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 2024-12-02.

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	13% 9% 3 8% 5% 4 5% 4 5% 7% 37%	101849
	Night	13% 10% 4 8% 5% 4 4 4 5% 6% 38%	97895
Transparency	Day	44% 6% 13% 24% 11% 1	105406
	Night	45% 6% 12% 24% 13% 1	102478
Seeing	Day	16% 10% 29% 26% 14% 4	16106
	Night	16% 9% 31% 32% 11% 1	26015

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	11% 19% 8% 13% 5% 4 5% 4 4 4 24%	2527
	11th to 20th	Day	7% 12% 4 14% 8% 9% 10% 7% 5% 4 18%	2530
	21st to end	Day	11% 18% 7% 14% 5% 4 4 3 5% 6% 22%	3007
February	1st to 10th	Day	8% 12% 4 10% 7% 5% 5% 5% 6% 6% 32%	1710
	11th to 20th	Day	9% 11% 4 9% 6% 6% 4 3 6% 7% 35%	1664
	21st to end	Day	12% 12% 4 10% 5% 4 4 4 6% 7% 32%	1542
March	1st to 10th	Day	13% 9% 3 6% 5% 3 3 3 4 6% 46%	3210
	11th to 20th	Day	10% 9% 3 9% 6% 5% 5% 4 5% 8% 39%	3480
	21st to end	Day	21% 12% 3 11% 5% 4 4 3 4 5% 26%	3965
April	1st to 10th	Day	17% 11% 4 7% 6% 4 3 4 5% 6% 33%	2280
	11th to 20th	Day	21% 10% 3 9% 5% 4 4 3 4 5% 31%	2254
	21st to end	Day	20% 11% 3 8% 5% 5% 4 4 5% 6% 29%	2384
May	1st to 10th	Day	18% 6% 2 6% 5% 4 4 3 5% 6% 41%	4397
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 5% 7% 41%	4399
	21st to end	Day	18% 8% 3 7% 5% 5% 4 3 5% 6% 36%	4824
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	2660
	11th to 20th	Day	12% 7% 3 8% 5% 6% 5% 4 6% 8% 36%	2660
	21st to end	Day	11% 5% 2 6% 5% 6% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 5% 8% 9% 37%	4440
	11th to 20th	Day	5% 4 1 4 3 4 4 4 7% 10% 55%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	4813
August	1st to 10th	Day	13% 6% 2 7% 4 4 5% 4 6% 8% 42%	4234
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 5% 8% 45%	4041
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 4 6% 8% 41%	4214
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 47%	2280
	11th to 20th	Day	13% 5% 2 6% 4 5% 4 4 5% 8% 45%	2280
	21st to end	Day	15% 7% 2 5% 3 4 5% 3 6% 6% 43%	2006
October	1st to 10th	Day	16% 7% 3 6% 4 3 4 3 6% 6% 43%	1900
	11th to 20th	Day	15% 11% 3 8% 5% 4 4 3 4 7% 35%	1900
	21st to end	Day	19% 11% 3 8% 4 3 3 3 4 5% 37%	1986
November	1st to 10th	Day	14% 10% 3 8% 5% 4 3 4 5% 6% 38%	1733
	11th to 20th	Day	13% 14% 5% 10% 6% 5% 4 3 6% 5% 28%	1600
	21st to end	Day	15% 12% 4 8% 5% 5% 4 3 5% 5% 35%	1600
December	1st to 10th	Day	10% 13% 6% 13% 8% 6% 6% 4 5% 5% 23%	1528
	11th to 20th	Day	11% 14% 7% 12% 7% 5% 5% 4 4 5% 26%	1918
	21st to end	Day	21% 18% 7% 14% 6% 4 3 2 3 4 18%	2783
January	1st to 10th	Night	10% 15% 8% 13% 6% 4 4 3 4 4 29%	4428
	11th to 20th	Night	8% 12% 5% 15% 7% 7% 8% 7% 6% 4 21%	4440
	21st to end	Night	12% 19% 8% 13% 6% 5% 4 3 4 5% 23%	4656
February	1st to 10th	Night	7% 12% 6% 10% 6% 5% 4 4 5% 5% 34%	2470
	11th to 20th	Night	7% 11% 3 9% 5% 5% 5% 3 5% 7% 38%	2403
	21st to end	Night	12% 10% 4 8% 6% 5% 5% 4 5% 6% 35%	1911
