Benton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Benton clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 8% 3 7% 5% 5% 5% 4 6% 8% 39%	126151
	Night	13% 8% 3 6% 4 4 4 4 5% 7% 41%	123242
Transparency	Day	41% 8% 12% 20% 16% 4	129902
	Night	40% 7% 11% 20% 17% 4	127559
Seeing	Day	17% 12% 23% 23% 17% 8%	15993
	Night	16% 10% 23% 29% 19% 3	25975

By Month

Percentage Cloud Cover SE (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	17% 11% 4 7% 4 3 3 3 4 5% 39%	4056
	11th to 20th	Day	13% 9% 7% 8% 4 3 3 2 3 4 44%	4060
	21st to end	Day	19% 13% 3 6% 4 3 3 2 4 7% 36%	4621
February	1st to 10th	Day	17% 11% 4 7% 5% 4 4 3 6% 6% 35%	2000
	11th to 20th	Day	22% 10% 3 7% 4 4 3 3 5% 6% 32%	1948
	21st to end	Day	20% 10% 3 7% 4 3 3 3 5% 5% 37%	1650
March	1st to 10th	Day	18% 9% 3 7% 5% 4 5% 4 5% 6% 34%	4545
	11th to 20th	Day	12% 7% 2 6% 4 4 3 3 5% 6% 48%	4980
	21st to end	Day	16% 8% 3 6% 5% 4 4 3 4 6% 42%	5465
April	1st to 10th	Day	11% 7% 2 6% 4 5% 6% 4 7% 7% 42%	2317
	11th to 20th	Day	14% 6% 3 7% 4 3 4 3 5% 7% 45%	2374
	21st to end	Day	12% 7% 3 6% 5% 5% 5% 4 6% 7% 41%	2560
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 6% 4 7% 8% 32%	5862
	11th to 20th	Day	15% 9% 4 9% 6% 7% 5% 5% 6% 6% 28%	5861
	21st to end	Day	13% 8% 3 8% 5% 4 5% 4 6% 7% 37%	6433
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 7% 5% 8% 9% 37%	2600
	11th to 20th	Day	5% 5% 2 7% 5% 7% 6% 6% 8% 10% 40%	2600
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 6% 8% 11% 38%	2600
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 9% 12% 38%	5900
	11th to 20th	Day	7% 5% 2 7% 6% 6% 7% 5% 9% 12% 34%	5900
	21st to end	Day	10% 8% 4 10% 7% 7% 7% 6% 8% 10% 23%	6424
August	1st to 10th	Day	6% 5% 2 6% 5% 6% 7% 5% 8% 11% 40%	4234
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 6% 7% 11% 40%	3922
	21st to end	Day	7% 4 2 6% 4 5% 6% 5% 6% 8% 48%	4312
September	1st to 10th	Day	10% 6% 2 8% 5% 5% 5% 5% 6% 9% 40%	2244
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 5% 5% 7% 41%	2200
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 5% 7% 46%	2200
October	1st to 10th	Day	11% 4 2 4 4 5% 4 3 5% 5% 54%	2123
	11th to 20th	Day	11% 6% 2 4 3 3 3 3 3 6% 56%	1990
	21st to end	Day	13% 6% 1 4 3 3 3 3 5% 6% 52%	2039
November	1st to 10th	Day	13% 5% 2 4 3 4 4 2 4 5% 54%	1762
	11th to 20th	Day	12% 8% 2 5% 3 4 3 3 6% 6% 48%	1800
	21st to end	Day	16% 8% 2 4 4 3 5% 3 5% 7% 44%	1800
December	1st to 10th	Day	14% 8% 2 6% 4 4 4 3 4 5% 45%	2930
	11th to 20th	Day	19% 13% 4 8% 5% 4 4 3 4 4 31%	3373
	21st to end	Day	15% 12% 4 8% 4 4 4 3 5% 7% 33%	4466
January	1st to 10th	Night	15% 10% 4 8% 4 4 3 2 4 5% 41%	5889
	11th to 20th	Night	18% 9% 3 6% 5% 3 3 2 3 5% 44%	5900
	21st to end	Night	16% 11% 3 7% 4 4 3 3 4 5% 41%	6334
February	1st to 10th	Night	16% 9% 3 7% 4 3 3 3 5% 6% 40%	2400
	11th to 20th	Night	22% 10% 3 6% 5% 4 4 4 3 4 35%	2338
