The W.M. Keck Observatory at Pacific Lutheran University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The W.M. Keck Observatory at Pacific Lutheran University clear sky chart from 2005-11-06 to 2026-07-31.

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	18% 9% 3 8% 5% 5% 4 4 5% 7% 33%	143137
	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	138483
Transparency	Day	51% 6% 14% 19% 9% 2	142855
	Night	53% 7% 14% 18% 7% 1	138347
Seeing	Day	23% 10% 20% 25% 17% 5%	17269
	Night	26% 8% 16% 27% 21% 3	25526

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	32% 13% 4 12% 8% 7% 5% 4 4 4 8%	4483
	11th to 20th	Day	19% 10% 4 10% 5% 5% 5% 3 5% 7% 27%	4716
	21st to end	Day	27% 11% 3 8% 5% 4 4 3 4 6% 26%	5423
February	1st to 10th	Day	29% 12% 4 8% 5% 5% 5% 4 6% 6% 15%	1890
	11th to 20th	Day	30% 13% 4 7% 6% 5% 4 3 4 5% 18%	1845
	21st to end	Day	29% 14% 4 8% 6% 4 4 3 5% 5% 18%	1628
March	1st to 10th	Day	17% 13% 4 10% 6% 6% 6% 5% 7% 7% 17%	4753
	11th to 20th	Day	20% 14% 4 9% 5% 5% 4 3 4 5% 26%	5110
	21st to end	Day	23% 13% 4 11% 7% 6% 6% 5% 6% 6% 12%	5777
April	1st to 10th	Day	26% 12% 3 8% 7% 5% 5% 4 6% 7% 18%	2520
	11th to 20th	Day	22% 10% 4 9% 5% 5% 4 4 5% 6% 26%	2512
	21st to end	Day	19% 11% 4 9% 6% 5% 5% 4 5% 6% 25%	2756
May	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 7% 42%	8195
	11th to 20th	Day	14% 10% 3 9% 5% 6% 5% 4 7% 8% 27%	8160
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 9% 32%	8993
June	1st to 10th	Day	19% 10% 3 7% 5% 4 5% 4 6% 8% 29%	2942
	11th to 20th	Day	19% 9% 3 8% 6% 5% 6% 5% 6% 8% 25%	3150
	21st to end	Day	13% 8% 2 7% 6% 5% 4 4 6% 7% 39%	3150
July	1st to 10th	Day	7% 4 1 4 3 3 3 2 4 5% 64%	6930
	11th to 20th	Day	5% 3 1 3 2 2 3 2 3 5% 70%	6536
	21st to end	Day	8% 4 2 4 3 3 3 3 5% 7% 58%	7015
August	1st to 10th	Day	5% 4 2 5% 3 3 3 3 5% 7% 62%	4419
	11th to 20th	Day	9% 5% 2 7% 5% 4 5% 4 7% 9% 42%	4156
	21st to end	Day	15% 8% 3 7% 4 4 3 3 5% 6% 43%	4459
September	1st to 10th	Day	11% 5% 2 6% 5% 4 4 3 5% 7% 48%	2400
	11th to 20th	Day	13% 7% 3 9% 5% 5% 4 4 6% 6% 39%	2400
	21st to end	Day	17% 6% 2 7% 4 4 4 3 6% 8% 39%	2110
October	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 6% 9% 37%	3280
	11th to 20th	Day	24% 9% 4 9% 5% 4 4 3 5% 7% 26%	3261
	21st to end	Day	26% 9% 3 9% 6% 5% 5% 4 5% 7% 22%	3378
November	1st to 10th	Day	30% 12% 4 8% 5% 5% 5% 4 5% 6% 18%	1698
	11th to 20th	Day	31% 11% 4 7% 6% 5% 5% 3 5% 6% 18%	1680
	21st to end	Day	29% 12% 4 8% 5% 4 4 4 6% 6% 19%	1680
December	1st to 10th	Day	22% 8% 3 7% 7% 6% 5% 4 6% 7% 25%	2700
	11th to 20th	Day	32% 13% 4 9% 7% 6% 5% 4 5% 5% 10%	3105
	21st to end	Day	30% 13% 5% 10% 7% 6% 5% 4 4 5% 11%	3927
January	1st to 10th	Night	32% 14% 5% 12% 7% 6% 4 3 4 3 9%	8355
	11th to 20th	Night	19% 8% 4 8% 4 4 4 3 5% 8% 33%	8182
	21st to end	Night	25% 10% 3 8% 4 4 4 3 4 5% 30%	8767
February	1st to 10th	Night	29% 12% 4 8% 5% 5% 4 3 4 7% 19%	2730
