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 2026-01-02.

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	12% 9% 3 8% 5% 4 5% 4 6% 7% 37%	127717
	Night	12% 9% 4 9% 5% 5% 4 3 5% 6% 38%	123928
Transparency	Day	46% 3 12% 26% 12% 1	131296
	Night	46% 3 12% 24% 14% 2	128426
Seeing	Day	17% 10% 30% 26% 13% 3	17529
	Night	16% 10% 33% 30% 10% 1	27461

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	6% 14% 6% 20% 11% 8% 7% 3 6% 6% 14%	3625
	11th to 20th	Day	6% 11% 5% 13% 8% 6% 9% 7% 12% 7% 15%	3756
	21st to end	Day	7% 14% 7% 15% 7% 5% 6% 3 4 6% 26%	4488
February	1st to 10th	Day	8% 13% 5% 13% 8% 7% 6% 6% 6% 5% 23%	1800
	11th to 20th	Day	9% 12% 4 9% 6% 7% 6% 4 6% 7% 30%	1754
	21st to end	Day	12% 11% 3 11% 6% 5% 5% 4 6% 7% 30%	1573
March	1st to 10th	Day	13% 8% 2 7% 4 3 3 3 5% 6% 45%	4839
	11th to 20th	Day	11% 7% 3 7% 3 3 4 4 5% 9% 44%	4980
	21st to end	Day	22% 12% 4 10% 6% 4 4 3 4 5% 26%	5525
April	1st to 10th	Day	17% 12% 4 8% 5% 4 4 3 6% 7% 30%	2400
	11th to 20th	Day	19% 12% 4 7% 5% 4 4 3 5% 6% 32%	2374
	21st to end	Day	21% 12% 4 7% 4 4 4 4 6% 6% 29%	2565
May	1st to 10th	Day	13% 7% 2 6% 3 3 4 3 6% 7% 44%	6041
	11th to 20th	Day	15% 7% 2 5% 3 4 4 3 5% 6% 45%	6319
	21st to end	Day	19% 7% 2 6% 4 4 4 3 6% 7% 38%	6935
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 6% 8% 46%	2800
	11th to 20th	Day	13% 7% 3 6% 5% 4 4 4 6% 8% 39%	2800
	21st to end	Day	12% 5% 2 7% 5% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	6360
	11th to 20th	Day	8% 5% 2 4 4 4 4 4 6% 8% 53%	6360
	21st to end	Day	8% 5% 2 7% 5% 4 5% 4 8% 9% 42%	6743
August	1st to 10th	Day	10% 7% 2 7% 4 4 5% 4 6% 8% 44%	4234
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 9% 40%	4171
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 48%	4416
September	1st to 10th	Day	12% 5% 2 6% 4 4 3 3 5% 6% 48%	2400
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 46%	2380
	21st to end	Day	13% 7% 3 6% 4 4 5% 3 6% 7% 44%	2095
October	1st to 10th	Day	14% 8% 2 5% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	15% 10% 2 7% 3 4 4 3 5% 6% 39%	2000
	21st to end	Day	17% 11% 3 7% 5% 3 3 2 5% 7% 37%	2196
November	1st to 10th	Day	12% 9% 3 8% 4 4 4 4 5% 7% 40%	1780
	11th to 20th	Day	11% 14% 4 12% 6% 4 5% 3 5% 6% 30%	1680
	21st to end	Day	12% 13% 5% 10% 7% 3 3 3 4 7% 34%	1680
December	1st to 10th	Day	7% 8% 4 12% 11% 8% 7% 4 6% 7% 27%	2690
	11th to 20th	Day	10% 15% 7% 13% 10% 8% 6% 4 4 5% 18%	3088
	21st to end	Day	18% 19% 7% 22% 6% 3 3 2 3 4 13%	4070
January	1st to 10th	Night	6% 11% 5% 19% 9% 8% 6% 4 5% 5% 20%	6363
	11th to 20th	Night	6% 11% 5% 13% 8% 7% 9% 8% 10% 6% 19%	6222
	21st to end	Night	10% 12% 7% 12% 6% 6% 5% 3 5% 6% 29%	6490
February	1st to 10th	Night	7% 14% 6% 10% 7% 6% 5% 4 5% 5% 31%	2600
	11th to 20th	Night	8% 11% 4 8% 6% 6% 5% 4 5% 6% 37%	2533
