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 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	16% 9% 3 8% 5% 5% 4 4 5% 7% 34%	131285
	Night	18% 10% 4 8% 5% 5% 4 4 5% 6% 30%	127447
Transparency	Day	50% 7% 15% 19% 8% 1	131000
	Night	55% 7% 13% 17% 7% 1	127309
Seeing	Day	22% 8% 19% 26% 19% 6%	16391
	Night	26% 8% 16% 27% 20% 3	24688

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	31% 13% 5% 12% 6% 6% 5% 4 5% 4 10%	3243
	11th to 20th	Day	18% 11% 4 13% 7% 5% 5% 4 5% 8% 21%	3616
	21st to end	Day	26% 12% 4 8% 6% 5% 4 3 4 6% 23%	4123
February	1st to 10th	Day	26% 14% 4 9% 7% 5% 5% 4 6% 6% 15%	1800
	11th to 20th	Day	24% 15% 4 10% 5% 5% 5% 4 4 6% 19%	1755
	21st to end	Day	23% 15% 4 9% 6% 5% 5% 4 5% 5% 20%	1550
March	1st to 10th	Day	17% 11% 5% 12% 8% 6% 6% 4 6% 7% 19%	4638
	11th to 20th	Day	17% 13% 4 10% 6% 5% 5% 3 5% 6% 27%	5000
	21st to end	Day	19% 12% 4 11% 7% 7% 7% 5% 6% 7% 15%	5660
April	1st to 10th	Day	21% 12% 4 9% 6% 5% 6% 5% 6% 6% 20%	2400
	11th to 20th	Day	18% 10% 3 7% 6% 4 4 4 6% 8% 29%	2397
	21st to end	Day	15% 10% 4 9% 6% 6% 5% 5% 6% 7% 27%	2644
May	1st to 10th	Day	13% 8% 3 7% 5% 4 4 4 5% 6% 42%	6320
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 4 6% 9% 30%	6277
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 3 6% 9% 32%	6903
June	1st to 10th	Day	17% 7% 2 7% 4 5% 5% 4 7% 8% 33%	2846
	11th to 20th	Day	16% 9% 3 9% 6% 6% 6% 5% 6% 7% 26%	3000
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 4 6% 7% 41%	3000
July	1st to 10th	Day	7% 5% 2 4 3 3 3 2 4 6% 63%	6780
	11th to 20th	Day	4 4 1 4 2 2 2 2 4 4 70%	6470
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 6% 58%	6874
August	1st to 10th	Day	4 3 2 5% 3 3 3 3 5% 7% 62%	4456
	11th to 20th	Day	7% 7% 3 7% 5% 5% 4 4 6% 8% 44%	4156
	21st to end	Day	13% 8% 3 8% 4 3 4 3 5% 5% 43%	4474
September	1st to 10th	Day	8% 6% 2 5% 4 4 4 3 5% 8% 50%	2400
	11th to 20th	Day	11% 9% 2 8% 5% 5% 4 3 5% 7% 40%	2400
	21st to end	Day	14% 7% 3 8% 5% 4 5% 4 5% 7% 38%	2113
October	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 5% 8% 11% 35%	3280
	11th to 20th	Day	23% 9% 4 8% 6% 5% 5% 4 6% 7% 23%	3261
	21st to end	Day	22% 12% 4 11% 6% 5% 5% 4 5% 7% 19%	3363
November	1st to 10th	Day	29% 11% 3 9% 7% 5% 4 4 5% 6% 17%	1628
	11th to 20th	Day	27% 12% 4 9% 6% 5% 4 3 5% 5% 21%	1600
	21st to end	Day	29% 12% 3 8% 5% 4 5% 3 5% 7% 18%	1600
December	1st to 10th	Day	19% 8% 3 12% 7% 7% 7% 4 6% 7% 19%	2620
	11th to 20th	Day	30% 12% 5% 10% 7% 5% 6% 4 5% 5% 11%	3025
	21st to end	Day	28% 14% 5% 12% 7% 6% 5% 3 4 5% 12%	3613
January	1st to 10th	Night	29% 15% 6% 12% 7% 6% 4 4 4 3 10%	6695
	11th to 20th	Night	19% 10% 4 8% 5% 5% 5% 4 5% 8% 27%	6282
	21st to end	Night	24% 10% 3 9% 5% 5% 4 4 5% 7% 24%	6837
February	1st to 10th	Night	26% 13% 4 9% 6% 5% 4 4 4 7% 19%	2600
	11th to 20th	Night	22% 13% 4 8% 6% 5% 4 3 5% 6% 25%	2532
