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 2024-02-05.

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 8% 7% 4 5% 5% 6% 7% 33%	78008
	Night	10% 10% 5% 9% 8% 4 6% 5% 6% 7% 30%	80837
Transparency	Day	33% 38% 10% 13% 5%	77709
	Night	36% 36% 10% 12% 5%	80718
Seeing	Day	16% 13% 34% 25% 11% 2	15243
	Night	16% 10% 31% 29% 12% 2	25007

By Month

Percentage Cloud Cover NW (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% 9% 15% 12% 6% 7% 6% 5% 6% 16%	2219
	11th to 20th	Day	4 8% 7% 20% 18% 8% 8% 5% 6% 5% 12%	2199
	21st to end	Day	8% 14% 6% 12% 11% 5% 6% 6% 7% 6% 19%	2698
February	1st to 10th	Day	7% 10% 5% 10% 10% 5% 10% 9% 7% 8% 20%	1472
	11th to 20th	Day	6% 10% 4 9% 8% 5% 9% 7% 7% 9% 25%	1505
	21st to end	Day	4 8% 6% 12% 9% 6% 7% 8% 9% 7% 24%	1430
March	1st to 10th	Day	11% 12% 5% 9% 9% 4 5% 5% 6% 6% 28%	1800
	11th to 20th	Day	10% 9% 5% 8% 6% 4 5% 6% 6% 6% 34%	1933
	21st to end	Day	8% 9% 4 7% 6% 4 5% 5% 6% 7% 38%	2235
April	1st to 10th	Day	8% 10% 5% 8% 7% 4 5% 5% 5% 6% 39%	2222
	11th to 20th	Day	12% 10% 4 8% 6% 3 4 5% 5% 7% 37%	2312
	21st to end	Day	9% 7% 3 6% 6% 3 5% 5% 6% 9% 41%	2340
May	1st to 10th	Day	15% 8% 3 6% 5% 3 6% 5% 6% 6% 37%	2611
	11th to 20th	Day	12% 8% 3 6% 4 3 4 5% 5% 6% 45%	2655
	21st to end	Day	19% 7% 4 6% 4 3 4 4 5% 7% 37%	2918
June	1st to 10th	Day	14% 7% 3 7% 5% 3 5% 6% 7% 7% 36%	2700
	11th to 20th	Day	19% 6% 3 5% 5% 4 5% 5% 6% 8% 35%	2800
	21st to end	Day	14% 7% 3 5% 5% 3 5% 7% 6% 8% 36%	2880
July	1st to 10th	Day	9% 4 2 5% 5% 4 5% 5% 6% 10% 46%	2790
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 6% 6% 8% 42%	2700
	21st to end	Day	11% 5% 2 4 4 3 5% 5% 6% 9% 45%	2939
August	1st to 10th	Day	10% 4 2 4 5% 3 4 6% 6% 9% 46%	2529
	11th to 20th	Day	12% 5% 2 4 5% 2 5% 5% 7% 9% 43%	2394
	21st to end	Day	18% 6% 2 5% 5% 3 5% 5% 6% 8% 38%	2574
September	1st to 10th	Day	12% 7% 4 6% 5% 3 4 6% 5% 9% 39%	2264
	11th to 20th	Day	18% 8% 2 6% 5% 2 4 4 6% 8% 37%	1991
	21st to end	Day	12% 7% 4 7% 6% 3 5% 5% 6% 10% 35%	1980
October	1st to 10th	Day	19% 13% 3 7% 6% 3 4 5% 5% 6% 29%	1882
	11th to 20th	Day	17% 17% 6% 9% 7% 4 5% 4 6% 6% 20%	1623
	21st to end	Day	21% 16% 5% 9% 8% 4 6% 4 4 5% 17%	1782
November	1st to 10th	Day	15% 20% 6% 12% 10% 3 5% 5% 3 4 16%	1501
	11th to 20th	Day	13% 15% 6% 13% 14% 5% 5% 5% 4 5% 16%	1349
	21st to end	Day	10% 15% 8% 12% 8% 5% 9% 6% 5% 7% 16%	1330
December	1st to 10th	Day	6% 13% 7% 15% 11% 5% 7% 8% 6% 6% 15%	1330
	11th to 20th	Day	10% 15% 7% 12% 11% 5% 6% 6% 6% 7% 17%	1679
	21st to end	Day	6% 9% 4 8% 8% 6% 7% 6% 6% 6% 34%	2442
January	1st to 10th	Night	5% 12% 6% 13% 11% 6% 9% 7% 6% 6% 19%	4737
	11th to 20th	Night	4 9% 6% 21% 14% 6% 8% 5% 8% 5% 14%	4709
	21st to end	Night	9% 15% 6% 12% 9% 4 6% 7% 6% 6% 19%	4967
February	1st to 10th	Night	6% 10% 4 8% 9% 6% 9% 7% 7% 7% 27%	2583
	11th to 20th	Night	5% 9% 4 9% 8% 6% 7% 8% 7% 8% 28%	2342
