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 2024-11-01.

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 8% 4 10% 5% 5% 4 4 6% 7% 44%	101282
	Night	1 8% 4 9% 5% 4 4 3 5% 7% 49%	97918
Transparency	Day	39% 7% 10% 20% 18% 7%	101037
	Night	34% 6% 13% 23% 19% 6%	97753
Seeing	Day	15% 22% 30% 21% 9% 2	15457
	Night	9% 12% 28% 34% 16% 2	25520

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% 4 19% 12% 4 3 3 5% 6% 36%	2543
	11th to 20th	Day	12% 8% 16% 9% 6% 3 3 3 6% 34%	2860
	21st to end	Day	1 10% 8% 16% 5% 3 3 2 4 6% 44%	3146
February	1st to 10th	Day	1 13% 8% 18% 7% 5% 3 2 4 5% 35%	1710
	11th to 20th	Day	1 11% 6% 15% 7% 5% 4 3 4 5% 38%	1736
	21st to end	Day	1 9% 8% 17% 9% 5% 3 3 3 6% 37%	1570
March	1st to 10th	Day	1 8% 9% 16% 6% 4 4 3 5% 5% 40%	3312
	11th to 20th	Day	10% 6% 12% 5% 4 4 4 6% 7% 41%	3490
	21st to end	Day	1 14% 9% 18% 7% 6% 4 3 5% 5% 27%	3917
April	1st to 10th	Day	2 13% 6% 10% 5% 4 4 4 6% 6% 41%	2280
	11th to 20th	Day	2 17% 6% 11% 6% 5% 5% 4 5% 6% 34%	2251
	21st to end	Day	2 15% 6% 10% 6% 5% 5% 4 6% 7% 35%	2280
May	1st to 10th	Day	3 14% 7% 10% 5% 5% 5% 4 7% 8% 32%	3979
	11th to 20th	Day	3 10% 3 7% 5% 5% 6% 5% 8% 8% 39%	4346
	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	4 8% 3 7% 4 5% 4 3 5% 5% 52%	1900
	11th to 20th	Day	3 10% 2 5% 3 2 3 3 5% 7% 59%	1900
	21st to end	Day	3 10% 3 9% 2 3 3 2 4 5% 57%	2090
November	1st to 10th	Day	2 8% 4 6% 3 3 3 3 5% 5% 58%	1727
	11th to 20th	Day	2 12% 3 6% 3 2 3 2 5% 6% 56%	1706
	21st to end	Day	1 8% 4 11% 5% 3 3 2 2 6% 55%	1520
December	1st to 10th	Day	1 13% 6% 14% 6% 4 4 2 3 4 43%	1520
	11th to 20th	Day	12% 4 13% 6% 4 3 3 4 6% 44%	1926
	21st to end	Day	1 9% 6% 16% 6% 5% 4 3 4 7% 39%	2794
January	1st to 10th	Night	6% 6% 21% 9% 4 3 3 4 6% 37%	4417
	11th to 20th	Night	11% 7% 18% 5% 8% 3 2 4 5% 35%	4133
	21st to end	Night	12% 6% 16% 6% 3 3 3 4 5% 41%	4521
February	1st to 10th	Night	9% 6% 15% 7% 5% 4 3 5% 7% 40%	2470
	11th to 20th	Night	1 10% 5% 14% 6% 4 4 4 6% 7% 38%	2329
	21st to end	Night	1 13% 6% 15% 7% 3 3 3 5% 6% 39%	1884
March	1st to 10th	Night	10% 11% 13% 6% 4 4 3 4 6% 40%	3554
	11th to 20th	Night	1 11% 5% 9% 4 3 3 3 5% 7% 50%	3490
	21st to end	Night	1 12% 6% 16% 7% 6% 5% 4 6% 5% 33%	3733
April	1st to 10th	Night	1 10% 4 10% 5% 4 5% 4 6% 8% 44%	1900
	11th to 20th	Night	1 12% 4 10% 4 4 4 3 6% 7% 44%	1877
	21st to end	Night	3 13% 4 7% 4 4 3 3 5% 7% 47%	1900
May	1st to 10th	Night	2 10% 5% 7% 5% 5% 4 5% 6% 8% 44%	2988
	11th to 20th	Night	4 7% 2 4 4 4 5% 5% 7% 10% 49%	2558
	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	3 6% 2 4 3 3 4 3 5% 7% 61%	2280
	11th to 20th	Night	2 6% 2 3 2 3 3 3 4 7% 64%	2280
	21st to end	Night	3 10% 3 7% 3 2 2 2 3 5% 60%	2508
November	1st to 10th	Night	1 8% 3 5% 3 2 3 2 5% 6% 62%	2337
	11th to 20th	Night	1 11% 5% 7% 4 2 3 3 4 5% 56%	2470
	21st to end	Night	1 8% 4 10% 4 3 2 3 4 6% 54%	2660
December	1st to 10th	Night	11% 5% 10% 6% 5% 3 3 4 5% 48%	2660
	11th to 20th	Night	10% 5% 10% 5% 3 4 3 7% 9% 43%	3271
	21st to end	Night	8% 5% 15% 9% 5% 4 3 4 7% 40%	4873

