Starved Rock State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starved Rock State Park clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 37%	126571
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	123033
Transparency	Day	44% 5% 12% 24% 14% 2	130091
	Night	45% 4 11% 22% 15% 2	127587
Seeing	Day	17% 10% 28% 25% 15% 5%	17080
	Night	18% 10% 30% 30% 12% 1	26843

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	11% 12% 7% 14% 7% 5% 6% 4 5% 5% 24%	3617
	11th to 20th	Day	9% 11% 6% 12% 7% 6% 6% 5% 7% 5% 26%	3620
	21st to end	Day	14% 17% 7% 10% 4 4 4 3 5% 6% 27%	4279
February	1st to 10th	Day	16% 14% 5% 9% 5% 5% 4 3 5% 6% 28%	1800
	11th to 20th	Day	14% 11% 4 8% 6% 6% 4 4 4 6% 33%	1948
	21st to end	Day	14% 14% 6% 10% 5% 4 4 3 5% 6% 32%	1650
March	1st to 10th	Day	18% 12% 3 6% 4 3 3 2 3 4 41%	4480
	11th to 20th	Day	9% 7% 3 7% 4 4 5% 4 6% 8% 44%	4750
	21st to end	Day	19% 11% 5% 11% 6% 5% 4 3 5% 6% 24%	5938
April	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 5% 5% 33%	2400
	11th to 20th	Day	15% 10% 4 7% 5% 4 4 4 5% 6% 37%	2374
	21st to end	Day	20% 9% 3 7% 4 4 5% 4 4 6% 34%	2443
May	1st to 10th	Day	16% 8% 3 7% 4 5% 5% 4 6% 8% 34%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 41%	6270
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 6% 9% 38%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 6% 9% 46%	2800
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 4 7% 9% 44%	2800
	21st to end	Day	10% 6% 2 7% 6% 6% 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	8% 7% 3 7% 5% 5% 6% 5% 8% 10% 38%	6360
	11th to 20th	Day	8% 6% 2 7% 4 4 5% 5% 7% 11% 41%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	6924
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4389
	11th to 20th	Day	9% 6% 2 5% 5% 4 4 4 8% 10% 43%	3849
	21st to end	Day	6% 5% 2 6% 4 4 4 4 7% 10% 49%	4312
September	1st to 10th	Day	11% 4 2 6% 4 4 4 4 6% 7% 50%	2400
	11th to 20th	Day	11% 4 2 4 3 3 4 4 6% 7% 54%	2310
	21st to end	Day	8% 6% 3 6% 5% 5% 3 4 6% 7% 47%	2200
October	1st to 10th	Day	11% 7% 2 7% 4 3 3 3 6% 6% 48%	2017
	11th to 20th	Day	17% 7% 3 7% 4 4 4 3 4 5% 42%	1900
	21st to end	Day	19% 10% 2 6% 4 3 5% 3 4 7% 37%	1924
November	1st to 10th	Day	11% 9% 2 6% 3 4 3 3 6% 6% 46%	1762
	11th to 20th	Day	16% 11% 5% 8% 4 4 4 3 4 7% 34%	1800
	21st to end	Day	18% 11% 4 8% 4 4 4 3 4 5% 35%	1698
December	1st to 10th	Day	9% 9% 3 7% 5% 4 3 3 4 7% 46%	2610
	11th to 20th	Day	12% 15% 6% 12% 6% 6% 5% 4 6% 6% 22%	3008
	21st to end	Day	26% 19% 8% 13% 5% 4 3 2 3 4 12%	3982
January	1st to 10th	Night	13% 14% 6% 12% 5% 4 4 3 4 4 31%	6348
	11th to 20th	Night	12% 15% 6% 13% 8% 6% 5% 4 5% 4 23%	6360
	21st to end	Night	17% 14% 6% 9% 4 4 4 3 4 4 32%	6697
February	1st to 10th	Night	15% 13% 4 9% 6% 4 4 3 5% 6% 31%	2598
	11th to 20th	Night	13% 11% 3 8% 5% 4 4 3 5% 6% 36%	2338
