San Isabel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Isabel Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	2 4 2 6% 4 5% 5% 4 7% 9% 51%	135990
	Night	2 4 2 5% 3 4 4 3 6% 9% 58%	132859
Transparency	Day	26% 6% 11% 24% 24% 9%	130936
	Night	22% 5% 13% 27% 25% 7%	128917
Seeing	Day	6% 9% 23% 34% 21% 8%	17051
	Night	4 5% 31% 43% 16% 1	27174

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	3 3 8% 4 3 3 3 5% 8% 62%	3646
	11th to 20th	Day	1 1 4 4 4 5% 4 7% 10% 60%	3939
	21st to end	Day	3 2 6% 3 3 4 3 6% 11% 58%	4510
February	1st to 10th	Day	3 2 5% 4 3 3 4 5% 7% 63%	1800
	11th to 20th	Day	3 1 4 3 3 4 3 6% 9% 64%	1899
	21st to end	Day	3 2 5% 4 4 5% 3 5% 7% 62%	1650
March	1st to 10th	Day	1 3 2 5% 4 3 4 4 6% 7% 62%	4578
	11th to 20th	Day	1 6% 3 8% 6% 6% 6% 5% 7% 11% 42%	5000
	21st to end	Day	1 4 2 5% 4 4 5% 4 7% 11% 53%	5744
April	1st to 10th	Day	2 4 2 4 4 5% 4 4 6% 10% 55%	2400
	11th to 20th	Day	3 7% 2 6% 6% 4 4 4 6% 8% 49%	2371
	21st to end	Day	3 7% 2 7% 5% 5% 5% 5% 8% 7% 47%	2400
May	1st to 10th	Day	3 7% 3 8% 6% 5% 6% 5% 7% 9% 40%	5606
	11th to 20th	Day	3 5% 3 6% 5% 5% 7% 6% 8% 11% 41%	6298
	21st to end	Day	2 4 2 5% 4 5% 6% 5% 8% 10% 48%	6925
June	1st to 10th	Day	3 4 2 5% 5% 5% 5% 6% 8% 10% 47%	2800
	11th to 20th	Day	4 4 2 5% 4 4 5% 4 6% 7% 55%	2800
	21st to end	Day	3 4 1 5% 5% 4 6% 6% 7% 8% 50%	2800
July	1st to 10th	Day	3 3 1 5% 4 4 6% 5% 7% 10% 51%	6339
	11th to 20th	Day	4 4 2 7% 5% 7% 7% 6% 10% 10% 38%	6340
	21st to end	Day	6% 6% 3 8% 6% 6% 6% 5% 8% 8% 38%	6548
August	1st to 10th	Day	5% 5% 2 7% 6% 7% 7% 6% 9% 12% 34%	5779
	11th to 20th	Day	3 4 2 6% 5% 5% 6% 5% 8% 10% 46%	5409
	21st to end	Day	5% 6% 3 8% 6% 6% 6% 5% 8% 9% 38%	5984
September	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 4 6% 8% 51%	2400
	11th to 20th	Day	5% 4 2 5% 4 5% 4 4 6% 6% 56%	2400
	21st to end	Day	4 4 2 5% 3 3 3 3 5% 7% 61%	2175
October	1st to 10th	Day	2 3 1 3 3 2 3 2 3 5% 72%	3270
	11th to 20th	Day	2 4 2 5% 4 5% 5% 4 6% 8% 56%	3250
	21st to end	Day	1 3 2 3 2 2 3 2 5% 7% 70%	3521
November	1st to 10th	Day	1 3 2 3 2 2 2 2 5% 7% 71%	1853
	11th to 20th	Day	1 3 1 3 2 2 2 3 5% 8% 71%	1890
	21st to end	Day	3 2 4 3 3 3 2 6% 9% 64%	1768
December	1st to 10th	Day	2 2 5% 2 2 2 2 4 11% 66%	2700
	11th to 20th	Day	2 1 3 3 3 3 3 6% 10% 66%	3106
	21st to end	Day	2 2 5% 3 4 4 4 7% 12% 58%	4092
January	1st to 10th	Night	2 2 8% 4 4 4 4 5% 9% 57%	6342
	11th to 20th	Night	2 1 4 3 4 3 3 6% 8% 68%	6085
	21st to end	Night	4 2 6% 3 4 4 3 6% 12% 56%	6468
February	1st to 10th	Night	3 2 5% 4 3 4 3 4 7% 65%	2600
	11th to 20th	Night	4 2 7% 2 4 4 3 5% 7% 62%	2386
	21st to end	Night	5% 2 5% 3 3 2 3 5% 7% 64%	1980
March	1st to 10th	Night	5% 2 6% 4 4 4 3 5% 7% 59%	5296
	11th to 20th	Night	1 7% 3 7% 4 4 4 4 5% 9% 51%	5000
