Eatonville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eatonville Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 11% 3 8% 5% 5% 4 3 5% 6% 34%	136426
	Night	16% 12% 4 8% 5% 4 4 3 5% 6% 33%	134449
Transparency	Day	54% 4 11% 17% 10% 2	136147
	Night	57% 5% 10% 16% 10% 2	134306
Seeing	Day	25% 12% 22% 26% 12% 3	16707
	Night	27% 9% 15% 26% 20% 3	25273

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	23% 21% 5% 10% 6% 6% 6% 3 4 5% 11%	4723
	11th to 20th	Day	16% 12% 3 6% 4 3 4 3 5% 7% 36%	4716
	21st to end	Day	16% 11% 4 9% 5% 5% 5% 4 5% 5% 31%	5425
February	1st to 10th	Day	22% 15% 5% 11% 5% 5% 5% 3 5% 6% 18%	1890
	11th to 20th	Day	23% 18% 4 9% 5% 5% 4 3 5% 5% 17%	1845
	21st to end	Day	24% 22% 4 9% 5% 5% 4 3 3 4 17%	1641
March	1st to 10th	Day	12% 13% 4 10% 7% 7% 6% 5% 8% 8% 18%	4743
	11th to 20th	Day	16% 18% 6% 10% 5% 5% 5% 4 4 3 25%	5110
	21st to end	Day	18% 17% 5% 11% 7% 6% 7% 5% 7% 7% 12%	5784
April	1st to 10th	Day	22% 17% 4 9% 6% 6% 5% 4 5% 6% 15%	2520
	11th to 20th	Day	20% 14% 4 10% 6% 5% 5% 4 5% 5% 23%	2448
	21st to end	Day	22% 15% 4 9% 5% 5% 4 4 4 4 22%	2625
May	1st to 10th	Day	16% 9% 3 7% 5% 4 5% 4 5% 6% 38%	6320
	11th to 20th	Day	16% 14% 4 11% 5% 6% 5% 4 5% 6% 23%	6277
	21st to end	Day	21% 9% 3 8% 5% 5% 5% 4 6% 8% 27%	6903
June	1st to 10th	Day	25% 10% 3 7% 5% 5% 4 3 5% 6% 27%	2800
	11th to 20th	Day	24% 12% 4 8% 6% 5% 5% 4 6% 6% 20%	2935
	21st to end	Day	13% 9% 3 7% 5% 5% 4 4 6% 7% 37%	3000
July	1st to 10th	Day	7% 4 1 4 2 3 2 2 4 5% 65%	6700
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 73%	6320
	21st to end	Day	8% 4 2 4 2 2 3 3 4 6% 62%	6874
August	1st to 10th	Day	6% 4 2 4 3 3 3 3 4 7% 63%	4419
	11th to 20th	Day	9% 6% 3 6% 5% 5% 5% 4 6% 7% 45%	4156
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 45%	4445
September	1st to 10th	Day	10% 6% 3 6% 3 4 5% 3 5% 7% 50%	2400
	11th to 20th	Day	13% 6% 3 7% 5% 4 4 4 5% 7% 41%	2400
	21st to end	Day	15% 8% 2 6% 4 5% 4 3 5% 6% 41%	2114
October	1st to 10th	Day	15% 8% 2 5% 3 4 4 3 5% 8% 44%	3280
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 3 5% 7% 28%	3261
	21st to end	Day	20% 12% 3 9% 5% 5% 5% 5% 5% 6% 25%	3377
November	1st to 10th	Day	25% 14% 4 8% 7% 5% 5% 5% 5% 5% 17%	1718
	11th to 20th	Day	26% 14% 5% 9% 6% 5% 4 3 6% 6% 18%	1680
	21st to end	Day	24% 15% 3 9% 5% 5% 5% 3 5% 5% 22%	1680
December	1st to 10th	Day	17% 12% 3 7% 4 5% 5% 4 4 5% 35%	2700
	11th to 20th	Day	22% 14% 5% 9% 6% 5% 6% 4 6% 6% 17%	3105
	21st to end	Day	23% 18% 5% 10% 6% 6% 5% 4 4 4 15%	4092
January	1st to 10th	Night	23% 20% 6% 11% 6% 5% 5% 3 4 4 11%	8172
	11th to 20th	Night	14% 13% 4 8% 4 3 3 3 4 7% 36%	8182
	21st to end	Night	17% 10% 4 9% 5% 5% 5% 3 5% 6% 32%	8754
February	1st to 10th	Night	22% 15% 4 10% 6% 5% 5% 3 4 6% 19%	2730
	11th to 20th	Night	21% 16% 4 9% 6% 5% 3 3 4 5% 24%	2662
