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 2024-03-23.

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% 3 6% 6% 3 5% 5% 6% 8% 39%	78753
	Night	14% 7% 3 5% 5% 3 5% 5% 6% 7% 40%	81135
Transparency	Day	41% 7% 13% 22% 14% 2	78463
	Night	41% 7% 13% 23% 15% 2	80944
Seeing	Day	18% 13% 26% 23% 15% 5%	14001
	Night	17% 11% 27% 31% 12% 1	23669

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% 7% 2 7% 5% 4 5% 4 4 6% 39%	2590
	11th to 20th	Day	14% 11% 4 8% 6% 4 5% 5% 5% 6% 32%	2840
	21st to end	Day	17% 12% 4 7% 7% 3 5% 4 5% 6% 31%	3124
February	1st to 10th	Day	14% 10% 3 7% 6% 3 5% 4 5% 7% 35%	1710
	11th to 20th	Day	15% 8% 3 6% 8% 3 4 4 4 7% 38%	1678
	21st to end	Day	19% 9% 4 6% 5% 3 4 4 4 4 38%	1570
March	1st to 10th	Day	14% 6% 3 5% 4 3 5% 4 4 6% 45%	2076
	11th to 20th	Day	16% 8% 3 5% 6% 3 5% 5% 4 7% 38%	2090
	21st to end	Day	17% 8% 3 7% 6% 3 4 4 5% 6% 36%	2217
April	1st to 10th	Day	13% 7% 3 7% 6% 3 5% 4 6% 8% 38%	2160
	11th to 20th	Day	14% 6% 2 5% 5% 3 5% 4 5% 7% 43%	2134
	21st to end	Day	19% 6% 3 5% 5% 3 5% 5% 5% 6% 39%	2171
May	1st to 10th	Day	16% 7% 3 5% 6% 4 4 6% 7% 7% 36%	2340
	11th to 20th	Day	19% 6% 3 6% 6% 3 5% 4 5% 8% 35%	2403
	21st to end	Day	13% 7% 3 5% 5% 3 4 5% 6% 9% 39%	2725
June	1st to 10th	Day	12% 5% 2 4 4 3 5% 6% 8% 10% 41%	2520
	11th to 20th	Day	11% 5% 3 5% 6% 3 7% 6% 7% 9% 38%	2520
	21st to end	Day	10% 4 2 5% 5% 3 5% 6% 7% 10% 44%	2520
July	1st to 10th	Day	8% 4 3 5% 6% 4 7% 7% 7% 8% 41%	2520
	11th to 20th	Day	7% 4 2 5% 6% 4 6% 6% 7% 10% 43%	2443
	21st to end	Day	9% 7% 2 6% 7% 3 7% 5% 7% 9% 37%	2547
August	1st to 10th	Day	8% 5% 2 6% 6% 5% 7% 7% 8% 9% 36%	2327
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 7% 9% 40%	2245
	21st to end	Day	10% 4 3 6% 6% 3 5% 5% 6% 10% 42%	2376
September	1st to 10th	Day	16% 5% 2 4 5% 3 4 4 5% 8% 44%	2160
	11th to 20th	Day	12% 4 1 4 4 3 4 6% 7% 8% 47%	2160
	21st to end	Day	18% 6% 3 5% 5% 3 4 4 6% 7% 39%	1880
October	1st to 10th	Day	15% 6% 2 4 6% 4 3 5% 5% 7% 44%	1800
	11th to 20th	Day	16% 5% 2 4 4 3 5% 4 5% 6% 45%	1800
	21st to end	Day	15% 6% 3 5% 4 2 4 4 5% 6% 46%	1980
November	1st to 10th	Day	11% 7% 2 5% 4 2 4 4 5% 7% 49%	1771
	11th to 20th	Day	16% 5% 2 4 5% 2 4 4 5% 7% 45%	1634
	21st to end	Day	16% 6% 3 6% 6% 2 4 5% 5% 8% 40%	1520
December	1st to 10th	Day	18% 9% 4 6% 4 3 5% 5% 6% 8% 32%	1520
	11th to 20th	Day	16% 8% 2 7% 6% 2 5% 5% 5% 6% 37%	1910
	21st to end	Day	18% 7% 3 7% 6% 4 5% 5% 7% 8% 30%	2772
January	1st to 10th	Night	12% 9% 3 7% 5% 3 5% 5% 6% 7% 37%	4353
	11th to 20th	Night	18% 8% 3 5% 6% 3 4 4 5% 6% 39%	4120
	21st to end	Night	19% 10% 4 6% 6% 3 5% 4 5% 6% 32%	4532
February	1st to 10th	Night	14% 7% 3 6% 5% 3 5% 5% 5% 7% 38%	2470
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 4 5% 6% 38%	2386
	21st to end	Night	17% 9% 3 6% 6% 4 5% 5% 5% 7% 34%	1884
