Fort Collins
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Collins clear sky chart from 2005-11-06 to 2025-08-05.

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	5% 5% 2 5% 4 4 4 4 6% 8% 52%	129841
	Night	6% 6% 2 5% 3 3 4 3 5% 7% 57%	127161
Transparency	Day	27% 7% 12% 23% 22% 9%	129538
	Night	26% 6% 12% 24% 23% 8%	126980
Seeing	Day	8% 8% 27% 34% 17% 5%	16711
	Night	8% 6% 28% 43% 14% 1	26684

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	1 7% 2 8% 4 3 3 3 6% 7% 56%	3638
	11th to 20th	Day	2 5% 3 10% 4 4 3 3 5% 7% 53%	3664
	21st to end	Day	1 5% 3 5% 3 3 3 4 5% 7% 61%	4510
February	1st to 10th	Day	3 7% 3 6% 4 4 4 3 6% 9% 52%	1800
	11th to 20th	Day	1 5% 2 6% 4 3 5% 3 7% 9% 55%	1901
	21st to end	Day	2 7% 3 7% 3 3 4 3 5% 6% 58%	1650
March	1st to 10th	Day	4 9% 2 5% 3 3 3 3 5% 9% 54%	4500
	11th to 20th	Day	6% 8% 3 6% 4 4 4 4 6% 8% 47%	4926
	21st to end	Day	5% 9% 2 5% 3 4 4 4 6% 10% 46%	5827
April	1st to 10th	Day	6% 7% 1 4 3 4 4 3 6% 9% 53%	2400
	11th to 20th	Day	11% 8% 2 5% 4 4 5% 3 5% 7% 45%	2371
	21st to end	Day	11% 8% 2 5% 4 4 4 5% 6% 9% 42%	2400
May	1st to 10th	Day	12% 8% 2 5% 4 4 5% 4 5% 7% 44%	5878
	11th to 20th	Day	9% 5% 2 5% 4 5% 6% 5% 7% 8% 44%	6298
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 45%	6925
June	1st to 10th	Day	5% 4 1 5% 5% 4 5% 6% 8% 10% 48%	2800
	11th to 20th	Day	7% 3 1 3 3 4 6% 4 8% 9% 52%	2800
	21st to end	Day	4 3 1 4 4 5% 6% 5% 8% 9% 51%	2800
July	1st to 10th	Day	4 3 1 5% 4 4 6% 5% 7% 9% 53%	6339
	11th to 20th	Day	4 3 1 5% 5% 6% 6% 6% 9% 10% 45%	6340
	21st to end	Day	6% 3 2 5% 5% 5% 6% 5% 7% 8% 47%	6870
August	1st to 10th	Day	11% 4 2 5% 4 6% 6% 5% 10% 9% 38%	4145
	11th to 20th	Day	4 3 1 5% 4 5% 6% 6% 8% 8% 50%	3763
	21st to end	Day	4 3 1 4 3 4 4 4 7% 8% 56%	4180
September	1st to 10th	Day	6% 3 1 4 2 3 4 4 6% 7% 59%	2280
	11th to 20th	Day	7% 2 1 3 3 4 4 4 5% 8% 59%	2261
	21st to end	Day	8% 4 1 4 2 3 3 3 3 6% 65%	2090
October	1st to 10th	Day	5% 4 1 2 2 2 2 2 4 4 72%	3180
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 7% 61%	3150
	21st to end	Day	5% 5% 2 3 3 3 3 3 5% 6% 63%	3287
November	1st to 10th	Day	4 4 2 3 3 3 3 3 5% 8% 63%	1763
	11th to 20th	Day	3 5% 1 3 3 3 2 3 5% 9% 63%	1800
	21st to end	Day	2 6% 2 5% 3 3 4 3 5% 8% 59%	1655
December	1st to 10th	Day	1 4 1 3 3 2 3 3 4 6% 71%	2620
	11th to 20th	Day	2 4 2 4 2 2 2 3 4 7% 67%	3026
	21st to end	Day	3 6% 3 6% 4 3 4 4 7% 7% 53%	4004
January	1st to 10th	Night	2 9% 3 7% 4 3 3 3 5% 7% 54%	6327
	11th to 20th	Night	1 4 2 8% 5% 4 3 3 5% 7% 57%	6340
	21st to end	Night	2 6% 3 7% 4 3 3 3 4 5% 61%	6486
February	1st to 10th	Night	2 9% 2 7% 4 3 4 3 4 7% 54%	2600
	11th to 20th	Night	2 7% 4 7% 5% 4 4 3 5% 7% 52%	2384
	21st to end	Night	2 9% 3 7% 4 3 3 2 4 6% 59%	1980
March	1st to 10th	Night	5% 10% 2 5% 4 3 3 3 4 5% 55%	5374
