Prairie State Vehicle Recreation Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prairie State Vehicle Recreation Area clear sky chart from 2005-11-06 to 2026-10-29.

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	7% 3 1 3 2 2 3 2 4 5% 66%	158051
	Night	9% 4 1 4 3 3 3 2 4 5% 63%	151564
Transparency	Day	21% 3 7% 18% 25% 25%	152989
	Night	26% 4 8% 19% 24% 21%	147612
Seeing	Day	9% 5% 16% 29% 29% 11%	17805
	Night	11% 2 14% 40% 30% 4	27075

By Month

Percentage Cloud Cover SW (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	22% 7% 2 4 3 3 4 4 5% 8% 37%	4722
	11th to 20th	Day	12% 6% 1 5% 3 3 3 2 3 4 58%	5255
	21st to end	Day	15% 6% 3 6% 4 4 4 4 5% 8% 41%	5843
February	1st to 10th	Day	19% 6% 2 6% 4 3 4 3 4 6% 44%	1890
	11th to 20th	Day	16% 6% 2 6% 4 5% 4 4 4 6% 44%	1983
	21st to end	Day	16% 5% 2 5% 3 3 4 3 5% 6% 48%	1730
March	1st to 10th	Day	14% 9% 4 8% 5% 4 4 4 5% 6% 37%	4784
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 5% 6% 48%	5110
	21st to end	Day	14% 6% 3 6% 4 4 4 4 6% 8% 40%	5820
April	1st to 10th	Day	12% 6% 2 4 3 4 4 3 5% 7% 49%	2520
	11th to 20th	Day	10% 4 1 4 3 3 4 3 5% 6% 56%	2489
	21st to end	Day	7% 4 1 3 3 2 3 3 5% 8% 62%	2520
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 5% 71%	7162
	11th to 20th	Day	2 1 1 2 1 1 2 2 3 4 83%	8158
	21st to end	Day	2 2 1 2 2 2 2 2 3 6% 77%	8992
June	1st to 10th	Day	4 3 1 3 2 2 2 3 5% 7% 68%	2940
	11th to 20th	Day	1 1 2 2 2 1 3 4 82%	2940
	21st to end	Day	2 1 1 1 1 1 1 1 3 3 84%	2940
July	1st to 10th	Day	1 1 1 1 1 1 3 91%	8219
	11th to 20th	Day	2 2 1 2 2 2 1 2 3 3 81%	8203
	21st to end	Day	1 1 2 1 2 2 2 3 4 84%	8304
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	7547
	11th to 20th	Day	1 2 1 2 2 2 3 6% 81%	7096
	21st to end	Day	3 2 1 2 2 2 2 2 3 5% 78%	7750
September	1st to 10th	Day	2 1 1 1 1 1 1 2 4 85%	2520
	11th to 20th	Day	2 2 1 2 1 3 3 2 3 5% 75%	2520
	21st to end	Day	3 2 1 3 2 2 2 1 3 4 79%	2267
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 75%	3380
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 5% 73%	3361
	21st to end	Day	5% 3 1 5% 3 3 3 3 5% 6% 64%	3639
November	1st to 10th	Day	10% 3 2 3 2 3 3 3 6% 8% 56%	1904
	11th to 20th	Day	13% 5% 2 4 4 4 4 3 5% 7% 49%	1890
