Benson Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Benson Lake Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	131472
	Night	13% 10% 4 8% 5% 4 4 3 5% 7% 37%	130662
Transparency	Day	45% 6% 13% 23% 12% 1	134935
	Night	46% 6% 13% 23% 12% 1	135274
Seeing	Day	19% 16% 33% 20% 9% 3	17886
	Night	18% 12% 34% 27% 9% 1	27925

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	9% 12% 4 10% 6% 6% 6% 4 6% 7% 29%	4691
	11th to 20th	Day	10% 13% 4 10% 6% 6% 6% 4 6% 7% 28%	4710
	21st to end	Day	8% 12% 5% 11% 5% 5% 5% 3 6% 7% 33%	5156
February	1st to 10th	Day	9% 12% 3 9% 6% 4 5% 3 6% 7% 34%	1811
	11th to 20th	Day	10% 13% 4 8% 5% 4 5% 3 5% 6% 36%	1985
	21st to end	Day	11% 14% 3 10% 5% 5% 4 4 5% 8% 32%	1730
March	1st to 10th	Day	12% 9% 2 5% 4 4 4 3 5% 6% 45%	4580
	11th to 20th	Day	11% 10% 3 9% 7% 6% 6% 4 5% 7% 33%	4774
	21st to end	Day	23% 12% 3 11% 5% 5% 4 2 3 4 27%	6099
April	1st to 10th	Day	15% 13% 4 9% 5% 5% 6% 4 5% 6% 28%	2412
	11th to 20th	Day	19% 10% 3 7% 5% 5% 4 4 5% 5% 32%	2374
	21st to end	Day	18% 9% 3 7% 5% 4 5% 4 5% 6% 32%	2590
May	1st to 10th	Day	13% 6% 2 7% 4 4 4 3 5% 5% 46%	6092
	11th to 20th	Day	11% 7% 3 7% 5% 5% 4 4 6% 8% 41%	6319
	21st to end	Day	14% 9% 2 8% 6% 5% 5% 4 5% 7% 36%	6935
June	1st to 10th	Day	12% 6% 2 7% 6% 5% 5% 4 6% 7% 42%	2800
	11th to 20th	Day	15% 6% 3 7% 6% 5% 5% 4 6% 7% 38%	2800
	21st to end	Day	13% 7% 2 7% 6% 5% 4 4 7% 9% 37%	2800
July	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 6% 9% 42%	6360
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 6% 8% 49%	6360
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 5% 6% 9% 45%	6924
August	1st to 10th	Day	8% 7% 2 7% 6% 6% 6% 5% 7% 8% 39%	4563
	11th to 20th	Day	12% 5% 2 6% 3 4 5% 3 5% 7% 47%	4172
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 5% 7% 42%	4312
September	1st to 10th	Day	12% 6% 2 8% 4 4 5% 3 6% 8% 42%	2400
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 5% 6% 8% 42%	2290
	21st to end	Day	12% 6% 3 8% 6% 5% 5% 4 7% 8% 37%	2200
October	1st to 10th	Day	15% 7% 3 7% 4 4 5% 4 6% 7% 39%	2103
	11th to 20th	Day	20% 10% 3 8% 5% 6% 4 3 6% 7% 28%	1927
	21st to end	Day	20% 10% 3 8% 5% 4 5% 4 4 7% 31%	1980
November	1st to 10th	Day	13% 10% 3 7% 5% 4 5% 5% 4 7% 37%	1852
	11th to 20th	Day	15% 11% 5% 10% 6% 5% 5% 3 5% 7% 28%	1843
	21st to end	Day	15% 13% 5% 8% 5% 4 5% 4 6% 7% 28%	1680
December	1st to 10th	Day	9% 13% 5% 12% 7% 6% 6% 5% 5% 6% 27%	2690
	11th to 20th	Day	13% 17% 4 9% 5% 5% 5% 4 5% 5% 27%	3088
	21st to end	Day	22% 21% 6% 10% 4 3 3 2 3 4 21%	4070
January	1st to 10th	Night	11% 12% 5% 11% 6% 5% 5% 3 5% 6% 31%	8232
	11th to 20th	Night	8% 15% 6% 13% 8% 6% 6% 4 5% 6% 23%	8280
	21st to end	Night	7% 11% 5% 11% 6% 5% 5% 4 6% 7% 33%	9058
February	1st to 10th	Night	10% 13% 5% 9% 6% 5% 4 4 5% 6% 34%	2807
	11th to 20th	Night	10% 11% 4 9% 6% 4 4 4 5% 7% 36%	2520
