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 2024-11-04.

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 8% 5% 5% 5% 4 5% 7% 34%	101247
	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 33%	97830
Transparency	Day	47% 8% 12% 20% 12% 1	104818
	Night	50% 7% 11% 19% 12% 1	102387
Seeing	Day	17% 14% 30% 23% 12% 4	15823
	Night	19% 10% 30% 29% 11% 1	25666

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	15% 23% 8% 14% 7% 5% 5% 3 3 4 13%	2527
	11th to 20th	Day	13% 18% 8% 18% 15% 9% 4 3 3 4 7%	2530
	21st to end	Day	22% 22% 9% 15% 7% 5% 3 3 2 3 8%	2783
February	1st to 10th	Day	12% 16% 6% 15% 11% 6% 6% 3 4 4 18%	1741
	11th to 20th	Day	13% 15% 6% 16% 8% 4 4 3 4 6% 21%	1848
	21st to end	Day	15% 16% 6% 12% 6% 4 5% 3 6% 6% 22%	1570
March	1st to 10th	Day	15% 14% 5% 8% 5% 5% 4 3 5% 6% 30%	3120
	11th to 20th	Day	12% 8% 6% 10% 6% 6% 5% 4 6% 7% 32%	3379
	21st to end	Day	20% 13% 6% 8% 5% 4 3 3 4 6% 28%	3962
April	1st to 10th	Day	18% 14% 4 7% 5% 4 3 3 5% 6% 30%	2280
	11th to 20th	Day	17% 11% 3 6% 4 4 4 3 5% 7% 36%	2309
	21st to end	Day	17% 10% 3 7% 5% 4 5% 3 5% 6% 35%	2470
May	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 3 5% 7% 37%	4082
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 8% 39%	4095
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	4824
June	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 6% 8% 43%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 9% 46%	2660
	21st to end	Day	11% 6% 2 7% 4 4 5% 4 6% 9% 41%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	4440
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 6% 10% 44%	4813
August	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 7% 8% 41%	4145
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 6% 8% 45%	4044
	21st to end	Day	8% 6% 2 7% 5% 4 5% 4 6% 8% 45%	4226
September	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 6% 7% 44%	2251
	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%	1699
	11th to 20th	Day	15% 19% 5% 11% 6% 4 5% 3 5% 6% 22%	1710
	21st to end	Day	20% 17% 5% 8% 4 5% 4 3 4 6% 24%	1710
December	1st to 10th	Day	14% 23% 7% 15% 7% 5% 4 3 3 5% 14%	1568
	11th to 20th	Day	14% 21% 9% 13% 7% 5% 5% 4 5% 5% 13%	1918
	21st to end	Day	21% 26% 7% 13% 6% 4 4 3 3 4 9%	2783
January	1st to 10th	Night	16% 22% 9% 14% 6% 4 4 2 3 4 15%	4428
	11th to 20th	Night	14% 19% 10% 18% 10% 7% 3 3 3 3 10%	4440
	21st to end	Night	22% 21% 9% 13% 6% 5% 4 2 4 4 11%	4884
February	1st to 10th	Night	14% 18% 5% 13% 9% 5% 4 3 4 5% 20%	2433
	11th to 20th	Night	12% 16% 6% 12% 6% 5% 6% 4 4 4 25%	2218
	21st to end	Night	16% 15% 4 10% 6% 5% 5% 3 5% 6% 25%	1884
March	1st to 10th	Night	16% 10% 5% 7% 5% 4 4 3 5% 6% 36%	3733
	11th to 20th	Night	9% 8% 3 6% 4 3 4 3 6% 8% 45%	3580
	21st to end	Night	17% 13% 5% 8% 5% 4 4 4 5% 6% 29%	3699
April	1st to 10th	Night	16% 11% 4 7% 5% 4 4 3 4 6% 37%	1900
	11th to 20th	Night	20% 8% 2 7% 4 3 4 4 5% 5% 38%	1820
	21st to end	Night	18% 8% 2 6% 3 4 4 4 5% 7% 38%	1710
May	1st to 10th	Night	14% 8% 4 8% 4 5% 4 4 4 7% 37%	2813
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2795
	21st to end	Night	16% 7% 2 8% 4 4 5% 4 4 6% 41%	2743
June	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 4 6% 9% 45%	1520
	11th to 20th	Night	9% 6% 3 6% 5% 4 3 4 6% 10% 44%	1520
	21st to end	Night	9% 6% 2 4 5% 5% 5% 5% 7% 8% 45%	1520
July	1st to 10th	Night	9% 7% 2 7% 5% 5% 5% 4 6% 8% 42%	2530
	11th to 20th	Night	7% 4 1 5% 3 4 4 3 6% 7% 56%	2530
	21st to end	Night	7% 5% 2 6% 4 5% 4 4 6% 7% 49%	2750
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 6% 7% 49%	2785
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 49%	2785
	21st to end	Night	9% 5% 3 7% 5% 5% 4 4 7% 7% 45%	3426
September	1st to 10th	Night	9% 4 2 6% 3 4 4 4 5% 7% 51%	1909
	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% 37%	2452
	11th to 20th	Night	15% 15% 5% 10% 5% 5% 4 4 7% 6% 24%	2470
	21st to end	Night	18% 16% 4 9% 5% 4 4 3 5% 6% 25%	2470
December	1st to 10th	Night	16% 21% 6% 13% 7% 5% 5% 3 4 5% 15%	2607
	11th to 20th	Night	16% 19% 9% 12% 6% 5% 4 3 4 5% 16%	3279
	21st to end	Night	27% 22% 6% 10% 5% 4 4 3 4 5% 12%	4884

