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 2024-11-04.

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% 38%	101258
	Night	15% 10% 3 7% 5% 4 4 4 5% 6% 37%	97815
Transparency	Day	43% 8% 13% 22% 13% 2	104841
	Night	45% 6% 12% 21% 14% 2	102373
Seeing	Day	15% 10% 29% 30% 13% 3	15797
	Night	18% 10% 29% 29% 13% 1	25631

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	13% 12% 7% 10% 6% 5% 5% 4 5% 6% 26%	2527
	11th to 20th	Day	13% 13% 7% 13% 6% 5% 6% 8% 8% 5% 16%	2530
	21st to end	Day	20% 22% 5% 11% 4 3 3 3 3 5% 22%	2818
February	1st to 10th	Day	13% 15% 5% 7% 6% 6% 6% 4 5% 5% 28%	1778
	11th to 20th	Day	14% 10% 4 7% 5% 5% 5% 4 6% 6% 34%	1848
	21st to end	Day	13% 12% 5% 10% 5% 4 5% 4 6% 5% 31%	1570
March	1st to 10th	Day	15% 11% 4 8% 4 4 4 4 5% 5% 35%	3120
	11th to 20th	Day	9% 6% 3 8% 5% 5% 5% 5% 7% 7% 40%	3259
	21st to end	Day	18% 12% 5% 12% 7% 6% 5% 3 5% 5% 22%	4165
April	1st to 10th	Day	17% 11% 3 7% 6% 4 4 4 5% 6% 33%	2280
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2254
	21st to end	Day	18% 8% 3 8% 4 4 4 4 6% 7% 33%	2405
May	1st to 10th	Day	17% 7% 3 6% 4 4 4 4 6% 7% 35%	4082
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 8% 42%	4237
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 8% 45%	4824
June	1st to 10th	Day	9% 5% 2 5% 5% 5% 5% 4 7% 7% 46%	2660
	11th to 20th	Day	10% 4 2 6% 5% 4 3 4 7% 8% 46%	2660
	21st to end	Day	10% 6% 2 5% 5% 5% 6% 4 7% 9% 40%	2660
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 6% 5% 7% 8% 37%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	4440
	21st to end	Day	7% 6% 2 7% 5% 5% 4 4 7% 8% 44%	4813
August	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 46%	4278
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 50%	3797
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	12% 4 2 6% 4 4 4 3 5% 8% 48%	2280
	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% 8% 3 6% 3 4 4 3 6% 7% 45%	1699
	11th to 20th	Day	15% 10% 3 9% 5% 4 5% 3 5% 8% 33%	1710
	21st to end	Day	18% 12% 3 9% 4 4 4 3 4 5% 35%	1677
December	1st to 10th	Day	15% 14% 4 8% 6% 5% 5% 5% 6% 6% 26%	1520
	11th to 20th	Day	13% 12% 4 9% 7% 5% 6% 5% 6% 6% 27%	1918
	21st to end	Day	20% 20% 6% 11% 6% 4 4 3 5% 5% 15%	2783
January	1st to 10th	Night	15% 16% 8% 10% 6% 5% 4 3 4 5% 25%	4428
	11th to 20th	Night	17% 17% 6% 14% 6% 4 5% 4 8% 4 16%	4440
	21st to end	Night	23% 18% 7% 8% 4 4 3 3 3 4 21%	4848
February	1st to 10th	Night	16% 14% 5% 8% 6% 4 3 3 5% 5% 32%	2400
	11th to 20th	Night	14% 11% 3 7% 5% 4 5% 4 5% 6% 36%	2218
	21st to end	Night	19% 12% 4 9% 5% 4 3 3 4 5% 34%	1884
March	1st to 10th	Night	18% 12% 3 7% 4 3 3 2 4 5% 38%	3733
	11th to 20th	Night	9% 7% 2 7% 4 4 4 3 5% 7% 47%	3716
	21st to end	Night	20% 11% 7% 10% 5% 4 4 3 4 5% 27%	3479
April	1st to 10th	Night	17% 8% 2 7% 5% 3 5% 3 5% 8% 37%	1900
	11th to 20th	Night	18% 9% 2 6% 4 5% 4 3 5% 5% 39%	1875
	21st to end	Night	18% 8% 2 6% 5% 4 5% 4 5% 5% 37%	1772
May	1st to 10th	Night	15% 8% 3 8% 6% 4 5% 4 6% 7% 34%	2813
	11th to 20th	Night	16% 7% 3 5% 4 3 5% 4 6% 8% 38%	2648
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 44%	2743
June	1st to 10th	Night	9% 5% 2 6% 5% 3 4 5% 5% 8% 48%	1520
	11th to 20th	Night	10% 6% 2 5% 3 4 4 5% 8% 8% 45%	1520
	21st to end	Night	10% 8% 2 6% 4 5% 5% 5% 7% 8% 41%	1520
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 6% 5% 7% 8% 40%	2530
