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 2024-01-16.

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	14% 9% 3 7% 6% 4 5% 5% 5% 7% 35%	74822
	Night	14% 9% 4 7% 6% 3 4 4 5% 7% 37%	75017
Transparency	Day	47% 7% 13% 21% 11% 1	74523
	Night	46% 7% 13% 22% 11% 1	74837
Seeing	Day	18% 16% 33% 21% 9% 3	14853
	Night	18% 11% 34% 27% 9% 1	24872

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	12% 11% 4 8% 6% 5% 6% 5% 7% 6% 31%	1517
	11th to 20th	Day	14% 14% 6% 11% 7% 5% 5% 4 5% 6% 24%	1485
	21st to end	Day	13% 13% 6% 9% 7% 4 5% 4 6% 5% 28%	1584
February	1st to 10th	Day	10% 11% 3 8% 7% 3 6% 4 5% 8% 34%	1552
	11th to 20th	Day	10% 13% 4 7% 6% 3 5% 4 5% 6% 37%	1694
	21st to end	Day	13% 13% 4 8% 6% 3 4 4 5% 8% 31%	1480
March	1st to 10th	Day	11% 10% 4 7% 8% 4 5% 5% 5% 5% 36%	1800
	11th to 20th	Day	12% 11% 4 8% 6% 3 4 4 4 6% 37%	1853
	21st to end	Day	17% 11% 4 9% 6% 4 6% 4 4 6% 29%	2376
April	1st to 10th	Day	16% 11% 5% 8% 5% 3 6% 5% 5% 6% 29%	2160
	11th to 20th	Day	20% 10% 4 6% 6% 3 5% 5% 4 5% 32%	2134
	21st to end	Day	18% 9% 4 7% 6% 3 4 5% 5% 7% 32%	2330
May	1st to 10th	Day	16% 7% 4 8% 7% 4 5% 5% 5% 6% 33%	2430
	11th to 20th	Day	16% 6% 3 6% 7% 4 5% 4 6% 7% 37%	2479
	21st to end	Day	12% 7% 2 6% 6% 3 6% 5% 6% 8% 38%	2725
June	1st to 10th	Day	11% 5% 2 5% 6% 3 5% 5% 5% 8% 43%	2520
	11th to 20th	Day	14% 5% 3 6% 6% 4 6% 4 6% 7% 39%	2520
	21st to end	Day	12% 6% 2 6% 6% 4 5% 5% 6% 9% 38%	2520
July	1st to 10th	Day	7% 4 2 5% 5% 3 6% 5% 7% 11% 46%	2520
	11th to 20th	Day	8% 5% 2 5% 6% 4 6% 5% 6% 9% 44%	2520
	21st to end	Day	8% 4 2 5% 6% 4 4 7% 7% 8% 45%	2743
August	1st to 10th	Day	9% 5% 3 5% 6% 4 6% 6% 7% 9% 41%	2507
	11th to 20th	Day	11% 5% 3 6% 5% 3 5% 5% 6% 8% 42%	2304
	21st to end	Day	14% 5% 2 5% 5% 3 5% 5% 5% 7% 44%	2376
September	1st to 10th	Day	12% 6% 3 7% 5% 3 4 4 6% 8% 42%	2160
	11th to 20th	Day	12% 5% 2 6% 5% 3 6% 6% 6% 8% 40%	2062
	21st to end	Day	13% 5% 3 7% 8% 3 6% 5% 7% 8% 34%	1980
October	1st to 10th	Day	16% 7% 3 6% 5% 3 4 6% 5% 8% 37%	1893
	11th to 20th	Day	21% 10% 4 7% 6% 5% 4 4 6% 7% 27%	1735
	21st to end	Day	21% 10% 3 7% 6% 4 4 5% 4 7% 29%	1782
November	1st to 10th	Day	13% 10% 3 7% 5% 3 4 6% 5% 7% 38%	1672
	11th to 20th	Day	15% 9% 6% 9% 7% 5% 5% 5% 6% 6% 27%	1668
	21st to end	Day	15% 11% 5% 6% 6% 3 6% 5% 6% 8% 29%	1520
December	1st to 10th	Day	12% 11% 4 9% 7% 3 6% 5% 5% 6% 32%	1520
	11th to 20th	Day	16% 13% 4 7% 8% 4 6% 5% 6% 5% 26%	1918
	21st to end	Day	22% 22% 6% 9% 6% 3 3 3 3 4 19%	2783
January	1st to 10th	Night	13% 11% 5% 8% 6% 4 5% 4 5% 6% 33%	2648
	11th to 20th	Night	15% 13% 5% 10% 8% 3 5% 4 4 5% 27%	2603
	21st to end	Night	11% 13% 6% 9% 7% 5% 5% 5% 6% 7% 26%	2772
February	1st to 10th	Night	10% 12% 5% 9% 7% 4 5% 5% 5% 6% 34%	2407
	11th to 20th	Night	10% 11% 4 7% 7% 4 5% 5% 5% 7% 36%	2151
	21st to end	Night	15% 13% 6% 8% 6% 3 4 4 4 6% 32%	1776
