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 2024-12-06.

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% 4 4 5% 4 6% 8% 38%	101480
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 40%	98285
Transparency	Day	43% 6% 12% 23% 15% 2	105071
	Night	43% 5% 11% 23% 16% 2	102822
Seeing	Day	15% 9% 27% 26% 17% 6%	15778
	Night	17% 9% 29% 31% 13% 1	25627

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	14% 13% 9% 14% 6% 4 4 4 4 6% 23%	2527
	11th to 20th	Day	14% 11% 6% 12% 7% 8% 6% 6% 7% 5% 18%	2530
	21st to end	Day	21% 22% 7% 10% 4 4 3 3 4 5% 18%	2988
February	1st to 10th	Day	16% 11% 5% 10% 5% 5% 4 4 4 6% 28%	1795
	11th to 20th	Day	15% 11% 3 10% 7% 5% 4 4 5% 5% 33%	1848
	21st to end	Day	17% 14% 5% 9% 5% 3 5% 2 4 5% 31%	1570
March	1st to 10th	Day	19% 18% 4 7% 4 3 4 2 3 4 32%	3120
	11th to 20th	Day	10% 9% 3 6% 4 3 3 4 6% 8% 44%	3274
	21st to end	Day	16% 10% 5% 9% 5% 5% 5% 4 6% 6% 29%	4165
April	1st to 10th	Day	14% 9% 3 8% 5% 4 4 3 7% 7% 34%	2280
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 5% 5% 38%	2254
	21st to end	Day	21% 7% 3 7% 4 4 5% 3 5% 6% 35%	2327
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 7% 9% 29%	4082
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 5% 8% 37%	4169
	21st to end	Day	9% 5% 2 7% 4 5% 5% 4 6% 9% 45%	4824
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 8% 45%	2660
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 7% 9% 47%	2660
	21st to end	Day	8% 5% 3 6% 5% 5% 5% 4 7% 9% 42%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 7% 11% 38%	4440
	11th to 20th	Day	6% 4 2 6% 5% 4 5% 5% 8% 11% 44%	4440
	21st to end	Day	9% 5% 2 7% 4 5% 6% 5% 7% 9% 41%	4813
August	1st to 10th	Day	7% 5% 1 5% 3 4 4 4 7% 9% 52%	4114
	11th to 20th	Day	8% 4 2 6% 5% 5% 5% 5% 8% 10% 41%	3729
	21st to end	Day	5% 4 1 5% 4 4 5% 4 6% 11% 50%	4180
September	1st to 10th	Day	11% 5% 3 6% 5% 4 3 4 5% 7% 46%	2280
	11th to 20th	Day	10% 5% 2 4 4 4 5% 4 6% 7% 49%	2158
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2090
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 5% 51%	2050
	11th to 20th	Day	16% 7% 3 6% 3 3 4 3 5% 7% 44%	1900
	21st to end	Day	18% 9% 3 6% 4 4 3 4 5% 7% 37%	1910
November	1st to 10th	Day	11% 7% 3 6% 3 3 4 3 5% 7% 50%	1762
	11th to 20th	Day	17% 10% 3 7% 4 3 4 3 5% 6% 40%	1800
	21st to end	Day	16% 10% 4 8% 4 4 4 4 5% 5% 37%	1800
December	1st to 10th	Day	17% 12% 4 10% 5% 4 5% 4 5% 6% 29%	1580
	11th to 20th	Day	14% 13% 4 12% 5% 4 5% 3 5% 7% 29%	1918
	21st to end	Day	22% 19% 7% 14% 5% 4 4 3 3 4 15%	2783
January	1st to 10th	Night	16% 14% 7% 10% 4 4 4 3 5% 5% 29%	4428
	11th to 20th	Night	14% 18% 6% 11% 6% 7% 6% 3 4 4 21%	4440
	21st to end	Night	24% 16% 7% 9% 4 3 3 2 4 5% 23%	4664
February	1st to 10th	Night	18% 11% 5% 9% 5% 4 4 3 5% 6% 30%	2382
