Cincinnati Observatory Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cincinnati Observatory Center clear sky chart from 2005-11-06 to 2025-06-10.

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% 36%	119870
	Night	15% 10% 3 7% 5% 4 4 3 5% 7% 36%	118330
Transparency	Day	45% 6% 11% 22% 14% 2	123551
	Night	46% 6% 10% 22% 14% 2	122786
Seeing	Day	19% 14% 27% 21% 13% 5%	16053
	Night	19% 11% 27% 27% 14% 2	25759

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% 16% 9% 11% 4 4 3 3 4 5% 25%	3617
	11th to 20th	Day	14% 15% 6% 12% 6% 7% 5% 3 3 4 26%	4036
	21st to end	Day	22% 14% 7% 8% 5% 5% 4 3 4 5% 24%	4466
February	1st to 10th	Day	19% 13% 4 8% 5% 3 4 4 4 6% 29%	1905
	11th to 20th	Day	18% 12% 5% 10% 5% 4 3 3 4 6% 30%	1948
	21st to end	Day	19% 13% 5% 8% 5% 4 3 3 4 6% 30%	1650
March	1st to 10th	Day	19% 14% 4 8% 5% 4 3 3 4 4 31%	4521
	11th to 20th	Day	14% 8% 3 7% 4 4 4 4 5% 8% 39%	4980
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 6% 7% 34%	5465
April	1st to 10th	Day	17% 10% 3 7% 4 4 5% 4 5% 7% 34%	2355
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 6% 6% 38%	2427
	21st to end	Day	18% 8% 3 6% 5% 5% 4 4 5% 7% 35%	2600
May	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 8% 28%	5862
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 5% 7% 33%	5861
	21st to end	Day	13% 7% 3 7% 5% 6% 6% 5% 8% 10% 32%	6433
June	1st to 10th	Day	13% 5% 2 7% 5% 4 5% 5% 6% 8% 40%	2587
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 6% 9% 44%	2639
	21st to end	Day	8% 6% 3 7% 6% 5% 5% 4 6% 9% 40%	2660
July	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 4 7% 10% 38%	4440
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 6% 10% 46%	4256
	21st to end	Day	11% 7% 3 8% 6% 6% 7% 6% 7% 10% 30%	4466
August	1st to 10th	Day	9% 5% 2 5% 4 4 6% 4 7% 10% 43%	4104
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	4044
	21st to end	Day	6% 4 1 5% 3 4 4 3 6% 9% 55%	4212
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 48%	2161
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 3 6% 8% 49%	2090
	21st to end	Day	16% 6% 2 6% 4 4 3 3 4 7% 46%	2090
October	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 50%	2090
	11th to 20th	Day	13% 7% 2 6% 5% 3 4 3 5% 7% 45%	1929
	21st to end	Day	21% 7% 2 5% 4 3 3 3 4 7% 39%	1950
November	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 4 6% 52%	1762
	11th to 20th	Day	14% 12% 4 8% 4 4 4 3 5% 6% 36%	1800
	21st to end	Day	18% 11% 4 8% 4 4 3 2 4 7% 35%	1800
December	1st to 10th	Day	20% 10% 3 7% 5% 4 3 2 4 5% 38%	2930
	11th to 20th	Day	18% 16% 4 9% 5% 4 5% 4 4 5% 26%	3373
	21st to end	Day	23% 19% 5% 9% 5% 5% 4 3 4 5% 18%	4361
January	1st to 10th	Night	16% 16% 6% 14% 6% 5% 4 3 5% 5% 19%	6333
	11th to 20th	Night	15% 16% 6% 13% 7% 5% 5% 3 4 4 21%	5900
	21st to end	Night	21% 13% 4 7% 5% 4 4 3 3 5% 30%	6490
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 4 4 4 28%	2490
