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 2025-07-06.

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% 42%	120001
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	118794
Transparency	Day	39% 6% 11% 24% 16% 4	123639
	Night	39% 5% 11% 24% 18% 4	123293
Seeing	Day	19% 14% 27% 23% 13% 5%	15779
	Night	18% 12% 29% 28% 12% 1	25340

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 4 4 3 4 4 5% 6% 49%	3758
	11th to 20th	Day	14% 9% 3 6% 4 3 4 3 4 5% 46%	4060
	21st to end	Day	14% 9% 2 6% 4 3 4 3 5% 6% 43%	4466
February	1st to 10th	Day	13% 8% 2 7% 5% 4 5% 5% 5% 6% 39%	1800
	11th to 20th	Day	18% 8% 2 6% 5% 4 4 3 5% 6% 39%	1783
	21st to end	Day	16% 8% 3 5% 5% 4 5% 3 5% 5% 41%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 4 4 3 5% 7% 41%	4907
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 49%	4980
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 5% 7% 40%	5510
April	1st to 10th	Day	14% 7% 3 6% 5% 4 4 4 6% 7% 41%	2400
	11th to 20th	Day	13% 6% 3 6% 4 3 4 3 5% 6% 46%	2374
	21st to end	Day	17% 6% 2 6% 4 3 4 4 5% 6% 42%	2400
May	1st to 10th	Day	19% 6% 2 7% 5% 4 6% 5% 7% 10% 31%	5852
	11th to 20th	Day	23% 6% 2 6% 4 4 4 4 7% 10% 31%	6088
	21st to end	Day	16% 5% 2 5% 4 4 4 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 4 2 6% 5% 5% 4 5% 6% 10% 43%	2800
	11th to 20th	Day	11% 5% 3 6% 4 4 5% 5% 7% 9% 41%	2800
	21st to end	Day	9% 4 2 5% 5% 4 5% 4 7% 10% 46%	2800
July	1st to 10th	Day	6% 4 2 6% 4 5% 6% 5% 8% 10% 43%	4510
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 4 7% 10% 40%	4255
	21st to end	Day	11% 6% 3 7% 6% 5% 6% 5% 7% 8% 37%	4466
August	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 5% 7% 9% 34%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 5% 7% 9% 42%	3916
	21st to end	Day	11% 4 2 5% 4 4 5% 4 5% 8% 48%	4180
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 4 7% 48%	2280
	21st to end	Day	21% 7% 2 5% 4 4 4 3 5% 7% 38%	1985
October	1st to 10th	Day	17% 5% 2 5% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Day	16% 5% 2 4 4 3 3 4 4 6% 49%	1900
	21st to end	Day	13% 5% 2 5% 3 3 4 3 4 6% 51%	2090
November	1st to 10th	Day	11% 5% 2 4 3 4 3 3 6% 7% 51%	1877
	11th to 20th	Day	15% 5% 3 4 3 3 4 3 6% 6% 49%	1746
	21st to end	Day	15% 6% 1 6% 3 4 4 4 5% 7% 45%	1600
December	1st to 10th	Day	12% 5% 2 6% 4 3 4 4 5% 6% 48%	2600
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 5% 6% 41%	2990
	21st to end	Day	18% 8% 2 7% 4 4 4 3 6% 8% 35%	3960
January	1st to 10th	Night	11% 7% 2 5% 4 3 4 4 5% 6% 49%	6183
	11th to 20th	Night	14% 6% 2 5% 3 3 4 2 4 6% 51%	5900
	21st to end	Night	15% 8% 3 6% 3 2 3 3 4 6% 48%	6490
February	1st to 10th	Night	15% 8% 3 6% 4 5% 4 4 5% 6% 41%	2600
	11th to 20th	Night	18% 7% 2 6% 4 3 4 4 5% 5% 41%	2501
	21st to end	Night	17% 8% 2 5% 5% 3 4 4 5% 6% 41%	1980