March	1st to 10th	Night	12% 10% 4 7% 6% 3 5% 4 4 6% 40%	2160
	11th to 20th	Night	13% 7% 3 7% 5% 3 5% 4 5% 7% 41%	2122
	21st to end	Night	15% 8% 3 7% 6% 4 5% 4 4 6% 37%	1980
April	1st to 10th	Night	19% 9% 4 7% 6% 3 5% 5% 5% 6% 31%	1800
	11th to 20th	Night	21% 8% 3 5% 4 3 5% 4 5% 6% 36%	1775
	21st to end	Night	17% 7% 3 7% 6% 4 6% 4 5% 7% 35%	1629
May	1st to 10th	Night	18% 8% 3 6% 7% 3 5% 5% 6% 7% 33%	1528
	11th to 20th	Night	17% 9% 3 6% 5% 3 3 4 5% 6% 38%	1416
	21st to end	Night	12% 5% 2 5% 5% 3 5% 6% 5% 9% 43%	1551
June	1st to 10th	Night	11% 5% 2 5% 5% 4 4 5% 5% 7% 47%	1440
	11th to 20th	Night	13% 7% 3 6% 5% 4 6% 5% 6% 6% 40%	1440
	21st to end	Night	12% 5% 3 4 5% 3 3 5% 6% 9% 45%	1440
July	1st to 10th	Night	7% 3 2 4 3 3 4 4 5% 10% 54%	1440
	11th to 20th	Night	10% 4 2 4 4 3 5% 6% 6% 8% 49%	1440
	21st to end	Night	7% 4 2 4 4 3 4 4 5% 10% 53%	1575
August	1st to 10th	Night	8% 5% 3 5% 6% 3 5% 6% 6% 7% 47%	1432
	11th to 20th	Night	8% 5% 2 4 5% 3 5% 5% 6% 7% 50%	1593
	21st to end	Night	12% 6% 2 4 5% 3 3 4 4 7% 51%	1980
September	1st to 10th	Night	10% 5% 2 5% 4 3 3 4 5% 7% 52%	1800
	11th to 20th	Night	12% 4 2 4 4 3 5% 4 5% 7% 50%	1880
	21st to end	Night	15% 6% 3 5% 5% 2 4 4 6% 8% 42%	1980
October	1st to 10th	Night	16% 6% 3 4 4 2 4 4 5% 7% 44%	2065
	11th to 20th	Night	17% 9% 4 6% 6% 4 4 5% 5% 7% 33%	2223
	21st to end	Night	20% 10% 4 5% 6% 3 4 4 4 7% 32%	2574
November	1st to 10th	Night	15% 8% 3 6% 4 3 4 4 5% 7% 39%	2406
	11th to 20th	Night	15% 10% 4 7% 7% 3 5% 5% 5% 7% 33%	2508
	21st to end	Night	15% 11% 4 8% 5% 3 5% 5% 5% 7% 31%	2660
December	1st to 10th	Night	13% 13% 5% 7% 6% 3 5% 5% 5% 6% 33%	2660
	11th to 20th	Night	13% 13% 5% 8% 7% 4 5% 4 4 5% 30%	3279
	21st to end	Night	24% 18% 7% 7% 5% 2 2 3 3 4 22%	4884

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	51% 7% 13% 18% 9% 1	1518
	11th to 20th	Day	61% 5% 11% 14% 7% 1	1485
	21st to end	Day	57% 4 15% 15% 8% 1	1584
February	1st to 10th	Day	48% 5% 14% 20% 12% 1	1552
	11th to 20th	Day	48% 6% 13% 23% 9% 1	1704
	21st to end	Day	53% 5% 12% 18% 11% 1	1480
March	1st to 10th	Day	50% 6% 11% 18% 13% 2	1800
	11th to 20th	Day	48% 5% 12% 22% 10% 2	1853
	21st to end	Day	56% 5% 12% 16% 10% 1	2376
April	1st to 10th	Day	55% 7% 12% 17% 9% 1	2160
	11th to 20th	Day	53% 6% 9% 20% 11% 1	2134
	21st to end	Day	51% 7% 9% 17% 13% 2	2330
May	1st to 10th	Day	50% 6% 12% 19% 11% 2	2430
	11th to 20th	Day	45% 8% 11% 21% 14% 2	2483
	21st to end	Day	43% 8% 12% 23% 11% 2	2725
June	1st to 10th	Day	37% 8% 14% 25% 15% 2	2520
	11th to 20th	Day	43% 7% 14% 19% 15% 1	2520
	21st to end	Day	41% 8% 16% 23% 12% 1	2520
July	1st to 10th	Day	30% 9% 16% 29% 16% 1	2451
	11th to 20th	Day	35% 7% 15% 25% 15% 3	2445
	21st to end	Day	33% 9% 15% 24% 17% 1	2591
August	1st to 10th	Day	37% 9% 16% 25% 12% 2	2492