	21st to end	Day	12% 4 1 4 2 3 4 4 5% 7% 52%	1753
December	1st to 10th	Day	8% 4 1 5% 3 4 4 3 5% 8% 55%	2700
	11th to 20th	Day	27% 7% 2 7% 4 4 4 3 6% 8% 28%	3108
	21st to end	Day	25% 8% 3 6% 5% 5% 6% 5% 8% 8% 23%	4092
January	1st to 10th	Night	17% 7% 3 6% 3 3 4 4 5% 8% 41%	8172
	11th to 20th	Night	12% 3 1 4 2 3 3 2 4 5% 60%	7729
	21st to end	Night	15% 6% 3 6% 4 4 4 3 5% 6% 45%	8371
February	1st to 10th	Night	18% 6% 1 4 3 3 3 3 4 5% 51%	2730
	11th to 20th	Night	14% 6% 2 5% 3 4 4 4 5% 8% 46%	2525
	21st to end	Night	15% 6% 1 3 4 3 3 3 4 5% 51%	2076
March	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 4 6% 40%	5327
	11th to 20th	Night	19% 6% 2 4 3 3 3 3 3 5% 49%	5110
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 7% 42%	5395
April	1st to 10th	Night	11% 6% 2 4 4 3 4 3 4 6% 51%	2100
	11th to 20th	Night	9% 4 2 4 4 3 3 3 5% 6% 56%	2079
	21st to end	Night	7% 3 1 3 2 2 2 3 4 7% 64%	2100
May	1st to 10th	Night	4 4 1 4 2 2 2 2 3 5% 70%	5806
	11th to 20th	Night	2 1 1 1 1 1 2 2 3 4 81%	4700
	21st to end	Night	2 2 1 2 1 2 2 2 3 5% 79%	5181
June	1st to 10th	Night	3 3 1 3 2 2 3 2 5% 5% 70%	1680
	11th to 20th	Night	1 1 1 2 1 1 1 2 4 84%	1680
	21st to end	Night	2 2 1 1 1 1 1 3 3 85%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	4740
	11th to 20th	Night	2 3 1 2 2 2 1 1 2 3 81%	4740
	21st to end	Night	1 1 1 2 1 1 2 2 3 4 82%	5806
August	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 86%	5275
	11th to 20th	Night	1 1 1 2 2 2 4 5% 82%	5783
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 79%	6499
September	1st to 10th	Night	1 1 1 1 1 2 1 2 3 86%	2100
	11th to 20th	Night	3 2 1 2 1 2 2 2 3 4 78%	2100
	21st to end	Night	2 1 2 2 2 2 2 3 5% 80%	2347
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 4 77%	4040
	11th to 20th	Night	3 2 1 2 2 1 2 2 3 5% 77%	4017
	21st to end	Night	6% 3 1 3 3 3 3 3 4 5% 67%	4349
November	1st to 10th	Night	11% 4 1 3 2 3 3 3 5% 8% 58%	2614
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 5% 8% 54%	2730
	21st to end	Night	12% 4 2 4 3 3 2 2 4 6% 58%	2861
December	1st to 10th	Night	9% 3 1 4 2 3 3 3 4 6% 61%	4700
	11th to 20th	Night	24% 7% 2 6% 4 4 5% 3 7% 8% 30%	5316
	21st to end	Night	24% 7% 3 8% 5% 5% 5% 4 5% 6% 29%	7106

