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 2026-10-29.

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	7% 5% 2 5% 4 4 4 4 6% 8% 51%	158581
	Night	6% 5% 2 6% 4 4 4 4 6% 8% 53%	151040
Transparency	Day	28% 7% 15% 26% 18% 5%	153345
	Night	28% 7% 15% 26% 19% 4	147288
Seeing	Day	11% 13% 33% 29% 12% 2	18681
	Night	9% 11% 34% 35% 10% 1	28569

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	2 6% 4 8% 4 5% 4 4 7% 9% 48%	4720
	11th to 20th	Day	1 5% 2 7% 6% 5% 5% 4 7% 8% 49%	4740
	21st to end	Day	1 3 2 8% 4 4 4 3 6% 7% 57%	5188
February	1st to 10th	Day	3 9% 3 7% 5% 5% 6% 5% 6% 8% 43%	1872
	11th to 20th	Day	3 8% 4 7% 5% 5% 5% 4 6% 8% 46%	2012
	21st to end	Day	2 7% 4 6% 4 4 6% 4 7% 8% 48%	1730
March	1st to 10th	Day	4 7% 3 6% 4 4 4 3 5% 7% 55%	4600
	11th to 20th	Day	6% 6% 2 5% 3 2 3 3 5% 8% 57%	4764
	21st to end	Day	12% 12% 3 7% 4 4 4 4 6% 8% 37%	6101
April	1st to 10th	Day	11% 9% 3 5% 4 4 4 3 6% 8% 44%	2520
	11th to 20th	Day	10% 7% 2 7% 5% 4 5% 5% 6% 7% 41%	2491
	21st to end	Day	13% 9% 2 7% 4 4 4 4 6% 7% 40%	2679
May	1st to 10th	Day	8% 7% 2 7% 4 4 5% 4 6% 8% 46%	7819
	11th to 20th	Day	13% 7% 2 4 3 3 4 4 6% 7% 47%	8191
	21st to end	Day	14% 6% 2 5% 4 4 5% 4 6% 8% 43%	9026
June	1st to 10th	Day	9% 4 1 4 4 4 5% 5% 7% 10% 47%	2940
	11th to 20th	Day	9% 4 1 4 3 4 4 5% 8% 9% 48%	2940
	21st to end	Day	6% 3 1 3 3 3 4 4 6% 7% 59%	2940
July	1st to 10th	Day	4 3 1 5% 4 5% 6% 6% 9% 11% 47%	8249
	11th to 20th	Day	6% 2 1 3 3 3 4 4 6% 8% 58%	8250
	21st to end	Day	6% 3 1 4 3 4 4 4 7% 10% 52%	8957
August	1st to 10th	Day	4 2 1 4 3 4 5% 5% 7% 10% 54%	8142
	11th to 20th	Day	6% 4 1 5% 4 4 6% 5% 8% 9% 48%	7304
	21st to end	Day	6% 3 1 4 3 3 4 4 7% 10% 55%	7751
September	1st to 10th	Day	6% 4 1 4 2 2 2 3 5% 7% 64%	2520
	11th to 20th	Day	9% 3 1 4 3 3 4 3 5% 7% 60%	2426
	21st to end	Day	9% 4 1 4 3 3 3 3 5% 6% 60%	2310
October	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 5% 71%	3540
	11th to 20th	Day	9% 4 1 3 3 3 3 3 4 6% 62%	3268
	21st to end	Day	7% 6% 2 4 3 3 4 3 6% 7% 55%	3262
November	1st to 10th	Day	5% 4 1 4 3 3 5% 4 6% 9% 56%	1853
	11th to 20th	Day	3 5% 2 5% 4 3 4 4 6% 8% 57%	1890
	21st to end	Day	2 6% 2 5% 4 3 4 4 7% 8% 54%	1688
December	1st to 10th	Day	1 4 3 6% 5% 4 4 3 6% 7% 57%	2700
	11th to 20th	Day	2 3 1 5% 5% 5% 6% 4 6% 8% 55%	3106
	21st to end	Day	4 8% 3 6% 4 5% 4 4 5% 7% 51%	4092
January	1st to 10th	Night	3 9% 3 8% 5% 5% 5% 4 5% 6% 46%	8201
	11th to 20th	Night	1 3 2 8% 5% 6% 6% 4 6% 9% 50%	8250
	21st to end	Night	1 3 2 8% 4 4 4 3 5% 7% 59%	9026
February	1st to 10th	Night	3 8% 4 8% 4 6% 4 4 6% 7% 45%	2767
	11th to 20th	Night	2 6% 3 8% 5% 4 5% 4 6% 8% 47%	2493
	21st to end	Night	3 7% 2 6% 4 4 4 3 7% 7% 52%	2076
March	1st to 10th	Night	4 7% 2 6% 3 3 3 3 5% 7% 55%	5494
	11th to 20th	Night	4 6% 2 6% 4 3 4 3 6% 9% 53%	5435
