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 2024-01-13.

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 5% 5% 3 5% 5% 6% 8% 42%	74678
	Night	14% 6% 2 5% 5% 3 4 5% 5% 7% 44%	75050
Transparency	Day	39% 6% 11% 23% 17% 3	74388
	Night	39% 6% 11% 23% 17% 3	74859
Seeing	Day	19% 12% 28% 24% 12% 4	13865
	Night	18% 11% 30% 29% 12% 1	23476

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% 6% 2 5% 4 3 4 5% 5% 8% 43%	1554
	11th to 20th	Day	15% 7% 4 5% 4 4 4 4 5% 7% 42%	1638
	21st to end	Day	14% 7% 3 6% 6% 3 5% 4 5% 6% 42%	1782
February	1st to 10th	Day	16% 6% 2 6% 5% 4 5% 5% 5% 8% 38%	1620
	11th to 20th	Day	19% 6% 2 5% 5% 2 4 5% 5% 7% 41%	1635
	21st to end	Day	19% 7% 3 6% 6% 2 4 4 4 6% 39%	1480
March	1st to 10th	Day	12% 6% 2 4 5% 2 4 4 4 7% 49%	1931
	11th to 20th	Day	17% 6% 2 5% 5% 3 4 5% 5% 7% 42%	1980
	21st to end	Day	17% 8% 3 6% 5% 4 4 5% 4 6% 39%	2218
April	1st to 10th	Day	13% 6% 3 5% 4 3 4 5% 5% 8% 44%	2160
	11th to 20th	Day	13% 5% 2 5% 5% 2 5% 4 4 6% 48%	2134
	21st to end	Day	19% 6% 2 5% 4 3 4 5% 6% 7% 41%	2160
May	1st to 10th	Day	15% 7% 3 5% 5% 3 6% 5% 6% 9% 37%	2280
	11th to 20th	Day	18% 6% 2 5% 5% 3 5% 6% 6% 8% 36%	2419
	21st to end	Day	14% 5% 3 5% 5% 3 6% 5% 6% 8% 41%	2725
June	1st to 10th	Day	12% 4 2 5% 5% 3 5% 6% 7% 10% 42%	2520
	11th to 20th	Day	12% 5% 3 5% 5% 3 5% 7% 6% 9% 40%	2520
	21st to end	Day	10% 4 2 4 5% 3 6% 6% 8% 11% 41%	2520
July	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 6% 7% 9% 40%	2520
	11th to 20th	Day	5% 5% 2 5% 5% 5% 6% 6% 7% 11% 43%	2457
	21st to end	Day	9% 5% 2 6% 5% 5% 7% 6% 8% 11% 35%	2547
August	1st to 10th	Day	10% 6% 3 6% 6% 4 7% 6% 7% 9% 35%	2327
	11th to 20th	Day	11% 5% 3 5% 6% 5% 6% 6% 7% 9% 37%	2196
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 6% 8% 10% 39%	2376
September	1st to 10th	Day	15% 4 2 4 5% 3 5% 5% 5% 7% 45%	2160
	11th to 20th	Day	11% 6% 2 4 4 3 4 4 6% 10% 46%	2160
	21st to end	Day	18% 7% 2 5% 4 3 5% 6% 5% 7% 39%	1901
October	1st to 10th	Day	16% 6% 2 4 5% 2 5% 3 5% 7% 46%	1800
	11th to 20th	Day	17% 4 2 4 4 2 3 4 4 7% 49%	1800
	21st to end	Day	14% 6% 2 5% 4 2 3 5% 4 6% 48%	1980
November	1st to 10th	Day	12% 5% 2 3 4 3 4 4 4 7% 52%	1750
	11th to 20th	Day	13% 6% 2 4 3 2 4 4 5% 8% 49%	1706
	21st to end	Day	15% 5% 2 3 4 3 4 4 5% 6% 48%	1520
December	1st to 10th	Day	16% 6% 2 4 5% 3 5% 5% 5% 7% 42%	1520
	11th to 20th	Day	16% 6% 2 5% 4 2 4 4 4 7% 47%	1910
	21st to end	Day	17% 7% 3 6% 6% 3 5% 5% 5% 7% 37%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 2 4 4 5% 7% 46%	2610
	11th to 20th	Night	17% 6% 3 4 5% 3 4 5% 6% 6% 41%	2367
	21st to end	Night	15% 7% 3 4 4 2 4 4 5% 6% 46%	2574
February	1st to 10th	Night	16% 7% 3 4 5% 3 5% 5% 6% 6% 40%	2340
	11th to 20th	Night	19% 6% 3 5% 4 3 4 4 3 7% 43%	2210
	21st to end	Night	18% 7% 3 5% 3 3 3 4 5% 7% 42%	1776
