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 2025-04-26.

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 4 4 5% 7% 44%	113942
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 46%	114610
Transparency	Day	38% 5% 11% 20% 19% 6%	117592
	Night	38% 5% 10% 19% 22% 7%	119099
Seeing	Day	17% 11% 25% 25% 16% 6%	16027
	Night	16% 9% 24% 31% 17% 2	26286

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% 5% 2 5% 3 3 3 2 2 4 56%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 3 2 3 4 58%	4060
	21st to end	Day	14% 8% 3 6% 4 4 4 3 4 4 46%	4466
February	1st to 10th	Day	14% 6% 3 5% 4 4 4 4 5% 7% 44%	1800
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 4 5% 45%	1768
	21st to end	Day	19% 7% 2 7% 4 4 4 3 4 5% 42%	1809
March	1st to 10th	Day	21% 6% 2 6% 3 4 4 3 4 6% 40%	4935
	11th to 20th	Day	11% 5% 2 5% 4 4 4 2 4 6% 54%	4980
	21st to end	Day	15% 5% 2 6% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	11% 3 1 4 3 3 4 3 5% 8% 53%	2305
	11th to 20th	Day	11% 5% 2 4 4 4 4 4 5% 7% 49%	2374
	21st to end	Day	14% 6% 2 5% 4 5% 5% 3 5% 6% 45%	2340
May	1st to 10th	Day	11% 7% 3 8% 6% 5% 6% 4 7% 9% 35%	4078
	11th to 20th	Day	17% 7% 3 8% 5% 4 5% 4 6% 7% 35%	4081
	21st to end	Day	8% 4 1 5% 4 5% 4 4 6% 9% 48%	4476
June	1st to 10th	Day	9% 6% 2 5% 5% 6% 6% 6% 8% 8% 39%	2470
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 4 7% 10% 44%	2470
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 6% 8% 11% 39%	2470
July	1st to 10th	Day	7% 5% 3 7% 7% 7% 7% 5% 8% 10% 34%	4120
	11th to 20th	Day	10% 7% 3 7% 6% 6% 6% 5% 9% 12% 30%	4120
	21st to end	Day	10% 8% 3 9% 7% 7% 7% 6% 7% 10% 27%	4466
August	1st to 10th	Day	21% 8% 3 8% 5% 5% 6% 5% 7% 9% 23%	4104
	11th to 20th	Day	8% 6% 2 8% 5% 7% 6% 5% 7% 10% 34%	3948
	21st to end	Day	7% 5% 1 4 4 4 4 4 6% 10% 51%	4180
September	1st to 10th	Day	9% 4 1 5% 4 5% 5% 4 6% 9% 47%	2280
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 6% 7% 41%	2118
	21st to end	Day	21% 7% 2 6% 3 4 4 3 6% 6% 38%	2066
October	1st to 10th	Day	16% 5% 3 5% 3 2 2 3 4 6% 49%	1900
	11th to 20th	Day	13% 3 1 4 3 3 4 3 4 5% 57%	1900
	21st to end	Day	11% 5% 2 6% 3 3 3 2 3 5% 58%	2090
November	1st to 10th	Day	15% 5% 2 4 3 3 2 2 4 5% 54%	1957
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 5% 6% 53%	1975
	21st to end	Day	14% 5% 1 5% 3 3 4 3 5% 6% 50%	1800
December	1st to 10th	Day	13% 6% 2 6% 3 3 3 2 4 5% 53%	2676
	11th to 20th	Day	17% 6% 3 6% 4 4 4 3 4 6% 45%	3373
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 45%	4466
January	1st to 10th	Night	13% 6% 2 4 3 3 3 2 4 5% 56%	5889
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 55%	5900
	21st to end	Night	15% 7% 2 6% 4 3 3 3 4 6% 47%	6490
February	1st to 10th	Night	17% 5% 2 5% 4 3 4 3 5% 6% 45%	2600
	11th to 20th	Night	21% 7% 2 4 4 4 4 3 4 6% 42%	2513
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 6% 48%	1820
