 20th	Day	26% 16% 24% 28% 6%	356
	21st to end	Day	33% 11% 29% 21% 5% 2	330
November	1st to 10th	Day	22% 19% 27% 23% 7% 2	300
	11th to 20th	Day	25% 22% 32% 17% 3	300
	21st to end	Day	26% 19% 37% 14% 4	300
December	1st to 10th	Day	26% 23% 25% 19% 6%	300
	11th to 20th	Day	22% 24% 31% 18% 5% 1	300
	21st to end	Day	29% 20% 32% 15% 3 1	330
January	1st to 10th	Night	23% 17% 32% 22% 7% 1	1000
	11th to 20th	Night	26% 15% 32% 21% 6% 1	1000
	21st to end	Night	23% 12% 33% 26% 6%	1100
February	1st to 10th	Night	21% 14% 35% 24% 5% 1	874
	11th to 20th	Night	22% 13% 33% 25% 7%	783
	21st to end	Night	24% 14% 34% 24% 3	660
March	1st to 10th	Night	20% 10% 30% 31% 9% 1	800
	11th to 20th	Night	17% 10% 32% 31% 9% 1	800
	21st to end	Night	22% 8% 27% 31% 11%	770
April	1st to 10th	Night	20% 9% 32% 31% 8% 1	600
	11th to 20th	Night	25% 10% 24% 30% 10% 1	593
	21st to end	Night	20% 7% 28% 33% 11% 1	600
May	1st to 10th	Night	19% 5% 25% 38% 13% 1	600
	11th to 20th	Night	19% 5% 23% 32% 20% 1	591
	21st to end	Night	15% 4 20% 35% 22% 4	650
June	1st to 10th	Night	12% 3 21% 41% 21% 2	600
	11th to 20th	Night	12% 3 23% 37% 23% 2	600
	21st to end	Night	10% 4 20% 40% 25% 3	600
July	1st to 10th	Night	9% 2 13% 35% 35% 5%	585
	11th to 20th	Night	8% 2 16% 38% 32% 4	582
	21st to end	Night	8% 2 14% 40% 33% 2	657
August	1st to 10th	Night	8% 2 12% 41% 31% 6%	597
	11th to 20th	Night	12% 3 10% 38% 31% 6%	594
	21st to end	Night	11% 3 12% 40% 30% 4	660
September	1st to 10th	Night	12% 4 16% 37% 25% 5%	600
	11th to 20th	Night	11% 4 15% 34% 31% 6%	785
	21st to end	Night	15% 7% 21% 28% 23% 6%	800
October	1st to 10th	Night	21% 5% 20% 32% 20% 3	773
	11th to 20th	Night	28% 11% 24% 25% 10% 1	760
	21st to end	Night	30% 12% 28% 22% 7% 1	836
November	1st to 10th	Night	24% 12% 27% 25% 11% 1	780
	11th to 20th	Night	30% 16% 30% 19% 5% 1	800
	21st to end	Night	29% 14% 30% 23% 4	800
December	1st to 10th	Night	28% 17% 30% 21% 5%	995
	11th to 20th	Night	28% 14% 34% 18% 4 1	1000
	21st to end	Night	32% 15% 32% 20% 2	1100

Page generated at 2025-10-01 01:39:03. Version ac07

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