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	72% 10% 10% 7% 1	2528
	11th to 20th	Day	82% 7% 7% 3 1	2530
	21st to end	Day	81% 6% 7% 4 2	2783
February	1st to 10th	Day	69% 7% 7% 13% 4	1741
	11th to 20th	Day	64% 6% 10% 13% 7%	1858
	21st to end	Day	62% 7% 12% 13% 5% 1	1570
March	1st to 10th	Day	53% 7% 12% 19% 7%	3120
	11th to 20th	Day	47% 9% 15% 19% 8% 1	3379
	21st to end	Day	57% 6% 11% 17% 7% 1	3962
April	1st to 10th	Day	54% 7% 10% 19% 11%	2280
	11th to 20th	Day	47% 6% 11% 20% 13% 2	2309
	21st to end	Day	48% 7% 12% 17% 14% 2	2470
May	1st to 10th	Day	42% 9% 13% 21% 15% 1	4120
	11th to 20th	Day	38% 10% 14% 24% 13% 2	4099
	21st to end	Day	34% 9% 12% 20% 22% 2	4837
June	1st to 10th	Day	36% 7% 13% 26% 16% 2	2660
	11th to 20th	Day	35% 7% 12% 24% 20% 1	2660
	21st to end	Day	37% 9% 13% 22% 17% 2	2660
July	1st to 10th	Day	42% 7% 13% 20% 15% 3	4371
	11th to 20th	Day	27% 9% 15% 30% 18% 1	4365
	21st to end	Day	33% 8% 14% 21% 22% 2	4662
August	1st to 10th	Day	37% 7% 13% 28% 15% 1	4130
	11th to 20th	Day	35% 8% 12% 28% 17% 1	4083
	21st to end	Day	35% 7% 13% 27% 17% 2	4225
September	1st to 10th	Day	37% 8% 14% 24% 15% 2	2251
	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	1699
	11th to 20th	Day	61% 7% 12% 15% 5% 1	1710
	21st to end	Day	62% 6% 14% 13% 4	1710
December	1st to 10th	Day	73% 7% 9% 8% 2	1568
	11th to 20th	Day	71% 9% 10% 9% 1	1915
	21st to end	Day	77% 7% 7% 7% 1	2783
January	1st to 10th	Night	73% 8% 8% 7% 4	4428
	11th to 20th	Night	80% 6% 7% 5% 3	4440
	21st to end	Night	76% 8% 8% 6% 2	4884
February	1st to 10th	Night	67% 7% 9% 12% 5%	2433
	11th to 20th	Night	61% 7% 9% 16% 7%	2219
	21st to end	Night	59% 7% 12% 14% 7% 1	1884
March	1st to 10th	Night	46% 8% 13% 25% 7% 1	3733
	11th to 20th	Night	34% 8% 16% 25% 15% 2	3580
	21st to end	Night	53% 8% 12% 17% 8% 1	3699
April	1st to 10th	Night	48% 7% 11% 21% 13% 1	1900
	11th to 20th	Night	47% 6% 9% 18% 17% 4	1820
	21st to end	Night	44% 6% 13% 19% 15% 3	1710
May	1st to 10th	Night	43% 9% 13% 19% 14% 1	2840
	11th to 20th	Night	41% 7% 9% 21% 18% 3	2796
	21st to end	Night	41% 8% 10% 18% 21% 2	2751
June	1st to 10th	Night	35% 7% 12% 29% 16% 3	1520
	11th to 20th	Night	34% 9% 14% 23% 19% 2	1520
	21st to end	Night	34% 8% 16% 24% 15% 2	1520
July	1st to 10th	Night	37% 8% 12% 25% 12% 7%	2490
	11th to 20th	Night	26% 7% 14% 28% 24% 1	2482
	21st to end	Night	33% 8% 13% 19% 24% 3	2662
August	1st to 10th	Night	32% 7% 14% 30% 17% 1	2776
	11th to 20th	Night	33% 7% 10% 32% 16% 1	2812
	21st to end	Night	36% 7% 11% 28% 17% 2	3426
September	1st to 10th	Night	31% 8% 14% 27% 18% 2	1909
	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% 12% 22% 12% 1	2452
	11th to 20th	Night	58% 8% 14% 15% 6% 1	2470
	21st to end	Night	59% 7% 15% 16% 3	2470
December	1st to 10th	Night	72% 7% 8% 9% 4	2607
	11th to 20th	Night	70% 7% 11% 9% 3	3281
	21st to end	Night	74% 7% 10% 6% 2	4884

