Aspen Science Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Aspen Science Center clear sky chart from 2005-11-06 to 2026-09-14.

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	2 6% 3 9% 5% 5% 5% 4 6% 8% 47%	157869
	Night	1 6% 3 8% 4 4 4 3 6% 8% 52%	150842
Transparency	Day	34% 7% 12% 21% 19% 7%	152775
	Night	30% 6% 14% 24% 20% 6%	146945
Seeing	Day	15% 22% 31% 21% 9% 2	18034
	Night	8% 12% 31% 32% 15% 2	28031

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	7% 5% 21% 11% 5% 3 2 4 5% 37%	4753
	11th to 20th	Day	6% 5% 11% 8% 6% 4 3 4 6% 47%	5340
	21st to end	Day	5% 5% 11% 7% 5% 4 3 4 6% 50%	5844
February	1st to 10th	Day	1 12% 7% 16% 7% 5% 3 3 4 5% 37%	1890
	11th to 20th	Day	1 11% 6% 15% 8% 5% 4 3 4 6% 36%	1924
	21st to end	Day	1 8% 7% 16% 9% 4 3 3 3 6% 40%	1730
March	1st to 10th	Day	7% 8% 14% 6% 4 4 3 4 6% 44%	4871
	11th to 20th	Day	8% 6% 14% 6% 5% 4 4 6% 8% 38%	5110
	21st to end	Day	1 13% 7% 15% 6% 5% 4 4 5% 6% 35%	5741
April	1st to 10th	Day	2 12% 6% 9% 5% 4 4 4 7% 6% 41%	2520
	11th to 20th	Day	2 15% 6% 10% 6% 5% 5% 4 5% 6% 36%	2491
	21st to end	Day	2 14% 6% 10% 6% 5% 5% 4 6% 7% 34%	2520
May	1st to 10th	Day	2 10% 5% 8% 5% 5% 6% 5% 8% 9% 38%	7391
	11th to 20th	Day	2 8% 3 7% 6% 5% 6% 6% 7% 8% 42%	8109
	21st to end	Day	2 5% 2 5% 4 5% 6% 5% 9% 10% 47%	9026
June	1st to 10th	Day	3 4 2 5% 4 5% 6% 4 6% 9% 51%	2940
	11th to 20th	Day	3 4 1 5% 4 3 3 4 6% 7% 59%	2940
	21st to end	Day	3 4 2 5% 5% 4 4 4 6% 8% 55%	2940
July	1st to 10th	Day	2 2 1 4 4 4 4 4 6% 8% 61%	8249
	11th to 20th	Day	2 3 1 5% 5% 5% 7% 6% 9% 10% 47%	8139
	21st to end	Day	3 4 2 7% 6% 6% 7% 6% 8% 9% 42%	8304
August	1st to 10th	Day	3 3 2 4 4 5% 6% 5% 8% 10% 50%	7549
	11th to 20th	Day	2 4 1 6% 5% 6% 6% 5% 8% 11% 48%	7264
	21st to end	Day	4 4 2 7% 6% 7% 7% 6% 9% 10% 38%	7751
September	1st to 10th	Day	5% 5% 2 6% 5% 4 4 4 5% 8% 52%	2520
	11th to 20th	Day	6% 6% 3 6% 5% 4 5% 4 5% 6% 51%	2437
	21st to end	Day	6% 6% 2 6% 3 3 5% 3 4 5% 57%	2132
October	1st to 10th	Day	3 5% 2 4 3 3 2 2 3 4 70%	3270
	11th to 20th	Day	2 7% 2 6% 4 4 5% 5% 7% 10% 48%	3250
	21st to end	Day	2 9% 3 7% 2 3 2 2 4 5% 61%	3548
November	1st to 10th	Day	1 8% 4 6% 3 3 3 3 5% 5% 58%	1913
	11th to 20th	Day	1 11% 3 6% 3 2 3 2 5% 6% 57%	1885
	21st to end	Day	1 9% 4 11% 5% 3 3 2 2 6% 55%	1680
December	1st to 10th	Day	1 7% 4 10% 6% 4 3 2 3 4 58%	2700
	11th to 20th	Day	9% 3 10% 5% 3 3 2 3 6% 56%	3106
	21st to end	Day	1 8% 6% 15% 6% 5% 4 3 5% 7% 39%	4092
January	1st to 10th	Night	6% 6% 19% 8% 5% 4 3 5% 7% 37%	8201
	11th to 20th	Night	7% 4 14% 5% 6% 3 2 3 5% 51%	7673
	21st to end	Night	7% 5% 12% 6% 4 4 3 4 5% 49%	8370
February	1st to 10th	Night	9% 5% 14% 6% 5% 4 3 5% 7% 42%	2730
	11th to 20th	Night	1 10% 6% 15% 7% 5% 4 4 6% 7% 36%	2581
	21st to end	Night	1 12% 6% 14% 7% 3 3 3 5% 6% 41%	2076
