Virgo Skyline Observatory.
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Virgo Skyline Observatory. clear sky chart from 2005-11-06 to 2024-01-15.

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 3 2 3 4 2 4 4 5% 8% 63%	74794
	Night	2 3 2 3 4 2 4 4 6% 8% 62%	74992
Transparency	Day	21% 5% 8% 21% 25% 19%	74501
	Night	21% 5% 10% 24% 25% 14%	74806
Seeing	Day	7% 17% 27% 26% 17% 5%	13923
	Night	4 8% 22% 36% 24% 5%	23409

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	2 3 2 4 5% 3 6% 7% 7% 11% 49%	1519
	11th to 20th	Day	1 3 2 5% 4 4 5% 5% 7% 9% 55%	1506
	21st to end	Day	3 5% 3 4 5% 3 5% 6% 8% 12% 46%	1782
February	1st to 10th	Day	2 4 2 3 5% 3 4 6% 8% 10% 54%	1639
	11th to 20th	Day	1 4 3 5% 5% 3 4 5% 7% 11% 51%	1750
	21st to end	Day	1 4 2 5% 5% 3 5% 5% 6% 10% 55%	1480
March	1st to 10th	Day	2 4 2 6% 6% 3 6% 7% 8% 10% 47%	1800
	11th to 20th	Day	2 5% 2 5% 5% 3 5% 6% 7% 11% 50%	1932
	21st to end	Day	1 3 1 4 5% 3 5% 6% 7% 10% 54%	2326
April	1st to 10th	Day	2 3 3 4 5% 3 5% 5% 7% 9% 54%	2160
	11th to 20th	Day	2 3 1 3 4 2 4 4 6% 9% 60%	2129
	21st to end	Day	2 4 1 3 3 2 3 4 6% 9% 63%	2160
May	1st to 10th	Day	2 4 2 4 4 2 4 4 4 8% 62%	2272
	11th to 20th	Day	3 3 2 4 5% 3 4 4 5% 7% 61%	2409
	21st to end	Day	4 4 2 5% 5% 3 5% 5% 5% 7% 54%	2723
June	1st to 10th	Day	2 3 1 3 3 3 4 4 6% 9% 63%	2520
	11th to 20th	Day	1 1 1 2 3 1 2 3 3 5% 77%	2520
	21st to end	Day	2 1 1 2 3 2 3 3 3 5% 76%	2520
July	1st to 10th	Day	1 1 1 2 2 1 3 3 4 4 79%	2520
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 4 76%	2520
	21st to end	Day	2 2 1 3 3 2 2 3 4 5% 71%	2689
August	1st to 10th	Day	2 2 1 2 3 3 3 4 4 5% 72%	2287
	11th to 20th	Day	1 1 1 2 2 1 3 3 4 4 78%	2122
	21st to end	Day	2 1 1 1 1 1 2 2 4 84%	2376
September	1st to 10th	Day	1 1 1 1 2 1 3 2 4 5% 79%	2160
	11th to 20th	Day	1 2 1 2 3 2 3 3 3 6% 75%	2121
	21st to end	Day	1 2 1 3 2 2 3 2 3 4 77%	1980
October	1st to 10th	Day	3 4 1 3 4 2 3 3 4 5% 69%	1848
	11th to 20th	Day	2 2 1 2 2 2 2 4 3 6% 74%	1800
	21st to end	Day	1 2 1 2 3 1 3 4 5% 6% 72%	1926
November	1st to 10th	Day	2 3 2 3 4 2 5% 4 7% 7% 62%	1674
	11th to 20th	Day	1 3 1 2 4 2 4 5% 6% 10% 62%	1710
	21st to end	Day	2 3 2 4 4 3 4 5% 6% 11% 57%	1672
December	1st to 10th	Day	2 4 2 4 4 3 5% 4 7% 11% 53%	1520
	11th to 20th	Day	3 6% 3 7% 8% 4 6% 8% 8% 9% 38%	1928
	21st to end	Day	1 4 4 7% 5% 4 6% 7% 8% 13% 39%	2794
January	1st to 10th	Night	2 5% 2 5% 5% 3 5% 5% 7% 14% 45%	2646
	11th to 20th	Night	1 4 2 5% 5% 3 6% 6% 7% 12% 50%	2540
	21st to end	Night	3 5% 3 6% 6% 3 4 6% 7% 11% 47%	2574
February	1st to 10th	Night	2 4 2 4 4 3 5% 5% 7% 10% 54%	2318
	11th to 20th	Night	1 3 2 4 4 2 4 6% 8% 12% 52%	2098
	21st to end	Night	1 4 3 5% 4 3 4 5% 6% 10% 55%	1776
March	1st to 10th	Night	1 5% 2 4 5% 4 5% 5% 8% 11% 51%	2160
