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 2024-11-12.

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% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	101914
	Night	12% 10% 4 9% 5% 5% 5% 4 5% 6% 34%	97418
Transparency	Day	49% 4 13% 25% 9% 1	105437
	Night	49% 4 13% 23% 10% 1	101983
Seeing	Day	17% 11% 31% 25% 12% 3	16162
	Night	16% 9% 33% 30% 11% 1	26049

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	7% 16% 6% 17% 7% 5% 5% 4 5% 6% 21%	2218
	11th to 20th	Day	6% 10% 5% 15% 9% 11% 12% 5% 7% 5% 14%	2375
	21st to end	Day	7% 14% 12% 18% 8% 5% 5% 4 5% 6% 17%	3016
February	1st to 10th	Day	9% 12% 4 12% 6% 6% 6% 5% 6% 7% 27%	1710
	11th to 20th	Day	8% 11% 4 9% 6% 6% 6% 4 6% 6% 33%	1664
	21st to end	Day	10% 10% 5% 11% 8% 6% 5% 3 6% 6% 29%	1430
March	1st to 10th	Day	10% 10% 4 8% 6% 5% 4 3 5% 7% 37%	3337
	11th to 20th	Day	7% 10% 7% 11% 5% 5% 4 3 5% 7% 36%	3480
	21st to end	Day	19% 13% 5% 9% 5% 3 4 3 4 6% 29%	3865
April	1st to 10th	Day	15% 12% 4 9% 6% 5% 5% 3 4 6% 32%	2280
	11th to 20th	Day	17% 11% 4 8% 5% 5% 5% 5% 5% 7% 30%	2333
	21st to end	Day	21% 10% 3 7% 4 4 4 4 4 5% 32%	2470
May	1st to 10th	Day	20% 9% 2 7% 4 4 4 3 5% 6% 35%	4386
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 39%	4399
	21st to end	Day	18% 9% 2 8% 5% 4 4 4 6% 8% 33%	4837
June	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 7% 9% 39%	2782
	11th to 20th	Day	16% 7% 2 7% 5% 4 5% 4 7% 7% 36%	2850
	21st to end	Day	13% 5% 2 7% 5% 5% 5% 5% 7% 9% 35%	2850
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	4750
	11th to 20th	Day	7% 4 1 5% 4 4 5% 4 7% 10% 47%	4671
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 44%	4814
August	1st to 10th	Day	9% 5% 2 7% 4 5% 5% 5% 7% 8% 41%	4201
	11th to 20th	Day	13% 5% 2 5% 4 5% 5% 5% 7% 10% 41%	4041
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 7% 40%	4413
September	1st to 10th	Day	11% 7% 2 5% 4 5% 4 4 5% 9% 43%	2280
	11th to 20th	Day	14% 7% 2 7% 4 5% 4 4 5% 7% 41%	2244
	21st to end	Day	18% 7% 2 6% 3 4 4 3 5% 7% 40%	2005
October	1st to 10th	Day	17% 10% 2 6% 4 4 5% 4 5% 6% 35%	1900
	11th to 20th	Day	16% 15% 3 8% 5% 4 4 3 4 6% 32%	1900
	21st to end	Day	19% 15% 5% 9% 4 3 4 4 4 6% 28%	2017
November	1st to 10th	Day	12% 16% 5% 10% 7% 5% 4 3 5% 5% 28%	1567
	11th to 20th	Day	12% 16% 6% 15% 8% 5% 5% 3 4 5% 22%	1528
	21st to end	Day	13% 13% 5% 12% 8% 6% 5% 4 5% 6% 22%	1520
December	1st to 10th	Day	11% 14% 5% 15% 10% 7% 5% 4 4 6% 19%	1520
	11th to 20th	Day	10% 16% 7% 15% 7% 6% 6% 5% 5% 4 19%	1819
	21st to end	Day	22% 19% 7% 13% 8% 4 3 2 3 5% 15%	2442
January	1st to 10th	Night	8% 13% 5% 15% 9% 6% 5% 4 5% 6% 25%	4737
	11th to 20th	Night	6% 12% 5% 14% 10% 9% 9% 4 5% 6% 20%	4593
	21st to end	Night	8% 17% 12% 15% 6% 5% 4 3 5% 5% 21%	4650
February	1st to 10th	Night	7% 10% 5% 11% 5% 6% 6% 5% 7% 7% 32%	2470
	11th to 20th	Night	6% 10% 4 10% 6% 5% 7% 5% 6% 6% 34%	2403
