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 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	26% 14% 3 5% 5% 3 4 3 4 5% 28%	75010
	Night	28% 15% 3 5% 4 2 3 3 4 5% 26%	74711
Transparency	Day	61% 6% 10% 15% 8% 1	74705
	Night	63% 6% 9% 13% 7% 1	74534
Seeing	Day	33% 14% 27% 20% 6% 1	13839
	Night	37% 10% 26% 21% 6%	22391

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	39% 18% 4 5% 5% 3 3 2 3 3 15%	1383
	11th to 20th	Day	34% 15% 5% 6% 5% 2 2 3 3 6% 20%	1456
	21st to end	Day	32% 18% 3 5% 3 2 3 3 5% 5% 22%	1584
February	1st to 10th	Day	32% 18% 5% 7% 3 2 4 4 3 5% 16%	1602
	11th to 20th	Day	27% 24% 4 6% 4 1 3 3 3 5% 20%	1575
	21st to end	Day	20% 27% 6% 8% 5% 2 4 4 3 4 18%	1415
March	1st to 10th	Day	26% 24% 5% 7% 4 3 3 3 3 4 19%	1800
	11th to 20th	Day	29% 24% 5% 6% 4 3 4 3 4 3 17%	1972
	21st to end	Day	29% 25% 4 7% 6% 3 4 4 3 3 12%	2284
April	1st to 10th	Day	26% 26% 4 5% 5% 2 4 4 4 4 15%	2160
	11th to 20th	Day	25% 21% 4 7% 6% 3 4 3 4 4 17%	2132
	21st to end	Day	30% 21% 4 6% 5% 3 3 4 3 4 17%	2429
May	1st to 10th	Day	24% 17% 4 6% 5% 4 4 3 4 5% 23%	2520
	11th to 20th	Day	24% 13% 3 6% 6% 4 5% 5% 5% 6% 24%	2477
	21st to end	Day	34% 11% 4 7% 5% 2 3 3 4 5% 21%	2723
June	1st to 10th	Day	34% 10% 3 5% 6% 3 4 4 5% 5% 22%	2551
	11th to 20th	Day	40% 12% 4 6% 5% 3 5% 3 4 4 14%	2700
	21st to end	Day	22% 8% 3 6% 6% 3 4 4 5% 7% 31%	2700
July	1st to 10th	Day	18% 6% 2 5% 5% 2 5% 4 4 6% 42%	2700
	11th to 20th	Day	16% 5% 2 4 4 2 4 4 4 6% 47%	2570
	21st to end	Day	12% 6% 2 3 3 2 3 3 3 5% 57%	2741
August	1st to 10th	Day	16% 6% 1 3 3 2 3 4 4 6% 51%	2507
	11th to 20th	Day	13% 4 2 4 3 2 3 3 5% 7% 54%	2328
	21st to end	Day	18% 6% 3 5% 5% 2 4 4 5% 7% 41%	2447
September	1st to 10th	Day	16% 7% 2 5% 4 2 4 3 3 6% 47%	2160
	11th to 20th	Day	20% 8% 3 5% 5% 2 4 4 4 5% 39%	2160
	21st to end	Day	27% 7% 2 4 5% 2 3 3 4 4 38%	1918
October	1st to 10th	Day	26% 9% 3 5% 4 2 4 4 5% 6% 34%	1800
	11th to 20th	Day	32% 9% 3 5% 5% 3 2 3 3 6% 29%	1800
	21st to end	Day	37% 12% 2 5% 4 2 3 2 3 4 26%	1822
November	1st to 10th	Day	40% 13% 3 5% 4 3 3 5% 4 4 17%	1548
	11th to 20th	Day	38% 19% 3 6% 3 2 3 2 3 2 20%	1520
	21st to end	Day	35% 17% 4 5% 4 2 2 3 3 5% 19%	1520
December	1st to 10th	Day	30% 17% 4 6% 4 3 3 2 3 4 24%	1520
	11th to 20th	Day	30% 19% 4 5% 5% 2 4 3 3 4 21%	1925
	21st to end	Day	29% 21% 5% 7% 7% 3 4 3 4 3 15%	2561
January	1st to 10th	Night	38% 19% 4 5% 4 2 3 3 3 3 15%	2776
	11th to 20th	Night	37% 13% 5% 5% 3 2 3 3 4 6% 19%	2548
	21st to end	Night	30% 19% 3 5% 4 3 4 3 3 5% 22%	2772
February	1st to 10th	Night	31% 21% 4 6% 4 3 3 4 4 5% 16%	2358
	11th to 20th	Night	25% 26% 4 6% 5% 2 3 2 3 3 20%	2272
	21st to end	Night	23% 26% 5% 7% 5% 3 3 3 3 4 19%	1840