	21st to end	Night	12% 10% 3 9% 6% 5% 6% 3 5% 6% 36%	2055
March	1st to 10th	Night	12% 8% 2 6% 3 3 3 3 4 7% 48%	5057
	11th to 20th	Night	9% 7% 3 6% 4 4 4 3 6% 9% 46%	4980
	21st to end	Night	23% 12% 3 10% 4 3 3 3 4 5% 30%	5390
April	1st to 10th	Night	14% 11% 3 7% 5% 5% 4 3 4 6% 38%	2000
	11th to 20th	Night	19% 10% 3 7% 5% 4 3 3 5% 6% 36%	1975
	21st to end	Night	20% 10% 3 7% 4 3 4 3 5% 6% 36%	1832
May	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	3872
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	3596
	21st to end	Night	17% 9% 3 6% 4 3 3 3 5% 7% 40%	3942
June	1st to 10th	Night	9% 4 2 5% 4 4 4 4 6% 8% 50%	1600
	11th to 20th	Night	11% 5% 2 7% 5% 5% 4 4 6% 8% 42%	1600
	21st to end	Night	10% 5% 2 4 4 3 3 3 5% 8% 53%	1600
July	1st to 10th	Night	7% 5% 2 5% 3 4 4 5% 7% 10% 48%	3618
	11th to 20th	Night	6% 5% 1 5% 3 3 4 3 6% 9% 55%	3617
	21st to end	Night	9% 5% 2 5% 3 4 4 4 5% 9% 51%	4124
August	1st to 10th	Night	10% 6% 2 7% 5% 4 4 3 5% 7% 48%	2926
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 5% 7% 51%	2888
	21st to end	Night	9% 5% 2 6% 4 3 4 3 5% 6% 54%	3484
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 5% 6% 57%	2000
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 8% 52%	2015
	21st to end	Night	10% 7% 2 5% 3 3 4 3 4 6% 55%	2284
October	1st to 10th	Night	12% 7% 2 5% 3 4 3 3 4 6% 50%	2400
	11th to 20th	Night	15% 8% 2 5% 4 3 4 3 6% 6% 44%	2400
	21st to end	Night	16% 10% 3 8% 4 4 4 3 5% 6% 38%	2642
November	1st to 10th	Night	11% 9% 3 7% 5% 3 4 3 5% 6% 44%	2734
	11th to 20th	Night	11% 13% 4 9% 5% 4 4 3 4 6% 36%	2940
	21st to end	Night	13% 12% 4 8% 5% 4 3 3 4 6% 37%	2940
December	1st to 10th	Night	6% 8% 4 12% 8% 8% 6% 4 6% 7% 30%	4720
	11th to 20th	Night	11% 15% 6% 14% 7% 5% 5% 4 4 5% 24%	5339
	21st to end	Night	20% 15% 7% 19% 7% 4 3 2 4 4 14%	7150

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	72% 2 7% 13% 5%	3626
	11th to 20th	Day	60% 2 9% 16% 13%	3756
	21st to end	Day	61% 2 13% 15% 8%	4488
February	1st to 10th	Day	60% 3 11% 16% 9% 1	1800
	11th to 20th	Day	54% 3 10% 23% 8% 2	1763
	21st to end	Day	54% 4 11% 20% 10% 1	1573
March	1st to 10th	Day	41% 3 14% 29% 12% 3	4839
	11th to 20th	Day	39% 2 12% 32% 13% 2	4980
	21st to end	Day	62% 2 12% 16% 7%	5527
April	1st to 10th	Day	54% 4 11% 20% 9% 1	2400
	11th to 20th	Day	54% 3 11% 19% 12% 1	2374
	21st to end	Day	56% 4 11% 20% 8% 1	2565
May	1st to 10th	Day	41% 2 10% 30% 17% 1	6079
	11th to 20th	Day	41% 3 10% 29% 16% 3	6323
	21st to end	Day	47% 3 10% 31% 10%	6949
June	1st to 10th	Day	37% 4 14% 30% 15% 1	2800
	11th to 20th	Day	43% 5% 13% 21% 17% 1	2800
	21st to end	Day	40% 5% 15% 26% 13% 1	2800
July	1st to 10th	Day	37% 4 17% 33% 8% 1	6291
	11th to 20th	Day	30% 4 15% 34% 16% 1	6285
	21st to end	Day	36% 6% 15% 29% 14% 1	6551
August	1st to 10th	Day	39% 4 13% 30% 12% 2	4220
