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 2024-03-18.

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% 4 6% 6% 3 5% 5% 5% 7% 36%	79204
	Night	16% 9% 4 6% 5% 3 4 4 5% 7% 37%	80544
Transparency	Day	47% 7% 11% 20% 13% 2	78918
	Night	47% 6% 11% 20% 14% 2	80350
Seeing	Day	19% 14% 27% 22% 13% 5%	14362
	Night	19% 11% 27% 27% 15% 2	24175

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	17% 16% 7% 8% 5% 4 4 4 4 6% 26%	2527
	11th to 20th	Day	17% 14% 7% 11% 8% 6% 6% 4 4 3 21%	2816
	21st to end	Day	29% 16% 9% 7% 5% 2 3 3 4 5% 18%	3124
February	1st to 10th	Day	20% 12% 5% 8% 6% 3 4 5% 4 6% 29%	1809
	11th to 20th	Day	17% 10% 5% 9% 6% 3 4 4 4 6% 31%	1848
	21st to end	Day	20% 12% 5% 8% 5% 3 3 4 4 6% 30%	1570
March	1st to 10th	Day	18% 11% 4 6% 6% 3 4 4 6% 6% 33%	1914
	11th to 20th	Day	19% 10% 3 7% 5% 3 4 5% 5% 7% 32%	2058
	21st to end	Day	22% 10% 4 6% 6% 3 4 4 5% 6% 29%	2178
April	1st to 10th	Day	16% 8% 3 6% 5% 3 5% 6% 5% 7% 36%	2119
	11th to 20th	Day	17% 8% 3 6% 5% 2 4 4 6% 6% 39%	2181
	21st to end	Day	20% 7% 3 5% 6% 4 4 5% 5% 7% 35%	2340
May	1st to 10th	Day	21% 7% 3 6% 6% 3 5% 5% 4 6% 34%	2340
	11th to 20th	Day	19% 8% 4 6% 6% 3 4 4 6% 6% 34%	2301
	21st to end	Day	9% 6% 4 6% 6% 4 6% 7% 7% 10% 35%	2531
June	1st to 10th	Day	13% 5% 2 6% 6% 3 5% 5% 6% 9% 41%	2340
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 6% 6% 9% 44%	2500
	21st to end	Day	9% 5% 3 6% 7% 4 6% 6% 6% 9% 39%	2520
July	1st to 10th	Day	13% 5% 3 5% 6% 4 6% 5% 7% 10% 37%	2520
	11th to 20th	Day	8% 5% 2 5% 6% 4 6% 6% 7% 10% 41%	2420
	21st to end	Day	10% 4 2 4 5% 3 7% 6% 7% 9% 40%	2547
August	1st to 10th	Day	9% 5% 1 5% 6% 3 5% 6% 7% 10% 43%	2327
	11th to 20th	Day	8% 6% 3 6% 6% 3 5% 7% 6% 11% 39%	2303
	21st to end	Day	8% 5% 2 5% 4 3 5% 5% 7% 10% 46%	2394
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 8% 47%	2048
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 4 6% 8% 49%	1980
	21st to end	Day	14% 5% 2 5% 4 3 4 3 4 7% 48%	1980
October	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 6% 48%	1980
	11th to 20th	Day	13% 7% 3 5% 5% 2 4 4 5% 7% 44%	1828
	21st to end	Day	22% 8% 3 5% 4 3 3 4 4 7% 38%	1848
November	1st to 10th	Day	9% 7% 2 5% 4 3 4 4 4 6% 53%	1672
	11th to 20th	Day	14% 10% 5% 6% 5% 3 4 4 5% 6% 38%	1710
	21st to end	Day	18% 9% 4 7% 5% 3 4 3 4 7% 36%	1710
December	1st to 10th	Day	22% 11% 6% 8% 7% 3 4 4 4 5% 25%	1710
	11th to 20th	Day	16% 15% 5% 8% 6% 3 4 4 4 5% 29%	2153
	21st to end	Day	22% 19% 5% 8% 7% 4 4 4 4 5% 17%	3058
January	1st to 10th	Night	17% 18% 6% 9% 6% 3 4 4 4 5% 22%	4427
	11th to 20th	Night	16% 16% 6% 12% 8% 4 4 4 3 4 23%	4120
	21st to end	Night	26% 17% 6% 7% 5% 2 3 3 3 5% 22%	4532
February	1st to 10th	Night	22% 11% 4 9% 5% 3 4 5% 4 4 29%	2366
	11th to 20th	Night	18% 10% 4 8% 6% 3 5% 4 5% 6% 31%	2218
	21st to end	Night	22% 14% 4 7% 5% 2 4 3 4 5% 30%	1884