	21st to end	Night	25% 12% 4 8% 6% 4 5% 3 6% 6% 21%	2079
March	1st to 10th	Night	15% 11% 4 10% 7% 7% 7% 6% 7% 8% 19%	5254
	11th to 20th	Night	18% 11% 4 7% 5% 5% 4 4 6% 6% 32%	4999
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 4 6% 5% 16%	5314
April	1st to 10th	Night	19% 10% 3 10% 7% 5% 5% 5% 6% 7% 22%	2000
	11th to 20th	Night	16% 10% 3 8% 6% 4 5% 4 5% 6% 32%	1949
	21st to end	Night	18% 10% 3 8% 7% 5% 3 4 5% 7% 28%	1755
May	1st to 10th	Night	13% 8% 3 7% 5% 5% 4 3 4 5% 44%	3640
	11th to 20th	Night	14% 9% 4 10% 6% 5% 5% 4 5% 7% 32%	3616
	21st to end	Night	12% 10% 3 10% 6% 7% 6% 5% 7% 7% 26%	3972
June	1st to 10th	Night	14% 8% 4 8% 5% 5% 5% 3 6% 7% 35%	1550
	11th to 20th	Night	14% 10% 4 9% 8% 5% 5% 4 5% 8% 29%	1400
	21st to end	Night	8% 6% 3 8% 6% 5% 4 4 5% 6% 44%	1400
July	1st to 10th	Night	8% 5% 2 5% 4 3 3 2 4 4 60%	3180
	11th to 20th	Night	2 2 1 3 3 2 2 2 4 5% 74%	3474
	21st to end	Night	5% 4 2 5% 4 3 3 3 6% 7% 59%	3969
August	1st to 10th	Night	4 2 1 4 3 3 4 2 4 5% 68%	2693
	11th to 20th	Night	9% 6% 2 6% 4 3 4 3 5% 6% 51%	2902
	21st to end	Night	13% 7% 3 6% 4 3 3 3 4 6% 48%	3440
September	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 6% 56%	2000
	11th to 20th	Night	13% 8% 3 6% 5% 4 4 4 5% 5% 43%	2000
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 5% 7% 40%	2284
October	1st to 10th	Night	12% 7% 3 7% 5% 4 4 4 6% 9% 39%	3920
	11th to 20th	Night	22% 8% 3 8% 4 4 4 4 5% 6% 30%	3897
	21st to end	Night	22% 13% 4 9% 5% 4 4 4 5% 6% 24%	4442
November	1st to 10th	Night	29% 10% 3 7% 6% 5% 5% 4 5% 5% 22%	2712
	11th to 20th	Night	28% 12% 4 8% 6% 4 4 3 5% 5% 22%	2800
	21st to end	Night	27% 10% 3 8% 7% 5% 5% 4 6% 6% 19%	2800
December	1st to 10th	Night	19% 9% 3 12% 6% 5% 5% 4 6% 7% 23%	4560
	11th to 20th	Night	28% 15% 5% 12% 6% 5% 5% 3 4 4 12%	5180
	21st to end	Night	27% 16% 5% 12% 6% 5% 4 4 4 4 12%	7320

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	78% 7% 7% 7% 2	3243
	11th to 20th	Day	59% 7% 11% 17% 5% 1	3640
	21st to end	Day	60% 5% 10% 16% 9%	4123
February	1st to 10th	Day	69% 9% 9% 8% 4 1	1800
	11th to 20th	Day	66% 8% 9% 10% 6%	1764
	21st to end	Day	67% 7% 11% 11% 5%	1550
March	1st to 10th	Day	62% 8% 14% 13% 4	4638
	11th to 20th	Day	59% 6% 12% 20% 2	5000
	21st to end	Day	68% 8% 14% 10% 1	5660
April	1st to 10th	Day	63% 9% 17% 10% 1	2400
	11th to 20th	Day	54% 8% 15% 15% 7% 1	2397
	21st to end	Day	57% 10% 15% 13% 5% 1	2644
May	1st to 10th	Day	43% 7% 16% 23% 10% 1	6320
	11th to 20th	Day	53% 7% 19% 19% 3	6280
	21st to end	Day	50% 7% 24% 16% 2	6904
June	1st to 10th	Day	50% 10% 19% 15% 6%	2846
	11th to 20th	Day	57% 10% 16% 12% 4	3000
	21st to end	Day	42% 10% 19% 20% 7% 2	3000
July	1st to 10th	Day	26% 4 17% 23% 20% 10%	6706
	11th to 20th	Day	20% 3 14% 31% 30% 3	6391
	21st to end	Day	28% 3 24% 32% 12% 1	6681
August	1st to 10th	Day	23% 5% 12% 39% 18% 2	4439
