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 2024-01-12.

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	16% 9% 3 6% 6% 3 5% 5% 5% 7% 36%	74864
	Night	15% 9% 3 6% 5% 3 4 4 5% 7% 38%	74847
Transparency	Day	46% 7% 12% 20% 13% 2	74587
	Night	45% 6% 11% 21% 14% 2	74643
Seeing	Day	21% 14% 26% 21% 13% 5%	13681
	Night	20% 11% 25% 27% 15% 2	23816

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% 13% 5% 10% 7% 3 5% 4 4 5% 25%	1706
	11th to 20th	Day	21% 15% 7% 8% 7% 3 4 4 5% 4 23%	1629
	21st to end	Day	19% 15% 6% 8% 6% 3 4 4 4 5% 26%	1782
February	1st to 10th	Day	22% 12% 5% 9% 6% 2 3 4 4 4 27%	1660
	11th to 20th	Day	19% 12% 4 9% 6% 3 5% 4 4 6% 27%	1748
	21st to end	Day	21% 13% 5% 6% 6% 3 3 3 4 5% 31%	1480
March	1st to 10th	Day	18% 11% 4 6% 5% 3 3 4 4 7% 35%	1860
	11th to 20th	Day	20% 9% 4 8% 6% 3 4 4 5% 5% 32%	1980
	21st to end	Day	21% 11% 4 6% 6% 3 5% 3 4 5% 31%	2178
April	1st to 10th	Day	14% 10% 4 5% 5% 3 4 4 4 7% 38%	2042
	11th to 20th	Day	19% 8% 3 6% 5% 2 3 3 4 7% 40%	2226
	21st to end	Day	19% 8% 3 7% 6% 4 4 3 6% 7% 34%	2340
May	1st to 10th	Day	21% 7% 3 6% 5% 4 4 4 5% 6% 35%	2340
	11th to 20th	Day	20% 9% 3 5% 6% 3 5% 5% 5% 6% 32%	2301
	21st to end	Day	9% 6% 3 6% 6% 5% 5% 7% 7% 10% 37%	2531
June	1st to 10th	Day	12% 5% 2 5% 5% 3 6% 6% 6% 9% 40%	2340
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 6% 6% 8% 42%	2340
	21st to end	Day	10% 5% 3 5% 5% 4 6% 6% 7% 9% 40%	2340
July	1st to 10th	Day	13% 5% 2 6% 6% 3 6% 5% 6% 8% 38%	2340
	11th to 20th	Day	10% 6% 3 4 6% 3 7% 6% 7% 10% 37%	2340
	21st to end	Day	12% 4 3 5% 5% 4 6% 6% 7% 10% 39%	2547
August	1st to 10th	Day	10% 5% 3 5% 5% 4 5% 6% 9% 10% 40%	2327
	11th to 20th	Day	9% 6% 2 4 7% 5% 6% 6% 8% 10% 37%	2303
	21st to end	Day	8% 5% 2 4 6% 3 5% 6% 6% 8% 48%	2466
September	1st to 10th	Day	15% 5% 2 4 5% 3 4 5% 5% 6% 46%	1998
	11th to 20th	Day	10% 4 2 5% 5% 2 5% 5% 6% 7% 48%	1980
	21st to end	Day	17% 6% 2 4 5% 2 3 4 5% 6% 46%	1980
October	1st to 10th	Day	13% 5% 2 4 5% 3 4 5% 5% 6% 48%	1963
	11th to 20th	Day	16% 8% 2 5% 4 3 4 4 3 6% 46%	1800
	21st to end	Day	22% 9% 3 5% 5% 2 3 3 4 6% 39%	1918
November	1st to 10th	Day	12% 6% 3 4 4 2 4 4 5% 6% 51%	1672
	11th to 20th	Day	15% 11% 5% 6% 4 2 4 4 4 6% 38%	1710
	21st to end	Day	18% 10% 4 6% 5% 2 4 4 4 6% 37%	1710
December	1st to 10th	Day	23% 14% 5% 7% 6% 3 4 4 3 5% 25%	1710
	11th to 20th	Day	17% 14% 6% 8% 6% 3 4 5% 5% 5% 27%	2153
	21st to end	Day	20% 20% 6% 9% 7% 3 4 4 4 5% 19%	3124
January	1st to 10th	Night	18% 14% 5% 11% 7% 3 5% 5% 4 4 25%	2459
	11th to 20th	Night	21% 16% 5% 10% 6% 3 3 4 4 5% 24%	2354
	21st to end	Night	18% 13% 5% 9% 6% 3 4 4 4 5% 29%	2574
February	1st to 10th	Night	22% 15% 5% 8% 5% 3 3 4 4 5% 26%	2295
	11th to 20th	Night	21% 11% 4 8% 6% 3 4 3 4 5% 30%	2098
	21st to end	Night	23% 15% 4 7% 5% 3 3 4 3 5% 29%	1776
