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 2024-01-13.

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	20% 9% 3 7% 6% 3 5% 5% 5% 7% 30%	75035
	Night	22% 8% 3 6% 6% 3 4 4 4 6% 32%	74677
Transparency	Day	35% 40% 9% 10% 4 1	74730
	Night	35% 41% 10% 10% 4 1	74500
Seeing	Day	21% 10% 20% 24% 19% 6%	13962
	Night	24% 8% 18% 27% 20% 3	22499

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	37% 11% 4 8% 6% 4 4 4 4 5% 13%	1392
	11th to 20th	Day	30% 11% 5% 9% 5% 3 4 4 4 5% 19%	1456
	21st to end	Day	29% 11% 5% 10% 6% 4 4 5% 4 5% 15%	1659
February	1st to 10th	Day	32% 11% 4 8% 5% 3 5% 5% 6% 6% 14%	1620
	11th to 20th	Day	31% 12% 3 7% 5% 4 5% 4 4 5% 19%	1575
	21st to end	Day	28% 12% 5% 7% 7% 3 4 5% 4 5% 19%	1387
March	1st to 10th	Day	25% 10% 4 9% 8% 5% 5% 4 5% 6% 18%	1858
	11th to 20th	Day	30% 11% 4 7% 6% 4 5% 4 5% 5% 19%	1980
	21st to end	Day	25% 13% 5% 8% 7% 4 7% 6% 5% 6% 14%	2237
April	1st to 10th	Day	27% 10% 4 8% 8% 3 5% 6% 6% 6% 18%	2160
	11th to 20th	Day	24% 10% 4 7% 6% 4 6% 5% 5% 6% 23%	2157
	21st to end	Day	20% 10% 4 8% 6% 4 6% 5% 5% 7% 25%	2358
May	1st to 10th	Day	17% 8% 3 6% 7% 4 6% 6% 6% 6% 33%	2520
	11th to 20th	Day	18% 9% 3 6% 6% 4 6% 5% 7% 8% 27%	2477
	21st to end	Day	20% 10% 3 8% 7% 4 6% 4 5% 7% 27%	2723
June	1st to 10th	Day	20% 8% 3 8% 6% 3 5% 5% 5% 8% 28%	2538
	11th to 20th	Day	20% 8% 4 8% 8% 4 6% 6% 5% 7% 23%	2700
	21st to end	Day	12% 7% 3 6% 5% 4 4 4 5% 7% 42%	2700
July	1st to 10th	Day	11% 6% 3 5% 4 2 4 4 4 6% 51%	2700
	11th to 20th	Day	10% 5% 2 4 5% 2 4 4 5% 7% 52%	2566
	21st to end	Day	7% 4 2 3 3 2 3 3 3 8% 62%	2741
August	1st to 10th	Day	8% 5% 2 4 4 3 3 4 5% 8% 53%	2435
	11th to 20th	Day	8% 4 2 4 4 2 3 4 5% 9% 57%	2298
	21st to end	Day	11% 5% 3 5% 5% 4 4 4 6% 7% 46%	2471
September	1st to 10th	Day	9% 6% 2 6% 5% 3 4 4 4 6% 50%	2160
	11th to 20th	Day	12% 8% 3 6% 6% 4 5% 6% 5% 7% 39%	2160
	21st to end	Day	17% 7% 2 7% 5% 3 4 5% 5% 8% 38%	1900
October	1st to 10th	Day	21% 9% 3 6% 6% 3 4 5% 5% 7% 30%	1800
	11th to 20th	Day	23% 9% 4 7% 6% 3 5% 5% 5% 6% 26%	1800
	21st to end	Day	28% 11% 3 6% 6% 4 4 4 5% 6% 24%	1891
November	1st to 10th	Day	30% 11% 5% 6% 6% 4 5% 4 7% 6% 17%	1529
	11th to 20th	Day	31% 11% 3 7% 6% 3 5% 4 4 5% 20%	1520
	21st to end	Day	29% 10% 5% 7% 6% 3 4 4 5% 7% 20%	1520
December	1st to 10th	Day	29% 8% 3 7% 8% 4 4 4 4 6% 23%	1520
	11th to 20th	Day	31% 13% 4 9% 9% 5% 5% 5% 5% 4 11%	1925
	21st to end	Day	26% 14% 5% 10% 8% 4 4 5% 5% 6% 12%	2602
January	1st to 10th	Night	39% 10% 4 6% 6% 4 4 4 4 5% 15%	2762
	11th to 20th	Night	31% 10% 4 7% 5% 3 4 4 5% 6% 20%	2548
	21st to end	Night	28% 11% 4 7% 6% 4 5% 4 5% 6% 21%	2696
February	1st to 10th	Night	30% 11% 4 7% 7% 3 4 4 4 6% 20%	2340