	21st to end	Night	22% 18% 4 8% 6% 5% 4 4 5% 5% 19%	2165
March	1st to 10th	Night	13% 15% 4 9% 6% 6% 6% 6% 7% 6% 21%	5366
	11th to 20th	Night	15% 13% 4 9% 6% 5% 5% 4 5% 6% 30%	5109
	21st to end	Night	22% 17% 5% 10% 5% 6% 5% 4 5% 6% 15%	5432
April	1st to 10th	Night	17% 16% 5% 10% 6% 6% 5% 4 5% 5% 21%	2100
	11th to 20th	Night	17% 15% 4 8% 5% 4 4 3 5% 5% 28%	2034
	21st to end	Night	21% 13% 3 9% 6% 4 4 3 5% 5% 26%	1775
May	1st to 10th	Night	17% 10% 3 7% 4 3 3 3 5% 5% 40%	3640
	11th to 20th	Night	14% 13% 4 8% 4 5% 4 3 5% 6% 32%	3616
	21st to end	Night	14% 11% 3 8% 4 5% 5% 4 6% 8% 31%	3972
June	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 5% 5% 7% 33%	1600
	11th to 20th	Night	18% 12% 4 8% 5% 4 4 3 5% 7% 30%	1458
	21st to end	Night	13% 6% 3 6% 5% 3 4 4 6% 6% 45%	1400
July	1st to 10th	Night	4 3 2 3 2 2 2 2 3 6% 72%	3242
	11th to 20th	Night	3 3 1 3 2 1 2 2 3 4 77%	3640
	21st to end	Night	6% 3 1 3 2 2 2 3 4 5% 69%	3969
August	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 71%	2732
	11th to 20th	Night	10% 7% 4 5% 4 3 3 2 4 6% 52%	2902
	21st to end	Night	15% 7% 2 6% 3 3 3 3 4 4 50%	3470
September	1st to 10th	Night	9% 6% 2 5% 3 3 4 3 4 5% 56%	2000
	11th to 20th	Night	14% 8% 2 7% 4 4 4 4 4 6% 44%	2000
	21st to end	Night	12% 7% 2 7% 4 4 4 3 5% 7% 44%	2281
October	1st to 10th	Night	13% 8% 3 6% 4 3 4 3 5% 7% 45%	3920
	11th to 20th	Night	19% 11% 3 7% 4 4 4 3 5% 6% 34%	3897
	21st to end	Night	15% 11% 5% 10% 5% 5% 5% 4 5% 6% 29%	4427
November	1st to 10th	Night	27% 11% 4 8% 5% 5% 5% 5% 5% 6% 20%	2796
	11th to 20th	Night	26% 13% 3 9% 6% 5% 5% 3 4 5% 20%	2940
	21st to end	Night	22% 14% 3 8% 6% 5% 5% 4 4 6% 22%	2940
December	1st to 10th	Night	15% 12% 3 7% 4 4 5% 4 5% 7% 33%	4700
	11th to 20th	Night	23% 17% 5% 10% 6% 5% 5% 4 5% 5% 15%	5320
	21st to end	Night	19% 16% 5% 10% 7% 6% 6% 4 5% 6% 17%	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	81% 3 4 7% 4	4723
	11th to 20th	Day	49% 4 8% 16% 18% 6%	4740
	21st to end	Day	60% 3 3 15% 18% 1	5426
February	1st to 10th	Day	70% 6% 7% 11% 6%	1890
	11th to 20th	Day	71% 6% 7% 9% 6% 1	1854
	21st to end	Day	73% 5% 8% 9% 5%	1641
March	1st to 10th	Day	63% 5% 14% 11% 6%	4743
	11th to 20th	Day	69% 3 5% 21% 2	5110
	21st to end	Day	75% 6% 11% 7% 2	5784
April	1st to 10th	Day	72% 7% 11% 9% 1	2520
	11th to 20th	Day	65% 5% 10% 13% 6% 1	2448
	21st to end	Day	68% 6% 10% 11% 3 1	2625
May	1st to 10th	Day	50% 4 13% 23% 10%	6320
	11th to 20th	Day	65% 7% 13% 12% 3	6280
	21st to end	Day	59% 6% 19% 13% 3	6904
June	1st to 10th	Day	60% 6% 14% 14% 6%	2800
	11th to 20th	Day	67% 6% 13% 11% 4	2935
	21st to end	Day	47% 7% 17% 19% 9% 1	3000
July	1st to 10th	Day	25% 3 14% 26% 17% 15%	6627
	11th to 20th	Day	18% 3 10% 27% 34% 8%	6245
	21st to end	Day	27% 2 19% 36% 15% 2	6681
August	1st to 10th	Day	24% 4 10% 30% 28% 4	4402
	11th to 20th	Day	44% 3 16% 25% 10% 2	4195
