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 2025-12-18.

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	16% 12% 5% 10% 5% 5% 4 4 5% 6% 28%	127505
	Night	17% 13% 5% 11% 6% 4 4 3 5% 6% 27%	123475
Transparency	Day	53% 8% 12% 18% 9% 1	131036
	Night	55% 8% 12% 16% 9% 1	128031
Seeing	Day	25% 18% 28% 17% 8% 3	16083
	Night	24% 17% 27% 23% 9% 1	25347

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	10% 18% 13% 22% 10% 3 3 2 3 3 11%	3597
	11th to 20th	Day	12% 18% 11% 23% 9% 5% 2 2 3 4 11%	3570
	21st to end	Day	15% 17% 8% 12% 9% 4 5% 3 5% 5% 18%	4466
February	1st to 10th	Day	16% 15% 5% 17% 9% 6% 5% 3 5% 4 15%	1800
	11th to 20th	Day	12% 17% 8% 17% 9% 5% 5% 3 3 5% 16%	1793
	21st to end	Day	19% 15% 4 14% 8% 5% 4 3 4 4 20%	1650
March	1st to 10th	Day	19% 16% 7% 13% 6% 5% 3 3 3 4 22%	4603
	11th to 20th	Day	11% 17% 7% 9% 5% 3 3 3 4 6% 32%	4980
	21st to end	Day	15% 14% 5% 14% 6% 5% 4 3 4 5% 25%	5694
April	1st to 10th	Day	18% 16% 6% 9% 5% 4 4 3 4 5% 27%	2400
	11th to 20th	Day	18% 13% 3 8% 4 5% 4 3 5% 5% 33%	2374
	21st to end	Day	19% 14% 3 6% 4 4 4 3 5% 6% 32%	2415
May	1st to 10th	Day	30% 11% 3 6% 4 3 4 3 4 5% 26%	6223
	11th to 20th	Day	19% 11% 3 8% 5% 4 4 4 5% 7% 29%	6318
	21st to end	Day	20% 11% 3 7% 4 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	18% 8% 2 6% 4 4 5% 4 7% 7% 33%	2800
	11th to 20th	Day	16% 8% 2 7% 5% 5% 6% 5% 6% 9% 31%	2800
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 8% 34%	2800
July	1st to 10th	Day	11% 6% 2 7% 4 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 6% 8% 11% 33%	6360
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	6910
August	1st to 10th	Day	18% 7% 3 7% 4 5% 6% 4 6% 8% 32%	4234
	11th to 20th	Day	18% 7% 3 8% 5% 6% 6% 4 7% 9% 28%	4047
	21st to end	Day	17% 8% 2 6% 4 4 5% 3 5% 8% 38%	4312
September	1st to 10th	Day	16% 7% 2 7% 4 4 4 3 6% 8% 39%	2400
	11th to 20th	Day	14% 7% 2 7% 4 5% 4 3 4 7% 44%	2400
	21st to end	Day	20% 8% 3 6% 5% 4 5% 4 5% 7% 33%	2130
October	1st to 10th	Day	18% 8% 3 8% 6% 4 5% 4 5% 6% 33%	2000
	11th to 20th	Day	20% 10% 3 7% 5% 6% 5% 3 4 6% 31%	2000
	21st to end	Day	23% 17% 3 7% 5% 4 4 3 5% 5% 25%	2127
November	1st to 10th	Day	13% 15% 4 8% 5% 4 4 3 4 6% 33%	1851
	11th to 20th	Day	15% 16% 6% 12% 5% 4 4 4 5% 6% 24%	1780
	21st to end	Day	18% 22% 6% 11% 6% 4 4 2 5% 5% 18%	1680
December	1st to 10th	Day	17% 21% 10% 18% 6% 4 3 3 4 4 10%	2680
	11th to 20th	Day	18% 17% 10% 15% 4 3 3 3 3 4 21%	3042
	21st to end	Day	18% 21% 7% 12% 6% 6% 4 3 4 5% 14%	3960
January	1st to 10th	Night	13% 14% 9% 28% 10% 4 4 3 3 3 10%	6349
	11th to 20th	Night	9% 17% 9% 20% 9% 5% 4 3 4 5% 14%	6297
	21st to end	Night	15% 13% 6% 14% 10% 7% 5% 3 4 5% 17%	6490
February	1st to 10th	Night	15% 16% 5% 14% 7% 5% 5% 3 5% 6% 18%	2600
	11th to 20th	Night	13% 14% 7% 17% 9% 4 6% 4 4 5% 17%	2493
