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 2024-03-18.

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 5% 5% 3 5% 5% 6% 8% 42%	78707
	Night	15% 6% 2 5% 5% 3 4 5% 5% 7% 44%	81083
Transparency	Day	27% 37% 9% 16% 10% 2	78418
	Night	27% 35% 8% 16% 11% 2	80891
Seeing	Day	19% 13% 26% 23% 13% 5%	14034
	Night	18% 11% 29% 28% 12% 1	23729

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	17% 5% 2 4 4 3 4 5% 5% 6% 46%	2622
	11th to 20th	Day	15% 10% 3 6% 6% 2 4 4 5% 6% 39%	2840
	21st to end	Day	17% 10% 3 7% 6% 2 4 4 4 6% 37%	3124
February	1st to 10th	Day	14% 7% 2 5% 6% 4 5% 6% 5% 6% 39%	1710
	11th to 20th	Day	17% 7% 2 5% 6% 3 4 4 5% 7% 40%	1691
	21st to end	Day	17% 8% 3 5% 5% 3 5% 4 5% 5% 40%	1570
March	1st to 10th	Day	13% 5% 2 4 5% 3 4 4 5% 7% 46%	2076
	11th to 20th	Day	16% 6% 2 4 5% 3 4 5% 5% 6% 42%	2058
	21st to end	Day	17% 6% 3 6% 6% 4 5% 4 5% 7% 38%	2196
April	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 4 6% 8% 42%	2160
	11th to 20th	Day	14% 4 3 6% 5% 2 4 4 5% 7% 47%	2134
	21st to end	Day	18% 6% 3 5% 5% 3 4 5% 5% 6% 42%	2160
May	1st to 10th	Day	16% 6% 2 6% 6% 3 5% 5% 6% 9% 38%	2330
	11th to 20th	Day	19% 5% 2 6% 5% 3 5% 5% 6% 8% 36%	2388
	21st to end	Day	14% 6% 3 4 5% 3 5% 5% 6% 8% 41%	2725
June	1st to 10th	Day	11% 4 2 5% 5% 4 4 5% 6% 10% 43%	2520
	11th to 20th	Day	11% 5% 3 5% 4 3 5% 7% 7% 9% 40%	2520
	21st to end	Day	9% 3 2 4 6% 3 5% 5% 7% 10% 45%	2520
July	1st to 10th	Day	8% 5% 2 5% 6% 4 6% 6% 7% 9% 42%	2520
	11th to 20th	Day	6% 3 2 4 6% 4 6% 6% 8% 10% 44%	2419
	21st to end	Day	10% 5% 2 5% 6% 4 6% 7% 8% 11% 37%	2547
August	1st to 10th	Day	9% 4 2 6% 7% 4 6% 6% 7% 9% 38%	2327
	11th to 20th	Day	12% 5% 2 5% 6% 4 5% 6% 7% 9% 39%	2231
	21st to end	Day	11% 5% 3 4 5% 3 5% 6% 6% 9% 43%	2376
September	1st to 10th	Day	15% 6% 2 4 4 3 4 4 5% 8% 44%	2160
	11th to 20th	Day	13% 4 2 4 4 3 4 4 4 7% 49%	2160
	21st to end	Day	18% 6% 2 5% 5% 3 4 4 5% 7% 40%	1881
October	1st to 10th	Day	17% 4 2 4 4 3 4 4 5% 6% 46%	1800
	11th to 20th	Day	17% 5% 2 3 5% 2 3 5% 4 6% 48%	1800
	21st to end	Day	14% 5% 2 5% 3 2 4 4 4 6% 49%	1980
November	1st to 10th	Day	12% 5% 2 3 4 3 3 4 5% 7% 51%	1781
	11th to 20th	Day	15% 4 2 4 3 2 4 4 6% 6% 50%	1659
	21st to end	Day	15% 4 2 5% 4 2 4 5% 5% 7% 46%	1520
December	1st to 10th	Day	15% 7% 3 6% 5% 3 4 6% 6% 7% 39%	1520
	11th to 20th	Day	16% 5% 2 4 5% 2 4 4 5% 7% 44%	1910
	21st to end	Day	19% 6% 2 6% 5% 4 5% 5% 7% 8% 34%	2772
January	1st to 10th	Night	13% 7% 2 5% 5% 3 4 5% 5% 7% 45%	4319
	11th to 20th	Night	19% 7% 2 4 4 2 3 3 4 6% 46%	4120
	21st to end	Night	19% 8% 3 5% 4 2 3 4 5% 6% 40%	4532
February	1st to 10th	Night	14% 7% 3 5% 5% 3 4 6% 5% 6% 42%	2470
	11th to 20th	Night	18% 6% 2 5% 4 3 5% 4 5% 6% 41%	2373
	21st to end	Night	18% 8% 2 4 6% 3 4 4 5% 6% 41%	1884
March	1st to 10th	Night	14% 5% 2 4 4 2 5% 5% 5% 6% 48%	2100