	21st to end	Night	1 5% 2 5% 4 4 4 4 6% 9% 57%	5219
April	1st to 10th	Night	1 4 2 4 3 4 4 3 6% 7% 64%	2000
	11th to 20th	Night	3 9% 3 6% 3 3 3 3 6% 8% 53%	1977
	21st to end	Night	4 8% 2 5% 4 4 3 3 5% 7% 53%	2000
May	1st to 10th	Night	3 9% 3 8% 5% 4 5% 4 6% 10% 43%	4366
	11th to 20th	Night	5% 7% 3 6% 4 3 4 4 7% 8% 49%	3617
	21st to end	Night	4 7% 3 6% 4 3 3 3 5% 8% 52%	3972
June	1st to 10th	Night	4 4 1 3 3 4 4 2 6% 10% 59%	1600
	11th to 20th	Night	3 3 1 4 3 3 2 4 5% 9% 64%	1600
	21st to end	Night	3 4 1 4 4 3 4 4 7% 9% 57%	1600
July	1st to 10th	Night	3 4 1 6% 4 4 4 4 6% 8% 56%	3640
	11th to 20th	Night	2 3 2 5% 4 5% 6% 5% 9% 12% 47%	3640
	21st to end	Night	4 4 2 6% 5% 5% 6% 5% 9% 12% 43%	4321
August	1st to 10th	Night	4 6% 2 6% 4 3 5% 5% 8% 11% 46%	4033
	11th to 20th	Night	3 3 1 5% 3 5% 6% 5% 9% 11% 48%	4437
	21st to end	Night	6% 7% 3 9% 6% 6% 6% 5% 8% 9% 35%	4993
September	1st to 10th	Night	4 5% 2 5% 4 4 5% 4 7% 8% 52%	2000
	11th to 20th	Night	5% 3 2 6% 4 4 4 3 6% 7% 57%	2000
	21st to end	Night	5% 5% 2 5% 3 3 3 2 4 6% 61%	2221
October	1st to 10th	Night	3 3 1 4 3 2 3 3 3 5% 69%	3920
	11th to 20th	Night	2 4 2 4 3 3 3 3 4 7% 64%	3898
	21st to end	Night	1 3 1 3 2 2 3 2 4 6% 72%	4252
November	1st to 10th	Night	1 3 1 3 2 2 3 2 5% 7% 71%	2657
	11th to 20th	Night	2 1 3 2 2 2 3 4 8% 73%	2730
	21st to end	Night	1 5% 2 4 3 3 2 2 5% 8% 66%	2850
December	1st to 10th	Night	2 1 4 2 2 2 2 4 6% 77%	4710
	11th to 20th	Night	2 1 3 2 2 3 3 6% 8% 71%	5321
	21st to end	Night	3 2 6% 4 4 4 4 6% 10% 57%	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	21% 3 12% 30% 25% 8%	3647
	11th to 20th	Day	19% 3 14% 22% 32% 10%	3939
	21st to end	Day	21% 3 10% 29% 25% 12%	4510
February	1st to 10th	Day	20% 4 16% 29% 20% 10%	1800
	11th to 20th	Day	16% 4 12% 29% 31% 7%	1909
	21st to end	Day	20% 5% 9% 23% 29% 14%	1650
March	1st to 10th	Day	18% 4 7% 18% 35% 17%	4578
	11th to 20th	Day	34% 4 10% 27% 21% 4	5000
	21st to end	Day	22% 7% 11% 42% 16% 2	5744
April	1st to 10th	Day	23% 6% 13% 29% 21% 8%	2400
	11th to 20th	Day	31% 6% 11% 25% 22% 6%	2374
	21st to end	Day	31% 6% 12% 30% 19% 3	2400
May	1st to 10th	Day	34% 8% 10% 22% 20% 6%	5606
	11th to 20th	Day	32% 8% 11% 22% 21% 5%	6302
	21st to end	Day	26% 8% 11% 20% 30% 6%	6926
June	1st to 10th	Day	28% 9% 13% 21% 21% 8%	2800
	11th to 20th	Day	26% 6% 9% 19% 23% 17%	2800
	21st to end	Day	27% 7% 9% 21% 22% 13%	2800
July	1st to 10th	Day	24% 6% 10% 15% 20% 26%	6271
	11th to 20th	Day	33% 9% 15% 22% 14% 6%	6265
	21st to end	Day	38% 7% 16% 14% 14% 11%	6360
August	1st to 10th	Day	40% 11% 17% 16% 12% 4	4208
	11th to 20th	Day	32% 8% 14% 15% 21% 10%	3913
	21st to end	Day	34% 7% 12% 17% 22% 8%	4311
September	1st to 10th	Day	30% 5% 10% 20% 24% 12%	2400