	11th to 20th	Night	7% 9% 2 4 3 3 3 3 5% 5% 56%	5055
	21st to end	Night	9% 8% 3 6% 4 3 4 3 5% 7% 49%	5131
April	1st to 10th	Night	5% 8% 2 4 3 3 4 3 5% 7% 56%	2000
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 6% 51%	1977
	21st to end	Night	15% 8% 2 5% 3 3 3 3 5% 7% 46%	2000
May	1st to 10th	Night	15% 7% 2 6% 3 4 4 4 5% 6% 45%	4094
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 8% 46%	3617
	21st to end	Night	14% 6% 2 4 3 3 3 3 5% 6% 51%	3972
June	1st to 10th	Night	7% 3 2 4 3 3 3 4 6% 9% 56%	1600
	11th to 20th	Night	5% 3 1 3 2 3 4 4 5% 8% 61%	1600
	21st to end	Night	5% 2 1 3 3 4 3 4 6% 9% 60%	1600
July	1st to 10th	Night	4 2 1 4 3 3 4 4 6% 8% 60%	3640
	11th to 20th	Night	4 2 2 3 3 3 4 4 8% 11% 54%	3640
	21st to end	Night	5% 3 1 4 4 4 5% 4 7% 8% 53%	3986
August	1st to 10th	Night	12% 3 1 5% 4 4 4 5% 7% 9% 45%	2883
	11th to 20th	Night	4 3 2 5% 4 4 5% 4 8% 10% 51%	3075
	21st to end	Night	3 2 1 4 4 4 5% 5% 8% 9% 56%	3487
September	1st to 10th	Night	7% 3 1 3 3 3 4 3 5% 6% 60%	1900
	11th to 20th	Night	8% 2 1 3 2 2 3 2 4 5% 67%	1919
	21st to end	Night	7% 2 1 3 3 3 3 2 4 6% 65%	2090
October	1st to 10th	Night	6% 3 1 3 2 2 2 2 3 4 72%	3787
	11th to 20th	Night	7% 4 2 5% 3 3 4 2 4 6% 59%	3778
	21st to end	Night	6% 6% 1 4 2 3 3 2 4 7% 63%	4239
November	1st to 10th	Night	3 5% 1 4 2 3 3 3 5% 6% 64%	2535
	11th to 20th	Night	3 5% 1 4 4 2 3 3 5% 7% 64%	2600
	21st to end	Night	3 6% 2 5% 3 3 3 3 5% 6% 60%	2740
December	1st to 10th	Night	1 4 2 4 3 2 2 2 2 5% 72%	4570
	11th to 20th	Night	2 5% 2 3 3 3 3 2 5% 10% 63%	5181
	21st to end	Night	4 8% 3 6% 4 4 4 3 5% 7% 52%	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	27% 5% 10% 32% 19% 6%	3639
	11th to 20th	Day	28% 5% 13% 22% 25% 7%	3664
	21st to end	Day	21% 6% 11% 25% 26% 11%	4510
February	1st to 10th	Day	30% 5% 14% 26% 19% 6%	1800
	11th to 20th	Day	24% 6% 12% 29% 23% 5%	1911
	21st to end	Day	26% 5% 10% 25% 24% 10%	1650
March	1st to 10th	Day	26% 6% 9% 19% 31% 9%	4500
	11th to 20th	Day	31% 7% 13% 22% 22% 4	4926
	21st to end	Day	29% 9% 14% 33% 13% 2	5827
April	1st to 10th	Day	27% 7% 12% 26% 20% 8%	2400
	11th to 20th	Day	37% 7% 12% 21% 18% 6%	2374
	21st to end	Day	35% 8% 13% 24% 17% 2	2400
May	1st to 10th	Day	34% 9% 11% 27% 15% 4	5878
	11th to 20th	Day	31% 10% 13% 19% 19% 9%	6302
	21st to end	Day	30% 10% 14% 24% 17% 3	6926
June	1st to 10th	Day	26% 10% 18% 23% 16% 6%	2800
	11th to 20th	Day	25% 9% 12% 21% 20% 14%	2800
	21st to end	Day	24% 9% 12% 21% 22% 13%	2800
July	1st to 10th	Day	23% 6% 10% 17% 26% 18%	6271
	11th to 20th	Day	28% 8% 13% 23% 17% 12%	6265
	21st to end	Day	30% 6% 11% 16% 20% 16%	6672
August	1st to 10th	Day	34% 10% 13% 20% 16% 8%	4131
	11th to 20th	Day	26% 9% 12% 17% 22% 13%	3799
	21st to end	Day	24% 6% 11% 19% 26% 14%	4179
September	1st to 10th	Day	22% 5% 9% 20% 27% 17%	2280
