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 2026-03-18.

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% 9% 3 8% 5% 4 5% 4 6% 8% 37%	131038
	Night	14% 9% 4 8% 5% 4 4 3 5% 7% 39%	130715
Transparency	Day	44% 6% 11% 23% 14% 2	134584
	Night	44% 5% 10% 23% 15% 2	135231
Seeing	Day	15% 9% 27% 26% 17% 6%	17461
	Night	17% 9% 29% 31% 13% 1	27441

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	12% 12% 6% 13% 7% 5% 4 4 5% 7% 26%	4691
	11th to 20th	Day	8% 11% 6% 14% 8% 8% 5% 5% 5% 5% 24%	4710
	21st to end	Day	15% 17% 5% 11% 7% 4 3 3 4 5% 24%	5554
February	1st to 10th	Day	16% 11% 5% 10% 5% 5% 4 4 4 6% 29%	1985
	11th to 20th	Day	14% 12% 3 10% 7% 5% 4 3 4 5% 32%	2048
	21st to end	Day	16% 13% 4 10% 5% 4 5% 2 4 5% 32%	1730
March	1st to 10th	Day	19% 15% 3 7% 4 3 3 2 3 3 38%	4580
	11th to 20th	Day	8% 10% 3 5% 4 4 4 4 6% 9% 44%	4760
	21st to end	Day	17% 11% 5% 8% 5% 5% 5% 4 5% 6% 30%	5967
April	1st to 10th	Day	15% 9% 3 8% 5% 4 4 3 7% 7% 34%	2400
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 5% 5% 38%	2374
	21st to end	Day	20% 7% 3 7% 4 4 5% 3 5% 7% 35%	2450
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	5862
	11th to 20th	Day	15% 7% 3 7% 4 5% 5% 4 5% 7% 38%	5991
	21st to end	Day	11% 6% 2 8% 4 5% 5% 4 7% 9% 38%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 8% 45%	2800
	11th to 20th	Day	9% 4 2 5% 5% 5% 5% 4 6% 9% 46%	2800
	21st to end	Day	8% 5% 3 6% 5% 5% 6% 4 7% 9% 43%	2800
July	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 11% 42%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 11% 36%	6360
	21st to end	Day	10% 6% 3 7% 4 5% 5% 5% 8% 10% 38%	6924
August	1st to 10th	Day	7% 5% 1 5% 3 4 4 4 6% 9% 52%	4244
	11th to 20th	Day	8% 4 2 6% 5% 5% 5% 5% 9% 10% 41%	3849
	21st to end	Day	5% 4 1 5% 4 4 5% 4 6% 11% 50%	4312
September	1st to 10th	Day	11% 5% 3 6% 5% 4 3 4 5% 7% 47%	2400
	11th to 20th	Day	10% 5% 2 4 4 4 5% 4 6% 7% 50%	2271
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2200
October	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 5% 6% 51%	2158
	11th to 20th	Day	15% 7% 3 6% 3 3 4 3 5% 7% 44%	2000
	21st to end	Day	17% 9% 3 6% 4 4 3 4 5% 7% 38%	2010
November	1st to 10th	Day	11% 7% 3 6% 3 3 4 3 5% 7% 49%	1852
	11th to 20th	Day	17% 10% 3 7% 4 2 4 3 5% 6% 40%	1890
	21st to end	Day	16% 11% 4 8% 4 4 4 3 4 5% 37%	1890
December	1st to 10th	Day	11% 10% 4 8% 4 4 4 3 5% 6% 42%	2723
	11th to 20th	Day	14% 15% 4 12% 5% 5% 5% 4 5% 6% 25%	3088
	21st to end	Day	27% 17% 6% 13% 5% 3 4 3 3 4 14%	4070
January	1st to 10th	Night	14% 13% 7% 11% 4 3 4 3 5% 5% 31%	8232
	11th to 20th	Night	10% 14% 6% 11% 7% 6% 6% 4 4 5% 26%	8280
	21st to end	Night	16% 12% 5% 9% 5% 4 4 3 4 5% 31%	8645
February	1st to 10th	Night	17% 11% 5% 9% 5% 4 4 3 5% 6% 30%	2632
	11th to 20th	Night	14% 10% 4 9% 5% 4 4 3 4 6% 36%	2458
	21st to end	Night	17% 10% 3 8% 5% 3 3 3 4 5% 37%	2076
