Virgil Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Virgil Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	17% 11% 5% 8% 6% 3 5% 5% 5% 7% 28%	74723
	Night	17% 12% 5% 8% 6% 3 5% 4 5% 6% 28%	75006
Transparency	Day	54% 8% 12% 17% 9% 1	74429
	Night	54% 8% 12% 16% 8% 1	74834
Seeing	Day	25% 18% 29% 18% 8% 3	13572
	Night	23% 17% 27% 23% 9% 1	22927

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	12% 16% 10% 13% 9% 4 6% 4 5% 5% 15%	1517
	11th to 20th	Day	17% 17% 8% 16% 10% 4 5% 3 4 5% 12%	1456
	21st to end	Day	11% 16% 9% 14% 11% 6% 5% 4 5% 5% 15%	1782
February	1st to 10th	Day	17% 14% 7% 15% 10% 4 6% 3 5% 5% 14%	1620
	11th to 20th	Day	13% 15% 10% 14% 10% 3 6% 4 3 5% 16%	1609
	21st to end	Day	20% 15% 5% 13% 10% 4 5% 3 4 4 18%	1480
March	1st to 10th	Day	15% 14% 7% 15% 6% 3 4 4 4 5% 22%	1840
	11th to 20th	Day	16% 16% 7% 9% 6% 3 4 4 4 5% 26%	1980
	21st to end	Day	20% 14% 5% 8% 6% 3 4 3 4 5% 27%	2268
April	1st to 10th	Day	16% 14% 7% 8% 6% 3 4 4 5% 5% 28%	2160
	11th to 20th	Day	17% 12% 3 7% 5% 3 5% 4 5% 6% 34%	2134
	21st to end	Day	20% 14% 4 5% 5% 3 4 4 5% 6% 31%	2173
May	1st to 10th	Day	19% 12% 4 6% 6% 3 5% 5% 4 6% 31%	2467
	11th to 20th	Day	19% 8% 3 6% 6% 3 5% 5% 5% 7% 33%	2478
	21st to end	Day	14% 8% 3 5% 5% 3 5% 5% 6% 8% 37%	2725
June	1st to 10th	Day	19% 7% 3 5% 5% 3 5% 5% 6% 7% 34%	2520
	11th to 20th	Day	16% 7% 2 6% 5% 3 6% 6% 6% 9% 32%	2520
	21st to end	Day	15% 7% 3 5% 6% 4 6% 6% 6% 8% 34%	2520
July	1st to 10th	Day	12% 6% 3 6% 5% 3 6% 6% 7% 9% 36%	2520
	11th to 20th	Day	11% 6% 3 5% 7% 4 6% 6% 6% 10% 36%	2520
	21st to end	Day	11% 6% 3 6% 7% 4 6% 6% 6% 8% 35%	2741
August	1st to 10th	Day	14% 5% 3 5% 6% 4 7% 7% 7% 10% 34%	2327
	11th to 20th	Day	15% 6% 3 7% 6% 4 6% 5% 7% 10% 30%	2236
	21st to end	Day	15% 7% 3 6% 6% 4 6% 5% 6% 9% 33%	2376
September	1st to 10th	Day	18% 6% 3 7% 5% 3 4 4 6% 8% 36%	2160
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 3 4 7% 42%	2160
	21st to end	Day	19% 7% 3 5% 6% 3 5% 5% 5% 7% 34%	1919
October	1st to 10th	Day	19% 9% 3 7% 7% 3 5% 5% 5% 6% 31%	1800
	11th to 20th	Day	20% 10% 3 6% 7% 5% 5% 4 5% 7% 29%	1800
	21st to end	Day	25% 17% 3 6% 6% 3 4 4 5% 4 23%	1915
November	1st to 10th	Day	15% 14% 4 7% 6% 3 5% 4 4 6% 34%	1671
	11th to 20th	Day	16% 14% 6% 10% 6% 3 4 5% 4 6% 25%	1611
	21st to end	Day	19% 20% 6% 9% 7% 3 4 3 5% 5% 20%	1520
December	1st to 10th	Day	20% 20% 8% 12% 6% 3 4 3 4 5% 15%	1520
	11th to 20th	Day	20% 15% 9% 14% 5% 2 3 3 4 5% 19%	1906
	21st to end	Day	22% 23% 9% 10% 7% 3 4 4 3 4 13%	2772
January	1st to 10th	Night	15% 15% 7% 14% 9% 4 7% 5% 4 4 14%	2649
	11th to 20th	Night	17% 16% 7% 14% 10% 5% 4 4 4 5% 15%	2556
	21st to end	Night	12% 14% 7% 12% 12% 7% 6% 5% 5% 5% 15%	2574
February	1st to 10th	Night	16% 14% 7% 13% 8% 4 5% 4 5% 5% 18%	2340
	11th to 20th	Night	14% 13% 8% 14% 10% 3 6% 5% 4 5% 18%	2237
