Tinley Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tinley Park clear sky chart from 2005-11-06 to 2026-09-14.

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% 5% 5% 4 6% 7% 36%	148961
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	142432
Transparency	Day	44% 6% 12% 23% 13% 2	152485
	Night	45% 6% 11% 22% 14% 2	146985
Seeing	Day	17% 13% 31% 23% 11% 4	18368
	Night	18% 11% 30% 28% 11% 1	28111

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	12% 14% 7% 12% 5% 5% 5% 4 5% 7% 23%	4691
	11th to 20th	Day	8% 12% 6% 14% 7% 5% 6% 5% 5% 4 27%	4710
	21st to end	Day	13% 17% 5% 11% 5% 4 3 3 5% 5% 28%	5290
February	1st to 10th	Day	15% 14% 5% 10% 6% 5% 4 4 5% 6% 26%	1966
	11th to 20th	Day	13% 13% 4 9% 6% 5% 3 3 5% 6% 34%	2048
	21st to end	Day	15% 13% 5% 10% 5% 4 4 4 6% 5% 29%	1730
March	1st to 10th	Day	18% 12% 3 6% 4 3 3 3 4 4 40%	4580
	11th to 20th	Day	8% 8% 4 7% 5% 5% 5% 4 6% 8% 41%	4753
	21st to end	Day	17% 14% 7% 12% 6% 5% 5% 4 5% 5% 20%	6099
April	1st to 10th	Day	17% 13% 3 8% 5% 5% 4 3 4 6% 31%	2520
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	2494
	21st to end	Day	19% 8% 3 7% 6% 4 5% 4 5% 7% 34%	2645
May	1st to 10th	Day	16% 8% 2 7% 5% 5% 5% 4 6% 8% 34%	7616
	11th to 20th	Day	12% 6% 3 8% 5% 5% 5% 4 6% 8% 38%	7918
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 6% 9% 39%	9047
June	1st to 10th	Day	10% 5% 3 6% 4 5% 5% 4 6% 8% 44%	2940
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 6% 8% 45%	2940
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	2940
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 6% 5% 7% 9% 38%	8280
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 5% 7% 11% 45%	8280
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 40%	8990
August	1st to 10th	Day	11% 7% 3 7% 5% 5% 4 4 6% 8% 41%	6277
	11th to 20th	Day	13% 7% 2 8% 4 5% 5% 4 6% 9% 37%	5537
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 6% 11% 48%	6080
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 8% 47%	2520
	11th to 20th	Day	10% 5% 1 4 3 3 4 3 5% 8% 52%	2291
	21st to end	Day	12% 6% 2 7% 4 3 5% 4 6% 8% 43%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 47%	2174
	11th to 20th	Day	18% 9% 3 6% 6% 4 3 3 4 7% 37%	1975
	21st to end	Day	18% 10% 3 7% 5% 4 4 3 5% 7% 35%	1980
November	1st to 10th	Day	12% 9% 3 6% 4 3 3 3 5% 7% 45%	1852
	11th to 20th	Day	15% 12% 3 8% 4 4 5% 3 5% 7% 34%	1890
	21st to end	Day	18% 12% 4 10% 4 3 4 3 4 5% 34%	1860
December	1st to 10th	Day	9% 11% 3 8% 6% 4 4 3 4 6% 41%	2690
	11th to 20th	Day	11% 17% 5% 11% 6% 5% 5% 4 6% 6% 24%	3088
	21st to end	Day	24% 22% 6% 13% 5% 4 4 3 3 4 11%	4070
January	1st to 10th	Night	12% 14% 6% 12% 5% 4 4 3 5% 4 29%	8232
	11th to 20th	Night	11% 14% 6% 14% 8% 6% 5% 5% 6% 4 22%	8280
	21st to end	Night	15% 13% 5% 8% 5% 5% 5% 3 4 5% 31%	8920
February	1st to 10th	Night	16% 14% 4 9% 6% 5% 4 2 4 5% 31%	2651