March	1st to 10th	Night	20% 7% 3 6% 4 4 3 3 4 5% 42%	4918
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 5% 7% 54%	4980
	21st to end	Night	15% 5% 1 6% 4 4 4 3 4 6% 49%	5467
April	1st to 10th	Night	15% 5% 2 4 3 3 3 3 5% 7% 49%	2095
	11th to 20th	Night	13% 5% 2 5% 4 2 3 3 4 7% 51%	1975
	21st to end	Night	11% 4 2 7% 4 5% 4 4 6% 7% 45%	1957
May	1st to 10th	Night	13% 7% 2 8% 4 4 5% 3 6% 7% 41%	2845
	11th to 20th	Night	18% 7% 3 7% 5% 4 5% 3 4 6% 38%	2812
	21st to end	Night	8% 4 2 6% 5% 4 4 4 7% 11% 46%	3079
June	1st to 10th	Night	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 38%	1710
	11th to 20th	Night	10% 6% 2 6% 6% 5% 5% 4 8% 9% 40%	1710
	21st to end	Night	7% 6% 2 6% 5% 5% 6% 5% 8% 12% 39%	1710
July	1st to 10th	Night	8% 7% 2 8% 6% 6% 7% 6% 8% 9% 33%	2840
	11th to 20th	Night	14% 7% 2 6% 5% 5% 5% 5% 8% 10% 32%	2840
	21st to end	Night	15% 11% 4 9% 6% 5% 6% 4 7% 8% 28%	3087
August	1st to 10th	Night	23% 6% 3 7% 5% 6% 6% 5% 7% 9% 23%	2825
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	2849
	21st to end	Night	6% 4 2 6% 4 4 4 4 7% 10% 50%	3476
September	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 6% 8% 46%	1900
	11th to 20th	Night	15% 6% 2 7% 5% 3 4 3 5% 7% 43%	2058
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 7% 37%	2114
October	1st to 10th	Night	14% 6% 3 6% 4 3 3 2 3 6% 50%	2280
	11th to 20th	Night	14% 6% 2 5% 3 2 2 3 4 5% 54%	2280
	21st to end	Night	12% 5% 2 4 3 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 4 53%	2341
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 4 5% 50%	2423
	21st to end	Night	13% 4 2 5% 4 3 3 3 5% 7% 51%	2600
December	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 4 5% 52%	4407
	11th to 20th	Night	17% 8% 4 8% 4 4 4 3 4 5% 40%	4822
	21st to end	Night	17% 5% 2 5% 3 3 3 3 4 5% 49%	6490

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	35% 2 3 20% 32% 8%	4057
	11th to 20th	Day	32% 2 4 16% 36% 10%	4060
	21st to end	Day	41% 4 4 21% 23% 8%	4466
February	1st to 10th	Day	40% 3 11% 21% 19% 5%	1800
	11th to 20th	Day	44% 3 6% 18% 22% 7%	1777
	21st to end	Day	45% 4 7% 17% 21% 6%	1809
March	1st to 10th	Day	46% 3 6% 16% 18% 11%	4935
	11th to 20th	Day	32% 3 7% 16% 25% 16%	4980
	21st to end	Day	40% 4 8% 20% 21% 7%	5465
April	1st to 10th	Day	30% 4 10% 24% 24% 8%	2305
	11th to 20th	Day	34% 4 11% 23% 20% 8%	2374
	21st to end	Day	40% 4 11% 23% 18% 5%	2340
May	1st to 10th	Day	45% 4 17% 18% 12% 2	4116
	11th to 20th	Day	47% 5% 12% 22% 12% 1	4085
	21st to end	Day	31% 5% 17% 23% 20% 4	4489
June	1st to 10th	Day	38% 8% 16% 23% 14% 1	2470
	11th to 20th	Day	34% 6% 17% 25% 16% 2	2470
	21st to end	Day	35% 8% 17% 23% 15% 1	2470
July	1st to 10th	Day	41% 10% 16% 18% 12% 2	4055
	11th to 20th	Day	43% 10% 24% 17% 5% 1	4051
	21st to end	Day	49% 10% 15% 17% 10%	4324
August	1st to 10th	Day	54% 8% 18% 14% 5%	4091
	11th to 20th	Day	40% 10% 19% 22% 9% 1	3987