March	1st to 10th	Night	18% 14% 4 6% 3 3 3 2 3 5% 38%	5493
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 8% 46%	5394
	21st to end	Night	18% 9% 3 9% 5% 5% 4 4 5% 7% 31%	4962
April	1st to 10th	Night	14% 8% 3 7% 6% 5% 4 3 5% 6% 40%	2000
	11th to 20th	Night	17% 9% 2 5% 4 4 3 3 5% 6% 42%	1975
	21st to end	Night	18% 8% 3 6% 4 4 5% 4 5% 6% 36%	1950
May	1st to 10th	Night	17% 11% 3 7% 4 4 4 3 5% 7% 34%	4033
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 9% 39%	3905
	21st to end	Night	13% 7% 3 8% 5% 5% 4 4 5% 7% 38%	3942
June	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 7% 7% 50%	1600
	11th to 20th	Night	8% 5% 2 6% 4 4 3 4 5% 8% 50%	1600
	21st to end	Night	6% 5% 2 5% 5% 4 6% 5% 8% 10% 44%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 9% 45%	3618
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 7% 10% 47%	3617
	21st to end	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 41%	3947
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 6% 9% 52%	2901
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 4 6% 8% 50%	3200
	21st to end	Night	6% 3 1 5% 3 4 3 4 5% 9% 56%	3586
September	1st to 10th	Night	11% 4 2 5% 3 3 3 3 5% 9% 53%	2000
	11th to 20th	Night	7% 4 2 5% 4 3 4 3 5% 8% 56%	2107
	21st to end	Night	10% 5% 2 6% 4 3 5% 4 5% 6% 50%	2200
October	1st to 10th	Night	10% 5% 2 4 3 4 4 3 5% 5% 56%	2238
	11th to 20th	Night	15% 6% 2 4 4 3 3 2 5% 6% 51%	2400
	21st to end	Night	17% 8% 3 7% 4 3 5% 3 5% 6% 39%	2828
November	1st to 10th	Night	10% 7% 2 5% 4 4 3 3 5% 6% 51%	2666
	11th to 20th	Night	13% 8% 3 7% 4 4 3 3 5% 7% 43%	2730
	21st to end	Night	19% 10% 4 7% 4 3 4 3 4 6% 37%	2730
December	1st to 10th	Night	13% 8% 3 8% 4 3 3 3 5% 6% 44%	4681
	11th to 20th	Night	16% 10% 4 10% 5% 5% 5% 3 4 6% 33%	5339
	21st to end	Night	27% 17% 6% 9% 5% 4 4 3 4 4 17%	7150

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% 4 12% 19% 7% 1	4692
	11th to 20th	Day	62% 4 8% 13% 12% 2	4710
	21st to end	Day	64% 3 8% 17% 6% 2	5554
February	1st to 10th	Day	57% 4 12% 17% 8% 2	1985
	11th to 20th	Day	55% 4 10% 20% 10% 1	2058
	21st to end	Day	54% 4 9% 20% 10% 2	1730
March	1st to 10th	Day	53% 3 6% 19% 17% 2	4580
	11th to 20th	Day	38% 4 10% 32% 15% 1	4760
	21st to end	Day	53% 5% 12% 20% 8% 1	5969
April	1st to 10th	Day	48% 6% 12% 20% 13% 1	2400
	11th to 20th	Day	47% 5% 11% 24% 10% 4	2374
	21st to end	Day	49% 5% 13% 20% 11% 2	2450
May	1st to 10th	Day	50% 7% 11% 21% 11% 1	5897
	11th to 20th	Day	44% 8% 10% 24% 12% 2	5995
	21st to end	Day	40% 8% 11% 22% 16% 2	6949
June	1st to 10th	Day	35% 9% 13% 25% 16% 3	2800
	11th to 20th	Day	33% 8% 11% 24% 21% 4	2800
	21st to end	Day	36% 8% 15% 21% 17% 3	2800
July	1st to 10th	Day	36% 6% 11% 30% 15% 2	6291
	11th to 20th	Day	40% 8% 14% 24% 13% 1	6285
	21st to end	Day	38% 8% 17% 23% 12% 1	6728
August	1st to 10th	Day	28% 7% 16% 35% 14% 1	4229
