Magnuson Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Magnuson Observing Site clear sky chart from 2005-11-06 to 2024-01-14.

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	13% 9% 4 7% 6% 3 5% 5% 6% 7% 36%	74717
	Night	12% 9% 4 7% 6% 3 4 4 5% 7% 39%	75061
Transparency	Day	48% 4 13% 23% 11% 1	74422
	Night	46% 3 12% 24% 13% 2	74877
Seeing	Day	17% 10% 30% 27% 13% 4	14778
	Night	16% 10% 33% 30% 11% 1	24794

By Month

Percentage Cloud Cover SE (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	9% 10% 6% 10% 9% 6% 8% 5% 6% 8% 22%	1517
	11th to 20th	Day	10% 13% 7% 12% 10% 4 7% 4 6% 7% 18%	1516
	21st to end	Day	10% 14% 6% 13% 10% 5% 7% 5% 6% 8% 17%	1782
February	1st to 10th	Day	8% 11% 6% 11% 10% 6% 7% 7% 6% 6% 23%	1620
	11th to 20th	Day	10% 11% 5% 8% 7% 5% 7% 5% 6% 7% 29%	1574
	21st to end	Day	13% 11% 4 10% 8% 4 6% 5% 5% 7% 27%	1410
March	1st to 10th	Day	14% 11% 5% 11% 8% 4 4 4 5% 6% 29%	1936
	11th to 20th	Day	14% 9% 4 9% 6% 3 5% 5% 4 8% 33%	1980
	21st to end	Day	19% 10% 4 8% 6% 3 5% 5% 5% 5% 30%	2200
April	1st to 10th	Day	18% 11% 5% 8% 5% 3 4 4 6% 7% 29%	2160
	11th to 20th	Day	19% 11% 4 6% 5% 3 4 4 4 6% 33%	2134
	21st to end	Day	22% 11% 4 7% 5% 3 4 5% 5% 5% 29%	2308
May	1st to 10th	Day	17% 9% 3 7% 6% 3 6% 5% 6% 8% 29%	2407
	11th to 20th	Day	17% 8% 3 6% 4 3 5% 5% 6% 7% 35%	2479
	21st to end	Day	17% 7% 2 6% 6% 3 4 4 5% 7% 38%	2725
June	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 4 5% 7% 47%	2520
	11th to 20th	Day	13% 6% 3 5% 6% 3 4 5% 6% 8% 42%	2520
	21st to end	Day	11% 5% 2 7% 6% 3 5% 5% 6% 9% 41%	2520
July	1st to 10th	Day	6% 4 2 5% 5% 4 5% 6% 6% 10% 48%	2520
	11th to 20th	Day	7% 4 2 4 5% 3 4 5% 7% 8% 50%	2520
	21st to end	Day	7% 3 2 5% 5% 3 5% 5% 7% 11% 47%	2671
August	1st to 10th	Day	8% 5% 2 6% 5% 4 6% 6% 7% 9% 43%	2328
	11th to 20th	Day	13% 6% 3 6% 5% 3 5% 5% 5% 8% 41%	2301
	21st to end	Day	9% 4 2 5% 4 2 4 5% 5% 8% 50%	2435
September	1st to 10th	Day	13% 5% 2 6% 5% 3 3 4 5% 6% 47%	2160
	11th to 20th	Day	10% 7% 2 5% 4 2 5% 4 6% 7% 48%	2142
	21st to end	Day	14% 7% 3 5% 5% 3 5% 4 6% 7% 42%	1886
October	1st to 10th	Day	15% 8% 2 5% 5% 2 4 4 5% 7% 42%	1800
	11th to 20th	Day	16% 10% 3 6% 4 3 4 4 5% 6% 39%	1800
	21st to end	Day	18% 11% 3 6% 6% 2 3 3 5% 7% 36%	1977
November	1st to 10th	Day	12% 9% 3 7% 4 3 4 5% 5% 7% 42%	1608
	11th to 20th	Day	11% 12% 6% 12% 7% 3 5% 4 5% 7% 29%	1520
	21st to end	Day	12% 12% 5% 7% 7% 3 3 4 4 7% 36%	1520
December	1st to 10th	Day	12% 10% 4 10% 10% 5% 7% 5% 4 6% 25%	1520
	11th to 20th	Day	10% 13% 6% 10% 7% 4 7% 5% 6% 6% 26%	1918
	21st to end	Day	19% 15% 7% 17% 6% 3 4 3 4 5% 15%	2783
January	1st to 10th	Night	9% 9% 5% 10% 10% 5% 6% 6% 6% 7% 29%	2648
	11th to 20th	Night	10% 11% 6% 12% 9% 5% 7% 5% 5% 8% 22%	2515
	21st to end	Night	12% 14% 6% 11% 8% 4 5% 5% 5% 7% 22%	2574
February	1st to 10th	Night	7% 13% 6% 10% 8% 5% 6% 5% 5% 6% 30%	2340
