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 2024-11-12.

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	101619
	Night	12% 9% 4 9% 5% 4 4 3 5% 6% 39%	97674
Transparency	Day	46% 3 12% 25% 12% 1	105240
	Night	45% 3 11% 25% 14% 2	102194
Seeing	Day	17% 10% 30% 27% 13% 4	15985
	Night	16% 10% 33% 30% 10% 1	25845

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	8% 16% 6% 20% 8% 6% 6% 4 6% 6% 14%	2527
	11th to 20th	Day	6% 13% 5% 12% 9% 8% 8% 5% 10% 8% 16%	2624
	21st to end	Day	10% 18% 7% 17% 9% 5% 5% 3 4 6% 16%	3135
February	1st to 10th	Day	7% 13% 5% 13% 8% 7% 7% 6% 6% 5% 23%	1710
	11th to 20th	Day	9% 12% 4 9% 6% 7% 6% 4 6% 7% 30%	1664
	21st to end	Day	12% 11% 4 11% 6% 6% 5% 4 5% 7% 28%	1496
March	1st to 10th	Day	14% 9% 3 8% 5% 3 3 3 5% 6% 42%	3367
	11th to 20th	Day	10% 8% 3 8% 4 4 4 4 5% 9% 41%	3480
	21st to end	Day	23% 13% 5% 12% 7% 4 4 3 4 4 22%	3863
April	1st to 10th	Day	18% 12% 4 8% 5% 4 4 3 6% 7% 29%	2280
	11th to 20th	Day	19% 12% 4 7% 5% 4 4 3 5% 5% 33%	2254
	21st to end	Day	21% 12% 4 8% 4 4 4 4 5% 5% 29%	2437
May	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 7% 9% 33%	4205
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 4 6% 7% 41%	4399
	21st to end	Day	18% 7% 2 6% 5% 4 4 4 6% 6% 38%	4824
June	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 5% 8% 46%	2660
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 6% 8% 40%	2660
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 6% 9% 41%	2660
July	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	5% 4 1 4 4 4 4 4 6% 8% 56%	4440
	21st to end	Day	6% 4 2 7% 5% 5% 6% 4 8% 10% 44%	4688
August	1st to 10th	Day	10% 7% 2 7% 4 4 4 4 6% 7% 44%	4104
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 9% 40%	4041
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 48%	4281
September	1st to 10th	Day	13% 5% 2 6% 4 4 3 3 5% 6% 49%	2280
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 47%	2261
	21st to end	Day	13% 7% 3 6% 4 4 4 3 6% 7% 43%	1990
October	1st to 10th	Day	14% 8% 2 5% 4 3 4 3 5% 7% 44%	1900
	11th to 20th	Day	15% 10% 2 6% 3 4 4 3 5% 6% 40%	1900
	21st to end	Day	18% 11% 3 7% 5% 3 3 2 5% 7% 37%	2086
November	1st to 10th	Day	13% 9% 3 8% 4 4 4 4 5% 7% 40%	1694
	11th to 20th	Day	11% 13% 4 13% 6% 4 5% 4 5% 6% 28%	1528
	21st to end	Day	12% 13% 4 8% 7% 3 3 3 4 7% 36%	1520
December	1st to 10th	Day	12% 11% 4 12% 8% 7% 6% 3 5% 7% 25%	1520
	11th to 20th	Day	10% 14% 5% 11% 6% 6% 6% 4 6% 6% 25%	1918
	21st to end	Day	18% 16% 7% 19% 5% 4 3 2 4 5% 15%	2783
January	1st to 10th	Night	9% 14% 7% 20% 7% 5% 5% 4 4 5% 20%	4428
	11th to 20th	Night	6% 13% 6% 14% 8% 6% 7% 6% 10% 6% 18%	4344