	11th to 20th	Night	17% 6% 2 4 4 3 4 5% 5% 6% 45%	2068
	21st to end	Night	16% 6% 3 6% 5% 3 4 4 5% 6% 41%	2160
April	1st to 10th	Night	14% 7% 3 5% 5% 3 5% 5% 5% 7% 41%	1800
	11th to 20th	Night	13% 5% 2 6% 5% 2 5% 4 6% 7% 45%	1775
	21st to end	Night	17% 5% 2 4 4 3 4 4 5% 7% 44%	1800
May	1st to 10th	Night	16% 6% 2 6% 4 3 5% 5% 5% 9% 39%	1645
	11th to 20th	Night	17% 8% 3 5% 4 2 4 6% 5% 8% 38%	1505
	21st to end	Night	13% 5% 3 5% 5% 3 5% 6% 7% 9% 41%	1550
June	1st to 10th	Night	11% 5% 2 4 4 3 5% 6% 6% 9% 45%	1440
	11th to 20th	Night	12% 4 2 5% 5% 3 6% 6% 6% 9% 42%	1440
	21st to end	Night	9% 3 3 4 5% 3 5% 6% 8% 10% 44%	1440
July	1st to 10th	Night	9% 5% 2 6% 7% 4 6% 6% 7% 10% 39%	1440
	11th to 20th	Night	7% 3 2 5% 5% 3 6% 6% 7% 10% 45%	1536
	21st to end	Night	8% 5% 2 4 5% 3 5% 6% 9% 12% 41%	1771
August	1st to 10th	Night	9% 5% 3 6% 7% 4 6% 7% 7% 10% 39%	1611
	11th to 20th	Night	10% 6% 3 6% 5% 4 5% 6% 5% 9% 41%	1652
	21st to end	Night	11% 5% 2 5% 5% 4 5% 5% 6% 9% 42%	1980
September	1st to 10th	Night	15% 5% 2 5% 5% 3 4 5% 5% 7% 44%	1800
	11th to 20th	Night	10% 5% 3 5% 4 2 4 3 4 6% 52%	1800
	21st to end	Night	19% 5% 3 5% 5% 2 4 4 5% 6% 42%	2075
October	1st to 10th	Night	21% 4 2 4 4 2 4 3 4 6% 46%	2160
	11th to 20th	Night	16% 4 2 4 4 2 3 3 3 5% 52%	2160
	21st to end	Night	15% 5% 2 4 4 3 3 3 4 6% 50%	2376
November	1st to 10th	Night	12% 5% 2 5% 4 2 3 3 4 6% 54%	2295
	11th to 20th	Night	14% 6% 2 4 3 2 4 4 5% 7% 49%	2517
	21st to end	Night	14% 3 2 4 5% 3 4 5% 5% 7% 47%	2660
December	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 4 5% 6% 41%	2660
	11th to 20th	Night	17% 6% 2 5% 4 3 5% 3 6% 7% 42%	3285
	21st to end	Night	17% 6% 2 5% 5% 3 5% 5% 6% 9% 36%	4884

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	27% 27% 7% 27% 9% 2	2623
	11th to 20th	Day	36% 27% 7% 15% 11% 4	2840
	21st to end	Day	40% 24% 10% 18% 8% 1	3124
February	1st to 10th	Day	28% 35% 8% 18% 10% 1	1710
	11th to 20th	Day	29% 36% 6% 17% 10% 2	1700
	21st to end	Day	30% 36% 8% 13% 10% 3	1570
March	1st to 10th	Day	23% 35% 10% 18% 13% 1	2076
	11th to 20th	Day	27% 36% 7% 14% 12% 4	2058
	21st to end	Day	32% 38% 7% 13% 9% 2	2196
April	1st to 10th	Day	24% 38% 10% 13% 11% 4	2160
	11th to 20th	Day	25% 37% 7% 14% 13% 5%	2134
	21st to end	Day	27% 37% 7% 14% 11% 3	2160
May	1st to 10th	Day	27% 39% 9% 12% 12% 1	2330
	11th to 20th	Day	30% 39% 8% 11% 10% 2	2392
	21st to end	Day	26% 38% 9% 17% 9% 1	2725
June	1st to 10th	Day	22% 39% 12% 16% 11% 1	2520
	11th to 20th	Day	26% 41% 10% 15% 8% 2	2520
	21st to end	Day	22% 39% 11% 17% 9% 1	2520
July	1st to 10th	Day	24% 41% 9% 15% 10% 2	2450
	11th to 20th	Day	21% 41% 10% 17% 9% 2	2345
	21st to end	Day	25% 42% 10% 12% 10% 1	2405
August	1st to 10th	Day	26% 40% 10% 16% 8%	2314
	11th to 20th	Day	27% 38% 9% 13% 12% 1	2231
	21st to end	Day	27% 39% 11% 14% 7% 3	2376