March	1st to 10th	Night	14% 4 2 4 4 2 4 4 4 6% 49%	2026
	11th to 20th	Night	17% 6% 2 5% 4 3 3 4 5% 7% 43%	1980
	21st to end	Night	15% 6% 3 7% 5% 3 3 5% 5% 6% 41%	2137
April	1st to 10th	Night	14% 6% 3 4 6% 3 5% 5% 5% 6% 41%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 4 5% 5% 7% 47%	1775
	21st to end	Night	19% 5% 2 4 5% 2 4 5% 6% 8% 42%	1800
May	1st to 10th	Night	16% 6% 3 5% 5% 2 4 5% 6% 7% 40%	1694
	11th to 20th	Night	17% 7% 3 4 4 3 4 5% 6% 8% 38%	1469
	21st to end	Night	13% 6% 1 5% 4 3 5% 5% 7% 11% 41%	1550
June	1st to 10th	Night	10% 5% 2 4 5% 3 5% 5% 7% 10% 44%	1440
	11th to 20th	Night	11% 6% 2 5% 5% 3 5% 5% 8% 9% 40%	1440
	21st to end	Night	9% 4 2 6% 6% 4 5% 6% 7% 10% 43%	1440
July	1st to 10th	Night	8% 4 3 6% 6% 5% 5% 6% 7% 10% 39%	1440
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 6% 6% 11% 46%	1499
	21st to end	Night	8% 4 3 4 5% 2 6% 6% 9% 11% 41%	1771
August	1st to 10th	Night	9% 4 2 5% 6% 4 5% 7% 7% 9% 40%	1611
	11th to 20th	Night	10% 7% 2 6% 7% 3 6% 6% 6% 8% 39%	1685
	21st to end	Night	9% 6% 3 6% 6% 3 4 5% 6% 9% 43%	1980
September	1st to 10th	Night	14% 6% 3 5% 5% 3 5% 4 4 8% 45%	1800
	11th to 20th	Night	10% 6% 3 5% 4 2 4 4 5% 7% 50%	1800
	21st to end	Night	16% 7% 3 5% 5% 3 4 5% 6% 8% 39%	2056
October	1st to 10th	Night	18% 6% 2 5% 4 2 4 4 4 6% 45%	2160
	11th to 20th	Night	15% 5% 2 4 5% 3 3 3 4 6% 51%	2160
	21st to end	Night	15% 5% 2 4 4 2 4 4 4 7% 49%	2376
November	1st to 10th	Night	13% 4 2 4 4 2 3 4 4 5% 54%	2325
	11th to 20th	Night	15% 5% 3 4 4 2 3 4 4 7% 50%	2470
	21st to end	Night	13% 4 2 3 5% 3 3 5% 6% 7% 48%	2660
December	1st to 10th	Night	17% 7% 3 5% 5% 2 4 5% 4 7% 42%	2660
	11th to 20th	Night	17% 5% 2 5% 4 3 4 4 5% 7% 44%	3285
	21st to end	Night	17% 6% 2 5% 5% 3 5% 5% 6% 7% 37%	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	41% 5% 10% 24% 15% 4	1555
	11th to 20th	Day	42% 5% 13% 23% 15% 2	1638
	21st to end	Day	43% 4 10% 25% 15% 4	1782
February	1st to 10th	Day	44% 5% 13% 24% 13% 2	1620
	11th to 20th	Day	42% 5% 8% 27% 15% 3	1644
	21st to end	Day	47% 4 11% 19% 16% 4	1480
March	1st to 10th	Day	36% 5% 10% 26% 21% 3	1931
	11th to 20th	Day	43% 4 10% 19% 18% 6%	1980
	21st to end	Day	46% 5% 9% 20% 17% 4	2218
April	1st to 10th	Day	38% 6% 11% 21% 18% 5%	2160
	11th to 20th	Day	37% 4 10% 22% 21% 7%	2134
	21st to end	Day	41% 5% 11% 22% 17% 4	2160
May	1st to 10th	Day	41% 7% 12% 20% 17% 3	2280
	11th to 20th	Day	45% 7% 12% 20% 14% 2	2423
	21st to end	Day	40% 8% 14% 24% 12% 2	2725
June	1st to 10th	Day	36% 8% 13% 26% 16% 2	2520
	11th to 20th	Day	38% 10% 11% 22% 16% 2	2520
	21st to end	Day	34% 9% 15% 23% 17% 2	2520
July	1st to 10th	Day	37% 9% 13% 22% 16% 3	2450
	11th to 20th	Day	32% 8% 16% 26% 15% 3	2382
	21st to end	Day	39% 10% 12% 20% 17% 2	2405
August	1st to 10th	Day	41% 9% 14% 23% 13% 1	2314
	11th to 20th	Day	41% 8% 13% 21% 15% 2	2196
