Valley Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Valley Vista Observatory clear sky chart from 2005-11-06 to 2026-07-01.

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	2 2 1 3 3 3 4 3 5% 7% 67%	146275
	Night	2 2 1 4 3 3 4 3 5% 7% 65%	143649
Transparency	Day	16% 5% 7% 14% 28% 30%	141226
	Night	17% 5% 7% 14% 28% 30%	139705
Seeing	Day	6% 12% 23% 31% 22% 7%	16805
	Night	4 5% 21% 41% 25% 3	26848

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	3 4 1 4 3 4 3 3 5% 6% 64%	5318
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 4 83%	5340
	21st to end	Day	5% 4 2 4 3 4 4 3 5% 8% 59%	5844
February	1st to 10th	Day	2 3 1 3 3 3 3 3 5% 6% 69%	1890
	11th to 20th	Day	3 4 1 3 3 3 3 4 4 6% 66%	2020
	21st to end	Day	3 2 1 3 2 3 3 3 4 5% 72%	1903
March	1st to 10th	Day	1 2 1 3 2 2 3 3 5% 7% 72%	5064
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 6% 70%	5110
	21st to end	Day	1 3 1 4 3 3 4 3 6% 9% 64%	5607
April	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 6% 74%	2394
	11th to 20th	Day	2 2 2 2 2 2 2 4 4 79%	2491
	21st to end	Day	1 2 1 1 2 2 2 2 3 5% 80%	2520
May	1st to 10th	Day	1 1 2 2 2 3 2 3 4 81%	7057
	11th to 20th	Day	1 1 2 1 2 2 2 4 5% 80%	7540
	21st to end	Day	1 1 1 2 1 1 2 2 3 4 83%	8370
June	1st to 10th	Day	2 2 1 2 2 3 4 3 5% 7% 71%	2730
	11th to 20th	Day	2 1 1 2 2 2 3 2 5% 5% 75%	2730
	21st to end	Day	3 3 2 4 4 5% 5% 4 7% 8% 54%	2730
July	1st to 10th	Day	2 2 1 3 4 5% 5% 5% 7% 9% 57%	5880
	11th to 20th	Day	2 2 1 4 4 5% 7% 6% 9% 11% 49%	5880
	21st to end	Day	4 3 2 6% 5% 6% 7% 6% 10% 11% 40%	6401
August	1st to 10th	Day	2 3 1 4 3 4 6% 5% 7% 11% 53%	5779
	11th to 20th	Day	2 3 1 4 4 5% 6% 5% 9% 12% 49%	5451
	21st to end	Day	3 3 1 5% 5% 5% 6% 5% 8% 10% 48%	5984
September	1st to 10th	Day	5% 4 2 6% 4 5% 6% 4 7% 8% 48%	2400
	11th to 20th	Day	3 3 1 5% 4 4 5% 5% 6% 7% 56%	2201
	21st to end	Day	3 2 1 3 3 3 3 2 5% 6% 68%	2195
October	1st to 10th	Day	1 1 2 2 2 2 2 4 6% 78%	3270
	11th to 20th	Day	2 2 1 2 2 2 2 2 2 3 80%	3250
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	3548
November	1st to 10th	Day	1 1 1 3 1 2 2 2 4 5% 77%	2059
	11th to 20th	Day	1 2 1 3 2 3 2 3 4 6% 73%	2100
	21st to end	Day	4 4 1 4 4 3 4 3 4 7% 64%	2071
December	1st to 10th	Day	4 4 1 3 2 2 3 3 4 5% 70%	3040
	11th to 20th	Day	1 3 2 4 3 3 3 2 3 6% 70%	3499
	21st to end	Day	2 2 1 3 2 3 3 2 4 7% 71%	4609
January	1st to 10th	Night	3 4 2 5% 4 3 4 4 5% 6% 61%	7605
	11th to 20th	Night	1 1 1 1 1 1 1 3 4 85%	7650
	21st to end	Night	6% 4 2 4 3 3 4 4 6% 7% 57%	8370
February	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 6% 69%	2730
	11th to 20th	Night	3 3 1 4 2 3 3 3 4 6% 68%	2480
	21st to end	Night	1 2 1 3 2 3 3 3 6% 6% 71%	1903
March	1st to 10th	Night	2 2 1 2 2 2 3 3 4 6% 73%	5050
