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 2024-12-24.

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	17% 11% 3 8% 5% 5% 4 3 5% 6% 33%	106391
	Night	17% 12% 3 8% 5% 4 4 3 5% 5% 32%	102336
Transparency	Day	55% 5% 11% 16% 10% 3	106153
	Night	57% 6% 11% 15% 9% 2	102220
Seeing	Day	25% 12% 21% 26% 13% 3	15070
	Night	27% 9% 14% 26% 20% 3	23584

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	7% 5% 1 4 2 3 3 3 4 5% 63%	4688
	11th to 20th	Day	6% 4 1 3 3 3 3 2 4 5% 68%	4420
	21st to end	Day	11% 5% 2 4 3 2 3 3 4 6% 58%	4790
August	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 7% 64%	4283
	11th to 20th	Day	9% 6% 3 6% 5% 5% 5% 4 6% 7% 45%	4026
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 45%	4309
September	1st to 10th	Day	10% 6% 3 6% 3 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	13% 6% 3 7% 5% 4 4 4 5% 7% 40%	2280
	21st to end	Day	16% 8% 3 6% 4 5% 4 3 5% 6% 40%	2008
October	1st to 10th	Day	15% 8% 2 5% 3 4 3 3 5% 8% 44%	3180
	11th to 20th	Day	22% 9% 3 7% 5% 5% 5% 3 5% 7% 28%	3161
	21st to end	Day	20% 12% 3 9% 5% 5% 5% 5% 5% 6% 25%	3273
November	1st to 10th	Day	25% 14% 4 9% 7% 5% 5% 5% 5% 5% 16%	1635
	11th to 20th	Day	25% 14% 4 9% 6% 5% 4 3 6% 6% 18%	1600
	21st to end	Day	24% 15% 3 9% 5% 5% 5% 2 6% 4 22%	1600
December	1st to 10th	Day	18% 14% 4 9% 5% 6% 5% 4 4 4 28%	1600
	11th to 20th	Day	20% 14% 4 10% 6% 4 5% 4 6% 6% 20%	2005
	21st to end	Day	21% 15% 4 10% 6% 7% 6% 5% 5% 5% 17%	2818
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	6% 2 2 3 2 2 2 2 4 4 71%	2268
	11th to 20th	Night	4 4 1 4 2 2 2 2 3 4 72%	2540
	21st to end	Night	8% 3 1 3 2 2 2 3 4 4 67%	2762
August	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 72%	2648
	11th to 20th	Night	10% 7% 4 5% 4 3 3 2 4 6% 52%	2812
	21st to end	Night	15% 7% 2 6% 3 3 2 3 4 4 51%	3364
September	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 4 5% 57%	1900
	11th to 20th	Night	14% 8% 2 7% 4 3 4 4 4 6% 43%	1900
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 7% 44%	2167
October	1st to 10th	Night	13% 8% 3 6% 4 3 4 3 5% 7% 45%	3800
	11th to 20th	Night	19% 11% 3 7% 4 4 4 3 5% 6% 34%	3777
	21st to end	Night	16% 11% 5% 10% 5% 5% 5% 4 5% 6% 29%	4290
November	1st to 10th	Night	27% 11% 4 8% 5% 5% 5% 5% 5% 6% 20%	2659
	11th to 20th	Night	27% 13% 3 8% 6% 5% 5% 3 4 5% 21%	2800
	21st to end	Night	22% 14% 3 8% 6% 5% 5% 4 4 6% 23%	2800
December	1st to 10th	Night	20% 14% 3 9% 5% 4 5% 4 4 6% 26%	2800
	11th to 20th	Night	21% 14% 4 10% 6% 5% 4 4 6% 5% 20%	3420
	21st to end	Night	20% 16% 4 10% 7% 6% 6% 4 5% 6% 17%	4910