March	1st to 10th	Night	17% 9% 4 6% 5% 3 4 4 5% 5% 37%	2098
	11th to 20th	Night	19% 9% 4 7% 5% 3 4 4 5% 7% 33%	1980
	21st to end	Night	18% 9% 4 7% 6% 3 4 4 5% 7% 34%	2178
April	1st to 10th	Night	14% 7% 3 5% 6% 3 4 5% 7% 7% 39%	1931
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 5% 6% 7% 40%	1681
	21st to end	Night	17% 7% 3 6% 6% 3 5% 5% 5% 7% 37%	1620
May	1st to 10th	Night	18% 8% 3 6% 4 3 4 4 5% 8% 36%	1620
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 4 7% 9% 36%	1592
	21st to end	Night	8% 5% 3 4 5% 3 5% 6% 8% 12% 42%	1744
June	1st to 10th	Night	6% 4 2 4 5% 4 6% 5% 6% 10% 47%	1620
	11th to 20th	Night	7% 3 2 5% 5% 3 4 6% 6% 11% 47%	1620
	21st to end	Night	9% 4 2 3 4 3 5% 6% 7% 11% 45%	1620
July	1st to 10th	Night	10% 5% 3 5% 5% 3 5% 5% 5% 9% 45%	1620
	11th to 20th	Night	6% 4 2 4 5% 3 4 6% 7% 10% 49%	1620
	21st to end	Night	7% 5% 2 4 5% 4 6% 5% 8% 8% 47%	1771
August	1st to 10th	Night	7% 4 2 5% 6% 3 6% 5% 7% 10% 46%	1611
	11th to 20th	Night	8% 5% 2 5% 6% 3 5% 6% 6% 10% 45%	1598
	21st to end	Night	7% 4 2 4 3 3 3 5% 6% 10% 53%	1886
September	1st to 10th	Night	11% 6% 2 5% 4 2 3 4 5% 7% 51%	1942
	11th to 20th	Night	9% 4 2 4 4 3 4 4 5% 8% 53%	1980
	21st to end	Night	13% 5% 2 5% 5% 2 4 5% 5% 5% 49%	1980
October	1st to 10th	Night	9% 5% 2 4 4 3 4 4 5% 8% 52%	1995
	11th to 20th	Night	16% 7% 2 4 4 3 4 4 6% 7% 44%	2160
	21st to end	Night	21% 8% 2 4 4 3 5% 3 4 6% 39%	2434
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 4 4 6% 49%	2406
	11th to 20th	Night	16% 9% 3 6% 5% 3 4 4 4 6% 40%	2470
	21st to end	Night	19% 11% 4 6% 5% 3 4 4 4 6% 35%	2470
December	1st to 10th	Night	23% 12% 5% 8% 5% 3 4 3 4 7% 27%	2470
	11th to 20th	Night	18% 12% 6% 8% 5% 2 4 3 4 6% 30%	3042
	21st to end	Night	20% 19% 6% 8% 7% 4 4 4 4 5% 18%	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	60% 5% 10% 15% 10%	1707
	11th to 20th	Day	65% 5% 9% 15% 5%	1629
	21st to end	Day	60% 5% 12% 16% 6% 1	1782
February	1st to 10th	Day	60% 5% 11% 15% 7% 1	1660
	11th to 20th	Day	58% 5% 10% 17% 8% 1	1758
	21st to end	Day	56% 5% 8% 20% 9% 2	1480
March	1st to 10th	Day	50% 4 10% 25% 10% 1	1860
	11th to 20th	Day	54% 7% 9% 19% 11% 2	1980
	21st to end	Day	56% 5% 8% 19% 10% 2	2178
April	1st to 10th	Day	45% 6% 12% 24% 12% 2	2042
	11th to 20th	Day	45% 5% 10% 20% 15% 4	2226
	21st to end	Day	50% 7% 11% 20% 10% 3	2340
May	1st to 10th	Day	50% 6% 12% 21% 9% 2	2340
	11th to 20th	Day	51% 8% 11% 17% 11% 2	2305
	21st to end	Day	40% 11% 16% 20% 13% 1	2531
June	1st to 10th	Day	38% 9% 13% 24% 15% 2	2340
	11th to 20th	Day	38% 10% 15% 21% 16% 1	2340
	21st to end	Day	39% 10% 14% 22% 13% 3	2340
July	1st to 10th	Day	43% 8% 12% 18% 16% 2	2275
	11th to 20th	Day	39% 10% 14% 20% 16% 1	2271
	21st to end	Day	38% 9% 15% 22% 13% 3	2405
August	1st to 10th	Day	36% 10% 16% 25% 13% 1	2314
	11th to 20th	Day	38% 9% 15% 25% 12% 1	2303