March	1st to 10th	Night	18% 10% 3 5% 5% 3 4 4 3 5% 38%	2266
	11th to 20th	Night	19% 8% 3 6% 4 3 4 4 4 7% 36%	2068
	21st to end	Night	20% 9% 3 7% 5% 3 4 5% 4 6% 34%	2178
April	1st to 10th	Night	14% 8% 3 6% 5% 3 4 5% 6% 8% 38%	1854
	11th to 20th	Night	14% 7% 3 5% 4 3 4 5% 6% 7% 40%	1726
	21st to end	Night	18% 7% 4 5% 5% 3 5% 4 5% 8% 37%	1620
May	1st to 10th	Night	17% 9% 3 6% 6% 3 5% 4 6% 7% 34%	1620
	11th to 20th	Night	16% 6% 3 6% 6% 3 5% 5% 6% 7% 37%	1592
	21st to end	Night	7% 5% 3 4 6% 3 5% 6% 8% 11% 42%	1744
June	1st to 10th	Night	7% 5% 2 5% 5% 2 5% 5% 6% 9% 49%	1620
	11th to 20th	Night	8% 4 3 4 5% 3 4 4 6% 11% 49%	1454
	21st to end	Night	6% 3 2 4 4 4 5% 6% 6% 10% 48%	1440
July	1st to 10th	Night	10% 5% 2 5% 6% 2 4 4 5% 9% 47%	1440
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 5% 6% 10% 50%	1536
	21st to end	Night	8% 4 2 5% 6% 3 4 4 5% 10% 48%	1771
August	1st to 10th	Night	7% 4 3 5% 5% 3 4 5% 7% 11% 48%	1611
	11th to 20th	Night	7% 5% 2 6% 5% 3 5% 6% 6% 10% 46%	1598
	21st to end	Night	5% 4 2 3 4 3 3 4 6% 10% 56%	1958
September	1st to 10th	Night	12% 5% 2 4 4 3 4 4 5% 7% 51%	1894
	11th to 20th	Night	9% 4 2 4 4 3 5% 4 5% 8% 53%	1980
	21st to end	Night	14% 4 2 4 4 2 3 4 4 6% 51%	1980
October	1st to 10th	Night	8% 4 2 4 6% 3 3 4 5% 6% 54%	1980
	11th to 20th	Night	16% 6% 2 5% 5% 3 4 5% 6% 6% 43%	2130
	21st to end	Night	19% 8% 3 5% 4 3 4 4 4 6% 40%	2506
November	1st to 10th	Night	13% 6% 3 5% 4 2 4 5% 4 6% 47%	2406
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 5% 7% 40%	2470
	21st to end	Night	20% 10% 4 6% 5% 3 4 5% 4 6% 35%	2470
December	1st to 10th	Night	21% 11% 5% 8% 6% 3 4 3 4 6% 30%	2470
	11th to 20th	Night	19% 12% 6% 7% 6% 2 4 4 5% 6% 30%	3042
	21st to end	Night	21% 18% 5% 10% 7% 3 5% 4 4 5% 17%	4573

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	59% 5% 12% 14% 10%	2528
	11th to 20th	Day	63% 8% 10% 13% 6%	2816
	21st to end	Day	70% 4 8% 14% 3 1	3124
February	1st to 10th	Day	57% 5% 10% 17% 10% 2	1809
	11th to 20th	Day	55% 4 10% 19% 10% 1	1858
	21st to end	Day	56% 5% 9% 19% 9% 2	1570
March	1st to 10th	Day	51% 6% 10% 22% 10% 1	1914
	11th to 20th	Day	52% 6% 10% 19% 12% 2	2058
	21st to end	Day	56% 6% 9% 20% 8% 1	2178
April	1st to 10th	Day	46% 7% 11% 22% 13% 1	2119
	11th to 20th	Day	45% 6% 10% 21% 15% 4	2181
	21st to end	Day	49% 6% 10% 22% 11% 2	2340
May	1st to 10th	Day	51% 7% 12% 20% 9% 1	2340
	11th to 20th	Day	49% 8% 11% 18% 12% 1	2305
	21st to end	Day	41% 11% 14% 20% 12% 1	2531
June	1st to 10th	Day	39% 8% 12% 23% 16% 2	2340
	11th to 20th	Day	36% 8% 15% 23% 18% 2	2500
	21st to end	Day	39% 9% 14% 21% 13% 4	2520
July	1st to 10th	Day	41% 10% 12% 18% 17% 2	2451
	11th to 20th	Day	36% 10% 12% 23% 17% 1	2346
	21st to end	Day	36% 10% 15% 23% 13% 3	2405
August	1st to 10th	Day	34% 10% 15% 26% 14% 2	2314
