Maple Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maple Valley Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	22% 10% 3 7% 4 4 4 3 5% 6% 32%	130866
	Night	25% 11% 3 7% 4 4 4 3 4 5% 30%	125911
Transparency	Day	55% 5% 13% 18% 9% 2	130584
	Night	58% 4 12% 16% 8% 1	125770
Seeing	Day	26% 10% 20% 26% 15% 4	15926
	Night	30% 9% 15% 26% 18% 2	24210

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	41% 15% 3 8% 3 4 3 2 4 5% 10%	3639
	11th to 20th	Day	20% 11% 3 9% 5% 4 5% 4 5% 6% 28%	3616
	21st to end	Day	31% 10% 3 7% 3 3 2 2 3 5% 30%	4114
February	1st to 10th	Day	33% 14% 4 8% 5% 4 4 3 3 6% 17%	1800
	11th to 20th	Day	34% 14% 3 8% 4 3 4 3 3 5% 19%	1755
	21st to end	Day	32% 15% 4 7% 5% 4 3 3 4 4 19%	1566
March	1st to 10th	Day	21% 15% 4 10% 6% 5% 6% 4 5% 6% 17%	4601
	11th to 20th	Day	22% 17% 5% 9% 5% 4 4 3 3 4 25%	5000
	21st to end	Day	30% 14% 4 9% 6% 6% 6% 5% 5% 5% 11%	5695
April	1st to 10th	Day	28% 15% 3 8% 5% 5% 5% 4 5% 6% 15%	2400
	11th to 20th	Day	23% 13% 3 7% 5% 5% 4 4 5% 5% 24%	2377
	21st to end	Day	24% 12% 4 8% 6% 5% 4 4 5% 6% 21%	2651
May	1st to 10th	Day	17% 9% 3 7% 4 4 4 4 5% 6% 36%	6320
	11th to 20th	Day	25% 12% 3 7% 4 4 5% 3 5% 6% 25%	6277
	21st to end	Day	24% 10% 3 7% 4 4 4 4 5% 8% 27%	6903
June	1st to 10th	Day	24% 9% 3 7% 5% 5% 4 4 6% 7% 26%	2800
	11th to 20th	Day	26% 12% 4 8% 5% 5% 4 4 5% 6% 20%	2985
	21st to end	Day	16% 8% 3 7% 5% 5% 4 4 5% 7% 37%	3000
July	1st to 10th	Day	10% 4 1 4 2 2 3 2 4 6% 61%	6780
	11th to 20th	Day	6% 4 2 4 2 2 2 2 3 5% 67%	6348
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 7% 55%	6874
August	1st to 10th	Day	7% 4 1 4 3 3 4 3 5% 6% 60%	4398
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	4026
	21st to end	Day	19% 9% 3 6% 3 3 3 2 4 5% 42%	4309
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 4 6% 50%	2280
	11th to 20th	Day	17% 8% 3 7% 5% 3 4 4 5% 7% 37%	2280
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 6% 37%	2015
October	1st to 10th	Day	20% 7% 2 5% 3 4 4 3 6% 10% 37%	3180
	11th to 20th	Day	32% 9% 2 7% 4 3 4 3 4 6% 26%	3161
	21st to end	Day	32% 11% 3 8% 4 4 4 3 5% 5% 19%	3237
November	1st to 10th	Day	37% 10% 3 7% 4 4 4 4 4 5% 17%	1630
	11th to 20th	Day	35% 11% 3 7% 4 4 3 3 4 5% 20%	1600
	21st to end	Day	35% 11% 4 6% 4 5% 4 3 4 4 21%	1600
December	1st to 10th	Day	25% 8% 3 5% 4 4 4 4 5% 5% 32%	2620
	11th to 20th	Day	37% 13% 4 9% 5% 4 3 3 5% 4 14%	3025
	21st to end	Day	34% 15% 4 9% 5% 5% 5% 3 3 3 13%	4004
January	1st to 10th	Night	39% 16% 3 8% 4 4 4 3 3 3 12%	6306
	11th to 20th	Night	21% 11% 3 6% 4 4 4 4 5% 7% 30%	6282
	21st to end	Night	29% 10% 3 6% 4 4 4 3 4 5% 30%	6843
February	1st to 10th	Night	32% 15% 4 7% 5% 4 4 3 3 5% 17%	2600
	11th to 20th	Night	31% 14% 3 7% 5% 3 4 3 4 4 23%	2532
	21st to end	Night	32% 16% 3 7% 4 3 4 4 5% 5% 19%	2063
