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 2025-05-11.

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 8% 4 10% 5% 5% 4 4 6% 7% 46%	117956
	Night	1 7% 4 9% 5% 4 4 3 5% 7% 51%	119928
Transparency	Day	37% 7% 10% 21% 19% 7%	117711
	Night	32% 6% 12% 24% 20% 6%	119763
Seeing	Day	15% 22% 31% 21% 9% 2	16098
	Night	8% 12% 29% 33% 16% 2	26308

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	6% 4 22% 12% 5% 4 2 5% 5% 36%	3657
	11th to 20th	Day	8% 6% 14% 9% 7% 5% 3 4 6% 37%	4100
	21st to end	Day	7% 6% 12% 6% 4 4 3 4 5% 49%	4510
February	1st to 10th	Day	1 12% 7% 17% 7% 5% 3 3 5% 5% 35%	1800
	11th to 20th	Day	1 11% 6% 15% 7% 5% 4 3 4 6% 37%	1830
	21st to end	Day	1 8% 8% 17% 9% 5% 3 3 3 6% 38%	1650
March	1st to 10th	Day	7% 8% 14% 6% 4 4 3 4 6% 43%	4766
	11th to 20th	Day	8% 6% 14% 7% 5% 5% 4 6% 8% 37%	5000
	21st to end	Day	1 13% 7% 15% 6% 5% 4 4 5% 6% 34%	5617
April	1st to 10th	Day	2 13% 6% 10% 5% 4 4 4 7% 6% 41%	2400
	11th to 20th	Day	2 16% 6% 11% 6% 5% 5% 4 5% 6% 35%	2371
	21st to end	Day	2 14% 6% 10% 6% 5% 5% 4 6% 8% 35%	2400
May	1st to 10th	Day	3 14% 7% 9% 5% 5% 5% 4 7% 8% 33%	4105
	11th to 20th	Day	3 10% 3 7% 5% 5% 6% 5% 8% 8% 39%	4348
	21st to end	Day	3 8% 2 6% 5% 5% 5% 5% 8% 10% 44%	4825
June	1st to 10th	Day	3 4 2 6% 4 5% 6% 4 6% 9% 50%	2660
	11th to 20th	Day	4 4 2 5% 4 4 3 4 6% 7% 58%	2660
	21st to end	Day	3 4 2 6% 5% 5% 4 5% 6% 8% 53%	2660
July	1st to 10th	Day	4 4 1 5% 4 4 4 3 4 5% 61%	4430
	11th to 20th	Day	2 4 2 5% 5% 5% 6% 6% 9% 11% 44%	4375
	21st to end	Day	5% 4 2 6% 6% 5% 6% 4 6% 7% 49%	4454
August	1st to 10th	Day	5% 5% 3 7% 6% 7% 7% 7% 8% 11% 36%	4092
	11th to 20th	Day	4 5% 2 7% 5% 6% 6% 5% 7% 8% 43%	3857
	21st to end	Day	4 4 3 7% 6% 6% 5% 5% 8% 9% 44%	4180
September	1st to 10th	Day	5% 5% 2 7% 5% 5% 4 4 5% 7% 51%	2280
	11th to 20th	Day	6% 6% 3 6% 5% 3 4 4 5% 6% 51%	2280
	21st to end	Day	6% 6% 2 5% 3 3 4 3 4 5% 58%	2026
October	1st to 10th	Day	2 5% 2 4 3 3 2 2 3 4 70%	3170
	11th to 20th	Day	2 7% 2 6% 4 4 5% 5% 7% 10% 48%	3150
	21st to end	Day	2 9% 3 7% 2 3 2 2 4 5% 61%	3438
November	1st to 10th	Day	1 8% 4 7% 3 3 3 3 5% 5% 57%	1820
	11th to 20th	Day	1 11% 3 6% 3 2 3 2 5% 6% 56%	1795
	21st to end	Day	1 9% 4 11% 5% 3 2 2 2 6% 56%	1600
December	1st to 10th	Day	1 7% 4 10% 5% 4 3 2 3 4 58%	2620
	11th to 20th	Day	9% 3 10% 5% 3 3 2 3 6% 55%	3026
	21st to end	Day	1 9% 6% 15% 6% 5% 4 3 5% 7% 38%	4004
January	1st to 10th	Night	7% 6% 20% 8% 4 3 3 4 6% 39%	6327
	11th to 20th	Night	9% 5% 17% 7% 8% 3 2 4 5% 41%	5903
	21st to end	Night	8% 5% 13% 6% 4 4 3 4 5% 48%	6468
February	1st to 10th	Night	9% 6% 14% 6% 5% 4 3 5% 7% 40%	2600
	11th to 20th	Night	1 10% 6% 14% 7% 5% 4 4 5% 7% 37%	2455
	21st to end	Night	1 12% 6% 14% 7% 3 3 3 5% 6% 41%	1980
March	1st to 10th	Night	10% 10% 12% 5% 4 4 3 4 6% 42%	5116
