Edmonds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edmonds Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	18% 8% 3 6% 4 4 4 3 5% 7% 39%	126504
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	123854
Transparency	Day	29% 39% 8% 14% 9% 2	130053
	Night	29% 38% 7% 13% 11% 2	128380
Seeing	Day	20% 14% 32% 23% 9% 2	16520
	Night	21% 14% 31% 25% 8% 1	25371

By Month

Percentage Cloud Cover NE (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	11% 9% 3 5% 3 3 3 3 5% 6% 48%	3597
	11th to 20th	Day	16% 9% 2 4 3 3 3 2 4 4 49%	3570
	21st to end	Day	17% 10% 3 6% 4 4 4 3 4 4 41%	4002
February	1st to 10th	Day	19% 9% 3 7% 4 4 4 4 5% 6% 35%	1910
	11th to 20th	Day	15% 7% 3 6% 4 4 4 4 5% 6% 43%	1948
	21st to end	Day	19% 9% 3 6% 4 3 4 4 4 5% 39%	1650
March	1st to 10th	Day	21% 13% 3 6% 3 3 3 3 3 5% 38%	4476
	11th to 20th	Day	15% 11% 3 6% 4 3 3 3 5% 7% 39%	4837
	21st to end	Day	25% 10% 3 7% 4 4 4 3 4 5% 32%	5666
April	1st to 10th	Day	20% 10% 3 5% 3 3 4 3 4 6% 39%	2400
	11th to 20th	Day	19% 7% 3 5% 4 4 4 3 4 6% 40%	2404
	21st to end	Day	20% 6% 3 5% 3 4 4 3 5% 7% 38%	2600
May	1st to 10th	Day	27% 11% 4 7% 3 3 4 3 5% 7% 25%	5886
	11th to 20th	Day	21% 11% 3 6% 5% 4 4 3 4 7% 32%	5861
	21st to end	Day	20% 8% 2 6% 4 4 4 3 6% 7% 35%	6848
June	1st to 10th	Day	20% 6% 2 5% 5% 4 4 4 6% 8% 35%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	15% 6% 3 5% 4 4 4 4 6% 8% 40%	2800
July	1st to 10th	Day	18% 8% 3 6% 4 4 4 4 6% 9% 34%	6360
	11th to 20th	Day	13% 8% 2 7% 4 4 5% 4 7% 10% 35%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 43%	6836
August	1st to 10th	Day	17% 6% 3 7% 5% 4 5% 4 6% 8% 34%	4235
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 5% 7% 8% 39%	4152
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 9% 49%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	2365
	11th to 20th	Day	14% 6% 2 6% 3 3 4 2 5% 7% 50%	2200
	21st to end	Day	20% 6% 2 5% 3 4 3 3 5% 6% 41%	2200
October	1st to 10th	Day	22% 5% 2 5% 3 3 3 3 5% 6% 44%	2200
	11th to 20th	Day	17% 5% 2 4 3 3 3 3 4 8% 47%	1986
	21st to end	Day	18% 6% 1 6% 4 3 4 3 5% 5% 46%	1980
November	1st to 10th	Day	17% 7% 2 5% 4 3 4 3 4 6% 45%	1851
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 5% 7% 46%	1863
	21st to end	Day	18% 7% 2 4 4 3 4 3 5% 5% 44%	1800
December	1st to 10th	Day	15% 10% 2 4 3 3 3 2 3 6% 49%	2803
	11th to 20th	Day	19% 7% 2 6% 3 3 3 3 3 5% 48%	2986
	21st to end	Day	19% 11% 4 7% 4 4 3 3 5% 5% 37%	3960
January	1st to 10th	Night	14% 6% 1 4 3 3 3 3 4 6% 52%	6349
	11th to 20th	Night	15% 10% 4 7% 4 3 3 2 4 5% 42%	6315
	21st to end	Night	17% 10% 4 8% 4 4 4 3 4 4 37%	6951
February	1st to 10th	Night	17% 9% 3 8% 5% 4 3 3 5% 5% 39%	2486
	11th to 20th	Night	18% 9% 3 5% 4 4 4 3 5% 5% 40%	2338
	21st to end	Night	17% 9% 2 7% 5% 4 3 3 4 5% 41%	1980
