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 2024-04-25.

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	3 4 2 4 5% 3 5% 5% 5% 7% 57%	83464
	Night	2 3 2 4 4 3 4 4 5% 7% 61%	85751
Transparency	Day	26% 4 6% 17% 28% 19%	83191
	Night	22% 4 7% 19% 30% 17%	85545
Seeing	Day	8% 13% 24% 27% 20% 8%	13650
	Night	4 10% 25% 35% 23% 3	24237

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	5% 2 5% 5% 2 4 5% 5% 6% 60%	2858
	11th to 20th	Day	1 3 2 2 3 2 3 4 4 8% 67%	2860
	21st to end	Day	3 8% 3 6% 6% 3 4 5% 6% 8% 48%	3146
February	1st to 10th	Day	1 4 2 4 3 2 4 4 5% 7% 65%	1710
	11th to 20th	Day	2 3 3 4 4 3 4 4 4 7% 63%	1797
	21st to end	Day	2 4 2 4 4 2 2 4 5% 6% 66%	1628
March	1st to 10th	Day	1 3 2 4 4 2 5% 6% 6% 8% 59%	3444
	11th to 20th	Day	5% 9% 3 6% 6% 4 4 5% 5% 8% 47%	3490
	21st to end	Day	1 3 3 4 4 2 4 5% 6% 8% 59%	3825
April	1st to 10th	Day	2 2 1 3 3 2 4 5% 6% 7% 66%	2265
	11th to 20th	Day	3 4 1 4 4 3 4 3 4 6% 66%	2251
	21st to end	Day	2 3 1 3 4 3 4 3 5% 6% 67%	2209
May	1st to 10th	Day	2 2 1 3 3 3 4 4 4 7% 68%	2160
	11th to 20th	Day	2 3 2 4 6% 3 5% 5% 5% 7% 58%	2251
	21st to end	Day	2 3 1 3 3 2 3 3 4 5% 70%	2530
June	1st to 10th	Day	2 3 2 4 4 3 4 5% 5% 8% 61%	2340
	11th to 20th	Day	2 2 1 3 3 2 4 3 4 6% 69%	2340
	21st to end	Day	4 4 2 5% 6% 4 5% 5% 7% 9% 49%	2340
July	1st to 10th	Day	6% 4 2 6% 7% 5% 7% 7% 7% 9% 40%	2340
	11th to 20th	Day	5% 4 2 7% 7% 5% 8% 8% 9% 10% 34%	2340
	21st to end	Day	6% 5% 3 7% 8% 6% 8% 10% 9% 10% 28%	2545
August	1st to 10th	Day	6% 4 3 6% 8% 5% 8% 8% 9% 10% 32%	2328
	11th to 20th	Day	7% 4 2 7% 8% 4 7% 6% 8% 9% 38%	2124
	21st to end	Day	5% 4 2 5% 7% 5% 8% 8% 8% 9% 40%	2376
September	1st to 10th	Day	8% 5% 3 6% 6% 4 5% 6% 6% 7% 45%	2160
	11th to 20th	Day	5% 5% 2 4 5% 4 5% 5% 5% 7% 53%	2151
	21st to end	Day	3 3 1 3 3 3 5% 4 4 7% 65%	1899
October	1st to 10th	Day	5% 3 1 3 5% 3 4 5% 4 5% 62%	1800
	11th to 20th	Day	2 2 1 3 3 2 3 2 2 5% 75%	1800
	21st to end	Day	2 1 1 3 2 1 3 2 4 6% 76%	1980
November	1st to 10th	Day	1 2 1 3 1 2 2 4 5% 7% 73%	1859
	11th to 20th	Day	2 2 1 3 3 2 2 3 4 6% 72%	1900
	21st to end	Day	2 4 2 4 4 2 5% 4 4 6% 63%	1722
December	1st to 10th	Day	3 4 2 3 3 2 3 5% 5% 7% 65%	1520
	11th to 20th	Day	3 5% 3 5% 5% 3 4 5% 5% 7% 55%	2030
	21st to end	Day	1 3 2 3 5% 3 5% 4 5% 7% 62%	3146
January	1st to 10th	Night	1 5% 3 7% 5% 3 4 5% 5% 8% 55%	4097
	11th to 20th	Night	2 2 1 2 2 2 3 4 4 5% 73%	4110
	21st to end	Night	2 7% 3 5% 5% 3 6% 5% 5% 8% 50%	4521
February	1st to 10th	Night	1 3 2 3 3 2 3 3 5% 7% 66%	2470
	11th to 20th	Night	2 3 2 4 4 2 3 4 5% 7% 63%	2267
	21st to end	Night	2 3 1 3 4 2 3 4 4 7% 67%	1822
March	1st to 10th	Night	1 4 1 3 3 2 3 3 5% 7% 68%	3430
	11th to 20th	Night	4 7% 3 6% 5% 3 4 3 5% 6% 54%	3490
