Suiattle Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Suiattle Valley clear sky chart from 2005-11-06 to 2025-12-03.

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	23% 14% 3 6% 4 4 4 3 4 5% 31%	131506
	Night	25% 17% 3 6% 4 3 3 2 4 5% 28%	127856
Transparency	Day	57% 5% 12% 16% 9% 1	131223
	Night	60% 5% 11% 15% 8% 1	127716
Seeing	Day	32% 14% 27% 20% 6% 1	16321
	Night	36% 10% 26% 22% 6%	24694

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	34% 26% 4 7% 3 3 2 2 3 3 11%	3314
	11th to 20th	Day	19% 14% 5% 7% 4 2 4 3 4 6% 32%	3616
	21st to end	Day	33% 13% 2 5% 2 3 3 3 4 3 29%	4004
February	1st to 10th	Day	29% 18% 5% 7% 3 4 4 3 4 5% 17%	1780
	11th to 20th	Day	26% 24% 4 7% 3 2 2 2 4 5% 20%	1755
	21st to end	Day	21% 28% 6% 8% 4 3 4 3 3 3 17%	1578
March	1st to 10th	Day	20% 23% 4 8% 4 5% 4 3 3 4 21%	4518
	11th to 20th	Day	15% 28% 5% 9% 5% 4 4 2 3 2 24%	4992
	21st to end	Day	26% 22% 4 10% 5% 5% 4 3 4 5% 13%	5747
April	1st to 10th	Day	24% 27% 4 6% 4 3 4 3 4 4 15%	2400
	11th to 20th	Day	22% 23% 3 7% 5% 4 3 2 5% 5% 20%	2372
	21st to end	Day	28% 21% 3 7% 5% 4 3 3 3 4 18%	2701
May	1st to 10th	Day	18% 16% 4 7% 4 5% 4 3 4 5% 32%	6320
	11th to 20th	Day	27% 18% 3 8% 5% 4 4 3 3 5% 20%	6277
	21st to end	Day	29% 12% 3 7% 5% 4 4 3 5% 6% 22%	6903
June	1st to 10th	Day	32% 10% 3 6% 5% 4 3 3 5% 6% 24%	2835
	11th to 20th	Day	37% 13% 3 8% 5% 4 5% 3 4 4 15%	3000
	21st to end	Day	21% 10% 2 7% 5% 4 4 3 6% 6% 31%	3000
July	1st to 10th	Day	11% 4 1 3 2 3 3 3 5% 6% 59%	6780
	11th to 20th	Day	7% 4 1 3 3 3 3 2 4 5% 65%	6440
	21st to end	Day	12% 6% 1 4 3 3 3 3 5% 7% 53%	6874
August	1st to 10th	Day	9% 4 1 3 2 3 3 3 4 7% 60%	4541
	11th to 20th	Day	18% 9% 3 7% 3 4 4 3 5% 7% 37%	4200
	21st to end	Day	19% 9% 3 6% 4 3 4 3 5% 5% 39%	4426
September	1st to 10th	Day	15% 7% 2 6% 3 4 4 3 3 6% 47%	2400
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 4 6% 39%	2400
	21st to end	Day	26% 7% 2 5% 4 3 3 3 4 5% 38%	2130
October	1st to 10th	Day	21% 8% 2 6% 3 4 4 4 7% 8% 34%	3280
	11th to 20th	Day	35% 11% 3 6% 4 3 2 2 3 4 26%	3261
	21st to end	Day	32% 15% 3 8% 4 4 4 3 4 5% 17%	3268
November	1st to 10th	Day	38% 15% 3 6% 3 4 3 3 4 4 17%	1714
	11th to 20th	Day	38% 20% 3 6% 3 2 2 2 3 2 19%	1680
	21st to end	Day	34% 18% 4 5% 4 3 3 3 3 5% 19%	1680
December	1st to 10th	Day	23% 14% 3 5% 4 3 4 3 4 5% 32%	2636
	11th to 20th	Day	32% 24% 5% 7% 4 3 3 2 3 3 14%	3025
	21st to end	Day	31% 24% 4 8% 5% 4 3 2 3 3 11%	3659
January	1st to 10th	Night	29% 26% 5% 7% 4 3 3 2 3 3 14%	6611
	11th to 20th	Night	22% 15% 4 6% 2 3 3 3 4 8% 31%	6282
	21st to end	Night	29% 14% 2 6% 3 3 3 3 3 4 30%	6952
February	1st to 10th	Night	28% 22% 4 8% 4 4 3 3 4 4 16%	2619
	11th to 20th	Night	24% 26% 4 7% 4 3 2 2 4 3 21%	2532