	11th to 20th	Night	1 8% 6% 12% 4 3 3 3 5% 6% 50%	5000
	21st to end	Night	1 13% 7% 13% 6% 5% 4 4 5% 6% 35%	5352
April	1st to 10th	Night	1 9% 4 10% 5% 4 5% 4 6% 8% 45%	2000
	11th to 20th	Night	1 11% 4 10% 4 4 4 3 6% 7% 45%	1977
	21st to end	Night	3 12% 4 7% 4 4 3 3 5% 7% 48%	2000
May	1st to 10th	Night	2 10% 5% 7% 5% 5% 4 5% 6% 8% 44%	3082
	11th to 20th	Night	4 7% 2 4 4 4 5% 5% 7% 10% 49%	2562
	21st to end	Night	2 6% 2 5% 3 3 4 3 6% 8% 57%	2762
June	1st to 10th	Night	1 3 1 4 3 3 4 3 6% 8% 64%	1520
	11th to 20th	Night	3 3 2 3 3 3 3 3 6% 8% 63%	1520
	21st to end	Night	1 2 3 3 3 5% 4 7% 8% 65%	1520
July	1st to 10th	Night	2 3 1 3 4 4 4 4 4 6% 65%	2540
	11th to 20th	Night	1 2 1 3 4 4 5% 5% 8% 11% 56%	2591
	21st to end	Night	2 3 1 4 4 4 5% 4 7% 7% 60%	3109
August	1st to 10th	Night	3 3 1 4 4 5% 6% 7% 10% 11% 45%	2847
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 9% 10% 55%	2981
	21st to end	Night	3 2 1 5% 4 4 5% 5% 7% 11% 53%	3487
September	1st to 10th	Night	3 3 1 4 3 3 4 4 5% 9% 60%	1900
	11th to 20th	Night	3 4 2 5% 3 3 3 3 4 7% 63%	1900
	21st to end	Night	3 4 1 3 3 2 3 3 5% 6% 67%	2148
October	1st to 10th	Night	2 4 1 2 2 2 3 2 3 4 76%	3800
	11th to 20th	Night	2 4 2 4 3 3 4 4 7% 10% 58%	3778
	21st to end	Night	3 7% 2 5% 3 2 2 2 3 5% 65%	4120
November	1st to 10th	Night	1 9% 3 6% 3 2 3 2 5% 6% 61%	2457
	11th to 20th	Night	1 10% 5% 7% 4 2 3 3 4 5% 57%	2601
	21st to end	Night	1 9% 4 10% 4 3 2 3 4 6% 54%	2800
December	1st to 10th	Night	6% 3 8% 6% 4 2 2 3 5% 61%	4570
	11th to 20th	Night	7% 3 8% 4 3 4 3 6% 9% 52%	5181
	21st to end	Night	1 12% 6% 14% 8% 5% 4 3 5% 6% 36%	6974

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% 9% 16% 15% 4	3658
	11th to 20th	Day	49% 3 13% 14% 19% 2	4100
	21st to end	Day	36% 5% 9% 21% 21% 7%	4510
February	1st to 10th	Day	52% 5% 13% 13% 12% 5%	1800
	11th to 20th	Day	49% 5% 8% 19% 15% 5%	1839
	21st to end	Day	48% 5% 9% 17% 15% 6%	1650
March	1st to 10th	Day	41% 6% 6% 20% 18% 10%	4766
	11th to 20th	Day	41% 10% 11% 25% 11% 2	5000
	21st to end	Day	51% 5% 13% 22% 8% 1	5617
April	1st to 10th	Day	42% 7% 11% 20% 15% 5%	2400
	11th to 20th	Day	50% 4 11% 17% 16% 2	2374
	21st to end	Day	48% 5% 9% 24% 12% 1	2400
May	1st to 10th	Day	48% 7% 10% 19% 15% 1	4105
	11th to 20th	Day	38% 9% 12% 27% 14% 1	4352
	21st to end	Day	31% 7% 10% 24% 22% 5%	4825
June	1st to 10th	Day	28% 5% 14% 25% 21% 7%	2660
	11th to 20th	Day	26% 4 7% 20% 28% 14%	2660
	21st to end	Day	29% 7% 8% 19% 25% 14%	2660
July	1st to 10th	Day	25% 7% 5% 9% 25% 29%	4361
	11th to 20th	Day	28% 10% 12% 22% 20% 8%	4300
	21st to end	Day	33% 7% 11% 14% 17% 18%	4314
August	1st to 10th	Day	38% 12% 15% 13% 15% 7%	4078
	11th to 20th	Day	37% 10% 11% 13% 21% 7%	3896
	21st to end	Day	33% 11% 10% 19% 21% 6%	4179
September	1st to 10th	Day	33% 7% 8% 15% 25% 11%	2280
