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 2025-05-11.

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 4 4 5% 7% 39%	114018
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	114953
Transparency	Day	42% 6% 10% 23% 16% 2	117716
	Night	42% 5% 10% 22% 18% 3	119384
Seeing	Day	16% 6% 22% 25% 20% 10%	15898
	Night	17% 5% 24% 33% 19% 3	25714

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	14% 12% 8% 9% 4 4 4 4 6% 6% 29%	3617
	11th to 20th	Day	14% 10% 4 14% 5% 4 4 3 4 4 35%	3793
	21st to end	Day	20% 10% 6% 5% 4 3 4 2 5% 6% 34%	4604
February	1st to 10th	Day	16% 13% 4 8% 5% 3 4 3 4 5% 35%	2000
	11th to 20th	Day	20% 9% 4 8% 5% 3 4 3 5% 6% 33%	1948
	21st to end	Day	17% 11% 3 8% 5% 4 5% 3 4 5% 34%	1650
March	1st to 10th	Day	18% 12% 4 7% 4 4 3 3 3 5% 36%	4480
	11th to 20th	Day	13% 10% 2 6% 4 4 4 3 4 6% 44%	4860
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 5% 7% 40%	5615
April	1st to 10th	Day	14% 7% 2 7% 4 5% 5% 4 6% 8% 40%	2400
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 5% 6% 41%	2374
	21st to end	Day	16% 8% 2 7% 5% 4 5% 3 5% 7% 38%	2538
May	1st to 10th	Day	20% 7% 3 6% 5% 5% 5% 5% 5% 8% 31%	4212
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 5% 6% 39%	4081
	21st to end	Day	11% 6% 3 7% 5% 6% 5% 4 6% 8% 39%	4476
June	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 4 6% 8% 41%	2497
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 8% 10% 45%	2660
	21st to end	Day	7% 4 2 5% 5% 4 5% 5% 8% 10% 43%	2660
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	4440
	11th to 20th	Day	7% 4 2 7% 5% 5% 5% 4 7% 10% 44%	4354
	21st to end	Day	14% 11% 4 10% 6% 6% 6% 5% 7% 7% 25%	4466
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 7% 10% 50%	4104
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 9% 39%	3769
	21st to end	Day	5% 3 2 5% 3 5% 4 4 6% 8% 55%	4180
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 5% 7% 43%	2218
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 6% 8% 45%	2090
	21st to end	Day	13% 5% 2 5% 4 4 4 3 6% 7% 47%	2090
October	1st to 10th	Day	9% 5% 2 4 4 2 4 3 5% 6% 56%	2090
	11th to 20th	Day	13% 6% 2 5% 2 3 4 4 5% 6% 51%	2026
	21st to end	Day	19% 5% 2 5% 4 3 3 3 4 5% 45%	1881
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 4 6% 53%	1762
	11th to 20th	Day	15% 8% 3 5% 4 3 4 2 5% 5% 45%	1800
	21st to end	Day	17% 9% 4 5% 4 3 4 3 5% 6% 41%	1800
December	1st to 10th	Day	17% 9% 3 7% 4 3 4 3 4 4 40%	2930
	11th to 20th	Day	17% 12% 5% 10% 4 5% 5% 4 4 5% 31%	3373
	21st to end	Day	19% 15% 5% 10% 5% 4 4 3 5% 6% 25%	4180
January	1st to 10th	Night	15% 15% 5% 12% 4 4 4 3 4 6% 28%	6348
	11th to 20th	Night	18% 12% 5% 9% 5% 6% 4 2 3 4 33%	6159
	21st to end	Night	19% 11% 3 7% 4 3 3 2 4 5% 39%	6334
February	1st to 10th	Night	22% 11% 3 8% 4 5% 4 4 5% 6% 28%	2400
	11th to 20th	Night	21% 8% 3 10% 5% 4 3 4 5% 4 34%	2338