	21st to end	Night	18% 11% 5% 13% 9% 5% 4 4 5% 6% 20%	1776
March	1st to 10th	Night	14% 13% 9% 12% 7% 3 3 4 5% 5% 23%	2105
	11th to 20th	Night	16% 15% 6% 9% 7% 2 4 4 5% 5% 27%	1979
	21st to end	Night	17% 13% 5% 8% 6% 3 4 5% 5% 5% 28%	2087
April	1st to 10th	Night	18% 14% 6% 6% 6% 3 5% 4 4 6% 29%	1800
	11th to 20th	Night	19% 11% 4 6% 5% 4 4 4 5% 6% 33%	1775
	21st to end	Night	19% 10% 3 6% 6% 4 4 4 5% 6% 34%	1800
May	1st to 10th	Night	20% 11% 4 7% 5% 3 4 5% 4 6% 30%	1492
	11th to 20th	Night	14% 6% 3 6% 5% 3 5% 7% 6% 9% 37%	1415
	21st to end	Night	12% 6% 3 7% 6% 3 4 5% 5% 8% 40%	1550
June	1st to 10th	Night	16% 7% 2 5% 5% 3 5% 5% 5% 8% 37%	1440
	11th to 20th	Night	15% 7% 2 5% 4 3 5% 4 7% 9% 39%	1440
	21st to end	Night	14% 5% 2 4 5% 3 6% 6% 5% 8% 42%	1440
July	1st to 10th	Night	8% 6% 3 5% 5% 3 5% 5% 6% 10% 43%	1440
	11th to 20th	Night	12% 4 2 4 5% 4 4 5% 7% 8% 45%	1440
	21st to end	Night	12% 6% 3 6% 5% 3 4 6% 7% 8% 41%	1574
August	1st to 10th	Night	10% 6% 2 6% 4 3 5% 5% 6% 8% 45%	1611
	11th to 20th	Night	11% 7% 4 8% 5% 4 5% 6% 6% 8% 38%	1645
	21st to end	Night	14% 6% 2 5% 5% 3 4 5% 5% 9% 41%	1980
September	1st to 10th	Night	15% 6% 3 5% 5% 3 5% 5% 5% 7% 42%	1800
	11th to 20th	Night	15% 6% 2 4 4 2 4 3 5% 7% 47%	1800
	21st to end	Night	16% 8% 2 4 7% 4 5% 5% 4 6% 38%	2038
October	1st to 10th	Night	20% 9% 3 6% 6% 3 4 4 5% 6% 33%	2160
	11th to 20th	Night	21% 11% 3 6% 4 3 4 3 4 7% 33%	2160
	21st to end	Night	26% 16% 4 7% 6% 2 3 4 4 5% 23%	2439
November	1st to 10th	Night	13% 14% 4 8% 5% 4 4 3 4 5% 35%	2407
	11th to 20th	Night	19% 13% 5% 8% 5% 3 5% 5% 5% 6% 27%	2565
	21st to end	Night	15% 17% 7% 9% 6% 3 5% 5% 5% 6% 23%	2660
December	1st to 10th	Night	21% 17% 10% 11% 6% 3 5% 3 4 5% 17%	2660
	11th to 20th	Night	18% 17% 7% 12% 7% 4 5% 4 4 4 18%	3288
	21st to end	Night	25% 16% 9% 9% 6% 3 4 4 4 5% 14%	4884

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	69% 6% 10% 11% 4	1518
	11th to 20th	Day	75% 6% 8% 7% 4	1456
	21st to end	Day	70% 6% 11% 9% 4 1	1782
February	1st to 10th	Day	73% 6% 8% 10% 3	1620
	11th to 20th	Day	70% 6% 8% 12% 5%	1618
	21st to end	Day	68% 6% 10% 11% 4 1	1480
March	1st to 10th	Day	62% 7% 13% 13% 5% 1	1843
	11th to 20th	Day	60% 6% 10% 13% 10% 1	1980
	21st to end	Day	59% 5% 9% 15% 11% 2	2268
April	1st to 10th	Day	56% 6% 9% 19% 9% 2	2160
	11th to 20th	Day	50% 7% 11% 18% 12% 1	2134
	21st to end	Day	54% 6% 9% 17% 11% 2	2173
May	1st to 10th	Day	54% 7% 10% 19% 10% 2	2467
	11th to 20th	Day	48% 8% 11% 20% 11% 2	2482
	21st to end	Day	43% 10% 16% 19% 11% 1	2725
June	1st to 10th	Day	47% 8% 12% 19% 13% 1	2520
	11th to 20th	Day	45% 8% 15% 20% 10% 1	2520
	21st to end	Day	45% 10% 14% 20% 11%	2520
July	1st to 10th	Day	43% 9% 13% 23% 11% 1	2450
	11th to 20th	Day	41% 9% 18% 22% 9%	2445
	21st to end	Day	44% 10% 14% 20% 11% 1	2588
August	1st to 10th	Day	41% 10% 18% 21% 9% 1	2314
