Seaman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seaman 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	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	125896
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	124130
Transparency	Day	38% 5% 12% 22% 18% 5%	129498
	Night	39% 5% 10% 20% 20% 6%	128597
Seeing	Day	15% 8% 23% 26% 19% 9%	16656
	Night	16% 7% 26% 33% 17% 2	26578

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% 5% 2 4 4 3 3 2 2 4 56%	4056
	11th to 20th	Day	12% 7% 2 4 3 3 3 2 3 4 57%	4060
	21st to end	Day	13% 7% 3 7% 4 4 4 3 4 5% 47%	4466
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 6% 44%	1800
	11th to 20th	Day	22% 6% 2 5% 3 4 3 2 3 4 46%	1833
	21st to end	Day	16% 7% 2 6% 5% 3 4 3 5% 7% 42%	1714
March	1st to 10th	Day	20% 8% 2 5% 4 4 4 4 5% 6% 39%	4935
	11th to 20th	Day	11% 5% 2 6% 3 3 3 2 4 6% 54%	4980
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 6% 48%	5465
April	1st to 10th	Day	11% 5% 2 4 3 4 3 4 5% 8% 52%	2382
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 6% 7% 50%	2374
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 7% 46%	2400
May	1st to 10th	Day	8% 6% 3 8% 5% 6% 6% 4 8% 10% 36%	5589
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 9% 29%	5861
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 9% 45%	6496
June	1st to 10th	Day	9% 4 2 7% 5% 6% 5% 5% 8% 10% 40%	2800
	11th to 20th	Day	8% 5% 2 5% 6% 4 6% 4 7% 10% 43%	2800
	21st to end	Day	7% 5% 2 6% 5% 4 6% 5% 8% 10% 42%	2717
July	1st to 10th	Day	8% 5% 3 7% 5% 6% 6% 5% 9% 12% 34%	5900
	11th to 20th	Day	10% 6% 2 8% 6% 6% 6% 5% 9% 12% 29%	5900
	21st to end	Day	8% 6% 2 8% 5% 5% 6% 5% 8% 11% 35%	6424
August	1st to 10th	Day	19% 9% 3 9% 6% 5% 5% 4 8% 10% 22%	4234
	11th to 20th	Day	9% 6% 2 8% 6% 5% 6% 5% 7% 10% 36%	3927
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 10% 47%	4312
September	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 8% 44%	2400
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 3 6% 8% 41%	2370
	21st to end	Day	19% 7% 2 7% 5% 5% 4 3 5% 7% 36%	2100
October	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 4 5% 49%	2000
	11th to 20th	Day	13% 5% 2 5% 3 2 3 4 3 5% 55%	2000
	21st to end	Day	12% 4 2 6% 4 4 3 2 3 4 56%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 6% 52%	1977
	11th to 20th	Day	13% 7% 2 4 3 4 3 3 4 6% 51%	1909
	21st to end	Day	13% 5% 2 4 4 3 3 2 5% 6% 53%	1670
December	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 5% 6% 52%	2600
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 5% 45%	2990
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 7% 45%	4255
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 5% 54%	5889
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 4 53%	5900
	21st to end	Night	14% 7% 2 5% 4 4 3 3 4 6% 47%	6490
February	1st to 10th	Night	18% 7% 2 6% 3 4 4 3 5% 5% 44%	2600
