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 2024-11-04.

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 6% 7% 33%	101494
	Night	15% 10% 3 7% 5% 5% 5% 4 5% 7% 35%	97527
Transparency	Day	46% 8% 14% 21% 10% 1	105095
	Night	46% 8% 14% 21% 11% 1	102014
Seeing	Day	20% 14% 30% 22% 10% 3	14441
	Night	19% 12% 31% 27% 10% 1	23771

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	17% 16% 4 8% 4 5% 3 3 4 5% 31%	2517
	11th to 20th	Day	11% 18% 7% 14% 8% 6% 4 3 4 5% 19%	2684
	21st to end	Day	21% 20% 6% 11% 6% 5% 4 3 4 5% 17%	3124
February	1st to 10th	Day	15% 17% 5% 11% 7% 5% 6% 4 5% 5% 21%	1710
	11th to 20th	Day	13% 14% 6% 12% 6% 4 4 4 5% 6% 26%	1663
	21st to end	Day	18% 14% 5% 10% 6% 5% 4 3 4 4 28%	1455
March	1st to 10th	Day	19% 12% 3 8% 6% 4 3 3 4 5% 32%	3432
	11th to 20th	Day	11% 15% 5% 8% 6% 5% 5% 4 5% 6% 29%	3480
	21st to end	Day	16% 9% 3 8% 5% 4 4 3 5% 5% 37%	3815
April	1st to 10th	Day	19% 13% 3 8% 5% 4 4 4 5% 6% 29%	2266
	11th to 20th	Day	20% 9% 3 6% 4 5% 4 4 5% 5% 35%	2301
	21st to end	Day	21% 8% 3 8% 5% 4 5% 3 5% 6% 34%	2470
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 6% 7% 32%	4158
	11th to 20th	Day	16% 10% 3 8% 6% 4 4 4 6% 8% 32%	4398
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 8% 46%	4837
June	1st to 10th	Day	16% 7% 3 6% 6% 5% 5% 5% 7% 8% 35%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	2660
	21st to end	Day	15% 6% 3 6% 5% 6% 5% 4 6% 8% 37%	2660
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 8% 11% 39%	4440
	11th to 20th	Day	11% 4 2 5% 4 5% 5% 5% 8% 10% 41%	4440
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 9% 41%	4594
August	1st to 10th	Day	16% 8% 2 6% 4 5% 4 4 6% 7% 37%	4104
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 7% 10% 36%	4044
	21st to end	Day	9% 5% 3 6% 5% 5% 5% 4 7% 9% 43%	4322
September	1st to 10th	Day	13% 5% 3 6% 5% 4 5% 4 6% 7% 42%	2280
	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% 3 8% 5% 6% 5% 3 6% 7% 32%	1621
	11th to 20th	Day	16% 13% 3 9% 7% 4 5% 5% 7% 6% 25%	1520
	21st to end	Day	19% 14% 4 10% 6% 4 5% 3 5% 7% 23%	1520
December	1st to 10th	Day	18% 15% 5% 11% 7% 5% 6% 4 7% 7% 16%	1520
	11th to 20th	Day	18% 14% 4 9% 5% 5% 4 4 6% 7% 25%	1906
	21st to end	Day	27% 17% 5% 12% 6% 5% 5% 3 3 4 13%	2772
January	1st to 10th	Night	18% 14% 5% 10% 6% 5% 5% 3 5% 5% 23%	4429
	11th to 20th	Night	15% 13% 5% 13% 8% 6% 6% 4 4 5% 20%	4272
	21st to end	Night	20% 13% 5% 9% 6% 6% 5% 4 5% 6% 22%	4532
February	1st to 10th	Night	13% 16% 4 10% 6% 6% 5% 4 5% 5% 24%	2470
	11th to 20th	Night	15% 13% 5% 10% 6% 5% 4 4 5% 6% 29%	2403
	21st to end	Night	14% 12% 4 10% 5% 6% 5% 4 6% 5% 30%	1994
March	1st to 10th	Night	17% 10% 4 8% 6% 6% 4 3 4 5% 33%	3408
	11th to 20th	Night	13% 10% 4 8% 6% 5% 4 4 5% 6% 35%	3479
	21st to end	Night	16% 7% 3 5% 4 4 5% 4 6% 6% 41%	3817
April	1st to 10th	Night	19% 11% 3 6% 5% 5% 4 4 4 7% 32%	1914
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 4 6% 38%	1844
	21st to end	Night	16% 10% 2 7% 5% 4 4 4 6% 7% 35%	1710
May	1st to 10th	Night	18% 7% 3 6% 5% 4 4 4 5% 6% 39%	2783
	11th to 20th	Night	16% 8% 2 7% 4 4 5% 4 5% 8% 38%	2495
	21st to end	Night	10% 5% 1 5% 4 4 4 3 5% 9% 50%	2738
June	1st to 10th	Night	12% 5% 2 7% 5% 5% 4 5% 6% 8% 40%	1520
	11th to 20th	Night	9% 5% 3 6% 5% 4 5% 4 6% 8% 43%	1520
	21st to end	Night	12% 6% 3 6% 5% 4 4 4 6% 7% 43%	1520