	11th to 20th	Day	40% 5% 16% 29% 10%	4210
	21st to end	Day	34% 5% 16% 30% 15% 1	4415
September	1st to 10th	Day	36% 4 13% 31% 16% 1	2400
	11th to 20th	Day	37% 4 16% 31% 12%	2380
	21st to end	Day	40% 3 10% 31% 15% 1	2095
October	1st to 10th	Day	27% 2 7% 40% 23% 1	3260
	11th to 20th	Day	38% 2 13% 24% 22% 2	3240
	21st to end	Day	43% 2 12% 32% 11%	3522
November	1st to 10th	Day	43% 4 13% 22% 15% 2	1780
	11th to 20th	Day	55% 3 12% 19% 10% 1	1680
	21st to end	Day	51% 3 16% 22% 7% 1	1680
December	1st to 10th	Day	55% 5% 17% 16% 7% 1	2690
	11th to 20th	Day	69% 3 12% 11% 5%	3085
	21st to end	Day	77% 2 7% 11% 3	4070
January	1st to 10th	Night	67% 2 9% 16% 6%	6363
	11th to 20th	Night	61% 3 10% 14% 12% 1	6222
	21st to end	Night	56% 3 12% 18% 11%	6490
February	1st to 10th	Night	54% 3 11% 21% 10% 1	2600
	11th to 20th	Night	48% 3 12% 25% 10% 2	2534
	21st to end	Night	49% 2 12% 22% 13% 2	2055
March	1st to 10th	Night	38% 2 9% 33% 14% 3	5057
	11th to 20th	Night	37% 2 11% 31% 19% 2	4980
	21st to end	Night	59% 3 11% 15% 11% 1	5390
April	1st to 10th	Night	49% 2 9% 24% 15% 1	2000
	11th to 20th	Night	49% 3 10% 21% 14% 2	1975
	21st to end	Night	50% 3 9% 23% 14% 2	1832
May	1st to 10th	Night	36% 3 8% 30% 22% 1	3899
	11th to 20th	Night	37% 3 10% 24% 19% 7%	3597
	21st to end	Night	47% 2 9% 29% 13% 1	3950
June	1st to 10th	Night	31% 4 14% 32% 16% 2	1600
	11th to 20th	Night	39% 5% 13% 22% 18% 3	1600
	21st to end	Night	30% 4 17% 29% 18% 3	1600
July	1st to 10th	Night	31% 4 21% 30% 13% 1	3580
	11th to 20th	Night	27% 4 17% 35% 15% 2	3572
	21st to end	Night	31% 5% 17% 30% 17% 1	4006
August	1st to 10th	Night	37% 3 12% 29% 16% 2	2916
	11th to 20th	Night	32% 5% 16% 31% 15% 1	2915
	21st to end	Night	32% 4 16% 28% 18% 2	3484
September	1st to 10th	Night	29% 3 10% 33% 22% 1	2000
	11th to 20th	Night	32% 3 16% 32% 16% 1	2015
	21st to end	Night	32% 3 11% 30% 22% 2	2284
October	1st to 10th	Night	25% 3 6% 34% 30% 3	3920
	11th to 20th	Night	29% 2 14% 27% 24% 4	3899
	21st to end	Night	46% 2 10% 26% 15% 1	4266
November	1st to 10th	Night	41% 3 14% 24% 15% 2	2734
	11th to 20th	Night	50% 3 12% 21% 12% 2	2940
	21st to end	Night	51% 2 13% 24% 9% 1	2940
December	1st to 10th	Night	52% 4 18% 20% 7%	4720
	11th to 20th	Night	62% 3 11% 17% 5% 1	5341
	21st to end	Night	77% 2 6% 11% 4	7150

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	16% 11% 39% 20% 10% 3	303
	11th to 20th	Day	17% 13% 45% 21% 3 1	300
	21st to end	Day	16% 11% 39% 25% 6% 2	330
February	1st to 10th	Day	14% 17% 41% 24% 4	300
	11th to 20th	Day	16% 18% 38% 21% 6% 1	300
	21st to end	Day	19% 17% 38% 24% 2	249
March	1st to 10th	Day	15% 13% 46% 21% 5% 1	300
	11th to 20th	Day	19% 12% 35% 25% 7% 1	300
	21st to end	Day	30% 13% 28% 21% 6% 1	355
April	1st to 10th	Day	27% 13% 35% 20% 4	500
	11th to 20th	Day	29% 15% 26% 22% 7% 1	492