	21st to end	Night	13% 13% 5% 8% 5% 4 4 4 5% 7% 33%	2076
March	1st to 10th	Night	13% 8% 2 7% 4 4 4 3 4 7% 44%	5493
	11th to 20th	Night	11% 10% 3 9% 5% 5% 5% 4 6% 9% 33%	5450
	21st to end	Night	20% 10% 4 9% 5% 5% 4 4 5% 6% 29%	5072
April	1st to 10th	Night	19% 11% 3 8% 6% 4 4 4 5% 6% 31%	2010
	11th to 20th	Night	21% 8% 3 6% 4 3 4 3 5% 6% 37%	1975
	21st to end	Night	17% 8% 3 8% 4 4 5% 3 5% 7% 36%	1809
May	1st to 10th	Night	14% 8% 3 7% 4 3 4 3 4 6% 44%	3798
	11th to 20th	Night	10% 9% 2 7% 4 4 4 3 5% 6% 45%	3596
	21st to end	Night	17% 7% 2 6% 5% 4 4 4 5% 6% 40%	3942
June	1st to 10th	Night	11% 5% 2 5% 5% 4 4 5% 5% 7% 47%	1600
	11th to 20th	Night	15% 7% 3 7% 4 4 6% 4 6% 6% 38%	1600
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 8% 43%	1600
July	1st to 10th	Night	12% 5% 2 5% 3 4 5% 3 5% 9% 46%	3618
	11th to 20th	Night	8% 3 2 4 3 3 4 4 5% 8% 56%	3617
	21st to end	Night	8% 5% 2 6% 4 4 5% 3 5% 8% 50%	3947
August	1st to 10th	Night	10% 6% 3 7% 4 4 4 4 5% 7% 46%	2597
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 49%	2876
	21st to end	Night	12% 6% 2 6% 5% 4 3 3 5% 6% 48%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 5% 6% 52%	2000
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 5% 7% 50%	2090
	21st to end	Night	14% 6% 3 6% 4 3 4 3 5% 7% 45%	2200
October	1st to 10th	Night	15% 6% 3 5% 3 3 5% 3 6% 6% 45%	2295
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 4 5% 6% 35%	2471
	21st to end	Night	19% 10% 3 6% 5% 4 4 3 4 7% 33%	2860
November	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	2666
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 3 5% 7% 34%	2772
	21st to end	Night	15% 12% 3 8% 5% 4 5% 4 5% 7% 30%	2940
December	1st to 10th	Night	13% 11% 4 7% 5% 5% 5% 4 6% 7% 33%	4720
	11th to 20th	Night	13% 16% 6% 12% 7% 5% 4 3 3 4 27%	5339
	21st to end	Night	23% 16% 5% 8% 5% 3 3 3 4 5% 25%	7150

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	52% 6% 15% 18% 8%	4692
	11th to 20th	Day	53% 7% 9% 22% 9% 1	4710
	21st to end	Day	50% 5% 12% 19% 14% 1	5156
February	1st to 10th	Day	48% 5% 14% 20% 12% 1	1811
	11th to 20th	Day	46% 6% 13% 24% 10% 1	1995
	21st to end	Day	51% 5% 11% 19% 11% 1	1730
March	1st to 10th	Day	39% 4 14% 20% 18% 5%	4580
	11th to 20th	Day	48% 6% 14% 20% 10% 2	4774
	21st to end	Day	61% 5% 9% 14% 11%	6101
April	1st to 10th	Day	55% 7% 11% 17% 9% 1	2412
	11th to 20th	Day	52% 6% 9% 21% 10% 1	2374
	21st to end	Day	51% 7% 9% 17% 14% 2	2590
May	1st to 10th	Day	38% 6% 10% 25% 18% 3	6127
	11th to 20th	Day	39% 8% 12% 26% 14% 2	6323
	21st to end	Day	45% 7% 9% 20% 16% 2	6949
June	1st to 10th	Day	38% 8% 14% 25% 15% 2	2800
	11th to 20th	Day	45% 7% 14% 19% 14% 1	2800
	21st to end	Day	41% 8% 15% 22% 12% 1	2800
July	1st to 10th	Day	37% 6% 15% 31% 10% 1	6291
	11th to 20th	Day	30% 6% 14% 35% 13% 2	6285
	21st to end	Day	34% 7% 13% 29% 17% 1	6728
August	1st to 10th	Day	40% 8% 15% 24% 13% 1	4548
