Champaign/Urbana Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Champaign/Urbana Astronomical Society Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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% 8% 3 6% 5% 3 5% 5% 5% 7% 38%	74824
	Night	14% 8% 3 6% 5% 3 4 4 5% 7% 40%	74976
Transparency	Day	45% 6% 12% 22% 13% 2	74525
	Night	44% 5% 11% 22% 15% 3	74796
Seeing	Day	15% 9% 26% 26% 18% 7%	14521
	Night	18% 9% 28% 31% 13% 1	24478

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	16% 13% 6% 9% 7% 4 5% 4 4 6% 26%	1517
	11th to 20th	Day	17% 16% 7% 9% 8% 3 5% 4 4 5% 22%	1477
	21st to end	Day	15% 14% 7% 11% 6% 3 5% 3 4 5% 26%	1705
February	1st to 10th	Day	18% 11% 6% 10% 6% 4 5% 5% 4 6% 28%	1701
	11th to 20th	Day	15% 11% 3 8% 9% 4 4 4 4 5% 31%	1748
	21st to end	Day	17% 14% 6% 9% 6% 2 5% 4 3 5% 29%	1480
March	1st to 10th	Day	17% 11% 5% 7% 5% 3 5% 4 5% 5% 34%	1800
	11th to 20th	Day	15% 11% 4 7% 6% 3 4 6% 5% 8% 33%	1858
	21st to end	Day	21% 12% 5% 8% 5% 3 5% 5% 5% 6% 26%	2376
April	1st to 10th	Day	14% 8% 5% 7% 6% 3 4 4 7% 7% 35%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 5% 5% 5% 38%	2134
	21st to end	Day	21% 7% 3 6% 5% 3 5% 3 5% 6% 35%	2204
May	1st to 10th	Day	17% 8% 3 6% 6% 4 6% 5% 6% 8% 32%	2340
	11th to 20th	Day	18% 7% 3 5% 5% 3 5% 5% 5% 6% 37%	2347
	21st to end	Day	11% 5% 2 7% 5% 4 7% 6% 6% 9% 37%	2725
June	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 6% 7% 8% 45%	2520
	11th to 20th	Day	9% 4 2 5% 5% 4 5% 5% 6% 9% 47%	2520
	21st to end	Day	8% 4 3 6% 6% 3 6% 6% 7% 9% 42%	2520
July	1st to 10th	Day	9% 5% 2 4 5% 3 5% 6% 8% 11% 41%	2520
	11th to 20th	Day	7% 4 2 5% 5% 4 6% 6% 8% 10% 44%	2520
	21st to end	Day	8% 4 2 5% 5% 4 5% 6% 7% 9% 47%	2743
August	1st to 10th	Day	7% 5% 2 5% 4 3 5% 5% 7% 10% 48%	2338
	11th to 20th	Day	8% 3 2 6% 6% 4 5% 6% 8% 9% 43%	2125
	21st to end	Day	7% 4 2 3 5% 3 4 5% 6% 10% 51%	2376
September	1st to 10th	Day	12% 5% 3 5% 6% 3 4 5% 5% 7% 44%	2160
	11th to 20th	Day	11% 4 2 4 5% 3 5% 5% 6% 7% 49%	2045
	21st to end	Day	9% 5% 2 5% 5% 3 4 4 5% 8% 49%	1980
October	1st to 10th	Day	12% 5% 2 6% 5% 3 4 4 5% 6% 49%	1943
	11th to 20th	Day	17% 7% 3 5% 4 2 3 5% 5% 7% 42%	1800
	21st to end	Day	19% 9% 3 6% 5% 3 3 4 5% 7% 37%	1810
November	1st to 10th	Day	11% 6% 3 5% 4 2 4 4 5% 7% 51%	1672
	11th to 20th	Day	17% 9% 4 6% 5% 2 4 4 4 6% 41%	1710
	21st to end	Day	17% 9% 4 7% 5% 3 4 4 4 5% 39%	1710
December	1st to 10th	Day	18% 11% 5% 9% 5% 3 5% 5% 5% 6% 29%	1539
	11th to 20th	Day	15% 12% 4 10% 6% 3 5% 4 4 7% 29%	1918
	21st to end	Day	22% 18% 7% 13% 6% 3 4 4 3 4 15%	2783
January	1st to 10th	Night	13% 12% 5% 9% 6% 3 5% 4 5% 5% 33%	2648
	11th to 20th	Night	17% 12% 6% 9% 7% 4 3 4 3 4 30%	2589
	21st to end	Night	15% 12% 6% 10% 6% 3 4 4 5% 5% 30%	2650
February	1st to 10th	Night	19% 11% 5% 8% 6% 3 4 4 5% 6% 29%	2256
