Grundy Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grundy Center clear sky chart from 2005-11-06 to 2026-08-15.

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% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	144374
	Night	11% 9% 3 8% 5% 4 4 3 5% 6% 41%	137903
Transparency	Day	43% 4 12% 25% 14% 1	147970
	Night	43% 4 12% 25% 15% 2	142386
Seeing	Day	16% 10% 30% 27% 13% 3	18370
	Night	16% 9% 33% 31% 11% 1	28149

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	6% 13% 6% 22% 8% 6% 5% 3 4 4 22%	4691
	11th to 20th	Day	5% 10% 4 12% 7% 8% 7% 6% 8% 6% 27%	5046
	21st to end	Day	8% 11% 5% 12% 6% 7% 7% 4 5% 6% 29%	5812
February	1st to 10th	Day	11% 11% 5% 11% 6% 6% 4 4 6% 8% 28%	1890
	11th to 20th	Day	12% 12% 4 8% 5% 4 6% 3 5% 7% 33%	1844
	21st to end	Day	12% 11% 3 10% 6% 5% 4 3 6% 6% 35%	1688
March	1st to 10th	Day	15% 9% 3 7% 4 3 3 2 3 5% 45%	4948
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 10% 45%	5090
	21st to end	Day	20% 14% 5% 10% 5% 5% 4 3 4 6% 25%	5665
April	1st to 10th	Day	19% 12% 4 7% 4 4 4 4 5% 6% 31%	2520
	11th to 20th	Day	18% 8% 3 7% 6% 5% 4 4 5% 6% 34%	2494
	21st to end	Day	21% 8% 3 7% 4 4 4 4 5% 7% 33%	2626
May	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 40%	7734
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 5% 7% 46%	8222
	21st to end	Day	15% 8% 3 7% 4 4 4 4 6% 7% 37%	9047
June	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 4 6% 9% 43%	2940
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 3 5% 9% 44%	2940
	21st to end	Day	11% 7% 2 7% 5% 6% 5% 5% 6% 8% 38%	2940
July	1st to 10th	Day	8% 6% 2 8% 5% 5% 6% 5% 7% 9% 38%	8280
	11th to 20th	Day	6% 5% 2 5% 4 3 4 3 6% 8% 55%	8280
	21st to end	Day	9% 5% 2 7% 6% 5% 6% 4 7% 10% 40%	8576
August	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 43%	4364
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	4224
	21st to end	Day	7% 5% 2 6% 4 4 4 4 6% 9% 49%	4348
September	1st to 10th	Day	11% 5% 2 6% 3 3 3 3 6% 7% 51%	2400
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 7% 48%	2398
	21st to end	Day	10% 6% 3 5% 4 4 4 4 4 8% 48%	2085
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 2 5% 6% 47%	2000
	11th to 20th	Day	16% 9% 2 6% 4 4 3 3 4 6% 43%	2000
	21st to end	Day	15% 10% 3 6% 4 4 4 4 5% 7% 41%	2200
November	1st to 10th	Day	10% 7% 3 8% 3 4 4 3 5% 8% 44%	1874
	11th to 20th	Day	12% 12% 3 10% 5% 3 4 3 5% 7% 35%	1680
	21st to end	Day	13% 12% 4 8% 4 4 5% 4 5% 7% 36%	1680
December	1st to 10th	Day	7% 9% 3 8% 5% 5% 4 4 6% 7% 42%	2690
	11th to 20th	Day	13% 13% 4 15% 7% 5% 5% 3 5% 5% 25%	3088
	21st to end	Day	21% 21% 9% 14% 5% 3 3 2 3 4 16%	4070
January	1st to 10th	Night	8% 12% 5% 18% 6% 5% 4 3 4 5% 27%	8232
	11th to 20th	Night	7% 11% 4 10% 8% 6% 8% 5% 7% 7% 28%	7943
	21st to end	Night	9% 12% 5% 10% 5% 4 4 3 5% 6% 35%	8402
February	1st to 10th	Night	9% 13% 5% 12% 7% 4 4 3 5% 7% 32%	2730
