Lord Fairfax Community College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lord Fairfax Community College clear sky chart from 2005-11-06 to 2025-12-03.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	126829
	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	123847
Transparency	Day	40% 5% 12% 24% 16% 3	130430
	Night	39% 5% 12% 24% 17% 3	128316
Seeing	Day	19% 12% 25% 24% 15% 6%	16316
	Night	17% 10% 25% 33% 14% 1	25946

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 6% 5% 5% 4 3 5% 5% 46%	3915
	11th to 20th	Day	11% 8% 3 7% 4 5% 4 3 4 5% 45%	4060
	21st to end	Day	13% 8% 3 8% 4 4 4 4 5% 6% 42%	4466
February	1st to 10th	Day	12% 10% 3 7% 6% 5% 4 4 6% 6% 39%	1800
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 39%	1753
	21st to end	Day	15% 9% 2 6% 5% 4 4 3 5% 5% 41%	1680
March	1st to 10th	Day	16% 9% 3 6% 5% 4 4 3 4 6% 42%	4935
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 7% 50%	4980
	21st to end	Day	15% 7% 2 7% 4 5% 4 4 6% 7% 39%	5465
April	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 4 6% 7% 40%	2383
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 4 5% 6% 46%	2374
	21st to end	Day	17% 5% 2 6% 5% 4 5% 4 5% 7% 40%	2445
May	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 5% 7% 9% 31%	5862
	11th to 20th	Day	20% 7% 3 7% 4 5% 5% 4 7% 9% 30%	5924
	21st to end	Day	15% 5% 2 6% 4 5% 5% 5% 7% 8% 38%	6934
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 4 5% 7% 8% 43%	2800
	11th to 20th	Day	10% 5% 2 6% 4 5% 6% 6% 6% 10% 39%	2800
	21st to end	Day	9% 4 1 4 5% 5% 5% 5% 7% 10% 45%	2800
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 4 7% 10% 45%	6346
	11th to 20th	Day	7% 5% 2 8% 6% 6% 7% 5% 9% 12% 33%	5900
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	6424
August	1st to 10th	Day	14% 6% 2 7% 5% 6% 6% 5% 7% 9% 34%	4234
	11th to 20th	Day	8% 4 2 7% 5% 6% 6% 5% 7% 9% 42%	4108
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 8% 48%	4312
September	1st to 10th	Day	14% 5% 2 6% 5% 4 4 3 5% 6% 47%	2400
	11th to 20th	Day	12% 5% 2 6% 4 3 4 3 5% 8% 47%	2345
	21st to end	Day	20% 8% 2 6% 5% 4 5% 3 5% 6% 38%	2115
October	1st to 10th	Day	16% 6% 2 5% 3 3 3 3 5% 7% 48%	2000
	11th to 20th	Day	14% 5% 2 6% 3 3 4 4 5% 6% 49%	2000
	21st to end	Day	13% 5% 2 5% 4 3 4 4 4 6% 50%	2200
November	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 5% 6% 52%	2038
	11th to 20th	Day	14% 6% 2 6% 4 3 4 4 5% 6% 46%	1785
	21st to end	Day	13% 7% 2 5% 4 4 4 4 4 6% 47%	1680
December	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 51%	2616
	11th to 20th	Day	17% 7% 3 7% 4 4 4 3 5% 6% 40%	2990
	21st to end	Day	14% 9% 2 7% 5% 5% 6% 4 6% 9% 34%	3960
January	1st to 10th	Night	9% 7% 2 8% 4 4 4 4 5% 6% 46%	6027
	11th to 20th	Night	13% 7% 2 5% 4 4 3 3 5% 5% 49%	5900
	21st to end	Night	13% 8% 3 6% 3 3 3 3 5% 5% 47%	6490
February	1st to 10th	Night	13% 8% 3 6% 5% 4 4 4 6% 6% 41%	2600