	21st to end	Day	29% 13% 27% 20% 8% 2	665
May	1st to 10th	Day	21% 10% 34% 23% 10% 2	703
	11th to 20th	Day	21% 12% 26% 26% 11% 4	725
	21st to end	Day	22% 9% 26% 26% 15% 3	789
June	1st to 10th	Day	12% 8% 26% 30% 19% 5%	730
	11th to 20th	Day	16% 6% 30% 32% 13% 3	730
	21st to end	Day	14% 9% 30% 29% 15% 3	730
July	1st to 10th	Day	8% 4 19% 36% 24% 9%	721
	11th to 20th	Day	9% 4 31% 30% 18% 8%	718
	21st to end	Day	7% 3 25% 38% 20% 7%	794
August	1st to 10th	Day	10% 5% 23% 31% 25% 7%	726
	11th to 20th	Day	14% 4 19% 34% 23% 5%	715
	21st to end	Day	11% 4 27% 32% 19% 6%	648
September	1st to 10th	Day	15% 7% 27% 27% 18% 6%	530
	11th to 20th	Day	12% 5% 29% 34% 15% 5%	510
	21st to end	Day	15% 12% 28% 26% 13% 6%	330
October	1st to 10th	Day	19% 9% 31% 24% 15% 3	330
	11th to 20th	Day	20% 17% 37% 19% 6%	330
	21st to end	Day	27% 15% 34% 17% 6% 1	363
November	1st to 10th	Day	18% 16% 35% 22% 6% 1	330
	11th to 20th	Day	21% 18% 40% 15% 5%	330
	21st to end	Day	21% 18% 37% 21% 4	330
December	1st to 10th	Day	15% 15% 40% 23% 5% 1	330
	11th to 20th	Day	18% 15% 40% 21% 6%	330
	21st to end	Day	28% 13% 36% 19% 3	363
January	1st to 10th	Night	12% 14% 40% 26% 7% 1	1006
	11th to 20th	Night	15% 13% 39% 28% 5% 1	1000
	21st to end	Night	17% 15% 37% 25% 6%	1100
February	1st to 10th	Night	15% 13% 39% 27% 5% 1	1000
	11th to 20th	Night	15% 15% 42% 22% 6% 1	979
	21st to end	Night	17% 15% 35% 24% 7%	735
March	1st to 10th	Night	17% 12% 37% 25% 8%	665
	11th to 20th	Night	18% 12% 36% 26% 8%	600
	21st to end	Night	24% 10% 32% 24% 7% 2	660
April	1st to 10th	Night	20% 15% 36% 25% 4	600
	11th to 20th	Night	24% 12% 31% 25% 6% 1	593
	21st to end	Night	25% 9% 33% 24% 7% 2	600
May	1st to 10th	Night	20% 9% 33% 29% 9% 1	600
	11th to 20th	Night	17% 9% 33% 31% 10% 1	591
	21st to end	Night	19% 8% 28% 33% 12% 1	650
June	1st to 10th	Night	11% 5% 29% 39% 16% 1	600
	11th to 20th	Night	14% 4 30% 40% 12%	600
	21st to end	Night	12% 3 27% 42% 15% 1	600
July	1st to 10th	Night	7% 2 20% 39% 31% 2	585
	11th to 20th	Night	5% 2 25% 41% 25% 2	582
	21st to end	Night	7% 2 17% 51% 20% 3	657
August	1st to 10th	Night	7% 2 18% 46% 24% 2	597
	11th to 20th	Night	10% 3 20% 39% 25% 4	594
	21st to end	Night	9% 2 24% 40% 23% 2	660
September	1st to 10th	Night	12% 4 24% 39% 17% 4	600
	11th to 20th	Night	12% 3 32% 35% 17% 2	600
	21st to end	Night	13% 9% 30% 31% 17% 1	684
October	1st to 10th	Night	15% 9% 32% 31% 12% 1	800
	11th to 20th	Night	20% 11% 37% 27% 6%	800
	21st to end	Night	23% 13% 31% 27% 6%	880
November	1st to 10th	Night	18% 13% 36% 26% 7% 1	888
	11th to 20th	Night	17% 15% 40% 23% 5%	1050
	21st to end	Night	19% 15% 37% 22% 7%	1050
December	1st to 10th	Night	16% 11% 40% 26% 6% 1	1050
	11th to 20th	Night	19% 13% 38% 23% 7%	1050
	21st to end	Night	25% 12% 34% 24% 5%	1155

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