	11th to 20th	Day	39% 8% 14% 26% 12% 1	2304
	21st to end	Day	38% 8% 15% 24% 15% 1	2376
September	1st to 10th	Day	40% 7% 13% 27% 13% 1	2160
	11th to 20th	Day	38% 9% 19% 26% 7%	2062
	21st to end	Day	44% 7% 15% 26% 7%	1980
October	1st to 10th	Day	44% 8% 14% 25% 8%	1893
	11th to 20th	Day	55% 6% 12% 19% 6% 1	1735
	21st to end	Day	56% 7% 13% 17% 7%	1782
November	1st to 10th	Day	45% 7% 12% 24% 11%	1672
	11th to 20th	Day	55% 7% 14% 14% 9% 1	1668
	21st to end	Day	52% 9% 16% 18% 4 1	1520
December	1st to 10th	Day	52% 7% 13% 21% 7%	1520
	11th to 20th	Day	56% 8% 12% 15% 7% 1	1915
	21st to end	Day	69% 5% 8% 13% 6%	2783
January	1st to 10th	Night	52% 6% 14% 19% 9% 1	2648
	11th to 20th	Night	59% 6% 12% 17% 6%	2603
	21st to end	Night	56% 6% 11% 17% 9% 1	2772
February	1st to 10th	Night	51% 5% 10% 20% 11% 2	2407
	11th to 20th	Night	47% 7% 13% 21% 10% 3	2152
	21st to end	Night	53% 5% 13% 19% 9% 1	1776
March	1st to 10th	Night	45% 6% 13% 22% 12% 2	2160
	11th to 20th	Night	43% 5% 13% 24% 14% 1	2122
	21st to end	Night	48% 7% 10% 21% 13% 2	1980
April	1st to 10th	Night	53% 6% 9% 16% 14% 1	1800
	11th to 20th	Night	48% 5% 11% 22% 11% 3	1775
	21st to end	Night	47% 7% 11% 20% 13% 2	1629
May	1st to 10th	Night	48% 7% 13% 19% 10% 2	1528
	11th to 20th	Night	47% 7% 9% 23% 12% 2	1417
	21st to end	Night	37% 8% 17% 22% 14% 2	1552
June	1st to 10th	Night	35% 8% 15% 26% 15% 2	1440
	11th to 20th	Night	42% 7% 13% 22% 15% 1	1440
	21st to end	Night	34% 8% 16% 28% 12% 2	1440
July	1st to 10th	Night	26% 8% 14% 31% 21% 1	1400
	11th to 20th	Night	31% 8% 17% 27% 13% 4	1392
	21st to end	Night	25% 8% 18% 29% 17% 2	1487
August	1st to 10th	Night	34% 8% 15% 27% 14% 2	1423
	11th to 20th	Night	31% 9% 13% 29% 15% 2	1593
	21st to end	Night	34% 6% 14% 26% 18% 2	1980
September	1st to 10th	Night	32% 7% 14% 31% 16% 1	1800
	11th to 20th	Night	32% 7% 17% 34% 10%	1880
	21st to end	Night	40% 6% 17% 28% 8%	1980
October	1st to 10th	Night	39% 8% 14% 28% 10% 1	2065
	11th to 20th	Night	49% 6% 14% 22% 8% 1	2223
	21st to end	Night	52% 7% 13% 20% 8%	2574
November	1st to 10th	Night	44% 6% 14% 23% 13% 1	2406
	11th to 20th	Night	50% 7% 14% 18% 10%	2508
	21st to end	Night	51% 7% 15% 22% 5% 1	2660
December	1st to 10th	Night	52% 7% 15% 20% 7%	2660
	11th to 20th	Night	55% 6% 13% 19% 7% 1	3281
	21st to end	Night	66% 5% 8% 15% 5%	4884

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	19% 15% 36% 21% 7% 1	270
	11th to 20th	Day	25% 18% 35% 16% 4 1	257
	21st to end	Day	23% 13% 42% 19% 3	264
February	1st to 10th	Day	17% 26% 42% 13% 3	240
	11th to 20th	Day	19% 31% 34% 13% 4	240
	21st to end	Day	26% 20% 46% 7% 1 1	198
March	1st to 10th	Day	16% 27% 41% 14% 2	240
	11th to 20th	Day	21% 20% 37% 16% 5%	262
	21st to end	Day	26% 21% 27% 17% 8% 1	462
April	1st to 10th	Day	26% 19% 31% 18% 5% 1	420
	11th to 20th	Day	28% 21% 25% 17% 8% 1	412
