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 2025-04-11.

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% 3 8% 5% 4 4 4 5% 7% 38%	113729
	Night	12% 9% 3 9% 5% 4 4 3 5% 6% 41%	114795
Transparency	Day	44% 4 13% 24% 13% 1	117366
	Night	43% 4 12% 24% 15% 2	119300
Seeing	Day	17% 9% 30% 27% 14% 3	16367
	Night	16% 9% 33% 31% 11% 1	26415

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	5% 12% 6% 23% 8% 7% 5% 3 4 5% 21%	3617
	11th to 20th	Day	6% 11% 4 11% 6% 8% 6% 6% 8% 6% 28%	3872
	21st to end	Day	10% 12% 6% 14% 5% 5% 5% 3 5% 6% 29%	4488
February	1st to 10th	Day	11% 11% 5% 11% 6% 5% 4 4 6% 8% 28%	1800
	11th to 20th	Day	12% 12% 4 8% 5% 5% 6% 3 5% 7% 33%	1754
	21st to end	Day	13% 11% 3 10% 6% 4 4 3 6% 6% 35%	1610
March	1st to 10th	Day	16% 9% 3 7% 4 3 3 2 3 5% 45%	4840
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 10% 45%	4980
	21st to end	Day	20% 14% 5% 10% 5% 5% 4 3 4 6% 25%	5543
April	1st to 10th	Day	19% 12% 3 7% 4 4 4 4 5% 6% 32%	2400
	11th to 20th	Day	18% 8% 3 7% 6% 5% 5% 4 4 6% 34%	2254
	21st to end	Day	22% 8% 3 7% 4 4 4 4 4 6% 34%	2376
May	1st to 10th	Day	20% 8% 2 7% 4 4 5% 4 6% 8% 32%	4145
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 7% 42%	4399
	21st to end	Day	14% 7% 3 6% 5% 4 4 4 6% 7% 41%	4824
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 43%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 3 5% 9% 44%	2660
	21st to end	Day	11% 7% 2 7% 5% 6% 5% 5% 7% 8% 38%	2660
July	1st to 10th	Day	9% 7% 2 8% 6% 6% 6% 5% 7% 9% 35%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 4 3 6% 8% 56%	4440
	21st to end	Day	6% 4 2 6% 6% 5% 6% 5% 8% 10% 42%	4595
August	1st to 10th	Day	9% 5% 2 6% 4 5% 6% 4 7% 9% 43%	4104
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	4041
	21st to end	Day	7% 5% 2 6% 4 4 4 3 6% 8% 49%	4215
September	1st to 10th	Day	11% 6% 2 6% 3 3 3 3 6% 7% 51%	2280
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 5% 7% 50%	2278
	21st to end	Day	11% 6% 3 5% 4 4 4 4 4 8% 48%	1981
October	1st to 10th	Day	13% 6% 2 7% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	16% 9% 2 6% 4 4 3 3 4 6% 43%	1900
	21st to end	Day	15% 10% 3 6% 4 4 4 3 4 6% 41%	2090
November	1st to 10th	Day	10% 7% 3 8% 3 4 4 3 5% 8% 44%	1783
	11th to 20th	Day	13% 11% 3 10% 5% 3 4 3 6% 8% 34%	1600
	21st to end	Day	13% 12% 4 7% 4 4 5% 4 5% 7% 36%	1600
December	1st to 10th	Day	7% 9% 3 7% 5% 4 4 4 6% 7% 43%	2610
	11th to 20th	Day	13% 13% 4 15% 7% 5% 5% 3 5% 5% 25%	3008
	21st to end	Day	21% 20% 9% 14% 4 3 3 2 3 4 16%	3982
January	1st to 10th	Night	7% 12% 5% 21% 7% 5% 4 3 4 5% 27%	6348
	11th to 20th	Night	9% 12% 4 10% 7% 5% 6% 4 7% 7% 27%	6107
	21st to end	Night	12% 13% 6% 11% 5% 4 4 3 5% 6% 32%	6490
February	1st to 10th	Night	9% 13% 5% 12% 7% 4 4 4 4 7% 32%	2600
	11th to 20th	Night	12% 9% 4 8% 5% 4 4 3 5% 6% 40%	2533
	21st to end	Night	13% 10% 4 7% 6% 4 4 3 4 5% 40%	2019