	21st to end	Night	17% 11% 4 9% 4 3 4 2 4 6% 36%	1980
March	1st to 10th	Night	17% 11% 4 6% 3 4 4 3 4 5% 40%	5373
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 47%	5238
	21st to end	Night	18% 12% 5% 11% 5% 5% 5% 4 4 5% 27%	4990
April	1st to 10th	Night	17% 11% 2 5% 5% 3 5% 3 5% 7% 38%	2000
	11th to 20th	Night	17% 8% 2 7% 4 3 3 3 4 6% 42%	1975
	21st to end	Night	17% 7% 2 7% 4 4 4 4 5% 8% 37%	1955
May	1st to 10th	Night	17% 9% 3 8% 5% 4 4 3 5% 7% 34%	4033
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 4 7% 10% 41%	3627
	21st to end	Night	13% 8% 3 7% 4 5% 4 4 6% 8% 39%	3942
June	1st to 10th	Night	8% 5% 2 6% 4 3 4 4 6% 7% 50%	1600
	11th to 20th	Night	8% 5% 3 6% 4 4 4 4 7% 10% 45%	1600
	21st to end	Night	8% 5% 3 7% 4 5% 5% 5% 6% 8% 44%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 10% 41%	3618
	11th to 20th	Night	7% 4 3 6% 4 4 4 4 6% 8% 50%	3617
	21st to end	Night	9% 6% 3 8% 5% 4 5% 5% 7% 8% 41%	3947
August	1st to 10th	Night	5% 5% 2 6% 3 4 5% 4 7% 9% 50%	2751
	11th to 20th	Night	7% 4 2 6% 4 4 5% 4 7% 8% 50%	3200
	21st to end	Night	6% 4 2 6% 4 3 4 3 5% 10% 53%	3586
September	1st to 10th	Night	9% 6% 2 5% 4 3 3 2 4 7% 57%	2000
	11th to 20th	Night	8% 5% 2 4 3 3 5% 3 5% 7% 55%	2069
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 8% 48%	2200
October	1st to 10th	Night	10% 5% 2 6% 3 4 4 3 5% 5% 53%	2228
	11th to 20th	Night	16% 7% 2 6% 3 3 3 2 4 6% 47%	2280
	21st to end	Night	19% 8% 3 6% 4 3 4 3 5% 6% 38%	2672
November	1st to 10th	Night	12% 7% 3 4 4 3 4 3 5% 6% 50%	2536
	11th to 20th	Night	14% 10% 3 8% 5% 4 4 3 5% 6% 38%	2600
	21st to end	Night	17% 10% 4 7% 4 4 3 3 4 6% 38%	2700
December	1st to 10th	Night	11% 9% 3 7% 5% 4 4 4 5% 6% 43%	4580
	11th to 20th	Night	16% 15% 4 10% 5% 3 3 3 4 5% 32%	5199
	21st to end	Night	24% 19% 6% 11% 6% 4 4 3 4 5% 16%	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	62% 3 11% 16% 7% 1	3618
	11th to 20th	Day	56% 5% 10% 11% 15% 4	3620
	21st to end	Day	59% 3 13% 18% 7%	4279
February	1st to 10th	Day	57% 5% 13% 16% 7% 2	1800
	11th to 20th	Day	51% 5% 11% 22% 10% 2	1958
	21st to end	Day	55% 3 11% 21% 8% 2	1650
March	1st to 10th	Day	48% 4 9% 25% 13% 3	4480
	11th to 20th	Day	38% 4 11% 33% 13% 1	4750
	21st to end	Day	60% 6% 12% 15% 7% 1	5940
April	1st to 10th	Day	52% 5% 11% 21% 11%	2400
	11th to 20th	Day	48% 4 11% 23% 11% 3	2374
	21st to end	Day	49% 6% 12% 20% 12% 2	2443
May	1st to 10th	Day	45% 6% 11% 22% 15% 1	5897
	11th to 20th	Day	41% 6% 10% 27% 13% 2	6274
	21st to end	Day	40% 6% 12% 24% 14% 4	6949
June	1st to 10th	Day	34% 7% 13% 27% 17% 3	2800
	11th to 20th	Day	34% 7% 11% 23% 21% 3	2800
	21st to end	Day	39% 7% 13% 24% 15% 2	2800
July	1st to 10th	Day	39% 6% 13% 27% 14% 1	6291
	11th to 20th	Day	35% 6% 15% 28% 15% 1	6285
	21st to end	Day	40% 7% 18% 20% 13% 2	6728
August	1st to 10th	Day	32% 6% 18% 31% 13% 1	4374
