Steilacoom
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Steilacoom 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	17% 9% 3 8% 5% 5% 4 4 5% 6% 33%	131714
	Night	19% 9% 3 8% 5% 4 4 3 5% 6% 34%	127642
Transparency	Day	29% 40% 10% 13% 6% 1	131430
	Night	32% 41% 9% 12% 5% 1	127503
Seeing	Day	20% 10% 20% 25% 18% 6%	16455
	Night	24% 9% 18% 27% 19% 2	24815

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% 11% 5% 12% 7% 6% 5% 3 4 5% 8%	3337
	11th to 20th	Day	20% 11% 5% 12% 5% 4 5% 4 5% 6% 23%	3616
	21st to end	Day	29% 12% 3 8% 5% 4 4 3 5% 5% 23%	4205
February	1st to 10th	Day	29% 13% 3 10% 5% 4 5% 4 7% 6% 15%	1800
	11th to 20th	Day	30% 14% 3 8% 5% 5% 5% 3 4 5% 18%	1755
	21st to end	Day	28% 14% 4 9% 6% 4 4 3 5% 5% 19%	1547
March	1st to 10th	Day	17% 12% 4 11% 8% 7% 6% 5% 7% 6% 17%	4667
	11th to 20th	Day	19% 15% 4 9% 5% 5% 5% 3 5% 5% 26%	5000
	21st to end	Day	23% 14% 4 11% 7% 6% 6% 5% 5% 6% 12%	5648
April	1st to 10th	Day	26% 11% 3 9% 7% 5% 5% 5% 6% 6% 18%	2400
	11th to 20th	Day	22% 10% 3 8% 5% 5% 5% 4 6% 6% 27%	2401
	21st to end	Day	19% 11% 4 9% 6% 5% 5% 4 6% 7% 26%	2620
May	1st to 10th	Day	14% 7% 2 7% 4 4 5% 4 6% 6% 41%	6320
	11th to 20th	Day	14% 10% 3 9% 6% 5% 6% 4 6% 7% 30%	6277
	21st to end	Day	16% 8% 3 8% 5% 5% 5% 4 7% 9% 29%	6903
June	1st to 10th	Day	19% 9% 3 9% 5% 4 5% 4 5% 8% 31%	2820
	11th to 20th	Day	18% 9% 3 10% 7% 5% 6% 4 6% 7% 25%	3000
	21st to end	Day	12% 8% 3 7% 5% 5% 4 3 6% 7% 41%	3000
July	1st to 10th	Day	7% 4 2 4 3 2 2 2 4 5% 66%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 71%	6422
	21st to end	Day	7% 4 2 5% 3 3 3 2 4 7% 59%	6874
August	1st to 10th	Day	5% 4 2 4 3 3 3 3 5% 7% 62%	4400
	11th to 20th	Day	9% 5% 2 8% 4 5% 5% 4 7% 9% 42%	4156
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 6% 44%	4477
September	1st to 10th	Day	9% 7% 2 8% 4 4 4 3 4 6% 49%	2400
	11th to 20th	Day	12% 8% 3 8% 5% 5% 5% 4 5% 7% 39%	2400
	21st to end	Day	16% 7% 2 7% 4 3 5% 3 5% 8% 38%	2109
October	1st to 10th	Day	15% 7% 3 6% 5% 4 4 5% 7% 10% 35%	3280
	11th to 20th	Day	23% 10% 4 8% 5% 4 4 3 6% 7% 26%	3261
	21st to end	Day	24% 9% 3 9% 6% 5% 6% 4 5% 6% 22%	3397
November	1st to 10th	Day	29% 12% 4 8% 5% 5% 5% 4 7% 6% 16%	1693
	11th to 20th	Day	30% 12% 3 8% 5% 5% 5% 4 4 5% 18%	1680
	21st to end	Day	29% 11% 4 8% 6% 4 4 3 5% 7% 19%	1680
December	1st to 10th	Day	21% 8% 3 9% 9% 6% 5% 5% 6% 7% 22%	2636
	11th to 20th	Day	32% 12% 4 10% 8% 7% 5% 4 5% 5% 9%	3025
	21st to end	Day	30% 14% 5% 11% 7% 5% 4 4 4 5% 10%	3728
January	1st to 10th	Night	33% 15% 6% 11% 6% 5% 4 3 3 4 10%	6584
	11th to 20th	Night	20% 8% 4 10% 5% 4 4 3 6% 7% 30%	6282
	21st to end	Night	27% 10% 3 8% 4 4 4 3 5% 5% 28%	6759
February	1st to 10th	Night	28% 13% 4 8% 6% 5% 3 3 4 6% 20%	2600
	11th to 20th	Night	26% 11% 3 8% 5% 5% 3 4 4 5% 25%	2532
	21st to end	Night	26% 12% 3 7% 5% 4 5% 4 5% 5% 24%	2082