March	1st to 10th	Night	15% 8% 3 5% 3 3 3 3 4 5% 49%	5057
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 49%	4980
	21st to end	Night	20% 12% 4 10% 5% 4 4 3 4 5% 28%	5386
April	1st to 10th	Night	14% 9% 2 8% 5% 4 3 3 5% 6% 41%	2000
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 3 5% 7% 39%	1881
	21st to end	Night	18% 7% 2 6% 3 4 3 4 4 7% 42%	1800
May	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 6% 6% 39%	2759
	11th to 20th	Night	11% 7% 2 7% 5% 4 4 4 5% 6% 46%	2506
	21st to end	Night	10% 7% 2 5% 5% 4 4 3 5% 7% 47%	2743
June	1st to 10th	Night	6% 6% 2 5% 5% 5% 6% 4 5% 9% 47%	1520
	11th to 20th	Night	8% 5% 2 6% 6% 4 4 4 7% 8% 46%	1520
	21st to end	Night	11% 5% 2 6% 3 4 4 3 6% 8% 48%	1520
July	1st to 10th	Night	8% 6% 2 6% 4 4 5% 5% 6% 9% 44%	2530
	11th to 20th	Night	3 3 1 5% 3 4 3 3 6% 8% 60%	2530
	21st to end	Night	5% 4 1 6% 4 4 5% 4 7% 9% 51%	2964
August	1st to 10th	Night	6% 4 2 5% 4 4 4 5% 6% 8% 53%	2836
	11th to 20th	Night	10% 5% 2 5% 3 2 4 3 4 7% 55%	2798
	21st to end	Night	8% 5% 2 5% 4 4 3 3 5% 7% 54%	3440
September	1st to 10th	Night	9% 5% 2 6% 4 3 3 2 5% 6% 56%	1900
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	1900
	21st to end	Night	10% 5% 1 5% 4 3 3 2 5% 6% 57%	2177
October	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 7% 52%	2280
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 46%	2280
	21st to end	Night	16% 9% 3 5% 3 3 4 3 5% 6% 41%	2508
November	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 5% 7% 51%	2513
	11th to 20th	Night	11% 9% 4 8% 5% 4 4 4 5% 5% 41%	2795
	21st to end	Night	14% 10% 3 8% 4 4 4 3 5% 6% 40%	2800
December	1st to 10th	Night	6% 7% 3 7% 5% 6% 6% 4 6% 7% 44%	4580
	11th to 20th	Night	13% 11% 5% 12% 5% 4 4 4 4 5% 33%	5199
	21st to end	Night	21% 17% 7% 16% 5% 4 3 3 3 4 17%	6996

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	67% 3 9% 15% 6%	3618
	11th to 20th	Day	54% 3 12% 15% 15% 1	3872
	21st to end	Day	57% 3 14% 18% 8% 1	4488
February	1st to 10th	Day	55% 4 14% 18% 9% 1	1800
	11th to 20th	Day	50% 4 11% 23% 10% 2	1763
	21st to end	Day	49% 4 13% 21% 11% 3	1610
March	1st to 10th	Day	43% 3 10% 27% 14% 2	4840
	11th to 20th	Day	36% 3 11% 33% 15% 2	4980
	21st to end	Day	61% 4 14% 15% 5% 1	5545
April	1st to 10th	Day	53% 4 11% 23% 9% 1	2400
	11th to 20th	Day	51% 4 10% 20% 13% 1	2254
	21st to end	Day	51% 4 10% 22% 12% 1	2376
May	1st to 10th	Day	49% 5% 14% 18% 13% 1	4183
	11th to 20th	Day	42% 5% 12% 28% 12% 1	4403
	21st to end	Day	42% 5% 12% 26% 14% 1	4837
June	1st to 10th	Day	37% 5% 16% 24% 16% 2	2660
	11th to 20th	Day	37% 5% 12% 23% 20% 2	2660
	21st to end	Day	41% 6% 14% 23% 14% 2	2660
July	1st to 10th	Day	43% 7% 13% 28% 8% 1	4371
	11th to 20th	Day	26% 5% 14% 34% 19% 2	4365
	21st to end	Day	33% 8% 19% 22% 17% 1	4450
August	1st to 10th	Day	36% 5% 16% 27% 14% 1	4090
