Longmont
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Longmont clear sky chart from 2005-11-06 to 2026-02-16.

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 5% 2 5% 4 4 4 4 6% 8% 54%	139756
	Night	5% 5% 2 5% 3 3 3 3 5% 7% 58%	138587
Transparency	Day	25% 6% 11% 24% 24% 10%	134695
	Night	25% 5% 11% 24% 25% 10%	134655
Seeing	Day	7% 6% 23% 35% 21% 7%	17307
	Night	7% 4 26% 44% 17% 2	27460

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	2 7% 3 7% 4 3 3 3 4 8% 56%	4720
	11th to 20th	Day	1 3 2 6% 4 4 5% 4 5% 7% 58%	4842
	21st to end	Day	1 4 2 7% 4 3 3 3 6% 7% 60%	5844
February	1st to 10th	Day	2 5% 4 7% 4 4 4 3 6% 7% 54%	1890
	11th to 20th	Day	1 5% 2 6% 4 4 3 4 6% 8% 57%	1956
	21st to end	Day	2 6% 2 5% 4 4 3 3 5% 6% 59%	1650
March	1st to 10th	Day	4 8% 2 4 2 3 3 3 5% 8% 60%	4500
	11th to 20th	Day	4 7% 3 7% 4 4 4 4 6% 7% 49%	4948
	21st to end	Day	4 7% 2 5% 4 4 5% 4 7% 9% 49%	5823
April	1st to 10th	Day	4 7% 2 4 4 4 4 3 5% 9% 55%	2400
	11th to 20th	Day	9% 9% 2 5% 3 4 4 4 6% 8% 45%	2371
	21st to end	Day	9% 8% 2 6% 4 5% 4 5% 6% 8% 45%	2400
May	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 6% 7% 44%	5799
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 5% 7% 9% 45%	6298
	21st to end	Day	8% 5% 1 4 4 5% 5% 5% 7% 9% 48%	6925
June	1st to 10th	Day	4 4 1 5% 3 5% 5% 5% 8% 10% 49%	2800
	11th to 20th	Day	5% 3 1 4 3 5% 6% 5% 7% 9% 52%	2800
	21st to end	Day	3 3 1 4 4 5% 6% 5% 9% 9% 51%	2800
July	1st to 10th	Day	3 3 1 4 3 4 5% 4 7% 9% 58%	6339
	11th to 20th	Day	3 3 1 5% 5% 6% 7% 6% 9% 10% 44%	6340
	21st to end	Day	5% 3 2 6% 5% 5% 6% 6% 7% 8% 49%	6818
August	1st to 10th	Day	7% 4 2 5% 5% 6% 6% 5% 7% 10% 44%	5779
	11th to 20th	Day	3 2 1 3 3 4 5% 5% 9% 10% 56%	5408
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 4 7% 8% 47%	5984
September	1st to 10th	Day	4 4 1 4 3 3 4 4 6% 7% 59%	2400
	11th to 20th	Day	5% 3 1 4 3 4 4 3 6% 7% 60%	2381
	21st to end	Day	6% 5% 1 3 2 3 3 2 4 6% 65%	2200
October	1st to 10th	Day	4 4 1 3 2 2 2 3 4 4 72%	3277
	11th to 20th	Day	5% 4 1 4 4 4 4 3 5% 6% 61%	3250
	21st to end	Day	4 6% 1 4 2 2 2 3 5% 7% 64%	3420
November	1st to 10th	Day	3 5% 2 3 3 2 3 3 6% 8% 63%	1853
	11th to 20th	Day	2 5% 1 4 2 3 3 3 5% 8% 63%	1890
	21st to end	Day	3 5% 2 5% 3 3 3 3 6% 8% 59%	1753
December	1st to 10th	Day	1 4 2 4 3 2 2 2 4 7% 69%	2700
	11th to 20th	Day	2 3 1 3 2 2 3 3 4 7% 69%	3106
	21st to end	Day	4 6% 2 6% 4 3 4 3 6% 7% 54%	4092
January	1st to 10th	Night	2 8% 3 6% 3 3 3 2 4 7% 58%	8201
	11th to 20th	Night	1 5% 3 8% 3 3 3 2 4 6% 60%	8191
	21st to end	Night	2 5% 3 8% 4 4 3 2 4 5% 61%	8370
February	1st to 10th	Night	1 8% 3 7% 4 3 4 3 5% 8% 54%	2730
	11th to 20th	Night	2 7% 4 7% 5% 4 4 3 5% 7% 52%	2440
	21st to end	Night	2 9% 2 6% 4 3 3 2 4 6% 58%	1980
March	1st to 10th	Night	5% 9% 2 5% 3 3 4 3 5% 6% 55%	5374