	21st to end	Night	5% 10% 5% 11% 9% 5% 6% 6% 5% 6% 33%	1841
March	1st to 10th	Night	9% 12% 5% 8% 8% 4 6% 5% 5% 6% 33%	2160
	11th to 20th	Night	8% 10% 4 7% 6% 5% 4 5% 6% 7% 37%	2024
	21st to end	Night	9% 9% 3 6% 6% 3 4 5% 5% 8% 41%	2120
April	1st to 10th	Night	9% 9% 3 8% 6% 4 5% 5% 5% 6% 41%	1738
	11th to 20th	Night	13% 9% 3 7% 6% 4 5% 5% 6% 6% 36%	1599
	21st to end	Night	8% 6% 3 7% 6% 4 5% 4 6% 8% 43%	1620
May	1st to 10th	Night	16% 9% 4 7% 5% 2 4 5% 6% 8% 34%	1346
	11th to 20th	Night	12% 7% 4 6% 6% 3 6% 5% 5% 5% 40%	1241
	21st to end	Night	17% 7% 4 6% 4 3 4 4 5% 7% 40%	1358
June	1st to 10th	Night	14% 7% 3 5% 5% 5% 5% 6% 6% 5% 38%	1260
	11th to 20th	Night	17% 7% 3 5% 5% 3 5% 6% 6% 6% 37%	1153
	21st to end	Night	15% 6% 3 5% 5% 3 5% 5% 6% 9% 38%	1080
July	1st to 10th	Night	9% 4 1 4 5% 3 5% 4 6% 10% 48%	1165
	11th to 20th	Night	10% 4 2 3 5% 3 5% 5% 7% 11% 45%	1260
	21st to end	Night	10% 4 2 5% 6% 3 4 5% 5% 8% 47%	1379
August	1st to 10th	Night	8% 3 2 4 4 3 5% 4 5% 8% 53%	1408
	11th to 20th	Night	11% 5% 2 4 4 2 3 5% 5% 8% 50%	1506
	21st to end	Night	15% 7% 2 5% 5% 3 3 4 5% 7% 43%	1782
September	1st to 10th	Night	11% 5% 3 5% 6% 4 4 4 6% 8% 44%	1695
	11th to 20th	Night	17% 7% 2 6% 5% 3 4 4 5% 8% 39%	1968
	21st to end	Night	13% 7% 3 5% 6% 3 5% 4 6% 8% 41%	1980
October	1st to 10th	Night	16% 10% 3 7% 7% 3 6% 5% 5% 5% 33%	2060
	11th to 20th	Night	16% 14% 5% 8% 6% 3 4 5% 4 6% 29%	2335
	21st to end	Night	17% 16% 6% 8% 7% 3 5% 4 4 5% 24%	2574
November	1st to 10th	Night	15% 16% 7% 12% 9% 5% 5% 4 5% 5% 17%	2576
	11th to 20th	Night	12% 15% 6% 11% 12% 6% 5% 5% 5% 6% 18%	2828
	21st to end	Night	11% 15% 7% 14% 10% 6% 6% 5% 6% 5% 16%	2850
December	1st to 10th	Night	7% 13% 7% 14% 12% 5% 7% 4 5% 6% 18%	2850
	11th to 20th	Night	9% 13% 6% 10% 8% 5% 6% 6% 6% 7% 23%	3518
	21st to end	Night	8% 10% 4 9% 8% 4 5% 5% 5% 6% 35%	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	55% 28% 7% 9% 1	2219
	11th to 20th	Day	60% 26% 4 8% 2	2220
	21st to end	Day	47% 29% 14% 8% 3	2698
February	1st to 10th	Day	39% 37% 9% 10% 4 1	1472
	11th to 20th	Day	33% 40% 6% 13% 7%	1513
	21st to end	Day	39% 38% 8% 10% 4	1430
March	1st to 10th	Day	37% 38% 9% 11% 5%	1800
	11th to 20th	Day	35% 39% 10% 12% 4	1935
	21st to end	Day	29% 39% 13% 14% 6%	2235
April	1st to 10th	Day	28% 39% 10% 14% 8% 1	2222
	11th to 20th	Day	33% 39% 8% 13% 7%	2312
	21st to end	Day	26% 40% 10% 17% 7%	2340
May	1st to 10th	Day	29% 39% 8% 16% 7%	2611
	11th to 20th	Day	25% 39% 9% 17% 8% 2	2659
	21st to end	Day	32% 39% 8% 13% 8%	2918
June	1st to 10th	Day	27% 41% 10% 15% 7% 1	2700
	11th to 20th	Day	31% 41% 12% 12% 5%	2800
	21st to end	Day	28% 40% 11% 16% 5%	2880
July	1st to 10th	Day	20% 40% 12% 20% 7% 1	2716
	11th to 20th	Day	24% 41% 12% 17% 6%	2619
	21st to end	Day	26% 42% 13% 17% 2	2776
August	1st to 10th	Day	22% 40% 14% 20% 4	2512