	21st to end	Day	32% 9% 15% 23% 17% 5%	2466
September	1st to 10th	Day	37% 7% 12% 22% 17% 5%	1998
	11th to 20th	Day	33% 7% 13% 22% 23% 2	1980
	21st to end	Day	39% 5% 9% 24% 20% 2	1980
October	1st to 10th	Day	35% 7% 12% 23% 21% 2	1963
	11th to 20th	Day	40% 6% 8% 27% 18% 2	1800
	21st to end	Day	48% 4 10% 22% 15% 2	1918
November	1st to 10th	Day	35% 5% 11% 27% 20% 3	1672
	11th to 20th	Day	47% 4 9% 20% 16% 4	1710
	21st to end	Day	49% 5% 11% 21% 14% 1	1710
December	1st to 10th	Day	62% 5% 10% 17% 5%	1710
	11th to 20th	Day	57% 7% 10% 16% 11%	2150
	21st to end	Day	67% 7% 9% 11% 6% 1	3124
January	1st to 10th	Night	63% 5% 8% 15% 9% 1	2459
	11th to 20th	Night	64% 5% 9% 15% 6% 1	2354
	21st to end	Night	58% 4 10% 19% 7% 2	2574
February	1st to 10th	Night	60% 5% 10% 17% 6% 1	2295
	11th to 20th	Night	57% 4 9% 18% 11% 1	2099
	21st to end	Night	59% 4 11% 15% 9% 3	1776
March	1st to 10th	Night	48% 5% 9% 21% 15% 2	2098
	11th to 20th	Night	51% 6% 10% 20% 11% 1	1980
	21st to end	Night	51% 6% 11% 21% 10% 2	2178
April	1st to 10th	Night	42% 6% 12% 23% 14% 2	1931
	11th to 20th	Night	42% 7% 10% 18% 18% 6%	1681
	21st to end	Night	46% 7% 11% 20% 13% 3	1620
May	1st to 10th	Night	46% 7% 12% 21% 10% 3	1620
	11th to 20th	Night	44% 9% 14% 18% 13% 2	1593
	21st to end	Night	32% 11% 18% 21% 15% 3	1745
June	1st to 10th	Night	31% 9% 14% 24% 19% 2	1620
	11th to 20th	Night	30% 9% 16% 20% 21% 4	1620
	21st to end	Night	30% 10% 18% 23% 16% 4	1620
July	1st to 10th	Night	36% 8% 13% 22% 17% 4	1575
	11th to 20th	Night	28% 10% 17% 23% 20% 2	1566
	21st to end	Night	31% 10% 15% 27% 15% 2	1672
August	1st to 10th	Night	30% 10% 17% 27% 14% 3	1600
	11th to 20th	Night	33% 11% 14% 24% 17% 1	1598
	21st to end	Night	26% 8% 16% 25% 20% 5%	1886
September	1st to 10th	Night	32% 7% 14% 23% 17% 7%	1942
	11th to 20th	Night	29% 8% 13% 22% 26% 3	1980
	21st to end	Night	35% 7% 9% 21% 27% 1	1980
October	1st to 10th	Night	30% 6% 13% 26% 22% 4	1995
	11th to 20th	Night	38% 5% 10% 25% 19% 2	2160
	21st to end	Night	46% 5% 9% 21% 15% 3	2434
November	1st to 10th	Night	37% 3 11% 23% 22% 4	2406
	11th to 20th	Night	47% 4 8% 20% 18% 4	2470
	21st to end	Night	51% 5% 9% 21% 12% 2	2470
December	1st to 10th	Night	59% 4 10% 19% 7% 1	2470
	11th to 20th	Night	55% 5% 11% 21% 8% 1	3044
	21st to end	Night	66% 7% 8% 13% 5%	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	30% 26% 29% 12% 3	270
	11th to 20th	Day	36% 23% 33% 8% 1	243
	21st to end	Day	34% 27% 26% 11% 2	264
February	1st to 10th	Day	36% 24% 28% 10% 2	240
	11th to 20th	Day	38% 30% 28% 3	240
	21st to end	Day	36% 27% 31% 5% 1	198
March	1st to 10th	Day	31% 24% 29% 12% 3	300
	11th to 20th	Day	29% 22% 32% 13% 5%	420
	21st to end	Day	34% 20% 26% 16% 4	462
April	1st to 10th	Day	22% 21% 29% 19% 9% 1	420
	11th to 20th	Day	26% 22% 23% 17% 10% 2	412
