Smithsonian NASM Public Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Smithsonian NASM Public Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	126687
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	123352
Transparency	Day	39% 6% 11% 24% 17% 3	130280
	Night	39% 5% 11% 23% 18% 4	127829
Seeing	Day	19% 13% 28% 24% 12% 4	16315
	Night	18% 11% 30% 29% 12% 1	25782

By Month

Percentage Cloud Cover SE (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	15% 4 1 4 4 3 4 3 4 6% 51%	3688
	11th to 20th	Day	15% 9% 3 6% 3 4 4 2 4 5% 47%	4060
	21st to end	Day	15% 7% 2 7% 5% 4 3 3 4 6% 43%	4466
February	1st to 10th	Day	15% 7% 2 7% 4 5% 5% 4 5% 7% 39%	1800
	11th to 20th	Day	19% 8% 2 5% 3 3 3 4 5% 7% 40%	1822
	21st to end	Day	19% 7% 2 6% 5% 3 4 3 4 6% 41%	1650
March	1st to 10th	Day	19% 5% 2 5% 4 3 4 3 5% 7% 43%	4833
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 4 6% 51%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	5574
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 5% 8% 43%	2400
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 4 5% 6% 48%	2374
	21st to end	Day	18% 6% 2 5% 3 4 4 4 6% 7% 42%	2400
May	1st to 10th	Day	20% 7% 3 6% 5% 4 5% 4 6% 9% 31%	5720
	11th to 20th	Day	21% 7% 2 6% 4 4 5% 4 6% 9% 31%	6175
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 5% 7% 9% 42%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 6% 7% 8% 41%	2800
	21st to end	Day	10% 5% 2 4 5% 4 5% 5% 8% 11% 43%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	6360
	11th to 20th	Day	10% 7% 3 7% 5% 6% 6% 5% 9% 12% 29%	6185
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 4 8% 10% 40%	6424
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 7% 10% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 4 6% 6% 5% 7% 9% 37%	3977
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 4 5% 6% 47%	2400
	11th to 20th	Day	11% 6% 2 5% 3 4 4 3 6% 10% 45%	2400
	21st to end	Day	20% 8% 2 7% 3 4 5% 4 5% 6% 37%	2110
October	1st to 10th	Day	15% 6% 2 5% 3 4 4 3 5% 6% 47%	2000
	11th to 20th	Day	16% 5% 2 5% 3 2 3 3 5% 6% 50%	2000
	21st to end	Day	13% 6% 2 6% 4 3 4 4 4 6% 50%	2200
November	1st to 10th	Day	12% 5% 2 4 3 4 4 3 5% 7% 51%	1864
	11th to 20th	Day	13% 6% 1 5% 3 4 3 4 6% 7% 49%	1795
	21st to end	Day	14% 6% 2 4 5% 4 3 3 5% 6% 48%	1600
December	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 6% 48%	2600
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 4 7% 44%	2990
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 5% 7% 39%	3960
January	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 6% 49%	6242
	11th to 20th	Night	14% 7% 2 4 3 3 3 3 4 5% 52%	5900
	21st to end	Night	15% 8% 3 6% 4 3 3 2 4 5% 48%	6490
February	1st to 10th	Night	15% 7% 2 5% 5% 4 5% 4 6% 6% 41%	2600
	11th to 20th	Night	20% 6% 2 5% 4 4 3 3 3 7% 43%	2463
