VLA
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VLA clear sky chart from 2005-11-06 to 2025-09-19.

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 4 2 5% 4 4 5% 4 6% 7% 59%	134004
	Night	2 3 1 4 3 4 4 4 6% 8% 61%	132546
Transparency	Day	24% 4 6% 16% 28% 22%	128977
	Night	21% 3 7% 18% 30% 20%	128576
Seeing	Day	7% 13% 24% 27% 20% 9%	15545
	Night	4 10% 25% 36% 23% 3	25989

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	1 6% 2 6% 3 3 4 4 4 6% 62%	4098
	11th to 20th	Day	1 2 2 3 2 2 3 3 4 7% 71%	4100
	21st to end	Day	2 7% 2 5% 4 3 3 3 5% 6% 58%	4510
February	1st to 10th	Day	1 3 2 4 3 3 4 4 5% 6% 65%	1800
	11th to 20th	Day	2 4 2 5% 3 4 4 3 4 7% 62%	1895
	21st to end	Day	2 4 2 5% 2 2 3 3 5% 6% 66%	1711
March	1st to 10th	Day	1 3 2 4 3 3 4 4 6% 8% 64%	4954
	11th to 20th	Day	3 7% 3 6% 4 4 3 3 5% 7% 54%	5000
	21st to end	Day	1 4 2 4 4 3 5% 4 6% 7% 59%	5486
April	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 7% 66%	2384
	11th to 20th	Day	3 5% 1 5% 3 3 3 2 4 6% 65%	2371
	21st to end	Day	2 3 1 3 3 3 3 3 5% 6% 68%	2400
May	1st to 10th	Day	1 2 1 3 3 4 5% 4 5% 6% 65%	5440
	11th to 20th	Day	2 3 1 4 4 4 4 3 5% 5% 66%	5726
	21st to end	Day	1 2 1 3 3 3 3 3 4 6% 72%	6423
June	1st to 10th	Day	2 4 2 5% 4 3 4 4 5% 7% 60%	2600
	11th to 20th	Day	2 2 1 3 3 3 4 3 5% 6% 69%	2600
	21st to end	Day	4 5% 2 7% 6% 5% 6% 4 7% 8% 47%	2600
July	1st to 10th	Day	3 3 1 5% 5% 5% 6% 4 6% 7% 55%	5879
	11th to 20th	Day	3 3 2 7% 6% 7% 8% 6% 8% 10% 41%	5880
	21st to end	Day	4 6% 2 9% 7% 8% 8% 7% 9% 9% 32%	6401
August	1st to 10th	Day	3 3 2 5% 5% 6% 7% 6% 9% 10% 43%	5779
	11th to 20th	Day	3 3 1 6% 5% 6% 6% 5% 8% 10% 46%	5367
	21st to end	Day	4 4 2 7% 6% 7% 8% 6% 8% 9% 39%	5984
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 7% 46%	2400
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 4 5% 7% 53%	2367
	21st to end	Day	3 3 1 4 2 3 4 3 4 7% 65%	2004
October	1st to 10th	Day	3 3 1 3 3 3 3 3 4 5% 69%	3170
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 6% 69%	3150
	21st to end	Day	1 1 1 2 1 1 2 1 3 5% 83%	3438
November	1st to 10th	Day	1 2 1 3 1 3 2 3 5% 7% 71%	1959
	11th to 20th	Day	2 2 1 3 2 2 2 3 4 6% 72%	2000
	21st to end	Day	2 4 2 4 4 3 4 3 4 6% 63%	1812
December	1st to 10th	Day	1 2 1 2 1 2 3 3 4 6% 74%	2620
	11th to 20th	Day	2 4 2 4 4 3 3 3 4 6% 66%	3186
	21st to end	Day	1 3 1 3 3 3 4 3 4 6% 68%	4510
January	1st to 10th	Night	1 6% 2 7% 3 4 5% 3 5% 7% 55%	5867
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 6% 74%	5880
	21st to end	Night	2 6% 3 5% 3 4 5% 4 5% 7% 57%	6468
February	1st to 10th	Night	1 4 2 4 3 3 3 3 5% 7% 66%	2600
	11th to 20th	Night	2 4 1 4 3 3 3 3 6% 7% 63%	2389
	21st to end	Night	2 3 1 4 3 3 3 3 5% 7% 67%	1915
March	1st to 10th	Night	1 4 1 4 3 3 4 3 5% 6% 68%	4940