March	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 4 5% 47%	4928
	11th to 20th	Night	11% 6% 2 5% 3 4 4 4 5% 7% 50%	4980
	21st to end	Night	12% 8% 2 5% 4 4 5% 4 5% 6% 44%	5422
April	1st to 10th	Night	15% 8% 2 5% 5% 4 5% 4 5% 6% 40%	2000
	11th to 20th	Night	13% 6% 2 7% 4 3 5% 3 6% 7% 44%	1975
	21st to end	Night	15% 5% 1 5% 4 4 4 4 6% 7% 45%	2000
May	1st to 10th	Night	22% 8% 3 7% 4 5% 4 4 5% 8% 29%	4086
	11th to 20th	Night	24% 8% 3 6% 4 4 4 4 5% 7% 31%	3791
	21st to end	Night	17% 6% 2 6% 4 5% 5% 5% 7% 8% 35%	3920
June	1st to 10th	Night	11% 6% 2 6% 4 4 4 5% 6% 9% 45%	1600
	11th to 20th	Night	13% 5% 2 6% 4 5% 5% 4 7% 8% 42%	1600
	21st to end	Night	8% 4 2 5% 5% 5% 5% 5% 8% 9% 44%	1600
July	1st to 10th	Night	6% 5% 2 7% 6% 6% 6% 5% 7% 10% 40%	2560
	11th to 20th	Night	8% 4 3 7% 6% 5% 6% 5% 8% 10% 39%	2700
	21st to end	Night	10% 7% 2 7% 5% 5% 5% 5% 7% 9% 38%	3087
August	1st to 10th	Night	16% 6% 3 7% 5% 5% 6% 4 6% 10% 32%	2825
	11th to 20th	Night	10% 6% 3 7% 5% 6% 5% 4 5% 9% 40%	2881
	21st to end	Night	12% 4 1 5% 4 4 4 4 5% 8% 48%	3476
September	1st to 10th	Night	15% 5% 1 6% 4 3 5% 4 5% 6% 45%	1900
	11th to 20th	Night	10% 6% 3 5% 4 3 4 3 4 6% 52%	1900
	21st to end	Night	21% 6% 2 6% 4 3 3 3 5% 6% 40%	2191
October	1st to 10th	Night	21% 5% 2 5% 3 3 4 3 4 6% 46%	2280
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 4 5% 53%	2280
	21st to end	Night	15% 5% 2 5% 3 4 3 2 4 7% 51%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 2 3 3 4 6% 54%	2419
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 7% 49%	2650
	21st to end	Night	13% 4 2 5% 5% 4 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 4 2 4 4 4 4 3 5% 7% 51%	4580
	11th to 20th	Night	20% 6% 2 5% 3 4 4 3 5% 7% 39%	5205
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 6% 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	35% 4 5% 28% 20% 7%	3759
	11th to 20th	Day	41% 4 7% 21% 18% 10%	4060
	21st to end	Day	42% 4 10% 27% 14% 3	4466
February	1st to 10th	Day	44% 6% 12% 24% 13% 2	1800
	11th to 20th	Day	46% 5% 8% 23% 16% 3	1792
	21st to end	Day	44% 5% 10% 21% 15% 4	1650
March	1st to 10th	Day	43% 4 8% 21% 16% 8%	4907
	11th to 20th	Day	34% 5% 9% 26% 20% 6%	4980
	21st to end	Day	43% 5% 10% 18% 18% 6%	5510
April	1st to 10th	Day	41% 6% 11% 19% 18% 5%	2400
	11th to 20th	Day	38% 5% 11% 22% 19% 6%	2374
	21st to end	Day	43% 5% 10% 23% 16% 4	2400
May	1st to 10th	Day	47% 8% 15% 18% 10% 2	5887
	11th to 20th	Day	47% 8% 10% 24% 9% 1	6092
	21st to end	Day	40% 8% 13% 22% 16% 2	6949
June	1st to 10th	Day	35% 8% 15% 25% 17% 1	2800
	11th to 20th	Day	37% 10% 13% 25% 13% 2	2800
	21st to end	Day	32% 9% 15% 26% 17% 2	2800
July	1st to 10th	Day	32% 10% 18% 23% 11% 5%	4440
	11th to 20th	Day	36% 8% 14% 29% 11% 2	4181
	21st to end	Day	43% 8% 10% 22% 17% 1	4324
August	1st to 10th	Day	43% 8% 15% 25% 8%	4091
	11th to 20th	Day	36% 8% 13% 27% 16% 1	3955
