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 2025-03-27.

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	19% 9% 3 8% 5% 5% 4 4 5% 6% 32%	113430
	Night	21% 9% 3 7% 5% 4 4 3 5% 6% 32%	114468
Transparency	Day	51% 7% 13% 18% 9% 2	113189
	Night	53% 8% 13% 17% 7% 1	114355
Seeing	Day	23% 11% 20% 25% 17% 5%	15449
	Night	26% 8% 16% 26% 21% 3	23980

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	34% 13% 3 11% 8% 6% 5% 3 4 4 8%	3453
	11th to 20th	Day	19% 10% 4 11% 6% 5% 5% 3 6% 7% 25%	3616
	21st to end	Day	29% 11% 3 8% 4 4 4 2 3 5% 27%	4186
February	1st to 10th	Day	29% 13% 4 9% 5% 4 5% 4 6% 6% 16%	1800
	11th to 20th	Day	31% 13% 4 7% 6% 4 4 3 4 5% 18%	1755
	21st to end	Day	29% 14% 4 8% 6% 4 4 3 5% 5% 18%	1553
March	1st to 10th	Day	17% 12% 4 11% 7% 7% 6% 5% 6% 7% 19%	3334
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 3 4 6% 34%	3600
	21st to end	Day	22% 13% 4 11% 6% 6% 6% 5% 6% 6% 15%	4007
April	1st to 10th	Day	27% 13% 3 8% 7% 5% 5% 4 6% 6% 17%	2280
	11th to 20th	Day	23% 11% 4 8% 5% 5% 4 4 5% 6% 25%	2270
	21st to end	Day	21% 11% 4 9% 6% 5% 5% 4 5% 6% 23%	2494
May	1st to 10th	Day	18% 9% 2 6% 4 4 4 4 5% 7% 36%	4420
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 3 7% 9% 37%	4377
	21st to end	Day	22% 10% 4 10% 6% 6% 6% 4 6% 7% 19%	4813
June	1st to 10th	Day	21% 10% 3 7% 5% 4 5% 4 6% 7% 27%	2662
	11th to 20th	Day	20% 10% 3 9% 6% 5% 6% 6% 5% 7% 22%	2850
	21st to end	Day	13% 7% 2 7% 5% 5% 4 4 6% 8% 40%	2850
July	1st to 10th	Day	7% 4 1 3 3 3 3 2 4 4 64%	4740
	11th to 20th	Day	6% 3 1 3 2 2 2 2 4 5% 68%	4466
	21st to end	Day	10% 5% 2 5% 3 3 4 3 4 6% 56%	4790
August	1st to 10th	Day	5% 4 2 4 3 3 3 3 5% 6% 63%	4283
	11th to 20th	Day	9% 6% 2 7% 5% 4 5% 4 6% 9% 42%	4026
	21st to end	Day	15% 7% 3 7% 4 3 3 3 5% 6% 43%	4322
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	2280
	11th to 20th	Day	13% 7% 4 9% 5% 5% 4 4 6% 7% 38%	2280
	21st to end	Day	17% 7% 2 7% 4 4 4 3 6% 8% 38%	2005
October	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 6% 10% 37%	3180
	11th to 20th	Day	25% 9% 4 9% 5% 4 4 3 5% 7% 26%	3161
	21st to end	Day	26% 9% 3 9% 6% 5% 5% 4 5% 7% 22%	3273
November	1st to 10th	Day	30% 12% 3 8% 5% 5% 5% 4 5% 5% 18%	1616
	11th to 20th	Day	30% 11% 4 7% 6% 5% 5% 3 5% 6% 18%	1600
	21st to end	Day	29% 12% 4 8% 5% 4 4 4 6% 6% 19%	1600
December	1st to 10th	Day	21% 8% 3 7% 7% 6% 5% 4 6% 7% 25%	2620
