Anacortes
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anacortes clear sky chart from 2005-11-06 to 2025-05-11.

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% 9% 3 7% 5% 4 4 4 5% 6% 34%	117514
	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	120145
Transparency	Day	51% 9% 14% 17% 8% 1	117266
	Night	55% 8% 13% 16% 6% 1	120039
Seeing	Day	22% 9% 20% 25% 18% 5%	15813
	Night	28% 8% 17% 24% 19% 4	24255

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	32% 16% 4 10% 6% 5% 5% 3 4 4 13%	3179
	11th to 20th	Day	18% 10% 3 8% 5% 5% 6% 4 6% 8% 28%	3323
	21st to end	Day	29% 12% 3 8% 4 4 5% 4 5% 4 22%	4075
February	1st to 10th	Day	27% 13% 4 9% 6% 5% 5% 4 5% 5% 17%	1800
	11th to 20th	Day	26% 12% 4 10% 5% 4 5% 3 4 5% 22%	1755
	21st to end	Day	24% 14% 5% 9% 6% 4 5% 3 5% 5% 20%	1526
March	1st to 10th	Day	19% 15% 4 10% 7% 5% 5% 4 5% 6% 19%	4644
	11th to 20th	Day	17% 14% 4 9% 5% 5% 5% 4 5% 6% 25%	5000
	21st to end	Day	23% 12% 4 10% 6% 6% 7% 5% 6% 5% 18%	5653
April	1st to 10th	Day	22% 11% 4 9% 6% 5% 6% 4 6% 7% 21%	2400
	11th to 20th	Day	17% 10% 3 7% 5% 5% 5% 5% 6% 6% 29%	2434
	21st to end	Day	21% 9% 3 9% 4 5% 4 4 6% 7% 29%	2655
May	1st to 10th	Day	16% 7% 3 7% 4 4 4 3 5% 6% 42%	4560
	11th to 20th	Day	11% 6% 2 5% 5% 4 4 4 6% 10% 42%	4380
	21st to end	Day	23% 10% 3 8% 4 4 4 3 5% 6% 29%	4813
June	1st to 10th	Day	18% 8% 3 6% 5% 4 5% 4 6% 7% 35%	2818
	11th to 20th	Day	16% 8% 3 7% 6% 6% 5% 5% 6% 6% 32%	2850
	21st to end	Day	11% 6% 2 5% 4 5% 5% 5% 5% 7% 43%	2850
July	1st to 10th	Day	5% 3 1 3 2 2 3 2 3 5% 69%	4740
	11th to 20th	Day	5% 3 1 3 3 2 2 2 4 5% 70%	4708
	21st to end	Day	10% 4 1 5% 3 2 3 2 4 6% 60%	4790
August	1st to 10th	Day	5% 3 1 3 3 3 3 3 5% 7% 65%	4351
	11th to 20th	Day	9% 5% 2 7% 4 4 5% 4 6% 8% 47%	4026
	21st to end	Day	14% 7% 2 6% 4 4 3 3 5% 7% 44%	4376
September	1st to 10th	Day	10% 5% 2 7% 3 3 4 4 5% 6% 51%	2280
	11th to 20th	Day	14% 8% 2 7% 4 5% 4 3 5% 7% 41%	2280
	21st to end	Day	19% 8% 2 7% 4 4 4 4 4 7% 36%	2003
October	1st to 10th	Day	18% 7% 2 7% 5% 4 6% 5% 7% 8% 30%	3180
	11th to 20th	Day	30% 10% 3 7% 4 4 4 4 5% 6% 23%	3161
	21st to end	Day	23% 11% 4 11% 7% 5% 5% 4 5% 6% 20%	3240
November	1st to 10th	Day	30% 11% 4 8% 6% 5% 6% 4 4 5% 17%	1568
	11th to 20th	Day	28% 12% 3 9% 6% 4 4 3 4 4 22%	1600
	21st to end	Day	29% 12% 3 9% 6% 4 5% 4 5% 5% 18%	1600
December	1st to 10th	Day	20% 9% 3 8% 8% 6% 6% 4 6% 7% 23%	2620
	11th to 20th	Day	30% 15% 5% 11% 6% 6% 5% 3 5% 4 11%	2778
	21st to end	Day	28% 16% 5% 11% 6% 5% 5% 3 5% 5% 11%	3498
January	1st to 10th	Night	30% 16% 5% 11% 6% 5% 5% 3 4 4 12%	6766
	11th to 20th	Night	20% 10% 4 7% 4 4 5% 4 6% 8% 28%	6560
	21st to end	Night	27% 12% 3 7% 4 4 4 3 5% 7% 25%	6881
February	1st to 10th	Night	29% 14% 5% 8% 5% 4 4 3 4 5% 21%	2600
	11th to 20th	Night	25% 15% 3 8% 5% 4 4 3 4 4 24%	2532
	21st to end	Night	28% 11% 3 8% 5% 5% 4 3 5% 5% 22%	2103
