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 2024-11-12.

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 3 2 4 5% 69%	105031
	Night	8% 3 1 3 2 2 2 2 4 5% 66%	102887
Transparency	Day	19% 2 7% 18% 24% 29%	104782
	Night	22% 2 9% 19% 23% 24%	102726
Seeing	Day	7% 4 17% 32% 28% 12%	15313
	Night	9% 2 13% 39% 31% 7%	24787

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	12% 9% 4 7% 6% 5% 5% 4 5% 7% 36%	2539
	11th to 20th	Day	13% 6% 3 8% 6% 6% 6% 4 6% 6% 35%	2768
	21st to end	Day	19% 7% 2 6% 5% 5% 5% 4 6% 8% 33%	3146
February	1st to 10th	Day	15% 5% 2 4 4 3 3 5% 5% 6% 49%	1778
	11th to 20th	Day	12% 5% 1 5% 3 3 4 3 6% 7% 50%	1850
	21st to end	Day	16% 5% 2 4 4 3 4 3 5% 6% 49%	1570
March	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 5% 7% 9% 36%	3138
	11th to 20th	Day	8% 4 2 4 3 2 3 2 4 6% 62%	3454
	21st to end	Day	15% 6% 2 5% 3 3 4 4 5% 8% 45%	4066
April	1st to 10th	Day	10% 5% 2 4 3 3 3 2 5% 6% 59%	2280
	11th to 20th	Day	8% 5% 1 3 3 3 2 2 5% 7% 60%	2249
	21st to end	Day	6% 3 1 3 2 3 3 3 4 6% 66%	2280
May	1st to 10th	Day	4 2 1 2 1 2 2 1 2 4 79%	3862
	11th to 20th	Day	2 1 2 1 1 2 2 4 5% 81%	4128
	21st to end	Day	3 1 2 1 1 2 2 3 4 81%	4813
June	1st to 10th	Day	2 2 1 2 2 2 3 2 3 6% 74%	2660
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	2660
July	1st to 10th	Day	1 1 1 1 2 92%	4420
	11th to 20th	Day	1 1 1 1 2 2 2 2 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4497
August	1st to 10th	Day	2 2 1 1 1 2 1 2 3 85%	3920
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 92%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 2 90%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 2 89%	2280
	11th to 20th	Day	2 2 1 1 1 1 1 2 3 5% 83%	2242
	21st to end	Day	2 1 1 1 1 1 2 2 3 4 83%	2090
October	1st to 10th	Day	1 2 1 2 2 1 2 2 4 4 79%	3258
	11th to 20th	Day	2 2 2 2 2 2 2 3 3 80%	3161
	21st to end	Day	3 2 1 3 2 3 3 3 4 6% 69%	3448
November	1st to 10th	Day	7% 3 1 2 4 3 3 3 4 8% 61%	1766
	11th to 20th	Day	8% 4 1 3 3 3 3 4 4 6% 60%	1719
	21st to end	Day	10% 4 1 4 3 3 3 3 4 6% 58%	1710
December	1st to 10th	Day	14% 5% 1 4 3 4 5% 3 4 6% 50%	1573
	11th to 20th	Day	21% 8% 2 6% 4 4 4 3 4 6% 37%	1928
	21st to end	Day	13% 7% 3 9% 6% 5% 5% 4 6% 8% 34%	2794
January	1st to 10th	Night	15% 8% 3 6% 3 3 3 2 4 6% 45%	4406
	11th to 20th	Night	18% 7% 3 7% 3 4 4 4 5% 7% 38%	4198
	21st to end	Night	20% 7% 2 5% 4 3 3 3 5% 8% 39%	4521
February	1st to 10th	Night	15% 6% 1 4 2 3 3 3 4 6% 52%	2399
	11th to 20th	Night	12% 4 1 3 3 2 2 3 5% 7% 57%	2218
	21st to end	Night	15% 5% 2 5% 3 3 2 3 4 6% 53%	1884
March	1st to 10th	Night	17% 5% 2 5% 3 4 5% 4 6% 7% 42%	3736
	11th to 20th	Night	10% 5% 2 3 3 2 2 2 4 6% 62%	3521