	21st to end	Day	43% 3 12% 23% 15% 4	4444
September	1st to 10th	Day	34% 5% 14% 26% 16% 5%	2400
	11th to 20th	Day	44% 7% 12% 18% 15% 4	2400
	21st to end	Day	45% 6% 14% 20% 12% 2	2114
October	1st to 10th	Day	42% 3 10% 30% 14% 1	3280
	11th to 20th	Day	60% 3 11% 15% 10% 1	3261
	21st to end	Day	66% 3 9% 11% 9% 2	3377
November	1st to 10th	Day	70% 5% 9% 11% 3 1	1718
	11th to 20th	Day	72% 5% 6% 9% 5% 2	1680
	21st to end	Day	67% 5% 5% 11% 10% 2	1680
December	1st to 10th	Day	56% 3 6% 15% 17% 3	2700
	11th to 20th	Day	74% 3 10% 9% 4	3108
	21st to end	Day	75% 5% 8% 9% 3	4092
January	1st to 10th	Night	81% 3 4 8% 4 1	8172
	11th to 20th	Night	48% 4 9% 16% 18% 5%	8220
	21st to end	Night	57% 4 5% 17% 15% 1	8754
February	1st to 10th	Night	69% 6% 9% 10% 5% 1	2730
	11th to 20th	Night	66% 5% 8% 10% 7% 3	2664
	21st to end	Night	70% 5% 9% 10% 5%	2165
March	1st to 10th	Night	66% 4 11% 12% 6% 1	5366
	11th to 20th	Night	62% 5% 11% 21% 2 1	5110
	21st to end	Night	75% 5% 9% 10% 1	5432
April	1st to 10th	Night	68% 7% 13% 10% 2	2100
	11th to 20th	Night	62% 6% 12% 13% 6% 1	2034
	21st to end	Night	66% 6% 12% 12% 4 1	1775
May	1st to 10th	Night	51% 4 13% 23% 10%	3637
	11th to 20th	Night	61% 6% 16% 14% 3	3616
	21st to end	Night	59% 10% 16% 14% 1	3972
June	1st to 10th	Night	54% 7% 15% 18% 5%	1600
	11th to 20th	Night	60% 7% 14% 16% 3	1458
	21st to end	Night	43% 8% 17% 22% 8% 2	1400
July	1st to 10th	Night	22% 3 14% 29% 14% 19%	3206
	11th to 20th	Night	17% 2 10% 29% 33% 9%	3592
	21st to end	Night	22% 3 19% 35% 16% 4	3857
August	1st to 10th	Night	21% 3 9% 36% 28% 4	2724
	11th to 20th	Night	43% 3 19% 22% 10% 3	2929
	21st to end	Night	42% 3 14% 24% 16% 2	3470
September	1st to 10th	Night	33% 6% 14% 25% 17% 5%	2000
	11th to 20th	Night	45% 5% 10% 20% 15% 5%	2000
	21st to end	Night	43% 7% 14% 20% 13% 4	2281
October	1st to 10th	Night	43% 4 13% 27% 13% 1	3920
	11th to 20th	Night	55% 3 12% 19% 9% 1	3897
	21st to end	Night	64% 3 9% 14% 10%	4427
November	1st to 10th	Night	69% 6% 9% 11% 4 1	2796
	11th to 20th	Night	70% 6% 7% 9% 5% 2	2940
	21st to end	Night	66% 6% 7% 9% 11% 1	2940
December	1st to 10th	Night	55% 4 8% 16% 16% 2	4700
	11th to 20th	Night	75% 4 8% 8% 3	5316
	21st to end	Night	75% 6% 9% 7% 3	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	39% 12% 19% 16% 10% 4	220
	11th to 20th	Day	29% 17% 17% 22% 13% 3	220
	21st to end	Day	33% 16% 18% 19% 10% 3	326
February	1st to 10th	Day	38% 13% 20% 17% 10% 1	430
	11th to 20th	Day	40% 15% 20% 16% 8% 1	425
	21st to end	Day	40% 16% 21% 19% 5% 1	399
March	1st to 10th	Day	31% 19% 24% 19% 6% 1	540
	11th to 20th	Day	35% 15% 19% 18% 12% 1	540
	21st to end	Day	33% 16% 28% 18% 5% 1	594
April	1st to 10th	Day	33% 16% 20% 20% 8% 3	540
	11th to 20th	Day	28% 21% 23% 18% 6% 3	517
	21st to end	Day	29% 15% 24% 22% 8% 2	500
May	1st to 10th	Day	22% 14% 28% 24% 11% 2	503
