Chicago
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chicago clear sky chart from 2005-11-06 to 2025-09-04.

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% 8% 3 7% 5% 4 5% 4 6% 7% 37%	126028
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 37%	122914
Transparency	Day	44% 7% 13% 22% 13% 2	129566
	Night	46% 6% 12% 21% 14% 2	127450
Seeing	Day	16% 10% 29% 29% 13% 3	16962
	Night	18% 10% 29% 29% 12% 1	26747

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% 10% 8% 12% 6% 5% 5% 5% 6% 6% 26%	3617
	11th to 20th	Day	10% 10% 6% 11% 6% 6% 6% 7% 7% 6% 27%	3620
	21st to end	Day	16% 17% 4 9% 3 3 3 3 5% 6% 31%	4045
February	1st to 10th	Day	13% 15% 5% 8% 6% 6% 6% 4 5% 5% 27%	1872
	11th to 20th	Day	14% 10% 4 8% 5% 5% 5% 4 5% 6% 34%	1948
	21st to end	Day	12% 12% 5% 10% 5% 4 5% 4 6% 5% 31%	1650
March	1st to 10th	Day	16% 10% 3 7% 4 4 4 3 4 4 41%	4480
	11th to 20th	Day	8% 6% 3 7% 4 5% 5% 4 6% 8% 43%	4661
	21st to end	Day	17% 14% 6% 12% 7% 6% 5% 3 5% 5% 21%	5967
April	1st to 10th	Day	17% 12% 3 7% 6% 4 4 3 5% 6% 32%	2400
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2374
	21st to end	Day	18% 8% 3 8% 4 4 5% 4 6% 7% 33%	2532
May	1st to 10th	Day	18% 7% 3 6% 4 4 5% 4 6% 8% 34%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 6% 5% 6% 7% 40%	6087
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 8% 42%	6935
June	1st to 10th	Day	9% 6% 2 5% 5% 5% 5% 4 6% 7% 46%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 4 7% 8% 45%	2800
	21st to end	Day	9% 6% 2 5% 5% 5% 6% 4 7% 9% 41%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 39%	6360
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 7% 9% 44%	6360
	21st to end	Day	9% 7% 2 7% 5% 5% 4 4 7% 9% 41%	6924
August	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 47%	4414
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 49%	3919
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 9% 51%	4312
September	1st to 10th	Day	12% 4 2 6% 4 4 4 3 5% 8% 48%	2316
	11th to 20th	Day	11% 5% 2 3 3 3 4 3 6% 8% 51%	2138
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	2090
October	1st to 10th	Day	13% 7% 2 7% 4 4 4 3 5% 5% 46%	2071
	11th to 20th	Day	17% 8% 4 8% 5% 4 4 3 4 6% 37%	1866
	21st to end	Day	18% 9% 4 7% 4 5% 5% 4 4 7% 33%	1881
November	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 6% 6% 44%	1762
	11th to 20th	Day	15% 11% 4 8% 6% 5% 5% 3 5% 7% 32%	1800
	21st to end	Day	17% 13% 3 10% 4 4 4 3 4 5% 34%	1765
December	1st to 10th	Day	9% 10% 3 7% 6% 5% 5% 4 5% 6% 39%	2610
	11th to 20th	Day	13% 15% 5% 11% 7% 6% 6% 5% 6% 6% 23%	3008
	21st to end	Day	22% 21% 6% 11% 6% 4 4 3 5% 5% 13%	3982
January	1st to 10th	Night	14% 14% 6% 12% 5% 5% 4 3 4 5% 28%	6348
	11th to 20th	Night	14% 14% 6% 12% 7% 5% 5% 4 7% 4 21%	6360
	21st to end	Night	18% 16% 6% 7% 4 4 3 3 3 4 32%	6932
February	1st to 10th	Night	16% 14% 5% 8% 6% 4 3 3 5% 5% 32%	2526