	21st to end	Night	17% 7% 3 5% 3 4 4 3 5% 7% 43%	1980
March	1st to 10th	Night	17% 6% 2 6% 3 3 3 3 4 5% 48%	5017
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 8% 50%	4980
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 45%	5343
April	1st to 10th	Night	15% 7% 3 5% 5% 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 4 4 5% 7% 46%	1975
	21st to end	Night	17% 5% 2 4 4 3 5% 4 7% 8% 42%	2000
May	1st to 10th	Night	20% 9% 3 7% 5% 4 5% 4 6% 8% 29%	4216
	11th to 20th	Night	22% 8% 2 5% 4 4 4 4 5% 8% 32%	3713
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 7% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 7% 10% 43%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 4 5% 4 7% 9% 40%	1600
	21st to end	Night	9% 5% 2 6% 6% 5% 5% 5% 7% 10% 42%	1600
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 7% 9% 39%	3598
	11th to 20th	Night	10% 8% 3 9% 6% 6% 6% 5% 8% 10% 30%	3768
	21st to end	Night	9% 6% 2 6% 5% 4 5% 5% 7% 9% 41%	4427
August	1st to 10th	Night	17% 5% 3 7% 5% 5% 5% 5% 7% 9% 33%	2915
	11th to 20th	Night	11% 8% 3 9% 6% 4 6% 4 6% 8% 35%	3037
	21st to end	Night	10% 5% 2 5% 4 3 4 3 5% 8% 50%	3586
September	1st to 10th	Night	13% 5% 3 6% 3 4 5% 3 4 7% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 3 3 4 3 5% 7% 49%	2000
	21st to end	Night	19% 8% 3 6% 5% 4 4 4 5% 7% 36%	2285
October	1st to 10th	Night	17% 7% 2 6% 3 3 4 3 4 6% 47%	2400
	11th to 20th	Night	15% 5% 2 4 4 3 3 2 4 6% 52%	2400
	21st to end	Night	15% 5% 1 4 3 3 4 3 4 7% 50%	2640
November	1st to 10th	Night	12% 4 2 5% 4 3 3 3 4 6% 53%	2476
	11th to 20th	Night	14% 6% 2 5% 4 3 3 4 4 6% 49%	2600
	21st to end	Night	13% 5% 2 4 5% 4 3 4 6% 7% 48%	2800
December	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 4 7% 52%	4580
	11th to 20th	Night	21% 6% 2 5% 3 4 4 3 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	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	34% 3 6% 28% 23% 6%	3689
	11th to 20th	Day	41% 3 8% 22% 20% 7%	4060
	21st to end	Day	43% 4 9% 28% 13% 3	4466
February	1st to 10th	Day	43% 5% 12% 25% 13% 2	1800
	11th to 20th	Day	43% 5% 8% 25% 16% 3	1831
	21st to end	Day	45% 4 11% 20% 16% 3	1650
March	1st to 10th	Day	40% 5% 8% 23% 17% 7%	4833
	11th to 20th	Day	35% 3 9% 25% 22% 5%	4980
	21st to end	Day	42% 5% 10% 19% 18% 6%	5574
April	1st to 10th	Day	40% 6% 11% 21% 18% 5%	2400
	11th to 20th	Day	37% 4 10% 22% 20% 6%	2374
	21st to end	Day	40% 5% 10% 23% 18% 4	2400
May	1st to 10th	Day	47% 8% 14% 18% 11% 2	5753
	11th to 20th	Day	47% 8% 11% 24% 9% 1	6179
	21st to end	Day	40% 7% 13% 22% 16% 2	6949
June	1st to 10th	Day	36% 8% 13% 26% 16% 2	2800
	11th to 20th	Day	38% 10% 11% 23% 16% 2	2800
	21st to end	Day	33% 9% 15% 24% 18% 2	2800
July	1st to 10th	Day	34% 10% 18% 20% 14% 4	6290
	11th to 20th	Day	44% 9% 17% 21% 7% 1	6110
	21st to end	Day	37% 8% 10% 23% 22% 1	6240
August	1st to 10th	Day	44% 8% 15% 24% 8%	4221