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	44% 6% 17% 20% 11% 3	4723
	11th to 20th	Day	32% 3 5% 15% 26% 19%	5255
	21st to end	Day	40% 5% 8% 18% 22% 7%	5843
February	1st to 10th	Day	41% 5% 9% 19% 17% 9%	1890
	11th to 20th	Day	40% 5% 11% 19% 18% 6%	1992
	21st to end	Day	38% 5% 11% 22% 18% 6%	1730
March	1st to 10th	Day	46% 8% 8% 18% 18% 2	4784
	11th to 20th	Day	36% 5% 11% 29% 11% 8%	5110
	21st to end	Day	40% 8% 14% 26% 11% 2	5820
April	1st to 10th	Day	34% 6% 12% 24% 18% 6%	2520
	11th to 20th	Day	29% 5% 12% 25% 21% 6%	2492
	21st to end	Day	22% 5% 14% 29% 25% 4	2520
May	1st to 10th	Day	18% 2 7% 27% 31% 14%	7162
	11th to 20th	Day	9% 2 4 24% 44% 16%	8161
	21st to end	Day	11% 3 7% 29% 41% 9%	8994
June	1st to 10th	Day	17% 4 11% 25% 31% 13%	2940
	11th to 20th	Day	8% 2 8% 19% 36% 28%	2940
	21st to end	Day	8% 1 4 12% 36% 39%	2940
July	1st to 10th	Day	4 1 7% 20% 67%	8151
	11th to 20th	Day	10% 1 2 5% 12% 69%	8128
	21st to end	Day	7% 1 2 11% 24% 54%	8119
August	1st to 10th	Day	7% 1 2 5% 23% 62%	5976
	11th to 20th	Day	8% 1 3 18% 34% 37%	5588
	21st to end	Day	12% 1 2 12% 39% 34%	6077
September	1st to 10th	Day	6% 1 3 10% 29% 50%	2520
	11th to 20th	Day	12% 3 6% 20% 37% 23%	2520
	21st to end	Day	12% 1 5% 18% 34% 28%	2267
October	1st to 10th	Day	13% 2 5% 11% 23% 46%	3380
	11th to 20th	Day	15% 3 5% 22% 35% 21%	3361
	21st to end	Day	23% 3 7% 18% 35% 15%	3639
November	1st to 10th	Day	26% 5% 11% 24% 23% 11%	1904
	11th to 20th	Day	35% 5% 13% 22% 20% 6%	1890
	21st to end	Day	30% 5% 13% 21% 21% 10%	1753
December	1st to 10th	Day	27% 4 10% 28% 23% 7%	2700
	11th to 20th	Day	52% 5% 11% 16% 13% 4	3108
	21st to end	Day	52% 10% 16% 14% 7% 2	4092
January	1st to 10th	Night	41% 6% 17% 21% 10% 5%	8172
	11th to 20th	Night	28% 4 7% 16% 30% 14%	7729
	21st to end	Night	40% 4 7% 17% 21% 10%	8371
February	1st to 10th	Night	38% 3 8% 21% 21% 9%	2730
	11th to 20th	Night	37% 5% 10% 22% 19% 7%	2526
	21st to end	Night	35% 5% 10% 25% 18% 7%	2076
March	1st to 10th	Night	43% 9% 8% 18% 17% 5%	5327
	11th to 20th	Night	39% 4 10% 24% 14% 9%	5110
	21st to end	Night	41% 7% 12% 23% 13% 3	5395
April	1st to 10th	Night	34% 5% 12% 21% 22% 6%	2100
	11th to 20th	Night	28% 5% 14% 26% 17% 10%	2079
	21st to end	Night	21% 5% 12% 31% 25% 6%	2100
May	1st to 10th	Night	18% 3 12% 24% 26% 16%	5804
	11th to 20th	Night	10% 2 6% 26% 39% 17%	4700
	21st to end	Night	11% 2 9% 30% 40% 8%	5182
June	1st to 10th	Night	17% 4 11% 28% 28% 12%	1680
	11th to 20th	Night	8% 2 7% 20% 34% 29%	1680
	21st to end	Night	8% 2 3 12% 34% 41%	1680
July	1st to 10th	Night	3 1 1 7% 26% 62%	4700
	11th to 20th	Night	13% 1 2 4 11% 69%	4692
	21st to end	Night	8% 1 3 12% 26% 50%	5679
August	1st to 10th	Night	8% 1 2 6% 25% 57%	4169
	11th to 20th	Night	8% 1 4 19% 35% 32%	4559
	21st to end	Night	12% 1 2 13% 38% 34%	5092
September	1st to 10th	Night	7% 1 3 9% 29% 50%	2100
	11th to 20th	Night	13% 2 6% 19% 38% 23%	2100
	21st to end	Night	10% 2 6% 17% 32% 33%	2347
October	1st to 10th	Night	13% 2 4 10% 20% 51%	4040
	11th to 20th	Night	12% 3 6% 26% 30% 24%	4017
	21st to end	Night	22% 3 5% 19% 31% 20%	4349
November	1st to 10th	Night	25% 5% 11% 23% 20% 15%	2614
	11th to 20th	Night	30% 3 12% 25% 22% 9%	2730
	21st to end	Night	29% 4 9% 23% 23% 12%	2861
December	1st to 10th	Night	25% 4 9% 27% 25% 10%	4700
	11th to 20th	Night	50% 6% 9% 17% 14% 4	5316
	21st to end	Night	54% 8% 12% 15% 9% 3	7106