	11th to 20th	Night	21% 7% 2 6% 4 3 3 3 4 5% 41%	2452
	21st to end	Night	15% 6% 1 4 4 3 4 3 6% 6% 47%	1912
March	1st to 10th	Night	18% 7% 3 6% 4 4 4 3 4 4 42%	4918
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 52%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 2 5% 6% 51%	5467
April	1st to 10th	Night	13% 5% 2 6% 4 3 3 4 6% 6% 48%	2016
	11th to 20th	Night	12% 6% 2 5% 4 4 4 2 5% 7% 49%	1975
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	2000
May	1st to 10th	Night	11% 8% 3 9% 6% 5% 5% 4 6% 9% 35%	4334
	11th to 20th	Night	20% 8% 3 7% 5% 5% 4 3 5% 7% 33%	4032
	21st to end	Night	16% 5% 2 6% 4 4 4 3 5% 9% 41%	4340
June	1st to 10th	Night	9% 5% 3 6% 4 4 5% 5% 9% 11% 40%	1600
	11th to 20th	Night	9% 6% 1 7% 6% 5% 5% 6% 8% 9% 39%	1600
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 6% 8% 11% 39%	1673
July	1st to 10th	Night	10% 6% 3 7% 6% 5% 6% 5% 8% 12% 33%	4058
	11th to 20th	Night	12% 6% 3 9% 7% 6% 7% 5% 8% 12% 25%	4057
	21st to end	Night	13% 7% 3 8% 5% 5% 5% 5% 7% 10% 34%	4427
August	1st to 10th	Night	22% 9% 3 8% 5% 4 5% 4 7% 8% 24%	2915
	11th to 20th	Night	9% 6% 2 8% 6% 6% 6% 5% 8% 10% 34%	3086
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 8% 10% 46%	3586
September	1st to 10th	Night	10% 5% 3 6% 3 4 4 4 6% 10% 46%	2000
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 3 4 7% 42%	2025
	21st to end	Night	19% 7% 3 8% 5% 4 4 3 4 7% 35%	2300
October	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 48%	2400
	11th to 20th	Night	14% 5% 2 5% 3 2 3 2 3 6% 54%	2400
	21st to end	Night	12% 4 2 5% 3 3 3 3 4 6% 56%	2640
November	1st to 10th	Night	14% 6% 2 6% 3 3 3 4 4 6% 50%	2366
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 5% 50%	2487
	21st to end	Night	13% 4 2 5% 3 3 4 3 5% 5% 52%	2727
December	1st to 10th	Night	11% 5% 2 7% 4 3 3 3 3 5% 53%	4580
	11th to 20th	Night	18% 7% 3 8% 5% 4 4 3 5% 5% 40%	5205
	21st to end	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	6693

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	35% 2 3 20% 31% 8%	4057
	11th to 20th	Day	33% 2 3 17% 36% 9%	4060
	21st to end	Day	41% 3 5% 22% 20% 8%	4466
February	1st to 10th	Day	40% 4 11% 22% 19% 5%	1800
	11th to 20th	Day	44% 2 7% 19% 22% 6%	1842
	21st to end	Day	42% 4 10% 16% 21% 6%	1714
March	1st to 10th	Day	46% 4 6% 18% 16% 10%	4935
	11th to 20th	Day	33% 3 7% 19% 24% 14%	4980
	21st to end	Day	37% 4 11% 23% 19% 6%	5465
April	1st to 10th	Day	32% 4 10% 26% 22% 7%	2382
	11th to 20th	Day	33% 4 12% 24% 22% 5%	2374
	21st to end	Day	37% 5% 13% 23% 18% 4	2400
May	1st to 10th	Day	42% 6% 20% 21% 10% 1	5621
	11th to 20th	Day	50% 6% 12% 22% 9%	5865
	21st to end	Day	36% 6% 13% 26% 17% 2	6511
June	1st to 10th	Day	37% 7% 14% 23% 17% 2	2800
	11th to 20th	Day	34% 8% 17% 25% 14% 1	2800
	21st to end	Day	33% 9% 18% 23% 15% 2	2717
July	1st to 10th	Day	38% 9% 21% 21% 9% 2	5835
	11th to 20th	Day	44% 11% 27% 12% 4 1	5831
