Charlottesville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charlottesville clear sky chart from 2005-11-06 to 2025-09-04.

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	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	125684
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	122995
Transparency	Day	37% 7% 12% 22% 17% 5%	129290
	Night	38% 6% 11% 22% 18% 5%	127458
Seeing	Day	18% 10% 25% 26% 15% 6%	16108
	Night	17% 7% 26% 34% 15% 2	25831

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	15% 5% 2 5% 4 3 4 3 4 5% 50%	4056
	11th to 20th	Day	13% 7% 2 6% 3 4 3 2 4 4 52%	4060
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 5% 47%	4466
February	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 4 5% 6% 42%	1800
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 5% 6% 42%	1753
	21st to end	Day	17% 7% 2 6% 4 4 5% 4 5% 6% 40%	1719
March	1st to 10th	Day	17% 6% 3 5% 4 4 5% 4 5% 6% 41%	4935
	11th to 20th	Day	10% 6% 2 6% 3 3 4 3 5% 7% 52%	4980
	21st to end	Day	15% 6% 2 7% 5% 4 4 3 6% 6% 41%	5465
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 6% 49%	2365
	11th to 20th	Day	13% 5% 1 5% 4 4 4 3 5% 7% 48%	2374
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 6% 42%	2425
May	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 5% 7% 10% 33%	5862
	11th to 20th	Day	21% 7% 2 6% 5% 4 4 4 6% 9% 32%	5861
	21st to end	Day	15% 5% 2 6% 3 4 4 4 7% 10% 40%	6770
June	1st to 10th	Day	10% 4 2 5% 5% 4 5% 5% 7% 10% 44%	2800
	11th to 20th	Day	11% 5% 1 6% 5% 6% 6% 5% 7% 10% 40%	2800
	21st to end	Day	8% 4 2 4 5% 5% 6% 5% 7% 10% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 6% 6% 5% 8% 10% 42%	6052
	11th to 20th	Day	8% 6% 2 8% 6% 7% 7% 6% 9% 13% 29%	5900
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 37%	6424
August	1st to 10th	Day	14% 5% 3 7% 4 5% 5% 4 9% 11% 32%	4234
	11th to 20th	Day	8% 4 2 7% 5% 6% 6% 5% 7% 10% 40%	4085
	21st to end	Day	8% 5% 1 5% 4 4 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 6% 48%	2316
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 6% 7% 45%	2205
	21st to end	Day	22% 7% 2 5% 4 4 4 4 5% 6% 37%	2023
October	1st to 10th	Day	15% 5% 2 5% 4 4 4 3 4 6% 49%	1900
	11th to 20th	Day	15% 4 2 3 3 3 4 3 4 5% 54%	1900
	21st to end	Day	13% 5% 2 4 3 3 4 3 3 6% 54%	2090
November	1st to 10th	Day	12% 4 1 4 3 3 4 3 5% 6% 54%	1957
	11th to 20th	Day	15% 5% 2 4 3 3 3 4 5% 5% 51%	1777
	21st to end	Day	14% 4 1 5% 3 4 4 3 5% 6% 51%	1600
December	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 4 7% 52%	2600
	11th to 20th	Day	19% 6% 2 5% 2 3 3 3 4 6% 47%	2990
	21st to end	Day	20% 7% 3 6% 4 4 3 4 5% 6% 39%	4028
January	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	14% 6% 2 4 2 2 3 2 4 4 56%	5900
	21st to end	Night	15% 6% 2 6% 3 3 3 2 3 5% 51%	6490
February	1st to 10th	Night	14% 7% 2 5% 4 4 5% 4 6% 6% 43%	2600
	11th to 20th	Night	21% 7% 2 5% 4 3 4 3 5% 5% 43%	2533
