Foggy Bottom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foggy Bottom Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	16% 12% 5% 10% 6% 5% 5% 4 5% 6% 27%	120389
	Night	16% 13% 5% 12% 6% 5% 4 3 4 6% 26%	118731
Transparency	Day	55% 7% 11% 17% 8% 1	123938
	Night	57% 6% 11% 16% 8% 1	123343
Seeing	Day	27% 19% 28% 17% 7% 2	15576
	Night	25% 18% 28% 21% 7% 1	24734

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	10% 18% 9% 23% 9% 3 3 2 3 4 15%	3597
	11th to 20th	Day	13% 18% 11% 21% 8% 5% 4 3 3 3 12%	3570
	21st to end	Day	14% 17% 8% 15% 8% 6% 5% 3 4 4 16%	4344
February	1st to 10th	Day	15% 18% 5% 16% 9% 6% 5% 3 4 4 15%	1800
	11th to 20th	Day	12% 17% 7% 18% 10% 5% 5% 3 4 4 17%	1823
	21st to end	Day	18% 18% 5% 13% 9% 4 4 4 3 4 19%	1650
March	1st to 10th	Day	19% 16% 7% 14% 7% 5% 3 3 3 3 20%	4476
	11th to 20th	Day	10% 17% 7% 9% 5% 3 3 3 4 6% 32%	4979
	21st to end	Day	17% 13% 6% 12% 5% 4 4 3 4 5% 28%	5780
April	1st to 10th	Day	17% 16% 4 11% 5% 3 4 3 4 5% 29%	2400
	11th to 20th	Day	18% 14% 3 7% 5% 4 4 4 4 5% 33%	2374
	21st to end	Day	19% 15% 3 7% 4 4 4 3 4 6% 31%	2400
May	1st to 10th	Day	30% 12% 3 7% 4 4 4 3 4 5% 24%	6291
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 4 6% 7% 30%	6318
	21st to end	Day	20% 10% 3 7% 4 4 5% 4 7% 7% 28%	6949
June	1st to 10th	Day	18% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	2800
	11th to 20th	Day	18% 7% 3 6% 5% 4 5% 4 7% 9% 31%	2800
	21st to end	Day	16% 7% 3 8% 5% 5% 6% 5% 7% 8% 30%	2800
July	1st to 10th	Day	11% 7% 2 6% 5% 5% 6% 5% 9% 10% 34%	4580
	11th to 20th	Day	9% 6% 2 7% 6% 6% 6% 6% 9% 11% 30%	4580
	21st to end	Day	12% 7% 2 8% 6% 5% 6% 5% 7% 8% 34%	4814
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 6% 5% 6% 8% 31%	4207
	11th to 20th	Day	15% 8% 3 8% 5% 5% 6% 4 8% 9% 29%	3867
	21st to end	Day	16% 8% 2 7% 4 4 5% 3 6% 9% 36%	4180
September	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 5% 6% 8% 37%	2280
	11th to 20th	Day	15% 7% 2 6% 4 3 3 3 5% 7% 43%	2266
	21st to end	Day	17% 7% 2 7% 5% 4 6% 5% 6% 6% 34%	2056
October	1st to 10th	Day	19% 10% 3 7% 5% 4 5% 4 6% 7% 29%	1900
	11th to 20th	Day	18% 11% 4 7% 4 4 5% 4 5% 7% 32%	1900
	21st to end	Day	22% 17% 2 7% 4 4 5% 3 5% 7% 23%	1976
November	1st to 10th	Day	14% 13% 3 8% 6% 4 5% 4 4 5% 33%	1761
	11th to 20th	Day	15% 19% 4 10% 5% 3 3 3 5% 5% 27%	1725
	21st to end	Day	18% 18% 5% 10% 6% 4 4 3 4 6% 20%	1600
December	1st to 10th	Day	16% 21% 10% 19% 7% 4 4 2 4 4 9%	2600
	11th to 20th	Day	18% 18% 9% 13% 6% 3 3 3 3 4 19%	2986
	21st to end	Day	17% 18% 9% 11% 6% 4 4 3 5% 5% 17%	3960
January	1st to 10th	Night	13% 15% 9% 29% 8% 5% 4 2 3 3 9%	6349
	11th to 20th	Night	9% 17% 8% 24% 9% 6% 5% 3 4 4 13%	6315
	21st to end	Night	14% 15% 6% 16% 9% 6% 5% 3 4 5% 17%	6610
February	1st to 10th	Night	14% 17% 5% 16% 8% 5% 3 4 4 5% 17%	2600
