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 2024-04-03.

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% 8% 7% 3 5% 5% 5% 6% 28%	78735
	Night	16% 12% 5% 9% 7% 3 5% 4 5% 6% 27%	81385
Transparency	Day	56% 7% 11% 17% 8% 1	78440
	Night	57% 7% 11% 16% 8% 1	81214
Seeing	Day	27% 19% 29% 17% 7% 2	13861
	Night	25% 18% 28% 21% 7% 1	23199

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	11% 20% 11% 14% 7% 3 4 4 4 4 18%	2517
	11th to 20th	Day	14% 18% 10% 17% 10% 4 4 4 3 4 14%	2520
	21st to end	Day	16% 20% 10% 12% 10% 4 5% 3 3 4 12%	3030
February	1st to 10th	Day	16% 17% 6% 13% 10% 4 6% 4 4 4 15%	1710
	11th to 20th	Day	11% 16% 8% 16% 10% 4 5% 4 4 5% 17%	1729
	21st to end	Day	18% 16% 7% 11% 10% 3 4 4 3 4 19%	1570
March	1st to 10th	Day	16% 13% 6% 15% 8% 4 4 4 4 4 23%	1897
	11th to 20th	Day	15% 17% 8% 9% 6% 3 3 3 3 5% 27%	2089
	21st to end	Day	20% 12% 5% 8% 6% 3 4 3 3 6% 30%	2426
April	1st to 10th	Day	17% 14% 6% 9% 5% 2 4 3 4 5% 30%	2184
	11th to 20th	Day	18% 12% 3 6% 6% 3 4 4 4 5% 34%	2134
	21st to end	Day	21% 14% 4 6% 5% 3 4 3 4 6% 31%	2160
May	1st to 10th	Day	19% 12% 3 7% 5% 3 5% 4 5% 6% 31%	2493
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 4 6% 6% 36%	2478
	21st to end	Day	13% 7% 3 7% 5% 3 5% 5% 7% 9% 35%	2725
June	1st to 10th	Day	19% 6% 3 7% 7% 4 5% 5% 6% 8% 31%	2520
	11th to 20th	Day	18% 6% 3 6% 5% 3 5% 6% 7% 9% 32%	2520
	21st to end	Day	16% 7% 3 7% 6% 4 6% 6% 7% 9% 31%	2520
July	1st to 10th	Day	12% 7% 3 6% 6% 3 6% 6% 7% 7% 36%	2520
	11th to 20th	Day	12% 5% 3 6% 6% 4 6% 6% 7% 10% 34%	2520
	21st to end	Day	14% 6% 3 6% 7% 4 6% 6% 7% 8% 33%	2744
August	1st to 10th	Day	13% 5% 3 6% 7% 5% 6% 6% 7% 9% 33%	2388
	11th to 20th	Day	15% 8% 3 7% 6% 5% 6% 6% 7% 9% 28%	2210
	21st to end	Day	15% 7% 2 6% 6% 3 6% 5% 6% 9% 34%	2376
September	1st to 10th	Day	16% 6% 3 6% 4 4 6% 6% 6% 8% 36%	2160
	11th to 20th	Day	16% 7% 2 6% 5% 2 4 3 5% 7% 43%	2147
	21st to end	Day	16% 7% 2 6% 6% 3 6% 6% 7% 6% 35%	1948
October	1st to 10th	Day	20% 10% 3 6% 6% 3 5% 4 6% 7% 29%	1800
	11th to 20th	Day	19% 10% 4 6% 5% 3 5% 5% 5% 7% 31%	1800
	21st to end	Day	25% 17% 3 6% 5% 3 4 5% 5% 6% 22%	1872
November	1st to 10th	Day	14% 13% 3 7% 7% 3 4 5% 4 6% 33%	1671
	11th to 20th	Day	15% 18% 5% 10% 6% 3 4 4 5% 5% 26%	1639
	21st to end	Day	18% 17% 6% 9% 8% 3 4 5% 4 6% 20%	1520
December	1st to 10th	Day	18% 20% 9% 13% 7% 3 4 4 4 5% 14%	1520
	11th to 20th	Day	20% 18% 9% 9% 8% 2 3 4 3 4 20%	1906
	21st to end	Day	22% 19% 10% 10% 6% 3 5% 4 5% 4 13%	2772
January	1st to 10th	Night	15% 16% 10% 16% 9% 4 5% 4 4 4 13%	4429
	11th to 20th	Night	11% 17% 9% 15% 11% 4 5% 4 4 4 17%	4440
	21st to end	Night	19% 16% 8% 15% 11% 4 5% 3 3 3 12%	4624
February	1st to 10th	Night	14% 15% 6% 15% 10% 4 4 4 4 5% 17%	2470
	11th to 20th	Night	12% 12% 8% 14% 12% 5% 6% 5% 5% 5% 16%	2335