	21st to end	Night	19% 6% 1 4 3 3 3 3 4 6% 47%	3586
April	1st to 10th	Night	9% 4 1 4 2 2 2 3 5% 7% 60%	1900
	11th to 20th	Night	7% 3 1 2 3 2 3 3 4 6% 65%	1879
	21st to end	Night	5% 3 1 2 2 2 3 2 3 5% 71%	1900
May	1st to 10th	Night	3 2 1 2 1 1 2 2 3 4 79%	3102
	11th to 20th	Night	3 1 1 1 1 1 1 3 4 82%	2770
	21st to end	Night	2 1 2 1 1 1 1 2 3 85%	2762
June	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 76%	1520
	11th to 20th	Night	1 1 1 1 1 2 2 3 88%	1520
	21st to end	Night	1 1 1 1 1 2 1 2 3 88%	1520
July	1st to 10th	Night	1 1 1 1 1 2 94%	2540
	11th to 20th	Night	2 2 1 1 1 2 2 2 3 3 81%	2540
	21st to end	Night	1 1 1 1 1 2 1 2 4 86%	3062
August	1st to 10th	Night	1 2 1 1 1 1 2 1 3 4 84%	3023
	11th to 20th	Night	1 1 1 1 1 1 2 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 3 90%	1900
	11th to 20th	Night	2 2 2 1 1 2 2 2 3 84%	1938
	21st to end	Night	2 1 1 1 1 1 2 1 3 3 84%	2090
October	1st to 10th	Night	2 1 2 1 2 2 1 2 4 83%	3706
	11th to 20th	Night	2 1 1 2 1 1 1 2 3 4 83%	3777
	21st to end	Night	4 2 1 2 1 1 2 2 3 5% 76%	4121
November	1st to 10th	Night	8% 4 1 3 3 2 2 2 4 6% 66%	2530
	11th to 20th	Night	6% 2 1 4 2 3 3 2 4 6% 67%	2483
	21st to end	Night	9% 3 1 4 2 3 3 3 5% 7% 60%	2470
December	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 6% 53%	2603
	11th to 20th	Night	20% 7% 2 6% 4 3 3 4 5% 7% 39%	3276
	21st to end	Night	15% 6% 3 7% 4 5% 5% 4 6% 7% 38%	4862

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	47% 4 19% 21% 7% 1	2539
	11th to 20th	Day	48% 4 15% 17% 10% 5%	2768
	21st to end	Day	49% 4 14% 20% 10% 2	3146
February	1st to 10th	Day	35% 5% 11% 26% 17% 6%	1778
	11th to 20th	Day	33% 3 10% 26% 22% 7%	1860
	21st to end	Day	35% 4 12% 29% 18% 2	1570
March	1st to 10th	Day	41% 4 17% 21% 14% 2	3138
	11th to 20th	Day	23% 3 12% 34% 16% 12%	3454
	21st to end	Day	37% 3 14% 27% 16% 3	4066
April	1st to 10th	Day	27% 3 12% 29% 21% 9%	2280
	11th to 20th	Day	24% 3 11% 28% 24% 10%	2252
	21st to end	Day	20% 3 10% 30% 30% 8%	2280
May	1st to 10th	Day	13% 1 7% 25% 35% 20%	3862
	11th to 20th	Day	9% 1 4 25% 42% 19%	4131
	21st to end	Day	10% 1 5% 27% 38% 19%	4814
June	1st to 10th	Day	13% 2 8% 22% 31% 24%	2660
	11th to 20th	Day	6% 1 4 15% 37% 37%	2660
	21st to end	Day	6% 1 2 7% 29% 54%	2660
July	1st to 10th	Day	3 1 1 3 12% 80%	4351
	11th to 20th	Day	7% 2 1 5% 15% 70%	4345
	21st to end	Day	6% 1 1 5% 26% 61%	4355
August	1st to 10th	Day	8% 1 1 5% 24% 60%	3905
	11th to 20th	Day	3 1 4 24% 67%	3760
	21st to end	Day	5% 1 1 7% 27% 59%	4179
September	1st to 10th	Day	6% 1 2 6% 27% 58%	2280
	11th to 20th	Day	8% 1 2 19% 38% 31%	2242
	21st to end	Day	7% 1 2 17% 38% 34%	2090
October	1st to 10th	Day	9% 2 4 12% 20% 54%	3258
