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 2024-03-01.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	78404
	Night	15% 10% 4 7% 5% 3 4 4 5% 6% 37%	81055
Transparency	Day	47% 6% 12% 21% 12% 2	78105
	Night	48% 6% 11% 20% 13% 2	80875
Seeing	Day	18% 12% 30% 24% 12% 5%	14752
	Night	18% 11% 30% 28% 11% 1	24799

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	14% 13% 8% 9% 6% 4 5% 5% 5% 6% 25%	2527
	11th to 20th	Day	14% 15% 7% 13% 8% 3 4 8% 7% 5% 18%	2530
	21st to end	Day	19% 25% 7% 9% 5% 2 3 3 4 5% 18%	2847
February	1st to 10th	Day	17% 13% 5% 8% 6% 4 5% 5% 4 6% 27%	1778
	11th to 20th	Day	14% 11% 4 7% 7% 3 4 4 5% 6% 34%	1848
	21st to end	Day	16% 13% 5% 9% 6% 2 4 4 5% 5% 29%	1570
March	1st to 10th	Day	16% 13% 6% 7% 7% 3 4 4 6% 5% 29%	1800
	11th to 20th	Day	14% 10% 5% 7% 6% 3 5% 5% 5% 7% 35%	1846
	21st to end	Day	21% 12% 5% 8% 6% 4 4 4 5% 6% 26%	2376
April	1st to 10th	Day	16% 11% 5% 7% 5% 4 5% 4 5% 6% 31%	2160
	11th to 20th	Day	18% 9% 2 5% 6% 3 5% 5% 5% 7% 35%	2134
	21st to end	Day	21% 7% 3 6% 6% 3 5% 4 5% 7% 33%	2267
May	1st to 10th	Day	17% 8% 3 7% 6% 4 5% 5% 6% 8% 31%	2340
	11th to 20th	Day	19% 8% 3 6% 5% 3 4 5% 6% 8% 33%	2383
	21st to end	Day	13% 6% 3 6% 7% 3 6% 5% 5% 7% 38%	2725
June	1st to 10th	Day	10% 5% 3 5% 5% 3 4 5% 6% 8% 45%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 6% 6% 8% 46%	2520
	21st to end	Day	11% 5% 3 6% 6% 4 6% 7% 7% 9% 36%	2520
July	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 6% 9% 46%	2520
	11th to 20th	Day	10% 4 2 4 5% 3 5% 6% 7% 10% 44%	2520
	21st to end	Day	7% 5% 3 5% 5% 4 6% 6% 7% 9% 43%	2743
August	1st to 10th	Day	6% 4 2 6% 6% 4 5% 5% 7% 9% 44%	2431
	11th to 20th	Day	9% 4 2 5% 6% 4 6% 5% 7% 9% 43%	2148
	21st to end	Day	8% 4 2 4 4 4 5% 6% 6% 10% 47%	2376
September	1st to 10th	Day	13% 5% 2 5% 4 3 4 5% 6% 8% 45%	2160
	11th to 20th	Day	12% 5% 2 4 4 3 4 5% 5% 8% 50%	2030
	21st to end	Day	11% 5% 2 6% 5% 2 5% 5% 6% 8% 44%	1980
October	1st to 10th	Day	13% 7% 3 6% 5% 3 4 5% 4 5% 46%	1958
	11th to 20th	Day	19% 8% 3 5% 6% 4 4 4 4 7% 36%	1778
	21st to end	Day	20% 10% 3 6% 7% 2 4 4 4 7% 34%	1782
November	1st to 10th	Day	12% 8% 3 5% 4 2 3 4 6% 7% 46%	1672
	11th to 20th	Day	16% 9% 4 8% 5% 3 5% 4 5% 7% 35%	1710
	21st to end	Day	18% 10% 4 8% 5% 2 4 3 4 5% 36%	1684
December	1st to 10th	Day	15% 13% 4 8% 7% 3 5% 4 5% 5% 30%	1520
	11th to 20th	Day	13% 14% 6% 9% 7% 4 4 4 6% 6% 28%	1918
	21st to end	Day	21% 22% 8% 12% 7% 3 4 3 3 4 13%	2783
January	1st to 10th	Night	15% 15% 8% 9% 6% 3 4 3 4 4 28%	4428
	11th to 20th	Night	15% 18% 8% 10% 7% 3 4 7% 6% 4 18%	4440
	21st to end	Night	23% 17% 8% 8% 5% 3 4 3 3 4 22%	4816
February	1st to 10th	Night	16% 14% 5% 7% 7% 4 4 3 3 6% 31%	2399
	11th to 20th	Night	14% 10% 4 7% 6% 3 4 4 5% 6% 37%	2218
	21st to end	Night	19% 11% 5% 7% 6% 2 4 3 4 5% 34%	1884