	21st to end	Night	9% 13% 5% 11% 6% 5% 6% 5% 6% 6% 29%	2023
March	1st to 10th	Night	9% 10% 3 8% 5% 4 4 4 5% 6% 43%	3532
	11th to 20th	Night	8% 9% 4 13% 7% 5% 4 4 5% 6% 34%	3479
	21st to end	Night	19% 16% 5% 11% 5% 3 3 3 3 4 28%	3765
April	1st to 10th	Night	16% 10% 4 10% 5% 4 4 3 6% 7% 33%	1900
	11th to 20th	Night	16% 11% 4 7% 5% 4 3 4 5% 5% 36%	1795
	21st to end	Night	20% 9% 3 8% 4 4 4 3 6% 4 35%	1710
May	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 5% 6% 38%	2564
	11th to 20th	Night	14% 7% 3 6% 4 4 3 3 6% 7% 43%	2506
	21st to end	Night	20% 8% 3 7% 4 4 4 3 4 6% 36%	2750
June	1st to 10th	Night	12% 6% 2 7% 5% 4 5% 3 6% 8% 41%	1394
	11th to 20th	Night	13% 6% 2 5% 4 3 5% 3 6% 8% 46%	1330
	21st to end	Night	8% 6% 2 5% 5% 3 5% 5% 7% 9% 45%	1330
July	1st to 10th	Night	7% 4 2 5% 3 5% 4 5% 9% 11% 45%	2220
	11th to 20th	Night	4 3 2 4 4 3 4 4 6% 9% 58%	2282
	21st to end	Night	5% 3 1 4 4 3 4 4 7% 9% 55%	2750
August	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 5% 8% 49%	2735
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 6% 9% 48%	2798
	21st to end	Night	13% 6% 3 6% 4 4 4 3 6% 7% 45%	3249
September	1st to 10th	Night	10% 5% 1 6% 3 4 3 4 5% 6% 51%	1900
	11th to 20th	Night	10% 5% 2 5% 4 4 3 3 6% 7% 50%	1933
	21st to end	Night	15% 7% 3 6% 4 3 4 2 4 6% 44%	2156
October	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 41%	2280
	11th to 20th	Night	16% 12% 3 7% 5% 4 4 3 4 6% 36%	2280
	21st to end	Night	19% 12% 4 9% 4 5% 5% 4 4 6% 28%	2579
November	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 5% 7% 33%	2731
	11th to 20th	Night	14% 12% 6% 12% 7% 5% 5% 4 5% 6% 25%	2675
	21st to end	Night	13% 10% 4 11% 7% 5% 5% 4 6% 7% 28%	2660
December	1st to 10th	Night	9% 13% 6% 15% 8% 7% 7% 4 6% 6% 18%	2660
	11th to 20th	Night	11% 14% 6% 14% 7% 7% 6% 4 6% 6% 20%	3374
	21st to end	Night	23% 19% 5% 12% 6% 5% 4 3 4 5% 16%	5225

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	63% 4 10% 17% 6%	2219
	11th to 20th	Day	69% 3 8% 16% 4	2375
	21st to end	Day	68% 4 7% 15% 6%	3016
February	1st to 10th	Day	56% 3 11% 20% 10% 1	1710
	11th to 20th	Day	49% 5% 13% 22% 10% 2	1673
	21st to end	Day	54% 3 13% 17% 12% 1	1430
March	1st to 10th	Day	48% 3 11% 26% 12%	3339
	11th to 20th	Day	48% 3 12% 26% 11%	3480
	21st to end	Day	58% 3 9% 23% 6%	3865
April	1st to 10th	Day	54% 3 8% 19% 14% 2	2280
	11th to 20th	Day	55% 5% 10% 20% 9% 1	2333
	21st to end	Day	54% 4 9% 22% 10% 1	2470
May	1st to 10th	Day	49% 4 10% 22% 14% 2	4398
	11th to 20th	Day	44% 3 14% 27% 10% 2	4403
	21st to end	Day	49% 4 12% 25% 9% 1	4837
June	1st to 10th	Day	40% 5% 18% 26% 10% 1	2782
	11th to 20th	Day	46% 5% 12% 24% 13% 1	2850
	21st to end	Day	42% 7% 15% 28% 8%	2850
July	1st to 10th	Day	39% 4 18% 27% 11%	4676
	11th to 20th	Day	31% 4 19% 36% 10% 1	4590
	21st to end	Day	34% 4 18% 33% 10% 1	4662