	11th to 20th	Night	26% 11% 3 7% 6% 3 4 4 4 5% 25%	2272
	21st to end	Night	25% 12% 4 6% 6% 3 6% 5% 5% 5% 24%	1868
March	1st to 10th	Night	26% 10% 4 7% 7% 2 5% 5% 4 7% 23%	2099
	11th to 20th	Night	27% 10% 4 7% 6% 3 5% 4 4 6% 23%	1979
	21st to end	Night	27% 10% 4 8% 7% 4 5% 6% 5% 6% 18%	2118
April	1st to 10th	Night	22% 11% 4 7% 6% 4 6% 5% 5% 6% 25%	1800
	11th to 20th	Night	18% 8% 3 7% 5% 3 5% 6% 6% 7% 31%	1752
	21st to end	Night	18% 9% 4 7% 6% 3 6% 5% 5% 7% 31%	1602
May	1st to 10th	Night	14% 7% 3 6% 5% 4 5% 5% 5% 6% 39%	1440
	11th to 20th	Night	14% 7% 3 7% 6% 4 7% 5% 6% 7% 34%	1416
	21st to end	Night	13% 7% 2 7% 6% 4 6% 5% 6% 8% 36%	1552
June	1st to 10th	Night	15% 5% 3 4 6% 4 5% 5% 6% 8% 40%	1418
	11th to 20th	Night	15% 6% 4 6% 5% 4 4 5% 6% 9% 36%	1260
	21st to end	Night	8% 5% 2 4 4 3 3 5% 6% 6% 53%	1260
July	1st to 10th	Night	7% 3 1 2 4 3 4 5% 4 7% 60%	1260
	11th to 20th	Night	5% 3 1 4 4 2 3 4 3 6% 66%	1390
	21st to end	Night	3 3 1 2 2 1 2 3 3 5% 74%	1576
August	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 7% 64%	1502
	11th to 20th	Night	5% 2 1 2 2 1 3 3 4 6% 71%	1602
	21st to end	Night	8% 5% 2 4 3 2 4 4 5% 7% 57%	1885
September	1st to 10th	Night	8% 3 2 4 4 3 4 3 4 7% 60%	1800
	11th to 20th	Night	14% 6% 3 5% 5% 3 3 4 4 6% 48%	1800
	21st to end	Night	16% 6% 2 5% 5% 3 4 4 4 6% 45%	2059
October	1st to 10th	Night	18% 6% 2 5% 5% 3 6% 4 4 7% 39%	2160
	11th to 20th	Night	22% 8% 3 5% 6% 3 4 4 4 5% 35%	2160
	21st to end	Night	26% 7% 2 6% 5% 2 4 4 5% 6% 33%	2464
November	1st to 10th	Night	32% 8% 3 6% 5% 4 5% 5% 5% 5% 23%	2546
	11th to 20th	Night	30% 10% 4 7% 6% 3 4 4 4 6% 23%	2660
	21st to end	Night	26% 10% 4 6% 7% 3 5% 5% 5% 5% 23%	2660
December	1st to 10th	Night	29% 9% 4 6% 5% 3 4 4 3 6% 25%	2660
	11th to 20th	Night	27% 13% 5% 9% 8% 4 6% 5% 4 5% 16%	3280
	21st to end	Night	28% 13% 6% 10% 7% 4 5% 4 4 4 16%	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	51% 42% 2 4 1	1392
	11th to 20th	Day	40% 46% 6% 4 3 2	1456
	21st to end	Day	43% 41% 7% 7% 2	1659
February	1st to 10th	Day	43% 42% 7% 6% 2	1620
	11th to 20th	Day	39% 41% 9% 7% 4	1584
	21st to end	Day	45% 40% 8% 5% 2	1387
March	1st to 10th	Day	44% 42% 9% 5%	1858
	11th to 20th	Day	46% 39% 7% 7% 2	1980
	21st to end	Day	47% 41% 7% 5%	2237
April	1st to 10th	Day	44% 39% 11% 6% 1	2160
	11th to 20th	Day	40% 39% 8% 8% 5% 1	2157
	21st to end	Day	39% 41% 10% 7% 3	2358
May	1st to 10th	Day	35% 41% 12% 9% 3	2520
	11th to 20th	Day	32% 41% 14% 10% 3	2480
	21st to end	Day	41% 40% 9% 8% 2	2724
June	1st to 10th	Day	40% 40% 11% 6% 3	2538
	11th to 20th	Day	40% 43% 10% 6% 1	2700
	21st to end	Day	27% 40% 14% 12% 5% 2	2700
July	1st to 10th	Day	20% 42% 10% 18% 9% 1	2626
