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 2024-03-28.

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% 4 6% 6% 3 5% 5% 6% 7% 37%	78678
	Night	15% 10% 4 7% 5% 3 4 5% 5% 6% 36%	81367
Transparency	Day	46% 8% 13% 20% 11% 2	78379
	Night	48% 7% 12% 20% 12% 2	81187
Seeing	Day	16% 10% 28% 30% 14% 3	14856
	Night	18% 10% 29% 29% 13% 2	24914

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% 8% 9% 8% 4 5% 5% 5% 6% 26%	2527
	11th to 20th	Day	13% 13% 7% 12% 8% 4 6% 10% 8% 5% 17%	2530
	21st to end	Day	20% 21% 6% 9% 5% 2 3 3 3 5% 22%	2818
February	1st to 10th	Day	14% 14% 6% 6% 7% 4 7% 5% 5% 5% 28%	1778
	11th to 20th	Day	14% 10% 4 7% 6% 4 5% 5% 6% 6% 35%	1848
	21st to end	Day	13% 11% 5% 9% 6% 3 5% 5% 5% 6% 31%	1570
March	1st to 10th	Day	14% 12% 4 9% 6% 3 4 6% 5% 5% 31%	1900
	11th to 20th	Day	13% 8% 3 7% 6% 3 5% 6% 5% 7% 37%	1947
	21st to end	Day	19% 11% 5% 8% 7% 4 6% 4 5% 6% 26%	2460
April	1st to 10th	Day	17% 9% 4 7% 7% 3 4 4 6% 6% 33%	2160
	11th to 20th	Day	16% 7% 3 6% 6% 4 6% 5% 6% 6% 36%	2134
	21st to end	Day	19% 8% 3 7% 5% 3 5% 5% 6% 7% 33%	2278
May	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 6% 6% 8% 33%	2340
	11th to 20th	Day	18% 7% 3 6% 5% 3 4 4 5% 7% 37%	2387
	21st to end	Day	13% 6% 3 6% 6% 3 5% 6% 6% 7% 39%	2725
June	1st to 10th	Day	9% 5% 3 4 5% 3 5% 5% 6% 7% 47%	2520
	11th to 20th	Day	10% 4 2 5% 6% 3 4 5% 7% 8% 46%	2520
	21st to end	Day	10% 5% 3 5% 5% 4 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	7% 4 2 5% 5% 3 5% 6% 6% 8% 50%	2520
	11th to 20th	Day	9% 4 2 4 5% 3 4 5% 6% 8% 49%	2520
	21st to end	Day	6% 3 2 5% 5% 4 5% 6% 7% 10% 46%	2743
August	1st to 10th	Day	7% 4 2 5% 5% 4 6% 5% 7% 9% 46%	2432
	11th to 20th	Day	8% 4 3 4 5% 3 4 6% 6% 9% 47%	2167
	21st to end	Day	7% 3 2 4 5% 3 5% 6% 6% 10% 50%	2376
September	1st to 10th	Day	13% 4 2 5% 5% 3 4 4 5% 8% 46%	2160
	11th to 20th	Day	12% 5% 2 3 4 3 4 4 6% 8% 50%	2026
	21st to end	Day	9% 5% 3 6% 6% 4 6% 5% 6% 9% 42%	1980
October	1st to 10th	Day	14% 7% 2 7% 5% 3 4 4 4 6% 44%	1962
	11th to 20th	Day	18% 8% 4 7% 6% 3 5% 4 4 6% 35%	1768
	21st to end	Day	19% 9% 4 6% 5% 4 5% 5% 4 7% 32%	1782
November	1st to 10th	Day	12% 8% 4 5% 4 3 4 4 5% 7% 44%	1672
	11th to 20th	Day	15% 9% 4 8% 6% 3 5% 4 4 8% 33%	1710
	21st to end	Day	18% 12% 3 8% 5% 2 5% 4 3 5% 36%	1677
December	1st to 10th	Day	15% 14% 5% 8% 7% 4 5% 5% 5% 7% 26%	1520
	11th to 20th	Day	13% 11% 5% 8% 7% 4 6% 6% 6% 6% 28%	1918
	21st to end	Day	20% 20% 7% 10% 7% 3 4 5% 4 5% 15%	2783
January	1st to 10th	Night	15% 15% 8% 9% 7% 4 4 4 4 5% 25%	4428
	11th to 20th	Night	17% 16% 7% 12% 7% 3 4 5% 8% 4 16%	4440
	21st to end	Night	23% 18% 8% 7% 6% 3 4 4 3 4 21%	4848
February	1st to 10th	Night	16% 13% 6% 7% 6% 3 4 4 4 5% 32%	2400
	11th to 20th	Night	14% 10% 3 7% 6% 3 5% 5% 5% 5% 36%	2218
	21st to end	Night	19% 11% 4 8% 5% 3 4 3 4 5% 34%	1884
March	1st to 10th	Night	16% 12% 5% 7% 6% 3 5% 3 4 5% 34%	2280
	11th to 20th	Night	11% 8% 3 6% 5% 3 4 5% 5% 8% 42%	2235
	21st to end	Night	21% 10% 4 6% 6% 3 3 4 5% 6% 31%	2063
April	1st to 10th	Night	17% 7% 3 6% 6% 3 5% 5% 5% 9% 36%	1800