March	1st to 10th	Night	24% 10% 2 5% 3 4 3 3 4 5% 37%	5364
	11th to 20th	Night	17% 11% 4 7% 4 4 4 3 4 5% 37%	5122
	21st to end	Night	24% 11% 2 6% 5% 4 4 3 4 4 32%	5248
April	1st to 10th	Night	19% 9% 3 7% 5% 5% 4 3 4 5% 37%	2000
	11th to 20th	Night	19% 9% 2 5% 4 4 4 3 5% 6% 37%	1945
	21st to end	Night	20% 7% 2 6% 5% 4 5% 3 5% 7% 37%	1800
May	1st to 10th	Night	27% 11% 3 8% 4 4 4 4 5% 6% 24%	4054
	11th to 20th	Night	21% 12% 4 8% 5% 4 5% 4 6% 6% 27%	4033
	21st to end	Night	21% 9% 3 8% 5% 5% 5% 5% 6% 7% 27%	4067
June	1st to 10th	Night	17% 8% 3 6% 6% 5% 4 3 5% 8% 34%	1600
	11th to 20th	Night	19% 8% 2 6% 4 4 5% 3 4 7% 38%	1600
	21st to end	Night	18% 8% 3 5% 4 3 4 4 7% 9% 36%	1600
July	1st to 10th	Night	19% 13% 4 7% 4 4 3 3 5% 8% 30%	3600
	11th to 20th	Night	20% 11% 2 5% 4 4 5% 3 6% 8% 32%	3600
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 4 5% 6% 39%	3969
August	1st to 10th	Night	23% 7% 3 7% 4 5% 5% 4 6% 7% 30%	2915
	11th to 20th	Night	10% 8% 3 7% 5% 6% 5% 5% 7% 8% 37%	2891
	21st to end	Night	9% 4 2 5% 4 3 3 4 6% 7% 53%	3586
September	1st to 10th	Night	13% 6% 3 6% 5% 4 4 3 5% 7% 44%	2035
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 6% 50%	2200
	21st to end	Night	20% 7% 3 5% 4 4 4 4 5% 5% 40%	2200
October	1st to 10th	Night	20% 7% 2 6% 3 4 4 3 4 6% 41%	2200
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 5% 6% 47%	2410
	21st to end	Night	18% 8% 3 6% 4 4 4 4 4 6% 40%	2860
November	1st to 10th	Night	16% 6% 2 6% 4 3 4 3 5% 6% 46%	2667
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 4 6% 45%	2692
	21st to end	Night	16% 6% 2 5% 4 3 4 4 4 7% 46%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 2 3 5% 47%	4373
	11th to 20th	Night	21% 9% 2 5% 4 3 3 3 4 5% 42%	5208
	21st to end	Night	17% 10% 4 6% 3 3 3 3 4 5% 40%	6996

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	26% 27% 5% 20% 20% 3	3598
	11th to 20th	Day	26% 27% 7% 17% 15% 7%	3600
	21st to end	Day	38% 29% 6% 13% 14% 2	4002
February	1st to 10th	Day	20% 63% 6% 6% 4 1	1910
	11th to 20th	Day	19% 61% 3 8% 8% 2	1957
	21st to end	Day	17% 62% 5% 8% 6% 2	1650
March	1st to 10th	Day	39% 29% 8% 7% 8% 10%	4479
	11th to 20th	Day	29% 32% 13% 18% 8% 1	4837
	21st to end	Day	43% 30% 4 9% 9% 5%	5666
April	1st to 10th	Day	21% 62% 4 6% 6% 1	2400
	11th to 20th	Day	16% 63% 4 9% 8% 1	2404
	21st to end	Day	18% 62% 4 8% 7% 1	2600
May	1st to 10th	Day	45% 33% 11% 8% 3	5897
	11th to 20th	Day	40% 32% 10% 12% 6%	5865
	21st to end	Day	37% 31% 7% 15% 10%	6848
June	1st to 10th	Day	17% 63% 4 9% 7%	2800
	11th to 20th	Day	16% 62% 5% 9% 6% 1	2800
	21st to end	Day	15% 61% 5% 11% 8%	2800
July	1st to 10th	Day	39% 29% 9% 13% 9% 1	6290
	11th to 20th	Day	36% 29% 12% 16% 6%	6285
	21st to end	Day	31% 27% 8% 23% 10% 1	6640
August	1st to 10th	Day	34% 38% 11% 13% 5%	4222
	11th to 20th	Day	27% 39% 12% 19% 4	4191