March	1st to 10th	Night	10% 10% 12% 5% 4 4 3 4 6% 43%	5230
	11th to 20th	Night	1 8% 6% 12% 4 3 3 2 5% 6% 51%	5110
	21st to end	Night	1 13% 6% 13% 6% 5% 4 4 5% 6% 36%	5470
April	1st to 10th	Night	1 9% 4 10% 5% 4 5% 4 6% 8% 45%	2100
	11th to 20th	Night	1 11% 4 10% 5% 4 4 3 6% 7% 45%	2077
	21st to end	Night	3 12% 4 7% 4 4 3 3 5% 7% 48%	2100
May	1st to 10th	Night	2 8% 3 6% 4 4 4 5% 7% 9% 47%	5553
	11th to 20th	Night	3 6% 2 5% 4 4 5% 5% 8% 9% 50%	4769
	21st to end	Night	2 5% 1 4 4 4 4 4 6% 9% 57%	5181
June	1st to 10th	Night	2 2 1 4 3 3 4 3 6% 8% 65%	1680
	11th to 20th	Night	3 3 2 3 3 3 3 3 5% 8% 65%	1680
	21st to end	Night	1 2 1 3 3 3 4 3 7% 8% 66%	1680
July	1st to 10th	Night	2 2 1 3 3 4 4 4 6% 7% 65%	4740
	11th to 20th	Night	1 1 1 3 4 4 5% 5% 7% 11% 58%	4833
	21st to end	Night	1 2 1 4 3 4 5% 5% 9% 11% 54%	5805
August	1st to 10th	Night	2 2 1 3 3 3 4 5% 7% 10% 61%	5273
	11th to 20th	Night	1 2 1 3 3 3 4 4 7% 11% 61%	5591
	21st to end	Night	3 3 1 5% 4 4 6% 6% 10% 15% 45%	6465
September	1st to 10th	Night	3 3 1 4 3 3 4 4 5% 9% 61%	2100
	11th to 20th	Night	3 4 2 5% 3 3 3 4 4 7% 62%	2030
	21st to end	Night	3 4 1 3 3 2 3 3 5% 7% 67%	2261
October	1st to 10th	Night	2 3 1 2 2 2 3 2 3 5% 76%	3920
	11th to 20th	Night	2 4 2 4 3 3 4 4 7% 10% 58%	3898
	21st to end	Night	3 7% 2 5% 3 2 2 2 3 5% 65%	4252
November	1st to 10th	Night	1 8% 3 5% 3 2 3 2 5% 6% 62%	2583
	11th to 20th	Night	1 10% 5% 7% 4 2 3 3 4 5% 57%	2731
	21st to end	Night	1 9% 4 10% 4 3 2 3 4 6% 53%	2940
December	1st to 10th	Night	7% 3 9% 6% 4 3 2 3 5% 60%	4710
	11th to 20th	Night	7% 3 8% 4 3 4 3 6% 9% 53%	5321
	21st to end	Night	1 12% 6% 14% 8% 5% 4 3 5% 6% 37%	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	50% 6% 10% 20% 12% 3	4756
	11th to 20th	Day	39% 4 16% 20% 18% 4	5340
	21st to end	Day	34% 6% 10% 22% 21% 7%	5844
February	1st to 10th	Day	50% 6% 13% 15% 12% 5%	1890
	11th to 20th	Day	49% 5% 8% 19% 15% 5%	1933
	21st to end	Day	46% 5% 9% 18% 16% 6%	1730
March	1st to 10th	Day	40% 6% 5% 21% 18% 10%	4871
	11th to 20th	Day	40% 10% 11% 25% 11% 2	5110
	21st to end	Day	50% 5% 13% 22% 8% 1	5741
April	1st to 10th	Day	41% 7% 11% 20% 16% 5%	2520
	11th to 20th	Day	48% 4 11% 17% 17% 2	2494
	21st to end	Day	48% 6% 9% 23% 12% 2	2520
May	1st to 10th	Day	37% 8% 13% 23% 17% 1	7391
	11th to 20th	Day	33% 9% 12% 25% 20% 2	8113
	21st to end	Day	25% 9% 13% 27% 23% 3	9027
June	1st to 10th	Day	27% 6% 13% 25% 23% 6%	2940
	11th to 20th	Day	24% 4 7% 20% 30% 15%	2940
	21st to end	Day	27% 7% 8% 18% 26% 15%	2940
July	1st to 10th	Day	19% 7% 6% 10% 26% 32%	8181
	11th to 20th	Day	26% 10% 13% 22% 19% 10%	8064
	21st to end	Day	33% 11% 16% 12% 15% 12%	8120
August	1st to 10th	Day	28% 9% 13% 13% 22% 14%	5978
	11th to 20th	Day	33% 12% 17% 17% 16% 5%	5723
	21st to end	Day	32% 14% 15% 20% 15% 4	6078
September	1st to 10th	Day	31% 7% 9% 17% 24% 11%	2520