	21st to end	Night	17% 13% 4 15% 8% 5% 5% 3 5% 5% 20%	1980
March	1st to 10th	Night	17% 12% 7% 14% 9% 4 3 3 5% 5% 22%	5235
	11th to 20th	Night	15% 16% 5% 9% 4 3 3 3 4 6% 32%	4979
	21st to end	Night	18% 14% 5% 13% 5% 4 4 3 4 4 25%	5223
April	1st to 10th	Night	18% 15% 5% 8% 5% 5% 4 3 4 6% 27%	2000
	11th to 20th	Night	20% 13% 4 7% 4 4 4 3 5% 6% 32%	1975
	21st to end	Night	18% 11% 3 6% 5% 5% 4 3 5% 6% 35%	2000
May	1st to 10th	Night	33% 11% 3 7% 4 4 4 3 4 5% 23%	3716
	11th to 20th	Night	19% 9% 3 7% 4 4 5% 4 5% 7% 34%	3575
	21st to end	Night	22% 11% 3 7% 4 4 4 4 5% 6% 30%	3926
June	1st to 10th	Night	17% 9% 2 6% 4 4 5% 4 5% 8% 36%	1600
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 8% 9% 38%	1600
	21st to end	Night	14% 6% 3 5% 4 5% 5% 5% 5% 8% 41%	1600
July	1st to 10th	Night	9% 8% 2 8% 5% 4 4 4 6% 10% 39%	3600
	11th to 20th	Night	14% 6% 2 5% 5% 4 4 3 6% 8% 41%	3600
	21st to end	Night	11% 7% 2 6% 4 4 3 4 6% 7% 46%	3926
August	1st to 10th	Night	21% 7% 2 6% 3 4 5% 3 5% 7% 37%	2915
	11th to 20th	Night	17% 8% 3 7% 3 4 4 3 5% 6% 40%	2967
	21st to end	Night	15% 9% 2 6% 4 4 4 3 4 8% 41%	3586
September	1st to 10th	Night	14% 7% 2 6% 4 4 5% 3 5% 6% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 7% 48%	2000
	21st to end	Night	17% 9% 2 6% 5% 5% 5% 3 5% 6% 36%	2266
October	1st to 10th	Night	19% 9% 3 8% 4 4 4 3 5% 6% 35%	2400
	11th to 20th	Night	20% 11% 3 6% 4 4 4 2 4 7% 34%	2400
	21st to end	Night	23% 16% 4 8% 5% 3 3 3 4 5% 25%	2711
November	1st to 10th	Night	13% 14% 4 9% 4 5% 4 2 4 5% 35%	2667
	11th to 20th	Night	17% 15% 5% 9% 4 4 4 4 5% 5% 26%	2836
	21st to end	Night	16% 19% 6% 10% 5% 4 4 4 5% 6% 21%	2940
December	1st to 10th	Night	15% 18% 8% 16% 6% 5% 5% 4 5% 5% 14%	4720
	11th to 20th	Night	21% 17% 7% 10% 5% 4 4 3 4 4 22%	5307
	21st to end	Night	21% 18% 8% 10% 7% 4 4 2 3 4 17%	6996

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	75% 5% 7% 10% 2	3598
	11th to 20th	Day	77% 6% 7% 7% 3	3600
	21st to end	Day	66% 7% 9% 13% 4	4466
February	1st to 10th	Day	71% 6% 9% 11% 3	1800
	11th to 20th	Day	70% 6% 8% 12% 4	1802
	21st to end	Day	66% 6% 10% 13% 5%	1650
March	1st to 10th	Day	64% 8% 10% 13% 5%	4606
	11th to 20th	Day	51% 7% 19% 14% 8% 2	4980
	21st to end	Day	58% 7% 8% 14% 12% 1	5694
April	1st to 10th	Day	57% 6% 8% 19% 8% 2	2400
	11th to 20th	Day	51% 7% 11% 17% 12% 1	2374
	21st to end	Day	52% 6% 9% 18% 12% 2	2415
May	1st to 10th	Day	60% 6% 9% 14% 9% 1	6234
	11th to 20th	Day	52% 9% 10% 18% 10% 1	6322
	21st to end	Day	49% 9% 12% 18% 11% 1	6949
June	1st to 10th	Day	46% 9% 12% 19% 13% 1	2800
	11th to 20th	Day	45% 9% 16% 20% 10% 1	2800
	21st to end	Day	45% 10% 14% 20% 11% 1	2800
July	1st to 10th	Day	39% 9% 15% 26% 10% 1	6290
	11th to 20th	Day	40% 10% 16% 24% 10% 1	6285
	21st to end	Day	39% 8% 12% 27% 13%	6712
August	1st to 10th	Day	47% 8% 15% 19% 10% 1	4221
