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 2024-11-03.

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 6% 5% 4 5% 4 6% 8% 39%	101230
	Night	14% 7% 2 6% 5% 4 4 4 6% 7% 40%	97705
Transparency	Day	41% 7% 13% 23% 14% 2	101031
	Night	41% 7% 13% 23% 14% 2	97574
Seeing	Day	18% 13% 26% 23% 15% 5%	14901
	Night	17% 11% 27% 31% 12% 1	24398

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 8% 5% 5% 5% 3 4 6% 39%	2590
	11th to 20th	Day	14% 12% 4 10% 5% 5% 5% 4 5% 6% 32%	2840
	21st to end	Day	17% 13% 4 8% 6% 4 5% 3 5% 6% 30%	3124
February	1st to 10th	Day	14% 10% 3 8% 5% 4 5% 3 5% 7% 35%	1710
	11th to 20th	Day	14% 9% 2 8% 6% 4 4 3 5% 6% 38%	1678
	21st to end	Day	19% 9% 3 7% 5% 4 4 3 4 4 38%	1570
March	1st to 10th	Day	21% 9% 3 6% 4 5% 4 4 5% 6% 33%	3421
	11th to 20th	Day	11% 7% 3 5% 5% 4 4 4 5% 7% 44%	3480
	21st to end	Day	18% 8% 3 7% 4 3 3 3 5% 6% 39%	3845
April	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 4 6% 8% 36%	2280
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 3 5% 7% 42%	2254
	21st to end	Day	18% 7% 2 6% 5% 4 4 4 5% 6% 39%	2292
May	1st to 10th	Day	25% 7% 2 7% 4 4 4 4 6% 6% 30%	4082
	11th to 20th	Day	22% 7% 3 6% 5% 5% 5% 4 6% 8% 29%	4267
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 10% 43%	4823
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 5% 8% 9% 41%	2660
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 4 7% 9% 39%	2660
	21st to end	Day	10% 5% 1 5% 4 4 5% 4 7% 9% 44%	2660
July	1st to 10th	Day	6% 3 2 6% 5% 5% 7% 5% 8% 10% 42%	4440
	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% 49%	1791
	11th to 20th	Day	15% 6% 2 5% 5% 3 4 4 5% 7% 45%	1634
	21st to end	Day	16% 7% 2 7% 5% 3 4 4 5% 7% 40%	1520
December	1st to 10th	Day	18% 10% 3 6% 4 4 4 5% 6% 8% 32%	1520
	11th to 20th	Day	16% 8% 2 8% 5% 3 5% 4 5% 6% 37%	1910
	21st to end	Day	18% 7% 3 8% 5% 5% 5% 4 7% 8% 30%	2772
January	1st to 10th	Night	12% 10% 3 8% 5% 4 5% 4 6% 7% 37%	4353
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 6% 39%	4120
	21st to end	Night	19% 10% 4 7% 5% 5% 4 3 5% 6% 32%	4532
February	1st to 10th	Night	14% 8% 3 7% 5% 4 5% 4 6% 7% 38%	2470
	11th to 20th	Night	17% 9% 3 6% 5% 4 4 4 5% 6% 37%	2386
	21st to end	Night	17% 10% 3 6% 5% 5% 4 4 5% 7% 34%	1884
March	1st to 10th	Night	21% 8% 3 6% 4 4 4 4 4 6% 37%	3430
	11th to 20th	Night	12% 7% 2 6% 5% 4 3 4 5% 7% 46%	3480
	21st to end	Night	13% 9% 2 7% 4 4 3 3 4 6% 46%	3786
April	1st to 10th	Night	14% 9% 3 8% 5% 5% 5% 5% 6% 5% 35%	1900
	11th to 20th	Night	13% 6% 3 7% 4 4 4 3 7% 8% 42%	1875
	21st to end	Night	16% 6% 2 5% 4 5% 4 4 6% 8% 41%	1900
May	1st to 10th	Night	25% 8% 3 6% 5% 4 4 3 5% 7% 30%	2816
	11th to 20th	Night	22% 9% 2 7% 5% 4 4 4 5% 7% 30%	2623
	21st to end	Night	11% 5% 2 6% 4 5% 6% 5% 7% 9% 39%	2732
June	1st to 10th	Night	10% 5% 2 4 5% 4 4 5% 6% 8% 47%	1520
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 5% 7% 8% 40%	1520
	21st to end	Night	8% 5% 2 5% 6% 4 6% 4 7% 9% 44%	1520
July	1st to 10th	Night	7% 5% 2 6% 6% 6% 6% 5% 7% 10% 41%	2520
	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	11% 7% 3 5% 3 3 3 3 5% 6% 52%	2331
	11th to 20th	Night	15% 7% 2 5% 4 4 5% 3 5% 7% 44%	2541
	21st to end	Night	14% 6% 2 6% 5% 4 4 4 6% 7% 41%	2660