	11th to 20th	Day	40% 8% 12% 26% 14% 1	4016
	21st to end	Day	34% 7% 12% 28% 18% 2	4312
September	1st to 10th	Day	37% 5% 13% 20% 20% 5%	2400
	11th to 20th	Day	34% 6% 9% 27% 21% 4	2400
	21st to end	Day	46% 6% 9% 19% 17% 3	2110
October	1st to 10th	Day	37% 4 6% 18% 29% 5%	3260
	11th to 20th	Day	25% 4 6% 29% 26% 9%	3240
	21st to end	Day	23% 3 8% 30% 28% 8%	3538
November	1st to 10th	Day	33% 4 12% 27% 20% 5%	1864
	11th to 20th	Day	34% 4 10% 24% 23% 5%	1795
	21st to end	Day	37% 4 9% 28% 18% 3	1600
December	1st to 10th	Day	36% 4 13% 28% 15% 4	2600
	11th to 20th	Day	41% 4 13% 29% 11% 2	2986
	21st to end	Day	43% 6% 14% 21% 15%	3960
January	1st to 10th	Night	36% 4 7% 25% 24% 5%	6242
	11th to 20th	Night	35% 3 8% 25% 22% 7%	5900
	21st to end	Night	40% 3 7% 28% 19% 3	6490
February	1st to 10th	Night	42% 5% 13% 24% 15% 2	2600
	11th to 20th	Night	44% 3 8% 22% 18% 5%	2465
	21st to end	Night	42% 4 10% 22% 17% 5%	1980
March	1st to 10th	Night	40% 4 7% 26% 15% 9%	5017
	11th to 20th	Night	33% 4 11% 26% 19% 7%	4980
	21st to end	Night	40% 4 8% 22% 20% 6%	5343
April	1st to 10th	Night	42% 6% 9% 19% 20% 4	2000
	11th to 20th	Night	38% 5% 11% 21% 20% 5%	1975
	21st to end	Night	38% 6% 11% 25% 16% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 9% 3	4246
	11th to 20th	Night	49% 7% 10% 21% 12% 1	3714
	21st to end	Night	44% 7% 14% 22% 11% 2	3927
June	1st to 10th	Night	35% 6% 12% 26% 17% 2	1600
	11th to 20th	Night	37% 8% 14% 25% 13% 3	1600
	21st to end	Night	35% 7% 14% 22% 19% 3	1600
July	1st to 10th	Night	39% 10% 17% 18% 14% 2	3560
	11th to 20th	Night	47% 9% 18% 17% 7% 1	3723
	21st to end	Night	36% 8% 13% 24% 19% 1	4297
August	1st to 10th	Night	45% 7% 17% 21% 9%	2904
	11th to 20th	Night	44% 7% 11% 25% 11% 1	3064
	21st to end	Night	33% 5% 11% 31% 18% 2	3586
September	1st to 10th	Night	38% 6% 10% 23% 18% 6%	2000
	11th to 20th	Night	34% 5% 9% 27% 21% 4	2000
	21st to end	Night	47% 6% 10% 18% 17% 3	2285
October	1st to 10th	Night	40% 4 9% 21% 20% 6%	3920
	11th to 20th	Night	25% 3 10% 23% 31% 8%	3899
	21st to end	Night	23% 3 9% 32% 25% 8%	4252
November	1st to 10th	Night	33% 4 9% 25% 23% 6%	2476
	11th to 20th	Night	36% 4 10% 23% 22% 6%	2600
	21st to end	Night	34% 6% 11% 25% 21% 3	2800
December	1st to 10th	Night	33% 4 12% 27% 21% 3	4580
	11th to 20th	Night	44% 4 12% 19% 16% 4	5208
	21st to end	Night	43% 6% 14% 21% 15% 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	29% 21% 33% 13% 4	220
	11th to 20th	Day	28% 25% 30% 13% 4	300
	21st to end	Day	22% 26% 34% 13% 5%	330
February	1st to 10th	Day	26% 24% 34% 12% 4	300
	11th to 20th	Day	36% 20% 36% 7% 1	369
	21st to end	Day	30% 18% 32% 18% 2	414
March	1st to 10th	Day	25% 22% 31% 18% 3 1	500
	11th to 20th	Day	28% 18% 32% 15% 5% 1	500
	21st to end	Day	26% 18% 33% 17% 5% 1	550
April	1st to 10th	Day	23% 22% 32% 16% 5% 1	500
	11th to 20th	Day	20% 20% 33% 18% 6% 3	492