	11th to 20th	Day	34% 7% 8% 18% 25% 9%	2280
	21st to end	Day	31% 5% 6% 24% 30% 6%	2026
October	1st to 10th	Day	21% 4 4 27% 35% 9%	3170
	11th to 20th	Day	28% 8% 10% 31% 19% 5%	3150
	21st to end	Day	26% 4 9% 34% 23% 3	3438
November	1st to 10th	Day	28% 5% 11% 24% 28% 3	1820
	11th to 20th	Day	29% 3 16% 25% 20% 6%	1795
	21st to end	Day	34% 3 9% 30% 16% 8%	1600
December	1st to 10th	Day	32% 4 10% 34% 16% 4	2620
	11th to 20th	Day	32% 4 16% 27% 20%	3024
	21st to end	Day	44% 6% 11% 16% 19% 4	4004
January	1st to 10th	Night	46% 4 12% 19% 16% 4	6327
	11th to 20th	Night	45% 3 12% 17% 18% 6%	5903
	21st to end	Night	40% 4 10% 18% 22% 7%	6468
February	1st to 10th	Night	44% 6% 13% 18% 14% 4	2600
	11th to 20th	Night	45% 5% 11% 21% 15% 4	2456
	21st to end	Night	44% 3 9% 21% 17% 6%	1980
March	1st to 10th	Night	42% 5% 8% 19% 19% 8%	5116
	11th to 20th	Night	34% 6% 11% 24% 22% 3	5000
	21st to end	Night	47% 7% 17% 22% 6% 1	5352
April	1st to 10th	Night	36% 8% 10% 24% 18% 3	2000
	11th to 20th	Night	38% 5% 12% 25% 18% 2	1977
	21st to end	Night	36% 5% 13% 29% 16%	2000
May	1st to 10th	Night	37% 7% 12% 25% 17% 2	3082
	11th to 20th	Night	28% 7% 16% 34% 14%	2563
	21st to end	Night	23% 6% 10% 29% 28% 4	2762
June	1st to 10th	Night	19% 5% 15% 29% 26% 6%	1520
	11th to 20th	Night	19% 4 12% 19% 30% 15%	1520
	21st to end	Night	17% 6% 10% 25% 24% 17%	1520
July	1st to 10th	Night	21% 6% 9% 10% 28% 25%	2500
	11th to 20th	Night	19% 8% 18% 30% 18% 8%	2543
	21st to end	Night	21% 8% 17% 23% 15% 15%	3011
August	1st to 10th	Night	26% 12% 23% 18% 13% 8%	2838
	11th to 20th	Night	22% 10% 22% 15% 21% 9%	3008
	21st to end	Night	26% 10% 13% 21% 23% 7%	3487
September	1st to 10th	Night	22% 7% 14% 19% 27% 11%	1900
	11th to 20th	Night	23% 5% 14% 21% 28% 9%	1900
	21st to end	Night	19% 5% 10% 32% 30% 4	2148
October	1st to 10th	Night	15% 4 6% 25% 41% 9%	3800
	11th to 20th	Night	19% 7% 11% 40% 18% 4	3778
	21st to end	Night	23% 4 9% 31% 29% 5%	4120
November	1st to 10th	Night	26% 5% 11% 27% 28% 4	2457
	11th to 20th	Night	29% 3 13% 27% 21% 6%	2601
	21st to end	Night	33% 4 10% 25% 21% 7%	2800
December	1st to 10th	Night	28% 4 10% 37% 18% 3	4570
	11th to 20th	Night	27% 5% 16% 28% 21% 2	5182
	21st to end	Night	47% 4 9% 17% 17% 5%	6974

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% 20% 29% 22% 9% 1	300
	11th to 20th	Day	17% 29% 27% 21% 5% 1	300
	21st to end	Day	12% 22% 37% 20% 7% 1	330
February	1st to 10th	Day	31% 32% 24% 12% 1 1	300
	11th to 20th	Day	27% 31% 26% 13% 3 1	300
	21st to end	Day	28% 32% 22% 16% 2	249
March	1st to 10th	Day	26% 30% 24% 16% 4	410
	11th to 20th	Day	21% 28% 32% 15% 4	500
	21st to end	Day	23% 34% 31% 10% 2	550
April	1st to 10th	Day	21% 32% 29% 13% 4 1	500
	11th to 20th	Day	28% 32% 28% 10% 2	492
	21st to end	Day	18% 32% 33% 14% 2 1	500
May	1st to 10th	Day	24% 33% 25% 13% 5%	503