	11th to 20th	Night	11% 9% 4 7% 5% 4 4 4 5% 6% 40%	2663
	21st to end	Night	12% 10% 4 7% 6% 5% 4 3 4 5% 40%	2117
March	1st to 10th	Night	15% 8% 3 5% 3 3 3 3 4 5% 49%	5170
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 48%	5090
	21st to end	Night	20% 12% 4 10% 5% 4 4 3 4 5% 29%	5505
April	1st to 10th	Night	15% 10% 3 8% 5% 4 3 3 5% 6% 40%	2100
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 3 5% 7% 39%	2075
	21st to end	Night	18% 7% 2 6% 3 4 3 4 4 7% 41%	1990
May	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 47%	5148
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 6% 49%	4670
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 41%	5139
June	1st to 10th	Night	6% 6% 2 5% 5% 5% 6% 4 5% 9% 47%	1680
	11th to 20th	Night	9% 5% 2 6% 6% 4 5% 4 7% 8% 46%	1680
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 7% 48%	1680
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 9% 44%	4708
	11th to 20th	Night	5% 4 2 5% 3 3 3 3 5% 7% 61%	4707
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 47%	5527
August	1st to 10th	Night	6% 3 2 5% 4 4 4 5% 6% 8% 53%	3016
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 4 7% 54%	2924
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 7% 54%	3549
September	1st to 10th	Night	9% 5% 2 6% 4 3 3 3 5% 6% 56%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 4 6% 53%	2000
	21st to end	Night	10% 5% 1 5% 4 3 3 2 5% 6% 57%	2292
October	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 7% 51%	2400
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 46%	2400
	21st to end	Night	16% 9% 3 5% 3 3 4 3 5% 6% 42%	2640
November	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 5% 7% 50%	2642
	11th to 20th	Night	11% 9% 4 8% 5% 4 4 3 5% 6% 40%	2935
	21st to end	Night	14% 10% 3 9% 4 4 4 3 4 6% 40%	2940
December	1st to 10th	Night	6% 8% 3 7% 5% 6% 5% 4 6% 7% 43%	4720
	11th to 20th	Night	13% 11% 5% 12% 5% 5% 4 4 4 5% 33%	5339
	21st to end	Night	21% 17% 7% 15% 5% 4 3 3 3 4 17%	7150

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	66% 3 10% 14% 6% 1	4692
	11th to 20th	Day	54% 3 10% 18% 13% 1	5046
	21st to end	Day	56% 3 13% 20% 7%	5812
February	1st to 10th	Day	55% 4 15% 18% 8% 1	1890
	11th to 20th	Day	50% 4 11% 23% 9% 2	1853
	21st to end	Day	49% 3 13% 21% 11% 3	1688
March	1st to 10th	Day	44% 3 10% 27% 14% 2	4948
	11th to 20th	Day	36% 3 11% 33% 15% 2	5090
	21st to end	Day	60% 5% 14% 15% 5% 1	5667
April	1st to 10th	Day	53% 5% 11% 22% 9% 1	2520
	11th to 20th	Day	51% 4 11% 21% 12% 1	2494
	21st to end	Day	51% 4 10% 22% 12% 1	2626
May	1st to 10th	Day	40% 4 11% 24% 20% 1	7772
	11th to 20th	Day	39% 4 10% 28% 17% 2	8226
	21st to end	Day	45% 5% 11% 25% 14% 1	9061
June	1st to 10th	Day	37% 5% 15% 25% 16% 2	2940
	11th to 20th	Day	37% 5% 12% 24% 20% 2	2940
	21st to end	Day	41% 6% 14% 23% 14% 2	2940
July	1st to 10th	Day	40% 7% 15% 29% 8% 1	8211
	11th to 20th	Day	27% 5% 13% 35% 19% 1	8205
