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 2024-03-23.

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	9% 5% 2 4 4 2 4 4 5% 7% 54%	78882
	Night	11% 6% 2 5% 4 3 4 4 5% 7% 50%	81043
Transparency	Day	30% 5% 9% 21% 23% 13%	78603
	Night	34% 5% 9% 19% 20% 13%	80844
Seeing	Day	10% 5% 18% 32% 26% 9%	14583
	Night	11% 3 9% 32% 34% 10%	24104

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	15% 7% 1 5% 5% 2 5% 4 5% 7% 44%	2741
	11th to 20th	Day	18% 7% 3 5% 6% 4 5% 4 5% 7% 36%	2860
	21st to end	Day	20% 8% 3 5% 5% 3 4 5% 6% 7% 33%	3146
February	1st to 10th	Day	19% 5% 3 4 4 2 3 4 4 6% 47%	1710
	11th to 20th	Day	13% 5% 2 5% 5% 4 5% 4 5% 6% 46%	1777
	21st to end	Day	14% 4 2 6% 5% 3 4 4 4 5% 49%	1570
March	1st to 10th	Day	18% 7% 2 5% 5% 3 5% 5% 6% 8% 36%	2044
	11th to 20th	Day	16% 7% 2 5% 4 3 5% 5% 5% 8% 40%	2090
	21st to end	Day	11% 5% 3 3 5% 3 5% 4 5% 6% 49%	2237
April	1st to 10th	Day	12% 5% 2 4 4 2 4 5% 5% 8% 48%	2160
	11th to 20th	Day	8% 5% 2 4 4 2 3 4 5% 8% 54%	2129
	21st to end	Day	6% 4 2 4 5% 2 4 4 5% 7% 56%	2160
May	1st to 10th	Day	5% 5% 2 5% 3 2 4 4 5% 9% 57%	2183
	11th to 20th	Day	5% 5% 2 4 3 2 3 3 4 9% 60%	2330
	21st to end	Day	6% 8% 3 5% 5% 3 4 4 5% 7% 51%	2723
June	1st to 10th	Day	4 6% 2 4 3 3 4 3 5% 10% 55%	2520
	11th to 20th	Day	3 4 2 3 3 2 3 4 5% 8% 64%	2520
	21st to end	Day	2 3 2 4 4 2 3 3 3 8% 65%	2520
July	1st to 10th	Day	2 4 2 4 3 2 4 3 5% 8% 63%	2504
	11th to 20th	Day	3 4 1 4 4 2 3 4 4 8% 62%	2340
	21st to end	Day	1 3 2 4 3 2 4 3 5% 8% 65%	2546
August	1st to 10th	Day	2 6% 2 5% 3 3 3 3 5% 9% 60%	2328
	11th to 20th	Day	1 4 2 4 3 1 3 4 3 9% 66%	2157
	21st to end	Day	1 5% 3 4 4 1 4 3 4 9% 62%	2376
September	1st to 10th	Day	2 3 1 3 3 2 3 3 3 5% 73%	2160
	11th to 20th	Day	3 5% 2 4 3 2 3 3 4 9% 62%	2160
	21st to end	Day	3 2 1 2 2 1 3 3 3 5% 74%	1917
October	1st to 10th	Day	3 2 1 2 3 2 4 3 4 7% 68%	1800
	11th to 20th	Day	5% 3 1 3 4 2 3 4 4 6% 64%	1800
	21st to end	Day	7% 5% 1 3 3 2 2 4 4 7% 62%	1980
November	1st to 10th	Day	8% 4 2 4 3 3 4 5% 5% 7% 55%	1836
	11th to 20th	Day	9% 4 2 3 4 3 4 5% 6% 7% 54%	1710
	21st to end	Day	11% 3 2 2 4 2 2 4 5% 7% 58%	1606
December	1st to 10th	Day	15% 6% 1 5% 3 3 4 5% 4 7% 48%	1520
	11th to 20th	Day	27% 8% 2 5% 4 2 3 4 5% 6% 34%	1928
	21st to end	Day	20% 7% 2 4 5% 3 4 5% 5% 7% 38%	2794
January	1st to 10th	Night	14% 6% 2 4 4 3 3 3 5% 6% 50%	4207
	11th to 20th	Night	20% 7% 2 4 4 4 5% 6% 6% 7% 36%	4110
	21st to end	Night	21% 7% 2 5% 5% 3 5% 5% 6% 7% 36%	4521
February	1st to 10th	Night	18% 6% 2 4 3 2 4 3 4 5% 50%	2470
	11th to 20th	Night	13% 6% 2 4 4 2 4 4 6% 7% 47%	2290
	21st to end	Night	13% 6% 2 3 3 2 3 3 5% 7% 52%	1884
March	1st to 10th	Night	18% 8% 3 6% 5% 2 4 5% 6% 5% 38%	2133
	11th to 20th	Night	16% 5% 2 4 4 3 4 3 5% 9% 44%	2090
