Center Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Center Valley Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	14% 9% 3 7% 5% 4 5% 4 6% 7% 37%	127134
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 37%	125028
Transparency	Day	45% 5% 11% 23% 14% 2	130728
	Night	46% 4 10% 22% 15% 2	129506
Seeing	Day	16% 9% 26% 24% 17% 7%	17219
	Night	17% 9% 28% 30% 15% 2	27125

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	14% 15% 7% 13% 6% 4 3 3 4 5% 25%	3697
	11th to 20th	Day	12% 13% 7% 15% 7% 5% 6% 4 4 4 22%	3700
	21st to end	Day	20% 15% 7% 10% 5% 5% 4 4 3 3 24%	4272
February	1st to 10th	Day	18% 13% 6% 9% 6% 4 4 3 5% 5% 28%	2000
	11th to 20th	Day	14% 11% 4 10% 5% 6% 3 3 6% 6% 31%	1948
	21st to end	Day	16% 14% 4 8% 4 4 4 3 4 6% 33%	1650
March	1st to 10th	Day	19% 15% 5% 7% 4 3 3 3 3 4 35%	4480
	11th to 20th	Day	12% 10% 2 6% 4 3 4 3 5% 7% 43%	4783
	21st to end	Day	17% 11% 3 8% 5% 5% 5% 4 5% 7% 31%	5772
April	1st to 10th	Day	15% 11% 3 8% 4 4 4 3 6% 7% 36%	2400
	11th to 20th	Day	15% 9% 3 7% 4 4 5% 3 6% 6% 38%	2374
	21st to end	Day	19% 8% 3 7% 4 4 5% 3 5% 7% 35%	2548
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 8% 28%	5862
	11th to 20th	Day	19% 7% 3 7% 4 4 4 3 5% 6% 37%	5861
	21st to end	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 9% 33%	6617
June	1st to 10th	Day	11% 7% 2 6% 4 5% 4 4 5% 8% 43%	2800
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 5% 7% 9% 44%	2800
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 7% 10% 42%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	11% 7% 3 7% 5% 6% 7% 5% 7% 9% 34%	6757
August	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 7% 10% 48%	4234
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 5% 7% 12% 41%	3929
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 9% 54%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 46%	2382
	11th to 20th	Day	9% 5% 2 4 4 4 5% 4 6% 7% 50%	2200
	21st to end	Day	13% 5% 2 5% 4 4 3 4 5% 6% 49%	2200
October	1st to 10th	Day	11% 6% 2 5% 5% 4 4 3 5% 8% 49%	2200
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 44%	2041
	21st to end	Day	18% 9% 3 7% 3 4 5% 4 4 5% 37%	1980
November	1st to 10th	Day	9% 7% 2 6% 4 3 4 3 4 7% 51%	1852
	11th to 20th	Day	16% 10% 3 7% 4 3 5% 3 4 6% 38%	1890
	21st to end	Day	18% 12% 4 8% 5% 4 3 2 5% 5% 35%	1890
December	1st to 10th	Day	13% 11% 3 8% 5% 4 3 3 3 5% 42%	3020
	11th to 20th	Day	15% 15% 5% 11% 5% 4 4 4 5% 5% 25%	3093
	21st to end	Day	25% 19% 7% 9% 4 3 4 3 4 5% 16%	4070
January	1st to 10th	Night	16% 15% 7% 13% 6% 4 4 3 5% 5% 22%	6488
	11th to 20th	Night	12% 18% 7% 14% 7% 7% 6% 3 3 4 20%	6500
	21st to end	Night	19% 13% 5% 8% 5% 4 4 3 4 5% 29%	6924
February	1st to 10th	Night	19% 11% 5% 10% 5% 4 4 3 5% 5% 28%	2401
	11th to 20th	Night	14% 12% 4 10% 6% 4 4 3 4 5% 32%	2338
	21st to end	Night	18% 13% 4 7% 4 4 3 3 4 6% 33%	1980
