Leesburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leesburg clear sky chart from 2005-11-06 to 2026-10-29.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	149487
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142569
Transparency	Day	39% 6% 11% 24% 16% 3	153084
	Night	40% 5% 11% 23% 17% 3	147043
Seeing	Day	20% 14% 27% 23% 13% 5%	17545
	Night	18% 12% 29% 29% 12% 1	27040

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	14% 5% 2 6% 4 4 4 4 5% 6% 46%	4878
	11th to 20th	Day	13% 9% 3 6% 3 3 3 3 4 5% 49%	5280
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 6% 7% 41%	5780
February	1st to 10th	Day	13% 8% 2 6% 5% 4 5% 5% 5% 6% 40%	1890
	11th to 20th	Day	18% 8% 2 6% 5% 4 4 3 5% 6% 39%	1875
	21st to end	Day	16% 9% 3 5% 5% 4 5% 3 5% 5% 40%	1730
March	1st to 10th	Day	18% 7% 2 5% 4 4 4 3 5% 7% 41%	5016
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 49%	5090
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 5% 7% 40%	5632
April	1st to 10th	Day	14% 7% 3 6% 4 4 4 4 6% 7% 42%	2520
	11th to 20th	Day	13% 6% 3 6% 4 3 4 3 5% 7% 46%	2494
	21st to end	Day	18% 6% 2 6% 4 3 4 4 5% 6% 42%	2520
May	1st to 10th	Day	17% 6% 2 7% 4 4 5% 4 6% 9% 35%	7592
	11th to 20th	Day	19% 5% 2 6% 4 4 5% 4 7% 10% 36%	7923
	21st to end	Day	23% 5% 2 5% 4 4 4 4 5% 7% 38%	9045
June	1st to 10th	Day	10% 4 2 6% 4 5% 5% 5% 6% 10% 44%	2940
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 42%	2940
	21st to end	Day	9% 4 2 5% 5% 4 5% 4 7% 10% 46%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 42%	8280
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 9% 12% 33%	7927
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	8340
August	1st to 10th	Day	13% 5% 3 8% 6% 6% 6% 5% 8% 10% 29%	6014
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	5764
	21st to end	Day	9% 4 2 5% 3 4 5% 5% 6% 9% 48%	6080
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 47%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 4 8% 48%	2520
	21st to end	Day	22% 7% 2 5% 4 4 4 3 5% 7% 36%	2193
October	1st to 10th	Day	16% 5% 2 5% 3 4 3 3 5% 6% 48%	2100
	11th to 20th	Day	16% 5% 2 4 4 3 3 4 4 6% 50%	2100
	21st to end	Day	13% 6% 2 5% 3 3 4 3 4 6% 52%	2280
November	1st to 10th	Day	11% 5% 2 4 3 4 3 3 6% 7% 52%	1973
	11th to 20th	Day	14% 5% 3 4 3 3 4 3 6% 6% 49%	1833
	21st to end	Day	14% 6% 1 6% 3 4 4 4 5% 7% 45%	1680
December	1st to 10th	Day	12% 5% 2 6% 4 3 4 4 5% 6% 48%	2680
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 5% 6% 42%	3070
	21st to end	Day	17% 8% 2 8% 4 4 4 3 6% 8% 35%	4048
January	1st to 10th	Night	10% 7% 2 6% 5% 4 5% 5% 6% 6% 44%	8011
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 4 6% 50%	7680
	21st to end	Night	13% 7% 2 5% 3 3 4 3 5% 6% 48%	8401
February	1st to 10th	Night	14% 7% 3 6% 4 5% 5% 4 5% 6% 42%	2730
	11th to 20th	Night	18% 7% 2 6% 4 3 4 4 5% 5% 41%	2629
