University of Colorado Alpine Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Colorado Alpine Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	3 5% 2 5% 4 4 5% 4 6% 9% 52%	158116
	Night	2 4 2 5% 3 3 4 3 5% 8% 60%	150912
Transparency	Day	26% 10% 13% 23% 20% 8%	153046
	Night	21% 8% 15% 27% 23% 7%	146991
Seeing	Day	14% 30% 31% 17% 7% 1	18235
	Night	10% 32% 27% 20% 9% 1	28159

By Month

Percentage Cloud Cover SW (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	5% 3 10% 4 3 3 2 4 6% 60%	4720
	11th to 20th	Day	2 1 4 4 4 3 2 4 7% 67%	4955
	21st to end	Day	5% 2 5% 4 2 4 3 4 7% 63%	5844
February	1st to 10th	Day	4 4 7% 4 3 4 4 6% 10% 54%	1890
	11th to 20th	Day	3 2 6% 4 3 4 3 7% 10% 58%	1985
	21st to end	Day	4 3 8% 4 2 3 3 6% 12% 55%	1730
March	1st to 10th	Day	1 6% 4 7% 3 4 4 4 5% 5% 57%	4617
	11th to 20th	Day	1 8% 6% 9% 4 4 4 4 5% 9% 47%	5110
	21st to end	Day	2 8% 3 5% 3 4 4 4 6% 10% 51%	5907
April	1st to 10th	Day	2 7% 2 6% 4 4 4 4 7% 9% 50%	2520
	11th to 20th	Day	7% 12% 4 7% 4 4 5% 3 5% 8% 42%	2491
	21st to end	Day	6% 11% 4 6% 4 5% 5% 5% 7% 8% 39%	2520
May	1st to 10th	Day	7% 13% 3 6% 4 4 5% 4 6% 8% 40%	7499
	11th to 20th	Day	7% 9% 2 6% 4 5% 5% 4 6% 8% 44%	8191
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 4 8% 9% 45%	9026
June	1st to 10th	Day	5% 4 2 5% 6% 5% 5% 5% 7% 10% 45%	2940
	11th to 20th	Day	6% 4 1 4 4 5% 5% 5% 6% 8% 51%	2940
	21st to end	Day	4 5% 2 5% 4 6% 6% 4 7% 9% 49%	2940
July	1st to 10th	Day	3 3 1 3 4 5% 6% 5% 8% 9% 53%	8249
	11th to 20th	Day	2 2 1 5% 4 5% 6% 6% 8% 9% 50%	8250
	21st to end	Day	5% 4 2 7% 6% 6% 7% 6% 8% 9% 40%	8723
August	1st to 10th	Day	3 4 1 5% 4 4 5% 5% 8% 11% 49%	7549
	11th to 20th	Day	4 3 1 4 4 5% 6% 5% 8% 10% 50%	7113
	21st to end	Day	5% 4 2 8% 5% 6% 7% 5% 7% 10% 40%	7751
September	1st to 10th	Day	5% 4 2 5% 4 5% 4 4 6% 6% 54%	2520
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 3 6% 7% 53%	2520
	21st to end	Day	7% 5% 1 4 3 3 3 3 4 7% 59%	2265
October	1st to 10th	Day	3 3 1 4 2 3 3 2 4 4 70%	3270
	11th to 20th	Day	4 4 2 5% 3 3 4 3 7% 8% 57%	3250
	21st to end	Day	3 5% 2 4 3 3 3 3 4 7% 62%	3458
November	1st to 10th	Day	2 4 2 4 3 3 3 3 5% 8% 64%	1853
	11th to 20th	Day	1 4 1 4 3 3 3 2 7% 9% 62%	1890
	21st to end	Day	1 4 3 7% 4 4 3 4 7% 7% 58%	1732
December	1st to 10th	Day	2 2 4 3 3 3 2 4 7% 71%	2700
	11th to 20th	Day	1 3 1 3 3 3 3 3 7% 10% 63%	3106
	21st to end	Day	6% 3 6% 3 3 4 4 7% 9% 55%	4092
January	1st to 10th	Night	3 2 8% 4 3 4 3 4 7% 61%	8201
	11th to 20th	Night	1 1 5% 3 4 3 3 5% 8% 69%	8093
	21st to end	Night	3 2 5% 4 3 3 3 4 7% 65%	8370
February	1st to 10th	Night	4 2 5% 4 3 3 2 6% 11% 60%	2730
	11th to 20th	Night	4 2 6% 4 3 3 3 6% 8% 62%	2520
	21st to end	Night	5% 3 6% 3 3 3 3 5% 11% 58%	2076
March	1st to 10th	Night	6% 3 6% 4 4 4 3 5% 8% 58%	5476