	11th to 20th	Day	46% 11% 15% 21% 7%	2236
	21st to end	Day	45% 9% 12% 22% 10% 1	2376
September	1st to 10th	Day	45% 7% 13% 23% 11% 1	2160
	11th to 20th	Day	43% 6% 10% 23% 17% 1	2160
	21st to end	Day	48% 7% 11% 21% 11% 1	1919
October	1st to 10th	Day	52% 8% 10% 18% 11% 1	1800
	11th to 20th	Day	53% 8% 12% 20% 7%	1800
	21st to end	Day	62% 7% 12% 11% 6% 1	1915
November	1st to 10th	Day	50% 8% 13% 17% 11% 2	1671
	11th to 20th	Day	57% 7% 13% 15% 6% 2	1611
	21st to end	Day	66% 6% 12% 13% 4	1520
December	1st to 10th	Day	70% 8% 11% 8% 3	1520
	11th to 20th	Day	69% 6% 13% 8% 4	1906
	21st to end	Day	76% 5% 8% 9% 2	2772
January	1st to 10th	Night	69% 7% 10% 9% 4	2649
	11th to 20th	Night	71% 7% 10% 9% 4	2556
	21st to end	Night	68% 6% 11% 10% 4 1	2574
February	1st to 10th	Night	67% 8% 10% 10% 5%	2340
	11th to 20th	Night	66% 8% 10% 11% 4 1	2239
	21st to end	Night	63% 6% 12% 12% 7% 1	1776
March	1st to 10th	Night	60% 6% 12% 16% 5% 1	2115
	11th to 20th	Night	57% 7% 10% 17% 8% 1	1980
	21st to end	Night	54% 7% 9% 15% 12% 2	2087
April	1st to 10th	Night	55% 6% 11% 18% 9% 2	1800
	11th to 20th	Night	50% 7% 12% 16% 14% 1	1775
	21st to end	Night	49% 7% 11% 18% 12% 3	1800
May	1st to 10th	Night	54% 6% 9% 17% 12% 2	1492
	11th to 20th	Night	41% 9% 13% 22% 13% 2	1416
	21st to end	Night	40% 10% 15% 22% 12% 2	1551
June	1st to 10th	Night	42% 7% 14% 22% 14% 1	1440
	11th to 20th	Night	40% 9% 16% 22% 11% 2	1440
	21st to end	Night	38% 9% 14% 25% 13% 1	1440
July	1st to 10th	Night	34% 11% 16% 22% 14% 2	1400
	11th to 20th	Night	34% 9% 15% 28% 13% 1	1392
	21st to end	Night	39% 9% 16% 21% 14% 2	1486
August	1st to 10th	Night	35% 10% 16% 26% 11% 1	1600
	11th to 20th	Night	41% 10% 17% 24% 8%	1645
	21st to end	Night	39% 8% 15% 24% 12% 1	1980
September	1st to 10th	Night	40% 9% 14% 23% 13% 2	1800
	11th to 20th	Night	36% 8% 11% 27% 17% 1	1800
	21st to end	Night	45% 7% 12% 21% 13% 1	2038
October	1st to 10th	Night	51% 8% 12% 19% 11% 1	2160
	11th to 20th	Night	50% 8% 14% 17% 9% 1	2160
	21st to end	Night	62% 6% 12% 13% 6% 1	2439
November	1st to 10th	Night	50% 7% 12% 18% 11% 2	2407
	11th to 20th	Night	56% 7% 11% 17% 7% 2	2565
	21st to end	Night	59% 8% 15% 14% 3	2660
December	1st to 10th	Night	69% 7% 12% 10% 2	2660
	11th to 20th	Night	67% 8% 10% 11% 3	3288
	21st to end	Night	71% 8% 10% 10% 1	4884

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	29% 24% 34% 9% 2 1	180
	11th to 20th	Day	38% 26% 30% 5% 1	164
	21st to end	Day	26% 33% 30% 8% 3	264
February	1st to 10th	Day	35% 33% 28% 4	240
	11th to 20th	Day	36% 33% 28% 3	276
	21st to end	Day	37% 24% 27% 10% 1	346
March	1st to 10th	Day	34% 23% 30% 11% 3	420
	11th to 20th	Day	34% 23% 26% 13% 3	420
	21st to end	Day	34% 24% 26% 10% 5% 1	462
April	1st to 10th	Day	32% 23% 27% 12% 4 1	420
	11th to 20th	Day	28% 26% 24% 11% 7% 3	412
	21st to end	Day	35% 17% 26% 16% 5% 2	420