	11th to 20th	Night	19% 11% 4 9% 5% 4 4 4 5% 6% 29%	2338
	21st to end	Night	22% 14% 4 7% 4 3 3 3 4 5% 31%	1980
March	1st to 10th	Night	20% 13% 2 6% 4 3 3 3 4 5% 37%	5332
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 5% 7% 44%	4980
	21st to end	Night	18% 9% 3 8% 4 5% 4 3 5% 5% 34%	5467
April	1st to 10th	Night	16% 10% 3 6% 4 4 4 4 6% 7% 36%	2060
	11th to 20th	Night	14% 8% 2 6% 4 4 4 4 6% 7% 40%	1920
	21st to end	Night	17% 7% 3 6% 4 5% 4 3 6% 9% 36%	1800
May	1st to 10th	Night	21% 10% 3 7% 5% 5% 5% 4 6% 7% 28%	4033
	11th to 20th	Night	17% 6% 3 7% 5% 4 5% 4 7% 9% 33%	4032
	21st to end	Night	13% 7% 2 7% 5% 4 5% 4 6% 9% 38%	4421
June	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 48%	1797
	11th to 20th	Night	8% 4 2 5% 4 4 4 3 7% 11% 48%	1534
	21st to end	Night	6% 4 2 5% 4 5% 5% 4 7% 10% 48%	1520
July	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 5% 8% 50%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 3 4 6% 10% 51%	2704
	21st to end	Night	11% 6% 2 7% 6% 5% 5% 4 7% 10% 37%	3087
August	1st to 10th	Night	8% 5% 3 6% 5% 4 4 4 6% 9% 45%	2825
	11th to 20th	Night	8% 6% 2 7% 4 3 5% 4 5% 9% 46%	2785
	21st to end	Night	3 3 1 3 3 3 4 3 6% 10% 60%	3440
September	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 52%	2000
	11th to 20th	Night	8% 4 2 5% 3 4 4 3 5% 8% 54%	2090
	21st to end	Night	15% 5% 2 5% 4 3 4 3 4 5% 49%	2090
October	1st to 10th	Night	8% 4 2 5% 4 4 3 3 5% 6% 55%	2090
	11th to 20th	Night	15% 6% 2 6% 4 3 5% 3 5% 6% 45%	2249
	21st to end	Night	18% 9% 3 6% 3 4 3 3 4 6% 41%	2646
November	1st to 10th	Night	13% 7% 3 6% 3 3 4 4 5% 6% 47%	2536
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 39%	2600
	21st to end	Night	20% 12% 3 7% 5% 3 4 3 4 6% 33%	2600
December	1st to 10th	Night	18% 11% 4 8% 4 3 3 2 4 5% 38%	4250
	11th to 20th	Night	20% 14% 6% 8% 5% 4 4 3 5% 5% 27%	4822
	21st to end	Night	23% 19% 6% 11% 5% 4 4 3 4 5% 17%	6559

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% 4 10% 17% 7%	3618
	11th to 20th	Day	60% 6% 8% 14% 10% 2	4036
	21st to end	Day	63% 4 9% 17% 7% 1	4466
February	1st to 10th	Day	57% 5% 10% 18% 9% 1	1905
	11th to 20th	Day	56% 4 10% 19% 10% 1	1958
	21st to end	Day	55% 5% 9% 20% 9% 2	1650
March	1st to 10th	Day	54% 6% 6% 19% 12% 2	4521
	11th to 20th	Day	42% 5% 9% 28% 15% 1	4980
	21st to end	Day	49% 6% 11% 21% 12% 2	5465
April	1st to 10th	Day	48% 7% 10% 21% 13% 1	2355
	11th to 20th	Day	46% 6% 10% 22% 14% 3	2427
	21st to end	Day	48% 6% 11% 23% 11% 2	2600
May	1st to 10th	Day	52% 8% 12% 17% 10% 1	5897
	11th to 20th	Day	49% 7% 11% 23% 10%	5865
	21st to end	Day	43% 9% 13% 22% 12% 1	6447
June	1st to 10th	Day	39% 8% 12% 22% 17% 2	2587
	11th to 20th	Day	35% 8% 14% 23% 18% 2	2639
	21st to end	Day	39% 8% 14% 21% 14% 4	2660
July	1st to 10th	Day	41% 7% 12% 20% 19% 2	4371
	11th to 20th	Day	31% 8% 11% 32% 18% 1	4182
