Hidden Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Valley Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 4 4 4 5% 7% 38%	131519
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 37%	129968
Transparency	Day	44% 6% 12% 25% 11% 1	135031
	Night	46% 6% 12% 23% 13% 1	134530
Seeing	Day	16% 10% 30% 26% 14% 4	17775
	Night	16% 9% 31% 32% 11% 1	27822

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% 14% 6% 18% 7% 6% 5% 3 4 4 24%	4691
	11th to 20th	Day	5% 13% 6% 14% 7% 6% 8% 8% 6% 5% 21%	4710
	21st to end	Day	6% 11% 6% 11% 5% 5% 6% 4 5% 8% 33%	5573
February	1st to 10th	Day	8% 12% 5% 10% 7% 5% 5% 5% 6% 6% 31%	1890
	11th to 20th	Day	9% 11% 4 9% 6% 5% 4 3 6% 7% 36%	1844
	21st to end	Day	11% 12% 4 10% 5% 4 4 4 6% 7% 33%	1700
March	1st to 10th	Day	13% 8% 2 5% 4 2 3 3 5% 7% 47%	4632
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 4 5% 8% 40%	4980
	21st to end	Day	22% 11% 3 10% 5% 4 4 4 4 6% 27%	5683
April	1st to 10th	Day	16% 12% 4 7% 6% 4 4 4 5% 6% 33%	2400
	11th to 20th	Day	20% 11% 3 9% 5% 4 4 3 4 5% 31%	2374
	21st to end	Day	20% 11% 3 8% 5% 5% 4 4 5% 6% 29%	2510
May	1st to 10th	Day	14% 5% 2 6% 4 4 3 3 5% 6% 49%	6317
	11th to 20th	Day	14% 8% 2 6% 4 4 5% 4 5% 6% 40%	6319
	21st to end	Day	18% 8% 3 6% 5% 4 4 3 5% 6% 37%	6935
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	2800
	11th to 20th	Day	13% 7% 3 8% 5% 6% 5% 4 6% 8% 35%	2800
	21st to end	Day	12% 6% 2 6% 5% 6% 5% 4 7% 9% 39%	2800
July	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 46%	6360
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 6% 9% 54%	6360
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 7% 8% 45%	6924
August	1st to 10th	Day	12% 6% 2 7% 4 4 5% 4 6% 8% 42%	4368
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 44%	4171
	21st to end	Day	11% 7% 3 6% 4 5% 5% 4 6% 8% 41%	4347
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 9% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 5% 8% 44%	2400
	21st to end	Day	15% 7% 2 5% 3 4 5% 3 5% 6% 44%	2112
October	1st to 10th	Day	16% 7% 3 6% 3 3 4 3 6% 6% 43%	2000
	11th to 20th	Day	15% 11% 3 8% 5% 4 4 3 4 7% 36%	2000
	21st to end	Day	19% 11% 4 8% 4 3 3 3 4 5% 37%	2090
November	1st to 10th	Day	14% 10% 3 8% 5% 4 3 4 5% 7% 38%	1821
	11th to 20th	Day	12% 14% 4 10% 6% 5% 4 4 6% 5% 29%	1680
	21st to end	Day	15% 12% 4 8% 5% 5% 4 3 5% 5% 34%	1680
December	1st to 10th	Day	7% 11% 6% 17% 13% 7% 5% 3 4 5% 23%	2690
	11th to 20th	Day	11% 17% 7% 11% 6% 4 4 3 4 5% 29%	3088
	21st to end	Day	20% 20% 7% 16% 6% 5% 4 2 3 3 14%	4070
January	1st to 10th	Night	8% 13% 6% 13% 8% 5% 4 3 4 5% 30%	8232
	11th to 20th	Night	6% 13% 5% 14% 8% 6% 9% 7% 6% 5% 21%	8280
	21st to end	Night	7% 12% 5% 11% 6% 5% 5% 3 5% 6% 34%	8623
February	1st to 10th	Night	7% 12% 6% 11% 6% 5% 4 4 6% 6% 33%	2730
	11th to 20th	Night	7% 10% 3 9% 5% 5% 5% 4 5% 7% 38%	2663