March	1st to 10th	Night	15% 12% 5% 8% 6% 3 5% 4 4 4 33%	2167
	11th to 20th	Night	13% 7% 4 6% 4 3 5% 4 6% 7% 42%	2124
	21st to end	Night	20% 10% 4 7% 6% 4 5% 5% 4 6% 31%	1985
April	1st to 10th	Night	15% 9% 4 7% 4 3 4 5% 5% 8% 38%	1800
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 4 5% 5% 39%	1775
	21st to end	Night	18% 8% 3 6% 5% 3 4 6% 5% 6% 36%	1692
May	1st to 10th	Night	17% 7% 3 7% 5% 3 5% 5% 6% 9% 34%	1620
	11th to 20th	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 41%	1507
	21st to end	Night	11% 5% 3 7% 6% 3 4 6% 7% 8% 41%	1551
June	1st to 10th	Night	10% 5% 2 5% 5% 2 3 5% 4 8% 50%	1440
	11th to 20th	Night	8% 6% 3 3 5% 3 4 4 6% 9% 47%	1440
	21st to end	Night	11% 5% 3 5% 5% 3 6% 5% 5% 9% 43%	1440
July	1st to 10th	Night	6% 4 1 5% 4 4 5% 5% 6% 10% 50%	1440
	11th to 20th	Night	7% 4 3 4 4 3 4 5% 6% 8% 53%	1440
	21st to end	Night	4 3 2 4 4 3 4 5% 7% 11% 51%	1575
August	1st to 10th	Night	5% 4 1 4 5% 3 6% 5% 7% 9% 50%	1506
	11th to 20th	Night	7% 3 2 4 4 3 5% 5% 6% 9% 53%	1750
	21st to end	Night	8% 3 1 4 4 3 5% 4 5% 9% 54%	1980
September	1st to 10th	Night	10% 6% 2 5% 4 3 3 4 4 7% 51%	1800
	11th to 20th	Night	9% 5% 2 4 4 2 4 4 5% 7% 55%	1911
	21st to end	Night	10% 4 3 7% 5% 3 4 5% 6% 7% 46%	1980
October	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 5% 6% 50%	2000
	11th to 20th	Night	17% 7% 3 6% 4 2 3 3 4 6% 45%	2181
	21st to end	Night	21% 8% 3 6% 4 2 3 4 4 7% 36%	2574
November	1st to 10th	Night	12% 7% 3 5% 5% 3 3 3 5% 6% 47%	2406
	11th to 20th	Night	14% 8% 4 8% 6% 3 4 4 4 7% 36%	2470
	21st to end	Night	18% 9% 4 6% 5% 3 4 3 5% 6% 38%	2493
December	1st to 10th	Night	16% 10% 4 9% 7% 3 4 5% 4 7% 32%	2660
	11th to 20th	Night	15% 14% 5% 8% 6% 3 4 4 4 5% 32%	3279
	21st to end	Night	23% 18% 6% 9% 7% 4 4 4 4 5% 16%	4884

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	59% 7% 14% 14% 6%	2528
	11th to 20th	Day	66% 4 10% 12% 8%	2530
	21st to end	Day	71% 4 7% 13% 5%	2847
February	1st to 10th	Day	59% 5% 12% 14% 8% 2	1778
	11th to 20th	Day	52% 4 11% 22% 10% 2	1858
	21st to end	Day	56% 6% 10% 19% 7% 2	1570
March	1st to 10th	Day	57% 5% 10% 20% 7% 1	1800
	11th to 20th	Day	48% 5% 13% 21% 11% 1	1846
	21st to end	Day	61% 5% 11% 17% 5% 1	2376
April	1st to 10th	Day	53% 6% 10% 19% 12%	2160
	11th to 20th	Day	48% 6% 10% 21% 11% 4	2134
	21st to end	Day	50% 6% 13% 17% 11% 2	2267
May	1st to 10th	Day	51% 7% 12% 22% 8%	2340
	11th to 20th	Day	49% 7% 12% 20% 12% 1	2387
	21st to end	Day	44% 7% 11% 19% 16% 2	2725
June	1st to 10th	Day	35% 7% 13% 25% 18% 2	2520
	11th to 20th	Day	35% 7% 12% 24% 20% 3	2520
	21st to end	Day	40% 10% 14% 20% 14% 2	2520
July	1st to 10th	Day	34% 8% 13% 26% 16% 3	2451
	11th to 20th	Day	33% 9% 15% 26% 14% 2	2445
	21st to end	Day	34% 9% 15% 24% 15% 3	2591
August	1st to 10th	Day	34% 9% 16% 28% 12% 2	2416
	11th to 20th	Day	34% 8% 15% 26% 15% 1	2148