March	1st to 10th	Night	19% 14% 4 6% 4 3 3 2 4 5% 37%	5373
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 46%	5175
	21st to end	Night	16% 9% 3 8% 5% 4 5% 4 5% 7% 34%	5201
April	1st to 10th	Night	13% 10% 4 7% 5% 4 4 4 5% 6% 39%	2000
	11th to 20th	Night	15% 8% 2 6% 6% 4 4 3 4 7% 41%	1975
	21st to end	Night	16% 7% 3 7% 4 4 4 5% 6% 6% 37%	1850
May	1st to 10th	Night	18% 11% 4 8% 5% 4 4 4 5% 7% 30%	4033
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 6% 9% 37%	4032
	21st to end	Night	13% 7% 3 8% 5% 4 5% 4 6% 9% 37%	4229
June	1st to 10th	Night	10% 5% 2 7% 4 3 4 4 4 8% 49%	1600
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 5% 8% 49%	1600
	21st to end	Night	6% 3 2 5% 5% 5% 6% 4 8% 9% 46%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 7% 9% 48%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	3617
	21st to end	Night	9% 6% 3 7% 4 5% 5% 4 7% 11% 39%	4094
August	1st to 10th	Night	8% 4 2 6% 4 4 4 3 5% 8% 51%	2915
	11th to 20th	Night	6% 5% 1 6% 4 4 6% 5% 6% 9% 48%	3104
	21st to end	Night	5% 3 2 5% 4 4 4 3 6% 8% 56%	3586
September	1st to 10th	Night	11% 6% 2 6% 3 4 4 4 6% 6% 50%	2000
	11th to 20th	Night	7% 4 1 5% 4 3 3 3 5% 7% 56%	2197
	21st to end	Night	12% 7% 2 5% 4 3 4 3 5% 6% 49%	2200
October	1st to 10th	Night	8% 4 1 5% 4 4 4 4 5% 6% 55%	2200
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	2353
	21st to end	Night	18% 8% 3 6% 4 4 3 3 4 7% 40%	2860
November	1st to 10th	Night	11% 7% 3 5% 4 3 3 3 5% 5% 50%	2666
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	2730
	21st to end	Night	20% 12% 4 7% 5% 4 3 4 4 5% 33%	2730
December	1st to 10th	Night	14% 10% 4 8% 5% 3 3 3 4 6% 39%	4380
	11th to 20th	Night	17% 12% 5% 11% 5% 4 4 3 4 5% 29%	5329
	21st to end	Night	27% 17% 6% 10% 5% 4 4 3 4 4 17%	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% 3 7% 20% 6% 1	3698
	11th to 20th	Day	64% 4 9% 11% 9% 2	3700
	21st to end	Day	66% 2 7% 17% 7% 2	4272
February	1st to 10th	Day	58% 5% 10% 16% 10% 1	2000
	11th to 20th	Day	54% 5% 11% 20% 10% 1	1958
	21st to end	Day	53% 4 10% 21% 10% 2	1650
March	1st to 10th	Day	55% 4 5% 20% 14% 3	4480
	11th to 20th	Day	41% 3 10% 32% 13% 1	4783
	21st to end	Day	53% 5% 13% 19% 9% 2	5774
April	1st to 10th	Day	48% 6% 12% 20% 14% 1	2400
	11th to 20th	Day	46% 6% 10% 24% 11% 3	2374
	21st to end	Day	48% 6% 10% 23% 10% 3	2548
May	1st to 10th	Day	52% 6% 12% 20% 9%	5897
	11th to 20th	Day	48% 5% 10% 26% 10% 1	5865
	21st to end	Day	44% 7% 13% 22% 13% 1	6631
June	1st to 10th	Day	38% 7% 11% 25% 16% 3	2800
	11th to 20th	Day	34% 9% 11% 23% 21% 3	2800
	21st to end	Day	36% 9% 15% 19% 17% 4	2800
July	1st to 10th	Day	37% 4 10% 29% 18% 2	6291
	11th to 20th	Day	40% 5% 17% 25% 13% 1	6285
	21st to end	Day	44% 5% 14% 22% 12% 2	6563
August	1st to 10th	Day	30% 6% 15% 33% 15% 1	4221
	11th to 20th	Day	35% 5% 13% 31% 15% 1	3967
