Circle City Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Circle City Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	143435
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	138739
Transparency	Day	39% 6% 12% 21% 18% 5%	147041
	Night	39% 5% 10% 19% 20% 6%	143205
Seeing	Day	16% 12% 25% 25% 16% 6%	17891
	Night	16% 9% 24% 31% 17% 2	28030

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	15% 5% 2 5% 3 3 3 2 3 5% 53%	5258
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 2 4 61%	5280
	21st to end	Day	17% 8% 3 6% 4 4 4 3 4 5% 43%	5780
February	1st to 10th	Day	14% 6% 3 5% 4 4 4 4 5% 7% 44%	1890
	11th to 20th	Day	21% 7% 2 5% 3 3 3 3 4 5% 45%	1859
	21st to end	Day	19% 7% 2 7% 4 4 4 3 4 5% 41%	1897
March	1st to 10th	Day	20% 6% 2 6% 3 4 4 3 5% 6% 40%	5045
	11th to 20th	Day	11% 5% 2 5% 3 4 4 2 4 6% 54%	5090
	21st to end	Day	15% 5% 2 6% 4 4 4 3 4 5% 48%	5586
April	1st to 10th	Day	11% 3 1 4 4 3 4 3 6% 9% 52%	2420
	11th to 20th	Day	11% 5% 2 4 4 4 4 4 5% 7% 50%	2494
	21st to end	Day	13% 6% 2 5% 4 5% 5% 3 5% 6% 47%	2520
May	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	7612
	11th to 20th	Day	15% 6% 2 7% 4 4 5% 4 6% 7% 40%	7626
	21st to end	Day	14% 6% 3 8% 5% 5% 5% 5% 6% 8% 36%	8391
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 6% 8% 9% 39%	2730
	11th to 20th	Day	8% 4 2 6% 5% 4 5% 5% 7% 10% 43%	2730
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	2730
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 6% 5% 9% 12% 36%	7680
	11th to 20th	Day	9% 6% 3 8% 6% 7% 7% 6% 9% 13% 29%	7680
	21st to end	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 10% 33%	8340
August	1st to 10th	Day	21% 8% 3 8% 5% 6% 5% 5% 7% 9% 22%	4364
	11th to 20th	Day	8% 6% 3 8% 5% 7% 6% 5% 7% 10% 35%	4202
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 10% 51%	4420
September	1st to 10th	Day	9% 4 1 5% 4 5% 5% 4 6% 9% 47%	2400
	11th to 20th	Day	14% 6% 2 7% 4 4 4 4 6% 7% 41%	2229
	21st to end	Day	20% 7% 2 6% 3 4 4 3 6% 7% 38%	2174
October	1st to 10th	Day	15% 5% 3 5% 4 3 2 3 4 7% 49%	2000
	11th to 20th	Day	14% 3 1 4 3 3 4 3 4 5% 57%	2000
	21st to end	Day	12% 5% 2 5% 3 3 3 2 3 5% 58%	2200
November	1st to 10th	Day	14% 5% 2 4 3 3 2 2 4 5% 56%	2057
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 6% 54%	2074
	21st to end	Day	13% 5% 1 5% 3 3 4 3 5% 6% 50%	1890
December	1st to 10th	Day	14% 6% 2 6% 3 3 3 3 4 5% 52%	2759
	11th to 20th	Day	17% 6% 3 6% 4 3 4 3 4 6% 45%	3463
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 45%	4565
January	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 4 5% 54%	7635
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 3 4 57%	7680
	21st to end	Night	17% 8% 2 6% 4 3 3 3 4 5% 44%	8401
February	1st to 10th	Night	17% 5% 2 5% 4 3 4 3 5% 5% 45%	2730
	11th to 20th	Night	20% 7% 2 4 4 4 4 3 4 6% 42%	2642
	21st to end	Night	16% 6% 2 5% 3 3 4 3 5% 6% 47%	1908