	21st to end	Night	23% 27% 4 7% 4 3 3 2 3 4 19%	2051
March	1st to 10th	Night	17% 25% 4 9% 4 4 3 3 4 5% 22%	5376
	11th to 20th	Night	20% 27% 4 6% 3 2 2 2 3 5% 26%	5004
	21st to end	Night	32% 24% 4 7% 4 3 3 2 3 4 14%	5227
April	1st to 10th	Night	24% 26% 4 6% 5% 4 3 3 4 5% 17%	2000
	11th to 20th	Night	26% 20% 3 6% 5% 3 3 3 3 5% 23%	1979
	21st to end	Night	33% 16% 3 7% 5% 3 3 2 3 5% 20%	1695
May	1st to 10th	Night	17% 19% 3 6% 3 4 3 2 3 4 37%	3640
	11th to 20th	Night	28% 18% 3 7% 4 3 3 3 4 5% 23%	3616
	21st to end	Night	31% 13% 3 6% 4 4 4 3 4 7% 21%	3972
June	1st to 10th	Night	29% 9% 2 7% 4 4 4 4 4 6% 27%	1560
	11th to 20th	Night	31% 13% 4 7% 3 4 3 3 5% 6% 22%	1400
	21st to end	Night	22% 8% 2 7% 4 4 3 3 5% 6% 35%	1400
July	1st to 10th	Night	13% 5% 1 4 2 2 2 2 3 5% 60%	3180
	11th to 20th	Night	7% 3 1 3 2 2 3 2 3 5% 70%	3502
	21st to end	Night	11% 6% 2 4 3 3 3 2 4 6% 57%	3969
August	1st to 10th	Night	8% 3 1 4 3 2 3 3 4 5% 64%	2609
	11th to 20th	Night	19% 11% 2 5% 3 3 2 3 4 6% 43%	2853
	21st to end	Night	21% 8% 2 5% 2 3 3 3 4 5% 44%	3490
September	1st to 10th	Night	15% 7% 3 5% 3 4 3 2 4 5% 49%	2000
	11th to 20th	Night	24% 10% 3 7% 4 3 2 2 4 5% 38%	2000
	21st to end	Night	24% 9% 3 6% 4 3 4 3 4 5% 35%	2265
October	1st to 10th	Night	21% 8% 2 5% 4 4 4 4 6% 7% 34%	3920
	11th to 20th	Night	35% 10% 2 5% 3 2 3 3 4 5% 29%	3897
	21st to end	Night	33% 16% 3 7% 4 3 3 3 3 4 22%	4533
November	1st to 10th	Night	38% 16% 3 6% 4 4 3 3 4 4 15%	2800
	11th to 20th	Night	37% 19% 2 6% 3 3 3 3 3 4 18%	2940
	21st to end	Night	34% 19% 3 6% 3 4 3 2 3 4 19%	2940
December	1st to 10th	Night	21% 15% 3 6% 3 3 4 3 4 5% 32%	4588
	11th to 20th	Night	32% 22% 4 7% 4 3 3 2 3 4 14%	5180
	21st to end	Night	32% 25% 4 9% 5% 4 3 2 3 4 10%	7274

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	80% 3 5% 9% 2 1	3314
	11th to 20th	Day	55% 4 11% 18% 10% 2	3640
	21st to end	Day	62% 3 6% 16% 13%	4004
February	1st to 10th	Day	71% 5% 8% 9% 6% 1	1780
	11th to 20th	Day	70% 5% 7% 8% 8% 1	1764
	21st to end	Day	73% 6% 6% 11% 4 1	1578
March	1st to 10th	Day	68% 4 7% 11% 10% 1	4518
	11th to 20th	Day	69% 3 8% 18% 2	4992
	21st to end	Day	76% 5% 9% 8% 3	5747
April	1st to 10th	Day	75% 7% 10% 8% 1	2400
	11th to 20th	Day	68% 6% 8% 10% 6% 1	2372
	21st to end	Day	73% 5% 8% 9% 4 1	2701
May	1st to 10th	Day	56% 6% 13% 20% 5%	6320
	11th to 20th	Day	68% 7% 14% 10% 1	6280
	21st to end	Day	63% 8% 17% 11% 1	6904
June	1st to 10th	Day	64% 6% 13% 11% 5% 1	2835
	11th to 20th	Day	74% 7% 8% 10% 2	3000
	21st to end	Day	54% 8% 13% 19% 6% 1	3000
July	1st to 10th	Day	28% 5% 20% 21% 19% 9%	6706
	11th to 20th	Day	23% 5% 16% 26% 28% 3	6362
	21st to end	Day	30% 5% 22% 31% 11% 2	6681
August	1st to 10th	Day	26% 4 11% 29% 28% 2	4524
	11th to 20th	Day	47% 6% 16% 20% 8% 2	4240