	21st to end	Day	15% 38% 11% 23% 13% 1	4312
September	1st to 10th	Day	16% 58% 4 14% 8%	2365
	11th to 20th	Day	15% 58% 5% 11% 10% 2	2200
	21st to end	Day	22% 57% 4 9% 7%	2200
October	1st to 10th	Day	23% 37% 7% 20% 11% 3	3590
	11th to 20th	Day	18% 36% 3 19% 20% 3	3202
	21st to end	Day	16% 36% 10% 24% 13% 1	3180
November	1st to 10th	Day	20% 54% 7% 9% 9% 1	1851
	11th to 20th	Day	18% 55% 4 10% 11% 2	1863
	21st to end	Day	17% 56% 5% 14% 6% 2	1800
December	1st to 10th	Day	24% 36% 8% 21% 10%	2803
	11th to 20th	Day	27% 32% 9% 12% 15% 5%	2986
	21st to end	Day	38% 27% 7% 14% 11% 3	3960
January	1st to 10th	Night	23% 26% 6% 28% 15% 2	6349
	11th to 20th	Night	33% 26% 8% 16% 12% 4	6360
	21st to end	Night	39% 30% 5% 12% 10% 4	6951
February	1st to 10th	Night	19% 62% 5% 8% 4 1	2486
	11th to 20th	Night	18% 61% 3 7% 10% 2	2340
	21st to end	Night	18% 62% 4 8% 6% 2	1980
March	1st to 10th	Night	39% 29% 6% 10% 10% 6%	5374
	11th to 20th	Night	35% 31% 11% 12% 9% 2	5123
	21st to end	Night	43% 29% 3 5% 14% 4	5248
April	1st to 10th	Night	22% 62% 3 7% 5% 2	2000
	11th to 20th	Night	17% 63% 5% 7% 7% 1	1945
	21st to end	Night	19% 62% 4 7% 7% 1	1800
May	1st to 10th	Night	45% 32% 11% 7% 4	4060
	11th to 20th	Night	43% 32% 9% 10% 4 1	4034
	21st to end	Night	41% 33% 8% 12% 6%	4068
June	1st to 10th	Night	17% 62% 4 9% 7%	1600
	11th to 20th	Night	16% 62% 4 11% 6% 1	1600
	21st to end	Night	16% 62% 4 10% 8% 1	1600
July	1st to 10th	Night	45% 26% 5% 12% 11%	3560
	11th to 20th	Night	40% 29% 11% 14% 6%	3552
	21st to end	Night	33% 27% 10% 19% 11% 1	3852
August	1st to 10th	Night	37% 39% 7% 10% 6% 1	2903
	11th to 20th	Night	26% 40% 10% 20% 4	2918
	21st to end	Night	14% 38% 12% 20% 17%	3586
September	1st to 10th	Night	16% 58% 5% 13% 7% 1	2035
	11th to 20th	Night	18% 57% 3 10% 11% 1	2200
	21st to end	Night	23% 58% 4 8% 7% 1	2200
October	1st to 10th	Night	25% 37% 7% 20% 10% 2	3590
	11th to 20th	Night	12% 36% 4 17% 28% 3	3920
	21st to end	Night	18% 36% 7% 21% 17% 1	4610
November	1st to 10th	Night	20% 54% 5% 10% 9% 1	2667
	11th to 20th	Night	18% 55% 4 9% 11% 3	2692
	21st to end	Night	16% 57% 6% 14% 6% 2	2600
December	1st to 10th	Night	26% 36% 7% 19% 12%	4373
	11th to 20th	Night	33% 31% 9% 10% 15% 1	5208
	21st to end	Night	36% 28% 7% 16% 13% 1	6996

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	21% 25% 35% 17% 3	200
	11th to 20th	Day	22% 27% 37% 11% 4	200
	21st to end	Day	18% 23% 40% 13% 5% 1	236
February	1st to 10th	Day	24% 21% 37% 16% 2	455
	11th to 20th	Day	25% 26% 37% 11% 1	496
	21st to end	Day	22% 23% 37% 14% 4	414
March	1st to 10th	Day	24% 22% 34% 17% 3	500
	11th to 20th	Day	20% 26% 33% 17% 4	500
	21st to end	Day	23% 22% 35% 15% 5% 1	550
April	1st to 10th	Day	26% 20% 34% 15% 4 1	500
	11th to 20th	Day	24% 20% 36% 16% 3	492
	21st to end	Day	24% 16% 34% 19% 6% 2	500
