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 2024-11-02.

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	4 6% 2 6% 4 4 5% 4 6% 9% 49%	101383
	Night	2 5% 2 5% 3 3 4 3 6% 8% 57%	97827
Transparency	Day	29% 10% 14% 22% 18% 7%	101129
	Night	23% 9% 16% 26% 20% 6%	97671
Seeing	Day	11% 30% 32% 18% 7% 2	15593
	Night	8% 33% 27% 20% 10% 1	25595

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	4 2 10% 5% 3 3 3 5% 9% 56%	2538
	11th to 20th	Day	4 2 7% 5% 3 3 2 6% 10% 58%	2643
	21st to end	Day	8% 4 7% 3 3 4 4 5% 9% 53%	3146
February	1st to 10th	Day	5% 4 8% 4 3 4 4 6% 10% 51%	1710
	11th to 20th	Day	3 2 5% 4 4 4 3 7% 9% 58%	1791
	21st to end	Day	4 3 8% 4 3 3 3 6% 12% 52%	1570
March	1st to 10th	Day	1 7% 4 8% 3 4 4 4 5% 6% 55%	3147
	11th to 20th	Day	1 11% 8% 10% 3 3 3 4 5% 9% 43%	3490
	21st to end	Day	2 11% 4 6% 4 4 4 5% 7% 9% 44%	4028
April	1st to 10th	Day	3 8% 2 5% 4 4 4 4 7% 9% 49%	2280
	11th to 20th	Day	7% 12% 3 7% 4 4 6% 4 5% 8% 40%	2251
	21st to end	Day	6% 12% 3 6% 4 5% 6% 5% 7% 8% 38%	2280
May	1st to 10th	Day	8% 14% 2 6% 4 4 5% 4 7% 9% 38%	4053
	11th to 20th	Day	8% 9% 2 6% 5% 4 6% 5% 7% 10% 39%	4388
	21st to end	Day	5% 5% 1 4 4 4 5% 4 8% 10% 50%	4825
June	1st to 10th	Day	6% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2660
	11th to 20th	Day	7% 4 1 5% 4 5% 5% 5% 6% 8% 49%	2660
	21st to end	Day	5% 5% 2 5% 4 6% 6% 4 7% 9% 47%	2660
July	1st to 10th	Day	5% 4 1 4 5% 6% 6% 4 6% 7% 52%	4430
	11th to 20th	Day	4 3 2 6% 6% 6% 7% 6% 8% 9% 43%	4430
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 7% 8% 46%	4680
August	1st to 10th	Day	6% 6% 2 7% 6% 6% 6% 5% 8% 10% 38%	4092
	11th to 20th	Day	5% 3 1 5% 5% 5% 5% 5% 7% 8% 50%	3784
	21st to end	Day	5% 4 2 6% 4 5% 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	6% 4 2 5% 4 4 4 4 6% 6% 55%	2280
	11th to 20th	Day	6% 4 2 5% 4 4 4 3 6% 7% 55%	2280
	21st to end	Day	7% 5% 1 4 3 3 3 3 4 7% 61%	2068
October	1st to 10th	Day	6% 6% 2 6% 4 4 5% 3 5% 6% 55%	1900
	11th to 20th	Day	3 4 2 4 3 3 4 2 6% 6% 62%	1900
	21st to end	Day	3 6% 2 5% 3 4 4 3 4 8% 59%	2039
November	1st to 10th	Day	1 3 1 4 3 3 3 3 5% 9% 65%	1682
	11th to 20th	Day	1 4 2 4 3 3 3 2 7% 9% 62%	1710
	21st to end	Day	1 4 3 7% 4 4 3 4 6% 7% 58%	1568
December	1st to 10th	Day	3 2 7% 5% 4 4 3 5% 10% 57%	1520
	11th to 20th	Day	1 5% 2 5% 4 3 3 3 9% 11% 53%	1926
	21st to end	Day	6% 3 8% 4 4 4 4 7% 9% 52%	2794
January	1st to 10th	Night	3 2 9% 5% 4 4 4 5% 8% 56%	4417
	11th to 20th	Night	2 1 5% 3 3 3 3 6% 9% 67%	4357
	21st to end	Night	1 6% 3 7% 4 3 3 3 4 8% 57%	4521
February	1st to 10th	Night	4 2 6% 4 3 4 2 6% 11% 57%	2470
	11th to 20th	Night	4 2 6% 4 3 3 3 6% 8% 62%	2274
	21st to end	Night	5% 3 6% 4 3 3 3 5% 11% 56%	1884
March	1st to 10th	Night	1 5% 3 5% 4 4 4 3 5% 8% 59%	3716