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	1907
March	1st to 10th	Night	17% 5% 2 5% 4 4 4 3 5% 5% 47%	4918
	11th to 20th	Night	10% 4 2 4 4 3 3 3 5% 8% 54%	4980
	21st to end	Night	12% 6% 2 5% 4 5% 5% 3 5% 6% 46%	5467
April	1st to 10th	Night	13% 8% 2 6% 4 4 3 3 6% 6% 45%	2035
	11th to 20th	Night	12% 5% 2 6% 3 3 4 3 6% 9% 48%	1975
	21st to end	Night	15% 5% 1 5% 4 4 4 3 5% 7% 46%	1991
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 6% 9% 33%	4033
	11th to 20th	Night	24% 6% 3 6% 4 5% 4 4 6% 7% 31%	4032
	21st to end	Night	16% 6% 2 6% 4 5% 4 4 6% 8% 38%	4066
June	1st to 10th	Night	10% 4 2 5% 4 4 5% 4 8% 9% 45%	1600
	11th to 20th	Night	11% 5% 2 6% 4 3 5% 6% 7% 9% 42%	1600
	21st to end	Night	8% 4 1 6% 5% 5% 5% 4 7% 9% 45%	1600
July	1st to 10th	Night	10% 6% 2 6% 5% 5% 5% 5% 8% 10% 37%	3873
	11th to 20th	Night	11% 10% 4 9% 6% 6% 6% 6% 8% 9% 25%	4057
	21st to end	Night	7% 6% 3 9% 6% 7% 7% 6% 8% 10% 31%	4427
August	1st to 10th	Night	15% 6% 3 7% 5% 6% 5% 4 7% 9% 32%	2915
	11th to 20th	Night	9% 7% 3 8% 6% 6% 6% 5% 7% 9% 35%	2931
	21st to end	Night	11% 5% 2 7% 5% 5% 4 4 6% 8% 43%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 6% 7% 46%	1931
	11th to 20th	Night	14% 5% 2 6% 3 4 4 3 5% 7% 47%	1971
	21st to end	Night	24% 6% 2 6% 3 4 4 3 5% 6% 38%	2156
October	1st to 10th	Night	18% 5% 2 5% 4 3 3 3 5% 6% 46%	2280
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 4 6% 50%	2280
	21st to end	Night	14% 4 2 4 3 3 3 3 5% 6% 54%	2508
November	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 5% 53%	2341
	11th to 20th	Night	14% 6% 2 6% 4 3 3 3 4 6% 50%	2615
	21st to end	Night	13% 4 2 5% 4 4 4 3 5% 6% 51%	2800
December	1st to 10th	Night	9% 6% 2 5% 3 4 4 3 5% 7% 53%	4580
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 4 5% 44%	5205
	21st to end	Night	18% 5% 2 5% 4 4 4 3 5% 7% 43%	6923

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	36% 5% 5% 24% 21% 10%	4057
	11th to 20th	Day	35% 4 6% 19% 22% 13%	4060
	21st to end	Day	39% 4 8% 25% 19% 5%	4466
February	1st to 10th	Day	42% 5% 11% 24% 15% 3	1800
	11th to 20th	Day	43% 5% 7% 21% 19% 5%	1762
	21st to end	Day	44% 6% 9% 17% 18% 6%	1719
March	1st to 10th	Day	42% 6% 8% 18% 17% 10%	4935
	11th to 20th	Day	31% 6% 8% 25% 21% 9%	4980
	21st to end	Day	43% 6% 10% 17% 17% 7%	5465
April	1st to 10th	Day	35% 6% 10% 22% 18% 8%	2365
	11th to 20th	Day	35% 6% 11% 21% 22% 6%	2374
	21st to end	Day	41% 6% 11% 23% 15% 4	2425
May	1st to 10th	Day	43% 10% 15% 19% 12% 1	5897
	11th to 20th	Day	47% 9% 12% 19% 12% 1	5865
	21st to end	Day	37% 8% 15% 19% 19% 2	6785
June	1st to 10th	Day	33% 9% 16% 25% 16% 1	2800
	11th to 20th	Day	38% 11% 14% 21% 15% 2	2800
	21st to end	Day	31% 10% 16% 26% 16% 2	2800
July	1st to 10th	Day	34% 10% 20% 19% 12% 4	5987
	11th to 20th	Day	42% 12% 19% 21% 6% 1	5831
	21st to end	Day	38% 9% 14% 24% 15%	6240
August	1st to 10th	Day	43% 9% 18% 24% 6%	4221