	11th to 20th	Day	26% 5% 10% 20% 28% 11%	2400
	21st to end	Day	22% 4 7% 24% 33% 9%	2175
October	1st to 10th	Day	16% 3 6% 19% 36% 19%	3270
	11th to 20th	Day	24% 4 8% 30% 26% 8%	3250
	21st to end	Day	15% 2 6% 29% 40% 7%	3521
November	1st to 10th	Day	14% 4 11% 25% 36% 11%	1853
	11th to 20th	Day	13% 2 9% 38% 29% 9%	1890
	21st to end	Day	18% 3 11% 30% 28% 11%	1768
December	1st to 10th	Day	15% 3 11% 35% 27% 10%	2700
	11th to 20th	Day	13% 5% 13% 36% 29% 5%	3104
	21st to end	Day	19% 4 11% 28% 28% 9%	4092
January	1st to 10th	Night	24% 4 9% 31% 26% 5%	6342
	11th to 20th	Night	15% 3 11% 29% 28% 14%	6085
	21st to end	Night	21% 5% 12% 25% 27% 10%	6468
February	1st to 10th	Night	18% 4 14% 33% 24% 7%	2600
	11th to 20th	Night	22% 3 12% 31% 27% 6%	2386
	21st to end	Night	20% 3 8% 28% 30% 11%	1980
March	1st to 10th	Night	23% 4 7% 19% 39% 8%	5296
	11th to 20th	Night	30% 4 9% 22% 29% 7%	5000
	21st to end	Night	23% 6% 17% 39% 14% 1	5219
April	1st to 10th	Night	20% 5% 13% 32% 25% 5%	2000
	11th to 20th	Night	29% 4 11% 30% 21% 4	1977
	21st to end	Night	30% 4 15% 30% 19% 2	2000
May	1st to 10th	Night	33% 8% 14% 24% 15% 5%	4363
	11th to 20th	Night	30% 9% 13% 24% 22% 2	3618
	21st to end	Night	28% 8% 13% 20% 27% 4	3972
June	1st to 10th	Night	21% 6% 21% 27% 14% 10%	1600
	11th to 20th	Night	17% 5% 13% 23% 24% 17%	1600
	21st to end	Night	22% 5% 15% 26% 21% 12%	1600
July	1st to 10th	Night	25% 6% 12% 19% 24% 15%	3600
	11th to 20th	Night	25% 9% 24% 25% 12% 4	3592
	21st to end	Night	29% 9% 23% 21% 8% 9%	4197
August	1st to 10th	Night	37% 11% 20% 18% 11% 4	2928
	11th to 20th	Night	25% 11% 22% 19% 18% 6%	3209
	21st to end	Night	32% 10% 18% 20% 18% 3	3597
September	1st to 10th	Night	28% 7% 12% 24% 21% 9%	2000
	11th to 20th	Night	24% 6% 12% 23% 21% 13%	2000
	21st to end	Night	25% 5% 8% 26% 29% 7%	2221
October	1st to 10th	Night	18% 4 8% 25% 33% 12%	3920
	11th to 20th	Night	19% 5% 13% 31% 26% 7%	3898
	21st to end	Night	14% 3 6% 31% 39% 7%	4252
November	1st to 10th	Night	14% 3 10% 29% 33% 10%	2657
	11th to 20th	Night	12% 3 10% 37% 30% 8%	2730
	21st to end	Night	18% 2 11% 30% 29% 9%	2850
December	1st to 10th	Night	11% 2 11% 39% 31% 6%	4710
	11th to 20th	Night	12% 4 11% 35% 35% 4	5322
	21st to end	Night	22% 5% 13% 23% 31% 6%	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	2 13% 24% 41% 15% 6%	303
	11th to 20th	Day	5% 11% 27% 41% 15% 1	300
	21st to end	Day	1 4 26% 49% 14% 7%	330
February	1st to 10th	Day	4 16% 34% 36% 9% 2	300
	11th to 20th	Day	1 16% 32% 40% 9% 2	300
	21st to end	Day	16% 37% 39% 7% 1	249
March	1st to 10th	Day	2 13% 28% 41% 13% 3	326
	11th to 20th	Day	8% 9% 30% 34% 16% 3	500
	21st to end	Day	6% 14% 35% 33% 11% 1	550
April	1st to 10th	Day	4 17% 28% 33% 16% 2	500
	11th to 20th	Day	11% 21% 27% 28% 11% 3	492