	21st to end	Night	21% 10% 4 7% 4 4 4 3 5% 4 33%	1980
March	1st to 10th	Night	19% 14% 3 7% 5% 4 3 3 4 4 35%	5373
	11th to 20th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 48%	5085
	21st to end	Night	13% 8% 3 8% 6% 4 4 4 5% 6% 37%	5344
April	1st to 10th	Night	12% 6% 3 6% 5% 4 4 4 6% 7% 43%	2000
	11th to 20th	Night	16% 6% 3 7% 5% 3 3 3 5% 6% 43%	1975
	21st to end	Night	14% 7% 3 8% 5% 4 4 4 5% 8% 37%	1860
May	1st to 10th	Night	19% 9% 3 8% 5% 4 6% 4 5% 5% 32%	2903
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 4 6% 7% 40%	2818
	21st to end	Night	8% 5% 2 7% 5% 4 5% 4 8% 9% 42%	3079
June	1st to 10th	Night	9% 6% 1 5% 4 4 4 4 7% 9% 47%	1678
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 52%	1520
	21st to end	Night	3 4 1 5% 5% 4 4 5% 8% 11% 48%	1520
July	1st to 10th	Night	7% 4 2 6% 4 5% 4 4 7% 10% 47%	2530
	11th to 20th	Night	6% 4 1 4 4 3 4 4 7% 10% 53%	2610
	21st to end	Night	9% 8% 4 11% 6% 6% 6% 5% 7% 7% 29%	3087
August	1st to 10th	Night	6% 3 2 6% 3 3 3 3 6% 8% 56%	2825
	11th to 20th	Night	5% 4 2 5% 5% 4 5% 4 8% 9% 49%	3056
	21st to end	Night	4 3 1 3 2 2 4 3 5% 9% 63%	3476
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 51%	1943
	11th to 20th	Night	10% 6% 2 6% 4 4 3 3 5% 8% 49%	2090
	21st to end	Night	11% 4 2 5% 4 3 4 3 5% 6% 52%	2090
October	1st to 10th	Night	10% 4 1 4 3 3 3 2 5% 6% 59%	2090
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 4 6% 51%	2152
	21st to end	Night	16% 6% 2 5% 4 3 4 2 4 6% 47%	2716
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 5% 4 51%	2536
	11th to 20th	Night	14% 8% 2 6% 5% 3 3 3 5% 7% 45%	2600
	21st to end	Night	17% 9% 4 6% 4 3 3 4 5% 5% 42%	2600
December	1st to 10th	Night	17% 12% 3 7% 4 4 3 3 4 4 39%	4250
	11th to 20th	Night	20% 10% 3 7% 3 3 3 3 5% 6% 36%	4822
	21st to end	Night	19% 15% 5% 11% 6% 5% 4 3 4 5% 23%	6766

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	55% 5% 10% 21% 8% 1	3618
	11th to 20th	Day	54% 4 6% 19% 15% 2	3793
	21st to end	Day	52% 3 9% 20% 12% 3	4604
February	1st to 10th	Day	53% 3 9% 21% 13% 2	2000
	11th to 20th	Day	53% 5% 8% 20% 11% 3	1958
	21st to end	Day	52% 4 8% 17% 14% 4	1650
March	1st to 10th	Day	51% 4 5% 16% 18% 5%	4480
	11th to 20th	Day	40% 5% 8% 25% 20% 2	4860
	21st to end	Day	43% 6% 11% 27% 13% 1	5617
April	1st to 10th	Day	41% 7% 11% 22% 17% 2	2400
	11th to 20th	Day	44% 5% 9% 22% 18% 3	2374
	21st to end	Day	46% 5% 10% 25% 12% 2	2538
May	1st to 10th	Day	49% 8% 14% 19% 9%	4250
	11th to 20th	Day	46% 7% 10% 23% 13%	4085
	21st to end	Day	40% 9% 12% 18% 20% 2	4489
June	1st to 10th	Day	39% 8% 10% 24% 17% 2	2497
	11th to 20th	Day	30% 9% 15% 25% 18% 2	2660
	21st to end	Day	34% 9% 15% 21% 18% 4	2660
July	1st to 10th	Day	39% 8% 14% 23% 13% 2	4371
	11th to 20th	Day	32% 9% 14% 31% 12% 2	4279
	21st to end	Day	55% 8% 12% 16% 7% 1	4324
August	1st to 10th	Day	26% 8% 16% 34% 16% 1	4091