	11th to 20th	Day	34% 7% 8% 18% 24% 9%	2437
	21st to end	Day	31% 5% 6% 23% 28% 6%	2132
October	1st to 10th	Day	21% 4 4 27% 35% 9%	3270
	11th to 20th	Day	28% 8% 10% 31% 19% 5%	3250
	21st to end	Day	26% 4 9% 35% 23% 3	3548
November	1st to 10th	Day	27% 5% 11% 26% 28% 3	1913
	11th to 20th	Day	29% 3 16% 27% 19% 6%	1885
	21st to end	Day	34% 3 9% 30% 15% 8%	1680
December	1st to 10th	Day	32% 5% 10% 33% 16% 3	2700
	11th to 20th	Day	31% 4 16% 28% 20% 1	3104
	21st to end	Day	43% 6% 11% 16% 20% 4	4092
January	1st to 10th	Night	44% 6% 13% 22% 12% 3	8202
	11th to 20th	Night	36% 3 13% 24% 19% 5%	7673
	21st to end	Night	37% 4 11% 20% 21% 7%	8370
February	1st to 10th	Night	42% 6% 13% 20% 14% 4	2730
	11th to 20th	Night	46% 5% 11% 20% 14% 3	2582
	21st to end	Night	44% 3 8% 22% 17% 6%	2076
March	1st to 10th	Night	41% 5% 8% 19% 19% 8%	5230
	11th to 20th	Night	33% 6% 11% 24% 22% 3	5110
	21st to end	Night	46% 7% 17% 22% 7% 1	5470
April	1st to 10th	Night	36% 8% 10% 25% 18% 3	2100
	11th to 20th	Night	38% 5% 12% 25% 18% 2	2077
	21st to end	Night	36% 5% 13% 29% 16% 1	2100
May	1st to 10th	Night	31% 9% 14% 32% 13% 1	5551
	11th to 20th	Night	25% 9% 16% 30% 20% 1	4770
	21st to end	Night	21% 7% 14% 31% 25% 2	5182
June	1st to 10th	Night	18% 5% 14% 29% 28% 6%	1680
	11th to 20th	Night	18% 4 11% 19% 31% 16%	1680
	21st to end	Night	16% 6% 10% 24% 25% 19%	1680
July	1st to 10th	Night	17% 6% 8% 11% 28% 32%	4700
	11th to 20th	Night	17% 8% 18% 32% 15% 10%	4785
	21st to end	Night	20% 11% 26% 20% 14% 9%	5677
August	1st to 10th	Night	19% 9% 18% 17% 24% 13%	4168
	11th to 20th	Night	19% 13% 27% 19% 16% 7%	4408
	21st to end	Night	23% 13% 26% 17% 16% 5%	5069
September	1st to 10th	Night	21% 7% 14% 20% 28% 10%	2100
	11th to 20th	Night	23% 6% 14% 22% 28% 8%	2030
	21st to end	Night	19% 5% 11% 32% 29% 5%	2261
October	1st to 10th	Night	15% 4 6% 25% 40% 9%	3920
	11th to 20th	Night	19% 7% 11% 40% 19% 4	3898
	21st to end	Night	23% 4 9% 31% 29% 5%	4252
November	1st to 10th	Night	25% 5% 11% 28% 28% 4	2583
	11th to 20th	Night	28% 4 14% 28% 20% 6%	2731
	21st to end	Night	34% 4 11% 25% 20% 7%	2940
December	1st to 10th	Night	29% 4 11% 37% 17% 3	4710
	11th to 20th	Night	27% 5% 16% 28% 21% 2	5322
	21st to end	Night	46% 5% 10% 17% 17% 5%	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	19% 21% 30% 21% 8% 1	330
	11th to 20th	Day	18% 28% 28% 20% 5% 1	330
	21st to end	Day	14% 22% 38% 18% 7% 1	363
February	1st to 10th	Day	29% 31% 25% 12% 2 1	330
	11th to 20th	Day	28% 30% 26% 12% 3 1	330
	21st to end	Day	29% 31% 23% 15% 2	273
March	1st to 10th	Day	25% 30% 24% 17% 4	445
	11th to 20th	Day	23% 28% 31% 15% 3	540
	21st to end	Day	22% 34% 32% 10% 2	594
April	1st to 10th	Day	21% 32% 29% 14% 4 1	540
	11th to 20th	Day	28% 31% 29% 10% 2	532
	21st to end	Day	20% 31% 32% 14% 2 1	540