	21st to end	Day	33% 7% 14% 27% 17% 2	4180
September	1st to 10th	Day	38% 7% 11% 19% 21% 4	2280
	11th to 20th	Day	35% 5% 8% 27% 22% 3	2280
	21st to end	Day	45% 7% 10% 19% 18% 2	1985
October	1st to 10th	Day	37% 4 8% 19% 28% 5%	3160
	11th to 20th	Day	26% 3 7% 28% 25% 9%	3140
	21st to end	Day	23% 3 10% 30% 28% 6%	3428
November	1st to 10th	Day	33% 4 11% 26% 21% 4	1877
	11th to 20th	Day	35% 4 11% 23% 22% 5%	1746
	21st to end	Day	38% 6% 11% 28% 16% 2	1600
December	1st to 10th	Day	35% 6% 11% 28% 15% 4	2600
	11th to 20th	Day	44% 4 13% 26% 11% 3	2986
	21st to end	Day	45% 5% 14% 20% 14% 1	3960
January	1st to 10th	Night	36% 4 7% 25% 24% 4	6183
	11th to 20th	Night	36% 3 8% 24% 22% 7%	5900
	21st to end	Night	38% 4 8% 30% 18% 3	6490
February	1st to 10th	Night	42% 5% 13% 24% 14% 2	2600
	11th to 20th	Night	44% 4 9% 22% 16% 4	2503
	21st to end	Night	44% 4 9% 22% 16% 5%	1980
March	1st to 10th	Night	39% 4 8% 26% 15% 8%	4928
	11th to 20th	Night	33% 5% 10% 26% 20% 6%	4980
	21st to end	Night	40% 5% 9% 22% 20% 5%	5422
April	1st to 10th	Night	42% 6% 9% 20% 19% 4	2000
	11th to 20th	Night	38% 7% 11% 20% 19% 6%	1975
	21st to end	Night	38% 6% 11% 26% 15% 4	2000
May	1st to 10th	Night	53% 7% 12% 17% 9% 4	4113
	11th to 20th	Night	51% 6% 10% 21% 10% 1	3792
	21st to end	Night	44% 8% 13% 23% 9% 2	3927
June	1st to 10th	Night	36% 7% 11% 25% 18% 3	1600
	11th to 20th	Night	38% 8% 14% 25% 14% 2	1600
	21st to end	Night	33% 8% 14% 24% 19% 3	1600
July	1st to 10th	Night	37% 8% 20% 18% 13% 4	2520
	11th to 20th	Night	38% 9% 18% 22% 12% 1	2651
	21st to end	Night	41% 9% 14% 20% 16% 1	2988
August	1st to 10th	Night	46% 8% 15% 23% 8% 1	2814
	11th to 20th	Night	40% 6% 12% 26% 15% 1	2908
	21st to end	Night	33% 5% 13% 30% 16% 2	3476
September	1st to 10th	Night	39% 7% 11% 20% 19% 5%	1900
	11th to 20th	Night	34% 5% 8% 28% 22% 3	1900
	21st to end	Night	46% 6% 10% 19% 17% 3	2191
October	1st to 10th	Night	41% 4 9% 21% 21% 4	3800
	11th to 20th	Night	25% 3 11% 22% 32% 7%	3779
	21st to end	Night	24% 3 9% 32% 26% 7%	4120
November	1st to 10th	Night	32% 5% 10% 25% 23% 6%	2419
	11th to 20th	Night	35% 4 8% 24% 22% 6%	2650
	21st to end	Night	36% 4 11% 25% 20% 3	2800
December	1st to 10th	Night	33% 4 13% 28% 20% 2	4580
	11th to 20th	Night	45% 3 12% 20% 16% 4	5208
	21st to end	Night	45% 7% 15% 19% 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	34% 23% 29% 13% 1	235
	11th to 20th	Day	35% 23% 28% 12% 1	300
	21st to end	Day	25% 31% 31% 11% 3 1	330
February	1st to 10th	Day	24% 30% 31% 12% 3	300
	11th to 20th	Day	41% 24% 28% 5% 2	330
	21st to end	Day	28% 21% 32% 16% 2	414
March	1st to 10th	Day	25% 23% 30% 19% 3	500
	11th to 20th	Day	26% 22% 29% 16% 4 2	500
	21st to end	Day	23% 20% 34% 17% 5% 1	550
April	1st to 10th	Day	25% 24% 28% 16% 6% 1	500
	11th to 20th	Day	19% 20% 29% 17% 12% 3	492
	21st to end	Day	25% 16% 31% 17% 9% 2	500