	11th to 20th	Day	34% 8% 15% 26% 16% 1	3885
	21st to end	Day	28% 6% 16% 31% 15% 4	4311
September	1st to 10th	Day	37% 5% 10% 24% 21% 4	2400
	11th to 20th	Day	33% 6% 9% 24% 24% 4	2271
	21st to end	Day	34% 5% 9% 24% 26% 2	2200
October	1st to 10th	Day	25% 4 7% 24% 38% 3	3509
	11th to 20th	Day	29% 4 9% 23% 33% 2	3240
	21st to end	Day	40% 3 9% 23% 24% 1	3222
November	1st to 10th	Day	35% 5% 13% 26% 18% 4	1852
	11th to 20th	Day	46% 4 9% 23% 14% 4	1890
	21st to end	Day	50% 4 10% 21% 14% 1	1890
December	1st to 10th	Day	45% 4 12% 28% 11%	2723
	11th to 20th	Day	59% 4 9% 22% 6% 1	3085
	21st to end	Day	75% 4 9% 8% 4	4070
January	1st to 10th	Night	56% 4 11% 22% 7%	8233
	11th to 20th	Night	60% 3 7% 15% 12% 2	8280
	21st to end	Night	56% 3 9% 21% 9% 1	8645
February	1st to 10th	Night	54% 4 11% 19% 10% 2	2632
	11th to 20th	Night	49% 4 11% 22% 13% 2	2459
	21st to end	Night	50% 3 9% 22% 13% 3	2076
March	1st to 10th	Night	51% 4 7% 22% 13% 4	5493
	11th to 20th	Night	37% 4 11% 28% 18% 2	5394
	21st to end	Night	52% 5% 10% 23% 8% 1	4962
April	1st to 10th	Night	44% 5% 9% 22% 17% 2	2000
	11th to 20th	Night	43% 5% 8% 23% 17% 4	1975
	21st to end	Night	48% 5% 10% 21% 12% 3	1950
May	1st to 10th	Night	49% 7% 11% 20% 10% 2	4060
	11th to 20th	Night	41% 7% 10% 26% 14% 2	3906
	21st to end	Night	43% 7% 13% 20% 15% 3	3950
June	1st to 10th	Night	33% 6% 10% 27% 19% 5%	1600
	11th to 20th	Night	31% 6% 10% 26% 23% 4	1600
	21st to end	Night	32% 8% 15% 21% 18% 5%	1600
July	1st to 10th	Night	36% 5% 11% 27% 18% 3	3580
	11th to 20th	Night	30% 8% 14% 28% 18% 2	3572
	21st to end	Night	38% 7% 15% 24% 13% 3	3832
August	1st to 10th	Night	27% 6% 16% 32% 17% 1	2892
	11th to 20th	Night	31% 7% 14% 29% 18% 1	3230
	21st to end	Night	25% 6% 13% 33% 19% 4	3586
September	1st to 10th	Night	30% 5% 10% 26% 21% 8%	2000
	11th to 20th	Night	26% 5% 9% 26% 29% 4	2107
	21st to end	Night	34% 5% 10% 23% 23% 5%	2200
October	1st to 10th	Night	20% 4 7% 26% 41% 3	3654
	11th to 20th	Night	25% 3 8% 26% 35% 3	3899
	21st to end	Night	37% 3 8% 23% 24% 5%	4566
November	1st to 10th	Night	34% 5% 10% 26% 21% 5%	2666
	11th to 20th	Night	41% 4 10% 24% 19% 2	2730
	21st to end	Night	50% 4 9% 22% 14% 1	2730
December	1st to 10th	Night	43% 4 12% 28% 13% 1	4681
	11th to 20th	Night	54% 4 11% 20% 11% 1	5341
	21st to end	Night	71% 4 9% 11% 5% 1	7150

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	21% 18% 31% 23% 6%	330
	11th to 20th	Day	22% 20% 37% 18% 2	330
	21st to end	Day	18% 17% 38% 23% 5%	363
February	1st to 10th	Day	23% 14% 38% 21% 5%	330
	11th to 20th	Day	25% 18% 36% 15% 5% 1	330
	21st to end	Day	27% 12% 41% 17% 3	273
March	1st to 10th	Day	21% 22% 35% 19% 3	330
	11th to 20th	Day	25% 18% 36% 16% 5%	337
	21st to end	Day	30% 12% 33% 16% 7% 1	550
April	1st to 10th	Day	21% 13% 34% 23% 7% 1	500
	11th to 20th	Day	23% 15% 28% 19% 14% 1	492