	11th to 20th	Day	11% 2 4 22% 41% 21%	3161
	21st to end	Day	18% 2 5% 18% 39% 19%	3448
November	1st to 10th	Day	23% 3 11% 26% 27% 11%	1766
	11th to 20th	Day	25% 4 12% 28% 25% 6%	1719
	21st to end	Day	28% 5% 10% 23% 26% 9%	1710
December	1st to 10th	Day	36% 6% 10% 21% 19% 9%	1573
	11th to 20th	Day	50% 5% 10% 19% 13% 4	1928
	21st to end	Day	48% 6% 17% 18% 8% 2	2794
January	1st to 10th	Night	42% 3 20% 24% 10% 2	4406
	11th to 20th	Night	46% 4 19% 15% 10% 6%	4198
	21st to end	Night	45% 3 14% 23% 12% 3	4521
February	1st to 10th	Night	33% 3 12% 24% 20% 8%	2399
	11th to 20th	Night	28% 2 11% 29% 24% 6%	2219
	21st to end	Night	34% 3 11% 30% 18% 4	1884
March	1st to 10th	Night	41% 2 15% 21% 18% 3	3736
	11th to 20th	Night	25% 2 13% 31% 20% 9%	3521
	21st to end	Night	39% 3 13% 28% 15% 3	3586
April	1st to 10th	Night	25% 3 13% 29% 21% 9%	1900
	11th to 20th	Night	21% 2 12% 29% 27% 9%	1879
	21st to end	Night	17% 3 11% 31% 33% 6%	1900
May	1st to 10th	Night	11% 1 7% 29% 35% 15%	3102
	11th to 20th	Night	9% 1 4 24% 44% 18%	2771
	21st to end	Night	8% 1 5% 28% 45% 11%	2762
June	1st to 10th	Night	13% 1 8% 24% 32% 21%	1520
	11th to 20th	Night	5% 1 4 17% 37% 36%	1520
	21st to end	Night	5% 1 2 9% 25% 58%	1520
July	1st to 10th	Night	2 1 2 13% 82%	2500
	11th to 20th	Night	10% 1 1 4 15% 69%	2492
	21st to end	Night	6% 1 1 4 25% 63%	2966
August	1st to 10th	Night	8% 1 1 7% 21% 62%	3014
	11th to 20th	Night	4 1 1 4 20% 71%	3156
	21st to end	Night	6% 1 1 6% 24% 63%	3498
September	1st to 10th	Night	5% 1 2 6% 25% 62%	1900
	11th to 20th	Night	9% 1 2 15% 40% 33%	1938
	21st to end	Night	8% 1 1 14% 37% 38%	2090
October	1st to 10th	Night	8% 1 3 10% 20% 57%	3706
	11th to 20th	Night	9% 1 3 22% 41% 23%	3777
	21st to end	Night	14% 1 3 18% 39% 25%	4121
November	1st to 10th	Night	22% 2 9% 25% 25% 16%	2530
	11th to 20th	Night	20% 2 11% 29% 27% 12%	2483
	21st to end	Night	24% 4 10% 24% 29% 9%	2470
December	1st to 10th	Night	34% 4 10% 22% 19% 12%	2603
	11th to 20th	Night	44% 5% 13% 18% 15% 4	3276
	21st to end	Night	46% 3 17% 23% 9% 3	4862

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	25% 3 18% 32% 17% 5%	180
	11th to 20th	Day	15% 4 11% 35% 23% 12%	317
	21st to end	Day	17% 2 15% 30% 26% 10%	407
February	1st to 10th	Day	17% 6% 13% 29% 22% 13%	403
	11th to 20th	Day	15% 5% 19% 33% 19% 9%	455
	21st to end	Day	13% 8% 20% 34% 21% 4	382
March	1st to 10th	Day	15% 8% 22% 30% 20% 4	460
	11th to 20th	Day	13% 8% 27% 24% 20% 7%	460
	21st to end	Day	17% 10% 24% 27% 19% 4	506
April	1st to 10th	Day	13% 11% 23% 26% 18% 9%	460
	11th to 20th	Day	11% 9% 25% 31% 17% 6%	452
	21st to end	Day	4 10% 23% 32% 19% 11%	460
May	1st to 10th	Day	5% 8% 30% 33% 17% 7%	463
