Lost Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lost Valley 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	6% 5% 1 3 2 2 2 2 3 5% 67%	139781
	Night	7% 5% 1 3 2 2 3 2 4 5% 65%	139513
Transparency	Day	22% 4 6% 20% 28% 21%	134828
	Night	23% 3 7% 21% 27% 19%	135442
Seeing	Day	11% 11% 31% 31% 11% 5%	17038
	Night	10% 5% 18% 33% 25% 9%	26653

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% 2 5% 3 3 3 3 4 7% 50%	4722
	11th to 20th	Day	7% 6% 2 5% 2 3 3 2 3 5% 63%	5040
	21st to end	Day	9% 6% 2 6% 4 4 4 4 5% 8% 48%	5843
February	1st to 10th	Day	14% 11% 2 4 2 2 3 3 4 6% 48%	2015
	11th to 20th	Day	10% 14% 2 5% 4 3 4 3 4 5% 46%	2050
	21st to end	Day	15% 12% 3 7% 3 3 3 2 5% 6% 41%	1730
March	1st to 10th	Day	10% 16% 4 7% 4 3 4 4 5% 6% 36%	4618
	11th to 20th	Day	11% 11% 2 6% 4 3 3 3 4 7% 47%	4961
	21st to end	Day	14% 15% 3 6% 3 3 4 3 4 6% 39%	6026
April	1st to 10th	Day	10% 11% 2 4 3 3 3 4 5% 6% 49%	2413
	11th to 20th	Day	10% 10% 2 5% 4 4 3 3 4 5% 50%	2369
	21st to end	Day	9% 8% 2 5% 3 3 3 3 5% 7% 52%	2400
May	1st to 10th	Day	7% 5% 2 3 2 2 2 2 4 4 68%	5633
	11th to 20th	Day	4 4 1 3 2 2 2 2 4 4 71%	5841
	21st to end	Day	4 2 1 3 2 1 2 2 3 5% 76%	6859
June	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 6% 66%	2800
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 4 82%	2800
	21st to end	Day	2 2 1 2 1 1 1 2 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	6320
	21st to end	Day	1 1 1 1 1 2 1 3 3 85%	6586
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 89%	5443
	11th to 20th	Day	1 1 1 1 1 1 2 93%	5364
	21st to end	Day	1 1 2 1 1 2 2 2 4 83%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 85%	2400
	11th to 20th	Day	2 2 1 2 2 2 2 2 2 4 79%	2330
	21st to end	Day	3 2 1 2 1 2 2 2 3 4 78%	2200
October	1st to 10th	Day	3 3 1 2 2 2 2 3 4 4 74%	3448
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 3 76%	3261
	21st to end	Day	5% 3 2 4 3 3 3 3 4 6% 64%	3497
November	1st to 10th	Day	7% 7% 2 4 3 2 3 3 4 7% 57%	1854
	11th to 20th	Day	9% 5% 1 4 3 3 3 4 5% 6% 59%	1890
	21st to end	Day	8% 6% 2 4 3 3 3 2 4 6% 58%	1890
December	1st to 10th	Day	7% 3 1 4 3 3 3 3 5% 6% 62%	2875
	11th to 20th	Day	17% 8% 2 5% 4 3 4 3 5% 6% 43%	3108
	21st to end	Day	14% 9% 3 5% 4 3 4 3 5% 8% 40%	4092
January	1st to 10th	Night	13% 7% 2 4 2 3 3 2 5% 8% 50%	8172
	11th to 20th	Night	8% 5% 1 3 2 3 3 3 5% 6% 61%	7918
	21st to end	Night	9% 8% 2 6% 4 4 3 3 6% 8% 48%	8371
February	1st to 10th	Night	14% 10% 2 4 2 3 3 3 5% 6% 50%	2603
	11th to 20th	Night	11% 9% 2 4 3 3 3 3 4 7% 50%	2458
	21st to end	Night	12% 9% 2 5% 3 3 3 2 4 6% 50%	2076
March	1st to 10th	Night	11% 14% 3 5% 3 3 4 4 6% 7% 40%	5496
	11th to 20th	Night	8% 9% 2 6% 3 3 4 2 4 6% 52%	5255
	21st to end	Night	15% 10% 2 5% 3 3 2 3 5% 8% 44%	5189
