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 2024-01-14.

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% 8% 4 6% 5% 3 5% 5% 5% 7% 38%	74863
	Night	15% 9% 4 6% 5% 3 4 4 5% 7% 39%	74914
Transparency	Day	45% 6% 11% 22% 13% 2	74569
	Night	45% 6% 11% 21% 15% 3	74729
Seeing	Day	16% 9% 25% 25% 18% 7%	14420
	Night	18% 8% 27% 30% 15% 2	24359

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	16% 13% 6% 9% 6% 4 4 4 4 6% 28%	1517
	11th to 20th	Day	19% 13% 6% 10% 7% 2 5% 3 4 5% 25%	1464
	21st to end	Day	13% 15% 8% 7% 7% 4 5% 4 5% 5% 27%	1661
February	1st to 10th	Day	19% 12% 7% 8% 6% 3 4 4 5% 5% 28%	1800
	11th to 20th	Day	15% 11% 5% 10% 6% 5% 4 4 6% 6% 30%	1748
	21st to end	Day	17% 14% 5% 8% 5% 3 4 4 3 6% 32%	1480
March	1st to 10th	Day	16% 12% 4 6% 6% 3 5% 4 4 6% 34%	1800
	11th to 20th	Day	18% 9% 4 7% 6% 2 4 3 4 7% 35%	1895
	21st to end	Day	20% 12% 5% 8% 5% 3 4 4 5% 6% 27%	2293
April	1st to 10th	Day	14% 9% 4 6% 6% 3 5% 3 6% 7% 37%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 5% 4 5% 6% 38%	2134
	21st to end	Day	21% 7% 3 6% 5% 3 5% 3 5% 7% 35%	2292
May	1st to 10th	Day	19% 8% 3 7% 6% 3 4 5% 5% 6% 34%	2340
	11th to 20th	Day	19% 8% 3 6% 5% 4 4 4 5% 6% 37%	2301
	21st to end	Day	9% 6% 3 6% 6% 4 7% 7% 7% 9% 37%	2603
June	1st to 10th	Day	11% 6% 3 5% 5% 4 4 5% 5% 8% 44%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 6% 7% 8% 45%	2520
	21st to end	Day	10% 4 3 6% 5% 4 6% 6% 6% 10% 41%	2520
July	1st to 10th	Day	11% 4 2 5% 6% 4 5% 6% 7% 10% 40%	2520
	11th to 20th	Day	8% 4 2 4 5% 3 5% 5% 7% 10% 46%	2520
	21st to end	Day	8% 4 2 5% 5% 4 6% 7% 8% 9% 43%	2684
August	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 7% 10% 44%	2327
	11th to 20th	Day	10% 4 2 6% 6% 3 6% 7% 7% 11% 39%	2176
	21st to end	Day	6% 4 2 4 5% 3 5% 5% 6% 9% 49%	2376
September	1st to 10th	Day	12% 6% 2 5% 5% 3 6% 5% 6% 8% 43%	2144
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 5% 6% 8% 47%	1980
	21st to end	Day	12% 4 2 5% 4 3 4 4 5% 6% 51%	1980
October	1st to 10th	Day	12% 6% 2 4 6% 3 4 4 5% 8% 47%	1980
	11th to 20th	Day	13% 6% 3 6% 5% 3 4 6% 5% 7% 43%	1838
	21st to end	Day	19% 9% 4 6% 5% 4 4 5% 4 5% 36%	1782
November	1st to 10th	Day	9% 6% 2 6% 4 2 4 4 4 7% 52%	1672
	11th to 20th	Day	16% 9% 4 6% 5% 2 4 4 4 6% 39%	1710
	21st to end	Day	18% 10% 4 7% 6% 3 4 3 5% 5% 36%	1710
December	1st to 10th	Day	18% 14% 5% 8% 6% 3 4 3 3 5% 31%	1710
	11th to 20th	Day	15% 15% 6% 10% 5% 2 3 4 4 6% 29%	1923
	21st to end	Day	21% 21% 9% 9% 5% 3 4 3 4 5% 17%	2783
January	1st to 10th	Night	17% 11% 5% 8% 6% 4 4 4 4 6% 29%	2648
	11th to 20th	Night	18% 14% 6% 9% 7% 4 4 4 3 5% 27%	2575
	21st to end	Night	15% 14% 5% 10% 7% 3 4 4 5% 5% 28%	2691
February	1st to 10th	Night	21% 10% 5% 9% 7% 3 4 4 5% 5% 28%	2160
	11th to 20th	Night	14% 11% 5% 9% 6% 3 4 4 5% 6% 32%	2098
