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 2026-10-29.

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%	149575
	Night	14% 7% 3 7% 5% 4 5% 4 6% 7% 40%	142470
Transparency	Day	41% 7% 12% 23% 14% 2	153171
	Night	40% 7% 12% 23% 15% 3	146945
Seeing	Day	19% 13% 26% 23% 14% 5%	17485
	Night	17% 12% 28% 30% 12% 1	26952

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% 5% 5% 4 5% 6% 36%	4818
	11th to 20th	Day	12% 11% 4 8% 4 4 4 3 5% 6% 39%	5280
	21st to end	Day	13% 7% 3 8% 6% 5% 5% 4 6% 6% 36%	5780
February	1st to 10th	Day	13% 11% 3 8% 5% 4 5% 4 5% 7% 36%	1890
	11th to 20th	Day	15% 9% 3 8% 6% 4 3 3 4 6% 37%	1860
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 4 38%	1730
March	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 4 6% 40%	5016
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 6% 7% 45%	5090
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 5% 6% 38%	5631
April	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 36%	2520
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 5% 7% 43%	2494
	21st to end	Day	17% 7% 2 7% 4 4 4 4 5% 6% 40%	2534
May	1st to 10th	Day	19% 7% 2 7% 5% 4 4 4 7% 8% 33%	7616
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 9% 34%	7980
	21st to end	Day	22% 7% 3 6% 4 4 4 4 5% 7% 35%	9045
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 5% 8% 9% 42%	2940
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 4 7% 9% 39%	2940
	21st to end	Day	10% 5% 1 6% 4 4 5% 4 7% 9% 44%	2940
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	8280
	11th to 20th	Day	8% 6% 2 6% 5% 6% 7% 5% 8% 11% 35%	8007
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	8340
August	1st to 10th	Day	13% 7% 3 8% 6% 6% 6% 5% 8% 10% 29%	6014
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 8% 9% 37%	5793
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 10% 47%	6080
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 8% 46%	2520
	11th to 20th	Day	12% 5% 1 6% 3 4 5% 4 7% 7% 46%	2520
	21st to end	Day	20% 8% 3 7% 4 4 3 3 5% 6% 36%	2192
October	1st to 10th	Day	14% 6% 2 5% 5% 4 3 4 5% 6% 47%	2100
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2100
	21st to end	Day	13% 6% 3 6% 3 3 4 3 5% 6% 48%	2280
November	1st to 10th	Day	11% 7% 1 5% 4 3 4 3 5% 7% 50%	1961
	11th to 20th	Day	15% 6% 2 5% 4 3 4 4 5% 7% 45%	1806
	21st to end	Day	15% 7% 3 7% 5% 4 4 4 5% 7% 40%	1680
December	1st to 10th	Day	14% 7% 4 7% 4 4 4 4 6% 8% 39%	2680
	11th to 20th	Day	20% 10% 2 7% 4 3 4 3 4 6% 37%	3070
	21st to end	Day	16% 10% 3 8% 5% 5% 5% 4 7% 8% 30%	4048
January	1st to 10th	Night	10% 10% 4 9% 6% 5% 5% 4 6% 7% 36%	8073
	11th to 20th	Night	13% 8% 3 8% 5% 4 4 3 5% 5% 42%	7680
	21st to end	Night	13% 8% 2 5% 4 4 5% 4 6% 7% 41%	8401
February	1st to 10th	Night	14% 9% 2 7% 5% 4 5% 4 6% 7% 38%	2730
	11th to 20th	Night	17% 9% 3 6% 5% 4 5% 4 5% 6% 36%	2644