May	1st to 10th	Day	29% 19% 30% 13% 5% 4	423
	11th to 20th	Day	22% 15% 34% 17% 9% 2	447
	21st to end	Day	19% 14% 30% 22% 12% 4	485
June	1st to 10th	Day	22% 15% 31% 21% 9% 2	450
	11th to 20th	Day	20% 15% 30% 23% 10% 2	450
	21st to end	Day	16% 16% 35% 24% 7% 2	450
July	1st to 10th	Day	13% 9% 29% 29% 15% 4	445
	11th to 20th	Day	12% 9% 28% 32% 15% 5%	444
	21st to end	Day	14% 8% 37% 26% 11% 3	488
August	1st to 10th	Day	13% 9% 29% 24% 19% 6%	446
	11th to 20th	Day	15% 10% 27% 26% 13% 8%	439
	21st to end	Day	13% 13% 29% 24% 13% 8%	495
September	1st to 10th	Day	19% 11% 22% 26% 15% 7%	450
	11th to 20th	Day	16% 11% 24% 22% 18% 8%	450
	21st to end	Day	21% 15% 26% 19% 14% 4	450
October	1st to 10th	Day	25% 14% 26% 14% 14% 5%	450
	11th to 20th	Day	25% 19% 29% 19% 7% 1	450
	21st to end	Day	36% 18% 26% 15% 5%	462
November	1st to 10th	Day	27% 19% 24% 19% 10% 1	365
	11th to 20th	Day	30% 26% 25% 16% 3	271
	21st to end	Day	35% 23% 32% 8% 2	180
December	1st to 10th	Day	34% 25% 29% 7% 4 1	180
	11th to 20th	Day	39% 21% 29% 9% 2	180
	21st to end	Day	34% 24% 35% 6% 1	198
January	1st to 10th	Night	27% 28% 30% 13% 2	760
	11th to 20th	Night	27% 22% 29% 18% 3	732
	21st to end	Night	21% 24% 35% 18% 2	792
February	1st to 10th	Night	27% 23% 34% 17% 1	720
	11th to 20th	Night	24% 27% 34% 13% 1	703
	21st to end	Night	30% 23% 29% 16% 3	592
March	1st to 10th	Night	25% 24% 31% 17% 2	720
	11th to 20th	Night	27% 20% 31% 19% 3	720
	21st to end	Night	25% 23% 28% 16% 7% 1	792
April	1st to 10th	Night	29% 19% 30% 18% 3	720
	11th to 20th	Night	28% 19% 27% 20% 6%	711
	21st to end	Night	26% 14% 31% 23% 6%	720
May	1st to 10th	Night	26% 13% 22% 30% 9%	412
	11th to 20th	Night	16% 11% 33% 30% 9% 1	354
	21st to end	Night	14% 9% 29% 32% 15% 1	389
June	1st to 10th	Night	18% 9% 26% 35% 11% 1	360
	11th to 20th	Night	16% 11% 26% 33% 14%	360
	21st to end	Night	15% 7% 25% 42% 11%	360
July	1st to 10th	Night	9% 4 24% 43% 19% 2	350
	11th to 20th	Night	12% 3 20% 43% 20% 2	348
	21st to end	Night	14% 4 23% 42% 16% 2	394
August	1st to 10th	Night	14% 3 22% 35% 24% 1	537
	11th to 20th	Night	14% 5% 18% 34% 25% 3	581
	21st to end	Night	14% 7% 19% 35% 23% 1	792
September	1st to 10th	Night	18% 7% 24% 32% 17% 3	720
	11th to 20th	Night	17% 9% 19% 28% 23% 4	720
	21st to end	Night	19% 11% 25% 27% 17% 2	720
October	1st to 10th	Night	25% 13% 22% 23% 16% 2	720
	11th to 20th	Night	27% 15% 27% 25% 5% 1	720
	21st to end	Night	33% 16% 28% 19% 4	792
November	1st to 10th	Night	21% 17% 30% 24% 8%	740
	11th to 20th	Night	28% 19% 30% 16% 6% 1	760
	21st to end	Night	26% 22% 29% 19% 4	760
December	1st to 10th	Night	31% 25% 27% 14% 3	760
	11th to 20th	Night	26% 26% 29% 16% 3	760
	21st to end	Night	31% 24% 30% 14% 2	836

Page generated at 2024-01-10 10:16:13. Version ac07

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