March	1st to 10th	Night	19% 7% 3 6% 4 4 3 3 4 5% 42%	5028
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 5% 7% 55%	5090
	21st to end	Night	15% 4 1 6% 4 4 4 3 4 6% 49%	5588
April	1st to 10th	Night	15% 4 2 5% 3 3 3 3 5% 8% 49%	2200
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 4 8% 51%	2075
	21st to end	Night	11% 4 2 6% 4 5% 5% 4 6% 8% 45%	2100
May	1st to 10th	Night	13% 7% 2 7% 4 5% 5% 4 6% 9% 39%	5297
	11th to 20th	Night	15% 6% 3 6% 4 3 4 3 5% 7% 44%	5234
	21st to end	Night	19% 6% 3 7% 5% 4 4 4 5% 8% 35%	5761
June	1st to 10th	Night	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 39%	1890
	11th to 20th	Night	9% 6% 2 7% 6% 5% 5% 4 8% 9% 39%	1890
	21st to end	Night	6% 6% 2 6% 4 5% 6% 5% 8% 12% 39%	1890
July	1st to 10th	Night	10% 6% 2 8% 5% 6% 6% 5% 8% 10% 34%	5278
	11th to 20th	Night	14% 7% 3 7% 6% 6% 6% 6% 8% 10% 27%	5277
	21st to end	Night	13% 8% 3 8% 5% 5% 6% 5% 7% 10% 30%	5736
August	1st to 10th	Night	23% 6% 3 7% 5% 6% 6% 5% 7% 9% 23%	3005
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	3033
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 10% 49%	3677
September	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 6% 8% 47%	2000
	11th to 20th	Night	14% 6% 2 7% 5% 3 4 3 5% 7% 43%	2167
	21st to end	Night	20% 7% 2 6% 5% 4 4 3 5% 7% 37%	2225
October	1st to 10th	Night	13% 6% 3 6% 4 3 3 2 3 6% 50%	2400
	11th to 20th	Night	14% 6% 2 5% 3 2 2 3 4 5% 54%	2400
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 5% 56%	2640
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 4 54%	2461
	11th to 20th	Night	13% 5% 2 6% 4 3 4 3 4 5% 51%	2544
	21st to end	Night	12% 4 2 5% 4 3 3 4 5% 7% 51%	2730
December	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 4 5% 51%	4542
	11th to 20th	Night	17% 8% 4 8% 4 4 4 3 4 5% 41%	4952
	21st to end	Night	17% 5% 2 5% 3 3 3 3 4 5% 49%	6633

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	36% 2 8% 21% 27% 6%	5259
	11th to 20th	Day	31% 2 4 13% 37% 13%	5280
	21st to end	Day	45% 3 3 20% 20% 9%	5780
February	1st to 10th	Day	40% 3 10% 21% 19% 6%	1890
	11th to 20th	Day	44% 3 6% 19% 21% 7%	1868
	21st to end	Day	45% 4 7% 17% 20% 6%	1897
March	1st to 10th	Day	45% 3 7% 16% 18% 11%	5045
	11th to 20th	Day	32% 3 7% 16% 25% 16%	5090
	21st to end	Day	40% 4 8% 21% 21% 7%	5586
April	1st to 10th	Day	30% 4 10% 25% 23% 8%	2420
	11th to 20th	Day	33% 4 11% 24% 20% 8%	2494
	21st to end	Day	39% 4 11% 24% 19% 4	2520
May	1st to 10th	Day	42% 5% 16% 22% 13% 2	7647
	11th to 20th	Day	42% 6% 13% 24% 15% 1	7630
	21st to end	Day	44% 6% 13% 18% 15% 3	8405
June	1st to 10th	Day	37% 8% 15% 23% 15% 2	2730
	11th to 20th	Day	34% 7% 17% 24% 16% 2	2730
	21st to end	Day	34% 8% 17% 25% 15% 1	2730
July	1st to 10th	Day	36% 10% 18% 26% 8% 1	7615
	11th to 20th	Day	42% 12% 24% 14% 7% 1	7611
	21st to end	Day	43% 10% 15% 19% 12%	8156
August	1st to 10th	Day	55% 9% 18% 13% 5%	4351