	21st to end	Night	19% 13% 5% 7% 5% 2 5% 4 4 5% 31%	1776
March	1st to 10th	Night	16% 10% 4 6% 6% 3 5% 4 5% 6% 36%	2160
	11th to 20th	Night	17% 8% 3 6% 6% 3 5% 4 5% 6% 38%	2063
	21st to end	Night	22% 8% 3 7% 5% 3 4 4 5% 6% 33%	2079
April	1st to 10th	Night	12% 8% 4 6% 6% 4 4 4 5% 7% 40%	1800
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 4 4 7% 41%	1775
	21st to end	Night	17% 8% 3 6% 5% 3 5% 5% 5% 6% 38%	1666
May	1st to 10th	Night	18% 8% 4 6% 6% 3 4 5% 6% 7% 33%	1620
	11th to 20th	Night	16% 7% 3 6% 5% 3 3 4 6% 8% 40%	1592
	21st to end	Night	8% 5% 2 6% 6% 3 5% 6% 7% 10% 41%	1669
June	1st to 10th	Night	9% 5% 2 6% 5% 2 4 5% 5% 8% 49%	1440
	11th to 20th	Night	8% 5% 3 5% 4 3 4 5% 5% 8% 50%	1440
	21st to end	Night	7% 3 2 4 6% 3 7% 5% 8% 10% 46%	1440
July	1st to 10th	Night	8% 5% 2 4 5% 3 5% 4 7% 10% 48%	1440
	11th to 20th	Night	7% 3 2 5% 3 2 4 4 6% 10% 54%	1440
	21st to end	Night	8% 4 3 4 5% 2 3 3 5% 9% 55%	1633
August	1st to 10th	Night	7% 3 2 5% 5% 2 6% 6% 6% 8% 51%	1611
	11th to 20th	Night	7% 4 1 3 5% 3 7% 6% 6% 9% 49%	1722
	21st to end	Night	7% 3 2 4 4 2 4 4 6% 9% 54%	1980
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 6% 6% 48%	1800
	11th to 20th	Night	8% 4 2 4 4 2 4 4 5% 8% 54%	1977
	21st to end	Night	11% 6% 2 4 4 2 4 4 5% 7% 51%	1980
October	1st to 10th	Night	8% 4 2 4 5% 3 4 6% 4 6% 54%	1980
	11th to 20th	Night	17% 6% 2 4 4 3 3 3 5% 6% 47%	2116
	21st to end	Night	19% 8% 3 5% 5% 3 4 3 4 6% 40%	2574
November	1st to 10th	Night	11% 7% 3 4 4 2 3 4 5% 6% 50%	2406
	11th to 20th	Night	14% 7% 3 7% 5% 3 4 4 5% 7% 41%	2470
	21st to end	Night	19% 10% 5% 6% 5% 3 4 4 4 5% 34%	2470
December	1st to 10th	Night	17% 11% 6% 8% 6% 2 4 4 5% 7% 30%	2470
	11th to 20th	Night	17% 12% 5% 9% 5% 2 4 4 4 6% 33%	3269
	21st to end	Night	24% 18% 6% 8% 6% 3 4 4 4 4 18%	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	58% 4 8% 17% 11% 1	1518
	11th to 20th	Day	62% 5% 10% 16% 7%	1464
	21st to end	Day	59% 4 10% 19% 5% 2	1661
February	1st to 10th	Day	59% 5% 10% 16% 9% 1	1800
	11th to 20th	Day	55% 5% 11% 19% 9% 1	1758
	21st to end	Day	56% 4 10% 20% 9% 2	1480
March	1st to 10th	Day	52% 5% 10% 22% 11%	1800
	11th to 20th	Day	50% 5% 13% 19% 11% 2	1895
	21st to end	Day	58% 5% 10% 19% 6% 1	2293
April	1st to 10th	Day	46% 6% 12% 21% 14% 1	2160
	11th to 20th	Day	46% 6% 10% 23% 11% 3	2134
	21st to end	Day	50% 6% 11% 20% 11% 3	2292
May	1st to 10th	Day	51% 6% 11% 22% 9% 1	2340
	11th to 20th	Day	48% 7% 11% 18% 14% 1	2305
	21st to end	Day	41% 11% 13% 19% 14% 2	2603
June	1st to 10th	Day	38% 7% 12% 25% 15% 3	2520
	11th to 20th	Day	33% 9% 12% 23% 20% 3	2520
	21st to end	Day	37% 9% 15% 20% 16% 4	2520
July	1st to 10th	Day	38% 9% 13% 22% 14% 3	2451
	11th to 20th	Day	31% 9% 16% 27% 17% 1	2445
	21st to end	Day	34% 10% 15% 23% 15% 3	2535
August	1st to 10th	Day	32% 9% 14% 29% 14% 2	2314
	11th to 20th	Day	37% 9% 13% 25% 15% 1	2176