	11th to 20th	Night	13% 13% 7% 16% 11% 6% 5% 4 4 5% 16%	2461
	21st to end	Night	15% 15% 5% 15% 9% 5% 5% 3 4 6% 17%	1980
March	1st to 10th	Night	17% 11% 6% 13% 10% 5% 4 4 4 5% 21%	5349
	11th to 20th	Night	15% 15% 6% 9% 4 3 3 2 4 5% 31%	4979
	21st to end	Night	20% 12% 5% 13% 5% 4 4 3 3 5% 26%	5151
April	1st to 10th	Night	18% 15% 4 9% 5% 5% 4 3 5% 6% 26%	2000
	11th to 20th	Night	18% 13% 3 6% 5% 5% 5% 3 4 6% 33%	1975
	21st to end	Night	19% 11% 4 7% 3 3 4 4 5% 5% 35%	2000
May	1st to 10th	Night	31% 13% 3 7% 4 4 3 3 4 5% 23%	3691
	11th to 20th	Night	14% 11% 3 7% 5% 5% 5% 4 6% 7% 32%	3575
	21st to end	Night	20% 11% 3 7% 4 4 5% 4 5% 6% 31%	3926
June	1st to 10th	Night	17% 8% 2 7% 5% 4 5% 4 6% 7% 36%	1600
	11th to 20th	Night	17% 8% 2 7% 4 4 5% 4 6% 9% 37%	1600
	21st to end	Night	13% 6% 2 7% 5% 5% 4 5% 6% 7% 42%	1600
July	1st to 10th	Night	10% 6% 2 7% 6% 5% 5% 5% 6% 8% 39%	2600
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 6% 8% 42%	2600
	21st to end	Night	13% 7% 3 7% 4 3 4 4 5% 7% 42%	2739
August	1st to 10th	Night	20% 8% 2 5% 3 4 4 3 5% 7% 38%	2719
	11th to 20th	Night	12% 9% 3 8% 5% 5% 4 3 5% 6% 40%	2928
	21st to end	Night	14% 7% 2 5% 4 4 4 4 6% 7% 43%	3476
September	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 7% 44%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 6% 49%	1913
	21st to end	Night	17% 7% 2 7% 5% 4 5% 4 5% 7% 38%	2122
October	1st to 10th	Night	19% 10% 3 8% 5% 4 4 3 5% 7% 33%	2280
	11th to 20th	Night	19% 11% 3 6% 4 4 4 4 5% 6% 32%	2280
	21st to end	Night	24% 15% 3 8% 5% 4 3 3 4 6% 26%	2622
November	1st to 10th	Night	12% 15% 4 9% 5% 4 4 3 5% 5% 34%	2537
	11th to 20th	Night	17% 14% 5% 9% 5% 4 5% 4 5% 6% 27%	2670
	21st to end	Night	15% 17% 6% 10% 6% 5% 5% 3 5% 6% 21%	2800
December	1st to 10th	Night	15% 18% 9% 15% 7% 6% 5% 4 5% 4 12%	4580
	11th to 20th	Night	22% 16% 7% 12% 5% 4 3 2 3 4 21%	5208
	21st to end	Night	20% 19% 8% 11% 6% 5% 3 3 4 5% 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	73% 4 8% 11% 3	3598
	11th to 20th	Day	77% 5% 6% 8% 4 1	3600
	21st to end	Day	68% 6% 9% 12% 5% 1	4344
February	1st to 10th	Day	72% 6% 7% 11% 3	1800
	11th to 20th	Day	71% 6% 7% 12% 4	1832
	21st to end	Day	67% 5% 9% 12% 6%	1650
March	1st to 10th	Day	68% 4 9% 11% 6% 1	4479
	11th to 20th	Day	53% 5% 18% 13% 8% 2	4979
	21st to end	Day	59% 6% 7% 13% 14% 1	5780
April	1st to 10th	Day	59% 4 7% 19% 8% 2	2400
	11th to 20th	Day	53% 6% 10% 19% 12% 1	2374
	21st to end	Day	54% 6% 9% 18% 11% 2	2400
May	1st to 10th	Day	62% 5% 9% 14% 9% 1	6302
	11th to 20th	Day	52% 8% 10% 20% 9% 1	6322
	21st to end	Day	51% 8% 12% 19% 9% 1	6949
June	1st to 10th	Day	50% 9% 11% 18% 12% 1	2800
	11th to 20th	Day	47% 9% 13% 22% 8% 1	2800
	21st to end	Day	48% 8% 15% 20% 9% 1	2800
July	1st to 10th	Day	40% 10% 16% 24% 10%	4510
	11th to 20th	Day	40% 10% 16% 25% 8%	4505
	21st to end	Day	45% 9% 14% 22% 10%	4661
