West Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Valley Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	126618
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	122767
Transparency	Day	42% 6% 13% 22% 15% 2	130160
	Night	41% 5% 12% 23% 16% 3	127311
Seeing	Day	21% 16% 28% 21% 11% 4	16092
	Night	20% 12% 29% 28% 10% 1	25330

By Month

Percentage Cloud Cover NE (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	13% 6% 2 5% 3 4 4 4 6% 7% 46%	3597
	11th to 20th	Day	13% 9% 4 11% 6% 5% 4 4 5% 6% 33%	3685
	21st to end	Day	17% 8% 2 7% 5% 5% 5% 4 5% 6% 37%	4466
February	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	1800
	11th to 20th	Day	14% 9% 3 6% 4 5% 4 4 6% 8% 37%	1864
	21st to end	Day	19% 9% 2 6% 5% 4 4 4 5% 6% 36%	1650
March	1st to 10th	Day	20% 5% 1 5% 4 4 5% 4 5% 6% 41%	4557
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 3 6% 7% 43%	4980
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 7% 42%	5740
April	1st to 10th	Day	17% 8% 3 6% 5% 5% 4 4 6% 7% 35%	2400
	11th to 20th	Day	16% 7% 2 7% 4 5% 4 3 5% 7% 40%	2374
	21st to end	Day	16% 6% 2 6% 5% 4 4 5% 6% 7% 39%	2400
May	1st to 10th	Day	27% 6% 2 5% 4 4 5% 4 6% 8% 29%	5921
	11th to 20th	Day	25% 8% 3 7% 4 4 4 3 5% 7% 31%	6318
	21st to end	Day	20% 6% 2 5% 4 4 4 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	2800
	11th to 20th	Day	16% 6% 2 5% 5% 5% 5% 4 8% 8% 37%	2800
	21st to end	Day	12% 5% 2 5% 4 5% 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	7% 6% 2 7% 6% 5% 6% 5% 7% 11% 38%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 11% 33%	6360
	21st to end	Day	11% 7% 2 7% 5% 4 5% 4 5% 9% 41%	6792
August	1st to 10th	Day	20% 6% 2 7% 4 5% 5% 4 7% 8% 33%	4234
	11th to 20th	Day	14% 5% 2 6% 4 5% 5% 5% 5% 9% 40%	3917
	21st to end	Day	14% 6% 2 4 3 4 4 4 6% 8% 46%	4312
September	1st to 10th	Day	16% 5% 2 6% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Day	13% 4 2 4 3 3 4 3 5% 7% 50%	2390
	21st to end	Day	20% 7% 3 6% 4 4 5% 3 5% 5% 38%	2163
October	1st to 10th	Day	18% 7% 2 4 4 4 4 4 5% 7% 42%	1930
	11th to 20th	Day	17% 5% 1 5% 3 5% 4 4 5% 8% 44%	1900
	21st to end	Day	19% 6% 2 5% 3 3 3 4 6% 7% 41%	2037
November	1st to 10th	Day	14% 6% 3 4 3 3 3 2 4 7% 52%	1761
	11th to 20th	Day	16% 6% 2 4 3 4 3 4 6% 7% 45%	1800
	21st to end	Day	18% 6% 3 4 4 4 4 4 6% 7% 41%	1615
December	1st to 10th	Day	15% 8% 2 6% 4 4 4 5% 6% 6% 39%	2600
	11th to 20th	Day	22% 9% 1 4 3 4 4 4 5% 6% 38%	2986
	21st to end	Day	18% 7% 3 6% 4 5% 5% 4 7% 7% 34%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	6349
	11th to 20th	Night	13% 8% 2 6% 4 3 5% 4 6% 7% 42%	6195
	21st to end	Night	18% 7% 2 5% 4 3 4 4 5% 6% 41%	6490
February	1st to 10th	Night	16% 9% 3 5% 4 3 4 4 4 7% 39%	2600
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 4 5% 6% 39%	2421
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 6% 40%	1980