	11th to 20th	Night	27% 12% 3 8% 6% 5% 4 4 4 5% 23%	2662
	21st to end	Night	26% 13% 3 7% 5% 5% 5% 4 5% 5% 24%	2177
March	1st to 10th	Night	17% 13% 4 9% 6% 5% 6% 5% 7% 7% 22%	5348
	11th to 20th	Night	22% 10% 3 8% 4 4 4 3 4 6% 32%	5109
	21st to end	Night	27% 14% 4 11% 7% 5% 5% 3 5% 5% 15%	5439
April	1st to 10th	Night	21% 11% 4 8% 5% 5% 5% 4 6% 6% 25%	2100
	11th to 20th	Night	17% 9% 3 8% 6% 5% 5% 4 6% 8% 30%	2053
	21st to end	Night	19% 8% 3 8% 6% 4 4 4 5% 7% 32%	1864
May	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 6% 47%	4724
	11th to 20th	Night	14% 8% 3 9% 5% 5% 6% 4 6% 7% 33%	4700
	21st to end	Night	8% 10% 3 8% 5% 5% 6% 4 7% 8% 37%	5178
June	1st to 10th	Night	15% 7% 2 6% 5% 4 5% 3 6% 8% 39%	1672
	11th to 20th	Night	15% 7% 3 6% 5% 5% 4 4 5% 9% 37%	1470
	21st to end	Night	10% 5% 2 6% 5% 3 4 4 7% 6% 49%	1470
July	1st to 10th	Night	4 2 1 2 2 3 3 2 4 7% 70%	3250
	11th to 20th	Night	2 1 1 3 2 2 2 2 3 4 78%	3641
	21st to end	Night	5% 2 1 3 2 2 2 3 4 7% 68%	4049
August	1st to 10th	Night	3 2 1 3 2 2 2 2 4 6% 72%	2732
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 5% 7% 54%	2902
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	3443
September	1st to 10th	Night	8% 4 1 4 3 5% 3 3 4 7% 59%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 4 5% 46%	2000
	21st to end	Night	16% 6% 2 6% 4 3 3 3 4 7% 45%	2285
October	1st to 10th	Night	13% 6% 2 5% 4 5% 5% 3 6% 7% 43%	3920
	11th to 20th	Night	24% 9% 3 6% 4 3 4 3 5% 6% 32%	3897
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 5% 6% 28%	4425
November	1st to 10th	Night	32% 9% 3 7% 5% 5% 5% 4 5% 6% 21%	2815
	11th to 20th	Night	31% 11% 3 7% 6% 5% 4 3 4 5% 21%	2940
	21st to end	Night	27% 11% 3 8% 6% 5% 4 3 6% 5% 22%	2940
December	1st to 10th	Night	23% 9% 3 7% 6% 4 5% 4 6% 7% 27%	4700
	11th to 20th	Night	32% 14% 4 11% 6% 5% 5% 3 4 4 12%	5320
	21st to end	Night	29% 14% 5% 10% 6% 5% 4 3 4 5% 14%	7224

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% 6% 2	4483
	11th to 20th	Day	55% 5% 10% 18% 11% 1	4740
	21st to end	Day	60% 4 8% 16% 13%	5424
February	1st to 10th	Day	67% 8% 10% 9% 5%	1890
	11th to 20th	Day	68% 7% 8% 11% 6% 1	1854
	21st to end	Day	67% 9% 9% 10% 4	1628
March	1st to 10th	Day	61% 7% 15% 13% 4	4753
	11th to 20th	Day	62% 5% 9% 21% 2	5110
	21st to end	Day	70% 9% 12% 9% 1	5777
April	1st to 10th	Day	65% 10% 14% 10% 1	2520
	11th to 20th	Day	58% 8% 13% 14% 7% 1	2512
	21st to end	Day	59% 9% 13% 13% 5%	2756
May	1st to 10th	Day	41% 6% 17% 27% 9%	8195
	11th to 20th	Day	55% 8% 19% 16% 2	8163
	21st to end	Day	49% 8% 23% 17% 4	8994
June	1st to 10th	Day	53% 9% 16% 15% 6%	2942
	11th to 20th	Day	57% 9% 15% 15% 5%	3150
	21st to end	Day	45% 8% 17% 20% 8% 1	3150
July	1st to 10th	Day	25% 4 16% 24% 18% 13%	6856
	11th to 20th	Day	20% 3 10% 29% 32% 6%	6459
	21st to end	Day	27% 2 21% 35% 13% 2	6822
August	1st to 10th	Day	24% 4 11% 37% 21% 3	4402