March	1st to 10th	Night	16% 12% 4 10% 7% 7% 6% 4 6% 7% 23%	5249
	11th to 20th	Night	18% 13% 4 8% 5% 5% 5% 3 5% 6% 28%	4999
	21st to end	Night	29% 13% 5% 9% 6% 5% 5% 3 5% 5% 16%	5321
April	1st to 10th	Night	23% 12% 4 8% 7% 5% 3 4 6% 6% 22%	2000
	11th to 20th	Night	20% 9% 3 8% 6% 5% 5% 4 5% 7% 29%	1913
	21st to end	Night	22% 11% 4 9% 5% 4 4 3 5% 5% 27%	1744
May	1st to 10th	Night	16% 7% 2 6% 4 3 3 2 4 6% 47%	2620
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 46%	2519
	21st to end	Night	20% 9% 4 8% 6% 6% 5% 3 6% 6% 27%	2762
June	1st to 10th	Night	18% 8% 3 8% 5% 5% 5% 4 5% 6% 33%	1359
	11th to 20th	Night	17% 8% 2 8% 5% 5% 4 5% 6% 7% 33%	1330
	21st to end	Night	13% 5% 2 7% 5% 5% 3 3 6% 8% 43%	1330
July	1st to 10th	Night	6% 3 2 3 2 2 2 2 3 5% 69%	2220
	11th to 20th	Night	5% 3 1 2 2 1 2 2 3 4 75%	2251
	21st to end	Night	10% 5% 1 4 3 3 3 2 4 4 62%	2762
August	1st to 10th	Night	4 2 1 2 2 2 3 3 4 5% 71%	2578
	11th to 20th	Night	10% 5% 2 6% 4 4 3 3 4 6% 54%	2812
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 8% 47%	3296
September	1st to 10th	Night	9% 4 1 5% 3 3 3 4 4 7% 58%	1900
	11th to 20th	Night	15% 6% 2 7% 4 4 4 3 4 6% 44%	1900
	21st to end	Night	16% 6% 3 7% 5% 4 4 4 5% 6% 41%	2175
October	1st to 10th	Night	16% 6% 2 8% 4 5% 4 4 5% 7% 38%	3800
	11th to 20th	Night	27% 8% 2 6% 4 4 4 3 5% 7% 31%	3777
	21st to end	Night	26% 10% 4 7% 5% 4 4 4 5% 6% 25%	4321
November	1st to 10th	Night	33% 10% 3 8% 5% 5% 4 3 4 4 19%	2725
	11th to 20th	Night	30% 12% 3 7% 5% 5% 4 3 4 5% 22%	2800
	21st to end	Night	30% 12% 2 7% 6% 5% 5% 4 4 5% 20%	2800
December	1st to 10th	Night	22% 10% 3 8% 5% 5% 6% 5% 7% 7% 23%	4560
	11th to 20th	Night	30% 16% 4 11% 6% 5% 4 3 4 5% 12%	5422
	21st to end	Night	27% 15% 5% 10% 6% 6% 5% 3 4 4 14%	7458

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	76% 8% 7% 6% 2	3179
	11th to 20th	Day	53% 8% 14% 17% 7% 1	3344
	21st to end	Day	64% 7% 8% 15% 5%	4075
February	1st to 10th	Day	68% 10% 9% 8% 4 2	1800
	11th to 20th	Day	65% 9% 10% 9% 7% 1	1764
	21st to end	Day	66% 10% 10% 10% 4 1	1526
March	1st to 10th	Day	62% 9% 13% 13% 3	4644
	11th to 20th	Day	58% 7% 13% 19% 2 1	5000
	21st to end	Day	66% 10% 13% 10% 1	5653
April	1st to 10th	Day	62% 11% 16% 9% 1	2400
	11th to 20th	Day	54% 9% 14% 14% 7% 2	2434
	21st to end	Day	57% 9% 14% 14% 5% 1	2655
May	1st to 10th	Day	44% 10% 14% 22% 10%	4560
	11th to 20th	Day	36% 14% 26% 19% 5%	4383
	21st to end	Day	56% 15% 18% 10% 1	4814
June	1st to 10th	Day	50% 10% 20% 13% 6% 1	2818
	11th to 20th	Day	54% 12% 18% 12% 4	2850
	21st to end	Day	40% 12% 21% 19% 7% 1	2850
July	1st to 10th	Day	21% 7% 11% 25% 23% 13%	4666
	11th to 20th	Day	18% 5% 11% 34% 29% 3	4627
	21st to end	Day	28% 6% 21% 31% 12% 2	4639
August	1st to 10th	Day	21% 8% 16% 38% 16% 1	4334
	11th to 20th	Day	36% 8% 23% 24% 7% 2	4065