March	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 5% 6% 41%	5281
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 4 6% 51%	4979
	21st to end	Night	16% 8% 3 6% 4 3 4 3 5% 5% 43%	5192
April	1st to 10th	Night	17% 8% 3 5% 5% 4 4 4 5% 7% 37%	2000
	11th to 20th	Night	15% 6% 2 7% 5% 4 4 4 6% 7% 40%	1975
	21st to end	Night	20% 4 2 5% 4 3 4 3 5% 6% 45%	2000
May	1st to 10th	Night	27% 8% 2 6% 4 4 5% 4 5% 7% 29%	4047
	11th to 20th	Night	24% 9% 3 7% 4 4 4 3 5% 6% 32%	3575
	21st to end	Night	22% 6% 2 5% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	16% 6% 2 6% 5% 4 3 3 6% 8% 42%	1600
	11th to 20th	Night	14% 7% 2 5% 4 4 5% 5% 7% 8% 41%	1600
	21st to end	Night	12% 5% 1 6% 4 5% 5% 3 7% 9% 44%	1600
July	1st to 10th	Night	10% 6% 3 7% 5% 6% 5% 4 6% 9% 39%	3600
	11th to 20th	Night	13% 7% 3 8% 5% 5% 6% 4 7% 9% 32%	3600
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 8% 45%	4055
August	1st to 10th	Night	20% 6% 2 8% 5% 4 5% 4 5% 8% 34%	2915
	11th to 20th	Night	16% 6% 2 7% 5% 4 4 4 5% 6% 41%	3097
	21st to end	Night	13% 5% 2 4 3 4 5% 4 5% 7% 48%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 4 6% 48%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 3 2 5% 7% 51%	2010
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 7% 41%	2233
October	1st to 10th	Night	21% 6% 2 5% 3 4 3 3 5% 6% 43%	2317
	11th to 20th	Night	17% 5% 2 4 3 4 3 3 5% 6% 48%	2280
	21st to end	Night	20% 6% 1 4 3 3 3 3 5% 5% 47%	2560
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 5% 52%	2537
	11th to 20th	Night	16% 7% 2 6% 4 3 3 2 4 7% 45%	2600
	21st to end	Night	15% 6% 2 6% 4 4 4 4 5% 7% 42%	2783
December	1st to 10th	Night	13% 8% 3 7% 4 4 4 4 5% 6% 43%	4580
	11th to 20th	Night	23% 7% 2 5% 3 4 4 3 5% 5% 40%	5208
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 6% 37%	6996

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	36% 5% 12% 25% 20% 2	3598
	11th to 20th	Day	49% 6% 10% 18% 12% 4	3715
	21st to end	Day	47% 6% 12% 21% 13% 1	4466
February	1st to 10th	Day	46% 7% 14% 20% 12% 1	1800
	11th to 20th	Day	45% 6% 11% 23% 13% 2	1873
	21st to end	Day	48% 6% 12% 22% 11% 3	1650
March	1st to 10th	Day	41% 6% 10% 24% 12% 6%	4560
	11th to 20th	Day	38% 7% 13% 27% 10% 4	4980
	21st to end	Day	42% 4 12% 16% 21% 4	5740
April	1st to 10th	Day	46% 7% 10% 20% 13% 5%	2400
	11th to 20th	Day	42% 7% 11% 22% 14% 4	2374
	21st to end	Day	41% 7% 12% 21% 15% 4	2400
May	1st to 10th	Day	51% 6% 15% 17% 9% 2	5930
	11th to 20th	Day	52% 7% 9% 20% 10% 1	6322
	21st to end	Day	42% 7% 13% 24% 13% 2	6949
June	1st to 10th	Day	40% 9% 14% 22% 14% 2	2800
	11th to 20th	Day	42% 9% 15% 22% 10% 2	2800
	21st to end	Day	37% 10% 15% 24% 14% 1	2800
July	1st to 10th	Day	39% 9% 14% 22% 14% 1	6290
	11th to 20th	Day	44% 6% 18% 22% 9% 1	6285
	21st to end	Day	40% 6% 11% 20% 22% 1	6596
August	1st to 10th	Day	48% 7% 14% 21% 10%	4221
	11th to 20th	Day	40% 6% 13% 29% 12% 1	3955