May	1st to 10th	Day	23% 33% 26% 13% 4	543
	11th to 20th	Day	14% 21% 31% 23% 9% 2	743
	21st to end	Day	15% 20% 32% 22% 10% 2	844
June	1st to 10th	Day	8% 16% 33% 27% 15% 3	780
	11th to 20th	Day	9% 20% 36% 22% 11% 1	780
	21st to end	Day	8% 19% 34% 23% 14% 2	780
July	1st to 10th	Day	7% 11% 31% 30% 18% 4	771
	11th to 20th	Day	6% 7% 29% 33% 20% 6%	735
	21st to end	Day	12% 11% 29% 29% 15% 3	619
August	1st to 10th	Day	13% 12% 32% 27% 13% 2	567
	11th to 20th	Day	6% 10% 40% 26% 14% 3	557
	21st to end	Day	6% 14% 36% 30% 11% 3	627
September	1st to 10th	Day	8% 17% 35% 26% 10% 4	570
	11th to 20th	Day	12% 19% 37% 22% 6% 3	543
	21st to end	Day	13% 21% 31% 21% 12% 3	392
October	1st to 10th	Day	16% 20% 25% 22% 15% 2	330
	11th to 20th	Day	19% 27% 27% 17% 8% 2	330
	21st to end	Day	17% 27% 31% 18% 7% 1	363
November	1st to 10th	Day	19% 29% 26% 18% 7% 1	330
	11th to 20th	Day	14% 28% 32% 18% 6% 2	330
	21st to end	Day	16% 26% 31% 21% 6% 1	330
December	1st to 10th	Day	18% 27% 28% 18% 7% 2	330
	11th to 20th	Day	18% 25% 35% 15% 5% 2	330
	21st to end	Day	21% 25% 35% 14% 3 1	363
January	1st to 10th	Night	10% 17% 31% 28% 12% 2	1050
	11th to 20th	Night	12% 14% 33% 28% 11% 2	850
	21st to end	Night	5% 16% 35% 32% 9% 2	924
February	1st to 10th	Night	10% 18% 39% 27% 6%	840
	11th to 20th	Night	10% 18% 41% 24% 8%	824
	21st to end	Night	13% 16% 41% 27% 3	692
March	1st to 10th	Night	13% 15% 35% 28% 8% 1	840
	11th to 20th	Night	8% 14% 34% 31% 12%	840
	21st to end	Night	9% 16% 39% 28% 7%	861
April	1st to 10th	Night	10% 20% 38% 26% 6%	630
	11th to 20th	Night	9% 20% 34% 27% 9%	623
	21st to end	Night	10% 16% 36% 28% 9% 2	630
May	1st to 10th	Night	9% 12% 37% 31% 10% 1	630
	11th to 20th	Night	9% 8% 30% 34% 17% 2	621
	21st to end	Night	8% 12% 29% 34% 15% 2	683
June	1st to 10th	Night	4 7% 26% 43% 19% 1	630
	11th to 20th	Night	5% 9% 31% 39% 15% 1	630
	21st to end	Night	2 4 30% 41% 23% 1	630
July	1st to 10th	Night	4 3 21% 40% 30% 2	615
	11th to 20th	Night	2 3 14% 44% 33% 3	612
	21st to end	Night	4 3 15% 38% 36% 4	690
August	1st to 10th	Night	6% 2 18% 41% 31% 1	627
	11th to 20th	Night	4 4 16% 46% 28% 2	624
	21st to end	Night	5% 2 18% 39% 32% 3	693
September	1st to 10th	Night	5% 4 19% 44% 27% 1	630
	11th to 20th	Night	5% 6% 21% 39% 25% 4	609
	21st to end	Night	6% 8% 21% 36% 22% 6%	725
October	1st to 10th	Night	7% 11% 30% 31% 18% 3	800
	11th to 20th	Night	9% 13% 29% 32% 14% 4	800
	21st to end	Night	10% 13% 29% 29% 15% 3	880
November	1st to 10th	Night	8% 14% 32% 31% 13% 2	943
	11th to 20th	Night	10% 14% 35% 29% 10% 2	1050
	21st to end	Night	10% 14% 29% 32% 13% 1	1050
December	1st to 10th	Night	10% 15% 36% 30% 8% 1	1050
	11th to 20th	Night	11% 18% 34% 26% 10% 1	1050
	21st to end	Night	12% 15% 40% 27% 5% 1	1155

Page generated at 2026-09-11 02:11:57. Version ac07

Copyright © 2026 Attilla Danko.