August	1st to 10th	Day	39% 3 16% 29% 12% 1	4186
	11th to 20th	Day	39% 4 21% 30% 6%	4080
	21st to end	Day	44% 3 16% 27% 9% 1	4412
September	1st to 10th	Day	38% 4 16% 31% 10%	2280
	11th to 20th	Day	42% 4 20% 26% 7%	2244
	21st to end	Day	44% 5% 14% 31% 7%	2005
October	1st to 10th	Day	40% 2 13% 33% 12%	3160
	11th to 20th	Day	47% 3 10% 23% 17% 1	3140
	21st to end	Day	56% 3 14% 21% 6%	3296
November	1st to 10th	Day	58% 4 10% 18% 9% 1	1567
	11th to 20th	Day	66% 3 12% 12% 7% 1	1528
	21st to end	Day	62% 5% 12% 16% 5% 1	1520
December	1st to 10th	Day	65% 5% 11% 15% 4	1520
	11th to 20th	Day	67% 3 9% 17% 3	1819
	21st to end	Day	74% 2 4 13% 5% 1	2442
January	1st to 10th	Night	60% 3 11% 20% 6%	4737
	11th to 20th	Night	65% 3 12% 14% 5%	4593
	21st to end	Night	66% 3 8% 18% 5%	4650
February	1st to 10th	Night	49% 4 12% 22% 12% 1	2470
	11th to 20th	Night	48% 5% 12% 22% 13% 1	2404
	21st to end	Night	52% 5% 11% 21% 10% 1	2023
March	1st to 10th	Night	42% 4 15% 22% 16% 2	3543
	11th to 20th	Night	51% 3 10% 24% 12% 1	3480
	21st to end	Night	61% 3 9% 16% 10% 1	3765
April	1st to 10th	Night	51% 4 10% 20% 15% 1	1900
	11th to 20th	Night	51% 4 8% 23% 13% 1	1795
	21st to end	Night	51% 3 9% 23% 13% 1	1710
May	1st to 10th	Night	48% 3 11% 26% 12% 1	2571
	11th to 20th	Night	41% 3 12% 26% 15% 3	2507
	21st to end	Night	50% 4 11% 23% 11% 1	2751
June	1st to 10th	Night	40% 4 21% 24% 11% 1	1394
	11th to 20th	Night	36% 5% 15% 25% 17% 2	1330
	21st to end	Night	34% 7% 17% 31% 10% 1	1330
July	1st to 10th	Night	32% 6% 22% 28% 12%	2185
	11th to 20th	Night	24% 3 17% 42% 13% 1	2240
	21st to end	Night	25% 5% 21% 37% 12% 1	2662
August	1st to 10th	Night	34% 3 19% 26% 15% 2	2726
	11th to 20th	Night	34% 4 18% 36% 8% 1	2825
	21st to end	Night	39% 4 19% 27% 10% 1	3249
September	1st to 10th	Night	33% 5% 16% 34% 11%	1900
	11th to 20th	Night	32% 5% 22% 31% 9%	1933
	21st to end	Night	41% 4 13% 32% 10%	2156
October	1st to 10th	Night	32% 2 10% 37% 16% 3	3800
	11th to 20th	Night	44% 2 11% 28% 15%	3779
	21st to end	Night	55% 3 12% 21% 8%	4251
November	1st to 10th	Night	51% 3 13% 22% 9% 1	2731
	11th to 20th	Night	60% 4 13% 15% 7% 1	2675
	21st to end	Night	53% 5% 18% 17% 6% 1	2660
December	1st to 10th	Night	63% 5% 14% 14% 3 1	2660
	11th to 20th	Night	63% 4 11% 15% 5% 1	3373
	21st to end	Night	72% 3 7% 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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 14% 34% 26% 12% 1	270
	11th to 20th	Day	16% 10% 38% 27% 9% 1	270
	21st to end	Day	13% 8% 36% 26% 15% 1	297
February	1st to 10th	Day	16% 13% 44% 20% 7% 1	270
	11th to 20th	Day	16% 19% 40% 14% 9% 2	270
	21st to end	Day	16% 12% 46% 21% 4 1	225
March	1st to 10th	Day	12% 14% 43% 22% 7% 3	270
	11th to 20th	Day	16% 11% 37% 22% 11% 3	270
	21st to end	Day	24% 15% 32% 19% 10% 2	323
April	1st to 10th	Day	22% 14% 33% 23% 6% 1	460
	11th to 20th	Day	23% 13% 27% 27% 9% 2	531