March	1st to 10th	Night	20% 16% 4 8% 5% 5% 5% 5% 6% 7% 20%	5292
	11th to 20th	Night	27% 15% 3 6% 4 4 4 3 4 4 28%	4999
	21st to end	Night	35% 15% 3 8% 4 4 5% 4 5% 5% 12%	5280
April	1st to 10th	Night	27% 13% 3 8% 6% 5% 5% 4 6% 6% 17%	2000
	11th to 20th	Night	22% 11% 3 7% 5% 4 5% 4 5% 6% 27%	1972
	21st to end	Night	26% 11% 3 9% 6% 4 5% 3 5% 6% 22%	1748
May	1st to 10th	Night	19% 10% 3 6% 4 4 3 3 4 4 40%	3640
	11th to 20th	Night	24% 12% 3 7% 4 5% 5% 4 5% 6% 25%	3616
	21st to end	Night	22% 14% 4 8% 5% 5% 5% 3 5% 6% 24%	3972
June	1st to 10th	Night	20% 10% 4 8% 5% 5% 5% 3 4 6% 31%	1600
	11th to 20th	Night	22% 11% 3 9% 7% 5% 4 4 5% 5% 26%	1410
	21st to end	Night	17% 7% 2 7% 4 4 4 4 5% 7% 41%	1400
July	1st to 10th	Night	10% 6% 2 4 3 3 2 2 3 5% 59%	3180
	11th to 20th	Night	4 3 1 3 2 2 3 3 3 4 73%	3594
	21st to end	Night	10% 4 2 4 3 3 3 3 4 6% 59%	3969
August	1st to 10th	Night	5% 3 2 4 2 2 2 2 4 7% 67%	2619
	11th to 20th	Night	14% 6% 3 6% 3 3 4 3 4 7% 46%	2812
	21st to end	Night	22% 7% 2 4 3 2 2 2 3 5% 47%	3363
September	1st to 10th	Night	12% 4 2 6% 3 3 4 3 4 6% 53%	1900
	11th to 20th	Night	19% 8% 4 6% 3 3 4 3 4 5% 41%	1900
	21st to end	Night	19% 8% 3 7% 3 4 4 2 4 6% 40%	2161
October	1st to 10th	Night	19% 6% 3 6% 4 3 5% 4 6% 8% 37%	3800
	11th to 20th	Night	30% 9% 2 6% 3 4 4 3 4 5% 30%	3777
	21st to end	Night	34% 13% 3 6% 4 3 4 3 4 5% 22%	4324
November	1st to 10th	Night	35% 10% 3 7% 4 4 4 3 5% 5% 20%	2665
	11th to 20th	Night	36% 10% 3 7% 4 4 4 3 4 3 22%	2800
	21st to end	Night	32% 12% 3 7% 5% 4 4 3 5% 5% 21%	2800
December	1st to 10th	Night	25% 9% 3 6% 4 4 4 3 4 6% 33%	4560
	11th to 20th	Night	36% 14% 4 9% 5% 4 4 3 4 4 14%	5180
	21st to end	Night	34% 16% 4 9% 5% 5% 5% 3 4 3 12%	6952

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	78% 3 5% 10% 3	3639
	11th to 20th	Day	57% 5% 11% 17% 9% 1	3640
	21st to end	Day	61% 2 6% 17% 13% 1	4114
February	1st to 10th	Day	72% 3 11% 9% 5% 1	1800
	11th to 20th	Day	72% 3 7% 10% 7% 1	1764
	21st to end	Day	72% 3 8% 12% 4	1566
March	1st to 10th	Day	66% 6% 11% 11% 6%	4601
	11th to 20th	Day	66% 4 9% 20% 2	5000
	21st to end	Day	74% 4 12% 8% 1	5695
April	1st to 10th	Day	72% 6% 13% 8% 1	2400
	11th to 20th	Day	63% 6% 11% 14% 6% 1	2377
	21st to end	Day	69% 5% 10% 11% 4 1	2651
May	1st to 10th	Day	50% 6% 13% 22% 8%	6320
	11th to 20th	Day	61% 7% 17% 13% 2	6280
	21st to end	Day	58% 5% 22% 13% 2	6904
June	1st to 10th	Day	60% 6% 15% 14% 5%	2800
	11th to 20th	Day	68% 5% 12% 12% 3	2985
	21st to end	Day	49% 5% 18% 18% 8% 2	3000
July	1st to 10th	Day	27% 5% 17% 22% 18% 11%	6706
	11th to 20th	Day	23% 3 12% 29% 31% 3	6272
	21st to end	Day	29% 4 21% 33% 11% 1	6681
August	1st to 10th	Day	25% 5% 10% 38% 19% 2	4381
	11th to 20th	Day	46% 6% 18% 20% 8% 1	4065
	21st to end	Day	47% 4 11% 26% 12% 1	4308
September	1st to 10th	Day	37% 4 15% 25% 14% 5%	2280
