Everett
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Everett clear sky chart from 2005-11-06 to 2025-11-03.

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	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	131149
	Night	23% 11% 3 8% 5% 4 4 3 5% 5% 29%	127584
Transparency	Day	53% 4 15% 19% 8% 1	130863
	Night	58% 4 13% 18% 7% 1	127447
Seeing	Day	23% 9% 20% 25% 17% 6%	16403
	Night	28% 9% 15% 25% 20% 4	24695

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	34% 18% 4 11% 6% 5% 3 3 4 4 8%	3179
	11th to 20th	Day	19% 9% 3 9% 7% 6% 6% 4 6% 7% 24%	3588
	21st to end	Day	28% 12% 4 8% 4 4 4 2 4 5% 24%	4075
February	1st to 10th	Day	30% 13% 4 9% 6% 5% 5% 3 5% 5% 15%	1800
	11th to 20th	Day	27% 15% 4 10% 6% 4 3 3 5% 4 20%	1755
	21st to end	Day	28% 15% 4 8% 6% 5% 4 3 4 5% 20%	1550
March	1st to 10th	Day	20% 14% 5% 9% 6% 6% 5% 4 6% 7% 18%	4603
	11th to 20th	Day	20% 14% 4 9% 5% 5% 4 3 5% 5% 25%	5000
	21st to end	Day	25% 13% 4 10% 6% 6% 7% 5% 6% 6% 13%	5678
April	1st to 10th	Day	23% 14% 4 9% 5% 6% 4 5% 6% 7% 17%	2400
	11th to 20th	Day	22% 10% 4 8% 6% 5% 5% 4 5% 7% 25%	2400
	21st to end	Day	19% 11% 3 9% 7% 6% 5% 4 6% 6% 23%	2660
May	1st to 10th	Day	16% 8% 2 7% 5% 4 3 3 5% 6% 40%	6320
	11th to 20th	Day	18% 10% 3 9% 5% 5% 5% 4 6% 7% 27%	6277
	21st to end	Day	22% 9% 3 7% 4 3 4 4 6% 8% 29%	6903
June	1st to 10th	Day	20% 8% 3 8% 5% 4 4 4 5% 8% 31%	2875
	11th to 20th	Day	22% 10% 3 9% 5% 5% 5% 4 6% 7% 24%	3000
	21st to end	Day	13% 7% 4 7% 5% 4 4 4 6% 8% 38%	3000
July	1st to 10th	Day	10% 4 1 3 2 3 3 3 4 6% 61%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 70%	6533
	21st to end	Day	8% 4 2 4 3 3 3 3 5% 7% 57%	6874
August	1st to 10th	Day	6% 3 1 3 3 3 4 3 5% 8% 62%	4492
	11th to 20th	Day	9% 5% 3 8% 6% 4 4 4 6% 9% 41%	4156
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 6% 41%	4476
September	1st to 10th	Day	10% 7% 3 7% 3 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	14% 8% 3 8% 5% 4 4 3 6% 6% 39%	2400
	21st to end	Day	18% 8% 3 7% 4 5% 4 3 5% 7% 37%	2115
October	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 7% 10% 32%	3280
	11th to 20th	Day	32% 11% 3 8% 4 3 3 3 4 6% 23%	3261
	21st to end	Day	25% 12% 5% 11% 6% 6% 4 4 5% 6% 17%	3336
November	1st to 10th	Day	34% 11% 4 8% 4 5% 5% 4 5% 6% 16%	1622
	11th to 20th	Day	33% 11% 3 7% 5% 3 4 3 5% 5% 21%	1600
	21st to end	Day	33% 12% 3 7% 5% 4 3 4 5% 5% 18%	1600
December	1st to 10th	Day	22% 10% 3 7% 6% 6% 6% 4 6% 7% 22%	2620
	11th to 20th	Day	33% 15% 5% 10% 5% 5% 4 3 4 5% 12%	3025
	21st to end	Day	30% 17% 5% 11% 6% 5% 4 3 4 4 11%	3516
January	1st to 10th	Night	34% 15% 4 12% 5% 4 4 3 4 3 10%	6766
	11th to 20th	Night	20% 9% 3 7% 4 5% 5% 4 6% 8% 29%	6306
	21st to end	Night	26% 11% 3 8% 5% 4 4 3 4 5% 28%	6881
February	1st to 10th	Night	30% 13% 3 9% 5% 4 4 3 4 5% 19%	2600
	11th to 20th	Night	28% 15% 4 8% 5% 4 4 3 4 4 22%	2532