	21st to end	Night	1 3 2 3 3 2 4 4 5% 8% 66%	3828
April	1st to 10th	Night	1 2 1 3 3 2 3 4 4 7% 70%	1914
	11th to 20th	Night	2 3 1 3 4 2 3 3 3 6% 70%	1877
	21st to end	Night	2 3 2 3 3 2 3 3 4 6% 70%	1850
May	1st to 10th	Night	2 3 2 3 3 2 3 4 4 7% 69%	1800
	11th to 20th	Night	3 4 3 4 3 3 4 4 5% 7% 62%	1640
	21st to end	Night	2 2 1 2 2 1 3 2 4 5% 75%	1746
June	1st to 10th	Night	4 3 2 3 4 2 4 4 5% 8% 61%	1620
	11th to 20th	Night	2 2 1 3 3 2 4 3 4 6% 70%	1620
	21st to end	Night	3 4 2 3 6% 3 6% 7% 7% 9% 51%	1620
July	1st to 10th	Night	4 4 2 5% 7% 5% 7% 7% 8% 10% 41%	1620
	11th to 20th	Night	3 4 3 6% 6% 4 8% 9% 11% 14% 34%	1620
	21st to end	Night	3 4 3 7% 8% 5% 7% 8% 8% 12% 35%	1773
August	1st to 10th	Night	2 3 2 6% 7% 4 6% 7% 8% 14% 42%	1611
	11th to 20th	Night	3 4 2 6% 6% 4 7% 8% 7% 11% 41%	1773
	21st to end	Night	3 3 2 5% 7% 3 6% 7% 8% 13% 43%	1980
September	1st to 10th	Night	5% 4 2 6% 6% 4 5% 6% 7% 8% 46%	1800
	11th to 20th	Night	5% 3 2 4 4 3 4 5% 6% 8% 58%	1805
	21st to end	Night	3 3 1 3 4 2 4 4 4 7% 65%	2060
October	1st to 10th	Night	4 3 2 4 3 2 3 4 5% 6% 65%	2160
	11th to 20th	Night	2 2 1 2 2 1 2 3 3 4 77%	2160
	21st to end	Night	2 2 1 2 2 2 2 3 3 5% 76%	2376
November	1st to 10th	Night	1 1 1 2 2 2 3 3 4 6% 75%	2220
	11th to 20th	Night	1 2 1 2 3 2 2 3 4 7% 74%	2280
	21st to end	Night	3 4 2 3 4 2 3 3 4 6% 66%	2430
December	1st to 10th	Night	2 3 2 3 3 2 4 4 5% 7% 65%	2660
	11th to 20th	Night	2 3 2 3 5% 4 5% 5% 7% 9% 55%	3180
	21st to end	Night	2 4 2 4 6% 2 3 3 5% 7% 62%	4521

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	25% 4 4 19% 37% 11%	2859
	11th to 20th	Day	17% 2 7% 31% 33% 11%	2860
	21st to end	Day	34% 3 4 19% 30% 10%	3146
February	1st to 10th	Day	19% 3 6% 19% 30% 23%	1710
	11th to 20th	Day	22% 2 3 20% 32% 20%	1807
	21st to end	Day	19% 2 3 14% 34% 28%	1628
March	1st to 10th	Day	23% 4 2 10% 37% 23%	3444
	11th to 20th	Day	36% 3 4 16% 29% 12%	3490
	21st to end	Day	23% 3 3 18% 41% 13%	3825
April	1st to 10th	Day	16% 4 4 16% 37% 24%	2265
	11th to 20th	Day	21% 3 2 11% 31% 31%	2254
	21st to end	Day	20% 3 3 16% 37% 20%	2209
May	1st to 10th	Day	18% 3 4 13% 35% 27%	2160
	11th to 20th	Day	26% 4 7% 14% 28% 21%	2254
	21st to end	Day	17% 3 6% 11% 31% 32%	2530
June	1st to 10th	Day	21% 4 4 16% 26% 28%	2340
	11th to 20th	Day	18% 3 3 8% 22% 45%	2340
	21st to end	Day	30% 6% 7% 14% 23% 20%	2340
July	1st to 10th	Day	38% 7% 12% 14% 16% 14%	2276
	11th to 20th	Day	41% 8% 17% 20% 9% 5%	2271
	21st to end	Day	45% 10% 20% 18% 6% 1	2404
August	1st to 10th	Day	42% 11% 17% 14% 12% 4	2313
	11th to 20th	Day	40% 7% 16% 16% 15% 6%	2124
	21st to end	Day	36% 9% 12% 20% 19% 4	2376
September	1st to 10th	Day	37% 5% 8% 17% 20% 13%	2160
	11th to 20th	Day	31% 5% 7% 16% 24% 18%	2151
	21st to end	Day	21% 3 6% 19% 30% 21%	1899
