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 2024-01-12.

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	24% 9% 3 6% 5% 3 4 4 5% 6% 31%	74877
	Night	27% 9% 3 6% 5% 3 4 4 4 5% 29%	74841
Transparency	Day	56% 6% 13% 17% 7% 1	74569
	Night	59% 6% 12% 15% 7% 1	74667
Seeing	Day	26% 8% 19% 25% 16% 6%	13971
	Night	33% 6% 12% 24% 20% 5%	22498

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	42% 12% 4 6% 5% 3 3 3 3 4 16%	1329
	11th to 20th	Day	32% 10% 5% 6% 6% 3 4 4 5% 6% 19%	1422
	21st to end	Day	35% 10% 4 8% 4 2 5% 4 5% 6% 17%	1606
February	1st to 10th	Day	36% 12% 4 6% 6% 3 4 4 4 5% 17%	1620
	11th to 20th	Day	30% 13% 4 8% 6% 3 3 4 4 5% 21%	1575
	21st to end	Day	30% 13% 4 7% 6% 3 5% 4 4 5% 19%	1390
March	1st to 10th	Day	28% 12% 4 6% 5% 4 5% 5% 5% 5% 20%	1831
	11th to 20th	Day	31% 11% 3 6% 5% 3 5% 4 4 5% 22%	1980
	21st to end	Day	31% 11% 5% 9% 6% 3 5% 4 5% 6% 15%	2256
April	1st to 10th	Day	27% 12% 4 7% 6% 4 6% 5% 6% 6% 17%	2160
	11th to 20th	Day	25% 9% 4 7% 7% 4 5% 5% 5% 5% 22%	2157
	21st to end	Day	23% 12% 3 8% 6% 5% 6% 5% 5% 6% 22%	2398
May	1st to 10th	Day	18% 7% 3 7% 6% 4 5% 5% 6% 6% 32%	2520
	11th to 20th	Day	20% 8% 3 6% 6% 3 6% 6% 6% 7% 30%	2477
	21st to end	Day	25% 10% 4 6% 6% 3 3 4 5% 6% 28%	2723
June	1st to 10th	Day	23% 8% 3 7% 5% 4 5% 5% 5% 7% 29%	2596
	11th to 20th	Day	25% 9% 3 8% 6% 3 4 5% 6% 7% 24%	2700
	21st to end	Day	14% 6% 2 6% 5% 3 5% 5% 6% 8% 39%	2700
July	1st to 10th	Day	12% 5% 3 5% 4 2 4 4 5% 7% 49%	2700
	11th to 20th	Day	12% 5% 2 3 4 2 4 4 4 6% 53%	2615
	21st to end	Day	10% 3 1 2 3 2 3 3 4 6% 63%	2741
August	1st to 10th	Day	11% 5% 2 3 4 2 4 4 4 6% 56%	2492
	11th to 20th	Day	9% 4 2 4 3 2 4 4 5% 7% 57%	2298
	21st to end	Day	13% 6% 2 5% 4 3 5% 4 4 8% 45%	2471
September	1st to 10th	Day	13% 6% 2 5% 5% 2 4 3 4 7% 50%	2160
	11th to 20th	Day	15% 7% 3 7% 6% 2 4 4 5% 7% 40%	2160
	21st to end	Day	21% 6% 3 5% 4 3 3 4 4 7% 39%	1908
October	1st to 10th	Day	21% 7% 3 7% 6% 3 5% 5% 6% 7% 30%	1800
	11th to 20th	Day	29% 9% 3 6% 4 3 4 4 4 7% 27%	1800
	21st to end	Day	34% 8% 3 5% 5% 3 4 4 5% 6% 23%	1855
November	1st to 10th	Day	37% 10% 3 7% 6% 2 4 5% 5% 5% 17%	1520
	11th to 20th	Day	37% 9% 4 5% 5% 3 4 3 3 4 22%	1520
	21st to end	Day	38% 10% 4 5% 4 3 4 4 4 5% 20%	1520
December	1st to 10th	Day	32% 13% 4 6% 5% 2 3 3 3 3 25%	1520
	11th to 20th	Day	34% 13% 4 7% 5% 3 4 4 3 6% 17%	1915
	21st to end	Day	29% 14% 5% 9% 6% 3 4 4 4 5% 14%	2442
January	1st to 10th	Night	42% 11% 4 6% 5% 3 4 3 3 4 16%	2836
	11th to 20th	Night	36% 12% 3 5% 5% 3 4 4 4 5% 21%	2563
	21st to end	Night	34% 10% 3 7% 6% 3 4 3 4 6% 20%	2747
February	1st to 10th	Night	36% 11% 3 6% 5% 4 5% 3 3 5% 20%	2340
	11th to 20th	Night	31% 12% 4 7% 5% 2 4 3 4 4 23%	2272
	21st to end	Night	30% 13% 3 7% 6% 3 4 4 5% 5% 21%	1866