	21st to end	Day	28% 4 14% 32% 18% 4	4311
September	1st to 10th	Day	35% 6% 11% 25% 18% 4	2382
	11th to 20th	Day	32% 5% 11% 26% 23% 3	2200
	21st to end	Day	36% 5% 11% 23% 24% 2	2200
October	1st to 10th	Day	23% 3 7% 32% 32% 2	3590
	11th to 20th	Day	32% 4 8% 27% 27% 2	3296
	21st to end	Day	41% 3 9% 22% 24% 1	3180
November	1st to 10th	Day	35% 4 12% 28% 18% 4	1852
	11th to 20th	Day	48% 4 8% 21% 14% 4	1890
	21st to end	Day	53% 4 8% 22% 12% 1	1890
December	1st to 10th	Day	47% 2 10% 27% 13%	3020
	11th to 20th	Day	61% 4 8% 19% 7%	3090
	21st to end	Day	72% 3 10% 10% 5%	4070
January	1st to 10th	Night	64% 4 8% 16% 8%	6488
	11th to 20th	Night	69% 3 7% 10% 8% 3	6500
	21st to end	Night	59% 3 8% 20% 9% 1	6924
February	1st to 10th	Night	58% 4 12% 17% 8% 1	2401
	11th to 20th	Night	54% 4 9% 18% 13% 1	2339
	21st to end	Night	53% 4 11% 19% 11% 2	1980
March	1st to 10th	Night	51% 3 6% 25% 12% 4	5373
	11th to 20th	Night	38% 5% 10% 27% 19% 1	5175
	21st to end	Night	49% 4 9% 28% 8% 1	5201
April	1st to 10th	Night	45% 6% 11% 23% 13% 2	2000
	11th to 20th	Night	44% 5% 9% 21% 17% 5%	1975
	21st to end	Night	45% 6% 10% 22% 14% 4	1850
May	1st to 10th	Night	53% 7% 11% 18% 10% 1	4060
	11th to 20th	Night	44% 5% 12% 27% 12%	4033
	21st to end	Night	43% 6% 15% 21% 13% 3	4237
June	1st to 10th	Night	34% 6% 10% 26% 21% 4	1600
	11th to 20th	Night	34% 6% 11% 24% 20% 5%	1600
	21st to end	Night	31% 8% 15% 23% 19% 4	1600
July	1st to 10th	Night	34% 5% 10% 29% 19% 3	3580
	11th to 20th	Night	34% 5% 16% 28% 16% 1	3572
	21st to end	Night	40% 4 16% 25% 13% 3	3976
August	1st to 10th	Night	31% 5% 14% 35% 14% 1	2904
	11th to 20th	Night	31% 6% 12% 31% 18% 1	3131
	21st to end	Night	26% 5% 13% 31% 22% 4	3586
September	1st to 10th	Night	34% 5% 12% 25% 20% 6%	2000
	11th to 20th	Night	27% 5% 9% 27% 27% 4	2197
	21st to end	Night	36% 6% 10% 21% 23% 4	2200
October	1st to 10th	Night	20% 4 6% 25% 42% 2	3590
	11th to 20th	Night	28% 3 7% 26% 34% 2	3837
	21st to end	Night	36% 2 8% 24% 25% 5%	4610
November	1st to 10th	Night	36% 4 9% 26% 22% 4	2666
	11th to 20th	Night	43% 4 10% 23% 17% 3	2730
	21st to end	Night	54% 4 9% 21% 11% 1	2730
December	1st to 10th	Night	48% 3 11% 26% 12%	4380
	11th to 20th	Night	58% 3 10% 18% 10% 1	5331
	21st to end	Night	72% 3 9% 11% 4	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	23% 15% 35% 20% 7% 1	330
	11th to 20th	Day	21% 24% 36% 16% 3 1	330
	21st to end	Day	22% 21% 33% 19% 6%	363
February	1st to 10th	Day	27% 12% 36% 20% 4	300
	11th to 20th	Day	24% 20% 34% 15% 6% 1	300
	21st to end	Day	24% 15% 39% 18% 4	249
March	1st to 10th	Day	22% 22% 34% 17% 4	300
	11th to 20th	Day	23% 17% 36% 17% 7%	407
	21st to end	Day	28% 15% 32% 18% 7%	550
April	1st to 10th	Day	19% 16% 32% 22% 10% 1	500
	11th to 20th	Day	19% 17% 26% 22% 12% 2	492
	21st to end	Day	24% 13% 33% 20% 8% 2	500