	21st to end	Day	30% 7% 14% 24% 18% 7%	2376
September	1st to 10th	Day	38% 7% 12% 24% 16% 4	2144
	11th to 20th	Day	34% 6% 11% 25% 21% 3	1980
	21st to end	Day	33% 5% 11% 25% 23% 2	1980
October	1st to 10th	Day	36% 4 11% 24% 20% 3	1980
	11th to 20th	Day	40% 6% 9% 24% 20% 2	1838
	21st to end	Day	50% 5% 10% 21% 13% 1	1782
November	1st to 10th	Day	34% 4 12% 27% 18% 5%	1672
	11th to 20th	Day	47% 5% 8% 21% 15% 4	1710
	21st to end	Day	51% 4 9% 22% 13% 1	1710
December	1st to 10th	Day	58% 4 10% 18% 10%	1710
	11th to 20th	Day	58% 5% 6% 22% 8% 1	1920
	21st to end	Day	70% 5% 9% 10% 5%	2783
January	1st to 10th	Night	56% 5% 8% 18% 12% 1	2648
	11th to 20th	Night	62% 4 11% 16% 7% 1	2575
	21st to end	Night	58% 5% 11% 17% 9% 1	2691
February	1st to 10th	Night	60% 4 12% 16% 7% 1	2160
	11th to 20th	Night	53% 4 10% 19% 12% 2	2099
	21st to end	Night	55% 4 11% 17% 10% 2	1776
March	1st to 10th	Night	51% 4 9% 21% 13% 2	2160
	11th to 20th	Night	46% 7% 12% 21% 12% 3	2063
	21st to end	Night	52% 5% 10% 22% 8% 2	2079
April	1st to 10th	Night	43% 6% 11% 25% 13% 2	1800
	11th to 20th	Night	44% 5% 10% 19% 17% 5%	1775
	21st to end	Night	47% 6% 10% 20% 14% 4	1666
May	1st to 10th	Night	50% 7% 10% 20% 11% 2	1620
	11th to 20th	Night	42% 8% 13% 22% 14% 1	1593
	21st to end	Night	36% 9% 16% 20% 15% 5%	1670
June	1st to 10th	Night	33% 6% 11% 25% 21% 3	1440
	11th to 20th	Night	32% 7% 12% 25% 20% 5%	1440
	21st to end	Night	31% 8% 15% 23% 19% 4	1440
July	1st to 10th	Night	31% 9% 15% 23% 18% 5%	1400
	11th to 20th	Night	26% 7% 15% 30% 20% 3	1392
	21st to end	Night	27% 5% 15% 30% 19% 4	1542
August	1st to 10th	Night	29% 9% 13% 29% 19% 2	1600
	11th to 20th	Night	28% 10% 13% 28% 19% 2	1722
	21st to end	Night	26% 7% 14% 25% 21% 7%	1980
September	1st to 10th	Night	35% 5% 13% 23% 18% 6%	1800
	11th to 20th	Night	28% 5% 10% 28% 24% 5%	1977
	21st to end	Night	33% 6% 10% 23% 24% 4	1980
October	1st to 10th	Night	30% 8% 11% 23% 25% 4	1980
	11th to 20th	Night	39% 4 8% 25% 21% 3	2116
	21st to end	Night	46% 4 10% 22% 16% 2	2574
November	1st to 10th	Night	35% 5% 9% 25% 21% 4	2406
	11th to 20th	Night	43% 4 10% 22% 17% 3	2470
	21st to end	Night	53% 5% 9% 22% 12% 1	2470
December	1st to 10th	Night	54% 5% 10% 19% 11% 1	2470
	11th to 20th	Night	53% 4 9% 20% 13% 1	3271
	21st to end	Night	70% 5% 9% 10% 6%	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	24% 14% 32% 22% 7% 1	270
	11th to 20th	Day	24% 16% 37% 18% 4 1	249
	21st to end	Day	21% 19% 34% 21% 5%	264
February	1st to 10th	Day	31% 12% 35% 18% 4	240
	11th to 20th	Day	25% 20% 33% 16% 6%	240
	21st to end	Day	26% 14% 40% 17% 3	198
March	1st to 10th	Day	23% 22% 34% 18% 4	240
	11th to 20th	Day	24% 14% 35% 19% 7% 1	335
	21st to end	Day	29% 13% 32% 17% 9%	462
April	1st to 10th	Day	17% 15% 33% 22% 12% 1	420
	11th to 20th	Day	19% 17% 25% 22% 14% 3	412
