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 2024-11-04.

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	16% 12% 4 9% 5% 5% 5% 4 6% 7% 28%	101498
	Night	16% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	97520
Transparency	Day	53% 7% 12% 19% 8% 1	105020
	Night	55% 7% 12% 17% 8% 1	102087
Seeing	Day	26% 19% 28% 17% 7% 2	14712
	Night	25% 18% 28% 21% 7% 1	23861

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% 21% 10% 15% 6% 4 4 3 4 4 18%	2517
	11th to 20th	Day	14% 18% 10% 19% 8% 5% 4 3 3 4 14%	2520
	21st to end	Day	16% 21% 9% 14% 9% 6% 4 3 3 4 12%	3030
February	1st to 10th	Day	15% 19% 5% 15% 9% 6% 5% 3 4 4 15%	1710
	11th to 20th	Day	11% 17% 7% 17% 9% 5% 5% 3 4 4 17%	1729
	21st to end	Day	18% 18% 5% 13% 8% 4 4 4 3 4 19%	1570
March	1st to 10th	Day	22% 16% 5% 13% 5% 4 3 3 3 4 23%	3116
	11th to 20th	Day	10% 21% 8% 11% 5% 4 3 3 4 5% 27%	3479
	21st to end	Day	18% 12% 6% 11% 4 3 3 3 4 6% 29%	4032
April	1st to 10th	Day	17% 16% 4 10% 5% 3 3 3 4 5% 29%	2280
	11th to 20th	Day	18% 13% 3 7% 5% 4 4 4 4 5% 33%	2254
	21st to end	Day	20% 15% 3 7% 4 4 4 3 4 6% 31%	2280
May	1st to 10th	Day	22% 11% 3 7% 4 4 4 3 5% 6% 31%	4385
	11th to 20th	Day	17% 9% 3 8% 5% 6% 5% 4 6% 7% 29%	4398
	21st to end	Day	12% 7% 2 6% 4 5% 5% 4 7% 9% 38%	4837
June	1st to 10th	Day	18% 7% 3 8% 6% 5% 5% 4 6% 8% 30%	2660
	11th to 20th	Day	17% 7% 3 6% 5% 5% 5% 4 7% 9% 32%	2660
	21st to end	Day	16% 7% 3 8% 5% 5% 6% 5% 7% 8% 30%	2660
July	1st to 10th	Day	11% 7% 2 6% 5% 5% 6% 5% 9% 10% 34%	4440
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 6% 9% 11% 31%	4440
	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% 14% 3 8% 6% 4 5% 4 4 5% 33%	1698
	11th to 20th	Day	15% 19% 4 11% 5% 3 3 3 5% 5% 26%	1639
	21st to end	Day	18% 18% 5% 10% 7% 4 4 3 4 6% 20%	1520
December	1st to 10th	Day	17% 22% 8% 14% 6% 4 4 3 4 5% 14%	1520
	11th to 20th	Day	19% 19% 8% 10% 6% 3 3 3 3 4 20%	1906
	21st to end	Day	21% 21% 9% 11% 5% 4 4 3 5% 4 13%	2772
January	1st to 10th	Night	15% 17% 9% 19% 6% 5% 4 3 4 4 13%	4429
	11th to 20th	Night	11% 18% 8% 17% 9% 5% 4 3 4 4 17%	4440
	21st to end	Night	19% 17% 7% 17% 9% 6% 4 2 3 3 12%	4624
February	1st to 10th	Night	14% 17% 5% 16% 9% 6% 4 3 4 5% 17%	2470
	11th to 20th	Night	12% 14% 7% 15% 11% 7% 5% 4 4 5% 16%	2335
	21st to end	Night	16% 14% 6% 15% 9% 5% 4 3 4 6% 18%	1884
March	1st to 10th	Night	19% 13% 5% 10% 6% 5% 4 3 4 5% 24%	3709
	11th to 20th	Night	12% 19% 8% 11% 5% 4 4 2 4 5% 25%	3479
	21st to end	Night	17% 14% 5% 12% 5% 4 4 3 3 4 29%	3599
April	1st to 10th	Night	17% 15% 4 8% 5% 5% 4 3 4 6% 27%	1900
	11th to 20th	Night	19% 12% 3 6% 5% 5% 5% 3 5% 6% 33%	1875
	21st to end	Night	20% 11% 4 7% 3 3 4 3 5% 5% 34%	1900
May	1st to 10th	Night	24% 11% 3 7% 3 4 3 3 5% 6% 30%	2583
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 6% 6% 33%	2495
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 5% 6% 8% 38%	2738
June	1st to 10th	Night	16% 8% 2 7% 5% 4 5% 4 6% 7% 36%	1520
	11th to 20th	Night	16% 8% 2 7% 4 4 5% 4 6% 9% 38%	1520
	21st to end	Night	13% 5% 2 7% 5% 5% 4 5% 6% 6% 42%	1520
July	1st to 10th	Night	10% 6% 2 7% 7% 5% 5% 5% 6% 8% 38%	2520
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	2520
