Beaver Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beaver Creek Observatory clear sky chart from 2005-11-06 to 2025-07-06.

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	14% 7% 2 7% 5% 4 5% 4 6% 8% 39%	120020
	Night	14% 7% 3 6% 5% 4 5% 4 6% 7% 40%	118762
Transparency	Day	41% 7% 12% 23% 15% 3	123657
	Night	40% 7% 12% 23% 15% 3	123262
Seeing	Day	19% 13% 26% 23% 14% 5%	15723
	Night	17% 11% 28% 30% 12% 1	25259

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% 7% 4 10% 5% 4 5% 4 4 6% 37%	3712
	11th to 20th	Day	12% 10% 4 9% 4 5% 5% 3 5% 6% 37%	4060
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 3 5% 6% 37%	4466
February	1st to 10th	Day	14% 11% 3 8% 5% 4 5% 3 5% 7% 35%	1800
	11th to 20th	Day	15% 9% 3 8% 6% 4 3 3 5% 6% 37%	1769
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 4 38%	1650
March	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 4 6% 40%	4907
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 6% 7% 45%	4980
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 5% 6% 38%	5509
April	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	2400
	11th to 20th	Day	13% 8% 2 6% 4 4 5% 3 5% 7% 42%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 6% 40%	2413
May	1st to 10th	Day	21% 7% 2 7% 5% 5% 5% 5% 8% 8% 29%	5862
	11th to 20th	Day	20% 7% 3 7% 5% 5% 5% 4 7% 9% 29%	6131
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 5% 6% 9% 35%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 5% 8% 9% 41%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 5% 6% 4 7% 9% 39%	2800
	21st to end	Day	10% 5% 1 6% 4 4 5% 4 7% 9% 45%	2800
July	1st to 10th	Day	6% 3 2 6% 5% 5% 7% 5% 8% 10% 43%	4510
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 4 7% 10% 43%	4293
	21st to end	Day	11% 8% 2 7% 6% 5% 5% 4 7% 8% 36%	4466
August	1st to 10th	Day	14% 7% 3 7% 5% 6% 6% 4 7% 9% 34%	4104
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	3930
	21st to end	Day	8% 5% 2 7% 5% 4 4 4 5% 8% 47%	4180
September	1st to 10th	Day	16% 5% 2 5% 4 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	11% 5% 1 5% 3 4 5% 4 7% 8% 47%	2280
	21st to end	Day	20% 7% 3 6% 4 4 3 4 6% 6% 37%	1984
October	1st to 10th	Day	15% 6% 2 5% 5% 4 3 4 5% 6% 45%	1900
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	1900
	21st to end	Day	14% 6% 3 6% 3 3 3 3 5% 6% 47%	2090
November	1st to 10th	Day	11% 7% 2 5% 3 3 4 3 5% 7% 50%	1866
	11th to 20th	Day	15% 6% 2 5% 5% 3 4 4 5% 7% 45%	1720
	21st to end	Day	15% 7% 2 7% 5% 4 4 4 5% 7% 40%	1600
December	1st to 10th	Day	13% 7% 4 7% 4 4 4 4 6% 8% 39%	2600
	11th to 20th	Day	20% 10% 2 7% 4 3 4 3 4 6% 37%	2990
	21st to end	Day	16% 10% 3 8% 5% 5% 5% 4 7% 8% 30%	3960
January	1st to 10th	Night	11% 9% 4 8% 5% 4 5% 3 6% 6% 39%	6231
	11th to 20th	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 41%	5900
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	6490
February	1st to 10th	Night	15% 8% 2 7% 5% 4 5% 4 6% 7% 38%	2600