	11th to 20th	Day	21% 6% 10% 21% 28% 15%	2261
	21st to end	Day	21% 5% 7% 20% 36% 10%	2090
October	1st to 10th	Day	17% 3 6% 18% 34% 23%	3180
	11th to 20th	Day	22% 4 11% 27% 27% 9%	3150
	21st to end	Day	22% 3 8% 26% 31% 9%	3287
November	1st to 10th	Day	22% 4 12% 26% 25% 10%	1763
	11th to 20th	Day	20% 4 10% 25% 32% 9%	1800
	21st to end	Day	23% 5% 9% 25% 29% 9%	1655
December	1st to 10th	Day	16% 3 8% 34% 28% 10%	2620
	11th to 20th	Day	18% 4 9% 34% 30% 4	3024
	21st to end	Day	27% 6% 12% 25% 21% 8%	4004
January	1st to 10th	Night	30% 5% 14% 24% 20% 7%	6327
	11th to 20th	Night	25% 6% 14% 20% 25% 9%	6340
	21st to end	Night	26% 5% 11% 24% 26% 8%	6486
February	1st to 10th	Night	29% 4 14% 28% 20% 5%	2600
	11th to 20th	Night	30% 6% 11% 27% 20% 5%	2384
	21st to end	Night	28% 3 8% 25% 24% 12%	1980
March	1st to 10th	Night	29% 5% 8% 17% 34% 7%	5374
	11th to 20th	Night	28% 6% 12% 16% 29% 10%	5055
	21st to end	Night	33% 7% 13% 29% 15% 2	5131
April	1st to 10th	Night	27% 6% 11% 26% 21% 8%	2000
	11th to 20th	Night	34% 6% 13% 23% 19% 6%	1977
	21st to end	Night	37% 7% 10% 25% 19% 3	2000
May	1st to 10th	Night	35% 10% 12% 25% 13% 5%	4092
	11th to 20th	Night	32% 10% 15% 21% 16% 6%	3618
	21st to end	Night	31% 9% 16% 24% 15% 4	3972
June	1st to 10th	Night	24% 9% 20% 27% 14% 7%	1600
	11th to 20th	Night	20% 8% 14% 22% 20% 17%	1600
	21st to end	Night	19% 7% 12% 25% 27% 11%	1600
July	1st to 10th	Night	20% 5% 11% 24% 29% 11%	3600
	11th to 20th	Night	21% 7% 20% 26% 18% 7%	3592
	21st to end	Night	27% 6% 15% 23% 20% 10%	3872
August	1st to 10th	Night	32% 8% 17% 22% 17% 5%	2874
	11th to 20th	Night	27% 8% 16% 22% 17% 11%	3105
	21st to end	Night	21% 6% 15% 23% 25% 10%	3487
September	1st to 10th	Night	23% 6% 10% 20% 21% 19%	1900
	11th to 20th	Night	19% 5% 11% 24% 26% 16%	1919
	21st to end	Night	21% 5% 9% 23% 30% 13%	2090
October	1st to 10th	Night	18% 3 8% 18% 31% 22%	3787
	11th to 20th	Night	28% 3 10% 26% 25% 8%	3778
	21st to end	Night	24% 3 7% 27% 28% 10%	4239
November	1st to 10th	Night	21% 4 11% 26% 26% 11%	2535
	11th to 20th	Night	20% 4 10% 28% 29% 9%	2600
	21st to end	Night	24% 4 11% 24% 29% 9%	2740
December	1st to 10th	Night	18% 3 10% 36% 28% 6%	4570
	11th to 20th	Night	20% 3 8% 38% 27% 4	5182
	21st to end	Night	31% 4 11% 23% 25% 6%	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	6% 10% 25% 42% 12% 5%	300
	11th to 20th	Day	5% 10% 37% 37% 9% 1	300
	21st to end	Day	4 11% 38% 36% 8% 2	330
February	1st to 10th	Day	6% 14% 42% 34% 3 1	300
	11th to 20th	Day	4 13% 42% 33% 8%	300
	21st to end	Day	5% 16% 35% 37% 6%	249
March	1st to 10th	Day	7% 15% 28% 38% 11% 2	300
	11th to 20th	Day	12% 14% 29% 33% 11% 2	465
	21st to end	Day	12% 14% 40% 26% 7% 1	550
April	1st to 10th	Day	12% 18% 33% 27% 8% 1	500
	11th to 20th	Day	16% 18% 37% 21% 6% 2	492
	21st to end	Day	18% 16% 29% 25% 10% 2	500
