Hensley Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hensley Lake 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	6% 3 1 3 2 2 2 2 3 5% 70%	135582
	Night	8% 3 1 3 2 2 2 2 3 5% 67%	133204
Transparency	Day	19% 2 7% 18% 26% 28%	130597
	Night	22% 2 9% 20% 25% 22%	129167
Seeing	Day	7% 4 17% 32% 28% 12%	16758
	Night	8% 2 13% 39% 31% 6%	26297

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	9% 6% 3 6% 4 4 4 3 5% 8% 47%	3647
	11th to 20th	Day	9% 4 2 6% 4 4 4 3 4 4 55%	3980
	21st to end	Day	14% 6% 2 5% 4 4 5% 4 5% 7% 44%	4510
February	1st to 10th	Day	15% 5% 2 4 4 3 4 5% 5% 6% 48%	1872
	11th to 20th	Day	12% 5% 1 5% 4 3 4 3 6% 7% 49%	1950
	21st to end	Day	15% 5% 2 4 4 3 4 3 5% 6% 49%	1650
March	1st to 10th	Day	14% 6% 2 6% 4 4 5% 5% 7% 10% 38%	4518
	11th to 20th	Day	13% 4 2 4 3 3 3 3 5% 7% 54%	4951
	21st to end	Day	14% 5% 2 4 3 3 3 4 5% 8% 49%	5831
April	1st to 10th	Day	9% 5% 2 4 3 3 3 2 5% 6% 59%	2400
	11th to 20th	Day	7% 4 1 3 3 3 3 2 5% 7% 61%	2369
	21st to end	Day	6% 3 1 3 2 3 3 3 4 6% 65%	2400
May	1st to 10th	Day	4 3 1 2 1 2 1 1 2 4 80%	5528
	11th to 20th	Day	2 1 2 1 1 2 2 3 5% 82%	5924
	21st to end	Day	2 1 2 1 1 2 2 3 4 83%	6902
June	1st to 10th	Day	2 2 1 2 2 2 3 3 3 6% 75%	2800
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	2800
July	1st to 10th	Day	1 1 1 1 2 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 1 1 1 2 3 3 87%	6457
August	1st to 10th	Day	1 1 1 1 1 1 1 2 90%	5546
	11th to 20th	Day	1 1 1 1 2 94%	5364
	21st to end	Day	1 1 1 1 1 1 1 3 4 85%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 2 89%	2400
	11th to 20th	Day	2 2 1 1 1 1 1 2 3 5% 82%	2360
	21st to end	Day	2 1 1 1 1 1 2 2 3 4 83%	2200
October	1st to 10th	Day	1 2 1 2 1 1 2 2 4 4 79%	3361
	11th to 20th	Day	2 2 2 1 2 2 2 3 3 80%	3261
	21st to end	Day	3 2 1 3 2 3 3 3 4 6% 69%	3558
November	1st to 10th	Day	7% 3 1 2 3 3 3 3 4 9% 62%	1856
	11th to 20th	Day	9% 4 1 3 3 3 3 4 5% 6% 58%	1890
	21st to end	Day	10% 4 1 4 3 3 3 3 4 6% 57%	1890
December	1st to 10th	Day	8% 4 1 4 3 4 5% 4 5% 7% 56%	2784
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 5% 8% 42%	3108
	21st to end	Day	15% 7% 3 8% 6% 5% 6% 4 7% 9% 30%	4092
January	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 5% 8% 52%	6320
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 5% 56%	5994
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 7% 50%	6468
February	1st to 10th	Night	16% 6% 1 5% 2 3 3 3 4 6% 52%	2525
	11th to 20th	Night	13% 4 1 4 3 2 3 3 5% 7% 56%	2338
	21st to end	Night	14% 5% 2 5% 3 3 3 3 4 6% 54%	1980
March	1st to 10th	Night	16% 5% 2 6% 4 4 4 4 6% 7% 44%	5376
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 53%	5044
