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 2025-07-06.

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% 52%	123823
	Night	6% 5% 2 5% 4 4 4 3 6% 7% 53%	123856
Transparency	Day	28% 8% 15% 26% 19% 4	123568
	Night	28% 7% 14% 26% 19% 4	123702
Seeing	Day	10% 13% 33% 29% 12% 2	16835
	Night	8% 12% 34% 35% 10% 1	26825

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	1 5% 4 9% 5% 5% 5% 4 6% 9% 47%	3638
	11th to 20th	Day	1 5% 3 7% 6% 5% 5% 3 5% 7% 51%	3640
	21st to end	Day	1 3 2 6% 4 3 4 3 5% 6% 63%	4004
February	1st to 10th	Day	4 9% 4 8% 5% 5% 6% 4 6% 8% 42%	1783
	11th to 20th	Day	3 8% 4 8% 5% 5% 5% 4 6% 8% 45%	1914
	21st to end	Day	2 8% 4 6% 4 4 6% 4 7% 8% 48%	1650
March	1st to 10th	Day	4 6% 3 6% 4 4 4 3 5% 7% 55%	4500
	11th to 20th	Day	7% 6% 2 5% 3 2 3 3 5% 8% 57%	4662
	21st to end	Day	12% 12% 3 7% 4 4 4 4 6% 8% 37%	5969
April	1st to 10th	Day	11% 9% 3 5% 4 4 4 3 6% 8% 44%	2400
	11th to 20th	Day	11% 7% 2 7% 5% 4 5% 5% 6% 7% 41%	2371
	21st to end	Day	13% 9% 2 6% 4 4 4 4 6% 7% 41%	2551
May	1st to 10th	Day	9% 7% 2 6% 4 4 4 4 6% 7% 47%	6026
	11th to 20th	Day	14% 8% 2 5% 3 4 4 4 6% 8% 42%	6298
	21st to end	Day	18% 6% 2 5% 4 4 4 3 6% 7% 41%	6925
June	1st to 10th	Day	9% 4 2 4 4 4 4 5% 8% 10% 47%	2800
	11th to 20th	Day	9% 4 1 4 4 4 4 5% 8% 10% 47%	2800
	21st to end	Day	5% 3 1 3 3 3 4 4 6% 8% 59%	2800
July	1st to 10th	Day	4 3 2 5% 4 5% 5% 5% 8% 10% 51%	4501
	11th to 20th	Day	3 2 1 3 3 4 5% 5% 8% 10% 56%	4430
	21st to end	Day	2 2 1 3 3 4 5% 4 7% 9% 61%	4802
August	1st to 10th	Day	5% 3 2 6% 4 5% 5% 5% 7% 10% 49%	4411
	11th to 20th	Day	5% 3 1 4 4 4 5% 5% 8% 9% 50%	3889
	21st to end	Day	5% 2 1 3 2 3 3 3 5% 7% 65%	4180
September	1st to 10th	Day	6% 4 1 4 2 2 2 3 5% 7% 63%	2280
	11th to 20th	Day	7% 3 1 4 3 3 3 3 5% 6% 63%	2195
	21st to end	Day	9% 4 1 4 3 3 2 3 4 6% 62%	2090
October	1st to 10th	Day	6% 3 1 3 2 2 2 3 3 5% 71%	3330
	11th to 20th	Day	9% 4 1 2 2 3 3 3 4 6% 63%	3072
	21st to end	Day	6% 6% 2 4 3 3 4 3 6% 7% 54%	3091
November	1st to 10th	Day	5% 4 1 4 3 3 5% 3 6% 9% 56%	1763
	11th to 20th	Day	3 5% 2 5% 4 3 4 4 6% 8% 57%	1800
	21st to end	Day	3 6% 2 5% 4 3 4 4 6% 9% 54%	1608
December	1st to 10th	Day	1 4 3 6% 5% 4 4 3 6% 7% 57%	2620
	11th to 20th	Day	2 3 1 5% 5% 5% 6% 4 6% 8% 55%	3026
	21st to end	Day	4 8% 3 6% 4 4 4 4 5% 7% 51%	4004
January	1st to 10th	Night	3 9% 3 8% 5% 4 5% 4 5% 6% 47%	6327
	11th to 20th	Night	1 4 3 8% 6% 6% 6% 4 5% 7% 50%	6340
	21st to end	Night	1 3 2 5% 4 3 4 3 5% 7% 62%	6974
February	1st to 10th	Night	3 9% 4 9% 4 6% 4 4 6% 7% 44%	2635
	11th to 20th	Night	2 7% 3 8% 5% 5% 5% 5% 6% 8% 46%	2372
	21st to end	Night	3 7% 2 7% 4 4 4 4 7% 7% 51%	1980
March	1st to 10th	Night	5% 7% 2 6% 3 3 3 3 5% 7% 55%	5374
	11th to 20th	Night	4 6% 2 6% 4 3 4 3 6% 9% 53%	5318
