Marysville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marysville Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	21% 9% 3 7% 4 4 4 3 5% 6% 33%	130838
	Night	25% 10% 3 7% 4 4 4 3 4 6% 29%	127248
Transparency	Day	53% 5% 15% 18% 8% 1	130551
	Night	58% 5% 13% 17% 7% 1	127112
Seeing	Day	26% 8% 19% 25% 16% 6%	16298
	Night	32% 6% 12% 25% 20% 5%	24576

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	38% 17% 4 8% 5% 4 4 3 4 3 9%	3179
	11th to 20th	Day	20% 10% 3 8% 6% 5% 5% 4 6% 8% 26%	3549
	21st to end	Day	31% 12% 3 8% 3 3 4 3 4 6% 24%	4068
February	1st to 10th	Day	33% 13% 4 7% 5% 4 4 4 4 5% 17%	1800
	11th to 20th	Day	29% 13% 3 9% 5% 4 3 4 4 5% 21%	1755
	21st to end	Day	31% 14% 4 8% 5% 4 4 2 4 4 19%	1550
March	1st to 10th	Day	23% 15% 4 9% 5% 5% 5% 4 5% 6% 20%	4601
	11th to 20th	Day	23% 14% 4 9% 5% 5% 4 3 4 6% 25%	5000
	21st to end	Day	29% 11% 3 10% 6% 6% 6% 4 6% 6% 14%	5693
April	1st to 10th	Day	27% 14% 3 7% 5% 6% 6% 4 6% 6% 17%	2400
	11th to 20th	Day	23% 10% 4 7% 6% 5% 5% 4 5% 5% 26%	2401
	21st to end	Day	23% 12% 3 9% 5% 6% 5% 3 6% 6% 23%	2665
May	1st to 10th	Day	17% 8% 3 6% 4 5% 4 3 5% 6% 39%	6320
	11th to 20th	Day	21% 11% 3 8% 5% 4 5% 4 6% 8% 27%	6277
	21st to end	Day	24% 9% 3 6% 4 4 3 3 6% 8% 30%	6903
June	1st to 10th	Day	21% 8% 3 8% 4 4 5% 4 5% 7% 31%	2885
	11th to 20th	Day	24% 9% 3 8% 5% 5% 5% 4 6% 7% 24%	3000
	21st to end	Day	14% 7% 2 7% 4 4 5% 4 6% 8% 38%	3000
July	1st to 10th	Day	10% 3 2 3 2 2 3 2 4 6% 63%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 70%	6555
	21st to end	Day	10% 4 2 4 3 3 4 3 5% 7% 56%	6874
August	1st to 10th	Day	6% 3 1 3 3 3 3 3 5% 7% 62%	4512
	11th to 20th	Day	10% 7% 3 8% 5% 4 4 4 5% 8% 43%	4156
	21st to end	Day	16% 7% 3 8% 3 4 4 3 4 7% 41%	4477
September	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 4 7% 49%	2400
	11th to 20th	Day	15% 7% 3 8% 5% 4 3 4 5% 7% 40%	2400
	21st to end	Day	21% 6% 2 6% 3 4 3 3 4 7% 39%	2118
October	1st to 10th	Day	19% 6% 2 7% 5% 4 5% 5% 7% 10% 31%	3211
	11th to 20th	Day	36% 9% 3 7% 4 3 3 3 4 6% 24%	3161
	21st to end	Day	30% 10% 3 9% 6% 5% 4 3 5% 6% 19%	3228
November	1st to 10th	Day	36% 10% 3 7% 5% 3 4 4 5% 5% 17%	1601
	11th to 20th	Day	36% 10% 3 6% 5% 4 3 2 4 4 21%	1600
	21st to end	Day	36% 11% 3 6% 4 4 4 3 4 5% 19%	1600
December	1st to 10th	Day	25% 10% 3 6% 4 5% 5% 4 6% 7% 25%	2620
	11th to 20th	Day	36% 17% 4 9% 4 4 3 3 3 5% 12%	3001
	21st to end	Day	35% 16% 5% 10% 5% 4 4 3 4 4 11%	3498
January	1st to 10th	Night	37% 14% 4 10% 6% 5% 4 3 4 5% 10%	6766
	11th to 20th	Night	22% 10% 3 6% 4 4 4 4 6% 10% 27%	6332
	21st to end	Night	29% 10% 3 7% 4 4 3 3 3 5% 28%	6888
February	1st to 10th	Night	33% 13% 3 7% 5% 5% 4 2 4 5% 20%	2600
	11th to 20th	Night	30% 13% 4 7% 5% 3 4 3 4 4 23%	2532
	21st to end	Night	31% 13% 3 7% 5% 4 4 3 4 5% 21%	2079