	11th to 20th	Day	25% 39% 18% 15% 3	2394
	21st to end	Day	28% 40% 13% 17% 2	2574
September	1st to 10th	Day	26% 39% 13% 16% 5% 1	2264
	11th to 20th	Day	27% 40% 13% 18% 2	1991
	21st to end	Day	28% 40% 14% 14% 4	1980
October	1st to 10th	Day	33% 40% 10% 14% 3	1882
	11th to 20th	Day	44% 39% 7% 9% 2	1623
	21st to end	Day	43% 42% 6% 7% 2	1782
November	1st to 10th	Day	43% 43% 6% 6% 2	1501
	11th to 20th	Day	44% 40% 8% 7% 2	1349
	21st to end	Day	42% 43% 8% 6% 1	1330
December	1st to 10th	Day	42% 42% 7% 8% 1	1330
	11th to 20th	Day	45% 36% 10% 8% 2	1680
	21st to end	Day	34% 29% 10% 16% 9% 2	2442
January	1st to 10th	Night	48% 31% 10% 8% 2	4737
	11th to 20th	Night	56% 27% 7% 9% 1	4750
	21st to end	Night	49% 28% 12% 9% 3	4967
February	1st to 10th	Night	35% 37% 10% 13% 5% 1	2583
	11th to 20th	Night	32% 39% 10% 13% 6%	2344
	21st to end	Night	37% 36% 9% 13% 5%	1841
March	1st to 10th	Night	34% 38% 9% 15% 4	2160
	11th to 20th	Night	32% 40% 10% 12% 6%	2024
	21st to end	Night	27% 38% 11% 18% 6% 1	2120
April	1st to 10th	Night	30% 37% 7% 15% 9% 2	1738
	11th to 20th	Night	34% 37% 6% 12% 10% 1	1599
	21st to end	Night	26% 38% 8% 17% 10% 1	1620
May	1st to 10th	Night	32% 39% 8% 14% 8%	1346
	11th to 20th	Night	28% 39% 8% 14% 10% 2	1241
	21st to end	Night	34% 38% 4 12% 10% 1	1358
June	1st to 10th	Night	29% 39% 9% 13% 8% 2	1260
	11th to 20th	Night	32% 41% 9% 14% 4 1	1153
	21st to end	Night	27% 40% 13% 15% 6%	1080
July	1st to 10th	Night	19% 39% 11% 24% 6% 1	1131
	11th to 20th	Night	18% 42% 14% 18% 8%	1218
	21st to end	Night	25% 42% 11% 19% 3	1302
August	1st to 10th	Night	18% 40% 11% 24% 7%	1401
	11th to 20th	Night	25% 38% 14% 18% 4 1	1506
	21st to end	Night	27% 38% 13% 16% 5%	1782
September	1st to 10th	Night	23% 39% 14% 18% 6%	1695
	11th to 20th	Night	28% 37% 13% 19% 4	1968
	21st to end	Night	28% 38% 10% 19% 5%	1980
October	1st to 10th	Night	33% 38% 10% 15% 4	2060
	11th to 20th	Night	38% 39% 8% 12% 3	2335
	21st to end	Night	40% 42% 8% 8% 2	2574
November	1st to 10th	Night	43% 42% 7% 6% 3	2576
	11th to 20th	Night	43% 42% 8% 6% 1	2828
	21st to end	Night	43% 43% 7% 6% 1	2850
December	1st to 10th	Night	43% 42% 8% 7% 1	2850
	11th to 20th	Night	41% 36% 11% 7% 5%	3516
	21st to end	Night	35% 27% 10% 13% 13% 2	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	11% 13% 44% 24% 7% 2	270
	11th to 20th	Day	6% 11% 46% 28% 8% 2	270
	21st to end	Day	16% 14% 30% 30% 8% 1	297
February	1st to 10th	Day	10% 15% 38% 25% 10% 1	252
	11th to 20th	Day	12% 13% 35% 25% 12% 3	240
	21st to end	Day	2 15% 41% 19% 21% 3	198
March	1st to 10th	Day	13% 14% 38% 24% 9% 3	240
	11th to 20th	Day	8% 15% 40% 30% 5% 3	240
	21st to end	Day	12% 15% 35% 25% 12% 2	321
April	1st to 10th	Day	14% 11% 41% 25% 7% 2	482
	11th to 20th	Day	17% 11% 30% 28% 11% 3	590
	21st to end	Day	11% 13% 38% 29% 9% 1	600
May	1st to 10th	Day	14% 13% 35% 28% 9% 1	603