December	1st to 10th	Night	17% 10% 3 7% 5% 4 4 4 5% 6% 36%	2660
	11th to 20th	Night	17% 7% 2 7% 5% 5% 5% 4 5% 6% 37%	3285
	21st to end	Night	17% 9% 2 6% 5% 5% 6% 5% 7% 8% 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	48% 7% 11% 21% 11% 2	3421
	11th to 20th	Day	35% 6% 13% 26% 17% 3	3480
	21st to end	Day	46% 6% 11% 20% 15% 3	3845
April	1st to 10th	Day	44% 7% 12% 17% 17% 4	2280
	11th to 20th	Day	39% 6% 12% 22% 18% 4	2254
	21st to end	Day	44% 6% 10% 21% 16% 3	2292
May	1st to 10th	Day	51% 8% 10% 20% 10% 1	4120
	11th to 20th	Day	49% 9% 14% 18% 8% 2	4271
	21st to end	Day	34% 8% 15% 25% 16% 2	4837
June	1st to 10th	Day	35% 8% 16% 23% 16% 1	2660
	11th to 20th	Day	38% 11% 13% 22% 13% 2	2660
	21st to end	Day	33% 8% 15% 27% 15% 2	2660
July	1st to 10th	Day	33% 10% 17% 24% 12% 4	4370
	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	39% 6% 11% 23% 18% 3	1900
	11th to 20th	Day	37% 6% 11% 25% 20% 2	1900
	21st to end	Day	37% 5% 11% 25% 20% 2	2090
November	1st to 10th	Day	34% 5% 13% 25% 19% 4	1791
	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	46% 7% 10% 26% 10% 1	3430
	11th to 20th	Night	36% 6% 11% 31% 14% 2	3480
	21st to end	Night	39% 5% 10% 23% 19% 4	3786
April	1st to 10th	Night	46% 6% 11% 18% 15% 3	1900
	11th to 20th	Night	38% 6% 12% 21% 17% 5%	1875
	21st to end	Night	39% 7% 12% 24% 14% 4	1900
May	1st to 10th	Night	53% 6% 9% 18% 11% 2	2843
	11th to 20th	Night	50% 9% 12% 18% 10% 1	2624
	21st to end	Night	35% 10% 17% 26% 10% 2	2739
June	1st to 10th	Night	34% 8% 12% 27% 17% 3	1520
	11th to 20th	Night	39% 9% 11% 25% 14% 2	1520
	21st to end	Night	34% 8% 14% 25% 17% 2	1520
July	1st to 10th	Night	37% 9% 19% 18% 12% 5%	2480
	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	41% 5% 11% 23% 17% 2	2280
	11th to 20th	Night	34% 5% 10% 28% 20% 3	2280
	21st to end	Night	35% 5% 11% 26% 20% 2	2508
November	1st to 10th	Night	33% 5% 11% 27% 21% 4	2331
	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% 21% 29% 19% 6% 1	506
April	1st to 10th	Day	22% 20% 29% 21% 7% 2	460
	11th to 20th	Day	16% 19% 32% 16% 14% 4	452
	21st to end	Day	25% 15% 29% 20% 10% 2	460
May	1st to 10th	Day	25% 14% 27% 21% 10% 3	463
	11th to 20th	Day	21% 13% 28% 22% 13% 4	487
	21st to end	Day	15% 8% 24% 25% 22% 7%	529
June	1st to 10th	Day	15% 8% 28% 24% 19% 7%	490
	11th to 20th	Day	13% 8% 26% 29% 18% 5%	490
	21st to end	Day	12% 6% 25% 30% 22% 5%	490
July	1st to 10th	Day	9% 4 20% 29% 25% 13%	485
	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% 26% 11% 2	539
November	1st to 10th	Day	16% 13% 29% 26% 13% 3	422
	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% 17% 36% 23% 5% 1	836
April	1st to 10th	Night	18% 19% 34% 24% 5%	760
	11th to 20th	Night	15% 13% 34% 29% 9%	751
	21st to end	Night	21% 13% 29% 29% 7%	760
May	1st to 10th	Night	20% 9% 25% 32% 13%	573
	11th to 20th	Night	19% 5% 27% 36% 12% 1	450
	21st to end	Night	14% 5% 22% 38% 20% 1	411
June	1st to 10th	Night	13% 2 26% 36% 23% 1	380
	11th to 20th	Night	9% 6% 22% 42% 20% 2	380
	21st to end	Night	8% 4 21% 51% 15% 1	380
July	1st to 10th	Night	12% 3 16% 37% 28% 4	370
	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	13% 9% 32% 34% 11% 1	749
	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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