	21st to end	Night	10% 11% 3 7% 4 4 4 4 7% 8% 38%	5094
April	1st to 10th	Night	12% 7% 2 5% 4 3 4 4 6% 7% 46%	2100
	11th to 20th	Night	11% 6% 3 4 3 4 5% 4 5% 7% 49%	2077
	21st to end	Night	11% 7% 2 5% 3 3 5% 3 6% 8% 48%	1939
May	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 6% 8% 51%	5125
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 7% 50%	4701
	21st to end	Night	16% 7% 2 4 3 3 3 3 5% 7% 45%	5181
June	1st to 10th	Night	8% 5% 1 4 3 4 4 4 5% 9% 52%	1680
	11th to 20th	Night	10% 4 2 4 3 5% 4 4 8% 9% 48%	1680
	21st to end	Night	7% 3 1 3 3 4 4 4 5% 8% 58%	1680
July	1st to 10th	Night	5% 4 1 4 3 4 4 4 7% 9% 55%	4740
	11th to 20th	Night	5% 2 1 3 2 3 4 3 6% 9% 61%	4740
	21st to end	Night	9% 3 1 5% 3 4 5% 4 7% 9% 49%	5152
August	1st to 10th	Night	6% 2 1 4 3 3 4 3 6% 7% 60%	4680
	11th to 20th	Night	7% 4 1 4 3 4 4 4 6% 9% 54%	5535
	21st to end	Night	5% 3 2 5% 4 4 5% 4 5% 8% 55%	6465
September	1st to 10th	Night	5% 3 1 3 3 2 3 2 5% 6% 66%	2100
	11th to 20th	Night	8% 3 1 4 3 2 3 3 6% 8% 59%	2194
	21st to end	Night	11% 4 1 4 2 2 2 3 4 5% 62%	2310
October	1st to 10th	Night	7% 3 1 2 2 2 2 2 3 4 72%	3861
	11th to 20th	Night	7% 4 2 4 3 3 4 3 5% 8% 59%	4065
	21st to end	Night	8% 6% 2 4 3 3 3 3 6% 7% 54%	4715
November	1st to 10th	Night	5% 6% 2 6% 4 3 4 4 6% 8% 52%	2665
	11th to 20th	Night	4 5% 2 5% 4 3 4 4 6% 7% 57%	2730
	21st to end	Night	3 5% 2 5% 4 4 3 5% 6% 7% 55%	2930
December	1st to 10th	Night	2 4 2 5% 3 3 4 4 6% 8% 58%	4710
	11th to 20th	Night	2 4 2 6% 5% 5% 6% 5% 6% 9% 51%	5321
	21st to end	Night	5% 8% 3 6% 4 4 5% 3 6% 7% 49%	7128

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	31% 7% 19% 27% 15% 2	4723
	11th to 20th	Day	29% 8% 16% 25% 18% 4	4740
	21st to end	Day	25% 7% 15% 29% 21% 4	5188
February	1st to 10th	Day	37% 11% 15% 21% 14% 2	1872
	11th to 20th	Day	35% 9% 15% 22% 15% 4	2022
	21st to end	Day	32% 8% 16% 26% 15% 4	1730
March	1st to 10th	Day	27% 6% 11% 25% 26% 5%	4600
	11th to 20th	Day	24% 7% 16% 29% 22% 2	4764
	21st to end	Day	42% 11% 15% 21% 10%	6101
April	1st to 10th	Day	36% 8% 18% 23% 11% 3	2520
	11th to 20th	Day	37% 10% 14% 22% 14% 3	2494
	21st to end	Day	40% 10% 17% 23% 10% 1	2679
May	1st to 10th	Day	32% 8% 14% 27% 17% 1	7819
	11th to 20th	Day	33% 7% 14% 23% 18% 5%	8195
	21st to end	Day	35% 9% 14% 26% 15% 1	9027
June	1st to 10th	Day	28% 12% 20% 25% 12% 2	2940
	11th to 20th	Day	28% 10% 20% 24% 15% 4	2940
	21st to end	Day	21% 8% 16% 28% 21% 5%	2940
July	1st to 10th	Day	25% 8% 16% 29% 20% 2	8181
	11th to 20th	Day	22% 6% 12% 27% 19% 14%	8175
	21st to end	Day	24% 7% 16% 27% 20% 6%	8759
August	1st to 10th	Day	21% 6% 14% 21% 23% 13%	6446
	11th to 20th	Day	32% 9% 19% 23% 13% 3	5759
	21st to end	Day	17% 6% 19% 28% 21% 8%	6078
September	1st to 10th	Day	21% 4 11% 28% 27% 10%	2520
	11th to 20th	Day	24% 7% 13% 26% 24% 7%	2426