October	1st to 10th	Day	24% 4 7% 17% 23% 24%	1800
	11th to 20th	Day	16% 2 3 11% 30% 38%	1800
	21st to end	Day	13% 2 5% 17% 40% 22%	1980
November	1st to 10th	Day	12% 3 3 23% 35% 24%	1859
	11th to 20th	Day	15% 2 4 18% 38% 23%	1900
	21st to end	Day	24% 2 4 15% 33% 21%	1722
December	1st to 10th	Day	18% 3 4 27% 30% 17%	1520
	11th to 20th	Day	30% 3 7% 19% 27% 14%	2029
	21st to end	Day	23% 2 5% 18% 29% 23%	3146
January	1st to 10th	Night	28% 3 5% 17% 34% 13%	4097
	11th to 20th	Night	15% 1 8% 28% 38% 10%	4110
	21st to end	Night	32% 2 6% 20% 31% 8%	4521
February	1st to 10th	Night	18% 3 7% 22% 28% 21%	2470
	11th to 20th	Night	21% 3 5% 23% 28% 21%	2267
	21st to end	Night	18% 3 4 18% 32% 25%	1822
March	1st to 10th	Night	18% 3 3 16% 44% 17%	3430
	11th to 20th	Night	32% 2 5% 20% 30% 11%	3490
	21st to end	Night	17% 3 4 27% 39% 10%	3828
April	1st to 10th	Night	15% 3 4 19% 37% 21%	1914
	11th to 20th	Night	18% 3 3 13% 35% 28%	1877
	21st to end	Night	17% 4 4 18% 37% 20%	1850
May	1st to 10th	Night	18% 3 6% 10% 38% 26%	1800
	11th to 20th	Night	23% 4 8% 16% 31% 18%	1641
	21st to end	Night	13% 3 5% 15% 33% 31%	1746
June	1st to 10th	Night	23% 3 5% 15% 25% 28%	1620
	11th to 20th	Night	17% 3 3 10% 24% 42%	1620
	21st to end	Night	29% 5% 10% 13% 19% 24%	1620
July	1st to 10th	Night	34% 9% 13% 16% 16% 12%	1575
	11th to 20th	Night	34% 11% 21% 21% 8% 5%	1566
	21st to end	Night	39% 9% 26% 19% 6% 1	1674
August	1st to 10th	Night	29% 10% 26% 18% 12% 5%	1602
	11th to 20th	Night	34% 8% 19% 18% 14% 6%	1773
	21st to end	Night	29% 8% 20% 23% 16% 4	1980
September	1st to 10th	Night	33% 6% 13% 18% 18% 12%	1800
	11th to 20th	Night	24% 4 11% 18% 24% 18%	1805
	21st to end	Night	20% 4 5% 19% 33% 19%	2060
October	1st to 10th	Night	20% 3 9% 21% 26% 21%	2160
	11th to 20th	Night	13% 2 3 16% 30% 36%	2160
	21st to end	Night	13% 3 4 19% 41% 20%	2376
November	1st to 10th	Night	12% 3 2 25% 36% 22%	2220
	11th to 20th	Night	13% 2 4 22% 35% 24%	2280
	21st to end	Night	21% 2 4 19% 34% 20%	2430
December	1st to 10th	Night	20% 4 6% 24% 32% 15%	2660
	11th to 20th	Night	24% 3 8% 21% 29% 14%	3180
	21st to end	Night	23% 3 5% 20% 29% 20%	4521

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% 17% 34% 29% 12% 1	270
	11th to 20th	Day	10% 18% 27% 26% 16% 3	270
	21st to end	Day	4 17% 29% 28% 19% 3	297
February	1st to 10th	Day	5% 17% 35% 26% 13% 3	270
	11th to 20th	Day	7% 19% 35% 30% 9%	270
	21st to end	Day	17% 31% 25% 21% 5% 2	283
March	1st to 10th	Day	12% 24% 33% 22% 8% 1	460
	11th to 20th	Day	15% 23% 31% 19% 10% 2	460
	21st to end	Day	12% 33% 32% 15% 7% 1	506
April	1st to 10th	Day	7% 24% 36% 20% 10% 3	460
	11th to 20th	Day	14% 25% 31% 19% 9% 2	452
	21st to end	Day	9% 24% 35% 21% 8% 2	437
May	1st to 10th	Day	5% 22% 36% 25% 10% 2	423