	21st to end	Night	14% 6% 3 4 3 2 3 4 4 6% 50%	2163
April	1st to 10th	Night	13% 5% 2 5% 4 2 4 6% 5% 7% 48%	1800
	11th to 20th	Night	10% 5% 2 4 4 2 4 5% 4 6% 53%	1779
	21st to end	Night	6% 5% 1 3 4 2 4 4 4 9% 57%	1800
May	1st to 10th	Night	5% 6% 2 5% 4 3 4 3 5% 7% 56%	1774
	11th to 20th	Night	3 6% 3 4 5% 4 5% 4 5% 9% 52%	1560
	21st to end	Night	5% 10% 3 6% 5% 3 4 3 5% 9% 49%	1552
June	1st to 10th	Night	4 4 3 5% 5% 3 4 3 4 8% 56%	1440
	11th to 20th	Night	1 5% 2 7% 4 2 4 4 5% 8% 58%	1440
	21st to end	Night	2 5% 3 7% 6% 3 3 4 4 7% 56%	1440
July	1st to 10th	Night	1 5% 3 7% 6% 3 3 4 4 8% 57%	1450
	11th to 20th	Night	1 5% 4 7% 5% 3 3 5% 6% 8% 53%	1620
	21st to end	Night	1 8% 5% 10% 9% 4 6% 5% 6% 9% 38%	1773
August	1st to 10th	Night	2 9% 5% 11% 7% 4 5% 4 4 8% 41%	1611
	11th to 20th	Night	1 6% 3 7% 6% 2 4 3 6% 10% 52%	1742
	21st to end	Night	1 8% 6% 7% 6% 2 3 3 4 6% 53%	1980
September	1st to 10th	Night	1 3 2 3 4 3 4 4 4 7% 65%	1800
	11th to 20th	Night	4 5% 2 5% 4 3 4 2 4 6% 60%	1800
	21st to end	Night	3 5% 1 3 3 1 3 2 3 6% 70%	2037
October	1st to 10th	Night	5% 5% 2 5% 4 2 2 2 3 5% 65%	2160
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 5% 7% 59%	2160
	21st to end	Night	6% 4 2 4 4 2 2 3 4 7% 61%	2376
November	1st to 10th	Night	8% 4 2 3 4 2 3 4 4 7% 59%	2241
	11th to 20th	Night	10% 4 2 3 4 3 3 5% 5% 10% 53%	2470
	21st to end	Night	8% 4 2 4 4 3 4 4 4 5% 60%	2572
December	1st to 10th	Night	16% 5% 2 3 4 2 4 3 4 6% 51%	2660
	11th to 20th	Night	29% 7% 3 4 3 2 3 3 5% 6% 35%	3276
	21st to end	Night	20% 6% 2 5% 5% 3 4 4 5% 6% 40%	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	38% 6% 13% 28% 11% 3	2741
	11th to 20th	Day	46% 8% 11% 15% 11% 8%	2860
	21st to end	Day	49% 7% 8% 18% 13% 5%	3146
February	1st to 10th	Day	38% 5% 6% 21% 20% 10%	1710
	11th to 20th	Day	36% 6% 9% 20% 20% 9%	1786
	21st to end	Day	36% 7% 9% 24% 18% 7%	1570
March	1st to 10th	Day	43% 8% 8% 20% 16% 4	2044
	11th to 20th	Day	41% 6% 10% 20% 17% 5%	2090
	21st to end	Day	35% 8% 14% 22% 17% 4	2237
April	1st to 10th	Day	34% 6% 13% 21% 20% 5%	2160
	11th to 20th	Day	28% 6% 14% 27% 19% 7%	2132
	21st to end	Day	28% 5% 13% 28% 21% 5%	2160
May	1st to 10th	Day	25% 5% 11% 27% 27% 5%	2183
	11th to 20th	Day	23% 4 11% 32% 22% 8%	2333
	21st to end	Day	32% 6% 15% 24% 20% 4	2724
June	1st to 10th	Day	25% 7% 10% 27% 24% 7%	2520
	11th to 20th	Day	20% 3 9% 24% 30% 13%	2520
	21st to end	Day	22% 2 5% 18% 31% 22%	2520
July	1st to 10th	Day	21% 1 3 13% 29% 32%	2435
	11th to 20th	Day	24% 2 6% 15% 25% 28%	2271
	21st to end	Day	20% 2 3 17% 30% 28%	2405
August	1st to 10th	Day	25% 2 3 17% 30% 23%	2312
	11th to 20th	Day	20% 2 2 18% 30% 28%	2157
	21st to end	Day	24% 2 3 17% 31% 23%	2376
September	1st to 10th	Day	17% 1 2 12% 31% 36%	2160
	11th to 20th	Day	23% 3 6% 23% 30% 15%	2160