	11th to 20th	Night	2 10% 5% 8% 3 2 3 3 5% 8% 53%	3490
	21st to end	Night	2 7% 3 5% 4 3 5% 4 5% 8% 55%	3616
April	1st to 10th	Night	2 6% 2 5% 3 3 4 4 5% 8% 58%	1900
	11th to 20th	Night	4 8% 2 6% 3 4 4 4 6% 7% 52%	1877
	21st to end	Night	7% 11% 2 5% 3 3 3 3 6% 8% 48%	1900
May	1st to 10th	Night	6% 10% 2 5% 3 4 4 4 5% 7% 50%	2913
	11th to 20th	Night	8% 9% 2 5% 4 4 4 4 8% 7% 44%	2517
	21st to end	Night	4 5% 1 4 4 3 4 4 5% 7% 60%	2762
June	1st to 10th	Night	4 3 2 4 3 3 3 4 7% 10% 57%	1520
	11th to 20th	Night	4 4 1 3 3 3 3 4 6% 7% 61%	1520
	21st to end	Night	3 3 1 4 3 3 4 4 7% 8% 60%	1520
July	1st to 10th	Night	3 5% 1 4 4 3 4 4 6% 7% 59%	2540
	11th to 20th	Night	3 2 1 4 3 3 5% 4 8% 11% 56%	2540
	21st to end	Night	4 3 1 3 3 3 5% 5% 7% 8% 59%	2878
August	1st to 10th	Night	4 5% 2 5% 4 4 5% 5% 8% 10% 48%	2847
	11th to 20th	Night	4 3 1 4 4 4 4 4 7% 9% 56%	3055
	21st to end	Night	3 4 1 5% 4 4 4 5% 7% 10% 54%	3487
September	1st to 10th	Night	6% 4 1 4 4 3 3 4 5% 6% 59%	1900
	11th to 20th	Night	6% 3 1 3 3 3 3 3 5% 6% 64%	1900
	21st to end	Night	6% 4 2 3 3 3 3 3 5% 5% 64%	2109
October	1st to 10th	Night	4 5% 1 4 3 3 4 3 4 6% 62%	2280
	11th to 20th	Night	2 4 1 3 3 2 3 2 5% 6% 68%	2280
	21st to end	Night	3 6% 3 6% 3 3 3 3 5% 7% 58%	2537
November	1st to 10th	Night	1 3 1 3 3 3 3 3 4 9% 67%	2419
	11th to 20th	Night	1 4 1 4 3 3 3 3 6% 9% 63%	2470
	21st to end	Night	2 4 2 5% 3 4 3 3 6% 8% 59%	2607
December	1st to 10th	Night	1 3 2 5% 2 2 3 3 4 10% 64%	2660
	11th to 20th	Night	1 5% 3 6% 4 4 4 4 6% 11% 53%	3271
	21st to end	Night	1 6% 2 6% 3 4 5% 4 7% 9% 53%	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	22% 14% 21% 21% 18% 3	2539
	11th to 20th	Day	26% 16% 23% 20% 11% 3	2643
	21st to end	Day	28% 10% 15% 25% 17% 5%	3146
February	1st to 10th	Day	29% 12% 20% 21% 13% 5%	1710
	11th to 20th	Day	23% 12% 16% 25% 19% 5%	1801
	21st to end	Day	29% 9% 16% 23% 18% 5%	1570
March	1st to 10th	Day	31% 8% 10% 26% 20% 5%	3147
	11th to 20th	Day	34% 10% 16% 27% 11% 2	3490
	21st to end	Day	35% 12% 15% 27% 10% 2	4028
April	1st to 10th	Day	30% 10% 14% 24% 16% 6%	2280
	11th to 20th	Day	41% 9% 12% 19% 15% 3	2254
	21st to end	Day	40% 11% 13% 23% 11% 1	2280
May	1st to 10th	Day	41% 11% 13% 18% 15% 1	4053
	11th to 20th	Day	35% 11% 15% 25% 14% 1	4392
	21st to end	Day	26% 8% 12% 28% 21% 3	4825
June	1st to 10th	Day	31% 12% 15% 21% 17% 5%	2660
	11th to 20th	Day	30% 10% 11% 15% 23% 11%	2660
	21st to end	Day	29% 10% 11% 19% 21% 9%	2660
July	1st to 10th	Day	25% 10% 9% 15% 19% 22%	4361
	11th to 20th	Day	28% 12% 13% 20% 18% 10%	4355
	21st to end	Day	30% 13% 11% 16% 13% 16%	4532
August	1st to 10th	Day	35% 13% 16% 17% 13% 6%	4078
	11th to 20th	Day	29% 10% 15% 15% 21% 10%	3821
	21st to end	Day	30% 10% 12% 15% 23% 10%	4179
September	1st to 10th	Day	27% 8% 10% 19% 24% 12%	2280