	21st to end	Day	35% 6% 11% 31% 16% 2	4312
September	1st to 10th	Day	40% 7% 11% 21% 20% 3	2400
	11th to 20th	Day	33% 6% 10% 25% 24% 3	2390
	21st to end	Day	48% 5% 10% 16% 19% 2	2163
October	1st to 10th	Day	39% 4 9% 20% 23% 5%	3190
	11th to 20th	Day	29% 4 11% 28% 23% 5%	3140
	21st to end	Day	27% 4 15% 25% 26% 3	3339
November	1st to 10th	Day	34% 4 15% 25% 20% 3	1761
	11th to 20th	Day	38% 6% 11% 25% 18% 2	1800
	21st to end	Day	40% 7% 14% 29% 9% 2	1615
December	1st to 10th	Day	45% 4 16% 24% 9% 2	2600
	11th to 20th	Day	47% 4 15% 25% 9% 1	2986
	21st to end	Day	45% 7% 14% 20% 13% 1	3960
January	1st to 10th	Night	37% 5% 12% 25% 19% 3	6349
	11th to 20th	Night	40% 5% 15% 20% 14% 6%	6240
	21st to end	Night	43% 5% 12% 21% 16% 3	6490
February	1st to 10th	Night	45% 5% 14% 25% 10% 2	2600
	11th to 20th	Night	45% 5% 11% 20% 16% 3	2423
	21st to end	Night	44% 5% 10% 21% 17% 3	1980
March	1st to 10th	Night	43% 5% 10% 22% 13% 8%	5291
	11th to 20th	Night	34% 6% 17% 26% 13% 5%	4980
	21st to end	Night	44% 4 7% 17% 23% 5%	5192
April	1st to 10th	Night	44% 6% 12% 21% 14% 3	2000
	11th to 20th	Night	42% 7% 10% 19% 19% 2	1975
	21st to end	Night	38% 7% 12% 21% 18% 5%	2000
May	1st to 10th	Night	54% 6% 11% 17% 10% 2	4054
	11th to 20th	Night	52% 7% 11% 16% 12% 1	3576
	21st to end	Night	45% 8% 10% 23% 12% 2	3927
June	1st to 10th	Night	40% 7% 11% 23% 18% 2	1600
	11th to 20th	Night	38% 9% 15% 24% 13% 1	1600
	21st to end	Night	35% 8% 15% 27% 13% 1	1600
July	1st to 10th	Night	43% 7% 14% 19% 15% 2	3560
	11th to 20th	Night	48% 5% 16% 20% 11% 1	3552
	21st to end	Night	35% 6% 14% 23% 21% 1	3936
August	1st to 10th	Night	48% 7% 17% 18% 10% 1	2904
	11th to 20th	Night	43% 6% 13% 23% 15% 1	3125
	21st to end	Night	34% 6% 10% 30% 19% 1	3586
September	1st to 10th	Night	37% 6% 11% 23% 21% 3	2000
	11th to 20th	Night	34% 6% 10% 23% 25% 3	2010
	21st to end	Night	43% 5% 10% 20% 21% 2	2233
October	1st to 10th	Night	42% 4 10% 21% 21% 2	3837
	11th to 20th	Night	28% 4 9% 30% 25% 4	3779
	21st to end	Night	28% 3 13% 31% 22% 3	4208
November	1st to 10th	Night	33% 5% 14% 24% 20% 4	2537
	11th to 20th	Night	40% 4 10% 24% 17% 4	2600
	21st to end	Night	40% 5% 13% 27% 13% 2	2783
December	1st to 10th	Night	42% 5% 13% 29% 9% 1	4580
	11th to 20th	Night	46% 4 12% 24% 12% 2	5208
	21st to end	Night	47% 6% 11% 20% 15% 1	6996

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	26% 33% 25% 14% 3 1	200
	11th to 20th	Day	34% 25% 30% 9% 2	225
	21st to end	Day	22% 27% 34% 13% 4 1	330
February	1st to 10th	Day	23% 30% 33% 11% 2 1	300
	11th to 20th	Day	29% 25% 35% 9% 1	412
	21st to end	Day	30% 20% 30% 17% 3	414
March	1st to 10th	Day	26% 22% 30% 18% 3	500
	11th to 20th	Day	24% 27% 30% 13% 4 1	500
	21st to end	Day	28% 24% 29% 13% 6% 1	550
April	1st to 10th	Day	28% 24% 28% 14% 5% 1	500
	11th to 20th	Day	22% 26% 28% 15% 6% 2	492