	21st to end	Day	40% 9% 19% 22% 10%	6240
August	1st to 10th	Day	56% 7% 18% 14% 5%	4221
	11th to 20th	Day	41% 8% 15% 26% 9% 1	3965
	21st to end	Day	32% 6% 16% 29% 15% 2	4312
September	1st to 10th	Day	37% 8% 15% 18% 18% 4	2400
	11th to 20th	Day	41% 5% 10% 22% 20% 2	2370
	21st to end	Day	48% 4 10% 21% 13% 4	2100
October	1st to 10th	Day	32% 5% 9% 23% 25% 5%	3260
	11th to 20th	Day	23% 3 5% 20% 30% 20%	3240
	21st to end	Day	26% 2 6% 24% 28% 14%	3538
November	1st to 10th	Day	35% 3 7% 25% 26% 5%	1977
	11th to 20th	Day	35% 3 9% 21% 25% 7%	1909
	21st to end	Day	34% 3 7% 24% 27% 5%	1670
December	1st to 10th	Day	35% 4 9% 23% 23% 7%	2600
	11th to 20th	Day	42% 4 10% 21% 18% 6%	2986
	21st to end	Day	40% 3 11% 24% 20% 1	4255
January	1st to 10th	Night	33% 3 5% 18% 31% 11%	5889
	11th to 20th	Night	36% 3 5% 17% 33% 7%	5900
	21st to end	Night	40% 3 6% 19% 27% 6%	6490
February	1st to 10th	Night	42% 3 9% 21% 21% 5%	2600
	11th to 20th	Night	45% 3 7% 16% 21% 6%	2454
	21st to end	Night	38% 4 7% 22% 20% 8%	1912
March	1st to 10th	Night	46% 4 6% 14% 21% 10%	4918
	11th to 20th	Night	34% 4 9% 13% 24% 17%	4980
	21st to end	Night	35% 3 9% 24% 22% 6%	5467
April	1st to 10th	Night	35% 5% 10% 19% 24% 7%	2016
	11th to 20th	Night	35% 5% 9% 24% 21% 7%	1975
	21st to end	Night	34% 5% 11% 28% 18% 5%	2000
May	1st to 10th	Night	45% 7% 16% 20% 11% 1	4364
	11th to 20th	Night	51% 5% 11% 20% 13% 1	4033
	21st to end	Night	40% 6% 13% 24% 16% 1	4347
June	1st to 10th	Night	35% 8% 18% 21% 16% 2	1600
	11th to 20th	Night	38% 9% 17% 19% 15% 2	1600
	21st to end	Night	35% 10% 14% 24% 14% 2	1673
July	1st to 10th	Night	41% 11% 21% 17% 8% 1	4015
	11th to 20th	Night	50% 14% 21% 11% 3 1	4006
	21st to end	Night	45% 9% 16% 20% 10%	4297
August	1st to 10th	Night	56% 7% 17% 15% 4	2904
	11th to 20th	Night	41% 11% 20% 20% 7% 1	3114
	21st to end	Night	31% 7% 12% 31% 17% 2	3586
September	1st to 10th	Night	34% 7% 16% 23% 15% 5%	2000
	11th to 20th	Night	41% 7% 10% 20% 19% 2	2025
	21st to end	Night	50% 5% 11% 17% 15% 3	2300
October	1st to 10th	Night	38% 5% 7% 23% 23% 4	3920
	11th to 20th	Night	25% 3 5% 18% 27% 23%	3899
	21st to end	Night	23% 3 7% 28% 24% 16%	4252
November	1st to 10th	Night	37% 4 6% 20% 28% 5%	2366
	11th to 20th	Night	37% 4 8% 16% 26% 10%	2487
	21st to end	Night	33% 3 7% 24% 26% 7%	2727
December	1st to 10th	Night	35% 3 7% 19% 29% 8%	4580
	11th to 20th	Night	48% 3 9% 16% 17% 7%	5208
	21st to end	Night	35% 4 11% 27% 20% 2	6693

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	20% 20% 36% 18% 6%	300
	11th to 20th	Day	19% 24% 32% 18% 7%	300
	21st to end	Day	18% 18% 41% 19% 4 1	330
February	1st to 10th	Day	19% 16% 37% 23% 5% 1	300
	11th to 20th	Day	32% 13% 37% 15% 3	380
	21st to end	Day	21% 14% 33% 25% 7%	414
March	1st to 10th	Day	20% 14% 33% 25% 7% 1	500
	11th to 20th	Day	19% 16% 34% 23% 7% 1	500
	21st to end	Day	19% 12% 36% 23% 9% 2	550
April	1st to 10th	Day	17% 15% 31% 25% 11% 1	500
