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 2024-11-10.

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	19% 9% 3 8% 5% 4 4 4 5% 6% 33%	105860
	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	101846
Transparency	Day	25% 49% 8% 11% 6% 1	105618
	Night	27% 50% 8% 10% 4 1	101734
Seeing	Day	20% 11% 20% 24% 18% 6%	15123
	Night	24% 9% 18% 27% 20% 2	23481

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	38% 11% 4 9% 6% 5% 5% 3 4 5% 10%	2321
	11th to 20th	Day	28% 11% 4 9% 4 3 4 3 4 5% 22%	2516
	21st to end	Day	38% 14% 4 10% 6% 4 4 3 4 4 9%	2925
February	1st to 10th	Day	30% 12% 4 10% 5% 4 5% 4 6% 6% 14%	1710
	11th to 20th	Day	31% 13% 3 8% 5% 5% 5% 3 4 5% 18%	1665
	21st to end	Day	27% 14% 4 9% 6% 4 4 4 4 5% 19%	1472
March	1st to 10th	Day	17% 12% 4 12% 8% 7% 6% 5% 6% 6% 18%	3248
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 35%	3490
	21st to end	Day	21% 14% 4 11% 6% 5% 6% 5% 5% 7% 16%	3935
April	1st to 10th	Day	26% 11% 3 9% 7% 5% 5% 5% 5% 5% 18%	2280
	11th to 20th	Day	23% 11% 3 8% 5% 5% 5% 4 5% 6% 25%	2279
	21st to end	Day	19% 11% 4 9% 6% 5% 5% 4 6% 7% 24%	2489
May	1st to 10th	Day	18% 8% 2 7% 5% 4 4 4 5% 6% 38%	4420
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 4 7% 8% 39%	4377
	21st to end	Day	21% 11% 3 9% 6% 5% 6% 4 6% 8% 21%	4813
June	1st to 10th	Day	20% 9% 3 9% 5% 4 5% 4 6% 8% 28%	2679
	11th to 20th	Day	19% 9% 3 10% 7% 5% 6% 4 6% 7% 24%	2850
	21st to end	Day	12% 8% 3 7% 4 4 4 3 6% 7% 41%	2850
July	1st to 10th	Day	7% 4 2 4 3 2 2 2 4 5% 66%	4740
	11th to 20th	Day	6% 3 1 3 3 2 2 2 4 5% 68%	4494
	21st to end	Day	9% 5% 2 5% 3 3 4 2 4 6% 56%	4790
August	1st to 10th	Day	5% 4 2 4 3 3 3 3 4 7% 63%	4264
	11th to 20th	Day	9% 5% 2 8% 4 5% 5% 4 7% 9% 43%	4026
	21st to end	Day	15% 7% 3 6% 4 4 3 3 5% 6% 44%	4340
September	1st to 10th	Day	9% 6% 2 7% 4 3 4 3 4 6% 50%	2280
	11th to 20th	Day	12% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	2280
	21st to end	Day	17% 7% 2 8% 4 3 5% 4 5% 8% 37%	2004
October	1st to 10th	Day	15% 7% 3 6% 5% 4 4 4 7% 10% 35%	3180
	11th to 20th	Day	23% 10% 4 8% 5% 4 4 3 6% 7% 26%	3161
	21st to end	Day	24% 10% 3 9% 6% 5% 6% 4 5% 6% 23%	3292
November	1st to 10th	Day	30% 12% 4 8% 5% 4 5% 3 6% 6% 16%	1603
	11th to 20th	Day	31% 12% 3 8% 5% 4 5% 4 4 5% 20%	1520
	21st to end	Day	29% 12% 3 8% 6% 4 4 3 5% 7% 20%	1520
December	1st to 10th	Day	28% 9% 3 8% 7% 4 4 3 5% 5% 23%	1520
	11th to 20th	Day	30% 13% 3 10% 8% 6% 4 4 5% 4 10%	1925
	21st to end	Day	26% 14% 5% 11% 7% 6% 4 4 5% 6% 12%	2602
January	1st to 10th	Night	33% 14% 4 10% 6% 6% 4 4 3 4 12%	4600
	11th to 20th	Night	28% 10% 4 8% 4 4 4 3 5% 7% 23%	4382
	21st to end	Night	36% 13% 4 9% 5% 5% 4 3 4 5% 13%	4734
February	1st to 10th	Night	29% 12% 4 8% 6% 5% 4 3 4 6% 19%	2470
	11th to 20th	Night	26% 11% 3 8% 5% 5% 3 4 4 5% 25%	2402
