Ashburn Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashburn Astronomical Observatory 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	15% 10% 3 8% 5% 5% 5% 4 5% 7% 33%	126420
	Night	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	122296
Transparency	Day	47% 7% 13% 20% 11% 1	130009
	Night	47% 7% 13% 20% 12% 1	126793
Seeing	Day	21% 14% 30% 22% 9% 3	15470
	Night	20% 12% 31% 27% 10% 1	24779

By Month

Percentage Cloud Cover NE (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% 12% 4 10% 6% 6% 4 3 5% 5% 31%	3597
	11th to 20th	Day	10% 18% 7% 17% 9% 6% 5% 3 4 4 17%	3815
	21st to end	Day	17% 18% 6% 14% 6% 5% 4 3 4 4 18%	4466
February	1st to 10th	Day	15% 17% 5% 11% 7% 6% 6% 4 4 4 21%	1800
	11th to 20th	Day	13% 14% 6% 13% 7% 4 4 4 5% 5% 25%	1753
	21st to end	Day	18% 15% 5% 10% 6% 5% 4 3 4 4 27%	1529
March	1st to 10th	Day	19% 10% 4 9% 6% 4 3 3 4 5% 32%	4932
	11th to 20th	Day	12% 12% 5% 8% 6% 5% 6% 4 6% 7% 29%	4980
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 4 5% 34%	5465
April	1st to 10th	Day	19% 13% 3 8% 5% 4 4 3 5% 6% 29%	2385
	11th to 20th	Day	20% 10% 3 7% 4 5% 4 3 5% 5% 35%	2424
	21st to end	Day	20% 8% 3 8% 5% 4 5% 3 5% 6% 34%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 31%	5952
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 3 5% 7% 34%	6318
	21st to end	Day	19% 6% 2 6% 4 4 5% 3 5% 8% 37%	6949
June	1st to 10th	Day	15% 7% 3 6% 6% 5% 5% 5% 6% 8% 35%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 39%	2800
	21st to end	Day	15% 6% 3 6% 5% 6% 4 4 6% 9% 37%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	6360
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 5% 7% 10% 44%	6360
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 9% 46%	6608
August	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 38%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 4 5% 4 7% 10% 36%	4174
	21st to end	Day	9% 5% 3 6% 4 5% 5% 4 7% 9% 43%	4459
September	1st to 10th	Day	13% 5% 3 6% 4 4 5% 4 6% 7% 43%	2316
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 45%	2201
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 5% 7% 38%	2030
October	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 5% 6% 35%	1900
	11th to 20th	Day	21% 8% 3 7% 6% 5% 5% 4 5% 6% 30%	1900
	21st to end	Day	23% 12% 4 9% 5% 5% 4 4 4 6% 24%	2090
November	1st to 10th	Day	15% 11% 4 8% 5% 6% 5% 3 6% 7% 32%	1677
	11th to 20th	Day	16% 13% 3 9% 7% 4 5% 5% 7% 6% 26%	1600
	21st to end	Day	18% 14% 4 10% 6% 5% 5% 3 4 6% 23%	1600
December	1st to 10th	Day	18% 17% 5% 12% 7% 5% 6% 4 6% 6% 13%	2600
	11th to 20th	Day	18% 18% 4 10% 5% 4 4 3 5% 5% 24%	2986
	21st to end	Day	25% 17% 5% 14% 5% 4 4 3 4 5% 15%	3960
January	1st to 10th	Night	15% 12% 4 11% 6% 6% 5% 4 5% 6% 25%	6349
	11th to 20th	Night	13% 13% 6% 14% 9% 6% 6% 4 5% 6% 19%	6066
	21st to end	Night	15% 14% 5% 11% 6% 5% 5% 4 5% 6% 24%	6490