	11th to 20th	Day	36% 6% 16% 30% 11%	4213
	21st to end	Day	41% 6% 15% 23% 14% 1	4311
September	1st to 10th	Day	39% 7% 14% 27% 13%	2400
	11th to 20th	Day	37% 9% 19% 27% 8%	2290
	21st to end	Day	42% 7% 15% 27% 10%	2200
October	1st to 10th	Day	31% 6% 13% 29% 21%	3428
	11th to 20th	Day	50% 5% 10% 22% 13%	3117
	21st to end	Day	50% 6% 15% 22% 6%	3180
November	1st to 10th	Day	45% 7% 12% 23% 11% 1	1852
	11th to 20th	Day	55% 7% 14% 15% 9% 1	1843
	21st to end	Day	54% 8% 15% 18% 4 1	1680
December	1st to 10th	Day	56% 7% 13% 19% 5%	2690
	11th to 20th	Day	58% 7% 10% 14% 11% 1	3085
	21st to end	Day	67% 4 6% 16% 7%	4070
January	1st to 10th	Night	52% 6% 9% 24% 8%	8233
	11th to 20th	Night	58% 7% 10% 14% 10% 1	8280
	21st to end	Night	49% 5% 10% 21% 14%	9058
February	1st to 10th	Night	51% 5% 10% 20% 11% 2	2807
	11th to 20th	Night	47% 6% 13% 21% 11% 2	2521
	21st to end	Night	51% 5% 13% 21% 9% 1	2076
March	1st to 10th	Night	39% 5% 18% 21% 15% 2	5493
	11th to 20th	Night	45% 6% 18% 20% 10% 1	5450
	21st to end	Night	54% 7% 12% 16% 11% 1	5072
April	1st to 10th	Night	52% 6% 9% 16% 15% 1	2010
	11th to 20th	Night	48% 5% 10% 23% 11% 3	1975
	21st to end	Night	46% 7% 12% 20% 14% 1	1809
May	1st to 10th	Night	39% 6% 13% 25% 15% 3	3825
	11th to 20th	Night	37% 8% 12% 24% 17% 2	3597
	21st to end	Night	43% 7% 12% 22% 13% 3	3950
June	1st to 10th	Night	35% 7% 15% 26% 16% 2	1600
	11th to 20th	Night	44% 7% 13% 21% 15% 1	1600
	21st to end	Night	37% 7% 15% 27% 12% 2	1600
July	1st to 10th	Night	35% 6% 18% 29% 10% 2	3580
	11th to 20th	Night	26% 6% 18% 35% 13% 2	3572
	21st to end	Night	31% 6% 15% 26% 20% 1	3832
August	1st to 10th	Night	37% 7% 15% 22% 18% 2	2588
	11th to 20th	Night	33% 8% 15% 29% 13% 2	2901
	21st to end	Night	37% 6% 13% 26% 17% 1	3586
September	1st to 10th	Night	31% 7% 15% 31% 15%	2000
	11th to 20th	Night	33% 7% 16% 33% 11%	2090
	21st to end	Night	38% 6% 16% 29% 11% 1	2200
October	1st to 10th	Night	31% 5% 11% 35% 16% 2	3750
	11th to 20th	Night	40% 5% 13% 27% 15%	4020
	21st to end	Night	50% 5% 11% 25% 9%	4610
November	1st to 10th	Night	45% 5% 14% 23% 12% 1	2666
	11th to 20th	Night	49% 6% 14% 19% 10% 1	2772
	21st to end	Night	52% 7% 14% 21% 5% 1	2940
December	1st to 10th	Night	48% 7% 21% 19% 4	4720
	11th to 20th	Night	60% 5% 10% 16% 9%	5341
	21st to end	Night	62% 5% 8% 16% 9%	7150

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	18% 17% 38% 21% 6% 1	330
	11th to 20th	Day	24% 18% 36% 16% 4 1	330
	21st to end	Day	19% 17% 45% 16% 3	363
February	1st to 10th	Day	16% 28% 39% 12% 4 1	330
	11th to 20th	Day	19% 28% 38% 12% 3	330
	21st to end	Day	26% 22% 43% 7% 2	273
March	1st to 10th	Day	22% 24% 40% 12% 2	330
	11th to 20th	Day	24% 23% 36% 13% 4	356
	21st to end	Day	29% 20% 29% 15% 6% 1	594
April	1st to 10th	Day	28% 21% 30% 17% 4 1	504
	11th to 20th	Day	28% 22% 27% 15% 7% 1	492