May	1st to 10th	Day	29% 16% 32% 17% 5%	503
	11th to 20th	Day	22% 12% 42% 17% 6% 2	527
	21st to end	Day	22% 10% 31% 27% 9% 2	573
June	1st to 10th	Day	20% 10% 30% 29% 10% 1	530
	11th to 20th	Day	17% 9% 37% 28% 9% 1	530
	21st to end	Day	14% 9% 33% 30% 12% 1	530
July	1st to 10th	Day	16% 7% 26% 33% 16% 2	525
	11th to 20th	Day	11% 6% 28% 33% 18% 3	524
	21st to end	Day	14% 6% 32% 34% 13% 2	576
August	1st to 10th	Day	13% 7% 29% 36% 14% 2	526
	11th to 20th	Day	14% 7% 24% 34% 18% 4	519
	21st to end	Day	13% 6% 30% 32% 14% 6%	583
September	1st to 10th	Day	18% 6% 28% 28% 15% 5%	530
	11th to 20th	Day	15% 8% 29% 25% 16% 7%	530
	21st to end	Day	25% 9% 28% 24% 10% 4	530
October	1st to 10th	Day	22% 12% 25% 23% 13% 6%	530
	11th to 20th	Day	20% 12% 34% 21% 12% 2	458
	21st to end	Day	22% 17% 28% 21% 9% 3	462
November	1st to 10th	Day	15% 16% 36% 21% 10% 2	425
	11th to 20th	Day	21% 20% 35% 19% 5%	421
	21st to end	Day	21% 24% 30% 18% 8% 1	400
December	1st to 10th	Day	20% 18% 37% 20% 5%	325
	11th to 20th	Day	26% 27% 33% 12% 3 1	200
	21st to end	Day	26% 18% 36% 12% 6% 2	220
January	1st to 10th	Night	21% 19% 40% 17% 4	800
	11th to 20th	Night	27% 21% 34% 16% 3	800
	21st to end	Night	21% 22% 36% 18% 4	880
February	1st to 10th	Night	22% 18% 37% 21% 2	800
	11th to 20th	Night	24% 22% 39% 14% 1	783
	21st to end	Night	21% 22% 35% 19% 3	660
March	1st to 10th	Night	23% 18% 36% 20% 4	800
	11th to 20th	Night	21% 15% 38% 21% 5% 1	800
	21st to end	Night	24% 17% 33% 21% 5%	880
April	1st to 10th	Night	23% 16% 34% 24% 4	800
	11th to 20th	Night	23% 16% 35% 23% 4	761
	21st to end	Night	22% 14% 33% 25% 6%	600
May	1st to 10th	Night	28% 11% 30% 25% 7%	600
	11th to 20th	Night	22% 11% 39% 22% 4 1	592
	21st to end	Night	23% 7% 23% 37% 9% 1	496
June	1st to 10th	Night	23% 5% 29% 31% 12%	400
	11th to 20th	Night	22% 5% 24% 39% 10% 1	400
	21st to end	Night	23% 5% 22% 39% 12%	400
July	1st to 10th	Night	19% 6% 23% 37% 14% 1	390
	11th to 20th	Night	16% 4 22% 40% 16% 2	388
	21st to end	Night	16% 4 23% 44% 12% 1	456
August	1st to 10th	Night	13% 5% 23% 45% 14% 1	598
	11th to 20th	Night	13% 5% 21% 44% 16% 1	598
	21st to end	Night	16% 4 20% 37% 20% 3	880
September	1st to 10th	Night	16% 4 25% 35% 18% 2	800
	11th to 20th	Night	18% 5% 25% 28% 21% 4	800
	21st to end	Night	24% 6% 22% 31% 14% 2	800
October	1st to 10th	Night	24% 10% 23% 26% 14% 3	800
	11th to 20th	Night	19% 14% 29% 25% 12% 2	800
	21st to end	Night	23% 14% 32% 22% 9%	880
November	1st to 10th	Night	20% 13% 31% 25% 10% 1	820
	11th to 20th	Night	19% 22% 32% 19% 7%	829
	21st to end	Night	17% 23% 33% 21% 6% 1	800
December	1st to 10th	Night	25% 19% 34% 18% 5% 1	800
	11th to 20th	Night	25% 22% 32% 18% 3 1	800
	21st to end	Night	25% 18% 37% 17% 3	880

Page generated at 2025-11-15 00:55:58. Version ac07

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