March	1st to 10th	Night	17% 12% 4 9% 6% 6% 7% 5% 6% 7% 21%	5211
	11th to 20th	Night	20% 10% 3 8% 4 4 5% 4 5% 6% 32%	4999
	21st to end	Night	26% 13% 4 11% 6% 6% 6% 4 5% 5% 14%	5326
April	1st to 10th	Night	21% 12% 4 8% 5% 5% 5% 4 5% 5% 25%	2000
	11th to 20th	Night	17% 8% 3 7% 5% 4 5% 5% 6% 7% 33%	1948
	21st to end	Night	18% 9% 3 7% 6% 4 6% 4 5% 7% 32%	1780
May	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 5% 45%	3640
	11th to 20th	Night	12% 7% 2 7% 5% 5% 6% 4 6% 7% 39%	3616
	21st to end	Night	8% 11% 3 8% 5% 5% 5% 4 7% 9% 36%	3972
June	1st to 10th	Night	14% 6% 2 5% 5% 5% 4 4 6% 7% 41%	1574
	11th to 20th	Night	13% 7% 3 6% 5% 5% 4 4 6% 8% 38%	1400
	21st to end	Night	8% 6% 3 6% 4 4 4 4 6% 5% 51%	1400
July	1st to 10th	Night	3 2 1 2 2 2 2 3 4 6% 72%	3180
	11th to 20th	Night	2 2 2 2 2 2 2 2 4 79%	3520
	21st to end	Night	4 3 1 4 2 2 2 3 5% 7% 68%	3969
August	1st to 10th	Night	3 2 1 3 3 3 3 2 3 5% 73%	2739
	11th to 20th	Night	7% 4 2 5% 3 4 4 3 5% 7% 56%	2902
	21st to end	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	3438
September	1st to 10th	Night	7% 3 2 4 3 4 3 3 4 7% 60%	2000
	11th to 20th	Night	14% 6% 2 6% 5% 4 4 3 4 6% 47%	2000
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 45%	2289
October	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	3920
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 5% 6% 33%	3897
	21st to end	Night	22% 9% 3 9% 6% 4 4 3 5% 6% 29%	4408
November	1st to 10th	Night	31% 8% 3 8% 5% 6% 4 4 5% 5% 22%	2822
	11th to 20th	Night	30% 11% 4 8% 5% 4 4 3 5% 5% 21%	2940
	21st to end	Night	26% 12% 3 7% 6% 5% 5% 4 5% 5% 22%	2940
December	1st to 10th	Night	22% 9% 3 8% 6% 5% 5% 4 5% 6% 26%	4588
	11th to 20th	Night	31% 15% 5% 11% 7% 5% 5% 4 4 4 12%	5180
	21st to end	Night	30% 13% 5% 11% 6% 5% 4 4 4 4 14%	7205

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	56% 33% 6% 3 2	3337
	11th to 20th	Day	42% 32% 9% 13% 3	3640
	21st to end	Day	45% 31% 6% 11% 6%	4205
February	1st to 10th	Day	27% 63% 4 3 2 1	1800
	11th to 20th	Day	31% 62% 2 2 2	1764
	21st to end	Day	24% 64% 4 6% 3	1547
March	1st to 10th	Day	40% 35% 13% 10% 2	4667
	11th to 20th	Day	39% 32% 10% 17% 2	5000
	21st to end	Day	48% 34% 11% 6% 1	5648
April	1st to 10th	Day	24% 65% 5% 6%	2400
	11th to 20th	Day	18% 65% 6% 8% 3	2401
	21st to end	Day	20% 64% 6% 8% 2	2620
May	1st to 10th	Day	26% 32% 12% 20% 10% 1	6320
	11th to 20th	Day	34% 34% 16% 15% 1	6280
	21st to end	Day	29% 36% 18% 14% 3	6904
June	1st to 10th	Day	14% 64% 9% 9% 3	2820
	11th to 20th	Day	19% 64% 7% 8% 3	3000
	21st to end	Day	16% 63% 8% 10% 3	3000
July	1st to 10th	Day	14% 26% 13% 19% 16% 12%	6706
	11th to 20th	Day	10% 26% 9% 24% 26% 4	6344
	21st to end	Day	20% 26% 17% 28% 8% 1	6681
August	1st to 10th	Day	13% 36% 6% 26% 17% 2	4383
	11th to 20th	Day	26% 40% 14% 15% 4 1	4195
