Seattle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seattle clear sky chart from 2005-11-06 to 2024-11-06.

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	17% 9% 3 8% 5% 4 4 4 5% 6% 34%	105720
	Night	19% 10% 3 8% 5% 4 4 4 5% 6% 31%	101944
Transparency	Day	50% 8% 14% 18% 8% 2	105477
	Night	54% 8% 13% 17% 7% 1	101833
Seeing	Day	22% 9% 19% 26% 18% 6%	15111
	Night	26% 8% 16% 27% 20% 3	23458

By Month

Percentage Cloud Cover NW (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	33% 14% 4 10% 5% 4 5% 4 5% 4 12%	2255
	11th to 20th	Day	24% 12% 4 10% 6% 4 4 3 4 6% 21%	2516
	21st to end	Day	34% 14% 4 10% 5% 5% 4 3 4 5% 10%	2871
February	1st to 10th	Day	27% 14% 4 9% 7% 5% 5% 4 6% 6% 14%	1710
	11th to 20th	Day	24% 15% 4 10% 5% 6% 5% 4 4 6% 19%	1665
	21st to end	Day	22% 15% 4 8% 6% 5% 5% 4 5% 5% 20%	1475
March	1st to 10th	Day	15% 11% 4 10% 8% 6% 6% 5% 6% 7% 21%	3219
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 5% 6% 36%	3490
	21st to end	Day	19% 12% 4 11% 6% 6% 7% 5% 6% 6% 18%	3947
April	1st to 10th	Day	21% 12% 4 9% 6% 5% 6% 5% 6% 6% 20%	2280
	11th to 20th	Day	18% 10% 3 7% 6% 4 4 4 6% 8% 28%	2275
	21st to end	Day	16% 11% 4 9% 6% 6% 5% 5% 5% 7% 26%	2512
May	1st to 10th	Day	15% 8% 2 7% 5% 5% 4 4 5% 6% 39%	4420
	11th to 20th	Day	16% 7% 3 5% 4 4 4 4 6% 9% 40%	4377
	21st to end	Day	22% 12% 4 9% 6% 5% 5% 3 5% 6% 23%	4813
June	1st to 10th	Day	18% 8% 2 7% 5% 5% 5% 5% 7% 8% 31%	2703
	11th to 20th	Day	16% 9% 3 9% 6% 6% 6% 5% 6% 7% 26%	2850
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 4 6% 7% 41%	2850
July	1st to 10th	Day	5% 5% 1 5% 3 3 3 3 4 5% 64%	4740
	11th to 20th	Day	5% 3 1 3 3 2 2 3 4 5% 69%	4528
	21st to end	Day	9% 5% 2 5% 3 3 3 3 4 5% 58%	4790
August	1st to 10th	Day	4 3 2 5% 3 3 3 3 5% 7% 63%	4319
	11th to 20th	Day	7% 7% 3 7% 5% 5% 4 4 6% 8% 44%	4026
	21st to end	Day	14% 8% 3 8% 4 3 4 3 5% 5% 43%	4337
September	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	12% 9% 3 8% 5% 5% 4 3 5% 7% 39%	2280
	21st to end	Day	14% 7% 3 8% 5% 3 5% 4 6% 7% 37%	2008
October	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 5% 8% 11% 34%	3180
	11th to 20th	Day	24% 9% 4 9% 6% 5% 4 4 6% 7% 23%	3161
	21st to end	Day	22% 12% 4 11% 6% 5% 5% 4 5% 7% 19%	3259
November	1st to 10th	Day	29% 11% 4 9% 7% 4 4 4 4 6% 17%	1570
	11th to 20th	Day	28% 12% 4 9% 6% 4 3 3 5% 5% 22%	1520
	21st to end	Day	30% 12% 3 7% 5% 4 5% 3 5% 7% 19%	1520
December	1st to 10th	Day	26% 10% 4 9% 6% 5% 5% 3 5% 6% 21%	1520
	11th to 20th	Day	29% 13% 5% 10% 8% 5% 6% 4 5% 5% 12%	1925
	21st to end	Day	25% 14% 5% 13% 7% 6% 5% 4 4 5% 12%	2529
January	1st to 10th	Night	32% 14% 5% 11% 7% 5% 5% 4 4 4 12%	4683
	11th to 20th	Night	26% 13% 5% 7% 4 4 4 3 4 8% 23%	4382
	21st to end	Night	32% 12% 4 12% 6% 5% 4 3 4 5% 12%	4786
February	1st to 10th	Night	27% 12% 4 9% 6% 5% 4 4 4 7% 19%	2470