	21st to end	Night	18% 5% 1 4 3 3 3 3 4 7% 50%	5143
April	1st to 10th	Night	9% 4 1 4 2 2 2 3 5% 7% 60%	2000
	11th to 20th	Night	6% 3 1 2 3 3 4 3 4 6% 64%	1979
	21st to end	Night	5% 3 1 2 2 3 3 2 3 6% 70%	2000
May	1st to 10th	Night	3 2 1 2 1 2 2 2 3 5% 79%	4454
	11th to 20th	Night	2 1 1 1 1 2 1 3 4 82%	3982
	21st to end	Night	1 1 2 1 1 1 1 2 3 88%	3972
June	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 76%	1600
	11th to 20th	Night	1 1 1 1 1 1 2 3 88%	1600
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	1600
July	1st to 10th	Night	1 1 1 95%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 2 87%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 4 88%	4397
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4275
	11th to 20th	Night	1 1 1 1 1 2 94%	4506
	21st to end	Night	2 1 1 1 1 2 2 2 3 85%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	2000
	11th to 20th	Night	2 2 2 1 1 2 2 2 3 83%	2040
	21st to end	Night	2 1 1 2 1 1 2 1 3 3 84%	2200
October	1st to 10th	Night	2 1 1 1 2 2 1 2 4 83%	3823
	11th to 20th	Night	2 1 1 1 1 1 1 2 3 4 82%	3897
	21st to end	Night	4 2 1 2 1 2 2 2 3 5% 76%	4253
November	1st to 10th	Night	7% 4 1 3 3 2 2 2 3 6% 67%	2660
	11th to 20th	Night	7% 3 2 4 3 3 3 2 4 6% 64%	2730
	21st to end	Night	9% 3 1 4 3 3 3 3 5% 6% 59%	2730
December	1st to 10th	Night	8% 3 1 3 3 3 3 3 4 6% 62%	4601
	11th to 20th	Night	16% 6% 2 6% 3 3 3 4 6% 9% 42%	5316
	21st to end	Night	14% 7% 3 8% 5% 5% 6% 4 7% 8% 33%	7106

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	36% 4 23% 22% 13% 2	3647
	11th to 20th	Day	34% 3 11% 15% 23% 15%	3980
	21st to end	Day	40% 4 11% 17% 25% 4	4510
February	1st to 10th	Day	35% 5% 11% 25% 18% 6%	1872
	11th to 20th	Day	33% 3 11% 26% 20% 6%	1960
	21st to end	Day	35% 3 12% 28% 18% 3	1650
March	1st to 10th	Day	39% 4 15% 21% 19% 3	4518
	11th to 20th	Day	31% 3 16% 30% 12% 8%	4951
	21st to end	Day	33% 3 14% 32% 15% 3	5831
April	1st to 10th	Day	26% 3 12% 29% 21% 8%	2400
	11th to 20th	Day	24% 3 11% 28% 24% 10%	2372
	21st to end	Day	20% 3 10% 30% 30% 7%	2400
May	1st to 10th	Day	12% 2 6% 24% 40% 17%	5528
	11th to 20th	Day	8% 2 4 30% 42% 16%	5927
	21st to end	Day	8% 1 4 22% 48% 18%	6904
June	1st to 10th	Day	12% 2 8% 22% 32% 23%	2800
	11th to 20th	Day	5% 1 4 14% 37% 39%	2800
	21st to end	Day	6% 1 2 7% 29% 55%	2800
July	1st to 10th	Day	2 1 1 2 14% 80%	6251
	11th to 20th	Day	5% 1 1 4 13% 76%	6245
	21st to end	Day	5% 1 1 8% 34% 50%	6270
August	1st to 10th	Day	8% 1 1 5% 24% 61%	4030
	11th to 20th	Day	3 1 4 25% 66%	3880
	21st to end	Day	5% 1 1 7% 28% 58%	4311
September	1st to 10th	Day	5% 1 3 7% 28% 56%	2400
	11th to 20th	Day	9% 1 2 19% 38% 30%	2360
	21st to end	Day	8% 1 3 17% 37% 34%	2200