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 6% 8% 52%	2530
	21st to end	Night	5% 5% 2 5% 5% 5% 6% 5% 8% 8% 45%	2750
August	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 3 7% 8% 46%	2653
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 5% 8% 52%	3030
	21st to end	Night	9% 4 2 5% 4 4 4 4 5% 8% 51%	3476
September	1st to 10th	Night	10% 5% 2 7% 4 3 4 3 5% 6% 51%	1900
	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	12% 8% 3 5% 4 5% 4 4 5% 6% 46%	2452
	11th to 20th	Night	15% 10% 3 7% 5% 4 4 3 6% 7% 36%	2470
	21st to end	Night	18% 9% 3 7% 5% 4 5% 3 5% 5% 36%	2498
December	1st to 10th	Night	17% 11% 4 9% 5% 5% 5% 4 5% 6% 27%	2660
	11th to 20th	Night	14% 14% 4 9% 5% 4 4 3 5% 6% 30%	3279
	21st to end	Night	23% 17% 5% 11% 6% 4 5% 3 4 5% 17%	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	58% 9% 15% 12% 6%	2528
	11th to 20th	Day	64% 8% 12% 12% 5%	2530
	21st to end	Day	67% 5% 10% 13% 5%	2818
February	1st to 10th	Day	58% 5% 13% 14% 8% 1	1778
	11th to 20th	Day	50% 6% 10% 22% 10% 1	1858
	21st to end	Day	53% 8% 11% 18% 8% 2	1570
March	1st to 10th	Day	49% 8% 11% 21% 9% 2	3120
	11th to 20th	Day	38% 11% 12% 28% 10% 1	3259
	21st to end	Day	60% 13% 13% 11% 3	4165
April	1st to 10th	Day	51% 8% 11% 19% 11%	2280
	11th to 20th	Day	46% 8% 10% 20% 12% 4	2254
	21st to end	Day	48% 9% 13% 17% 12% 2	2405
May	1st to 10th	Day	43% 10% 13% 23% 10% 1	4120
	11th to 20th	Day	38% 8% 14% 27% 13% 1	4241
	21st to end	Day	33% 9% 13% 21% 20% 4	4837
June	1st to 10th	Day	35% 9% 13% 23% 18% 2	2660
	11th to 20th	Day	34% 8% 12% 23% 20% 3	2660
	21st to end	Day	39% 10% 15% 20% 14% 2	2660
July	1st to 10th	Day	44% 7% 13% 21% 12% 2	4371
	11th to 20th	Day	29% 8% 17% 29% 17% 1	4365
	21st to end	Day	36% 8% 17% 16% 20% 2	4662
August	1st to 10th	Day	34% 8% 15% 27% 14% 1	4263
	11th to 20th	Day	31% 8% 11% 33% 16% 1	3835
	21st to end	Day	30% 7% 14% 26% 20% 3	4179
September	1st to 10th	Day	35% 7% 13% 23% 18% 3	2280
	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	38% 6% 15% 23% 15% 2	1699
	11th to 20th	Day	50% 6% 16% 19% 9% 1	1710
	21st to end	Day	52% 6% 12% 21% 9%	1677
December	1st to 10th	Day	56% 7% 16% 15% 6%	1520
	11th to 20th	Day	54% 7% 14% 20% 5%	1915
	21st to end	Day	73% 8% 8% 9% 2	2783
January	1st to 10th	Night	59% 7% 14% 15% 6% 1	4428
	11th to 20th	Night	67% 7% 12% 10% 4	4440
	21st to end	Night	67% 6% 10% 13% 5%	4848
February	1st to 10th	Night	56% 4 12% 16% 10% 2	2400
	11th to 20th	Night	48% 5% 12% 21% 13% 1	2219
	21st to end	Night	54% 4 11% 18% 11% 3	1884
March	1st to 10th	Night	47% 6% 12% 23% 9% 3	3733
	11th to 20th	Night	35% 8% 11% 27% 17% 2	3716
	21st to end	Night	58% 8% 10% 18% 6% 1	3479
April	1st to 10th	Night	44% 7% 13% 22% 13% 1	1900
	11th to 20th	Night	46% 6% 9% 19% 17% 4	1875
	21st to end	Night	48% 7% 10% 20% 14% 2	1772
May	1st to 10th	Night	46% 8% 12% 20% 13% 2	2840
	11th to 20th	Night	41% 8% 14% 26% 10% 1	2649
	21st to end	Night	37% 9% 11% 16% 21% 6%	2751
June	1st to 10th	Night	32% 7% 14% 24% 19% 3	1520
	11th to 20th	Night	33% 7% 13% 23% 23% 2	1520
	21st to end	Night	38% 9% 14% 21% 16% 2	1520
July	1st to 10th	Night	40% 6% 16% 21% 11% 5%	2490
	11th to 20th	Night	27% 7% 14% 27% 22% 2	2482
	21st to end	Night	31% 9% 17% 18% 21% 4	2662
August	1st to 10th	Night	34% 8% 14% 28% 16% 1	2644
	11th to 20th	Night	30% 7% 14% 28% 21% 1	3058