March	1st to 10th	Night	30% 12% 3 5% 5% 3 5% 5% 5% 6% 22%	2128
	11th to 20th	Night	33% 11% 4 6% 5% 3 3 4 3 5% 22%	1979
	21st to end	Night	32% 12% 4 7% 5% 3 4 5% 5% 5% 17%	2099
April	1st to 10th	Night	27% 11% 4 6% 7% 4 5% 4 5% 7% 19%	1800
	11th to 20th	Night	26% 10% 4 6% 5% 3 4 5% 5% 5% 25%	1752
	21st to end	Night	26% 10% 4 7% 6% 3 6% 5% 5% 6% 22%	1562
May	1st to 10th	Night	19% 7% 3 6% 6% 3 4 4 4 5% 39%	1440
	11th to 20th	Night	20% 7% 3 6% 5% 3 5% 5% 6% 8% 34%	1416
	21st to end	Night	23% 8% 4 7% 5% 3 5% 5% 4 6% 31%	1552
June	1st to 10th	Night	23% 8% 3 6% 5% 2 5% 5% 4 6% 32%	1360
	11th to 20th	Night	28% 9% 3 6% 7% 4 4 3 4 6% 26%	1260
	21st to end	Night	15% 5% 2 4 4 3 4 6% 7% 8% 42%	1260
July	1st to 10th	Night	13% 4 2 5% 4 2 5% 4 3 6% 52%	1260
	11th to 20th	Night	9% 4 2 3 4 2 3 4 5% 6% 57%	1341
	21st to end	Night	9% 3 2 4 4 1 3 2 4 5% 64%	1576
August	1st to 10th	Night	9% 3 2 4 3 2 3 3 4 7% 60%	1444
	11th to 20th	Night	8% 4 2 3 3 2 3 4 4 7% 63%	1602
	21st to end	Night	11% 5% 2 6% 3 2 4 4 4 6% 52%	1885
September	1st to 10th	Night	11% 4 2 5% 4 2 4 4 5% 5% 55%	1800
	11th to 20th	Night	19% 6% 3 5% 6% 2 4 4 4 5% 42%	1800
	21st to end	Night	20% 7% 3 6% 5% 2 3 4 4 5% 41%	2050
October	1st to 10th	Night	20% 8% 3 6% 6% 3 4 4 4 6% 35%	2160
	11th to 20th	Night	28% 7% 3 5% 4 3 3 4 4 6% 33%	2160
	21st to end	Night	31% 8% 2 6% 5% 3 3 3 4 6% 29%	2499
November	1st to 10th	Night	38% 10% 4 6% 5% 3 4 3 4 5% 20%	2554
	11th to 20th	Night	36% 9% 3 5% 5% 3 4 4 3 4 23%	2660
	21st to end	Night	37% 10% 3 6% 5% 3 4 4 4 5% 20%	2660
December	1st to 10th	Night	31% 11% 3 7% 4 2 4 4 5% 4 25%	2660
	11th to 20th	Night	33% 13% 4 8% 5% 3 4 4 4 5% 16%	3284
	21st to end	Night	31% 15% 5% 9% 6% 4 5% 4 4 4 12%	5214

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	76% 4 5% 10% 4 1	1329
	11th to 20th	Day	67% 6% 11% 8% 6% 2	1422
	21st to end	Day	69% 5% 10% 12% 4	1606
February	1st to 10th	Day	71% 6% 9% 9% 3 2	1620
	11th to 20th	Day	68% 6% 9% 9% 8% 1	1584
	21st to end	Day	70% 5% 9% 10% 5% 1	1390
March	1st to 10th	Day	67% 5% 11% 11% 4	1831
	11th to 20th	Day	69% 6% 8% 13% 4 1	1980
	21st to end	Day	72% 7% 10% 10% 1	2256
April	1st to 10th	Day	68% 7% 14% 10% 1	2160
	11th to 20th	Day	65% 6% 10% 13% 6% 1	2157
	21st to end	Day	65% 7% 11% 11% 5% 1	2398
May	1st to 10th	Day	54% 7% 15% 17% 6%	2520
	11th to 20th	Day	55% 8% 18% 15% 4	2480
	21st to end	Day	61% 7% 15% 15% 2	2724
June	1st to 10th	Day	57% 8% 16% 13% 6% 1	2596
	11th to 20th	Day	62% 6% 16% 12% 3	2700
	21st to end	Day	45% 8% 19% 20% 7% 1	2700
July	1st to 10th	Day	37% 8% 15% 26% 12% 2	2626
	11th to 20th	Day	34% 7% 14% 31% 13% 1	2534
	21st to end	Day	25% 5% 15% 36% 16% 3	2589
August	1st to 10th	Day	32% 7% 18% 29% 12% 2	2475
	11th to 20th	Day	28% 6% 21% 31% 11% 3	2298
	21st to end	Day	40% 7% 15% 30% 8% 1	2471
September	1st to 10th	Day	38% 4 15% 26% 12% 4	2160