	21st to end	Night	28% 13% 3 8% 4 5% 4 4 4 5% 21%	2078
March	1st to 10th	Night	17% 15% 4 11% 6% 6% 5% 5% 6% 6% 18%	5288
	11th to 20th	Night	22% 13% 4 8% 4 4 4 3 4 5% 28%	4999
	21st to end	Night	29% 15% 4 10% 5% 5% 5% 4 4 4 14%	5284
April	1st to 10th	Night	25% 12% 4 9% 6% 5% 5% 4 6% 6% 18%	2000
	11th to 20th	Night	21% 11% 3 7% 6% 4 5% 3 5% 5% 30%	1949
	21st to end	Night	23% 11% 3 8% 6% 6% 5% 3 6% 6% 24%	1740
May	1st to 10th	Night	17% 9% 3 7% 4 3 3 3 3 4 42%	3640
	11th to 20th	Night	24% 10% 3 7% 4 4 3 3 5% 5% 32%	3616
	21st to end	Night	25% 9% 3 8% 5% 4 5% 4 6% 7% 25%	3972
June	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 4 5% 6% 32%	1520
	11th to 20th	Night	24% 11% 3 9% 5% 4 4 3 6% 5% 26%	1400
	21st to end	Night	15% 6% 3 6% 4 5% 4 4 7% 8% 39%	1400
July	1st to 10th	Night	13% 5% 2 4 3 3 2 2 3 4 59%	3180
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 5% 73%	3410
	21st to end	Night	9% 5% 1 4 3 3 3 3 5% 6% 58%	3969
August	1st to 10th	Night	4 3 1 3 3 3 3 2 4 6% 68%	2657
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 4 6% 50%	2902
	21st to end	Night	15% 6% 3 7% 4 3 3 3 3 5% 47%	3438
September	1st to 10th	Night	9% 5% 2 6% 3 3 5% 3 5% 6% 54%	2000
	11th to 20th	Night	15% 9% 3 7% 6% 3 4 3 4 5% 41%	2000
	21st to end	Night	17% 8% 4 8% 4 4 3 3 5% 6% 39%	2280
October	1st to 10th	Night	15% 7% 2 8% 5% 5% 5% 3 6% 7% 35%	3920
	11th to 20th	Night	27% 10% 2 6% 4 4 4 3 5% 6% 29%	3897
	21st to end	Night	29% 14% 4 9% 5% 4 3 3 4 5% 20%	4467
November	1st to 10th	Night	34% 11% 4 7% 5% 4 4 3 4 5% 19%	2718
	11th to 20th	Night	34% 12% 3 7% 4 4 3 3 4 4 21%	2800
	21st to end	Night	32% 12% 3 8% 5% 4 5% 4 4 5% 19%	2800
December	1st to 10th	Night	24% 10% 3 8% 5% 6% 5% 5% 6% 6% 24%	4560
	11th to 20th	Night	32% 14% 4 10% 6% 5% 5% 3 4 5% 13%	5180
	21st to end	Night	29% 17% 5% 11% 6% 5% 5% 3 4 4 11%	7435

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	81% 3 6% 8% 2	3179
	11th to 20th	Day	56% 6% 11% 20% 8% 1	3612
	21st to end	Day	63% 2 8% 17% 10%	4075
February	1st to 10th	Day	71% 3 10% 10% 4 1	1800
	11th to 20th	Day	69% 2 10% 10% 8% 1	1764
	21st to end	Day	71% 1 9% 13% 5%	1550
March	1st to 10th	Day	66% 3 10% 16% 6%	4603
	11th to 20th	Day	63% 4 13% 18% 2	5000
	21st to end	Day	71% 4 13% 10% 1	5678
April	1st to 10th	Day	69% 4 15% 11% 1	2400
	11th to 20th	Day	62% 2 11% 16% 7% 1	2400
	21st to end	Day	64% 4 12% 15% 5% 1	2660
May	1st to 10th	Day	47% 3 14% 25% 11%	6320
	11th to 20th	Day	56% 6% 18% 17% 3	6280
	21st to end	Day	55% 4 23% 16% 1	6904
June	1st to 10th	Day	56% 3 17% 18% 5% 1	2875
	11th to 20th	Day	62% 4 16% 15% 3	3000
	21st to end	Day	46% 4 21% 21% 6% 1	3000
July	1st to 10th	Day	26% 4 18% 23% 20% 8%	6706
	11th to 20th	Day	19% 3 16% 32% 28% 2	6453
	21st to end	Day	28% 6% 24% 31% 10% 1	6681
August	1st to 10th	Day	23% 4 15% 40% 17% 1	4475
	11th to 20th	Day	40% 8% 20% 24% 7% 2	4195
