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 2026-01-02.

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	12% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	127880
	Night	11% 10% 4 10% 6% 5% 5% 4 5% 6% 33%	123727
Transparency	Day	49% 3 13% 24% 10% 1	131386
	Night	51% 3 12% 22% 11% 1	128308
Seeing	Day	18% 11% 32% 24% 12% 3	17716
	Night	16% 9% 33% 30% 11% 1	27677

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	5% 13% 5% 17% 8% 6% 6% 4 5% 6% 26%	3185
	11th to 20th	Day	5% 11% 7% 15% 7% 9% 10% 6% 10% 6% 13%	3370
	21st to end	Day	5% 14% 11% 17% 7% 5% 5% 4 5% 6% 20%	4327
February	1st to 10th	Day	9% 13% 5% 12% 7% 7% 6% 5% 6% 7% 26%	1800
	11th to 20th	Day	7% 11% 4 9% 6% 6% 6% 5% 6% 6% 34%	1754
	21st to end	Day	9% 10% 5% 11% 8% 6% 5% 3 6% 7% 29%	1503
March	1st to 10th	Day	10% 9% 3 7% 5% 5% 4 4 6% 9% 39%	4795
	11th to 20th	Day	8% 9% 6% 10% 6% 5% 5% 4 7% 9% 33%	4980
	21st to end	Day	20% 12% 4 8% 4 3 4 3 4 5% 32%	5543
April	1st to 10th	Day	15% 12% 4 9% 6% 5% 5% 3 4 6% 32%	2400
	11th to 20th	Day	16% 11% 4 8% 5% 5% 5% 5% 4 7% 30%	2457
	21st to end	Day	21% 10% 3 7% 4 4 4 4 4 5% 32%	2600
May	1st to 10th	Day	16% 8% 2 5% 3 4 3 3 4 5% 47%	6292
	11th to 20th	Day	15% 10% 3 6% 4 4 4 3 5% 7% 40%	6319
	21st to end	Day	13% 8% 3 8% 4 4 4 4 6% 8% 37%	6949
June	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2929
	11th to 20th	Day	16% 7% 2 7% 5% 4 5% 4 7% 7% 36%	3000
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 35%	3000
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 40%	6799
	11th to 20th	Day	13% 5% 2 5% 3 4 4 4 6% 10% 43%	6695
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 7% 10% 43%	6926
August	1st to 10th	Day	10% 5% 2 7% 4 5% 5% 5% 7% 8% 41%	4334
	11th to 20th	Day	13% 5% 2 5% 4 5% 5% 5% 7% 10% 41%	4171
	21st to end	Day	16% 7% 3 6% 4 4 4 3 6% 7% 40%	4552
September	1st to 10th	Day	11% 7% 2 5% 4 5% 4 4 5% 9% 42%	2400
	11th to 20th	Day	14% 7% 2 7% 4 5% 4 4 5% 7% 40%	2362
	21st to end	Day	17% 7% 2 6% 3 4 4 4 5% 7% 41%	2110
October	1st to 10th	Day	17% 10% 2 6% 4 4 5% 4 6% 6% 36%	2000
	11th to 20th	Day	16% 14% 3 8% 5% 4 4 3 5% 6% 31%	2000
	21st to end	Day	19% 15% 5% 9% 4 3 4 4 4 6% 27%	2122
November	1st to 10th	Day	12% 17% 4 11% 7% 5% 4 3 5% 5% 28%	1647
	11th to 20th	Day	12% 16% 6% 14% 7% 5% 4 3 4 5% 24%	1680
	21st to end	Day	13% 14% 5% 14% 8% 5% 5% 4 5% 5% 21%	1680
December	1st to 10th	Day	10% 15% 5% 17% 17% 6% 4 3 3 5% 15%	2690
	11th to 20th	Day	9% 17% 8% 14% 5% 5% 5% 5% 5% 4 23%	2934
	21st to end	Day	18% 19% 6% 21% 8% 4 3 2 3 4 12%	3575
January	1st to 10th	Night	6% 10% 5% 18% 10% 7% 5% 4 5% 6% 26%	6803
	11th to 20th	Night	6% 11% 5% 12% 9% 9% 9% 7% 8% 6% 17%	6521
	21st to end	Night	6% 15% 11% 15% 6% 5% 5% 4 6% 6% 21%	6650
February	1st to 10th	Night	7% 11% 4 11% 5% 6% 6% 5% 7% 7% 31%	2600
	11th to 20th	Night	6% 9% 4 9% 6% 5% 7% 5% 6% 7% 34%	2533
