Sola Virtus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sola Virtus Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	15% 7% 2 6% 4 4 5% 4 6% 8% 40%	126788
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	123245
Transparency	Day	27% 38% 7% 15% 11% 2	130379
	Night	26% 38% 7% 15% 11% 2	127724
Seeing	Day	20% 13% 28% 23% 12% 4	16252
	Night	18% 11% 30% 28% 11% 1	25618

By Month

Percentage Cloud Cover SE (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% 5% 2 5% 4 3 4 3 6% 6% 48%	3597
	11th to 20th	Day	13% 8% 3 9% 6% 4 4 3 4 5% 41%	3950
	21st to end	Day	15% 8% 3 7% 4 4 4 3 5% 6% 41%	4466
February	1st to 10th	Day	13% 9% 3 7% 4 4 5% 4 6% 7% 38%	1800
	11th to 20th	Day	15% 9% 3 6% 5% 4 4 4 6% 6% 38%	1833
	21st to end	Day	17% 10% 2 7% 5% 3 3 2 4 6% 39%	1650
March	1st to 10th	Day	19% 5% 2 5% 4 4 4 3 4 5% 46%	4709
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 6% 7% 46%	4980
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 6% 41%	5642
April	1st to 10th	Day	15% 8% 2 6% 4 5% 4 5% 6% 7% 38%	2400
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 5% 7% 43%	2374
	21st to end	Day	17% 6% 2 6% 4 4 3 4 6% 7% 41%	2400
May	1st to 10th	Day	25% 7% 2 6% 4 4 4 4 6% 8% 30%	5757
	11th to 20th	Day	24% 7% 3 6% 4 4 4 4 6% 8% 30%	6318
	21st to end	Day	17% 5% 2 6% 4 4 5% 4 7% 9% 36%	6934
June	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 5% 7% 10% 38%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 8% 8% 38%	2800
	21st to end	Day	10% 3 2 6% 5% 5% 5% 4 7% 10% 42%	2800
July	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 40%	6360
	11th to 20th	Day	10% 7% 3 8% 6% 6% 7% 6% 9% 10% 29%	6360
	21st to end	Day	9% 7% 2 7% 5% 5% 5% 5% 7% 10% 38%	6481
August	1st to 10th	Day	17% 6% 2 6% 5% 5% 6% 5% 6% 8% 33%	4234
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 8% 9% 37%	3957
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 47%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	12% 6% 1 5% 4 4 5% 3 5% 8% 48%	2400
	21st to end	Day	20% 8% 3 6% 4 4 4 4 4 8% 36%	2125
October	1st to 10th	Day	17% 7% 2 5% 3 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 6% 6% 47%	2000
	21st to end	Day	15% 6% 2 6% 3 3 4 4 4 6% 46%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 5% 7% 51%	1804
	11th to 20th	Day	15% 6% 2 4 3 4 4 4 5% 6% 48%	1795
	21st to end	Day	15% 7% 2 5% 4 4 4 3 4 7% 45%	1600
December	1st to 10th	Day	12% 6% 2 7% 5% 4 4 4 6% 7% 44%	2600
	11th to 20th	Day	19% 10% 2 5% 3 4 3 3 5% 6% 39%	2990
	21st to end	Day	15% 8% 2 7% 5% 4 5% 4 6% 7% 37%	3960
January	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 47%	6349
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 4 6% 49%	5992
	21st to end	Night	16% 8% 2 6% 3 3 3 3 5% 5% 46%	6490
February	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 5% 7% 40%	2600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 5% 41%	2451