	11th to 20th	Day	45% 6% 13% 18% 15% 2	2160
	21st to end	Day	47% 6% 17% 18% 10% 2	1908
October	1st to 10th	Day	54% 7% 13% 19% 7%	1800
	11th to 20th	Day	59% 5% 13% 18% 5% 1	1800
	21st to end	Day	64% 5% 10% 13% 8%	1855
November	1st to 10th	Day	69% 7% 12% 8% 4	1520
	11th to 20th	Day	68% 6% 8% 10% 6% 2	1520
	21st to end	Day	68% 5% 5% 11% 9% 1	1520
December	1st to 10th	Day	66% 5% 9% 10% 9% 1	1520
	11th to 20th	Day	70% 4 12% 10% 3 1	1918
	21st to end	Day	73% 7% 10% 8% 2	2442
January	1st to 10th	Night	75% 5% 6% 9% 3 1	2836
	11th to 20th	Night	68% 6% 10% 10% 5% 1	2563
	21st to end	Night	68% 4 9% 13% 5% 1	2747
February	1st to 10th	Night	69% 4 9% 11% 4 1	2340
	11th to 20th	Night	67% 5% 11% 10% 6% 1	2274
	21st to end	Night	65% 5% 10% 14% 5% 1	1866
March	1st to 10th	Night	63% 6% 11% 13% 5%	2128
	11th to 20th	Night	67% 5% 10% 13% 4 1	1980
	21st to end	Night	70% 6% 14% 9% 1	2099
April	1st to 10th	Night	66% 8% 12% 12% 2	1800
	11th to 20th	Night	62% 6% 12% 12% 6% 1	1752
	21st to end	Night	64% 8% 10% 11% 6% 1	1562
May	1st to 10th	Night	48% 8% 16% 19% 8% 1	1440
	11th to 20th	Night	49% 8% 21% 15% 7%	1416
	21st to end	Night	56% 7% 17% 17% 2	1552
June	1st to 10th	Night	54% 6% 18% 16% 5% 1	1360
	11th to 20th	Night	60% 8% 17% 12% 3	1260
	21st to end	Night	40% 8% 23% 21% 5% 3	1260
July	1st to 10th	Night	35% 6% 16% 26% 12% 4	1225
	11th to 20th	Night	30% 8% 16% 30% 16% 1	1299
	21st to end	Night	24% 5% 12% 35% 20% 5%	1488
August	1st to 10th	Night	26% 7% 16% 31% 17% 3	1436
	11th to 20th	Night	24% 6% 20% 33% 15% 3	1602
	21st to end	Night	34% 6% 18% 31% 10% 1	1885
September	1st to 10th	Night	32% 6% 15% 29% 13% 4	1800
	11th to 20th	Night	47% 6% 10% 19% 15% 3	1800
	21st to end	Night	47% 6% 13% 22% 10% 2	2050
October	1st to 10th	Night	51% 5% 13% 23% 7%	2160
	11th to 20th	Night	55% 6% 16% 16% 7% 1	2160
	21st to end	Night	58% 7% 12% 15% 8%	2499
November	1st to 10th	Night	69% 5% 11% 11% 3 1	2554
	11th to 20th	Night	68% 5% 9% 11% 5% 2	2660
	21st to end	Night	67% 6% 7% 11% 8% 1	2660
December	1st to 10th	Night	64% 5% 11% 13% 7% 1	2660
	11th to 20th	Night	72% 6% 9% 9% 3 1	3280
	21st to end	Night	74% 7% 8% 7% 3	5214

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% 8% 16% 13% 15% 9%	180
	11th to 20th	Day	40% 13% 11% 16% 12% 8%	162
	21st to end	Day	44% 7% 12% 16% 14% 7%	198
February	1st to 10th	Day	48% 7% 18% 16% 10% 2	340
	11th to 20th	Day	43% 9% 19% 19% 9% 1	335
	21st to end	Day	37% 13% 18% 23% 7% 2	306
March	1st to 10th	Day	34% 14% 24% 17% 10% 2	420
	11th to 20th	Day	34% 16% 13% 17% 15% 4	420
	21st to end	Day	38% 11% 23% 16% 10% 2	462
April	1st to 10th	Day	32% 14% 21% 20% 10% 3	420
	11th to 20th	Day	29% 12% 24% 18% 10% 6%	412
	21st to end	Day	25% 15% 24% 23% 10% 3	420
May	1st to 10th	Day	17% 13% 25% 29% 14% 3	423