	21st to end	Day	25% 5% 10% 31% 25% 4	2310
October	1st to 10th	Day	18% 4 10% 19% 32% 17%	3540
	11th to 20th	Day	24% 4 11% 33% 23% 4	3268
	21st to end	Day	28% 6% 14% 32% 16% 5%	3262
November	1st to 10th	Day	23% 8% 17% 25% 21% 6%	1853
	11th to 20th	Day	24% 8% 14% 28% 21% 6%	1890
	21st to end	Day	26% 7% 16% 31% 16% 4	1688
December	1st to 10th	Day	26% 5% 19% 29% 19% 1	2700
	11th to 20th	Day	25% 7% 20% 31% 15% 2	3104
	21st to end	Day	33% 6% 10% 32% 18% 2	4092
January	1st to 10th	Night	38% 7% 12% 29% 12% 2	8202
	11th to 20th	Night	29% 7% 17% 24% 17% 5%	8250
	21st to end	Night	24% 8% 14% 27% 23% 4	9026
February	1st to 10th	Night	36% 8% 16% 23% 14% 3	2767
	11th to 20th	Night	32% 9% 14% 25% 15% 4	2493
	21st to end	Night	29% 7% 15% 26% 16% 6%	2076
March	1st to 10th	Night	27% 7% 10% 26% 24% 6%	5494
	11th to 20th	Night	27% 7% 18% 25% 23% 2	5435
	21st to end	Night	40% 10% 20% 19% 11% 1	5094
April	1st to 10th	Night	35% 10% 14% 25% 14% 3	2100
	11th to 20th	Night	33% 8% 15% 24% 17% 3	2077
	21st to end	Night	33% 10% 17% 25% 14% 1	1939
May	1st to 10th	Night	29% 9% 12% 27% 22% 1	5126
	11th to 20th	Night	32% 7% 14% 28% 16% 2	4702
	21st to end	Night	37% 7% 13% 26% 17% 1	5182
June	1st to 10th	Night	29% 10% 21% 24% 15% 2	1680
	11th to 20th	Night	31% 10% 21% 20% 16% 3	1680
	21st to end	Night	23% 9% 16% 27% 20% 5%	1680
July	1st to 10th	Night	25% 6% 18% 28% 22% 2	4700
	11th to 20th	Night	19% 5% 14% 27% 19% 15%	4692
	21st to end	Night	30% 7% 15% 28% 17% 4	5038
August	1st to 10th	Night	22% 7% 13% 21% 25% 12%	3700
	11th to 20th	Night	29% 9% 23% 21% 15% 3	4356
	21st to end	Night	21% 6% 19% 28% 21% 5%	5069
September	1st to 10th	Night	19% 5% 10% 30% 27% 9%	2100
	11th to 20th	Night	23% 7% 14% 24% 25% 7%	2194
	21st to end	Night	25% 5% 8% 29% 25% 8%	2310
October	1st to 10th	Night	18% 4 8% 22% 32% 16%	3861
	11th to 20th	Night	25% 4 10% 29% 25% 5%	4065
	21st to end	Night	29% 4 13% 28% 18% 8%	4715
November	1st to 10th	Night	28% 7% 16% 22% 21% 6%	2665
	11th to 20th	Night	26% 7% 14% 26% 22% 6%	2730
	21st to end	Night	26% 8% 15% 28% 18% 5%	2930
December	1st to 10th	Night	24% 6% 17% 32% 18% 3	4710
	11th to 20th	Night	28% 8% 18% 31% 13% 2	5322
	21st to end	Night	34% 6% 11% 32% 16% 2	7128

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	5% 13% 32% 30% 15% 5%	330
	11th to 20th	Day	12% 16% 35% 27% 7% 2	330
	21st to end	Day	7% 23% 43% 21% 5% 1	363
February	1st to 10th	Day	8% 19% 47% 21% 5%	330
	11th to 20th	Day	9% 16% 46% 22% 7%	330
	21st to end	Day	10% 23% 38% 23% 4 1	273
March	1st to 10th	Day	5% 20% 41% 22% 9% 2	330
	11th to 20th	Day	9% 19% 39% 24% 8% 2	360
	21st to end	Day	19% 19% 36% 19% 7% 1	594
April	1st to 10th	Day	17% 24% 36% 19% 5%	540
	11th to 20th	Day	19% 23% 37% 17% 3	532
	21st to end	Day	19% 17% 31% 24% 9% 1	699