	21st to end	Day	37% 6% 19% 25% 11% 1	8389
August	1st to 10th	Day	36% 6% 16% 27% 14% 1	4350
	11th to 20th	Day	40% 6% 16% 26% 12% 1	4263
	21st to end	Day	32% 5% 15% 29% 18% 2	4347
September	1st to 10th	Day	32% 4 14% 27% 20% 3	2400
	11th to 20th	Day	33% 5% 14% 31% 15% 1	2398
	21st to end	Day	35% 5% 10% 31% 18% 1	2085
October	1st to 10th	Day	24% 2 6% 32% 34% 1	3260
	11th to 20th	Day	32% 2 10% 30% 23% 3	3240
	21st to end	Day	41% 3 10% 30% 14% 3	3538
November	1st to 10th	Day	39% 4 13% 26% 16% 2	1874
	11th to 20th	Day	49% 4 13% 22% 13% 1	1680
	21st to end	Day	48% 5% 16% 23% 8% 1	1680
December	1st to 10th	Day	41% 3 14% 35% 6% 1	2690
	11th to 20th	Day	61% 3 13% 13% 9%	3085
	21st to end	Day	74% 3 7% 12% 3	4070
January	1st to 10th	Night	59% 4 13% 16% 7% 1	8233
	11th to 20th	Night	53% 3 12% 17% 14% 1	7943
	21st to end	Night	50% 3 14% 22% 11% 1	8402
February	1st to 10th	Night	52% 4 12% 20% 11% 1	2730
	11th to 20th	Night	44% 4 11% 26% 13% 2	2664
	21st to end	Night	47% 3 11% 21% 15% 3	2117
March	1st to 10th	Night	39% 3 7% 30% 19% 2	5170
	11th to 20th	Night	35% 3 9% 32% 18% 3	5090
	21st to end	Night	59% 4 14% 15% 7% 1	5505
April	1st to 10th	Night	45% 3 11% 27% 13% 2	2100
	11th to 20th	Night	45% 4 10% 22% 16% 3	2075
	21st to end	Night	43% 4 12% 27% 13% 2	1990
May	1st to 10th	Night	36% 4 8% 25% 26% 1	5175
	11th to 20th	Night	37% 4 10% 25% 19% 5%	4671
	21st to end	Night	45% 4 9% 25% 15% 2	5149
June	1st to 10th	Night	33% 7% 15% 27% 16% 3	1680
	11th to 20th	Night	34% 5% 13% 20% 23% 5%	1680
	21st to end	Night	35% 4 14% 27% 16% 4	1680
July	1st to 10th	Night	34% 6% 17% 28% 13% 1	4670
	11th to 20th	Night	23% 5% 14% 33% 22% 3	4662
	21st to end	Night	34% 5% 18% 30% 13% 1	5403
August	1st to 10th	Night	28% 6% 16% 34% 15% 2	3006
	11th to 20th	Night	30% 5% 15% 36% 13% 1	2951
	21st to end	Night	30% 5% 15% 28% 19% 4	3549
September	1st to 10th	Night	30% 4 9% 29% 26% 2	2000
	11th to 20th	Night	33% 4 10% 32% 21% 1	2000
	21st to end	Night	29% 3 12% 30% 24% 2	2292
October	1st to 10th	Night	23% 3 6% 29% 37% 3	3920
	11th to 20th	Night	28% 3 9% 26% 27% 7%	3899
	21st to end	Night	43% 3 10% 25% 16% 3	4252
November	1st to 10th	Night	35% 4 14% 27% 17% 4	2642
	11th to 20th	Night	45% 3 12% 25% 13% 1	2935
	21st to end	Night	47% 3 13% 26% 11% 1	2940
December	1st to 10th	Night	40% 4 14% 33% 9%	4720
	11th to 20th	Night	53% 4 12% 24% 6% 1	5341
	21st to end	Night	72% 4 8% 11% 5%	7150

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	18% 11% 40% 22% 7% 2	330
	11th to 20th	Day	18% 15% 44% 19% 4	330
	21st to end	Day	17% 14% 36% 26% 4 2	363
February	1st to 10th	Day	15% 20% 40% 20% 5% 1	330
	11th to 20th	Day	19% 18% 35% 20% 6% 1	330
	21st to end	Day	22% 15% 39% 22% 2	273
March	1st to 10th	Day	17% 17% 42% 18% 5% 1	330
	11th to 20th	Day	25% 15% 29% 25% 5% 1	330
	21st to end	Day	31% 16% 31% 16% 5% 1	394
April	1st to 10th	Day	27% 15% 36% 16% 6%	540
	11th to 20th	Day	24% 19% 30% 20% 6% 1	532