	11th to 20th	Day	39% 5% 23% 24% 8% 2	4195
	21st to end	Day	44% 5% 11% 28% 10% 1	4473
September	1st to 10th	Day	35% 6% 17% 25% 12% 5%	2400
	11th to 20th	Day	45% 8% 14% 18% 14% 2	2400
	21st to end	Day	45% 8% 18% 18% 9% 2	2113
October	1st to 10th	Day	42% 6% 17% 28% 6%	3280
	11th to 20th	Day	60% 7% 11% 15% 7%	3261
	21st to end	Day	65% 5% 13% 9% 7% 1	3363
November	1st to 10th	Day	66% 8% 12% 10% 4	1628
	11th to 20th	Day	66% 8% 10% 9% 6% 2	1600
	21st to end	Day	65% 10% 7% 10% 7% 1	1600
December	1st to 10th	Day	60% 5% 9% 17% 10%	2620
	11th to 20th	Day	72% 8% 11% 8% 1	3028
	21st to end	Day	73% 8% 10% 6% 2	3613
January	1st to 10th	Night	78% 8% 6% 7% 2	6695
	11th to 20th	Night	53% 5% 14% 21% 6%	6320
	21st to end	Night	58% 6% 10% 18% 8%	6837
February	1st to 10th	Night	67% 8% 11% 9% 6%	2600
	11th to 20th	Night	61% 10% 12% 10% 7% 1	2534
	21st to end	Night	63% 8% 12% 12% 4	2079
March	1st to 10th	Night	59% 9% 15% 13% 4	5254
	11th to 20th	Night	53% 8% 14% 23% 2	5000
	21st to end	Night	69% 9% 11% 10%	5314
April	1st to 10th	Night	61% 11% 15% 12% 2	2000
	11th to 20th	Night	52% 10% 15% 15% 7% 1	1949
	21st to end	Night	57% 7% 16% 14% 5% 1	1755
May	1st to 10th	Night	43% 7% 12% 29% 9%	3637
	11th to 20th	Night	53% 8% 19% 17% 3	3616
	21st to end	Night	58% 10% 18% 14% 1	3972
June	1st to 10th	Night	48% 8% 18% 19% 6%	1550
	11th to 20th	Night	55% 10% 16% 16% 3	1400
	21st to end	Night	41% 8% 21% 20% 7% 2	1400
July	1st to 10th	Night	30% 3 16% 21% 17% 12%	3145
	11th to 20th	Night	17% 3 15% 29% 35% 2	3430
	21st to end	Night	26% 3 22% 32% 14% 2	3857
August	1st to 10th	Night	20% 4 12% 40% 22% 2	2685
	11th to 20th	Night	35% 6% 24% 23% 10% 2	2929
	21st to end	Night	41% 4 15% 29% 10% 1	3440
September	1st to 10th	Night	30% 7% 18% 26% 15% 4	2000
	11th to 20th	Night	43% 8% 10% 22% 13% 4	2000
	21st to end	Night	45% 7% 13% 23% 10% 2	2284
October	1st to 10th	Night	43% 5% 19% 29% 4	3920
	11th to 20th	Night	56% 6% 13% 19% 6%	3897
	21st to end	Night	63% 5% 12% 13% 7%	4442
November	1st to 10th	Night	64% 8% 13% 11% 3	2712
	11th to 20th	Night	65% 8% 11% 9% 5% 2	2800
	21st to end	Night	63% 10% 10% 9% 8% 1	2800
December	1st to 10th	Night	58% 5% 10% 19% 8%	4560
	11th to 20th	Night	73% 8% 10% 7% 2	5176
	21st to end	Night	74% 8% 9% 8% 2	7320

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	41% 8% 17% 16% 14% 5%	200
	11th to 20th	Day	31% 9% 17% 21% 16% 6%	200
	21st to end	Day	36% 14% 16% 18% 10% 6%	274
February	1st to 10th	Day	39% 9% 20% 18% 11% 3	400
	11th to 20th	Day	39% 10% 24% 17% 7% 2	395
	21st to end	Day	33% 14% 21% 21% 9% 2	364
March	1st to 10th	Day	31% 13% 24% 21% 9% 2	500
	11th to 20th	Day	29% 13% 20% 21% 14% 4	500
	21st to end	Day	31% 10% 29% 18% 10% 2	550
April	1st to 10th	Day	30% 13% 22% 19% 11% 4	500
	11th to 20th	Day	22% 15% 25% 22% 10% 6%	492
	21st to end	Day	21% 11% 25% 27% 14% 3	500
May	1st to 10th	Day	15% 10% 24% 32% 16% 4	503