	11th to 20th	Day	5% 6% 25% 33% 24% 8%	488
	21st to end	Day	5% 7% 16% 35% 28% 9%	529
June	1st to 10th	Day	2 3 22% 31% 31% 11%	490
	11th to 20th	Day	2 4 20% 36% 28% 11%	490
	21st to end	Day	1 2 14% 38% 29% 15%	490
July	1st to 10th	Day	1 8% 36% 41% 14%	486
	11th to 20th	Day	1 5% 29% 44% 21%	484
	21st to end	Day	1 1 6% 31% 41% 20%	531
August	1st to 10th	Day	5% 11% 38% 32% 14%	487
	11th to 20th	Day	1 1 13% 34% 35% 17%	477
	21st to end	Day	1 1 9% 30% 40% 19%	539
September	1st to 10th	Day	1 2 10% 36% 34% 17%	490
	11th to 20th	Day	3 2 20% 34% 30% 10%	490
	21st to end	Day	2 3 15% 31% 36% 13%	490
October	1st to 10th	Day	2 3 18% 33% 28% 16%	490
	11th to 20th	Day	3 5% 15% 28% 34% 14%	490
	21st to end	Day	5% 4 13% 30% 35% 14%	539
November	1st to 10th	Day	9% 3 16% 27% 30% 16%	302
	11th to 20th	Day	5% 4 12% 32% 34% 14%	273
	21st to end	Day	9% 3 14% 30% 34% 11%	270
December	1st to 10th	Day	15% 2 9% 40% 24% 10%	205
	11th to 20th	Day	14% 4 16% 33% 21% 13%	180
	21st to end	Day	22% 4 16% 31% 20% 8%	198
January	1st to 10th	Night	18% 2 11% 35% 24% 10%	760
	11th to 20th	Night	12% 3 10% 39% 28% 9%	760
	21st to end	Night	18% 2 8% 38% 28% 6%	836
February	1st to 10th	Night	17% 3 10% 36% 25% 9%	760
	11th to 20th	Night	13% 3 14% 33% 29% 7%	744
	21st to end	Night	17% 2 12% 45% 21% 4	628
March	1st to 10th	Night	16% 3 15% 38% 23% 4	760
	11th to 20th	Night	13% 3 17% 34% 26% 6%	760
	21st to end	Night	14% 3 19% 38% 23% 2	836
April	1st to 10th	Night	10% 5% 20% 34% 26% 3	760
	11th to 20th	Night	6% 7% 23% 36% 22% 5%	751
	21st to end	Night	6% 5% 25% 35% 24% 6%	760
May	1st to 10th	Night	4 5% 25% 40% 22% 4	721
	11th to 20th	Night	5% 3 24% 46% 21% 1	524
	21st to end	Night	4 4 26% 38% 27% 1	411
June	1st to 10th	Night	4 4 24% 47% 17% 3	380
	11th to 20th	Night	1 3 21% 52% 23%	380
	21st to end	Night	1 18% 51% 29% 1	380
July	1st to 10th	Night	12% 51% 36% 1	370
	11th to 20th	Night	2 6% 50% 40% 2	368
	21st to end	Night	1 1 6% 33% 51% 8%	590
August	1st to 10th	Night	3 9% 43% 38% 6%	664
	11th to 20th	Night	9% 43% 41% 6%	752
	21st to end	Night	2 5% 37% 49% 7%	836
September	1st to 10th	Night	1 7% 36% 49% 8%	760
	11th to 20th	Night	2 1 12% 40% 36% 8%	760
	21st to end	Night	2 1 10% 34% 43% 10%	760
October	1st to 10th	Night	4 1 10% 38% 39% 8%	760
	11th to 20th	Night	4 2 8% 36% 40% 10%	760
	21st to end	Night	5% 1 9% 35% 33% 16%	836
November	1st to 10th	Night	9% 2 9% 33% 35% 12%	780
	11th to 20th	Night	6% 1 9% 43% 32% 9%	764
	21st to end	Night	9% 1 10% 38% 33% 9%	760
December	1st to 10th	Night	16% 1 11% 36% 28% 8%	760
	11th to 20th	Night	18% 1 9% 39% 27% 5%	760
	21st to end	Night	19% 2 11% 41% 21% 5%	836

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