Washington State Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington State Star Party clear sky chart from 2005-11-06 to 2026-04-02.

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	6% 7% 3 7% 4 4 4 3 5% 7% 49%	135299
	Night	8% 9% 3 7% 5% 4 4 3 5% 7% 45%	135212
Transparency	Day	34% 6% 11% 23% 20% 7%	135027
	Night	38% 6% 11% 23% 16% 5%	135062
Seeing	Day	9% 9% 22% 29% 22% 9%	15954
	Night	11% 5% 23% 37% 21% 3	25106

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	13% 17% 6% 16% 7% 5% 4 2 4 4 21%	4723
	11th to 20th	Day	8% 9% 3 7% 5% 5% 4 4 5% 7% 42%	4716
	21st to end	Day	15% 16% 6% 12% 5% 4 4 2 4 6% 28%	5186
February	1st to 10th	Day	11% 14% 4 11% 7% 6% 5% 4 6% 5% 27%	1801
	11th to 20th	Day	8% 13% 5% 10% 6% 4 4 4 7% 8% 32%	1865
	21st to end	Day	7% 12% 4 9% 7% 5% 5% 4 6% 8% 33%	1730
March	1st to 10th	Day	4 7% 3 9% 5% 6% 5% 5% 6% 8% 41%	4618
	11th to 20th	Day	5% 8% 2 8% 5% 5% 5% 4 6% 7% 46%	4885
	21st to end	Day	8% 7% 3 8% 7% 6% 7% 5% 7% 9% 34%	6028
April	1st to 10th	Day	8% 8% 3 7% 6% 4 6% 5% 6% 9% 38%	2413
	11th to 20th	Day	6% 7% 3 6% 5% 5% 5% 5% 6% 9% 45%	2372
	21st to end	Day	5% 7% 2 7% 7% 5% 4 4 6% 8% 45%	2643
May	1st to 10th	Day	5% 5% 2 5% 4 3 4 3 5% 8% 56%	6320
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 5% 7% 49%	6277
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 4 4 8% 49%	6903
June	1st to 10th	Day	8% 6% 2 6% 5% 4 5% 5% 6% 7% 45%	2800
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 6% 7% 50%	2800
	21st to end	Day	4 5% 2 5% 3 3 3 3 5% 8% 61%	2800
July	1st to 10th	Day	1 2 1 2 2 2 2 2 4 5% 78%	6320
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 80%	6320
	21st to end	Day	3 2 1 3 2 2 3 2 4 6% 73%	6874
August	1st to 10th	Day	2 2 1 3 3 2 3 2 4 6% 72%	4541
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 7% 70%	4302
	21st to end	Day	4 5% 2 5% 3 3 3 3 4 6% 63%	4312
September	1st to 10th	Day	3 2 1 3 2 3 3 2 5% 7% 69%	2400
	11th to 20th	Day	4 4 2 5% 4 4 4 4 6% 7% 56%	2375
	21st to end	Day	6% 5% 2 5% 3 4 4 3 4 7% 57%	2190
October	1st to 10th	Day	4 4 2 4 3 3 3 3 5% 8% 60%	3280
	11th to 20th	Day	6% 6% 2 6% 5% 6% 6% 4 6% 8% 46%	3214
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 5% 7% 46%	3200
November	1st to 10th	Day	13% 11% 3 9% 6% 4 5% 4 6% 6% 33%	1834
	11th to 20th	Day	10% 16% 4 9% 6% 5% 5% 3 5% 6% 30%	1680
	21st to end	Day	12% 14% 6% 12% 6% 4 5% 4 4 9% 24%	1680
December	1st to 10th	Day	9% 11% 4 14% 7% 5% 5% 4 6% 8% 28%	2700
	11th to 20th	Day	15% 20% 7% 16% 5% 5% 4 3 4 4 17%	3105
	21st to end	Day	11% 22% 6% 14% 6% 6% 5% 4 4 5% 16%	4092
January	1st to 10th	Night	14% 19% 7% 12% 5% 4 4 3 4 5% 22%	8172
	11th to 20th	Night	8% 9% 3 8% 5% 4 4 3 5% 8% 44%	8182
	21st to end	Night	12% 15% 5% 12% 5% 4 4 3 4 5% 32%	8994
February	1st to 10th	Night	9% 13% 5% 9% 6% 5% 5% 5% 6% 7% 30%	2819
	11th to 20th	Night	10% 11% 3 8% 5% 4 5% 5% 6% 8% 35%	2641