	21st to end	Night	26% 12% 3 7% 5% 4 5% 4 5% 5% 23%	1981
March	1st to 10th	Night	19% 13% 4 10% 7% 5% 6% 5% 5% 7% 18%	3623
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 41%	3489
	21st to end	Night	26% 14% 4 10% 6% 5% 5% 4 4 4 16%	3717
April	1st to 10th	Night	21% 12% 3 8% 5% 5% 6% 5% 5% 5% 24%	1900
	11th to 20th	Night	17% 9% 3 7% 5% 4 5% 5% 6% 7% 32%	1850
	21st to end	Night	18% 10% 3 7% 6% 4 6% 4 5% 7% 30%	1691
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 4 5% 41%	2540
	11th to 20th	Night	9% 5% 2 6% 4 5% 6% 4 6% 7% 46%	2516
	21st to end	Night	10% 12% 3 9% 5% 6% 6% 4 6% 7% 32%	2762
June	1st to 10th	Night	15% 6% 2 5% 4 5% 4 4 6% 7% 40%	1496
	11th to 20th	Night	14% 7% 3 7% 4 5% 4 4 6% 8% 37%	1330
	21st to end	Night	8% 6% 3 5% 4 3 4 4 6% 5% 52%	1330
July	1st to 10th	Night	4 2 1 2 2 3 2 3 3 5% 73%	2220
	11th to 20th	Night	3 2 1 2 3 2 2 2 2 5% 76%	2462
	21st to end	Night	4 4 2 4 2 2 2 2 4 6% 69%	2762
August	1st to 10th	Night	3 2 1 3 2 3 2 2 3 5% 74%	2655
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 7% 57%	2812
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	3333
September	1st to 10th	Night	7% 3 2 4 3 4 3 2 4 7% 60%	1900
	11th to 20th	Night	15% 6% 2 6% 4 4 3 3 4 6% 46%	1900
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 45%	2174
October	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	3800
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 5% 6% 32%	3777
	21st to end	Night	22% 9% 3 9% 5% 4 4 3 5% 6% 29%	4271
November	1st to 10th	Night	31% 8% 3 8% 4 5% 4 4 5% 5% 22%	2676
	11th to 20th	Night	29% 11% 3 8% 5% 4 4 3 5% 5% 23%	2660
	21st to end	Night	26% 11% 3 7% 6% 5% 5% 4 5% 5% 23%	2660
December	1st to 10th	Night	29% 11% 3 7% 5% 4 4 4 4 6% 25%	2660
	11th to 20th	Night	27% 13% 4 10% 7% 5% 5% 4 5% 4 16%	3280
	21st to end	Night	28% 14% 5% 11% 5% 5% 4 3 4 4 15%	5031

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	45% 45% 5% 3 1 1	2321
	11th to 20th	Day	37% 41% 9% 11% 1	2540
	21st to end	Day	51% 43% 3 3	2925
February	1st to 10th	Day	26% 66% 4 2 2	1710
	11th to 20th	Day	29% 65% 2 3 1	1674
	21st to end	Day	22% 67% 4 5% 3	1472
March	1st to 10th	Day	31% 47% 11% 9% 2	3248
	11th to 20th	Day	22% 44% 11% 21% 1	3490
	21st to end	Day	36% 46% 10% 7% 1	3935
April	1st to 10th	Day	22% 68% 4 5%	2280
	11th to 20th	Day	19% 68% 5% 6% 2	2279
	21st to end	Day	20% 67% 5% 6% 2	2489
May	1st to 10th	Day	23% 44% 10% 14% 8% 1	4420
	11th to 20th	Day	20% 46% 19% 14% 2	4380
	21st to end	Day	32% 49% 13% 5% 1	4814
June	1st to 10th	Day	14% 68% 9% 6% 3	2679
	11th to 20th	Day	18% 67% 6% 6% 3	2850
	21st to end	Day	14% 66% 7% 9% 3	2850
July	1st to 10th	Day	11% 36% 4 13% 20% 16%	4666
	11th to 20th	Day	7% 37% 5% 18% 27% 5%	4416
	21st to end	Day	19% 36% 9% 24% 10% 1	4639
August	1st to 10th	Day	12% 37% 5% 26% 17% 3	4247
	11th to 20th	Day	25% 41% 14% 15% 4 1	4065
