Cincinnati Astronomical Society Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cincinnati Astronomical Society Dark Sky Site clear sky chart from 2005-11-06 to 2025-10-19.

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	15% 9% 3 7% 5% 4 4 4 5% 7% 35%	126471
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 36%	123262
Transparency	Day	46% 6% 11% 22% 14% 2	130145
	Night	46% 6% 10% 21% 14% 2	127656
Seeing	Day	21% 15% 26% 21% 12% 5%	16080
	Night	20% 12% 25% 27% 15% 2	26104

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% 20% 6% 12% 5% 3 4 3 4 5% 22%	4056
	11th to 20th	Day	14% 17% 7% 12% 5% 4 4 3 3 3 26%	4060
	21st to end	Day	22% 14% 7% 8% 5% 5% 4 3 4 5% 24%	4466
February	1st to 10th	Day	20% 13% 4 9% 5% 3 3 3 5% 4 29%	1845
	11th to 20th	Day	19% 13% 4 11% 5% 5% 4 3 4 6% 28%	1948
	21st to end	Day	20% 13% 5% 7% 5% 3 2 3 4 6% 32%	1650
March	1st to 10th	Day	17% 15% 5% 8% 4 4 3 3 3 5% 31%	4652
	11th to 20th	Day	13% 7% 3 8% 4 4 4 4 6% 7% 40%	4980
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 34%	5465
April	1st to 10th	Day	15% 11% 3 6% 5% 4 5% 3 4 7% 36%	2270
	11th to 20th	Day	18% 9% 3 7% 5% 4 3 3 5% 7% 39%	2478
	21st to end	Day	18% 8% 2 8% 5% 5% 4 3 6% 6% 35%	2600
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 7% 7% 28%	5862
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 4 6% 6% 31%	5861
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 9% 32%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 6% 5% 6% 9% 40%	2600
	11th to 20th	Day	10% 6% 2 7% 4 5% 6% 4 6% 9% 41%	2600
	21st to end	Day	10% 6% 2 6% 5% 6% 6% 5% 7% 9% 41%	2600
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	5900
	11th to 20th	Day	11% 7% 3 6% 5% 5% 6% 5% 7% 11% 34%	5900
	21st to end	Day	11% 7% 3 9% 6% 6% 6% 5% 7% 9% 31%	6424
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 8% 11% 41%	4234
	11th to 20th	Day	8% 6% 2 5% 5% 6% 6% 4 7% 10% 39%	4174
	21st to end	Day	5% 4 1 5% 3 4 4 4 6% 8% 57%	4477
September	1st to 10th	Day	14% 5% 2 5% 4 4 3 4 5% 6% 48%	2220
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 7% 50%	2200
	21st to end	Day	18% 7% 2 6% 4 3 4 3 5% 6% 43%	2200
October	1st to 10th	Day	12% 5% 1 5% 4 4 4 4 5% 7% 50%	2181
	11th to 20th	Day	14% 7% 2 5% 3 4 4 3 3 6% 49%	1980
	21st to end	Day	20% 9% 2 5% 4 3 3 2 4 6% 40%	2024
November	1st to 10th	Day	12% 7% 2 5% 4 3 4 3 5% 6% 50%	1762
	11th to 20th	Day	15% 12% 4 7% 4 3 4 4 4 7% 37%	1800
	21st to end	Day	18% 12% 3 7% 4 3 3 3 4 6% 36%	1800
December	1st to 10th	Day	20% 12% 3 8% 5% 4 3 3 4 5% 33%	2930
	11th to 20th	Day	21% 15% 5% 8% 5% 5% 4 4 5% 5% 25%	3373
	21st to end	Day	20% 19% 6% 11% 5% 4 4 3 4 4 21%	4466
January	1st to 10th	Night	16% 16% 6% 16% 7% 5% 4 3 4 4 18%	5889
	11th to 20th	Night	16% 16% 6% 14% 6% 4 4 3 4 5% 22%	5900
	21st to end	Night	20% 12% 5% 8% 5% 5% 4 3 3 4 31%	6490
February	1st to 10th	Night	20% 15% 4 8% 4 4 3 3 5% 6% 28%	2550