	21st to end	Night	12% 9% 3 9% 7% 5% 5% 4 6% 6% 35%	2105
March	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 8% 46%	5310
	11th to 20th	Night	9% 8% 3 9% 6% 5% 5% 4 6% 9% 38%	4980
	21st to end	Night	23% 11% 4 10% 4 3 3 3 4 6% 29%	5244
April	1st to 10th	Night	18% 10% 3 7% 4 4 4 4 6% 6% 35%	2000
	11th to 20th	Night	22% 9% 3 6% 3 4 3 4 5% 6% 34%	1975
	21st to end	Night	20% 8% 3 5% 4 4 4 3 6% 6% 35%	1889
May	1st to 10th	Night	14% 7% 2 5% 4 4 3 4 5% 6% 46%	3596
	11th to 20th	Night	13% 6% 3 6% 4 3 3 3 4 6% 50%	3596
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 6% 39%	3942
June	1st to 10th	Night	10% 6% 1 5% 4 3 5% 5% 7% 9% 45%	1600
	11th to 20th	Night	12% 7% 3 6% 3 4 5% 3 7% 8% 43%	1600
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 6% 8% 47%	1600
July	1st to 10th	Night	9% 4 2 5% 4 4 5% 4 7% 8% 48%	3618
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 5% 9% 54%	3617
	21st to end	Night	7% 5% 3 6% 4 4 4 4 6% 8% 49%	3947
August	1st to 10th	Night	11% 5% 2 7% 5% 4 5% 3 4 7% 46%	2788
	11th to 20th	Night	12% 6% 2 6% 4 3 3 4 5% 6% 49%	2888
	21st to end	Night	12% 6% 3 7% 4 4 4 3 4 7% 46%	3550
September	1st to 10th	Night	10% 5% 2 7% 3 4 4 3 5% 8% 51%	2000
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 8% 49%	2000
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 6% 51%	2264
October	1st to 10th	Night	13% 6% 2 6% 3 3 4 3 5% 6% 49%	2400
	11th to 20th	Night	16% 8% 2 7% 4 4 4 3 5% 7% 41%	2400
	21st to end	Night	17% 10% 3 6% 5% 4 3 3 5% 6% 36%	2750
November	1st to 10th	Night	13% 11% 3 7% 4 4 4 3 5% 7% 40%	2692
	11th to 20th	Night	13% 11% 4 10% 5% 5% 4 3 5% 6% 34%	2940
	21st to end	Night	15% 11% 4 9% 4 4 4 4 5% 6% 35%	2940
December	1st to 10th	Night	8% 9% 4 14% 8% 7% 5% 4 6% 6% 29%	4720
	11th to 20th	Night	11% 18% 6% 14% 6% 5% 4 4 4 6% 23%	5339
	21st to end	Night	22% 16% 6% 15% 6% 4 3 3 3 5% 18%	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	63% 5% 9% 15% 8%	4692
	11th to 20th	Day	61% 5% 7% 16% 11%	4710
	21st to end	Day	50% 5% 14% 22% 10%	5573
February	1st to 10th	Day	51% 6% 13% 19% 10% 1	1890
	11th to 20th	Day	47% 5% 13% 25% 10% 1	1853
	21st to end	Day	49% 7% 12% 22% 9% 1	1700
March	1st to 10th	Day	37% 4 17% 25% 13% 4	4632
	11th to 20th	Day	41% 5% 14% 27% 11% 1	4980
	21st to end	Day	58% 5% 10% 17% 10% 1	5685
April	1st to 10th	Day	50% 7% 10% 20% 13% 1	2400
	11th to 20th	Day	54% 5% 10% 18% 11% 2	2374
	21st to end	Day	54% 6% 12% 18% 9% 1	2510
May	1st to 10th	Day	36% 5% 11% 30% 17% 2	6355
	11th to 20th	Day	42% 7% 11% 23% 14% 3	6323
	21st to end	Day	46% 6% 9% 27% 11% 1	6949
June	1st to 10th	Day	36% 7% 14% 29% 12% 2	2800
	11th to 20th	Day	45% 8% 13% 21% 12% 1	2800
	21st to end	Day	38% 8% 17% 26% 10% 1	2800
July	1st to 10th	Day	33% 8% 16% 34% 9%	6291
	11th to 20th	Day	27% 6% 15% 36% 15% 1	6285
	21st to end	Day	34% 8% 14% 28% 16% 1	6728
August	1st to 10th	Day	38% 7% 13% 26% 14% 1	4353