	11th to 20th	Day	49% 6% 11% 17% 14% 3	2280
	21st to end	Day	50% 4 16% 17% 10% 3	2015
October	1st to 10th	Day	44% 5% 12% 30% 8% 1	3180
	11th to 20th	Day	60% 4 12% 15% 9% 1	3161
	21st to end	Day	67% 4 10% 9% 8% 2	3237
November	1st to 10th	Day	71% 4 10% 11% 4	1630
	11th to 20th	Day	70% 5% 8% 9% 6% 2	1600
	21st to end	Day	69% 5% 5% 10% 9% 3	1600
December	1st to 10th	Day	55% 4 7% 18% 15% 2	2620
	11th to 20th	Day	76% 3 8% 9% 3	3028
	21st to end	Day	77% 2 10% 8% 2	4004
January	1st to 10th	Night	78% 4 5% 8% 4 1	6306
	11th to 20th	Night	54% 5% 11% 20% 10% 1	6320
	21st to end	Night	59% 2 10% 16% 11% 2	6843
February	1st to 10th	Night	72% 4 10% 8% 6% 1	2600
	11th to 20th	Night	66% 5% 10% 10% 7% 3	2534
	21st to end	Night	69% 5% 9% 11% 5%	2063
March	1st to 10th	Night	63% 5% 13% 13% 6%	5292
	11th to 20th	Night	62% 4 12% 19% 3	5000
	21st to end	Night	74% 4 12% 9% 1	5280
April	1st to 10th	Night	69% 8% 12% 10% 1	2000
	11th to 20th	Night	61% 5% 12% 14% 8% 2	1972
	21st to end	Night	66% 6% 10% 11% 5% 1	1748
May	1st to 10th	Night	50% 4 12% 23% 10%	3637
	11th to 20th	Night	61% 6% 17% 13% 2 1	3616
	21st to end	Night	64% 5% 17% 12% 1	3972
June	1st to 10th	Night	59% 4 15% 16% 5%	1600
	11th to 20th	Night	62% 6% 16% 13% 2	1410
	21st to end	Night	47% 5% 20% 20% 7% 2	1400
July	1st to 10th	Night	30% 5% 16% 20% 15% 14%	3145
	11th to 20th	Night	19% 3 13% 29% 35% 1	3547
	21st to end	Night	27% 5% 20% 33% 13% 2	3857
August	1st to 10th	Night	20% 4 11% 39% 23% 2	2611
	11th to 20th	Night	40% 6% 21% 22% 8% 2	2839
	21st to end	Night	44% 3 12% 28% 12% 1	3363
September	1st to 10th	Night	35% 4 15% 26% 15% 5%	1900
	11th to 20th	Night	50% 5% 9% 19% 13% 5%	1900
	21st to end	Night	49% 5% 12% 19% 12% 3	2161
October	1st to 10th	Night	46% 4 13% 29% 8% 1	3800
	11th to 20th	Night	59% 3 10% 16% 11%	3777
	21st to end	Night	66% 4 10% 11% 8%	4324
November	1st to 10th	Night	69% 5% 10% 11% 4	2665
	11th to 20th	Night	70% 3 9% 10% 5% 3	2800
	21st to end	Night	69% 4 7% 9% 10% 1	2800
December	1st to 10th	Night	55% 4 7% 16% 16% 2	4560
	11th to 20th	Night	76% 3 8% 9% 4	5176
	21st to end	Night	78% 4 8% 7% 2	6952

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	42% 12% 16% 16% 10% 5%	200
	11th to 20th	Day	33% 14% 17% 22% 11% 4	200
	21st to end	Day	40% 13% 15% 16% 11% 5%	265
February	1st to 10th	Day	41% 11% 21% 16% 9% 2	400
	11th to 20th	Day	45% 11% 22% 14% 8% 1	395
	21st to end	Day	37% 15% 21% 23% 5% 1	372
March	1st to 10th	Day	33% 16% 22% 19% 9% 1	500
	11th to 20th	Day	32% 16% 19% 20% 9% 3	500
	21st to end	Day	35% 15% 25% 16% 8% 1	550
April	1st to 10th	Day	34% 16% 21% 16% 11% 1	500
	11th to 20th	Day	25% 18% 28% 17% 8% 4	492
	21st to end	Day	26% 15% 25% 23% 10% 2	500
May	1st to 10th	Day	19% 13% 25% 30% 12% 2	503
	11th to 20th	Day	29% 10% 20% 24% 14% 4	528