	21st to end	Night	9% 12% 5% 11% 6% 5% 6% 5% 6% 6% 29%	2126
March	1st to 10th	Night	10% 8% 3 7% 4 4 4 4 5% 7% 44%	5088
	11th to 20th	Night	8% 11% 4 13% 7% 6% 5% 5% 5% 6% 29%	4979
	21st to end	Night	18% 15% 5% 10% 5% 3 3 3 4 5% 29%	5387
April	1st to 10th	Night	16% 10% 4 9% 5% 4 4 3 6% 7% 33%	2000
	11th to 20th	Night	15% 11% 4 7% 5% 4 3 4 5% 5% 36%	1890
	21st to end	Night	20% 9% 3 8% 4 4 4 3 6% 4 35%	1800
May	1st to 10th	Night	16% 7% 2 5% 3 3 4 2 5% 5% 47%	3668
	11th to 20th	Night	14% 8% 3 6% 4 4 3 3 5% 6% 45%	3596
	21st to end	Night	16% 8% 3 7% 4 4 5% 3 5% 7% 38%	3949
June	1st to 10th	Night	12% 7% 2 7% 5% 4 5% 3 6% 8% 42%	1467
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 6% 8% 45%	1400
	21st to end	Night	9% 6% 2 5% 5% 4 5% 5% 7% 9% 44%	1400
July	1st to 10th	Night	7% 4 2 5% 4 4 5% 5% 8% 10% 46%	3180
	11th to 20th	Night	8% 5% 2 4 3 3 4 4 5% 9% 53%	3268
	21st to end	Night	9% 5% 2 6% 4 4 4 4 7% 8% 46%	3948
August	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 5% 8% 49%	2822
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 6% 9% 48%	2888
	21st to end	Night	13% 6% 3 6% 4 4 4 3 6% 7% 45%	3352
September	1st to 10th	Night	10% 5% 2 6% 3 4 3 4 5% 7% 50%	2000
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 7% 49%	2035
	21st to end	Night	14% 7% 3 6% 4 3 4 2 4 6% 45%	2270
October	1st to 10th	Night	14% 8% 3 6% 5% 4 4 3 5% 6% 42%	2400
	11th to 20th	Night	16% 12% 3 7% 5% 4 4 3 4 6% 36%	2400
	21st to end	Night	19% 12% 4 9% 4 5% 5% 4 4 6% 27%	2715
November	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 5% 7% 32%	2871
	11th to 20th	Night	14% 12% 6% 12% 6% 4 4 4 5% 6% 27%	2940
	21st to end	Night	12% 10% 5% 13% 8% 5% 5% 4 5% 7% 27%	2940
December	1st to 10th	Night	8% 12% 6% 21% 12% 8% 6% 4 5% 5% 14%	4720
	11th to 20th	Night	9% 14% 6% 16% 7% 6% 5% 5% 6% 5% 20%	5476
	21st to end	Night	20% 19% 7% 16% 6% 4 3 2 4 4 15%	7645

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	60% 3 8% 16% 13%	3186
	11th to 20th	Day	66% 2 6% 20% 6%	3400
	21st to end	Day	65% 3 10% 17% 6%	4327
February	1st to 10th	Day	57% 3 10% 19% 10% 1	1800
	11th to 20th	Day	48% 4 12% 23% 10% 2	1763
	21st to end	Day	54% 3 14% 18% 11% 1	1503
March	1st to 10th	Day	43% 2 13% 24% 13% 4	4797
	11th to 20th	Day	49% 2 13% 24% 11%	4980
	21st to end	Day	56% 2 8% 22% 11%	5543
April	1st to 10th	Day	54% 3 8% 19% 14% 2	2400
	11th to 20th	Day	55% 5% 10% 20% 9% 1	2457
	21st to end	Day	54% 4 9% 22% 10% 1	2600
May	1st to 10th	Day	40% 3 11% 26% 19% 1	6302
	11th to 20th	Day	46% 2 12% 23% 13% 3	6323
	21st to end	Day	45% 4 9% 27% 15% 1	6949
June	1st to 10th	Day	40% 5% 18% 26% 10% 1	2929
	11th to 20th	Day	47% 5% 13% 23% 12% 1	3000
	21st to end	Day	42% 7% 14% 28% 9% 1	3000
July	1st to 10th	Day	38% 3 21% 29% 8%	6726
	11th to 20th	Day	37% 3 19% 33% 8%	6614
	21st to end	Day	37% 3 18% 31% 10%	6728