May	1st to 10th	Day	21% 12% 28% 25% 10% 3	503
	11th to 20th	Day	21% 7% 26% 24% 15% 6%	527
	21st to end	Day	12% 6% 26% 27% 19% 10%	653
June	1st to 10th	Day	12% 4 26% 32% 19% 6%	730
	11th to 20th	Day	12% 5% 23% 31% 21% 8%	730
	21st to end	Day	12% 5% 25% 28% 21% 9%	730
July	1st to 10th	Day	11% 2 15% 26% 26% 21%	721
	11th to 20th	Day	7% 3 18% 33% 25% 14%	718
	21st to end	Day	10% 1 16% 29% 31% 12%	727
August	1st to 10th	Day	10% 2 13% 24% 34% 19%	526
	11th to 20th	Day	13% 3 15% 27% 29% 13%	519
	21st to end	Day	8% 2 11% 27% 36% 16%	583
September	1st to 10th	Day	9% 4 15% 28% 32% 12%	530
	11th to 20th	Day	8% 2 13% 24% 34% 18%	530
	21st to end	Day	16% 7% 18% 22% 26% 11%	530
October	1st to 10th	Day	11% 5% 22% 29% 23% 9%	530
	11th to 20th	Day	11% 13% 34% 23% 16% 3	435
	21st to end	Day	25% 16% 34% 22% 4	363
November	1st to 10th	Day	13% 13% 36% 23% 12% 3	330
	11th to 20th	Day	23% 17% 28% 19% 11% 2	330
	21st to end	Day	24% 14% 35% 22% 5%	330
December	1st to 10th	Day	15% 21% 39% 20% 4 1	330
	11th to 20th	Day	22% 14% 38% 19% 5% 2	330
	21st to end	Day	31% 14% 32% 19% 4	363
January	1st to 10th	Night	21% 14% 33% 26% 4	1050
	11th to 20th	Night	23% 15% 38% 20% 3	1050
	21st to end	Night	20% 15% 38% 25% 3	1091
February	1st to 10th	Night	23% 12% 38% 24% 2	801
	11th to 20th	Night	20% 15% 38% 24% 3	783
	21st to end	Night	25% 12% 38% 22% 3	660
March	1st to 10th	Night	22% 14% 33% 26% 5%	800
	11th to 20th	Night	19% 13% 37% 25% 6% 1	800
	21st to end	Night	23% 11% 33% 27% 5% 1	769
April	1st to 10th	Night	16% 11% 35% 29% 8% 1	600
	11th to 20th	Night	18% 11% 31% 30% 10% 1	593
	21st to end	Night	20% 8% 37% 26% 10%	600
May	1st to 10th	Night	19% 7% 32% 32% 10%	600
	11th to 20th	Night	17% 6% 22% 34% 17% 3	591
	21st to end	Night	10% 4 23% 39% 22% 3	650
June	1st to 10th	Night	12% 3 21% 43% 20% 1	600
	11th to 20th	Night	11% 3 20% 43% 22% 1	600
	21st to end	Night	9% 1 21% 38% 30% 2	600
July	1st to 10th	Night	10% 1 10% 36% 36% 7%	585
	11th to 20th	Night	7% 2 15% 38% 35% 4	582
	21st to end	Night	9% 1 9% 45% 32% 4	657
August	1st to 10th	Night	8% 2 10% 38% 39% 3	597
	11th to 20th	Night	7% 1 11% 42% 35% 5%	594
	21st to end	Night	8% 2 10% 34% 41% 6%	660
September	1st to 10th	Night	13% 3 12% 36% 32% 5%	600
	11th to 20th	Night	9% 3 14% 33% 37% 4	797
	21st to end	Night	13% 6% 18% 35% 26% 3	800
October	1st to 10th	Night	9% 4 23% 36% 25% 4	800
	11th to 20th	Night	19% 9% 25% 30% 16% 1	800
	21st to end	Night	21% 10% 33% 28% 7%	880
November	1st to 10th	Night	15% 8% 33% 32% 11% 1	820
	11th to 20th	Night	17% 13% 33% 28% 8%	840
	21st to end	Night	24% 13% 27% 28% 8%	840
December	1st to 10th	Night	23% 10% 38% 23% 5%	840
	11th to 20th	Night	23% 11% 32% 26% 7% 1	1040
	21st to end	Night	32% 12% 31% 21% 4	1155

Page generated at 2026-01-29 01:21:41. Version ac07

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