Bluegrass Astro Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bluegrass Astro Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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% 8% 3 7% 5% 4 5% 4 6% 7% 38%	148679
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 40%	142481
Transparency	Day	42% 6% 11% 23% 15% 2	152333
	Night	42% 6% 11% 22% 17% 3	146889
Seeing	Day	16% 7% 22% 25% 20% 10%	17796
	Night	17% 5% 24% 33% 19% 3	27415

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	15% 12% 7% 9% 5% 4 4 3 5% 5% 30%	4691
	11th to 20th	Day	12% 9% 4 13% 4 3 4 2 3 4 40%	4935
	21st to end	Day	21% 10% 6% 6% 5% 4 4 3 4 5% 32%	5960
February	1st to 10th	Day	16% 13% 4 8% 5% 3 4 3 4 5% 35%	2100
	11th to 20th	Day	19% 9% 4 8% 5% 4 4 3 5% 6% 33%	2048
	21st to end	Day	17% 11% 3 8% 5% 4 5% 4 4 5% 34%	1730
March	1st to 10th	Day	19% 12% 4 7% 4 4 3 3 3 5% 36%	4580
	11th to 20th	Day	13% 9% 2 7% 4 4 4 3 4 7% 44%	4967
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 5% 7% 40%	5739
April	1st to 10th	Day	13% 6% 2 7% 4 4 5% 4 6% 8% 41%	2520
	11th to 20th	Day	14% 7% 3 7% 4 5% 4 4 5% 6% 41%	2494
	21st to end	Day	16% 7% 2 7% 5% 4 5% 3 5% 7% 39%	2665
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 6% 8% 31%	7616
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 6% 7% 38%	7626
	21st to end	Day	17% 8% 4 9% 5% 5% 5% 4 6% 8% 29%	8391
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 4 7% 8% 41%	2760
	11th to 20th	Day	6% 5% 2 5% 5% 5% 5% 5% 8% 10% 44%	2940
	21st to end	Day	8% 5% 2 5% 5% 4 5% 5% 8% 11% 42%	2940
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 4 7% 9% 38%	8280
	11th to 20th	Day	8% 6% 2 8% 5% 6% 6% 5% 8% 11% 34%	8124
	21st to end	Day	9% 8% 3 8% 6% 6% 7% 5% 8% 10% 30%	8340
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	6014
	11th to 20th	Day	7% 5% 2 7% 5% 5% 6% 6% 8% 10% 39%	5555
	21st to end	Day	4 3 1 4 3 4 4 3 5% 8% 61%	6080
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 5% 7% 44%	2452
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 6% 8% 45%	2233
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	9% 5% 2 4 4 2 4 3 4 6% 56%	2200
	11th to 20th	Day	12% 6% 2 5% 2 3 4 4 5% 6% 53%	2132
	21st to end	Day	20% 6% 2 6% 4 3 3 3 4 5% 44%	1980
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 7% 52%	1852
	11th to 20th	Day	15% 9% 3 5% 4 3 3 2 5% 5% 45%	1890
	21st to end	Day	16% 9% 4 6% 4 3 4 3 5% 6% 40%	1890
December	1st to 10th	Day	17% 9% 3 8% 4 3 3 3 4 5% 40%	3020
	11th to 20th	Day	17% 12% 5% 10% 4 5% 5% 4 4 5% 31%	3463
	21st to end	Day	19% 14% 5% 10% 5% 4 4 3 5% 6% 25%	4272
January	1st to 10th	Night	14% 14% 4 10% 4 4 4 3 4 6% 32%	8232
	11th to 20th	Night	16% 10% 4 9% 5% 5% 4 3 4 4 36%	8023
	21st to end	Night	18% 11% 3 8% 4 3 3 2 4 5% 37%	8203
February	1st to 10th	Night	21% 11% 3 8% 4 5% 5% 4 5% 6% 28%	2520
	11th to 20th	Night	21% 8% 3 9% 5% 4 3 4 5% 4 35%	2458