	21st to end	Day	24% 11% 35% 20% 9% 2	500
May	1st to 10th	Day	20% 10% 31% 26% 10% 2	503
	11th to 20th	Day	19% 10% 24% 26% 16% 5%	579
	21st to end	Day	13% 5% 31% 27% 15% 9%	789
June	1st to 10th	Day	10% 6% 25% 31% 22% 7%	730
	11th to 20th	Day	10% 4 26% 30% 23% 6%	730
	21st to end	Day	9% 7% 26% 32% 19% 7%	730
July	1st to 10th	Day	11% 3 14% 28% 27% 17%	721
	11th to 20th	Day	8% 3 18% 32% 29% 11%	718
	21st to end	Day	9% 2 15% 32% 31% 10%	794
August	1st to 10th	Day	7% 2 13% 35% 31% 12%	616
	11th to 20th	Day	7% 3 16% 29% 31% 13%	519
	21st to end	Day	5% 3 14% 31% 34% 14%	583
September	1st to 10th	Day	10% 6% 16% 30% 30% 8%	530
	11th to 20th	Day	8% 3 13% 28% 32% 16%	530
	21st to end	Day	12% 7% 24% 23% 22% 12%	530
October	1st to 10th	Day	10% 5% 25% 32% 23% 7%	488
	11th to 20th	Day	15% 15% 32% 24% 12% 2	330
	21st to end	Day	26% 17% 34% 18% 5% 1	363
November	1st to 10th	Day	14% 15% 29% 28% 11% 4	330
	11th to 20th	Day	22% 16% 33% 19% 8% 3	330
	21st to end	Day	25% 8% 37% 24% 5%	330
December	1st to 10th	Day	17% 15% 42% 17% 6% 3	330
	11th to 20th	Day	18% 14% 39% 23% 5% 1	330
	21st to end	Day	27% 16% 36% 17% 4	363
January	1st to 10th	Night	19% 14% 39% 25% 4	1050
	11th to 20th	Night	22% 16% 37% 22% 4	1050
	21st to end	Night	22% 13% 36% 25% 3	1155
February	1st to 10th	Night	23% 12% 37% 24% 4	952
	11th to 20th	Night	20% 15% 36% 26% 3	823
	21st to end	Night	23% 13% 40% 22% 2	692
March	1st to 10th	Night	24% 13% 35% 24% 4	840
	11th to 20th	Night	20% 13% 37% 25% 5%	794
	21st to end	Night	24% 12% 34% 24% 6%	660
April	1st to 10th	Night	18% 14% 36% 27% 6%	600
	11th to 20th	Night	21% 11% 33% 27% 9%	593
	21st to end	Night	23% 10% 34% 26% 7% 1	600
May	1st to 10th	Night	18% 7% 33% 33% 9%	600
	11th to 20th	Night	18% 7% 24% 36% 15% 1	591
	21st to end	Night	12% 4 26% 36% 20% 3	650
June	1st to 10th	Night	10% 3 23% 45% 19%	600
	11th to 20th	Night	10% 3 21% 45% 21% 1	600
	21st to end	Night	7% 4 23% 42% 24% 1	600
July	1st to 10th	Night	11% 1 12% 36% 34% 5%	585
	11th to 20th	Night	5% 2 16% 43% 31% 3	582
	21st to end	Night	6% 2 12% 45% 33% 2	657
August	1st to 10th	Night	7% 2 11% 43% 34% 3	597
	11th to 20th	Night	6% 1 13% 40% 35% 4	594
	21st to end	Night	7% 1 12% 38% 37% 4	660
September	1st to 10th	Night	13% 2 13% 38% 31% 4	600
	11th to 20th	Night	7% 4 13% 41% 31% 4	707
	21st to end	Night	13% 6% 21% 31% 27% 3	800
October	1st to 10th	Night	11% 4 23% 40% 20% 3	800
	11th to 20th	Night	18% 10% 28% 31% 13% 1	800
	21st to end	Night	21% 12% 35% 25% 6%	880
November	1st to 10th	Night	14% 8% 39% 29% 10%	820
	11th to 20th	Night	18% 13% 33% 28% 7% 1	840
	21st to end	Night	25% 14% 29% 27% 5%	840
December	1st to 10th	Night	23% 11% 35% 26% 4	1024
	11th to 20th	Night	23% 11% 33% 27% 6%	1050
	21st to end	Night	31% 13% 32% 21% 2	1155

Page generated at 2026-03-15 01:23:56. Version ac07

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