	21st to end	Day	38% 9% 12% 23% 14% 3	2376
September	1st to 10th	Day	38% 5% 14% 20% 18% 4	2160
	11th to 20th	Day	34% 6% 9% 26% 21% 4	2160
	21st to end	Day	44% 6% 9% 21% 18% 3	1901
October	1st to 10th	Day	39% 5% 9% 25% 19% 3	1800
	11th to 20th	Day	36% 5% 9% 25% 22% 3	1800
	21st to end	Day	37% 4 8% 26% 22% 3	1980
November	1st to 10th	Day	33% 4 11% 27% 20% 5%	1750
	11th to 20th	Day	34% 4 10% 24% 23% 5%	1706
	21st to end	Day	37% 4 9% 28% 18% 3	1520
December	1st to 10th	Day	41% 5% 14% 26% 13% 2	1520
	11th to 20th	Day	38% 4 10% 32% 14% 3	1906
	21st to end	Day	45% 7% 13% 24% 11% 1	2772
January	1st to 10th	Night	39% 4 9% 24% 19% 5%	2610
	11th to 20th	Night	43% 5% 12% 23% 16% 2	2367
	21st to end	Night	39% 3 9% 27% 20% 2	2574
February	1st to 10th	Night	42% 5% 12% 24% 15% 2	2340
	11th to 20th	Night	43% 4 8% 21% 18% 5%	2212
	21st to end	Night	43% 4 10% 22% 17% 5%	1776
March	1st to 10th	Night	37% 4 10% 27% 19% 3	2026
	11th to 20th	Night	40% 5% 12% 21% 18% 4	1980
	21st to end	Night	42% 6% 9% 21% 16% 6%	2137
April	1st to 10th	Night	41% 6% 10% 19% 20% 4	1800
	11th to 20th	Night	37% 5% 11% 21% 20% 6%	1775
	21st to end	Night	39% 6% 11% 24% 15% 6%	1800
May	1st to 10th	Night	41% 7% 13% 21% 13% 4	1694
	11th to 20th	Night	43% 7% 11% 18% 17% 3	1470
	21st to end	Night	36% 8% 19% 21% 13% 2	1551
June	1st to 10th	Night	35% 7% 13% 27% 17% 2	1440
	11th to 20th	Night	37% 8% 15% 24% 13% 3	1440
	21st to end	Night	35% 7% 15% 22% 19% 3	1440
July	1st to 10th	Night	39% 10% 16% 18% 13% 4	1400
	11th to 20th	Night	32% 9% 15% 25% 17% 2	1451
	21st to end	Night	31% 8% 16% 27% 16% 1	1672
August	1st to 10th	Night	36% 8% 17% 23% 15% 1	1600
	11th to 20th	Night	40% 9% 14% 22% 13% 2	1685
	21st to end	Night	38% 7% 13% 24% 14% 3	1980
September	1st to 10th	Night	39% 7% 10% 22% 17% 5%	1800
	11th to 20th	Night	33% 5% 9% 27% 22% 4	1800
	21st to end	Night	43% 6% 10% 20% 18% 4	2056
October	1st to 10th	Night	40% 5% 9% 24% 19% 3	2160
	11th to 20th	Night	36% 4 9% 27% 21% 3	2160
	21st to end	Night	36% 4 9% 26% 22% 4	2376
November	1st to 10th	Night	34% 4 9% 26% 22% 6%	2325
	11th to 20th	Night	36% 4 10% 22% 23% 6%	2470
	21st to end	Night	33% 6% 11% 26% 21% 3	2660
December	1st to 10th	Night	42% 5% 11% 24% 15% 2	2660
	11th to 20th	Night	40% 4 11% 23% 19% 3	3288
	21st to end	Night	44% 7% 14% 23% 11% 1	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	27% 22% 32% 14% 4 1	198
	11th to 20th	Day	26% 26% 30% 13% 5%	246
	21st to end	Day	23% 28% 33% 12% 5%	264
February	1st to 10th	Day	27% 23% 33% 12% 5%	240
	11th to 20th	Day	32% 20% 40% 7% 1	302
	21st to end	Day	31% 18% 32% 17% 2	346
March	1st to 10th	Day	23% 23% 31% 20% 3	420
	11th to 20th	Day	28% 18% 32% 15% 5% 1	420
	21st to end	Day	25% 18% 32% 18% 6% 2	462
April	1st to 10th	Day	21% 21% 33% 18% 5% 1	420
	11th to 20th	Day	18% 19% 33% 19% 7% 4	412