	11th to 20th	Day	32% 12% 4 9% 6% 6% 5% 4 5% 5% 11%	3025
	21st to end	Day	31% 13% 5% 11% 7% 6% 5% 4 4 5% 11%	3843
January	1st to 10th	Night	34% 14% 5% 11% 6% 6% 4 3 4 3 9%	6449
	11th to 20th	Night	20% 8% 4 8% 5% 4 4 3 5% 7% 31%	6282
	21st to end	Night	27% 10% 3 7% 4 4 3 3 4 5% 30%	6776
February	1st to 10th	Night	29% 12% 4 8% 5% 5% 4 3 4 7% 19%	2600
	11th to 20th	Night	28% 12% 3 7% 6% 5% 3 3 4 5% 24%	2532
	21st to end	Night	26% 12% 2 7% 5% 6% 5% 4 5% 5% 24%	2076
March	1st to 10th	Night	18% 14% 4 9% 7% 5% 5% 5% 6% 6% 19%	3747
	11th to 20th	Night	18% 8% 3 6% 4 4 4 2 4 7% 40%	3599
	21st to end	Night	26% 15% 4 10% 7% 5% 5% 4 4 5% 15%	3783
April	1st to 10th	Night	22% 11% 4 8% 5% 5% 5% 4 6% 6% 23%	1900
	11th to 20th	Night	17% 9% 3 8% 6% 5% 5% 4 7% 7% 30%	1855
	21st to end	Night	20% 9% 3 8% 6% 5% 4 4 5% 7% 29%	1686
May	1st to 10th	Night	19% 8% 3 7% 5% 4 3 4 4 6% 39%	2540
	11th to 20th	Night	9% 6% 2 7% 5% 5% 6% 4 6% 8% 42%	2516
	21st to end	Night	11% 12% 3 10% 6% 5% 6% 4 6% 7% 30%	2762
June	1st to 10th	Night	16% 6% 2 6% 5% 4 5% 3 5% 8% 38%	1513
	11th to 20th	Night	16% 8% 4 7% 5% 5% 3 4 5% 9% 34%	1330
	21st to end	Night	10% 5% 2 5% 5% 3 4 3 7% 6% 49%	1330
July	1st to 10th	Night	5% 2 1 3 2 3 2 2 4 5% 71%	2220
	11th to 20th	Night	3 2 1 3 2 2 2 3 2 4 75%	2491
	21st to end	Night	6% 3 1 3 2 2 2 3 4 6% 67%	2762
August	1st to 10th	Night	3 2 1 3 2 2 2 2 4 6% 73%	2648
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 5% 7% 55%	2812
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	3338
September	1st to 10th	Night	8% 4 1 4 3 4 3 3 4 7% 60%	1900
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 4 5% 46%	1900
	21st to end	Night	16% 6% 2 6% 4 3 3 3 4 7% 45%	2170
October	1st to 10th	Night	13% 6% 2 5% 4 5% 5% 3 6% 7% 44%	3800
	11th to 20th	Night	24% 9% 3 6% 4 3 4 3 5% 6% 32%	3777
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 5% 6% 28%	4288
November	1st to 10th	Night	32% 9% 3 6% 5% 5% 5% 4 5% 5% 22%	2678
	11th to 20th	Night	31% 11% 3 7% 6% 5% 4 3 4 5% 22%	2800
	21st to end	Night	27% 11% 3 8% 6% 5% 4 3 6% 5% 22%	2800
December	1st to 10th	Night	22% 9% 3 7% 6% 4 5% 4 6% 7% 28%	4560
	11th to 20th	Night	31% 14% 4 11% 6% 5% 5% 3 4 4 12%	5180
	21st to end	Night	29% 14% 5% 10% 6% 5% 4 3 4 5% 14%	7068

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	79% 7% 6% 6% 2	3453
	11th to 20th	Day	56% 7% 13% 18% 6% 1	3640
	21st to end	Day	59% 5% 9% 16% 12%	4186