	11th to 20th	Night	20% 12% 3 10% 5% 4 3 3 4 5% 30%	2338
	21st to end	Night	22% 14% 4 7% 4 4 3 3 4 5% 30%	1980
March	1st to 10th	Night	18% 13% 3 6% 4 4 3 3 4 6% 37%	5199
	11th to 20th	Night	13% 8% 3 5% 4 3 4 4 6% 8% 42%	4980
	21st to end	Night	16% 10% 4 9% 6% 5% 4 3 5% 6% 32%	5467
April	1st to 10th	Night	16% 9% 3 6% 6% 5% 4 3 6% 7% 37%	2145
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 6% 7% 40%	1869
	21st to end	Night	17% 8% 2 6% 5% 4 4 4 5% 8% 37%	1800
May	1st to 10th	Night	21% 11% 3 7% 5% 4 4 4 5% 7% 28%	4033
	11th to 20th	Night	16% 7% 3 6% 5% 4 5% 4 8% 9% 33%	4032
	21st to end	Night	14% 7% 3 6% 4 4 5% 4 7% 9% 37%	4421
June	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 4 6% 9% 47%	1800
	11th to 20th	Night	8% 4 2 6% 4 4 5% 4 7% 12% 46%	1800
	21st to end	Night	8% 5% 1 4 4 4 5% 5% 8% 11% 45%	1800
July	1st to 10th	Night	9% 5% 3 7% 4 4 4 4 5% 8% 46%	4058
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 4 7% 10% 39%	4057
	21st to end	Night	8% 7% 3 8% 5% 6% 6% 4 7% 9% 38%	4427
August	1st to 10th	Night	7% 5% 2 7% 4 4 5% 4 7% 9% 44%	2915
	11th to 20th	Night	10% 7% 2 7% 5% 4 5% 4 5% 8% 42%	2875
	21st to end	Night	4 3 1 3 3 4 4 4 6% 10% 58%	3417
September	1st to 10th	Night	10% 6% 2 6% 4 3 3 3 5% 7% 52%	2158
	11th to 20th	Night	9% 4 2 4 4 4 4 3 5% 7% 54%	2200
	21st to end	Night	14% 5% 2 7% 4 4 4 3 5% 5% 47%	2200
October	1st to 10th	Night	9% 5% 2 4 3 3 4 3 5% 8% 54%	2217
	11th to 20th	Night	14% 7% 2 5% 3 4 4 3 5% 6% 47%	2377
	21st to end	Night	19% 8% 2 4 3 4 5% 3 4 6% 41%	2570
November	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 5% 6% 48%	2536
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 5% 6% 39%	2600
	21st to end	Night	19% 12% 4 6% 5% 4 4 3 4 6% 34%	2600
December	1st to 10th	Night	19% 11% 4 8% 4 3 3 2 4 5% 36%	4250
	11th to 20th	Night	20% 14% 6% 9% 5% 3 3 3 4 6% 27%	4822
	21st to end	Night	21% 18% 6% 10% 5% 5% 4 3 4 5% 18%	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	65% 4 9% 15% 6%	4057
	11th to 20th	Day	63% 4 7% 15% 10% 2	4060
	21st to end	Day	63% 4 8% 17% 7% 1	4466
February	1st to 10th	Day	58% 5% 11% 16% 9% 1	1845
	11th to 20th	Day	58% 5% 10% 17% 10% 1	1958
	21st to end	Day	54% 5% 9% 21% 10% 2	1650
March	1st to 10th	Day	55% 4 7% 21% 11% 3	4652
	11th to 20th	Day	41% 6% 9% 26% 16% 2	4980
	21st to end	Day	48% 5% 11% 21% 13% 2	5465
April	1st to 10th	Day	47% 5% 12% 23% 11% 2	2270
	11th to 20th	Day	45% 6% 10% 22% 14% 3	2478
	21st to end	Day	48% 7% 11% 20% 11% 2	2600
May	1st to 10th	Day	52% 7% 12% 18% 10% 1	5897
	11th to 20th	Day	51% 7% 10% 22% 9% 1	5865
	21st to end	Day	44% 9% 13% 20% 13% 2	6447
June	1st to 10th	Day	38% 9% 13% 23% 16% 2	2600
	11th to 20th	Day	38% 9% 14% 21% 16% 1	2600
	21st to end	Day	38% 9% 14% 22% 14% 3	2600
July	1st to 10th	Day	39% 6% 11% 25% 17% 1	5835
	11th to 20th	Day	41% 9% 13% 25% 11% 1	5831