	21st to end	Day	30% 37% 7% 18% 9% 1	4476
September	1st to 10th	Day	18% 59% 9% 9% 4 1	2400
	11th to 20th	Day	20% 60% 5% 7% 7%	2400
	21st to end	Day	19% 59% 8% 10% 4	2109
October	1st to 10th	Day	25% 39% 11% 22% 3	3280
	11th to 20th	Day	37% 37% 9% 9% 7%	3261
	21st to end	Day	40% 37% 8% 8% 5% 1	3397
November	1st to 10th	Day	32% 55% 7% 5% 1	1693
	11th to 20th	Day	29% 56% 5% 6% 3 1	1680
	21st to end	Day	26% 57% 5% 6% 5%	1680
December	1st to 10th	Day	39% 39% 4 11% 6%	2636
	11th to 20th	Day	50% 38% 6% 6% 1	3028
	21st to end	Day	55% 32% 9% 4 1	3728
January	1st to 10th	Night	59% 31% 4 4 1 1	6584
	11th to 20th	Night	35% 31% 10% 19% 4	6320
	21st to end	Night	44% 32% 6% 13% 5%	6759
February	1st to 10th	Night	26% 63% 5% 3 3	2600
	11th to 20th	Night	27% 63% 3 4 2 1	2534
	21st to end	Night	21% 64% 6% 6% 2	2082
March	1st to 10th	Night	38% 35% 13% 10% 4	5211
	11th to 20th	Night	34% 32% 14% 19% 1 1	5000
	21st to end	Night	48% 36% 8% 7%	5326
April	1st to 10th	Night	21% 66% 6% 6%	2000
	11th to 20th	Night	19% 65% 6% 7% 3	1948
	21st to end	Night	19% 64% 6% 9% 2	1780
May	1st to 10th	Night	28% 31% 8% 25% 6% 1	3637
	11th to 20th	Night	29% 36% 17% 15% 2	3616
	21st to end	Night	31% 37% 18% 13% 1	3972
June	1st to 10th	Night	13% 63% 9% 12% 3	1574
	11th to 20th	Night	14% 64% 9% 11% 2	1400
	21st to end	Night	13% 62% 10% 12% 3	1400
July	1st to 10th	Night	13% 27% 15% 19% 13% 12%	3145
	11th to 20th	Night	7% 26% 11% 22% 31% 3	3475
	21st to end	Night	15% 27% 19% 27% 11% 1	3857
August	1st to 10th	Night	9% 36% 6% 28% 19% 2	2731
	11th to 20th	Night	22% 40% 20% 12% 4 1	2929
	21st to end	Night	25% 37% 10% 19% 8% 1	3438
September	1st to 10th	Night	14% 59% 10% 10% 6%	2000
	11th to 20th	Night	20% 59% 6% 9% 6% 1	2000
	21st to end	Night	18% 59% 7% 12% 4	2289
October	1st to 10th	Night	23% 37% 16% 22% 2	3920
	11th to 20th	Night	36% 37% 10% 14% 3	3897
	21st to end	Night	39% 37% 8% 11% 5% 1	4408
November	1st to 10th	Night	30% 56% 6% 7% 1	2822
	11th to 20th	Night	30% 56% 5% 5% 3 2	2940
	21st to end	Night	27% 56% 5% 5% 5% 1	2940
December	1st to 10th	Night	40% 38% 5% 13% 4	4588
	11th to 20th	Night	52% 37% 5% 4 1	5176
	21st to end	Night	52% 33% 8% 6% 1	7205

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	33% 16% 18% 18% 11% 5%	200
	11th to 20th	Day	33% 11% 21% 18% 15% 5%	200
	21st to end	Day	30% 13% 22% 17% 14% 4	304
February	1st to 10th	Day	33% 16% 16% 20% 11% 4	400
	11th to 20th	Day	36% 15% 19% 16% 10% 2	395
	21st to end	Day	31% 18% 21% 20% 9% 2	363
March	1st to 10th	Day	28% 14% 23% 23% 10% 2	500
	11th to 20th	Day	26% 16% 18% 22% 13% 5%	500
	21st to end	Day	27% 17% 23% 21% 9% 3	550
April	1st to 10th	Day	28% 16% 19% 23% 10% 4	500
	11th to 20th	Day	22% 14% 29% 21% 8% 6%	492
	21st to end	Day	20% 13% 26% 24% 14% 3	500
May	1st to 10th	Day	17% 10% 24% 28% 17% 4	503
	11th to 20th	Day	22% 9% 21% 23% 18% 7%	528