March	1st to 10th	Night	15% 6% 3 5% 4 3 5% 5% 5% 6% 45%	2102
	11th to 20th	Night	17% 7% 3 5% 5% 3 5% 5% 4 7% 40%	2090
	21st to end	Night	15% 7% 2 7% 5% 3 4 4 5% 7% 41%	2183
April	1st to 10th	Night	14% 8% 4 7% 6% 4 5% 5% 6% 5% 36%	1800
	11th to 20th	Night	13% 5% 3 6% 5% 3 4 4 6% 9% 42%	1775
	21st to end	Night	17% 6% 2 4 5% 3 5% 5% 5% 8% 41%	1800
May	1st to 10th	Night	17% 6% 3 5% 5% 3 4 5% 6% 7% 39%	1623
	11th to 20th	Night	16% 6% 3 5% 6% 3 5% 5% 6% 9% 36%	1487
	21st to end	Night	12% 5% 2 5% 6% 3 5% 6% 6% 9% 40%	1550
June	1st to 10th	Night	11% 5% 3 4 4 3 4 5% 6% 9% 47%	1440
	11th to 20th	Night	11% 6% 3 7% 5% 3 6% 6% 6% 8% 40%	1440
	21st to end	Night	7% 5% 3 5% 6% 2 7% 5% 7% 9% 44%	1440
July	1st to 10th	Night	10% 5% 3 5% 6% 3 7% 6% 6% 9% 41%	1440
	11th to 20th	Night	6% 4 2 5% 4 4 6% 6% 7% 9% 47%	1513
	21st to end	Night	9% 4 2 4 4 4 5% 7% 9% 12% 40%	1771
August	1st to 10th	Night	9% 5% 2 6% 6% 3 5% 6% 7% 10% 40%	1611
	11th to 20th	Night	10% 5% 3 6% 5% 4 6% 6% 6% 7% 43%	1637
	21st to end	Night	10% 5% 2 5% 5% 3 4 4 6% 9% 45%	1980
September	1st to 10th	Night	13% 5% 2 5% 5% 3 4 5% 6% 7% 45%	1800
	11th to 20th	Night	10% 6% 2 4 4 3 4 4 5% 7% 52%	1800
	21st to end	Night	17% 6% 3 4 5% 3 4 4 5% 6% 42%	2076
October	1st to 10th	Night	19% 5% 2 5% 5% 3 4 4 4 6% 44%	2160
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 6% 48%	2160
	21st to end	Night	16% 6% 2 4 3 2 4 4 5% 6% 46%	2376
November	1st to 10th	Night	11% 6% 3 4 4 2 3 4 5% 7% 52%	2306
	11th to 20th	Night	15% 7% 2 4 4 3 5% 4 5% 7% 44%	2541
	21st to end	Night	14% 5% 2 6% 6% 3 4 5% 5% 8% 42%	2660
December	1st to 10th	Night	17% 9% 3 6% 5% 3 4 5% 5% 7% 36%	2660
	11th to 20th	Night	17% 6% 2 6% 6% 3 5% 5% 5% 6% 38%	3285
	21st to end	Night	17% 8% 2 6% 6% 4 6% 6% 7% 9% 30%	4884

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	43% 6% 12% 27% 10% 3	2591
	11th to 20th	Day	49% 6% 12% 18% 12% 3	2840
	21st to end	Day	51% 7% 11% 21% 9% 1	3124
February	1st to 10th	Day	48% 5% 13% 22% 10% 2	1710
	11th to 20th	Day	46% 4 11% 23% 13% 2	1687
	21st to end	Day	49% 4 11% 19% 13% 4	1570
March	1st to 10th	Day	40% 6% 12% 24% 17% 2	2076
	11th to 20th	Day	44% 5% 13% 18% 15% 5%	2090
	21st to end	Day	48% 5% 11% 19% 15% 3	2217
April	1st to 10th	Day	43% 6% 12% 18% 17% 4	2160
	11th to 20th	Day	39% 6% 12% 21% 18% 4	2134
	21st to end	Day	44% 6% 10% 20% 16% 3	2171
May	1st to 10th	Day	44% 8% 12% 19% 14% 2	2340
	11th to 20th	Day	46% 8% 12% 18% 13% 3	2407
	21st to end	Day	40% 8% 15% 24% 12% 1	2725
June	1st to 10th	Day	35% 8% 16% 23% 16% 1	2520
	11th to 20th	Day	39% 12% 13% 21% 13% 2	2520
	21st to end	Day	33% 8% 16% 27% 15% 1	2520
July	1st to 10th	Day	37% 11% 13% 22% 15% 3	2450
	11th to 20th	Day	32% 9% 15% 25% 17% 2	2368
	21st to end	Day	39% 10% 13% 20% 15% 2	2405
August	1st to 10th	Day	39% 10% 15% 24% 12%	2314
	11th to 20th	Day	38% 9% 14% 23% 15% 2	2245