	21st to end	Night	15% 8% 2 7% 4 5% 4 4 4 6% 41%	1980
March	1st to 10th	Night	16% 7% 3 5% 4 3 3 3 4 5% 47%	5126
	11th to 20th	Night	10% 6% 3 5% 4 4 4 3 5% 6% 51%	4980
	21st to end	Night	13% 8% 2 6% 4 4 3 3 4 6% 45%	5289
April	1st to 10th	Night	17% 8% 2 6% 5% 4 4 3 5% 7% 38%	2000
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 5% 8% 42%	1975
	21st to end	Night	18% 7% 2 5% 3 3 3 4 5% 6% 44%	2000
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 4 5% 7% 29%	4181
	11th to 20th	Night	24% 10% 3 6% 4 3 3 3 5% 8% 30%	3575
	21st to end	Night	19% 6% 2 5% 3 4 4 4 7% 8% 37%	3920
June	1st to 10th	Night	12% 5% 3 5% 5% 4 4 4 5% 8% 45%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 4 8% 9% 40%	1600
	21st to end	Night	11% 5% 1 5% 4 5% 5% 5% 7% 10% 43%	1600
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 4 7% 10% 38%	3598
	11th to 20th	Night	10% 8% 3 9% 6% 5% 6% 6% 8% 9% 30%	3597
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 6% 8% 44%	4353
August	1st to 10th	Night	19% 5% 2 6% 4 4 5% 4 5% 10% 35%	2915
	11th to 20th	Night	12% 8% 2 8% 4 4 5% 3 6% 8% 39%	3057
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 8% 48%	3586
September	1st to 10th	Night	13% 7% 2 6% 3 3 4 3 5% 7% 47%	2000
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 7% 52%	2000
	21st to end	Night	20% 8% 2 6% 4 4 4 3 5% 6% 40%	2270
October	1st to 10th	Night	19% 6% 3 5% 4 3 3 3 5% 5% 45%	2400
	11th to 20th	Night	16% 5% 2 5% 2 3 4 3 5% 6% 51%	2400
	21st to end	Night	16% 5% 1 4 3 4 3 3 5% 7% 49%	2640
November	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 4 6% 53%	2537
	11th to 20th	Night	15% 7% 2 5% 4 3 4 3 4 6% 47%	2603
	21st to end	Night	13% 5% 2 5% 5% 4 4 3 6% 7% 47%	2800
December	1st to 10th	Night	11% 7% 2 5% 4 4 4 3 5% 8% 48%	4580
	11th to 20th	Night	21% 7% 2 5% 3 4 4 4 5% 6% 39%	5205
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 6% 36%	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	25% 27% 6% 20% 19% 3	3598
	11th to 20th	Day	35% 27% 4 15% 14% 6%	3950
	21st to end	Day	32% 30% 8% 18% 11% 2	4466
February	1st to 10th	Day	18% 63% 6% 8% 4 1	1800
	11th to 20th	Day	18% 61% 4 9% 7% 1	1842
	21st to end	Day	17% 62% 4 9% 6% 2	1650
March	1st to 10th	Day	30% 30% 6% 15% 13% 6%	4709
	11th to 20th	Day	26% 30% 7% 21% 12% 4	4980
	21st to end	Day	31% 29% 7% 13% 16% 4	5642
April	1st to 10th	Day	20% 62% 3 7% 7% 1	2400
	11th to 20th	Day	15% 63% 5% 8% 6% 1	2374
	21st to end	Day	16% 62% 4 9% 8% 1	2400
May	1st to 10th	Day	40% 30% 10% 13% 6% 1	5790
	11th to 20th	Day	38% 31% 8% 17% 6%	6322
	21st to end	Day	32% 31% 9% 16% 12% 1	6949
June	1st to 10th	Day	17% 63% 4 10% 7% 1	2800
	11th to 20th	Day	16% 63% 4 10% 6% 1	2800
	21st to end	Day	14% 62% 6% 10% 8% 1	2800
July	1st to 10th	Day	29% 29% 13% 15% 12% 2	6290
	11th to 20th	Day	38% 28% 14% 15% 5%	6285
	21st to end	Day	30% 26% 8% 18% 18% 1	6295