	21st to end	Night	21% 10% 4 8% 4 4 4 3 5% 4 34%	2076
March	1st to 10th	Night	19% 14% 3 7% 5% 4 3 3 4 5% 34%	5493
	11th to 20th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 48%	5198
	21st to end	Night	13% 8% 3 8% 6% 4 4 4 5% 6% 37%	5463
April	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 6% 7% 44%	2100
	11th to 20th	Night	15% 6% 3 7% 5% 3 4 3 5% 7% 42%	2075
	21st to end	Night	14% 7% 3 7% 5% 4 4 4 5% 8% 38%	1953
May	1st to 10th	Night	16% 9% 3 8% 6% 5% 5% 4 6% 7% 30%	5237
	11th to 20th	Night	12% 6% 3 8% 6% 6% 5% 4 6% 8% 36%	5234
	21st to end	Night	16% 8% 4 8% 5% 5% 5% 4 7% 8% 31%	5761
June	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 7% 9% 46%	1854
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 6% 9% 50%	1680
	21st to end	Night	4 4 1 5% 5% 4 5% 5% 8% 11% 47%	1680
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 4 4 6% 9% 46%	4708
	11th to 20th	Night	7% 6% 2 6% 5% 4 6% 5% 8% 10% 41%	4852
	21st to end	Night	7% 7% 3 9% 6% 6% 6% 5% 8% 10% 32%	5736
August	1st to 10th	Night	7% 5% 2 7% 4 4 4 4 6% 9% 48%	4135
	11th to 20th	Night	4 3 2 5% 4 4 5% 4 8% 10% 52%	4486
	21st to end	Night	3 3 1 3 2 2 4 3 5% 9% 65%	5046
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 5% 7% 52%	2147
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 49%	2234
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 7% 52%	2200
October	1st to 10th	Night	9% 4 1 4 3 3 3 3 5% 6% 60%	2200
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 4 6% 52%	2265
	21st to end	Night	17% 6% 2 5% 4 3 4 2 4 6% 47%	2859
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 5% 5% 51%	2666
	11th to 20th	Night	13% 8% 2 6% 5% 3 3 3 4 7% 46%	2730
	21st to end	Night	17% 9% 4 6% 4 3 2 3 5% 5% 41%	2730
December	1st to 10th	Night	17% 12% 3 7% 4 4 3 3 4 4 39%	4380
	11th to 20th	Night	20% 10% 3 7% 3 3 3 3 4 6% 37%	4952
	21st to end	Night	19% 15% 5% 11% 6% 5% 4 3 4 5% 23%	6915

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	56% 4 10% 21% 8% 1	4692
	11th to 20th	Day	48% 3 6% 23% 17% 2	4935
	21st to end	Day	55% 3 8% 21% 10% 3	5960
February	1st to 10th	Day	53% 3 9% 22% 12% 1	2100
	11th to 20th	Day	53% 5% 8% 21% 11% 3	2058
	21st to end	Day	52% 4 8% 18% 13% 4	1730
March	1st to 10th	Day	51% 4 5% 16% 18% 5%	4580
	11th to 20th	Day	41% 5% 8% 24% 20% 2	4967
	21st to end	Day	43% 6% 11% 27% 13% 1	5741
April	1st to 10th	Day	40% 7% 13% 22% 16% 2	2520
	11th to 20th	Day	43% 5% 10% 22% 17% 3	2494
	21st to end	Day	45% 5% 10% 25% 12% 2	2665
May	1st to 10th	Day	48% 8% 13% 21% 10% 1	7651
	11th to 20th	Day	44% 7% 12% 22% 14% 1	7630
	21st to end	Day	51% 8% 11% 15% 13% 2	8405
June	1st to 10th	Day	39% 8% 10% 23% 18% 2	2760
	11th to 20th	Day	32% 9% 15% 25% 17% 2	2940
	21st to end	Day	34% 9% 15% 22% 17% 4	2940
July	1st to 10th	Day	40% 8% 14% 24% 13% 1	8211
	11th to 20th	Day	39% 12% 16% 24% 9% 1	8049
	21st to end	Day	44% 10% 17% 20% 8% 1	8156