	21st to end	Day	48% 5% 11% 24% 11% 1	4425
September	1st to 10th	Day	40% 4 13% 22% 15% 6%	2400
	11th to 20th	Day	48% 6% 9% 18% 15% 4	2400
	21st to end	Day	50% 5% 15% 16% 11% 3	2130
October	1st to 10th	Day	47% 7% 15% 24% 7%	3280
	11th to 20th	Day	64% 4 9% 13% 10% 1	3261
	21st to end	Day	68% 5% 9% 9% 7%	3268
November	1st to 10th	Day	72% 7% 8% 9% 4	1714
	11th to 20th	Day	75% 3 5% 8% 7% 2	1680
	21st to end	Day	70% 5% 5% 10% 8% 2	1680
December	1st to 10th	Day	57% 4 8% 18% 12% 1	2636
	11th to 20th	Day	77% 4 8% 9% 2 1	3028
	21st to end	Day	79% 4 7% 7% 3	3659
January	1st to 10th	Night	78% 3 4 10% 4 1	6611
	11th to 20th	Night	53% 5% 11% 17% 12% 1	6320
	21st to end	Night	61% 3 9% 15% 11% 1	6952
February	1st to 10th	Night	73% 4 6% 9% 6% 1	2619
	11th to 20th	Night	69% 5% 7% 10% 6% 3	2534
	21st to end	Night	72% 4 8% 11% 4 1	2051
March	1st to 10th	Night	64% 6% 9% 9% 10% 1	5376
	11th to 20th	Night	64% 4 11% 17% 3	5005
	21st to end	Night	77% 6% 10% 7% 1	5227
April	1st to 10th	Night	73% 6% 12% 8% 1	2000
	11th to 20th	Night	65% 6% 11% 10% 7% 1	1979
	21st to end	Night	70% 5% 8% 11% 5%	1695
May	1st to 10th	Night	53% 8% 12% 21% 7%	3637
	11th to 20th	Night	62% 11% 13% 12% 1	3616
	21st to end	Night	64% 10% 15% 11%	3972
June	1st to 10th	Night	59% 9% 13% 12% 6%	1560
	11th to 20th	Night	64% 8% 13% 12% 3	1400
	21st to end	Night	51% 9% 16% 17% 6% 1	1400
July	1st to 10th	Night	30% 5% 18% 21% 21% 6%	3145
	11th to 20th	Night	18% 5% 19% 24% 32% 2	3457
	21st to end	Night	30% 5% 21% 28% 12% 3	3857
August	1st to 10th	Night	23% 5% 14% 33% 23% 2	2601
	11th to 20th	Night	43% 7% 20% 18% 10% 2	2879
	21st to end	Night	44% 5% 13% 26% 11% 2	3490
September	1st to 10th	Night	39% 6% 13% 22% 16% 4	2000
	11th to 20th	Night	51% 5% 8% 18% 15% 4	2000
	21st to end	Night	53% 6% 11% 17% 10% 3	2265
October	1st to 10th	Night	48% 5% 13% 27% 6%	3920
	11th to 20th	Night	60% 4 10% 15% 11% 1	3897
	21st to end	Night	68% 4 8% 11% 9% 1	4533
November	1st to 10th	Night	74% 4 9% 8% 4 1	2800
	11th to 20th	Night	73% 4 8% 8% 6% 2	2940
	21st to end	Night	72% 4 5% 9% 8% 2	2940
December	1st to 10th	Night	56% 5% 9% 17% 11% 2	4588
	11th to 20th	Night	76% 4 6% 11% 3	5176
	21st to end	Night	80% 4 7% 6% 3	7274

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	48% 14% 21% 15% 4	200
	11th to 20th	Day	38% 15% 28% 15% 6%	200
	21st to end	Day	42% 13% 21% 17% 6% 1	220
February	1st to 10th	Day	42% 17% 24% 13% 3	380
	11th to 20th	Day	45% 14% 24% 15% 2	395
	21st to end	Day	37% 19% 25% 16% 3	379
March	1st to 10th	Day	36% 20% 26% 15% 3	500
	11th to 20th	Day	37% 16% 23% 19% 4 1	500
	21st to end	Day	43% 18% 23% 14% 2	550
April	1st to 10th	Day	43% 16% 21% 15% 5%	500
	11th to 20th	Day	36% 20% 23% 17% 4 1	492
	21st to end	Day	43% 13% 22% 15% 4 1	500