August	1st to 10th	Day	49% 8% 14% 18% 10%	4193
	11th to 20th	Day	47% 9% 18% 20% 6%	3906
	21st to end	Day	44% 7% 13% 29% 7% 1	4180
September	1st to 10th	Day	43% 9% 12% 25% 10% 1	2280
	11th to 20th	Day	40% 7% 12% 24% 17% 1	2266
	21st to end	Day	48% 8% 11% 21% 11% 1	2056
October	1st to 10th	Day	45% 8% 10% 26% 10% 1	3160
	11th to 20th	Day	46% 6% 13% 20% 14%	3140
	21st to end	Day	45% 8% 15% 24% 8% 1	3240
November	1st to 10th	Day	50% 7% 12% 18% 11% 1	1761
	11th to 20th	Day	60% 5% 10% 15% 8% 2	1725
	21st to end	Day	64% 7% 14% 11% 4	1600
December	1st to 10th	Day	78% 6% 9% 6% 2	2600
	11th to 20th	Day	70% 5% 10% 10% 6%	2986
	21st to end	Day	69% 5% 9% 11% 6%	3960
January	1st to 10th	Night	78% 5% 8% 6% 2	6349
	11th to 20th	Night	73% 6% 8% 8% 5% 1	6360
	21st to end	Night	67% 5% 9% 9% 9% 1	6610
February	1st to 10th	Night	68% 6% 9% 12% 5%	2600
	11th to 20th	Night	69% 6% 9% 11% 5%	2463
	21st to end	Night	66% 6% 11% 11% 6% 1	1980
March	1st to 10th	Night	64% 6% 12% 12% 7%	5359
	11th to 20th	Night	55% 5% 18% 14% 5% 3	4980
	21st to end	Night	59% 6% 6% 18% 11% 1	5151
April	1st to 10th	Night	59% 6% 10% 17% 7% 2	2000
	11th to 20th	Night	53% 6% 10% 18% 13% 1	1975
	21st to end	Night	50% 7% 10% 18% 13% 3	2000
May	1st to 10th	Night	63% 6% 8% 12% 9% 2	3697
	11th to 20th	Night	48% 9% 14% 19% 9% 1	3576
	21st to end	Night	53% 7% 11% 17% 11% 1	3927
June	1st to 10th	Night	45% 7% 12% 22% 13% 1	1600
	11th to 20th	Night	43% 8% 15% 22% 12% 1	1600
	21st to end	Night	39% 9% 15% 25% 11% 1	1600
July	1st to 10th	Night	41% 8% 15% 22% 14% 1	2560
	11th to 20th	Night	38% 9% 15% 26% 12% 1	2552
	21st to end	Night	40% 7% 11% 26% 14% 1	2651
August	1st to 10th	Night	46% 7% 15% 23% 8%	2709
	11th to 20th	Night	44% 7% 15% 26% 8%	2955
	21st to end	Night	38% 7% 14% 31% 10%	3476
September	1st to 10th	Night	41% 5% 15% 25% 13% 1	1900
	11th to 20th	Night	36% 6% 12% 28% 17% 1	1913
	21st to end	Night	45% 7% 11% 23% 12% 1	2122
October	1st to 10th	Night	51% 8% 11% 21% 9%	3800
	11th to 20th	Night	46% 7% 14% 18% 14% 3	3779
	21st to end	Night	47% 6% 14% 25% 7%	4308
November	1st to 10th	Night	50% 7% 12% 17% 12% 2	2537
	11th to 20th	Night	57% 5% 11% 16% 8% 2	2670
	21st to end	Night	61% 8% 15% 13% 4	2800
December	1st to 10th	Night	70% 8% 11% 9% 1	4580
	11th to 20th	Night	66% 6% 8% 11% 8%	5208
	21st to end	Night	72% 5% 6% 9% 7% 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	33% 29% 25% 11% 3	200
	11th to 20th	Day	43% 26% 26% 5% 1 1	200
	21st to end	Day	32% 30% 29% 8% 1	303
February	1st to 10th	Day	42% 30% 23% 5%	300
	11th to 20th	Day	39% 34% 22% 5%	370
	21st to end	Day	38% 24% 29% 7% 1	414
March	1st to 10th	Day	33% 26% 28% 11% 2	500
	11th to 20th	Day	35% 25% 27% 11% 2	500
	21st to end	Day	38% 24% 23% 11% 4 1	550
April	1st to 10th	Day	37% 24% 26% 10% 4	500
	11th to 20th	Day	34% 26% 23% 9% 7% 2	492