	21st to end	Night	15% 10% 3 7% 5% 5% 4 4 5% 7% 35%	2076
March	1st to 10th	Night	17% 7% 3 6% 4 3 4 3 4 6% 44%	5041
	11th to 20th	Night	10% 6% 2 6% 5% 4 4 4 5% 7% 47%	5090
	21st to end	Night	11% 8% 2 7% 5% 4 5% 3 5% 7% 43%	5542
April	1st to 10th	Night	15% 9% 3 8% 5% 5% 4 4 6% 4 36%	2100
	11th to 20th	Night	13% 6% 3 7% 5% 4 4 3 7% 8% 41%	2075
	21st to end	Night	16% 6% 2 5% 3 4 4 4 6% 8% 42%	2100
May	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 3 6% 7% 32%	5240
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 5% 7% 9% 32%	4877
	21st to end	Night	22% 7% 3 7% 4 4 5% 4 5% 6% 32%	5108
June	1st to 10th	Night	10% 5% 3 4 5% 4 4 4 6% 8% 47%	1680
	11th to 20th	Night	11% 6% 2 8% 5% 5% 5% 5% 6% 8% 39%	1680
	21st to end	Night	8% 6% 2 5% 6% 4 6% 4 7% 9% 43%	1680
July	1st to 10th	Night	11% 6% 3 6% 6% 5% 5% 4 6% 9% 41%	4678
	11th to 20th	Night	8% 7% 3 9% 5% 6% 6% 5% 7% 9% 34%	4932
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 4 7% 9% 42%	5736
August	1st to 10th	Night	13% 7% 3 9% 6% 6% 6% 5% 7% 10% 29%	4135
	11th to 20th	Night	11% 6% 2 8% 5% 5% 6% 5% 6% 8% 38%	4206
	21st to end	Night	9% 5% 2 7% 5% 4 5% 4 6% 8% 45%	5045
September	1st to 10th	Night	12% 5% 1 6% 4 4 4 4 5% 7% 47%	2100
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 50%	2100
	21st to end	Night	18% 7% 3 5% 5% 4 4 4 5% 6% 38%	2424
October	1st to 10th	Night	18% 5% 2 5% 4 4 4 3 4 6% 46%	2520
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 50%	2520
	21st to end	Night	15% 6% 2 5% 3 3 4 3 6% 6% 49%	2737
November	1st to 10th	Night	10% 6% 3 5% 3 3 4 3 5% 7% 51%	2556
	11th to 20th	Night	15% 8% 2 5% 4 4 5% 3 5% 7% 44%	2809
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 7% 41%	2940
December	1st to 10th	Night	11% 8% 3 6% 4 4 4 4 5% 8% 44%	4720
	11th to 20th	Night	19% 7% 2 7% 5% 5% 5% 4 5% 6% 35%	5345
	21st to end	Night	16% 9% 3 8% 5% 5% 6% 5% 6% 8% 29%	7150

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	44% 8% 10% 23% 12% 3	4819
	11th to 20th	Day	43% 5% 9% 20% 17% 6%	5280
	21st to end	Day	43% 7% 11% 26% 12% 2	5780
February	1st to 10th	Day	47% 5% 12% 23% 11% 2	1890
	11th to 20th	Day	48% 4 10% 22% 13% 2	1869
	21st to end	Day	48% 4 11% 20% 12% 3	1730
March	1st to 10th	Day	43% 6% 10% 22% 14% 6%	5016
	11th to 20th	Day	35% 5% 13% 26% 15% 5%	5090
	21st to end	Day	45% 6% 10% 19% 17% 4	5631
April	1st to 10th	Day	44% 7% 12% 17% 17% 3	2520
	11th to 20th	Day	38% 6% 13% 22% 17% 4	2494
	21st to end	Day	43% 6% 11% 22% 16% 3	2534
May	1st to 10th	Day	47% 6% 11% 22% 11% 1	7651
	11th to 20th	Day	43% 8% 13% 26% 9% 1	7984
	21st to end	Day	46% 7% 11% 19% 16% 2	9061
June	1st to 10th	Day	35% 8% 15% 23% 18% 1	2940
	11th to 20th	Day	38% 11% 13% 23% 13% 2	2940
	21st to end	Day	34% 7% 15% 27% 16% 1	2940
July	1st to 10th	Day	38% 9% 13% 23% 14% 3	8210
	11th to 20th	Day	39% 8% 15% 27% 10% 1	7932
	21st to end	Day	38% 8% 11% 24% 19% 1	8156
August	1st to 10th	Day	48% 9% 15% 20% 8%	6001