	11th to 20th	Day	36% 9% 13% 25% 16% 1	3806
	21st to end	Day	26% 6% 11% 37% 16% 3	4179
September	1st to 10th	Day	40% 7% 11% 19% 18% 4	2218
	11th to 20th	Day	36% 6% 11% 23% 22% 3	2090
	21st to end	Day	35% 6% 9% 25% 21% 3	2090
October	1st to 10th	Day	20% 3 7% 23% 44% 3	3480
	11th to 20th	Day	23% 5% 10% 26% 33% 4	3332
	21st to end	Day	31% 3 7% 29% 22% 8%	3081
November	1st to 10th	Day	34% 3 9% 27% 22% 5%	1762
	11th to 20th	Day	42% 3 7% 22% 20% 5%	1800
	21st to end	Day	45% 4 9% 23% 18% 2	1800
December	1st to 10th	Day	47% 3 8% 23% 18% 1	2930
	11th to 20th	Day	57% 4 8% 17% 13% 2	3370
	21st to end	Day	59% 6% 11% 17% 6%	4180
January	1st to 10th	Night	56% 5% 9% 20% 9% 1	6348
	11th to 20th	Night	56% 4 6% 17% 13% 3	6159
	21st to end	Night	49% 3 8% 19% 19% 2	6334
February	1st to 10th	Night	56% 4 10% 17% 12% 1	2400
	11th to 20th	Night	53% 4 7% 20% 14% 2	2339
	21st to end	Night	53% 4 10% 17% 13% 3	1980
March	1st to 10th	Night	53% 4 6% 19% 14% 4	5373
	11th to 20th	Night	37% 5% 8% 25% 20% 6%	5085
	21st to end	Night	46% 6% 10% 23% 13% 1	5344
April	1st to 10th	Night	39% 6% 11% 23% 18% 3	2000
	11th to 20th	Night	42% 4 9% 19% 20% 5%	1975
	21st to end	Night	44% 7% 9% 23% 14% 2	1860
May	1st to 10th	Night	51% 8% 10% 23% 7% 2	2930
	11th to 20th	Night	41% 8% 12% 25% 12% 1	2819
	21st to end	Night	34% 10% 15% 22% 17% 3	3087
June	1st to 10th	Night	32% 7% 13% 26% 18% 4	1678
	11th to 20th	Night	27% 7% 15% 24% 23% 4	1520
	21st to end	Night	27% 10% 17% 21% 19% 6%	1520
July	1st to 10th	Night	32% 8% 13% 29% 15% 2	2490
	11th to 20th	Night	26% 8% 14% 36% 14% 3	2562
	21st to end	Night	49% 9% 13% 18% 9% 1	2988
August	1st to 10th	Night	24% 7% 15% 36% 17% 1	2814
	11th to 20th	Night	28% 9% 16% 29% 17% 1	3085
	21st to end	Night	18% 6% 13% 36% 23% 4	3476
September	1st to 10th	Night	32% 6% 11% 22% 23% 5%	1943
	11th to 20th	Night	33% 6% 10% 25% 23% 4	2090
	21st to end	Night	32% 6% 8% 24% 24% 6%	2090
October	1st to 10th	Night	19% 4 6% 19% 47% 5%	3480
	11th to 20th	Night	23% 3 7% 24% 36% 6%	3573
	21st to end	Night	29% 4 7% 21% 31% 9%	4466
November	1st to 10th	Night	36% 3 7% 25% 24% 5%	2536
	11th to 20th	Night	39% 4 7% 21% 21% 7%	2600
	21st to end	Night	45% 4 7% 24% 18% 2	2600
December	1st to 10th	Night	50% 4 9% 20% 17% 1	4250
	11th to 20th	Night	49% 4 10% 19% 16% 2	4824
	21st to end	Night	64% 5% 8% 16% 6% 1	6766

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	23% 15% 33% 18% 9% 1	300
	11th to 20th	Day	23% 16% 37% 18% 5%	300
	21st to end	Day	23% 12% 43% 15% 6%	330
February	1st to 10th	Day	24% 12% 41% 19% 4 1	300
	11th to 20th	Day	29% 16% 36% 16% 4 1	300
	21st to end	Day	26% 12% 42% 16% 4	249
March	1st to 10th	Day	21% 15% 37% 19% 7% 1	300
	11th to 20th	Day	21% 13% 38% 20% 6% 2	445
	21st to end	Day	23% 14% 35% 20% 6% 1	550
April	1st to 10th	Day	19% 13% 28% 27% 11% 2	500
	11th to 20th	Day	18% 11% 30% 24% 13% 4	492