	21st to end	Night	13% 16% 8% 14% 6% 6% 4 3 4 6% 18%	4532
February	1st to 10th	Night	7% 14% 6% 10% 7% 6% 5% 4 5% 5% 31%	2470
	11th to 20th	Night	8% 11% 3 8% 6% 6% 5% 4 5% 6% 37%	2403
	21st to end	Night	12% 10% 3 9% 6% 5% 6% 3 5% 6% 35%	1956
March	1st to 10th	Night	11% 8% 3 7% 3 3 4 3 4 7% 46%	3515
	11th to 20th	Night	10% 7% 3 7% 4 4 4 3 5% 8% 45%	3480
	21st to end	Night	24% 12% 3 11% 4 3 3 2 4 4 28%	3768
April	1st to 10th	Night	14% 11% 3 7% 5% 5% 4 3 4 6% 38%	1900
	11th to 20th	Night	19% 11% 3 6% 4 4 3 3 5% 6% 35%	1875
	21st to end	Night	19% 10% 3 6% 4 3 4 3 5% 6% 36%	1741
May	1st to 10th	Night	16% 7% 3 7% 4 4 3 4 4 5% 43%	2698
	11th to 20th	Night	12% 6% 2 5% 3 4 3 4 5% 6% 49%	2506
	21st to end	Night	12% 8% 3 6% 4 4 3 3 5% 7% 44%	2743
June	1st to 10th	Night	9% 4 2 5% 4 4 4 4 6% 8% 51%	1520
	11th to 20th	Night	11% 5% 2 7% 5% 5% 4 4 7% 8% 43%	1520
	21st to end	Night	10% 5% 2 5% 4 3 3 3 6% 8% 53%	1520
July	1st to 10th	Night	9% 6% 2 6% 3 4 3 4 7% 9% 46%	2530
	11th to 20th	Night	3 2 1 4 3 4 4 3 6% 9% 60%	2530
	21st to end	Night	4 4 2 4 3 4 4 4 6% 10% 56%	2871
August	1st to 10th	Night	10% 6% 2 7% 5% 4 4 3 5% 7% 48%	2836
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 5% 7% 51%	2798
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 6% 53%	3377
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 5% 6% 58%	1900
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 53%	1914
	21st to end	Night	10% 7% 2 5% 3 3 3 3 4 6% 54%	2170
October	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 4 6% 50%	2280
	11th to 20th	Night	15% 8% 2 5% 4 3 4 3 6% 6% 44%	2280
	21st to end	Night	16% 10% 3 8% 4 4 4 3 5% 6% 38%	2510
November	1st to 10th	Night	11% 8% 3 7% 5% 3 4 4 5% 5% 45%	2601
	11th to 20th	Night	10% 13% 5% 10% 5% 4 4 3 4 6% 35%	2675
	21st to end	Night	13% 12% 4 7% 5% 4 3 3 4 6% 39%	2660
December	1st to 10th	Night	10% 11% 5% 11% 7% 6% 5% 4 6% 6% 28%	2660
	11th to 20th	Night	12% 12% 5% 12% 7% 5% 5% 4 5% 6% 27%	3279
	21st to end	Night	20% 15% 6% 15% 7% 5% 4 3 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	72% 3 9% 12% 5%	2528
	11th to 20th	Day	62% 2 10% 17% 8%	2624
	21st to end	Day	71% 3 11% 12% 4 1	3135
February	1st to 10th	Day	60% 3 11% 17% 8% 1	1710
	11th to 20th	Day	54% 3 10% 23% 8% 2	1673
	21st to end	Day	56% 4 11% 18% 10% 1	1496
March	1st to 10th	Day	44% 3 11% 28% 13% 2	3367
	11th to 20th	Day	42% 3 9% 32% 13% 2	3480
	21st to end	Day	67% 2 11% 14% 6%	3863
April	1st to 10th	Day	55% 4 11% 20% 9% 1	2280
	11th to 20th	Day	53% 3 11% 19% 12% 1	2254
	21st to end	Day	56% 4 10% 20% 8% 1	2437
May	1st to 10th	Day	50% 2 11% 24% 12% 1	4243
	11th to 20th	Day	42% 3 10% 32% 12% 1	4403