	21st to end	Day	44% 5% 13% 27% 11% 1	4475
September	1st to 10th	Day	39% 3 15% 27% 12% 4	2400
	11th to 20th	Day	47% 4 14% 20% 14% 1	2400
	21st to end	Day	48% 4 18% 19% 10% 2	2115
October	1st to 10th	Day	46% 5% 17% 26% 5%	3280
	11th to 20th	Day	64% 2 10% 16% 8%	3261
	21st to end	Day	66% 5% 11% 10% 7%	3336
November	1st to 10th	Day	70% 3 12% 10% 4	1622
	11th to 20th	Day	68% 3 10% 11% 6% 2	1600
	21st to end	Day	69% 4 6% 12% 8% 1	1600
December	1st to 10th	Day	57% 5% 11% 16% 12%	2620
	11th to 20th	Day	75% 5% 9% 9% 2	3028
	21st to end	Day	78% 3 9% 8% 2	3516
January	1st to 10th	Night	80% 3 7% 7% 3	6766
	11th to 20th	Night	53% 6% 12% 21% 7% 1	6344
	21st to end	Night	61% 2 10% 18% 9%	6881
February	1st to 10th	Night	69% 3 9% 12% 5% 1	2600
	11th to 20th	Night	67% 3 10% 11% 7% 1	2534
	21st to end	Night	66% 2 11% 15% 5% 1	2078
March	1st to 10th	Night	65% 4 11% 15% 5%	5288
	11th to 20th	Night	61% 3 16% 18% 2 1	5000
	21st to end	Night	75% 4 13% 8% 1	5284
April	1st to 10th	Night	67% 5% 14% 13% 2	2000
	11th to 20th	Night	58% 3 15% 15% 8% 1	1949
	21st to end	Night	63% 3 12% 16% 5% 1	1740
May	1st to 10th	Night	48% 2 12% 29% 8%	3637
	11th to 20th	Night	56% 4 18% 18% 4	3616
	21st to end	Night	60% 6% 20% 13% 1	3972
June	1st to 10th	Night	54% 4 16% 20% 6% 1	1520
	11th to 20th	Night	62% 4 15% 17% 3	1400
	21st to end	Night	45% 3 21% 23% 5% 3	1400
July	1st to 10th	Night	31% 3 15% 22% 18% 11%	3145
	11th to 20th	Night	16% 3 16% 30% 34% 1	3367
	21st to end	Night	27% 5% 22% 31% 14% 2	3857
August	1st to 10th	Night	20% 3 15% 41% 19% 2	2649
	11th to 20th	Night	37% 6% 22% 24% 9% 2	2929
	21st to end	Night	41% 4 15% 30% 9% 1	3438
September	1st to 10th	Night	34% 4 16% 29% 13% 4	2000
	11th to 20th	Night	47% 4 11% 22% 14% 3	2000
	21st to end	Night	49% 4 14% 23% 9% 1	2280
October	1st to 10th	Night	46% 6% 19% 25% 4	3920
	11th to 20th	Night	57% 3 14% 19% 7%	3897
	21st to end	Night	67% 4 11% 12% 6%	4467
November	1st to 10th	Night	68% 3 12% 13% 3 1	2718
	11th to 20th	Night	69% 2 10% 12% 5% 2	2800
	21st to end	Night	68% 3 8% 12% 8% 1	2800
December	1st to 10th	Night	56% 6% 12% 18% 8%	4560
	11th to 20th	Night	75% 4 9% 9% 2	5176
	21st to end	Night	79% 3 8% 8% 2	7435

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	39% 10% 16% 17% 11% 8%	200
	11th to 20th	Day	33% 13% 17% 18% 16% 5%	200
	21st to end	Day	39% 10% 16% 17% 13% 5%	252
February	1st to 10th	Day	39% 12% 21% 17% 9% 3	400
	11th to 20th	Day	37% 12% 24% 17% 9% 2	395
	21st to end	Day	33% 17% 18% 22% 8% 2	364
March	1st to 10th	Day	29% 16% 26% 16% 12% 2	500
	11th to 20th	Day	28% 16% 18% 19% 15% 5%	500
	21st to end	Day	32% 13% 27% 18% 9% 1	550
April	1st to 10th	Day	27% 17% 22% 20% 11% 3	500
	11th to 20th	Day	22% 15% 28% 19% 10% 6%	492
	21st to end	Day	21% 15% 25% 24% 12% 4	500
May	1st to 10th	Day	13% 11% 26% 33% 14% 3	503