	11th to 20th	Night	1 7% 4 6% 3 3 4 3 5% 9% 55%	5110
	21st to end	Night	2 6% 2 5% 3 3 4 4 5% 8% 57%	5294
April	1st to 10th	Night	2 6% 2 5% 3 3 4 4 5% 8% 58%	2100
	11th to 20th	Night	4 8% 2 6% 3 4 4 3 5% 7% 53%	2077
	21st to end	Night	6% 11% 2 5% 3 3 3 3 6% 8% 49%	2100
May	1st to 10th	Night	8% 13% 3 5% 3 3 4 3 5% 8% 46%	5431
	11th to 20th	Night	7% 8% 2 6% 4 4 4 4 6% 7% 50%	4701
	21st to end	Night	4 7% 2 5% 4 4 4 3 5% 7% 57%	5181
June	1st to 10th	Night	4 3 2 4 3 3 3 4 6% 10% 58%	1680
	11th to 20th	Night	4 4 1 3 3 3 3 4 6% 7% 63%	1680
	21st to end	Night	3 4 1 4 3 3 4 4 7% 8% 61%	1680
July	1st to 10th	Night	2 3 1 3 3 3 4 4 7% 8% 62%	4740
	11th to 20th	Night	3 3 1 3 2 3 4 4 7% 10% 61%	4740
	21st to end	Night	3 3 1 4 3 4 6% 6% 9% 11% 50%	5381
August	1st to 10th	Night	3 3 1 4 3 3 5% 5% 8% 10% 55%	5273
	11th to 20th	Night	3 2 1 3 3 4 4 4 7% 9% 59%	5743
	21st to end	Night	3 4 2 5% 4 5% 5% 5% 8% 11% 48%	6465
September	1st to 10th	Night	5% 4 1 4 4 3 3 4 4 6% 60%	2100
	11th to 20th	Night	6% 3 1 3 3 3 3 3 5% 7% 62%	2100
	21st to end	Night	6% 4 2 4 3 3 3 3 5% 5% 62%	2308
October	1st to 10th	Night	3 3 1 2 2 2 2 2 3 5% 75%	3920
	11th to 20th	Night	2 5% 1 3 3 3 3 3 5% 8% 64%	3898
	21st to end	Night	2 4 2 4 3 3 3 3 4 6% 67%	4307
November	1st to 10th	Night	1 4 1 3 2 3 3 3 4 9% 67%	2665
	11th to 20th	Night	1 4 1 4 3 3 3 3 6% 9% 64%	2730
	21st to end	Night	2 4 2 5% 3 4 3 3 6% 8% 60%	2883
December	1st to 10th	Night	2 2 3 2 2 2 2 3 6% 77%	4710
	11th to 20th	Night	3 2 4 3 3 3 3 5% 9% 65%	5321
	21st to end	Night	6% 2 6% 3 4 5% 4 6% 9% 54%	7128

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	23% 9% 14% 30% 20% 4	4723
	11th to 20th	Day	18% 10% 16% 24% 22% 9%	4955
	21st to end	Day	20% 6% 12% 26% 26% 11%	5844
February	1st to 10th	Day	26% 12% 19% 23% 15% 5%	1890
	11th to 20th	Day	22% 11% 16% 26% 20% 5%	1995
	21st to end	Day	27% 8% 15% 23% 21% 6%	1730
March	1st to 10th	Day	27% 7% 9% 22% 25% 10%	4617
	11th to 20th	Day	29% 8% 13% 29% 18% 3	5110
	21st to end	Day	28% 10% 18% 31% 11% 1	5907
April	1st to 10th	Day	30% 9% 13% 25% 17% 5%	2520
	11th to 20th	Day	40% 9% 12% 20% 16% 4	2494
	21st to end	Day	39% 11% 13% 23% 12% 2	2520
May	1st to 10th	Day	38% 11% 12% 23% 15% 2	7499
	11th to 20th	Day	32% 9% 12% 21% 22% 3	8195
	21st to end	Day	29% 10% 12% 25% 20% 3	9027
June	1st to 10th	Day	30% 12% 15% 21% 18% 5%	2940
	11th to 20th	Day	27% 10% 11% 15% 24% 13%	2940
	21st to end	Day	27% 10% 11% 19% 22% 10%	2940
July	1st to 10th	Day	19% 10% 9% 15% 22% 25%	8181
	11th to 20th	Day	21% 11% 13% 23% 18% 14%	8175
	21st to end	Day	32% 14% 16% 14% 12% 10%	8528
August	1st to 10th	Day	26% 11% 14% 17% 16% 14%	5978
	11th to 20th	Day	27% 12% 19% 18% 17% 7%	5606
	21st to end	Day	30% 12% 19% 17% 16% 7%	6078
September	1st to 10th	Day	27% 9% 10% 20% 23% 11%	2520