	11th to 20th	Night	1 3 1 4 3 3 4 5% 9% 11% 54%	2026
	21st to end	Night	1 3 2 3 3 3 4 5% 6% 9% 60%	2029
April	1st to 10th	Night	2 3 2 4 4 3 5% 5% 8% 10% 56%	1800
	11th to 20th	Night	1 3 1 3 4 2 4 5% 6% 8% 63%	1779
	21st to end	Night	3 3 1 2 3 2 3 3 6% 7% 66%	1800
May	1st to 10th	Night	3 3 2 3 4 3 4 3 5% 7% 64%	1687
	11th to 20th	Night	2 3 1 2 4 2 3 4 5% 8% 66%	1480
	21st to end	Night	3 4 2 4 4 2 4 4 5% 8% 60%	1552
June	1st to 10th	Night	4 3 1 3 3 2 3 4 4 5% 68%	1440
	11th to 20th	Night	1 1 1 2 2 3 2 3 5% 80%	1440
	21st to end	Night	2 2 1 2 2 1 2 2 3 4 80%	1440
July	1st to 10th	Night	1 1 1 1 1 3 2 3 4 83%	1440
	11th to 20th	Night	1 2 1 2 2 1 3 3 4 5% 78%	1440
	21st to end	Night	3 2 1 2 2 2 3 3 4 5% 74%	1624
August	1st to 10th	Night	2 2 1 3 3 2 3 3 4 4 73%	1650
	11th to 20th	Night	1 1 1 2 2 1 2 3 3 5% 79%	1780
	21st to end	Night	2 2 1 2 2 1 1 2 2 4 83%	1980
September	1st to 10th	Night	1 1 1 1 1 1 2 3 4 85%	1800
	11th to 20th	Night	2 1 1 2 1 1 3 2 4 5% 78%	1837
	21st to end	Night	1 2 1 2 2 1 2 3 3 5% 77%	1980
October	1st to 10th	Night	3 2 2 3 3 2 3 3 4 5% 71%	2110
	11th to 20th	Night	1 3 1 2 2 1 2 3 3 5% 76%	2160
	21st to end	Night	2 2 1 2 2 2 2 3 4 7% 74%	2430
November	1st to 10th	Night	2 3 1 2 3 2 3 4 6% 9% 64%	2405
	11th to 20th	Night	1 3 1 3 3 2 4 4 7% 11% 61%	2470
	21st to end	Night	2 3 2 4 4 3 4 5% 7% 10% 58%	2503
December	1st to 10th	Night	3 4 2 4 5% 3 4 5% 6% 11% 53%	2660
	11th to 20th	Night	4 6% 3 6% 6% 3 6% 6% 8% 13% 39%	3276
	21st to end	Night	2 6% 3 6% 7% 5% 7% 9% 10% 10% 35%	4862

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	28% 8% 16% 31% 15% 1	1519
	11th to 20th	Day	28% 8% 13% 25% 20% 7%	1506
	21st to end	Day	31% 7% 11% 27% 20% 4	1782
February	1st to 10th	Day	26% 7% 11% 28% 20% 7%	1639
	11th to 20th	Day	28% 5% 11% 29% 22% 6%	1760
	21st to end	Day	25% 7% 14% 30% 19% 5%	1480
March	1st to 10th	Day	31% 8% 15% 27% 18% 2	1800
	11th to 20th	Day	30% 6% 13% 23% 22% 5%	1932
	21st to end	Day	25% 8% 20% 29% 14% 4	2326
April	1st to 10th	Day	29% 7% 13% 24% 20% 7%	2160
	11th to 20th	Day	23% 6% 9% 28% 26% 8%	2132
	21st to end	Day	21% 6% 11% 30% 26% 5%	2160
May	1st to 10th	Day	23% 5% 7% 27% 31% 6%	2272
	11th to 20th	Day	26% 5% 6% 26% 29% 8%	2412
	21st to end	Day	31% 5% 9% 24% 24% 7%	2724
June	1st to 10th	Day	21% 5% 7% 22% 32% 12%	2520
	11th to 20th	Day	13% 3 5% 21% 35% 23%	2520
	21st to end	Day	15% 3 2 9% 30% 40%	2520
July	1st to 10th	Day	11% 4 2 9% 22% 52%	2451
	11th to 20th	Day	13% 6% 2 10% 21% 49%	2445
	21st to end	Day	17% 4 2 8% 23% 46%	2539
August	1st to 10th	Day	16% 4 3 10% 22% 44%	2271
	11th to 20th	Day	12% 4 2 6% 29% 47%	2122
	21st to end	Day	9% 2 1 7% 31% 49%	2376
September	1st to 10th	Day	11% 3 2 7% 30% 47%	2160
	11th to 20th	Day	13% 4 2 17% 35% 28%	2121
	21st to end	Day	13% 3 3 17% 37% 27%	1980