	21st to end	Day	43% 7% 15% 25% 10%	4375
September	1st to 10th	Day	32% 11% 18% 22% 13% 4	2280
	11th to 20th	Day	44% 8% 16% 16% 15% 1	2280
	21st to end	Day	46% 10% 17% 16% 9% 1	2003
October	1st to 10th	Day	47% 8% 19% 21% 4	3180
	11th to 20th	Day	61% 8% 12% 12% 7%	3161
	21st to end	Day	65% 7% 11% 11% 6%	3240
November	1st to 10th	Day	66% 10% 12% 8% 3 1	1568
	11th to 20th	Day	66% 8% 9% 10% 5% 2	1600
	21st to end	Day	66% 9% 9% 8% 7% 1	1600
December	1st to 10th	Day	57% 9% 10% 15% 8%	2620
	11th to 20th	Day	73% 10% 7% 7% 2	2781
	21st to end	Day	75% 9% 9% 5% 1	3498
January	1st to 10th	Night	75% 9% 7% 7% 2	6766
	11th to 20th	Night	53% 9% 15% 17% 6% 1	6600
	21st to end	Night	60% 9% 11% 16% 5%	6881
February	1st to 10th	Night	67% 8% 10% 8% 5% 2	2600
	11th to 20th	Night	63% 9% 10% 11% 5% 2	2534
	21st to end	Night	63% 8% 10% 14% 4 1	2103
March	1st to 10th	Night	59% 9% 14% 13% 5%	5249
	11th to 20th	Night	56% 7% 14% 20% 2	5000
	21st to end	Night	70% 9% 11% 9% 1	5321
April	1st to 10th	Night	63% 11% 13% 11% 2	2000
	11th to 20th	Night	56% 9% 14% 13% 7% 2	1913
	21st to end	Night	60% 6% 13% 14% 6% 1	1744
May	1st to 10th	Night	39% 8% 16% 27% 10%	2620
	11th to 20th	Night	33% 14% 26% 20% 7%	2519
	21st to end	Night	55% 15% 17% 11% 2	2762
June	1st to 10th	Night	53% 8% 17% 16% 5% 2	1359
	11th to 20th	Night	52% 10% 19% 16% 3	1330
	21st to end	Night	41% 8% 22% 22% 5% 3	1330
July	1st to 10th	Night	22% 5% 10% 27% 23% 14%	2185
	11th to 20th	Night	16% 4 12% 31% 35% 2	2209
	21st to end	Night	28% 4 14% 28% 22% 3	2674
August	1st to 10th	Night	16% 6% 16% 41% 20% 2	2570
	11th to 20th	Night	36% 5% 24% 24% 10% 2	2839
	21st to end	Night	38% 6% 17% 28% 10% 1	3296
September	1st to 10th	Night	28% 9% 16% 27% 14% 5%	1900
	11th to 20th	Night	43% 7% 14% 17% 16% 3	1900
	21st to end	Night	43% 9% 15% 23% 9% 1	2175
October	1st to 10th	Night	47% 6% 22% 21% 5%	3800
	11th to 20th	Night	55% 7% 15% 18% 5%	3777
	21st to end	Night	62% 7% 13% 12% 6%	4321
November	1st to 10th	Night	68% 6% 12% 10% 3 1	2725
	11th to 20th	Night	66% 7% 11% 9% 5% 2	2800
	21st to end	Night	66% 9% 11% 8% 6% 1	2800
December	1st to 10th	Night	59% 9% 11% 15% 5%	4560
	11th to 20th	Night	72% 9% 9% 7% 2	5419
	21st to end	Night	72% 10% 9% 7% 1	7458

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	37% 15% 17% 16% 10% 6%	200
	11th to 20th	Day	30% 17% 18% 16% 14% 7%	200
	21st to end	Day	40% 11% 18% 14% 14% 3	252
February	1st to 10th	Day	39% 9% 21% 19% 10% 2	400
	11th to 20th	Day	34% 12% 22% 20% 11% 2	395
	21st to end	Day	32% 14% 22% 18% 11% 2	352
March	1st to 10th	Day	31% 12% 24% 20% 12% 1	500
	11th to 20th	Day	30% 10% 19% 20% 17% 4	500
	21st to end	Day	31% 11% 23% 22% 11% 2	550
April	1st to 10th	Day	27% 13% 20% 23% 12% 3	500
	11th to 20th	Day	22% 12% 25% 23% 13% 5%	492
	21st to end	Day	26% 11% 22% 24% 14% 4	500
May	1st to 10th	Day	16% 9% 23% 30% 20% 3	503