	11th to 20th	Night	8% 8% 2 4 3 3 3 3 4 7% 55%	5048
	21st to end	Night	8% 9% 3 6% 4 4 3 3 5% 7% 50%	5135
April	1st to 10th	Night	5% 8% 1 5% 3 4 4 3 5% 7% 55%	2000
	11th to 20th	Night	10% 7% 2 4 3 4 4 3 5% 7% 51%	1977
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 7% 47%	2000
May	1st to 10th	Night	13% 6% 2 5% 3 3 4 4 6% 7% 46%	4166
	11th to 20th	Night	9% 6% 2 7% 5% 4 4 3 6% 9% 46%	3617
	21st to end	Night	11% 6% 2 4 3 3 3 3 4 7% 55%	3972
June	1st to 10th	Night	6% 3 1 3 3 3 3 4 6% 10% 57%	1600
	11th to 20th	Night	5% 3 2 4 3 3 3 3 5% 7% 62%	1600
	21st to end	Night	4 3 1 4 3 3 4 4 6% 7% 61%	1600
July	1st to 10th	Night	3 2 1 3 3 3 4 3 6% 8% 63%	3640
	11th to 20th	Night	4 2 1 3 3 3 4 5% 8% 11% 56%	3640
	21st to end	Night	4 4 1 4 4 4 5% 4 7% 9% 53%	4051
August	1st to 10th	Night	7% 4 2 5% 4 4 5% 4 7% 9% 50%	4033
	11th to 20th	Night	3 2 1 3 3 3 4 4 7% 10% 58%	4441
	21st to end	Night	7% 5% 2 6% 3 4 4 4 6% 9% 49%	4993
September	1st to 10th	Night	6% 3 1 4 3 3 3 4 5% 6% 62%	2000
	11th to 20th	Night	7% 2 1 3 2 2 4 3 4 7% 64%	2017
	21st to end	Night	7% 2 1 3 3 3 3 3 4 6% 65%	2200
October	1st to 10th	Night	5% 3 1 2 1 2 2 2 3 5% 74%	3910
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 7% 61%	3898
	21st to end	Night	6% 6% 2 3 2 2 3 2 3 6% 65%	4348
November	1st to 10th	Night	3 4 1 3 3 3 3 3 4 7% 66%	2665
	11th to 20th	Night	2 5% 2 4 3 3 2 3 5% 7% 64%	2730
	21st to end	Night	3 6% 2 4 3 3 3 3 5% 7% 61%	2861
December	1st to 10th	Night	1 4 1 3 3 2 2 2 3 6% 72%	4710
	11th to 20th	Night	1 4 2 4 2 2 3 3 5% 9% 65%	5321
	21st to end	Night	6% 9% 2 6% 4 3 3 3 4 7% 53%	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	26% 5% 9% 30% 24% 7%	4723
	11th to 20th	Day	21% 5% 11% 29% 24% 9%	4842
	21st to end	Day	22% 5% 9% 28% 25% 11%	5844
February	1st to 10th	Day	29% 4 12% 27% 20% 7%	1890
	11th to 20th	Day	24% 4 12% 28% 27% 5%	1966
	21st to end	Day	24% 5% 9% 24% 26% 12%	1650
March	1st to 10th	Day	24% 4 9% 20% 31% 12%	4500
	11th to 20th	Day	29% 7% 10% 24% 24% 5%	4948
	21st to end	Day	26% 8% 13% 35% 16% 2	5823
April	1st to 10th	Day	25% 5% 13% 25% 22% 9%	2400
	11th to 20th	Day	35% 5% 11% 21% 19% 8%	2374
	21st to end	Day	34% 8% 12% 24% 21% 2	2400
May	1st to 10th	Day	33% 7% 13% 26% 15% 5%	5799
	11th to 20th	Day	30% 10% 12% 20% 19% 9%	6302
	21st to end	Day	29% 9% 13% 25% 18% 5%	6926
June	1st to 10th	Day	25% 10% 16% 24% 18% 7%	2800
	11th to 20th	Day	25% 8% 11% 18% 23% 14%	2800
	21st to end	Day	22% 9% 11% 21% 24% 13%	2800
July	1st to 10th	Day	19% 7% 8% 17% 29% 20%	6271
	11th to 20th	Day	27% 9% 13% 23% 17% 11%	6265
	21st to end	Day	29% 8% 11% 16% 18% 17%	6621
August	1st to 10th	Day	35% 10% 14% 17% 15% 9%	4208
	11th to 20th	Day	23% 8% 14% 19% 21% 15%	3912
	21st to end	Day	25% 7% 10% 18% 26% 14%	4311
September	1st to 10th	Day	21% 5% 9% 21% 26% 18%	2400