	21st to end	Day	26% 16% 30% 20% 6% 1	420
May	1st to 10th	Day	23% 18% 28% 20% 9% 2	423
	11th to 20th	Day	21% 14% 28% 26% 8% 3	447
	21st to end	Day	18% 7% 24% 29% 19% 3	485
June	1st to 10th	Day	14% 8% 26% 30% 16% 6%	450
	11th to 20th	Day	17% 7% 28% 27% 16% 5%	450
	21st to end	Day	11% 5% 29% 34% 17% 4	450
July	1st to 10th	Day	14% 2 17% 37% 19% 12%	445
	11th to 20th	Day	5% 3 21% 35% 29% 7%	444
	21st to end	Day	11% 4 23% 34% 22% 7%	488
August	1st to 10th	Day	12% 4 20% 32% 24% 9%	446
	11th to 20th	Day	13% 4 16% 33% 25% 10%	439
	21st to end	Day	10% 3 22% 32% 20% 13%	495
September	1st to 10th	Day	17% 7% 21% 25% 17% 14%	450
	11th to 20th	Day	13% 4 21% 36% 17% 9%	450
	21st to end	Day	24% 11% 23% 27% 10% 5%	450
October	1st to 10th	Day	22% 7% 24% 25% 17% 6%	450
	11th to 20th	Day	17% 15% 27% 23% 15% 2	450
	21st to end	Day	19% 11% 38% 20% 9% 2	495
November	1st to 10th	Day	16% 14% 32% 24% 10% 3	404
	11th to 20th	Day	18% 16% 34% 22% 8% 2	366
	21st to end	Day	23% 22% 29% 17% 9% 1	180
December	1st to 10th	Day	18% 24% 37% 15% 4 2	180
	11th to 20th	Day	26% 21% 29% 18% 7%	180
	21st to end	Day	26% 17% 33% 19% 5% 1	198
January	1st to 10th	Night	20% 17% 39% 20% 3	760
	11th to 20th	Night	22% 13% 33% 27% 5%	729
	21st to end	Night	18% 17% 38% 25% 2	792
February	1st to 10th	Night	20% 16% 33% 27% 3	720
	11th to 20th	Night	26% 20% 34% 19% 1	703
	21st to end	Night	24% 18% 36% 18% 3	592
March	1st to 10th	Night	19% 16% 40% 23% 2	720
	11th to 20th	Night	23% 15% 37% 19% 5% 1	720
	21st to end	Night	19% 16% 39% 22% 4 1	792
April	1st to 10th	Night	19% 20% 36% 21% 4	720
	11th to 20th	Night	17% 14% 33% 29% 6%	711
	21st to end	Night	23% 10% 33% 26% 8%	720
May	1st to 10th	Night	20% 10% 31% 27% 12%	614
	11th to 20th	Night	19% 7% 26% 37% 10%	408
	21st to end	Night	15% 5% 22% 39% 17% 2	389
June	1st to 10th	Night	11% 2 31% 35% 19% 1	360
	11th to 20th	Night	14% 4 26% 39% 16% 1	360
	21st to end	Night	11% 3 25% 45% 17% 1	360
July	1st to 10th	Night	8% 3 20% 43% 25% 2	350
	11th to 20th	Night	7% 3 16% 43% 29% 3	407
	21st to end	Night	9% 2 20% 41% 26% 3	591
August	1st to 10th	Night	10% 1 13% 43% 31% 2	537
	11th to 20th	Night	14% 4 14% 40% 26% 2	621
	21st to end	Night	11% 3 19% 35% 27% 4	792
September	1st to 10th	Night	16% 3 20% 30% 27% 5%	720
	11th to 20th	Night	11% 4 16% 39% 25% 5%	720
	21st to end	Night	20% 5% 24% 32% 18% 2	720
October	1st to 10th	Night	20% 6% 22% 28% 20% 5%	720
	11th to 20th	Night	17% 10% 26% 32% 14% 1	720
	21st to end	Night	19% 11% 33% 25% 12% 1	792
November	1st to 10th	Night	14% 9% 34% 29% 12% 2	740
	11th to 20th	Night	18% 13% 32% 28% 8% 1	760
	21st to end	Night	17% 14% 32% 27% 9% 1	760
December	1st to 10th	Night	22% 13% 33% 23% 8% 1	760
	11th to 20th	Night	18% 11% 39% 26% 5%	760
	21st to end	Night	21% 12% 32% 31% 3	836

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