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 2024-11-10.

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	24% 14% 3 6% 4 3 3 3 4 5% 30%	105755
	Night	27% 16% 3 6% 3 3 3 3 4 5% 28%	101963
Transparency	Day	58% 5% 10% 16% 9% 2	105514
	Night	61% 6% 10% 14% 8% 1	101850
Seeing	Day	32% 14% 27% 20% 6% 1	14994
	Night	37% 10% 26% 22% 6%	23367

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	35% 27% 5% 7% 3 3 2 2 3 3 10%	2312
	11th to 20th	Day	26% 14% 7% 8% 4 2 2 2 3 5% 25%	2516
	21st to end	Day	44% 18% 3 6% 3 3 2 2 4 3 13%	2794
February	1st to 10th	Day	31% 19% 5% 7% 3 4 4 3 3 5% 16%	1691
	11th to 20th	Day	26% 24% 4 7% 3 2 2 2 4 5% 20%	1665
	21st to end	Day	20% 28% 6% 8% 4 3 4 3 3 4 17%	1501
March	1st to 10th	Day	21% 25% 4 8% 4 4 3 2 3 4 21%	3138
	11th to 20th	Day	18% 22% 4 6% 3 3 3 2 3 3 34%	3482
	21st to end	Day	27% 23% 3 8% 5% 5% 4 3 3 4 16%	4008
April	1st to 10th	Day	25% 27% 4 6% 4 3 4 3 4 4 15%	2280
	11th to 20th	Day	23% 23% 3 7% 5% 4 3 2 4 4 19%	2252
	21st to end	Day	30% 22% 3 7% 5% 4 3 3 3 4 16%	2565
May	1st to 10th	Day	21% 19% 4 6% 4 4 4 2 4 4 29%	4420
	11th to 20th	Day	22% 14% 2 6% 4 4 4 3 4 7% 29%	4377
	21st to end	Day	39% 14% 3 7% 5% 4 4 2 3 4 14%	4813
June	1st to 10th	Day	33% 10% 3 6% 5% 4 4 3 5% 5% 21%	2693
	11th to 20th	Day	38% 13% 3 7% 4 4 4 3 4 4 15%	2850
	21st to end	Day	22% 10% 2 7% 5% 4 4 3 6% 6% 31%	2850
July	1st to 10th	Day	11% 5% 1 4 3 3 3 3 4 5% 59%	4740
	11th to 20th	Day	9% 4 1 3 3 2 3 2 3 4 65%	4498
	21st to end	Day	14% 6% 1 4 3 3 3 2 4 6% 53%	4790
August	1st to 10th	Day	9% 4 1 3 2 3 3 3 4 7% 61%	4401
	11th to 20th	Day	18% 9% 3 7% 3 4 4 3 5% 7% 37%	4068
	21st to end	Day	19% 9% 3 7% 4 3 4 3 5% 5% 38%	4290
September	1st to 10th	Day	16% 7% 2 6% 3 4 4 3 3 6% 47%	2280
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 3 4 5% 38%	2280
	21st to end	Day	26% 8% 2 5% 4 3 3 3 4 4 38%	2024
October	1st to 10th	Day	22% 8% 2 5% 3 4 4 4 7% 8% 33%	3180
	11th to 20th	Day	35% 11% 3 6% 4 4 2 2 3 4 26%	3161
	21st to end	Day	32% 15% 3 7% 4 4 4 3 4 5% 18%	3167
November	1st to 10th	Day	39% 14% 3 6% 3 4 3 3 4 4 17%	1623
	11th to 20th	Day	37% 20% 3 6% 3 2 2 2 3 2 20%	1520
	21st to end	Day	35% 18% 4 5% 4 2 3 3 3 4 19%	1520
December	1st to 10th	Day	30% 18% 3 6% 4 3 3 2 3 4 24%	1520
	11th to 20th	Day	29% 20% 4 6% 4 3 3 2 4 4 21%	1925
	21st to end	Day	28% 22% 5% 8% 6% 4 3 2 4 3 15%	2561
January	1st to 10th	Night	33% 28% 4 7% 4 3 3 2 2 3 11%	4627
	11th to 20th	Night	31% 15% 5% 7% 2 3 3 2 4 7% 22%	4382
	21st to end	Night	40% 17% 3 7% 4 3 4 2 3 4 14%	4862