	11th to 20th	Night	9% 10% 4 7% 7% 5% 6% 5% 5% 7% 37%	2273
	21st to end	Night	13% 9% 3 9% 7% 4 6% 5% 4 6% 34%	1844
March	1st to 10th	Night	14% 10% 4 7% 5% 3 4 5% 6% 7% 36%	2039
	11th to 20th	Night	14% 10% 4 7% 5% 4 5% 4 5% 7% 36%	1980
	21st to end	Night	17% 12% 4 6% 5% 4 4 5% 5% 7% 32%	2156
April	1st to 10th	Night	14% 11% 4 6% 6% 4 4 4 4 6% 38%	1800
	11th to 20th	Night	20% 10% 3 6% 5% 3 3 4 5% 7% 34%	1775
	21st to end	Night	20% 9% 3 6% 4 3 4 4 5% 6% 36%	1650
May	1st to 10th	Night	18% 7% 3 6% 5% 2 4 4 5% 7% 40%	1551
	11th to 20th	Night	15% 7% 2 4 4 2 4 5% 6% 7% 42%	1416
	21st to end	Night	14% 7% 3 6% 5% 3 4 4 5% 9% 40%	1551
June	1st to 10th	Night	8% 4 2 4 4 3 4 5% 6% 8% 51%	1440
	11th to 20th	Night	10% 4 2 5% 6% 4 3 5% 6% 9% 44%	1440
	21st to end	Night	9% 4 1 4 4 2 3 3 5% 8% 55%	1440
July	1st to 10th	Night	7% 3 2 4 4 2 3 5% 6% 9% 57%	1440
	11th to 20th	Night	3 3 2 4 4 3 4 4 6% 9% 58%	1440
	21st to end	Night	5% 3 2 3 3 2 4 4 4 10% 60%	1643
August	1st to 10th	Night	6% 4 2 5% 6% 3 4 4 7% 8% 51%	1611
	11th to 20th	Night	10% 5% 2 4 4 2 4 5% 5% 8% 52%	1600
	21st to end	Night	7% 4 2 4 4 3 4 3 5% 6% 58%	1920
September	1st to 10th	Night	9% 5% 2 4 4 2 4 4 5% 6% 57%	1800
	11th to 20th	Night	11% 5% 2 4 4 2 4 3 4 7% 53%	1813
	21st to end	Night	11% 7% 3 4 3 2 4 3 3 7% 53%	2055
October	1st to 10th	Night	13% 7% 2 5% 4 3 4 4 4 6% 48%	2160
	11th to 20th	Night	16% 8% 2 4 4 3 4 4 5% 7% 43%	2160
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 5% 6% 38%	2377
November	1st to 10th	Night	11% 7% 3 6% 5% 3 4 4 5% 6% 46%	2467
	11th to 20th	Night	11% 12% 5% 9% 6% 3 4 4 4 6% 35%	2660
	21st to end	Night	13% 11% 4 6% 6% 3 3 3 4 6% 39%	2660
December	1st to 10th	Night	10% 11% 5% 9% 8% 5% 6% 5% 6% 7% 29%	2660
	11th to 20th	Night	12% 11% 5% 10% 9% 4 5% 5% 5% 6% 27%	3279
	21st to end	Night	20% 14% 6% 14% 8% 3 4 4 4 5% 18%	4884

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	59% 4 12% 17% 8%	1518
	11th to 20th	Day	64% 3 10% 14% 8%	1516
	21st to end	Day	65% 4 12% 12% 7% 1	1782
February	1st to 10th	Day	60% 3 12% 17% 8% 1	1620
	11th to 20th	Day	55% 4 11% 21% 8% 2	1583
	21st to end	Day	57% 5% 10% 18% 10% 1	1410
March	1st to 10th	Day	55% 4 13% 19% 8% 1	1936
	11th to 20th	Day	51% 4 13% 22% 9% 1	1980
	21st to end	Day	56% 4 12% 20% 7% 1	2200
April	1st to 10th	Day	55% 4 11% 20% 9% 1	2160
	11th to 20th	Day	54% 3 11% 19% 12% 1	2134
	21st to end	Day	56% 4 11% 20% 8% 1	2308
May	1st to 10th	Day	53% 4 14% 21% 7% 1	2407
	11th to 20th	Day	47% 5% 12% 24% 10% 1	2483
	21st to end	Day	47% 4 13% 23% 12% 1	2725
June	1st to 10th	Day	37% 4 14% 30% 14% 1	2520
	11th to 20th	Day	41% 5% 13% 22% 18% 1	2520
	21st to end	Day	39% 6% 16% 27% 12% 1	2520
July	1st to 10th	Day	32% 5% 16% 33% 12% 1	2451
	11th to 20th	Day	32% 5% 14% 29% 17% 3	2445
	21st to end	Day	30% 5% 16% 32% 15% 2	2523