	11th to 20th	Day	21% 7% 17% 28% 22% 5%	489
	21st to end	Day	21% 8% 22% 28% 16% 6%	529
June	1st to 10th	Day	21% 7% 25% 30% 15% 3	648
	11th to 20th	Day	21% 9% 24% 33% 10% 3	680
	21st to end	Day	11% 6% 25% 30% 24% 5%	680
July	1st to 10th	Day	11% 3 20% 35% 25% 7%	671
	11th to 20th	Day	7% 6% 19% 35% 26% 7%	649
	21st to end	Day	7% 3 14% 33% 32% 11%	531
August	1st to 10th	Day	10% 3 10% 29% 36% 11%	487
	11th to 20th	Day	6% 2 14% 29% 39% 10%	477
	21st to end	Day	10% 6% 18% 32% 26% 8%	539
September	1st to 10th	Day	14% 4 17% 32% 25% 7%	490
	11th to 20th	Day	16% 6% 17% 30% 27% 5%	490
	21st to end	Day	22% 7% 19% 24% 21% 7%	490
October	1st to 10th	Day	22% 6% 19% 25% 18% 9%	490
	11th to 20th	Day	31% 10% 19% 17% 16% 7%	490
	21st to end	Day	31% 12% 20% 20% 13% 5%	419
November	1st to 10th	Day	34% 16% 22% 16% 12% 2	200
	11th to 20th	Day	35% 12% 18% 16% 13% 7%	200
	21st to end	Day	31% 14% 22% 16% 9% 10%	200
December	1st to 10th	Day	37% 16% 13% 9% 12% 14%	200
	11th to 20th	Day	40% 16% 19% 16% 9% 1	200
	21st to end	Day	40% 10% 21% 21% 5% 2	220
January	1st to 10th	Night	40% 13% 17% 16% 12% 2	900
	11th to 20th	Night	37% 11% 14% 19% 18% 1	862
	21st to end	Night	36% 9% 17% 22% 14% 3	880
February	1st to 10th	Night	37% 11% 17% 23% 11% 2	800
	11th to 20th	Night	33% 10% 17% 24% 15% 1	784
	21st to end	Night	35% 11% 19% 21% 11% 3	660
March	1st to 10th	Night	31% 13% 20% 22% 13% 2	800
	11th to 20th	Night	32% 11% 19% 20% 16% 3	800
	21st to end	Night	34% 11% 20% 24% 9% 2	880
April	1st to 10th	Night	31% 9% 17% 26% 15% 2	800
	11th to 20th	Night	25% 9% 20% 28% 15% 3	725
	21st to end	Night	29% 8% 19% 26% 15% 3	544
May	1st to 10th	Night	18% 7% 16% 33% 23% 2	400
	11th to 20th	Night	21% 5% 15% 25% 28% 5%	375
	21st to end	Night	20% 5% 18% 29% 23% 4	411
June	1st to 10th	Night	22% 5% 22% 28% 22% 2	380
	11th to 20th	Night	21% 7% 19% 33% 17% 2	380
	21st to end	Night	16% 4 15% 33% 28% 4	380
July	1st to 10th	Night	12% 3 14% 34% 34% 4	370
	11th to 20th	Night	11% 3 14% 33% 35% 5%	368
	21st to end	Night	7% 2 13% 32% 38% 8%	416
August	1st to 10th	Night	9% 3 7% 24% 46% 11%	401
	11th to 20th	Night	9% 2 6% 24% 45% 15%	564
	21st to end	Night	13% 3 12% 30% 34% 8%	712
September	1st to 10th	Night	11% 3 11% 36% 32% 8%	760
	11th to 20th	Night	19% 4 12% 21% 32% 12%	760
	21st to end	Night	20% 6% 15% 25% 27% 7%	760
October	1st to 10th	Night	22% 5% 15% 23% 27% 8%	760
	11th to 20th	Night	31% 9% 11% 22% 22% 6%	760
	21st to end	Night	32% 9% 18% 24% 13% 4	836
November	1st to 10th	Night	37% 12% 19% 22% 11%	780
	11th to 20th	Night	39% 10% 17% 18% 14% 2	800
	21st to end	Night	35% 11% 20% 17% 13% 4	800
December	1st to 10th	Night	31% 9% 16% 23% 17% 4	800
	11th to 20th	Night	39% 12% 22% 19% 8% 1	857
	21st to end	Night	39% 13% 23% 17% 7% 1	990

Page generated at 2025-05-08 06:34:00. Version ac07

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