	21st to end	Day	47% 8% 12% 18% 13% 2	6240
August	1st to 10th	Day	34% 8% 15% 32% 10% 1	4221
	11th to 20th	Day	36% 7% 11% 29% 16%	4213
	21st to end	Day	25% 7% 15% 30% 21% 3	4476
September	1st to 10th	Day	35% 6% 11% 22% 19% 5%	2220
	11th to 20th	Day	32% 6% 13% 22% 25% 2	2200
	21st to end	Day	42% 5% 9% 22% 20% 2	2200
October	1st to 10th	Day	23% 4 8% 30% 33% 2	3558
	11th to 20th	Day	29% 4 8% 30% 26% 2	3220
	21st to end	Day	35% 3 10% 26% 23% 3	3313
November	1st to 10th	Day	35% 5% 10% 27% 20% 3	1762
	11th to 20th	Day	48% 4 9% 20% 16% 4	1800
	21st to end	Day	50% 5% 10% 20% 14% 1	1800
December	1st to 10th	Day	54% 5% 9% 23% 8%	2930
	11th to 20th	Day	60% 6% 9% 17% 7%	3370
	21st to end	Day	66% 6% 9% 13% 6% 1	4466
January	1st to 10th	Night	68% 6% 7% 13% 6%	5889
	11th to 20th	Night	65% 4 8% 13% 8% 2	5900
	21st to end	Night	57% 4 8% 20% 9% 1	6490
February	1st to 10th	Night	58% 5% 10% 18% 8% 1	2550
	11th to 20th	Night	58% 4 9% 17% 11% 1	2339
	21st to end	Night	57% 4 11% 15% 10% 3	1980
March	1st to 10th	Night	49% 5% 6% 26% 12% 2	5199
	11th to 20th	Night	38% 6% 10% 29% 17% 1	4980
	21st to end	Night	52% 5% 7% 22% 12% 1	5467
April	1st to 10th	Night	45% 6% 12% 22% 14% 2	2145
	11th to 20th	Night	41% 7% 10% 20% 17% 6%	1869
	21st to end	Night	45% 7% 11% 21% 14% 2	1800
May	1st to 10th	Night	54% 7% 11% 17% 8% 3	4060
	11th to 20th	Night	45% 9% 14% 22% 9% 1	4033
	21st to end	Night	39% 10% 16% 22% 10% 3	4429
June	1st to 10th	Night	32% 9% 14% 24% 20% 2	1800
	11th to 20th	Night	31% 9% 16% 20% 21% 3	1800
	21st to end	Night	30% 10% 17% 23% 17% 3	1800
July	1st to 10th	Night	36% 7% 11% 24% 19% 3	4015
	11th to 20th	Night	38% 9% 15% 24% 13% 1	4006
	21st to end	Night	41% 8% 12% 22% 13% 3	4297
August	1st to 10th	Night	33% 9% 14% 31% 11% 1	2904
	11th to 20th	Night	39% 8% 9% 25% 18% 1	2902
	21st to end	Night	21% 7% 17% 30% 22% 3	3417
September	1st to 10th	Night	31% 7% 13% 23% 20% 7%	2158
	11th to 20th	Night	28% 7% 12% 22% 28% 3	2200
	21st to end	Night	38% 6% 9% 20% 26% 1	2200
October	1st to 10th	Night	24% 4 8% 25% 36% 3	3620
	11th to 20th	Night	27% 4 8% 26% 29% 5%	3876
	21st to end	Night	33% 4 8% 22% 26% 7%	4231
November	1st to 10th	Night	37% 3 11% 23% 22% 3	2536
	11th to 20th	Night	47% 4 8% 20% 18% 4	2600
	21st to end	Night	52% 5% 9% 21% 12% 1	2600
December	1st to 10th	Night	52% 3 11% 24% 9%	4250
	11th to 20th	Night	59% 4 12% 18% 7% 1	4824
	21st to end	Night	67% 6% 8% 14% 4	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	32% 25% 28% 12% 3	300
	11th to 20th	Day	38% 25% 29% 7% 1	300
	21st to end	Day	35% 26% 27% 10% 2	330
February	1st to 10th	Day	33% 24% 29% 12% 2	300
	11th to 20th	Day	40% 27% 27% 3 2	300
	21st to end	Day	37% 26% 32% 6%	249
March	1st to 10th	Day	31% 25% 28% 13% 4	368
	11th to 20th	Day	29% 22% 31% 14% 4	500
	21st to end	Day	34% 21% 26% 15% 3	550
April	1st to 10th	Day	25% 23% 27% 16% 8% 1	500