May	1st to 10th	Day	22% 18% 28% 18% 10% 3	503
	11th to 20th	Day	23% 12% 28% 20% 13% 4	527
	21st to end	Day	19% 9% 23% 24% 20% 5%	573
June	1st to 10th	Day	13% 9% 29% 26% 18% 5%	530
	11th to 20th	Day	15% 8% 26% 30% 17% 5%	530
	21st to end	Day	10% 5% 28% 32% 18% 7%	530
July	1st to 10th	Day	11% 4 18% 30% 26% 11%	505
	11th to 20th	Day	6% 3 20% 33% 30% 8%	484
	21st to end	Day	12% 4 21% 33% 22% 8%	532
August	1st to 10th	Day	11% 6% 19% 27% 24% 13%	486
	11th to 20th	Day	11% 5% 21% 31% 22% 10%	479
	21st to end	Day	10% 3 20% 31% 22% 13%	539
September	1st to 10th	Day	18% 7% 19% 25% 21% 10%	490
	11th to 20th	Day	14% 5% 21% 29% 19% 12%	490
	21st to end	Day	27% 10% 24% 24% 12% 3	490
October	1st to 10th	Day	21% 8% 20% 25% 19% 6%	490
	11th to 20th	Day	17% 15% 24% 27% 14% 2	490
	21st to end	Day	16% 13% 33% 24% 11% 3	539
November	1st to 10th	Day	17% 13% 32% 25% 9% 4	455
	11th to 20th	Day	20% 14% 35% 21% 9% 1	346
	21st to end	Day	24% 22% 31% 16% 6% 2	200
December	1st to 10th	Day	19% 23% 40% 14% 4 1	200
	11th to 20th	Day	24% 26% 26% 19% 6%	200
	21st to end	Day	25% 22% 29% 19% 5% 1	220
January	1st to 10th	Night	22% 22% 36% 17% 3	800
	11th to 20th	Night	25% 16% 31% 24% 4	800
	21st to end	Night	20% 18% 40% 21% 2	880
February	1st to 10th	Night	20% 17% 37% 24% 2	800
	11th to 20th	Night	26% 22% 35% 15% 2	783
	21st to end	Night	25% 17% 37% 18% 3	660
March	1st to 10th	Night	21% 20% 35% 21% 3	800
	11th to 20th	Night	24% 19% 32% 20% 5%	800
	21st to end	Night	20% 19% 37% 20% 4 1	880
April	1st to 10th	Night	20% 21% 35% 21% 4	800
	11th to 20th	Night	18% 12% 37% 25% 7% 1	791
	21st to end	Night	20% 13% 31% 28% 9%	800
May	1st to 10th	Night	21% 8% 27% 32% 11%	627
	11th to 20th	Night	23% 5% 25% 35% 11% 1	493
	21st to end	Night	16% 5% 22% 39% 18% 1	433
June	1st to 10th	Night	10% 5% 27% 37% 22%	400
	11th to 20th	Night	13% 6% 20% 42% 18% 2	400
	21st to end	Night	9% 3 23% 46% 18% 1	400
July	1st to 10th	Night	11% 3 19% 39% 24% 3	380
	11th to 20th	Night	7% 1 16% 40% 33% 3	469
	21st to end	Night	9% 3 17% 42% 27% 3	624
August	1st to 10th	Night	12% 1 12% 42% 30% 3	567
	11th to 20th	Night	11% 4 13% 42% 28% 2	621
	21st to end	Night	12% 3 16% 39% 26% 4	836
September	1st to 10th	Night	16% 3 20% 30% 27% 4	760
	11th to 20th	Night	12% 3 19% 34% 28% 4	760
	21st to end	Night	24% 5% 21% 30% 20% 1	760
October	1st to 10th	Night	22% 6% 22% 29% 19% 3	760
	11th to 20th	Night	16% 12% 26% 32% 14% 1	760
	21st to end	Night	18% 11% 31% 29% 11%	836
November	1st to 10th	Night	14% 12% 32% 31% 10% 1	780
	11th to 20th	Night	18% 14% 33% 27% 8% 1	800
	21st to end	Night	17% 15% 33% 27% 7% 1	800
December	1st to 10th	Night	22% 15% 34% 22% 6%	800
	11th to 20th	Night	20% 13% 39% 24% 4	800
	21st to end	Night	21% 15% 34% 29% 3	880

Page generated at 2025-07-03 01:30:15. Version ac07

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