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 2025-05-11.

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	13% 10% 3 8% 5% 4 4 4 6% 7% 36%	113618
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	115519
Transparency	Day	45% 6% 12% 22% 13% 2	117189
	Night	46% 5% 11% 22% 14% 2	120091
Seeing	Day	18% 12% 31% 23% 12% 4	16399
	Night	18% 11% 30% 28% 11% 1	26384

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% 12% 8% 13% 5% 5% 5% 4 6% 6% 24%	3617
	11th to 20th	Day	11% 14% 6% 13% 6% 5% 5% 5% 6% 5% 26%	3620
	21st to end	Day	16% 20% 5% 8% 4 3 3 2 5% 5% 29%	4088
February	1st to 10th	Day	16% 14% 5% 9% 6% 6% 4 4 4 6% 26%	1872
	11th to 20th	Day	14% 13% 4 9% 6% 5% 3 3 5% 6% 33%	1948
	21st to end	Day	15% 13% 5% 10% 5% 4 4 4 6% 5% 30%	1650
March	1st to 10th	Day	18% 12% 3 6% 4 3 3 3 4 4 40%	4480
	11th to 20th	Day	8% 8% 3 7% 5% 5% 5% 4 6% 8% 41%	4650
	21st to end	Day	18% 15% 7% 12% 6% 5% 5% 4 5% 5% 20%	5967
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 4 3 4 6% 31%	2400
	11th to 20th	Day	17% 9% 2 6% 5% 5% 4 4 5% 6% 35%	2374
	21st to end	Day	19% 8% 3 7% 6% 4 4 3 5% 7% 34%	2519
May	1st to 10th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 8% 34%	4212
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 40%	4233
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 9% 44%	4824
June	1st to 10th	Day	11% 5% 3 6% 4 4 5% 4 7% 8% 45%	2660
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 6% 8% 46%	2660
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 9% 36%	2660
July	1st to 10th	Day	11% 8% 3 7% 6% 6% 5% 4 7% 9% 33%	4440
	11th to 20th	Day	7% 4 2 5% 5% 4 4 5% 6% 11% 47%	4440
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 5% 7% 8% 41%	4813
August	1st to 10th	Day	7% 6% 2 6% 4 5% 4 4 6% 9% 45%	4277
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 45%	3765
	21st to end	Day	6% 5% 2 5% 4 5% 5% 4 6% 10% 48%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 47%	2280
	11th to 20th	Day	11% 5% 1 4 4 4 4 3 6% 8% 51%	2142
	21st to end	Day	12% 6% 2 7% 4 3 4 4 6% 8% 43%	2090
October	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 4 6% 47%	2066
	11th to 20th	Day	18% 9% 3 6% 6% 5% 4 3 4 7% 37%	1876
	21st to end	Day	19% 10% 3 7% 5% 3 4 3 5% 7% 34%	1881
November	1st to 10th	Day	12% 9% 3 6% 3 3 3 3 6% 7% 45%	1762
	11th to 20th	Day	15% 11% 3 9% 4 4 5% 3 5% 7% 34%	1800
	21st to end	Day	18% 11% 4 10% 5% 3 4 3 4 5% 34%	1772
December	1st to 10th	Day	9% 11% 3 8% 6% 4 4 3 4 6% 42%	2610
	11th to 20th	Day	11% 17% 5% 11% 6% 5% 5% 4 6% 6% 23%	3008
	21st to end	Day	24% 22% 7% 12% 5% 4 4 3 4 4 11%	3982
January	1st to 10th	Night	14% 14% 6% 12% 5% 4 4 3 5% 4 29%	6348
	11th to 20th	Night	13% 15% 7% 12% 7% 5% 5% 5% 5% 4 21%	6360
	21st to end	Night	18% 14% 6% 8% 4 4 4 3 4 4 31%	6886
February	1st to 10th	Night	16% 15% 5% 8% 6% 5% 4 2 4 5% 31%	2525
	11th to 20th	Night	14% 12% 4 8% 5% 4 4 3 5% 5% 36%	2338