	21st to end	Night	10% 11% 3 8% 4 4 4 4 7% 8% 37%	4984
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	12% 6% 3 4 3 4 5% 4 5% 7% 48%	1977
	21st to end	Night	11% 7% 2 5% 4 3 4 3 6% 7% 48%	1847
May	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 7% 51%	3951
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 7% 48%	3617
	21st to end	Night	19% 8% 2 5% 3 3 3 3 5% 6% 43%	3972
June	1st to 10th	Night	8% 5% 2 4 3 4 4 4 5% 9% 53%	1600
	11th to 20th	Night	10% 4 2 4 3 5% 4 4 8% 9% 48%	1600
	21st to end	Night	6% 3 1 3 3 4 4 4 5% 8% 59%	1600
July	1st to 10th	Night	5% 3 1 4 3 3 4 4 7% 8% 59%	2580
	11th to 20th	Night	2 2 1 3 3 4 5% 3 8% 10% 60%	2540
	21st to end	Night	3 2 1 3 3 3 4 4 6% 9% 61%	2761
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 8% 8% 50%	2528
	11th to 20th	Night	5% 4 1 4 3 4 4 4 6% 10% 55%	2946
	21st to end	Night	3 3 1 5% 4 3 4 4 4 6% 63%	3487
September	1st to 10th	Night	6% 3 1 3 3 2 3 2 5% 6% 66%	1900
	11th to 20th	Night	6% 3 1 4 3 2 3 3 6% 8% 62%	1985
	21st to end	Night	11% 4 1 3 2 2 2 2 4 5% 62%	2090
October	1st to 10th	Night	7% 3 1 3 2 2 2 2 3 4 72%	3631
	11th to 20th	Night	7% 4 2 4 3 3 4 3 5% 8% 60%	3823
	21st to end	Night	7% 6% 2 4 3 3 3 3 6% 7% 54%	4467
November	1st to 10th	Night	5% 6% 2 6% 4 3 4 4 7% 8% 52%	2535
	11th to 20th	Night	4 6% 2 5% 4 3 3 4 6% 6% 58%	2600
	21st to end	Night	3 5% 2 5% 4 4 4 4 6% 7% 56%	2790
December	1st to 10th	Night	2 4 2 5% 3 3 4 4 6% 8% 58%	4570
	11th to 20th	Night	2 4 2 6% 5% 5% 6% 5% 6% 9% 51%	5181
	21st to end	Night	5% 9% 3 6% 4 4 5% 3 6% 7% 48%	6974

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	33% 8% 18% 26% 14% 1	3639
	11th to 20th	Day	30% 9% 17% 23% 18% 4	3640
	21st to end	Day	20% 8% 16% 31% 21% 5%	4004
February	1st to 10th	Day	38% 11% 15% 20% 13% 3	1783
	11th to 20th	Day	36% 9% 16% 21% 14% 4	1924
	21st to end	Day	32% 8% 16% 25% 15% 4	1650
March	1st to 10th	Day	27% 7% 11% 25% 26% 4	4500
	11th to 20th	Day	24% 6% 16% 29% 22% 2	4662
	21st to end	Day	42% 11% 15% 21% 10%	5969
April	1st to 10th	Day	35% 9% 18% 24% 11% 3	2400
	11th to 20th	Day	37% 11% 15% 22% 13% 3	2374
	21st to end	Day	39% 10% 18% 24% 9%	2551
May	1st to 10th	Day	31% 10% 13% 26% 18% 2	6026
	11th to 20th	Day	36% 9% 16% 21% 13% 4	6302
	21st to end	Day	37% 10% 15% 23% 14% 1	6926
June	1st to 10th	Day	29% 12% 21% 25% 12% 2	2800
	11th to 20th	Day	28% 10% 20% 23% 15% 4	2800
	21st to end	Day	20% 8% 16% 29% 21% 5%	2800
July	1st to 10th	Day	24% 8% 14% 32% 21% 2	4432
	11th to 20th	Day	20% 7% 13% 29% 24% 7%	4355
	21st to end	Day	16% 7% 13% 27% 27% 10%	4651
August	1st to 10th	Day	27% 9% 18% 20% 18% 8%	4396
	11th to 20th	Day	26% 9% 17% 28% 16% 4	3928
	21st to end	Day	18% 5% 11% 26% 28% 12%	4179
September	1st to 10th	Day	21% 5% 11% 26% 27% 10%	2280
	11th to 20th	Day	22% 7% 12% 26% 25% 8%	2195