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 7% 73%	5110
	21st to end	Night	1 2 1 3 3 3 4 3 5% 7% 68%	5610
April	1st to 10th	Night	1 2 2 2 3 2 3 4 6% 74%	2225
	11th to 20th	Night	1 2 1 3 2 2 2 3 4 5% 76%	2077
	21st to end	Night	1 1 1 2 2 3 2 1 3 4 80%	2100
May	1st to 10th	Night	2 1 1 2 2 2 3 2 4 4 78%	5890
	11th to 20th	Night	1 1 1 3 2 2 2 2 4 5% 78%	5335
	21st to end	Night	1 1 2 2 1 2 2 3 4 82%	5837
June	1st to 10th	Night	3 2 1 4 2 3 3 2 4 6% 71%	1890
	11th to 20th	Night	2 2 1 3 2 3 2 3 5% 5% 74%	1890
	21st to end	Night	3 3 1 4 5% 4 5% 6% 7% 9% 53%	1890
July	1st to 10th	Night	3 3 2 5% 5% 5% 7% 6% 8% 10% 47%	4100
	11th to 20th	Night	3 4 2 7% 7% 7% 9% 7% 9% 12% 34%	4100
	21st to end	Night	3 5% 2 8% 7% 7% 9% 7% 10% 14% 26%	4472
August	1st to 10th	Night	4 4 2 7% 6% 6% 7% 7% 9% 12% 37%	4033
	11th to 20th	Night	3 4 2 8% 7% 8% 8% 7% 9% 13% 30%	4394
	21st to end	Night	3 4 2 6% 5% 7% 7% 7% 9% 11% 39%	4993
September	1st to 10th	Night	6% 5% 2 6% 5% 6% 5% 5% 7% 9% 44%	2000
	11th to 20th	Night	5% 4 1 4 3 4 5% 4 6% 7% 58%	2193
	21st to end	Night	2 2 1 3 4 3 3 3 5% 4 69%	2205
October	1st to 10th	Night	1 1 1 2 2 2 2 2 3 4 78%	3920
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 76%	3898
	21st to end	Night	1 1 1 1 1 2 2 3 4 86%	4252
November	1st to 10th	Night	2 1 1 2 1 2 2 2 3 6% 79%	2460
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 5% 73%	2520
	21st to end	Night	3 3 2 3 3 3 3 3 5% 7% 65%	2545
December	1st to 10th	Night	4 2 1 2 2 2 2 2 4 5% 75%	4370
	11th to 20th	Night	1 2 1 3 3 3 4 3 5% 6% 69%	4941
	21st to end	Night	3 2 1 3 2 2 3 3 5% 6% 70%	6611

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	20% 4 5% 16% 35% 20%	5321
	11th to 20th	Day	7% 3 3 9% 43% 35%	5340
	21st to end	Day	22% 6% 5% 14% 28% 25%	5844
February	1st to 10th	Day	17% 4 4 13% 29% 33%	1890
	11th to 20th	Day	19% 4 6% 15% 30% 26%	2029
	21st to end	Day	16% 3 4 12% 28% 38%	1903
March	1st to 10th	Day	12% 4 5% 7% 33% 38%	5064
	11th to 20th	Day	11% 5% 6% 15% 33% 29%	5110
	21st to end	Day	14% 6% 8% 11% 34% 28%	5607
April	1st to 10th	Day	11% 3 4 10% 37% 34%	2394
	11th to 20th	Day	10% 2 3 8% 29% 48%	2494
	21st to end	Day	10% 2 3 9% 30% 46%	2520
May	1st to 10th	Day	8% 3 4 9% 27% 50%	7057
	11th to 20th	Day	7% 2 4 10% 27% 48%	7543
	21st to end	Day	7% 2 3 5% 17% 66%	8371
June	1st to 10th	Day	13% 4 4 7% 27% 44%	2730
	11th to 20th	Day	11% 3 5% 6% 20% 55%	2730
	21st to end	Day	24% 7% 8% 12% 19% 30%	2730
July	1st to 10th	Day	19% 9% 12% 18% 25% 16%	5817
	11th to 20th	Day	21% 12% 18% 24% 20% 6%	5811
	21st to end	Day	30% 13% 20% 20% 12% 5%	6217
August	1st to 10th	Day	27% 11% 16% 23% 18% 5%	4208
	11th to 20th	Day	25% 9% 12% 22% 26% 6%	3944
	21st to end	Day	29% 12% 15% 21% 16% 8%	4311
September	1st to 10th	Day	31% 9% 10% 15% 20% 15%	2400
	11th to 20th	Day	24% 6% 8% 17% 25% 20%	2201