March	1st to 10th	Night	9% 7% 2 7% 4 4 4 3 6% 8% 45%	3673
	11th to 20th	Night	9% 8% 3 9% 6% 5% 5% 4 5% 7% 40%	3480
	21st to end	Night	24% 11% 4 12% 5% 3 3 2 3 4 28%	3664
April	1st to 10th	Night	18% 10% 3 7% 4 4 4 4 6% 6% 35%	1900
	11th to 20th	Night	23% 9% 3 6% 4 4 4 3 5% 6% 34%	1875
	21st to end	Night	20% 9% 3 5% 4 4 4 3 6% 6% 35%	1795
May	1st to 10th	Night	17% 8% 3 6% 4 5% 4 4 5% 7% 37%	2506
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2506
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 6% 7% 41%	2743
June	1st to 10th	Night	10% 6% 1 5% 4 3 5% 5% 7% 8% 45%	1520
	11th to 20th	Night	11% 6% 3 6% 3 4 5% 3 7% 8% 45%	1520
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 7% 8% 47%	1520
July	1st to 10th	Night	12% 4 2 5% 4 4 4 3 7% 8% 47%	2530
	11th to 20th	Night	5% 4 1 5% 4 3 4 4 5% 9% 57%	2530
	21st to end	Night	5% 4 2 5% 4 3 4 4 6% 8% 55%	2750
August	1st to 10th	Night	11% 5% 2 7% 5% 4 5% 4 5% 7% 46%	2702
	11th to 20th	Night	12% 5% 2 6% 4 3 3 4 5% 6% 50%	2798
	21st to end	Night	12% 7% 3 7% 4 4 4 3 4 6% 45%	3441
September	1st to 10th	Night	9% 5% 2 6% 3 4 4 3 5% 8% 51%	1900
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 8% 50%	1900
	21st to end	Night	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2151
October	1st to 10th	Night	13% 7% 2 6% 3 3 3 3 5% 6% 49%	2280
	11th to 20th	Night	16% 8% 2 7% 4 4 4 3 5% 7% 41%	2280
	21st to end	Night	18% 10% 3 6% 5% 4 3 3 5% 6% 36%	2612
November	1st to 10th	Night	14% 11% 3 7% 4 3 4 3 5% 7% 40%	2561
	11th to 20th	Night	14% 10% 4 10% 5% 5% 4 3 5% 6% 33%	2800
	21st to end	Night	14% 11% 4 9% 4 4 4 4 5% 6% 35%	2800
December	1st to 10th	Night	11% 13% 5% 12% 7% 4 4 3 5% 6% 30%	2687
	11th to 20th	Night	12% 15% 5% 11% 7% 5% 4 4 5% 6% 26%	3279
	21st to end	Night	24% 16% 5% 11% 5% 3 3 3 4 5% 21%	4884

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	62% 7% 11% 16% 4	2528
	11th to 20th	Day	65% 7% 8% 14% 6%	2530
	21st to end	Day	62% 5% 14% 14% 4	3007
February	1st to 10th	Day	50% 6% 14% 19% 10% 1	1710
	11th to 20th	Day	47% 5% 14% 24% 9% 1	1673
	21st to end	Day	50% 7% 12% 20% 10% 2	1542
March	1st to 10th	Day	41% 4 15% 23% 15% 2	3210
	11th to 20th	Day	42% 5% 11% 29% 11% 1	3480
	21st to end	Day	61% 5% 10% 19% 6% 1	3965
April	1st to 10th	Day	50% 7% 10% 20% 12% 1	2280
	11th to 20th	Day	54% 5% 10% 18% 12% 2	2254
	21st to end	Day	55% 6% 12% 18% 9% 1	2384
May	1st to 10th	Day	42% 6% 11% 27% 13% 2	4438
	11th to 20th	Day	40% 8% 12% 25% 14% 1	4403
	21st to end	Day	47% 7% 10% 22% 12% 2	4837
June	1st to 10th	Day	36% 7% 14% 29% 12% 2	2660
	11th to 20th	Day	44% 7% 13% 22% 13% 1	2660
	21st to end	Day	37% 8% 17% 27% 10% 1	2660
July	1st to 10th	Day	40% 9% 16% 24% 10% 1	4371
	11th to 20th	Day	23% 7% 17% 34% 17% 1	4365
	21st to end	Day	32% 9% 15% 26% 17% 1	4662
August	1st to 10th	Day	38% 7% 13% 26% 14% 1	4219