	11th to 20th	Night	14% 12% 4 8% 4 4 4 3 5% 6% 36%	2458
	21st to end	Night	17% 12% 4 9% 4 3 4 3 4 5% 35%	2076
March	1st to 10th	Night	15% 12% 4 7% 4 3 4 3 4 5% 40%	5493
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 5% 7% 46%	5452
	21st to end	Night	17% 10% 6% 10% 6% 6% 5% 3 4 5% 27%	5077
April	1st to 10th	Night	15% 10% 3 7% 4 4 3 4 5% 8% 37%	2100
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 5% 39%	2075
	21st to end	Night	16% 8% 3 7% 4 4 5% 5% 5% 7% 36%	1974
May	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 4 5% 7% 37%	5237
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 9% 44%	4954
	21st to end	Night	13% 8% 3 8% 4 4 4 3 5% 7% 40%	5139
June	1st to 10th	Night	10% 5% 2 6% 4 3 3 4 4 8% 49%	1680
	11th to 20th	Night	9% 7% 2 5% 5% 4 4 4 7% 9% 45%	1680
	21st to end	Night	10% 6% 3 5% 5% 5% 5% 4 5% 9% 42%	1680
July	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 4 6% 10% 43%	4708
	11th to 20th	Night	6% 5% 2 6% 4 4 4 3 6% 8% 52%	4707
	21st to end	Night	7% 6% 2 7% 5% 5% 5% 5% 7% 9% 45%	5117
August	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	3882
	11th to 20th	Night	11% 7% 3 7% 4 5% 5% 3 6% 8% 41%	4492
	21st to end	Night	7% 4 2 5% 3 4 4 3 4 9% 53%	5046
September	1st to 10th	Night	9% 6% 2 6% 4 3 3 3 5% 7% 52%	2100
	11th to 20th	Night	8% 5% 1 4 3 3 4 3 5% 7% 56%	2154
	21st to end	Night	10% 5% 2 8% 4 4 4 4 7% 6% 46%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 6% 52%	2223
	11th to 20th	Night	16% 7% 2 7% 3 3 3 3 4 6% 45%	2424
	21st to end	Night	20% 8% 3 7% 4 3 4 3 4 7% 37%	2860
November	1st to 10th	Night	12% 8% 2 6% 5% 3 3 3 5% 6% 46%	2666
	11th to 20th	Night	14% 9% 4 9% 5% 4 4 3 5% 7% 36%	2730
	21st to end	Night	18% 10% 4 8% 4 4 4 2 5% 5% 36%	2756
December	1st to 10th	Night	11% 9% 3 8% 5% 4 4 3 5% 6% 42%	4720
	11th to 20th	Night	16% 15% 5% 9% 5% 4 4 3 5% 5% 29%	5339
	21st to end	Night	23% 19% 6% 10% 5% 4 4 3 4 5% 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	60% 5% 15% 13% 6% 1	4692
	11th to 20th	Day	58% 4 8% 15% 11% 3	4710
	21st to end	Day	57% 4 11% 19% 8% 1	5290
February	1st to 10th	Day	59% 5% 12% 14% 8% 2	1966
	11th to 20th	Day	53% 4 11% 21% 10% 2	2058
	21st to end	Day	55% 6% 10% 19% 7% 2	1730
March	1st to 10th	Day	48% 4 8% 25% 12% 3	4580
	11th to 20th	Day	39% 6% 13% 32% 10% 1	4753
	21st to end	Day	64% 7% 11% 12% 6% 1	6101
April	1st to 10th	Day	55% 6% 10% 18% 11%	2520
	11th to 20th	Day	48% 7% 10% 21% 11% 3	2494
	21st to end	Day	49% 6% 13% 19% 11% 2	2645
May	1st to 10th	Day	46% 7% 11% 23% 11% 1	7651
	11th to 20th	Day	42% 7% 12% 23% 15% 1	7922
	21st to end	Day	41% 7% 11% 21% 16% 4	9061
June	1st to 10th	Day	36% 8% 12% 24% 18% 2	2940
	11th to 20th	Day	34% 7% 12% 25% 20% 2	2940
	21st to end	Day	39% 10% 14% 21% 14% 2	2940
July	1st to 10th	Day	38% 9% 14% 23% 15% 1	8211
	11th to 20th	Day	31% 8% 14% 31% 15% 1	8205
	21st to end	Day	37% 8% 14% 23% 16% 2	8794