	11th to 20th	Day	4 15% 33% 26% 16% 5%	448
	21st to end	Day	6% 16% 33% 29% 13% 2	485
June	1st to 10th	Day	4 8% 27% 34% 21% 6%	450
	11th to 20th	Day	4 10% 24% 32% 24% 7%	450
	21st to end	Day	6% 4 21% 35% 26% 9%	450
July	1st to 10th	Day	7% 3 10% 32% 33% 15%	446
	11th to 20th	Day	8% 2 6% 23% 34% 26%	444
	21st to end	Day	7% 2 5% 21% 33% 32%	487
August	1st to 10th	Day	6% 2 8% 26% 35% 22%	447
	11th to 20th	Day	9% 4 10% 25% 34% 17%	437
	21st to end	Day	7% 3 9% 26% 33% 21%	495
September	1st to 10th	Day	9% 4 10% 29% 34% 15%	450
	11th to 20th	Day	7% 3 18% 36% 26% 10%	450
	21st to end	Day	5% 7% 24% 31% 24% 9%	369
October	1st to 10th	Day	6% 9% 24% 24% 26% 11%	270
	11th to 20th	Day	5% 8% 25% 30% 22% 10%	270
	21st to end	Day	6% 11% 19% 32% 25% 6%	297
November	1st to 10th	Day	7% 10% 36% 29% 15% 4	270
	11th to 20th	Day	5% 10% 18% 34% 27% 6%	270
	21st to end	Day	3 24% 26% 27% 15% 4	270
December	1st to 10th	Day	8% 10% 30% 31% 17% 3	270
	11th to 20th	Day	6% 19% 28% 30% 17% 1	270
	21st to end	Day	12% 18% 32% 27% 11% 1	297
January	1st to 10th	Night	5% 10% 28% 36% 18% 3	760
	11th to 20th	Night	5% 10% 30% 35% 18% 2	760
	21st to end	Night	5% 14% 29% 34% 18% 2	836
February	1st to 10th	Night	4 14% 31% 33% 16% 2	760
	11th to 20th	Night	4 15% 38% 34% 9% 1	744
	21st to end	Night	5% 23% 36% 29% 7%	628
March	1st to 10th	Night	4 19% 34% 33% 10% 1	760
	11th to 20th	Night	5% 14% 33% 33% 15% 1	760
	21st to end	Night	5% 18% 34% 33% 9%	836
April	1st to 10th	Night	2 18% 39% 32% 8% 1	584
	11th to 20th	Night	3 21% 37% 30% 9% 1	563
	21st to end	Night	3 16% 42% 28% 10%	555
May	1st to 10th	Night	3 9% 40% 39% 8%	540
	11th to 20th	Night	4 8% 29% 40% 17% 2	531
	21st to end	Night	3 6% 35% 43% 12% 1	584
June	1st to 10th	Night	5% 6% 27% 45% 16% 1	540
	11th to 20th	Night	3 4 27% 46% 19% 1	540
	21st to end	Night	4 3 16% 44% 31% 2	540
July	1st to 10th	Night	4 2 9% 38% 40% 7%	525
	11th to 20th	Night	5% 1 5% 27% 54% 8%	522
	21st to end	Night	5% 2 5% 19% 56% 13%	591
August	1st to 10th	Night	2 2 4 19% 58% 14%	537
	11th to 20th	Night	4 1 6% 27% 53% 8%	534
	21st to end	Night	3 1 5% 26% 55% 9%	594
September	1st to 10th	Night	6% 10% 36% 44% 5%	540
	11th to 20th	Night	6% 1 8% 42% 39% 3	545
	21st to end	Night	4 2 14% 40% 36% 3	720
October	1st to 10th	Night	5% 5% 23% 41% 24% 3	720
	11th to 20th	Night	2 3 23% 47% 22% 3	720
	21st to end	Night	3 9% 21% 35% 26% 6%	792
November	1st to 10th	Night	2 7% 22% 40% 25% 4	740
	11th to 20th	Night	2 9% 21% 42% 23% 4	760
	21st to end	Night	5% 15% 23% 33% 21% 3	897
December	1st to 10th	Night	5% 13% 25% 41% 16% 2	950
	11th to 20th	Night	5% 16% 30% 34% 15% 1	893
	21st to end	Night	5% 16% 27% 38% 12% 2	836

Page generated at 2024-04-22 07:25:45. Version ac07

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