	21st to end	Day	37% 7% 15% 25% 13% 3	2376
September	1st to 10th	Day	39% 7% 12% 20% 19% 4	2160
	11th to 20th	Day	33% 6% 11% 26% 21% 3	2160
	21st to end	Day	44% 5% 10% 19% 19% 2	1880
October	1st to 10th	Day	40% 6% 12% 24% 17% 2	1800
	11th to 20th	Day	38% 6% 11% 24% 20% 1	1800
	21st to end	Day	39% 5% 11% 24% 19% 2	1980
November	1st to 10th	Day	34% 5% 13% 25% 18% 4	1771
	11th to 20th	Day	37% 5% 14% 22% 19% 3	1634
	21st to end	Day	41% 6% 13% 27% 13% 1	1520
December	1st to 10th	Day	48% 7% 13% 22% 9% 1	1520
	11th to 20th	Day	43% 7% 13% 28% 7% 2	1906
	21st to end	Day	46% 9% 19% 17% 9%	2772
January	1st to 10th	Night	42% 5% 15% 23% 12% 2	4353
	11th to 20th	Night	44% 6% 15% 23% 11% 2	4120
	21st to end	Night	49% 6% 12% 24% 8% 1	4532
February	1st to 10th	Night	43% 6% 14% 23% 12% 1	2470
	11th to 20th	Night	46% 4 12% 20% 16% 2	2388
	21st to end	Night	47% 5% 13% 18% 12% 5%	1884
March	1st to 10th	Night	39% 5% 12% 27% 15% 2	2102
	11th to 20th	Night	43% 5% 11% 22% 16% 3	2090
	21st to end	Night	43% 6% 12% 21% 15% 4	2183
April	1st to 10th	Night	46% 6% 10% 18% 15% 4	1800
	11th to 20th	Night	38% 6% 12% 21% 17% 6%	1775
	21st to end	Night	40% 7% 12% 23% 14% 4	1800
May	1st to 10th	Night	43% 7% 10% 22% 13% 4	1623
	11th to 20th	Night	43% 9% 12% 18% 17% 2	1488
	21st to end	Night	37% 10% 16% 24% 11% 1	1551
June	1st to 10th	Night	33% 8% 12% 27% 17% 3	1440
	11th to 20th	Night	40% 10% 11% 24% 14% 2	1440
	21st to end	Night	33% 9% 14% 25% 17% 2	1440
July	1st to 10th	Night	39% 11% 15% 20% 12% 4	1400
	11th to 20th	Night	31% 9% 17% 22% 18% 3	1465
	21st to end	Night	29% 10% 21% 21% 16% 2	1672
August	1st to 10th	Night	37% 9% 16% 24% 13% 1	1600
	11th to 20th	Night	39% 8% 14% 21% 16% 2	1637
	21st to end	Night	35% 8% 14% 24% 16% 3	1980
September	1st to 10th	Night	37% 7% 13% 20% 18% 4	1800
	11th to 20th	Night	33% 5% 11% 27% 23% 3	1800
	21st to end	Night	42% 7% 10% 21% 18% 3	2076
October	1st to 10th	Night	42% 5% 11% 23% 17% 2	2160
	11th to 20th	Night	35% 5% 11% 28% 19% 2	2160
	21st to end	Night	37% 5% 11% 26% 20% 2	2376
November	1st to 10th	Night	33% 5% 11% 27% 20% 4	2306
	11th to 20th	Night	37% 5% 11% 23% 19% 4	2541
	21st to end	Night	39% 6% 13% 23% 16% 2	2660
December	1st to 10th	Night	47% 6% 12% 23% 11% 1	2660
	11th to 20th	Night	44% 6% 11% 23% 14% 2	3288
	21st to end	Night	44% 10% 18% 18% 9%	4884

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	28% 27% 25% 16% 4	202
	11th to 20th	Day	29% 29% 27% 11% 3	270
	21st to end	Day	23% 29% 33% 11% 3 1	297
February	1st to 10th	Day	25% 24% 37% 11% 4	270
	11th to 20th	Day	31% 28% 32% 8% 1	285
	21st to end	Day	29% 22% 28% 18% 2	382
March	1st to 10th	Day	22% 23% 29% 21% 5%	460
	11th to 20th	Day	24% 23% 29% 17% 5% 1	460
	21st to end	Day	23% 22% 29% 19% 6% 1	470
April	1st to 10th	Day	21% 20% 29% 21% 7% 2	420
	11th to 20th	Day	15% 18% 32% 16% 15% 4	412