March	1st to 10th	Night	27% 18% 5% 6% 4 2 4 3 4 6% 22%	2160
	11th to 20th	Night	30% 21% 3 5% 4 3 3 4 3 5% 19%	1984
	21st to end	Night	31% 21% 4 6% 6% 2 4 3 4 4 16%	2071
April	1st to 10th	Night	24% 25% 4 5% 6% 2 4 3 4 5% 18%	1800
	11th to 20th	Night	28% 20% 3 6% 5% 2 3 3 3 5% 21%	1779
	21st to end	Night	34% 15% 4 6% 6% 3 4 3 3 4 19%	1527
May	1st to 10th	Night	24% 15% 3 5% 4 3 4 4 3 6% 29%	1440
	11th to 20th	Night	25% 9% 4 7% 6% 2 5% 5% 5% 6% 25%	1416
	21st to end	Night	32% 13% 4 4 4 3 4 4 5% 6% 23%	1552
June	1st to 10th	Night	30% 9% 2 6% 5% 2 5% 5% 4 6% 25%	1404
	11th to 20th	Night	34% 11% 4 7% 4 2 4 3 4 6% 21%	1260
	21st to end	Night	21% 7% 2 6% 4 3 4 4 5% 6% 37%	1260
July	1st to 10th	Night	19% 8% 2 5% 4 2 4 3 3 6% 44%	1260
	11th to 20th	Night	15% 5% 2 4 5% 2 4 4 4 6% 49%	1386
	21st to end	Night	12% 4 2 2 3 2 3 3 4 6% 60%	1576
August	1st to 10th	Night	13% 5% 1 4 3 2 2 3 5% 6% 56%	1432
	11th to 20th	Night	12% 5% 1 3 3 2 3 3 4 7% 57%	1568
	21st to end	Night	15% 8% 2 4 4 2 3 4 5% 7% 47%	1908
September	1st to 10th	Night	17% 6% 3 4 4 3 3 3 4 4 50%	1800
	11th to 20th	Night	24% 9% 2 6% 4 2 2 3 3 6% 39%	1800
	21st to end	Night	24% 10% 2 5% 5% 3 4 4 4 5% 34%	2037
October	1st to 10th	Night	27% 9% 3 5% 4 3 3 3 4 5% 34%	2160
	11th to 20th	Night	33% 10% 3 5% 4 2 4 3 3 3 30%	2160
	21st to end	Night	34% 12% 2 5% 4 2 3 3 3 4 27%	2533
November	1st to 10th	Night	40% 14% 3 5% 4 3 4 3 4 4 16%	2526
	11th to 20th	Night	38% 17% 2 5% 4 2 3 3 2 4 20%	2660
	21st to end	Night	37% 18% 3 4 4 3 3 3 3 3 19%	2660
December	1st to 10th	Night	29% 16% 4 5% 3 2 3 4 4 5% 25%	2660
	11th to 20th	Night	32% 19% 4 7% 5% 2 3 3 3 4 16%	3280
	21st to end	Night	31% 21% 4 7% 6% 3 4 4 4 4 13%	5086

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% 3 5% 10% 2 2	1383
	11th to 20th	Day	69% 5% 8% 9% 7% 3	1456
	21st to end	Day	66% 5% 9% 11% 9% 1	1584
February	1st to 10th	Day	72% 6% 8% 9% 5% 1	1602
	11th to 20th	Day	70% 5% 8% 7% 8% 1	1584
	21st to end	Day	72% 6% 6% 11% 5% 1	1415
March	1st to 10th	Day	73% 4 7% 11% 4 1	1800
	11th to 20th	Day	75% 5% 7% 9% 4	1972
	21st to end	Day	81% 5% 7% 6% 1	2284
April	1st to 10th	Day	75% 7% 10% 7% 1	2160
	11th to 20th	Day	73% 6% 6% 9% 6% 1	2132
	21st to end	Day	75% 5% 8% 7% 4 1	2429
May	1st to 10th	Day	66% 6% 11% 13% 4	2520
	11th to 20th	Day	62% 8% 14% 13% 3	2480
	21st to end	Day	68% 7% 12% 11% 2	2724
June	1st to 10th	Day	67% 5% 13% 9% 5% 1	2551
	11th to 20th	Day	76% 7% 8% 8% 2	2700
	21st to end	Day	53% 8% 12% 19% 6% 1	2700
July	1st to 10th	Day	45% 7% 12% 23% 11% 2	2626
	11th to 20th	Day	38% 7% 12% 28% 14% 1	2492
	21st to end	Day	30% 4 12% 34% 16% 5%	2589
August	1st to 10th	Day	36% 6% 14% 24% 17% 3	2490
	11th to 20th	Day	31% 6% 17% 29% 14% 3	2328
	21st to end	Day	44% 6% 12% 26% 10% 1	2447