	11th to 20th	Day	36% 7% 13% 34% 10%	4210
	21st to end	Day	38% 7% 15% 27% 12% 1	4346
September	1st to 10th	Day	35% 7% 14% 30% 14%	2400
	11th to 20th	Day	37% 7% 18% 30% 8% 1	2400
	21st to end	Day	39% 6% 13% 32% 10%	2112
October	1st to 10th	Day	28% 3 9% 38% 20%	3260
	11th to 20th	Day	39% 5% 11% 26% 17% 2	3240
	21st to end	Day	43% 4 16% 27% 9%	3354
November	1st to 10th	Day	46% 6% 14% 19% 13% 2	1821
	11th to 20th	Day	56% 5% 12% 19% 7% 1	1680
	21st to end	Day	51% 5% 15% 21% 7% 1	1680
December	1st to 10th	Day	65% 5% 10% 16% 4	2690
	11th to 20th	Day	59% 4 9% 19% 8% 1	3085
	21st to end	Day	76% 4 7% 10% 4	4070
January	1st to 10th	Night	57% 4 13% 17% 9%	8233
	11th to 20th	Night	60% 6% 9% 14% 11%	8280
	21st to end	Night	50% 5% 11% 21% 13%	8623
February	1st to 10th	Night	51% 5% 12% 21% 10% 2	2730
	11th to 20th	Night	45% 6% 12% 26% 10% 1	2664
	21st to end	Night	48% 6% 13% 22% 10% 1	2105
March	1st to 10th	Night	36% 5% 15% 24% 14% 5%	5310
	11th to 20th	Night	41% 7% 15% 23% 12% 1	4980
	21st to end	Night	59% 4 10% 15% 12% 1	5244
April	1st to 10th	Night	48% 7% 8% 19% 17% 2	2000
	11th to 20th	Night	50% 6% 8% 19% 15% 2	1975
	21st to end	Night	47% 6% 10% 21% 15% 1	1889
May	1st to 10th	Night	37% 6% 11% 24% 21% 1	3620
	11th to 20th	Night	34% 6% 11% 25% 20% 6%	3597
	21st to end	Night	43% 7% 11% 26% 13% 1	3950
June	1st to 10th	Night	32% 9% 14% 27% 17% 1	1600
	11th to 20th	Night	36% 8% 14% 23% 18% 1	1600
	21st to end	Night	32% 8% 14% 32% 12% 2	1600
July	1st to 10th	Night	31% 8% 18% 32% 11% 1	3580
	11th to 20th	Night	26% 6% 16% 35% 16% 1	3572
	21st to end	Night	31% 8% 15% 26% 18% 2	3832
August	1st to 10th	Night	37% 7% 12% 27% 15% 3	2779
	11th to 20th	Night	33% 7% 14% 31% 13% 2	2915
	21st to end	Night	37% 7% 15% 26% 13% 2	3550
September	1st to 10th	Night	31% 6% 12% 34% 16% 1	2000
	11th to 20th	Night	32% 6% 15% 31% 15%	2000
	21st to end	Night	34% 5% 14% 32% 14% 1	2264
October	1st to 10th	Night	26% 3 7% 35% 25% 4	3920
	11th to 20th	Night	33% 6% 11% 26% 22% 1	3899
	21st to end	Night	47% 5% 10% 24% 13%	4436
November	1st to 10th	Night	43% 5% 12% 25% 13% 1	2692
	11th to 20th	Night	50% 6% 14% 18% 11% 1	2940
	21st to end	Night	50% 5% 15% 22% 7% 1	2940
December	1st to 10th	Night	53% 8% 16% 19% 4	4720
	11th to 20th	Night	63% 5% 11% 15% 6%	5341
	21st to end	Night	70% 4 8% 12% 6%	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	14% 12% 36% 25% 11% 3	330
	11th to 20th	Day	16% 12% 39% 25% 7% 1	330
	21st to end	Day	14% 9% 38% 28% 9% 1	363
February	1st to 10th	Day	15% 13% 42% 21% 7% 2	330
	11th to 20th	Day	14% 15% 41% 22% 8%	330
	21st to end	Day	16% 15% 45% 21% 3	273
March	1st to 10th	Day	16% 12% 40% 22% 9% 1	306
	11th to 20th	Day	19% 12% 36% 23% 8% 2	300
	21st to end	Day	25% 11% 32% 21% 8% 3	422
April	1st to 10th	Day	24% 17% 29% 24% 6%	500
	11th to 20th	Day	28% 13% 26% 23% 8% 1	492