	21st to end	Day	26% 10% 23% 25% 13% 3	573
June	1st to 10th	Day	25% 9% 20% 28% 15% 3	530
	11th to 20th	Day	24% 14% 22% 27% 11% 2	715
	21st to end	Day	17% 7% 19% 33% 20% 4	730
July	1st to 10th	Day	13% 5% 17% 36% 26% 2	721
	11th to 20th	Day	9% 6% 18% 39% 23% 6%	538
	21st to end	Day	9% 3 14% 38% 29% 5%	575
August	1st to 10th	Day	13% 5% 13% 34% 27% 9%	504
	11th to 20th	Day	8% 3 14% 35% 31% 7%	477
	21st to end	Day	17% 7% 22% 30% 18% 6%	539
September	1st to 10th	Day	16% 5% 23% 36% 16% 4	490
	11th to 20th	Day	19% 8% 20% 30% 20% 4	490
	21st to end	Day	20% 11% 19% 28% 19% 3	490
October	1st to 10th	Day	24% 10% 21% 21% 19% 5%	490
	11th to 20th	Day	32% 9% 15% 24% 14% 4	490
	21st to end	Day	36% 14% 17% 20% 10% 4	416
November	1st to 10th	Day	39% 12% 16% 20% 12% 1	233
	11th to 20th	Day	38% 17% 17% 10% 14% 5%	200
	21st to end	Day	36% 12% 15% 20% 9% 9%	200
December	1st to 10th	Day	43% 9% 12% 16% 13% 9%	200
	11th to 20th	Day	45% 8% 23% 17% 7% 2	200
	21st to end	Day	40% 14% 15% 20% 10% 1	220
January	1st to 10th	Night	43% 14% 17% 17% 8% 1	800
	11th to 20th	Night	36% 11% 15% 26% 11% 1	800
	21st to end	Night	36% 9% 15% 28% 11% 1	880
February	1st to 10th	Night	38% 11% 20% 23% 8% 1	800
	11th to 20th	Night	37% 10% 15% 27% 10% 1	784
	21st to end	Night	38% 12% 17% 23% 8% 1	660
March	1st to 10th	Night	32% 13% 18% 23% 13% 1	800
	11th to 20th	Night	33% 12% 18% 25% 11% 1	800
	21st to end	Night	36% 13% 16% 25% 8% 1	880
April	1st to 10th	Night	33% 11% 18% 25% 11% 2	800
	11th to 20th	Night	26% 11% 19% 29% 12% 2	784
	21st to end	Night	30% 12% 16% 26% 14% 2	548
May	1st to 10th	Night	22% 12% 14% 31% 20% 1	400
	11th to 20th	Night	23% 7% 13% 29% 26% 2	394
	21st to end	Night	24% 8% 15% 24% 26% 3	433
June	1st to 10th	Night	22% 8% 16% 28% 25% 2	400
	11th to 20th	Night	24% 12% 15% 30% 19% 1	400
	21st to end	Night	18% 4 12% 34% 30% 3	400
July	1st to 10th	Night	13% 4 11% 32% 36% 4	390
	11th to 20th	Night	11% 3 8% 37% 38% 3	388
	21st to end	Night	9% 3 8% 30% 45% 5%	438
August	1st to 10th	Night	10% 3 8% 26% 45% 8%	418
	11th to 20th	Night	11% 1 7% 25% 45% 11%	564
	21st to end	Night	16% 5% 9% 27% 38% 5%	753
September	1st to 10th	Night	13% 3 10% 38% 32% 4	760
	11th to 20th	Night	20% 5% 10% 25% 34% 6%	760
	21st to end	Night	22% 6% 12% 31% 26% 3	760
October	1st to 10th	Night	23% 6% 13% 28% 26% 3	760
	11th to 20th	Night	30% 9% 13% 27% 18% 4	760
	21st to end	Night	34% 9% 16% 27% 12% 2	836
November	1st to 10th	Night	38% 10% 20% 25% 7%	780
	11th to 20th	Night	40% 11% 16% 23% 9% 1	800
	21st to end	Night	36% 10% 19% 23% 12%	800
December	1st to 10th	Night	32% 9% 20% 26% 12% 1	800
	11th to 20th	Night	42% 10% 20% 20% 7% 1	800
	21st to end	Night	47% 11% 18% 18% 5%	880

Page generated at 2025-08-01 22:32:23. Version ac07

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