September	1st to 10th	Day	41% 4 12% 22% 15% 6%	2160
	11th to 20th	Day	48% 6% 9% 17% 16% 4	2160
	21st to end	Day	51% 5% 14% 15% 12% 3	1918
October	1st to 10th	Day	52% 7% 12% 19% 8% 1	1800
	11th to 20th	Day	59% 5% 10% 15% 10% 1	1800
	21st to end	Day	65% 4 7% 12% 10% 1	1822
November	1st to 10th	Day	73% 7% 7% 9% 5%	1548
	11th to 20th	Day	74% 4 5% 8% 7% 2	1520
	21st to end	Day	70% 5% 5% 10% 9% 2	1520
December	1st to 10th	Day	67% 4 7% 11% 9% 1	1520
	11th to 20th	Day	70% 4 10% 12% 4 1	1928
	21st to end	Day	76% 5% 6% 9% 4	2561
January	1st to 10th	Night	77% 4 4 11% 3 2	2776
	11th to 20th	Night	69% 5% 6% 11% 6% 3	2548
	21st to end	Night	67% 4 7% 11% 8% 2	2772
February	1st to 10th	Night	74% 4 6% 10% 5% 1	2358
	11th to 20th	Night	71% 5% 7% 9% 5% 3	2274
	21st to end	Night	71% 4 8% 11% 5% 1	1840
March	1st to 10th	Night	65% 7% 9% 12% 6% 1	2160
	11th to 20th	Night	70% 5% 10% 11% 5%	1985
	21st to end	Night	75% 6% 11% 8% 1	2071
April	1st to 10th	Night	72% 6% 12% 9% 1	1800
	11th to 20th	Night	68% 6% 10% 10% 6% 1	1779
	21st to end	Night	72% 5% 7% 10% 5% 1	1527
May	1st to 10th	Night	59% 9% 12% 15% 4	1440
	11th to 20th	Night	59% 8% 16% 13% 4	1416
	21st to end	Night	62% 11% 13% 13% 1	1552
June	1st to 10th	Night	61% 10% 13% 12% 5%	1404
	11th to 20th	Night	65% 9% 14% 9% 3	1260
	21st to end	Night	48% 10% 16% 18% 6% 2	1260
July	1st to 10th	Night	45% 8% 13% 21% 10% 3	1225
	11th to 20th	Night	37% 9% 13% 27% 13% 1	1341
	21st to end	Night	26% 6% 14% 32% 17% 4	1488
August	1st to 10th	Night	30% 8% 15% 26% 17% 3	1424
	11th to 20th	Night	28% 8% 19% 26% 18% 3	1568
	21st to end	Night	40% 5% 15% 27% 11% 2	1908
September	1st to 10th	Night	39% 7% 12% 23% 16% 3	1800
	11th to 20th	Night	51% 4 8% 18% 16% 4	1800
	21st to end	Night	53% 6% 10% 17% 10% 3	2037
October	1st to 10th	Night	54% 4 11% 21% 9%	2160
	11th to 20th	Night	61% 4 9% 15% 9% 2	2160
	21st to end	Night	63% 5% 7% 14% 11%	2533
November	1st to 10th	Night	74% 4 9% 8% 4 1	2526
	11th to 20th	Night	72% 4 8% 8% 7% 2	2660
	21st to end	Night	72% 4 4 9% 9% 3	2660
December	1st to 10th	Night	64% 6% 8% 12% 9% 1	2660
	11th to 20th	Night	74% 5% 7% 9% 5%	3276
	21st to end	Night	76% 4 8% 8% 4	5086

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	46% 14% 21% 14% 4	180
	11th to 20th	Day	42% 13% 26% 14% 5%	164
	21st to end	Day	44% 13% 22% 17% 5%	176
February	1st to 10th	Day	47% 15% 24% 12% 2	322
	11th to 20th	Day	47% 14% 23% 13% 2	335
	21st to end	Day	36% 18% 26% 16% 3	318
March	1st to 10th	Day	39% 21% 26% 12% 3	420
	11th to 20th	Day	39% 15% 23% 19% 3	420
	21st to end	Day	45% 17% 22% 13% 2	462
April	1st to 10th	Day	45% 15% 20% 15% 5%	420
	11th to 20th	Day	39% 20% 21% 16% 4 1	412
	21st to end	Day	46% 14% 22% 14% 4	420
May	1st to 10th	Day	30% 18% 31% 18% 3	423