August	1st to 10th	Day	38% 8% 13% 26% 15% 1	6262
	11th to 20th	Day	42% 8% 12% 26% 12% 1	5574
	21st to end	Day	29% 6% 14% 32% 16% 2	6079
September	1st to 10th	Day	34% 6% 13% 26% 17% 3	2520
	11th to 20th	Day	30% 5% 12% 28% 22% 3	2291
	21st to end	Day	36% 8% 9% 26% 20% 1	2200
October	1st to 10th	Day	26% 4 7% 31% 29% 2	3538
	11th to 20th	Day	35% 6% 11% 25% 23% 1	3191
	21st to end	Day	43% 4 9% 26% 16% 3	3180
November	1st to 10th	Day	39% 5% 14% 25% 16% 2	1852
	11th to 20th	Day	49% 5% 12% 21% 12% 1	1890
	21st to end	Day	54% 5% 9% 22% 10%	1860
December	1st to 10th	Day	45% 4 15% 26% 9%	2690
	11th to 20th	Day	59% 6% 11% 18% 6% 1	3085
	21st to end	Day	78% 4 6% 8% 4	4070
January	1st to 10th	Night	57% 5% 11% 19% 8%	8233
	11th to 20th	Night	62% 5% 10% 11% 10% 1	8280
	21st to end	Night	55% 5% 11% 20% 9% 1	8920
February	1st to 10th	Night	56% 4 11% 17% 10% 2	2651
	11th to 20th	Night	49% 5% 10% 22% 13% 1	2459
	21st to end	Night	52% 5% 11% 19% 11% 2	2076
March	1st to 10th	Night	48% 4 10% 24% 11% 3	5493
	11th to 20th	Night	38% 5% 12% 28% 14% 3	5452
	21st to end	Night	57% 6% 9% 19% 8%	5077
April	1st to 10th	Night	46% 5% 13% 23% 12% 2	2100
	11th to 20th	Night	45% 6% 9% 21% 16% 4	2075
	21st to end	Night	47% 7% 9% 22% 13% 2	1974
May	1st to 10th	Night	45% 6% 10% 21% 15% 3	5264
	11th to 20th	Night	35% 7% 15% 25% 17% 2	4955
	21st to end	Night	41% 6% 10% 18% 19% 6%	5149
June	1st to 10th	Night	34% 6% 12% 25% 20% 3	1680
	11th to 20th	Night	33% 6% 14% 22% 23% 2	1680
	21st to end	Night	38% 9% 14% 21% 16% 3	1680
July	1st to 10th	Night	35% 7% 13% 25% 15% 3	4670
	11th to 20th	Night	28% 7% 13% 28% 23% 2	4662
	21st to end	Night	33% 8% 14% 24% 18% 3	5002
August	1st to 10th	Night	36% 8% 14% 26% 16% 1	3873
	11th to 20th	Night	40% 8% 14% 23% 14% 1	4521
	21st to end	Night	28% 6% 12% 35% 16% 2	5046
September	1st to 10th	Night	31% 5% 12% 27% 22% 3	2100
	11th to 20th	Night	27% 5% 8% 29% 28% 3	2154
	21st to end	Night	36% 6% 10% 27% 18% 2	2200
October	1st to 10th	Night	22% 4 8% 36% 29% 2	3639
	11th to 20th	Night	35% 4 7% 24% 28% 2	3947
	21st to end	Night	39% 4 9% 24% 20% 4	4610
November	1st to 10th	Night	39% 5% 10% 25% 18% 3	2666
	11th to 20th	Night	48% 5% 12% 22% 12% 2	2730
	21st to end	Night	51% 4 12% 22% 11%	2756
December	1st to 10th	Night	43% 5% 14% 27% 11% 1	4720
	11th to 20th	Night	56% 5% 12% 16% 10% 1	5341
	21st to end	Night	71% 5% 8% 10% 6%	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	22% 15% 38% 19% 5%	330
	11th to 20th	Day	24% 23% 37% 10% 4 1	330
	21st to end	Day	22% 22% 36% 16% 2 1	363
February	1st to 10th	Day	23% 20% 42% 13% 2	330
	11th to 20th	Day	28% 20% 35% 14% 2 1	330
	21st to end	Day	25% 25% 38% 10% 2	273
March	1st to 10th	Day	23% 25% 38% 12% 2	330
	11th to 20th	Day	27% 18% 35% 16% 5%	372
	21st to end	Day	30% 19% 29% 16% 6% 1	594
April	1st to 10th	Day	28% 16% 31% 18% 6%	540
	11th to 20th	Day	26% 20% 25% 19% 9% 1	532