	11th to 20th	Day	37% 8% 14% 25% 15% 1	2303
	21st to end	Day	32% 9% 15% 22% 17% 6%	2394
September	1st to 10th	Day	36% 7% 12% 23% 17% 5%	2048
	11th to 20th	Day	32% 7% 11% 26% 21% 2	1980
	21st to end	Day	37% 6% 9% 25% 22% 2	1980
October	1st to 10th	Day	36% 7% 10% 24% 21% 2	1980
	11th to 20th	Day	39% 5% 9% 25% 19% 3	1828
	21st to end	Day	47% 4 11% 21% 15% 2	1848
November	1st to 10th	Day	34% 5% 10% 28% 21% 3	1672
	11th to 20th	Day	48% 4 8% 21% 16% 3	1710
	21st to end	Day	50% 4 9% 22% 14% 1	1710
December	1st to 10th	Day	62% 4 10% 17% 6%	1710
	11th to 20th	Day	57% 5% 9% 18% 11%	2150
	21st to end	Day	69% 6% 9% 10% 6% 1	3058
January	1st to 10th	Night	62% 8% 9% 14% 7%	4427
	11th to 20th	Night	65% 5% 9% 14% 6%	4120
	21st to end	Night	66% 4 8% 15% 6% 1	4532
February	1st to 10th	Night	58% 5% 11% 19% 7% 1	2366
	11th to 20th	Night	53% 5% 10% 19% 11% 1	2219
	21st to end	Night	57% 4 10% 16% 10% 2	1884
March	1st to 10th	Night	50% 5% 7% 23% 14% 2	2266
	11th to 20th	Night	48% 5% 12% 21% 12% 2	2068
	21st to end	Night	52% 6% 11% 21% 9% 2	2178
April	1st to 10th	Night	43% 7% 11% 25% 12% 2	1854
	11th to 20th	Night	41% 7% 10% 17% 19% 6%	1726
	21st to end	Night	46% 6% 12% 20% 13% 3	1620
May	1st to 10th	Night	49% 7% 10% 21% 10% 3	1620
	11th to 20th	Night	44% 9% 13% 19% 13% 2	1593
	21st to end	Night	33% 10% 16% 22% 15% 3	1745
June	1st to 10th	Night	31% 8% 11% 27% 20% 3	1620
	11th to 20th	Night	30% 9% 14% 23% 20% 4	1454
	21st to end	Night	30% 9% 17% 22% 17% 5%	1440
July	1st to 10th	Night	35% 8% 14% 21% 19% 4	1400
	11th to 20th	Night	29% 8% 15% 26% 19% 3	1487
	21st to end	Night	31% 8% 13% 30% 16% 2	1672
August	1st to 10th	Night	29% 9% 16% 29% 15% 2	1600
	11th to 20th	Night	31% 10% 13% 28% 17% 1	1598
	21st to end	Night	24% 8% 16% 26% 21% 5%	1958
September	1st to 10th	Night	32% 7% 12% 25% 17% 7%	1894
	11th to 20th	Night	28% 8% 10% 25% 26% 2	1980
	21st to end	Night	35% 5% 9% 23% 26% 3	1980
October	1st to 10th	Night	30% 6% 11% 28% 23% 4	1980
	11th to 20th	Night	39% 7% 8% 24% 21% 2	2130
	21st to end	Night	46% 4 10% 21% 17% 3	2506
November	1st to 10th	Night	37% 4 11% 21% 23% 4	2406
	11th to 20th	Night	45% 4 9% 20% 19% 3	2470
	21st to end	Night	52% 5% 10% 20% 12% 1	2470
December	1st to 10th	Night	56% 5% 11% 18% 9% 1	2470
	11th to 20th	Night	55% 5% 10% 20% 9% 1	3044
	21st to end	Night	68% 6% 9% 12% 5%	4573

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% 4	270
	11th to 20th	Day	31% 26% 33% 8% 1	270
	21st to end	Day	33% 24% 29% 11% 2	297
February	1st to 10th	Day	30% 26% 30% 13% 1	270
	11th to 20th	Day	32% 29% 30% 7% 1	270
	21st to end	Day	32% 26% 36% 5%	225
March	1st to 10th	Day	26% 32% 29% 12% 1	284
	11th to 20th	Day	26% 22% 32% 16% 4	449
	21st to end	Day	34% 17% 29% 14% 6%	462
April	1st to 10th	Day	21% 20% 30% 19% 8% 1	420
	11th to 20th	Day	23% 21% 26% 17% 9% 3	412
