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 2024-10-25.

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%	100925
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	97800
Transparency	Day	40% 5% 12% 21% 18% 4	100737
	Night	40% 5% 11% 20% 19% 5%	97658
Seeing	Day	16% 11% 25% 25% 17% 6%	15400
	Night	16% 9% 24% 31% 18% 2	25555

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	19% 5% 2 4 3 3 3 2 3 5% 51%	2836
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 3 5% 53%	2840
	21st to end	Day	19% 10% 3 5% 4 3 3 3 4 4 42%	3124
February	1st to 10th	Day	15% 6% 3 5% 4 4 4 4 5% 7% 43%	1710
	11th to 20th	Day	21% 7% 2 4 3 3 3 3 3 5% 46%	1677
	21st to end	Day	20% 7% 2 7% 4 4 4 3 4 5% 40%	1721
March	1st to 10th	Day	23% 7% 2 6% 3 4 4 3 4 6% 37%	3435
	11th to 20th	Day	11% 4 2 4 3 4 4 3 4 6% 55%	3480
	21st to end	Day	20% 6% 2 6% 4 4 3 2 4 5% 43%	3815
April	1st to 10th	Day	11% 3 1 4 3 3 4 3 5% 8% 53%	2190
	11th to 20th	Day	11% 5% 2 4 4 4 4 4 5% 8% 49%	2254
	21st to end	Day	14% 5% 2 5% 4 5% 5% 3 5% 6% 46%	2280
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%	2027
November	1st to 10th	Day	15% 5% 2 4 3 3 2 2 4 5% 56%	1857
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 5% 6% 53%	1877
	21st to end	Day	14% 5% 1 5% 3 4 4 3 5% 6% 50%	1710
December	1st to 10th	Day	19% 7% 2 6% 3 3 4 3 5% 7% 42%	1568
	11th to 20th	Day	16% 5% 2 4 3 2 2 2 4 5% 56%	2153
	21st to end	Day	18% 6% 2 5% 4 3 4 4 5% 7% 42%	3124
January	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 5% 5% 48%	4109
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 5% 47%	4120
	21st to end	Night	20% 7% 2 6% 3 3 3 3 4 5% 44%	4532
February	1st to 10th	Night	17% 5% 2 5% 4 3 4 3 6% 6% 45%	2470
	11th to 20th	Night	20% 7% 2 4 4 3 4 3 4 6% 43%	2384
	21st to end	Night	15% 6% 2 5% 3 4 4 4 6% 6% 46%	1732
March	1st to 10th	Night	24% 8% 3 6% 4 4 3 3 5% 5% 33%	3418
	11th to 20th	Night	14% 5% 2 5% 4 3 3 2 4 5% 52%	3480
	21st to end	Night	19% 5% 1 5% 3 3 3 3 4 5% 48%	3817
April	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 7% 50%	1990
	11th to 20th	Night	13% 5% 2 5% 4 2 3 3 4 7% 51%	1875
	21st to end	Night	12% 4 2 6% 4 4 5% 4 6% 7% 46%	1900
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%	2435
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 4 54%	2221
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 4 5% 51%	2301
	21st to end	Night	14% 4 2 5% 4 3 4 3 5% 7% 50%	2470
December	1st to 10th	Night	18% 7% 2 7% 4 4 3 3 4 5% 42%	2557
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	3042
	21st to end	Night	18% 5% 2 4 4 3 3 3 4 6% 46%	4532