July	1st to 10th	Night	7% 6% 2 5% 4 4 4 4 7% 8% 50%	2520
	11th to 20th	Night	12% 4 2 4 3 3 4 3 6% 9% 50%	2520
	21st to end	Night	8% 4 2 4 3 4 4 4 6% 8% 52%	2955
August	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 5% 8% 48%	2825
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 50%	2785
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 7% 49%	3302
September	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 4 6% 7% 46%	1900
	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% 5% 4 5% 4 5% 7% 36%	2527
	11th to 20th	Night	16% 11% 3 8% 6% 5% 4 4 6% 7% 28%	2660
	21st to end	Night	20% 11% 4 7% 5% 5% 6% 4 6% 7% 24%	2660
December	1st to 10th	Night	19% 15% 5% 8% 6% 5% 4 5% 6% 6% 20%	2660
	11th to 20th	Night	15% 18% 4 10% 6% 6% 5% 4 6% 6% 21%	3288
	21st to end	Night	28% 16% 4 10% 5% 5% 6% 3 4 5% 15%	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	55% 9% 17% 11% 7% 1	2518
	11th to 20th	Day	67% 7% 11% 10% 4	2684
	21st to end	Day	69% 7% 11% 8% 4	3124
February	1st to 10th	Day	64% 7% 11% 14% 4 1	1710
	11th to 20th	Day	57% 7% 12% 15% 7% 1	1672
	21st to end	Day	59% 6% 12% 16% 7% 1	1455
March	1st to 10th	Day	52% 7% 14% 17% 10%	3435
	11th to 20th	Day	51% 8% 14% 17% 8% 1	3480
	21st to end	Day	45% 6% 13% 20% 15% 2	3815
April	1st to 10th	Day	54% 6% 12% 17% 10% 2	2266
	11th to 20th	Day	49% 5% 11% 21% 13% 1	2301
	21st to end	Day	50% 7% 11% 18% 12% 3	2470
May	1st to 10th	Day	46% 8% 13% 18% 13% 2	4170
	11th to 20th	Day	47% 10% 14% 19% 9% 1	4402
	21st to end	Day	35% 8% 15% 24% 15% 3	4837
June	1st to 10th	Day	43% 9% 13% 20% 14% 1	2660
	11th to 20th	Day	38% 8% 13% 23% 15% 1	2660
	21st to end	Day	40% 9% 15% 22% 13% 1	2660
July	1st to 10th	Day	36% 7% 17% 24% 14% 1	4370
	11th to 20th	Day	33% 7% 16% 34% 9%	4365
	21st to end	Day	34% 10% 16% 23% 16% 1	4447
August	1st to 10th	Day	43% 8% 14% 22% 12% 1	4091
	11th to 20th	Day	39% 8% 16% 28% 9%	4083
	21st to end	Day	34% 7% 15% 34% 9% 1	4322
September	1st to 10th	Day	38% 9% 14% 28% 10% 1	2280
	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% 8% 16% 18% 8% 1	1621
	11th to 20th	Day	55% 9% 14% 12% 8% 1	1520
	21st to end	Day	60% 7% 15% 15% 3	1520
December	1st to 10th	Day	64% 10% 14% 10% 3	1520
	11th to 20th	Day	57% 11% 15% 11% 6%	1906
	21st to end	Day	73% 7% 9% 8% 2	2772
January	1st to 10th	Night	59% 9% 16% 12% 4	4429
	11th to 20th	Night	62% 10% 13% 11% 3 1	4272
	21st to end	Night	61% 9% 14% 11% 4 1	4532
February	1st to 10th	Night	59% 8% 11% 17% 4 1	2470
	11th to 20th	Night	55% 7% 12% 17% 8%	2405
	21st to end	Night	52% 7% 14% 18% 7% 1	1994
March	1st to 10th	Night	51% 6% 12% 22% 8% 1	3418
	11th to 20th	Night	47% 8% 15% 20% 9% 2	3480
	21st to end	Night	39% 8% 13% 24% 16% 2	3817
April	1st to 10th	Night	51% 6% 11% 19% 13% 1	1914
	11th to 20th	Night	46% 7% 10% 19% 17% 2	1844
	21st to end	Night	46% 8% 12% 17% 15% 2	1710
May	1st to 10th	Night	44% 8% 12% 17% 16% 3	2790
	11th to 20th	Night	42% 9% 15% 21% 11% 2	2496
	21st to end	Night	30% 9% 13% 25% 21% 2	2739
June	1st to 10th	Night	39% 9% 12% 23% 16% 2	1520
	11th to 20th	Night	36% 7% 12% 22% 21% 2	1520
	21st to end	Night	38% 7% 14% 24% 16% 1	1520
July	1st to 10th	Night	31% 6% 15% 26% 20% 2	2480
	11th to 20th	Night	31% 6% 13% 34% 15% 1	2472
	21st to end	Night	28% 8% 15% 31% 17% 1	2861
August	1st to 10th	Night	35% 5% 14% 28% 15% 2	2814
	11th to 20th	Night	32% 6% 17% 31% 12% 1	2812