	11th to 20th	Day	47% 11% 15% 21% 5%	4086
	21st to end	Day	43% 7% 11% 30% 7% 1	4312
September	1st to 10th	Day	42% 7% 12% 26% 12% 1	2400
	11th to 20th	Day	40% 6% 10% 26% 17% 1	2400
	21st to end	Day	50% 8% 11% 20% 10% 1	2130
October	1st to 10th	Day	42% 9% 11% 26% 11% 1	3260
	11th to 20th	Day	46% 7% 13% 18% 15% 1	3240
	21st to end	Day	44% 8% 17% 23% 8% 1	3417
November	1st to 10th	Day	50% 7% 13% 18% 10% 1	1851
	11th to 20th	Day	57% 7% 13% 15% 6% 2	1780
	21st to end	Day	67% 6% 12% 12% 3	1680
December	1st to 10th	Day	75% 8% 8% 7% 2	2680
	11th to 20th	Day	69% 5% 9% 10% 7%	3042
	21st to end	Day	71% 5% 10% 10% 4	3960
January	1st to 10th	Night	76% 8% 8% 6% 2	6349
	11th to 20th	Night	68% 8% 10% 10% 4 1	6342
	21st to end	Night	67% 6% 8% 9% 9% 1	6490
February	1st to 10th	Night	65% 8% 10% 12% 5%	2600
	11th to 20th	Night	66% 8% 11% 11% 4 1	2495
	21st to end	Night	63% 7% 12% 12% 6% 1	1980
March	1st to 10th	Night	61% 7% 13% 12% 7%	5245
	11th to 20th	Night	51% 7% 19% 15% 5% 4	4980
	21st to end	Night	57% 8% 7% 17% 10% 1	5223
April	1st to 10th	Night	57% 6% 10% 17% 8% 1	2000
	11th to 20th	Night	51% 6% 12% 16% 14% 1	1975
	21st to end	Night	48% 7% 11% 19% 13% 3	2000
May	1st to 10th	Night	62% 7% 8% 13% 8% 3	3722
	11th to 20th	Night	47% 10% 13% 19% 10% 1	3576
	21st to end	Night	49% 10% 12% 16% 11% 1	3927
June	1st to 10th	Night	43% 7% 14% 21% 14% 1	1600
	11th to 20th	Night	41% 9% 16% 22% 11% 2	1600
	21st to end	Night	38% 9% 15% 25% 12% 1	1600
July	1st to 10th	Night	39% 8% 17% 22% 13% 1	3560
	11th to 20th	Night	39% 9% 13% 25% 13% 1	3552
	21st to end	Night	35% 7% 13% 27% 17% 1	3809
August	1st to 10th	Night	46% 8% 14% 22% 9% 1	2904
	11th to 20th	Night	42% 8% 15% 26% 9%	2994
	21st to end	Night	40% 7% 12% 30% 10% 1	3586
September	1st to 10th	Night	39% 9% 14% 23% 14% 1	2000
	11th to 20th	Night	35% 8% 11% 29% 16% 1	2000
	21st to end	Night	47% 7% 12% 19% 12% 1	2266
October	1st to 10th	Night	50% 8% 9% 22% 11% 1	3920
	11th to 20th	Night	43% 7% 13% 18% 13% 5%	3899
	21st to end	Night	45% 7% 14% 26% 8%	4371
November	1st to 10th	Night	49% 6% 12% 20% 11% 2	2667
	11th to 20th	Night	56% 8% 11% 17% 7% 2	2836
	21st to end	Night	60% 8% 14% 14% 3	2940
December	1st to 10th	Night	66% 9% 11% 12% 1	4720
	11th to 20th	Night	64% 7% 9% 11% 8%	5307
	21st to end	Night	70% 6% 8% 9% 6% 1	6996

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	31% 27% 32% 9% 2 1	200
	11th to 20th	Day	39% 28% 28% 5% 1 1	200
	21st to end	Day	29% 32% 29% 7% 2	330
February	1st to 10th	Day	35% 32% 27% 5%	300
	11th to 20th	Day	38% 31% 27% 3	340
	21st to end	Day	36% 24% 30% 9% 1	414
March	1st to 10th	Day	35% 23% 29% 10% 3	500
	11th to 20th	Day	34% 23% 26% 13% 3	500
	21st to end	Day	34% 26% 27% 9% 4 1	550
April	1st to 10th	Day	36% 23% 25% 11% 4 1	500
	11th to 20th	Day	32% 27% 23% 10% 6% 3	492
	21st to end	Day	32% 20% 26% 16% 4 1	500