	11th to 20th	Day	40% 10% 19% 22% 9% 1	4241
	21st to end	Day	29% 6% 14% 34% 15% 2	4420
September	1st to 10th	Day	33% 7% 15% 20% 20% 5%	2400
	11th to 20th	Day	41% 6% 12% 17% 21% 4	2229
	21st to end	Day	46% 5% 10% 21% 15% 5%	2174
October	1st to 10th	Day	33% 4 8% 21% 26% 9%	3260
	11th to 20th	Day	20% 2 5% 21% 27% 25%	3240
	21st to end	Day	25% 3 6% 24% 24% 17%	3538
November	1st to 10th	Day	32% 3 8% 22% 27% 8%	2057
	11th to 20th	Day	32% 3 8% 22% 27% 8%	2074
	21st to end	Day	34% 4 8% 23% 25% 6%	1890
December	1st to 10th	Day	37% 3 7% 19% 27% 7%	2759
	11th to 20th	Day	42% 4 9% 18% 22% 6%	3460
	21st to end	Day	40% 3 10% 26% 19% 2	4565
January	1st to 10th	Night	34% 3 8% 21% 27% 7%	7636
	11th to 20th	Night	33% 2 5% 15% 35% 10%	7680
	21st to end	Night	44% 3 4 18% 24% 8%	8401
February	1st to 10th	Night	40% 3 8% 22% 21% 6%	2730
	11th to 20th	Night	45% 3 8% 16% 21% 7%	2644
	21st to end	Night	38% 4 8% 19% 23% 9%	1908
March	1st to 10th	Night	45% 3 6% 14% 23% 9%	5028
	11th to 20th	Night	32% 3 6% 15% 26% 18%	5090
	21st to end	Night	37% 3 8% 23% 23% 6%	5588
April	1st to 10th	Night	34% 4 10% 20% 24% 8%	2200
	11th to 20th	Night	32% 4 10% 24% 22% 8%	2075
	21st to end	Night	36% 5% 12% 25% 18% 4	2100
May	1st to 10th	Night	41% 6% 17% 21% 14% 2	5324
	11th to 20th	Night	40% 6% 12% 22% 19% 1	5235
	21st to end	Night	46% 6% 12% 20% 13% 2	5771
June	1st to 10th	Night	39% 8% 15% 21% 15% 2	1890
	11th to 20th	Night	38% 8% 16% 20% 16% 1	1890
	21st to end	Night	35% 10% 16% 23% 14% 3	1890
July	1st to 10th	Night	41% 11% 17% 21% 9% 1	5235
	11th to 20th	Night	48% 13% 19% 11% 7% 1	5226
	21st to end	Night	47% 8% 17% 16% 11% 1	5606
August	1st to 10th	Night	55% 9% 18% 13% 5%	2994
	11th to 20th	Night	37% 11% 21% 23% 7% 1	3060
	21st to end	Night	30% 6% 13% 29% 20% 3	3677
September	1st to 10th	Night	34% 7% 15% 22% 17% 5%	2000
	11th to 20th	Night	39% 7% 11% 19% 21% 3	2167
	21st to end	Night	47% 5% 12% 17% 15% 4	2225
October	1st to 10th	Night	37% 4 7% 22% 25% 6%	3920
	11th to 20th	Night	25% 2 6% 18% 28% 20%	3899
	21st to end	Night	26% 3 6% 24% 25% 16%	4252
November	1st to 10th	Night	34% 4 7% 20% 26% 9%	2461
	11th to 20th	Night	36% 3 7% 16% 26% 11%	2544
	21st to end	Night	33% 4 7% 23% 26% 7%	2730
December	1st to 10th	Night	37% 3 6% 19% 27% 8%	4542
	11th to 20th	Night	47% 3 9% 17% 17% 8%	4954
	21st to end	Night	38% 3 12% 24% 21% 2	6633

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	26% 26% 32% 13% 2	330
	11th to 20th	Day	27% 23% 34% 12% 3	330
	21st to end	Day	22% 26% 38% 12% 3	363
February	1st to 10th	Day	20% 20% 42% 15% 4	330
	11th to 20th	Day	37% 23% 31% 7% 1	330
	21st to end	Day	26% 19% 33% 18% 4	440
March	1st to 10th	Day	22% 18% 32% 22% 6%	540
	11th to 20th	Day	20% 21% 37% 16% 5% 1	540
	21st to end	Day	21% 21% 36% 17% 5% 1	594
April	1st to 10th	Day	19% 17% 33% 21% 8% 1	540
	11th to 20th	Day	13% 19% 31% 24% 9% 3	532