	21st to end	Night	32% 7% 13% 32% 14% 1	3302
September	1st to 10th	Night	34% 8% 14% 27% 14% 1	1900
	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% 22% 11% 1	2527
	11th to 20th	Night	52% 8% 15% 14% 9% 1	2660
	21st to end	Night	55% 10% 18% 13% 4	2660
December	1st to 10th	Night	62% 8% 15% 12% 4	2660
	11th to 20th	Night	61% 9% 13% 13% 4	3288
	21st to end	Night	70% 9% 11% 7% 3	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	23% 23% 28% 19% 7%	180
	11th to 20th	Day	22% 32% 28% 13% 4 1	227
	21st to end	Day	27% 23% 34% 14% 2	297
February	1st to 10th	Day	28% 27% 29% 13% 3	270
	11th to 20th	Day	23% 29% 39% 9% 1	270
	21st to end	Day	37% 19% 34% 9%	262
March	1st to 10th	Day	24% 19% 37% 17% 3	460
	11th to 20th	Day	25% 22% 31% 18% 3 1	460
	21st to end	Day	30% 16% 33% 16% 4 1	506
April	1st to 10th	Day	24% 18% 35% 16% 6%	460
	11th to 20th	Day	29% 19% 27% 15% 8% 3	452
	21st to end	Day	26% 15% 35% 15% 7% 2	460
May	1st to 10th	Day	24% 12% 30% 22% 8% 5%	463
	11th to 20th	Day	18% 13% 32% 24% 10% 3	487
	21st to end	Day	14% 10% 30% 28% 14% 4	529
June	1st to 10th	Day	18% 11% 33% 27% 10% 1	490
	11th to 20th	Day	15% 12% 29% 29% 13% 3	490
	21st to end	Day	21% 7% 30% 28% 10% 3	490
July	1st to 10th	Day	11% 7% 28% 31% 18% 5%	485
	11th to 20th	Day	14% 6% 28% 28% 16% 7%	484
	21st to end	Day	7% 7% 32% 36% 11% 6%	532
August	1st to 10th	Day	14% 5% 28% 29% 15% 8%	486
	11th to 20th	Day	13% 8% 22% 29% 22% 7%	479
	21st to end	Day	14% 8% 27% 27% 16% 8%	539
September	1st to 10th	Day	14% 9% 26% 28% 15% 9%	490
	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	24% 20% 28% 20% 7%	276
	11th to 20th	Day	26% 21% 34% 15% 3 1	180
	21st to end	Day	27% 22% 37% 12% 3	180
December	1st to 10th	Day	29% 17% 34% 15% 4 1	180
	11th to 20th	Day	29% 22% 29% 16% 4	180
	21st to end	Day	31% 24% 31% 12% 2	198
January	1st to 10th	Night	23% 20% 35% 16% 5%	760
	11th to 20th	Night	27% 16% 33% 19% 4 1	760
	21st to end	Night	19% 14% 38% 23% 5%	836
February	1st to 10th	Night	21% 18% 39% 20% 2	760
	11th to 20th	Night	23% 17% 37% 18% 4	743
	21st to end	Night	21% 17% 39% 19% 4	628
March	1st to 10th	Night	20% 16% 38% 23% 3	760
	11th to 20th	Night	21% 19% 34% 21% 4 1	760
	21st to end	Night	20% 13% 34% 26% 7%	836
April	1st to 10th	Night	24% 16% 33% 23% 4	760
	11th to 20th	Night	18% 15% 36% 22% 8% 1	720
	21st to end	Night	21% 8% 34% 27% 9%	570
May	1st to 10th	Night	20% 10% 26% 31% 12% 2	546
	11th to 20th	Night	16% 7% 28% 35% 12% 1	374
	21st to end	Night	11% 6% 26% 38% 17% 1	411
June	1st to 10th	Night	12% 8% 30% 37% 13% 1	380
	11th to 20th	Night	11% 8% 27% 39% 15% 1	380
	21st to end	Night	13% 5% 27% 40% 13% 1	380
July	1st to 10th	Night	7% 4 26% 42% 19% 1	370
	11th to 20th	Night	9% 5% 25% 38% 23%	368
	21st to end	Night	10% 2 23% 44% 20% 1	544
August	1st to 10th	Night	9% 4 18% 42% 24% 3	567
	11th to 20th	Night	8% 2 18% 42% 26% 3	564
	21st to end	Night	11% 4 27% 34% 22% 2	727
September	1st to 10th	Night	12% 6% 25% 35% 18% 4	760
	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% 26% 9% 1	755
	11th to 20th	Night	22% 16% 33% 22% 6% 2	760
	21st to end	Night	26% 14% 30% 25% 5%	760
December	1st to 10th	Night	28% 17% 35% 18% 3	760
	11th to 20th	Night	22% 16% 38% 20% 3 1	760
	21st to end	Night	31% 14% 35% 18% 2	836

Page generated at 2024-11-01 22:47:30. Version ac07

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