August	1st to 10th	Day	37% 5% 15% 30% 10% 3	2314
	11th to 20th	Day	40% 5% 14% 29% 12% 1	2301
	21st to end	Day	32% 5% 13% 30% 19% 1	2435
September	1st to 10th	Day	38% 4 12% 30% 15% 1	2160
	11th to 20th	Day	37% 4 14% 33% 12% 1	2142
	21st to end	Day	42% 3 11% 31% 13% 1	1886
October	1st to 10th	Day	42% 3 11% 28% 14% 1	1800
	11th to 20th	Day	45% 3 13% 23% 15% 1	1800
	21st to end	Day	49% 3 12% 24% 11% 1	1977
November	1st to 10th	Day	42% 4 13% 22% 16% 2	1608
	11th to 20th	Day	56% 3 12% 19% 10% 1	1520
	21st to end	Day	49% 3 17% 23% 7% 1	1520
December	1st to 10th	Day	59% 4 16% 12% 8% 1	1520
	11th to 20th	Day	59% 3 17% 15% 7%	1915
	21st to end	Day	72% 2 8% 13% 4	2783
January	1st to 10th	Night	54% 4 12% 20% 10% 1	2648
	11th to 20th	Night	61% 4 12% 15% 8%	2515
	21st to end	Night	61% 4 11% 16% 8% 1	2574
February	1st to 10th	Night	55% 3 11% 21% 9% 1	2340
	11th to 20th	Night	47% 4 12% 25% 11% 2	2274
	21st to end	Night	51% 2 10% 22% 12% 2	1844
March	1st to 10th	Night	47% 4 14% 26% 8% 1	2039
	11th to 20th	Night	48% 3 13% 22% 13% 1	1980
	21st to end	Night	53% 4 12% 20% 8% 2	2156
April	1st to 10th	Night	49% 2 10% 23% 15% 1	1800
	11th to 20th	Night	50% 3 10% 20% 14% 3	1775
	21st to end	Night	49% 3 9% 23% 14% 2	1650
May	1st to 10th	Night	45% 4 12% 24% 14% 1	1551
	11th to 20th	Night	40% 6% 14% 25% 13% 2	1417
	21st to end	Night	43% 3 14% 23% 16% 1	1552
June	1st to 10th	Night	31% 4 15% 33% 16% 2	1440
	11th to 20th	Night	37% 6% 14% 22% 19% 3	1440
	21st to end	Night	27% 4 18% 30% 18% 3	1440
July	1st to 10th	Night	25% 3 15% 36% 19% 2	1400
	11th to 20th	Night	23% 6% 17% 32% 18% 4	1392
	21st to end	Night	22% 4 15% 37% 19% 3	1552
August	1st to 10th	Night	31% 3 15% 31% 17% 3	1601
	11th to 20th	Night	31% 5% 15% 29% 17% 2	1600
	21st to end	Night	28% 4 13% 30% 20% 4	1920
September	1st to 10th	Night	29% 4 10% 32% 23% 2	1800
	11th to 20th	Night	32% 3 15% 32% 18% 1	1813
	21st to end	Night	34% 3 12% 31% 19% 1	2055
October	1st to 10th	Night	38% 3 9% 30% 17% 2	2160
	11th to 20th	Night	42% 3 13% 25% 17% 1	2160
	21st to end	Night	48% 3 11% 24% 13% 1	2377
November	1st to 10th	Night	40% 3 14% 24% 16% 3	2467
	11th to 20th	Night	50% 4 12% 21% 12% 1	2660
	21st to end	Night	48% 2 14% 25% 9% 1	2660
December	1st to 10th	Night	55% 4 15% 18% 7% 1	2660
	11th to 20th	Night	57% 3 14% 19% 6% 1	3281
	21st to end	Night	71% 2 7% 14% 5%	4884

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	17% 11% 39% 20% 10% 3	270
	11th to 20th	Day	20% 10% 42% 22% 4 2	249
	21st to end	Day	18% 11% 39% 25% 6% 2	264
February	1st to 10th	Day	13% 17% 44% 24% 3	240
	11th to 20th	Day	18% 16% 39% 19% 6% 1	240
	21st to end	Day	22% 15% 36% 25% 2	198
March	1st to 10th	Day	14% 15% 48% 17% 6% 1	240
	11th to 20th	Day	19% 11% 35% 25% 9% 2	240
	21st to end	Day	31% 12% 27% 22% 7% 1	286
April	1st to 10th	Day	28% 12% 35% 20% 5%	420