	11th to 20th	Night	15% 9% 3 7% 4 4 5% 4 5% 5% 40%	2533
	21st to end	Night	14% 9% 3 6% 5% 5% 4 3 5% 6% 40%	1947
March	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 4 5% 47%	4918
	11th to 20th	Night	8% 6% 2 6% 4 4 4 3 5% 9% 49%	4980
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 4 5% 6% 45%	5467
April	1st to 10th	Night	14% 9% 2 7% 5% 4 4 4 6% 7% 39%	2015
	11th to 20th	Night	12% 6% 1 7% 6% 4 4 4 6% 8% 43%	1975
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 5% 7% 44%	1971
May	1st to 10th	Night	19% 8% 3 8% 5% 4 5% 4 5% 8% 31%	4033
	11th to 20th	Night	21% 9% 2 6% 4 5% 5% 4 6% 8% 31%	3952
	21st to end	Night	16% 7% 3 6% 4 5% 5% 5% 7% 8% 34%	3920
June	1st to 10th	Night	10% 4 2 5% 4 5% 5% 5% 6% 9% 45%	1600
	11th to 20th	Night	10% 5% 3 5% 4 4 5% 4 6% 9% 43%	1600
	21st to end	Night	8% 4 2 6% 4 5% 5% 4 9% 11% 43%	1600
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 6% 5% 8% 11% 40%	3598
	11th to 20th	Night	5% 6% 3 9% 7% 7% 8% 6% 8% 11% 30%	4052
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	4427
August	1st to 10th	Night	13% 5% 2 8% 6% 5% 5% 5% 7% 10% 34%	2915
	11th to 20th	Night	8% 6% 2 7% 6% 6% 5% 5% 6% 9% 41%	2908
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 6% 9% 47%	3586
September	1st to 10th	Night	12% 5% 2 5% 4 4 6% 3 6% 6% 46%	2000
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 4 6% 51%	2050
	21st to end	Night	18% 7% 3 5% 3 4 4 3 4 6% 42%	2284
October	1st to 10th	Night	18% 5% 2 4 3 3 4 3 4 5% 49%	2400
	11th to 20th	Night	14% 4 2 5% 3 3 4 3 5% 6% 52%	2400
	21st to end	Night	13% 5% 2 5% 3 4 4 3 5% 6% 51%	2640
November	1st to 10th	Night	9% 7% 2 5% 4 3 4 3 5% 6% 53%	2477
	11th to 20th	Night	13% 7% 3 5% 4 3 4 3 6% 7% 46%	2832
	21st to end	Night	12% 5% 2 5% 5% 4 4 4 6% 7% 45%	2940
December	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 51%	4609
	11th to 20th	Night	16% 7% 4 7% 4 4 4 4 5% 6% 39%	5205
	21st to end	Night	13% 8% 3 7% 5% 5% 6% 5% 7% 8% 34%	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	40% 3 5% 25% 20% 7%	3916
	11th to 20th	Day	42% 2 8% 21% 17% 10%	4060
	21st to end	Day	43% 3 9% 27% 15% 4	4466
February	1st to 10th	Day	46% 4 12% 22% 13% 3	1800
	11th to 20th	Day	46% 4 8% 21% 17% 2	1762
	21st to end	Day	44% 3 11% 21% 17% 4	1680
March	1st to 10th	Day	44% 4 8% 22% 15% 8%	4935
	11th to 20th	Day	33% 4 9% 28% 21% 5%	4980
	21st to end	Day	45% 4 11% 18% 17% 6%	5465
April	1st to 10th	Day	42% 5% 11% 20% 18% 5%	2383
	11th to 20th	Day	39% 4 10% 22% 20% 5%	2374
	21st to end	Day	44% 5% 10% 24% 15% 4	2445
May	1st to 10th	Day	47% 7% 14% 20% 10% 2	5897
	11th to 20th	Day	50% 6% 11% 24% 9% 1	5928
	21st to end	Day	42% 6% 13% 22% 16% 2	6949
June	1st to 10th	Day	38% 5% 14% 25% 17% 1	2800
	11th to 20th	Day	39% 7% 16% 23% 14% 2	2800
	21st to end	Day	34% 6% 16% 26% 16% 2	2800
July	1st to 10th	Day	33% 10% 16% 23% 15% 4	6276
	11th to 20th	Day	39% 10% 17% 25% 8% 1	5831