October	1st to 10th	Day	19% 4 6% 17% 30% 24%	1848
	11th to 20th	Day	14% 3 5% 24% 34% 20%	1800
	21st to end	Day	15% 5% 6% 27% 31% 16%	1926
November	1st to 10th	Day	23% 6% 10% 24% 28% 10%	1674
	11th to 20th	Day	17% 6% 15% 25% 31% 6%	1710
	21st to end	Day	23% 6% 11% 27% 26% 7%	1672
December	1st to 10th	Day	26% 7% 13% 28% 21% 6%	1520
	11th to 20th	Day	40% 9% 10% 21% 15% 5%	1928
	21st to end	Day	35% 7% 16% 24% 16% 2	2794
January	1st to 10th	Night	30% 6% 15% 31% 17% 2	2646
	11th to 20th	Night	27% 7% 16% 29% 15% 7%	2540
	21st to end	Night	31% 6% 13% 25% 21% 4	2574
February	1st to 10th	Night	26% 7% 12% 28% 21% 6%	2318
	11th to 20th	Night	25% 5% 13% 33% 20% 4	2099
	21st to end	Night	25% 7% 15% 31% 18% 5%	1776
March	1st to 10th	Night	27% 6% 16% 26% 21% 4	2160
	11th to 20th	Night	22% 9% 17% 25% 21% 6%	2026
	21st to end	Night	22% 7% 21% 30% 16% 4	2029
April	1st to 10th	Night	24% 7% 16% 30% 17% 6%	1800
	11th to 20th	Night	21% 6% 15% 31% 20% 7%	1779
	21st to end	Night	19% 6% 14% 35% 22% 3	1800
May	1st to 10th	Night	22% 6% 9% 28% 31% 5%	1687
	11th to 20th	Night	18% 5% 12% 31% 27% 8%	1480
	21st to end	Night	25% 4 13% 31% 24% 4	1552
June	1st to 10th	Night	20% 5% 9% 26% 29% 11%	1440
	11th to 20th	Night	11% 4 5% 22% 36% 23%	1440
	21st to end	Night	12% 3 3 15% 33% 34%	1440
July	1st to 10th	Night	8% 3 1 10% 32% 47%	1400
	11th to 20th	Night	11% 4 3 11% 27% 44%	1392
	21st to end	Night	16% 4 3 8% 27% 42%	1534
August	1st to 10th	Night	15% 4 4 15% 24% 38%	1641
	11th to 20th	Night	11% 3 1 11% 33% 40%	1780
	21st to end	Night	9% 2 2 9% 34% 45%	1980
September	1st to 10th	Night	7% 3 2 8% 35% 46%	1800
	11th to 20th	Night	12% 4 4 20% 34% 26%	1837
	21st to end	Night	13% 4 3 19% 36% 25%	1980
October	1st to 10th	Night	18% 3 7% 21% 29% 22%	2110
	11th to 20th	Night	12% 3 6% 24% 36% 19%	2160
	21st to end	Night	13% 3 7% 27% 36% 14%	2430
November	1st to 10th	Night	17% 6% 10% 28% 26% 12%	2405
	11th to 20th	Night	18% 6% 11% 29% 29% 6%	2470
	21st to end	Night	21% 6% 10% 29% 26% 8%	2503
December	1st to 10th	Night	26% 8% 13% 27% 21% 5%	2660
	11th to 20th	Night	34% 8% 15% 22% 17% 4	3276
	21st to end	Night	37% 6% 15% 23% 15% 3	4862

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	18% 18% 29% 21% 13% 2	180
	11th to 20th	Day	9% 21% 30% 20% 15% 5%	202
	21st to end	Day	8% 14% 30% 27% 18% 4	374
February	1st to 10th	Day	11% 18% 27% 25% 16% 3	349
	11th to 20th	Day	8% 21% 28% 27% 14% 1	415
	21st to end	Day	9% 20% 32% 27% 12% 1	346
March	1st to 10th	Day	10% 25% 32% 22% 10% 1	420
	11th to 20th	Day	10% 25% 26% 20% 14% 3	420
	21st to end	Day	11% 28% 27% 23% 10%	462
April	1st to 10th	Day	12% 26% 28% 20% 11% 3	420
	11th to 20th	Day	16% 26% 26% 20% 9% 3	412
	21st to end	Day	10% 24% 27% 23% 11% 4	420
May	1st to 10th	Day	13% 23% 29% 22% 10% 4	423