August	1st to 10th	Day	32% 9% 16% 28% 14% 1	6001
	11th to 20th	Day	35% 9% 17% 25% 13% 1	5592
	21st to end	Day	21% 5% 13% 41% 17% 2	6079
September	1st to 10th	Day	38% 7% 12% 20% 19% 4	2452
	11th to 20th	Day	35% 6% 10% 23% 22% 3	2233
	21st to end	Day	35% 6% 9% 24% 22% 4	2200
October	1st to 10th	Day	20% 3 7% 23% 43% 3	3590
	11th to 20th	Day	22% 4 10% 26% 33% 4	3438
	21st to end	Day	32% 3 7% 29% 22% 8%	3180
November	1st to 10th	Day	34% 4 9% 26% 22% 4	1852
	11th to 20th	Day	42% 3 7% 23% 19% 5%	1890
	21st to end	Day	46% 3 9% 23% 17% 2	1890
December	1st to 10th	Day	47% 3 8% 23% 18% 1	3020
	11th to 20th	Day	56% 4 8% 17% 13% 2	3460
	21st to end	Day	59% 6% 11% 17% 6%	4272
January	1st to 10th	Night	52% 5% 11% 23% 8% 1	8233
	11th to 20th	Night	51% 4 5% 19% 17% 4	8023
	21st to end	Night	51% 3 8% 19% 16% 2	8203
February	1st to 10th	Night	56% 5% 10% 17% 11% 1	2520
	11th to 20th	Night	53% 4 7% 21% 14% 2	2459
	21st to end	Night	53% 4 9% 17% 13% 3	2076
March	1st to 10th	Night	53% 4 6% 19% 14% 4	5493
	11th to 20th	Night	37% 5% 8% 25% 19% 5%	5198
	21st to end	Night	46% 6% 10% 23% 13% 1	5463
April	1st to 10th	Night	38% 6% 11% 23% 18% 3	2100
	11th to 20th	Night	42% 4 10% 19% 19% 5%	2075
	21st to end	Night	44% 7% 9% 24% 15% 2	1953
May	1st to 10th	Night	51% 7% 10% 21% 10% 1	5264
	11th to 20th	Night	45% 8% 12% 20% 15%	5235
	21st to end	Night	48% 8% 13% 19% 11% 1	5771
June	1st to 10th	Night	32% 7% 13% 25% 19% 4	1854
	11th to 20th	Night	29% 7% 14% 23% 23% 4	1680
	21st to end	Night	28% 10% 17% 21% 19% 5%	1680
July	1st to 10th	Night	34% 8% 13% 29% 15% 1	4670
	11th to 20th	Night	35% 11% 17% 26% 10% 1	4807
	21st to end	Night	42% 10% 17% 20% 10% 1	5606
August	1st to 10th	Night	31% 8% 15% 28% 16% 1	4124
	11th to 20th	Night	24% 9% 23% 29% 13% 1	4515
	21st to end	Night	17% 6% 12% 40% 22% 3	5046
September	1st to 10th	Night	31% 6% 11% 23% 24% 5%	2147
	11th to 20th	Night	33% 6% 10% 25% 23% 4	2234
	21st to end	Night	32% 6% 9% 23% 25% 6%	2200
October	1st to 10th	Night	19% 4 6% 19% 47% 5%	3590
	11th to 20th	Night	23% 3 7% 24% 36% 7%	3686
	21st to end	Night	30% 4 7% 21% 31% 8%	4609
November	1st to 10th	Night	36% 3 7% 25% 23% 5%	2666
	11th to 20th	Night	39% 4 7% 22% 21% 7%	2730
	21st to end	Night	45% 4 7% 24% 17% 2	2730
December	1st to 10th	Night	50% 4 9% 20% 16% 1	4380
	11th to 20th	Night	49% 4 10% 20% 15% 2	4954
	21st to end	Night	64% 5% 9% 15% 6% 1	6915

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	22% 15% 33% 19% 9% 1	330
	11th to 20th	Day	22% 18% 38% 17% 5%	330
	21st to end	Day	23% 13% 42% 15% 6%	363
February	1st to 10th	Day	22% 12% 42% 18% 5% 1	330
	11th to 20th	Day	28% 16% 36% 15% 4 1	330
	21st to end	Day	26% 12% 41% 16% 4	273
March	1st to 10th	Day	22% 15% 39% 17% 6% 1	330
	11th to 20th	Day	21% 14% 39% 19% 6% 2	482
	21st to end	Day	22% 14% 37% 20% 6% 1	594
April	1st to 10th	Day	17% 13% 29% 27% 11% 2	540
	11th to 20th	Day	17% 12% 30% 24% 13% 4	532