	21st to end	Day	21% 7% 29% 28% 12% 4	500
May	1st to 10th	Day	20% 7% 28% 27% 13% 6%	503
	11th to 20th	Day	17% 3 23% 29% 19% 9%	487
	21st to end	Day	13% 2 17% 29% 22% 18%	529
June	1st to 10th	Day	14% 2 15% 26% 26% 17%	517
	11th to 20th	Day	7% 2 14% 32% 31% 15%	680
	21st to end	Day	9% 3 17% 30% 26% 15%	680
July	1st to 10th	Day	12% 1 10% 22% 31% 23%	671
	11th to 20th	Day	9% 1 10% 30% 29% 20%	621
	21st to end	Day	12% 2 6% 25% 33% 22%	532
August	1st to 10th	Day	11% 2 7% 22% 34% 24%	486
	11th to 20th	Day	12% 1 9% 27% 36% 15%	479
	21st to end	Day	10% 1 6% 21% 35% 27%	539
September	1st to 10th	Day	13% 1 8% 23% 37% 18%	490
	11th to 20th	Day	13% 1 8% 22% 36% 19%	490
	21st to end	Day	17% 3 17% 23% 26% 14%	490
October	1st to 10th	Day	11% 2 13% 34% 30% 10%	490
	11th to 20th	Day	13% 7% 29% 28% 15% 8%	488
	21st to end	Day	23% 8% 25% 27% 15% 1	330
November	1st to 10th	Day	18% 7% 25% 32% 14% 4	300
	11th to 20th	Day	20% 10% 31% 28% 9% 1	300
	21st to end	Day	24% 7% 30% 26% 12% 1	300
December	1st to 10th	Day	20% 16% 30% 24% 8% 2	300
	11th to 20th	Day	20% 13% 30% 26% 10% 1	300
	21st to end	Day	27% 15% 29% 22% 6% 1	330
January	1st to 10th	Night	20% 9% 35% 30% 6%	1000
	11th to 20th	Night	25% 9% 35% 25% 5%	916
	21st to end	Night	22% 8% 38% 30% 2	880
February	1st to 10th	Night	27% 6% 35% 28% 3	800
	11th to 20th	Night	25% 8% 40% 25% 2	783
	21st to end	Night	26% 8% 38% 25% 3	660
March	1st to 10th	Night	21% 8% 35% 29% 7%	800
	11th to 20th	Night	22% 7% 33% 30% 7% 1	800
	21st to end	Night	19% 8% 30% 34% 8%	834
April	1st to 10th	Night	15% 6% 35% 33% 10% 1	600
	11th to 20th	Night	19% 5% 31% 32% 13% 1	593
	21st to end	Night	18% 5% 28% 38% 11% 1	600
May	1st to 10th	Night	22% 2 26% 34% 16% 2	600
	11th to 20th	Night	13% 2 16% 42% 22% 4	563
	21st to end	Night	6% 2 14% 40% 32% 7%	617
June	1st to 10th	Night	12% 1 11% 39% 34% 4	570
	11th to 20th	Night	7% 1 10% 41% 37% 4	570
	21st to end	Night	4 2 11% 44% 33% 6%	570
July	1st to 10th	Night	11% 1 5% 29% 43% 10%	555
	11th to 20th	Night	10% 1 5% 31% 45% 7%	552
	21st to end	Night	11% 5% 37% 40% 7%	624
August	1st to 10th	Night	8% 1 4 31% 50% 7%	567
	11th to 20th	Night	7% 1 5% 36% 44% 7%	564
	21st to end	Night	7% 1 3 30% 50% 9%	627
September	1st to 10th	Night	13% 1 5% 35% 40% 6%	613
	11th to 20th	Night	12% 2 7% 33% 41% 6%	760
	21st to end	Night	13% 2 16% 31% 34% 5%	760
October	1st to 10th	Night	11% 2 13% 36% 31% 6%	760
	11th to 20th	Night	16% 3 22% 37% 20% 2	760
	21st to end	Night	18% 4 28% 34% 14% 1	836
November	1st to 10th	Night	13% 5% 27% 37% 16% 2	780
	11th to 20th	Night	16% 6% 32% 33% 12%	800
	21st to end	Night	21% 6% 29% 33% 10% 1	800
December	1st to 10th	Night	26% 5% 30% 32% 7% 1	800
	11th to 20th	Night	26% 4 31% 31% 8%	800
	21st to end	Night	28% 9% 28% 31% 4	1000

Page generated at 2025-05-08 09:33:55. Version ac07

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