	11th to 20th	Night	22% 13% 4 8% 6% 5% 4 3 5% 6% 25%	2402
	21st to end	Night	25% 13% 4 8% 6% 4 5% 3 6% 6% 21%	1979
March	1st to 10th	Night	17% 12% 4 10% 7% 7% 8% 6% 6% 7% 17%	3653
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 6% 7% 41%	3489
	21st to end	Night	22% 15% 5% 10% 6% 5% 5% 4 5% 5% 18%	3705
April	1st to 10th	Night	20% 10% 3 10% 7% 5% 5% 5% 6% 7% 23%	1900
	11th to 20th	Night	16% 10% 3 8% 6% 4 5% 5% 5% 6% 31%	1851
	21st to end	Night	19% 11% 3 9% 7% 6% 3 4 5% 6% 27%	1667
May	1st to 10th	Night	15% 9% 3 6% 5% 4 4 4 3 6% 41%	2540
	11th to 20th	Night	12% 8% 3 9% 5% 4 4 4 5% 7% 39%	2516
	21st to end	Night	15% 11% 4 11% 6% 7% 6% 5% 6% 5% 24%	2762
June	1st to 10th	Night	14% 9% 4 8% 5% 5% 5% 3 6% 7% 33%	1473
	11th to 20th	Night	15% 10% 4 9% 7% 5% 5% 4 5% 8% 28%	1330
	21st to end	Night	9% 6% 3 8% 6% 4 4 4 5% 6% 44%	1330
July	1st to 10th	Night	6% 5% 2 5% 4 2 3 2 4 4 62%	2220
	11th to 20th	Night	3 2 1 3 2 2 2 2 4 4 74%	2430
	21st to end	Night	7% 6% 2 5% 3 3 3 3 5% 6% 56%	2762
August	1st to 10th	Night	3 2 1 4 3 3 3 2 4 5% 68%	2610
	11th to 20th	Night	9% 6% 2 6% 4 3 4 3 5% 7% 52%	2812
	21st to end	Night	14% 7% 3 6% 4 3 3 3 4 6% 48%	3335
September	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 6% 57%	1900
	11th to 20th	Night	13% 8% 3 6% 5% 4 4 4 5% 5% 42%	1900
	21st to end	Night	14% 8% 3 7% 4 4 4 3 5% 7% 40%	2170
October	1st to 10th	Night	12% 7% 3 7% 5% 4 4 4 6% 9% 39%	3800
	11th to 20th	Night	23% 8% 3 8% 4 4 5% 4 5% 6% 30%	3777
	21st to end	Night	23% 13% 4 9% 5% 4 4 4 5% 6% 24%	4304
November	1st to 10th	Night	29% 10% 3 7% 5% 5% 5% 4 5% 5% 22%	2628
	11th to 20th	Night	27% 11% 4 8% 5% 4 4 3 5% 5% 23%	2660
	21st to end	Night	27% 9% 3 8% 6% 5% 4 4 6% 6% 20%	2660
December	1st to 10th	Night	26% 11% 4 9% 6% 4 4 3 4 5% 24%	2660
	11th to 20th	Night	25% 14% 5% 12% 7% 5% 5% 4 5% 5% 15%	3280
	21st to end	Night	26% 15% 5% 11% 6% 6% 4 4 4 5% 14%	5118

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	75% 9% 6% 7% 2 1	2255
	11th to 20th	Day	62% 9% 10% 14% 4 1	2540
	21st to end	Day	76% 7% 9% 6% 2	2871
February	1st to 10th	Day	70% 9% 10% 7% 4 1	1710
	11th to 20th	Day	66% 9% 9% 10% 6%	1674
	21st to end	Day	67% 7% 11% 10% 5%	1475
March	1st to 10th	Day	58% 10% 14% 13% 3	3219
	11th to 20th	Day	49% 8% 14% 26% 2 1	3490
	21st to end	Day	64% 11% 13% 11% 1	3947
April	1st to 10th	Day	64% 10% 16% 9% 1	2280
	11th to 20th	Day	56% 9% 14% 13% 7% 1	2275
	21st to end	Day	59% 10% 14% 12% 5% 1	2512
May	1st to 10th	Day	44% 9% 15% 20% 10% 1	4420
	11th to 20th	Day	42% 9% 24% 20% 5%	4380
	21st to end	Day	63% 10% 18% 9% 2	4814
June	1st to 10th	Day	52% 10% 19% 13% 6%	2703
	11th to 20th	Day	58% 11% 16% 11% 4	2850
	21st to end	Day	41% 11% 19% 20% 7% 2	2850
July	1st to 10th	Day	25% 6% 9% 22% 24% 14%	4666
	11th to 20th	Day	20% 4 9% 29% 34% 4	4449
	21st to end	Day	30% 4 16% 33% 16% 2	4639
August	1st to 10th	Day	23% 6% 12% 40% 18% 2	4302
