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 2024-11-11.

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% 12% 4 9% 5% 5% 4 4 5% 7% 28%	101563
	Night	17% 13% 5% 10% 5% 4 4 3 5% 6% 28%	97580
Transparency	Day	52% 8% 12% 18% 9% 1	105112
	Night	54% 8% 12% 17% 8% 1	102120
Seeing	Day	25% 18% 28% 18% 8% 3	14725
	Night	23% 17% 27% 23% 9% 1	23939

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	13% 20% 14% 14% 6% 4 4 2 3 3 16%	2517
	11th to 20th	Day	13% 19% 10% 20% 7% 6% 3 2 3 4 14%	2520
	21st to end	Day	18% 21% 9% 12% 10% 5% 4 2 3 4 12%	3124
February	1st to 10th	Day	17% 16% 5% 16% 9% 5% 5% 3 5% 4 15%	1710
	11th to 20th	Day	13% 17% 9% 16% 8% 4 5% 3 3 5% 16%	1701
	21st to end	Day	19% 15% 4 14% 8% 5% 4 3 4 4 20%	1570
March	1st to 10th	Day	22% 17% 5% 12% 4 3 3 3 4 4 23%	3201
	11th to 20th	Day	11% 21% 7% 10% 5% 4 3 3 4 5% 27%	3480
	21st to end	Day	18% 14% 5% 12% 5% 4 3 3 4 6% 27%	3974
April	1st to 10th	Day	17% 16% 5% 8% 5% 4 4 3 4 5% 28%	2280
	11th to 20th	Day	18% 13% 3 7% 5% 5% 4 3 5% 5% 33%	2254
	21st to end	Day	19% 14% 3 6% 4 4 4 3 5% 6% 31%	2294
May	1st to 10th	Day	24% 10% 3 6% 4 3 4 3 5% 5% 32%	4345
	11th to 20th	Day	19% 9% 3 8% 6% 5% 5% 4 6% 8% 27%	4398
	21st to end	Day	13% 8% 2 6% 5% 4 5% 4 6% 8% 40%	4837
June	1st to 10th	Day	19% 8% 2 6% 4 4 5% 4 7% 8% 33%	2660
	11th to 20th	Day	15% 7% 2 7% 5% 5% 6% 5% 6% 9% 32%	2660
	21st to end	Day	15% 7% 3 6% 5% 5% 5% 5% 7% 8% 34%	2660
July	1st to 10th	Day	11% 6% 2 6% 4 5% 6% 5% 8% 11% 35%	4440
	11th to 20th	Day	9% 6% 2 7% 6% 6% 6% 6% 9% 11% 33%	4440
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 5% 7% 8% 34%	4798
August	1st to 10th	Day	19% 7% 3 7% 4 5% 6% 4 6% 8% 31%	4104
	11th to 20th	Day	18% 7% 3 8% 5% 6% 6% 4 7% 9% 27%	3921
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 5% 7% 38%	4180
September	1st to 10th	Day	17% 7% 2 7% 4 4 4 3 6% 8% 37%	2280
	11th to 20th	Day	15% 7% 2 7% 4 5% 4 3 4 7% 43%	2280
	21st to end	Day	20% 8% 3 6% 5% 4 5% 4 5% 7% 33%	2024
October	1st to 10th	Day	19% 9% 3 8% 6% 4 5% 4 5% 6% 31%	1900
	11th to 20th	Day	20% 10% 3 7% 5% 6% 4 3 4 6% 31%	1900
	21st to end	Day	23% 17% 3 7% 5% 4 4 3 5% 5% 24%	2021
November	1st to 10th	Day	13% 15% 3 8% 5% 4 4 4 5% 6% 34%	1761
	11th to 20th	Day	16% 15% 5% 12% 5% 4 4 4 4 6% 25%	1611
	21st to end	Day	18% 20% 6% 11% 6% 4 4 2 5% 5% 20%	1520
December	1st to 10th	Day	19% 21% 7% 13% 5% 4 3 3 4 5% 15%	1520
	11th to 20th	Day	20% 16% 8% 14% 5% 3 3 3 4 5% 19%	1906
	21st to end	Day	22% 25% 8% 11% 5% 4 3 3 3 4 13%	2772
January	1st to 10th	Night	16% 16% 8% 19% 6% 4 5% 3 4 4 14%	4429
	11th to 20th	Night	12% 19% 9% 16% 9% 5% 4 3 3 4 16%	4436