	11th to 20th	Day	20% 40% 9% 20% 11% 1	2488
	21st to end	Day	14% 39% 10% 22% 12% 3	2589
August	1st to 10th	Day	20% 39% 13% 20% 7% 1	2418
	11th to 20th	Day	18% 37% 13% 19% 9% 3	2298
	21st to end	Day	23% 39% 11% 20% 6% 1	2471
September	1st to 10th	Day	19% 38% 11% 19% 9% 4	2160
	11th to 20th	Day	26% 40% 11% 14% 8% 2	2160
	21st to end	Day	31% 39% 11% 11% 6% 1	1900
October	1st to 10th	Day	34% 41% 8% 12% 5%	1800
	11th to 20th	Day	37% 39% 11% 10% 3 1	1800
	21st to end	Day	39% 40% 9% 9% 4	1891
November	1st to 10th	Day	42% 43% 6% 6% 3	1529
	11th to 20th	Day	43% 43% 7% 5% 3	1520
	21st to end	Day	41% 45% 6% 6% 2	1520
December	1st to 10th	Day	40% 42% 8% 7% 3	1520
	11th to 20th	Day	48% 40% 5% 5% 1	1928
	21st to end	Day	53% 33% 9% 4 1	2602
January	1st to 10th	Night	48% 42% 6% 4 1	2762
	11th to 20th	Night	40% 45% 7% 6% 2 1	2548
	21st to end	Night	40% 41% 7% 8% 3 1	2696
February	1st to 10th	Night	42% 40% 9% 7% 2	2340
	11th to 20th	Night	36% 42% 9% 8% 4 1	2274
	21st to end	Night	43% 40% 7% 6% 3	1868
March	1st to 10th	Night	41% 39% 10% 9% 1	2099
	11th to 20th	Night	43% 39% 7% 8% 3	1980
	21st to end	Night	45% 42% 7% 6%	2118
April	1st to 10th	Night	39% 42% 10% 7% 1	1800
	11th to 20th	Night	32% 42% 12% 8% 5% 1	1752
	21st to end	Night	35% 42% 12% 7% 3 1	1602
May	1st to 10th	Night	31% 43% 10% 11% 4	1440
	11th to 20th	Night	30% 40% 13% 11% 4 1	1416
	21st to end	Night	36% 42% 11% 10% 2	1552
June	1st to 10th	Night	32% 41% 11% 11% 3	1418
	11th to 20th	Night	32% 43% 17% 7% 1	1260
	21st to end	Night	23% 40% 17% 12% 6% 2	1260
July	1st to 10th	Night	15% 41% 13% 19% 8% 4	1225
	11th to 20th	Night	14% 40% 11% 23% 11% 1	1345
	21st to end	Night	8% 40% 10% 23% 14% 4	1488
August	1st to 10th	Night	16% 39% 13% 21% 11% 1	1494
	11th to 20th	Night	11% 39% 14% 21% 12% 4	1602
	21st to end	Night	20% 39% 12% 20% 9% 1	1885
September	1st to 10th	Night	14% 39% 12% 19% 11% 5%	1800
	11th to 20th	Night	24% 39% 10% 14% 10% 3	1800
	21st to end	Night	27% 40% 11% 14% 8% 1	2059
October	1st to 10th	Night	29% 40% 10% 14% 7% 1	2160
	11th to 20th	Night	33% 40% 10% 13% 3 1	2160
	21st to end	Night	33% 41% 9% 11% 6%	2464
November	1st to 10th	Night	39% 44% 7% 6% 3	2546
	11th to 20th	Night	41% 42% 8% 6% 3	2660
	21st to end	Night	39% 44% 9% 5% 2	2660
December	1st to 10th	Night	39% 43% 8% 8% 3	2660
	11th to 20th	Night	45% 41% 8% 4 1	3276
	21st to end	Night	53% 32% 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% 17% 17% 18% 12% 6%	180
	11th to 20th	Day	35% 10% 21% 16% 12% 5%	164
	21st to end	Day	32% 11% 21% 18% 15% 3	251
February	1st to 10th	Day	36% 14% 15% 20% 11% 3	340
	11th to 20th	Day	38% 15% 19% 15% 11% 1	335
	21st to end	Day	30% 20% 20% 20% 9% 2	305