	11th to 20th	Day	28% 8% 12% 21% 24% 8%	2520
	21st to end	Day	25% 5% 9% 24% 30% 7%	2265
October	1st to 10th	Day	18% 4 6% 19% 37% 15%	3270
	11th to 20th	Day	23% 9% 10% 29% 25% 5%	3250
	21st to end	Day	20% 6% 10% 33% 27% 4	3458
November	1st to 10th	Day	19% 7% 16% 23% 30% 5%	1853
	11th to 20th	Day	18% 6% 17% 30% 22% 7%	1890
	21st to end	Day	24% 6% 14% 28% 19% 8%	1732
December	1st to 10th	Day	16% 6% 13% 36% 23% 6%	2700
	11th to 20th	Day	14% 9% 15% 38% 22% 2	3104
	21st to end	Day	23% 9% 14% 22% 26% 5%	4092
January	1st to 10th	Night	23% 8% 12% 32% 20% 5%	8202
	11th to 20th	Night	16% 10% 16% 25% 23% 10%	8093
	21st to end	Night	17% 8% 13% 23% 28% 12%	8370
February	1st to 10th	Night	21% 11% 19% 26% 20% 5%	2730
	11th to 20th	Night	21% 8% 19% 29% 20% 3	2520
	21st to end	Night	22% 8% 16% 24% 22% 8%	2076
March	1st to 10th	Night	24% 7% 11% 19% 29% 10%	5476
	11th to 20th	Night	26% 7% 10% 24% 30% 4	5110
	21st to end	Night	22% 8% 20% 36% 12% 1	5294
April	1st to 10th	Night	23% 10% 15% 29% 19% 5%	2100
	11th to 20th	Night	30% 8% 15% 26% 18% 3	2077
	21st to end	Night	31% 8% 14% 28% 19% 1	2100
May	1st to 10th	Night	33% 8% 14% 31% 12% 1	5429
	11th to 20th	Night	30% 7% 13% 29% 19% 3	4702
	21st to end	Night	25% 8% 16% 29% 21% 2	5182
June	1st to 10th	Night	21% 8% 19% 29% 18% 6%	1680
	11th to 20th	Night	18% 7% 12% 22% 26% 15%	1680
	21st to end	Night	19% 7% 12% 23% 25% 13%	1680
July	1st to 10th	Night	17% 8% 6% 15% 33% 21%	4700
	11th to 20th	Night	15% 9% 16% 29% 21% 11%	4692
	21st to end	Night	22% 11% 25% 20% 12% 10%	5264
August	1st to 10th	Night	21% 10% 16% 18% 25% 9%	4168
	11th to 20th	Night	20% 11% 27% 21% 14% 6%	4526
	21st to end	Night	23% 11% 26% 18% 19% 3	5069
September	1st to 10th	Night	23% 6% 12% 22% 26% 10%	2100
	11th to 20th	Night	21% 7% 11% 26% 25% 11%	2100
	21st to end	Night	23% 5% 10% 27% 27% 8%	2308
October	1st to 10th	Night	14% 4 7% 25% 37% 14%	3920
	11th to 20th	Night	18% 6% 11% 36% 23% 6%	3898
	21st to end	Night	18% 5% 9% 32% 31% 6%	4307
November	1st to 10th	Night	16% 7% 15% 27% 28% 7%	2665
	11th to 20th	Night	17% 6% 15% 32% 23% 6%	2730
	21st to end	Night	22% 8% 12% 29% 22% 7%	2883
December	1st to 10th	Night	11% 6% 14% 36% 27% 6%	4710
	11th to 20th	Night	15% 7% 14% 37% 25% 2	5322
	21st to end	Night	23% 10% 16% 22% 26% 4	7128

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	21% 42% 22% 10% 4 1	330
	11th to 20th	Day	18% 52% 18% 10% 1	330
	21st to end	Day	16% 48% 26% 7% 2 1	363
February	1st to 10th	Day	22% 54% 18% 4 1 1	330
	11th to 20th	Day	22% 55% 17% 5% 2	330
	21st to end	Day	27% 50% 17% 5%	273
March	1st to 10th	Day	19% 49% 23% 8% 1	334
	11th to 20th	Day	24% 35% 31% 9% 1	540
	21st to end	Day	20% 45% 28% 6% 2	594
April	1st to 10th	Day	19% 43% 30% 8% 1	540
	11th to 20th	Day	24% 43% 26% 5% 2	532