	21st to end	Day	28% 6% 14% 34% 15% 2	4180
September	1st to 10th	Day	34% 7% 15% 19% 19% 5%	2280
	11th to 20th	Day	42% 5% 12% 17% 21% 4	2118
	21st to end	Day	46% 5% 9% 20% 15% 5%	2066
October	1st to 10th	Day	33% 4 8% 20% 26% 10%	3160
	11th to 20th	Day	20% 2 5% 21% 27% 26%	3140
	21st to end	Day	25% 3 6% 25% 24% 17%	3428
November	1st to 10th	Day	33% 3 8% 21% 27% 8%	1957
	11th to 20th	Day	33% 3 8% 21% 27% 7%	1975
	21st to end	Day	35% 4 8% 22% 25% 6%	1800
December	1st to 10th	Day	36% 3 7% 20% 27% 8%	2676
	11th to 20th	Day	42% 4 9% 18% 21% 6%	3370
	21st to end	Day	41% 3 10% 26% 19% 2	4466
January	1st to 10th	Night	32% 3 4 18% 34% 9%	5889
	11th to 20th	Night	34% 3 6% 15% 35% 8%	5900
	21st to end	Night	40% 3 5% 18% 27% 6%	6490
February	1st to 10th	Night	41% 3 8% 22% 20% 6%	2600
	11th to 20th	Night	44% 3 8% 15% 21% 8%	2515
	21st to end	Night	37% 4 8% 19% 23% 9%	1820
March	1st to 10th	Night	45% 3 6% 13% 23% 10%	4918
	11th to 20th	Night	32% 3 6% 14% 26% 19%	4980
	21st to end	Night	38% 3 7% 22% 23% 6%	5467
April	1st to 10th	Night	35% 4 9% 19% 24% 8%	2095
	11th to 20th	Night	33% 4 10% 23% 22% 8%	1975
	21st to end	Night	36% 6% 12% 24% 18% 5%	1957
May	1st to 10th	Night	42% 4 17% 21% 13% 2	2872
	11th to 20th	Night	47% 5% 10% 19% 17% 1	2813
	21st to end	Night	31% 6% 17% 25% 18% 2	3087
June	1st to 10th	Night	39% 8% 16% 21% 14% 2	1710
	11th to 20th	Night	38% 7% 17% 21% 16% 2	1710
	21st to end	Night	36% 10% 16% 22% 13% 3	1710
July	1st to 10th	Night	43% 11% 17% 15% 13% 2	2795
	11th to 20th	Night	44% 11% 23% 16% 5% 1	2786
	21st to end	Night	54% 8% 14% 15% 10%	2988
August	1st to 10th	Night	54% 9% 18% 13% 5%	2814
	11th to 20th	Night	37% 11% 21% 23% 7% 1	2876
	21st to end	Night	29% 6% 14% 29% 20% 3	3476
September	1st to 10th	Night	35% 7% 15% 21% 17% 6%	1900
	11th to 20th	Night	40% 6% 11% 18% 22% 3	2058
	21st to end	Night	47% 5% 11% 16% 16% 4	2114
October	1st to 10th	Night	37% 4 7% 21% 25% 6%	3800
	11th to 20th	Night	25% 2 6% 18% 29% 21%	3779
	21st to end	Night	25% 3 6% 25% 25% 16%	4120
November	1st to 10th	Night	35% 4 7% 20% 25% 9%	2341
	11th to 20th	Night	37% 3 7% 16% 26% 12%	2423
	21st to end	Night	33% 3 7% 23% 26% 7%	2600
December	1st to 10th	Night	36% 3 6% 19% 28% 8%	4407
	11th to 20th	Night	47% 4 8% 17% 16% 8%	4824
	21st to end	Night	38% 3 11% 24% 21% 2	6490

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	28% 25% 32% 13% 2	300
	11th to 20th	Day	26% 24% 34% 13% 3	300
	21st to end	Day	20% 25% 39% 12% 3	330
February	1st to 10th	Day	19% 20% 42% 14% 4	300
	11th to 20th	Day	38% 22% 30% 8% 1	300
	21st to end	Day	24% 20% 34% 18% 4	408
March	1st to 10th	Day	23% 19% 32% 20% 6%	500
	11th to 20th	Day	20% 21% 37% 16% 5% 1	500
	21st to end	Day	22% 22% 34% 17% 5% 1	550
April	1st to 10th	Day	20% 18% 33% 20% 7% 1	500
	11th to 20th	Day	14% 19% 31% 23% 9% 4	492
	21st to end	Day	18% 12% 29% 29% 11% 1	480