	11th to 20th	Night	13% 9% 3 9% 6% 4 4 4 4 6% 37%	2218
	21st to end	Night	18% 11% 3 8% 5% 3 4 3 4 5% 35%	1884
March	1st to 10th	Night	18% 15% 3 6% 4 3 4 3 4 5% 35%	3733
	11th to 20th	Night	11% 7% 2 6% 4 3 4 3 5% 6% 48%	3711
	21st to end	Night	19% 9% 3 9% 5% 5% 3 4 5% 7% 31%	3466
April	1st to 10th	Night	13% 8% 3 7% 6% 5% 4 3 5% 7% 40%	1900
	11th to 20th	Night	17% 9% 2 5% 4 4 3 3 4 6% 43%	1875
	21st to end	Night	19% 8% 3 6% 4 4 4 4 5% 6% 36%	1853
May	1st to 10th	Night	16% 11% 3 7% 4 4 5% 3 6% 8% 34%	2813
	11th to 20th	Night	17% 7% 3 6% 4 4 4 3 5% 7% 39%	2724
	21st to end	Night	10% 5% 2 7% 5% 5% 4 4 6% 8% 45%	2743
June	1st to 10th	Night	8% 5% 2 6% 4 3 4 3 7% 7% 50%	1520
	11th to 20th	Night	8% 5% 1 6% 4 4 4 4 5% 8% 51%	1520
	21st to end	Night	7% 5% 2 5% 5% 4 6% 5% 7% 10% 44%	1520
July	1st to 10th	Night	11% 5% 2 6% 4 4 4 5% 7% 9% 45%	2530
	11th to 20th	Night	4 3 1 3 3 4 4 4 6% 10% 58%	2530
	21st to end	Night	7% 7% 2 7% 4 4 5% 4 6% 10% 43%	2750
August	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 9% 53%	2811
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 4 6% 8% 50%	3100
	21st to end	Night	6% 3 1 5% 3 4 4 3 5% 9% 56%	3476
September	1st to 10th	Night	11% 4 2 5% 3 3 2 3 5% 9% 53%	1900
	11th to 20th	Night	7% 5% 2 5% 4 3 4 3 5% 8% 55%	2001
	21st to end	Night	10% 5% 2 5% 4 3 5% 4 6% 7% 49%	2090
October	1st to 10th	Night	10% 5% 2 4 3 4 5% 3 5% 5% 55%	2126
	11th to 20th	Night	15% 6% 1 4 4 3 3 2 4 6% 51%	2280
	21st to end	Night	17% 8% 3 7% 4 3 4 3 5% 6% 39%	2686
November	1st to 10th	Night	11% 7% 2 5% 4 4 3 3 5% 6% 51%	2536
	11th to 20th	Night	13% 8% 3 8% 4 4 3 3 5% 7% 43%	2600
	21st to end	Night	19% 10% 4 7% 4 3 4 3 4 6% 37%	2600
December	1st to 10th	Night	17% 10% 4 9% 5% 4 3 3 6% 6% 33%	2712
	11th to 20th	Night	17% 10% 4 9% 5% 4 4 3 3 5% 35%	3279
	21st to end	Night	24% 19% 7% 10% 5% 4 4 2 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	61% 5% 8% 19% 7% 1	2528
	11th to 20th	Day	67% 4 11% 11% 7%	2530
	21st to end	Day	70% 4 8% 13% 4	2988
February	1st to 10th	Day	58% 4 11% 16% 9% 2	1795
	11th to 20th	Day	54% 4 10% 21% 10% 1	1858
	21st to end	Day	56% 5% 9% 20% 9% 2	1570
March	1st to 10th	Day	57% 4 7% 17% 13% 2	3120
	11th to 20th	Day	37% 5% 13% 27% 17% 1	3274
	21st to end	Day	53% 6% 14% 22% 5% 1	4165
April	1st to 10th	Day	48% 6% 12% 21% 14% 1	2280
	11th to 20th	Day	46% 5% 11% 24% 10% 4	2254
	21st to end	Day	50% 5% 12% 20% 11% 2	2327
May	1st to 10th	Day	47% 9% 13% 23% 8%	4120
	11th to 20th	Day	44% 8% 11% 26% 10%	4173
	21st to end	Day	34% 8% 12% 20% 23% 3	4837
June	1st to 10th	Day	34% 9% 13% 24% 16% 3	2660
	11th to 20th	Day	31% 8% 11% 25% 21% 4	2660
	21st to end	Day	36% 8% 14% 21% 17% 3	2660
July	1st to 10th	Day	40% 6% 11% 29% 12% 2	4371