	21st to end	Day	47% 2 10% 28% 12% 1	4837
June	1st to 10th	Day	37% 4 14% 30% 15% 1	2660
	11th to 20th	Day	42% 5% 13% 21% 18% 1	2660
	21st to end	Day	39% 6% 16% 26% 12% 1	2660
July	1st to 10th	Day	41% 5% 19% 26% 8% 1	4371
	11th to 20th	Day	27% 5% 14% 34% 19% 2	4365
	21st to end	Day	33% 7% 15% 29% 16% 1	4541
August	1st to 10th	Day	39% 4 13% 31% 12% 2	4090
	11th to 20th	Day	40% 5% 16% 29% 10%	4080
	21st to end	Day	34% 5% 16% 29% 15% 1	4280
September	1st to 10th	Day	36% 4 12% 31% 16% 1	2280
	11th to 20th	Day	37% 4 14% 32% 12%	2261
	21st to end	Day	41% 3 10% 30% 15% 1	1990
October	1st to 10th	Day	28% 2 7% 40% 23% 1	3160
	11th to 20th	Day	37% 2 13% 24% 22% 1	3140
	21st to end	Day	43% 2 13% 32% 10%	3412
November	1st to 10th	Day	43% 4 14% 22% 15% 2	1694
	11th to 20th	Day	56% 3 12% 18% 10% 1	1528
	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	68% 2 8% 16% 6%	4428
	11th to 20th	Night	62% 3 9% 17% 8%	4344
	21st to end	Night	68% 3 9% 14% 5% 1	4532
February	1st to 10th	Night	54% 3 10% 21% 10% 1	2470
	11th to 20th	Night	47% 3 12% 25% 11% 2	2404
	21st to end	Night	50% 2 11% 22% 13% 2	1956
March	1st to 10th	Night	40% 2 10% 33% 14% 1	3515
	11th to 20th	Night	39% 2 8% 29% 20% 2	3480
	21st to end	Night	61% 3 9% 17% 8% 2	3768
April	1st to 10th	Night	49% 2 9% 24% 14% 1	1900
	11th to 20th	Night	49% 3 10% 21% 15% 3	1875
	21st to end	Night	49% 3 9% 23% 14% 2	1741
May	1st to 10th	Night	44% 3 8% 21% 22% 1	2725
	11th to 20th	Night	36% 4 10% 27% 17% 6%	2507
	21st to end	Night	42% 2 10% 28% 17% 1	2751
June	1st to 10th	Night	31% 4 14% 32% 16% 2	1520
	11th to 20th	Night	37% 5% 13% 22% 19% 3	1520
	21st to end	Night	29% 4 17% 29% 18% 3	1520
July	1st to 10th	Night	34% 4 20% 28% 12% 1	2490
	11th to 20th	Night	21% 4 17% 38% 18% 2	2482
	21st to end	Night	24% 6% 17% 31% 20% 2	2780
August	1st to 10th	Night	37% 3 12% 29% 16% 2	2826
	11th to 20th	Night	32% 5% 16% 31% 15% 1	2825
	21st to end	Night	32% 4 16% 27% 18% 2	3377
September	1st to 10th	Night	28% 4 10% 33% 23% 1	1900
	11th to 20th	Night	31% 3 15% 33% 17% 1	1914
	21st to end	Night	33% 3 12% 31% 21% 1	2170
October	1st to 10th	Night	24% 3 6% 34% 30% 3	3800
	11th to 20th	Night	28% 2 14% 28% 24% 4	3779
	21st to end	Night	46% 2 10% 27% 15% 1	4134
November	1st to 10th	Night	41% 3 14% 24% 15% 2	2601
	11th to 20th	Night	50% 4 12% 21% 12% 1	2675
	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	18% 12% 44% 22% 3 1	270
	21st to end	Day	17% 11% 37% 27% 6% 2	297
February	1st to 10th	Day	12% 17% 41% 25% 4	270
	11th to 20th	Day	16% 17% 39% 21% 6% 1	270
	21st to end	Day	20% 17% 38% 24% 2	225
March	1st to 10th	Day	14% 14% 47% 19% 6% 1	270
	11th to 20th	Day	19% 11% 35% 25% 8% 1	270