	21st to end	Day	16% 12% 29% 30% 12% 1	540
May	1st to 10th	Day	14% 18% 32% 23% 9% 4	543
	11th to 20th	Day	15% 9% 24% 28% 19% 5%	567
	21st to end	Day	18% 6% 21% 26% 22% 7%	617
June	1st to 10th	Day	11% 7% 25% 31% 21% 6%	570
	11th to 20th	Day	9% 7% 21% 32% 24% 8%	570
	21st to end	Day	7% 3 19% 37% 25% 9%	570
July	1st to 10th	Day	10% 2 10% 30% 34% 14%	565
	11th to 20th	Day	7% 3 11% 28% 38% 14%	564
	21st to end	Day	8% 3 15% 34% 28% 12%	620
August	1st to 10th	Day	16% 4 15% 30% 24% 11%	566
	11th to 20th	Day	9% 4 11% 31% 31% 14%	559
	21st to end	Day	13% 3 12% 32% 23% 17%	619
September	1st to 10th	Day	12% 3 17% 27% 27% 14%	530
	11th to 20th	Day	17% 4 15% 28% 25% 11%	530
	21st to end	Day	24% 5% 21% 29% 17% 4	530
October	1st to 10th	Day	17% 6% 21% 25% 22% 8%	530
	11th to 20th	Day	14% 11% 24% 29% 17% 5%	530
	21st to end	Day	15% 11% 28% 27% 15% 4	583
November	1st to 10th	Day	19% 9% 28% 28% 12% 3	535
	11th to 20th	Day	18% 17% 27% 27% 9% 1	514
	21st to end	Day	13% 22% 30% 23% 10% 1	330
December	1st to 10th	Day	26% 18% 34% 17% 5%	247
	11th to 20th	Day	22% 16% 33% 22% 7% 1	330
	21st to end	Day	23% 17% 34% 18% 8% 1	363
January	1st to 10th	Night	19% 19% 30% 26% 5%	840
	11th to 20th	Night	20% 14% 33% 27% 5%	840
	21st to end	Night	20% 14% 35% 28% 3	924
February	1st to 10th	Night	20% 13% 35% 28% 3	840
	11th to 20th	Night	25% 18% 35% 21% 1	823
	21st to end	Night	19% 15% 37% 25% 4	692
March	1st to 10th	Night	17% 15% 35% 29% 5%	840
	11th to 20th	Night	20% 15% 32% 25% 8%	840
	21st to end	Night	17% 14% 38% 25% 5%	924
April	1st to 10th	Night	19% 16% 31% 27% 7%	840
	11th to 20th	Night	15% 13% 32% 29% 11% 1	831
	21st to end	Night	12% 8% 34% 35% 10%	840
May	1st to 10th	Night	12% 11% 32% 35% 9%	651
	11th to 20th	Night	16% 5% 22% 39% 16% 1	621
	21st to end	Night	14% 3 14% 41% 25% 2	683
June	1st to 10th	Night	12% 4 16% 42% 25% 2	630
	11th to 20th	Night	12% 5% 15% 34% 29% 5%	630
	21st to end	Night	9% 2 15% 36% 36% 2	630
July	1st to 10th	Night	8% 3 10% 35% 39% 5%	615
	11th to 20th	Night	10% 2 7% 35% 38% 7%	612
	21st to end	Night	8% 2 12% 36% 36% 5%	690
August	1st to 10th	Night	15% 3 8% 36% 34% 4	627
	11th to 20th	Night	12% 3 9% 32% 38% 6%	666
	21st to end	Night	11% 4 12% 29% 38% 6%	917
September	1st to 10th	Night	11% 3 12% 33% 36% 5%	800
	11th to 20th	Night	16% 4 13% 29% 34% 4	800
	21st to end	Night	23% 4 15% 33% 23% 2	800
October	1st to 10th	Night	15% 4 17% 31% 28% 5%	800
	11th to 20th	Night	17% 8% 25% 29% 20% 2	800
	21st to end	Night	15% 10% 22% 32% 20% 1	880
November	1st to 10th	Night	16% 8% 24% 35% 15% 2	820
	11th to 20th	Night	16% 12% 27% 33% 11%	840
	21st to end	Night	14% 12% 29% 32% 12% 1	840
December	1st to 10th	Night	22% 13% 29% 30% 6%	840
	11th to 20th	Night	19% 9% 29% 35% 8%	840
	21st to end	Night	22% 12% 26% 32% 8%	924

Page generated at 2026-08-27 04:07:12. Version ac07

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