	21st to end	Day	36% 19% 26% 12% 5% 1	500
May	1st to 10th	Day	33% 21% 26% 12% 7% 2	503
	11th to 20th	Day	25% 14% 35% 15% 8% 2	527
	21st to end	Day	21% 15% 30% 23% 9% 2	573
June	1st to 10th	Day	22% 16% 29% 23% 9% 1	530
	11th to 20th	Day	20% 14% 32% 23% 9% 1	530
	21st to end	Day	21% 17% 35% 20% 7% 1	530
July	1st to 10th	Day	13% 13% 33% 27% 12% 3	525
	11th to 20th	Day	14% 10% 31% 31% 12% 3	524
	21st to end	Day	15% 9% 39% 24% 10% 3	532
August	1st to 10th	Day	14% 13% 28% 25% 14% 5%	486
	11th to 20th	Day	18% 8% 29% 28% 11% 5%	479
	21st to end	Day	15% 15% 29% 24% 12% 5%	539
September	1st to 10th	Day	17% 13% 28% 22% 14% 6%	490
	11th to 20th	Day	14% 13% 24% 21% 17% 10%	490
	21st to end	Day	20% 17% 26% 21% 11% 4	490
October	1st to 10th	Day	26% 15% 28% 14% 12% 5%	490
	11th to 20th	Day	27% 21% 27% 17% 7% 2	490
	21st to end	Day	36% 20% 27% 14% 3	479
November	1st to 10th	Day	29% 22% 25% 16% 8%	395
	11th to 20th	Day	33% 28% 22% 13% 4	325
	21st to end	Day	37% 23% 35% 6%	200
December	1st to 10th	Day	39% 22% 29% 9% 1 1	200
	11th to 20th	Day	39% 25% 30% 6% 1	200
	21st to end	Day	35% 26% 31% 7% 1	220
January	1st to 10th	Night	29% 28% 31% 10% 1	800
	11th to 20th	Night	29% 27% 29% 13% 2 1	800
	21st to end	Night	23% 29% 31% 16% 2	880
February	1st to 10th	Night	29% 25% 31% 14% 1	800
	11th to 20th	Night	27% 29% 30% 13% 1	783
	21st to end	Night	31% 26% 29% 13% 2	660
March	1st to 10th	Night	28% 25% 30% 16% 2	800
	11th to 20th	Night	29% 24% 29% 16% 2	800
	21st to end	Night	30% 22% 24% 18% 4	880
April	1st to 10th	Night	29% 20% 32% 18% 2	800
	11th to 20th	Night	29% 20% 28% 16% 7%	791
	21st to end	Night	29% 16% 27% 22% 5%	800
May	1st to 10th	Night	29% 13% 26% 25% 6%	445
	11th to 20th	Night	20% 11% 36% 27% 6% 1	394
	21st to end	Night	17% 9% 33% 33% 8% 1	433
June	1st to 10th	Night	19% 8% 31% 31% 11% 1	400
	11th to 20th	Night	20% 10% 26% 33% 11%	400
	21st to end	Night	15% 8% 29% 38% 11% 1	400
July	1st to 10th	Night	10% 6% 26% 42% 14% 1	390
	11th to 20th	Night	12% 5% 28% 40% 14% 2	388
	21st to end	Night	15% 4 24% 42% 13% 1	416
August	1st to 10th	Night	15% 5% 24% 36% 18% 1	500
	11th to 20th	Night	14% 5% 23% 33% 22% 3	642
	21st to end	Night	17% 7% 23% 33% 19% 1	836
September	1st to 10th	Night	16% 9% 28% 29% 16% 1	760
	11th to 20th	Night	18% 9% 20% 28% 20% 4	760
	21st to end	Night	20% 13% 24% 27% 15% 1	760
October	1st to 10th	Night	25% 14% 25% 21% 14% 2	760
	11th to 20th	Night	26% 16% 30% 22% 6%	760
	21st to end	Night	34% 17% 29% 17% 3	836
November	1st to 10th	Night	24% 20% 31% 19% 6%	780
	11th to 20th	Night	28% 23% 29% 14% 6% 1	800
	21st to end	Night	29% 24% 30% 14% 3	800
December	1st to 10th	Night	33% 26% 27% 12% 2	800
	11th to 20th	Night	32% 25% 28% 14% 2	800
	21st to end	Night	32% 29% 26% 12% 1	880

Page generated at 2025-07-18 01:09:58. Version ac07

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