	11th to 20th	Day	13% 21% 31% 25% 10% 2	647
	21st to end	Day	15% 20% 30% 22% 11% 2	734
June	1st to 10th	Day	7% 15% 31% 28% 16% 3	680
	11th to 20th	Day	9% 19% 35% 24% 12% 1	680
	21st to end	Day	6% 17% 34% 25% 16% 2	680
July	1st to 10th	Day	8% 9% 30% 31% 19% 4	671
	11th to 20th	Day	7% 6% 28% 33% 22% 6%	639
	21st to end	Day	14% 11% 29% 30% 14% 2	531
August	1st to 10th	Day	14% 10% 31% 28% 14% 2	487
	11th to 20th	Day	7% 10% 40% 26% 13% 4	477
	21st to end	Day	6% 13% 35% 31% 11% 3	539
September	1st to 10th	Day	9% 17% 32% 27% 10% 5%	490
	11th to 20th	Day	12% 18% 37% 22% 7% 3	490
	21st to end	Day	13% 20% 31% 21% 12% 3	359
October	1st to 10th	Day	16% 20% 23% 22% 16% 3	300
	11th to 20th	Day	16% 27% 28% 18% 9% 3	300
	21st to end	Day	16% 28% 30% 17% 7% 1	330
November	1st to 10th	Day	20% 30% 25% 17% 8% 1	300
	11th to 20th	Day	15% 29% 30% 18% 7% 2	300
	21st to end	Day	16% 26% 29% 21% 6% 1	300
December	1st to 10th	Day	18% 25% 28% 20% 7% 2	300
	11th to 20th	Day	18% 26% 34% 16% 5% 2	300
	21st to end	Day	20% 27% 34% 15% 3 1	330
January	1st to 10th	Night	11% 17% 29% 29% 12% 3	1000
	11th to 20th	Night	12% 14% 31% 29% 12% 2	810
	21st to end	Night	5% 16% 33% 34% 9% 2	880
February	1st to 10th	Night	10% 18% 36% 28% 7%	800
	11th to 20th	Night	10% 17% 40% 25% 9%	784
	21st to end	Night	13% 16% 40% 28% 3	660
March	1st to 10th	Night	13% 15% 34% 29% 8% 1	800
	11th to 20th	Night	7% 14% 32% 33% 13% 1	800
	21st to end	Night	10% 17% 37% 30% 7%	820
April	1st to 10th	Night	9% 20% 38% 26% 6%	600
	11th to 20th	Night	10% 20% 32% 28% 9% 1	593
	21st to end	Night	10% 16% 34% 29% 9% 2	600
May	1st to 10th	Night	10% 12% 36% 32% 10% 1	600
	11th to 20th	Night	9% 8% 27% 36% 18% 2	563
	21st to end	Night	8% 12% 27% 35% 17% 2	617
June	1st to 10th	Night	4 7% 24% 44% 20% 2	570
	11th to 20th	Night	6% 9% 27% 41% 16% 1	570
	21st to end	Night	2 4 26% 42% 25% 1	570
July	1st to 10th	Night	4 3 16% 41% 33% 3	555
	11th to 20th	Night	2 2 11% 45% 37% 3	552
	21st to end	Night	4 3 12% 38% 39% 4	624
August	1st to 10th	Night	7% 2 13% 42% 35% 2	567
	11th to 20th	Night	4 4 14% 44% 31% 3	564
	21st to end	Night	5% 2 16% 38% 36% 4	627
September	1st to 10th	Night	5% 4 13% 46% 30% 2	570
	11th to 20th	Night	5% 6% 19% 40% 26% 4	570
	21st to end	Night	6% 9% 19% 36% 23% 7%	689
October	1st to 10th	Night	7% 11% 28% 32% 19% 3	760
	11th to 20th	Night	8% 12% 28% 34% 14% 4	760
	21st to end	Night	11% 14% 26% 30% 16% 3	836
November	1st to 10th	Night	8% 15% 29% 32% 14% 2	897
	11th to 20th	Night	11% 14% 33% 29% 11% 2	1000
	21st to end	Night	10% 15% 28% 33% 14% 1	1000
December	1st to 10th	Night	10% 14% 35% 30% 9% 1	1000
	11th to 20th	Night	11% 18% 32% 27% 11% 1	1000
	21st to end	Night	12% 15% 38% 28% 5% 1	1100

Page generated at 2025-05-08 07:33:54. Version ac07

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