April	1st to 10th	Night	10% 7% 1 3 3 3 2 3 5% 6% 55%	2010
	11th to 20th	Night	7% 7% 1 5% 4 3 3 3 4 5% 59%	1979
	21st to end	Night	6% 5% 2 3 2 3 3 3 5% 6% 62%	2000
May	1st to 10th	Night	4 3 1 3 2 2 3 2 3 5% 73%	4358
	11th to 20th	Night	3 2 1 2 1 2 2 2 4 5% 77%	4074
	21st to end	Night	2 1 1 2 1 1 1 1 2 4 84%	3999
June	1st to 10th	Night	4 3 1 3 2 2 3 2 3 6% 72%	1600
	11th to 20th	Night	1 1 2 1 1 2 2 3 3 84%	1600
	21st to end	Night	1 1 1 1 1 1 2 2 2 88%	1600
July	1st to 10th	Night	1 1 1 2 94%	3640
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 2 85%	3640
	21st to end	Night	1 1 1 1 1 1 2 2 4 85%	4275
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	4382
	11th to 20th	Night	1 1 1 1 1 1 2 93%	4506
	21st to end	Night	2 1 1 1 1 2 2 2 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 2 89%	2000
	11th to 20th	Night	2 2 1 2 1 1 1 2 3 3 82%	2069
	21st to end	Night	2 1 1 1 2 1 2 3 3 82%	2200
October	1st to 10th	Night	3 2 1 2 1 2 2 2 2 3 82%	3749
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 4 80%	3897
	21st to end	Night	4 2 1 3 2 2 2 2 3 5% 75%	4310
November	1st to 10th	Night	8% 5% 1 3 2 2 2 2 4 6% 64%	2665
	11th to 20th	Night	8% 4 2 4 2 2 3 3 5% 7% 60%	2730
	21st to end	Night	9% 6% 2 3 3 3 3 3 4 6% 58%	2730
December	1st to 10th	Night	7% 4 1 4 2 3 3 2 4 6% 63%	4525
	11th to 20th	Night	17% 7% 2 4 3 3 4 3 6% 9% 42%	5316
	21st to end	Night	11% 8% 3 7% 5% 5% 5% 4 7% 8% 37%	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	34% 4 11% 27% 19% 6%	4723
	11th to 20th	Day	26% 3 6% 12% 27% 26%	5040
	21st to end	Day	34% 4 8% 19% 25% 11%	5843
February	1st to 10th	Day	38% 4 6% 23% 19% 11%	2015
	11th to 20th	Day	39% 5% 6% 22% 21% 7%	2060
	21st to end	Day	45% 4 7% 24% 16% 3	1730
March	1st to 10th	Day	46% 6% 8% 15% 19% 6%	4618
	11th to 20th	Day	35% 6% 14% 26% 11% 7%	4961
	21st to end	Day	44% 7% 11% 23% 13% 2	6026
April	1st to 10th	Day	34% 6% 12% 25% 18% 3	2413
	11th to 20th	Day	36% 5% 10% 25% 21% 3	2372
	21st to end	Day	31% 5% 11% 33% 17% 2	2400
May	1st to 10th	Day	19% 5% 8% 33% 30% 4	5633
	11th to 20th	Day	16% 5% 8% 39% 31% 3	5844
	21st to end	Day	13% 3 5% 38% 39% 2	6861
June	1st to 10th	Day	18% 5% 11% 27% 31% 8%	2800
	11th to 20th	Day	9% 3 7% 22% 41% 19%	2800
	21st to end	Day	9% 2 2 14% 41% 32%	2800
July	1st to 10th	Day	3 1 1 6% 20% 69%	6251
	11th to 20th	Day	6% 2 2 6% 26% 58%	6245
	21st to end	Day	6% 2 2 15% 37% 37%	6396
August	1st to 10th	Day	8% 2 2 8% 39% 41%	3961
	11th to 20th	Day	4 2 1 8% 32% 54%	3880
	21st to end	Day	7% 2 1 10% 37% 43%	4311
September	1st to 10th	Day	7% 3 2 10% 37% 40%	2400
	11th to 20th	Day	11% 3 3 23% 38% 22%	2330
	21st to end	Day	12% 3 3 20% 38% 24%	2200
October	1st to 10th	Day	15% 3 3 12% 27% 40%	3448