	11th to 20th	Day	22% 10% 20% 25% 18% 6%	528
	21st to end	Day	21% 12% 18% 26% 16% 7%	573
June	1st to 10th	Day	20% 8% 22% 28% 18% 4	605
	11th to 20th	Day	20% 9% 25% 28% 15% 3	730
	21st to end	Day	14% 4 21% 33% 21% 6%	730
July	1st to 10th	Day	11% 3 19% 36% 26% 5%	721
	11th to 20th	Day	8% 5% 18% 35% 27% 8%	615
	21st to end	Day	8% 2 15% 36% 30% 9%	575
August	1st to 10th	Day	10% 4 12% 37% 27% 11%	527
	11th to 20th	Day	8% 3 14% 35% 28% 11%	517
	21st to end	Day	13% 4 21% 31% 21% 9%	583
September	1st to 10th	Day	15% 5% 20% 33% 19% 8%	530
	11th to 20th	Day	16% 8% 19% 27% 26% 5%	530
	21st to end	Day	21% 9% 22% 27% 16% 5%	530
October	1st to 10th	Day	23% 9% 20% 22% 19% 7%	530
	11th to 20th	Day	31% 10% 18% 21% 15% 6%	530
	21st to end	Day	32% 13% 20% 18% 13% 4	449
November	1st to 10th	Day	44% 11% 18% 15% 10% 2	224
	11th to 20th	Day	41% 13% 12% 14% 11% 10%	200
	21st to end	Day	34% 14% 17% 14% 10% 13%	200
December	1st to 10th	Day	40% 10% 12% 11% 16% 13%	200
	11th to 20th	Day	43% 14% 19% 14% 11% 1	200
	21st to end	Day	43% 10% 15% 22% 6% 2	220
January	1st to 10th	Night	40% 13% 14% 17% 13% 2	900
	11th to 20th	Night	35% 11% 13% 20% 18% 2	805
	21st to end	Night	35% 10% 14% 23% 15% 3	880
February	1st to 10th	Night	36% 10% 16% 25% 11% 2	800
	11th to 20th	Night	33% 11% 16% 23% 16% 2	784
	21st to end	Night	33% 10% 19% 24% 12% 2	660
March	1st to 10th	Night	31% 12% 18% 24% 14% 2	800
	11th to 20th	Night	31% 16% 15% 20% 15% 2	800
	21st to end	Night	30% 15% 21% 23% 9% 1	880
April	1st to 10th	Night	28% 12% 18% 25% 15% 2	800
	11th to 20th	Night	25% 10% 21% 26% 14% 3	761
	21st to end	Night	26% 10% 20% 27% 14% 3	540
May	1st to 10th	Night	20% 9% 15% 33% 22% 3	400
	11th to 20th	Night	21% 7% 13% 29% 27% 3	394
	21st to end	Night	22% 7% 16% 26% 24% 5%	433
June	1st to 10th	Night	22% 8% 17% 31% 19% 4	400
	11th to 20th	Night	26% 8% 15% 32% 18% 2	400
	21st to end	Night	19% 4 11% 36% 27% 4	400
July	1st to 10th	Night	14% 3 11% 33% 36% 4	390
	11th to 20th	Night	8% 4 12% 37% 35% 4	388
	21st to end	Night	8% 4 10% 30% 43% 6%	438
August	1st to 10th	Night	9% 2 7% 29% 44% 9%	420
	11th to 20th	Night	10% 2 8% 25% 42% 14%	594
	21st to end	Night	12% 5% 12% 29% 35% 7%	775
September	1st to 10th	Night	12% 2 10% 33% 35% 7%	800
	11th to 20th	Night	19% 5% 12% 24% 30% 11%	800
	21st to end	Night	21% 6% 15% 26% 25% 6%	800
October	1st to 10th	Night	20% 7% 12% 27% 26% 7%	800
	11th to 20th	Night	29% 8% 14% 23% 21% 5%	800
	21st to end	Night	33% 10% 16% 23% 15% 4	880
November	1st to 10th	Night	39% 12% 15% 21% 12% 1	788
	11th to 20th	Night	40% 9% 19% 18% 12% 3	800
	21st to end	Night	36% 9% 15% 20% 15% 5%	800
December	1st to 10th	Night	33% 8% 14% 24% 18% 4	800
	11th to 20th	Night	40% 12% 16% 19% 11% 1	800
	21st to end	Night	41% 13% 18% 18% 9% 1	985

Page generated at 2025-10-30 23:02:41. Version ac07

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