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 2024-07-16.

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% 12% 3 8% 5% 5% 5% 4 5% 6% 29%	90326
	Night	18% 12% 4 8% 5% 5% 4 4 5% 6% 29%	89220
Transparency	Day	58% 6% 11% 16% 8% 1	90049
	Night	59% 6% 11% 15% 7% 2	89077
Seeing	Day	25% 12% 21% 26% 13% 3	14536
	Night	27% 9% 14% 26% 20% 3	23002

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	27% 20% 4 9% 6% 5% 5% 2 3 4 12%	2539
	11th to 20th	Day	25% 15% 3 7% 5% 4 4 3 4 6% 24%	2516
	21st to end	Day	23% 13% 5% 12% 7% 6% 6% 4 6% 4 14%	2922
February	1st to 10th	Day	23% 15% 5% 11% 5% 5% 5% 3 5% 6% 18%	1710
	11th to 20th	Day	25% 18% 4 9% 5% 5% 4 3 5% 6% 17%	1665
	21st to end	Day	24% 22% 4 9% 5% 4 4 3 3 4 17%	1489
March	1st to 10th	Day	11% 13% 4 11% 8% 8% 7% 5% 6% 7% 19%	3221
	11th to 20th	Day	15% 14% 4 8% 5% 4 4 3 4 3 36%	3490
	21st to end	Day	18% 16% 5% 12% 8% 6% 6% 4 6% 5% 14%	3946
April	1st to 10th	Day	24% 18% 4 9% 6% 5% 5% 4 6% 6% 13%	2280
	11th to 20th	Day	21% 14% 4 10% 7% 5% 5% 4 5% 5% 22%	2255
	21st to end	Day	23% 15% 4 10% 6% 5% 4 4 4 4 20%	2494
May	1st to 10th	Day	21% 11% 3 7% 5% 4 4 3 4 5% 33%	4420
	11th to 20th	Day	14% 11% 3 9% 5% 5% 5% 4 6% 7% 32%	4377
	21st to end	Day	27% 11% 4 9% 6% 5% 6% 4 5% 6% 17%	4813
June	1st to 10th	Day	26% 10% 3 7% 5% 5% 4 4 5% 6% 25%	2660
	11th to 20th	Day	25% 12% 4 8% 6% 5% 5% 4 6% 6% 19%	2788
	21st to end	Day	13% 9% 3 7% 5% 4 4 4 6% 7% 37%	2850
July	1st to 10th	Day	10% 7% 2 6% 3 4 3 3 5% 6% 49%	2824
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 6% 7% 50%	2593
	21st to end	Day	8% 4 1 3 2 2 3 3 5% 6% 62%	2741
August	1st to 10th	Day	9% 6% 2 5% 3 3 3 3 6% 8% 52%	2441
	11th to 20th	Day	8% 4 2 3 3 3 3 3 5% 7% 57%	2298
	21st to end	Day	10% 6% 2 5% 5% 4 5% 3 6% 8% 45%	2453
September	1st to 10th	Day	11% 6% 3 5% 3 4 4 3 5% 7% 51%	2160
	11th to 20th	Day	13% 6% 3 8% 5% 4 4 4 5% 7% 41%	2160
	21st to end	Day	16% 9% 3 6% 4 5% 4 3 5% 6% 39%	1904
October	1st to 10th	Day	19% 11% 2 7% 4 4 4 3 5% 7% 34%	1800
	11th to 20th	Day	20% 9% 3 8% 5% 5% 4 3 5% 7% 31%	1800
	21st to end	Day	25% 11% 2 9% 4 4 4 4 4 5% 28%	1886
November	1st to 10th	Day	26% 13% 4 9% 7% 5% 5% 5% 5% 5% 16%	1552
	11th to 20th	Day	26% 14% 4 9% 5% 5% 4 3 5% 6% 19%	1520
	21st to end	Day	25% 15% 3 9% 5% 4 5% 2 5% 4 23%	1520
December	1st to 10th	Day	18% 14% 4 9% 5% 6% 5% 4 4 4 27%	1520
	11th to 20th	Day	20% 14% 4 10% 6% 4 5% 4 6% 6% 21%	1925
	21st to end	Day	21% 15% 4 10% 6% 7% 6% 5% 5% 5% 16%	2794
January	1st to 10th	Night	26% 21% 6% 11% 6% 5% 5% 3 4 4 11%	4406
	11th to 20th	Night	21% 15% 4 8% 5% 5% 4 4 5% 7% 22%	4382
	21st to end	Night	23% 14% 4 11% 6% 6% 6% 4 6% 6% 14%	4738
February	1st to 10th	Night	24% 15% 4 10% 6% 4 5% 4 4 6% 18%	2470
	11th to 20th	Night	22% 16% 4 9% 6% 5% 3 3 4 4 23%	2402
	21st to end	Night	22% 19% 3 8% 6% 5% 5% 4 4 5% 18%	1965