	21st to end	Night	18% 12% 4 8% 4 3 4 3 5% 5% 35%	1980
March	1st to 10th	Night	15% 12% 4 7% 4 3 4 3 4 5% 40%	5373
	11th to 20th	Night	10% 7% 2 7% 4 4 4 3 5% 7% 47%	5334
	21st to end	Night	17% 11% 6% 10% 6% 5% 5% 3 4 5% 27%	4967
April	1st to 10th	Night	15% 10% 4 6% 4 4 3 4 5% 8% 38%	2000
	11th to 20th	Night	18% 8% 2 7% 4 4 4 3 5% 5% 39%	1975
	21st to end	Night	16% 9% 3 8% 4 4 5% 4 5% 6% 36%	1880
May	1st to 10th	Night	17% 7% 2 8% 5% 4 4 4 6% 8% 34%	2903
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 8% 40%	2668
	21st to end	Night	10% 7% 2 7% 5% 4 4 4 6% 6% 44%	2743
June	1st to 10th	Night	9% 5% 2 6% 4 3 4 4 5% 8% 49%	1520
	11th to 20th	Night	8% 7% 2 4 5% 4 4 4 7% 9% 46%	1520
	21st to end	Night	10% 6% 3 5% 5% 5% 5% 4 5% 9% 42%	1520
July	1st to 10th	Night	10% 5% 2 7% 5% 6% 5% 4 6% 9% 40%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 3 4 6% 7% 54%	2530
	21st to end	Night	4 5% 2 6% 4 5% 5% 5% 8% 10% 45%	2750
August	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 6% 9% 47%	2656
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 5% 8% 52%	3061
	21st to end	Night	8% 5% 2 5% 3 4 4 3 5% 10% 52%	3476
September	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 5% 6% 53%	1900
	11th to 20th	Night	8% 5% 1 4 3 4 4 3 5% 7% 55%	2018
	21st to end	Night	10% 5% 2 8% 4 4 4 4 6% 7% 45%	2090
October	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 6% 52%	2112
	11th to 20th	Night	17% 7% 2 7% 3 3 3 2 4 6% 46%	2303
	21st to end	Night	20% 8% 3 7% 4 3 4 3 4 7% 37%	2717
November	1st to 10th	Night	12% 8% 2 6% 5% 3 3 3 5% 6% 47%	2536
	11th to 20th	Night	14% 9% 4 10% 5% 4 4 3 5% 7% 35%	2600
	21st to end	Night	18% 10% 3 8% 4 4 4 2 5% 5% 36%	2625
December	1st to 10th	Night	11% 9% 3 8% 5% 3 4 3 5% 6% 43%	4580
	11th to 20th	Night	16% 15% 5% 9% 5% 4 4 3 5% 5% 29%	5199
	21st to end	Night	23% 19% 6% 10% 5% 4 4 3 4 5% 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	58% 6% 12% 16% 8% 1	3618
	11th to 20th	Day	59% 4 10% 10% 13% 4	3620
	21st to end	Day	58% 4 11% 18% 8% 1	4088
February	1st to 10th	Day	59% 5% 12% 14% 8% 2	1872
	11th to 20th	Day	53% 4 11% 21% 9% 2	1958
	21st to end	Day	55% 6% 10% 19% 7% 2	1650
March	1st to 10th	Day	48% 4 8% 24% 12% 3	4480
	11th to 20th	Day	39% 6% 13% 32% 10% 1	4650
	21st to end	Day	64% 7% 11% 11% 6% 1	5969
April	1st to 10th	Day	54% 6% 10% 19% 11%	2400
	11th to 20th	Day	47% 7% 10% 21% 11% 4	2374
	21st to end	Day	49% 6% 13% 19% 11% 2	2519
May	1st to 10th	Day	46% 8% 12% 24% 10% 1	4250
	11th to 20th	Day	40% 7% 12% 28% 12%	4237
	21st to end	Day	35% 7% 12% 21% 20% 4	4837
June	1st to 10th	Day	35% 7% 13% 24% 18% 2	2660
	11th to 20th	Day	34% 7% 12% 24% 20% 3	2660
	21st to end	Day	40% 10% 14% 20% 14% 2	2660
July	1st to 10th	Day	44% 8% 13% 20% 12% 2	4371
	11th to 20th	Day	29% 8% 17% 29% 16% 1	4365
	21st to end	Day	37% 10% 15% 17% 19% 2	4662