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	19% 18% 37% 20% 4 1	270
	11th to 20th	Day	23% 19% 41% 12% 4	270
	21st to end	Day	26% 17% 35% 19% 3	297
February	1st to 10th	Day	21% 27% 30% 17% 3 2	270
	11th to 20th	Day	19% 31% 34% 15% 1	270
	21st to end	Day	23% 34% 33% 8% 2	225
March	1st to 10th	Day	21% 28% 38% 9% 3	270
	11th to 20th	Day	22% 18% 32% 19% 9% 1	398
	21st to end	Day	28% 16% 30% 18% 5% 2	506
April	1st to 10th	Day	24% 20% 31% 19% 5% 1	460
	11th to 20th	Day	27% 21% 27% 16% 6% 3	452
	21st to end	Day	24% 16% 30% 20% 9% 1	460
May	1st to 10th	Day	22% 18% 29% 21% 7% 2	463
	11th to 20th	Day	18% 14% 29% 23% 11% 5%	488
	21st to end	Day	14% 12% 28% 26% 15% 4	734
June	1st to 10th	Day	12% 11% 33% 29% 11% 4	680
	11th to 20th	Day	14% 12% 31% 28% 12% 4	680
	21st to end	Day	15% 9% 35% 25% 12% 4	680
July	1st to 10th	Day	8% 7% 26% 30% 20% 8%	671
	11th to 20th	Day	10% 9% 26% 30% 18% 7%	668
	21st to end	Day	9% 5% 31% 32% 17% 7%	739
August	1st to 10th	Day	11% 5% 25% 24% 22% 13%	515
	11th to 20th	Day	11% 6% 23% 28% 24% 9%	479
	21st to end	Day	12% 6% 23% 29% 21% 10%	539
September	1st to 10th	Day	13% 8% 23% 29% 19% 9%	490
	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	22% 18% 36% 17% 6% 1	279
	11th to 20th	Day	22% 27% 33% 13% 5%	270
	21st to end	Day	28% 16% 41% 12% 3	270
December	1st to 10th	Day	24% 29% 28% 13% 6% 1	270
	11th to 20th	Day	22% 27% 34% 13% 4	270
	21st to end	Day	24% 28% 35% 11% 2	297
January	1st to 10th	Night	21% 16% 33% 23% 6% 1	950
	11th to 20th	Night	24% 14% 34% 23% 5% 1	950
	21st to end	Night	22% 10% 34% 30% 4	1045
February	1st to 10th	Night	22% 15% 35% 25% 3	789
	11th to 20th	Night	20% 15% 35% 25% 4	743
	21st to end	Night	24% 16% 38% 19% 3	628
March	1st to 10th	Night	18% 14% 35% 26% 6%	760
	11th to 20th	Night	17% 14% 36% 25% 6% 1	760
	21st to end	Night	21% 12% 33% 27% 7% 1	769
April	1st to 10th	Night	19% 12% 36% 27% 5%	570
	11th to 20th	Night	23% 12% 33% 25% 6% 1	563
	21st to end	Night	23% 11% 32% 27% 7% 1	570
May	1st to 10th	Night	18% 9% 32% 32% 8% 1	570
	11th to 20th	Night	18% 8% 31% 31% 12%	561
	21st to end	Night	13% 7% 26% 34% 18% 1	617
June	1st to 10th	Night	11% 6% 29% 39% 15% 1	570
	11th to 20th	Night	11% 5% 32% 36% 16%	570
	21st to end	Night	11% 5% 27% 38% 18% 2	570
July	1st to 10th	Night	8% 3 20% 41% 26% 3	555
	11th to 20th	Night	7% 2 22% 43% 24% 3	552
	21st to end	Night	8% 1 20% 42% 28% 1	624
August	1st to 10th	Night	9% 2 18% 43% 26% 2	567
	11th to 20th	Night	10% 2 18% 41% 24% 4	564
	21st to end	Night	10% 3 18% 41% 25% 3	627
September	1st to 10th	Night	9% 4 21% 39% 22% 4	579
	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% 31% 26% 9%	755
	11th to 20th	Night	23% 15% 34% 22% 6% 1	760
	21st to end	Night	25% 14% 31% 25% 4	760
December	1st to 10th	Night	25% 15% 32% 23% 5%	897
	11th to 20th	Night	25% 15% 36% 20% 4 1	950
	21st to end	Night	31% 12% 37% 18% 1	1045

Page generated at 2024-11-01 22:47:50. Version ac07

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