	11th to 20th	Day	29% 13% 23% 21% 9% 4	528
	21st to end	Day	26% 12% 22% 27% 11% 3	573
June	1st to 10th	Day	25% 10% 23% 27% 12% 3	530
	11th to 20th	Day	25% 12% 28% 25% 7% 3	665
	21st to end	Day	14% 9% 24% 35% 15% 4	730
July	1st to 10th	Day	14% 7% 23% 38% 16% 3	694
	11th to 20th	Day	8% 7% 24% 38% 19% 3	524
	21st to end	Day	9% 3 19% 41% 22% 6%	575
August	1st to 10th	Day	11% 7% 17% 40% 18% 6%	527
	11th to 20th	Day	8% 3 19% 42% 20% 8%	517
	21st to end	Day	11% 11% 23% 33% 15% 7%	583
September	1st to 10th	Day	14% 7% 23% 37% 15% 5%	530
	11th to 20th	Day	16% 10% 21% 31% 19% 2	530
	21st to end	Day	18% 12% 24% 28% 15% 3	530
October	1st to 10th	Day	22% 14% 20% 22% 19% 3	530
	11th to 20th	Day	28% 14% 19% 21% 14% 5%	530
	21st to end	Day	33% 13% 20% 20% 10% 4	477
November	1st to 10th	Day	33% 19% 17% 19% 9% 2	258
	11th to 20th	Day	35% 17% 13% 16% 12% 6%	220
	21st to end	Day	27% 14% 22% 17% 12% 8%	220
December	1st to 10th	Day	40% 14% 9% 15% 14% 9%	220
	11th to 20th	Day	42% 13% 21% 16% 6% 1	220
	21st to end	Day	40% 12% 16% 19% 11% 2	242
January	1st to 10th	Night	42% 13% 15% 16% 12% 2	840
	11th to 20th	Night	32% 12% 15% 24% 16% 3	840
	21st to end	Night	31% 10% 14% 26% 15% 3	924
February	1st to 10th	Night	34% 11% 17% 27% 10% 1	840
	11th to 20th	Night	33% 11% 14% 27% 14% 1	824
	21st to end	Night	35% 12% 17% 25% 10% 1	692
March	1st to 10th	Night	30% 14% 17% 25% 14% 1	840
	11th to 20th	Night	34% 14% 16% 23% 13% 1	840
	21st to end	Night	35% 11% 21% 23% 9% 1	924
April	1st to 10th	Night	29% 12% 17% 24% 16% 2	840
	11th to 20th	Night	27% 10% 21% 25% 15% 2	810
	21st to end	Night	28% 11% 13% 31% 14% 2	575
May	1st to 10th	Night	24% 9% 15% 33% 19% 1	400
	11th to 20th	Night	21% 9% 12% 28% 27% 2	394
	21st to end	Night	24% 7% 15% 24% 26% 3	433
June	1st to 10th	Night	19% 9% 16% 26% 27% 3	400
	11th to 20th	Night	22% 8% 18% 31% 21% 1	400
	21st to end	Night	15% 5% 14% 31% 33% 3	400
July	1st to 10th	Night	11% 3 9% 34% 39% 4	390
	11th to 20th	Night	9% 3 8% 38% 39% 3	388
	21st to end	Night	6% 2 10% 29% 48% 5%	438
August	1st to 10th	Night	11% 3 8% 25% 43% 9%	469
	11th to 20th	Night	9% 4 5% 26% 45% 11%	594
	21st to end	Night	13% 5% 9% 32% 37% 4	794
September	1st to 10th	Night	12% 4 10% 33% 35% 6%	800
	11th to 20th	Night	16% 6% 12% 25% 34% 7%	800
	21st to end	Night	18% 7% 12% 32% 27% 4	800
October	1st to 10th	Night	21% 7% 12% 30% 25% 4	800
	11th to 20th	Night	27% 9% 15% 26% 20% 4	800
	21st to end	Night	28% 10% 20% 25% 14% 3	880
November	1st to 10th	Night	35% 11% 17% 28% 9% 1	820
	11th to 20th	Night	37% 12% 16% 21% 12% 2	840
	21st to end	Night	34% 9% 18% 22% 14% 4	840
December	1st to 10th	Night	31% 10% 16% 26% 15% 3	840
	11th to 20th	Night	39% 12% 17% 23% 8% 1	840
	21st to end	Night	42% 13% 17% 18% 8%	924

Page generated at 2026-04-13 23:44:24. Version ac07

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