	21st to end	Night	21% 9% 3 6% 4 4 4 3 5% 6% 35%	1980
March	1st to 10th	Night	19% 11% 3 6% 4 4 4 3 5% 6% 34%	5292
	11th to 20th	Night	13% 6% 2 4 3 3 4 4 6% 7% 49%	4980
	21st to end	Night	12% 9% 3 7% 4 5% 4 3 5% 6% 42%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 4 6% 8% 47%	2099
	11th to 20th	Night	13% 7% 2 5% 3 4 3 3 5% 7% 49%	1975
	21st to end	Night	13% 6% 2 6% 4 4 3 4 5% 7% 45%	1838
May	1st to 10th	Night	12% 8% 2 8% 6% 6% 5% 5% 7% 9% 32%	4033
	11th to 20th	Night	17% 10% 4 9% 6% 5% 5% 4 5% 7% 28%	4032
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	4421
June	1st to 10th	Night	6% 5% 3 7% 6% 6% 6% 5% 7% 11% 39%	1800
	11th to 20th	Night	4 4 1 5% 5% 5% 5% 4 8% 12% 47%	1800
	21st to end	Night	4 5% 2 7% 4 5% 6% 4 8% 11% 45%	1800
July	1st to 10th	Night	8% 5% 2 7% 6% 6% 6% 5% 8% 11% 36%	4058
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 13% 38%	4057
	21st to end	Night	10% 8% 3 8% 6% 6% 7% 5% 9% 11% 25%	4427
August	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 4 8% 11% 43%	2915
	11th to 20th	Night	8% 5% 2 6% 4 5% 4 4 6% 10% 45%	3122
	21st to end	Night	6% 4 2 5% 4 5% 5% 5% 6% 9% 48%	3586
September	1st to 10th	Night	10% 5% 3 7% 6% 4 6% 5% 6% 7% 41%	2135
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 3 6% 8% 43%	2200
	21st to end	Night	11% 6% 3 5% 4 4 4 3 5% 7% 48%	2200
October	1st to 10th	Night	11% 4 2 4 3 3 3 2 4 6% 58%	2124
	11th to 20th	Night	14% 4 1 5% 3 4 4 2 4 4 54%	2187
	21st to end	Night	13% 6% 2 4 2 3 3 3 5% 5% 54%	2555
November	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 5% 53%	2536
	11th to 20th	Night	13% 7% 2 5% 4 2 3 3 4 7% 49%	2600
	21st to end	Night	14% 9% 3 5% 4 4 4 4 4 6% 44%	2600
December	1st to 10th	Night	15% 9% 2 6% 4 4 3 3 4 5% 42%	4250
	11th to 20th	Night	22% 11% 3 7% 4 3 4 3 4 6% 35%	4822
	21st to end	Night	15% 11% 4 8% 5% 4 5% 3 5% 6% 34%	6490

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	48% 4 7% 18% 22% 1	4057
	11th to 20th	Day	44% 5% 5% 16% 27% 3	4060
	21st to end	Day	49% 4 7% 18% 18% 4	4621
February	1st to 10th	Day	50% 5% 7% 18% 16% 3	2000
	11th to 20th	Day	54% 4 6% 16% 15% 4	1958
	21st to end	Day	50% 5% 7% 13% 20% 5%	1650
March	1st to 10th	Day	48% 5% 9% 13% 18% 7%	4545
	11th to 20th	Day	38% 4 6% 16% 23% 13%	4980
	21st to end	Day	43% 4 8% 27% 16% 2	5465
April	1st to 10th	Day	39% 9% 11% 21% 17% 4	2317
	11th to 20th	Day	39% 5% 8% 19% 24% 5%	2374
	21st to end	Day	41% 7% 11% 26% 13% 2	2560
May	1st to 10th	Day	46% 10% 14% 20% 10% 1	5897
	11th to 20th	Day	53% 10% 13% 18% 6% 1	5865
	21st to end	Day	44% 9% 10% 22% 14% 2	6447
June	1st to 10th	Day	39% 12% 16% 22% 10% 1	2600
	11th to 20th	Day	35% 12% 17% 23% 11% 2	2600
	21st to end	Day	36% 14% 17% 19% 13% 3	2600
July	1st to 10th	Day	34% 13% 20% 22% 11% 1	5835
	11th to 20th	Day	38% 13% 26% 16% 5% 1	5831
	21st to end	Day	53% 12% 18% 14% 4	6240
August	1st to 10th	Day	33% 12% 17% 30% 8%	4221