	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% 33%	2453
	11th to 20th	Night	17% 14% 5% 9% 5% 4 5% 4 5% 6% 26%	2536
	21st to end	Night	15% 16% 6% 10% 6% 5% 5% 3 5% 7% 22%	2660
December	1st to 10th	Night	17% 19% 7% 13% 6% 5% 4 3 4 4 16%	2660
	11th to 20th	Night	19% 14% 6% 14% 6% 5% 4 3 4 5% 18%	3288
	21st to end	Night	24% 17% 10% 11% 5% 4 3 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	66% 4 11% 12% 7% 1	3119
	11th to 20th	Day	58% 6% 15% 11% 9% 1	3479
	21st to end	Day	57% 5% 8% 16% 13% 1	4032
April	1st to 10th	Day	58% 4 7% 20% 8% 2	2280
	11th to 20th	Day	52% 6% 10% 19% 11% 1	2254
	21st to end	Day	54% 6% 9% 18% 11% 2	2280
May	1st to 10th	Day	53% 6% 10% 18% 11% 1	4399
	11th to 20th	Day	51% 9% 12% 20% 6% 1	4402
	21st to end	Day	39% 10% 14% 24% 12% 1	4837
June	1st to 10th	Day	50% 9% 11% 18% 11% 1	2660
	11th to 20th	Day	46% 9% 13% 22% 9% 1	2660
	21st to end	Day	48% 9% 15% 20% 9%	2660
July	1st to 10th	Day	40% 10% 16% 24% 10%	4370
	11th to 20th	Day	40% 10% 16% 25% 8%	4365
	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% 8% 12% 18% 11% 1	1698
	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% 6% 11% 14% 7%	3719
	11th to 20th	Night	60% 6% 12% 17% 5%	3480
	21st to end	Night	56% 6% 7% 18% 11% 2	3599
April	1st to 10th	Night	58% 6% 10% 17% 7% 2	1900
	11th to 20th	Night	52% 6% 10% 18% 13% 1	1875
	21st to end	Night	51% 7% 9% 18% 13% 3	1900
May	1st to 10th	Night	54% 7% 11% 16% 9% 3	2589
	11th to 20th	Night	48% 9% 16% 18% 7% 1	2496
	21st to end	Night	41% 9% 14% 19% 15% 1	2739
June	1st to 10th	Night	44% 7% 13% 23% 13% 1	1520
	11th to 20th	Night	42% 8% 15% 22% 12% 1	1520
	21st to end	Night	39% 9% 15% 26% 11% 1	1520
July	1st to 10th	Night	41% 8% 14% 22% 14% 1	2480
	11th to 20th	Night	38% 8% 15% 26% 12% 1	2472
	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	51% 7% 12% 16% 12% 2	2453
	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% 27% 11% 4	460
	11th to 20th	Day	33% 25% 23% 10% 7% 2	452
	21st to end	Day	36% 18% 27% 12% 5% 1	460
May	1st to 10th	Day	31% 21% 27% 13% 7% 2	463
	11th to 20th	Day	24% 13% 36% 16% 8% 2	487
	21st to end	Day	18% 15% 32% 24% 9% 3	529
June	1st to 10th	Day	22% 16% 30% 22% 9% 1	490
	11th to 20th	Day	19% 14% 32% 24% 10% 1	490
	21st to end	Day	20% 18% 35% 19% 7% 1	490
July	1st to 10th	Day	13% 12% 32% 27% 12% 3	485
	11th to 20th	Day	14% 9% 30% 31% 11% 3	484
	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% 21% 24% 17% 8%	374
	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	28% 20% 33% 18% 3	760
	11th to 20th	Night	28% 19% 29% 16% 7%	751
	21st to end	Night	29% 17% 27% 22% 6%	760
May	1st to 10th	Night	28% 13% 27% 26% 6%	423
	11th to 20th	Night	19% 11% 36% 27% 7% 1	374
	21st to end	Night	16% 9% 33% 33% 9% 1	411
June	1st to 10th	Night	17% 8% 31% 32% 11% 1	380
	11th to 20th	Night	19% 10% 26% 34% 11%	380
	21st to end	Night	15% 8% 29% 38% 10% 1	380
July	1st to 10th	Night	10% 6% 26% 42% 14% 1	370
	11th to 20th	Night	11% 4 29% 40% 14% 2	368
	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%	755
	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 2024-11-01 22:47:52. Version ac07

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