	11th to 20th	Day	27% 8% 12% 21% 24% 9%	2280
	21st to end	Day	24% 5% 9% 23% 32% 7%	2068
October	1st to 10th	Day	30% 7% 9% 19% 27% 7%	1900
	11th to 20th	Day	21% 7% 10% 28% 26% 7%	1900
	21st to end	Day	25% 6% 13% 27% 21% 7%	2039
November	1st to 10th	Day	18% 8% 17% 23% 30% 6%	1682
	11th to 20th	Day	19% 6% 17% 30% 22% 6%	1710
	21st to end	Day	25% 6% 14% 27% 19% 9%	1568
December	1st to 10th	Day	26% 10% 20% 26% 14% 4	1520
	11th to 20th	Day	20% 12% 16% 32% 18% 2	1924
	21st to end	Day	27% 10% 15% 21% 22% 5%	2794
January	1st to 10th	Night	26% 11% 18% 26% 16% 3	4417
	11th to 20th	Night	18% 16% 22% 24% 14% 6%	4357
	21st to end	Night	25% 11% 18% 25% 16% 5%	4521
February	1st to 10th	Night	23% 12% 19% 25% 17% 4	2470
	11th to 20th	Night	21% 8% 20% 28% 19% 3	2274
	21st to end	Night	24% 8% 17% 24% 20% 7%	1884
March	1st to 10th	Night	24% 8% 14% 24% 24% 6%	3716
	11th to 20th	Night	31% 7% 13% 30% 16% 3	3490
	21st to end	Night	24% 10% 20% 34% 11% 1	3616
April	1st to 10th	Night	24% 10% 15% 28% 18% 5%	1900
	11th to 20th	Night	31% 8% 15% 26% 16% 3	1877
	21st to end	Night	32% 8% 14% 29% 17% 1	1900
May	1st to 10th	Night	29% 10% 15% 31% 13% 2	2913
	11th to 20th	Night	32% 10% 16% 27% 15%	2518
	21st to end	Night	21% 8% 16% 30% 22% 3	2762
June	1st to 10th	Night	21% 7% 20% 28% 17% 6%	1520
	11th to 20th	Night	19% 7% 13% 22% 25% 14%	1520
	21st to end	Night	19% 7% 13% 24% 25% 12%	1520
July	1st to 10th	Night	22% 8% 9% 18% 23% 20%	2500
	11th to 20th	Night	17% 10% 14% 32% 19% 8%	2492
	21st to end	Night	19% 9% 16% 26% 14% 16%	2788
August	1st to 10th	Night	28% 11% 19% 19% 19% 4	2838
	11th to 20th	Night	22% 8% 23% 18% 19% 9%	3084
	21st to end	Night	23% 9% 15% 20% 28% 5%	3487
September	1st to 10th	Night	24% 6% 12% 21% 27% 11%	1900
	11th to 20th	Night	20% 7% 11% 25% 26% 12%	1900
	21st to end	Night	22% 5% 10% 27% 29% 8%	2109
October	1st to 10th	Night	22% 6% 11% 26% 26% 9%	2280
	11th to 20th	Night	16% 5% 14% 30% 25% 10%	2280
	21st to end	Night	25% 6% 14% 26% 23% 6%	2537
November	1st to 10th	Night	16% 7% 16% 26% 27% 8%	2419
	11th to 20th	Night	18% 7% 16% 32% 22% 6%	2470
	21st to end	Night	23% 8% 12% 28% 22% 8%	2607
December	1st to 10th	Night	19% 10% 23% 26% 17% 6%	2660
	11th to 20th	Night	21% 10% 18% 28% 20% 2	3272
	21st to end	Night	24% 12% 16% 22% 20% 6%	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	13% 44% 25% 12% 5% 1	270
	11th to 20th	Day	10% 57% 19% 11% 1	270
	21st to end	Day	5% 55% 29% 8% 3 1	297
February	1st to 10th	Day	13% 61% 21% 2 1 1	270
	11th to 20th	Day	15% 59% 19% 5% 2	270
	21st to end	Day	20% 55% 19% 6%	225
March	1st to 10th	Day	15% 50% 25% 9% 1	274
	11th to 20th	Day	15% 37% 35% 11% 2	460
	21st to end	Day	16% 45% 30% 6% 2	506
April	1st to 10th	Day	16% 43% 31% 9% 1	460
	11th to 20th	Day	22% 42% 28% 6% 2	452