	11th to 20th	Night	18% 9% 3 6% 5% 4 4 4 5% 6% 37%	2515
	21st to end	Night	16% 10% 3 7% 5% 5% 4 4 5% 7% 34%	1980
March	1st to 10th	Night	17% 7% 3 6% 4 3 4 3 4 6% 44%	4930
	11th to 20th	Night	10% 6% 2 6% 5% 4 3 4 5% 7% 48%	4980
	21st to end	Night	12% 8% 2 7% 4 4 4 3 5% 7% 43%	5422
April	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 5% 6% 4 35%	2000
	11th to 20th	Night	13% 6% 3 7% 5% 4 4 3 7% 8% 41%	1975
	21st to end	Night	16% 6% 2 5% 4 4 4 4 6% 8% 42%	2000
May	1st to 10th	Night	24% 8% 3 7% 6% 4 5% 3 6% 7% 28%	4036
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 4 6% 8% 30%	3759
	21st to end	Night	17% 6% 3 7% 5% 5% 6% 5% 6% 8% 34%	3920
June	1st to 10th	Night	11% 5% 3 4 5% 4 4 4 6% 8% 46%	1600
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 5% 7% 8% 40%	1600
	21st to end	Night	7% 6% 2 5% 6% 4 6% 4 7% 9% 44%	1600
July	1st to 10th	Night	7% 5% 2 6% 6% 6% 6% 5% 7% 10% 41%	2560
	11th to 20th	Night	6% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	2663
	21st to end	Night	12% 6% 3 6% 4 6% 5% 4 7% 9% 38%	3087
August	1st to 10th	Night	13% 7% 3 9% 6% 5% 5% 4 6% 9% 34%	2825
	11th to 20th	Night	10% 6% 2 8% 5% 5% 5% 4 6% 7% 41%	2866
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 47%	3476
September	1st to 10th	Night	13% 5% 1 6% 4 4 4 4 5% 6% 46%	1900
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 52%	1900
	21st to end	Night	18% 6% 3 5% 5% 4 4 3 5% 6% 40%	2192
October	1st to 10th	Night	18% 5% 2 5% 4 4 4 3 4 6% 44%	2280
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 50%	2280
	21st to end	Night	16% 6% 2 5% 3 3 4 3 6% 6% 48%	2508
November	1st to 10th	Night	10% 6% 3 5% 3 3 3 3 5% 7% 52%	2431
	11th to 20th	Night	15% 8% 2 5% 4 4 5% 3 5% 7% 43%	2675
	21st to end	Night	14% 6% 2 6% 5% 4 4 4 6% 7% 41%	2800
December	1st to 10th	Night	11% 8% 3 5% 4 4 4 4 5% 8% 44%	4580
	11th to 20th	Night	19% 7% 2 7% 5% 5% 5% 4 5% 6% 35%	5205
	21st to end	Night	16% 9% 3 8% 5% 5% 6% 5% 6% 8% 29%	6996

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	42% 9% 9% 23% 13% 3	3713
	11th to 20th	Day	44% 6% 10% 19% 17% 4	4060
	21st to end	Day	44% 7% 11% 22% 13% 2	4466
February	1st to 10th	Day	48% 5% 13% 23% 10% 2	1800
	11th to 20th	Day	48% 4 10% 23% 13% 2	1778
	21st to end	Day	48% 5% 11% 20% 13% 4	1650
March	1st to 10th	Day	42% 6% 9% 22% 14% 6%	4907
	11th to 20th	Day	35% 5% 13% 26% 15% 5%	4980
	21st to end	Day	45% 6% 10% 18% 17% 4	5509
April	1st to 10th	Day	45% 7% 12% 17% 16% 4	2400
	11th to 20th	Day	39% 6% 12% 22% 17% 4	2374
	21st to end	Day	43% 6% 10% 22% 17% 3	2413
May	1st to 10th	Day	49% 7% 13% 19% 10% 2	5897
	11th to 20th	Day	48% 8% 12% 22% 8% 1	6135
	21st to end	Day	41% 8% 13% 21% 14% 1	6949
June	1st to 10th	Day	35% 8% 16% 23% 17% 1	2800
	11th to 20th	Day	39% 11% 13% 22% 13% 2	2800
	21st to end	Day	33% 7% 15% 27% 16% 2	2800
July	1st to 10th	Day	33% 10% 16% 24% 12% 4	4440
	11th to 20th	Day	34% 8% 16% 28% 13% 1	4218