	21st to end	Night	30% 6% 12% 32% 17% 3	3476
September	1st to 10th	Night	32% 7% 12% 25% 21% 3	1900
	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	39% 5% 11% 24% 17% 3	2452
	11th to 20th	Night	47% 6% 14% 21% 10% 1	2470
	21st to end	Night	49% 6% 13% 23% 9%	2498
December	1st to 10th	Night	53% 8% 14% 16% 8% 1	2660
	11th to 20th	Night	53% 8% 13% 19% 7% 1	3281
	21st to end	Night	69% 7% 10% 11% 3	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% 13% 34% 25% 3 1	270
	11th to 20th	Day	21% 16% 36% 23% 3 1	270
	21st to end	Day	23% 14% 37% 20% 6%	297
February	1st to 10th	Day	18% 16% 40% 21% 4	270
	11th to 20th	Day	22% 19% 35% 21% 3	270
	21st to end	Day	18% 18% 42% 20% 2	225
March	1st to 10th	Day	17% 18% 40% 23% 1	270
	11th to 20th	Day	21% 14% 39% 22% 4	313
	21st to end	Day	24% 17% 31% 21% 6% 1	506
April	1st to 10th	Day	22% 16% 33% 23% 5%	460
	11th to 20th	Day	18% 15% 33% 23% 10% 2	452
	21st to end	Day	21% 11% 34% 24% 8% 1	460
May	1st to 10th	Day	22% 10% 31% 27% 8% 2	463
	11th to 20th	Day	19% 6% 31% 30% 11% 2	578
	21st to end	Day	13% 5% 28% 33% 16% 4	734
June	1st to 10th	Day	10% 6% 28% 38% 16% 3	680
	11th to 20th	Day	11% 5% 30% 35% 17% 3	680
	21st to end	Day	12% 7% 30% 35% 13% 4	680
July	1st to 10th	Day	10% 3 20% 34% 24% 9%	671
	11th to 20th	Day	11% 4 23% 38% 19% 6%	668
	21st to end	Day	7% 3 24% 42% 19% 4	739
August	1st to 10th	Day	9% 3 17% 39% 24% 8%	596
	11th to 20th	Day	9% 4 22% 34% 23% 8%	479
	21st to end	Day	6% 4 15% 42% 27% 6%	539
September	1st to 10th	Day	14% 5% 20% 34% 23% 4	490
	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	14% 14% 33% 30% 9% 1	279
	11th to 20th	Day	21% 19% 34% 20% 6% 1	270
	21st to end	Day	25% 13% 39% 21% 2	270
December	1st to 10th	Day	18% 22% 33% 23% 4	270
	11th to 20th	Day	19% 20% 37% 18% 6%	270
	21st to end	Day	21% 21% 39% 18% 2	297
January	1st to 10th	Night	21% 16% 37% 23% 3	950
	11th to 20th	Night	23% 16% 36% 20% 4 1	950
	21st to end	Night	22% 12% 37% 25% 4	1045
February	1st to 10th	Night	22% 11% 38% 24% 5%	880
	11th to 20th	Night	19% 14% 37% 27% 3	743
	21st to end	Night	24% 14% 37% 21% 3	628
March	1st to 10th	Night	21% 13% 35% 24% 8%	760
	11th to 20th	Night	16% 10% 38% 29% 7% 1	760
	21st to end	Night	23% 11% 30% 25% 10% 1	640
April	1st to 10th	Night	19% 12% 36% 26% 6% 1	570
	11th to 20th	Night	21% 10% 31% 27% 10% 1	563
	21st to end	Night	20% 9% 32% 27% 11% 1	570
May	1st to 10th	Night	19% 6% 30% 34% 11% 1	570
	11th to 20th	Night	17% 4 26% 34% 18% 1	561
	21st to end	Night	14% 3 19% 38% 24% 2	617
June	1st to 10th	Night	10% 4 20% 42% 23% 1	570
	11th to 20th	Night	10% 4 24% 42% 19% 1	570
	21st to end	Night	11% 4 23% 38% 23% 1	570
July	1st to 10th	Night	8% 4 11% 42% 29% 6%	555
	11th to 20th	Night	8% 3 18% 38% 28% 5%	552
	21st to end	Night	5% 1 18% 46% 27% 2	624
August	1st to 10th	Night	7% 2 12% 46% 27% 5%	567
	11th to 20th	Night	7% 2 17% 36% 31% 6%	564
	21st to end	Night	9% 3 15% 38% 32% 4	627
September	1st to 10th	Night	11% 4 18% 35% 27% 5%	570
	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% 14% 35% 23% 10% 1	755
	11th to 20th	Night	19% 19% 34% 22% 5% 1	760
	21st to end	Night	22% 15% 35% 22% 6%	788
December	1st to 10th	Night	23% 16% 37% 20% 4	950
	11th to 20th	Night	23% 15% 35% 23% 4	950
	21st to end	Night	29% 16% 34% 18% 3	1045

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