	21st to end	Day	36% 8% 10% 24% 20% 1	6240
August	1st to 10th	Day	44% 7% 15% 24% 8%	4221
	11th to 20th	Day	35% 8% 13% 28% 15% 1	4147
	21st to end	Day	32% 6% 15% 28% 16% 1	4312
September	1st to 10th	Day	39% 4 13% 19% 21% 4	2400
	11th to 20th	Day	37% 4 8% 28% 20% 3	2345
	21st to end	Day	48% 5% 7% 22% 17% 2	2115
October	1st to 10th	Day	32% 4 9% 21% 28% 6%	3260
	11th to 20th	Day	27% 3 8% 30% 24% 8%	3240
	21st to end	Day	24% 3 9% 29% 30% 5%	3538
November	1st to 10th	Day	33% 3 11% 24% 25% 4	2038
	11th to 20th	Day	39% 3 10% 21% 22% 5%	1785
	21st to end	Day	39% 3 11% 27% 17% 2	1680
December	1st to 10th	Day	34% 4 14% 28% 16% 4	2616
	11th to 20th	Day	46% 3 12% 24% 12% 3	2986
	21st to end	Day	46% 6% 15% 19% 14% 1	3960
January	1st to 10th	Night	37% 4 7% 26% 22% 3	6027
	11th to 20th	Night	38% 3 8% 24% 19% 7%	5900
	21st to end	Night	39% 3 9% 30% 17% 3	6490
February	1st to 10th	Night	43% 4 12% 25% 14% 1	2600
	11th to 20th	Night	45% 3 9% 23% 16% 4	2535
	21st to end	Night	47% 3 10% 21% 15% 5%	1947
March	1st to 10th	Night	39% 4 8% 27% 15% 8%	4918
	11th to 20th	Night	32% 4 12% 27% 19% 5%	4980
	21st to end	Night	39% 4 11% 23% 18% 4	5467
April	1st to 10th	Night	44% 5% 10% 20% 18% 4	2015
	11th to 20th	Night	39% 4 12% 22% 18% 5%	1975
	21st to end	Night	40% 5% 10% 27% 15% 4	1971
May	1st to 10th	Night	51% 6% 14% 16% 9% 3	4060
	11th to 20th	Night	51% 6% 11% 23% 9% 1	3953
	21st to end	Night	45% 6% 16% 23% 8% 3	3927
June	1st to 10th	Night	35% 7% 11% 28% 16% 2	1600
	11th to 20th	Night	37% 7% 15% 26% 14% 1	1600
	21st to end	Night	33% 7% 17% 25% 17% 2	1600
July	1st to 10th	Night	36% 10% 18% 20% 13% 3	3560
	11th to 20th	Night	44% 13% 17% 17% 8% 1	4002
	21st to end	Night	35% 8% 16% 23% 18%	4297
August	1st to 10th	Night	44% 7% 16% 25% 8% 1	2904
	11th to 20th	Night	37% 8% 13% 25% 16%	2935
	21st to end	Night	33% 6% 15% 30% 15% 1	3586
September	1st to 10th	Night	37% 5% 12% 22% 19% 5%	2000
	11th to 20th	Night	34% 4 8% 30% 21% 2	2050
	21st to end	Night	44% 5% 10% 23% 17% 2	2284
October	1st to 10th	Night	38% 4 10% 23% 22% 3	3920
	11th to 20th	Night	25% 4 10% 24% 32% 6%	3899
	21st to end	Night	24% 3 10% 32% 27% 5%	4252
November	1st to 10th	Night	33% 3 11% 25% 23% 5%	2477
	11th to 20th	Night	38% 3 7% 25% 21% 6%	2832
	21st to end	Night	38% 4 11% 25% 19% 3	2940
December	1st to 10th	Night	33% 4 13% 30% 18% 2	4609
	11th to 20th	Night	45% 4 12% 20% 16% 3	5208
	21st to end	Night	45% 6% 14% 20% 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	30% 24% 25% 16% 4	268
	11th to 20th	Day	27% 24% 29% 15% 4	300
	21st to end	Day	23% 25% 31% 15% 4 1	330
February	1st to 10th	Day	27% 24% 32% 11% 6%	300
	11th to 20th	Day	39% 25% 28% 7%	300
	21st to end	Day	28% 18% 30% 21% 2	404
March	1st to 10th	Day	25% 24% 24% 23% 4 1	500
	11th to 20th	Day	23% 22% 30% 16% 7% 2	500
	21st to end	Day	22% 19% 32% 21% 5% 1	550
April	1st to 10th	Day	24% 19% 28% 20% 8% 1	500
	11th to 20th	Day	18% 19% 27% 23% 9% 4	492