	21st to end	Day	42% 8% 10% 21% 17% 2	4324
August	1st to 10th	Day	44% 8% 15% 24% 8%	4091
	11th to 20th	Day	37% 7% 12% 29% 14% 1	3969
	21st to end	Day	35% 6% 13% 27% 17% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 21% 4	2280
	11th to 20th	Day	33% 6% 11% 26% 21% 3	2280
	21st to end	Day	46% 5% 10% 19% 18% 2	1984
October	1st to 10th	Day	38% 4 8% 21% 24% 5%	3160
	11th to 20th	Day	28% 6% 10% 28% 20% 7%	3140
	21st to end	Day	26% 3 12% 29% 27% 3	3428
November	1st to 10th	Day	33% 5% 13% 25% 19% 4	1866
	11th to 20th	Day	37% 5% 14% 22% 19% 3	1720
	21st to end	Day	41% 7% 12% 26% 13% 1	1600
December	1st to 10th	Day	40% 8% 16% 27% 6% 3	2600
	11th to 20th	Day	47% 5% 14% 24% 8% 2	2986
	21st to end	Day	46% 8% 18% 14% 12% 1	3960
January	1st to 10th	Night	38% 9% 11% 23% 17% 2	6231
	11th to 20th	Night	39% 7% 12% 21% 15% 6%	5900
	21st to end	Night	40% 5% 12% 26% 15% 2	6490
February	1st to 10th	Night	43% 6% 14% 23% 12% 1	2600
	11th to 20th	Night	46% 4 12% 20% 15% 2	2517
	21st to end	Night	47% 6% 12% 18% 12% 4	1980
March	1st to 10th	Night	40% 6% 9% 26% 12% 7%	4930
	11th to 20th	Night	34% 6% 12% 29% 13% 5%	4980
	21st to end	Night	39% 6% 11% 20% 20% 3	5422
April	1st to 10th	Night	47% 6% 10% 18% 15% 3	2000
	11th to 20th	Night	39% 6% 12% 21% 17% 5%	1975
	21st to end	Night	38% 7% 12% 24% 15% 4	2000
May	1st to 10th	Night	55% 6% 11% 16% 10% 3	4063
	11th to 20th	Night	48% 8% 11% 22% 9% 1	3760
	21st to end	Night	45% 8% 14% 24% 8% 1	3927
June	1st to 10th	Night	34% 8% 11% 26% 17% 3	1600
	11th to 20th	Night	39% 9% 11% 25% 14% 2	1600
	21st to end	Night	34% 8% 13% 25% 18% 2	1600
July	1st to 10th	Night	37% 9% 19% 19% 12% 5%	2520
	11th to 20th	Night	38% 8% 17% 21% 14% 1	2615
	21st to end	Night	39% 8% 15% 19% 17% 1	2988
August	1st to 10th	Night	46% 7% 16% 22% 8% 1	2814
	11th to 20th	Night	39% 7% 12% 28% 14% 1	2893
	21st to end	Night	34% 6% 14% 29% 15% 2	3476
September	1st to 10th	Night	37% 7% 12% 20% 20% 4	1900
	11th to 20th	Night	33% 5% 10% 27% 23% 2	1900
	21st to end	Night	44% 7% 10% 20% 17% 2	2192
October	1st to 10th	Night	39% 6% 10% 22% 20% 2	3800
	11th to 20th	Night	25% 5% 11% 25% 27% 7%	3779
	21st to end	Night	26% 4 11% 30% 25% 4	4120
November	1st to 10th	Night	32% 4 11% 26% 22% 4	2431
	11th to 20th	Night	38% 6% 11% 23% 19% 5%	2675
	21st to end	Night	39% 6% 13% 23% 16% 2	2800
December	1st to 10th	Night	37% 6% 15% 30% 11% 1	4580
	11th to 20th	Night	47% 5% 13% 20% 13% 2	5208
	21st to end	Night	46% 10% 15% 17% 11%	6996

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	32% 26% 24% 14% 4	225
	11th to 20th	Day	32% 27% 28% 10% 3	300
	21st to end	Day	23% 29% 34% 11% 3 1	330
February	1st to 10th	Day	28% 23% 36% 10% 3	300
	11th to 20th	Day	36% 26% 30% 8% 1	316
	21st to end	Day	28% 22% 29% 18% 2	414
March	1st to 10th	Day	23% 23% 29% 20% 4	500
	11th to 20th	Day	25% 22% 29% 17% 6% 1	500
	21st to end	Day	24% 22% 29% 18% 6% 1	550
April	1st to 10th	Day	25% 20% 27% 20% 6% 2	500