	21st to end	Day	29% 18% 32% 13% 6% 2	420
May	1st to 10th	Day	27% 17% 30% 15% 8% 2	423
	11th to 20th	Day	23% 10% 35% 20% 8% 4	447
	21st to end	Day	11% 11% 27% 28% 17% 6%	485
June	1st to 10th	Day	13% 10% 25% 29% 17% 7%	450
	11th to 20th	Day	14% 9% 26% 29% 15% 7%	450
	21st to end	Day	11% 11% 31% 22% 16% 10%	450
July	1st to 10th	Day	15% 6% 14% 23% 20% 21%	445
	11th to 20th	Day	9% 7% 23% 28% 21% 11%	444
	21st to end	Day	13% 4 21% 29% 20% 13%	488
August	1st to 10th	Day	12% 4 18% 29% 24% 13%	446
	11th to 20th	Day	11% 6% 21% 31% 22% 9%	439
	21st to end	Day	8% 5% 22% 29% 23% 12%	495
September	1st to 10th	Day	13% 6% 19% 26% 26% 10%	450
	11th to 20th	Day	10% 3 18% 29% 30% 10%	450
	21st to end	Day	21% 9% 20% 23% 23% 4	450
October	1st to 10th	Day	13% 8% 27% 29% 16% 7%	450
	11th to 20th	Day	18% 16% 32% 21% 10% 3	450
	21st to end	Day	29% 14% 32% 20% 5%	433
November	1st to 10th	Day	17% 19% 31% 25% 7% 1	270
	11th to 20th	Day	30% 17% 30% 17% 5% 1	270
	21st to end	Day	24% 22% 34% 14% 5%	270
December	1st to 10th	Day	33% 21% 30% 10% 6%	270
	11th to 20th	Day	26% 24% 33% 13% 4	270
	21st to end	Day	31% 26% 29% 11% 3	297
January	1st to 10th	Night	30% 20% 28% 18% 4	760
	11th to 20th	Night	31% 20% 29% 17% 3	725
	21st to end	Night	26% 20% 32% 20% 2	792
February	1st to 10th	Night	32% 17% 29% 20% 2	720
	11th to 20th	Night	32% 19% 30% 18% 1	703
	21st to end	Night	31% 19% 33% 16% 1	592
March	1st to 10th	Night	24% 19% 29% 23% 4	720
	11th to 20th	Night	26% 17% 32% 18% 7%	720
	21st to end	Night	25% 14% 30% 24% 7%	792
April	1st to 10th	Night	21% 15% 32% 25% 7%	671
	11th to 20th	Night	17% 14% 32% 23% 12% 1	533
	21st to end	Night	22% 9% 33% 28% 6%	540
May	1st to 10th	Night	22% 10% 30% 25% 11% 1	540
	11th to 20th	Night	18% 8% 27% 31% 17% 1	531
	21st to end	Night	9% 5% 21% 32% 29% 3	584
June	1st to 10th	Night	7% 4 23% 41% 23% 2	540
	11th to 20th	Night	8% 4 20% 38% 27% 3	540
	21st to end	Night	9% 3 23% 35% 26% 3	540
July	1st to 10th	Night	12% 3 14% 31% 29% 10%	525
	11th to 20th	Night	6% 3 15% 35% 37% 4	522
	21st to end	Night	9% 2 14% 37% 34% 4	591
August	1st to 10th	Night	8% 2 10% 41% 36% 4	537
	11th to 20th	Night	7% 3 14% 37% 33% 7%	534
	21st to end	Night	8% 1 15% 34% 35% 7%	594
September	1st to 10th	Night	13% 3 14% 32% 32% 6%	682
	11th to 20th	Night	10% 4 15% 30% 35% 6%	720
	21st to end	Night	13% 7% 18% 35% 25% 2	720
October	1st to 10th	Night	11% 7% 23% 32% 24% 4	720
	11th to 20th	Night	20% 10% 26% 29% 13% 1	720
	21st to end	Night	24% 13% 26% 26% 10% 1	792
November	1st to 10th	Night	19% 11% 24% 27% 18% 1	740
	11th to 20th	Night	23% 17% 26% 24% 9% 1	760
	21st to end	Night	26% 12% 26% 26% 9% 1	760
December	1st to 10th	Night	29% 18% 27% 21% 5%	760
	11th to 20th	Night	26% 16% 29% 22% 6% 1	760
	21st to end	Night	29% 17% 29% 22% 3	836

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