	11th to 20th	Night	18% 9% 2 5% 5% 4 4 4 5% 5% 38%	1775
	21st to end	Night	19% 8% 3 6% 5% 3 4 5% 4 6% 37%	1679
May	1st to 10th	Night	16% 7% 3 7% 7% 4 5% 5% 6% 6% 34%	1620
	11th to 20th	Night	17% 8% 3 3 5% 2 5% 5% 5% 8% 39%	1506
	21st to end	Night	12% 5% 4 5% 5% 3 5% 4 6% 8% 42%	1551
June	1st to 10th	Night	9% 5% 2 5% 5% 2 3 6% 5% 8% 49%	1440
	11th to 20th	Night	10% 5% 3 4 4 3 4 6% 7% 8% 46%	1440
	21st to end	Night	10% 6% 2 6% 4 3 6% 5% 7% 8% 42%	1440
July	1st to 10th	Night	7% 3 1 3 4 3 6% 6% 7% 10% 50%	1440
	11th to 20th	Night	8% 3 2 5% 3 2 5% 5% 6% 9% 51%	1440
	21st to end	Night	5% 4 2 5% 4 4 5% 6% 7% 9% 49%	1575
August	1st to 10th	Night	6% 4 1 6% 4 4 5% 5% 7% 8% 50%	1503
	11th to 20th	Night	7% 3 2 4 5% 3 4 5% 6% 9% 53%	1732
	21st to end	Night	8% 3 2 4 4 3 4 4 5% 7% 54%	1980
September	1st to 10th	Night	10% 5% 2 6% 5% 2 4 4 5% 7% 50%	1800
	11th to 20th	Night	11% 4 2 4 4 3 3 4 5% 7% 54%	1914
	21st to end	Night	11% 5% 3 6% 5% 3 4 4 6% 8% 46%	1980
October	1st to 10th	Night	13% 5% 2 5% 5% 3 3 4 6% 6% 47%	1994
	11th to 20th	Night	17% 7% 3 5% 5% 3 4 4 4 6% 42%	2191
	21st to end	Night	22% 8% 3 6% 5% 2 4 4 4 7% 35%	2574
November	1st to 10th	Night	12% 8% 3 5% 4 3 5% 5% 5% 5% 46%	2406
	11th to 20th	Night	15% 10% 4 6% 6% 3 5% 4 5% 7% 36%	2470
	21st to end	Night	18% 8% 4 6% 6% 3 4 5% 5% 6% 36%	2498
December	1st to 10th	Night	17% 11% 4 8% 6% 4 5% 6% 5% 7% 27%	2660
	11th to 20th	Night	15% 13% 5% 8% 6% 3 4 4 5% 6% 30%	3279
	21st to end	Night	24% 17% 5% 9% 7% 3 5% 4 5% 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	55% 6% 12% 19% 7% 1	1900
	11th to 20th	Day	45% 8% 12% 23% 10% 1	1947
	21st to end	Day	59% 8% 11% 16% 5% 1	2460
April	1st to 10th	Day	51% 8% 11% 19% 11%	2160
	11th to 20th	Day	47% 8% 10% 19% 11% 4	2134
	21st to end	Day	49% 8% 13% 16% 12% 2	2278
May	1st to 10th	Day	49% 8% 14% 21% 8%	2340
	11th to 20th	Day	47% 7% 12% 21% 12% 1	2391
	21st to end	Day	43% 9% 12% 20% 15% 2	2725
June	1st to 10th	Day	35% 9% 13% 23% 18% 2	2520
	11th to 20th	Day	35% 8% 12% 23% 20% 3	2520
	21st to end	Day	39% 10% 15% 20% 14% 2	2520
July	1st to 10th	Day	33% 7% 13% 27% 16% 4	2451
	11th to 20th	Day	32% 9% 16% 27% 15% 2	2445
	21st to end	Day	31% 10% 18% 23% 15% 3	2591
August	1st to 10th	Day	33% 10% 16% 28% 12% 2	2417
	11th to 20th	Day	32% 10% 15% 26% 16% 1	2167
	21st to end	Day	29% 9% 15% 26% 17% 4	2376
September	1st to 10th	Day	37% 7% 14% 22% 17% 4	2160
	11th to 20th	Day	31% 7% 13% 29% 17% 3	2026
	21st to end	Day	37% 9% 12% 23% 18% 1	1980
October	1st to 10th	Day	40% 7% 11% 24% 15% 2	1962
	11th to 20th	Day	50% 6% 10% 21% 12% 1	1768
	21st to end	Day	52% 6% 12% 20% 9% 1	1782
November	1st to 10th	Day	39% 6% 15% 22% 16% 2	1672
	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	53% 5% 12% 21% 9% 1	2280
	11th to 20th	Night	40% 7% 13% 22% 15% 3	2235
	21st to end	Night	54% 7% 11% 18% 8% 1	2063
April	1st to 10th	Night	44% 7% 14% 22% 13% 1	1800
	11th to 20th	Night	47% 5% 9% 19% 16% 4	1775
	21st to end	Night	48% 7% 10% 20% 14% 2	1679
May	1st to 10th	Night	48% 7% 11% 22% 12%	1620
	11th to 20th	Night	44% 6% 11% 22% 15% 2	1507