	11th to 20th	Day	37% 9% 15% 25% 14%	4124
	21st to end	Day	31% 7% 18% 26% 16% 1	4312
September	1st to 10th	Day	36% 7% 13% 17% 22% 4	2316
	11th to 20th	Day	36% 7% 9% 26% 19% 4	2205
	21st to end	Day	47% 6% 10% 17% 16% 3	2023
October	1st to 10th	Day	34% 4 7% 19% 31% 5%	3160
	11th to 20th	Day	24% 4 7% 24% 30% 11%	3140
	21st to end	Day	21% 3 10% 30% 24% 12%	3428
November	1st to 10th	Day	31% 5% 9% 24% 27% 5%	1957
	11th to 20th	Day	35% 4 10% 21% 22% 8%	1777
	21st to end	Day	33% 5% 11% 24% 23% 4	1600
December	1st to 10th	Day	34% 4 10% 25% 18% 8%	2600
	11th to 20th	Day	38% 4 14% 24% 15% 5%	2986
	21st to end	Day	44% 6% 11% 22% 15% 1	4028
January	1st to 10th	Night	31% 4 6% 21% 27% 10%	5889
	11th to 20th	Night	33% 3 6% 21% 26% 12%	5900
	21st to end	Night	38% 3 6% 26% 21% 6%	6490
February	1st to 10th	Night	40% 6% 11% 24% 17% 3	2600
	11th to 20th	Night	44% 3 8% 19% 19% 7%	2535
	21st to end	Night	41% 5% 10% 20% 17% 7%	1907
March	1st to 10th	Night	37% 6% 8% 21% 20% 9%	4918
	11th to 20th	Night	28% 5% 9% 22% 28% 8%	4980
	21st to end	Night	38% 5% 9% 20% 22% 6%	5467
April	1st to 10th	Night	39% 5% 10% 18% 21% 7%	2035
	11th to 20th	Night	33% 6% 13% 20% 22% 6%	1975
	21st to end	Night	37% 5% 13% 24% 16% 5%	1991
May	1st to 10th	Night	47% 8% 16% 17% 9% 3	4060
	11th to 20th	Night	48% 9% 11% 21% 9% 1	4033
	21st to end	Night	42% 8% 15% 23% 11% 2	4073
June	1st to 10th	Night	33% 10% 13% 28% 16% 1	1600
	11th to 20th	Night	36% 10% 18% 21% 13% 2	1600
	21st to end	Night	33% 10% 15% 21% 18% 3	1600
July	1st to 10th	Night	39% 11% 19% 14% 14% 2	3830
	11th to 20th	Night	51% 11% 17% 16% 4 1	4006
	21st to end	Night	42% 11% 14% 21% 12%	4297
August	1st to 10th	Night	46% 8% 16% 22% 6% 1	2904
	11th to 20th	Night	42% 9% 14% 23% 12% 1	2958
	21st to end	Night	38% 7% 14% 29% 11% 1	3586
September	1st to 10th	Night	38% 7% 11% 21% 15% 7%	1931
	11th to 20th	Night	37% 6% 12% 22% 20% 3	1971
	21st to end	Night	46% 6% 10% 19% 16% 3	2156
October	1st to 10th	Night	42% 4 9% 19% 20% 7%	3800
	11th to 20th	Night	25% 4 8% 22% 30% 11%	3779
	21st to end	Night	20% 3 8% 32% 27% 10%	4120
November	1st to 10th	Night	34% 5% 9% 21% 25% 7%	2341
	11th to 20th	Night	36% 4 8% 20% 24% 7%	2615
	21st to end	Night	34% 4 10% 24% 23% 6%	2800
December	1st to 10th	Night	31% 4 9% 27% 24% 4	4580
	11th to 20th	Night	43% 3 12% 18% 17% 7%	5208
	21st to end	Night	39% 6% 12% 25% 16% 1	6923

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	26% 19% 33% 16% 6%	300
	11th to 20th	Day	25% 28% 27% 16% 3	300
	21st to end	Day	23% 22% 33% 18% 3 1	330
February	1st to 10th	Day	23% 21% 38% 14% 3	300
	11th to 20th	Day	39% 22% 28% 9% 1	300
	21st to end	Day	27% 18% 30% 22% 3	409
March	1st to 10th	Day	23% 18% 31% 22% 6% 1	500
	11th to 20th	Day	22% 19% 30% 21% 7% 1	500
	21st to end	Day	23% 17% 30% 24% 6% 1	550
April	1st to 10th	Day	24% 19% 28% 20% 9% 1	500
	11th to 20th	Day	19% 15% 29% 22% 11% 4	492