	21st to end	Day	46% 8% 14% 17% 12% 3	4324
August	1st to 10th	Day	33% 8% 16% 31% 11% 1	4091
	11th to 20th	Day	34% 7% 12% 32% 15% 1	4083
	21st to end	Day	26% 6% 15% 29% 21% 3	4211
September	1st to 10th	Day	35% 6% 11% 23% 19% 5%	2161
	11th to 20th	Day	32% 7% 11% 26% 21% 3	2090
	21st to end	Day	39% 6% 9% 24% 21% 2	2090
October	1st to 10th	Day	23% 4 8% 32% 30% 2	3480
	11th to 20th	Day	30% 4 9% 30% 26% 2	3182
	21st to end	Day	36% 3 10% 26% 23% 2	3187
November	1st to 10th	Day	35% 5% 10% 28% 20% 3	1762
	11th to 20th	Day	49% 4 8% 21% 15% 3	1800
	21st to end	Day	51% 4 9% 21% 14% 1	1800
December	1st to 10th	Day	51% 3 10% 26% 10%	2930
	11th to 20th	Day	61% 5% 8% 18% 8%	3370
	21st to end	Day	68% 5% 10% 12% 5%	4361
January	1st to 10th	Night	65% 7% 8% 13% 7%	6333
	11th to 20th	Night	65% 5% 8% 11% 8% 2	5900
	21st to end	Night	57% 4 9% 20% 9% 1	6490
February	1st to 10th	Night	58% 4 11% 19% 7% 1	2490
	11th to 20th	Night	55% 4 10% 18% 11% 1	2339
	21st to end	Night	56% 4 10% 17% 10% 2	1980
March	1st to 10th	Night	50% 4 6% 27% 11% 2	5332
	11th to 20th	Night	39% 6% 11% 27% 16% 1	4980
	21st to end	Night	51% 5% 8% 24% 11% 1	5467
April	1st to 10th	Night	45% 7% 11% 23% 12% 2	2060
	11th to 20th	Night	41% 6% 10% 20% 18% 5%	1920
	21st to end	Night	45% 6% 12% 21% 14% 3	1800
May	1st to 10th	Night	53% 9% 10% 18% 8% 2	4060
	11th to 20th	Night	45% 8% 14% 23% 9% 1	4033
	21st to end	Night	40% 8% 16% 22% 11% 3	4429
June	1st to 10th	Night	32% 8% 11% 26% 21% 2	1797
	11th to 20th	Night	29% 9% 14% 23% 21% 4	1534
	21st to end	Night	30% 9% 17% 22% 17% 5%	1520
July	1st to 10th	Night	33% 7% 11% 24% 19% 6%	2490
	11th to 20th	Night	28% 7% 13% 31% 20% 2	2655
	21st to end	Night	41% 7% 13% 22% 13% 4	2988
August	1st to 10th	Night	33% 8% 14% 32% 12% 1	2814
	11th to 20th	Night	34% 7% 11% 28% 18% 1	2812
	21st to end	Night	20% 6% 16% 32% 23% 3	3440
September	1st to 10th	Night	31% 6% 12% 25% 20% 7%	2000
	11th to 20th	Night	28% 8% 10% 24% 27% 3	2090
	21st to end	Night	37% 5% 9% 22% 24% 2	2090
October	1st to 10th	Night	22% 4 7% 28% 37% 3	3480
	11th to 20th	Night	27% 4 9% 25% 33% 2	3735
	21st to end	Night	33% 3 9% 23% 28% 5%	4359
November	1st to 10th	Night	38% 4 11% 21% 22% 3	2536
	11th to 20th	Night	45% 4 9% 21% 18% 3	2600
	21st to end	Night	53% 5% 10% 20% 11% 1	2600
December	1st to 10th	Night	50% 4 11% 24% 11%	4250
	11th to 20th	Night	59% 4 12% 17% 7% 1	4824
	21st to end	Night	70% 5% 9% 13% 4	6559

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% 27% 31% 12% 4	300
	11th to 20th	Day	29% 27% 34% 9% 2	300
	21st to end	Day	30% 24% 32% 12% 2	330
February	1st to 10th	Day	30% 24% 32% 12% 1	300
	11th to 20th	Day	33% 30% 29% 7% 1	300
	21st to end	Day	30% 26% 38% 6%	249
March	1st to 10th	Day	26% 31% 29% 13% 1	315
	11th to 20th	Day	26% 23% 30% 15% 4	500
	21st to end	Day	34% 18% 29% 14% 5%	550
April	1st to 10th	Day	25% 20% 31% 17% 7% 1	500