August	1st to 10th	Day	34% 8% 15% 28% 14% 1	4262
	11th to 20th	Day	32% 8% 12% 32% 15% 1	3802
	21st to end	Day	31% 7% 13% 27% 19% 3	4179
September	1st to 10th	Day	35% 6% 13% 24% 17% 4	2280
	11th to 20th	Day	31% 6% 12% 27% 21% 3	2142
	21st to end	Day	37% 8% 9% 25% 20% 1	2090
October	1st to 10th	Day	26% 4 7% 31% 29% 2	3430
	11th to 20th	Day	34% 6% 11% 25% 23% 1	3092
	21st to end	Day	43% 4 10% 25% 15% 3	3081
November	1st to 10th	Day	39% 5% 14% 24% 16% 2	1762
	11th to 20th	Day	50% 6% 12% 20% 12% 1	1800
	21st to end	Day	54% 5% 9% 22% 10%	1772
December	1st to 10th	Day	44% 4 16% 27% 9%	2610
	11th to 20th	Day	59% 5% 11% 18% 6% 1	3005
	21st to end	Day	78% 4 6% 8% 3	3982
January	1st to 10th	Night	58% 5% 10% 18% 9%	6348
	11th to 20th	Night	64% 5% 10% 9% 11% 1	6360
	21st to end	Night	57% 4 11% 16% 11% 1	6886
February	1st to 10th	Night	56% 4 11% 17% 10% 2	2525
	11th to 20th	Night	50% 5% 10% 21% 14% 1	2339
	21st to end	Night	52% 5% 11% 18% 11% 2	1980
March	1st to 10th	Night	48% 4 10% 25% 11% 3	5373
	11th to 20th	Night	37% 6% 12% 29% 15% 3	5334
	21st to end	Night	58% 6% 9% 19% 8%	4967
April	1st to 10th	Night	45% 5% 13% 23% 13% 2	2000
	11th to 20th	Night	45% 6% 9% 21% 16% 4	1975
	21st to end	Night	48% 7% 9% 21% 14% 2	1880
May	1st to 10th	Night	46% 6% 13% 21% 12% 2	2930
	11th to 20th	Night	40% 8% 15% 26% 10% 1	2669
	21st to end	Night	37% 8% 12% 17% 20% 6%	2751
June	1st to 10th	Night	33% 6% 12% 25% 20% 4	1520
	11th to 20th	Night	32% 6% 14% 23% 23% 2	1520
	21st to end	Night	38% 9% 13% 21% 16% 3	1520
July	1st to 10th	Night	40% 8% 15% 22% 11% 5%	2490
	11th to 20th	Night	26% 8% 13% 27% 24% 2	2482
	21st to end	Night	31% 9% 17% 19% 20% 4	2662
August	1st to 10th	Night	31% 9% 16% 27% 16% 1	2647
	11th to 20th	Night	30% 7% 14% 28% 20% 1	3090
	21st to end	Night	29% 6% 12% 32% 17% 3	3476
September	1st to 10th	Night	32% 5% 12% 25% 23% 3	1900
	11th to 20th	Night	28% 5% 9% 28% 27% 3	2018
	21st to end	Night	37% 6% 10% 28% 16% 2	2090
October	1st to 10th	Night	22% 4 8% 36% 28% 2	3528
	11th to 20th	Night	35% 4 7% 24% 28% 2	3826
	21st to end	Night	39% 4 9% 24% 20% 4	4467
November	1st to 10th	Night	38% 5% 10% 26% 19% 3	2536
	11th to 20th	Night	48% 5% 12% 21% 12% 2	2600
	21st to end	Night	50% 4 12% 22% 11%	2625
December	1st to 10th	Night	42% 5% 14% 27% 11% 1	4580
	11th to 20th	Night	56% 5% 12% 16% 10% 1	5201
	21st to end	Night	71% 5% 8% 10% 6%	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	22% 14% 38% 20% 5%	300
	11th to 20th	Day	25% 20% 38% 11% 4 1	300
	21st to end	Day	22% 20% 37% 16% 3 1	330
February	1st to 10th	Day	25% 20% 41% 12% 2	300
	11th to 20th	Day	29% 19% 34% 15% 2 1	300
	21st to end	Day	24% 24% 38% 11% 2	249
March	1st to 10th	Day	23% 25% 39% 12% 2	300
	11th to 20th	Day	26% 18% 34% 16% 5%	340
	21st to end	Day	31% 17% 28% 17% 6% 1	550
April	1st to 10th	Day	27% 16% 31% 19% 7%	500
	11th to 20th	Day	27% 18% 24% 20% 9% 1	492