	11th to 20th	Day	11% 24% 29% 19% 10% 6%	448
	21st to end	Day	10% 19% 24% 24% 18% 6%	485
June	1st to 10th	Day	8% 19% 27% 23% 17% 6%	450
	11th to 20th	Day	9% 19% 31% 23% 13% 5%	450
	21st to end	Day	3 12% 30% 30% 18% 7%	450
July	1st to 10th	Day	1 13% 32% 28% 20% 6%	446
	11th to 20th	Day	2 12% 28% 28% 22% 8%	444
	21st to end	Day	3 8% 29% 30% 23% 8%	487
August	1st to 10th	Day	4 14% 25% 26% 24% 8%	447
	11th to 20th	Day	2 10% 29% 26% 23% 11%	437
	21st to end	Day	2 9% 30% 31% 23% 5%	495
September	1st to 10th	Day	3 8% 30% 29% 23% 7%	450
	11th to 20th	Day	3 16% 26% 29% 21% 5%	450
	21st to end	Day	4 13% 28% 33% 19% 3	450
October	1st to 10th	Day	5% 16% 24% 28% 20% 7%	450
	11th to 20th	Day	7% 11% 23% 29% 24% 6%	450
	21st to end	Day	5% 15% 24% 27% 21% 7%	441
November	1st to 10th	Day	10% 14% 20% 24% 20% 11%	270
	11th to 20th	Day	5% 13% 20% 31% 22% 10%	270
	21st to end	Day	6% 13% 27% 26% 21% 7%	252
December	1st to 10th	Day	11% 16% 21% 25% 19% 8%	180
	11th to 20th	Day	6% 17% 29% 25% 16% 8%	180
	21st to end	Day	15% 13% 26% 25% 17% 5%	198
January	1st to 10th	Night	6% 13% 25% 28% 20% 8%	760
	11th to 20th	Night	4 11% 25% 37% 17% 6%	736
	21st to end	Night	5% 7% 28% 36% 18% 6%	792
February	1st to 10th	Night	6% 9% 25% 34% 22% 4	720
	11th to 20th	Night	3 11% 29% 32% 19% 6%	704
	21st to end	Night	6% 15% 26% 36% 14% 4	592
March	1st to 10th	Night	6% 12% 33% 30% 15% 4	720
	11th to 20th	Night	4 14% 26% 29% 23% 4	720
	21st to end	Night	4 16% 31% 33% 15% 2	792
April	1st to 10th	Night	5% 11% 29% 34% 19% 2	720
	11th to 20th	Night	6% 14% 25% 33% 17% 4	711
	21st to end	Night	5% 11% 25% 34% 20% 5%	720
May	1st to 10th	Night	6% 10% 27% 32% 20% 4	607
	11th to 20th	Night	4 8% 22% 40% 23% 3	418
	21st to end	Night	4 6% 20% 37% 30% 3	389
June	1st to 10th	Night	5% 3 22% 41% 26% 3	360
	11th to 20th	Night	1 5% 20% 43% 30% 1	360
	21st to end	Night	2 2 18% 46% 31% 2	360
July	1st to 10th	Night	1 1 11% 55% 31% 1	350
	11th to 20th	Night	1 1 14% 52% 31% 1	348
	21st to end	Night	3 1 8% 43% 39% 5%	442
August	1st to 10th	Night	4 1 11% 41% 38% 5%	576
	11th to 20th	Night	2 2 14% 45% 32% 4	712
	21st to end	Night	2 2 15% 47% 31% 3	792
September	1st to 10th	Night	1 2 13% 40% 38% 6%	720
	11th to 20th	Night	2 6% 14% 35% 35% 8%	720
	21st to end	Night	3 4 19% 33% 33% 8%	720
October	1st to 10th	Night	3 5% 20% 38% 26% 9%	720
	11th to 20th	Night	4 7% 12% 38% 31% 7%	720
	21st to end	Night	3 7% 18% 33% 26% 13%	792
November	1st to 10th	Night	4 9% 20% 31% 28% 8%	740
	11th to 20th	Night	4 7% 25% 39% 19% 7%	760
	21st to end	Night	6% 9% 23% 31% 25% 6%	760
December	1st to 10th	Night	7% 10% 26% 34% 18% 6%	760
	11th to 20th	Night	6% 12% 25% 35% 17% 5%	760
	21st to end	Night	7% 11% 30% 35% 14% 3	836

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