	21st to end	Day	29% 38% 7% 17% 8%	4339
September	1st to 10th	Day	16% 62% 8% 9% 5% 1	2280
	11th to 20th	Day	20% 63% 5% 5% 7%	2280
	21st to end	Day	18% 62% 8% 8% 4	2004
October	1st to 10th	Day	24% 40% 11% 21% 3	3180
	11th to 20th	Day	37% 38% 9% 9% 6%	3161
	21st to end	Day	39% 38% 8% 8% 5% 1	3292
November	1st to 10th	Day	30% 58% 6% 5% 1	1603
	11th to 20th	Day	24% 61% 5% 5% 3 1	1520
	21st to end	Day	22% 62% 5% 6% 5%	1520
December	1st to 10th	Day	26% 61% 3 5% 4 1	1520
	11th to 20th	Day	36% 55% 4 4 1	1928
	21st to end	Day	44% 44% 8% 3 1	2602
January	1st to 10th	Night	47% 43% 4 3 2 1	4600
	11th to 20th	Night	33% 41% 10% 14% 2	4420
	21st to end	Night	49% 44% 4 3 1	4734
February	1st to 10th	Night	24% 66% 4 2 3	2470
	11th to 20th	Night	25% 66% 3 4 2	2404
	21st to end	Night	19% 67% 6% 5% 3	1981
March	1st to 10th	Night	34% 47% 8% 8% 2	3623
	11th to 20th	Night	17% 44% 17% 21% 1 1	3490
	21st to end	Night	40% 48% 6% 6%	3717
April	1st to 10th	Night	20% 69% 6% 5%	1900
	11th to 20th	Night	18% 68% 5% 6% 3	1850
	21st to end	Night	19% 67% 5% 7% 2	1691
May	1st to 10th	Night	24% 44% 9% 18% 5%	2540
	11th to 20th	Night	16% 49% 17% 15% 2	2516
	21st to end	Night	29% 50% 14% 5% 1	2762
June	1st to 10th	Night	13% 66% 8% 11% 3	1496
	11th to 20th	Night	14% 66% 9% 9% 2	1330
	21st to end	Night	11% 65% 9% 11% 3	1330
July	1st to 10th	Night	11% 36% 6% 15% 14% 17%	2185
	11th to 20th	Night	7% 36% 4 18% 32% 3	2417
	21st to end	Night	16% 37% 11% 22% 13% 2	2674
August	1st to 10th	Night	9% 37% 5% 28% 20% 2	2647
	11th to 20th	Night	22% 41% 21% 12% 4 1	2839
	21st to end	Night	25% 38% 10% 19% 8%	3333
September	1st to 10th	Night	13% 62% 8% 10% 6%	1900
	11th to 20th	Night	20% 61% 5% 8% 5% 1	1900
	21st to end	Night	17% 61% 6% 11% 4 1	2174
October	1st to 10th	Night	22% 38% 16% 22% 2	3800
	11th to 20th	Night	36% 38% 10% 13% 3	3777
	21st to end	Night	39% 38% 7% 10% 5% 1	4271
November	1st to 10th	Night	27% 59% 6% 7% 1	2676
	11th to 20th	Night	24% 61% 5% 5% 3 2	2660
	21st to end	Night	22% 61% 5% 5% 6% 1	2660
December	1st to 10th	Night	27% 61% 3 6% 3 1	2660
	11th to 20th	Night	33% 57% 5% 4 2	3276
	21st to end	Night	41% 44% 8% 5% 1	5031

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	32% 16% 17% 18% 12% 6%	180
	11th to 20th	Day	35% 11% 21% 16% 13% 4	180
	21st to end	Day	32% 13% 22% 17% 13% 3	277
February	1st to 10th	Day	35% 15% 15% 21% 11% 4	370
	11th to 20th	Day	37% 16% 19% 16% 10% 2	365
	21st to end	Day	30% 19% 21% 20% 8% 2	336
March	1st to 10th	Day	27% 15% 24% 24% 9% 1	460
	11th to 20th	Day	26% 14% 18% 23% 14% 5%	460
	21st to end	Day	26% 17% 23% 22% 9% 3	506
April	1st to 10th	Day	28% 16% 20% 22% 10% 4	460
	11th to 20th	Day	24% 14% 29% 20% 7% 6%	452
	21st to end	Day	21% 14% 28% 23% 13% 2	460
May	1st to 10th	Day	17% 10% 24% 28% 17% 4	463
	11th to 20th	Day	21% 8% 20% 24% 19% 8%	488