	21st to end	Day	24% 5% 10% 32% 25% 4	2090
October	1st to 10th	Day	18% 4 11% 18% 32% 18%	3330
	11th to 20th	Day	23% 4 11% 34% 23% 4	3072
	21st to end	Day	28% 6% 15% 32% 15% 4	3091
November	1st to 10th	Day	23% 8% 17% 24% 22% 6%	1763
	11th to 20th	Day	24% 8% 13% 27% 21% 6%	1800
	21st to end	Day	26% 7% 16% 31% 16% 4	1608
December	1st to 10th	Day	26% 5% 19% 29% 19% 1	2620
	11th to 20th	Day	25% 8% 20% 31% 15% 2	3024
	21st to end	Day	33% 6% 10% 32% 18% 2	4004
January	1st to 10th	Night	37% 9% 12% 27% 14% 2	6327
	11th to 20th	Night	32% 9% 15% 22% 17% 6%	6340
	21st to end	Night	20% 9% 14% 28% 24% 4	6974
February	1st to 10th	Night	37% 8% 16% 22% 14% 3	2635
	11th to 20th	Night	33% 9% 15% 24% 14% 4	2372
	21st to end	Night	30% 8% 15% 26% 16% 6%	1980
March	1st to 10th	Night	27% 7% 10% 26% 24% 6%	5374
	11th to 20th	Night	26% 7% 18% 24% 23% 2	5318
	21st to end	Night	40% 10% 20% 19% 10% 1	4984
April	1st to 10th	Night	35% 10% 14% 24% 14% 3	2000
	11th to 20th	Night	34% 8% 15% 24% 16% 3	1977
	21st to end	Night	33% 10% 17% 26% 13% 1	1847
May	1st to 10th	Night	31% 10% 11% 28% 19% 2	3951
	11th to 20th	Night	32% 9% 16% 29% 12% 2	3618
	21st to end	Night	40% 8% 12% 23% 16% 1	3972
June	1st to 10th	Night	29% 10% 21% 24% 15% 2	1600
	11th to 20th	Night	31% 10% 21% 20% 14% 3	1600
	21st to end	Night	22% 9% 16% 28% 20% 5%	1600
July	1st to 10th	Night	22% 6% 16% 29% 24% 3	2540
	11th to 20th	Night	18% 6% 16% 32% 22% 6%	2492
	21st to end	Night	18% 7% 14% 31% 24% 6%	2674
August	1st to 10th	Night	29% 9% 16% 20% 18% 8%	2520
	11th to 20th	Night	25% 8% 19% 25% 19% 4	2973
	21st to end	Night	21% 6% 12% 24% 29% 8%	3487
September	1st to 10th	Night	20% 5% 9% 28% 28% 9%	1900
	11th to 20th	Night	21% 7% 13% 26% 27% 7%	1985
	21st to end	Night	25% 5% 8% 29% 26% 8%	2090
October	1st to 10th	Night	18% 4 8% 20% 32% 17%	3631
	11th to 20th	Night	24% 5% 11% 30% 26% 5%	3823
	21st to end	Night	29% 5% 13% 28% 17% 9%	4467
November	1st to 10th	Night	28% 7% 16% 22% 21% 6%	2535
	11th to 20th	Night	25% 7% 14% 26% 23% 6%	2600
	21st to end	Night	26% 8% 16% 27% 19% 5%	2790
December	1st to 10th	Night	24% 6% 17% 32% 18% 3	4570
	11th to 20th	Night	28% 8% 18% 32% 13% 2	5182
	21st to end	Night	34% 6% 11% 32% 16% 2	6974

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	4 12% 32% 31% 16% 5%	300
	11th to 20th	Day	9% 16% 35% 29% 8% 3	300
	21st to end	Day	5% 24% 42% 22% 5% 1	330
February	1st to 10th	Day	9% 20% 46% 19% 5%	300
	11th to 20th	Day	9% 17% 47% 21% 6%	300
	21st to end	Day	10% 22% 39% 23% 4 1	249
March	1st to 10th	Day	4 21% 42% 22% 9% 2	300
	11th to 20th	Day	9% 18% 38% 26% 8% 2	329
	21st to end	Day	19% 19% 35% 19% 7% 1	550
April	1st to 10th	Day	16% 24% 36% 19% 5%	500
	11th to 20th	Day	19% 24% 37% 17% 3	492
	21st to end	Day	18% 16% 32% 25% 9% 1	651
May	1st to 10th	Day	14% 16% 31% 25% 12% 2	703