	11th to 20th	Day	40% 4 20% 26% 8% 2	4195
	21st to end	Day	43% 5% 12% 27% 13% 1	4458
September	1st to 10th	Day	35% 6% 18% 25% 13% 5%	2400
	11th to 20th	Day	42% 9% 14% 18% 14% 2	2400
	21st to end	Day	43% 8% 17% 19% 11% 1	2110
October	1st to 10th	Day	43% 4 14% 30% 8%	3280
	11th to 20th	Day	57% 4 14% 16% 9%	3261
	21st to end	Day	64% 4 11% 13% 8% 1	3378
November	1st to 10th	Day	66% 8% 9% 12% 3	1698
	11th to 20th	Day	67% 10% 8% 8% 6% 2	1680
	21st to end	Day	63% 8% 10% 10% 8%	1680
December	1st to 10th	Day	57% 5% 7% 18% 12% 1	2700
	11th to 20th	Day	72% 6% 11% 9% 1	3108
	21st to end	Day	73% 7% 11% 7% 2	3927
January	1st to 10th	Night	81% 5% 6% 6% 2 1	8355
	11th to 20th	Night	51% 6% 10% 21% 11%	8220
	21st to end	Night	57% 4 8% 20% 10%	8767
February	1st to 10th	Night	67% 7% 11% 9% 6%	2730
	11th to 20th	Night	62% 9% 10% 10% 7% 1	2664
	21st to end	Night	62% 9% 12% 12% 5%	2177
March	1st to 10th	Night	59% 6% 14% 15% 6%	5348
	11th to 20th	Night	55% 6% 14% 22% 2 1	5110
	21st to end	Night	72% 8% 10% 10% 1	5439
April	1st to 10th	Night	59% 11% 16% 11% 2	2100
	11th to 20th	Night	53% 11% 15% 14% 7% 1	2053
	21st to end	Night	53% 11% 16% 14% 5%	1864
May	1st to 10th	Night	38% 7% 15% 31% 8%	4721
	11th to 20th	Night	51% 10% 20% 16% 3	4700
	21st to end	Night	47% 10% 25% 16% 1	5182
June	1st to 10th	Night	46% 10% 18% 21% 5%	1672
	11th to 20th	Night	47% 10% 20% 19% 4	1470
	21st to end	Night	37% 7% 23% 22% 8% 2	1470
July	1st to 10th	Night	19% 4 18% 30% 15% 14%	3215
	11th to 20th	Night	14% 4 12% 30% 35% 6%	3595
	21st to end	Night	20% 4 21% 37% 15% 3	3937
August	1st to 10th	Night	17% 4 11% 41% 24% 2	2724
	11th to 20th	Night	34% 4 27% 23% 9% 2	2929
	21st to end	Night	38% 5% 15% 32% 10% 1	3443
September	1st to 10th	Night	28% 8% 18% 26% 15% 4	2000
	11th to 20th	Night	41% 8% 12% 21% 14% 4	2000
	21st to end	Night	41% 8% 15% 23% 11% 2	2285
October	1st to 10th	Night	42% 5% 19% 30% 5%	3920
	11th to 20th	Night	55% 5% 14% 20% 6%	3897
	21st to end	Night	59% 4 12% 16% 8%	4425
November	1st to 10th	Night	64% 8% 13% 11% 4	2815
	11th to 20th	Night	66% 7% 10% 10% 4 2	2940
	21st to end	Night	63% 8% 11% 10% 7%	2940
December	1st to 10th	Night	57% 4 10% 21% 8% 1	4700
	11th to 20th	Night	74% 5% 11% 7% 2	5316
	21st to end	Night	70% 9% 11% 9% 2	7224

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	35% 11% 18% 20% 12% 4	220
	11th to 20th	Day	30% 11% 18% 20% 17% 5%	220
	21st to end	Day	34% 12% 16% 18% 16% 5%	324
February	1st to 10th	Day	34% 12% 19% 19% 13% 3	430
	11th to 20th	Day	42% 13% 19% 15% 11% 1	425
	21st to end	Day	35% 18% 20% 19% 7% 2	394
March	1st to 10th	Day	32% 15% 24% 19% 9% 2	540
	11th to 20th	Day	33% 12% 18% 21% 13% 3	540
	21st to end	Day	30% 14% 25% 20% 9% 1	594
April	1st to 10th	Day	31% 13% 19% 22% 10% 4	540
	11th to 20th	Day	25% 15% 27% 19% 8% 6%	532
	21st to end	Day	21% 15% 26% 24% 12% 3	540