	11th to 20th	Day	40% 6% 15% 26% 12% 1	4080
	21st to end	Day	32% 5% 15% 29% 17% 2	4214
September	1st to 10th	Day	32% 4 14% 27% 21% 3	2280
	11th to 20th	Day	33% 5% 13% 32% 16% 1	2278
	21st to end	Day	35% 5% 10% 31% 17% 1	1981
October	1st to 10th	Day	24% 2 6% 32% 34% 1	3160
	11th to 20th	Day	31% 2 10% 30% 24% 3	3140
	21st to end	Day	41% 2 11% 30% 14% 3	3428
November	1st to 10th	Day	39% 4 14% 25% 15% 2	1783
	11th to 20th	Day	49% 4 13% 22% 12% 1	1600
	21st to end	Day	47% 5% 16% 23% 8% 1	1600
December	1st to 10th	Day	40% 3 14% 36% 6% 1	2610
	11th to 20th	Day	61% 3 13% 12% 9%	3005
	21st to end	Day	74% 3 7% 12% 3	3982
January	1st to 10th	Night	60% 3 11% 17% 8% 1	6348
	11th to 20th	Night	53% 3 13% 15% 14% 1	6107
	21st to end	Night	54% 3 11% 19% 12%	6490
February	1st to 10th	Night	53% 4 11% 20% 12% 1	2600
	11th to 20th	Night	45% 4 11% 25% 13% 1	2534
	21st to end	Night	47% 3 11% 21% 16% 3	2019
March	1st to 10th	Night	39% 3 7% 30% 19% 2	5057
	11th to 20th	Night	34% 3 9% 32% 18% 3	4980
	21st to end	Night	59% 4 14% 15% 7% 1	5386
April	1st to 10th	Night	44% 3 11% 27% 13% 2	2000
	11th to 20th	Night	46% 4 10% 21% 17% 3	1881
	21st to end	Night	42% 4 12% 28% 13% 2	1800
May	1st to 10th	Night	44% 5% 10% 19% 21% 1	2786
	11th to 20th	Night	39% 4 10% 28% 16% 4	2507
	21st to end	Night	36% 5% 12% 28% 16% 3	2751
June	1st to 10th	Night	33% 7% 15% 27% 16% 3	1520
	11th to 20th	Night	33% 5% 14% 21% 21% 6%	1520
	21st to end	Night	34% 4 14% 28% 16% 4	1520
July	1st to 10th	Night	35% 6% 17% 28% 12% 2	2490
	11th to 20th	Night	22% 5% 16% 36% 18% 3	2482
	21st to end	Night	28% 6% 18% 26% 20% 2	2870
August	1st to 10th	Night	28% 6% 15% 35% 15% 2	2826
	11th to 20th	Night	30% 5% 15% 36% 13% 1	2825
	21st to end	Night	30% 5% 15% 27% 19% 4	3440
September	1st to 10th	Night	30% 4 9% 28% 26% 2	1900
	11th to 20th	Night	32% 4 10% 32% 22% 1	1900
	21st to end	Night	29% 3 12% 31% 23% 2	2177
October	1st to 10th	Night	22% 3 6% 29% 37% 3	3800
	11th to 20th	Night	27% 3 9% 27% 28% 7%	3779
	21st to end	Night	44% 3 10% 25% 16% 3	4120
November	1st to 10th	Night	35% 4 14% 26% 18% 4	2513
	11th to 20th	Night	45% 3 12% 25% 13% 2	2795
	21st to end	Night	46% 3 13% 26% 11% 1	2800
December	1st to 10th	Night	39% 4 15% 33% 9%	4580
	11th to 20th	Night	53% 4 12% 24% 6% 1	5201
	21st to end	Night	72% 4 8% 11% 5%	6996

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% 12% 40% 21% 7% 2	300
	11th to 20th	Day	19% 13% 44% 19% 4	300
	21st to end	Day	19% 13% 36% 25% 5% 2	330
February	1st to 10th	Day	16% 20% 40% 20% 4 1	300
	11th to 20th	Day	19% 19% 35% 20% 6% 1	300
	21st to end	Day	23% 14% 38% 22% 2	249
March	1st to 10th	Day	17% 17% 43% 17% 5% 1	300
	11th to 20th	Day	24% 14% 30% 25% 6% 1	300
	21st to end	Day	31% 16% 30% 17% 5% 1	360
April	1st to 10th	Day	28% 15% 35% 16% 6%	500
	11th to 20th	Day	24% 19% 28% 21% 7% 1	452
	21st to end	Day	27% 13% 33% 17% 9% 1	556