	21st to end	Day	15% 3 5% 23% 36% 18%	1917
October	1st to 10th	Day	17% 5% 8% 20% 27% 24%	1800
	11th to 20th	Day	22% 4 8% 22% 21% 23%	1800
	21st to end	Day	24% 4 8% 22% 22% 20%	1980
November	1st to 10th	Day	27% 6% 7% 24% 25% 11%	1836
	11th to 20th	Day	29% 5% 11% 21% 25% 9%	1710
	21st to end	Day	26% 4 10% 18% 29% 12%	1606
December	1st to 10th	Day	36% 5% 10% 20% 17% 12%	1520
	11th to 20th	Day	50% 7% 8% 15% 14% 6%	1928
	21st to end	Day	44% 5% 12% 22% 13% 4	2794
January	1st to 10th	Night	35% 6% 17% 27% 12% 4	4207
	11th to 20th	Night	46% 7% 14% 14% 10% 8%	4110
	21st to end	Night	48% 6% 11% 17% 12% 7%	4521
February	1st to 10th	Night	39% 3 7% 18% 19% 13%	2470
	11th to 20th	Night	36% 5% 10% 19% 22% 8%	2292
	21st to end	Night	31% 4 10% 30% 18% 6%	1884
March	1st to 10th	Night	46% 6% 11% 17% 15% 5%	2133
	11th to 20th	Night	38% 4 9% 22% 20% 7%	2090
	21st to end	Night	35% 5% 13% 23% 18% 6%	2163
April	1st to 10th	Night	34% 5% 14% 21% 20% 6%	1800
	11th to 20th	Night	32% 5% 11% 28% 17% 8%	1779
	21st to end	Night	25% 6% 12% 32% 20% 6%	1800
May	1st to 10th	Night	29% 3 9% 25% 24% 10%	1774
	11th to 20th	Night	29% 5% 12% 25% 22% 7%	1561
	21st to end	Night	34% 6% 13% 23% 21% 4	1552
June	1st to 10th	Night	28% 6% 9% 23% 24% 9%	1440
	11th to 20th	Night	26% 4 8% 15% 29% 18%	1440
	21st to end	Night	27% 5% 4 15% 27% 23%	1440
July	1st to 10th	Night	27% 3 1 10% 26% 32%	1410
	11th to 20th	Night	30% 3 5% 12% 23% 27%	1566
	21st to end	Night	41% 4 3 11% 20% 21%	1674
August	1st to 10th	Night	42% 4 3 12% 20% 20%	1602
	11th to 20th	Night	30% 3 2 12% 26% 27%	1742
	21st to end	Night	34% 3 3 11% 24% 24%	1980
September	1st to 10th	Night	21% 3 3 10% 31% 33%	1800
	11th to 20th	Night	28% 3 4 20% 30% 15%	1800
	21st to end	Night	19% 2 3 20% 34% 20%	2037
October	1st to 10th	Night	25% 3 7% 17% 22% 27%	2160
	11th to 20th	Night	28% 3 6% 20% 21% 22%	2160
	21st to end	Night	26% 3 5% 21% 22% 22%	2376
November	1st to 10th	Night	27% 4 8% 24% 23% 14%	2241
	11th to 20th	Night	29% 4 11% 22% 24% 10%	2470
	21st to end	Night	28% 4 8% 20% 25% 16%	2572
December	1st to 10th	Night	35% 4 9% 20% 18% 13%	2660
	11th to 20th	Night	50% 5% 10% 15% 16% 5%	3276
	21st to end	Night	44% 7% 13% 18% 13% 5%	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	27% 7% 15% 27% 18% 6%	304
	11th to 20th	Day	18% 4 21% 26% 26% 5%	370
	21st to end	Day	16% 3 21% 30% 23% 7%	407
February	1st to 10th	Day	20% 7% 23% 20% 20% 10%	370
	11th to 20th	Day	15% 13% 22% 29% 17% 5%	419
	21st to end	Day	12% 10% 26% 32% 17% 3	382
March	1st to 10th	Day	25% 10% 21% 27% 14% 2	460
	11th to 20th	Day	22% 7% 20% 26% 18% 6%	460
	21st to end	Day	16% 10% 27% 29% 15% 3	471
April	1st to 10th	Day	14% 9% 25% 32% 16% 4	420
	11th to 20th	Day	10% 14% 25% 32% 14% 5%	412
	21st to end	Day	6% 10% 27% 31% 20% 6%	420
May	1st to 10th	Day	8% 9% 24% 35% 17% 6%	423