	21st to end	Night	16% 13% 6% 13% 10% 4 5% 4 4 6% 18%	1884
March	1st to 10th	Night	15% 12% 7% 12% 9% 4 4 5% 4 5% 24%	2268
	11th to 20th	Night	14% 15% 7% 10% 7% 3 4 4 5% 5% 26%	2089
	21st to end	Night	17% 13% 5% 8% 6% 3 5% 4 3 6% 29%	2171
April	1st to 10th	Night	17% 13% 5% 8% 6% 4 5% 4 4 6% 27%	1821
	11th to 20th	Night	18% 11% 3 6% 5% 4 5% 4 4 6% 33%	1775
	21st to end	Night	20% 11% 4 7% 4 3 4 5% 5% 5% 33%	1800
May	1st to 10th	Night	20% 12% 3 6% 4 3 4 5% 4 6% 33%	1481
	11th to 20th	Night	13% 7% 2 5% 6% 4 5% 5% 7% 9% 37%	1415
	21st to end	Night	12% 7% 3 6% 6% 4 5% 7% 5% 7% 40%	1550
June	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 6% 6% 8% 36%	1440
	11th to 20th	Night	16% 7% 2 6% 4 2 5% 5% 6% 8% 39%	1440
	21st to end	Night	13% 5% 2 6% 5% 3 4 6% 6% 7% 43%	1440
July	1st to 10th	Night	9% 5% 2 5% 6% 4 5% 6% 6% 9% 43%	1440
	11th to 20th	Night	11% 4 2 5% 5% 4 5% 6% 5% 8% 44%	1440
	21st to end	Night	12% 5% 2 6% 6% 2 4 6% 5% 8% 44%	1574
August	1st to 10th	Night	10% 5% 2 5% 5% 3 5% 5% 6% 10% 44%	1547
	11th to 20th	Night	11% 7% 3 6% 6% 5% 4 5% 6% 7% 38%	1671
	21st to end	Night	14% 6% 3 5% 5% 2 4 5% 6% 8% 42%	1980
September	1st to 10th	Night	14% 6% 3 5% 5% 3 4 5% 4 7% 44%	1800
	11th to 20th	Night	17% 6% 2 4 4 2 4 4 4 7% 48%	1812
	21st to end	Night	16% 6% 2 5% 5% 3 5% 5% 5% 7% 40%	2010
October	1st to 10th	Night	19% 9% 3 7% 6% 3 5% 4 5% 7% 34%	2160
	11th to 20th	Night	20% 11% 3 6% 6% 3 5% 5% 5% 6% 31%	2160
	21st to end	Night	26% 15% 3 7% 6% 3 4 4 3 6% 24%	2484
November	1st to 10th	Night	12% 14% 5% 9% 6% 3 5% 4 4 5% 33%	2407
	11th to 20th	Night	18% 13% 5% 8% 6% 3 5% 5% 5% 6% 26%	2536
	21st to end	Night	15% 15% 7% 9% 7% 4 5% 5% 4 7% 22%	2660
December	1st to 10th	Night	18% 17% 9% 12% 7% 4 5% 4 4 4 16%	2660
	11th to 20th	Night	19% 14% 7% 13% 8% 4 5% 4 4 5% 18%	3288
	21st to end	Night	24% 16% 11% 10% 6% 3 4 3 4 5% 14%	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	67% 5% 11% 12% 4	2518
	11th to 20th	Day	74% 6% 7% 9% 3 1	2520
	21st to end	Day	75% 5% 9% 7% 3 1	3030
February	1st to 10th	Day	72% 6% 8% 11% 3	1710
	11th to 20th	Day	69% 6% 7% 13% 4	1738
	21st to end	Day	67% 5% 9% 12% 6%	1570
March	1st to 10th	Day	64% 4 11% 14% 5% 1	1900
	11th to 20th	Day	60% 5% 10% 14% 10% 2	2089
	21st to end	Day	57% 5% 8% 15% 13% 2	2426
April	1st to 10th	Day	58% 5% 7% 20% 8% 3	2184
	11th to 20th	Day	51% 6% 11% 20% 12% 1	2134
	21st to end	Day	55% 6% 9% 18% 11% 2	2160
May	1st to 10th	Day	55% 6% 8% 18% 11% 2	2493
	11th to 20th	Day	48% 7% 10% 22% 11% 1	2482
	21st to end	Day	44% 10% 15% 20% 10% 1	2725
June	1st to 10th	Day	50% 9% 11% 18% 11% 1	2520
	11th to 20th	Day	47% 8% 12% 23% 9% 1	2520
	21st to end	Day	48% 8% 15% 19% 9% 1	2520
July	1st to 10th	Day	43% 10% 12% 23% 11% 1	2450
	11th to 20th	Day	42% 10% 16% 24% 7%	2445
	21st to end	Day	46% 9% 13% 21% 10%	2591
August	1st to 10th	Day	44% 9% 16% 20% 10% 1	2374