	21st to end	Day	18% 9% 23% 26% 17% 7%	573
June	1st to 10th	Day	19% 8% 23% 24% 19% 7%	550
	11th to 20th	Day	17% 12% 24% 28% 15% 4	730
	21st to end	Day	10% 9% 21% 30% 23% 7%	730
July	1st to 10th	Day	9% 3 19% 32% 29% 7%	721
	11th to 20th	Day	6% 4 16% 35% 30% 9%	571
	21st to end	Day	6% 3 10% 31% 38% 12%	575
August	1st to 10th	Day	6% 5% 15% 31% 33% 12%	527
	11th to 20th	Day	7% 3 13% 31% 32% 14%	517
	21st to end	Day	12% 8% 17% 28% 26% 9%	583
September	1st to 10th	Day	10% 6% 22% 30% 25% 7%	530
	11th to 20th	Day	14% 7% 19% 26% 24% 9%	530
	21st to end	Day	17% 9% 21% 29% 20% 4	530
October	1st to 10th	Day	21% 9% 21% 22% 18% 8%	530
	11th to 20th	Day	27% 11% 16% 23% 18% 6%	530
	21st to end	Day	31% 15% 21% 16% 12% 5%	484
November	1st to 10th	Day	34% 15% 22% 16% 11% 2	245
	11th to 20th	Day	37% 12% 20% 15% 10% 6%	220
	21st to end	Day	30% 12% 24% 17% 9% 9%	220
December	1st to 10th	Day	38% 9% 17% 14% 14% 8%	204
	11th to 20th	Day	39% 15% 18% 18% 9% 2	200
	21st to end	Day	36% 13% 21% 17% 11% 2	220
January	1st to 10th	Night	40% 13% 14% 17% 13% 2	860
	11th to 20th	Night	34% 10% 14% 23% 18% 2	800
	21st to end	Night	29% 9% 18% 25% 16% 2	880
February	1st to 10th	Night	33% 13% 19% 22% 12% 2	800
	11th to 20th	Night	30% 11% 20% 23% 14% 2	784
	21st to end	Night	32% 13% 21% 22% 11% 2	660
March	1st to 10th	Night	26% 14% 20% 25% 14% 2	800
	11th to 20th	Night	28% 13% 18% 26% 14% 2	800
	21st to end	Night	29% 11% 23% 25% 10% 1	880
April	1st to 10th	Night	24% 10% 23% 27% 15% 2	800
	11th to 20th	Night	18% 11% 26% 29% 14% 2	760
	21st to end	Night	21% 9% 21% 33% 14% 2	580
May	1st to 10th	Night	15% 10% 21% 37% 17% 1	400
	11th to 20th	Night	16% 6% 21% 30% 24% 3	394
	21st to end	Night	11% 8% 20% 35% 23% 3	433
June	1st to 10th	Night	13% 7% 25% 32% 23%	400
	11th to 20th	Night	13% 9% 26% 34% 18% 1	400
	21st to end	Night	10% 5% 21% 37% 26% 2	400
July	1st to 10th	Night	7% 3 16% 39% 32% 3	390
	11th to 20th	Night	5% 2 15% 44% 32% 3	388
	21st to end	Night	4 2 12% 42% 38% 3	438
August	1st to 10th	Night	5% 2 13% 34% 39% 7%	476
	11th to 20th	Night	7% 1 10% 31% 47% 5%	594
	21st to end	Night	10% 4 15% 34% 33% 4	775
September	1st to 10th	Night	10% 3 15% 35% 32% 5%	800
	11th to 20th	Night	15% 3 15% 26% 35% 6%	800
	21st to end	Night	17% 6% 18% 32% 24% 3	800
October	1st to 10th	Night	20% 5% 17% 29% 26% 4	800
	11th to 20th	Night	26% 9% 15% 25% 20% 5%	800
	21st to end	Night	27% 9% 20% 26% 16% 2	880
November	1st to 10th	Night	33% 10% 19% 26% 12%	820
	11th to 20th	Night	33% 12% 20% 22% 12% 1	840
	21st to end	Night	31% 10% 21% 21% 15% 2	840
December	1st to 10th	Night	29% 8% 18% 26% 17% 2	808
	11th to 20th	Night	36% 11% 19% 22% 12% 1	800
	21st to end	Night	41% 13% 18% 19% 9% 1	935

Page generated at 2025-11-29 22:00:51. Version ac07

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