	21st to end	Day	23% 10% 30% 22% 12% 2	500
May	1st to 10th	Day	18% 14% 33% 21% 11% 3	503
	11th to 20th	Day	22% 9% 27% 25% 13% 4	527
	21st to end	Day	19% 5% 25% 25% 20% 6%	573
June	1st to 10th	Day	11% 4 28% 29% 20% 8%	530
	11th to 20th	Day	13% 6% 24% 31% 17% 9%	530
	21st to end	Day	10% 5% 22% 34% 22% 8%	530
July	1st to 10th	Day	11% 2 18% 30% 30% 10%	525
	11th to 20th	Day	7% 3 14% 34% 28% 13%	524
	21st to end	Day	10% 3 16% 35% 25% 11%	576
August	1st to 10th	Day	14% 5% 17% 28% 26% 10%	526
	11th to 20th	Day	14% 2 16% 29% 26% 13%	519
	21st to end	Day	9% 3 14% 33% 25% 15%	583
September	1st to 10th	Day	13% 5% 15% 28% 22% 17%	502
	11th to 20th	Day	14% 3 18% 26% 29% 10%	490
	21st to end	Day	29% 8% 18% 26% 15% 4	490
October	1st to 10th	Day	20% 5% 18% 33% 17% 7%	490
	11th to 20th	Day	15% 12% 23% 28% 18% 4	490
	21st to end	Day	15% 10% 31% 29% 11% 3	539
November	1st to 10th	Day	16% 10% 26% 32% 13% 4	495
	11th to 20th	Day	20% 11% 31% 24% 12% 2	350
	21st to end	Day	19% 15% 33% 24% 9% 2	200
December	1st to 10th	Day	20% 16% 34% 22% 7% 2	200
	11th to 20th	Day	25% 10% 32% 23% 11% 1	200
	21st to end	Day	26% 12% 32% 21% 7% 2	235
January	1st to 10th	Night	18% 12% 39% 28% 4	800
	11th to 20th	Night	20% 10% 35% 31% 4	800
	21st to end	Night	17% 10% 41% 29% 3	880
February	1st to 10th	Night	17% 10% 38% 32% 3	800
	11th to 20th	Night	26% 13% 37% 23% 1	783
	21st to end	Night	19% 13% 38% 27% 3	660
March	1st to 10th	Night	16% 14% 36% 30% 4	800
	11th to 20th	Night	21% 11% 35% 27% 6% 1	800
	21st to end	Night	18% 13% 36% 28% 5%	880
April	1st to 10th	Night	17% 14% 36% 26% 6%	800
	11th to 20th	Night	14% 12% 33% 31% 10% 1	791
	21st to end	Night	17% 7% 30% 37% 9%	791
May	1st to 10th	Night	18% 7% 27% 33% 14%	600
	11th to 20th	Night	22% 3 21% 36% 18% 1	591
	21st to end	Night	17% 2 17% 38% 24% 3	502
June	1st to 10th	Night	10% 2 16% 45% 26% 1	400
	11th to 20th	Night	11% 2 16% 42% 28% 1	400
	21st to end	Night	9% 3 18% 47% 21% 3	400
July	1st to 10th	Night	13% 2 11% 34% 35% 6%	504
	11th to 20th	Night	6% 2 10% 36% 40% 5%	582
	21st to end	Night	9% 2 11% 41% 32% 5%	657
August	1st to 10th	Night	14% 2 8% 40% 32% 4	597
	11th to 20th	Night	14% 3 9% 35% 36% 4	624
	21st to end	Night	12% 4 11% 37% 32% 4	880
September	1st to 10th	Night	15% 2 12% 34% 31% 6%	773
	11th to 20th	Night	14% 4 12% 35% 31% 4	760
	21st to end	Night	26% 3 17% 31% 22% 1	760
October	1st to 10th	Night	20% 5% 17% 31% 24% 3	760
	11th to 20th	Night	17% 7% 23% 35% 17% 1	760
	21st to end	Night	15% 7% 25% 39% 13% 1	836
November	1st to 10th	Night	15% 6% 26% 38% 13% 2	780
	11th to 20th	Night	18% 7% 30% 35% 10%	800
	21st to end	Night	15% 7% 32% 35% 9% 1	800
December	1st to 10th	Night	19% 7% 36% 30% 7% 1	800
	11th to 20th	Night	18% 7% 32% 38% 6%	800
	21st to end	Night	20% 7% 30% 39% 4	880

Page generated at 2025-09-01 01:51:07. Version ac07

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