	11th to 20th	Day	19% 19% 30% 16% 13% 4	492
	21st to end	Day	25% 15% 29% 19% 10% 2	500
May	1st to 10th	Day	24% 15% 28% 21% 10% 3	503
	11th to 20th	Day	21% 13% 28% 21% 13% 3	527
	21st to end	Day	16% 9% 24% 24% 20% 7%	573
June	1st to 10th	Day	15% 8% 28% 24% 19% 7%	530
	11th to 20th	Day	15% 8% 25% 29% 18% 5%	530
	21st to end	Day	12% 5% 24% 30% 23% 6%	530
July	1st to 10th	Day	9% 4 21% 30% 24% 12%	505
	11th to 20th	Day	6% 5% 20% 28% 30% 11%	484
	21st to end	Day	11% 4 19% 34% 24% 8%	532
August	1st to 10th	Day	10% 5% 18% 28% 27% 12%	486
	11th to 20th	Day	11% 4 20% 30% 23% 12%	479
	21st to end	Day	9% 4 17% 32% 24% 14%	539
September	1st to 10th	Day	18% 6% 18% 24% 23% 11%	490
	11th to 20th	Day	11% 3 20% 30% 24% 12%	490
	21st to end	Day	24% 10% 23% 22% 17% 5%	490
October	1st to 10th	Day	19% 7% 23% 23% 21% 6%	490
	11th to 20th	Day	17% 14% 24% 28% 14% 3	490
	21st to end	Day	16% 13% 33% 25% 11% 2	539
November	1st to 10th	Day	16% 13% 31% 25% 13% 3	449
	11th to 20th	Day	19% 19% 30% 21% 11% 1	320
	21st to end	Day	23% 22% 32% 17% 6% 1	200
December	1st to 10th	Day	25% 20% 34% 16% 5% 1	200
	11th to 20th	Day	27% 21% 26% 20% 6%	200
	21st to end	Day	21% 20% 31% 20% 8%	220
January	1st to 10th	Night	18% 21% 39% 20% 3	800
	11th to 20th	Night	23% 18% 32% 25% 3	800
	21st to end	Night	17% 19% 39% 24% 1	880
February	1st to 10th	Night	19% 19% 33% 27% 2	800
	11th to 20th	Night	26% 22% 33% 19% 1	783
	21st to end	Night	23% 20% 36% 18% 3	660
March	1st to 10th	Night	21% 17% 37% 21% 4	800
	11th to 20th	Night	24% 19% 30% 22% 6% 1	800
	21st to end	Night	19% 17% 37% 23% 4 1	880
April	1st to 10th	Night	20% 19% 33% 24% 5%	800
	11th to 20th	Night	16% 13% 34% 28% 8%	791
	21st to end	Night	21% 13% 30% 29% 7%	800
May	1st to 10th	Night	21% 8% 26% 32% 12%	603
	11th to 20th	Night	19% 5% 27% 36% 12% 1	474
	21st to end	Night	15% 5% 22% 37% 19% 1	433
June	1st to 10th	Night	14% 3 26% 36% 22% 1	400
	11th to 20th	Night	10% 6% 22% 41% 19% 2	400
	21st to end	Night	8% 4 21% 50% 16% 1	400
July	1st to 10th	Night	12% 3 16% 37% 28% 3	380
	11th to 20th	Night	7% 3 12% 41% 33% 3	446
	21st to end	Night	11% 1 13% 42% 30% 3	624
August	1st to 10th	Night	11% 2 9% 45% 31% 3	567
	11th to 20th	Night	12% 3 13% 41% 28% 3	606
	21st to end	Night	11% 3 14% 41% 27% 4	836
September	1st to 10th	Night	15% 2 17% 37% 26% 3	760
	11th to 20th	Night	11% 3 17% 37% 28% 3	760
	21st to end	Night	21% 5% 18% 34% 19% 2	760
October	1st to 10th	Night	20% 6% 20% 31% 20% 3	760
	11th to 20th	Night	16% 9% 29% 33% 12% 1	760
	21st to end	Night	20% 8% 31% 30% 10%	836
November	1st to 10th	Night	12% 9% 32% 34% 11% 1	780
	11th to 20th	Night	18% 16% 29% 29% 8% 1	800
	21st to end	Night	16% 17% 32% 28% 6%	800
December	1st to 10th	Night	21% 16% 32% 25% 6%	800
	11th to 20th	Night	19% 13% 37% 27% 4	800
	21st to end	Night	20% 12% 34% 31% 2	880

Page generated at 2025-07-03 01:30:14. Version ac07

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