	21st to end	Day	17% 4 7% 18% 32% 22%	2195
October	1st to 10th	Day	9% 3 5% 10% 22% 50%	3270
	11th to 20th	Day	12% 2 3 11% 30% 42%	3250
	21st to end	Day	6% 1 3 10% 34% 46%	3548
November	1st to 10th	Day	10% 3 5% 17% 36% 29%	2059
	11th to 20th	Day	13% 4 5% 17% 33% 27%	2100
	21st to end	Day	20% 4 5% 19% 30% 22%	2071
December	1st to 10th	Day	17% 4 5% 16% 34% 23%	3040
	11th to 20th	Day	18% 3 4 12% 36% 26%	3498
	21st to end	Day	13% 3 6% 16% 41% 20%	4609
January	1st to 10th	Night	20% 6% 6% 16% 36% 15%	7605
	11th to 20th	Night	6% 2 2 12% 40% 37%	7650
	21st to end	Night	25% 6% 5% 14% 24% 27%	8370
February	1st to 10th	Night	15% 4 6% 14% 26% 35%	2730
	11th to 20th	Night	18% 4 5% 16% 32% 26%	2481
	21st to end	Night	13% 4 6% 11% 30% 36%	1903
March	1st to 10th	Night	11% 4 4 9% 33% 39%	5050
	11th to 20th	Night	9% 4 6% 18% 36% 26%	5110
	21st to end	Night	14% 4 7% 16% 32% 26%	5610
April	1st to 10th	Night	12% 3 5% 13% 35% 32%	2225
	11th to 20th	Night	12% 2 4 10% 30% 43%	2077
	21st to end	Night	10% 2 3 10% 32% 43%	2100
May	1st to 10th	Night	10% 4 4 9% 29% 44%	5888
	11th to 20th	Night	9% 3 4 10% 25% 49%	5336
	21st to end	Night	8% 2 2 5% 19% 63%	5838
June	1st to 10th	Night	14% 4 4 8% 27% 43%	1890
	11th to 20th	Night	13% 3 5% 6% 20% 54%	1890
	21st to end	Night	24% 7% 9% 12% 18% 30%	1890
July	1st to 10th	Night	26% 9% 15% 19% 18% 13%	4055
	11th to 20th	Night	35% 12% 14% 20% 13% 6%	4046
	21st to end	Night	40% 14% 21% 14% 8% 4	4344
August	1st to 10th	Night	35% 13% 16% 19% 11% 5%	2928
	11th to 20th	Night	33% 12% 14% 17% 20% 4	3177
	21st to end	Night	34% 12% 20% 17% 12% 5%	3597
September	1st to 10th	Night	32% 11% 12% 13% 18% 13%	2000
	11th to 20th	Night	23% 7% 8% 18% 25% 19%	2193
	21st to end	Night	17% 5% 7% 16% 34% 20%	2205
October	1st to 10th	Night	11% 3 6% 9% 22% 49%	3920
	11th to 20th	Night	11% 4 4 13% 23% 44%	3898
	21st to end	Night	5% 2 3 9% 31% 50%	4252
November	1st to 10th	Night	8% 3 5% 17% 37% 30%	2460
	11th to 20th	Night	14% 3 5% 18% 33% 28%	2520
	21st to end	Night	19% 5% 6% 18% 30% 23%	2545
December	1st to 10th	Night	14% 4 4 16% 40% 21%	4370
	11th to 20th	Night	16% 5% 5% 12% 35% 27%	4941
	21st to end	Night	15% 4 5% 15% 38% 24%	6611

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	10% 15% 28% 33% 12% 2	330
	11th to 20th	Day	6% 8% 28% 39% 15% 3	330
	21st to end	Day	10% 13% 28% 35% 12% 2	363
February	1st to 10th	Day	5% 16% 26% 30% 19% 3	330
	11th to 20th	Day	17% 19% 30% 24% 11%	408
	21st to end	Day	11% 26% 34% 20% 8%	446
March	1st to 10th	Day	9% 25% 34% 24% 7% 2	540
	11th to 20th	Day	10% 21% 29% 26% 11% 3	540
	21st to end	Day	9% 25% 30% 24% 10% 3	594
April	1st to 10th	Day	6% 19% 33% 25% 14% 3	540
	11th to 20th	Day	6% 26% 32% 25% 10% 2	532
	21st to end	Day	6% 24% 32% 26% 9% 3	540