August	1st to 10th	Day	34% 37% 8% 16% 5%	4221
	11th to 20th	Day	24% 38% 8% 21% 9%	3996
	21st to end	Day	20% 36% 7% 21% 15% 1	4312
September	1st to 10th	Day	16% 58% 5% 10% 8% 2	2400
	11th to 20th	Day	17% 57% 5% 12% 8% 2	2400
	21st to end	Day	24% 57% 4 8% 6%	2125
October	1st to 10th	Day	23% 36% 4 11% 22% 4	3260
	11th to 20th	Day	14% 36% 6% 19% 17% 8%	3240
	21st to end	Day	10% 35% 7% 21% 22% 4	3538
November	1st to 10th	Day	17% 56% 7% 9% 8% 3	1804
	11th to 20th	Day	13% 57% 4 12% 12% 1	1795
	21st to end	Day	15% 57% 6% 14% 6% 2	1600
December	1st to 10th	Day	27% 36% 10% 17% 7% 3	2600
	11th to 20th	Day	33% 31% 9% 20% 7% 1	2986
	21st to end	Day	34% 29% 11% 13% 11% 1	3960
January	1st to 10th	Night	27% 28% 6% 19% 17% 4	6349
	11th to 20th	Night	26% 27% 8% 18% 15% 6%	5992
	21st to end	Night	31% 28% 7% 19% 14% 2	6490
February	1st to 10th	Night	17% 62% 6% 11% 3 1	2600
	11th to 20th	Night	18% 60% 3 8% 9% 1	2453
	21st to end	Night	16% 62% 5% 8% 6% 2	1980
March	1st to 10th	Night	32% 30% 5% 18% 7% 9%	5126
	11th to 20th	Night	22% 29% 10% 25% 10% 5%	4980
	21st to end	Night	30% 29% 5% 13% 20% 2	5289
April	1st to 10th	Night	19% 62% 3 9% 6% 1	2000
	11th to 20th	Night	15% 63% 5% 8% 7% 1	1975
	21st to end	Night	16% 62% 4 9% 8% 1	2000
May	1st to 10th	Night	42% 30% 8% 11% 7% 2	4211
	11th to 20th	Night	40% 30% 8% 16% 6%	3576
	21st to end	Night	34% 31% 8% 17% 8% 1	3927
June	1st to 10th	Night	16% 63% 3 10% 8%	1600
	11th to 20th	Night	16% 63% 5% 9% 7%	1600
	21st to end	Night	14% 62% 5% 11% 6% 1	1600
July	1st to 10th	Night	31% 30% 13% 13% 12% 2	3560
	11th to 20th	Night	40% 28% 14% 13% 5%	3552
	21st to end	Night	30% 27% 8% 19% 16%	4225
August	1st to 10th	Night	34% 37% 12% 12% 4 1	2904
	11th to 20th	Night	26% 37% 6% 21% 9%	3084
	21st to end	Night	21% 36% 5% 25% 12% 1	3586
September	1st to 10th	Night	15% 58% 6% 12% 8% 2	2000
	11th to 20th	Night	17% 57% 5% 10% 10% 2	2000
	21st to end	Night	23% 58% 5% 8% 7%	2270
October	1st to 10th	Night	29% 36% 7% 11% 16% 2	3920
	11th to 20th	Night	13% 35% 7% 18% 22% 5%	3899
	21st to end	Night	12% 35% 7% 24% 18% 4	4252
November	1st to 10th	Night	15% 56% 7% 12% 8% 3	2537
	11th to 20th	Night	16% 56% 3 10% 12% 2	2603
	21st to end	Night	14% 57% 6% 14% 8% 1	2800
December	1st to 10th	Night	23% 36% 10% 21% 10% 1	4580
	11th to 20th	Night	32% 31% 9% 14% 11% 3	5208
	21st to end	Night	36% 29% 8% 12% 14% 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	29% 28% 28% 13% 3 1	200
	11th to 20th	Day	34% 25% 27% 13% 1	277
	21st to end	Day	21% 31% 33% 10% 6%	330
February	1st to 10th	Day	26% 23% 38% 10% 3	300
	11th to 20th	Day	31% 27% 35% 6% 2	380
	21st to end	Day	30% 20% 33% 15% 2	414
March	1st to 10th	Day	24% 24% 31% 18% 3	500
	11th to 20th	Day	22% 24% 32% 15% 4 2	500
	21st to end	Day	25% 20% 33% 16% 5% 1	550
April	1st to 10th	Day	24% 21% 33% 15% 5% 1	500
	11th to 20th	Day	22% 22% 27% 18% 8% 3	492