	11th to 20th	Day	35% 10% 25% 22% 8% 1	448
	21st to end	Day	38% 14% 23% 18% 5% 1	485
June	1st to 10th	Day	34% 17% 23% 20% 6% 1	481
	11th to 20th	Day	40% 17% 26% 12% 4	630
	21st to end	Day	23% 13% 29% 28% 7% 1	630
July	1st to 10th	Day	18% 13% 36% 27% 6% 1	621
	11th to 20th	Day	16% 13% 36% 25% 7% 2	491
	21st to end	Day	13% 8% 35% 30% 10% 4	487
August	1st to 10th	Day	18% 7% 30% 30% 12% 2	447
	11th to 20th	Day	11% 11% 31% 31% 14% 3	437
	21st to end	Day	18% 14% 33% 26% 9% 1	495
September	1st to 10th	Day	21% 16% 31% 25% 6% 1	450
	11th to 20th	Day	24% 14% 28% 25% 7%	450
	21st to end	Day	32% 13% 28% 18% 9%	450
October	1st to 10th	Day	32% 13% 24% 21% 8% 2	450
	11th to 20th	Day	39% 11% 24% 17% 8% 1	450
	21st to end	Day	43% 9% 28% 14% 5% 1	337
November	1st to 10th	Day	41% 11% 30% 14% 3	210
	11th to 20th	Day	49% 12% 14% 17% 8% 1	180
	21st to end	Day	47% 7% 25% 12% 7% 2	180
December	1st to 10th	Day	47% 11% 16% 16% 9% 1	180
	11th to 20th	Day	44% 13% 26% 13% 4	180
	21st to end	Day	44% 14% 24% 15% 4	198
January	1st to 10th	Night	53% 11% 25% 11%	821
	11th to 20th	Night	45% 10% 29% 14% 2	728
	21st to end	Night	42% 9% 31% 17% 1	792
February	1st to 10th	Night	44% 10% 33% 13%	720
	11th to 20th	Night	39% 12% 31% 16% 1	704
	21st to end	Night	40% 11% 32% 16% 2	592
March	1st to 10th	Night	36% 12% 30% 19% 3	720
	11th to 20th	Night	42% 13% 26% 16% 3	720
	21st to end	Night	42% 13% 29% 13% 3	792
April	1st to 10th	Night	39% 11% 26% 21% 4	720
	11th to 20th	Night	39% 12% 26% 20% 3	711
	21st to end	Night	44% 9% 28% 17% 3	447
May	1st to 10th	Night	29% 11% 32% 23% 6%	360
	11th to 20th	Night	30% 7% 26% 31% 7%	354
	21st to end	Night	35% 7% 23% 29% 6%	389
June	1st to 10th	Night	33% 8% 28% 23% 8%	360
	11th to 20th	Night	36% 10% 24% 21% 8%	360
	21st to end	Night	23% 5% 29% 34% 8% 1	360
July	1st to 10th	Night	21% 9% 18% 35% 18%	350
	11th to 20th	Night	16% 5% 20% 40% 18%	348
	21st to end	Night	9% 6% 15% 42% 27% 1	394
August	1st to 10th	Night	15% 4 12% 40% 29%	358
	11th to 20th	Night	14% 2 14% 47% 22%	500
	21st to end	Night	19% 6% 22% 40% 13%	720
September	1st to 10th	Night	19% 4 30% 34% 12%	720
	11th to 20th	Night	27% 8% 19% 33% 14%	720
	21st to end	Night	31% 6% 23% 33% 7%	720
October	1st to 10th	Night	33% 6% 25% 29% 7%	720
	11th to 20th	Night	39% 6% 27% 23% 4	720
	21st to end	Night	40% 11% 31% 16% 1	792
November	1st to 10th	Night	50% 11% 28% 10% 1	740
	11th to 20th	Night	51% 12% 22% 14% 1	760
	21st to end	Night	47% 12% 26% 13% 2	760
December	1st to 10th	Night	36% 13% 34% 15% 2	760
	11th to 20th	Night	48% 14% 28% 9% 1	760
	21st to end	Night	50% 14% 26% 9% 1	899

Page generated at 2024-01-09 22:25:03. Version ac07

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