May	1st to 10th	Day	15% 13% 28% 29% 11% 2	655
	11th to 20th	Day	19% 6% 22% 32% 16% 4	725
	21st to end	Day	11% 8% 27% 30% 20% 4	789
June	1st to 10th	Day	5% 6% 22% 35% 25% 7%	730
	11th to 20th	Day	7% 6% 27% 35% 22% 4	730
	21st to end	Day	4 5% 25% 37% 22% 7%	730
July	1st to 10th	Day	7% 3 16% 35% 30% 10%	721
	11th to 20th	Day	4 2 16% 38% 29% 10%	718
	21st to end	Day	5% 3 15% 33% 35% 9%	786
August	1st to 10th	Day	7% 3 19% 41% 23% 6%	504
	11th to 20th	Day	4 3 22% 40% 24% 7%	477
	21st to end	Day	5% 3 26% 34% 24% 8%	539
September	1st to 10th	Day	7% 3 22% 37% 23% 7%	490
	11th to 20th	Day	6% 4 28% 32% 26% 5%	471
	21st to end	Day	10% 4 23% 30% 24% 9%	300
October	1st to 10th	Day	11% 4 24% 31% 22% 9%	300
	11th to 20th	Day	4 9% 32% 35% 17% 4	300
	21st to end	Day	8% 10% 29% 39% 12% 2	330
November	1st to 10th	Day	7% 10% 30% 35% 14% 5%	300
	11th to 20th	Day	7% 11% 28% 43% 9% 3	300
	21st to end	Day	8% 10% 26% 46% 8% 2	300
December	1st to 10th	Day	7% 12% 27% 37% 14% 3	300
	11th to 20th	Day	5% 10% 32% 45% 8% 1	300
	21st to end	Day	7% 12% 32% 39% 7% 3	330
January	1st to 10th	Night	5% 10% 31% 42% 10% 2	1000
	11th to 20th	Night	4 14% 36% 37% 8% 1	1000
	21st to end	Night	5% 8% 34% 45% 7% 1	885
February	1st to 10th	Night	7% 7% 47% 33% 6% 1	800
	11th to 20th	Night	5% 9% 40% 40% 6%	784
	21st to end	Night	6% 11% 37% 42% 3	660
March	1st to 10th	Night	6% 7% 38% 42% 7% 1	800
	11th to 20th	Night	9% 8% 32% 41% 9% 1	800
	21st to end	Night	10% 9% 33% 40% 7% 1	729
April	1st to 10th	Night	9% 11% 37% 38% 6%	600
	11th to 20th	Night	15% 9% 33% 35% 8% 1	593
	21st to end	Night	19% 7% 26% 37% 10% 1	600
May	1st to 10th	Night	16% 8% 27% 34% 13% 2	600
	11th to 20th	Night	19% 4 20% 38% 18% 2	591
	21st to end	Night	11% 4 26% 36% 22% 1	650
June	1st to 10th	Night	8% 3 18% 49% 21% 2	600
	11th to 20th	Night	8% 3 22% 52% 15%	600
	21st to end	Night	6% 2 17% 50% 24% 2	600
July	1st to 10th	Night	8% 2 11% 44% 32% 3	585
	11th to 20th	Night	3 1 10% 52% 32% 2	582
	21st to end	Night	7% 1 9% 44% 37% 2	657
August	1st to 10th	Night	6% 2 11% 46% 31% 3	579
	11th to 20th	Night	5% 2 10% 51% 29% 3	564
	21st to end	Night	3 1 13% 46% 34% 3	627
September	1st to 10th	Night	8% 2 16% 45% 24% 5%	570
	11th to 20th	Night	8% 2 18% 46% 23% 4	570
	21st to end	Night	6% 3 23% 46% 21% 1	570
October	1st to 10th	Night	11% 4 23% 42% 17% 3	747
	11th to 20th	Night	8% 5% 26% 46% 14% 1	760
	21st to end	Night	11% 7% 29% 41% 10% 2	906
November	1st to 10th	Night	5% 6% 30% 45% 12% 1	975
	11th to 20th	Night	5% 11% 32% 44% 6% 2	1000
	21st to end	Night	7% 7% 31% 44% 10% 1	1000
December	1st to 10th	Night	4 9% 33% 46% 8%	1000
	11th to 20th	Night	6% 8% 35% 43% 7% 1	1000
	21st to end	Night	7% 9% 34% 43% 7% 1	1100

Page generated at 2025-08-01 23:32:24. Version ac07

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