March	1st to 10th	Night	20% 16% 4 10% 5% 6% 5% 4 5% 5% 18%	5292
	11th to 20th	Night	27% 14% 4 8% 4 3 3 3 4 5% 27%	4999
	21st to end	Night	34% 14% 4 10% 5% 4 4 3 4 4 14%	5281
April	1st to 10th	Night	28% 12% 3 7% 7% 5% 5% 4 5% 6% 18%	2000
	11th to 20th	Night	25% 10% 4 6% 5% 4 4 4 5% 5% 28%	1948
	21st to end	Night	26% 10% 4 8% 5% 4 5% 4 5% 5% 23%	1735
May	1st to 10th	Night	18% 10% 3 7% 4 3 3 3 3 4 43%	3640
	11th to 20th	Night	24% 9% 3 8% 4 4 3 2 4 6% 32%	3616
	21st to end	Night	25% 8% 3 8% 4 4 5% 4 6% 7% 27%	3972
June	1st to 10th	Night	22% 8% 2 7% 5% 4 4 4 4 6% 33%	1510
	11th to 20th	Night	26% 10% 3 8% 5% 4 4 2 4 6% 27%	1400
	21st to end	Night	17% 6% 2 5% 4 4 5% 4 7% 7% 40%	1400
July	1st to 10th	Night	14% 4 2 4 2 2 3 1 2 5% 62%	3180
	11th to 20th	Night	5% 3 1 2 2 2 2 2 3 4 74%	3401
	21st to end	Night	10% 4 2 5% 3 3 4 2 4 5% 58%	3969
August	1st to 10th	Night	5% 2 1 3 3 2 3 3 4 6% 68%	2636
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 5% 50%	2902
	21st to end	Night	17% 6% 2 6% 3 3 3 3 4 5% 47%	3438
September	1st to 10th	Night	11% 4 2 5% 3 3 4 3 5% 5% 54%	2000
	11th to 20th	Night	18% 7% 3 6% 5% 3 3 3 4 5% 41%	2000
	21st to end	Night	20% 7% 3 7% 4 3 3 3 4 5% 40%	2279
October	1st to 10th	Night	18% 8% 3 7% 5% 5% 5% 3 5% 8% 33%	3836
	11th to 20th	Night	31% 8% 3 5% 3 3 3 3 5% 7% 29%	3777
	21st to end	Night	35% 12% 3 7% 4 4 3 3 4 5% 21%	4332
November	1st to 10th	Night	37% 11% 3 6% 4 4 4 2 4 5% 19%	2692
	11th to 20th	Night	37% 11% 3 6% 4 4 3 3 4 4 22%	2800
	21st to end	Night	35% 11% 3 7% 4 3 4 4 4 5% 19%	2800
December	1st to 10th	Night	25% 10% 3 7% 4 4 4 4 7% 9% 23%	4560
	11th to 20th	Night	35% 14% 4 10% 5% 4 4 3 4 5% 13%	5198
	21st to end	Night	34% 18% 5% 10% 5% 5% 4 2 4 4 10%	7458

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	80% 5% 6% 7% 2	3179
	11th to 20th	Day	56% 5% 13% 19% 6% 1	3573
	21st to end	Day	63% 3 7% 16% 9%	4068
February	1st to 10th	Day	71% 6% 9% 9% 4 2	1800
	11th to 20th	Day	68% 5% 9% 9% 8% 1	1764
	21st to end	Day	70% 5% 9% 11% 5% 1	1550
March	1st to 10th	Day	65% 6% 10% 14% 5%	4601
	11th to 20th	Day	63% 5% 12% 18% 2	5000
	21st to end	Day	71% 6% 13% 10% 1	5693
April	1st to 10th	Day	68% 7% 15% 10% 1	2400
	11th to 20th	Day	61% 6% 11% 14% 6% 1	2401
	21st to end	Day	63% 7% 12% 13% 5%	2665
May	1st to 10th	Day	47% 5% 13% 24% 10%	6320
	11th to 20th	Day	56% 7% 19% 15% 2	6280
	21st to end	Day	55% 5% 23% 16% 1	6904
June	1st to 10th	Day	54% 8% 16% 15% 6% 1	2885
	11th to 20th	Day	61% 6% 16% 13% 3	3000
	21st to end	Day	46% 7% 19% 20% 7% 1	3000
July	1st to 10th	Day	26% 3 21% 21% 20% 8%	6706
	11th to 20th	Day	20% 3 16% 31% 27% 2	6474
	21st to end	Day	30% 2 25% 31% 10% 1	6681
August	1st to 10th	Day	24% 4 14% 39% 17% 1	4495
	11th to 20th	Day	41% 4 23% 24% 7% 2	4195
	21st to end	Day	45% 4 12% 27% 11% 1	4476