February	1st to 10th	Day	67% 9% 10% 9% 5% 1	1800
	11th to 20th	Day	69% 7% 8% 10% 5% 1	1764
	21st to end	Day	66% 9% 9% 11% 4	1553
March	1st to 10th	Day	58% 10% 15% 13% 4	3334
	11th to 20th	Day	52% 7% 12% 26% 2 1	3600
	21st to end	Day	66% 12% 12% 10% 1	4007
April	1st to 10th	Day	66% 11% 13% 9% 1	2280
	11th to 20th	Day	59% 9% 12% 12% 7% 1	2270
	21st to end	Day	62% 10% 12% 11% 4	2494
May	1st to 10th	Day	46% 10% 13% 21% 9%	4420
	11th to 20th	Day	42% 13% 23% 18% 3	4380
	21st to end	Day	61% 13% 16% 8% 2	4814
June	1st to 10th	Day	56% 10% 17% 12% 5%	2662
	11th to 20th	Day	60% 9% 15% 11% 4	2850
	21st to end	Day	43% 9% 18% 20% 8% 2	2850
July	1st to 10th	Day	26% 5% 8% 21% 22% 18%	4666
	11th to 20th	Day	20% 4 7% 26% 35% 7%	4389
	21st to end	Day	31% 3 12% 34% 18% 2	4639
August	1st to 10th	Day	23% 4 10% 38% 21% 3	4266
	11th to 20th	Day	40% 5% 20% 26% 8% 2	4065
	21st to end	Day	43% 5% 12% 27% 13% 1	4321
September	1st to 10th	Day	34% 6% 17% 26% 13% 5%	2280
	11th to 20th	Day	43% 10% 14% 17% 13% 3	2280
	21st to end	Day	44% 9% 17% 18% 11% 1	2005
October	1st to 10th	Day	43% 4 14% 30% 8% 1	3180
	11th to 20th	Day	57% 4 14% 16% 8% 1	3161
	21st to end	Day	63% 4 11% 13% 8% 1	3273
November	1st to 10th	Day	67% 9% 9% 12% 3	1616
	11th to 20th	Day	66% 10% 9% 7% 6% 2	1600
	21st to end	Day	63% 9% 10% 9% 8%	1600
December	1st to 10th	Day	56% 5% 7% 19% 12% 1	2620
	11th to 20th	Day	72% 6% 11% 9% 1	3028
	21st to end	Day	73% 7% 11% 7% 2	3843
January	1st to 10th	Night	80% 7% 6% 5% 2 1	6449
	11th to 20th	Night	51% 8% 12% 23% 5% 1	6320
	21st to end	Night	57% 6% 9% 18% 10% 1	6776
February	1st to 10th	Night	67% 7% 11% 9% 6%	2600
	11th to 20th	Night	62% 9% 10% 10% 7% 1	2534
	21st to end	Night	62% 9% 12% 12% 5%	2076
March	1st to 10th	Night	60% 9% 13% 14% 4	3747
	11th to 20th	Night	46% 9% 18% 25% 2 1	3600
	21st to end	Night	69% 12% 10% 8%	3783
April	1st to 10th	Night	61% 13% 15% 10% 2	1900
	11th to 20th	Night	53% 11% 15% 13% 8% 1	1855
	21st to end	Night	55% 12% 15% 13% 5% 1	1686
May	1st to 10th	Night	45% 13% 11% 21% 9%	2540
	11th to 20th	Night	38% 17% 22% 18% 4 1	2516
	21st to end	Night	54% 18% 16% 10% 2	2762
June	1st to 10th	Night	46% 11% 18% 20% 5%	1513
	11th to 20th	Night	50% 11% 22% 15% 2	1330
	21st to end	Night	36% 8% 24% 22% 8% 2	1330
July	1st to 10th	Night	21% 5% 9% 28% 17% 20%	2185
	11th to 20th	Night	17% 6% 7% 27% 38% 7%	2445
	21st to end	Night	21% 6% 13% 35% 22% 4	2674