March	1st to 10th	Day	28% 15% 24% 23% 9% 1	420
	11th to 20th	Day	28% 14% 19% 21% 14% 4	420
	21st to end	Day	27% 17% 23% 21% 9% 3	462
April	1st to 10th	Day	29% 16% 19% 22% 10% 5%	420
	11th to 20th	Day	25% 13% 28% 20% 7% 7%	412
	21st to end	Day	20% 14% 28% 24% 12% 2	420
May	1st to 10th	Day	17% 10% 22% 29% 17% 4	423
	11th to 20th	Day	22% 8% 21% 24% 19% 7%	448
	21st to end	Day	18% 8% 24% 26% 18% 7%	485
June	1st to 10th	Day	20% 9% 24% 22% 18% 7%	468
	11th to 20th	Day	17% 12% 25% 26% 15% 4	630
	21st to end	Day	10% 8% 21% 30% 24% 7%	630
July	1st to 10th	Day	10% 3 18% 32% 29% 8%	621
	11th to 20th	Day	7% 4 16% 34% 30% 9%	487
	21st to end	Day	5% 2 11% 30% 39% 12%	487
August	1st to 10th	Day	6% 5% 15% 27% 33% 14%	447
	11th to 20th	Day	6% 2 10% 32% 35% 15%	437
	21st to end	Day	11% 9% 19% 28% 25% 9%	495
September	1st to 10th	Day	10% 7% 24% 31% 22% 6%	450
	11th to 20th	Day	14% 8% 18% 26% 25% 10%	450
	21st to end	Day	18% 10% 20% 28% 19% 4	450
October	1st to 10th	Day	23% 10% 18% 22% 19% 8%	450
	11th to 20th	Day	26% 11% 15% 23% 18% 6%	450
	21st to end	Day	32% 16% 19% 16% 13% 5%	406
November	1st to 10th	Day	35% 15% 20% 16% 11% 2	201
	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% 21% 18% 2	728
	21st to end	Night	30% 8% 18% 25% 16% 2	792
February	1st to 10th	Night	34% 11% 18% 22% 13% 2	720
	11th to 20th	Night	29% 12% 19% 23% 14% 2	704
	21st to end	Night	31% 13% 22% 21% 11% 2	592
March	1st to 10th	Night	27% 14% 19% 24% 14% 2	720
	11th to 20th	Night	29% 13% 17% 25% 14% 2	720
	21st to end	Night	29% 11% 22% 26% 11% 1	792
April	1st to 10th	Night	25% 9% 23% 26% 15% 2	720
	11th to 20th	Night	19% 11% 24% 28% 16% 2	684
	21st to end	Night	21% 8% 22% 35% 13% 2	522
May	1st to 10th	Night	16% 9% 22% 34% 18% 1	360
	11th to 20th	Night	17% 5% 19% 31% 25% 3	354
	21st to end	Night	11% 8% 20% 36% 23% 3	389
June	1st to 10th	Night	13% 6% 26% 32% 23%	360
	11th to 20th	Night	13% 9% 25% 33% 19% 1	360
	21st to end	Night	11% 5% 19% 36% 27% 2	360
July	1st to 10th	Night	7% 3 15% 37% 34% 3	350
	11th to 20th	Night	5% 2 15% 45% 30% 3	348
	21st to end	Night	4 2 12% 40% 39% 2	394
August	1st to 10th	Night	6% 2 13% 33% 40% 7%	428
	11th to 20th	Night	6% 1 9% 31% 48% 5%	534
	21st to end	Night	9% 4 16% 35% 33% 4	697
September	1st to 10th	Night	10% 4 16% 35% 31% 4	720
	11th to 20th	Night	14% 3 14% 25% 37% 6%	720
	21st to end	Night	17% 6% 18% 33% 23% 3	720
October	1st to 10th	Night	20% 4 16% 28% 27% 5%	720
	11th to 20th	Night	26% 7% 15% 25% 21% 5%	720
	21st to end	Night	29% 8% 19% 25% 16% 2	792
November	1st to 10th	Night	33% 9% 20% 27% 11%	740
	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 2024-01-09 22:25:05. Version ac07

Copyright 2024 Attilla Danko