	11th to 20th	Day	26% 24% 23% 16% 9% 2	492
	21st to end	Day	26% 18% 32% 15% 6% 2	500
May	1st to 10th	Day	26% 19% 29% 16% 7% 2	503
	11th to 20th	Day	24% 11% 32% 19% 9% 4	527
	21st to end	Day	14% 13% 28% 26% 15% 5%	573
June	1st to 10th	Day	12% 12% 25% 29% 16% 6%	530
	11th to 20th	Day	13% 9% 25% 32% 15% 7%	530
	21st to end	Day	10% 10% 31% 23% 16% 9%	530
July	1st to 10th	Day	15% 7% 16% 24% 19% 19%	525
	11th to 20th	Day	10% 7% 24% 28% 21% 10%	524
	21st to end	Day	13% 4 21% 29% 20% 14%	576
August	1st to 10th	Day	11% 5% 20% 29% 24% 12%	526
	11th to 20th	Day	10% 6% 21% 31% 22% 8%	519
	21st to end	Day	7% 5% 22% 31% 24% 11%	583
September	1st to 10th	Day	12% 7% 18% 27% 27% 9%	530
	11th to 20th	Day	9% 3 17% 30% 30% 11%	530
	21st to end	Day	23% 9% 19% 23% 22% 4	530
October	1st to 10th	Day	12% 8% 25% 30% 18% 7%	530
	11th to 20th	Day	16% 15% 31% 22% 11% 5%	522
	21st to end	Day	27% 13% 33% 21% 6%	473
November	1st to 10th	Day	17% 18% 33% 24% 7% 1	300
	11th to 20th	Day	30% 16% 31% 16% 5% 1	300
	21st to end	Day	27% 23% 32% 13% 4	300
December	1st to 10th	Day	33% 21% 31% 10% 5%	300
	11th to 20th	Day	28% 24% 32% 12% 4	300
	21st to end	Day	32% 25% 29% 12% 3	330
January	1st to 10th	Night	30% 20% 28% 17% 4	800
	11th to 20th	Night	31% 21% 29% 17% 2	800
	21st to end	Night	26% 20% 32% 20% 2	880
February	1st to 10th	Night	31% 18% 29% 20% 2	800
	11th to 20th	Night	32% 20% 29% 18% 1	783
	21st to end	Night	30% 19% 33% 17% 1	660
March	1st to 10th	Night	24% 19% 28% 25% 4	800
	11th to 20th	Night	26% 17% 31% 18% 7%	800
	21st to end	Night	24% 15% 30% 24% 7%	880
April	1st to 10th	Night	22% 15% 32% 23% 7%	745
	11th to 20th	Night	17% 15% 34% 23% 10% 1	593
	21st to end	Night	22% 10% 34% 28% 7% 1	600
May	1st to 10th	Night	22% 11% 29% 26% 12% 1	600
	11th to 20th	Night	17% 8% 26% 31% 17% 1	591
	21st to end	Night	10% 6% 22% 32% 27% 3	650
June	1st to 10th	Night	8% 4 23% 41% 22% 2	600
	11th to 20th	Night	9% 3 19% 39% 27% 3	600
	21st to end	Night	9% 3 23% 34% 27% 4	600
July	1st to 10th	Night	12% 4 14% 32% 29% 9%	585
	11th to 20th	Night	6% 3 15% 36% 36% 4	582
	21st to end	Night	9% 3 13% 37% 34% 4	657
August	1st to 10th	Night	8% 2 12% 39% 35% 4	597
	11th to 20th	Night	8% 3 14% 37% 32% 6%	594
	21st to end	Night	8% 1 15% 35% 35% 7%	660
September	1st to 10th	Night	13% 3 15% 33% 32% 5%	758
	11th to 20th	Night	9% 4 15% 31% 36% 7%	800
	21st to end	Night	15% 7% 18% 35% 24% 2	800
October	1st to 10th	Night	11% 7% 21% 32% 25% 4	800
	11th to 20th	Night	18% 10% 25% 31% 14% 2	793
	21st to end	Night	23% 12% 26% 27% 11%	836
November	1st to 10th	Night	19% 11% 25% 27% 17% 1	780
	11th to 20th	Night	23% 16% 27% 23% 9% 1	800
	21st to end	Night	27% 13% 26% 26% 8% 1	800
December	1st to 10th	Night	30% 18% 27% 20% 5%	800
	11th to 20th	Night	27% 16% 29% 22% 6% 1	800
	21st to end	Night	29% 17% 29% 22% 3	880

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