March	1st to 10th	Night	11% 14% 5% 10% 7% 7% 7% 5% 8% 7% 19%	3648
	11th to 20th	Night	13% 10% 3 7% 5% 4 4 3 5% 5% 40%	3489
	21st to end	Night	23% 17% 4 10% 6% 6% 5% 4 5% 4 14%	3706
April	1st to 10th	Night	18% 17% 5% 10% 6% 6% 5% 4 5% 5% 20%	1900
	11th to 20th	Night	18% 15% 4 8% 6% 5% 4 3 5% 5% 27%	1874
	21st to end	Night	22% 13% 3 9% 6% 4 4 3 5% 6% 23%	1686
May	1st to 10th	Night	21% 12% 3 8% 5% 3 3 3 4 4 35%	2540
	11th to 20th	Night	12% 10% 3 8% 4 5% 4 3 6% 6% 39%	2516
	21st to end	Night	19% 14% 3 8% 4 6% 5% 4 6% 6% 25%	2762
June	1st to 10th	Night	19% 9% 3 7% 5% 4 5% 5% 5% 7% 31%	1520
	11th to 20th	Night	19% 12% 4 8% 5% 4 4 3 6% 7% 29%	1385
	21st to end	Night	12% 6% 3 6% 4 3 4 4 6% 6% 45%	1330
July	1st to 10th	Night	10% 4 3 5% 3 2 3 3 5% 7% 55%	1352
	11th to 20th	Night	7% 6% 2 6% 3 2 3 2 4 6% 60%	1488
	21st to end	Night	4 4 1 4 3 2 2 3 5% 5% 67%	1576
August	1st to 10th	Night	7% 5% 1 4 3 3 3 2 4 6% 62%	1495
	11th to 20th	Night	6% 4 1 3 3 2 2 2 4 6% 66%	1602
	21st to end	Night	9% 5% 2 6% 3 3 3 3 5% 5% 55%	1902
September	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 5% 57%	1800
	11th to 20th	Night	14% 8% 2 7% 4 3 4 4 4 6% 44%	1800
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 7% 43%	2052
October	1st to 10th	Night	17% 9% 4 7% 5% 4 4 3 5% 5% 39%	2160
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 4 5% 5% 33%	2160
	21st to end	Night	21% 10% 4 7% 4 4 3 4 5% 6% 33%	2470
November	1st to 10th	Night	27% 11% 4 8% 5% 5% 4 5% 5% 6% 20%	2522
	11th to 20th	Night	27% 12% 3 8% 6% 5% 5% 3 4 5% 22%	2660
	21st to end	Night	23% 14% 3 8% 6% 5% 5% 4 4 5% 23%	2660
December	1st to 10th	Night	20% 14% 3 9% 5% 4 5% 4 4 5% 25%	2660
	11th to 20th	Night	21% 14% 4 10% 6% 5% 4 4 6% 5% 20%	3280
	21st to end	Night	20% 16% 4 10% 7% 6% 6% 4 5% 6% 17%	4862

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	79% 6% 5% 6% 3 1	2539
	11th to 20th	Day	63% 6% 11% 11% 6% 2	2540
	21st to end	Day	76% 5% 5% 8% 5% 1	2922
February	1st to 10th	Day	70% 6% 8% 10% 5%	1710
	11th to 20th	Day	71% 6% 7% 10% 5% 1	1674
	21st to end	Day	72% 6% 8% 9% 4	1489
March	1st to 10th	Day	64% 7% 13% 12% 5%	3221
	11th to 20th	Day	56% 4 7% 29% 3 1	3490
	21st to end	Day	73% 8% 11% 7% 2	3946
April	1st to 10th	Day	73% 8% 10% 8% 1	2280
	11th to 20th	Day	66% 6% 10% 11% 7% 1	2255
	21st to end	Day	70% 6% 10% 10% 3 1	2494
May	1st to 10th	Day	56% 5% 12% 17% 10%	4420
	11th to 20th	Day	52% 10% 18% 16% 4	4380
	21st to end	Day	70% 8% 13% 8% 1	4814
June	1st to 10th	Day	62% 6% 14% 11% 6%	2660
	11th to 20th	Day	68% 6% 13% 10% 4	2788
	21st to end	Day	46% 7% 17% 19% 9% 1	2850
July	1st to 10th	Day	38% 6% 12% 23% 15% 5%	2751
	11th to 20th	Day	32% 6% 11% 28% 21% 2	2518
	21st to end	Day	24% 5% 11% 34% 22% 5%	2589
August	1st to 10th	Day	31% 7% 16% 27% 17% 3	2424
	11th to 20th	Day	28% 5% 15% 33% 16% 4	2298