	11th to 20th	Day	34% 6% 14% 30% 16% 1	3885
	21st to end	Day	31% 6% 12% 32% 17% 3	4311
September	1st to 10th	Day	33% 5% 11% 26% 20% 5%	2400
	11th to 20th	Day	29% 5% 12% 29% 22% 4	2310
	21st to end	Day	35% 5% 10% 28% 21% 2	2200
October	1st to 10th	Day	25% 3 7% 29% 34% 2	3329
	11th to 20th	Day	30% 3 10% 26% 29% 2	3140
	21st to end	Day	40% 4 9% 27% 16% 3	3149
November	1st to 10th	Day	37% 4 15% 25% 16% 2	1762
	11th to 20th	Day	51% 4 11% 22% 11% 1	1800
	21st to end	Day	52% 4 11% 23% 9%	1698
December	1st to 10th	Day	41% 3 14% 33% 9%	2610
	11th to 20th	Day	61% 5% 11% 16% 6% 1	3005
	21st to end	Day	77% 3 7% 8% 4	3982
January	1st to 10th	Night	56% 4 11% 19% 9%	6348
	11th to 20th	Night	64% 3 11% 9% 13% 1	6360
	21st to end	Night	57% 3 10% 18% 11% 1	6697
February	1st to 10th	Night	54% 5% 11% 19% 11% 1	2598
	11th to 20th	Night	48% 4 11% 23% 12% 1	2339
	21st to end	Night	50% 4 10% 21% 12% 2	1980
March	1st to 10th	Night	47% 3 10% 24% 11% 5%	5373
	11th to 20th	Night	37% 4 12% 29% 15% 3	5238
	21st to end	Night	59% 4 10% 19% 7% 1	4990
April	1st to 10th	Night	45% 5% 11% 23% 13% 2	2000
	11th to 20th	Night	44% 3 9% 24% 15% 5%	1975
	21st to end	Night	45% 5% 13% 23% 11% 3	1955
May	1st to 10th	Night	48% 5% 12% 17% 15% 3	4060
	11th to 20th	Night	37% 7% 14% 25% 13% 4	3628
	21st to end	Night	41% 7% 11% 24% 15% 3	3950
June	1st to 10th	Night	32% 6% 14% 26% 19% 3	1600
	11th to 20th	Night	33% 6% 12% 24% 22% 3	1600
	21st to end	Night	36% 8% 15% 22% 16% 3	1600
July	1st to 10th	Night	39% 6% 12% 24% 17% 2	3580
	11th to 20th	Night	32% 5% 13% 28% 20% 1	3572
	21st to end	Night	38% 8% 15% 20% 16% 3	3832
August	1st to 10th	Night	28% 6% 19% 27% 19% 1	2742
	11th to 20th	Night	30% 6% 17% 30% 17% 1	3230
	21st to end	Night	28% 5% 12% 34% 18% 3	3586
September	1st to 10th	Night	29% 4 10% 29% 23% 5%	2000
	11th to 20th	Night	28% 5% 10% 29% 24% 4	2069
	21st to end	Night	33% 5% 12% 27% 20% 3	2200
October	1st to 10th	Night	23% 3 6% 33% 31% 4	3696
	11th to 20th	Night	30% 3 8% 23% 32% 3	3779
	21st to end	Night	40% 3 10% 24% 19% 4	4397
November	1st to 10th	Night	36% 4 10% 28% 17% 4	2536
	11th to 20th	Night	47% 4 11% 22% 14% 2	2600
	21st to end	Night	49% 4 11% 24% 12% 1	2700
December	1st to 10th	Night	42% 3 14% 30% 10% 1	4580
	11th to 20th	Night	55% 3 12% 18% 10%	5201
	21st to end	Night	71% 4 9% 10% 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	23% 14% 35% 20% 8% 1	300
	11th to 20th	Day	22% 19% 38% 15% 5% 1	300
	21st to end	Day	22% 18% 36% 18% 6%	330
February	1st to 10th	Day	21% 18% 39% 19% 3	300
	11th to 20th	Day	20% 21% 37% 17% 4	300
	21st to end	Day	23% 19% 39% 16% 3	249
March	1st to 10th	Day	25% 20% 36% 16% 2 1	300
	11th to 20th	Day	28% 17% 35% 14% 5%	300
	21st to end	Day	29% 17% 32% 16% 5% 1	534
April	1st to 10th	Day	28% 12% 33% 19% 7% 1	500
	11th to 20th	Day	21% 20% 27% 21% 9% 2	492
	21st to end	Day	24% 14% 31% 21% 8% 1	500