May	1st to 10th	Day	13% 17% 32% 24% 9% 5%	463
	11th to 20th	Day	15% 9% 24% 28% 20% 6%	487
	21st to end	Day	16% 5% 21% 26% 25% 8%	529
June	1st to 10th	Day	11% 7% 23% 31% 22% 6%	490
	11th to 20th	Day	10% 6% 19% 30% 26% 9%	490
	21st to end	Day	8% 3 20% 37% 24% 8%	490
July	1st to 10th	Day	10% 1 11% 31% 33% 14%	485
	11th to 20th	Day	7% 2 11% 28% 39% 13%	484
	21st to end	Day	7% 3 15% 35% 29% 11%	532
August	1st to 10th	Day	14% 4 16% 30% 24% 13%	486
	11th to 20th	Day	10% 4 12% 29% 29% 15%	479
	21st to end	Day	13% 3 11% 30% 25% 19%	539
September	1st to 10th	Day	12% 2 16% 27% 28% 16%	490
	11th to 20th	Day	18% 4 12% 28% 26% 11%	490
	21st to end	Day	23% 5% 21% 30% 17% 4	490
October	1st to 10th	Day	19% 6% 21% 25% 21% 8%	490
	11th to 20th	Day	13% 11% 24% 30% 18% 5%	490
	21st to end	Day	14% 10% 27% 29% 17% 4	539
November	1st to 10th	Day	20% 9% 27% 28% 13% 3	495
	11th to 20th	Day	19% 17% 26% 27% 10% 1	475
	21st to end	Day	14% 21% 31% 22% 11% 1	300
December	1st to 10th	Day	24% 19% 36% 16% 5%	224
	11th to 20th	Day	21% 16% 34% 21% 7% 1	300
	21st to end	Day	24% 16% 33% 18% 9% 1	330
January	1st to 10th	Night	20% 19% 31% 25% 6% 1	800
	11th to 20th	Night	20% 14% 33% 28% 6%	800
	21st to end	Night	20% 13% 36% 28% 3	880
February	1st to 10th	Night	20% 13% 35% 28% 4	800
	11th to 20th	Night	26% 18% 34% 21% 1	783
	21st to end	Night	18% 15% 37% 26% 4	660
March	1st to 10th	Night	17% 15% 35% 28% 5%	800
	11th to 20th	Night	21% 14% 33% 24% 8% 1	800
	21st to end	Night	18% 15% 37% 25% 5%	880
April	1st to 10th	Night	20% 16% 31% 26% 7%	800
	11th to 20th	Night	16% 13% 32% 28% 11% 1	791
	21st to end	Night	13% 8% 34% 35% 10%	783
May	1st to 10th	Night	12% 12% 31% 35% 10%	589
	11th to 20th	Night	17% 5% 21% 39% 17% 2	561
	21st to end	Night	12% 2 12% 43% 28% 3	617
June	1st to 10th	Night	12% 4 15% 42% 26% 2	570
	11th to 20th	Night	12% 5% 14% 34% 31% 6%	570
	21st to end	Night	9% 2 15% 36% 35% 2	570
July	1st to 10th	Night	8% 3 11% 34% 39% 6%	555
	11th to 20th	Night	10% 3 7% 36% 38% 7%	552
	21st to end	Night	8% 2 12% 38% 37% 4	624
August	1st to 10th	Night	14% 3 8% 36% 35% 4	567
	11th to 20th	Night	13% 3 9% 32% 36% 6%	602
	21st to end	Night	11% 4 11% 29% 39% 6%	836
September	1st to 10th	Night	11% 2 12% 32% 38% 5%	760
	11th to 20th	Night	17% 4 13% 28% 34% 4	760
	21st to end	Night	23% 4 15% 33% 23% 2	760
October	1st to 10th	Night	16% 4 17% 31% 28% 5%	760
	11th to 20th	Night	16% 8% 25% 28% 21% 2	760
	21st to end	Night	14% 9% 22% 33% 21% 1	836
November	1st to 10th	Night	16% 8% 24% 35% 16% 2	780
	11th to 20th	Night	17% 12% 26% 34% 12%	800
	21st to end	Night	15% 12% 28% 33% 13% 1	800
December	1st to 10th	Night	22% 13% 28% 31% 6%	800
	11th to 20th	Night	19% 9% 29% 35% 8%	800
	21st to end	Night	22% 13% 25% 32% 8%	880

Page generated at 2025-04-23 09:25:07. Version ac07

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