	21st to end	Night	16% 9% 2 5% 5% 3 4 4 5% 6% 42%	2076
March	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 4 5% 47%	5039
	11th to 20th	Night	11% 6% 2 5% 3 4 4 4 5% 7% 50%	5090
	21st to end	Night	12% 8% 2 6% 4 4 5% 4 5% 7% 44%	5542
April	1st to 10th	Night	15% 8% 2 5% 5% 4 5% 4 5% 6% 41%	2100
	11th to 20th	Night	13% 5% 2 7% 4 3 5% 3 6% 7% 44%	2075
	21st to end	Night	15% 5% 1 5% 4 4 4 4 6% 7% 45%	2100
May	1st to 10th	Night	20% 7% 2 7% 4 5% 4 4 5% 8% 34%	5304
	11th to 20th	Night	20% 7% 2 6% 4 4 4 4 6% 8% 35%	4923
	21st to end	Night	23% 7% 2 6% 4 4 4 4 5% 7% 33%	5108
June	1st to 10th	Night	10% 6% 2 5% 4 4 4 5% 6% 8% 45%	1680
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 4 6% 9% 41%	1680
	21st to end	Night	8% 5% 2 6% 5% 4 5% 5% 8% 9% 43%	1680
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	4678
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 5% 7% 10% 32%	5011
	21st to end	Night	8% 6% 2 7% 5% 5% 6% 5% 7% 9% 40%	5736
August	1st to 10th	Night	14% 6% 3 8% 6% 6% 6% 5% 7% 11% 28%	4135
	11th to 20th	Night	12% 6% 3 8% 5% 6% 5% 5% 6% 9% 36%	4236
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 6% 8% 45%	5045
September	1st to 10th	Night	14% 5% 1 5% 4 3 5% 4 5% 7% 46%	2100
	11th to 20th	Night	10% 6% 3 5% 4 3 4 3 4 7% 51%	2100
	21st to end	Night	21% 6% 3 7% 5% 4 3 3 5% 5% 38%	2422
October	1st to 10th	Night	20% 5% 2 5% 3 3 3 2 4 5% 48%	2520
	11th to 20th	Night	16% 4 2 4 3 3 3 3 3 5% 53%	2520
	21st to end	Night	14% 5% 2 5% 3 4 3 2 4 6% 52%	2737
November	1st to 10th	Night	11% 6% 2 5% 3 2 3 3 4 6% 54%	2543
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 7% 49%	2783
	21st to end	Night	13% 4 2 5% 4 4 4 4 5% 7% 47%	2940
December	1st to 10th	Night	11% 4 2 5% 4 4 4 3 5% 7% 51%	4720
	11th to 20th	Night	20% 6% 2 5% 3 4 4 3 5% 7% 40%	5345
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 6% 8% 34%	7150

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	38% 4 8% 26% 18% 6%	4879
	11th to 20th	Day	39% 3 6% 21% 20% 11%	5280
	21st to end	Day	41% 4 9% 30% 12% 3	5780
February	1st to 10th	Day	42% 6% 12% 24% 14% 2	1890
	11th to 20th	Day	46% 5% 9% 23% 16% 3	1884
	21st to end	Day	45% 5% 10% 22% 15% 4	1730
March	1st to 10th	Day	43% 4 8% 21% 16% 8%	5016
	11th to 20th	Day	34% 5% 9% 27% 20% 6%	5090
	21st to end	Day	43% 5% 11% 18% 18% 6%	5632
April	1st to 10th	Day	40% 6% 11% 20% 18% 4	2520
	11th to 20th	Day	37% 5% 11% 22% 19% 5%	2494
	21st to end	Day	43% 5% 9% 23% 16% 3	2520
May	1st to 10th	Day	45% 7% 13% 21% 13% 2	7627
	11th to 20th	Day	42% 7% 11% 28% 11% 1	7927
	21st to end	Day	45% 6% 10% 18% 17% 3	9061
June	1st to 10th	Day	34% 7% 14% 24% 18% 2	2940
	11th to 20th	Day	36% 10% 13% 25% 14% 2	2940
	21st to end	Day	32% 8% 15% 26% 16% 2	2940
July	1st to 10th	Day	36% 9% 15% 22% 14% 3	8210
	11th to 20th	Day	40% 9% 13% 27% 10% 1	7853
	21st to end	Day	37% 7% 10% 23% 21% 1	8156
August	1st to 10th	Day	47% 10% 15% 21% 7%	6001