September	1st to 10th	Day	25% 37% 7% 12% 14% 4	2160
	11th to 20th	Day	22% 38% 6% 17% 14% 3	2160
	21st to end	Day	28% 39% 7% 11% 14% 2	1881
October	1st to 10th	Day	24% 37% 8% 17% 13% 2	1800
	11th to 20th	Day	26% 37% 8% 16% 13% 1	1800
	21st to end	Day	25% 38% 5% 16% 14% 1	1980
November	1st to 10th	Day	18% 41% 7% 20% 13% 2	1781
	11th to 20th	Day	24% 40% 8% 14% 12% 3	1659
	21st to end	Day	24% 41% 8% 16% 10% 1	1520
December	1st to 10th	Day	26% 42% 8% 17% 7% 1	1520
	11th to 20th	Day	27% 33% 7% 22% 8% 2	1906
	21st to end	Day	32% 30% 13% 18% 7%	2772
January	1st to 10th	Night	29% 27% 9% 24% 11% 1	4319
	11th to 20th	Night	31% 24% 9% 20% 13% 3	4120
	21st to end	Night	37% 24% 8% 22% 9%	4532
February	1st to 10th	Night	28% 35% 7% 18% 11% 1	2470
	11th to 20th	Night	30% 36% 8% 15% 9% 3	2375
	21st to end	Night	30% 35% 7% 15% 10% 3	1884
March	1st to 10th	Night	23% 35% 9% 20% 12% 2	2100
	11th to 20th	Night	27% 37% 7% 14% 13% 3	2068
	21st to end	Night	29% 37% 7% 14% 9% 4	2160
April	1st to 10th	Night	27% 38% 7% 12% 13% 3	1800
	11th to 20th	Night	25% 37% 8% 11% 14% 5%	1775
	21st to end	Night	25% 38% 9% 16% 9% 4	1800
May	1st to 10th	Night	27% 39% 8% 13% 10% 3	1645
	11th to 20th	Night	28% 40% 7% 12% 11% 2	1506
	21st to end	Night	23% 41% 12% 14% 9% 1	1551
June	1st to 10th	Night	22% 39% 8% 18% 11% 2	1440
	11th to 20th	Night	25% 39% 10% 14% 10% 2	1440
	21st to end	Night	23% 39% 10% 16% 11% 2	1440
July	1st to 10th	Night	26% 41% 11% 13% 6% 3	1400
	11th to 20th	Night	22% 42% 10% 13% 11% 2	1487
	21st to end	Night	18% 42% 14% 15% 10% 1	1672
August	1st to 10th	Night	26% 39% 10% 15% 9%	1600
	11th to 20th	Night	29% 37% 8% 14% 10% 2	1652
	21st to end	Night	29% 39% 10% 13% 8% 2	1980
September	1st to 10th	Night	24% 38% 7% 12% 14% 4	1800
	11th to 20th	Night	22% 37% 7% 18% 15% 2	1800
	21st to end	Night	28% 38% 6% 11% 13% 3	2075
October	1st to 10th	Night	26% 37% 6% 16% 12% 2	2160
	11th to 20th	Night	23% 37% 8% 17% 13% 1	2160
	21st to end	Night	25% 37% 6% 16% 15% 2	2376
November	1st to 10th	Night	19% 41% 5% 17% 16% 3	2295
	11th to 20th	Night	23% 40% 6% 16% 12% 3	2517
	21st to end	Night	24% 40% 7% 14% 13% 2	2660
December	1st to 10th	Night	25% 40% 7% 18% 10% 1	2660
	11th to 20th	Night	29% 33% 7% 16% 14% 2	3288
	21st to end	Night	32% 30% 14% 16% 8% 1	4884

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	31% 23% 30% 14% 1	211
	11th to 20th	Day	34% 24% 28% 13% 1	270
	21st to end	Day	25% 32% 30% 10% 3 1	297
February	1st to 10th	Day	23% 27% 33% 13% 4	270
	11th to 20th	Day	37% 26% 30% 5% 2	298
	21st to end	Day	29% 22% 31% 16% 2	382
March	1st to 10th	Day	25% 23% 29% 20% 3	460
	11th to 20th	Day	25% 23% 29% 16% 4 2	449
	21st to end	Day	22% 21% 34% 17% 5% 1	462
April	1st to 10th	Day	21% 24% 29% 18% 6% 1	420
	11th to 20th	Day	18% 17% 29% 19% 14% 3	412
	21st to end	Day	25% 16% 31% 16% 9% 2	420