	21st to end	Day	27% 14% 28% 20% 9% 2	610
May	1st to 10th	Day	20% 12% 29% 27% 10% 2	703
	11th to 20th	Day	19% 13% 25% 26% 13% 4	725
	21st to end	Day	18% 11% 26% 24% 16% 4	789
June	1st to 10th	Day	11% 8% 28% 32% 18% 3	730
	11th to 20th	Day	15% 10% 35% 28% 10% 3	730
	21st to end	Day	15% 7% 30% 27% 15% 6%	730
July	1st to 10th	Day	9% 5% 20% 28% 28% 10%	721
	11th to 20th	Day	10% 6% 26% 29% 20% 9%	718
	21st to end	Day	6% 4 27% 33% 22% 8%	794
August	1st to 10th	Day	10% 5% 22% 31% 24% 8%	726
	11th to 20th	Day	12% 5% 19% 30% 26% 7%	715
	21st to end	Day	15% 5% 22% 29% 23% 5%	605
September	1st to 10th	Day	13% 8% 25% 28% 20% 5%	530
	11th to 20th	Day	12% 5% 28% 31% 20% 4	530
	21st to end	Day	18% 10% 24% 27% 16% 5%	407
October	1st to 10th	Day	17% 10% 31% 25% 14% 3	330
	11th to 20th	Day	20% 20% 31% 22% 6% 1	330
	21st to end	Day	25% 15% 35% 16% 7% 2	363
November	1st to 10th	Day	20% 15% 34% 22% 9% 1	330
	11th to 20th	Day	19% 19% 34% 20% 6%	330
	21st to end	Day	24% 14% 38% 18% 6%	330
December	1st to 10th	Day	16% 12% 39% 24% 8% 1	330
	11th to 20th	Day	19% 15% 37% 22% 6% 1	330
	21st to end	Day	26% 12% 39% 18% 4 1	363
January	1st to 10th	Night	13% 13% 33% 31% 9% 1	1050
	11th to 20th	Night	17% 11% 35% 31% 6% 1	1050
	21st to end	Night	17% 9% 34% 32% 7% 1	1155
February	1st to 10th	Night	13% 12% 38% 29% 7%	1050
	11th to 20th	Night	13% 11% 41% 28% 7%	1029
	21st to end	Night	16% 13% 34% 30% 7%	721
March	1st to 10th	Night	15% 10% 38% 30% 6% 1	773
	11th to 20th	Night	16% 10% 36% 30% 7%	600
	21st to end	Night	22% 9% 30% 27% 10% 2	660
April	1st to 10th	Night	21% 14% 33% 26% 5% 1	600
	11th to 20th	Night	27% 10% 29% 28% 6%	593
	21st to end	Night	24% 9% 33% 26% 8% 1	600
May	1st to 10th	Night	21% 9% 32% 31% 6% 1	600
	11th to 20th	Night	19% 7% 31% 31% 11% 1	591
	21st to end	Night	14% 6% 26% 39% 14%	650
June	1st to 10th	Night	11% 5% 31% 39% 13% 1	600
	11th to 20th	Night	14% 5% 32% 40% 10% 1	600
	21st to end	Night	9% 5% 25% 44% 16% 1	600
July	1st to 10th	Night	8% 2 21% 39% 26% 3	585
	11th to 20th	Night	8% 5% 23% 41% 21% 2	582
	21st to end	Night	5% 3 21% 46% 22% 3	657
August	1st to 10th	Night	7% 3 16% 46% 26% 2	597
	11th to 20th	Night	12% 3 20% 36% 26% 3	594
	21st to end	Night	13% 3 22% 35% 24% 2	660
September	1st to 10th	Night	12% 4 24% 38% 18% 4	600
	11th to 20th	Night	11% 4 28% 36% 19% 2	600
	21st to end	Night	13% 9% 28% 33% 15% 1	744
October	1st to 10th	Night	16% 9% 28% 32% 15% 1	800
	11th to 20th	Night	20% 10% 34% 29% 7%	800
	21st to end	Night	22% 10% 31% 28% 9%	880
November	1st to 10th	Night	17% 11% 35% 29% 8%	846
	11th to 20th	Night	18% 13% 35% 25% 8%	1050
	21st to end	Night	20% 13% 36% 25% 6%	1050
December	1st to 10th	Night	17% 10% 36% 28% 8% 1	1050
	11th to 20th	Night	20% 12% 35% 24% 8% 1	1050
	21st to end	Night	26% 10% 33% 25% 5%	1155

Page generated at 2026-02-28 00:40:24. Version ac07

Copyright © 2026 Attilla Danko.