February	1st to 10th	Night	30% 22% 4 8% 4 4 3 3 3 4 16%	2489
	11th to 20th	Night	24% 26% 4 7% 4 3 3 2 4 3 20%	2402
	21st to end	Night	23% 27% 4 8% 4 3 3 2 3 4 19%	1952
March	1st to 10th	Night	20% 25% 5% 8% 4 3 3 3 4 5% 21%	3736
	11th to 20th	Night	21% 20% 2 4 2 2 3 2 3 6% 35%	3494
	21st to end	Night	30% 22% 4 8% 5% 3 3 2 3 4 16%	3644
April	1st to 10th	Night	24% 25% 4 6% 5% 4 3 3 4 5% 17%	1900
	11th to 20th	Night	26% 21% 3 6% 5% 3 3 3 3 5% 22%	1879
	21st to end	Night	34% 16% 3 8% 5% 3 4 2 3 4 18%	1611
May	1st to 10th	Night	21% 19% 2 5% 3 3 3 2 3 5% 35%	2540
	11th to 20th	Night	23% 13% 2 6% 4 3 3 3 4 6% 32%	2516
	21st to end	Night	41% 15% 3 5% 4 3 3 2 3 5% 16%	2762
June	1st to 10th	Night	30% 9% 2 7% 4 4 4 4 4 6% 25%	1482
	11th to 20th	Night	33% 12% 4 7% 3 4 3 3 5% 5% 21%	1330
	21st to end	Night	21% 8% 2 7% 4 4 3 3 5% 6% 36%	1330
July	1st to 10th	Night	13% 6% 2 4 2 2 2 2 2 4 61%	2220
	11th to 20th	Night	8% 3 1 3 3 2 2 2 3 4 69%	2444
	21st to end	Night	14% 5% 1 3 3 2 3 2 4 6% 57%	2762
August	1st to 10th	Night	7% 3 1 3 3 2 2 3 5% 5% 65%	2529
	11th to 20th	Night	18% 11% 2 5% 3 3 2 3 4 6% 43%	2765
	21st to end	Night	22% 8% 2 5% 3 3 3 3 4 5% 44%	3384
September	1st to 10th	Night	16% 6% 3 5% 3 4 3 2 4 4 50%	1900
	11th to 20th	Night	24% 10% 2 7% 4 3 2 2 4 5% 37%	1900
	21st to end	Night	24% 9% 3 6% 4 3 4 3 4 5% 34%	2151
October	1st to 10th	Night	22% 8% 2 5% 4 4 4 4 6% 7% 34%	3800
	11th to 20th	Night	35% 10% 2 5% 3 2 3 3 4 5% 29%	3777
	21st to end	Night	33% 16% 3 7% 4 3 3 2 3 4 22%	4392
November	1st to 10th	Night	39% 16% 3 6% 4 3 3 3 4 4 16%	2655
	11th to 20th	Night	36% 18% 2 6% 3 3 3 2 3 4 20%	2660
	21st to end	Night	36% 19% 2 5% 3 4 3 2 3 3 19%	2660
December	1st to 10th	Night	28% 17% 3 5% 3 3 3 3 4 5% 25%	2660
	11th to 20th	Night	31% 20% 4 8% 4 3 3 2 3 4 16%	3280
	21st to end	Night	30% 21% 4 8% 5% 4 3 3 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	83% 2 4 8% 2 1	2312
	11th to 20th	Day	65% 4 8% 13% 8% 3	2540
	21st to end	Day	78% 4 6% 7% 5% 1	2794
February	1st to 10th	Day	72% 6% 8% 9% 5% 1	1691
	11th to 20th	Day	70% 5% 8% 8% 8% 1	1674
	21st to end	Day	73% 6% 6% 11% 4 1	1501
March	1st to 10th	Day	70% 4 6% 12% 7% 1	3138
	11th to 20th	Day	59% 3 9% 25% 3	3482
	21st to end	Day	76% 5% 8% 8% 3	4008
April	1st to 10th	Day	75% 7% 10% 7% 1	2280
	11th to 20th	Day	70% 6% 7% 9% 7% 1	2252
	21st to end	Day	75% 5% 7% 7% 4 1	2565
May	1st to 10th	Day	60% 7% 9% 17% 7%	4420
	11th to 20th	Day	56% 9% 19% 15% 2	4380
	21st to end	Day	77% 6% 10% 6% 1	4814