	11th to 20th	Day	34% 10% 19% 24% 12% 1	3960
	21st to end	Day	32% 10% 12% 29% 15% 1	4311
September	1st to 10th	Day	40% 11% 13% 16% 16% 5%	2244
	11th to 20th	Day	41% 9% 12% 20% 16% 1	2200
	21st to end	Day	37% 8% 13% 17% 19% 6%	2200
October	1st to 10th	Day	26% 6% 7% 16% 34% 11%	3513
	11th to 20th	Day	22% 4 6% 21% 31% 16%	3282
	21st to end	Day	24% 5% 8% 28% 26% 10%	3341
November	1st to 10th	Day	32% 5% 8% 19% 29% 7%	1762
	11th to 20th	Day	36% 5% 7% 21% 25% 7%	1800
	21st to end	Day	40% 5% 7% 20% 24% 4	1800
December	1st to 10th	Day	41% 5% 6% 18% 26% 3	2930
	11th to 20th	Day	55% 5% 7% 16% 14% 3	3370
	21st to end	Day	47% 5% 12% 24% 11% 1	4466
January	1st to 10th	Night	46% 4 8% 19% 20% 3	5889
	11th to 20th	Night	44% 5% 6% 16% 22% 8%	5900
	21st to end	Night	46% 5% 6% 18% 20% 6%	6334
February	1st to 10th	Night	46% 5% 9% 18% 19% 4	2400
	11th to 20th	Night	54% 5% 6% 15% 15% 5%	2339
	21st to end	Night	51% 5% 7% 16% 17% 5%	1980
March	1st to 10th	Night	50% 5% 7% 15% 19% 4	5292
	11th to 20th	Night	33% 5% 7% 19% 22% 13%	4980
	21st to end	Night	43% 4 8% 24% 16% 3	5467
April	1st to 10th	Night	34% 7% 13% 21% 21% 4	2099
	11th to 20th	Night	35% 5% 9% 23% 22% 6%	1975
	21st to end	Night	37% 6% 13% 28% 16% 1	1838
May	1st to 10th	Night	44% 11% 17% 19% 9% 1	4060
	11th to 20th	Night	54% 9% 14% 17% 7%	4033
	21st to end	Night	41% 10% 14% 23% 12% 1	4429
June	1st to 10th	Night	37% 12% 17% 21% 12% 2	1800
	11th to 20th	Night	27% 12% 21% 27% 12% 2	1800
	21st to end	Night	31% 10% 20% 21% 14% 4	1800
July	1st to 10th	Night	37% 11% 21% 23% 7% 1	4015
	11th to 20th	Night	35% 10% 29% 19% 4 2	4006
	21st to end	Night	47% 13% 19% 16% 4 1	4297
August	1st to 10th	Night	31% 10% 17% 31% 11%	2904
	11th to 20th	Night	32% 9% 22% 25% 11% 1	3150
	21st to end	Night	28% 10% 15% 30% 15% 1	3586
September	1st to 10th	Night	38% 10% 15% 18% 14% 5%	2135
	11th to 20th	Night	37% 9% 15% 20% 19% 1	2200
	21st to end	Night	35% 7% 12% 20% 20% 6%	2200
October	1st to 10th	Night	22% 6% 9% 20% 29% 14%	3514
	11th to 20th	Night	23% 3 6% 18% 33% 17%	3634
	21st to end	Night	22% 4 4 28% 29% 12%	4203
November	1st to 10th	Night	34% 5% 8% 21% 23% 9%	2536
	11th to 20th	Night	37% 4 6% 20% 25% 9%	2600
	21st to end	Night	42% 5% 6% 21% 22% 5%	2600
December	1st to 10th	Night	44% 5% 6% 18% 24% 3	4250
	11th to 20th	Night	50% 5% 6% 16% 20% 2	4824
	21st to end	Night	49% 5% 11% 22% 12% 1	6490

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	28% 24% 34% 12% 3	300
	11th to 20th	Day	26% 27% 30% 13% 4	300
	21st to end	Day	22% 28% 36% 11% 2	330
February	1st to 10th	Day	25% 24% 33% 13% 5%	300
	11th to 20th	Day	39% 27% 25% 9%	300
	21st to end	Day	37% 23% 30% 9%	249
March	1st to 10th	Day	28% 25% 28% 13% 5% 1	326
	11th to 20th	Day	25% 22% 31% 16% 6%	500
	21st to end	Day	25% 19% 34% 16% 5%	550
April	1st to 10th	Day	21% 19% 29% 20% 11% 1	500