	11th to 20th	Night	14% 11% 3 8% 5% 4 5% 4 5% 6% 35%	2338
	21st to end	Night	18% 11% 4 8% 4 4 3 3 3 5% 35%	1980
March	1st to 10th	Night	16% 11% 4 7% 4 4 3 2 4 5% 40%	5373
	11th to 20th	Night	10% 7% 2 7% 4 3 4 3 5% 8% 47%	5314
	21st to end	Night	18% 11% 6% 10% 5% 5% 5% 3 4 5% 28%	4975
April	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 5% 8% 36%	2000
	11th to 20th	Night	18% 9% 2 6% 4 5% 4 3 5% 5% 38%	1975
	21st to end	Night	17% 9% 2 7% 5% 4 5% 4 5% 5% 37%	1865
May	1st to 10th	Night	19% 8% 3 8% 5% 4 4 4 6% 6% 34%	4033
	11th to 20th	Night	14% 8% 3 7% 4 3 4 4 5% 7% 40%	3803
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 6% 41%	3942
June	1st to 10th	Night	9% 5% 2 6% 5% 3 4 5% 5% 8% 48%	1600
	11th to 20th	Night	11% 6% 2 5% 4 4 4 5% 7% 8% 44%	1600
	21st to end	Night	9% 7% 2 6% 4 5% 5% 5% 7% 8% 42%	1600
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 5% 7% 8% 41%	3618
	11th to 20th	Night	7% 5% 3 6% 4 4 5% 4 6% 8% 49%	3617
	21st to end	Night	9% 5% 2 6% 4 5% 5% 5% 7% 7% 44%	3947
August	1st to 10th	Night	8% 5% 2 7% 4 5% 5% 3 7% 8% 47%	2737
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 5% 8% 51%	3128
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 8% 52%	3586
September	1st to 10th	Night	10% 5% 2 7% 4 3 4 3 5% 6% 52%	1930
	11th to 20th	Night	10% 4 2 5% 3 4 3 3 5% 8% 54%	2021
	21st to end	Night	12% 6% 3 7% 5% 4 4 3 6% 7% 45%	2090
October	1st to 10th	Night	13% 5% 2 5% 4 4 3 4 6% 6% 49%	2105
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 4 6% 43%	2313
	21st to end	Night	21% 8% 2 7% 5% 4 3 3 5% 7% 35%	2717
November	1st to 10th	Night	13% 8% 3 5% 4 4 4 4 5% 5% 45%	2536
	11th to 20th	Night	15% 11% 4 7% 5% 4 4 3 6% 6% 35%	2600
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 5% 35%	2630
December	1st to 10th	Night	12% 10% 3 8% 4 4 5% 3 5% 6% 39%	4580
	11th to 20th	Night	16% 15% 4 9% 5% 4 4 3 5% 6% 28%	5199
	21st to end	Night	23% 18% 5% 10% 5% 4 5% 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	57% 6% 14% 14% 8%	3618
	11th to 20th	Day	55% 7% 12% 10% 12% 4	3620
	21st to end	Day	55% 4 13% 19% 8%	4045
February	1st to 10th	Day	58% 5% 13% 14% 8% 1	1872
	11th to 20th	Day	51% 6% 10% 22% 9% 1	1958
	21st to end	Day	52% 8% 11% 18% 9% 2	1650
March	1st to 10th	Day	46% 6% 9% 24% 11% 4	4480
	11th to 20th	Day	36% 8% 13% 32% 10% 1	4661
	21st to end	Day	63% 10% 11% 10% 6%	5969
April	1st to 10th	Day	52% 7% 11% 19% 11%	2400
	11th to 20th	Day	46% 8% 11% 20% 11% 3	2374
	21st to end	Day	48% 8% 13% 18% 11% 2	2532
May	1st to 10th	Day	44% 9% 13% 21% 11% 2	5897
	11th to 20th	Day	39% 8% 13% 25% 14% 1	6091
	21st to end	Day	39% 7% 12% 24% 15% 3	6949
June	1st to 10th	Day	35% 9% 13% 23% 18% 2	2800
	11th to 20th	Day	35% 8% 12% 24% 19% 3	2800
	21st to end	Day	38% 9% 15% 20% 15% 3	2800
July	1st to 10th	Day	40% 7% 14% 24% 14% 2	6291
	11th to 20th	Day	35% 8% 15% 26% 15% 1	6285