	11th to 20th	Day	41% 7% 15% 26% 11% 1	5832
	21st to end	Day	32% 6% 16% 28% 16% 1	6080
September	1st to 10th	Day	37% 6% 13% 21% 20% 4	2520
	11th to 20th	Day	34% 6% 11% 26% 20% 3	2520
	21st to end	Day	48% 5% 10% 18% 18% 2	2192
October	1st to 10th	Day	37% 4 8% 22% 25% 5%	3360
	11th to 20th	Day	28% 6% 10% 28% 20% 7%	3340
	21st to end	Day	26% 3 12% 29% 27% 3	3618
November	1st to 10th	Day	33% 5% 13% 25% 20% 4	1961
	11th to 20th	Day	38% 5% 13% 21% 20% 3	1806
	21st to end	Day	41% 7% 13% 26% 12% 1	1680
December	1st to 10th	Day	40% 7% 17% 26% 6% 3	2680
	11th to 20th	Day	47% 5% 14% 24% 8% 2	3066
	21st to end	Day	47% 8% 18% 15% 12%	4048
January	1st to 10th	Night	42% 8% 12% 23% 13% 1	8074
	11th to 20th	Night	39% 7% 10% 20% 18% 6%	7680
	21st to end	Night	37% 6% 11% 28% 15% 2	8401
February	1st to 10th	Night	43% 7% 13% 23% 13% 1	2730
	11th to 20th	Night	47% 5% 12% 20% 15% 2	2646
	21st to end	Night	46% 6% 13% 18% 12% 4	2076
March	1st to 10th	Night	40% 6% 9% 26% 12% 7%	5041
	11th to 20th	Night	34% 6% 12% 29% 13% 5%	5090
	21st to end	Night	39% 6% 11% 21% 20% 3	5542
April	1st to 10th	Night	46% 6% 10% 19% 15% 3	2100
	11th to 20th	Night	39% 6% 12% 21% 17% 5%	2075
	21st to end	Night	38% 7% 13% 24% 14% 4	2100
May	1st to 10th	Night	51% 5% 10% 19% 12% 2	5267
	11th to 20th	Night	44% 9% 13% 24% 10% 1	4878
	21st to end	Night	50% 7% 11% 20% 11% 1	5115
June	1st to 10th	Night	34% 8% 11% 26% 19% 3	1680
	11th to 20th	Night	40% 9% 11% 24% 14% 2	1680
	21st to end	Night	35% 8% 13% 25% 17% 2	1680
July	1st to 10th	Night	41% 8% 14% 19% 15% 3	4640
	11th to 20th	Night	45% 7% 14% 20% 13% 1	4887
	21st to end	Night	37% 7% 13% 22% 19% 1	5606
August	1st to 10th	Night	49% 9% 14% 19% 8%	4124
	11th to 20th	Night	42% 7% 15% 24% 11% 1	4233
	21st to end	Night	36% 6% 15% 29% 14% 1	5045
September	1st to 10th	Night	36% 7% 13% 21% 20% 4	2100
	11th to 20th	Night	34% 5% 10% 27% 22% 2	2100
	21st to end	Night	46% 7% 10% 18% 16% 3	2424
October	1st to 10th	Night	38% 5% 10% 23% 21% 2	4040
	11th to 20th	Night	26% 5% 11% 25% 27% 7%	4019
	21st to end	Night	25% 4 11% 30% 25% 4	4349
November	1st to 10th	Night	33% 5% 11% 26% 22% 4	2556
	11th to 20th	Night	38% 5% 11% 23% 19% 4	2809
	21st to end	Night	40% 6% 13% 23% 16% 2	2940
December	1st to 10th	Night	37% 6% 16% 30% 11% 1	4720
	11th to 20th	Night	47% 6% 12% 20% 13% 2	5348
	21st to end	Night	46% 10% 15% 17% 11%	7150

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	31% 27% 23% 15% 3	248
	11th to 20th	Day	33% 28% 27% 9% 3	330
	21st to end	Day	25% 31% 31% 10% 3 1	363
February	1st to 10th	Day	27% 23% 36% 11% 3	330
	11th to 20th	Day	36% 25% 31% 7% 1	347
	21st to end	Day	28% 22% 30% 17% 2	446
March	1st to 10th	Day	24% 23% 30% 19% 4	540
	11th to 20th	Day	26% 22% 29% 17% 5% 1	540
	21st to end	Day	24% 22% 31% 17% 5% 1	594
April	1st to 10th	Day	24% 21% 27% 20% 6% 1	540
	11th to 20th	Day	19% 18% 33% 15% 12% 3	532