May	1st to 10th	Day	7% 20% 33% 25% 12% 3	543
	11th to 20th	Day	6% 17% 30% 29% 15% 3	568
	21st to end	Day	3 12% 36% 34% 13% 2	617
June	1st to 10th	Day	4 7% 26% 38% 20% 4	570
	11th to 20th	Day	2 6% 23% 41% 25% 5%	570
	21st to end	Day	5% 4 15% 39% 31% 6%	570
July	1st to 10th	Day	4 2 9% 35% 41% 10%	527
	11th to 20th	Day	5% 1 5% 27% 47% 15%	524
	21st to end	Day	6% 1 6% 26% 42% 18%	575
August	1st to 10th	Day	5% 1 5% 24% 42% 24%	527
	11th to 20th	Day	4 2 6% 33% 41% 13%	517
	21st to end	Day	6% 1 7% 28% 38% 21%	583
September	1st to 10th	Day	8% 2 13% 36% 32% 9%	530
	11th to 20th	Day	4 4 13% 35% 34% 10%	530
	21st to end	Day	5% 7% 19% 37% 22% 11%	525
October	1st to 10th	Day	3 6% 21% 32% 26% 12%	330
	11th to 20th	Day	9% 8% 21% 33% 20% 9%	330
	21st to end	Day	4 7% 21% 32% 26% 10%	363
November	1st to 10th	Day	5% 10% 26% 34% 19% 6%	330
	11th to 20th	Day	4 8% 25% 33% 26% 5%	330
	21st to end	Day	7% 18% 26% 33% 15% 2	330
December	1st to 10th	Day	7% 10% 25% 35% 20% 3	330
	11th to 20th	Day	5% 12% 28% 37% 16% 3	330
	21st to end	Day	10% 13% 27% 35% 11% 4	363
January	1st to 10th	Night	5% 5% 26% 43% 19% 3	840
	11th to 20th	Night	4 4 23% 46% 20% 3	840
	21st to end	Night	7% 8% 26% 42% 15% 2	924
February	1st to 10th	Night	3 8% 25% 42% 20% 3	840
	11th to 20th	Night	6% 11% 28% 41% 13% 1	824
	21st to end	Night	3 12% 34% 40% 11% 1	692
March	1st to 10th	Night	5% 12% 30% 41% 11% 1	840
	11th to 20th	Night	5% 8% 30% 37% 18% 2	840
	21st to end	Night	3 10% 27% 42% 17% 2	924
April	1st to 10th	Night	3 6% 34% 42% 14% 1	755
	11th to 20th	Night	3 11% 33% 39% 14% 1	623
	21st to end	Night	1 10% 34% 42% 11% 2	630
May	1st to 10th	Night	2 8% 31% 44% 14% 1	630
	11th to 20th	Night	3 6% 27% 47% 17% 1	621
	21st to end	Night	2 5% 27% 48% 18%	683
June	1st to 10th	Night	5% 2 16% 55% 20% 2	630
	11th to 20th	Night	2 2 19% 52% 24% 1	630
	21st to end	Night	4 2 11% 45% 37% 1	630
July	1st to 10th	Night	6% 2 7% 42% 39% 5%	585
	11th to 20th	Night	5% 1 8% 33% 47% 5%	582
	21st to end	Night	5% 1 5% 31% 50% 8%	657
August	1st to 10th	Night	5% 2 6% 23% 53% 11%	597
	11th to 20th	Night	6% 2 6% 26% 52% 8%	594
	21st to end	Night	5% 1 7% 26% 53% 8%	660
September	1st to 10th	Night	8% 2 7% 36% 44% 3	600
	11th to 20th	Night	6% 2 10% 34% 44% 5%	793
	21st to end	Night	3 2 13% 42% 35% 5%	800
October	1st to 10th	Night	3 2 16% 44% 31% 3	800
	11th to 20th	Night	5% 4 18% 43% 28% 4	800
	21st to end	Night	3 4 13% 43% 32% 6%	880
November	1st to 10th	Night	2 4 18% 43% 27% 6%	820
	11th to 20th	Night	4 5% 19% 43% 26% 3	840
	21st to end	Night	5% 6% 27% 40% 20% 3	840
December	1st to 10th	Night	6% 5% 23% 44% 20% 3	840
	11th to 20th	Night	3 6% 24% 44% 21% 2	840
	21st to end	Night	6% 8% 25% 41% 16% 4	924

Page generated at 2026-06-28 00:10:49. Version ac07

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