	11th to 20th	Day	21% 5% 9% 21% 27% 16%	2381
	21st to end	Day	21% 4 7% 20% 36% 12%	2200
October	1st to 10th	Day	16% 3 6% 16% 34% 25%	3277
	11th to 20th	Day	22% 5% 10% 25% 29% 9%	3250
	21st to end	Day	20% 3 7% 25% 35% 10%	3420
November	1st to 10th	Day	19% 5% 9% 26% 29% 13%	1853
	11th to 20th	Day	19% 3 9% 26% 34% 9%	1890
	21st to end	Day	23% 3 11% 24% 29% 10%	1753
December	1st to 10th	Day	18% 4 8% 35% 26% 10%	2700
	11th to 20th	Day	16% 3 8% 33% 34% 6%	3104
	21st to end	Day	28% 3 11% 25% 24% 9%	4092
January	1st to 10th	Night	27% 3 7% 32% 23% 7%	8202
	11th to 20th	Night	24% 5% 11% 23% 24% 13%	8191
	21st to end	Night	26% 4 9% 24% 27% 9%	8370
February	1st to 10th	Night	29% 3 10% 27% 25% 6%	2730
	11th to 20th	Night	31% 4 11% 27% 22% 6%	2440
	21st to end	Night	28% 2 7% 23% 26% 13%	1980
March	1st to 10th	Night	29% 3 9% 15% 35% 9%	5374
	11th to 20th	Night	29% 4 10% 17% 27% 12%	5048
	21st to end	Night	32% 5% 11% 30% 19% 3	5135
April	1st to 10th	Night	28% 6% 12% 22% 23% 10%	2000
	11th to 20th	Night	32% 5% 13% 23% 20% 7%	1977
	21st to end	Night	36% 5% 10% 25% 20% 4	2000
May	1st to 10th	Night	34% 7% 14% 25% 13% 7%	4163
	11th to 20th	Night	32% 9% 13% 22% 17% 7%	3618
	21st to end	Night	28% 7% 17% 26% 16% 6%	3972
June	1st to 10th	Night	22% 7% 17% 31% 15% 8%	1600
	11th to 20th	Night	21% 6% 12% 21% 21% 19%	1600
	21st to end	Night	20% 5% 13% 22% 29% 11%	1600
July	1st to 10th	Night	18% 6% 10% 21% 33% 12%	3600
	11th to 20th	Night	20% 8% 19% 26% 20% 7%	3592
	21st to end	Night	24% 7% 15% 22% 20% 12%	3936
August	1st to 10th	Night	33% 7% 15% 23% 17% 5%	2928
	11th to 20th	Night	22% 8% 19% 21% 19% 12%	3213
	21st to end	Night	21% 6% 14% 23% 26% 9%	3597
September	1st to 10th	Night	22% 5% 10% 22% 22% 19%	2000
	11th to 20th	Night	20% 4 10% 24% 25% 18%	2017
	21st to end	Night	21% 3 8% 20% 32% 14%	2200
October	1st to 10th	Night	16% 3 7% 19% 31% 24%	3910
	11th to 20th	Night	23% 4 9% 26% 28% 10%	3898
	21st to end	Night	23% 3 6% 24% 33% 12%	4348
November	1st to 10th	Night	19% 4 9% 25% 28% 14%	2665
	11th to 20th	Night	20% 3 9% 27% 32% 9%	2730
	21st to end	Night	23% 4 11% 22% 30% 9%	2861
December	1st to 10th	Night	16% 4 8% 35% 31% 7%	4710
	11th to 20th	Night	18% 3 8% 33% 33% 5%	5322
	21st to end	Night	31% 4 10% 23% 26% 7%	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	5% 8% 18% 44% 18% 5%	330
	11th to 20th	Day	6% 6% 28% 43% 15% 1	330
	21st to end	Day	3 4 29% 45% 17% 2	363
February	1st to 10th	Day	5% 9% 37% 39% 8% 2	330
	11th to 20th	Day	3 6% 34% 46% 9% 2	315
	21st to end	Day	5% 9% 36% 42% 8% 1	249
March	1st to 10th	Day	5% 9% 32% 34% 17% 3	300
	11th to 20th	Day	11% 11% 30% 35% 11% 3	474
	21st to end	Day	10% 13% 37% 27% 11% 2	550
April	1st to 10th	Day	10% 14% 32% 30% 12% 2	500
	11th to 20th	Day	13% 17% 32% 28% 8% 2	492
	21st to end	Day	15% 12% 28% 29% 14% 2	500