	21st to end	Day	39% 5% 14% 28% 12% 2	2453
September	1st to 10th	Day	33% 6% 14% 27% 16% 5%	2160
	11th to 20th	Day	45% 8% 12% 16% 15% 4	2160
	21st to end	Day	47% 6% 14% 17% 13% 3	1904
October	1st to 10th	Day	52% 5% 10% 20% 11% 2	1800
	11th to 20th	Day	55% 6% 14% 14% 10% 1	1800
	21st to end	Day	62% 5% 7% 14% 11% 1	1886
November	1st to 10th	Day	70% 6% 9% 10% 3 1	1552
	11th to 20th	Day	70% 6% 7% 8% 6% 3	1520
	21st to end	Day	67% 6% 4 11% 10% 3	1520
December	1st to 10th	Day	64% 6% 9% 10% 8% 3	1520
	11th to 20th	Day	67% 5% 11% 11% 5% 1	1928
	21st to end	Day	70% 7% 10% 9% 4	2794
January	1st to 10th	Night	81% 4 5% 6% 3 1	4406
	11th to 20th	Night	63% 5% 11% 15% 5% 2	4420
	21st to end	Night	73% 7% 7% 7% 5% 1	4738
February	1st to 10th	Night	70% 6% 9% 10% 5% 1	2470
	11th to 20th	Night	66% 6% 9% 10% 6% 3	2404
	21st to end	Night	71% 6% 9% 10% 5% 1	1965
March	1st to 10th	Night	66% 5% 12% 11% 5%	3648
	11th to 20th	Night	50% 6% 14% 27% 2 1	3490
	21st to end	Night	76% 7% 9% 8% 1	3706
April	1st to 10th	Night	69% 8% 12% 9% 2	1900
	11th to 20th	Night	62% 7% 12% 12% 7% 1	1874
	21st to end	Night	67% 6% 11% 12% 4 1	1686
May	1st to 10th	Night	55% 6% 11% 18% 10%	2540
	11th to 20th	Night	50% 9% 18% 18% 5% 1	2516
	21st to end	Night	65% 12% 11% 10% 2	2762
June	1st to 10th	Night	56% 8% 15% 16% 5%	1520
	11th to 20th	Night	62% 7% 14% 14% 3	1385
	21st to end	Night	42% 8% 17% 22% 9% 2	1330
July	1st to 10th	Night	33% 6% 15% 23% 17% 7%	1316
	11th to 20th	Night	31% 5% 11% 28% 21% 4	1440
	21st to end	Night	21% 6% 9% 35% 21% 8%	1488
August	1st to 10th	Night	28% 5% 14% 30% 20% 4	1487
	11th to 20th	Night	25% 6% 15% 33% 17% 5%	1602
	21st to end	Night	34% 5% 14% 31% 15% 2	1902
September	1st to 10th	Night	32% 6% 13% 25% 18% 5%	1800
	11th to 20th	Night	45% 6% 9% 20% 15% 5%	1800
	21st to end	Night	43% 7% 13% 19% 14% 4	2052
October	1st to 10th	Night	48% 6% 13% 21% 11% 1	2160
	11th to 20th	Night	55% 5% 11% 18% 10% 2	2160
	21st to end	Night	55% 5% 11% 16% 12% 1	2470
November	1st to 10th	Night	67% 7% 10% 11% 5% 1	2522
	11th to 20th	Night	68% 7% 8% 9% 6% 2	2660
	21st to end	Night	66% 7% 6% 9% 12% 2	2660
December	1st to 10th	Night	64% 7% 9% 10% 8% 2	2660
	11th to 20th	Night	67% 7% 11% 10% 5% 1	3276
	21st to end	Night	72% 8% 9% 8% 3	4862

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	41% 12% 17% 15% 11% 5%	180
	11th to 20th	Day	31% 21% 15% 22% 11% 1	180
	21st to end	Day	35% 18% 18% 18% 9% 2	274
February	1st to 10th	Day	40% 12% 19% 17% 10% 2	370
	11th to 20th	Day	42% 14% 19% 17% 7% 1	365
	21st to end	Day	38% 16% 20% 19% 5% 1	343
March	1st to 10th	Day	30% 20% 25% 18% 6% 1	460
	11th to 20th	Day	30% 15% 20% 19% 14% 2	460
	21st to end	Day	33% 16% 26% 18% 6% 1	506
April	1st to 10th	Day	35% 17% 20% 18% 8% 2	460
	11th to 20th	Day	29% 22% 24% 17% 6% 4	452
	21st to end	Day	31% 16% 24% 21% 7% 2	460