	21st to end	Day	25% 15% 32% 20% 7% 2	500
May	1st to 10th	Day	24% 17% 29% 20% 8% 2	503
	11th to 20th	Day	22% 14% 28% 24% 9% 2	527
	21st to end	Day	18% 9% 25% 28% 17% 3	573
June	1st to 10th	Day	13% 9% 27% 29% 15% 7%	530
	11th to 20th	Day	17% 7% 26% 29% 16% 4	530
	21st to end	Day	11% 5% 28% 32% 18% 5%	530
July	1st to 10th	Day	14% 2 17% 38% 20% 11%	525
	11th to 20th	Day	6% 3 21% 36% 27% 7%	524
	21st to end	Day	12% 3 21% 35% 22% 7%	576
August	1st to 10th	Day	14% 4 21% 31% 22% 8%	526
	11th to 20th	Day	14% 4 17% 34% 23% 9%	519
	21st to end	Day	11% 3 22% 32% 20% 11%	583
September	1st to 10th	Day	16% 6% 23% 26% 16% 12%	530
	11th to 20th	Day	13% 5% 22% 35% 17% 8%	530
	21st to end	Day	27% 10% 23% 26% 9% 5%	530
October	1st to 10th	Day	20% 7% 24% 26% 16% 7%	530
	11th to 20th	Day	17% 17% 28% 22% 14% 3	530
	21st to end	Day	17% 13% 37% 21% 9% 2	583
November	1st to 10th	Day	14% 15% 34% 24% 9% 3	446
	11th to 20th	Day	18% 16% 35% 22% 8% 2	395
	21st to end	Day	24% 23% 29% 16% 9% 1	200
December	1st to 10th	Day	19% 23% 39% 15% 4 2	200
	11th to 20th	Day	26% 20% 31% 17% 8%	200
	21st to end	Day	25% 17% 32% 20% 5%	220
January	1st to 10th	Night	20% 19% 39% 19% 3	800
	11th to 20th	Night	22% 15% 33% 25% 5%	800
	21st to end	Night	19% 17% 37% 25% 2	880
February	1st to 10th	Night	20% 16% 34% 27% 3	800
	11th to 20th	Night	27% 20% 33% 19% 1	783
	21st to end	Night	25% 17% 37% 19% 3	660
March	1st to 10th	Night	20% 16% 38% 23% 2	800
	11th to 20th	Night	22% 17% 37% 19% 5% 1	800
	21st to end	Night	19% 17% 39% 21% 4 1	880
April	1st to 10th	Night	20% 20% 36% 21% 4	800
	11th to 20th	Night	18% 15% 33% 28% 6%	791
	21st to end	Night	21% 11% 34% 26% 8%	800
May	1st to 10th	Night	21% 10% 31% 27% 11%	682
	11th to 20th	Night	20% 8% 27% 35% 9%	454
	21st to end	Night	15% 6% 24% 36% 16% 1	433
June	1st to 10th	Night	11% 3 32% 36% 18% 1	400
	11th to 20th	Night	15% 5% 27% 39% 15% 1	400
	21st to end	Night	11% 3 26% 43% 17% 1	400
July	1st to 10th	Night	8% 3 21% 42% 23% 2	390
	11th to 20th	Night	7% 3 16% 43% 29% 2	455
	21st to end	Night	9% 2 20% 42% 25% 3	657
August	1st to 10th	Night	11% 2 13% 43% 29% 2	597
	11th to 20th	Night	14% 4 15% 40% 25% 2	691
	21st to end	Night	11% 3 19% 36% 27% 4	880
September	1st to 10th	Night	15% 3 22% 30% 26% 4	800
	11th to 20th	Night	13% 4 16% 39% 25% 4	800
	21st to end	Night	23% 5% 23% 32% 17% 1	800
October	1st to 10th	Night	19% 6% 22% 28% 21% 5%	800
	11th to 20th	Night	17% 11% 26% 32% 14% 1	800
	21st to end	Night	18% 10% 33% 26% 12% 1	880
November	1st to 10th	Night	13% 10% 35% 29% 12% 2	789
	11th to 20th	Night	19% 13% 32% 28% 8% 1	800
	21st to end	Night	17% 14% 33% 26% 9% 1	800
December	1st to 10th	Night	22% 13% 33% 24% 7% 1	800
	11th to 20th	Night	19% 11% 38% 27% 5%	800
	21st to end	Night	21% 12% 32% 31% 4	880

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