	21st to end	Day	30% 8% 14% 26% 18% 4	2376
September	1st to 10th	Day	37% 6% 14% 23% 16% 4	2160
	11th to 20th	Day	32% 6% 12% 28% 19% 3	2030
	21st to end	Day	36% 8% 9% 26% 20% 2	1980
October	1st to 10th	Day	40% 6% 12% 25% 16% 3	1958
	11th to 20th	Day	49% 6% 10% 21% 13% 1	1778
	21st to end	Day	51% 5% 12% 22% 9% 2	1782
November	1st to 10th	Day	38% 5% 15% 24% 16% 2	1672
	11th to 20th	Day	49% 6% 13% 21% 12% 1	1710
	21st to end	Day	53% 5% 9% 22% 10%	1684
December	1st to 10th	Day	57% 5% 13% 18% 8% 1	1520
	11th to 20th	Day	56% 5% 11% 20% 7% 1	1915
	21st to end	Day	77% 4 7% 9% 3	2783
January	1st to 10th	Night	60% 5% 11% 15% 8% 1	4428
	11th to 20th	Night	68% 4 12% 11% 5%	4440
	21st to end	Night	67% 5% 9% 13% 6%	4816
February	1st to 10th	Night	57% 4 11% 17% 10% 2	2399
	11th to 20th	Night	48% 5% 10% 21% 14% 1	2219
	21st to end	Night	53% 5% 10% 18% 11% 3	1884
March	1st to 10th	Night	53% 5% 11% 21% 9% 1	2167
	11th to 20th	Night	41% 6% 13% 20% 16% 3	2124
	21st to end	Night	55% 5% 11% 19% 10% 1	1985
April	1st to 10th	Night	45% 5% 13% 23% 12% 2	1800
	11th to 20th	Night	46% 6% 8% 21% 16% 4	1775
	21st to end	Night	48% 7% 9% 20% 14% 2	1692
May	1st to 10th	Night	46% 6% 13% 24% 11%	1620
	11th to 20th	Night	43% 7% 12% 22% 15% 1	1508
	21st to end	Night	39% 9% 14% 19% 16% 3	1552
June	1st to 10th	Night	32% 7% 12% 25% 20% 4	1440
	11th to 20th	Night	32% 6% 14% 23% 23% 2	1440
	21st to end	Night	38% 9% 14% 21% 15% 3	1440
July	1st to 10th	Night	30% 8% 15% 27% 16% 5%	1400
	11th to 20th	Night	27% 7% 14% 26% 23% 3	1392
	21st to end	Night	24% 8% 18% 28% 17% 6%	1487
August	1st to 10th	Night	28% 8% 17% 29% 16% 1	1497
	11th to 20th	Night	28% 7% 17% 25% 20% 2	1750
	21st to end	Night	28% 7% 12% 28% 21% 5%	1980
September	1st to 10th	Night	33% 5% 12% 24% 22% 4	1800
	11th to 20th	Night	29% 5% 9% 29% 26% 3	1911
	21st to end	Night	36% 6% 10% 28% 17% 3	1980
October	1st to 10th	Night	35% 5% 10% 26% 21% 3	2000
	11th to 20th	Night	42% 4 9% 25% 19% 2	2181
	21st to end	Night	48% 5% 11% 21% 13% 1	2574
November	1st to 10th	Night	38% 5% 10% 25% 19% 3	2406
	11th to 20th	Night	48% 5% 12% 22% 12% 2	2470
	21st to end	Night	49% 4 12% 23% 11%	2493
December	1st to 10th	Night	52% 6% 11% 20% 9% 1	2660
	11th to 20th	Night	55% 5% 12% 18% 10% 1	3281
	21st to end	Night	71% 5% 9% 11% 4	4884

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% 37% 21% 5%	270
	11th to 20th	Day	26% 19% 36% 13% 5% 1	270
	21st to end	Day	23% 19% 36% 18% 3 1	297
February	1st to 10th	Day	26% 19% 40% 13% 2	270
	11th to 20th	Day	29% 17% 35% 16% 3 1	270
	21st to end	Day	23% 24% 38% 13% 2	225
March	1st to 10th	Day	24% 23% 39% 12% 2	240
	11th to 20th	Day	27% 14% 35% 17% 6%	276
	21st to end	Day	31% 16% 27% 18% 7% 1	462
April	1st to 10th	Day	24% 15% 32% 21% 8%	420
	11th to 20th	Day	26% 17% 25% 20% 10% 1	412