May	1st to 10th	Day	24% 12% 30% 23% 9% 2	653
	11th to 20th	Day	24% 9% 28% 25% 11% 2	675
	21st to end	Day	15% 10% 27% 29% 16% 4	734
June	1st to 10th	Day	9% 6% 28% 35% 19% 4	680
	11th to 20th	Day	13% 6% 30% 33% 15% 3	680
	21st to end	Day	11% 7% 32% 32% 14% 5%	680
July	1st to 10th	Day	10% 2 19% 35% 23% 11%	671
	11th to 20th	Day	8% 3 26% 35% 21% 6%	668
	21st to end	Day	7% 2 24% 37% 24% 6%	739
August	1st to 10th	Day	9% 2 24% 34% 25% 7%	676
	11th to 20th	Day	13% 3 22% 32% 23% 8%	665
	21st to end	Day	12% 3 21% 35% 25% 5%	561
September	1st to 10th	Day	14% 6% 22% 29% 23% 7%	490
	11th to 20th	Day	10% 3 26% 33% 24% 5%	488
	21st to end	Day	15% 11% 27% 29% 16% 3	300
October	1st to 10th	Day	16% 6% 32% 25% 17% 4	300
	11th to 20th	Day	21% 20% 34% 17% 9%	300
	21st to end	Day	22% 15% 34% 19% 8% 2	330
November	1st to 10th	Day	16% 11% 42% 22% 7% 2	300
	11th to 20th	Day	20% 19% 34% 20% 6% 1	300
	21st to end	Day	22% 12% 41% 22% 2 1	300
December	1st to 10th	Day	14% 14% 44% 20% 7% 1	300
	11th to 20th	Day	18% 15% 36% 23% 7% 1	300
	21st to end	Day	25% 14% 41% 17% 3	330
January	1st to 10th	Night	14% 12% 40% 28% 7%	1000
	11th to 20th	Night	17% 13% 36% 29% 5% 1	1000
	21st to end	Night	19% 11% 37% 28% 5%	1100
February	1st to 10th	Night	17% 13% 38% 26% 6% 1	1000
	11th to 20th	Night	17% 14% 39% 25% 5% 1	979
	21st to end	Night	19% 13% 39% 23% 5%	699
March	1st to 10th	Night	19% 14% 37% 23% 6% 1	665
	11th to 20th	Night	18% 9% 38% 27% 9%	600
	21st to end	Night	23% 11% 37% 21% 6% 1	660
April	1st to 10th	Night	19% 13% 39% 25% 4	600
	11th to 20th	Night	20% 10% 36% 27% 7%	565
	21st to end	Night	22% 10% 34% 26% 7% 1	570
May	1st to 10th	Night	19% 7% 34% 32% 8% 1	570
	11th to 20th	Night	17% 5% 34% 35% 9%	561
	21st to end	Night	16% 5% 27% 37% 15%	617
June	1st to 10th	Night	9% 3 29% 41% 17% 1	570
	11th to 20th	Night	10% 3 32% 43% 12% 1	570
	21st to end	Night	12% 5% 27% 39% 17% 1	570
July	1st to 10th	Night	7% 2 14% 47% 27% 2	555
	11th to 20th	Night	5% 2 24% 41% 26% 3	552
	21st to end	Night	7% 1 19% 47% 24% 2	624
August	1st to 10th	Night	8% 2 17% 45% 27% 2	567
	11th to 20th	Night	11% 2 16% 40% 26% 4	564
	21st to end	Night	11% 2 20% 39% 26% 3	627
September	1st to 10th	Night	11% 3 22% 39% 22% 3	570
	11th to 20th	Night	15% 2 22% 39% 19% 3	570
	21st to end	Night	12% 7% 29% 36% 16% 1	659
October	1st to 10th	Night	14% 6% 31% 34% 13% 1	760
	11th to 20th	Night	18% 10% 35% 28% 8%	760
	21st to end	Night	22% 11% 30% 28% 8%	836
November	1st to 10th	Night	16% 10% 38% 28% 8% 1	780
	11th to 20th	Night	17% 13% 42% 22% 7%	995
	21st to end	Night	18% 12% 38% 24% 8%	1000
December	1st to 10th	Night	16% 13% 39% 27% 5% 1	1000
	11th to 20th	Night	22% 10% 35% 27% 6% 1	1000
	21st to end	Night	26% 11% 33% 24% 6%	1100

Page generated at 2025-04-08 08:17:30. Version ac07

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