	11th to 20th	Day	49% 10% 15% 20% 7%	2210
	21st to end	Day	46% 9% 12% 22% 9% 1	2376
September	1st to 10th	Day	44% 9% 13% 24% 10% 1	2160
	11th to 20th	Day	41% 7% 11% 23% 17% 1	2147
	21st to end	Day	47% 8% 11% 22% 11% 1	1948
October	1st to 10th	Day	53% 8% 11% 18% 10% 1	1800
	11th to 20th	Day	51% 8% 13% 21% 6% 1	1800
	21st to end	Day	62% 8% 12% 11% 7% 1	1872
November	1st to 10th	Day	50% 8% 12% 18% 11% 1	1671
	11th to 20th	Day	62% 5% 10% 15% 7% 2	1639
	21st to end	Day	64% 7% 14% 11% 4	1520
December	1st to 10th	Day	72% 5% 11% 8% 3	1520
	11th to 20th	Day	69% 5% 13% 9% 3	1906
	21st to end	Day	75% 6% 7% 8% 4	2772
January	1st to 10th	Night	71% 7% 12% 7% 3	4429
	11th to 20th	Night	70% 6% 7% 11% 5%	4440
	21st to end	Night	77% 4 7% 8% 3 1	4624
February	1st to 10th	Night	68% 7% 9% 11% 5%	2470
	11th to 20th	Night	69% 6% 9% 11% 5%	2337
	21st to end	Night	65% 6% 11% 11% 6% 1	1884
March	1st to 10th	Night	62% 5% 11% 15% 6% 1	2278
	11th to 20th	Night	60% 6% 11% 16% 7% 1	2090
	21st to end	Night	56% 6% 8% 16% 12% 2	2171
April	1st to 10th	Night	58% 7% 10% 16% 7% 1	1821
	11th to 20th	Night	51% 6% 10% 19% 13% 1	1775
	21st to end	Night	52% 7% 9% 18% 12% 3	1800
May	1st to 10th	Night	53% 7% 9% 15% 13% 2	1481
	11th to 20th	Night	42% 8% 13% 22% 13% 2	1416
	21st to end	Night	41% 9% 16% 22% 10% 2	1551
June	1st to 10th	Night	44% 7% 13% 23% 13% 1	1440
	11th to 20th	Night	42% 8% 15% 22% 12% 1	1440
	21st to end	Night	38% 9% 15% 27% 11% 1	1440
July	1st to 10th	Night	35% 10% 15% 25% 15% 2	1400
	11th to 20th	Night	35% 10% 16% 27% 11% 1	1392
	21st to end	Night	37% 9% 15% 23% 14% 1	1486
August	1st to 10th	Night	35% 10% 16% 28% 10% 1	1537
	11th to 20th	Night	41% 10% 17% 22% 9%	1671
	21st to end	Night	38% 8% 16% 25% 13% 1	1980
September	1st to 10th	Night	42% 6% 15% 25% 12% 1	1800
	11th to 20th	Night	37% 6% 12% 27% 17% 1	1812
	21st to end	Night	43% 8% 12% 24% 13% 1	2010
October	1st to 10th	Night	50% 8% 13% 18% 11% 1	2160
	11th to 20th	Night	51% 8% 14% 18% 8% 1	2160
	21st to end	Night	62% 6% 11% 13% 7% 1	2484
November	1st to 10th	Night	51% 7% 12% 16% 12% 2	2407
	11th to 20th	Night	58% 6% 12% 16% 7% 2	2536
	21st to end	Night	60% 7% 15% 14% 4	2660
December	1st to 10th	Night	70% 6% 12% 10% 2	2660
	11th to 20th	Night	67% 7% 10% 11% 4	3288
	21st to end	Night	72% 6% 8% 10% 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	32% 26% 27% 12% 3	180
	11th to 20th	Day	43% 26% 24% 6% 1 1	180
	21st to end	Day	29% 29% 31% 9% 1	272
February	1st to 10th	Day	41% 30% 24% 5%	270
	11th to 20th	Day	37% 34% 24% 5%	336
	21st to end	Day	39% 23% 29% 7% 2	382
March	1st to 10th	Day	32% 25% 28% 12% 2	460
	11th to 20th	Day	34% 26% 27% 11% 2	460
	21st to end	Day	38% 24% 22% 11% 4 1	506
April	1st to 10th	Day	35% 23% 28% 10% 4	428
	11th to 20th	Day	32% 24% 24% 11% 8% 2	412
	21st to end	Day	37% 18% 26% 12% 5% 1	420