	21st to end	Day	23% 11% 32% 21% 12% 1	650
May	1st to 10th	Day	21% 15% 31% 22% 8% 3	653
	11th to 20th	Day	19% 12% 29% 25% 12% 2	675
	21st to end	Day	18% 10% 28% 29% 11% 3	734
June	1st to 10th	Day	14% 11% 30% 28% 13% 4	680
	11th to 20th	Day	17% 12% 33% 27% 8% 3	680
	21st to end	Day	16% 9% 30% 25% 14% 6%	680
July	1st to 10th	Day	11% 5% 25% 28% 22% 9%	671
	11th to 20th	Day	11% 8% 29% 25% 18% 9%	668
	21st to end	Day	8% 5% 27% 31% 20% 8%	739
August	1st to 10th	Day	12% 6% 24% 32% 20% 5%	676
	11th to 20th	Day	14% 5% 24% 30% 21% 7%	665
	21st to end	Day	17% 7% 25% 27% 19% 5%	681
September	1st to 10th	Day	12% 9% 31% 28% 16% 3	490
	11th to 20th	Day	16% 7% 32% 26% 14% 4	454
	21st to end	Day	23% 13% 31% 20% 11% 3	300
October	1st to 10th	Day	23% 14% 30% 18% 12% 3	300
	11th to 20th	Day	24% 15% 34% 22% 4 1	300
	21st to end	Day	29% 17% 33% 16% 4 1	330
November	1st to 10th	Day	20% 19% 32% 22% 7% 1	300
	11th to 20th	Day	23% 19% 36% 16% 5%	273
	21st to end	Day	22% 16% 38% 20% 3	270
December	1st to 10th	Day	19% 13% 39% 25% 5%	270
	11th to 20th	Day	20% 17% 31% 27% 5%	270
	21st to end	Day	27% 11% 32% 22% 6% 2	297
January	1st to 10th	Night	13% 9% 38% 27% 11% 2	950
	11th to 20th	Night	15% 8% 40% 28% 9% 1	950
	21st to end	Night	16% 10% 37% 29% 7%	1045
February	1st to 10th	Night	12% 8% 38% 30% 11% 1	950
	11th to 20th	Night	12% 12% 40% 25% 9% 2	929
	21st to end	Night	13% 9% 38% 27% 10% 2	767
March	1st to 10th	Night	16% 11% 37% 27% 9% 1	648
	11th to 20th	Night	14% 11% 39% 29% 6% 1	570
	21st to end	Night	21% 9% 29% 26% 12% 3	627
April	1st to 10th	Night	19% 10% 35% 28% 7% 1	570
	11th to 20th	Night	19% 10% 35% 27% 8% 1	563
	21st to end	Night	24% 7% 28% 31% 11%	570
May	1st to 10th	Night	20% 9% 33% 29% 8% 1	570
	11th to 20th	Night	18% 6% 33% 31% 11% 1	561
	21st to end	Night	16% 8% 29% 35% 12%	617
June	1st to 10th	Night	14% 8% 30% 35% 12% 1	570
	11th to 20th	Night	15% 6% 35% 36% 8%	570
	21st to end	Night	12% 5% 26% 39% 17% 1	570
July	1st to 10th	Night	6% 3 24% 41% 23% 2	555
	11th to 20th	Night	8% 5% 26% 41% 19% 2	552
	21st to end	Night	6% 3 26% 42% 22% 2	624
August	1st to 10th	Night	10% 3 24% 41% 21% 1	567
	11th to 20th	Night	13% 3 23% 37% 23% 2	564
	21st to end	Night	12% 6% 25% 37% 19% 2	627
September	1st to 10th	Night	12% 5% 26% 38% 16% 2	570
	11th to 20th	Night	11% 5% 31% 34% 17% 2	570
	21st to end	Night	17% 10% 34% 25% 12% 2	636
October	1st to 10th	Night	22% 9% 31% 29% 9%	760
	11th to 20th	Night	23% 10% 34% 26% 6% 1	760
	21st to end	Night	26% 12% 32% 24% 5%	841
November	1st to 10th	Night	20% 9% 34% 29% 7% 1	975
	11th to 20th	Night	19% 13% 35% 24% 7% 1	956
	21st to end	Night	18% 14% 38% 24% 6% 1	950
December	1st to 10th	Night	16% 11% 35% 28% 10% 1	950
	11th to 20th	Night	20% 10% 38% 24% 8% 1	950
	21st to end	Night	24% 9% 34% 25% 6% 1	1045

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