May	1st to 10th	Day	31% 20% 29% 12% 5% 3	503
	11th to 20th	Day	24% 15% 34% 17% 9% 2	527
	21st to end	Day	20% 14% 30% 21% 10% 3	573
June	1st to 10th	Day	21% 16% 30% 22% 9% 2	530
	11th to 20th	Day	20% 15% 30% 24% 10% 2	530
	21st to end	Day	17% 16% 35% 24% 6% 2	530
July	1st to 10th	Day	12% 10% 33% 28% 13% 4	525
	11th to 20th	Day	10% 11% 29% 31% 14% 4	524
	21st to end	Day	15% 9% 37% 27% 11% 3	576
August	1st to 10th	Day	13% 10% 28% 25% 19% 5%	526
	11th to 20th	Day	16% 11% 26% 27% 13% 6%	519
	21st to end	Day	14% 13% 29% 23% 13% 8%	583
September	1st to 10th	Day	18% 12% 21% 26% 15% 7%	530
	11th to 20th	Day	15% 9% 23% 22% 21% 10%	530
	21st to end	Day	23% 15% 27% 19% 12% 4	530
October	1st to 10th	Day	24% 15% 26% 16% 14% 6%	530
	11th to 20th	Day	28% 18% 28% 18% 7% 2	530
	21st to end	Day	33% 18% 27% 16% 5%	542
November	1st to 10th	Day	28% 21% 24% 17% 8% 1	425
	11th to 20th	Day	30% 25% 25% 16% 3	320
	21st to end	Day	38% 24% 30% 6% 1	220
December	1st to 10th	Day	34% 27% 28% 7% 3	220
	11th to 20th	Day	40% 21% 29% 7% 2	214
	21st to end	Day	33% 22% 37% 7% 1	220
January	1st to 10th	Night	27% 30% 30% 12% 2	800
	11th to 20th	Night	27% 23% 30% 17% 3	800
	21st to end	Night	21% 25% 35% 17% 2	880
February	1st to 10th	Night	27% 23% 32% 18% 1	800
	11th to 20th	Night	26% 27% 34% 12% 1	783
	21st to end	Night	30% 23% 29% 16% 3	660
March	1st to 10th	Night	26% 25% 31% 17% 2	800
	11th to 20th	Night	26% 22% 31% 18% 3	800
	21st to end	Night	26% 24% 29% 15% 6% 1	880
April	1st to 10th	Night	31% 19% 30% 18% 3	800
	11th to 20th	Night	29% 20% 26% 19% 6%	791
	21st to end	Night	25% 15% 31% 23% 6%	800
May	1st to 10th	Night	26% 14% 21% 30% 8%	457
	11th to 20th	Night	18% 11% 31% 29% 9% 1	394
	21st to end	Night	15% 10% 28% 32% 14% 1	433
June	1st to 10th	Night	19% 10% 26% 34% 10% 1	400
	11th to 20th	Night	17% 11% 25% 33% 14%	400
	21st to end	Night	16% 7% 25% 41% 12%	400
July	1st to 10th	Night	9% 4 24% 44% 17% 2	390
	11th to 20th	Night	12% 3 19% 45% 19% 2	388
	21st to end	Night	13% 4 23% 42% 17% 2	438
August	1st to 10th	Night	14% 4 23% 34% 25% 1	597
	11th to 20th	Night	14% 6% 18% 33% 26% 3	647
	21st to end	Night	15% 8% 19% 34% 23% 1	880
September	1st to 10th	Night	18% 7% 24% 32% 17% 2	800
	11th to 20th	Night	15% 8% 18% 29% 25% 4	800
	21st to end	Night	20% 11% 25% 26% 16% 2	800
October	1st to 10th	Night	24% 13% 22% 23% 17% 2	800
	11th to 20th	Night	26% 15% 27% 25% 6% 1	800
	21st to end	Night	32% 17% 28% 20% 4	880
November	1st to 10th	Night	23% 17% 30% 23% 7%	820
	11th to 20th	Night	29% 19% 30% 16% 5% 1	840
	21st to end	Night	27% 22% 28% 18% 4	840
December	1st to 10th	Night	30% 25% 28% 14% 3	840
	11th to 20th	Night	27% 26% 28% 16% 2	829
	21st to end	Night	30% 24% 29% 14% 2	880

Page generated at 2025-12-15 01:13:55. Version ac07

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