June	1st to 10th	Day	67% 5% 12% 10% 5% 1	2693
	11th to 20th	Day	75% 7% 7% 9% 3	2850
	21st to end	Day	53% 8% 12% 19% 6% 1	2850
July	1st to 10th	Day	29% 5% 9% 23% 22% 12%	4666
	11th to 20th	Day	24% 4 8% 27% 33% 4	4420
	21st to end	Day	32% 4 15% 32% 14% 3	4639
August	1st to 10th	Day	25% 4 11% 30% 28% 2	4384
	11th to 20th	Day	47% 6% 16% 20% 8% 2	4108
	21st to end	Day	48% 5% 10% 24% 11% 1	4289
September	1st to 10th	Day	40% 4 13% 21% 16% 6%	2280
	11th to 20th	Day	49% 6% 9% 16% 16% 4	2280
	21st to end	Day	51% 5% 15% 15% 11% 3	2024
October	1st to 10th	Day	48% 7% 14% 24% 7%	3180
	11th to 20th	Day	64% 3 9% 13% 9% 1	3161
	21st to end	Day	68% 5% 10% 10% 8%	3167
November	1st to 10th	Day	72% 7% 7% 9% 4	1623
	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	82% 3 3 7% 3 1	4627
	11th to 20th	Night	64% 5% 6% 12% 11% 2	4420
	21st to end	Night	77% 4 6% 6% 5% 1	4862
February	1st to 10th	Night	74% 4 6% 9% 5% 1	2489
	11th to 20th	Night	70% 5% 7% 9% 6% 3	2404
	21st to end	Night	72% 3 8% 11% 5% 1	1952
March	1st to 10th	Night	66% 7% 8% 9% 9% 1	3736
	11th to 20th	Night	54% 4 14% 23% 5%	3495
	21st to end	Night	75% 7% 10% 7% 1	3644
April	1st to 10th	Night	73% 6% 12% 9% 1	1900
	11th to 20th	Night	66% 6% 10% 9% 7% 1	1879
	21st to end	Night	72% 5% 7% 10% 5%	1611
May	1st to 10th	Night	53% 11% 9% 21% 5%	2540
	11th to 20th	Night	51% 14% 17% 16% 2	2516
	21st to end	Night	72% 11% 9% 8%	2762
June	1st to 10th	Night	61% 10% 13% 12% 5%	1482
	11th to 20th	Night	65% 8% 14% 10% 3	1330
	21st to end	Night	49% 10% 16% 17% 6% 2	1330
July	1st to 10th	Night	30% 5% 8% 23% 25% 8%	2185
	11th to 20th	Night	22% 5% 9% 23% 38% 3	2399
	21st to end	Night	30% 6% 17% 26% 18% 4	2674
August	1st to 10th	Night	22% 5% 14% 33% 23% 2	2521
	11th to 20th	Night	43% 7% 20% 19% 10% 2	2791
	21st to end	Night	44% 5% 13% 26% 11% 1	3384
September	1st to 10th	Night	39% 7% 12% 22% 16% 4	1900
	11th to 20th	Night	52% 5% 7% 17% 15% 4	1900
	21st to end	Night	53% 7% 11% 17% 10% 3	2151
October	1st to 10th	Night	48% 6% 13% 27% 6%	3800
	11th to 20th	Night	60% 4 10% 15% 11% 1	3777
	21st to end	Night	68% 4 8% 10% 9% 1	4392
November	1st to 10th	Night	74% 4 9% 8% 4 1	2655
	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	47% 14% 21% 14% 4	180
	11th to 20th	Day	41% 14% 27% 14% 4	180
	21st to end	Day	45% 14% 20% 16% 5%	198
February	1st to 10th	Day	45% 16% 24% 12% 3	351
	11th to 20th	Day	45% 15% 24% 13% 2	365
	21st to end	Day	37% 20% 25% 15% 3	350
March	1st to 10th	Day	37% 22% 26% 13% 3	460
	11th to 20th	Day	37% 16% 23% 20% 5% 1	460