May	1st to 10th	Day	19% 10% 26% 25% 16% 4	543
	11th to 20th	Day	27% 11% 19% 24% 14% 5%	568
	21st to end	Day	22% 10% 22% 27% 17% 4	617
June	1st to 10th	Day	23% 8% 22% 26% 15% 6%	572
	11th to 20th	Day	18% 12% 25% 29% 13% 3	780
	21st to end	Day	14% 8% 19% 32% 22% 4	780
July	1st to 10th	Day	11% 5% 19% 34% 26% 5%	771
	11th to 20th	Day	7% 4 22% 35% 26% 6%	599
	21st to end	Day	7% 4 14% 35% 33% 7%	616
August	1st to 10th	Day	10% 4 12% 36% 28% 10%	527
	11th to 20th	Day	8% 3 14% 36% 30% 9%	517
	21st to end	Day	14% 7% 19% 31% 21% 8%	583
September	1st to 10th	Day	12% 5% 23% 31% 23% 7%	530
	11th to 20th	Day	15% 7% 19% 32% 20% 6%	530
	21st to end	Day	18% 9% 24% 27% 19% 4	530
October	1st to 10th	Day	25% 9% 18% 23% 17% 7%	530
	11th to 20th	Day	29% 12% 15% 23% 16% 6%	530
	21st to end	Day	32% 14% 18% 20% 11% 5%	478
November	1st to 10th	Day	34% 15% 17% 20% 12% 2	247
	11th to 20th	Day	36% 16% 15% 13% 13% 7%	220
	21st to end	Day	30% 11% 20% 19% 10% 10%	220
December	1st to 10th	Day	43% 11% 10% 11% 10% 14%	220
	11th to 20th	Day	40% 17% 14% 17% 10% 2	220
	21st to end	Day	37% 12% 16% 16% 15% 4	242
January	1st to 10th	Night	41% 12% 14% 17% 12% 2	874
	11th to 20th	Night	34% 9% 12% 22% 19% 3	840
	21st to end	Night	31% 8% 15% 25% 18% 3	924
February	1st to 10th	Night	36% 8% 16% 25% 14% 2	840
	11th to 20th	Night	33% 9% 16% 24% 16% 2	824
	21st to end	Night	35% 10% 19% 24% 11% 1	692
March	1st to 10th	Night	31% 12% 17% 24% 15% 2	840
	11th to 20th	Night	31% 12% 18% 23% 14% 2	840
	21st to end	Night	30% 12% 22% 25% 10% 1	924
April	1st to 10th	Night	27% 9% 20% 26% 17% 2	840
	11th to 20th	Night	22% 11% 21% 27% 17% 2	805
	21st to end	Night	21% 10% 17% 34% 15% 2	604
May	1st to 10th	Night	17% 9% 18% 34% 21% 1	420
	11th to 20th	Night	21% 6% 18% 29% 24% 2	414
	21st to end	Night	16% 7% 18% 29% 26% 3	455
June	1st to 10th	Night	16% 8% 20% 29% 26% 1	420
	11th to 20th	Night	17% 7% 20% 35% 20% 1	420
	21st to end	Night	12% 4 19% 36% 28% 1	420
July	1st to 10th	Night	9% 2 13% 36% 36% 4	410
	11th to 20th	Night	6% 4 13% 42% 33% 2	408
	21st to end	Night	5% 2 10% 36% 45% 1	458
August	1st to 10th	Night	6% 3 10% 30% 44% 6%	469
	11th to 20th	Night	7% 2 8% 28% 46% 8%	594
	21st to end	Night	11% 4 11% 34% 35% 4	780
September	1st to 10th	Night	11% 3 12% 35% 34% 6%	800
	11th to 20th	Night	17% 4 12% 26% 32% 8%	800
	21st to end	Night	18% 5% 16% 32% 26% 4	800
October	1st to 10th	Night	21% 5% 14% 28% 27% 5%	800
	11th to 20th	Night	28% 7% 14% 26% 20% 5%	800
	21st to end	Night	30% 7% 18% 25% 17% 3	880
November	1st to 10th	Night	36% 9% 19% 25% 11%	820
	11th to 20th	Night	36% 11% 16% 21% 13% 2	840
	21st to end	Night	32% 9% 18% 21% 15% 4	840
December	1st to 10th	Night	32% 8% 14% 26% 17% 3	840
	11th to 20th	Night	39% 10% 16% 22% 11% 2	840
	21st to end	Night	42% 10% 17% 20% 10% 1	951

Page generated at 2026-07-28 00:25:25. Version ac07

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