	21st to end	Day	26% 12% 32% 19% 8% 3	420
May	1st to 10th	Day	22% 12% 28% 26% 9% 3	423
	11th to 20th	Day	22% 7% 28% 24% 15% 5%	447
	21st to end	Day	12% 4 23% 28% 21% 11%	557
June	1st to 10th	Day	12% 4 25% 33% 20% 7%	630
	11th to 20th	Day	11% 5% 24% 30% 21% 9%	630
	21st to end	Day	12% 6% 25% 28% 20% 9%	630
July	1st to 10th	Day	11% 1 15% 26% 24% 22%	621
	11th to 20th	Day	7% 3 16% 34% 25% 14%	618
	21st to end	Day	10% 1 16% 28% 32% 13%	625
August	1st to 10th	Day	10% 2 11% 25% 33% 20%	446
	11th to 20th	Day	14% 2 14% 28% 30% 13%	439
	21st to end	Day	8% 2 11% 27% 35% 17%	495
September	1st to 10th	Day	10% 3 15% 27% 31% 13%	450
	11th to 20th	Day	9% 3 14% 24% 33% 16%	450
	21st to end	Day	14% 8% 19% 22% 26% 12%	450
October	1st to 10th	Day	12% 5% 24% 31% 19% 8%	450
	11th to 20th	Day	13% 13% 37% 22% 13% 1	365
	21st to end	Day	27% 14% 34% 23% 2	297
November	1st to 10th	Day	13% 12% 33% 24% 14% 4	270
	11th to 20th	Day	21% 16% 29% 20% 12% 2	270
	21st to end	Day	22% 14% 34% 24% 6%	270
December	1st to 10th	Day	15% 22% 35% 21% 5% 1	270
	11th to 20th	Day	22% 13% 36% 20% 6% 2	270
	21st to end	Day	30% 15% 31% 19% 4	297
January	1st to 10th	Night	21% 15% 33% 26% 5%	950
	11th to 20th	Night	25% 13% 37% 21% 4	920
	21st to end	Night	19% 15% 36% 26% 4	936
February	1st to 10th	Night	25% 12% 37% 24% 3	720
	11th to 20th	Night	20% 15% 37% 25% 3	703
	21st to end	Night	27% 12% 38% 21% 3	592
March	1st to 10th	Night	22% 14% 34% 26% 5%	720
	11th to 20th	Night	19% 12% 36% 25% 6% 1	720
	21st to end	Night	24% 9% 33% 28% 5% 1	693
April	1st to 10th	Night	14% 11% 36% 29% 9% 1	540
	11th to 20th	Night	19% 10% 29% 30% 11% 1	533
	21st to end	Night	21% 7% 37% 25% 9%	540
May	1st to 10th	Night	20% 7% 33% 30% 10%	540
	11th to 20th	Night	17% 6% 22% 35% 16% 3	531
	21st to end	Night	9% 3 22% 39% 24% 3	584
June	1st to 10th	Night	12% 2 21% 43% 20% 1	540
	11th to 20th	Night	9% 3 21% 42% 23% 1	540
	21st to end	Night	9% 1 21% 38% 29% 1	540
July	1st to 10th	Night	10% 1 10% 35% 37% 7%	525
	11th to 20th	Night	6% 2 15% 37% 35% 4	522
	21st to end	Night	9% 1 10% 45% 31% 4	591
August	1st to 10th	Night	8% 2 10% 37% 40% 3	537
	11th to 20th	Night	7% 1 11% 41% 35% 5%	534
	21st to end	Night	8% 2 11% 31% 41% 6%	594
September	1st to 10th	Night	14% 3 12% 35% 32% 5%	540
	11th to 20th	Night	10% 3 15% 33% 36% 4	717
	21st to end	Night	11% 6% 18% 35% 26% 3	720
October	1st to 10th	Night	10% 4 23% 35% 24% 4	720
	11th to 20th	Night	21% 8% 26% 29% 14% 1	720
	21st to end	Night	22% 10% 33% 28% 7%	792
November	1st to 10th	Night	15% 8% 33% 31% 12% 1	740
	11th to 20th	Night	17% 13% 32% 29% 9% 1	760
	21st to end	Night	23% 13% 27% 28% 8%	760
December	1st to 10th	Night	23% 11% 37% 24% 5% 1	760
	11th to 20th	Night	23% 10% 32% 26% 7% 1	940
	21st to end	Night	30% 13% 31% 21% 5%	1045

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