	21st to end	Day	20% 34% 34% 10% 1	540
May	1st to 10th	Day	20% 31% 32% 13% 4	652
	11th to 20th	Day	19% 21% 37% 16% 6% 1	775
	21st to end	Day	14% 23% 31% 22% 8% 1	844
June	1st to 10th	Day	7% 20% 34% 25% 11% 3	780
	11th to 20th	Day	9% 25% 35% 21% 8% 1	780
	21st to end	Day	8% 22% 35% 20% 13% 2	780
July	1st to 10th	Day	7% 9% 34% 31% 16% 4	771
	11th to 20th	Day	5% 8% 36% 30% 16% 4	768
	21st to end	Day	7% 10% 32% 32% 16% 4	768
August	1st to 10th	Day	10% 17% 35% 29% 8% 1	567
	11th to 20th	Day	7% 15% 39% 25% 12% 2	557
	21st to end	Day	9% 16% 36% 26% 10% 2	627
September	1st to 10th	Day	7% 18% 37% 27% 9% 2	570
	11th to 20th	Day	8% 20% 42% 21% 8% 1	570
	21st to end	Day	12% 21% 34% 21% 10% 2	354
October	1st to 10th	Day	9% 29% 30% 20% 9% 3	330
	11th to 20th	Day	14% 39% 29% 12% 4 2	330
	21st to end	Day	16% 44% 26% 10% 3	363
November	1st to 10th	Day	14% 47% 24% 9% 5%	330
	11th to 20th	Day	20% 44% 26% 7% 2 1	330
	21st to end	Day	17% 43% 28% 10% 1	330
December	1st to 10th	Day	23% 45% 22% 8% 2 1	330
	11th to 20th	Day	21% 52% 20% 6%	330
	21st to end	Day	26% 46% 18% 7% 2	363
January	1st to 10th	Night	15% 44% 23% 13% 4 1	1050
	11th to 20th	Night	16% 49% 21% 10% 4 1	997
	21st to end	Night	11% 48% 27% 10% 3 1	924
February	1st to 10th	Night	14% 53% 22% 9% 3	840
	11th to 20th	Night	11% 50% 25% 11% 2	824
	21st to end	Night	13% 52% 24% 9% 2	692
March	1st to 10th	Night	10% 45% 29% 13% 4	840
	11th to 20th	Night	14% 33% 29% 18% 5%	840
	21st to end	Night	11% 41% 31% 13% 2 1	789
April	1st to 10th	Night	13% 35% 34% 16% 2	630
	11th to 20th	Night	12% 38% 28% 16% 7%	623
	21st to end	Night	13% 28% 33% 18% 6% 1	630
May	1st to 10th	Night	12% 20% 37% 20% 9% 1	630
	11th to 20th	Night	17% 12% 32% 28% 10% 1	621
	21st to end	Night	9% 16% 32% 28% 13% 1	683
June	1st to 10th	Night	6% 9% 30% 34% 18% 2	630
	11th to 20th	Night	6% 14% 39% 30% 11% 1	630
	21st to end	Night	5% 11% 26% 36% 20% 2	630
July	1st to 10th	Night	5% 5% 19% 38% 28% 4	615
	11th to 20th	Night	2 4 21% 37% 32% 4	612
	21st to end	Night	5% 4 15% 39% 32% 5%	690
August	1st to 10th	Night	4 7% 22% 41% 24% 2	627
	11th to 20th	Night	6% 5% 20% 41% 23% 4	624
	21st to end	Night	4 7% 21% 37% 26% 5%	693
September	1st to 10th	Night	7% 6% 28% 35% 20% 4	630
	11th to 20th	Night	8% 14% 28% 31% 16% 3	630
	21st to end	Night	8% 17% 32% 28% 13% 3	644
October	1st to 10th	Night	7% 27% 29% 24% 11% 2	800
	11th to 20th	Night	9% 34% 31% 18% 7% 2	800
	21st to end	Night	10% 40% 31% 13% 5% 2	911
November	1st to 10th	Night	8% 47% 28% 12% 4 1	1025
	11th to 20th	Night	12% 48% 29% 8% 3	1050
	21st to end	Night	9% 44% 32% 13% 2	1050
December	1st to 10th	Night	16% 44% 28% 10% 2	1050
	11th to 20th	Night	15% 48% 24% 10% 3	1050
	21st to end	Night	14% 48% 25% 10% 3	1155

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