	21st to end	Day	23% 17% 35% 15% 9% 2	420
May	1st to 10th	Day	24% 16% 28% 22% 7% 3	423
	11th to 20th	Day	23% 9% 33% 22% 11% 3	529
	21st to end	Day	15% 11% 31% 25% 12% 6%	679
June	1st to 10th	Day	11% 9% 33% 29% 13% 6%	630
	11th to 20th	Day	13% 7% 33% 27% 17% 4	630
	21st to end	Day	11% 11% 33% 28% 12% 5%	630
July	1st to 10th	Day	8% 5% 24% 31% 18% 14%	621
	11th to 20th	Day	12% 5% 27% 29% 17% 10%	618
	21st to end	Day	8% 5% 28% 33% 19% 6%	684
August	1st to 10th	Day	7% 3 23% 32% 23% 11%	550
	11th to 20th	Day	8% 7% 17% 31% 25% 12%	439
	21st to end	Day	8% 3 20% 38% 21% 10%	495
September	1st to 10th	Day	15% 6% 23% 29% 21% 6%	450
	11th to 20th	Day	11% 5% 19% 30% 24% 11%	450
	21st to end	Day	13% 10% 26% 25% 18% 8%	450
October	1st to 10th	Day	15% 9% 27% 28% 16% 4	428
	11th to 20th	Day	21% 22% 34% 16% 5% 1	270
	21st to end	Day	33% 11% 37% 14% 5%	297
November	1st to 10th	Day	16% 15% 38% 21% 7% 3	270
	11th to 20th	Day	27% 14% 36% 16% 6% 1	270
	21st to end	Day	25% 11% 41% 19% 3	270
December	1st to 10th	Day	19% 24% 37% 15% 5%	270
	11th to 20th	Day	19% 20% 40% 18% 4	270
	21st to end	Day	26% 22% 37% 13% 2	297
January	1st to 10th	Night	21% 18% 34% 24% 3	950
	11th to 20th	Night	23% 18% 34% 21% 4 1	950
	21st to end	Night	22% 15% 34% 25% 4	1045
February	1st to 10th	Night	24% 13% 37% 22% 3	879
	11th to 20th	Night	20% 16% 35% 25% 4	743
	21st to end	Night	26% 17% 35% 19% 4	628
March	1st to 10th	Night	22% 15% 37% 21% 6%	723
	11th to 20th	Night	17% 12% 38% 26% 6% 1	720
	21st to end	Night	25% 13% 30% 25% 7% 1	599
April	1st to 10th	Night	20% 12% 35% 25% 6% 1	540
	11th to 20th	Night	23% 12% 31% 25% 8% 1	533
	21st to end	Night	22% 9% 34% 26% 8%	540
May	1st to 10th	Night	19% 7% 35% 29% 9%	540
	11th to 20th	Night	19% 6% 29% 32% 14% 1	531
	21st to end	Night	13% 6% 26% 36% 19% 1	584
June	1st to 10th	Night	10% 4 26% 44% 15% 1	540
	11th to 20th	Night	11% 4 30% 41% 13% 1	540
	21st to end	Night	11% 5% 27% 38% 19% 1	540
July	1st to 10th	Night	8% 2 20% 41% 26% 4	525
	11th to 20th	Night	8% 3 20% 38% 27% 3	522
	21st to end	Night	5% 3 19% 47% 24% 2	591
August	1st to 10th	Night	6% 2 11% 48% 29% 4	537
	11th to 20th	Night	8% 2 14% 37% 33% 4	534
	21st to end	Night	8% 2 21% 36% 30% 3	594
September	1st to 10th	Night	12% 4 18% 35% 26% 4	540
	11th to 20th	Night	9% 6% 18% 36% 26% 5%	651
	21st to end	Night	12% 7% 22% 33% 22% 3	720
October	1st to 10th	Night	14% 6% 26% 34% 19% 2	720
	11th to 20th	Night	20% 15% 30% 26% 8%	720
	21st to end	Night	27% 13% 29% 23% 7%	792
November	1st to 10th	Night	17% 13% 38% 21% 9% 1	740
	11th to 20th	Night	19% 19% 32% 24% 5% 1	760
	21st to end	Night	23% 12% 37% 21% 6%	783
December	1st to 10th	Night	23% 14% 37% 22% 4	950
	11th to 20th	Night	23% 15% 35% 23% 4	950
	21st to end	Night	30% 15% 33% 19% 2	1045

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