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	38% 2 4 25% 24% 6%	2837
	11th to 20th	Day	36% 3 5% 17% 33% 6%	2840
	21st to end	Day	46% 4 4 17% 22% 7%	3124
February	1st to 10th	Day	40% 3 10% 21% 20% 5%	1710
	11th to 20th	Day	43% 3 6% 19% 22% 7%	1686
	21st to end	Day	46% 4 8% 17% 19% 6%	1721
March	1st to 10th	Day	48% 3 7% 18% 15% 8%	3435
	11th to 20th	Day	31% 3 8% 19% 26% 13%	3480
	21st to end	Day	45% 3 7% 20% 20% 5%	3815
April	1st to 10th	Day	29% 4 10% 24% 25% 8%	2190
	11th to 20th	Day	34% 4 11% 22% 21% 8%	2254
	21st to end	Day	39% 4 11% 24% 18% 5%	2280
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	37% 3 9% 23% 24% 5%	1900
	11th to 20th	Day	30% 4 7% 26% 23% 10%	1900
	21st to end	Day	32% 3 6% 23% 26% 11%	2027
November	1st to 10th	Day	33% 3 8% 21% 28% 9%	1857
	11th to 20th	Day	33% 3 8% 21% 27% 8%	1877
	21st to end	Day	35% 4 8% 23% 24% 7%	1710
December	1st to 10th	Day	43% 3 10% 21% 19% 4	1568
	11th to 20th	Day	33% 2 7% 25% 25% 9%	2150
	21st to end	Day	42% 3 9% 22% 21% 3	3124
January	1st to 10th	Night	39% 3 6% 21% 25% 6%	4109
	11th to 20th	Night	39% 3 8% 17% 27% 6%	4120
	21st to end	Night	44% 3 5% 18% 23% 7%	4532
February	1st to 10th	Night	40% 3 8% 22% 21% 6%	2470
	11th to 20th	Night	44% 3 8% 15% 22% 8%	2386
	21st to end	Night	38% 4 8% 20% 21% 8%	1732
March	1st to 10th	Night	54% 3 7% 15% 15% 6%	3418
	11th to 20th	Night	35% 3 7% 14% 24% 17%	3480
	21st to end	Night	40% 3 7% 21% 22% 6%	3817
April	1st to 10th	Night	35% 4 9% 18% 25% 8%	1990
	11th to 20th	Night	34% 4 10% 23% 22% 8%	1875
	21st to end	Night	36% 5% 12% 24% 18% 5%	1900
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% 23% 24% 5%	2280
	11th to 20th	Night	34% 3 7% 22% 24% 9%	2280
	21st to end	Night	31% 3 6% 21% 27% 12%	2435
November	1st to 10th	Night	35% 3 7% 19% 26% 10%	2221
	11th to 20th	Night	36% 3 7% 16% 25% 12%	2301
	21st to end	Night	34% 3 7% 23% 26% 7%	2470
December	1st to 10th	Night	45% 4 8% 20% 19% 5%	2557
	11th to 20th	Night	39% 2 8% 20% 23% 9%	3044
	21st to end	Night	40% 3 7% 24% 24% 3	4532

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% 23% 35% 14% 2	270
	11th to 20th	Day	26% 23% 34% 13% 3	270
	21st to end	Day	21% 25% 39% 13% 3	297
February	1st to 10th	Day	20% 20% 40% 15% 4	270
	11th to 20th	Day	36% 22% 32% 8% 1	270
	21st to end	Day	26% 20% 34% 16% 3	376
March	1st to 10th	Day	22% 19% 33% 20% 6%	460
	11th to 20th	Day	20% 22% 38% 15% 5% 1	460
	21st to end	Day	23% 21% 33% 17% 5% 1	506
April	1st to 10th	Day	21% 18% 32% 20% 7% 2	460
	11th to 20th	Day	14% 19% 30% 23% 10% 4	452
	21st to end	Day	18% 12% 30% 29% 11% 1	460
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% 16% 4	514
November	1st to 10th	Day	20% 9% 27% 27% 13% 4	455
	11th to 20th	Day	19% 17% 25% 28% 10% 1	437
	21st to end	Day	13% 20% 32% 21% 12% 1	270
December	1st to 10th	Day	25% 17% 37% 15% 6%	202
	11th to 20th	Day	21% 17% 33% 21% 8% 1	270
	21st to end	Day	24% 15% 34% 16% 9% 1	297
January	1st to 10th	Night	20% 18% 30% 26% 6% 1	760
	11th to 20th	Night	20% 13% 33% 28% 6%	760
	21st to end	Night	20% 13% 35% 28% 3	836
February	1st to 10th	Night	20% 13% 35% 29% 4	760
	11th to 20th	Night	25% 18% 35% 21% 1	743
	21st to end	Night	18% 15% 37% 25% 4	628
March	1st to 10th	Night	17% 14% 35% 28% 5%	760
	11th to 20th	Night	21% 14% 33% 23% 8% 1	760
	21st to end	Night	18% 15% 36% 26% 5%	836
April	1st to 10th	Night	19% 17% 31% 26% 7%	760
	11th to 20th	Night	16% 13% 31% 28% 11% 1	751
	21st to end	Night	13% 8% 34% 34% 10%	760
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% 20% 1	812
November	1st to 10th	Night	16% 8% 24% 34% 16% 3	740
	11th to 20th	Night	16% 12% 26% 34% 12%	760
	21st to end	Night	15% 10% 27% 33% 14% 1	760
December	1st to 10th	Night	22% 12% 28% 31% 7%	760
	11th to 20th	Night	20% 9% 29% 35% 8%	760
	21st to end	Night	23% 12% 25% 31% 8%	836

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