	11th to 20th	Day	39% 5% 23% 23% 8% 2	4065
	21st to end	Day	45% 5% 11% 28% 10% 1	4336
September	1st to 10th	Day	34% 6% 16% 25% 13% 5%	2280
	11th to 20th	Day	46% 8% 14% 17% 14% 2	2280
	21st to end	Day	46% 9% 17% 17% 9% 2	2008
October	1st to 10th	Day	42% 6% 17% 29% 6%	3180
	11th to 20th	Day	60% 7% 11% 14% 7%	3161
	21st to end	Day	65% 5% 13% 9% 7% 1	3259
November	1st to 10th	Day	66% 8% 12% 10% 4	1570
	11th to 20th	Day	65% 8% 10% 9% 6% 2	1520
	21st to end	Day	64% 10% 7% 10% 8% 1	1520
December	1st to 10th	Day	64% 8% 8% 12% 8% 1	1520
	11th to 20th	Day	69% 11% 10% 8% 2 1	1928
	21st to end	Day	70% 11% 11% 6% 2	2529
January	1st to 10th	Night	75% 10% 6% 6% 2 1	4683
	11th to 20th	Night	60% 7% 11% 18% 4 1	4420
	21st to end	Night	73% 8% 8% 8% 3 1	4786
February	1st to 10th	Night	67% 8% 11% 9% 5%	2470
	11th to 20th	Night	61% 10% 12% 10% 7% 1	2404
	21st to end	Night	63% 8% 12% 12% 5%	1979
March	1st to 10th	Night	59% 12% 14% 12% 3	3653
	11th to 20th	Night	42% 11% 17% 27% 2 1	3490
	21st to end	Night	66% 12% 12% 10%	3705
April	1st to 10th	Night	60% 11% 15% 12% 2	1900
	11th to 20th	Night	53% 10% 14% 14% 7% 1	1851
	21st to end	Night	59% 7% 15% 13% 5% 1	1667
May	1st to 10th	Night	45% 9% 13% 24% 9% 1	2540
	11th to 20th	Night	43% 10% 23% 19% 5% 1	2516
	21st to end	Night	63% 12% 14% 9% 2	2762
June	1st to 10th	Night	50% 9% 17% 18% 5%	1473
	11th to 20th	Night	55% 10% 17% 15% 3	1330
	21st to end	Night	40% 9% 21% 20% 8% 3	1330
July	1st to 10th	Night	28% 5% 10% 21% 19% 17%	2185
	11th to 20th	Night	18% 4 9% 29% 39% 3	2386
	21st to end	Night	29% 5% 13% 30% 20% 3	2674
August	1st to 10th	Night	20% 5% 11% 40% 23% 2	2602
	11th to 20th	Night	35% 6% 24% 24% 9% 2	2839
	21st to end	Night	41% 4 15% 29% 10% 1	3335
September	1st to 10th	Night	29% 8% 16% 27% 15% 5%	1900
	11th to 20th	Night	44% 8% 10% 21% 13% 4	1900
	21st to end	Night	45% 7% 13% 23% 10% 2	2170
October	1st to 10th	Night	43% 5% 19% 29% 4	3800
	11th to 20th	Night	56% 6% 13% 19% 6%	3777
	21st to end	Night	63% 5% 11% 13% 7%	4304
November	1st to 10th	Night	63% 9% 13% 11% 3	2628
	11th to 20th	Night	64% 8% 11% 9% 5% 2	2660
	21st to end	Night	62% 10% 10% 10% 8% 1	2660
December	1st to 10th	Night	64% 6% 10% 12% 7% 1	2660
	11th to 20th	Night	67% 12% 11% 7% 3	3276
	21st to end	Night	69% 11% 9% 8% 2	5118

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	40% 8% 16% 14% 16% 6%	180
	11th to 20th	Day	34% 9% 16% 19% 16% 6%	180
	21st to end	Day	39% 15% 16% 18% 9% 4	249
February	1st to 10th	Day	40% 9% 19% 19% 10% 3	370
	11th to 20th	Day	39% 11% 23% 17% 8% 2	365
	21st to end	Day	32% 15% 21% 21% 9% 2	337
March	1st to 10th	Day	30% 13% 24% 21% 9% 2	460
	11th to 20th	Day	27% 13% 19% 22% 14% 4	460
	21st to end	Day	31% 11% 28% 18% 10% 2	506
April	1st to 10th	Day	31% 13% 22% 19% 11% 4	460
	11th to 20th	Day	23% 16% 25% 20% 10% 6%	452
	21st to end	Day	22% 12% 27% 25% 13% 2	460