	21st to end	Day	23% 18% 35% 15% 8% 1	500
May	1st to 10th	Day	24% 16% 29% 21% 8% 2	503
	11th to 20th	Day	21% 9% 32% 23% 11% 3	574
	21st to end	Day	14% 12% 31% 25% 12% 6%	734
June	1st to 10th	Day	12% 9% 32% 29% 12% 5%	680
	11th to 20th	Day	13% 7% 34% 27% 16% 3	680
	21st to end	Day	11% 12% 34% 26% 12% 4	680
July	1st to 10th	Day	9% 5% 25% 30% 17% 13%	671
	11th to 20th	Day	12% 5% 29% 29% 15% 10%	668
	21st to end	Day	8% 5% 27% 34% 19% 6%	739
August	1st to 10th	Day	7% 4 24% 32% 23% 11%	595
	11th to 20th	Day	9% 7% 19% 31% 23% 11%	479
	21st to end	Day	8% 2 19% 40% 21% 10%	539
September	1st to 10th	Day	14% 6% 23% 30% 21% 6%	490
	11th to 20th	Day	10% 4 19% 31% 25% 11%	490
	21st to end	Day	15% 10% 25% 25% 18% 7%	490
October	1st to 10th	Day	14% 10% 27% 27% 17% 4	466
	11th to 20th	Day	21% 21% 32% 17% 7% 1	300
	21st to end	Day	31% 12% 36% 15% 5%	330
November	1st to 10th	Day	18% 15% 37% 21% 7% 2	300
	11th to 20th	Day	25% 16% 37% 15% 5% 1	300
	21st to end	Day	25% 13% 41% 18% 3	300
December	1st to 10th	Day	19% 24% 37% 14% 5%	300
	11th to 20th	Day	21% 20% 39% 17% 3	300
	21st to end	Day	28% 21% 35% 14% 2	330
January	1st to 10th	Night	20% 19% 34% 23% 3	1000
	11th to 20th	Night	23% 18% 35% 20% 4 1	1000
	21st to end	Night	22% 15% 35% 24% 3	1100
February	1st to 10th	Night	24% 14% 37% 22% 3	925
	11th to 20th	Night	21% 17% 35% 24% 4	783
	21st to end	Night	25% 17% 36% 19% 4	660
March	1st to 10th	Night	22% 16% 36% 21% 5%	800
	11th to 20th	Night	17% 13% 39% 25% 6% 1	800
	21st to end	Night	25% 14% 31% 24% 7% 1	665
April	1st to 10th	Night	21% 13% 36% 24% 5% 1	600
	11th to 20th	Night	23% 13% 32% 24% 7% 1	593
	21st to end	Night	22% 11% 33% 27% 8%	600
May	1st to 10th	Night	19% 7% 34% 30% 9%	600
	11th to 20th	Night	19% 5% 29% 33% 13% 1	563
	21st to end	Night	13% 6% 26% 36% 18%	617
June	1st to 10th	Night	10% 5% 27% 43% 14% 1	570
	11th to 20th	Night	11% 4 31% 41% 13% 1	570
	21st to end	Night	11% 5% 28% 38% 18% 1	570
July	1st to 10th	Night	8% 3 20% 41% 24% 4	555
	11th to 20th	Night	9% 3 21% 37% 28% 3	552
	21st to end	Night	5% 3 19% 47% 25% 2	624
August	1st to 10th	Night	6% 2 12% 48% 28% 4	567
	11th to 20th	Night	9% 2 15% 38% 32% 4	564
	21st to end	Night	7% 2 20% 38% 30% 3	627
September	1st to 10th	Night	12% 4 18% 35% 27% 4	570
	11th to 20th	Night	9% 5% 18% 37% 27% 5%	688
	21st to end	Night	13% 7% 23% 33% 21% 3	760
October	1st to 10th	Night	13% 6% 26% 35% 18% 2	760
	11th to 20th	Night	20% 14% 30% 26% 9%	760
	21st to end	Night	26% 14% 29% 23% 8%	836
November	1st to 10th	Night	17% 14% 38% 21% 9% 1	780
	11th to 20th	Night	19% 19% 33% 24% 5% 1	800
	21st to end	Night	23% 14% 37% 20% 6%	825
December	1st to 10th	Night	23% 15% 37% 21% 4	1000
	11th to 20th	Night	23% 15% 35% 22% 4	1000
	21st to end	Night	30% 16% 33% 19% 2	1100

Page generated at 2025-05-08 08:34:00. Version ac07

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