	11th to 20th	Day	39% 9% 15% 25% 12% 1	5803
	21st to end	Day	31% 7% 17% 28% 17% 1	6080
September	1st to 10th	Day	36% 7% 12% 21% 20% 4	2520
	11th to 20th	Day	36% 5% 9% 27% 21% 3	2520
	21st to end	Day	47% 7% 10% 18% 17% 2	2193
October	1st to 10th	Day	36% 4 8% 19% 28% 5%	3360
	11th to 20th	Day	27% 3 7% 28% 25% 9%	3340
	21st to end	Day	23% 3 10% 29% 28% 7%	3618
November	1st to 10th	Day	33% 4 11% 26% 21% 4	1973
	11th to 20th	Day	35% 4 11% 23% 22% 5%	1833
	21st to end	Day	39% 5% 11% 28% 16% 2	1680
December	1st to 10th	Day	36% 6% 12% 28% 15% 4	2680
	11th to 20th	Day	44% 4 13% 26% 10% 3	3066
	21st to end	Day	45% 5% 14% 20% 14% 1	4048
January	1st to 10th	Night	39% 4 9% 25% 20% 3	8012
	11th to 20th	Night	37% 3 7% 22% 24% 7%	7680
	21st to end	Night	37% 3 7% 31% 18% 3	8401
February	1st to 10th	Night	42% 5% 13% 24% 15% 2	2730
	11th to 20th	Night	44% 4 9% 22% 16% 4	2631
	21st to end	Night	43% 4 10% 22% 16% 5%	2076
March	1st to 10th	Night	40% 4 8% 25% 15% 8%	5039
	11th to 20th	Night	33% 5% 10% 26% 20% 6%	5090
	21st to end	Night	40% 4 9% 22% 20% 5%	5542
April	1st to 10th	Night	42% 6% 10% 20% 19% 4	2100
	11th to 20th	Night	38% 6% 11% 21% 19% 5%	2075
	21st to end	Night	38% 6% 12% 26% 14% 4	2100
May	1st to 10th	Night	49% 6% 11% 20% 11% 3	5331
	11th to 20th	Night	46% 6% 12% 25% 11% 1	4924
	21st to end	Night	50% 7% 11% 18% 12% 2	5115
June	1st to 10th	Night	36% 7% 10% 25% 19% 2	1680
	11th to 20th	Night	38% 8% 14% 24% 14% 2	1680
	21st to end	Night	34% 8% 14% 24% 18% 3	1680
July	1st to 10th	Night	42% 8% 14% 19% 15% 2	4640
	11th to 20th	Night	46% 10% 14% 18% 11% 1	4965
	21st to end	Night	38% 8% 14% 21% 17% 1	5606
August	1st to 10th	Night	49% 9% 14% 20% 8% 1	4124
	11th to 20th	Night	44% 7% 14% 21% 13% 1	4263
	21st to end	Night	36% 6% 14% 28% 15% 1	5045
September	1st to 10th	Night	37% 7% 11% 21% 19% 5%	2100
	11th to 20th	Night	35% 5% 8% 27% 22% 3	2100
	21st to end	Night	48% 6% 9% 18% 16% 3	2422
October	1st to 10th	Night	40% 4 9% 21% 22% 4	4040
	11th to 20th	Night	26% 3 11% 22% 31% 8%	4019
	21st to end	Night	24% 3 9% 31% 27% 7%	4349
November	1st to 10th	Night	32% 5% 10% 24% 23% 5%	2543
	11th to 20th	Night	35% 4 8% 24% 22% 6%	2783
	21st to end	Night	37% 4 11% 25% 20% 3	2940
December	1st to 10th	Night	33% 4 13% 28% 20% 2	4720
	11th to 20th	Night	44% 3 12% 20% 16% 4	5348
	21st to end	Night	45% 7% 15% 19% 14% 1	7150

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	34% 22% 28% 14% 2	259
	11th to 20th	Day	34% 25% 28% 11% 1	330
	21st to end	Day	25% 31% 31% 10% 2 1	363
February	1st to 10th	Day	25% 28% 31% 13% 3	330
	11th to 20th	Day	41% 23% 28% 5% 2	362
	21st to end	Day	28% 22% 32% 17% 2	446
March	1st to 10th	Day	26% 23% 30% 18% 3	540
	11th to 20th	Day	27% 22% 29% 16% 4 2	540
	21st to end	Day	23% 20% 34% 17% 5% 1	594
April	1st to 10th	Day	25% 24% 27% 17% 6% 1	540
	11th to 20th	Day	19% 19% 30% 17% 11% 3	532