May	1st to 10th	Day	15% 10% 23% 32% 16% 4	463
	11th to 20th	Day	20% 6% 22% 25% 18% 8%	488
	21st to end	Day	20% 7% 22% 27% 17% 7%	529
June	1st to 10th	Day	23% 8% 20% 26% 18% 6%	533
	11th to 20th	Day	23% 9% 20% 29% 17% 3	680
	21st to end	Day	13% 5% 19% 35% 21% 7%	680
July	1st to 10th	Day	10% 3 19% 32% 27% 9%	671
	11th to 20th	Day	8% 4 13% 34% 32% 9%	549
	21st to end	Day	8% 2 11% 29% 38% 11%	531
August	1st to 10th	Day	7% 3 13% 30% 34% 13%	487
	11th to 20th	Day	7% 2 12% 30% 36% 13%	477
	21st to end	Day	15% 5% 19% 30% 24% 7%	539
September	1st to 10th	Day	11% 5% 22% 36% 20% 5%	490
	11th to 20th	Day	16% 7% 18% 29% 23% 7%	490
	21st to end	Day	19% 11% 19% 28% 20% 4	490
October	1st to 10th	Day	21% 9% 20% 23% 20% 7%	490
	11th to 20th	Day	28% 9% 16% 25% 16% 5%	490
	21st to end	Day	31% 13% 18% 21% 14% 4	425
November	1st to 10th	Day	34% 13% 21% 20% 10% 2	212
	11th to 20th	Day	36% 13% 17% 14% 14% 7%	180
	21st to end	Day	34% 11% 17% 18% 8% 12%	180
December	1st to 10th	Day	37% 11% 16% 14% 14% 8%	180
	11th to 20th	Day	36% 13% 23% 17% 9% 1	180
	21st to end	Day	36% 12% 15% 21% 13% 3	198
January	1st to 10th	Night	42% 11% 14% 16% 15% 2	835
	11th to 20th	Night	36% 10% 13% 22% 17% 2	760
	21st to end	Night	33% 9% 15% 26% 15% 2	836
February	1st to 10th	Night	34% 9% 18% 25% 10% 3	760
	11th to 20th	Night	30% 10% 17% 27% 15% 1	744
	21st to end	Night	35% 10% 17% 23% 12% 2	628
March	1st to 10th	Night	29% 12% 16% 26% 15% 2	760
	11th to 20th	Night	27% 12% 20% 26% 14% 2	760
	21st to end	Night	30% 10% 20% 27% 11% 2	836
April	1st to 10th	Night	27% 9% 21% 27% 14% 3	760
	11th to 20th	Night	22% 11% 20% 28% 16% 3	723
	21st to end	Night	26% 9% 19% 31% 13% 2	527
May	1st to 10th	Night	18% 9% 18% 33% 20% 1	380
	11th to 20th	Night	18% 5% 16% 32% 26% 5%	374
	21st to end	Night	20% 4 15% 31% 25% 4	411
June	1st to 10th	Night	18% 6% 19% 33% 22% 3	380
	11th to 20th	Night	20% 10% 22% 26% 21% 2	380
	21st to end	Night	13% 5% 15% 34% 30% 3	380
July	1st to 10th	Night	10% 4 11% 31% 39% 5%	370
	11th to 20th	Night	8% 2 14% 38% 34% 4	368
	21st to end	Night	7% 3 10% 33% 44% 4	416
August	1st to 10th	Night	8% 10% 29% 43% 10%	420
	11th to 20th	Night	10% 1 7% 27% 45% 10%	564
	21st to end	Night	14% 3 12% 31% 36% 4	738
September	1st to 10th	Night	10% 3 14% 35% 32% 5%	760
	11th to 20th	Night	18% 4 11% 24% 36% 7%	760
	21st to end	Night	20% 6% 15% 31% 25% 4	760
October	1st to 10th	Night	21% 4 16% 28% 25% 5%	760
	11th to 20th	Night	27% 8% 14% 26% 21% 4	760
	21st to end	Night	31% 8% 19% 24% 16% 2	836
November	1st to 10th	Night	35% 9% 18% 27% 10%	764
	11th to 20th	Night	34% 10% 20% 21% 14% 2	760
	21st to end	Night	34% 9% 19% 21% 15% 3	760
December	1st to 10th	Night	33% 5% 18% 26% 16% 2	760
	11th to 20th	Night	38% 11% 19% 21% 11% 1	760
	21st to end	Night	41% 12% 18% 20% 9% 1	908

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