	11th to 20th	Day	36% 7% 13% 34% 10%	4080
	21st to end	Day	39% 7% 15% 27% 12% 1	4213
September	1st to 10th	Day	35% 6% 14% 30% 14%	2280
	11th to 20th	Day	37% 6% 17% 31% 8% 1	2280
	21st to end	Day	40% 6% 13% 31% 10%	2006
October	1st to 10th	Day	28% 3 9% 38% 20%	3160
	11th to 20th	Day	39% 5% 11% 26% 17% 2	3140
	21st to end	Day	43% 4 16% 27% 9%	3250
November	1st to 10th	Day	46% 6% 13% 20% 13% 2	1733
	11th to 20th	Day	57% 5% 12% 18% 7% 1	1600
	21st to end	Day	50% 5% 16% 21% 6% 1	1600
December	1st to 10th	Day	62% 7% 12% 15% 4	1528
	11th to 20th	Day	60% 6% 13% 14% 5% 1	1915
	21st to end	Day	72% 4 8% 12% 4	2783
January	1st to 10th	Night	58% 5% 11% 18% 6% 1	4428
	11th to 20th	Night	62% 7% 11% 13% 7%	4440
	21st to end	Night	64% 5% 9% 17% 5% 1	4656
February	1st to 10th	Night	50% 5% 12% 21% 10% 2	2470
	11th to 20th	Night	45% 6% 13% 26% 9% 1	2404
	21st to end	Night	48% 6% 12% 22% 10% 2	1911
March	1st to 10th	Night	37% 6% 14% 28% 13% 2	3673
	11th to 20th	Night	41% 7% 10% 27% 14% 2	3480
	21st to end	Night	61% 3 8% 19% 8% 1	3664
April	1st to 10th	Night	48% 7% 8% 19% 16% 2	1900
	11th to 20th	Night	50% 6% 8% 19% 15% 2	1875
	21st to end	Night	47% 6% 10% 21% 15% 1	1795
May	1st to 10th	Night	43% 8% 13% 16% 18% 1	2530
	11th to 20th	Night	33% 5% 8% 31% 19% 4	2507
	21st to end	Night	40% 8% 11% 23% 16% 1	2751
June	1st to 10th	Night	32% 9% 13% 28% 17% 1	1520
	11th to 20th	Night	34% 9% 14% 24% 19% 1	1520
	21st to end	Night	32% 8% 14% 32% 12% 3	1520
July	1st to 10th	Night	33% 7% 19% 26% 14% 1	2490
	11th to 20th	Night	23% 6% 15% 35% 19% 1	2482
	21st to end	Night	25% 8% 15% 29% 20% 3	2662
August	1st to 10th	Night	37% 6% 12% 28% 14% 3	2693
	11th to 20th	Night	33% 7% 14% 31% 13% 2	2825
	21st to end	Night	38% 7% 15% 25% 13% 2	3441
September	1st to 10th	Night	30% 7% 12% 33% 16% 1	1900
	11th to 20th	Night	31% 6% 14% 33% 15%	1900
	21st to end	Night	34% 5% 14% 31% 14% 1	2151
October	1st to 10th	Night	26% 3 7% 35% 25% 4	3800
	11th to 20th	Night	32% 6% 12% 27% 22% 1	3779
	21st to end	Night	47% 5% 10% 24% 13%	4298
November	1st to 10th	Night	44% 5% 12% 24% 14% 1	2561
	11th to 20th	Night	51% 6% 14% 17% 11% 1	2800
	21st to end	Night	49% 6% 16% 22% 7% 1	2800
December	1st to 10th	Night	55% 6% 14% 20% 5%	2687
	11th to 20th	Night	58% 6% 13% 17% 5% 1	3281
	21st to end	Night	67% 4 10% 14% 5%	4884

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% 13% 33% 25% 12% 3	270
	11th to 20th	Day	19% 10% 37% 26% 7% 1	270
	21st to end	Day	16% 6% 38% 29% 10% 1	297
February	1st to 10th	Day	14% 12% 41% 22% 8% 2	270
	11th to 20th	Day	16% 15% 39% 21% 9%	270
	21st to end	Day	17% 13% 44% 24% 3	225
March	1st to 10th	Day	17% 12% 38% 21% 11% 1	270
	11th to 20th	Day	19% 10% 35% 24% 9% 2	270
	21st to end	Day	23% 11% 32% 21% 9% 4	384
April	1st to 10th	Day	24% 16% 29% 24% 6%	460
	11th to 20th	Day	29% 12% 25% 24% 8% 1	452