	11th to 20th	Day	15% 3 2 17% 41% 22%	3261
	21st to end	Day	21% 5% 4 17% 37% 16%	3497
November	1st to 10th	Day	27% 5% 7% 20% 25% 16%	1854
	11th to 20th	Day	25% 4 7% 23% 30% 11%	1890
	21st to end	Day	29% 3 6% 19% 28% 15%	1890
December	1st to 10th	Day	23% 4 6% 24% 29% 14%	2875
	11th to 20th	Day	42% 4 10% 16% 20% 8%	3108
	21st to end	Day	42% 3 12% 23% 13% 6%	4092
January	1st to 10th	Night	34% 2 13% 23% 21% 7%	8172
	11th to 20th	Night	25% 3 5% 16% 31% 20%	7918
	21st to end	Night	34% 4 7% 16% 24% 14%	8371
February	1st to 10th	Night	36% 3 6% 24% 19% 12%	2603
	11th to 20th	Night	33% 4 8% 23% 24% 7%	2459
	21st to end	Night	36% 4 10% 25% 17% 7%	2076
March	1st to 10th	Night	41% 4 8% 19% 19% 9%	5496
	11th to 20th	Night	32% 5% 12% 30% 13% 8%	5255
	21st to end	Night	38% 5% 18% 23% 14% 3	5189
April	1st to 10th	Night	29% 5% 15% 29% 17% 5%	2010
	11th to 20th	Night	27% 6% 12% 29% 21% 6%	1979
	21st to end	Night	22% 5% 14% 39% 19% 2	2000
May	1st to 10th	Night	15% 4 10% 40% 26% 4	4355
	11th to 20th	Night	10% 4 10% 43% 30% 4	4075
	21st to end	Night	7% 2 6% 42% 40% 2	3999
June	1st to 10th	Night	15% 3 12% 29% 34% 7%	1600
	11th to 20th	Night	7% 3 6% 25% 40% 19%	1600
	21st to end	Night	6% 2 3 13% 42% 34%	1600
July	1st to 10th	Night	2 1 1 5% 21% 71%	3600
	11th to 20th	Night	8% 2 2 6% 29% 53%	3592
	21st to end	Night	6% 2 1 17% 35% 38%	4153
August	1st to 10th	Night	9% 2 3 11% 32% 43%	3179
	11th to 20th	Night	4 2 1 8% 27% 58%	3256
	21st to end	Night	8% 1 1 8% 36% 46%	3608
September	1st to 10th	Night	5% 2 1 14% 35% 44%	2000
	11th to 20th	Night	10% 2 4 20% 44% 20%	2069
	21st to end	Night	9% 3 3 24% 38% 23%	2200
October	1st to 10th	Night	10% 2 4 11% 30% 43%	3749
	11th to 20th	Night	11% 2 3 18% 39% 27%	3897
	21st to end	Night	14% 3 3 21% 39% 21%	4310
November	1st to 10th	Night	23% 3 8% 22% 26% 18%	2665
	11th to 20th	Night	23% 4 7% 21% 32% 13%	2730
	21st to end	Night	28% 4 6% 19% 31% 12%	2730
December	1st to 10th	Night	24% 3 6% 23% 27% 17%	4525
	11th to 20th	Night	39% 4 10% 18% 21% 8%	5316
	21st to end	Night	43% 4 11% 20% 16% 5%	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	30% 12% 25% 20% 11% 2	220
	11th to 20th	Day	17% 10% 26% 29% 14% 5%	325
	21st to end	Day	19% 10% 26% 27% 14% 4	473
February	1st to 10th	Day	25% 12% 23% 24% 13% 3	496
	11th to 20th	Day	20% 18% 33% 19% 8% 2	535
	21st to end	Day	17% 16% 42% 18% 6% 1	446
March	1st to 10th	Day	22% 17% 37% 17% 5% 1	540
	11th to 20th	Day	22% 16% 28% 24% 8% 2	540
	21st to end	Day	24% 15% 33% 21% 6%	594
April	1st to 10th	Day	17% 14% 33% 27% 7% 2	505
	11th to 20th	Day	20% 18% 29% 25% 6% 2	492
	21st to end	Day	15% 10% 33% 27% 11% 4	500
May	1st to 10th	Day	17% 16% 31% 24% 8% 4	503
	11th to 20th	Day	15% 13% 28% 27% 13% 5%	528
	21st to end	Day	14% 9% 27% 31% 11% 8%	573