	21st to end	Day	27% 17% 27% 19% 8% 2	420
May	1st to 10th	Day	21% 21% 33% 15% 7% 2	513
	11th to 20th	Day	20% 18% 35% 18% 7% 3	625
	21st to end	Day	17% 15% 33% 22% 8% 5%	679
June	1st to 10th	Day	11% 18% 36% 23% 10% 3	630
	11th to 20th	Day	17% 13% 36% 22% 10% 2	630
	21st to end	Day	15% 12% 31% 28% 10% 4	630
July	1st to 10th	Day	8% 10% 31% 33% 13% 5%	621
	11th to 20th	Day	9% 12% 35% 25% 13% 6%	618
	21st to end	Day	8% 9% 35% 29% 13% 7%	684
August	1st to 10th	Day	10% 7% 32% 29% 16% 6%	626
	11th to 20th	Day	15% 8% 22% 27% 19% 8%	490
	21st to end	Day	18% 11% 23% 24% 18% 5%	495
September	1st to 10th	Day	17% 11% 26% 27% 14% 6%	450
	11th to 20th	Day	12% 9% 30% 31% 15% 3	450
	21st to end	Day	18% 16% 29% 22% 13% 3	450
October	1st to 10th	Day	22% 16% 28% 20% 12% 2	363
	11th to 20th	Day	23% 28% 31% 15% 3	270
	21st to end	Day	31% 21% 34% 12% 2	297
November	1st to 10th	Day	18% 19% 39% 16% 8% 1	270
	11th to 20th	Day	19% 27% 35% 14% 4 1	270
	21st to end	Day	21% 21% 39% 14% 5%	270
December	1st to 10th	Day	19% 19% 40% 16% 6% 1	270
	11th to 20th	Day	23% 23% 35% 14% 4	270
	21st to end	Day	31% 20% 35% 11% 3	297
January	1st to 10th	Night	17% 15% 40% 24% 4	950
	11th to 20th	Night	20% 15% 37% 20% 7% 1	930
	21st to end	Night	19% 11% 40% 23% 7% 1	990
February	1st to 10th	Night	17% 15% 40% 23% 5%	900
	11th to 20th	Night	19% 17% 36% 22% 6%	756
	21st to end	Night	21% 16% 38% 22% 2	592
March	1st to 10th	Night	16% 14% 36% 25% 8%	720
	11th to 20th	Night	16% 16% 41% 20% 6% 1	704
	21st to end	Night	19% 11% 32% 29% 8% 1	594
April	1st to 10th	Night	22% 15% 33% 25% 5%	540
	11th to 20th	Night	26% 9% 31% 27% 6%	533
	21st to end	Night	21% 11% 33% 27% 8% 1	540
May	1st to 10th	Night	21% 12% 33% 27% 7% 1	540
	11th to 20th	Night	20% 12% 34% 28% 6% 1	531
	21st to end	Night	14% 7% 32% 36% 12% 1	584
June	1st to 10th	Night	12% 8% 30% 39% 11%	540
	11th to 20th	Night	17% 8% 27% 38% 10% 1	540
	21st to end	Night	13% 6% 28% 38% 14% 1	540
July	1st to 10th	Night	8% 4 26% 44% 18%	525
	11th to 20th	Night	10% 4 30% 39% 17%	522
	21st to end	Night	8% 6% 29% 38% 17% 2	591
August	1st to 10th	Night	10% 2 27% 42% 17% 2	537
	11th to 20th	Night	10% 4 25% 35% 25% 2	534
	21st to end	Night	13% 6% 25% 38% 18% 1	594
September	1st to 10th	Night	13% 6% 26% 31% 20% 4	540
	11th to 20th	Night	13% 6% 32% 30% 17% 2	620
	21st to end	Night	19% 9% 30% 26% 15% 1	720
October	1st to 10th	Night	18% 11% 32% 25% 13% 1	720
	11th to 20th	Night	20% 15% 33% 24% 7%	720
	21st to end	Night	26% 11% 33% 24% 7%	792
November	1st to 10th	Night	20% 11% 38% 23% 7% 1	740
	11th to 20th	Night	20% 17% 36% 21% 5%	798
	21st to end	Night	21% 16% 34% 24% 6%	950
December	1st to 10th	Night	22% 15% 37% 21% 5%	950
	11th to 20th	Night	20% 15% 34% 24% 6% 1	950
	21st to end	Night	27% 13% 36% 20% 4	1045

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