	21st to end	Day	25% 15% 33% 20% 6% 2	500
May	1st to 10th	Day	26% 17% 27% 18% 9% 2	503
	11th to 20th	Day	22% 14% 28% 22% 11% 2	527
	21st to end	Day	20% 6% 24% 26% 19% 5%	573
June	1st to 10th	Day	14% 11% 31% 23% 15% 5%	530
	11th to 20th	Day	18% 7% 26% 29% 15% 5%	530
	21st to end	Day	14% 6% 26% 34% 15% 6%	530
July	1st to 10th	Day	14% 3 19% 30% 24% 10%	525
	11th to 20th	Day	7% 3 21% 35% 27% 8%	524
	21st to end	Day	12% 5% 23% 35% 21% 5%	576
August	1st to 10th	Day	12% 4 22% 31% 22% 8%	526
	11th to 20th	Day	13% 4 18% 34% 21% 10%	519
	21st to end	Day	11% 5% 22% 30% 21% 10%	583
September	1st to 10th	Day	18% 7% 21% 23% 20% 11%	530
	11th to 20th	Day	15% 4 20% 30% 22% 9%	530
	21st to end	Day	27% 10% 22% 25% 13% 4	530
October	1st to 10th	Day	22% 7% 24% 24% 15% 7%	530
	11th to 20th	Day	19% 15% 27% 23% 12% 3	530
	21st to end	Day	17% 15% 37% 21% 8% 2	583
November	1st to 10th	Day	16% 14% 31% 27% 11% 2	415
	11th to 20th	Day	18% 18% 34% 20% 9%	395
	21st to end	Day	26% 20% 32% 16% 6% 1	200
December	1st to 10th	Day	24% 23% 37% 13% 3 2	200
	11th to 20th	Day	29% 23% 30% 14% 6%	200
	21st to end	Day	24% 20% 33% 16% 8%	220
January	1st to 10th	Night	19% 18% 42% 18% 3	800
	11th to 20th	Night	23% 18% 33% 23% 3	800
	21st to end	Night	18% 17% 44% 20% 1	880
February	1st to 10th	Night	21% 17% 38% 22% 2	800
	11th to 20th	Night	25% 21% 37% 16% 1	783
	21st to end	Night	22% 22% 37% 17% 3	660
March	1st to 10th	Night	20% 19% 38% 20% 4	800
	11th to 20th	Night	21% 19% 35% 20% 5% 1	800
	21st to end	Night	21% 15% 39% 20% 5%	880
April	1st to 10th	Night	22% 18% 37% 19% 4	800
	11th to 20th	Night	18% 14% 34% 27% 6%	791
	21st to end	Night	22% 12% 33% 26% 7%	800
May	1st to 10th	Night	22% 10% 28% 31% 8% 1	673
	11th to 20th	Night	20% 8% 32% 31% 8%	394
	21st to end	Night	18% 4 21% 41% 16% 1	433
June	1st to 10th	Night	13% 4 25% 40% 18% 1	400
	11th to 20th	Night	14% 4 24% 44% 15% 1	400
	21st to end	Night	12% 3 23% 46% 17%	400
July	1st to 10th	Night	11% 1 18% 40% 27% 2	390
	11th to 20th	Night	8% 2 15% 45% 28% 3	388
	21st to end	Night	10% 2 17% 44% 26% 2	622
August	1st to 10th	Night	13% 2 13% 44% 26% 2	597
	11th to 20th	Night	13% 2 17% 38% 28% 2	698
	21st to end	Night	14% 2 16% 40% 25% 3	880
September	1st to 10th	Night	15% 3 21% 33% 24% 3	800
	11th to 20th	Night	14% 3 17% 37% 26% 4	800
	21st to end	Night	24% 4 21% 31% 19% 2	800
October	1st to 10th	Night	21% 6% 23% 27% 20% 4	800
	11th to 20th	Night	19% 10% 28% 31% 12% 1	800
	21st to end	Night	19% 11% 33% 25% 12%	880
November	1st to 10th	Night	14% 9% 34% 32% 10% 1	789
	11th to 20th	Night	19% 15% 31% 27% 7% 1	800
	21st to end	Night	16% 17% 35% 25% 7%	800
December	1st to 10th	Night	22% 15% 36% 20% 6%	800
	11th to 20th	Night	19% 15% 39% 22% 5%	800
	21st to end	Night	20% 16% 35% 27% 2	880

Page generated at 2025-10-31 02:02:37. Version ac07

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