	21st to end	Day	25% 20% 31% 13% 8% 2	420
May	1st to 10th	Day	26% 19% 29% 16% 5% 4	423
	11th to 20th	Day	21% 11% 34% 21% 8% 4	447
	21st to end	Day	13% 8% 28% 28% 15% 9%	485
June	1st to 10th	Day	15% 8% 23% 30% 17% 7%	450
	11th to 20th	Day	12% 8% 27% 31% 17% 5%	610
	21st to end	Day	10% 7% 30% 29% 18% 7%	630
July	1st to 10th	Day	13% 5% 18% 26% 23% 16%	621
	11th to 20th	Day	9% 7% 23% 31% 22% 9%	519
	21st to end	Day	11% 3 20% 31% 22% 12%	488
August	1st to 10th	Day	11% 3 17% 30% 23% 15%	446
	11th to 20th	Day	13% 4 20% 32% 21% 10%	439
	21st to end	Day	9% 5% 18% 28% 26% 13%	495
September	1st to 10th	Day	14% 5% 19% 28% 26% 9%	450
	11th to 20th	Day	11% 3 18% 24% 31% 12%	450
	21st to end	Day	18% 9% 22% 25% 19% 8%	450
October	1st to 10th	Day	12% 8% 28% 30% 17% 6%	450
	11th to 20th	Day	18% 16% 31% 23% 10% 2	450
	21st to end	Day	26% 14% 38% 19% 4	363
November	1st to 10th	Day	15% 19% 33% 21% 9% 3	270
	11th to 20th	Day	21% 24% 27% 19% 6% 1	270
	21st to end	Day	26% 18% 37% 15% 3	270
December	1st to 10th	Day	25% 23% 33% 13% 6%	270
	11th to 20th	Day	22% 23% 39% 11% 4	270
	21st to end	Day	33% 22% 32% 10% 2 1	297
January	1st to 10th	Night	24% 20% 31% 21% 4	949
	11th to 20th	Night	30% 18% 32% 17% 3	760
	21st to end	Night	26% 18% 33% 20% 3	836
February	1st to 10th	Night	29% 13% 32% 23% 2	760
	11th to 20th	Night	25% 20% 32% 21% 2	743
	21st to end	Night	29% 18% 33% 19% 1	628
March	1st to 10th	Night	23% 18% 32% 22% 5%	760
	11th to 20th	Night	25% 15% 32% 22% 6% 1	752
	21st to end	Night	25% 13% 30% 26% 7%	792
April	1st to 10th	Night	19% 14% 34% 25% 8%	594
	11th to 20th	Night	20% 10% 33% 25% 11% 1	533
	21st to end	Night	20% 11% 37% 23% 9% 1	540
May	1st to 10th	Night	22% 11% 30% 24% 11% 1	540
	11th to 20th	Night	18% 6% 26% 32% 17% 2	531
	21st to end	Night	10% 3 23% 34% 26% 4	584
June	1st to 10th	Night	10% 4 21% 41% 22% 2	540
	11th to 20th	Night	10% 3 17% 41% 25% 3	540
	21st to end	Night	7% 2 26% 39% 24% 3	540
July	1st to 10th	Night	13% 2 14% 29% 33% 9%	525
	11th to 20th	Night	8% 2 17% 34% 35% 3	522
	21st to end	Night	11% 2 14% 36% 34% 4	591
August	1st to 10th	Night	9% 1 14% 37% 36% 3	537
	11th to 20th	Night	9% 2 15% 38% 30% 6%	534
	21st to end	Night	6% 1 15% 34% 37% 6%	594
September	1st to 10th	Night	14% 2 17% 31% 32% 4	634
	11th to 20th	Night	10% 3 17% 31% 33% 5%	720
	21st to end	Night	15% 6% 22% 31% 24% 3	720
October	1st to 10th	Night	11% 6% 23% 35% 22% 4	720
	11th to 20th	Night	19% 10% 26% 31% 13% 1	720
	21st to end	Night	24% 12% 31% 23% 10% 1	792
November	1st to 10th	Night	17% 11% 29% 27% 16% 1	740
	11th to 20th	Night	21% 17% 28% 24% 10% 1	760
	21st to end	Night	26% 12% 27% 28% 6% 1	760
December	1st to 10th	Night	26% 15% 31% 23% 5%	760
	11th to 20th	Night	26% 13% 32% 23% 6% 1	760
	21st to end	Night	31% 16% 30% 20% 3	864

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