May	1st to 10th	Day	23% 13% 28% 24% 11% 2	463
	11th to 20th	Day	29% 12% 23% 23% 10% 4	488
	21st to end	Day	27% 12% 21% 27% 11% 2	529
June	1st to 10th	Day	26% 11% 23% 26% 12% 3	490
	11th to 20th	Day	26% 12% 28% 25% 6% 3	618
	21st to end	Day	13% 9% 25% 34% 15% 4	680
July	1st to 10th	Day	14% 7% 22% 38% 16% 3	646
	11th to 20th	Day	8% 8% 24% 37% 20% 3	465
	21st to end	Day	9% 3 19% 41% 23% 6%	487
August	1st to 10th	Day	12% 7% 16% 40% 19% 6%	447
	11th to 20th	Day	7% 3 17% 42% 21% 9%	437
	21st to end	Day	11% 11% 24% 33% 15% 6%	495
September	1st to 10th	Day	16% 6% 24% 35% 14% 4	450
	11th to 20th	Day	16% 9% 21% 31% 21% 2	450
	21st to end	Day	19% 13% 22% 27% 16% 3	450
October	1st to 10th	Day	24% 14% 18% 22% 20% 3	450
	11th to 20th	Day	27% 14% 17% 21% 15% 5%	450
	21st to end	Day	34% 11% 17% 20% 11% 5%	401
November	1st to 10th	Day	33% 20% 16% 19% 10% 2	212
	11th to 20th	Day	33% 18% 12% 16% 15% 7%	180
	21st to end	Day	29% 15% 19% 15% 12% 9%	180
December	1st to 10th	Day	38% 15% 10% 15% 14% 8%	180
	11th to 20th	Day	39% 13% 22% 17% 7% 2	180
	21st to end	Day	38% 14% 16% 19% 13% 1	198
January	1st to 10th	Night	43% 13% 13% 16% 13% 2	760
	11th to 20th	Night	33% 12% 14% 23% 15% 3	760
	21st to end	Night	33% 11% 13% 25% 14% 3	836
February	1st to 10th	Night	35% 11% 15% 26% 11% 1	760
	11th to 20th	Night	34% 10% 13% 27% 14% 1	744
	21st to end	Night	36% 12% 17% 24% 11% 1	628
March	1st to 10th	Night	29% 14% 16% 24% 14% 1	760
	11th to 20th	Night	32% 14% 15% 24% 14% 1	760
	21st to end	Night	35% 11% 21% 22% 9% 1	836
April	1st to 10th	Night	31% 13% 16% 23% 15% 2	760
	11th to 20th	Night	27% 10% 21% 25% 15% 2	746
	21st to end	Night	29% 11% 14% 31% 14% 2	546
May	1st to 10th	Night	24% 9% 15% 31% 19% 1	380
	11th to 20th	Night	20% 9% 12% 28% 28% 2	374
	21st to end	Night	25% 7% 15% 24% 26% 3	411
June	1st to 10th	Night	20% 9% 17% 25% 26% 3	380
	11th to 20th	Night	23% 8% 18% 31% 20% 1	380
	21st to end	Night	14% 5% 14% 32% 32% 3	380
July	1st to 10th	Night	12% 3 10% 33% 39% 4	370
	11th to 20th	Night	10% 3 8% 38% 38% 3	360
	21st to end	Night	6% 2 10% 28% 48% 5%	394
August	1st to 10th	Night	12% 3 9% 24% 43% 10%	421
	11th to 20th	Night	8% 3 6% 26% 46% 12%	534
	21st to end	Night	12% 6% 9% 32% 37% 4	714
September	1st to 10th	Night	13% 4 10% 34% 34% 6%	720
	11th to 20th	Night	16% 6% 11% 24% 35% 7%	720
	21st to end	Night	19% 7% 12% 31% 28% 4	720
October	1st to 10th	Night	22% 7% 12% 30% 26% 4	720
	11th to 20th	Night	26% 9% 14% 25% 22% 5%	720
	21st to end	Night	28% 10% 17% 26% 15% 4	792
November	1st to 10th	Night	34% 11% 17% 28% 9% 1	740
	11th to 20th	Night	36% 12% 15% 22% 12% 2	760
	21st to end	Night	34% 9% 17% 20% 15% 4	760
December	1st to 10th	Night	31% 10% 16% 27% 14% 3	760
	11th to 20th	Night	38% 11% 16% 24% 9% 1	760
	21st to end	Night	44% 13% 16% 19% 8%	836

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