	11th to 20th	Day	20% 12% 26% 26% 14% 2	725
	21st to end	Day	17% 12% 36% 22% 11% 2	789
June	1st to 10th	Day	9% 8% 29% 33% 17% 3	730
	11th to 20th	Day	12% 10% 37% 29% 10% 1	730
	21st to end	Day	8% 9% 30% 36% 16% 1	730
July	1st to 10th	Day	5% 4 28% 40% 20% 3	697
	11th to 20th	Day	4 4 30% 44% 17% 1	668
	21st to end	Day	3 5% 24% 42% 23% 2	739
August	1st to 10th	Day	5% 4 29% 42% 18% 2	676
	11th to 20th	Day	5% 5% 28% 41% 16% 4	553
	21st to end	Day	3 9% 27% 39% 17% 4	539
September	1st to 10th	Day	6% 6% 34% 34% 16% 4	490
	11th to 20th	Day	9% 8% 26% 36% 16% 5%	405
	21st to end	Day	13% 13% 34% 23% 14% 4	300
October	1st to 10th	Day	16% 10% 31% 25% 13% 6%	300
	11th to 20th	Day	16% 17% 31% 21% 10% 5%	300
	21st to end	Day	12% 19% 34% 22% 12% 2	330
November	1st to 10th	Day	9% 20% 35% 24% 9% 3	300
	11th to 20th	Day	12% 16% 34% 24% 12% 1	300
	21st to end	Day	8% 16% 33% 29% 12% 1	300
December	1st to 10th	Day	8% 19% 38% 25% 8% 2	300
	11th to 20th	Day	9% 17% 45% 25% 5%	300
	21st to end	Day	8% 20% 35% 27% 8% 2	330
January	1st to 10th	Night	6% 14% 38% 30% 11% 1	1000
	11th to 20th	Night	5% 22% 38% 27% 7% 2	1000
	21st to end	Night	5% 19% 41% 29% 5%	1100
February	1st to 10th	Night	6% 17% 46% 25% 6% 1	835
	11th to 20th	Night	6% 16% 37% 34% 7%	784
	21st to end	Night	8% 11% 42% 32% 7% 1	660
March	1st to 10th	Night	8% 12% 40% 34% 6% 1	800
	11th to 20th	Night	8% 10% 35% 38% 7% 1	782
	21st to end	Night	14% 13% 30% 35% 7% 1	660
April	1st to 10th	Night	14% 11% 35% 33% 7%	600
	11th to 20th	Night	15% 15% 37% 28% 5%	593
	21st to end	Night	16% 11% 28% 35% 10% 1	600
May	1st to 10th	Night	12% 10% 31% 32% 14% 1	600
	11th to 20th	Night	16% 10% 26% 35% 12% 1	591
	21st to end	Night	19% 7% 35% 28% 10%	650
June	1st to 10th	Night	10% 8% 30% 37% 15% 1	600
	11th to 20th	Night	12% 8% 35% 38% 7%	600
	21st to end	Night	7% 7% 30% 45% 11%	600
July	1st to 10th	Night	5% 4 25% 44% 21% 1	570
	11th to 20th	Night	4 3 24% 54% 15% 1	552
	21st to end	Night	2 2 25% 55% 16%	624
August	1st to 10th	Night	5% 4 23% 53% 15% 1	567
	11th to 20th	Night	4 2 24% 48% 20% 1	564
	21st to end	Night	3 4 23% 49% 20% 1	627
September	1st to 10th	Night	7% 4 25% 44% 19% 1	570
	11th to 20th	Night	7% 6% 25% 44% 16% 2	570
	21st to end	Night	13% 8% 27% 38% 13% 1	570
October	1st to 10th	Night	14% 10% 26% 37% 12% 1	656
	11th to 20th	Night	11% 12% 31% 34% 12% 1	780
	21st to end	Night	11% 14% 33% 32% 9% 1	1045
November	1st to 10th	Night	7% 13% 38% 34% 6% 2	975
	11th to 20th	Night	8% 16% 36% 31% 8% 1	1000
	21st to end	Night	6% 17% 39% 32% 6%	1000
December	1st to 10th	Night	6% 17% 43% 28% 7%	1000
	11th to 20th	Night	6% 12% 40% 34% 8% 1	1000
	21st to end	Night	10% 17% 38% 28% 7%	1100

Page generated at 2025-07-02 23:30:15. Version ac07

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