	11th to 20th	Day	32% 8% 14% 29% 17% 1	4365
	21st to end	Day	36% 9% 17% 21% 16% 2	4662
August	1st to 10th	Day	28% 7% 16% 34% 15% 1	4099
	11th to 20th	Day	35% 8% 15% 26% 16% 1	3765
	21st to end	Day	28% 6% 16% 31% 14% 4	4179
September	1st to 10th	Day	38% 5% 10% 24% 20% 5%	2280
	11th to 20th	Day	33% 6% 9% 24% 23% 4	2158
	21st to end	Day	34% 5% 9% 24% 24% 2	2090
October	1st to 10th	Day	25% 4 7% 24% 37% 3	3401
	11th to 20th	Day	29% 3 9% 23% 34% 2	3140
	21st to end	Day	40% 3 9% 23% 24% 1	3122
November	1st to 10th	Day	35% 5% 14% 25% 18% 4	1762
	11th to 20th	Day	46% 4 9% 23% 14% 4	1800
	21st to end	Day	50% 4 10% 22% 13% 1	1800
December	1st to 10th	Day	56% 5% 11% 19% 8% 1	1580
	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	57% 4 11% 18% 9%	4428
	11th to 20th	Night	67% 3 8% 14% 7%	4440
	21st to end	Night	65% 3 9% 15% 8%	4664
February	1st to 10th	Night	55% 4 11% 18% 10% 2	2382
	11th to 20th	Night	48% 4 11% 21% 13% 2	2219
	21st to end	Night	52% 3 9% 21% 11% 3	1884
March	1st to 10th	Night	52% 6% 8% 22% 11% 1	3733
	11th to 20th	Night	36% 5% 11% 26% 20% 2	3711
	21st to end	Night	52% 5% 12% 23% 7% 1	3466
April	1st to 10th	Night	44% 6% 9% 22% 17% 2	1900
	11th to 20th	Night	43% 5% 8% 23% 17% 4	1875
	21st to end	Night	49% 5% 10% 20% 13% 3	1853
May	1st to 10th	Night	47% 7% 14% 23% 8%	2840
	11th to 20th	Night	42% 8% 12% 27% 10% 1	2725
	21st to end	Night	35% 7% 13% 20% 21% 4	2751
June	1st to 10th	Night	32% 6% 10% 27% 18% 5%	1520
	11th to 20th	Night	30% 6% 11% 26% 23% 4	1520
	21st to end	Night	33% 8% 15% 21% 18% 5%	1520
July	1st to 10th	Night	36% 5% 13% 25% 16% 4	2490
	11th to 20th	Night	21% 6% 13% 34% 24% 2	2482
	21st to end	Night	36% 7% 15% 22% 16% 4	2662
August	1st to 10th	Night	27% 6% 16% 32% 18% 1	2802
	11th to 20th	Night	31% 6% 13% 29% 19% 1	3130
	21st to end	Night	26% 6% 13% 33% 18% 4	3476
September	1st to 10th	Night	30% 5% 11% 25% 21% 8%	1900
	11th to 20th	Night	26% 5% 9% 27% 27% 5%	2001
	21st to end	Night	34% 5% 10% 24% 22% 5%	2090
October	1st to 10th	Night	20% 4 7% 26% 41% 3	3542
	11th to 20th	Night	25% 3 8% 26% 35% 3	3779
	21st to end	Night	37% 3 8% 23% 24% 5%	4424
November	1st to 10th	Night	34% 5% 10% 26% 21% 5%	2536
	11th to 20th	Night	41% 4 10% 24% 19% 2	2600
	21st to end	Night	50% 4 9% 22% 14% 1	2600
December	1st to 10th	Night	52% 4 12% 20% 11% 1	2712
	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	25% 17% 36% 19% 3	270
	21st to end	Day	18% 15% 36% 25% 6%	297
February	1st to 10th	Day	26% 14% 34% 21% 4	270
	11th to 20th	Day	24% 16% 35% 17% 6% 1	270
	21st to end	Day	28% 11% 40% 18% 4	225
March	1st to 10th	Day	20% 23% 33% 20% 4	270
	11th to 20th	Day	24% 16% 38% 17% 5%	285
	21st to end	Day	30% 11% 33% 17% 8% 1	506
April	1st to 10th	Day	19% 13% 36% 24% 7% 1	460
	11th to 20th	Day	22% 14% 29% 19% 15% 2	452