August	1st to 10th	Day	39% 3 16% 29% 12% 1	4319
	11th to 20th	Day	39% 4 21% 30% 6%	4210
	21st to end	Day	44% 3 16% 27% 8% 1	4551
September	1st to 10th	Day	39% 4 17% 30% 9%	2400
	11th to 20th	Day	43% 4 20% 25% 7%	2362
	21st to end	Day	42% 5% 14% 31% 7%	2110
October	1st to 10th	Day	39% 2 14% 32% 12% 1	3260
	11th to 20th	Day	47% 3 10% 22% 17% 1	3240
	21st to end	Day	56% 3 14% 21% 6%	3401
November	1st to 10th	Day	58% 4 10% 17% 9% 1	1647
	11th to 20th	Day	65% 3 11% 14% 6% 1	1680
	21st to end	Day	63% 4 12% 15% 5% 1	1680
December	1st to 10th	Day	74% 3 8% 13% 2	2690
	11th to 20th	Day	63% 2 6% 18% 11%	2934
	21st to end	Day	79% 2 4 11% 4	3575
January	1st to 10th	Night	60% 2 9% 16% 13%	6803
	11th to 20th	Night	64% 2 9% 15% 9%	6565
	21st to end	Night	63% 2 11% 18% 6%	6650
February	1st to 10th	Night	50% 4 12% 21% 12% 1	2600
	11th to 20th	Night	47% 4 12% 22% 13% 1	2534
	21st to end	Night	52% 4 11% 21% 10% 1	2126
March	1st to 10th	Night	41% 3 16% 20% 17% 3	5099
	11th to 20th	Night	56% 2 8% 21% 11%	4980
	21st to end	Night	60% 2 9% 17% 11% 1	5387
April	1st to 10th	Night	51% 3 9% 20% 15% 1	2000
	11th to 20th	Night	51% 4 9% 23% 12% 1	1890
	21st to end	Night	52% 3 9% 22% 13% 1	1800
May	1st to 10th	Night	40% 2 12% 28% 17%	3675
	11th to 20th	Night	41% 3 11% 23% 18% 4	3597
	21st to end	Night	47% 3 9% 26% 13% 1	3950
June	1st to 10th	Night	40% 4 20% 25% 10% 1	1467
	11th to 20th	Night	37% 4 15% 25% 17% 1	1400
	21st to end	Night	35% 6% 17% 31% 10% 1	1400
July	1st to 10th	Night	33% 5% 21% 30% 10%	3145
	11th to 20th	Night	30% 3 20% 36% 10% 1	3226
	21st to end	Night	35% 4 20% 30% 11% 1	3832
August	1st to 10th	Night	35% 3 19% 26% 15% 2	2813
	11th to 20th	Night	34% 3 18% 36% 8% 1	2915
	21st to end	Night	39% 3 20% 27% 10% 1	3352
September	1st to 10th	Night	33% 5% 17% 33% 11%	2000
	11th to 20th	Night	33% 5% 22% 31% 9%	2035
	21st to end	Night	40% 3 12% 33% 11%	2270
October	1st to 10th	Night	31% 2 11% 37% 16% 3	3920
	11th to 20th	Night	44% 2 11% 27% 15%	3899
	21st to end	Night	56% 3 11% 22% 8%	4387
November	1st to 10th	Night	51% 3 13% 23% 9% 1	2871
	11th to 20th	Night	59% 4 13% 16% 7% 1	2940
	21st to end	Night	55% 5% 16% 17% 6% 1	2940
December	1st to 10th	Night	72% 3 10% 12% 2	4720
	11th to 20th	Night	65% 3 9% 14% 9% 1	5475
	21st to end	Night	75% 2 6% 13% 5%	7645

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	13% 15% 34% 25% 12% 1	303
	11th to 20th	Day	16% 11% 37% 28% 8% 1	300
	21st to end	Day	15% 10% 37% 24% 13% 1	330
February	1st to 10th	Day	17% 14% 44% 18% 7% 1	300
	11th to 20th	Day	15% 19% 41% 14% 9% 2	300
	21st to end	Day	16% 12% 47% 20% 4 1	249
March	1st to 10th	Day	14% 14% 43% 20% 6% 2	300
	11th to 20th	Day	18% 12% 38% 21% 10% 2	300
	21st to end	Day	24% 16% 31% 18% 9% 1	358
April	1st to 10th	Day	23% 14% 34% 22% 6% 1	500
	11th to 20th	Day	22% 13% 29% 26% 8% 1	575