	21st to end	Day	25% 16% 31% 18% 9% 2	540
May	1st to 10th	Day	22% 19% 29% 18% 9% 3	543
	11th to 20th	Day	22% 13% 28% 21% 13% 4	567
	21st to end	Day	21% 9% 22% 24% 19% 5%	617
June	1st to 10th	Day	12% 9% 29% 27% 18% 5%	570
	11th to 20th	Day	15% 9% 26% 29% 16% 5%	570
	21st to end	Day	11% 5% 29% 31% 18% 6%	570
July	1st to 10th	Day	13% 4 18% 28% 26% 11%	565
	11th to 20th	Day	6% 3 20% 34% 29% 8%	564
	21st to end	Day	11% 5% 21% 33% 23% 8%	620
August	1st to 10th	Day	12% 6% 19% 27% 24% 12%	566
	11th to 20th	Day	11% 4 23% 31% 21% 9%	559
	21st to end	Day	10% 3 23% 31% 22% 11%	627
September	1st to 10th	Day	16% 7% 21% 27% 20% 9%	570
	11th to 20th	Day	14% 5% 20% 30% 19% 11%	570
	21st to end	Day	28% 9% 24% 23% 12% 3	570
October	1st to 10th	Day	19% 8% 21% 27% 19% 7%	570
	11th to 20th	Day	19% 15% 25% 26% 13% 2	570
	21st to end	Day	16% 14% 32% 25% 12% 2	615
November	1st to 10th	Day	16% 15% 32% 25% 9% 3	491
	11th to 20th	Day	22% 15% 34% 21% 8% 1	373
	21st to end	Day	25% 22% 30% 16% 5% 1	220
December	1st to 10th	Day	21% 23% 39% 14% 3 1	220
	11th to 20th	Day	23% 25% 29% 19% 5%	220
	21st to end	Day	26% 22% 29% 18% 4 1	242
January	1st to 10th	Night	21% 21% 37% 18% 3	840
	11th to 20th	Night	25% 17% 31% 23% 4	840
	21st to end	Night	20% 18% 40% 20% 2	924
February	1st to 10th	Night	20% 17% 37% 24% 2	840
	11th to 20th	Night	26% 22% 35% 16% 2	823
	21st to end	Night	25% 17% 37% 18% 3	692
March	1st to 10th	Night	22% 19% 35% 21% 3	840
	11th to 20th	Night	24% 19% 32% 20% 5%	840
	21st to end	Night	20% 19% 37% 20% 4 1	924
April	1st to 10th	Night	20% 20% 35% 21% 4	840
	11th to 20th	Night	17% 13% 37% 26% 7%	831
	21st to end	Night	19% 13% 31% 28% 8%	840
May	1st to 10th	Night	21% 9% 28% 31% 11%	658
	11th to 20th	Night	22% 6% 26% 34% 12% 1	518
	21st to end	Night	17% 5% 23% 38% 17% 1	455
June	1st to 10th	Night	10% 5% 27% 36% 22%	420
	11th to 20th	Night	13% 6% 21% 41% 18% 1	420
	21st to end	Night	8% 3 23% 46% 18% 1	420
July	1st to 10th	Night	11% 3 19% 39% 26% 3	410
	11th to 20th	Night	7% 1 16% 41% 33% 3	520
	21st to end	Night	9% 3 17% 42% 26% 3	690
August	1st to 10th	Night	12% 2 12% 41% 31% 3	627
	11th to 20th	Night	11% 4 13% 42% 28% 2	687
	21st to end	Night	12% 3 18% 39% 24% 3	924
September	1st to 10th	Night	15% 3 21% 32% 26% 4	840
	11th to 20th	Night	12% 3 19% 35% 28% 3	840
	21st to end	Night	24% 5% 21% 30% 19% 1	840
October	1st to 10th	Night	21% 5% 22% 29% 19% 3	840
	11th to 20th	Night	18% 12% 25% 31% 14% 1	840
	21st to end	Night	17% 11% 31% 31% 11%	913
November	1st to 10th	Night	14% 13% 32% 30% 10% 1	820
	11th to 20th	Night	18% 15% 33% 26% 7% 1	840
	21st to end	Night	17% 15% 33% 27% 7%	840
December	1st to 10th	Night	22% 16% 34% 22% 6%	840
	11th to 20th	Night	21% 13% 38% 25% 4	840
	21st to end	Night	20% 15% 34% 28% 3	924

Page generated at 2026-10-26 04:37:51. Version ac07

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