May	1st to 10th	Day	14% 16% 32% 25% 11% 2	753
	11th to 20th	Day	20% 13% 27% 25% 13% 2	775
	21st to end	Day	16% 12% 35% 23% 11% 2	844
June	1st to 10th	Day	9% 9% 30% 31% 17% 3	780
	11th to 20th	Day	13% 12% 37% 28% 9% 1	780
	21st to end	Day	9% 9% 30% 35% 15% 1	780
July	1st to 10th	Day	5% 4 27% 41% 20% 3	771
	11th to 20th	Day	5% 4 29% 43% 17% 1	768
	21st to end	Day	5% 6% 24% 42% 22% 2	849
August	1st to 10th	Day	5% 5% 29% 42% 18% 2	776
	11th to 20th	Day	6% 5% 28% 40% 16% 4	641
	21st to end	Day	3 9% 29% 39% 16% 4	627
September	1st to 10th	Day	6% 8% 35% 33% 15% 4	570
	11th to 20th	Day	11% 9% 28% 34% 14% 4	476
	21st to end	Day	12% 11% 36% 23% 14% 4	360
October	1st to 10th	Day	14% 10% 31% 27% 14% 5%	360
	11th to 20th	Day	18% 18% 31% 20% 9% 4	360
	21st to end	Day	13% 20% 33% 22% 11% 2	387
November	1st to 10th	Day	9% 20% 34% 25% 9% 3	330
	11th to 20th	Day	12% 15% 36% 24% 12% 2	330
	21st to end	Day	8% 16% 33% 29% 12% 2	330
December	1st to 10th	Day	8% 19% 39% 25% 7% 2	330
	11th to 20th	Day	11% 16% 45% 24% 5%	330
	21st to end	Day	9% 20% 36% 27% 7% 2	363
January	1st to 10th	Night	6% 14% 38% 30% 11% 1	1050
	11th to 20th	Night	6% 22% 38% 26% 7% 1	1050
	21st to end	Night	5% 19% 41% 29% 5%	1155
February	1st to 10th	Night	6% 17% 46% 25% 5% 1	877
	11th to 20th	Night	6% 15% 37% 34% 7%	824
	21st to end	Night	8% 11% 42% 31% 7% 1	692
March	1st to 10th	Night	8% 11% 40% 34% 6% 1	840
	11th to 20th	Night	9% 10% 36% 37% 7% 1	821
	21st to end	Night	14% 14% 31% 34% 7% 1	693
April	1st to 10th	Night	14% 11% 36% 32% 7%	630
	11th to 20th	Night	15% 15% 37% 28% 5%	623
	21st to end	Night	16% 11% 28% 35% 9%	630
May	1st to 10th	Night	12% 9% 31% 33% 14% 1	630
	11th to 20th	Night	16% 10% 26% 35% 12%	621
	21st to end	Night	19% 7% 35% 29% 10%	683
June	1st to 10th	Night	9% 8% 30% 37% 14% 1	630
	11th to 20th	Night	12% 8% 35% 38% 7%	630
	21st to end	Night	7% 7% 30% 45% 11%	630
July	1st to 10th	Night	5% 5% 24% 45% 20% 1	615
	11th to 20th	Night	4 3 25% 54% 14% 1	612
	21st to end	Night	3 2 24% 55% 15%	690
August	1st to 10th	Night	4 4 24% 52% 15% 1	627
	11th to 20th	Night	5% 2 25% 47% 20% 1	624
	21st to end	Night	3 4 23% 49% 19% 1	693
September	1st to 10th	Night	7% 4 25% 44% 18% 1	630
	11th to 20th	Night	8% 6% 26% 43% 15% 1	630
	21st to end	Night	14% 7% 26% 38% 14% 1	630
October	1st to 10th	Night	13% 9% 26% 38% 13% 1	726
	11th to 20th	Night	12% 12% 31% 32% 12%	862
	21st to end	Night	11% 13% 34% 32% 9% 1	1141
November	1st to 10th	Night	8% 13% 38% 33% 6% 2	1025
	11th to 20th	Night	8% 16% 35% 31% 8% 1	1050
	21st to end	Night	6% 16% 39% 32% 7%	1050
December	1st to 10th	Night	6% 16% 43% 28% 6%	1050
	11th to 20th	Night	7% 12% 41% 33% 7%	1050
	21st to end	Night	10% 17% 39% 28% 7%	1155

Page generated at 2026-10-26 02:37:52. Version ac07

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