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% 5% 10% 16% 17% 3	2544
	11th to 20th	Day	54% 3 15% 16% 10% 2	2860
	21st to end	Day	44% 4 10% 24% 14% 5%	3146
February	1st to 10th	Day	53% 5% 13% 13% 12% 4	1710
	11th to 20th	Day	48% 4 8% 19% 16% 5%	1745
	21st to end	Day	49% 5% 9% 17% 15% 5%	1570
March	1st to 10th	Day	46% 5% 6% 22% 15% 5%	3312
	11th to 20th	Day	40% 10% 11% 29% 8% 2	3490
	21st to end	Day	60% 5% 9% 16% 10% 1	3917
April	1st to 10th	Day	43% 7% 10% 19% 15% 5%	2280
	11th to 20th	Day	51% 4 11% 16% 15% 2	2254
	21st to end	Day	49% 5% 9% 24% 12% 1	2280
May	1st to 10th	Day	49% 7% 9% 19% 15% 1	3979
	11th to 20th	Day	38% 9% 12% 27% 14% 1	4350
	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	35% 6% 6% 23% 23% 7%	1900
	11th to 20th	Day	27% 5% 7% 29% 24% 8%	1900
	21st to end	Day	32% 4 11% 30% 19% 5%	2090
November	1st to 10th	Day	27% 5% 11% 24% 29% 3	1727
	11th to 20th	Day	30% 3 16% 26% 19% 5%	1706
	21st to end	Day	35% 3 9% 30% 14% 9%	1520
December	1st to 10th	Day	46% 5% 13% 19% 13% 3	1520
	11th to 20th	Day	42% 4 9% 26% 17% 1	1924
	21st to end	Day	46% 5% 7% 14% 22% 6%	2794
January	1st to 10th	Night	48% 4 12% 19% 14% 3	4417
	11th to 20th	Night	51% 3 14% 19% 11% 2	4133
	21st to end	Night	48% 3 10% 21% 12% 5%	4521
February	1st to 10th	Night	45% 6% 14% 18% 14% 4	2470
	11th to 20th	Night	44% 5% 11% 21% 15% 3	2330
	21st to end	Night	46% 3 9% 21% 16% 5%	1884
March	1st to 10th	Night	46% 5% 8% 20% 17% 4	3554
	11th to 20th	Night	35% 6% 12% 31% 13% 3	3490
	21st to end	Night	50% 6% 15% 19% 8% 1	3733
April	1st to 10th	Night	37% 8% 11% 23% 18% 3	1900
	11th to 20th	Night	39% 5% 12% 24% 17% 2	1877
	21st to end	Night	38% 5% 13% 28% 15%	1900
May	1st to 10th	Night	37% 7% 12% 25% 17% 2	2988
	11th to 20th	Night	29% 7% 16% 34% 14%	2559
	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	25% 6% 10% 27% 24% 8%	2280
	11th to 20th	Night	22% 5% 9% 33% 24% 7%	2280
	21st to end	Night	30% 3 11% 29% 21% 6%	2508
November	1st to 10th	Night	25% 5% 11% 27% 28% 4	2337
	11th to 20th	Night	29% 3 14% 28% 20% 5%	2470
	21st to end	Night	33% 4 10% 25% 20% 8%	2660
December	1st to 10th	Night	39% 5% 16% 20% 16% 4	2660
	11th to 20th	Night	38% 5% 10% 23% 20% 4	3272
	21st to end	Night	45% 3 10% 15% 20% 7%	4873

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% 19% 27% 23% 10% 1	270
	11th to 20th	Day	18% 30% 25% 21% 6% 1	270
	21st to end	Day	12% 22% 36% 22% 7% 1	297
February	1st to 10th	Day	28% 33% 24% 13% 1 1	270
	11th to 20th	Day	26% 31% 26% 13% 4 1	270
	21st to end	Day	30% 34% 20% 14% 2	225
March	1st to 10th	Day	27% 29% 24% 16% 4	374
	11th to 20th	Day	20% 28% 32% 15% 4	460
	21st to end	Day	24% 35% 31% 9% 2	506
April	1st to 10th	Day	21% 32% 29% 13% 3 1	460
	11th to 20th	Day	29% 31% 28% 10% 2	452
	21st to end	Day	18% 32% 32% 13% 3 1	460
May	1st to 10th	Day	25% 33% 25% 13% 4	463
	11th to 20th	Day	13% 21% 31% 24% 10% 2	646
	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	21% 29% 24% 17% 9% 1	270
	11th to 20th	Day	14% 30% 27% 19% 7% 2	270
	21st to end	Day	15% 26% 28% 23% 7% 1	270
December	1st to 10th	Day	19% 27% 27% 19% 6% 2	270
	11th to 20th	Day	18% 26% 32% 17% 6% 2	270
	21st to end	Day	20% 28% 33% 15% 3 1	297
January	1st to 10th	Night	11% 16% 28% 30% 12% 2	950
	11th to 20th	Night	12% 14% 31% 28% 12% 2	770
	21st to end	Night	5% 16% 31% 35% 10% 3	836
February	1st to 10th	Night	10% 17% 36% 29% 7%	760
	11th to 20th	Night	10% 17% 39% 26% 9%	744
	21st to end	Night	13% 16% 39% 29% 3	628
March	1st to 10th	Night	13% 15% 32% 31% 9% 1	760
	11th to 20th	Night	7% 14% 31% 34% 14% 1	760
	21st to end	Night	10% 17% 35% 30% 8%	779
April	1st to 10th	Night	10% 20% 35% 28% 7%	570
	11th to 20th	Night	10% 20% 30% 29% 10% 1	563
	21st to end	Night	10% 16% 32% 30% 10% 2	570
May	1st to 10th	Night	10% 12% 35% 32% 10% 1	570
	11th to 20th	Night	9% 8% 26% 36% 19% 2	561
	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	9% 14% 28% 33% 15% 2	853
	11th to 20th	Night	11% 13% 31% 31% 11% 3	950
	21st to end	Night	10% 15% 26% 34% 15% 1	950
December	1st to 10th	Night	11% 15% 35% 30% 9% 1	950
	11th to 20th	Night	11% 18% 30% 28% 11% 1	950
	21st to end	Night	12% 15% 38% 29% 5% 1	1045

Page generated at 2024-10-29 09:20:48. Version ac07

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