	21st to end	Night	7% 12% 3 9% 6% 5% 5% 5% 7% 7% 33%	2076
March	1st to 10th	Night	5% 7% 2 6% 5% 4 5% 5% 8% 9% 42%	5496
	11th to 20th	Night	6% 9% 3 7% 5% 3 5% 4 5% 7% 47%	5320
	21st to end	Night	8% 9% 3 9% 7% 6% 6% 5% 8% 9% 31%	5176
April	1st to 10th	Night	7% 9% 2 8% 6% 4 5% 4 5% 8% 40%	2010
	11th to 20th	Night	6% 8% 2 6% 4 5% 4 5% 6% 6% 48%	1979
	21st to end	Night	6% 6% 3 5% 5% 4 6% 4 6% 8% 47%	1754
May	1st to 10th	Night	3 4 2 6% 3 4 4 3 5% 7% 59%	3640
	11th to 20th	Night	5% 6% 3 5% 3 4 4 3 5% 6% 55%	3616
	21st to end	Night	5% 4 1 4 4 4 4 4 6% 9% 56%	3972
June	1st to 10th	Night	7% 6% 1 5% 4 4 4 4 7% 7% 50%	1600
	11th to 20th	Night	5% 4 2 5% 4 3 4 5% 6% 7% 54%	1600
	21st to end	Night	3 2 2 5% 4 4 4 3 5% 7% 61%	1600
July	1st to 10th	Night	2 2 1 2 1 2 2 2 3 5% 80%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 3 3 86%	3640
	21st to end	Night	1 1 1 2 2 2 3 2 4 5% 76%	3969
August	1st to 10th	Night	2 3 1 3 3 2 3 2 4 6% 71%	2609
	11th to 20th	Night	2 2 1 4 3 3 3 3 5% 7% 68%	2734
	21st to end	Night	3 4 1 4 3 3 4 3 4 5% 66%	3608
September	1st to 10th	Night	3 2 1 3 3 3 3 3 4 8% 68%	2000
	11th to 20th	Night	4 4 2 5% 4 3 4 3 5% 7% 60%	2025
	21st to end	Night	5% 4 2 4 3 4 4 5% 5% 7% 58%	2205
October	1st to 10th	Night	5% 3 1 3 3 4 5% 4 7% 8% 58%	3920
	11th to 20th	Night	5% 5% 2 5% 5% 5% 5% 5% 6% 8% 49%	3928
	21st to end	Night	10% 7% 3 7% 5% 4 5% 4 5% 7% 44%	4600
November	1st to 10th	Night	14% 9% 3 7% 4 5% 5% 4 6% 8% 35%	2681
	11th to 20th	Night	11% 12% 3 9% 6% 4 5% 5% 6% 6% 34%	2940
	21st to end	Night	12% 14% 5% 10% 6% 5% 5% 4 5% 5% 28%	2940
December	1st to 10th	Night	7% 9% 3 9% 5% 4 4 3 6% 8% 40%	4700
	11th to 20th	Night	16% 15% 7% 16% 7% 6% 5% 4 5% 6% 13%	5320
	21st to end	Night	16% 19% 6% 11% 6% 5% 4 3 4 5% 20%	7106

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	67% 5% 7% 12% 8%	4723
	11th to 20th	Day	42% 6% 15% 28% 8% 2	4740
	21st to end	Day	59% 6% 10% 18% 7%	5187
February	1st to 10th	Day	56% 9% 13% 15% 5% 1	1801
	11th to 20th	Day	48% 8% 14% 19% 10% 1	1874
	21st to end	Day	47% 9% 14% 20% 9% 1	1730
March	1st to 10th	Day	39% 6% 12% 21% 21% 1	4618
	11th to 20th	Day	35% 5% 11% 32% 15% 1	4885
	21st to end	Day	43% 9% 18% 19% 10% 1	6028
April	1st to 10th	Day	41% 8% 15% 22% 13% 2	2413
	11th to 20th	Day	34% 7% 14% 22% 18% 5%	2372
	21st to end	Day	35% 6% 12% 25% 18% 4	2643
May	1st to 10th	Day	25% 4 8% 22% 35% 7%	6320
	11th to 20th	Day	31% 6% 13% 27% 20% 4	6280
	21st to end	Day	31% 7% 14% 30% 17% 2	6904
June	1st to 10th	Day	35% 6% 11% 21% 21% 6%	2800
	11th to 20th	Day	30% 6% 11% 26% 22% 5%	2800
	21st to end	Day	23% 4 10% 29% 26% 9%	2800
July	1st to 10th	Day	11% 2 6% 19% 31% 32%	6251
	11th to 20th	Day	11% 2 3 17% 32% 35%	6245
	21st to end	Day	13% 3 5% 24% 34% 21%	6681
August	1st to 10th	Day	14% 4 6% 19% 36% 22%	4524
	11th to 20th	Day	14% 4 9% 36% 30% 7%	4344