	21st to end	Day	38% 7% 16% 21% 16% 2	6728
August	1st to 10th	Day	34% 8% 15% 28% 14% 1	4399
	11th to 20th	Day	31% 8% 11% 33% 16% 1	3957
	21st to end	Day	30% 7% 14% 26% 20% 3	4311
September	1st to 10th	Day	35% 7% 13% 24% 18% 3	2316
	11th to 20th	Day	30% 6% 13% 29% 19% 3	2138
	21st to end	Day	38% 8% 11% 23% 18% 1	2090
October	1st to 10th	Day	27% 4 7% 36% 24% 2	3448
	11th to 20th	Day	36% 5% 11% 26% 21%	3082
	21st to end	Day	46% 4 10% 25% 13% 3	3081
November	1st to 10th	Day	39% 6% 15% 23% 15% 2	1762
	11th to 20th	Day	51% 6% 15% 18% 10% 1	1800
	21st to end	Day	53% 5% 12% 21% 9%	1765
December	1st to 10th	Day	44% 5% 19% 25% 7%	2610
	11th to 20th	Day	59% 7% 13% 16% 5%	3005
	21st to end	Day	75% 6% 6% 9% 3	3982
January	1st to 10th	Night	57% 5% 11% 18% 8%	6348
	11th to 20th	Night	63% 5% 11% 8% 11% 1	6360
	21st to end	Night	57% 4 13% 16% 10% 1	6932
February	1st to 10th	Night	56% 4 12% 16% 11% 1	2526
	11th to 20th	Night	49% 5% 12% 21% 12% 1	2339
	21st to end	Night	53% 4 12% 18% 11% 3	1980
March	1st to 10th	Night	47% 4 11% 24% 11% 2	5373
	11th to 20th	Night	36% 6% 12% 30% 14% 2	5314
	21st to end	Night	57% 6% 9% 18% 9% 1	4975
April	1st to 10th	Night	45% 7% 13% 22% 13% 1	2000
	11th to 20th	Night	46% 6% 10% 19% 16% 3	1975
	21st to end	Night	47% 6% 10% 21% 14% 2	1865
May	1st to 10th	Night	48% 6% 12% 16% 14% 4	4060
	11th to 20th	Night	40% 7% 13% 24% 13% 3	3804
	21st to end	Night	43% 7% 10% 20% 17% 4	3950
June	1st to 10th	Night	33% 7% 14% 25% 19% 3	1600
	11th to 20th	Night	34% 7% 12% 23% 22% 2	1600
	21st to end	Night	37% 8% 14% 22% 16% 2	1600
July	1st to 10th	Night	39% 6% 14% 26% 12% 4	3580
	11th to 20th	Night	31% 7% 14% 26% 21% 2	3572
	21st to end	Night	36% 7% 14% 23% 17% 3	3832
August	1st to 10th	Night	34% 7% 14% 28% 16% 1	2728
	11th to 20th	Night	30% 7% 14% 27% 21% 1	3156
	21st to end	Night	30% 6% 12% 32% 17% 3	3586
September	1st to 10th	Night	31% 7% 12% 26% 21% 3	1930
	11th to 20th	Night	29% 5% 10% 27% 27% 2	2021
	21st to end	Night	37% 6% 11% 27% 16% 2	2090
October	1st to 10th	Night	22% 4 9% 37% 26% 2	3508
	11th to 20th	Night	37% 4 9% 23% 25% 2	3836
	21st to end	Night	41% 4 10% 24% 18% 3	4467
November	1st to 10th	Night	40% 5% 11% 25% 17% 3	2536
	11th to 20th	Night	48% 6% 13% 21% 10% 2	2600
	21st to end	Night	50% 6% 12% 22% 9%	2630
December	1st to 10th	Night	43% 5% 16% 25% 9% 2	4580
	11th to 20th	Night	56% 6% 13% 16% 9% 1	5201
	21st to end	Night	70% 5% 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% 36% 24% 3 1	300
	11th to 20th	Day	21% 18% 36% 21% 3 1	300
	21st to end	Day	23% 15% 38% 18% 5%	330
February	1st to 10th	Day	18% 17% 41% 20% 4	300
	11th to 20th	Day	22% 21% 34% 19% 3	300
	21st to end	Day	20% 18% 42% 18% 2	249
March	1st to 10th	Day	18% 18% 40% 23% 1	300
	11th to 20th	Day	22% 16% 38% 20% 4	346
	21st to end	Day	25% 17% 32% 19% 6% 1	550
April	1st to 10th	Day	24% 17% 32% 21% 5%	500
	11th to 20th	Day	19% 16% 33% 22% 10% 1	492