October	1st to 10th	Day	9% 2 4 12% 21% 53%	3361
	11th to 20th	Day	11% 2 4 22% 41% 20%	3261
	21st to end	Day	18% 2 5% 18% 39% 18%	3558
November	1st to 10th	Day	22% 3 11% 27% 27% 10%	1856
	11th to 20th	Day	27% 4 12% 29% 23% 5%	1890
	21st to end	Day	29% 5% 10% 23% 25% 8%	1890
December	1st to 10th	Day	29% 5% 10% 28% 22% 6%	2784
	11th to 20th	Day	42% 5% 13% 21% 16% 3	3108
	21st to end	Day	49% 8% 19% 17% 6% 2	4092
January	1st to 10th	Night	32% 3 23% 24% 15% 2	6320
	11th to 20th	Night	33% 3 13% 14% 28% 9%	5994
	21st to end	Night	37% 3 10% 22% 22% 7%	6468
February	1st to 10th	Night	34% 3 11% 24% 21% 8%	2525
	11th to 20th	Night	29% 3 11% 29% 23% 6%	2339
	21st to end	Night	33% 3 11% 30% 19% 5%	1980
March	1st to 10th	Night	39% 2 12% 22% 21% 3	5376
	11th to 20th	Night	34% 3 14% 27% 15% 6%	5044
	21st to end	Night	36% 2 17% 26% 16% 3	5143
April	1st to 10th	Night	25% 3 14% 30% 20% 8%	2000
	11th to 20th	Night	22% 2 12% 29% 27% 9%	1979
	21st to end	Night	18% 3 11% 31% 32% 6%	2000
May	1st to 10th	Night	11% 2 8% 30% 36% 14%	4451
	11th to 20th	Night	8% 1 4 28% 44% 14%	3983
	21st to end	Night	6% 1 4 22% 53% 13%	3972
June	1st to 10th	Night	13% 1 8% 24% 33% 20%	1600
	11th to 20th	Night	5% 1 4 16% 36% 38%	1600
	21st to end	Night	5% 1 2 9% 25% 58%	1600
July	1st to 10th	Night	2 2 13% 83%	3600
	11th to 20th	Night	7% 1 1 3 12% 76%	3592
	21st to end	Night	5% 1 1 6% 37% 51%	4272
August	1st to 10th	Night	7% 1 1 6% 21% 63%	3109
	11th to 20th	Night	4 1 1 4 21% 70%	3256
	21st to end	Night	6% 1 1 5% 24% 63%	3608
September	1st to 10th	Night	5% 1 2 7% 26% 60%	2000
	11th to 20th	Night	9% 1 2 15% 40% 32%	2040
	21st to end	Night	9% 2 2 14% 36% 37%	2200
October	1st to 10th	Night	8% 1 3 11% 21% 56%	3823
	11th to 20th	Night	10% 1 3 22% 41% 22%	3897
	21st to end	Night	14% 1 4 19% 38% 24%	4253
November	1st to 10th	Night	21% 2 9% 26% 26% 16%	2660
	11th to 20th	Night	22% 2 12% 28% 25% 10%	2730
	21st to end	Night	25% 4 10% 24% 28% 9%	2730
December	1st to 10th	Night	25% 3 13% 29% 23% 7%	4601
	11th to 20th	Night	39% 5% 16% 20% 17% 4	5316
	21st to end	Night	48% 5% 17% 21% 7% 2	7106

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% 3 20% 31% 17% 5%	202
	11th to 20th	Day	14% 4 13% 35% 23% 11%	345
	21st to end	Day	16% 2 16% 30% 25% 10%	440
February	1st to 10th	Day	18% 7% 15% 27% 21% 12%	437
	11th to 20th	Day	15% 6% 20% 34% 18% 8%	495
	21st to end	Day	12% 8% 20% 35% 21% 4	414
March	1st to 10th	Day	16% 7% 22% 31% 20% 4	500
	11th to 20th	Day	15% 8% 27% 24% 19% 7%	500
	21st to end	Day	16% 10% 24% 27% 19% 5%	550
April	1st to 10th	Day	12% 11% 23% 28% 18% 8%	500
	11th to 20th	Day	11% 9% 25% 31% 18% 6%	492
	21st to end	Day	5% 10% 23% 32% 19% 11%	500
May	1st to 10th	Day	4 8% 30% 33% 17% 7%	503