February	1st to 10th	Night	13% 16% 4 11% 6% 6% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	15% 13% 5% 10% 6% 5% 4 4 5% 6% 28%	2533
	21st to end	Night	13% 12% 4 10% 5% 6% 5% 4 6% 5% 29%	2095
March	1st to 10th	Night	16% 11% 4 9% 6% 5% 5% 4 4 6% 32%	4908
	11th to 20th	Night	10% 8% 3 8% 5% 6% 5% 5% 7% 8% 36%	4979
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 6% 37%	5467
April	1st to 10th	Night	19% 10% 3 7% 5% 5% 4 4 4 7% 31%	2015
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 4 6% 38%	1942
	21st to end	Night	16% 9% 2 7% 5% 5% 5% 4 6% 7% 35%	1800
May	1st to 10th	Night	24% 8% 3 6% 4 4 4 3 5% 5% 34%	3989
	11th to 20th	Night	14% 8% 2 7% 5% 4 5% 4 5% 7% 39%	3575
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 8% 41%	3926
June	1st to 10th	Night	12% 5% 2 7% 5% 5% 4 5% 6% 8% 41%	1600
	11th to 20th	Night	9% 5% 3 7% 5% 4 5% 4 6% 8% 43%	1600
	21st to end	Night	11% 6% 3 6% 4 5% 4 4 6% 7% 43%	1600
July	1st to 10th	Night	7% 6% 2 5% 5% 4 5% 4 6% 9% 47%	3600
	11th to 20th	Night	10% 4 2 5% 4 4 5% 3 6% 8% 50%	3600
	21st to end	Night	9% 5% 1 4 3 4 4 3 5% 8% 53%	4240
August	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 5% 8% 48%	2915
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 50%	2875
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 7% 49%	3406
September	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 4 6% 7% 47%	1931
	11th to 20th	Night	11% 4 2 4 4 3 3 3 5% 7% 54%	1976
	21st to end	Night	16% 7% 2 7% 4 4 3 4 5% 6% 42%	2149
October	1st to 10th	Night	15% 6% 2 7% 5% 5% 5% 3 5% 7% 40%	2280
	11th to 20th	Night	19% 8% 3 7% 5% 5% 4 3 6% 6% 33%	2280
	21st to end	Night	20% 10% 4 6% 5% 5% 5% 4 5% 6% 30%	2508
November	1st to 10th	Night	19% 8% 2 6% 4 4 5% 4 5% 7% 36%	2618
	11th to 20th	Night	16% 12% 3 9% 6% 5% 4 4 6% 7% 28%	2800
	21st to end	Night	20% 11% 4 7% 5% 5% 6% 5% 6% 7% 24%	2800
December	1st to 10th	Night	18% 14% 5% 11% 7% 6% 5% 5% 6% 7% 17%	4580
	11th to 20th	Night	15% 18% 5% 10% 6% 5% 5% 4 5% 6% 22%	5208
	21st to end	Night	24% 15% 5% 13% 6% 5% 5% 3 4 4 16%	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	53% 7% 15% 13% 11% 1	3598
	11th to 20th	Day	70% 6% 10% 9% 4 1	3848
	21st to end	Day	67% 6% 11% 9% 7%	4466
February	1st to 10th	Day	64% 7% 10% 14% 4 1	1800
	11th to 20th	Day	59% 7% 12% 15% 7% 1	1762
	21st to end	Day	60% 6% 11% 16% 7% 1	1529
March	1st to 10th	Day	52% 6% 14% 14% 14%	4935
	11th to 20th	Day	50% 7% 17% 17% 7% 2	4980
	21st to end	Day	50% 5% 11% 19% 14% 1	5465
April	1st to 10th	Day	54% 6% 11% 17% 10% 2	2385
	11th to 20th	Day	49% 5% 11% 21% 13% 1	2424
	21st to end	Day	50% 7% 11% 18% 12% 3	2600
May	1st to 10th	Day	49% 7% 13% 16% 11% 4	5961
	11th to 20th	Day	47% 9% 11% 16% 16% 1	6322
	21st to end	Day	44% 7% 13% 22% 12% 3	6949
June	1st to 10th	Day	43% 8% 13% 20% 15% 1	2800
	11th to 20th	Day	39% 8% 14% 24% 14% 1	2800
	21st to end	Day	41% 8% 15% 22% 13% 1	2800
July	1st to 10th	Day	38% 6% 16% 24% 14% 1	6290