	21st to end	Day	21% 13% 34% 23% 8% 1	500
May	1st to 10th	Day	22% 11% 31% 27% 8% 1	503
	11th to 20th	Day	20% 7% 31% 30% 11% 2	623
	21st to end	Day	14% 6% 29% 31% 15% 4	789
June	1st to 10th	Day	11% 6% 29% 36% 15% 3	730
	11th to 20th	Day	11% 5% 30% 35% 16% 2	730
	21st to end	Day	11% 6% 31% 35% 13% 4	730
July	1st to 10th	Day	10% 3 21% 35% 23% 8%	721
	11th to 20th	Day	12% 4 24% 37% 18% 6%	718
	21st to end	Day	7% 4 25% 41% 19% 4	794
August	1st to 10th	Day	8% 3 18% 40% 24% 8%	642
	11th to 20th	Day	9% 4 22% 34% 23% 7%	519
	21st to end	Day	6% 4 17% 42% 25% 6%	583
September	1st to 10th	Day	13% 5% 20% 34% 23% 5%	502
	11th to 20th	Day	9% 4 16% 34% 28% 8%	490
	21st to end	Day	14% 11% 24% 26% 21% 4	490
October	1st to 10th	Day	16% 8% 26% 31% 15% 4	471
	11th to 20th	Day	18% 22% 28% 25% 8%	300
	21st to end	Day	28% 14% 32% 22% 3	330
November	1st to 10th	Day	16% 14% 32% 29% 8% 1	300
	11th to 20th	Day	20% 20% 36% 18% 5% 1	300
	21st to end	Day	25% 14% 39% 20% 2	300
December	1st to 10th	Day	18% 22% 33% 22% 4	300
	11th to 20th	Day	21% 21% 36% 17% 5%	300
	21st to end	Day	24% 20% 38% 17% 2	330
January	1st to 10th	Night	20% 16% 37% 23% 3	1000
	11th to 20th	Night	23% 17% 36% 19% 4 1	1000
	21st to end	Night	22% 12% 37% 24% 4	1100
February	1st to 10th	Night	22% 12% 37% 23% 5%	926
	11th to 20th	Night	19% 15% 37% 26% 3	783
	21st to end	Night	23% 14% 38% 21% 3	660
March	1st to 10th	Night	21% 13% 35% 24% 7%	800
	11th to 20th	Night	17% 10% 38% 28% 7% 1	800
	21st to end	Night	23% 12% 30% 25% 9% 1	673
April	1st to 10th	Night	20% 12% 36% 25% 6% 1	600
	11th to 20th	Night	21% 11% 31% 27% 9% 1	593
	21st to end	Night	20% 9% 31% 28% 10% 1	600
May	1st to 10th	Night	19% 6% 30% 34% 11% 1	600
	11th to 20th	Night	17% 4 27% 34% 18% 1	591
	21st to end	Night	15% 3 19% 38% 22% 2	650
June	1st to 10th	Night	10% 4 21% 42% 22% 1	600
	11th to 20th	Night	11% 4 24% 41% 19% 1	600
	21st to end	Night	10% 4 24% 38% 23% 1	600
July	1st to 10th	Night	9% 4 11% 41% 29% 6%	585
	11th to 20th	Night	8% 3 18% 39% 27% 5%	582
	21st to end	Night	5% 1 18% 47% 26% 2	657
August	1st to 10th	Night	7% 2 13% 45% 27% 5%	597
	11th to 20th	Night	8% 2 17% 37% 31% 6%	594
	21st to end	Night	9% 2 17% 37% 31% 4	660
September	1st to 10th	Night	11% 4 18% 35% 27% 6%	579
	11th to 20th	Night	10% 4 18% 37% 27% 5%	691
	21st to end	Night	14% 7% 25% 28% 22% 4	760
October	1st to 10th	Night	16% 6% 23% 33% 20% 2	760
	11th to 20th	Night	20% 14% 29% 27% 9% 1	760
	21st to end	Night	25% 14% 30% 22% 8%	836
November	1st to 10th	Night	18% 13% 35% 22% 10% 1	780
	11th to 20th	Night	20% 19% 34% 22% 5% 1	800
	21st to end	Night	23% 16% 34% 22% 5%	830
December	1st to 10th	Night	23% 16% 37% 19% 4	1000
	11th to 20th	Night	23% 16% 35% 23% 4	1000
	21st to end	Night	29% 16% 33% 18% 3	1100

Page generated at 2025-09-01 00:51:03. Version ac07

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