September	1st to 10th	Day	38% 4 16% 25% 12% 5%	2400
	11th to 20th	Day	45% 6% 14% 19% 14% 2	2400
	21st to end	Day	47% 5% 19% 18% 9% 2	2118
October	1st to 10th	Day	48% 4 18% 25% 5%	3211
	11th to 20th	Day	64% 3 10% 15% 8%	3161
	21st to end	Day	67% 3 12% 11% 8%	3228
November	1st to 10th	Day	69% 7% 12% 9% 4	1601
	11th to 20th	Day	68% 6% 8% 10% 6% 2	1600
	21st to end	Day	69% 6% 6% 11% 9% 1	1600
December	1st to 10th	Day	57% 4 10% 17% 12%	2620
	11th to 20th	Day	77% 3 9% 8% 2	3004
	21st to end	Day	79% 5% 9% 6% 1	3498
January	1st to 10th	Night	78% 5% 7% 8% 2	6766
	11th to 20th	Night	53% 5% 16% 20% 6% 1	6370
	21st to end	Night	61% 4 9% 17% 8%	6888
February	1st to 10th	Night	69% 4 9% 11% 6% 1	2600
	11th to 20th	Night	66% 5% 11% 10% 7% 2	2534
	21st to end	Night	66% 5% 10% 14% 5% 1	2079
March	1st to 10th	Night	64% 6% 12% 12% 5%	5292
	11th to 20th	Night	62% 4 15% 17% 2	5000
	21st to end	Night	74% 5% 12% 8%	5281
April	1st to 10th	Night	67% 8% 13% 11% 2	2000
	11th to 20th	Night	59% 6% 13% 13% 8% 2	1948
	21st to end	Night	63% 8% 11% 12% 5% 1	1735
May	1st to 10th	Night	47% 4 13% 28% 8%	3637
	11th to 20th	Night	55% 6% 19% 16% 4	3616
	21st to end	Night	58% 8% 20% 12% 1	3972
June	1st to 10th	Night	53% 6% 18% 16% 6% 1	1510
	11th to 20th	Night	59% 7% 16% 15% 3	1400
	21st to end	Night	42% 7% 22% 21% 5% 3	1400
July	1st to 10th	Night	29% 3 16% 23% 19% 11%	3145
	11th to 20th	Night	17% 4 16% 29% 33% 1	3359
	21st to end	Night	29% 3 20% 32% 13% 2	3857
August	1st to 10th	Night	19% 4 15% 42% 19% 2	2628
	11th to 20th	Night	38% 3 23% 24% 9% 2	2929
	21st to end	Night	41% 4 17% 28% 9% 1	3438
September	1st to 10th	Night	32% 6% 16% 28% 13% 4	2000
	11th to 20th	Night	47% 5% 11% 20% 14% 3	2000
	21st to end	Night	48% 5% 14% 22% 9% 2	2279
October	1st to 10th	Night	49% 4 18% 25% 4	3836
	11th to 20th	Night	56% 3 14% 18% 7%	3777
	21st to end	Night	67% 4 11% 11% 6%	4332
November	1st to 10th	Night	69% 5% 12% 11% 3 1	2692
	11th to 20th	Night	69% 5% 9% 10% 5% 2	2800
	21st to end	Night	68% 6% 7% 10% 8% 1	2800
December	1st to 10th	Night	57% 4 10% 22% 7%	4560
	11th to 20th	Night	76% 4 9% 9% 2	5194
	21st to end	Night	79% 5% 7% 6% 2	7458

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	40% 8% 16% 14% 14% 9%	200
	11th to 20th	Day	36% 12% 12% 18% 15% 9%	200
	21st to end	Day	42% 9% 13% 15% 13% 8%	245
February	1st to 10th	Day	45% 7% 18% 18% 10% 3	400
	11th to 20th	Day	42% 8% 20% 20% 9% 2	395
	21st to end	Day	38% 13% 18% 22% 7% 2	364
March	1st to 10th	Day	33% 14% 22% 18% 11% 2	500
	11th to 20th	Day	33% 15% 14% 17% 16% 5%	500
	21st to end	Day	36% 12% 24% 16% 10% 2	550
April	1st to 10th	Day	32% 14% 20% 21% 10% 3	500
	11th to 20th	Day	26% 13% 25% 20% 11% 5%	492
	21st to end	Day	26% 14% 22% 23% 12% 4	500
May	1st to 10th	Day	17% 13% 24% 29% 14% 3	503
	11th to 20th	Day	29% 7% 19% 23% 18% 5%	528