	21st to end	Day	24% 11% 32% 21% 11% 1	700
May	1st to 10th	Day	20% 15% 31% 22% 9% 3	703
	11th to 20th	Day	20% 12% 30% 25% 11% 2	725
	21st to end	Day	17% 11% 28% 29% 11% 3	789
June	1st to 10th	Day	14% 11% 30% 29% 12% 4	730
	11th to 20th	Day	17% 12% 34% 27% 8% 2	730
	21st to end	Day	17% 10% 31% 24% 14% 5%	730
July	1st to 10th	Day	11% 6% 25% 28% 22% 8%	721
	11th to 20th	Day	12% 8% 29% 25% 17% 9%	718
	21st to end	Day	9% 5% 28% 30% 19% 8%	794
August	1st to 10th	Day	12% 6% 24% 31% 20% 5%	726
	11th to 20th	Day	14% 5% 24% 29% 20% 7%	715
	21st to end	Day	16% 8% 25% 28% 19% 5%	732
September	1st to 10th	Day	12% 10% 31% 28% 16% 3	530
	11th to 20th	Day	18% 7% 32% 26% 13% 4	492
	21st to end	Day	21% 12% 29% 20% 12% 5%	330
October	1st to 10th	Day	22% 14% 33% 17% 11% 3	330
	11th to 20th	Day	26% 16% 33% 21% 3 1	330
	21st to end	Day	30% 17% 33% 16% 4 1	363
November	1st to 10th	Day	22% 18% 32% 21% 6% 1	330
	11th to 20th	Day	25% 20% 35% 15% 5% 1	330
	21st to end	Day	22% 15% 38% 22% 2	330
December	1st to 10th	Day	19% 12% 41% 24% 5%	330
	11th to 20th	Day	21% 18% 32% 25% 4	330
	21st to end	Day	26% 11% 35% 20% 6% 2	363
January	1st to 10th	Night	13% 10% 38% 27% 11% 2	1006
	11th to 20th	Night	15% 8% 39% 28% 9% 1	1000
	21st to end	Night	16% 11% 38% 28% 7%	1100
February	1st to 10th	Night	13% 9% 38% 29% 10% 1	1000
	11th to 20th	Night	12% 13% 40% 25% 9% 2	979
	21st to end	Night	14% 9% 38% 28% 10% 2	806
March	1st to 10th	Night	16% 11% 37% 28% 9% 1	682
	11th to 20th	Night	14% 12% 40% 28% 6% 1	600
	21st to end	Night	21% 10% 29% 26% 11% 3	660
April	1st to 10th	Night	19% 10% 34% 29% 7% 1	600
	11th to 20th	Night	20% 10% 35% 27% 8% 1	593
	21st to end	Night	24% 7% 29% 30% 10%	600
May	1st to 10th	Night	20% 8% 33% 30% 8% 1	600
	11th to 20th	Night	18% 6% 33% 31% 10% 1	591
	21st to end	Night	16% 8% 28% 37% 11%	650
June	1st to 10th	Night	14% 8% 29% 36% 12% 1	600
	11th to 20th	Night	15% 6% 35% 36% 8%	600
	21st to end	Night	12% 5% 27% 38% 16% 1	600
July	1st to 10th	Night	7% 3 25% 41% 22% 2	585
	11th to 20th	Night	8% 5% 26% 40% 18% 2	582
	21st to end	Night	6% 3 26% 42% 21% 2	657
August	1st to 10th	Night	10% 4 23% 41% 22% 1	597
	11th to 20th	Night	14% 3 23% 36% 22% 2	594
	21st to end	Night	12% 5% 25% 38% 18% 2	660
September	1st to 10th	Night	12% 5% 26% 38% 16% 3	600
	11th to 20th	Night	12% 5% 31% 34% 16% 2	600
	21st to end	Night	17% 9% 33% 26% 12% 3	670
October	1st to 10th	Night	21% 10% 31% 28% 9%	800
	11th to 20th	Night	23% 11% 34% 26% 6% 1	800
	21st to end	Night	26% 13% 32% 24% 5%	885
November	1st to 10th	Night	20% 10% 34% 29% 7%	1025
	11th to 20th	Night	20% 14% 34% 24% 7% 1	1050
	21st to end	Night	18% 15% 38% 23% 6%	1050
December	1st to 10th	Night	15% 11% 36% 28% 9% 1	1050
	11th to 20th	Night	21% 11% 37% 23% 7% 1	1050
	21st to end	Night	24% 9% 35% 24% 6% 2	1155

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