	21st to end	Day	44% 17% 23% 13% 2	506
April	1st to 10th	Day	45% 16% 21% 15% 4	460
	11th to 20th	Day	37% 20% 23% 15% 4 1	452
	21st to end	Day	46% 14% 22% 15% 4	460
May	1st to 10th	Day	30% 18% 31% 18% 3	463
	11th to 20th	Day	33% 12% 24% 22% 7% 1	488
	21st to end	Day	40% 15% 22% 18% 5% 1	529
June	1st to 10th	Day	34% 17% 23% 20% 5% 1	523
	11th to 20th	Day	40% 17% 25% 13% 5%	680
	21st to end	Day	23% 14% 29% 27% 7% 1	680
July	1st to 10th	Day	17% 13% 35% 28% 6% 1	671
	11th to 20th	Day	15% 12% 34% 28% 8% 3	533
	21st to end	Day	14% 8% 35% 29% 11% 4	531
August	1st to 10th	Day	17% 7% 31% 30% 12% 3	487
	11th to 20th	Day	13% 11% 31% 29% 13% 3	477
	21st to end	Day	19% 14% 31% 25% 9% 1	539
September	1st to 10th	Day	20% 15% 30% 25% 8% 2	490
	11th to 20th	Day	23% 15% 28% 25% 8%	490
	21st to end	Day	31% 13% 28% 19% 9%	490
October	1st to 10th	Day	32% 13% 24% 21% 9% 2	490
	11th to 20th	Day	40% 11% 23% 17% 8% 1	490
	21st to end	Day	42% 10% 29% 14% 5% 1	372
November	1st to 10th	Day	42% 12% 30% 13% 3	231
	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	44% 11% 30% 14% 1	760
	21st to end	Night	42% 10% 31% 16% 1	836
February	1st to 10th	Night	43% 11% 32% 13% 1	760
	11th to 20th	Night	38% 12% 32% 16% 1	744
	21st to end	Night	41% 11% 31% 16% 2	628
March	1st to 10th	Night	35% 12% 30% 19% 3	760
	11th to 20th	Night	41% 12% 26% 17% 3	760
	21st to end	Night	41% 13% 30% 13% 3	836
April	1st to 10th	Night	39% 11% 25% 21% 3	760
	11th to 20th	Night	38% 13% 27% 19% 3	751
	21st to end	Night	43% 9% 28% 17% 3	471
May	1st to 10th	Night	28% 11% 32% 23% 5%	380
	11th to 20th	Night	29% 8% 25% 31% 7%	374
	21st to end	Night	36% 8% 23% 28% 6%	411
June	1st to 10th	Night	33% 8% 27% 24% 8%	380
	11th to 20th	Night	36% 10% 24% 21% 8%	380
	21st to end	Night	24% 5% 29% 34% 8% 1	380
July	1st to 10th	Night	20% 8% 18% 35% 18%	370
	11th to 20th	Night	15% 5% 19% 39% 21%	368
	21st to end	Night	10% 6% 16% 41% 27%	416
August	1st to 10th	Night	14% 4 12% 41% 30%	378
	11th to 20th	Night	15% 2 14% 45% 23% 1	529
	21st to end	Night	19% 6% 21% 40% 13%	761
September	1st to 10th	Night	19% 4 29% 34% 13%	760
	11th to 20th	Night	28% 8% 18% 32% 14%	760
	21st to end	Night	31% 6% 23% 33% 7%	760
October	1st to 10th	Night	32% 6% 25% 29% 7%	760
	11th to 20th	Night	40% 6% 27% 23% 4	760
	21st to end	Night	41% 11% 31% 16% 2	836
November	1st to 10th	Night	49% 11% 28% 11% 1	778
	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 2024-11-07 03:55:48. Version ac07

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