	21st to end	Day	20% 7% 29% 29% 11% 4	540
May	1st to 10th	Day	19% 7% 29% 27% 12% 5%	543
	11th to 20th	Day	16% 4 25% 29% 18% 8%	567
	21st to end	Day	15% 3 18% 29% 20% 15%	617
June	1st to 10th	Day	14% 2 16% 28% 25% 16%	600
	11th to 20th	Day	7% 3 16% 32% 29% 14%	780
	21st to end	Day	9% 3 16% 29% 26% 17%	780
July	1st to 10th	Day	12% 2 9% 22% 32% 23%	771
	11th to 20th	Day	9% 2 10% 30% 30% 19%	716
	21st to end	Day	12% 2 7% 25% 33% 21%	620
August	1st to 10th	Day	11% 2 8% 22% 34% 23%	566
	11th to 20th	Day	11% 2 9% 27% 36% 15%	559
	21st to end	Day	8% 1 6% 24% 36% 25%	627
September	1st to 10th	Day	12% 1 9% 23% 36% 19%	570
	11th to 20th	Day	13% 1 8% 21% 37% 20%	542
	21st to end	Day	16% 3 18% 24% 26% 13%	530
October	1st to 10th	Day	11% 2 13% 33% 31% 10%	530
	11th to 20th	Day	12% 7% 28% 29% 16% 8%	528
	21st to end	Day	24% 10% 26% 26% 14% 1	363
November	1st to 10th	Day	18% 7% 27% 31% 13% 4	330
	11th to 20th	Day	21% 12% 30% 27% 9% 1	330
	21st to end	Day	23% 10% 30% 26% 11% 1	330
December	1st to 10th	Day	21% 15% 32% 23% 8% 2	330
	11th to 20th	Day	19% 14% 32% 25% 9% 1	330
	21st to end	Day	27% 16% 30% 21% 5% 1	363
January	1st to 10th	Night	20% 9% 35% 30% 5%	1050
	11th to 20th	Night	25% 10% 36% 25% 5%	962
	21st to end	Night	22% 9% 38% 29% 2	924
February	1st to 10th	Night	26% 6% 35% 29% 3	840
	11th to 20th	Night	25% 9% 39% 25% 2	823
	21st to end	Night	26% 8% 38% 25% 2	692
March	1st to 10th	Night	21% 8% 36% 28% 7%	840
	11th to 20th	Night	22% 8% 33% 30% 6% 1	840
	21st to end	Night	18% 8% 31% 34% 8%	876
April	1st to 10th	Night	15% 6% 35% 34% 10% 1	630
	11th to 20th	Night	19% 4 32% 31% 13% 1	623
	21st to end	Night	17% 5% 28% 38% 11% 1	630
May	1st to 10th	Night	21% 3 26% 34% 15% 2	630
	11th to 20th	Night	14% 3 17% 41% 21% 4	621
	21st to end	Night	9% 2 14% 39% 29% 6%	683
June	1st to 10th	Night	12% 1 11% 40% 32% 3	630
	11th to 20th	Night	8% 2 11% 40% 35% 4	630
	21st to end	Night	4 2 11% 43% 34% 6%	630
July	1st to 10th	Night	10% 1 6% 28% 45% 10%	615
	11th to 20th	Night	10% 1 5% 30% 46% 8%	612
	21st to end	Night	10% 5% 36% 40% 8%	690
August	1st to 10th	Night	7% 1 6% 30% 49% 7%	627
	11th to 20th	Night	7% 1 5% 35% 45% 7%	624
	21st to end	Night	6% 1 3 33% 48% 9%	693
September	1st to 10th	Night	12% 1 6% 35% 40% 6%	677
	11th to 20th	Night	12% 2 7% 32% 42% 6%	813
	21st to end	Night	13% 2 16% 32% 33% 5%	800
October	1st to 10th	Night	11% 2 13% 36% 32% 6%	800
	11th to 20th	Night	15% 3 22% 38% 20% 2	800
	21st to end	Night	19% 4 28% 34% 14% 1	880
November	1st to 10th	Night	13% 5% 27% 37% 15% 2	820
	11th to 20th	Night	15% 7% 32% 34% 12%	840
	21st to end	Night	22% 6% 28% 33% 10% 1	840
December	1st to 10th	Night	26% 5% 30% 32% 7%	840
	11th to 20th	Night	25% 5% 32% 30% 8%	840
	21st to end	Night	29% 9% 29% 30% 4	1050

Page generated at 2026-09-11 04:11:56. Version ac07

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