	11th to 20th	Night	14% 9% 4 8% 7% 3 5% 4 4 7% 36%	2098
	21st to end	Night	20% 10% 4 7% 6% 2 3 4 4 5% 35%	1776
March	1st to 10th	Night	17% 12% 4 6% 5% 3 5% 4 4 5% 36%	2160
	11th to 20th	Night	16% 8% 4 6% 5% 3 5% 4 5% 7% 38%	2113
	21st to end	Night	22% 9% 3 6% 6% 3 4 5% 4 7% 32%	1980
April	1st to 10th	Night	13% 6% 4 6% 7% 4 4 4 5% 7% 41%	1800
	11th to 20th	Night	17% 8% 3 5% 4 3 4 4 4 6% 43%	1775
	21st to end	Night	20% 8% 3 6% 4 3 4 4 5% 6% 36%	1756
May	1st to 10th	Night	15% 7% 3 5% 5% 4 5% 3 6% 8% 38%	1620
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 4 5% 6% 41%	1543
	21st to end	Night	10% 5% 2 6% 7% 4 5% 6% 6% 8% 42%	1551
June	1st to 10th	Night	9% 4 3 5% 5% 2 4 4 7% 7% 51%	1440
	11th to 20th	Night	7% 5% 2 5% 5% 3 5% 5% 5% 8% 52%	1440
	21st to end	Night	7% 4 3 5% 5% 3 6% 7% 7% 10% 44%	1440
July	1st to 10th	Night	8% 4 2 4 4 3 4 5% 6% 10% 49%	1440
	11th to 20th	Night	5% 3 1 3 4 2 4 5% 6% 9% 58%	1440
	21st to end	Night	4 4 2 5% 4 2 5% 5% 5% 9% 55%	1575
August	1st to 10th	Night	6% 4 2 4 4 3 5% 6% 5% 9% 52%	1592
	11th to 20th	Night	5% 4 2 4 5% 3 5% 6% 7% 8% 50%	1776
	21st to end	Night	6% 3 1 5% 4 3 4 4 5% 8% 56%	1980
September	1st to 10th	Night	12% 4 2 5% 3 2 3 4 5% 10% 51%	1800
	11th to 20th	Night	8% 4 2 4 4 2 4 4 5% 8% 55%	1895
	21st to end	Night	9% 5% 2 4 4 2 4 5% 6% 7% 52%	1980
October	1st to 10th	Night	10% 5% 2 3 4 3 4 4 5% 6% 54%	2014
	11th to 20th	Night	16% 6% 2 4 4 2 3 3 5% 6% 50%	2160
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 4 6% 38%	2544
November	1st to 10th	Night	10% 6% 2 4 4 3 4 4 4 6% 52%	2406
	11th to 20th	Night	13% 7% 3 7% 5% 3 4 4 5% 7% 44%	2470
	21st to end	Night	19% 9% 4 6% 5% 2 4 4 3 6% 38%	2470
December	1st to 10th	Night	17% 10% 4 8% 6% 3 4 4 6% 7% 32%	2636
	11th to 20th	Night	17% 10% 5% 8% 6% 3 4 4 3 6% 35%	3279
	21st to end	Night	24% 19% 7% 9% 6% 3 4 3 3 4 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	59% 4 8% 17% 10% 1	1518
	11th to 20th	Day	65% 4 10% 14% 6% 1	1477
	21st to end	Day	61% 4 12% 15% 7% 1	1705
February	1st to 10th	Day	59% 4 11% 16% 8% 2	1701
	11th to 20th	Day	56% 4 10% 20% 8% 1	1758
	21st to end	Day	58% 4 9% 19% 8% 2	1480
March	1st to 10th	Day	52% 5% 11% 22% 10% 1	1800
	11th to 20th	Day	50% 6% 12% 21% 9% 1	1858
	21st to end	Day	59% 5% 11% 19% 5% 1	2376
April	1st to 10th	Day	47% 6% 12% 21% 14% 1	2160
	11th to 20th	Day	47% 5% 11% 24% 10% 4	2134
	21st to end	Day	50% 5% 12% 19% 11% 2	2204
May	1st to 10th	Day	49% 7% 13% 21% 9% 1	2340
	11th to 20th	Day	47% 8% 12% 19% 14% 1	2351
	21st to end	Day	41% 9% 13% 19% 15% 3	2725
June	1st to 10th	Day	34% 9% 13% 25% 16% 3	2520
	11th to 20th	Day	32% 8% 11% 24% 20% 4	2520
	21st to end	Day	37% 8% 15% 21% 16% 4	2520
July	1st to 10th	Day	35% 8% 14% 27% 13% 4	2451
	11th to 20th	Day	33% 8% 16% 27% 15% 2	2445
	21st to end	Day	31% 8% 15% 27% 17% 2	2591
August	1st to 10th	Day	31% 8% 15% 32% 13% 2	2323