	21st to end	Night	22% 17% 7% 15% 10% 6% 4 2 3 3 13%	4532
February	1st to 10th	Night	15% 16% 6% 14% 7% 5% 5% 4 5% 5% 19%	2470
	11th to 20th	Night	13% 14% 7% 16% 9% 4 6% 4 4 5% 18%	2365
	21st to end	Night	18% 12% 5% 15% 7% 5% 4 3 5% 5% 21%	1884
March	1st to 10th	Night	19% 15% 6% 11% 5% 4 3 3 4 5% 25%	3637
	11th to 20th	Night	14% 20% 7% 10% 6% 3 4 3 4 5% 25%	3479
	21st to end	Night	17% 14% 5% 12% 5% 4 4 3 4 4 28%	3643
April	1st to 10th	Night	18% 15% 5% 7% 5% 4 4 3 4 7% 28%	1900
	11th to 20th	Night	20% 12% 4 7% 4 4 4 3 5% 6% 32%	1875
	21st to end	Night	18% 11% 3 6% 5% 5% 4 3 5% 6% 35%	1900
May	1st to 10th	Night	26% 10% 3 7% 4 4 4 3 5% 6% 28%	2594
	11th to 20th	Night	19% 8% 3 7% 4 4 5% 4 5% 7% 33%	2495
	21st to end	Night	13% 7% 3 7% 5% 4 5% 5% 6% 8% 38%	2738
June	1st to 10th	Night	16% 9% 2 6% 4 4 5% 4 5% 8% 36%	1520
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 8% 9% 38%	1520
	21st to end	Night	14% 6% 3 5% 4 5% 5% 5% 5% 8% 40%	1520
July	1st to 10th	Night	10% 8% 2 8% 5% 4 4 4 6% 8% 39%	2520
	11th to 20th	Night	12% 6% 2 4 5% 4 4 3 7% 8% 43%	2520
	21st to end	Night	13% 8% 2 6% 4 5% 4 5% 6% 7% 41%	2739
August	1st to 10th	Night	21% 8% 2 6% 3 4 5% 3 5% 7% 36%	2825
	11th to 20th	Night	17% 8% 3 7% 3 4 4 3 5% 6% 40%	2874
	21st to end	Night	15% 9% 2 6% 4 4 4 3 4 8% 41%	3476
September	1st to 10th	Night	15% 7% 2 6% 4 4 5% 3 5% 6% 43%	1900
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 7% 48%	1900
	21st to end	Night	17% 9% 2 6% 5% 6% 4 4 5% 6% 36%	2153
October	1st to 10th	Night	20% 10% 3 8% 4 4 4 3 5% 6% 33%	2280
	11th to 20th	Night	20% 11% 3 6% 4 4 4 2 4 7% 34%	2280
	21st to end	Night	24% 16% 4 8% 4 3 3 3 4 5% 25%	2575
November	1st to 10th	Night	13% 14% 4 8% 4 5% 4 2 5% 5% 36%	2537
	11th to 20th	Night	18% 14% 4 8% 4 4 5% 4 5% 6% 27%	2572
	21st to end	Night	15% 18% 6% 10% 5% 4 4 4 5% 6% 22%	2660
December	1st to 10th	Night	20% 19% 9% 11% 6% 4 4 3 4 5% 16%	2660
	11th to 20th	Night	18% 18% 6% 13% 6% 5% 5% 4 4 4 18%	3288
	21st to end	Night	25% 17% 9% 10% 5% 4 4 3 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% 7% 10% 12% 2	2518
	11th to 20th	Day	73% 6% 9% 9% 4	2520
	21st to end	Day	75% 6% 9% 7% 2	3124
February	1st to 10th	Day	70% 7% 9% 11% 3	1710
	11th to 20th	Day	69% 6% 8% 12% 4	1710
	21st to end	Day	66% 6% 10% 13% 5% 1	1570
March	1st to 10th	Day	63% 7% 10% 14% 5%	3204
	11th to 20th	Day	55% 9% 14% 13% 8% 1	3480
	21st to end	Day	56% 7% 9% 16% 11% 1	3974
April	1st to 10th	Day	57% 6% 8% 19% 8% 2	2280
	11th to 20th	Day	51% 7% 11% 17% 12% 1	2254
	21st to end	Day	53% 6% 9% 17% 12% 2	2294
May	1st to 10th	Day	52% 6% 11% 18% 11% 2	4359
	11th to 20th	Day	51% 11% 13% 18% 6% 1	4402