	21st to end	Night	38% 8% 15% 19% 16% 3	1552
June	1st to 10th	Night	32% 7% 14% 25% 19% 3	1440
	11th to 20th	Night	33% 7% 12% 23% 23% 2	1440
	21st to end	Night	38% 9% 14% 22% 16% 2	1440
July	1st to 10th	Night	28% 8% 16% 27% 15% 5%	1400
	11th to 20th	Night	28% 8% 15% 24% 21% 3	1392
	21st to end	Night	25% 10% 17% 25% 17% 5%	1487
August	1st to 10th	Night	29% 8% 16% 30% 16% 2	1494
	11th to 20th	Night	27% 8% 17% 26% 19% 2	1732
	21st to end	Night	28% 7% 11% 29% 19% 5%	1980
September	1st to 10th	Night	33% 7% 13% 24% 20% 3	1800
	11th to 20th	Night	30% 5% 11% 28% 25% 2	1914
	21st to end	Night	36% 6% 12% 27% 16% 2	1980
October	1st to 10th	Night	36% 6% 10% 26% 19% 2	1994
	11th to 20th	Night	42% 5% 11% 23% 18% 2	2191
	21st to end	Night	49% 5% 12% 21% 11% 1	2574
November	1st to 10th	Night	39% 5% 11% 24% 17% 3	2406
	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	25% 17% 28% 22% 7% 1	490
April	1st to 10th	Day	22% 15% 33% 24% 6%	420
	11th to 20th	Day	18% 15% 32% 23% 11% 2	412
	21st to end	Day	22% 11% 33% 24% 9% 1	420
May	1st to 10th	Day	22% 10% 30% 27% 9% 2	423
	11th to 20th	Day	20% 6% 32% 29% 11% 2	533
	21st to end	Day	14% 5% 27% 33% 17% 5%	679
June	1st to 10th	Day	10% 4 28% 38% 17% 3	630
	11th to 20th	Day	12% 4 29% 35% 18% 3	630
	21st to end	Day	12% 6% 28% 37% 13% 4	630
July	1st to 10th	Day	9% 3 19% 34% 25% 10%	621
	11th to 20th	Day	12% 3 21% 38% 20% 6%	618
	21st to end	Day	7% 3 25% 42% 20% 4	684
August	1st to 10th	Day	8% 2 15% 41% 26% 8%	551
	11th to 20th	Day	8% 4 19% 36% 25% 8%	439
	21st to end	Day	7% 4 16% 40% 27% 6%	495
September	1st to 10th	Day	14% 5% 19% 34% 24% 4	450
	11th to 20th	Day	10% 5% 16% 34% 28% 8%	450
	21st to end	Day	12% 11% 25% 27% 22% 5%	450
October	1st to 10th	Day	17% 7% 26% 30% 16% 4	432
	11th to 20th	Day	19% 22% 29% 24% 6%	270
	21st to end	Day	30% 13% 32% 22% 2	297
November	1st to 10th	Day	14% 14% 32% 31% 9% 1	270
	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	24% 11% 29% 26% 10% 1	628
April	1st to 10th	Night	19% 12% 35% 27% 6% 1	540
	11th to 20th	Night	21% 9% 31% 28% 10% 1	533
	21st to end	Night	21% 8% 31% 28% 11% 1	540
May	1st to 10th	Night	19% 5% 30% 34% 11% 1	540
	11th to 20th	Night	18% 4 26% 33% 18% 1	531
	21st to end	Night	15% 2 18% 38% 25% 2	584
June	1st to 10th	Night	10% 4 19% 42% 24% 1	540
	11th to 20th	Night	11% 4 22% 42% 20% 1	540
	21st to end	Night	11% 3 23% 38% 24% 1	540
July	1st to 10th	Night	8% 3 10% 42% 31% 6%	525
	11th to 20th	Night	9% 3 17% 40% 27% 5%	522
	21st to end	Night	5% 1 19% 46% 27% 2	591
August	1st to 10th	Night	7% 2 12% 47% 27% 6%	537
	11th to 20th	Night	7% 2 15% 37% 32% 7%	534
	21st to end	Night	9% 3 15% 37% 32% 4	594
September	1st to 10th	Night	11% 4 18% 35% 26% 6%	540
	11th to 20th	Night	10% 4 18% 36% 27% 6%	654
	21st to end	Night	13% 7% 25% 28% 23% 4	720
October	1st to 10th	Night	16% 6% 23% 32% 21% 2	720
	11th to 20th	Night	21% 14% 30% 27% 8% 1	720
	21st to end	Night	26% 13% 30% 22% 8%	792
November	1st to 10th	Night	17% 14% 35% 22% 11% 1	740
	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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