	11th to 20th	Day	22% 6% 23% 24% 17% 8%	528
	21st to end	Day	19% 7% 22% 27% 17% 8%	573
June	1st to 10th	Day	22% 8% 19% 27% 19% 6%	576
	11th to 20th	Day	22% 9% 20% 30% 17% 3	730
	21st to end	Day	13% 5% 19% 35% 22% 7%	730
July	1st to 10th	Day	10% 3 18% 33% 27% 9%	721
	11th to 20th	Day	8% 4 13% 35% 32% 9%	592
	21st to end	Day	8% 2 11% 28% 39% 11%	575
August	1st to 10th	Day	7% 4 13% 30% 34% 12%	527
	11th to 20th	Day	9% 2 12% 30% 34% 13%	517
	21st to end	Day	14% 5% 18% 31% 24% 8%	583
September	1st to 10th	Day	11% 5% 21% 35% 23% 5%	530
	11th to 20th	Day	16% 7% 18% 30% 23% 7%	530
	21st to end	Day	19% 10% 20% 28% 19% 4	530
October	1st to 10th	Day	21% 8% 20% 24% 20% 7%	530
	11th to 20th	Day	28% 9% 18% 24% 16% 5%	530
	21st to end	Day	31% 13% 19% 21% 13% 4	463
November	1st to 10th	Day	34% 13% 21% 19% 11% 3	228
	11th to 20th	Day	36% 15% 17% 14% 13% 6%	200
	21st to end	Day	34% 10% 16% 21% 10% 11%	200
December	1st to 10th	Day	35% 10% 15% 16% 16% 10%	200
	11th to 20th	Day	39% 13% 22% 18% 9% 1	200
	21st to end	Day	39% 13% 14% 20% 12% 3	220
January	1st to 10th	Night	42% 11% 14% 17% 14% 2	883
	11th to 20th	Night	34% 10% 13% 24% 17% 2	800
	21st to end	Night	32% 9% 15% 27% 15% 3	880
February	1st to 10th	Night	34% 10% 19% 25% 10% 2	800
	11th to 20th	Night	30% 10% 17% 27% 15% 1	784
	21st to end	Night	36% 10% 17% 24% 12% 2	660
March	1st to 10th	Night	29% 12% 16% 26% 15% 2	800
	11th to 20th	Night	28% 12% 20% 25% 13% 2	800
	21st to end	Night	31% 10% 20% 27% 10% 2	880
April	1st to 10th	Night	27% 9% 20% 28% 14% 2	800
	11th to 20th	Night	21% 11% 20% 30% 16% 2	761
	21st to end	Night	25% 9% 19% 31% 14% 2	555
May	1st to 10th	Night	18% 9% 18% 34% 21% 1	400
	11th to 20th	Night	18% 5% 17% 30% 25% 4	394
	21st to end	Night	20% 4 15% 31% 25% 4	433
June	1st to 10th	Night	17% 5% 19% 33% 24% 3	400
	11th to 20th	Night	20% 10% 22% 27% 21% 2	400
	21st to end	Night	12% 6% 15% 34% 31% 3	400
July	1st to 10th	Night	11% 4 11% 31% 39% 4	390
	11th to 20th	Night	8% 2 14% 39% 34% 4	388
	21st to end	Night	7% 3 10% 32% 45% 4	438
August	1st to 10th	Night	9% 10% 29% 43% 9%	443
	11th to 20th	Night	11% 1 7% 27% 44% 9%	594
	21st to end	Night	13% 3 12% 31% 37% 4	777
September	1st to 10th	Night	10% 3 14% 34% 33% 6%	800
	11th to 20th	Night	18% 4 12% 25% 35% 7%	800
	21st to end	Night	20% 6% 15% 32% 25% 4	800
October	1st to 10th	Night	21% 4 17% 29% 25% 5%	800
	11th to 20th	Night	27% 8% 15% 26% 20% 4	800
	21st to end	Night	31% 7% 19% 25% 15% 2	880
November	1st to 10th	Night	36% 9% 18% 27% 10%	788
	11th to 20th	Night	36% 10% 19% 21% 13% 2	800
	21st to end	Night	34% 9% 18% 21% 14% 4	800
December	1st to 10th	Night	32% 5% 17% 26% 18% 2	800
	11th to 20th	Night	38% 11% 19% 21% 10% 1	800
	21st to end	Night	41% 12% 18% 19% 9% 1	960

Page generated at 2025-10-30 23:02:43. Version ac07

Copyright © 2025 Attilla Danko.