	11th to 20th	Day	35% 8% 14% 27% 14% 2	2125
	21st to end	Day	28% 7% 14% 26% 17% 7%	2376
September	1st to 10th	Day	39% 5% 10% 23% 18% 5%	2160
	11th to 20th	Day	34% 6% 9% 25% 22% 4	2045
	21st to end	Day	33% 5% 10% 25% 25% 2	1980
October	1st to 10th	Day	36% 6% 11% 25% 20% 3	1943
	11th to 20th	Day	42% 5% 10% 21% 21% 2	1800
	21st to end	Day	47% 4 10% 23% 14% 1	1810
November	1st to 10th	Day	33% 5% 14% 25% 18% 4	1672
	11th to 20th	Day	45% 4 9% 24% 14% 4	1710
	21st to end	Day	49% 4 10% 22% 14% 1	1710
December	1st to 10th	Day	57% 5% 11% 19% 8% 1	1539
	11th to 20th	Day	55% 4 9% 25% 6% 1	1915
	21st to end	Day	74% 4 8% 9% 4	2783
January	1st to 10th	Night	53% 4 9% 20% 14% 1	2648
	11th to 20th	Night	60% 4 10% 17% 8%	2589
	21st to end	Night	56% 3 12% 18% 10% 1	2650
February	1st to 10th	Night	57% 4 11% 17% 9% 2	2256
	11th to 20th	Night	50% 4 11% 21% 12% 2	2099
	21st to end	Night	53% 3 10% 20% 11% 3	1776
March	1st to 10th	Night	50% 5% 9% 21% 13% 1	2160
	11th to 20th	Night	46% 4 13% 22% 13% 2	2113
	21st to end	Night	52% 6% 12% 20% 9% 2	1980
April	1st to 10th	Night	43% 6% 10% 23% 16% 2	1800
	11th to 20th	Night	43% 5% 7% 23% 18% 4	1775
	21st to end	Night	49% 5% 10% 20% 12% 3	1756
May	1st to 10th	Night	44% 7% 13% 25% 10% 1	1620
	11th to 20th	Night	42% 7% 12% 22% 16% 1	1544
	21st to end	Night	38% 9% 16% 18% 15% 4	1552
June	1st to 10th	Night	32% 7% 10% 27% 19% 5%	1440
	11th to 20th	Night	30% 6% 11% 26% 23% 4	1440
	21st to end	Night	33% 8% 16% 21% 18% 5%	1440
July	1st to 10th	Night	33% 7% 14% 26% 15% 4	1400
	11th to 20th	Night	22% 6% 15% 32% 22% 3	1392
	21st to end	Night	25% 6% 15% 31% 20% 3	1487
August	1st to 10th	Night	28% 8% 14% 28% 20% 2	1583
	11th to 20th	Night	31% 8% 13% 28% 19% 2	1776
	21st to end	Night	26% 7% 13% 25% 21% 7%	1980
September	1st to 10th	Night	31% 6% 11% 26% 19% 8%	1800
	11th to 20th	Night	27% 5% 9% 28% 27% 4	1895
	21st to end	Night	31% 5% 10% 25% 23% 5%	1980
October	1st to 10th	Night	32% 5% 11% 24% 24% 4	2014
	11th to 20th	Night	36% 4 9% 27% 22% 3	2160
	21st to end	Night	47% 4 8% 21% 17% 2	2544
November	1st to 10th	Night	34% 5% 10% 26% 20% 5%	2406
	11th to 20th	Night	41% 4 11% 24% 19% 2	2470
	21st to end	Night	49% 4 9% 23% 15% 1	2470
December	1st to 10th	Night	53% 4 12% 20% 10% 1	2636
	11th to 20th	Night	52% 3 11% 21% 12% 1	3281
	21st to end	Night	72% 4 6% 11% 6% 1	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% 16% 31% 22% 7%	270
	11th to 20th	Day	27% 17% 35% 19% 3	254
	21st to end	Day	16% 16% 38% 24% 5%	264
February	1st to 10th	Day	28% 15% 34% 19% 4	240
	11th to 20th	Day	26% 18% 33% 17% 5% 2	240
	21st to end	Day	29% 11% 40% 16% 4	198
March	1st to 10th	Day	20% 23% 33% 20% 4	240
	11th to 20th	Day	25% 13% 39% 17% 5%	254
	21st to end	Day	32% 10% 32% 16% 8% 1	462
April	1st to 10th	Day	18% 13% 35% 25% 8% 1	420