	11th to 20th	Day	13% 12% 28% 28% 15% 3	492
	21st to end	Day	15% 9% 26% 28% 19% 3	500
May	1st to 10th	Day	10% 11% 35% 24% 13% 6%	503
	11th to 20th	Day	17% 6% 23% 28% 18% 8%	527
	21st to end	Day	16% 6% 16% 25% 24% 13%	573
June	1st to 10th	Day	11% 2 16% 30% 27% 13%	530
	11th to 20th	Day	11% 3 18% 25% 26% 18%	530
	21st to end	Day	8% 3 11% 30% 34% 14%	530
July	1st to 10th	Day	10% 1 8% 26% 35% 20%	525
	11th to 20th	Day	9% 2 6% 27% 35% 21%	524
	21st to end	Day	8% 2 10% 31% 32% 17%	576
August	1st to 10th	Day	14% 3 13% 27% 28% 16%	526
	11th to 20th	Day	12% 2 10% 25% 29% 21%	519
	21st to end	Day	11% 2 11% 28% 26% 23%	583
September	1st to 10th	Day	13% 2 14% 22% 27% 21%	530
	11th to 20th	Day	15% 5% 14% 25% 27% 14%	530
	21st to end	Day	22% 6% 19% 28% 18% 7%	530
October	1st to 10th	Day	16% 4 20% 24% 25% 12%	530
	11th to 20th	Day	16% 6% 22% 30% 21% 5%	530
	21st to end	Day	13% 8% 25% 28% 20% 6%	583
November	1st to 10th	Day	18% 7% 24% 31% 15% 5%	503
	11th to 20th	Day	17% 9% 32% 28% 12% 2	454
	21st to end	Day	17% 11% 27% 26% 16% 4	270
December	1st to 10th	Day	20% 12% 41% 18% 9% 2	200
	11th to 20th	Day	18% 9% 37% 21% 14% 2	200
	21st to end	Day	21% 12% 33% 19% 12% 2	284
January	1st to 10th	Night	19% 12% 37% 26% 5% 1	800
	11th to 20th	Night	19% 11% 35% 28% 7%	800
	21st to end	Night	18% 8% 42% 28% 4	880
February	1st to 10th	Night	21% 8% 39% 29% 3	800
	11th to 20th	Night	25% 13% 41% 20% 1	783
	21st to end	Night	19% 10% 37% 31% 2	660
March	1st to 10th	Night	16% 13% 35% 32% 5%	800
	11th to 20th	Night	20% 11% 38% 25% 7% 1	800
	21st to end	Night	19% 9% 37% 29% 6%	880
April	1st to 10th	Night	15% 12% 37% 31% 6%	800
	11th to 20th	Night	15% 9% 35% 31% 10%	791
	21st to end	Night	13% 6% 31% 42% 9% 1	800
May	1st to 10th	Night	13% 8% 33% 35% 12%	735
	11th to 20th	Night	18% 7% 21% 38% 15% 1	591
	21st to end	Night	13% 2 15% 39% 30% 2	620
June	1st to 10th	Night	9% 4 18% 41% 27% 1	400
	11th to 20th	Night	10% 4 15% 41% 27% 4	400
	21st to end	Night	6% 3 13% 45% 31% 3	473
July	1st to 10th	Night	10% 2 10% 37% 38% 4	585
	11th to 20th	Night	8% 2 6% 39% 37% 7%	582
	21st to end	Night	9% 2 11% 40% 33% 5%	657
August	1st to 10th	Night	13% 2 9% 38% 35% 3	597
	11th to 20th	Night	12% 2 8% 36% 37% 5%	715
	21st to end	Night	13% 3 9% 32% 38% 6%	880
September	1st to 10th	Night	12% 2 10% 34% 37% 5%	800
	11th to 20th	Night	16% 3 13% 34% 30% 5%	800
	21st to end	Night	23% 4 15% 34% 25% 1	800
October	1st to 10th	Night	16% 4 16% 33% 27% 4	800
	11th to 20th	Night	16% 6% 23% 35% 19% 2	800
	21st to end	Night	14% 5% 26% 32% 21% 2	880
November	1st to 10th	Night	16% 4 26% 34% 17% 3	789
	11th to 20th	Night	17% 7% 29% 34% 12% 1	800
	21st to end	Night	14% 7% 30% 33% 15% 1	800
December	1st to 10th	Night	19% 8% 33% 32% 8%	800
	11th to 20th	Night	18% 7% 32% 35% 8%	800
	21st to end	Night	17% 8% 32% 33% 9%	880

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