June	1st to 10th	Day	5% 12% 27% 35% 14% 8%	530
	11th to 20th	Day	5% 10% 30% 35% 12% 7%	530
	21st to end	Day	2 8% 33% 35% 13% 8%	530
July	1st to 10th	Day	2 4 30% 40% 14% 10%	526
	11th to 20th	Day	2 2 25% 44% 14% 14%	524
	21st to end	Day	2 4 25% 46% 11% 12%	575
August	1st to 10th	Day	2 7% 36% 34% 13% 9%	527
	11th to 20th	Day	2 7% 29% 41% 11% 10%	517
	21st to end	Day	2 5% 28% 44% 15% 5%	583
September	1st to 10th	Day	4 7% 35% 39% 12% 4	530
	11th to 20th	Day	6% 10% 36% 35% 11% 2	530
	21st to end	Day	6% 12% 33% 37% 11% 1	530
October	1st to 10th	Day	6% 12% 32% 38% 11% 1	530
	11th to 20th	Day	6% 12% 33% 37% 11% 2	530
	21st to end	Day	8% 10% 34% 33% 14% 2	546
November	1st to 10th	Day	10% 12% 31% 29% 17% 2	330
	11th to 20th	Day	12% 13% 32% 25% 14% 4	330
	21st to end	Day	11% 14% 30% 28% 13% 4	330
December	1st to 10th	Day	14% 7% 30% 28% 16% 4	278
	11th to 20th	Day	16% 9% 27% 28% 15% 5%	220
	21st to end	Day	24% 11% 30% 20% 10% 5%	242
January	1st to 10th	Night	19% 8% 22% 25% 16% 10%	840
	11th to 20th	Night	12% 7% 28% 30% 16% 8%	840
	21st to end	Night	18% 7% 25% 30% 15% 6%	924
February	1st to 10th	Night	21% 6% 20% 31% 15% 7%	840
	11th to 20th	Night	15% 8% 23% 28% 18% 8%	824
	21st to end	Night	17% 7% 24% 35% 12% 4	692
March	1st to 10th	Night	19% 9% 19% 33% 15% 5%	840
	11th to 20th	Night	15% 6% 22% 27% 22% 8%	840
	21st to end	Night	18% 5% 18% 36% 18% 4	924
April	1st to 10th	Night	14% 6% 19% 37% 18% 5%	804
	11th to 20th	Night	11% 6% 24% 31% 22% 6%	791
	21st to end	Night	9% 6% 20% 36% 22% 8%	800
May	1st to 10th	Night	7% 5% 20% 36% 25% 7%	715
	11th to 20th	Night	8% 3 19% 34% 31% 6%	591
	21st to end	Night	6% 2 20% 31% 30% 10%	447
June	1st to 10th	Night	5% 1 14% 39% 35% 8%	400
	11th to 20th	Night	1 3 16% 37% 35% 8%	400
	21st to end	Night	1 2 14% 37% 39% 8%	400
July	1st to 10th	Night	1 2 13% 28% 49% 8%	390
	11th to 20th	Night	3 1 9% 32% 44% 11%	388
	21st to end	Night	1 1 10% 27% 42% 18%	564
August	1st to 10th	Night	3 2 11% 31% 42% 10%	743
	11th to 20th	Night	1 9% 30% 49% 11%	792
	21st to end	Night	2 1 9% 29% 46% 13%	880
September	1st to 10th	Night	2 1 10% 32% 42% 13%	800
	11th to 20th	Night	3 2 10% 36% 35% 14%	800
	21st to end	Night	3 1 16% 35% 32% 13%	800
October	1st to 10th	Night	5% 2 14% 45% 23% 10%	800
	11th to 20th	Night	6% 3 19% 34% 26% 12%	800
	21st to end	Night	5% 4 19% 30% 26% 16%	880
November	1st to 10th	Night	10% 3 15% 34% 25% 12%	820
	11th to 20th	Night	10% 5% 22% 35% 20% 8%	840
	21st to end	Night	12% 7% 23% 32% 19% 8%	840
December	1st to 10th	Night	15% 5% 21% 37% 16% 7%	840
	11th to 20th	Night	20% 5% 24% 32% 15% 5%	840
	21st to end	Night	20% 10% 23% 27% 15% 4	924

Page generated at 2026-03-29 23:18:41. Version ac07

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