	11th to 20th	Day	28% 13% 27% 23% 8% 1	412
	21st to end	Day	29% 13% 26% 20% 9% 2	568
May	1st to 10th	Day	22% 10% 36% 23% 8% 1	603
	11th to 20th	Day	21% 12% 28% 25% 10% 4	625
	21st to end	Day	22% 8% 26% 25% 16% 3	679
June	1st to 10th	Day	12% 7% 24% 31% 20% 6%	630
	11th to 20th	Day	15% 6% 29% 34% 13% 3	630
	21st to end	Day	12% 8% 29% 30% 17% 4	630
July	1st to 10th	Day	7% 4 18% 36% 26% 10%	621
	11th to 20th	Day	9% 4 31% 31% 18% 7%	618
	21st to end	Day	7% 3 26% 37% 19% 6%	684
August	1st to 10th	Day	9% 4 22% 31% 25% 8%	626
	11th to 20th	Day	14% 4 18% 35% 24% 6%	615
	21st to end	Day	12% 4 26% 33% 19% 6%	554
September	1st to 10th	Day	16% 7% 26% 28% 17% 5%	450
	11th to 20th	Day	12% 6% 31% 32% 14% 5%	432
	21st to end	Day	17% 14% 30% 26% 9% 3	270
October	1st to 10th	Day	22% 9% 29% 24% 14% 3	270
	11th to 20th	Day	19% 19% 36% 19% 7%	270
	21st to end	Day	28% 17% 33% 16% 5% 2	297
November	1st to 10th	Day	17% 15% 34% 24% 7% 1	270
	11th to 20th	Day	19% 19% 39% 18% 6%	270
	21st to end	Day	20% 17% 36% 23% 3	270
December	1st to 10th	Day	15% 15% 38% 25% 6% 1	270
	11th to 20th	Day	18% 14% 39% 23% 7%	270
	21st to end	Day	27% 14% 36% 20% 3	297
January	1st to 10th	Night	13% 14% 40% 26% 7% 1	950
	11th to 20th	Night	16% 12% 38% 28% 5% 1	918
	21st to end	Night	19% 14% 35% 24% 7%	990
February	1st to 10th	Night	15% 13% 39% 27% 5% 1	900
	11th to 20th	Night	15% 15% 41% 22% 6% 1	879
	21st to end	Night	18% 16% 34% 24% 8%	660
March	1st to 10th	Night	18% 13% 35% 26% 9%	599
	11th to 20th	Night	19% 10% 35% 28% 8%	540
	21st to end	Night	23% 11% 31% 25% 8% 2	594
April	1st to 10th	Night	19% 14% 37% 25% 4	540
	11th to 20th	Night	26% 11% 31% 25% 6% 1	533
	21st to end	Night	24% 8% 33% 25% 8% 2	540
May	1st to 10th	Night	21% 8% 33% 29% 8% 1	540
	11th to 20th	Night	17% 9% 33% 31% 9% 1	531
	21st to end	Night	18% 8% 28% 33% 13% 1	584
June	1st to 10th	Night	10% 5% 28% 41% 15% 1	540
	11th to 20th	Night	13% 4 29% 41% 12%	540
	21st to end	Night	10% 4 27% 42% 16% 1	540
July	1st to 10th	Night	7% 2 20% 37% 32% 2	525
	11th to 20th	Night	4 3 26% 40% 25% 2	522
	21st to end	Night	5% 2 17% 53% 20% 2	591
August	1st to 10th	Night	7% 2 18% 46% 26% 2	537
	11th to 20th	Night	10% 3 20% 37% 26% 4	534
	21st to end	Night	8% 3 25% 40% 22% 2	594
September	1st to 10th	Night	13% 4 23% 40% 17% 4	540
	11th to 20th	Night	12% 3 32% 34% 17% 2	540
	21st to end	Night	14% 10% 33% 30% 14%	615
October	1st to 10th	Night	16% 8% 33% 29% 13% 1	720
	11th to 20th	Night	20% 11% 35% 27% 6%	720
	21st to end	Night	23% 13% 30% 27% 6% 1	792
November	1st to 10th	Night	17% 13% 35% 26% 7% 1	801
	11th to 20th	Night	16% 15% 41% 22% 5%	950
	21st to end	Night	18% 14% 38% 23% 7%	950
December	1st to 10th	Night	16% 11% 39% 27% 6% 1	950
	11th to 20th	Night	19% 10% 40% 24% 7%	950
	21st to end	Night	24% 13% 35% 23% 5%	1045

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