	21st to end	Day	31% 13% 26% 21% 7% 1	321
April	1st to 10th	Day	28% 12% 35% 19% 5%	460
	11th to 20th	Day	29% 14% 26% 22% 7% 1	452
	21st to end	Day	29% 14% 26% 20% 9% 2	617
May	1st to 10th	Day	22% 11% 36% 22% 8% 1	653
	11th to 20th	Day	20% 12% 28% 26% 10% 4	675
	21st to end	Day	21% 9% 27% 24% 15% 3	734
June	1st to 10th	Day	12% 8% 26% 30% 19% 5%	680
	11th to 20th	Day	15% 6% 30% 33% 13% 3	680
	21st to end	Day	13% 9% 30% 29% 16% 4	680
July	1st to 10th	Day	9% 4 18% 35% 25% 9%	671
	11th to 20th	Day	8% 4 31% 30% 19% 8%	668
	21st to end	Day	7% 3 25% 38% 19% 7%	739
August	1st to 10th	Day	9% 5% 23% 31% 25% 7%	676
	11th to 20th	Day	14% 4 19% 35% 23% 5%	665
	21st to end	Day	11% 4 27% 33% 18% 6%	601
September	1st to 10th	Day	16% 7% 26% 28% 18% 6%	490
	11th to 20th	Day	12% 6% 30% 33% 14% 5%	471
	21st to end	Day	16% 14% 29% 26% 11% 4	300
October	1st to 10th	Day	20% 9% 30% 23% 14% 3	300
	11th to 20th	Day	19% 17% 38% 19% 7%	300
	21st to end	Day	27% 16% 35% 16% 5% 2	330
November	1st to 10th	Day	20% 14% 34% 24% 7% 1	300
	11th to 20th	Day	18% 19% 39% 18% 6%	273
	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	15% 12% 38% 28% 5% 1	950
	21st to end	Night	18% 15% 35% 25% 7%	1045
February	1st to 10th	Night	15% 13% 39% 28% 6% 1	950
	11th to 20th	Night	15% 15% 41% 23% 6% 1	929
	21st to end	Night	18% 15% 35% 25% 7%	700
March	1st to 10th	Night	17% 13% 36% 25% 8%	632
	11th to 20th	Night	18% 11% 35% 27% 8%	570
	21st to end	Night	23% 11% 31% 25% 8% 2	627
April	1st to 10th	Night	20% 15% 37% 25% 3	570
	11th to 20th	Night	25% 12% 32% 25% 6% 1	563
	21st to end	Night	24% 9% 33% 25% 8% 2	570
May	1st to 10th	Night	20% 9% 34% 29% 8% 1	570
	11th to 20th	Night	17% 9% 33% 32% 9% 1	561
	21st to end	Night	18% 8% 28% 33% 12% 1	617
June	1st to 10th	Night	10% 5% 29% 39% 15% 1	570
	11th to 20th	Night	13% 4 30% 41% 12%	570
	21st to end	Night	11% 3 26% 42% 16% 1	570
July	1st to 10th	Night	7% 2 21% 37% 31% 2	555
	11th to 20th	Night	4 3 25% 41% 26% 2	552
	21st to end	Night	5% 2 17% 52% 21% 3	624
August	1st to 10th	Night	7% 2 18% 46% 24% 2	567
	11th to 20th	Night	11% 3 19% 38% 25% 4	564
	21st to end	Night	9% 2 25% 40% 22% 2	627
September	1st to 10th	Night	12% 4 23% 40% 17% 4	570
	11th to 20th	Night	12% 3 32% 34% 16% 2	570
	21st to end	Night	13% 10% 31% 30% 15%	650
October	1st to 10th	Night	15% 9% 32% 30% 13% 1	760
	11th to 20th	Night	19% 11% 36% 28% 6%	760
	21st to end	Night	22% 13% 31% 27% 6%	836
November	1st to 10th	Night	17% 13% 36% 26% 7% 1	845
	11th to 20th	Night	16% 15% 41% 22% 6%	956
	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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