	21st to end	Day	27% 14% 29% 20% 8% 2	420
May	1st to 10th	Day	24% 14% 27% 21% 10% 3	423
	11th to 20th	Day	21% 13% 29% 22% 13% 3	447
	21st to end	Day	16% 7% 22% 25% 22% 8%	485
June	1st to 10th	Day	14% 8% 28% 25% 19% 7%	450
	11th to 20th	Day	14% 8% 27% 27% 18% 6%	450
	21st to end	Day	12% 5% 24% 31% 22% 5%	450
July	1st to 10th	Day	10% 4 19% 28% 25% 14%	445
	11th to 20th	Day	6% 4 20% 27% 31% 12%	444
	21st to end	Day	11% 4 19% 33% 25% 9%	488
August	1st to 10th	Day	10% 4 17% 27% 28% 13%	446
	11th to 20th	Day	11% 3 18% 30% 24% 13%	439
	21st to end	Day	9% 3 17% 31% 24% 15%	495
September	1st to 10th	Day	18% 5% 18% 23% 23% 12%	450
	11th to 20th	Day	11% 4 20% 30% 24% 12%	450
	21st to end	Day	21% 9% 23% 23% 18% 5%	450
October	1st to 10th	Day	18% 7% 23% 24% 22% 6%	450
	11th to 20th	Day	17% 14% 25% 28% 14% 2	450
	21st to end	Day	17% 14% 32% 25% 9% 2	495
November	1st to 10th	Day	16% 12% 29% 26% 13% 3	414
	11th to 20th	Day	20% 19% 28% 21% 12% 1	294
	21st to end	Day	19% 23% 32% 19% 6% 1	180
December	1st to 10th	Day	26% 21% 32% 16% 5% 1	180
	11th to 20th	Day	26% 22% 26% 20% 7%	180
	21st to end	Day	22% 19% 31% 20% 9%	198
January	1st to 10th	Night	18% 19% 39% 21% 3	760
	11th to 20th	Night	23% 17% 31% 25% 3	760
	21st to end	Night	18% 19% 37% 24% 2	836
February	1st to 10th	Night	18% 19% 33% 28% 2	760
	11th to 20th	Night	25% 21% 34% 20% 1	743
	21st to end	Night	24% 20% 35% 18% 3	628
March	1st to 10th	Night	20% 17% 36% 22% 4	760
	11th to 20th	Night	24% 19% 30% 22% 6% 1	760
	21st to end	Night	18% 16% 37% 23% 5% 1	801
April	1st to 10th	Night	18% 19% 33% 24% 5%	720
	11th to 20th	Night	16% 12% 34% 29% 9%	711
	21st to end	Night	22% 13% 29% 29% 7%	720
May	1st to 10th	Night	19% 9% 25% 33% 13%	543
	11th to 20th	Night	19% 4 26% 38% 12% 1	426
	21st to end	Night	15% 5% 21% 38% 20% 1	389
June	1st to 10th	Night	13% 3 25% 36% 23% 1	360
	11th to 20th	Night	10% 6% 21% 41% 21% 2	360
	21st to end	Night	8% 4 21% 51% 16% 1	360
July	1st to 10th	Night	13% 2 15% 37% 29% 4	350
	11th to 20th	Night	7% 3 11% 41% 34% 4	421
	21st to end	Night	10% 1 13% 42% 31% 3	591
August	1st to 10th	Night	11% 1 9% 45% 31% 3	537
	11th to 20th	Night	12% 3 12% 41% 29% 3	573
	21st to end	Night	11% 3 14% 40% 28% 4	792
September	1st to 10th	Night	16% 2 16% 37% 26% 3	720
	11th to 20th	Night	11% 3 18% 36% 28% 3	720
	21st to end	Night	20% 5% 18% 36% 20% 2	720
October	1st to 10th	Night	19% 6% 20% 31% 20% 3	720
	11th to 20th	Night	16% 9% 29% 33% 12% 1	720
	21st to end	Night	21% 9% 30% 30% 10% 1	792
November	1st to 10th	Night	13% 9% 31% 35% 11% 1	740
	11th to 20th	Night	17% 16% 29% 29% 8% 1	760
	21st to end	Night	16% 16% 32% 29% 7%	760
December	1st to 10th	Night	22% 16% 32% 24% 6%	760
	11th to 20th	Night	18% 13% 38% 26% 4	760
	21st to end	Night	20% 12% 34% 31% 2	836

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