	21st to end	Day	25% 11% 34% 20% 8% 2	460
May	1st to 10th	Day	18% 10% 32% 27% 10% 2	463
	11th to 20th	Day	20% 9% 24% 27% 16% 5%	536
	21st to end	Day	11% 5% 31% 28% 16% 9%	734
June	1st to 10th	Day	9% 6% 26% 30% 23% 7%	680
	11th to 20th	Day	10% 4 27% 31% 23% 7%	680
	21st to end	Day	9% 8% 28% 31% 18% 7%	680
July	1st to 10th	Day	11% 3 14% 28% 26% 17%	671
	11th to 20th	Day	7% 3 18% 32% 29% 12%	668
	21st to end	Day	9% 2 15% 32% 31% 11%	739
August	1st to 10th	Day	7% 2 13% 34% 31% 13%	572
	11th to 20th	Day	8% 4 17% 29% 30% 12%	479
	21st to end	Day	5% 3 13% 30% 35% 14%	539
September	1st to 10th	Day	11% 5% 16% 30% 30% 8%	490
	11th to 20th	Day	9% 3 12% 28% 32% 15%	490
	21st to end	Day	12% 7% 25% 22% 22% 11%	490
October	1st to 10th	Day	10% 5% 26% 31% 21% 7%	450
	11th to 20th	Day	15% 16% 33% 22% 11% 3	300
	21st to end	Day	27% 17% 32% 18% 5% 1	330
November	1st to 10th	Day	15% 12% 29% 29% 11% 4	300
	11th to 20th	Day	21% 16% 33% 20% 7% 3	300
	21st to end	Day	24% 8% 38% 25% 5%	300
December	1st to 10th	Day	18% 16% 38% 17% 7% 3	285
	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	950
	21st to end	Night	23% 12% 35% 26% 4	1045
February	1st to 10th	Night	24% 11% 37% 24% 4	862
	11th to 20th	Night	19% 14% 37% 27% 3	743
	21st to end	Night	25% 12% 38% 22% 3	628
March	1st to 10th	Night	23% 13% 35% 25% 4	760
	11th to 20th	Night	19% 12% 37% 26% 6%	727
	21st to end	Night	25% 10% 34% 25% 6%	627
April	1st to 10th	Night	17% 14% 36% 27% 6%	570
	11th to 20th	Night	21% 11% 32% 27% 9%	563
	21st to end	Night	24% 10% 34% 25% 7% 1	570
May	1st to 10th	Night	18% 6% 33% 34% 9%	570
	11th to 20th	Night	18% 6% 23% 37% 15% 1	561
	21st to end	Night	11% 3 26% 36% 21% 3	617
June	1st to 10th	Night	10% 3 22% 45% 19%	570
	11th to 20th	Night	9% 3 22% 44% 21% 1	570
	21st to end	Night	7% 4 24% 41% 23% 1	570
July	1st to 10th	Night	11% 1 12% 36% 34% 5%	555
	11th to 20th	Night	5% 2 16% 43% 30% 3	552
	21st to end	Night	5% 3 13% 45% 32% 2	624
August	1st to 10th	Night	7% 2 11% 43% 34% 4	567
	11th to 20th	Night	6% 1 13% 40% 35% 4	564
	21st to end	Night	8% 1 13% 37% 37% 4	627
September	1st to 10th	Night	13% 2 13% 38% 31% 4	570
	11th to 20th	Night	7% 4 14% 40% 31% 4	671
	21st to end	Night	13% 6% 22% 30% 26% 3	760
October	1st to 10th	Night	12% 4 23% 39% 20% 3	760
	11th to 20th	Night	18% 10% 29% 29% 14% 1	760
	21st to end	Night	22% 12% 35% 26% 6%	836
November	1st to 10th	Night	14% 8% 38% 29% 10%	780
	11th to 20th	Night	18% 13% 33% 29% 7% 1	800
	21st to end	Night	25% 13% 30% 27% 5%	800
December	1st to 10th	Night	24% 11% 34% 26% 4 1	953
	11th to 20th	Night	23% 10% 32% 29% 6%	950
	21st to end	Night	30% 14% 31% 22% 3	1045

Page generated at 2024-12-03 05:45:09. Version ac07

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