	21st to end	Day	19% 9% 23% 26% 17% 7%	529
June	1st to 10th	Day	21% 8% 23% 24% 17% 7%	509
	11th to 20th	Day	18% 12% 24% 27% 15% 4	680
	21st to end	Day	10% 10% 21% 30% 23% 7%	680
July	1st to 10th	Day	9% 3 19% 32% 29% 7%	671
	11th to 20th	Day	6% 4 16% 35% 29% 10%	529
	21st to end	Day	5% 4 11% 31% 38% 11%	531
August	1st to 10th	Day	6% 5% 14% 30% 33% 13%	487
	11th to 20th	Day	7% 3 11% 30% 34% 15%	477
	21st to end	Day	12% 9% 18% 27% 25% 8%	539
September	1st to 10th	Day	10% 7% 23% 31% 23% 6%	490
	11th to 20th	Day	15% 8% 19% 26% 24% 9%	490
	21st to end	Day	18% 10% 21% 28% 20% 4	490
October	1st to 10th	Day	22% 10% 20% 22% 19% 8%	490
	11th to 20th	Day	27% 11% 16% 22% 18% 6%	490
	21st to end	Day	30% 15% 20% 16% 13% 5%	445
November	1st to 10th	Day	35% 15% 20% 16% 11% 3	221
	11th to 20th	Day	37% 12% 21% 12% 12% 7%	180
	21st to end	Day	29% 12% 24% 15% 8% 11%	180
December	1st to 10th	Day	40% 10% 19% 13% 12% 7%	180
	11th to 20th	Day	38% 16% 18% 18% 9% 2	180
	21st to end	Day	34% 13% 22% 18% 12% 2	198
January	1st to 10th	Night	41% 14% 14% 17% 13% 2	814
	11th to 20th	Night	36% 10% 13% 22% 18% 2	760
	21st to end	Night	31% 9% 18% 25% 15% 2	836
February	1st to 10th	Night	33% 11% 18% 22% 13% 2	760
	11th to 20th	Night	29% 12% 19% 23% 15% 2	744
	21st to end	Night	32% 13% 21% 21% 11% 2	628
March	1st to 10th	Night	26% 14% 19% 25% 14% 2	760
	11th to 20th	Night	27% 13% 17% 26% 14% 2	760
	21st to end	Night	29% 11% 23% 25% 10% 1	836
April	1st to 10th	Night	25% 10% 23% 26% 15% 2	760
	11th to 20th	Night	18% 11% 25% 28% 15% 2	722
	21st to end	Night	21% 9% 22% 34% 13% 2	551
May	1st to 10th	Night	16% 10% 21% 35% 18% 1	380
	11th to 20th	Night	16% 6% 20% 31% 25% 3	374
	21st to end	Night	11% 8% 20% 35% 23% 3	411
June	1st to 10th	Night	13% 7% 26% 32% 22%	380
	11th to 20th	Night	13% 9% 26% 33% 18% 1	380
	21st to end	Night	10% 5% 20% 37% 26% 2	380
July	1st to 10th	Night	7% 3 16% 38% 33% 3	370
	11th to 20th	Night	5% 2 15% 43% 32% 3	368
	21st to end	Night	4 2 13% 41% 38% 2	416
August	1st to 10th	Night	5% 2 13% 34% 40% 7%	452
	11th to 20th	Night	6% 1 10% 31% 47% 5%	564
	21st to end	Night	10% 4 16% 34% 32% 4	736
September	1st to 10th	Night	10% 3 16% 35% 32% 4	760
	11th to 20th	Night	16% 3 14% 25% 36% 6%	760
	21st to end	Night	17% 6% 18% 32% 23% 3	760
October	1st to 10th	Night	20% 4 16% 28% 27% 4	760
	11th to 20th	Night	26% 7% 15% 25% 21% 5%	760
	21st to end	Night	28% 9% 20% 25% 16% 2	836
November	1st to 10th	Night	33% 10% 19% 26% 12%	778
	11th to 20th	Night	32% 11% 19% 22% 13% 1	760
	21st to end	Night	31% 9% 21% 21% 16% 2	760
December	1st to 10th	Night	30% 8% 19% 26% 16% 1	760
	11th to 20th	Night	36% 11% 19% 21% 12% 1	760
	21st to end	Night	40% 13% 17% 19% 9% 1	885

Page generated at 2024-11-07 03:55:44. Version ac07

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