	11th to 20th	Day	24% 22% 27% 16% 8% 3	492
	21st to end	Day	25% 20% 31% 14% 9% 2	500
May	1st to 10th	Day	25% 19% 30% 16% 6% 4	503
	11th to 20th	Day	22% 12% 32% 21% 8% 5%	527
	21st to end	Day	14% 10% 30% 25% 13% 7%	573
June	1st to 10th	Day	15% 9% 25% 29% 16% 6%	526
	11th to 20th	Day	12% 7% 26% 32% 18% 5%	659
	21st to end	Day	10% 7% 30% 29% 17% 7%	680
July	1st to 10th	Day	14% 5% 18% 26% 22% 15%	671
	11th to 20th	Day	8% 6% 24% 30% 22% 9%	563
	21st to end	Day	12% 4 21% 30% 22% 11%	532
August	1st to 10th	Day	11% 4 18% 30% 22% 15%	486
	11th to 20th	Day	12% 4 22% 32% 20% 10%	479
	21st to end	Day	9% 4 17% 28% 28% 14%	539
September	1st to 10th	Day	13% 5% 18% 29% 26% 9%	490
	11th to 20th	Day	11% 3 18% 25% 31% 12%	490
	21st to end	Day	20% 8% 22% 24% 19% 7%	490
October	1st to 10th	Day	11% 7% 26% 31% 19% 6%	490
	11th to 20th	Day	16% 16% 30% 22% 12% 4	490
	21st to end	Day	24% 14% 37% 20% 6%	399
November	1st to 10th	Day	15% 18% 35% 21% 9% 2	300
	11th to 20th	Day	22% 24% 29% 18% 6% 1	300
	21st to end	Day	26% 19% 38% 14% 3	300
December	1st to 10th	Day	24% 23% 35% 13% 5%	300
	11th to 20th	Day	24% 22% 39% 11% 4	300
	21st to end	Day	33% 21% 32% 11% 2 1	330
January	1st to 10th	Night	24% 20% 31% 21% 4	998
	11th to 20th	Night	30% 18% 32% 18% 3	800
	21st to end	Night	25% 18% 34% 21% 3	880
February	1st to 10th	Night	29% 13% 33% 23% 2	800
	11th to 20th	Night	27% 20% 32% 20% 2	783
	21st to end	Night	28% 18% 33% 20% 1	660
March	1st to 10th	Night	23% 18% 31% 23% 5%	800
	11th to 20th	Night	24% 15% 32% 22% 6% 1	800
	21st to end	Night	24% 13% 32% 24% 6%	880
April	1st to 10th	Night	21% 15% 34% 23% 8%	660
	11th to 20th	Night	20% 11% 34% 24% 10% 1	593
	21st to end	Night	19% 11% 37% 24% 9% 1	600
May	1st to 10th	Night	22% 11% 30% 26% 11% 1	600
	11th to 20th	Night	17% 7% 25% 31% 17% 2	591
	21st to end	Night	11% 3 24% 34% 24% 3	650
June	1st to 10th	Night	11% 4 21% 42% 21% 2	599
	11th to 20th	Night	10% 3 17% 42% 25% 3	570
	21st to end	Night	7% 2 26% 38% 24% 3	570
July	1st to 10th	Night	12% 2 15% 31% 32% 8%	555
	11th to 20th	Night	7% 2 18% 35% 34% 3	552
	21st to end	Night	11% 2 13% 38% 32% 4	624
August	1st to 10th	Night	9% 2 14% 37% 35% 4	567
	11th to 20th	Night	9% 3 14% 39% 29% 6%	564
	21st to end	Night	6% 1 14% 33% 39% 7%	627
September	1st to 10th	Night	14% 3 17% 31% 32% 4	670
	11th to 20th	Night	9% 3 16% 32% 34% 5%	760
	21st to end	Night	17% 6% 22% 31% 23% 2	760
October	1st to 10th	Night	10% 6% 22% 35% 23% 4	760
	11th to 20th	Night	18% 10% 26% 30% 14% 2	760
	21st to end	Night	22% 12% 31% 23% 11%	836
November	1st to 10th	Night	17% 11% 29% 26% 15% 1	780
	11th to 20th	Night	21% 16% 28% 24% 9% 1	800
	21st to end	Night	26% 12% 27% 27% 6% 1	800
December	1st to 10th	Night	27% 15% 31% 23% 5%	800
	11th to 20th	Night	26% 13% 32% 22% 6% 1	800
	21st to end	Night	31% 16% 29% 20% 3	910

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