	11th to 20th	Day	28% 7% 17% 24% 18% 6%	448
	21st to end	Day	26% 8% 17% 27% 15% 7%	485
June	1st to 10th	Day	22% 9% 24% 26% 16% 3	526
	11th to 20th	Day	24% 9% 25% 26% 13% 3	630
	21st to end	Day	15% 6% 20% 34% 20% 5%	630
July	1st to 10th	Day	15% 4 21% 32% 25% 3	621
	11th to 20th	Day	11% 6% 19% 35% 23% 6%	533
	21st to end	Day	8% 3 18% 38% 24% 10%	487
August	1st to 10th	Day	12% 4 11% 36% 24% 11%	447
	11th to 20th	Day	10% 2 15% 37% 24% 13%	437
	21st to end	Day	15% 5% 23% 31% 17% 9%	495
September	1st to 10th	Day	18% 5% 22% 33% 15% 6%	450
	11th to 20th	Day	16% 9% 19% 28% 22% 6%	450
	21st to end	Day	23% 9% 19% 28% 14% 8%	450
October	1st to 10th	Day	26% 8% 19% 23% 17% 7%	450
	11th to 20th	Day	34% 7% 16% 18% 17% 7%	450
	21st to end	Day	37% 10% 15% 18% 13% 6%	370
November	1st to 10th	Day	41% 10% 20% 16% 9% 4	196
	11th to 20th	Day	44% 8% 8% 13% 15% 12%	180
	21st to end	Day	39% 9% 16% 13% 11% 12%	180
December	1st to 10th	Day	48% 9% 9% 8% 13% 13%	180
	11th to 20th	Day	44% 12% 12% 16% 12% 3	180
	21st to end	Day	44% 7% 14% 21% 12% 3	198
January	1st to 10th	Night	49% 8% 10% 16% 14% 3	850
	11th to 20th	Night	43% 8% 10% 16% 19% 3	737
	21st to end	Night	40% 6% 12% 22% 16% 4	792
February	1st to 10th	Night	43% 7% 11% 26% 11% 3	720
	11th to 20th	Night	40% 7% 11% 25% 16% 2	704
	21st to end	Night	39% 6% 13% 24% 15% 3	592
March	1st to 10th	Night	38% 9% 13% 23% 15% 2	720
	11th to 20th	Night	39% 11% 11% 22% 14% 3	720
	21st to end	Night	39% 10% 15% 23% 11% 1	792
April	1st to 10th	Night	33% 9% 16% 25% 15% 3	720
	11th to 20th	Night	31% 10% 16% 24% 16% 4	684
	21st to end	Night	31% 9% 17% 25% 16% 3	482
May	1st to 10th	Night	21% 7% 13% 31% 24% 3	360
	11th to 20th	Night	22% 5% 9% 27% 32% 5%	354
	21st to end	Night	25% 5% 13% 26% 24% 6%	389
June	1st to 10th	Night	27% 4 14% 30% 22% 4	360
	11th to 20th	Night	29% 6% 11% 32% 19% 3	360
	21st to end	Night	18% 4 10% 35% 28% 5%	360
July	1st to 10th	Night	13% 3 8% 34% 36% 5%	350
	11th to 20th	Night	11% 2 11% 34% 35% 6%	348
	21st to end	Night	8% 2 9% 31% 41% 8%	394
August	1st to 10th	Night	11% 3 5% 25% 44% 14%	370
	11th to 20th	Night	9% 2 6% 24% 42% 18%	534
	21st to end	Night	13% 6% 9% 29% 35% 9%	697
September	1st to 10th	Night	13% 4 9% 34% 33% 7%	720
	11th to 20th	Night	23% 5% 9% 22% 31% 11%	720
	21st to end	Night	26% 5% 12% 26% 26% 6%	720
October	1st to 10th	Night	24% 5% 9% 27% 28% 8%	720
	11th to 20th	Night	33% 6% 12% 21% 22% 6%	720
	21st to end	Night	37% 6% 14% 23% 15% 5%	792
November	1st to 10th	Night	44% 8% 13% 24% 11% 1	740
	11th to 20th	Night	42% 7% 14% 20% 13% 4	760
	21st to end	Night	44% 5% 12% 19% 15% 5%	760
December	1st to 10th	Night	37% 6% 11% 25% 16% 5%	760
	11th to 20th	Night	46% 8% 12% 20% 12% 1	762
	21st to end	Night	48% 7% 14% 19% 11% 1	935

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