	11th to 20th	Day	14% 15% 33% 25% 12% 2	625
	21st to end	Day	20% 11% 31% 25% 10% 2	679
June	1st to 10th	Day	18% 14% 31% 21% 13% 3	630
	11th to 20th	Day	23% 12% 29% 25% 9% 2	630
	21st to end	Day	16% 11% 27% 30% 13% 3	630
July	1st to 10th	Day	11% 14% 29% 30% 12% 4	621
	11th to 20th	Day	15% 10% 31% 27% 14% 3	618
	21st to end	Day	12% 10% 29% 31% 13% 6%	684
August	1st to 10th	Day	13% 9% 28% 29% 16% 4	626
	11th to 20th	Day	14% 11% 28% 25% 17% 5%	615
	21st to end	Day	19% 8% 34% 25% 12% 1	693
September	1st to 10th	Day	17% 17% 34% 22% 8% 1	554
	11th to 20th	Day	22% 14% 34% 21% 8% 1	281
	21st to end	Day	17% 13% 34% 27% 7% 1	270
October	1st to 10th	Day	26% 12% 36% 16% 9% 2	270
	11th to 20th	Day	23% 17% 38% 17% 5% 1	270
	21st to end	Day	31% 15% 30% 18% 5% 2	297
November	1st to 10th	Day	22% 16% 32% 23% 7%	270
	11th to 20th	Day	21% 17% 38% 17% 6% 1	270
	21st to end	Day	19% 16% 41% 17% 6% 1	270
December	1st to 10th	Day	9% 14% 37% 27% 10% 3	270
	11th to 20th	Day	14% 14% 46% 20% 6%	270
	21st to end	Day	10% 8% 34% 33% 12% 3	297
January	1st to 10th	Night	14% 11% 34% 31% 10% 1	950
	11th to 20th	Night	14% 11% 37% 29% 9% 1	950
	21st to end	Night	18% 9% 36% 28% 9% 1	1045
February	1st to 10th	Night	11% 9% 37% 31% 11% 1	923
	11th to 20th	Night	9% 10% 35% 29% 14% 3	879
	21st to end	Night	9% 10% 34% 31% 14% 2	657
March	1st to 10th	Night	14% 9% 25% 35% 15% 2	720
	11th to 20th	Night	12% 13% 32% 30% 11% 2	584
	21st to end	Night	15% 7% 29% 30% 16% 4	594
April	1st to 10th	Night	14% 6% 29% 34% 15% 3	540
	11th to 20th	Night	16% 7% 26% 33% 15% 3	533
	21st to end	Night	11% 7% 31% 36% 14% 1	540
May	1st to 10th	Night	18% 8% 28% 32% 11% 1	540
	11th to 20th	Night	17% 10% 26% 31% 15% 2	531
	21st to end	Night	21% 8% 27% 30% 14% 1	584
June	1st to 10th	Night	18% 7% 27% 30% 16% 3	540
	11th to 20th	Night	22% 7% 24% 30% 15% 1	433
	21st to end	Night	19% 3 23% 38% 17% 1	360
July	1st to 10th	Night	11% 7% 25% 37% 19% 1	431
	11th to 20th	Night	11% 6% 29% 33% 19% 2	522
	21st to end	Night	13% 5% 24% 34% 20% 5%	591
August	1st to 10th	Night	10% 7% 25% 33% 23% 2	537
	11th to 20th	Night	14% 6% 23% 27% 25% 5%	534
	21st to end	Night	18% 8% 28% 28% 18% 1	594
September	1st to 10th	Night	14% 6% 31% 34% 13% 2	540
	11th to 20th	Night	20% 8% 28% 35% 9% 1	540
	21st to end	Night	16% 8% 32% 31% 12% 1	540
October	1st to 10th	Night	22% 9% 31% 27% 10% 1	620
	11th to 20th	Night	26% 11% 31% 21% 9% 1	895
	21st to end	Night	26% 14% 29% 22% 7% 1	990
November	1st to 10th	Night	21% 14% 32% 23% 9% 1	925
	11th to 20th	Night	20% 13% 37% 24% 6% 1	950
	21st to end	Night	19% 17% 36% 22% 5% 1	950
December	1st to 10th	Night	13% 11% 35% 29% 10% 2	950
	11th to 20th	Night	19% 12% 36% 24% 8% 2	950
	21st to end	Night	16% 8% 30% 30% 13% 3	1045

Page generated at 2024-02-02 04:20:29. Version ac07

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