	21st to end	Day	27% 9% 17% 26% 14% 6%	573
June	1st to 10th	Day	21% 9% 23% 28% 16% 3	615
	11th to 20th	Day	24% 9% 25% 26% 13% 3	730
	21st to end	Day	16% 5% 21% 33% 20% 5%	730
July	1st to 10th	Day	14% 3 21% 33% 25% 4	721
	11th to 20th	Day	9% 6% 19% 37% 23% 7%	623
	21st to end	Day	8% 3 17% 39% 22% 10%	575
August	1st to 10th	Day	13% 4 12% 36% 24% 11%	527
	11th to 20th	Day	11% 3 15% 36% 22% 13%	517
	21st to end	Day	14% 5% 23% 32% 17% 10%	583
September	1st to 10th	Day	17% 5% 20% 32% 18% 8%	530
	11th to 20th	Day	16% 8% 19% 28% 23% 5%	530
	21st to end	Day	23% 9% 20% 28% 14% 7%	530
October	1st to 10th	Day	26% 8% 19% 22% 17% 7%	503
	11th to 20th	Day	34% 7% 16% 19% 17% 7%	490
	21st to end	Day	36% 9% 16% 20% 13% 6%	407
November	1st to 10th	Day	41% 9% 21% 17% 9% 5%	217
	11th to 20th	Day	45% 10% 9% 12% 15% 11%	200
	21st to end	Day	38% 8% 17% 14% 11% 13%	200
December	1st to 10th	Day	45% 9% 9% 8% 14% 16%	200
	11th to 20th	Day	47% 11% 13% 15% 12% 3	200
	21st to end	Day	47% 6% 14% 20% 11% 2	220
January	1st to 10th	Night	48% 8% 10% 17% 14% 3	900
	11th to 20th	Night	40% 8% 10% 19% 19% 3	812
	21st to end	Night	39% 6% 12% 23% 16% 4	880
February	1st to 10th	Night	41% 7% 11% 27% 11% 3	800
	11th to 20th	Night	39% 7% 10% 26% 16% 2	784
	21st to end	Night	40% 7% 13% 23% 14% 3	660
March	1st to 10th	Night	37% 10% 13% 24% 15% 1	800
	11th to 20th	Night	39% 11% 12% 22% 14% 3	800
	21st to end	Night	39% 10% 16% 23% 11% 1	880
April	1st to 10th	Night	33% 9% 15% 25% 15% 3	800
	11th to 20th	Night	29% 10% 17% 26% 15% 4	760
	21st to end	Night	31% 9% 16% 25% 16% 4	535
May	1st to 10th	Night	22% 7% 14% 31% 24% 3	400
	11th to 20th	Night	23% 6% 10% 27% 30% 5%	394
	21st to end	Night	25% 6% 12% 26% 25% 6%	433
June	1st to 10th	Night	27% 4 13% 31% 23% 4	400
	11th to 20th	Night	28% 6% 12% 33% 20% 3	400
	21st to end	Night	20% 4 10% 35% 27% 5%	400
July	1st to 10th	Night	13% 3 9% 35% 35% 5%	390
	11th to 20th	Night	10% 2 11% 35% 36% 6%	388
	21st to end	Night	9% 2 10% 32% 40% 8%	438
August	1st to 10th	Night	10% 3 5% 26% 44% 13%	412
	11th to 20th	Night	11% 2 6% 24% 41% 17%	594
	21st to end	Night	13% 6% 9% 29% 35% 9%	775
September	1st to 10th	Night	13% 3 8% 33% 34% 9%	800
	11th to 20th	Night	23% 4 8% 23% 31% 11%	800
	21st to end	Night	25% 4 12% 27% 26% 6%	800
October	1st to 10th	Night	24% 4 10% 28% 27% 7%	772
	11th to 20th	Night	33% 6% 12% 22% 21% 6%	760
	21st to end	Night	38% 6% 14% 23% 15% 4	836
November	1st to 10th	Night	44% 8% 13% 24% 11% 1	780
	11th to 20th	Night	44% 7% 14% 19% 13% 4	800
	21st to end	Night	43% 5% 12% 20% 15% 5%	800
December	1st to 10th	Night	36% 6% 11% 25% 18% 5%	800
	11th to 20th	Night	46% 7% 13% 20% 12% 1	803
	21st to end	Night	49% 8% 14% 19% 11% 1	990

Page generated at 2025-09-30 22:39:06. Version ac07

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