May	1st to 10th	Day	23% 17% 28% 18% 10% 3	423
	11th to 20th	Day	22% 12% 28% 21% 13% 4	447
	21st to end	Day	19% 7% 22% 25% 21% 6%	485
June	1st to 10th	Day	14% 8% 27% 27% 19% 5%	450
	11th to 20th	Day	15% 8% 27% 28% 16% 6%	450
	21st to end	Day	10% 6% 30% 31% 17% 6%	450
July	1st to 10th	Day	12% 4 18% 28% 27% 11%	445
	11th to 20th	Day	6% 2 19% 34% 31% 8%	444
	21st to end	Day	12% 4 21% 32% 22% 9%	488
August	1st to 10th	Day	11% 6% 19% 26% 24% 14%	446
	11th to 20th	Day	11% 5% 18% 31% 24% 11%	439
	21st to end	Day	10% 3 20% 32% 22% 13%	495
September	1st to 10th	Day	18% 7% 19% 24% 21% 11%	450
	11th to 20th	Day	14% 5% 21% 30% 19% 12%	450
	21st to end	Day	23% 10% 24% 25% 14% 4	450
October	1st to 10th	Day	21% 7% 19% 26% 20% 6%	450
	11th to 20th	Day	17% 15% 25% 27% 13% 2	450
	21st to end	Day	17% 13% 34% 23% 10% 3	495
November	1st to 10th	Day	17% 12% 31% 25% 10% 4	419
	11th to 20th	Day	20% 14% 33% 22% 10% 1	319
	21st to end	Day	22% 21% 33% 17% 6% 2	180
December	1st to 10th	Day	18% 24% 39% 14% 4 1	180
	11th to 20th	Day	23% 28% 24% 20% 5%	180
	21st to end	Day	24% 23% 29% 18% 5% 1	198
January	1st to 10th	Night	21% 21% 37% 18% 3	760
	11th to 20th	Night	25% 16% 31% 24% 4	760
	21st to end	Night	20% 18% 39% 21% 2	836
February	1st to 10th	Night	19% 17% 37% 25% 3	760
	11th to 20th	Night	25% 22% 35% 16% 2	743
	21st to end	Night	26% 18% 36% 18% 3	628
March	1st to 10th	Night	21% 18% 36% 21% 3	760
	11th to 20th	Night	24% 18% 32% 20% 5%	752
	21st to end	Night	21% 16% 38% 20% 4 1	792
April	1st to 10th	Night	19% 20% 34% 22% 4	720
	11th to 20th	Night	17% 11% 37% 27% 8% 1	711
	21st to end	Night	21% 12% 30% 28% 9%	720
May	1st to 10th	Night	21% 8% 28% 31% 12%	565
	11th to 20th	Night	22% 5% 25% 36% 11% 1	444
	21st to end	Night	15% 5% 20% 41% 18% 1	389
June	1st to 10th	Night	10% 4 27% 36% 24%	360
	11th to 20th	Night	12% 5% 20% 41% 19% 2	360
	21st to end	Night	8% 3 23% 47% 18% 1	360
July	1st to 10th	Night	10% 3 19% 39% 25% 3	350
	11th to 20th	Night	7% 1 15% 40% 34% 3	443
	21st to end	Night	9% 2 17% 42% 27% 3	591
August	1st to 10th	Night	11% 1 12% 42% 30% 3	537
	11th to 20th	Night	11% 4 12% 42% 29% 3	588
	21st to end	Night	12% 3 15% 39% 27% 4	792
September	1st to 10th	Night	17% 3 19% 30% 27% 4	720
	11th to 20th	Night	11% 3 20% 33% 28% 4	720
	21st to end	Night	22% 5% 21% 30% 21% 1	720
October	1st to 10th	Night	22% 6% 21% 29% 19% 3	720
	11th to 20th	Night	17% 12% 26% 31% 14% 1	720
	21st to end	Night	19% 12% 31% 27% 11%	792
November	1st to 10th	Night	14% 12% 32% 30% 11% 1	740
	11th to 20th	Night	18% 14% 33% 27% 8% 1	760
	21st to end	Night	17% 15% 33% 28% 8% 1	760
December	1st to 10th	Night	23% 16% 34% 21% 6%	760
	11th to 20th	Night	19% 13% 40% 24% 4	760
	21st to end	Night	20% 14% 34% 28% 3	836

Page generated at 2024-03-15 14:01:42. Version ac07

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