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	34% 8% 14% 23% 16% 5%	220
	11th to 20th	Day	17% 6% 16% 25% 25% 10%	397
	21st to end	Day	17% 4 17% 28% 23% 11%	473
February	1st to 10th	Day	21% 9% 13% 23% 24% 10%	430
	11th to 20th	Day	19% 9% 24% 23% 18% 8%	499
	21st to end	Day	16% 7% 22% 28% 23% 4	446
March	1st to 10th	Day	20% 10% 22% 27% 16% 4	540
	11th to 20th	Day	20% 10% 19% 23% 20% 9%	540
	21st to end	Day	15% 9% 23% 27% 19% 7%	594
April	1st to 10th	Day	14% 8% 21% 28% 21% 9%	540
	11th to 20th	Day	12% 7% 27% 23% 22% 9%	532
	21st to end	Day	6% 6% 19% 34% 21% 14%	540
May	1st to 10th	Day	6% 8% 24% 27% 24% 11%	543
	11th to 20th	Day	6% 5% 21% 27% 29% 12%	568
	21st to end	Day	4 4 18% 28% 28% 18%	617
June	1st to 10th	Day	3 4 19% 31% 31% 11%	570
	11th to 20th	Day	1 5% 19% 35% 35% 6%	570
	21st to end	Day	1 2 15% 34% 37% 11%	570
July	1st to 10th	Day	1 1 13% 36% 40% 10%	566
	11th to 20th	Day	1 7% 37% 43% 11%	564
	21st to end	Day	1 7% 38% 42% 12%	619
August	1st to 10th	Day	2 1 11% 35% 39% 12%	567
	11th to 20th	Day	1 2 10% 35% 41% 12%	557
	21st to end	Day	2 1 8% 31% 45% 13%	627
September	1st to 10th	Day	2 1 12% 34% 39% 13%	570
	11th to 20th	Day	4 3 16% 33% 31% 13%	570
	21st to end	Day	3 1 16% 32% 32% 16%	570
October	1st to 10th	Day	2 4 18% 27% 30% 18%	570
	11th to 20th	Day	6% 5% 14% 25% 30% 20%	570
	21st to end	Day	8% 7% 14% 26% 29% 15%	616
November	1st to 10th	Day	12% 6% 13% 25% 27% 17%	380
	11th to 20th	Day	16% 4 12% 29% 28% 10%	330
	21st to end	Day	12% 3 17% 25% 34% 9%	258
December	1st to 10th	Day	16% 5% 15% 28% 23% 14%	220
	11th to 20th	Day	21% 5% 16% 24% 23% 11%	220
	21st to end	Day	25% 9% 20% 25% 16% 5%	242
January	1st to 10th	Night	21% 4 16% 31% 22% 5%	840
	11th to 20th	Night	14% 4 12% 39% 26% 5%	840
	21st to end	Night	18% 2 12% 39% 24% 5%	924
February	1st to 10th	Night	20% 4 14% 37% 21% 3	840
	11th to 20th	Night	17% 4 19% 36% 20% 4	824
	21st to end	Night	17% 3 18% 42% 17% 2	692
March	1st to 10th	Night	19% 5% 19% 33% 21% 3	840
	11th to 20th	Night	19% 4 13% 36% 25% 3	840
	21st to end	Night	16% 3 20% 38% 22% 2	924
April	1st to 10th	Night	12% 4 20% 39% 25% 1	840
	11th to 20th	Night	9% 5% 22% 37% 24% 4	831
	21st to end	Night	8% 3 17% 38% 31% 3	840
May	1st to 10th	Night	6% 3 22% 40% 26% 3	803
	11th to 20th	Night	5% 2 14% 47% 31% 1	414
	21st to end	Night	5% 1 16% 44% 33% 1	455
June	1st to 10th	Night	3 2 19% 45% 30% 1	420
	11th to 20th	Night	1 1 17% 52% 28% 1	420
	21st to end	Night	2 1 13% 49% 35% 1	420
July	1st to 10th	Night	10% 50% 39% 1	410
	11th to 20th	Night	1 8% 55% 34%	408
	21st to end	Night	1 4 48% 44% 3	690
August	1st to 10th	Night	2 8% 50% 38% 2	627
	11th to 20th	Night	1 1 8% 49% 38% 3	793
	21st to end	Night	2 1 6% 44% 46% 2	924
September	1st to 10th	Night	1 1 9% 39% 46% 4	840
	11th to 20th	Night	4 2 13% 38% 38% 5%	840
	21st to end	Night	2 1 13% 39% 38% 7%	840
October	1st to 10th	Night	6% 2 9% 43% 33% 7%	840
	11th to 20th	Night	6% 2 12% 37% 38% 5%	840
	21st to end	Night	7% 2 12% 37% 33% 10%	912
November	1st to 10th	Night	11% 3 11% 35% 32% 7%	820
	11th to 20th	Night	13% 2 13% 42% 27% 5%	840
	21st to end	Night	14% 2 13% 39% 28% 5%	840
December	1st to 10th	Night	15% 3 15% 35% 25% 7%	840
	11th to 20th	Night	26% 3 12% 32% 23% 4	840
	21st to end	Night	25% 3 18% 33% 18% 2	924

Page generated at 2026-10-26 01:37:46. Version ac07

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