	21st to end	Day	16% 35% 35% 11% 2	460
May	1st to 10th	Day	19% 31% 33% 13% 4	562
	11th to 20th	Day	17% 20% 38% 18% 6% 1	675
	21st to end	Day	14% 22% 31% 23% 9% 1	734
June	1st to 10th	Day	6% 18% 35% 26% 12% 3	680
	11th to 20th	Day	8% 24% 36% 22% 9% 2	680
	21st to end	Day	7% 19% 36% 21% 14% 3	680
July	1st to 10th	Day	7% 8% 32% 32% 17% 4	671
	11th to 20th	Day	5% 7% 37% 30% 16% 4	668
	21st to end	Day	7% 9% 30% 33% 17% 4	666
August	1st to 10th	Day	9% 13% 35% 32% 9% 1	487
	11th to 20th	Day	7% 14% 39% 26% 13% 2	477
	21st to end	Day	8% 15% 37% 27% 11% 2	539
September	1st to 10th	Day	7% 16% 37% 28% 10% 3	490
	11th to 20th	Day	6% 17% 45% 22% 8% 1	490
	21st to end	Day	12% 19% 35% 21% 10% 3	300
October	1st to 10th	Day	8% 29% 31% 19% 9% 4	300
	11th to 20th	Day	10% 41% 31% 12% 4 2	300
	21st to end	Day	14% 44% 28% 11% 3	330
November	1st to 10th	Day	8% 51% 24% 10% 6%	273
	11th to 20th	Day	14% 47% 29% 8% 2 1	270
	21st to end	Day	9% 46% 33% 11% 1	270
December	1st to 10th	Day	15% 49% 24% 9% 3	270
	11th to 20th	Day	8% 61% 24% 7%	270
	21st to end	Day	16% 51% 21% 8% 3	297
January	1st to 10th	Night	11% 46% 24% 14% 4 1	950
	11th to 20th	Night	12% 51% 21% 10% 4 1	903
	21st to end	Night	7% 51% 28% 10% 3 1	836
February	1st to 10th	Night	10% 55% 23% 9% 3	760
	11th to 20th	Night	9% 51% 26% 12% 2	744
	21st to end	Night	8% 55% 26% 10% 2	628
March	1st to 10th	Night	7% 46% 28% 14% 4	760
	11th to 20th	Night	8% 35% 31% 20% 5%	760
	21st to end	Night	9% 42% 32% 15% 3 1	714
April	1st to 10th	Night	12% 35% 35% 16% 2	570
	11th to 20th	Night	10% 38% 28% 17% 7%	563
	21st to end	Night	13% 28% 33% 18% 7% 1	570
May	1st to 10th	Night	12% 20% 38% 19% 10% 1	570
	11th to 20th	Night	16% 10% 32% 30% 11% 1	561
	21st to end	Night	8% 16% 30% 29% 14% 1	617
June	1st to 10th	Night	5% 8% 30% 35% 20% 2	570
	11th to 20th	Night	5% 14% 37% 31% 12% 1	570
	21st to end	Night	4 9% 24% 38% 22% 3	570
July	1st to 10th	Night	5% 3 18% 38% 31% 4	555
	11th to 20th	Night	1 3 22% 36% 32% 5%	552
	21st to end	Night	5% 4 12% 39% 34% 5%	624
August	1st to 10th	Night	5% 5% 19% 43% 26% 3	567
	11th to 20th	Night	7% 5% 18% 41% 25% 5%	564
	21st to end	Night	5% 6% 19% 37% 28% 5%	627
September	1st to 10th	Night	7% 4 27% 36% 22% 4	570
	11th to 20th	Night	8% 13% 26% 33% 17% 3	570
	21st to end	Night	8% 16% 30% 29% 14% 3	589
October	1st to 10th	Night	6% 27% 28% 24% 12% 2	760
	11th to 20th	Night	7% 35% 31% 19% 7% 2	760
	21st to end	Night	9% 40% 31% 13% 6% 2	865
November	1st to 10th	Night	6% 47% 29% 13% 5% 1	931
	11th to 20th	Night	9% 49% 30% 9% 3	950
	21st to end	Night	6% 44% 34% 14% 2	950
December	1st to 10th	Night	13% 47% 29% 9% 2	950
	11th to 20th	Night	8% 51% 26% 11% 3	950
	21st to end	Night	10% 49% 26% 11% 4	1045

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