	21st to end	Day	26% 19% 32% 15% 5% 3	500
May	1st to 10th	Day	29% 19% 26% 16% 6% 3	503
	11th to 20th	Day	23% 17% 31% 20% 8% 2	527
	21st to end	Day	21% 9% 28% 23% 15% 4	573
June	1st to 10th	Day	16% 16% 28% 22% 15% 3	530
	11th to 20th	Day	21% 12% 27% 22% 14% 3	530
	21st to end	Day	14% 11% 32% 25% 14% 4	530
July	1st to 10th	Day	11% 6% 23% 30% 23% 7%	525
	11th to 20th	Day	8% 5% 25% 32% 24% 6%	524
	21st to end	Day	13% 6% 28% 33% 16% 5%	576
August	1st to 10th	Day	12% 6% 22% 32% 20% 8%	526
	11th to 20th	Day	13% 5% 20% 32% 21% 7%	519
	21st to end	Day	14% 6% 25% 27% 17% 10%	583
September	1st to 10th	Day	18% 9% 20% 24% 21% 7%	530
	11th to 20th	Day	13% 4 25% 27% 20% 11%	530
	21st to end	Day	26% 13% 21% 22% 14% 5%	530
October	1st to 10th	Day	25% 8% 25% 22% 14% 7%	502
	11th to 20th	Day	19% 16% 28% 24% 12% 2	490
	21st to end	Day	21% 17% 31% 19% 11% 1	511
November	1st to 10th	Day	16% 16% 30% 24% 11% 3	395
	11th to 20th	Day	21% 22% 30% 23% 5% 1	400
	21st to end	Day	25% 21% 35% 13% 6%	215
December	1st to 10th	Day	29% 25% 30% 12% 5%	200
	11th to 20th	Day	34% 25% 26% 13% 4	200
	21st to end	Day	26% 21% 31% 15% 5% 2	220
January	1st to 10th	Night	19% 21% 38% 20% 3	800
	11th to 20th	Night	24% 18% 32% 23% 3	800
	21st to end	Night	18% 19% 38% 23% 2	880
February	1st to 10th	Night	22% 17% 36% 24% 2	800
	11th to 20th	Night	25% 22% 36% 16% 1	783
	21st to end	Night	24% 18% 36% 19% 2	660
March	1st to 10th	Night	22% 21% 32% 23% 4	800
	11th to 20th	Night	23% 18% 33% 20% 5% 1	800
	21st to end	Night	22% 16% 37% 20% 4	880
April	1st to 10th	Night	22% 20% 33% 20% 4	800
	11th to 20th	Night	19% 15% 36% 23% 6%	791
	21st to end	Night	22% 12% 33% 26% 7%	800
May	1st to 10th	Night	25% 10% 30% 27% 8%	607
	11th to 20th	Night	19% 8% 33% 30% 9%	394
	21st to end	Night	19% 6% 21% 41% 12% 1	433
June	1st to 10th	Night	18% 5% 25% 36% 17%	400
	11th to 20th	Night	15% 8% 23% 42% 12% 1	400
	21st to end	Night	11% 4 27% 44% 14%	400
July	1st to 10th	Night	12% 3 19% 41% 23% 2	390
	11th to 20th	Night	10% 4 21% 39% 24% 2	388
	21st to end	Night	11% 4 18% 42% 23% 1	492
August	1st to 10th	Night	13% 3 15% 43% 25% 2	597
	11th to 20th	Night	15% 5% 14% 36% 27% 2	726
	21st to end	Night	15% 3 18% 36% 25% 2	880
September	1st to 10th	Night	17% 4 24% 34% 20% 2	800
	11th to 20th	Night	15% 4 17% 36% 23% 5%	800
	21st to end	Night	22% 5% 21% 31% 20% 1	800
October	1st to 10th	Night	21% 8% 24% 26% 18% 3	773
	11th to 20th	Night	18% 11% 28% 32% 11% 1	760
	21st to end	Night	23% 11% 28% 28% 9%	836
November	1st to 10th	Night	15% 11% 35% 28% 12% 1	780
	11th to 20th	Night	20% 17% 27% 28% 8% 1	800
	21st to end	Night	18% 16% 34% 26% 7%	800
December	1st to 10th	Night	24% 17% 32% 22% 5% 1	800
	11th to 20th	Night	22% 16% 32% 25% 5%	800
	21st to end	Night	22% 15% 32% 28% 3	880

Page generated at 2025-10-01 01:39:03. Version ac07

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