	21st to end	Day	8% 13% 30% 32% 15% 3	500
May	1st to 10th	Day	8% 10% 29% 35% 15% 3	554
	11th to 20th	Day	9% 9% 18% 37% 21% 6%	725
	21st to end	Day	7% 9% 21% 33% 24% 6%	789
June	1st to 10th	Day	5% 8% 20% 33% 25% 10%	730
	11th to 20th	Day	7% 8% 22% 36% 19% 8%	730
	21st to end	Day	4 5% 22% 35% 26% 8%	730
July	1st to 10th	Day	8% 3 12% 30% 36% 12%	721
	11th to 20th	Day	7% 3 9% 28% 38% 15%	718
	21st to end	Day	9% 4 13% 27% 33% 14%	644
August	1st to 10th	Day	10% 4 16% 30% 26% 13%	527
	11th to 20th	Day	9% 5% 20% 25% 25% 17%	517
	21st to end	Day	8% 5% 18% 28% 26% 15%	583
September	1st to 10th	Day	8% 4 17% 29% 27% 14%	530
	11th to 20th	Day	7% 6% 24% 32% 23% 8%	530
	21st to end	Day	5% 7% 15% 29% 25% 19%	337
October	1st to 10th	Day	4 10% 21% 24% 24% 17%	330
	11th to 20th	Day	8% 14% 24% 25% 20% 9%	330
	21st to end	Day	5% 13% 23% 36% 19% 4	363
November	1st to 10th	Day	5% 11% 26% 36% 17% 5%	330
	11th to 20th	Day	3 10% 23% 42% 16% 5%	330
	21st to end	Day	3 7% 26% 46% 13% 4	330
December	1st to 10th	Day	2 12% 31% 36% 12% 6%	330
	11th to 20th	Day	3 11% 27% 44% 12% 3	330
	21st to end	Day	4 15% 28% 39% 10% 4	363
January	1st to 10th	Night	1 6% 38% 42% 10% 2	1006
	11th to 20th	Night	1 8% 43% 38% 9% 2	888
	21st to end	Night	2 4 38% 46% 10% 1	880
February	1st to 10th	Night	1 5% 46% 41% 7%	800
	11th to 20th	Night	2 5% 49% 38% 7%	784
	21st to end	Night	2 4 47% 42% 4	660
March	1st to 10th	Night	3 5% 43% 41% 8% 1	800
	11th to 20th	Night	4 4 33% 50% 9%	800
	21st to end	Night	4 6% 41% 44% 5%	765
April	1st to 10th	Night	3 7% 42% 44% 5%	600
	11th to 20th	Night	8% 8% 37% 38% 8% 1	593
	21st to end	Night	8% 6% 32% 46% 9%	600
May	1st to 10th	Night	8% 6% 29% 45% 12% 1	600
	11th to 20th	Night	9% 4 22% 44% 19% 1	591
	21st to end	Night	9% 5% 25% 40% 20% 2	650
June	1st to 10th	Night	5% 5% 18% 45% 24% 3	600
	11th to 20th	Night	4 2 26% 52% 15% 2	600
	21st to end	Night	4 2 17% 46% 29% 2	600
July	1st to 10th	Night	6% 2 9% 42% 37% 3	585
	11th to 20th	Night	4 2 6% 40% 44% 4	582
	21st to end	Night	6% 3 5% 36% 47% 4	657
August	1st to 10th	Night	9% 2 8% 40% 39% 3	597
	11th to 20th	Night	7% 2 8% 45% 35% 3	594
	21st to end	Night	7% 3 7% 45% 35% 2	660
September	1st to 10th	Night	5% 3 10% 48% 30% 5%	600
	11th to 20th	Night	6% 2 16% 50% 24% 3	600
	21st to end	Night	7% 3 21% 47% 19% 2	630
October	1st to 10th	Night	5% 4 27% 43% 17% 3	800
	11th to 20th	Night	3 3 33% 45% 15% 2	800
	21st to end	Night	4 4 31% 48% 12% 2	880
November	1st to 10th	Night	2 6% 36% 44% 11% 1	1017
	11th to 20th	Night	2 7% 38% 44% 8% 1	1050
	21st to end	Night	3 6% 35% 46% 9% 1	1050
December	1st to 10th	Night	1 6% 42% 42% 9%	1050
	11th to 20th	Night	2 6% 42% 42% 8%	1050
	21st to end	Night	1 8% 43% 39% 7% 1	1155

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