	11th to 20th	Day	19% 19% 29% 21% 10% 1	492
	21st to end	Day	19% 13% 30% 25% 11% 2	500
May	1st to 10th	Day	16% 13% 34% 20% 12% 5%	503
	11th to 20th	Day	14% 8% 27% 28% 15% 8%	527
	21st to end	Day	13% 9% 21% 28% 18% 12%	573
June	1st to 10th	Day	9% 6% 22% 28% 23% 10%	530
	11th to 20th	Day	6% 5% 16% 31% 27% 15%	530
	21st to end	Day	8% 3 19% 29% 24% 17%	530
July	1st to 10th	Day	9% 4 15% 26% 28% 18%	525
	11th to 20th	Day	8% 3 10% 25% 35% 20%	524
	21st to end	Day	12% 3 9% 25% 31% 20%	576
August	1st to 10th	Day	11% 3 15% 26% 26% 19%	526
	11th to 20th	Day	10% 3 17% 29% 28% 14%	519
	21st to end	Day	12% 3 12% 24% 31% 18%	583
September	1st to 10th	Day	10% 3 14% 31% 27% 15%	530
	11th to 20th	Day	15% 4 12% 24% 30% 15%	530
	21st to end	Day	15% 7% 14% 28% 29% 7%	530
October	1st to 10th	Day	12% 7% 18% 32% 23% 8%	502
	11th to 20th	Day	12% 13% 25% 29% 19% 2	490
	21st to end	Day	18% 10% 24% 28% 18% 3	488
November	1st to 10th	Day	21% 10% 29% 25% 13% 2	300
	11th to 20th	Day	18% 16% 34% 25% 8%	300
	21st to end	Day	15% 18% 36% 21% 8% 1	300
December	1st to 10th	Day	25% 23% 32% 16% 4	300
	11th to 20th	Day	25% 19% 31% 19% 6% 1	300
	21st to end	Day	25% 20% 32% 16% 6% 1	330
January	1st to 10th	Night	23% 22% 28% 23% 4	800
	11th to 20th	Night	25% 16% 30% 24% 5%	800
	21st to end	Night	22% 16% 35% 25% 3	880
February	1st to 10th	Night	20% 15% 34% 27% 5%	800
	11th to 20th	Night	28% 21% 31% 19% 2	783
	21st to end	Night	28% 21% 30% 19% 2	660
March	1st to 10th	Night	20% 16% 33% 26% 5%	800
	11th to 20th	Night	21% 17% 32% 22% 7% 1	800
	21st to end	Night	20% 14% 32% 26% 8%	880
April	1st to 10th	Night	14% 18% 29% 29% 9%	699
	11th to 20th	Night	16% 13% 33% 28% 10% 1	593
	21st to end	Night	16% 9% 28% 34% 13% 1	600
May	1st to 10th	Night	11% 7% 30% 34% 17% 2	600
	11th to 20th	Night	15% 7% 21% 30% 23% 4	591
	21st to end	Night	12% 3 15% 35% 29% 6%	650
June	1st to 10th	Night	7% 4 15% 37% 33% 4	600
	11th to 20th	Night	5% 2 13% 35% 35% 11%	600
	21st to end	Night	7% 1 10% 33% 40% 9%	600
July	1st to 10th	Night	11% 3 9% 27% 41% 9%	585
	11th to 20th	Night	9% 2 6% 26% 45% 12%	582
	21st to end	Night	9% 2 9% 25% 46% 10%	657
August	1st to 10th	Night	9% 3 7% 26% 47% 8%	597
	11th to 20th	Night	8% 3 8% 33% 40% 7%	594
	21st to end	Night	11% 2 8% 30% 40% 10%	660
September	1st to 10th	Night	10% 3 11% 31% 39% 6%	735
	11th to 20th	Night	12% 3 11% 33% 36% 5%	800
	21st to end	Night	14% 3 15% 34% 32% 2	800
October	1st to 10th	Night	12% 5% 19% 29% 28% 6%	773
	11th to 20th	Night	15% 6% 25% 32% 20% 2	760
	21st to end	Night	16% 9% 21% 36% 17% 2	836
November	1st to 10th	Night	14% 8% 21% 37% 17% 2	780
	11th to 20th	Night	16% 12% 29% 31% 11% 1	800
	21st to end	Night	19% 12% 28% 31% 10%	800
December	1st to 10th	Night	24% 16% 26% 28% 7%	800
	11th to 20th	Night	23% 12% 30% 29% 7%	800
	21st to end	Night	22% 16% 27% 30% 5%	880

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