	11th to 20th	Day	9% 8% 25% 31% 21% 6%	448
	21st to end	Day	11% 8% 20% 32% 21% 7%	485
June	1st to 10th	Day	6% 5% 20% 35% 24% 9%	450
	11th to 20th	Day	4 6% 22% 39% 25% 4	450
	21st to end	Day	3 2 18% 40% 29% 8%	450
July	1st to 10th	Day	5% 1 12% 39% 35% 9%	446
	11th to 20th	Day	3 1 6% 41% 38% 11%	444
	21st to end	Day	2 8% 40% 38% 13%	487
August	1st to 10th	Day	5% 12% 42% 34% 6%	447
	11th to 20th	Day	2 11% 41% 35% 11%	437
	21st to end	Day	4 1 8% 37% 38% 12%	495
September	1st to 10th	Day	2 2 11% 36% 33% 15%	450
	11th to 20th	Day	5% 2 18% 36% 28% 12%	450
	21st to end	Day	4 5% 12% 31% 38% 11%	450
October	1st to 10th	Day	4 4 17% 31% 30% 14%	450
	11th to 20th	Day	6% 3 18% 28% 32% 14%	450
	21st to end	Day	10% 4 16% 26% 30% 14%	495
November	1st to 10th	Day	11% 4 16% 24% 31% 14%	432
	11th to 20th	Day	9% 4 13% 34% 29% 11%	270
	21st to end	Day	9% 3 17% 33% 30% 8%	221
December	1st to 10th	Day	17% 8% 15% 23% 27% 10%	180
	11th to 20th	Day	17% 7% 17% 24% 25% 10%	180
	21st to end	Day	21% 7% 21% 23% 22% 7%	198
January	1st to 10th	Night	20% 4 11% 34% 25% 6%	760
	11th to 20th	Night	16% 3 12% 34% 27% 8%	760
	21st to end	Night	17% 4 9% 36% 29% 4	836
February	1st to 10th	Night	20% 6% 11% 31% 25% 6%	760
	11th to 20th	Night	16% 4 15% 34% 24% 6%	744
	21st to end	Night	16% 5% 15% 38% 23% 3	628
March	1st to 10th	Night	20% 6% 12% 35% 24% 3	760
	11th to 20th	Night	18% 3 13% 32% 27% 6%	760
	21st to end	Night	16% 4 16% 36% 25% 3	800
April	1st to 10th	Night	14% 4 13% 38% 27% 3	720
	11th to 20th	Night	12% 7% 15% 35% 25% 6%	711
	21st to end	Night	8% 4 18% 33% 28% 9%	720
May	1st to 10th	Night	8% 3 14% 36% 31% 7%	694
	11th to 20th	Night	5% 2 8% 34% 39% 12%	498
	21st to end	Night	9% 5% 12% 31% 35% 7%	389
June	1st to 10th	Night	6% 1 7% 39% 38% 9%	360
	11th to 20th	Night	3 1 9% 39% 42% 6%	360
	21st to end	Night	5% 3 33% 49% 10%	360
July	1st to 10th	Night	2 1 1 25% 58% 13%	360
	11th to 20th	Night	4 1 19% 53% 23%	522
	21st to end	Night	6% 2 19% 53% 20%	591
August	1st to 10th	Night	9% 1 2 27% 48% 13%	537
	11th to 20th	Night	4 1 2 30% 48% 15%	674
	21st to end	Night	5% 1 2 24% 50% 19%	792
September	1st to 10th	Night	3 4 25% 50% 19%	720
	11th to 20th	Night	6% 1 5% 30% 44% 15%	720
	21st to end	Night	5% 2 6% 25% 46% 16%	720
October	1st to 10th	Night	6% 2 6% 34% 37% 15%	720
	11th to 20th	Night	8% 2 6% 28% 42% 14%	720
	21st to end	Night	8% 1 7% 31% 34% 18%	792
November	1st to 10th	Night	10% 1 10% 31% 37% 12%	740
	11th to 20th	Night	12% 1 8% 35% 37% 7%	760
	21st to end	Night	9% 3 11% 40% 30% 8%	760
December	1st to 10th	Night	18% 3 10% 36% 27% 7%	760
	11th to 20th	Night	22% 3 11% 38% 22% 4	760
	21st to end	Night	20% 5% 15% 37% 19% 4	836

Page generated at 2024-03-19 20:31:05. Version ac07

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