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	26% 4 7% 21% 20% 22%	4615
	11th to 20th	Day	21% 4 6% 22% 37% 10%	4345
	21st to end	Day	30% 3 10% 33% 20% 3	4639
August	1st to 10th	Day	23% 5% 10% 30% 29% 4	4266
	11th to 20th	Day	44% 3 15% 26% 10% 2	4065
	21st to end	Day	44% 3 12% 23% 15% 4	4308
September	1st to 10th	Day	33% 6% 14% 26% 16% 5%	2280
	11th to 20th	Day	45% 8% 12% 17% 14% 4	2280
	21st to end	Day	46% 6% 15% 18% 12% 2	2008
October	1st to 10th	Day	42% 3 9% 30% 14% 1	3180
	11th to 20th	Day	60% 3 11% 14% 10% 1	3161
	21st to end	Day	66% 3 9% 11% 9% 2	3273
November	1st to 10th	Day	71% 6% 9% 11% 3 1	1635
	11th to 20th	Day	71% 6% 7% 8% 6% 3	1600
	21st to end	Day	67% 5% 5% 10% 10% 2	1600
December	1st to 10th	Day	63% 5% 9% 11% 9% 3	1600
	11th to 20th	Day	68% 5% 11% 11% 5% 1	2008
	21st to end	Day	70% 7% 10% 9% 4	2818
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	22% 3 9% 24% 15% 26%	2232
	11th to 20th	Night	20% 3 7% 23% 36% 11%	2492
	21st to end	Night	25% 4 10% 33% 23% 5%	2674
August	1st to 10th	Night	20% 3 8% 36% 29% 4	2640
	11th to 20th	Night	42% 3 19% 23% 9% 3	2839
	21st to end	Night	42% 3 14% 25% 16% 1	3364
September	1st to 10th	Night	32% 6% 13% 25% 18% 5%	1900
	11th to 20th	Night	46% 6% 9% 20% 14% 5%	1900
	21st to end	Night	43% 7% 14% 19% 13% 4	2167
October	1st to 10th	Night	42% 4 13% 27% 13% 1	3800
	11th to 20th	Night	55% 3 12% 19% 9% 1	3777
	21st to end	Night	64% 3 9% 14% 10%	4290
November	1st to 10th	Night	68% 6% 10% 11% 4 1	2659
	11th to 20th	Night	69% 7% 8% 9% 5% 2	2800
	21st to end	Night	66% 6% 6% 9% 11% 1	2800
December	1st to 10th	Night	62% 6% 9% 11% 9% 2	2800
	11th to 20th	Night	67% 7% 11% 10% 5% 1	3416
	21st to end	Night	72% 7% 9% 8% 3	4910

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% 7% 24% 38% 20% 3	484
	21st to end	Day	10% 3 20% 40% 22% 5%	531
August	1st to 10th	Day	11% 6% 17% 41% 18% 6%	487
	11th to 20th	Day	7% 4 17% 42% 21% 8%	477
	21st to end	Day	12% 11% 23% 32% 14% 7%	539
September	1st to 10th	Day	15% 6% 23% 36% 14% 5%	490
	11th to 20th	Day	17% 10% 22% 30% 20% 2	490
	21st to end	Day	18% 13% 23% 28% 16% 3	490
October	1st to 10th	Day	23% 14% 19% 22% 20% 3	490
	11th to 20th	Day	28% 14% 18% 20% 15% 5%	490
	21st to end	Day	33% 12% 19% 20% 11% 5%	439
November	1st to 10th	Day	34% 19% 17% 19% 10% 3	235
	11th to 20th	Day	35% 18% 13% 16% 14% 6%	200
	21st to end	Day	28% 15% 19% 18% 13% 9%	200
December	1st to 10th	Day	37% 14% 10% 16% 15% 10%	200
	11th to 20th	Day	41% 13% 23% 16% 7% 2	200
	21st to end	Day	39% 13% 16% 19% 12%	204
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	368
	21st to end	Night	6% 2 10% 30% 47% 5%	416
August	1st to 10th	Night	11% 3 8% 25% 44% 9%	445
	11th to 20th	Night	9% 3 5% 26% 45% 12%	564
	21st to end	Night	13% 6% 9% 32% 36% 4	754
September	1st to 10th	Night	12% 4 10% 33% 35% 6%	760
	11th to 20th	Night	16% 6% 12% 24% 35% 7%	760
	21st to end	Night	18% 7% 13% 31% 28% 4	760
October	1st to 10th	Night	21% 7% 12% 30% 26% 4	760
	11th to 20th	Night	26% 9% 14% 25% 21% 4	760
	21st to end	Night	28% 10% 18% 26% 14% 4	836
November	1st to 10th	Night	34% 11% 17% 28% 9% 1	780
	11th to 20th	Night	37% 12% 15% 21% 12% 2	800
	21st to end	Night	34% 9% 17% 21% 15% 4	800
December	1st to 10th	Night	30% 10% 16% 26% 16% 3	800
	11th to 20th	Night	38% 12% 17% 24% 9% 1	800
	21st to end	Night	44% 13% 16% 19% 8%	850

Page generated at 2024-12-21 01:13:52. Version ac07

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