May	1st to 10th	Day	30% 21% 28% 12% 7% 2	423
	11th to 20th	Day	25% 14% 37% 15% 8% 2	447
	21st to end	Day	19% 14% 32% 22% 10% 3	485
June	1st to 10th	Day	22% 16% 29% 22% 9% 1	450
	11th to 20th	Day	20% 14% 32% 22% 10% 1	450
	21st to end	Day	20% 17% 36% 19% 7% 1	450
July	1st to 10th	Day	14% 12% 30% 27% 13% 3	445
	11th to 20th	Day	15% 9% 29% 32% 12% 3	444
	21st to end	Day	15% 8% 40% 25% 9% 3	488
August	1st to 10th	Day	13% 12% 29% 26% 15% 5%	446
	11th to 20th	Day	18% 7% 29% 28% 12% 5%	439
	21st to end	Day	15% 15% 29% 25% 11% 6%	495
September	1st to 10th	Day	18% 13% 27% 22% 15% 6%	450
	11th to 20th	Day	15% 14% 26% 21% 16% 8%	450
	21st to end	Day	19% 17% 27% 21% 12% 4	450
October	1st to 10th	Day	27% 15% 28% 13% 12% 5%	450
	11th to 20th	Day	26% 22% 27% 17% 7% 1	450
	21st to end	Day	38% 20% 26% 13% 3	441
November	1st to 10th	Day	29% 21% 25% 17% 8%	365
	11th to 20th	Day	34% 28% 22% 12% 3	299
	21st to end	Day	36% 23% 36% 6%	180
December	1st to 10th	Day	38% 22% 29% 9% 1 1	180
	11th to 20th	Day	39% 24% 30% 7%	180
	21st to end	Day	36% 27% 31% 5% 1	198
January	1st to 10th	Night	28% 28% 32% 11% 1	760
	11th to 20th	Night	29% 26% 28% 14% 2 1	760
	21st to end	Night	23% 29% 31% 16% 2	836
February	1st to 10th	Night	28% 25% 32% 14% 1	760
	11th to 20th	Night	26% 30% 30% 13% 1	743
	21st to end	Night	31% 26% 28% 13% 2	628
March	1st to 10th	Night	27% 25% 31% 16% 2	760
	11th to 20th	Night	29% 24% 29% 17% 2	760
	21st to end	Night	30% 22% 23% 19% 5%	836
April	1st to 10th	Night	27% 20% 33% 17% 3	729
	11th to 20th	Night	28% 19% 29% 16% 8%	711
	21st to end	Night	29% 17% 27% 22% 6%	720
May	1st to 10th	Night	28% 12% 27% 26% 7%	401
	11th to 20th	Night	19% 11% 37% 27% 6% 1	354
	21st to end	Night	16% 7% 34% 33% 9% 1	389
June	1st to 10th	Night	18% 7% 31% 32% 12% 1	360
	11th to 20th	Night	19% 9% 26% 34% 11%	360
	21st to end	Night	14% 7% 29% 39% 9% 1	360
July	1st to 10th	Night	10% 6% 26% 42% 15% 1	350
	11th to 20th	Night	11% 4 29% 40% 15% 2	348
	21st to end	Night	15% 4 24% 43% 12% 2	394
August	1st to 10th	Night	13% 5% 26% 37% 19% 1	473
	11th to 20th	Night	13% 5% 23% 35% 22% 3	607
	21st to end	Night	16% 7% 24% 34% 18% 1	792
September	1st to 10th	Night	16% 9% 28% 30% 16% 1	720
	11th to 20th	Night	19% 10% 22% 28% 17% 4	720
	21st to end	Night	19% 13% 25% 27% 16% 1	720
October	1st to 10th	Night	25% 14% 25% 21% 13% 2	720
	11th to 20th	Night	26% 16% 30% 21% 6%	720
	21st to end	Night	36% 17% 28% 17% 2	792
November	1st to 10th	Night	23% 20% 32% 19% 6%	740
	11th to 20th	Night	28% 23% 28% 13% 6% 1	760
	21st to end	Night	28% 24% 31% 14% 3	760
December	1st to 10th	Night	34% 26% 27% 12% 2	760
	11th to 20th	Night	30% 25% 29% 13% 2	760
	21st to end	Night	33% 29% 26% 11% 1	836

Page generated at 2024-03-31 06:08:53. Version ac07

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