May	1st to 10th	Day	13% 9% 26% 35% 14% 3	638
	11th to 20th	Day	17% 5% 21% 33% 19% 6%	725
	21st to end	Day	11% 6% 25% 29% 24% 5%	789
June	1st to 10th	Day	5% 4 18% 34% 29% 11%	730
	11th to 20th	Day	6% 3 23% 38% 23% 6%	730
	21st to end	Day	5% 4 19% 37% 26% 8%	730
July	1st to 10th	Day	6% 2 14% 30% 36% 12%	721
	11th to 20th	Day	4 2 13% 34% 35% 13%	718
	21st to end	Day	6% 2 13% 27% 40% 13%	759
August	1st to 10th	Day	7% 1 22% 34% 27% 9%	527
	11th to 20th	Day	3 3 18% 35% 30% 11%	517
	21st to end	Day	7% 3 17% 34% 30% 9%	583
September	1st to 10th	Day	7% 3 17% 33% 29% 12%	530
	11th to 20th	Day	5% 2 21% 35% 28% 10%	511
	21st to end	Day	8% 3 19% 30% 24% 16%	330
October	1st to 10th	Day	10% 5% 17% 26% 27% 15%	330
	11th to 20th	Day	3 5% 24% 43% 17% 8%	330
	21st to end	Day	8% 5% 27% 36% 20% 4	363
November	1st to 10th	Day	6% 7% 22% 42% 18% 7%	330
	11th to 20th	Day	5% 5% 26% 43% 16% 5%	330
	21st to end	Day	8% 5% 23% 45% 16% 3	330
December	1st to 10th	Day	3 9% 24% 43% 15% 5%	330
	11th to 20th	Day	3 7% 28% 49% 12% 2	330
	21st to end	Day	6% 8% 30% 44% 10% 3	363
January	1st to 10th	Night	4 8% 32% 43% 11% 2	1050
	11th to 20th	Night	3 8% 37% 40% 11% 1	1030
	21st to end	Night	5% 4 33% 50% 8%	924
February	1st to 10th	Night	6% 4 43% 40% 7% 1	840
	11th to 20th	Night	4 6% 37% 44% 9%	804
	21st to end	Night	5% 6% 39% 46% 4	660
March	1st to 10th	Night	4 4 36% 45% 10% 1	800
	11th to 20th	Night	9% 5% 28% 44% 12% 1	800
	21st to end	Night	8% 7% 30% 46% 7% 1	733
April	1st to 10th	Night	8% 6% 36% 44% 6%	600
	11th to 20th	Night	13% 6% 33% 37% 9% 1	593
	21st to end	Night	16% 4 29% 37% 12% 1	600
May	1st to 10th	Night	15% 5% 26% 39% 15% 1	600
	11th to 20th	Night	17% 3 17% 41% 19% 3	591
	21st to end	Night	10% 4 22% 39% 23% 2	650
June	1st to 10th	Night	7% 2 13% 48% 26% 3	600
	11th to 20th	Night	6% 2 19% 54% 19% 1	600
	21st to end	Night	5% 3 13% 48% 29% 2	600
July	1st to 10th	Night	7% 1 10% 40% 37% 5%	585
	11th to 20th	Night	3 1 7% 45% 40% 4	582
	21st to end	Night	6% 1 7% 36% 45% 4	657
August	1st to 10th	Night	6% 1 10% 41% 38% 3	597
	11th to 20th	Night	4 1 7% 47% 38% 4	594
	21st to end	Night	4 1 9% 44% 37% 6%	660
September	1st to 10th	Night	6% 2 9% 46% 31% 6%	600
	11th to 20th	Night	7% 1 13% 47% 28% 4	600
	21st to end	Night	8% 2 15% 49% 24% 3	600
October	1st to 10th	Night	10% 2 18% 45% 20% 4	790
	11th to 20th	Night	7% 3 21% 50% 17% 2	800
	21st to end	Night	10% 3 24% 47% 14% 3	940
November	1st to 10th	Night	5% 3 29% 47% 13% 2	1025
	11th to 20th	Night	5% 6% 33% 45% 9% 1	1050
	21st to end	Night	6% 4 29% 47% 13% 1	1050
December	1st to 10th	Night	5% 6% 32% 46% 11%	1050
	11th to 20th	Night	5% 4 38% 43% 9% 1	1050
	21st to end	Night	6% 4 37% 42% 8% 1	1155

Page generated at 2026-02-12 23:42:18. Version ac07

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