	21st to end	Day	25% 13% 35% 17% 8% 1	646
May	1st to 10th	Day	21% 12% 31% 24% 10% 2	753
	11th to 20th	Day	24% 10% 28% 24% 12% 2	775
	21st to end	Day	15% 10% 28% 28% 15% 3	844
June	1st to 10th	Day	10% 7% 28% 34% 18% 4	780
	11th to 20th	Day	13% 7% 31% 32% 14% 3	780
	21st to end	Day	12% 7% 34% 30% 13% 4	780
July	1st to 10th	Day	10% 3 19% 35% 23% 11%	771
	11th to 20th	Day	8% 3 25% 35% 21% 8%	768
	21st to end	Day	8% 3 23% 37% 23% 5%	849
August	1st to 10th	Day	10% 3 24% 33% 24% 7%	776
	11th to 20th	Day	13% 3 21% 32% 24% 7%	736
	21st to end	Day	12% 3 21% 34% 25% 5%	606
September	1st to 10th	Day	13% 7% 22% 28% 23% 7%	530
	11th to 20th	Day	10% 3 25% 33% 24% 5%	528
	21st to end	Day	14% 10% 25% 28% 18% 5%	330
October	1st to 10th	Day	15% 6% 32% 26% 16% 5%	330
	11th to 20th	Day	21% 20% 35% 16% 8%	330
	21st to end	Day	22% 15% 35% 19% 7% 2	363
November	1st to 10th	Day	15% 13% 43% 20% 6% 2	330
	11th to 20th	Day	19% 19% 36% 19% 6% 1	330
	21st to end	Day	23% 11% 41% 22% 2 1	330
December	1st to 10th	Day	15% 15% 43% 19% 8% 1	330
	11th to 20th	Day	18% 15% 35% 23% 7% 1	330
	21st to end	Day	25% 14% 41% 16% 3	363
January	1st to 10th	Night	14% 12% 39% 27% 7%	1050
	11th to 20th	Night	16% 14% 37% 28% 5% 1	1050
	21st to end	Night	18% 11% 37% 28% 5%	1155
February	1st to 10th	Night	16% 13% 38% 26% 6% 1	1050
	11th to 20th	Night	16% 14% 39% 26% 5% 1	1029
	21st to end	Night	18% 14% 40% 23% 5%	733
March	1st to 10th	Night	19% 14% 37% 23% 6% 1	698
	11th to 20th	Night	19% 9% 37% 26% 9%	630
	21st to end	Night	22% 12% 37% 22% 6% 1	693
April	1st to 10th	Night	19% 13% 39% 24% 4	630
	11th to 20th	Night	20% 11% 36% 27% 6%	623
	21st to end	Night	22% 10% 34% 26% 7% 1	630
May	1st to 10th	Night	18% 6% 34% 33% 7% 1	630
	11th to 20th	Night	17% 6% 34% 33% 10%	621
	21st to end	Night	17% 5% 27% 37% 14%	683
June	1st to 10th	Night	9% 3 29% 42% 16% 1	630
	11th to 20th	Night	10% 4 33% 42% 11% 1	630
	21st to end	Night	13% 5% 27% 38% 16% 1	630
July	1st to 10th	Night	7% 2 14% 47% 27% 2	615
	11th to 20th	Night	6% 2 22% 41% 26% 3	612
	21st to end	Night	9% 2 20% 45% 23% 2	690
August	1st to 10th	Night	8% 1 16% 45% 27% 2	627
	11th to 20th	Night	12% 2 16% 40% 26% 4	606
	21st to end	Night	10% 2 20% 40% 26% 2	660
September	1st to 10th	Night	10% 3 23% 38% 23% 3	600
	11th to 20th	Night	14% 2 23% 39% 20% 3	600
	21st to end	Night	12% 6% 28% 36% 17% 1	694
October	1st to 10th	Night	14% 6% 30% 35% 13% 1	800
	11th to 20th	Night	18% 10% 37% 28% 7%	800
	21st to end	Night	22% 12% 31% 27% 8%	880
November	1st to 10th	Night	15% 11% 39% 28% 7%	820
	11th to 20th	Night	17% 14% 41% 22% 6%	1045
	21st to end	Night	18% 13% 38% 23% 8%	1050
December	1st to 10th	Night	16% 13% 39% 26% 5% 1	1050
	11th to 20th	Night	22% 10% 35% 27% 5% 1	1050
	21st to end	Night	26% 11% 33% 23% 6%	1155

Page generated at 2026-08-12 02:37:15. Version ac07

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