	21st to end	Day	25% 14% 30% 20% 9% 1	540
May	1st to 10th	Day	24% 16% 28% 20% 9% 3	543
	11th to 20th	Day	20% 14% 28% 21% 13% 3	567
	21st to end	Day	20% 8% 24% 23% 19% 6%	617
June	1st to 10th	Day	14% 8% 29% 24% 18% 7%	570
	11th to 20th	Day	15% 9% 26% 28% 17% 5%	570
	21st to end	Day	12% 5% 25% 29% 22% 5%	570
July	1st to 10th	Day	10% 4 19% 30% 25% 12%	565
	11th to 20th	Day	7% 5% 20% 29% 29% 10%	564
	21st to end	Day	10% 5% 20% 33% 23% 8%	620
August	1st to 10th	Day	11% 5% 17% 29% 26% 11%	566
	11th to 20th	Day	12% 4 22% 30% 22% 10%	559
	21st to end	Day	8% 4 20% 31% 23% 13%	627
September	1st to 10th	Day	16% 6% 19% 25% 22% 11%	570
	11th to 20th	Day	13% 4 19% 29% 24% 11%	570
	21st to end	Day	25% 10% 24% 22% 16% 4	570
October	1st to 10th	Day	17% 8% 23% 25% 20% 6%	570
	11th to 20th	Day	19% 15% 24% 27% 13% 2	570
	21st to end	Day	16% 14% 32% 25% 12% 1	615
November	1st to 10th	Day	17% 14% 31% 24% 12% 3	484
	11th to 20th	Day	21% 19% 29% 19% 10% 1	346
	21st to end	Day	25% 22% 31% 16% 5% 1	220
December	1st to 10th	Day	25% 21% 34% 15% 4 1	220
	11th to 20th	Day	27% 22% 26% 20% 5%	220
	21st to end	Day	25% 20% 30% 18% 7%	242
January	1st to 10th	Night	18% 21% 39% 20% 3	840
	11th to 20th	Night	23% 19% 31% 24% 3	840
	21st to end	Night	18% 19% 39% 23% 1	924
February	1st to 10th	Night	19% 19% 33% 27% 2	840
	11th to 20th	Night	26% 22% 32% 19% 1	823
	21st to end	Night	23% 20% 36% 18% 3	692
March	1st to 10th	Night	21% 17% 37% 21% 4	840
	11th to 20th	Night	24% 19% 30% 21% 5%	840
	21st to end	Night	19% 17% 37% 22% 4 1	924
April	1st to 10th	Night	20% 18% 34% 24% 5%	840
	11th to 20th	Night	16% 13% 35% 28% 9%	831
	21st to end	Night	20% 13% 30% 30% 7%	840
May	1st to 10th	Night	21% 9% 27% 32% 12%	633
	11th to 20th	Night	18% 5% 28% 36% 12% 1	498
	21st to end	Night	18% 5% 22% 36% 18% 1	455
June	1st to 10th	Night	13% 3 26% 35% 22% 1	420
	11th to 20th	Night	10% 7% 22% 41% 18% 1	420
	21st to end	Night	9% 4 21% 50% 15% 1	420
July	1st to 10th	Night	12% 3 16% 37% 29% 3	410
	11th to 20th	Night	8% 4 12% 42% 32% 3	494
	21st to end	Night	10% 2 14% 42% 29% 3	690
August	1st to 10th	Night	11% 2 9% 43% 31% 3	627
	11th to 20th	Night	11% 3 14% 41% 27% 3	670
	21st to end	Night	11% 3 15% 41% 26% 4	924
September	1st to 10th	Night	14% 2 18% 39% 25% 3	840
	11th to 20th	Night	11% 3 17% 39% 27% 3	840
	21st to end	Night	22% 6% 19% 34% 18% 2	840
October	1st to 10th	Night	19% 6% 20% 33% 20% 3	840
	11th to 20th	Night	17% 9% 29% 31% 12% 1	840
	21st to end	Night	19% 8% 30% 31% 11%	913
November	1st to 10th	Night	13% 10% 32% 34% 10% 1	820
	11th to 20th	Night	18% 16% 29% 28% 7%	840
	21st to end	Night	17% 17% 33% 27% 6%	840
December	1st to 10th	Night	21% 16% 32% 25% 6%	840
	11th to 20th	Night	19% 13% 37% 26% 4	840
	21st to end	Night	21% 13% 34% 30% 2	924

Page generated at 2026-10-26 04:37:49. Version ac07

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