	11th to 20th	Night	3 6% 2 6% 4 3 3 3 5% 6% 60%	5000
	21st to end	Night	1 3 1 3 2 2 4 3 5% 8% 67%	5489
April	1st to 10th	Night	1 2 1 3 3 2 3 4 5% 7% 69%	2015
	11th to 20th	Night	2 3 1 4 3 2 3 3 3 5% 69%	1977
	21st to end	Night	2 3 1 3 3 3 3 2 5% 5% 71%	2000
May	1st to 10th	Night	2 3 1 4 4 3 3 3 5% 6% 67%	4540
	11th to 20th	Night	2 3 1 3 2 3 3 2 4 6% 72%	4172
	21st to end	Night	1 1 1 2 2 2 3 2 4 6% 75%	4474
June	1st to 10th	Night	3 4 1 4 3 4 3 4 5% 8% 62%	1800
	11th to 20th	Night	2 2 1 3 3 3 4 3 4 6% 70%	1800
	21st to end	Night	3 4 1 5% 5% 5% 5% 6% 7% 8% 49%	1800
July	1st to 10th	Night	3 3 1 6% 6% 6% 5% 5% 7% 9% 49%	4100
	11th to 20th	Night	2 3 2 6% 4 6% 7% 6% 11% 13% 41%	4100
	21st to end	Night	3 4 2 8% 6% 6% 7% 7% 10% 13% 35%	4472
August	1st to 10th	Night	1 3 1 6% 6% 6% 7% 6% 10% 12% 41%	4033
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 6% 9% 13% 41%	4478
	21st to end	Night	3 4 2 7% 7% 7% 8% 6% 10% 15% 32%	4993
September	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 4 7% 9% 47%	2000
	11th to 20th	Night	4 3 2 4 3 4 4 3 5% 8% 59%	1986
	21st to end	Night	3 3 1 3 3 3 4 3 4 7% 66%	2175
October	1st to 10th	Night	2 3 1 3 3 2 2 3 4 5% 70%	3800
	11th to 20th	Night	2 3 1 4 3 3 3 3 5% 7% 66%	3778
	21st to end	Night	1 1 1 2 1 2 2 2 4 6% 78%	4120
November	1st to 10th	Night	2 2 1 2 2 2 3 2 5% 6% 74%	2340
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 6% 74%	2400
	21st to end	Night	3 4 2 4 4 3 3 3 4 6% 66%	2559
December	1st to 10th	Night	1 2 1 3 2 3 4 3 4 7% 70%	4570
	11th to 20th	Night	1 2 1 3 3 4 4 3 6% 8% 65%	5048
	21st to end	Night	1 3 1 4 4 3 3 2 5% 7% 67%	6468

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	24% 4 3 20% 39% 11%	4099
	11th to 20th	Day	14% 2 5% 22% 33% 24%	4100
	21st to end	Day	27% 3 3 14% 28% 26%	4510
February	1st to 10th	Day	19% 3 6% 18% 30% 24%	1800
	11th to 20th	Day	23% 2 3 21% 31% 19%	1905
	21st to end	Day	19% 2 3 13% 33% 31%	1711
March	1st to 10th	Day	19% 4 2 8% 34% 33%	4954
	11th to 20th	Day	29% 3 3 14% 33% 19%	5000
	21st to end	Day	24% 3 2 17% 39% 15%	5486
April	1st to 10th	Day	16% 4 4 16% 37% 23%	2384
	11th to 20th	Day	22% 3 2 11% 31% 32%	2374
	21st to end	Day	19% 3 3 15% 38% 22%	2400
May	1st to 10th	Day	20% 3 3 16% 33% 25%	5440
	11th to 20th	Day	19% 3 3 10% 34% 30%	5729
	21st to end	Day	14% 3 2 8% 22% 50%	6424
June	1st to 10th	Day	22% 4 4 17% 25% 29%	2600
	11th to 20th	Day	18% 3 3 8% 23% 46%	2600
	21st to end	Day	33% 6% 7% 13% 21% 20%	2600
July	1st to 10th	Day	27% 5% 8% 14% 19% 27%	5816
	11th to 20th	Day	35% 4 18% 22% 13% 7%	5811
	21st to end	Day	45% 4 17% 20% 10% 3	6217
August	1st to 10th	Day	37% 7% 17% 21% 13% 5%	4208
	11th to 20th	Day	31% 4 13% 22% 22% 8%	3883
	21st to end	Day	43% 6% 14% 18% 16% 4	4311
September	1st to 10th	Day	36% 5% 8% 17% 21% 13%	2400