	21st to end	Day	37% 11% 14% 23% 15% 1	4837
June	1st to 10th	Day	46% 9% 12% 19% 13% 1	2660
	11th to 20th	Day	44% 9% 16% 20% 10% 1	2660
	21st to end	Day	45% 10% 14% 20% 10%	2660
July	1st to 10th	Day	39% 9% 17% 25% 10% 1	4370
	11th to 20th	Day	39% 11% 16% 24% 10%	4365
	21st to end	Day	44% 10% 14% 22% 10%	4645
August	1st to 10th	Day	48% 8% 15% 19% 9%	4091
	11th to 20th	Day	47% 12% 15% 21% 5%	3960
	21st to end	Day	44% 7% 11% 30% 7% 1	4180
September	1st to 10th	Day	43% 7% 13% 24% 12% 1	2280
	11th to 20th	Day	41% 6% 11% 24% 17% 1	2280
	21st to end	Day	50% 7% 11% 20% 11% 1	2024
October	1st to 10th	Day	43% 9% 11% 25% 11% 1	3160
	11th to 20th	Day	46% 7% 13% 18% 15% 1	3140
	21st to end	Day	44% 8% 17% 23% 8% 1	3311
November	1st to 10th	Day	49% 7% 13% 18% 11% 1	1761
	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	70% 8% 12% 8% 3	4429
	11th to 20th	Night	69% 8% 9% 11% 3	4436
	21st to end	Night	77% 4 7% 9% 2	4532
February	1st to 10th	Night	65% 8% 10% 11% 5%	2470
	11th to 20th	Night	65% 8% 11% 11% 4 1	2367
	21st to end	Night	62% 6% 12% 12% 7% 1	1884
March	1st to 10th	Night	59% 7% 11% 15% 7%	3647
	11th to 20th	Night	57% 8% 14% 16% 5%	3480
	21st to end	Night	53% 9% 9% 18% 9% 2	3643
April	1st to 10th	Night	56% 6% 11% 17% 9% 1	1900
	11th to 20th	Night	51% 7% 12% 16% 14% 1	1875
	21st to end	Night	48% 7% 11% 18% 12% 3	1900
May	1st to 10th	Night	54% 8% 11% 17% 8% 3	2600
	11th to 20th	Night	47% 12% 15% 18% 8% 1	2496
	21st to end	Night	38% 12% 15% 18% 16% 2	2739
June	1st to 10th	Night	42% 8% 14% 22% 14% 1	1520
	11th to 20th	Night	39% 9% 16% 23% 11% 2	1520
	21st to end	Night	38% 9% 14% 25% 13% 1	1520
July	1st to 10th	Night	40% 9% 16% 21% 14% 1	2480
	11th to 20th	Night	36% 9% 14% 27% 13% 1	2472
	21st to end	Night	41% 8% 12% 23% 15% 1	2651
August	1st to 10th	Night	47% 8% 15% 22% 8% 1	2814
	11th to 20th	Night	43% 8% 15% 25% 9%	2901
	21st to end	Night	40% 7% 12% 30% 10% 1	3476
September	1st to 10th	Night	39% 9% 14% 23% 14% 1	1900
	11th to 20th	Night	36% 8% 11% 27% 17% 1	1900
	21st to end	Night	47% 7% 12% 20% 12% 1	2153
October	1st to 10th	Night	51% 8% 10% 21% 10%	3800
	11th to 20th	Night	43% 7% 13% 18% 13% 5%	3779
	21st to end	Night	45% 7% 15% 26% 7%	4235
November	1st to 10th	Night	49% 7% 12% 20% 11% 2	2537
	11th to 20th	Night	56% 7% 11% 16% 7% 2	2572
	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% 27% 29% 5% 1 1	180
	21st to end	Day	28% 31% 31% 8% 3	297
February	1st to 10th	Day	33% 33% 29% 6%	270
	11th to 20th	Day	35% 33% 28% 4	308
	21st to end	Day	37% 23% 29% 10% 1	382
March	1st to 10th	Day	34% 22% 28% 11% 3	460
	11th to 20th	Day	34% 23% 26% 13% 3	460