	11th to 20th	Day	32% 7% 14% 32% 14% 1	6285
	21st to end	Day	31% 8% 16% 30% 15%	6419
August	1st to 10th	Day	43% 8% 15% 22% 12% 1	4221
	11th to 20th	Day	39% 7% 16% 28% 9%	4213
	21st to end	Day	34% 7% 15% 34% 9% 1	4459
September	1st to 10th	Day	37% 9% 14% 28% 10% 1	2316
	11th to 20th	Day	33% 8% 15% 28% 14%	2201
	21st to end	Day	43% 8% 14% 25% 9%	2030
October	1st to 10th	Day	37% 7% 15% 27% 13% 1	3160
	11th to 20th	Day	39% 6% 17% 23% 14%	3140
	21st to end	Day	51% 7% 15% 21% 5%	3428
November	1st to 10th	Day	49% 7% 16% 18% 9% 1	1677
	11th to 20th	Day	55% 9% 14% 13% 8% 1	1600
	21st to end	Day	61% 7% 15% 15% 3	1600
December	1st to 10th	Day	68% 8% 13% 8% 2	2600
	11th to 20th	Day	61% 8% 13% 11% 8%	2986
	21st to end	Day	72% 7% 7% 8% 6% 1	3960
January	1st to 10th	Night	54% 8% 15% 16% 7%	6349
	11th to 20th	Night	62% 8% 13% 12% 4	6108
	21st to end	Night	58% 8% 13% 11% 10% 1	6490
February	1st to 10th	Night	59% 7% 12% 17% 5% 1	2600
	11th to 20th	Night	56% 7% 11% 17% 8%	2535
	21st to end	Night	52% 7% 14% 19% 7% 1	2095
March	1st to 10th	Night	52% 6% 11% 17% 12% 2	4918
	11th to 20th	Night	42% 7% 19% 21% 9% 2	4980
	21st to end	Night	45% 6% 11% 20% 16% 2	5467
April	1st to 10th	Night	52% 6% 10% 18% 13% 1	2015
	11th to 20th	Night	46% 7% 9% 19% 17% 2	1942
	21st to end	Night	46% 7% 12% 17% 16% 2	1800
May	1st to 10th	Night	50% 6% 11% 14% 13% 6%	3996
	11th to 20th	Night	42% 9% 13% 20% 14% 2	3576
	21st to end	Night	41% 7% 13% 23% 14% 2	3927
June	1st to 10th	Night	38% 8% 12% 24% 16% 2	1600
	11th to 20th	Night	37% 6% 12% 23% 20% 2	1600
	21st to end	Night	38% 7% 14% 24% 16% 1	1600
July	1st to 10th	Night	33% 6% 15% 25% 19% 2	3560
	11th to 20th	Night	30% 6% 14% 32% 17% 1	3552
	21st to end	Night	28% 7% 12% 30% 21% 1	4114
August	1st to 10th	Night	34% 5% 15% 28% 16% 2	2904
	11th to 20th	Night	32% 6% 16% 31% 13% 1	2902
	21st to end	Night	32% 7% 14% 33% 14% 1	3406
September	1st to 10th	Night	34% 8% 14% 28% 14% 1	1931
	11th to 20th	Night	30% 6% 12% 33% 19% 1	1976
	21st to end	Night	42% 7% 12% 27% 12%	2149
October	1st to 10th	Night	32% 7% 10% 28% 23% 1	3800
	11th to 20th	Night	40% 7% 15% 28% 10%	3779
	21st to end	Night	40% 7% 16% 27% 9%	4120
November	1st to 10th	Night	45% 6% 14% 21% 12% 1	2618
	11th to 20th	Night	52% 8% 15% 15% 10% 1	2800
	21st to end	Night	55% 10% 18% 13% 4	2800
December	1st to 10th	Night	63% 7% 17% 11% 3	4580
	11th to 20th	Night	61% 8% 10% 12% 8%	5208
	21st to end	Night	69% 8% 11% 7% 5%	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	24% 25% 28% 18% 6%	200
	11th to 20th	Day	24% 30% 28% 14% 4 1	253
	21st to end	Day	28% 23% 34% 13% 2	330
February	1st to 10th	Day	30% 27% 28% 13% 3	300
	11th to 20th	Day	27% 29% 35% 8% 1	300
	21st to end	Day	38% 18% 35% 9%	288
March	1st to 10th	Day	25% 19% 36% 16% 3	500
	11th to 20th	Day	26% 22% 31% 18% 3 1	500
	21st to end	Day	32% 16% 31% 16% 4 1	550
April	1st to 10th	Day	27% 19% 34% 15% 5%	500
