Cowles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cowles 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	3 5% 2 6% 4 4 5% 4 6% 7% 53%	135408
	Night	3 4 2 5% 4 4 4 4 6% 8% 58%	133412
Transparency	Day	25% 7% 11% 18% 25% 14%	130428
	Night	21% 7% 11% 19% 27% 14%	129396
Seeing	Day	17% 21% 30% 22% 9% 1	16707
	Night	11% 17% 31% 27% 13% 2	26874

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 9% 3 3 3 3 5% 7% 60%	3893
	11th to 20th	Day	1 3 1 9% 4 2 2 2 5% 5% 65%	4100
	21st to end	Day	1 6% 4 5% 4 4 4 4 5% 5% 56%	4510
February	1st to 10th	Day	1 5% 3 7% 4 4 4 4 7% 6% 55%	1852
	11th to 20th	Day	1 7% 3 6% 5% 3 4 3 6% 6% 57%	1949
	21st to end	Day	1 8% 3 5% 4 4 4 4 4 6% 57%	1650
March	1st to 10th	Day	1 5% 4 6% 4 3 3 3 6% 7% 59%	4633
	11th to 20th	Day	2 11% 4 7% 5% 4 4 4 6% 7% 47%	5000
	21st to end	Day	1 6% 3 5% 4 4 4 4 6% 8% 56%	5722
April	1st to 10th	Day	2 6% 2 5% 4 5% 3 3 6% 7% 56%	2400
	11th to 20th	Day	3 8% 2 6% 4 3 3 3 4 6% 58%	2371
	21st to end	Day	3 7% 2 5% 4 3 4 3 5% 7% 56%	2400
May	1st to 10th	Day	7% 9% 2 5% 5% 4 4 3 5% 5% 50%	5440
	11th to 20th	Day	5% 5% 1 4 3 3 3 3 4 5% 61%	5731
	21st to end	Day	2 2 1 4 3 3 4 4 6% 7% 65%	6925
June	1st to 10th	Day	3 3 2 5% 4 4 5% 5% 7% 8% 55%	2800
	11th to 20th	Day	3 2 1 4 3 3 4 4 5% 7% 64%	2800
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 7% 7% 46%	2800
July	1st to 10th	Day	3 3 2 6% 5% 5% 7% 5% 7% 8% 49%	6339
	11th to 20th	Day	3 3 1 6% 6% 8% 10% 9% 12% 11% 32%	5982
	21st to end	Day	6% 6% 3 11% 8% 8% 8% 6% 7% 8% 28%	6401
August	1st to 10th	Day	4 3 2 6% 6% 7% 7% 6% 8% 11% 40%	5573
	11th to 20th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 9% 40%	5366
	21st to end	Day	4 5% 2 8% 6% 7% 8% 6% 8% 10% 37%	5984
September	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 6% 7% 42%	2400
	11th to 20th	Day	7% 6% 2 6% 4 5% 4 4 6% 6% 50%	2400
	21st to end	Day	4 3 2 5% 3 4 4 4 5% 6% 60%	2195
October	1st to 10th	Day	6% 4 2 5% 3 3 3 4 6% 6% 58%	3270
	11th to 20th	Day	10% 6% 2 4 2 3 3 3 4 6% 57%	3250
	21st to end	Day	3 3 1 3 2 2 2 1 3 4 76%	3548
November	1st to 10th	Day	2 4 1 4 2 2 3 4 4 6% 67%	1943
	11th to 20th	Day	1 4 2 4 3 3 4 3 5% 7% 65%	1890
	21st to end	Day	3 6% 1 5% 3 3 3 4 5% 6% 61%	1890
December	1st to 10th	Day	1 2 1 3 2 2 2 3 4 7% 72%	2803
	11th to 20th	Day	1 3 1 3 4 4 4 3 6% 7% 63%	3106
	21st to end	Day	1 4 2 4 3 3 3 3 5% 6% 67%	4092
January	1st to 10th	Night	4 2 9% 4 4 4 3 6% 7% 58%	6138
	11th to 20th	Night	1 2 1 6% 4 3 3 3 7% 6% 63%	5880
	21st to end	Night	1 4 2 5% 5% 4 5% 4 7% 7% 57%	6468
February	1st to 10th	Night	1 6% 2 7% 3 3 3 3 6% 7% 59%	2545
	11th to 20th	Night	1 6% 2 6% 4 4 4 3 5% 8% 57%	2338
	21st to end	Night	2 6% 2 4 4 3 4 2 5% 6% 61%	1980
March	1st to 10th	Night	1 5% 2 5% 4 3 3 4 5% 7% 62%	5242
	11th to 20th	Night	2 10% 2 6% 4 4 3 3 5% 6% 56%	5000