	21st to end	Day	26% 14% 27% 20% 10% 2	564
May	1st to 10th	Day	20% 13% 30% 26% 9% 2	653
	11th to 20th	Day	18% 13% 26% 26% 12% 4	675
	21st to end	Day	18% 11% 26% 24% 17% 5%	734
June	1st to 10th	Day	11% 7% 28% 32% 19% 4	680
	11th to 20th	Day	13% 10% 35% 29% 11% 3	680
	21st to end	Day	14% 6% 30% 28% 16% 6%	680
July	1st to 10th	Day	9% 5% 20% 28% 28% 11%	671
	11th to 20th	Day	9% 6% 26% 29% 21% 9%	668
	21st to end	Day	6% 4 27% 33% 23% 8%	739
August	1st to 10th	Day	10% 5% 22% 32% 24% 7%	676
	11th to 20th	Day	12% 4 19% 30% 26% 7%	665
	21st to end	Day	15% 5% 23% 29% 23% 5%	560
September	1st to 10th	Day	12% 7% 26% 29% 20% 5%	490
	11th to 20th	Day	12% 5% 27% 32% 20% 4	490
	21st to end	Day	19% 11% 25% 26% 15% 4	373
October	1st to 10th	Day	18% 10% 30% 25% 14% 3	300
	11th to 20th	Day	20% 19% 30% 24% 6% 1	300
	21st to end	Day	25% 15% 36% 16% 6% 2	330
November	1st to 10th	Day	21% 12% 33% 23% 10% 1	300
	11th to 20th	Day	20% 18% 36% 20% 6%	300
	21st to end	Day	24% 12% 41% 16% 7%	300
December	1st to 10th	Day	17% 11% 38% 24% 9% 1	273
	11th to 20th	Day	20% 13% 37% 22% 7% 1	270
	21st to end	Day	27% 11% 38% 20% 3 1	297
January	1st to 10th	Night	13% 12% 34% 31% 9% 1	950
	11th to 20th	Night	17% 9% 35% 31% 6% 1	950
	21st to end	Night	18% 8% 33% 32% 7% 1	1045
February	1st to 10th	Night	12% 11% 38% 31% 7% 1	950
	11th to 20th	Night	13% 11% 41% 27% 7% 1	929
	21st to end	Night	17% 13% 33% 30% 7%	655
March	1st to 10th	Night	15% 9% 37% 31% 6% 1	726
	11th to 20th	Night	16% 9% 36% 31% 7%	570
	21st to end	Night	21% 9% 30% 27% 11% 2	627
April	1st to 10th	Night	22% 13% 34% 25% 5% 1	570
	11th to 20th	Night	27% 9% 29% 28% 6%	563
	21st to end	Night	23% 9% 32% 26% 8% 1	570
May	1st to 10th	Night	22% 9% 32% 31% 5% 1	570
	11th to 20th	Night	19% 7% 31% 31% 11% 1	561
	21st to end	Night	14% 6% 26% 39% 14%	617
June	1st to 10th	Night	11% 5% 31% 39% 14% 1	570
	11th to 20th	Night	13% 4 32% 41% 10% 1	570
	21st to end	Night	9% 5% 25% 44% 16% 1	570
July	1st to 10th	Night	8% 2 21% 39% 26% 3	555
	11th to 20th	Night	7% 4 23% 41% 22% 3	552
	21st to end	Night	5% 3 20% 46% 23% 3	624
August	1st to 10th	Night	7% 3 17% 46% 25% 2	567
	11th to 20th	Night	12% 2 20% 36% 27% 3	564
	21st to end	Night	14% 4 22% 35% 24% 2	627
September	1st to 10th	Night	11% 4 24% 38% 18% 4	570
	11th to 20th	Night	11% 4 28% 36% 19% 2	570
	21st to end	Night	14% 9% 29% 32% 15% 1	707
October	1st to 10th	Night	17% 9% 27% 32% 15% 1	760
	11th to 20th	Night	20% 10% 33% 29% 7%	760
	21st to end	Night	22% 10% 30% 28% 8%	836
November	1st to 10th	Night	17% 10% 34% 30% 8%	805
	11th to 20th	Night	18% 13% 36% 26% 7%	1000
	21st to end	Night	19% 12% 36% 26% 7%	1000
December	1st to 10th	Night	17% 10% 36% 29% 8% 1	960
	11th to 20th	Night	21% 10% 35% 25% 8% 1	950
	21st to end	Night	26% 10% 34% 25% 5%	1045

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