	11th to 20th	Day	22% 15% 28% 19% 16% 2	412
	21st to end	Day	26% 11% 34% 20% 8% 2	420
May	1st to 10th	Day	19% 10% 33% 26% 9% 2	423
	11th to 20th	Day	20% 9% 25% 26% 16% 3	493
	21st to end	Day	12% 5% 29% 28% 16% 10%	679
June	1st to 10th	Day	9% 5% 24% 31% 24% 8%	630
	11th to 20th	Day	10% 4 26% 30% 23% 7%	630
	21st to end	Day	9% 8% 26% 32% 18% 7%	630
July	1st to 10th	Day	10% 3 13% 27% 28% 19%	621
	11th to 20th	Day	7% 2 17% 32% 29% 12%	618
	21st to end	Day	9% 2 15% 31% 31% 11%	684
August	1st to 10th	Day	7% 2 11% 34% 32% 13%	528
	11th to 20th	Day	8% 4 16% 28% 31% 13%	439
	21st to end	Day	5% 3 14% 31% 33% 14%	495
September	1st to 10th	Day	12% 6% 16% 29% 29% 8%	450
	11th to 20th	Day	9% 4 12% 29% 32% 14%	450
	21st to end	Day	10% 7% 25% 23% 24% 12%	450
October	1st to 10th	Day	11% 5% 26% 32% 19% 7%	413
	11th to 20th	Day	17% 16% 34% 23% 9% 1	270
	21st to end	Day	28% 16% 34% 17% 4 1	297
November	1st to 10th	Day	13% 12% 31% 27% 12% 5%	270
	11th to 20th	Day	20% 16% 33% 20% 8% 3	270
	21st to end	Day	22% 8% 39% 26% 5%	270
December	1st to 10th	Day	18% 15% 39% 17% 8% 3	270
	11th to 20th	Day	16% 14% 39% 23% 7% 1	270
	21st to end	Day	25% 17% 37% 16% 5%	297
January	1st to 10th	Night	19% 14% 38% 25% 4	950
	11th to 20th	Night	23% 14% 35% 23% 4	925
	21st to end	Night	22% 12% 35% 27% 4	990
February	1st to 10th	Night	25% 11% 37% 23% 4	816
	11th to 20th	Night	20% 15% 36% 26% 3	703
	21st to end	Night	25% 12% 38% 22% 3	592
March	1st to 10th	Night	23% 13% 34% 25% 4	720
	11th to 20th	Night	20% 11% 36% 26% 6%	689
	21st to end	Night	25% 10% 33% 25% 6%	594
April	1st to 10th	Night	17% 13% 36% 28% 7%	540
	11th to 20th	Night	21% 10% 31% 27% 10%	533
	21st to end	Night	24% 9% 33% 25% 7% 1	540
May	1st to 10th	Night	18% 6% 33% 34% 9%	540
	11th to 20th	Night	18% 6% 23% 36% 15% 1	531
	21st to end	Night	11% 3 25% 36% 22% 3	584
June	1st to 10th	Night	10% 2 22% 45% 20%	540
	11th to 20th	Night	9% 2 21% 44% 22% 1	540
	21st to end	Night	7% 4 23% 41% 24% 1	540
July	1st to 10th	Night	11% 1 12% 35% 36% 6%	525
	11th to 20th	Night	5% 1 16% 44% 31% 4	522
	21st to end	Night	5% 3 13% 45% 33% 2	591
August	1st to 10th	Night	7% 2 11% 44% 33% 4	537
	11th to 20th	Night	6% 1 12% 39% 36% 5%	534
	21st to end	Night	8% 1 13% 36% 37% 4	594
September	1st to 10th	Night	14% 2 13% 37% 30% 4	540
	11th to 20th	Night	8% 4 15% 40% 30% 4	635
	21st to end	Night	12% 6% 22% 30% 27% 3	720
October	1st to 10th	Night	12% 4 23% 38% 19% 3	720
	11th to 20th	Night	19% 10% 30% 29% 13% 1	720
	21st to end	Night	23% 12% 34% 25% 6%	792
November	1st to 10th	Night	14% 8% 38% 29% 11%	740
	11th to 20th	Night	18% 13% 32% 29% 7% 1	760
	21st to end	Night	24% 13% 29% 28% 5%	760
December	1st to 10th	Night	24% 11% 33% 26% 5% 1	926
	11th to 20th	Night	23% 10% 32% 29% 6%	950
	21st to end	Night	30% 14% 31% 22% 3	1045

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