	21st to end	Night	1 4 2 5% 3 3 3 2 4 7% 64%	5227
April	1st to 10th	Night	1 4 2 5% 4 4 3 3 5% 9% 60%	2000
	11th to 20th	Night	2 6% 2 5% 3 4 3 3 5% 7% 60%	1977
	21st to end	Night	3 4 1 4 4 4 4 2 6% 6% 62%	2000
May	1st to 10th	Night	4 8% 2 5% 4 3 4 4 6% 7% 53%	4540
	11th to 20th	Night	4 5% 2 4 3 3 4 3 5% 7% 61%	4163
	21st to end	Night	2 2 2 4 3 4 4 3 5% 7% 64%	3972
June	1st to 10th	Night	2 3 2 5% 5% 5% 5% 5% 8% 8% 51%	1600
	11th to 20th	Night	2 2 1 4 3 3 4 4 4 7% 66%	1600
	21st to end	Night	7% 5% 2 3 5% 5% 6% 5% 7% 8% 47%	1600
July	1st to 10th	Night	3 6% 3 6% 6% 6% 5% 5% 7% 7% 46%	3640
	11th to 20th	Night	2 3 1 6% 4 5% 6% 6% 11% 15% 40%	3980
	21st to end	Night	6% 4 2 6% 5% 5% 7% 7% 10% 11% 37%	4472
August	1st to 10th	Night	2 4 2 6% 5% 6% 7% 6% 9% 12% 41%	4237
	11th to 20th	Night	2 3 1 6% 4 5% 7% 6% 9% 13% 43%	4483
	21st to end	Night	5% 5% 2 7% 5% 7% 8% 7% 10% 13% 32%	4993
September	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 8% 8% 51%	2000
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 5% 6% 55%	2000
	21st to end	Night	5% 4 2 4 3 3 3 3 5% 6% 63%	2200
October	1st to 10th	Night	4 3 1 3 2 2 2 2 3 4 73%	3920
	11th to 20th	Night	10% 6% 1 3 2 2 2 3 4 5% 63%	3898
	21st to end	Night	3 2 1 2 1 1 2 2 4 6% 75%	4252
November	1st to 10th	Night	2 2 1 3 3 3 3 2 4 6% 71%	2554
	11th to 20th	Night	1 2 1 2 2 4 3 3 5% 7% 70%	2730
	21st to end	Night	3 6% 1 4 4 4 3 3 6% 6% 60%	2730
December	1st to 10th	Night	1 3 1 3 2 2 2 3 4 7% 73%	4604
	11th to 20th	Night	1 3 1 3 3 3 3 3 6% 8% 65%	5321
	21st to end	Night	1 4 2 5% 3 3 3 3 5% 5% 66%	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	22% 5% 9% 20% 34% 10%	3894
	11th to 20th	Day	18% 7% 10% 21% 22% 23%	4100
	21st to end	Day	24% 7% 8% 13% 31% 17%	4510
February	1st to 10th	Day	27% 7% 10% 17% 24% 15%	1852
	11th to 20th	Day	27% 6% 8% 22% 23% 14%	1959
	21st to end	Day	26% 5% 7% 15% 27% 21%	1650
March	1st to 10th	Day	21% 5% 7% 14% 32% 21%	4633
	11th to 20th	Day	26% 7% 12% 14% 27% 13%	5000
	21st to end	Day	20% 6% 10% 33% 27% 5%	5722
April	1st to 10th	Day	25% 5% 9% 21% 27% 13%	2400
	11th to 20th	Day	26% 6% 6% 18% 30% 14%	2374
	21st to end	Day	25% 6% 9% 22% 31% 8%	2400
May	1st to 10th	Day	30% 6% 10% 14% 29% 10%	5440
	11th to 20th	Day	21% 4 9% 14% 34% 18%	5735
	21st to end	Day	15% 5% 10% 15% 29% 25%	6926
June	1st to 10th	Day	22% 8% 12% 20% 23% 15%	2800
	11th to 20th	Day	18% 5% 8% 15% 25% 29%	2800
	21st to end	Day	32% 7% 11% 17% 20% 14%	2800
July	1st to 10th	Day	26% 9% 12% 13% 18% 21%	6271
	11th to 20th	Day	31% 16% 22% 22% 6% 4	5910
	21st to end	Day	44% 12% 18% 14% 9% 2	6217
August	1st to 10th	Day	39% 13% 19% 15% 9% 4	4057
	11th to 20th	Day	30% 9% 17% 23% 14% 7%	3882
	21st to end	Day	38% 11% 16% 22% 12% 2	4311
September	1st to 10th	Day	37% 7% 13% 19% 18% 6%	2400