	21st to end	Day	23% 4 9% 26% 31% 7%	4311
September	1st to 10th	Day	16% 4 10% 27% 32% 11%	2400
	11th to 20th	Day	25% 7% 12% 24% 25% 7%	2375
	21st to end	Day	27% 6% 15% 30% 17% 5%	2190
October	1st to 10th	Day	23% 5% 12% 37% 21% 2	3280
	11th to 20th	Day	35% 7% 16% 23% 19% 1	3214
	21st to end	Day	36% 7% 14% 26% 14% 4	3200
November	1st to 10th	Day	50% 7% 16% 17% 9% 1	1834
	11th to 20th	Day	53% 9% 13% 16% 7% 1	1680
	21st to end	Day	57% 10% 13% 11% 7% 1	1680
December	1st to 10th	Day	52% 9% 16% 18% 5%	2700
	11th to 20th	Day	72% 6% 7% 9% 6%	3108
	21st to end	Day	70% 8% 8% 9% 4	4092
January	1st to 10th	Night	64% 6% 11% 13% 6%	8172
	11th to 20th	Night	40% 6% 15% 30% 10%	8220
	21st to end	Night	55% 6% 12% 19% 8%	8994
February	1st to 10th	Night	50% 10% 17% 15% 7% 1	2819
	11th to 20th	Night	45% 8% 17% 19% 10% 1	2643
	21st to end	Night	46% 9% 15% 19% 10% 1	2076
March	1st to 10th	Night	34% 7% 12% 25% 19% 3	5496
	11th to 20th	Night	36% 5% 11% 31% 16% 1	5321
	21st to end	Night	45% 9% 16% 21% 8% 1	5176
April	1st to 10th	Night	40% 6% 15% 26% 11% 2	2010
	11th to 20th	Night	33% 6% 12% 24% 20% 4	1979
	21st to end	Night	33% 7% 12% 27% 19% 3	1754
May	1st to 10th	Night	25% 4 8% 19% 39% 6%	3637
	11th to 20th	Night	29% 4 9% 29% 26% 2	3616
	21st to end	Night	23% 6% 17% 36% 17% 1	3972
June	1st to 10th	Night	32% 5% 9% 25% 22% 6%	1600
	11th to 20th	Night	26% 6% 12% 28% 23% 4	1600
	21st to end	Night	23% 5% 10% 29% 26% 7%	1600
July	1st to 10th	Night	10% 2 5% 21% 30% 33%	3600
	11th to 20th	Night	6% 1 3 20% 31% 38%	3592
	21st to end	Night	11% 3 6% 22% 34% 24%	3857
August	1st to 10th	Night	14% 3 5% 18% 35% 24%	2601
	11th to 20th	Night	16% 4 8% 35% 28% 8%	2758
	21st to end	Night	20% 5% 7% 26% 30% 12%	3608
September	1st to 10th	Night	16% 4 8% 26% 31% 15%	2000
	11th to 20th	Night	23% 5% 10% 26% 27% 9%	2025
	21st to end	Night	24% 6% 12% 31% 19% 7%	2205
October	1st to 10th	Night	22% 5% 7% 42% 22% 1	3920
	11th to 20th	Night	32% 7% 13% 26% 20% 2	3928
	21st to end	Night	39% 6% 12% 26% 14% 3	4600
November	1st to 10th	Night	45% 8% 17% 18% 9% 2	2681
	11th to 20th	Night	49% 9% 14% 17% 9% 2	2940
	21st to end	Night	55% 9% 13% 15% 8% 1	2940
December	1st to 10th	Night	43% 9% 19% 25% 5%	4700
	11th to 20th	Night	71% 8% 10% 7% 3	5316
	21st to end	Night	64% 8% 10% 10% 7% 1	7106

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	23% 8% 24% 31% 12% 3	220
	11th to 20th	Day	19% 8% 16% 32% 20% 5%	220
	21st to end	Day	21% 4 15% 36% 19% 5%	242
February	1st to 10th	Day	15% 8% 22% 38% 15% 4	341
	11th to 20th	Day	14% 6% 25% 32% 18% 4	436
	21st to end	Day	11% 9% 28% 30% 19% 3	446
March	1st to 10th	Day	9% 8% 27% 31% 19% 5%	540
	11th to 20th	Day	10% 12% 25% 25% 20% 8%	540
	21st to end	Day	10% 13% 28% 29% 16% 3	594
April	1st to 10th	Day	12% 13% 23% 27% 20% 5%	505
	11th to 20th	Day	8% 16% 31% 24% 16% 7%	492
	21st to end	Day	6% 14% 24% 27% 21% 8%	500