May	1st to 10th	Day	29% 18% 30% 18% 4	503
	11th to 20th	Day	36% 12% 24% 21% 7% 1	528
	21st to end	Day	38% 15% 23% 17% 5% 1	573
June	1st to 10th	Day	32% 17% 23% 20% 7% 1	565
	11th to 20th	Day	38% 17% 27% 13% 5%	730
	21st to end	Day	24% 13% 29% 27% 7% 1	730
July	1st to 10th	Day	17% 13% 35% 28% 7% 1	721
	11th to 20th	Day	15% 13% 34% 28% 8% 3	576
	21st to end	Day	13% 8% 35% 29% 11% 4	575
August	1st to 10th	Day	17% 7% 30% 30% 12% 3	527
	11th to 20th	Day	14% 11% 32% 28% 13% 3	517
	21st to end	Day	18% 13% 31% 26% 10% 2	583
September	1st to 10th	Day	19% 15% 29% 26% 9% 2	530
	11th to 20th	Day	22% 14% 29% 26% 8% 1	530
	21st to end	Day	30% 13% 29% 19% 9%	530
October	1st to 10th	Day	30% 14% 25% 21% 9% 2	530
	11th to 20th	Day	41% 11% 23% 17% 8% 1	530
	21st to end	Day	42% 11% 28% 14% 5%	407
November	1st to 10th	Day	40% 13% 29% 15% 3	256
	11th to 20th	Day	49% 15% 14% 15% 7%	220
	21st to end	Day	45% 7% 27% 13% 7% 1	220
December	1st to 10th	Day	44% 9% 15% 17% 14%	204
	11th to 20th	Day	47% 13% 25% 13% 4	200
	21st to end	Day	46% 13% 22% 16% 3	220
January	1st to 10th	Night	51% 11% 26% 11%	867
	11th to 20th	Night	42% 11% 31% 15% 1	800
	21st to end	Night	40% 10% 30% 17% 2	880
February	1st to 10th	Night	41% 12% 33% 13% 1	800
	11th to 20th	Night	38% 12% 33% 16% 1	784
	21st to end	Night	42% 11% 30% 15% 2	660
March	1st to 10th	Night	35% 12% 30% 20% 3	800
	11th to 20th	Night	42% 12% 26% 17% 3	800
	21st to end	Night	42% 13% 29% 13% 2	880
April	1st to 10th	Night	39% 12% 25% 21% 3	800
	11th to 20th	Night	37% 12% 28% 20% 3	791
	21st to end	Night	42% 9% 28% 17% 4	495
May	1st to 10th	Night	28% 11% 32% 24% 5%	400
	11th to 20th	Night	31% 8% 24% 30% 7%	394
	21st to end	Night	35% 7% 23% 28% 6%	433
June	1st to 10th	Night	32% 8% 27% 25% 9%	400
	11th to 20th	Night	36% 10% 24% 22% 9%	400
	21st to end	Night	25% 5% 28% 34% 8% 1	400
July	1st to 10th	Night	19% 8% 18% 35% 19%	390
	11th to 20th	Night	14% 5% 19% 41% 21%	388
	21st to end	Night	10% 6% 15% 40% 28%	438
August	1st to 10th	Night	14% 4 13% 39% 29% 1	398
	11th to 20th	Night	17% 2 13% 45% 23% 1	557
	21st to end	Night	19% 6% 21% 40% 14%	801
September	1st to 10th	Night	19% 4 28% 34% 15% 1	800
	11th to 20th	Night	28% 7% 18% 32% 15%	800
	21st to end	Night	31% 6% 24% 33% 7%	800
October	1st to 10th	Night	32% 7% 26% 29% 7%	800
	11th to 20th	Night	39% 6% 27% 23% 4	800
	21st to end	Night	40% 11% 31% 16% 1	880
November	1st to 10th	Night	49% 11% 28% 11% 1	820
	11th to 20th	Night	52% 12% 22% 14% 1	840
	21st to end	Night	46% 12% 27% 14% 2	840
December	1st to 10th	Night	35% 13% 33% 17% 2	808
	11th to 20th	Night	48% 14% 28% 9% 1	800
	21st to end	Night	51% 14% 26% 9% 1	950

Page generated at 2025-11-29 22:00:52. Version ac07

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