	11th to 20th	Day	31% 7% 10% 17% 24% 11%	2400
	21st to end	Day	23% 5% 9% 22% 33% 10%	2195
October	1st to 10th	Day	22% 5% 8% 16% 33% 17%	3270
	11th to 20th	Day	28% 5% 5% 18% 30% 15%	3250
	21st to end	Day	14% 2 5% 14% 40% 25%	3548
November	1st to 10th	Day	16% 4 9% 19% 37% 16%	1943
	11th to 20th	Day	17% 4 8% 23% 33% 14%	1890
	21st to end	Day	23% 3 7% 18% 29% 20%	1890
December	1st to 10th	Day	12% 4 8% 23% 35% 19%	2803
	11th to 20th	Day	18% 5% 8% 15% 41% 13%	3104
	21st to end	Day	16% 4 9% 21% 34% 16%	4092
January	1st to 10th	Night	21% 7% 10% 18% 30% 14%	6138
	11th to 20th	Night	15% 8% 11% 21% 24% 21%	5880
	21st to end	Night	22% 7% 8% 15% 29% 18%	6468
February	1st to 10th	Night	23% 6% 12% 20% 23% 16%	2545
	11th to 20th	Night	26% 5% 10% 24% 22% 13%	2338
	21st to end	Night	21% 7% 7% 14% 32% 19%	1980
March	1st to 10th	Night	19% 4 10% 12% 34% 20%	5242
	11th to 20th	Night	24% 6% 11% 17% 30% 12%	5000
	21st to end	Night	16% 6% 11% 31% 30% 5%	5227
April	1st to 10th	Night	20% 6% 9% 23% 29% 13%	2000
	11th to 20th	Night	24% 5% 9% 19% 29% 15%	1977
	21st to end	Night	20% 8% 10% 23% 31% 8%	2000
May	1st to 10th	Night	24% 7% 13% 18% 29% 9%	4537
	11th to 20th	Night	20% 7% 9% 21% 27% 16%	4164
	21st to end	Night	16% 7% 12% 12% 27% 26%	3972
June	1st to 10th	Night	25% 8% 15% 19% 18% 15%	1600
	11th to 20th	Night	17% 5% 9% 15% 24% 29%	1600
	21st to end	Night	31% 8% 12% 16% 19% 15%	1600
July	1st to 10th	Night	31% 9% 13% 16% 13% 18%	3600
	11th to 20th	Night	24% 14% 29% 22% 5% 6%	3929
	21st to end	Night	33% 12% 25% 20% 9% 1	4344
August	1st to 10th	Night	36% 14% 21% 16% 9% 5%	3077
	11th to 20th	Night	26% 12% 21% 21% 14% 6%	3243
	21st to end	Night	35% 14% 19% 21% 9% 2	3597
September	1st to 10th	Night	27% 8% 17% 24% 17% 7%	2000
	11th to 20th	Night	26% 8% 11% 17% 26% 12%	2000
	21st to end	Night	21% 6% 9% 22% 31% 10%	2200
October	1st to 10th	Night	16% 4 6% 21% 38% 15%	3920
	11th to 20th	Night	24% 4 6% 20% 30% 16%	3898
	21st to end	Night	11% 3 6% 18% 39% 24%	4252
November	1st to 10th	Night	13% 3 8% 24% 36% 15%	2554
	11th to 20th	Night	14% 4 8% 24% 35% 15%	2730
	21st to end	Night	24% 5% 6% 19% 26% 20%	2730
December	1st to 10th	Night	13% 5% 5% 23% 35% 18%	4604
	11th to 20th	Night	16% 5% 7% 17% 36% 20%	5322
	21st to end	Night	17% 6% 10% 19% 33% 16%	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	23% 31% 26% 15% 4	303
	11th to 20th	Day	30% 27% 26% 14% 3	300
	21st to end	Day	17% 30% 31% 15% 5% 2	330
February	1st to 10th	Day	27% 29% 30% 11% 2	300
	11th to 20th	Day	24% 38% 25% 12% 1	300
	21st to end	Day	39% 28% 24% 8% 1	249
March	1st to 10th	Day	26% 28% 33% 12% 1	355
	11th to 20th	Day	31% 29% 28% 11% 2 1	500
	21st to end	Day	31% 34% 27% 6% 2	550
April	1st to 10th	Day	24% 34% 32% 7% 2	500
	11th to 20th	Day	30% 37% 25% 7% 1	492
	21st to end	Day	24% 39% 29% 7% 1	500