May	1st to 10th	Day	20% 14% 33% 22% 10% 3	503
	11th to 20th	Day	21% 12% 27% 25% 12% 3	702
	21st to end	Day	17% 9% 31% 23% 13% 7%	789
June	1st to 10th	Day	11% 8% 28% 29% 18% 6%	730
	11th to 20th	Day	10% 5% 29% 32% 19% 4	730
	21st to end	Day	11% 8% 31% 30% 14% 5%	730
July	1st to 10th	Day	9% 5% 18% 30% 24% 15%	721
	11th to 20th	Day	10% 4 23% 31% 20% 12%	718
	21st to end	Day	8% 2 21% 35% 27% 7%	794
August	1st to 10th	Day	6% 3 19% 34% 26% 11%	710
	11th to 20th	Day	11% 4 16% 29% 28% 13%	519
	21st to end	Day	8% 3 17% 32% 29% 10%	583
September	1st to 10th	Day	13% 6% 18% 30% 27% 7%	530
	11th to 20th	Day	9% 4 16% 28% 28% 14%	530
	21st to end	Day	12% 10% 26% 23% 22% 8%	530
October	1st to 10th	Day	13% 8% 26% 29% 20% 5%	396
	11th to 20th	Day	21% 19% 31% 21% 7% 1	300
	21st to end	Day	32% 13% 28% 22% 4 1	330
November	1st to 10th	Day	17% 13% 39% 19% 8% 4	300
	11th to 20th	Day	26% 16% 34% 18% 5% 2	300
	21st to end	Day	27% 9% 37% 22% 5%	300
December	1st to 10th	Day	18% 18% 39% 17% 7% 2	300
	11th to 20th	Day	20% 15% 39% 23% 3	300
	21st to end	Day	32% 18% 34% 14% 2	330
January	1st to 10th	Night	20% 14% 36% 24% 5% 1	1000
	11th to 20th	Night	21% 16% 36% 21% 5%	1000
	21st to end	Night	21% 13% 34% 27% 4	1100
February	1st to 10th	Night	23% 13% 37% 23% 4	998
	11th to 20th	Night	19% 15% 36% 25% 4	783
	21st to end	Night	23% 15% 37% 23% 3	660
March	1st to 10th	Night	23% 14% 34% 23% 6%	800
	11th to 20th	Night	18% 12% 35% 30% 5% 1	715
	21st to end	Night	27% 11% 34% 22% 5% 1	660
April	1st to 10th	Night	22% 12% 36% 25% 5%	600
	11th to 20th	Night	23% 12% 31% 27% 6%	593
	21st to end	Night	22% 11% 31% 28% 8%	600
May	1st to 10th	Night	19% 9% 36% 28% 7% 1	600
	11th to 20th	Night	17% 7% 32% 31% 11% 1	591
	21st to end	Night	14% 6% 28% 36% 15% 1	650
June	1st to 10th	Night	10% 4 27% 45% 14% 1	600
	11th to 20th	Night	11% 5% 26% 42% 16%	600
	21st to end	Night	10% 4 28% 40% 18% 1	600
July	1st to 10th	Night	8% 2 15% 39% 31% 4	585
	11th to 20th	Night	8% 3 19% 40% 27% 3	582
	21st to end	Night	5% 2 14% 48% 28% 2	657
August	1st to 10th	Night	6% 2 15% 43% 29% 4	597
	11th to 20th	Night	9% 2 11% 42% 32% 4	594
	21st to end	Night	7% 2 16% 42% 30% 3	660
September	1st to 10th	Night	13% 4 17% 36% 28% 3	600
	11th to 20th	Night	10% 3 18% 36% 30% 3	669
	21st to end	Night	12% 7% 23% 34% 22% 2	800
October	1st to 10th	Night	11% 7% 26% 35% 19% 2	773
	11th to 20th	Night	21% 12% 29% 26% 11%	760
	21st to end	Night	22% 12% 32% 25% 8%	836
November	1st to 10th	Night	15% 11% 37% 26% 11% 1	780
	11th to 20th	Night	21% 13% 34% 25% 7% 1	800
	21st to end	Night	23% 12% 34% 26% 6%	900
December	1st to 10th	Night	22% 12% 35% 25% 6%	1000
	11th to 20th	Night	23% 12% 33% 27% 5% 1	1000
	21st to end	Night	30% 15% 31% 21% 3	1100

Page generated at 2025-10-01 00:39:04. Version ac07

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