	21st to end	Day	33% 26% 27% 9% 4 1	506
April	1st to 10th	Day	34% 23% 27% 12% 4 1	460
	11th to 20th	Day	30% 27% 23% 11% 6% 3	452
	21st to end	Day	33% 18% 26% 17% 4 2	460
May	1st to 10th	Day	29% 19% 30% 13% 5% 3	463
	11th to 20th	Day	22% 15% 34% 17% 9% 2	487
	21st to end	Day	18% 14% 30% 23% 11% 3	529
June	1st to 10th	Day	21% 16% 31% 21% 10% 1	490
	11th to 20th	Day	18% 15% 30% 25% 10% 2	490
	21st to end	Day	18% 16% 35% 23% 7% 2	490
July	1st to 10th	Day	13% 10% 32% 28% 14% 4	485
	11th to 20th	Day	11% 10% 29% 31% 14% 4	484
	21st to end	Day	15% 8% 37% 26% 11% 3	532
August	1st to 10th	Day	14% 11% 28% 24% 18% 6%	486
	11th to 20th	Day	16% 11% 27% 26% 13% 7%	479
	21st to end	Day	14% 13% 28% 24% 13% 8%	539
September	1st to 10th	Day	19% 12% 21% 26% 15% 7%	490
	11th to 20th	Day	16% 10% 22% 22% 20% 9%	490
	21st to end	Day	23% 15% 26% 19% 13% 4	490
October	1st to 10th	Day	25% 15% 26% 15% 13% 5%	490
	11th to 20th	Day	26% 18% 29% 19% 7% 2	490
	21st to end	Day	34% 18% 26% 16% 5%	502
November	1st to 10th	Day	26% 20% 25% 18% 9% 1	395
	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% 23% 29% 18% 3	760
	21st to end	Night	21% 24% 35% 17% 2	836
February	1st to 10th	Night	26% 22% 34% 18% 1	760
	11th to 20th	Night	24% 27% 34% 12% 1	743
	21st to end	Night	30% 23% 28% 16% 3	628
March	1st to 10th	Night	26% 25% 31% 17% 2	760
	11th to 20th	Night	26% 22% 31% 19% 3	760
	21st to end	Night	25% 24% 28% 16% 6% 1	836
April	1st to 10th	Night	29% 19% 31% 18% 3	760
	11th to 20th	Night	28% 19% 27% 19% 6%	751
	21st to end	Night	25% 15% 30% 23% 6%	760
May	1st to 10th	Night	25% 14% 22% 30% 8%	434
	11th to 20th	Night	17% 11% 32% 29% 10% 1	374
	21st to end	Night	14% 9% 29% 32% 15% 1	411
June	1st to 10th	Night	18% 10% 26% 35% 11% 1	380
	11th to 20th	Night	16% 11% 26% 34% 14%	380
	21st to end	Night	15% 7% 26% 41% 11%	380
July	1st to 10th	Night	9% 4 25% 43% 18% 2	370
	11th to 20th	Night	12% 3 19% 45% 20% 2	368
	21st to end	Night	13% 4 23% 41% 17% 2	416
August	1st to 10th	Night	15% 4 22% 34% 24% 1	567
	11th to 20th	Night	14% 5% 18% 33% 25% 3	614
	21st to end	Night	14% 8% 19% 34% 23% 1	836
September	1st to 10th	Night	19% 7% 24% 31% 16% 3	760
	11th to 20th	Night	16% 9% 18% 28% 24% 4	760
	21st to end	Night	20% 12% 24% 26% 16% 2	760
October	1st to 10th	Night	24% 13% 22% 22% 16% 2	760
	11th to 20th	Night	26% 15% 27% 26% 6% 1	760
	21st to end	Night	32% 16% 28% 19% 4	836
November	1st to 10th	Night	21% 17% 31% 23% 8%	780
	11th to 20th	Night	28% 19% 30% 16% 6% 1	763
	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 2024-11-08 06:44:50. Version ac07

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