May	1st to 10th	Day	19% 34% 37% 8% 2 1	503
	11th to 20th	Day	18% 23% 34% 18% 6% 1	626
	21st to end	Day	12% 23% 38% 21% 5%	789
June	1st to 10th	Day	7% 14% 35% 33% 11% 1	730
	11th to 20th	Day	9% 17% 34% 27% 12% 1	730
	21st to end	Day	9% 15% 30% 31% 13% 3	730
July	1st to 10th	Day	7% 6% 25% 36% 21% 4	721
	11th to 20th	Day	7% 2 16% 44% 26% 5%	571
	21st to end	Day	8% 3 16% 43% 24% 6%	575
August	1st to 10th	Day	6% 7% 25% 42% 17% 3	527
	11th to 20th	Day	8% 6% 29% 40% 16% 2	517
	21st to end	Day	6% 7% 32% 37% 16% 2	583
September	1st to 10th	Day	9% 7% 32% 35% 14% 2	530
	11th to 20th	Day	8% 15% 37% 28% 10% 2	530
	21st to end	Day	7% 22% 32% 27% 8% 3	330
October	1st to 10th	Day	15% 25% 27% 22% 9% 2	330
	11th to 20th	Day	19% 26% 29% 17% 6% 2	330
	21st to end	Day	23% 21% 32% 17% 6% 2	363
November	1st to 10th	Day	20% 30% 29% 18% 3	330
	11th to 20th	Day	18% 25% 34% 18% 6%	330
	21st to end	Day	19% 32% 29% 12% 6%	330
December	1st to 10th	Day	25% 27% 27% 15% 6%	330
	11th to 20th	Day	21% 30% 31% 14% 4	330
	21st to end	Day	31% 33% 24% 10% 2	363
January	1st to 10th	Night	11% 23% 34% 21% 9% 2	919
	11th to 20th	Night	13% 24% 35% 20% 7% 2	800
	21st to end	Night	8% 25% 37% 20% 8% 2	880
February	1st to 10th	Night	14% 23% 40% 18% 5% 1	800
	11th to 20th	Night	14% 28% 36% 17% 4	784
	21st to end	Night	16% 31% 39% 13% 1	660
March	1st to 10th	Night	14% 26% 36% 19% 5% 1	800
	11th to 20th	Night	14% 21% 31% 24% 9%	800
	21st to end	Night	16% 24% 38% 18% 4	773
April	1st to 10th	Night	14% 27% 38% 19% 3	600
	11th to 20th	Night	15% 26% 31% 22% 5%	593
	21st to end	Night	12% 25% 36% 23% 5%	600
May	1st to 10th	Night	9% 18% 38% 24% 10%	600
	11th to 20th	Night	12% 17% 31% 27% 13% 1	591
	21st to end	Night	10% 18% 32% 33% 7%	650
June	1st to 10th	Night	5% 7% 35% 38% 16%	600
	11th to 20th	Night	3 11% 36% 38% 12% 1	600
	21st to end	Night	8% 7% 26% 37% 21% 1	600
July	1st to 10th	Night	5% 5% 15% 42% 30% 4	585
	11th to 20th	Night	3 1 11% 44% 36% 6%	582
	21st to end	Night	7% 2 11% 33% 39% 7%	657
August	1st to 10th	Night	5% 1 10% 40% 35% 8%	597
	11th to 20th	Night	6% 3 13% 46% 28% 4	594
	21st to end	Night	5% 2 17% 39% 31% 6%	660
September	1st to 10th	Night	6% 2 23% 41% 25% 3	600
	11th to 20th	Night	6% 5% 24% 41% 23% 2	600
	21st to end	Night	6% 10% 26% 42% 16% 1	600
October	1st to 10th	Night	9% 12% 32% 33% 13% 1	800
	11th to 20th	Night	11% 13% 35% 26% 12% 3	800
	21st to end	Night	11% 18% 30% 25% 14% 4	880
November	1st to 10th	Night	12% 17% 33% 27% 11% 2	914
	11th to 20th	Night	9% 18% 37% 26% 9% 2	1050
	21st to end	Night	15% 22% 31% 22% 9% 2	1050
December	1st to 10th	Night	14% 23% 34% 22% 6% 1	1050
	11th to 20th	Night	15% 25% 32% 20% 7% 1	1050
	21st to end	Night	18% 24% 38% 17% 4	1155

Page generated at 2025-12-29 23:14:04. Version ac07

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