	21st to end	Day	22% 17% 36% 15% 8% 1	540
May	1st to 10th	Day	24% 17% 29% 21% 7% 2	543
	11th to 20th	Day	21% 12% 31% 22% 11% 3	664
	21st to end	Day	15% 11% 33% 24% 11% 5%	844
June	1st to 10th	Day	13% 9% 34% 28% 12% 5%	780
	11th to 20th	Day	12% 9% 35% 26% 14% 3	780
	21st to end	Day	11% 11% 36% 27% 11% 4	780
July	1st to 10th	Day	8% 6% 26% 32% 17% 12%	771
	11th to 20th	Day	11% 6% 27% 29% 16% 10%	768
	21st to end	Day	9% 7% 27% 34% 18% 6%	849
August	1st to 10th	Day	8% 4 23% 33% 22% 10%	687
	11th to 20th	Day	10% 8% 19% 32% 22% 10%	559
	21st to end	Day	7% 3 21% 38% 21% 10%	627
September	1st to 10th	Day	13% 6% 25% 29% 21% 6%	570
	11th to 20th	Day	9% 4 18% 31% 25% 11%	542
	21st to end	Day	14% 10% 25% 26% 18% 7%	530
October	1st to 10th	Day	13% 10% 27% 29% 17% 4	504
	11th to 20th	Day	20% 21% 32% 20% 7% 1	330
	21st to end	Day	29% 12% 38% 16% 4	363
November	1st to 10th	Day	18% 17% 36% 20% 6% 2	330
	11th to 20th	Day	26% 18% 35% 15% 5% 1	330
	21st to end	Day	26% 14% 41% 17% 3	330
December	1st to 10th	Day	19% 24% 37% 15% 5%	330
	11th to 20th	Day	22% 20% 38% 16% 3	330
	21st to end	Day	29% 21% 36% 13% 1	363
January	1st to 10th	Night	20% 19% 34% 23% 3	1050
	11th to 20th	Night	23% 19% 35% 19% 3 1	1050
	21st to end	Night	22% 16% 35% 23% 3	1155
February	1st to 10th	Night	24% 14% 37% 22% 3	971
	11th to 20th	Night	20% 17% 35% 24% 3	823
	21st to end	Night	24% 18% 36% 18% 3	692
March	1st to 10th	Night	22% 17% 36% 20% 5%	840
	11th to 20th	Night	17% 14% 39% 24% 5% 1	840
	21st to end	Night	25% 14% 32% 23% 6% 1	698
April	1st to 10th	Night	21% 15% 35% 23% 5% 1	630
	11th to 20th	Night	23% 13% 31% 24% 8% 1	623
	21st to end	Night	21% 10% 34% 27% 7% 1	630
May	1st to 10th	Night	19% 8% 35% 29% 8%	630
	11th to 20th	Night	18% 6% 31% 32% 13%	621
	21st to end	Night	13% 6% 27% 36% 17%	683
June	1st to 10th	Night	11% 6% 26% 43% 14% 1	630
	11th to 20th	Night	11% 5% 32% 39% 12% 1	630
	21st to end	Night	10% 5% 28% 39% 18% 1	630
July	1st to 10th	Night	8% 3 20% 41% 24% 3	615
	11th to 20th	Night	8% 4 20% 37% 28% 3	612
	21st to end	Night	6% 3 19% 46% 24% 2	690
August	1st to 10th	Night	7% 2 12% 48% 28% 4	627
	11th to 20th	Night	9% 3 15% 38% 31% 4	624
	21st to end	Night	7% 2 20% 38% 29% 3	693
September	1st to 10th	Night	11% 4 20% 34% 28% 3	630
	11th to 20th	Night	8% 5% 17% 38% 28% 4	733
	21st to end	Night	13% 7% 23% 34% 22% 3	800
October	1st to 10th	Night	13% 6% 25% 36% 19% 2	800
	11th to 20th	Night	20% 14% 30% 28% 9%	800
	21st to end	Night	25% 14% 30% 23% 7%	880
November	1st to 10th	Night	17% 14% 38% 21% 9% 1	820
	11th to 20th	Night	19% 19% 34% 23% 5%	840
	21st to end	Night	24% 15% 35% 20% 6%	866
December	1st to 10th	Night	23% 15% 37% 21% 4	1050
	11th to 20th	Night	24% 16% 35% 22% 4	1050
	21st to end	Night	31% 16% 33% 19% 2	1155

Page generated at 2026-09-11 03:11:56. Version ac07

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