	11th to 20th	Day	30% 5% 6% 16% 25% 18%	2367
	21st to end	Day	20% 3 5% 18% 30% 24%	2004
October	1st to 10th	Day	18% 4 4 12% 29% 33%	3170
	11th to 20th	Day	18% 3 3 15% 37% 25%	3150
	21st to end	Day	8% 1 3 14% 38% 36%	3438
November	1st to 10th	Day	13% 3 3 22% 35% 23%	1959
	11th to 20th	Day	15% 2 4 18% 38% 24%	2000
	21st to end	Day	24% 2 4 16% 32% 22%	1812
December	1st to 10th	Day	12% 3 3 25% 39% 19%	2620
	11th to 20th	Day	22% 2 4 15% 42% 15%	3185
	21st to end	Day	18% 2 4 23% 33% 20%	4510
January	1st to 10th	Night	27% 3 3 19% 37% 10%	5867
	11th to 20th	Night	12% 3 5% 20% 41% 19%	5880
	21st to end	Night	26% 3 4 15% 27% 25%	6468
February	1st to 10th	Night	18% 3 7% 21% 28% 23%	2600
	11th to 20th	Night	21% 3 5% 23% 29% 20%	2389
	21st to end	Night	18% 3 3 17% 32% 27%	1915
March	1st to 10th	Night	18% 3 2 12% 43% 22%	4940
	11th to 20th	Night	26% 2 3 20% 31% 17%	5000
	21st to end	Night	16% 2 3 25% 42% 12%	5489
April	1st to 10th	Night	15% 3 4 20% 37% 21%	2015
	11th to 20th	Night	19% 3 3 13% 35% 28%	1977
	21st to end	Night	16% 3 4 16% 38% 22%	2000
May	1st to 10th	Night	20% 2 6% 15% 38% 19%	4537
	11th to 20th	Night	15% 2 5% 12% 38% 27%	4173
	21st to end	Night	11% 2 2 9% 22% 53%	4474
June	1st to 10th	Night	22% 3 5% 16% 25% 28%	1800
	11th to 20th	Night	17% 3 3 9% 24% 43%	1800
	21st to end	Night	31% 5% 10% 12% 18% 24%	1800
July	1st to 10th	Night	30% 6% 11% 14% 16% 22%	4055
	11th to 20th	Night	31% 5% 24% 19% 14% 6%	4046
	21st to end	Night	38% 5% 28% 18% 9% 2	4344
August	1st to 10th	Night	31% 5% 24% 23% 12% 4	2928
	11th to 20th	Night	33% 6% 17% 21% 17% 6%	3238
	21st to end	Night	38% 6% 22% 20% 12% 3	3597
September	1st to 10th	Night	32% 6% 13% 18% 19% 12%	2000
	11th to 20th	Night	23% 4 10% 19% 25% 18%	1986
	21st to end	Night	20% 4 5% 18% 34% 20%	2175
October	1st to 10th	Night	16% 3 5% 14% 33% 29%	3800
	11th to 20th	Night	18% 3 3 22% 32% 23%	3778
	21st to end	Night	9% 2 2 15% 40% 31%	4120
November	1st to 10th	Night	13% 3 2 24% 35% 22%	2340
	11th to 20th	Night	13% 2 4 21% 35% 26%	2400
	21st to end	Night	21% 2 4 21% 32% 20%	2559
December	1st to 10th	Night	15% 4 3 27% 31% 20%	4570
	11th to 20th	Night	19% 2 5% 19% 38% 17%	5048
	21st to end	Night	19% 2 4 25% 33% 17%	6468

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	6% 16% 34% 31% 12% 1	300
	11th to 20th	Day	9% 17% 27% 29% 16% 3	300
	21st to end	Day	4 16% 30% 28% 18% 4	330
February	1st to 10th	Day	5% 17% 35% 27% 13% 4	300
	11th to 20th	Day	8% 19% 34% 31% 8%	300
	21st to end	Day	15% 28% 25% 23% 5% 3	310
March	1st to 10th	Day	13% 24% 32% 21% 9% 2	500
	11th to 20th	Day	14% 23% 32% 19% 10% 2	500
	21st to end	Day	12% 32% 31% 16% 7% 1	550
April	1st to 10th	Day	8% 22% 35% 20% 11% 4	500
	11th to 20th	Day	14% 26% 31% 19% 9% 2	492
	21st to end	Day	9% 23% 35% 22% 9% 2	500