	21st to end	Day	25% 14% 28% 20% 12% 2	500
May	1st to 10th	Day	22% 14% 29% 19% 12% 4	503
	11th to 20th	Day	22% 12% 27% 22% 13% 4	527
	21st to end	Day	19% 7% 23% 27% 18% 6%	573
June	1st to 10th	Day	16% 8% 24% 26% 19% 7%	530
	11th to 20th	Day	14% 6% 25% 27% 20% 8%	530
	21st to end	Day	11% 5% 25% 28% 23% 7%	530
July	1st to 10th	Day	10% 2 17% 32% 27% 12%	525
	11th to 20th	Day	7% 2 19% 32% 28% 12%	524
	21st to end	Day	13% 3 19% 31% 26% 9%	576
August	1st to 10th	Day	11% 4 18% 26% 28% 12%	526
	11th to 20th	Day	11% 4 16% 31% 25% 13%	519
	21st to end	Day	9% 2 19% 30% 25% 15%	583
September	1st to 10th	Day	16% 6% 17% 23% 24% 13%	530
	11th to 20th	Day	14% 3 15% 31% 24% 14%	530
	21st to end	Day	25% 11% 19% 24% 17% 5%	530
October	1st to 10th	Day	18% 7% 18% 28% 19% 11%	530
	11th to 20th	Day	14% 15% 26% 25% 16% 4	530
	21st to end	Day	16% 13% 32% 24% 11% 3	583
November	1st to 10th	Day	16% 13% 29% 27% 13% 3	524
	11th to 20th	Day	20% 16% 28% 26% 10% 1	325
	21st to end	Day	20% 20% 33% 21% 5% 1	220
December	1st to 10th	Day	21% 19% 33% 18% 8%	204
	11th to 20th	Day	25% 13% 30% 24% 8%	200
	21st to end	Day	20% 14% 30% 25% 10%	220
January	1st to 10th	Night	17% 19% 35% 25% 4	800
	11th to 20th	Night	21% 14% 30% 29% 5%	800
	21st to end	Night	18% 16% 37% 28% 2	880
February	1st to 10th	Night	17% 16% 35% 31% 2	800
	11th to 20th	Night	24% 21% 31% 22% 1	783
	21st to end	Night	23% 16% 34% 24% 3	660
March	1st to 10th	Night	18% 18% 35% 25% 4	800
	11th to 20th	Night	22% 19% 29% 24% 5% 1	800
	21st to end	Night	17% 17% 36% 24% 5% 1	880
April	1st to 10th	Night	20% 19% 32% 25% 5%	800
	11th to 20th	Night	16% 13% 34% 27% 10% 1	791
	21st to end	Night	18% 12% 29% 33% 8%	771
May	1st to 10th	Night	23% 5% 24% 32% 16% 1	600
	11th to 20th	Night	20% 5% 23% 37% 16% 1	563
	21st to end	Night	15% 3 18% 42% 20% 2	433
June	1st to 10th	Night	10% 3 23% 41% 23% 1	400
	11th to 20th	Night	10% 5% 16% 44% 23% 3	400
	21st to end	Night	9% 4 16% 48% 22% 2	400
July	1st to 10th	Night	10% 2 12% 40% 31% 5%	390
	11th to 20th	Night	7% 1 8% 40% 39% 4	578
	21st to end	Night	8% 2 8% 45% 33% 4	657
August	1st to 10th	Night	10% 2 7% 45% 33% 3	597
	11th to 20th	Night	11% 2 9% 43% 32% 4	614
	21st to end	Night	12% 2 12% 41% 30% 4	880
September	1st to 10th	Night	16% 2 11% 36% 30% 5%	800
	11th to 20th	Night	13% 2 14% 36% 31% 4	800
	21st to end	Night	22% 5% 17% 33% 21% 2	800
October	1st to 10th	Night	21% 5% 14% 32% 24% 3	800
	11th to 20th	Night	16% 9% 24% 35% 15% 1	800
	21st to end	Night	17% 11% 24% 37% 11% 1	880
November	1st to 10th	Night	14% 9% 26% 37% 12% 1	820
	11th to 20th	Night	18% 15% 28% 31% 9% 1	840
	21st to end	Night	16% 14% 30% 31% 8% 1	840
December	1st to 10th	Night	19% 14% 30% 31% 7%	809
	11th to 20th	Night	18% 10% 34% 35% 4	800
	21st to end	Night	16% 13% 28% 39% 4	880

Page generated at 2025-11-30 01:00:56. Version ac07

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