	21st to end	Day	28% 17% 28% 18% 7% 2	500
May	1st to 10th	Day	20% 21% 32% 16% 7% 2	603
	11th to 20th	Day	20% 18% 35% 18% 7% 2	725
	21st to end	Day	19% 15% 33% 21% 7% 5%	789
June	1st to 10th	Day	12% 18% 36% 22% 9% 3	730
	11th to 20th	Day	19% 13% 36% 21% 10% 2	730
	21st to end	Day	17% 13% 33% 26% 9% 3	730
July	1st to 10th	Day	8% 10% 32% 32% 13% 5%	721
	11th to 20th	Day	9% 12% 35% 26% 14% 5%	718
	21st to end	Day	8% 8% 34% 29% 14% 6%	794
August	1st to 10th	Day	10% 8% 31% 29% 17% 6%	726
	11th to 20th	Day	15% 8% 25% 27% 19% 7%	576
	21st to end	Day	17% 11% 24% 23% 18% 6%	583
September	1st to 10th	Day	16% 12% 27% 25% 13% 6%	530
	11th to 20th	Day	12% 8% 28% 32% 17% 3	530
	21st to end	Day	16% 15% 27% 26% 13% 3	530
October	1st to 10th	Day	21% 17% 29% 20% 12% 2	433
	11th to 20th	Day	24% 25% 33% 16% 2	330
	21st to end	Day	28% 21% 35% 14% 3 1	363
November	1st to 10th	Day	21% 18% 39% 15% 6% 1	330
	11th to 20th	Day	22% 25% 34% 14% 4 2	330
	21st to end	Day	24% 22% 38% 13% 4	330
December	1st to 10th	Day	17% 21% 41% 15% 5% 1	330
	11th to 20th	Day	23% 25% 34% 14% 4	330
	21st to end	Day	33% 18% 34% 12% 3 1	363
January	1st to 10th	Night	17% 15% 40% 24% 4	1050
	11th to 20th	Night	19% 16% 38% 20% 6% 1	1050
	21st to end	Night	18% 12% 41% 23% 6% 1	1155
February	1st to 10th	Night	17% 15% 39% 22% 6%	1050
	11th to 20th	Night	18% 18% 37% 22% 6%	885
	21st to end	Night	20% 16% 39% 23% 2	692
March	1st to 10th	Night	17% 15% 36% 25% 7%	840
	11th to 20th	Night	18% 18% 40% 19% 5%	821
	21st to end	Night	19% 12% 34% 27% 7% 1	693
April	1st to 10th	Night	23% 15% 33% 25% 4	603
	11th to 20th	Night	27% 11% 29% 26% 6%	593
	21st to end	Night	21% 12% 32% 27% 7% 1	600
May	1st to 10th	Night	19% 12% 33% 28% 7% 1	600
	11th to 20th	Night	19% 12% 34% 29% 6% 1	591
	21st to end	Night	14% 7% 32% 35% 11%	650
June	1st to 10th	Night	12% 9% 32% 38% 10%	600
	11th to 20th	Night	18% 8% 28% 37% 9% 1	600
	21st to end	Night	16% 7% 28% 37% 12% 1	600
July	1st to 10th	Night	8% 4 27% 44% 17%	585
	11th to 20th	Night	9% 4 30% 40% 17%	582
	21st to end	Night	9% 6% 29% 39% 16% 1	657
August	1st to 10th	Night	10% 2 27% 41% 18% 2	597
	11th to 20th	Night	11% 4 25% 36% 23% 2	594
	21st to end	Night	13% 6% 24% 38% 18% 1	660
September	1st to 10th	Night	13% 6% 27% 31% 20% 3	600
	11th to 20th	Night	13% 7% 29% 30% 19% 2	690
	21st to end	Night	18% 8% 29% 28% 16% 1	800
October	1st to 10th	Night	18% 11% 32% 25% 13% 1	800
	11th to 20th	Night	21% 16% 33% 23% 7%	800
	21st to end	Night	25% 11% 31% 25% 7%	880
November	1st to 10th	Night	21% 11% 38% 23% 6% 1	820
	11th to 20th	Night	20% 18% 36% 21% 5%	882
	21st to end	Night	22% 16% 34% 23% 5%	1050
December	1st to 10th	Night	22% 15% 38% 21% 5%	1050
	11th to 20th	Night	20% 16% 34% 23% 6% 1	1050
	21st to end	Night	28% 13% 35% 20% 4	1155

Page generated at 2026-03-30 01:18:44. Version ac07

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