May	1st to 10th	Day	5% 21% 34% 27% 11% 2	503
	11th to 20th	Day	6% 15% 32% 25% 16% 5%	528
	21st to end	Day	6% 15% 33% 29% 15% 2	573
June	1st to 10th	Day	4 8% 28% 34% 20% 7%	530
	11th to 20th	Day	4 8% 24% 32% 25% 7%	530
	21st to end	Day	5% 4 20% 33% 26% 10%	530
July	1st to 10th	Day	6% 3 11% 33% 32% 15%	526
	11th to 20th	Day	7% 2 6% 24% 34% 27%	524
	21st to end	Day	8% 2 5% 21% 34% 31%	575
August	1st to 10th	Day	6% 2 7% 26% 36% 24%	527
	11th to 20th	Day	9% 3 9% 25% 34% 20%	517
	21st to end	Day	7% 3 8% 27% 33% 22%	583
September	1st to 10th	Day	8% 3 9% 29% 35% 15%	530
	11th to 20th	Day	7% 4 19% 35% 26% 10%	523
	21st to end	Day	4 7% 22% 30% 26% 11%	404
October	1st to 10th	Day	6% 8% 22% 23% 27% 13%	300
	11th to 20th	Day	6% 9% 23% 29% 22% 11%	300
	21st to end	Day	6% 11% 19% 32% 25% 8%	330
November	1st to 10th	Day	8% 9% 34% 30% 15% 4	300
	11th to 20th	Day	5% 11% 17% 34% 27% 6%	300
	21st to end	Day	3 23% 26% 27% 15% 6%	300
December	1st to 10th	Day	7% 9% 30% 31% 19% 4	300
	11th to 20th	Day	5% 17% 26% 30% 17% 4	300
	21st to end	Day	11% 18% 31% 26% 12% 2	330
January	1st to 10th	Night	5% 10% 28% 37% 17% 3	800
	11th to 20th	Night	4 10% 31% 36% 17% 2	800
	21st to end	Night	4 13% 30% 34% 17% 2	880
February	1st to 10th	Night	4 14% 33% 33% 15% 2	800
	11th to 20th	Night	4 15% 38% 33% 8% 1	784
	21st to end	Night	5% 22% 35% 30% 7%	660
March	1st to 10th	Night	4 18% 35% 33% 10% 1	800
	11th to 20th	Night	5% 15% 33% 33% 14% 1	800
	21st to end	Night	4 17% 35% 34% 9%	880
April	1st to 10th	Night	2 18% 38% 33% 9%	615
	11th to 20th	Night	3 20% 38% 30% 8% 1	593
	21st to end	Night	3 16% 43% 29% 10%	600
May	1st to 10th	Night	3 9% 40% 40% 8%	600
	11th to 20th	Night	3 8% 30% 40% 16% 2	591
	21st to end	Night	3 6% 35% 44% 11% 1	650
June	1st to 10th	Night	5% 6% 26% 46% 16% 1	600
	11th to 20th	Night	3 4 27% 47% 19% 1	600
	21st to end	Night	5% 3 16% 45% 31% 2	600
July	1st to 10th	Night	4 2 9% 39% 39% 6%	585
	11th to 20th	Night	5% 1 4 29% 53% 7%	582
	21st to end	Night	5% 2 5% 22% 54% 12%	657
August	1st to 10th	Night	3 2 4 22% 56% 13%	597
	11th to 20th	Night	4 3 6% 27% 53% 8%	594
	21st to end	Night	3 1 5% 27% 55% 9%	660
September	1st to 10th	Night	6% 1 9% 38% 43% 4	600
	11th to 20th	Night	6% 1 9% 44% 37% 3	600
	21st to end	Night	4 2 14% 40% 36% 3	760
October	1st to 10th	Night	4 4 22% 42% 24% 3	760
	11th to 20th	Night	3 4 23% 46% 22% 3	760
	21st to end	Night	3 8% 21% 36% 26% 6%	836
November	1st to 10th	Night	3 6% 22% 41% 24% 4	780
	11th to 20th	Night	2 9% 21% 42% 22% 4	800
	21st to end	Night	5% 15% 23% 33% 20% 3	945
December	1st to 10th	Night	5% 12% 25% 41% 16% 2	1000
	11th to 20th	Night	5% 15% 30% 35% 15% 1	940
	21st to end	Night	5% 15% 28% 38% 13% 2	880

Page generated at 2025-09-15 23:49:15. Version ac07

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