	11th to 20th	Day	28% 19% 27% 15% 7% 3	492
	21st to end	Day	26% 16% 35% 15% 7% 2	500
May	1st to 10th	Day	24% 13% 31% 21% 7% 4	503
	11th to 20th	Day	18% 13% 32% 23% 10% 3	527
	21st to end	Day	16% 11% 29% 27% 13% 4	573
June	1st to 10th	Day	18% 12% 34% 26% 9% 1	530
	11th to 20th	Day	15% 12% 30% 28% 12% 2	530
	21st to end	Day	21% 8% 31% 27% 10% 3	530
July	1st to 10th	Day	11% 7% 30% 31% 17% 5%	525
	11th to 20th	Day	13% 7% 29% 28% 15% 7%	524
	21st to end	Day	7% 8% 34% 35% 11% 6%	576
August	1st to 10th	Day	13% 5% 28% 31% 15% 8%	526
	11th to 20th	Day	13% 7% 22% 29% 21% 7%	519
	21st to end	Day	14% 8% 28% 27% 16% 7%	583
September	1st to 10th	Day	13% 9% 25% 28% 15% 9%	502
	11th to 20th	Day	10% 6% 26% 26% 18% 14%	490
	21st to end	Day	16% 12% 27% 26% 13% 6%	490
October	1st to 10th	Day	22% 12% 29% 18% 15% 3	490
	11th to 20th	Day	22% 19% 33% 18% 6% 1	490
	21st to end	Day	30% 19% 27% 20% 5%	539
November	1st to 10th	Day	26% 20% 27% 20% 7%	290
	11th to 20th	Day	27% 21% 34% 15% 4 1	200
	21st to end	Day	26% 25% 35% 12% 3	200
December	1st to 10th	Day	31% 18% 33% 15% 4 1	200
	11th to 20th	Day	33% 22% 28% 14% 4	200
	21st to end	Day	31% 23% 31% 12% 2	220
January	1st to 10th	Night	23% 21% 35% 16% 5%	800
	11th to 20th	Night	27% 16% 34% 19% 4 1	800
	21st to end	Night	20% 15% 38% 23% 5%	880
February	1st to 10th	Night	22% 18% 38% 20% 2	800
	11th to 20th	Night	23% 18% 37% 18% 4	783
	21st to end	Night	21% 18% 39% 19% 3	660
March	1st to 10th	Night	21% 16% 38% 22% 3	800
	11th to 20th	Night	20% 19% 35% 21% 4 1	800
	21st to end	Night	21% 14% 33% 25% 7%	880
April	1st to 10th	Night	25% 16% 32% 23% 4	800
	11th to 20th	Night	18% 15% 36% 22% 8% 1	758
	21st to end	Night	21% 9% 34% 27% 9%	600
May	1st to 10th	Night	21% 9% 26% 30% 11% 2	575
	11th to 20th	Night	16% 9% 29% 35% 12% 1	394
	21st to end	Night	12% 6% 27% 37% 17% 1	433
June	1st to 10th	Night	12% 8% 30% 37% 13% 1	400
	11th to 20th	Night	11% 9% 27% 38% 15% 1	400
	21st to end	Night	13% 6% 28% 40% 14% 1	400
July	1st to 10th	Night	7% 4 26% 42% 19% 1	390
	11th to 20th	Night	9% 5% 24% 39% 22%	388
	21st to end	Night	10% 2 24% 44% 19% 1	573
August	1st to 10th	Night	9% 4 18% 42% 26% 3	597
	11th to 20th	Night	9% 2 18% 42% 26% 3	594
	21st to end	Night	10% 4 27% 34% 22% 2	765
September	1st to 10th	Night	12% 6% 25% 34% 19% 4	773
	11th to 20th	Night	12% 6% 21% 31% 24% 6%	760
	21st to end	Night	19% 8% 21% 33% 16% 4	760
October	1st to 10th	Night	19% 7% 30% 27% 16% 1	760
	11th to 20th	Night	23% 14% 29% 26% 8% 1	760
	21st to end	Night	27% 11% 32% 25% 4	836
November	1st to 10th	Night	24% 12% 29% 25% 8% 1	780
	11th to 20th	Night	22% 16% 33% 22% 6% 2	800
	21st to end	Night	26% 15% 30% 24% 5%	800
December	1st to 10th	Night	28% 17% 35% 18% 3	800
	11th to 20th	Night	23% 17% 38% 19% 3 1	800
	21st to end	Night	30% 14% 35% 18% 2	880

Page generated at 2025-09-01 01:51:08. Version ac07

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