May	1st to 10th	Day	7% 11% 23% 35% 20% 5%	503
	11th to 20th	Day	13% 9% 22% 25% 19% 12%	528
	21st to end	Day	8% 9% 22% 24% 24% 13%	573
June	1st to 10th	Day	7% 10% 25% 30% 19% 10%	530
	11th to 20th	Day	6% 11% 29% 30% 18% 5%	530
	21st to end	Day	3 7% 22% 26% 30% 12%	530
July	1st to 10th	Day	4 5% 18% 36% 30% 8%	526
	11th to 20th	Day	2 4 21% 30% 31% 12%	524
	21st to end	Day	1 4 14% 32% 32% 17%	575
August	1st to 10th	Day	3 5% 15% 30% 30% 17%	527
	11th to 20th	Day	2 3 16% 27% 32% 19%	517
	21st to end	Day	5% 6% 18% 28% 27% 16%	583
September	1st to 10th	Day	4 5% 15% 30% 29% 18%	530
	11th to 20th	Day	5% 7% 18% 30% 28% 12%	530
	21st to end	Day	8% 12% 19% 30% 24% 8%	530
October	1st to 10th	Day	7% 8% 26% 29% 22% 8%	530
	11th to 20th	Day	9% 8% 22% 27% 25% 8%	508
	21st to end	Day	13% 9% 28% 31% 15% 3	363
November	1st to 10th	Day	15% 12% 23% 30% 18% 1	319
	11th to 20th	Day	13% 15% 24% 25% 17% 6%	220
	21st to end	Day	15% 7% 21% 32% 21% 5%	220
December	1st to 10th	Day	17% 9% 16% 27% 22% 10%	220
	11th to 20th	Day	16% 9% 31% 30% 11% 2	220
	21st to end	Day	18% 9% 21% 33% 15% 4	242
January	1st to 10th	Night	23% 5% 25% 31% 13% 2	840
	11th to 20th	Night	20% 4 22% 33% 18% 3	840
	21st to end	Night	18% 3 19% 39% 18% 2	924
February	1st to 10th	Night	14% 5% 26% 38% 15% 2	840
	11th to 20th	Night	14% 5% 24% 37% 19% 2	824
	21st to end	Night	11% 6% 30% 35% 15% 3	692
March	1st to 10th	Night	12% 6% 23% 39% 18% 2	840
	11th to 20th	Night	8% 7% 27% 36% 21% 1	840
	21st to end	Night	10% 7% 30% 37% 14% 2	924
April	1st to 10th	Night	10% 7% 21% 38% 22% 2	804
	11th to 20th	Night	9% 8% 30% 34% 16% 3	791
	21st to end	Night	8% 7% 31% 34% 17% 3	554
May	1st to 10th	Night	6% 6% 28% 38% 21% 2	400
	11th to 20th	Night	10% 6% 20% 35% 26% 4	394
	21st to end	Night	8% 4 18% 39% 27% 4	433
June	1st to 10th	Night	7% 6% 24% 41% 21% 2	400
	11th to 20th	Night	6% 7% 31% 39% 16% 2	400
	21st to end	Night	4 3 27% 36% 28% 2	400
July	1st to 10th	Night	3 3 20% 43% 29% 2	390
	11th to 20th	Night	2 2 24% 42% 28% 1	388
	21st to end	Night	2 2 18% 38% 37% 3	438
August	1st to 10th	Night	2 2 16% 35% 40% 5%	398
	11th to 20th	Night	3 1 13% 36% 38% 9%	488
	21st to end	Night	4 3 16% 41% 33% 4	880
September	1st to 10th	Night	3 2 19% 37% 33% 7%	800
	11th to 20th	Night	6% 4 17% 30% 36% 8%	800
	21st to end	Night	6% 4 20% 37% 29% 4	800
October	1st to 10th	Night	10% 5% 16% 40% 27% 1	800
	11th to 20th	Night	10% 6% 23% 34% 23% 4	800
	21st to end	Night	14% 4 24% 37% 16% 4	880
November	1st to 10th	Night	18% 7% 21% 41% 12%	820
	11th to 20th	Night	16% 10% 26% 31% 14% 3	840
	21st to end	Night	18% 5% 21% 33% 19% 3	840
December	1st to 10th	Night	15% 6% 20% 38% 17% 3	840
	11th to 20th	Night	21% 7% 21% 38% 13% 1	840
	21st to end	Night	22% 5% 25% 37% 10% 1	924

Page generated at 2026-03-29 23:18:44. Version ac07

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