	11th to 20th	Day	4 7% 25% 33% 23% 7%	528
	21st to end	Day	5% 6% 16% 36% 28% 8%	573
June	1st to 10th	Day	2 3 20% 32% 31% 11%	530
	11th to 20th	Day	2 4 21% 35% 27% 11%	530
	21st to end	Day	1 3 15% 37% 30% 15%	530
July	1st to 10th	Day	1 9% 36% 40% 13%	526
	11th to 20th	Day	1 5% 30% 43% 21%	524
	21st to end	Day	1 1 6% 33% 41% 19%	575
August	1st to 10th	Day	4 1 11% 37% 33% 14%	527
	11th to 20th	Day	1 1 12% 33% 35% 18%	517
	21st to end	Day	1 1 9% 30% 40% 19%	583
September	1st to 10th	Day	1 2 11% 36% 33% 17%	530
	11th to 20th	Day	4 2 19% 34% 30% 11%	530
	21st to end	Day	2 3 15% 33% 34% 13%	530
October	1st to 10th	Day	2 3 19% 32% 28% 16%	530
	11th to 20th	Day	5% 6% 15% 28% 33% 14%	530
	21st to end	Day	5% 4 13% 30% 35% 14%	583
November	1st to 10th	Day	8% 3 15% 26% 32% 17%	332
	11th to 20th	Day	8% 4 16% 31% 29% 11%	330
	21st to end	Day	10% 3 13% 32% 34% 9%	330
December	1st to 10th	Day	13% 2 10% 37% 25% 13%	250
	11th to 20th	Day	13% 4 15% 35% 21% 14%	220
	21st to end	Day	22% 4 16% 33% 18% 7%	242
January	1st to 10th	Night	18% 2 12% 36% 24% 9%	804
	11th to 20th	Night	12% 3 11% 40% 27% 8%	800
	21st to end	Night	17% 2 9% 38% 28% 5%	880
February	1st to 10th	Night	17% 3 12% 36% 24% 9%	800
	11th to 20th	Night	14% 3 15% 34% 28% 7%	784
	21st to end	Night	16% 2 12% 46% 20% 4	660
March	1st to 10th	Night	16% 4 16% 38% 23% 4	800
	11th to 20th	Night	14% 3 17% 35% 25% 6%	800
	21st to end	Night	14% 4 19% 38% 23% 3	880
April	1st to 10th	Night	10% 5% 21% 35% 26% 3	800
	11th to 20th	Night	6% 7% 24% 37% 21% 5%	791
	21st to end	Night	6% 5% 24% 35% 24% 6%	800
May	1st to 10th	Night	4 5% 25% 41% 22% 4	759
	11th to 20th	Night	5% 4 25% 46% 20% 1	552
	21st to end	Night	4 4 25% 40% 26% 1	433
June	1st to 10th	Night	4 4 24% 48% 18% 3	400
	11th to 20th	Night	1 4 21% 53% 22%	400
	21st to end	Night	1 1 18% 51% 30% 1	400
July	1st to 10th	Night	12% 52% 35% 1	390
	11th to 20th	Night	2 6% 49% 41% 2	388
	21st to end	Night	1 1 6% 35% 50% 8%	621
August	1st to 10th	Night	3 9% 43% 39% 5%	699
	11th to 20th	Night	9% 43% 41% 6%	792
	21st to end	Night	2 5% 38% 49% 6%	880
September	1st to 10th	Night	1 7% 37% 48% 8%	800
	11th to 20th	Night	3 1 12% 40% 36% 8%	800
	21st to end	Night	2 1 10% 36% 41% 10%	800
October	1st to 10th	Night	4 1 10% 38% 39% 8%	800
	11th to 20th	Night	5% 2 8% 36% 40% 10%	800
	21st to end	Night	5% 1 9% 36% 34% 15%	880
November	1st to 10th	Night	9% 2 9% 32% 35% 13%	820
	11th to 20th	Night	7% 2 10% 43% 29% 9%	840
	21st to end	Night	10% 1 11% 38% 32% 8%	840
December	1st to 10th	Night	15% 1 11% 36% 29% 8%	840
	11th to 20th	Night	17% 2 9% 40% 27% 6%	840
	21st to end	Night	19% 2 12% 42% 19% 5%	924

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