August	1st to 10th	Night	17% 5% 10% 42% 24% 2	2640
	11th to 20th	Night	34% 4 28% 24% 8% 2	2839
	21st to end	Night	37% 5% 15% 32% 10%	3338
September	1st to 10th	Night	27% 9% 16% 27% 16% 5%	1900
	11th to 20th	Night	42% 8% 12% 21% 13% 4	1900
	21st to end	Night	41% 8% 15% 22% 12% 2	2170
October	1st to 10th	Night	42% 5% 18% 30% 5%	3800
	11th to 20th	Night	55% 5% 14% 20% 6%	3777
	21st to end	Night	60% 4 11% 16% 8% 1	4288
November	1st to 10th	Night	63% 8% 13% 11% 4 1	2678
	11th to 20th	Night	66% 7% 11% 10% 5% 2	2800
	21st to end	Night	62% 8% 12% 10% 8%	2800
December	1st to 10th	Night	56% 4 10% 21% 8% 1	4560
	11th to 20th	Night	73% 6% 12% 7% 2	5176
	21st to end	Night	70% 9% 11% 8% 2	7068

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% 12% 17% 20% 13% 5%	200
	11th to 20th	Day	31% 12% 19% 20% 15% 4	200
	21st to end	Day	34% 13% 17% 16% 15% 5%	298
February	1st to 10th	Day	34% 13% 19% 19% 13% 3	400
	11th to 20th	Day	43% 13% 17% 14% 11% 2	395
	21st to end	Day	35% 17% 19% 20% 7% 2	367
March	1st to 10th	Day	31% 14% 25% 20% 9% 2	500
	11th to 20th	Day	29% 13% 19% 22% 14% 3	500
	21st to end	Day	31% 15% 24% 20% 8% 2	531
April	1st to 10th	Day	33% 15% 20% 20% 10% 3	460
	11th to 20th	Day	26% 17% 27% 17% 7% 6%	452
	21st to end	Day	23% 17% 27% 20% 11% 2	460
May	1st to 10th	Day	21% 11% 27% 24% 14% 3	463
	11th to 20th	Day	26% 10% 19% 24% 15% 6%	488
	21st to end	Day	23% 11% 23% 25% 16% 3	529
June	1st to 10th	Day	25% 8% 22% 24% 16% 5%	492
	11th to 20th	Day	20% 13% 24% 27% 13% 3	680
	21st to end	Day	12% 9% 20% 32% 23% 4	680
July	1st to 10th	Day	11% 5% 20% 33% 26% 5%	671
	11th to 20th	Day	8% 4 21% 33% 28% 6%	515
	21st to end	Day	8% 4 14% 36% 33% 6%	531
August	1st to 10th	Day	10% 4 12% 35% 29% 10%	487
	11th to 20th	Day	6% 3 13% 36% 31% 10%	477
	21st to end	Day	14% 8% 20% 30% 20% 8%	539
September	1st to 10th	Day	12% 6% 24% 32% 21% 6%	490
	11th to 20th	Day	16% 8% 19% 31% 20% 6%	490
	21st to end	Day	18% 9% 24% 27% 20% 3	490
October	1st to 10th	Day	27% 9% 18% 21% 18% 7%	490
	11th to 20th	Day	29% 11% 13% 23% 17% 6%	490
	21st to end	Day	32% 14% 18% 19% 12% 6%	439
November	1st to 10th	Day	35% 15% 16% 19% 13% 3	225
	11th to 20th	Day	34% 18% 16% 11% 14% 8%	200
	21st to end	Day	30% 12% 20% 18% 10% 11%	200
December	1st to 10th	Day	40% 12% 11% 12% 11% 16%	200
	11th to 20th	Day	39% 16% 14% 19% 11% 2	200
	21st to end	Day	39% 13% 16% 16% 15% 2	220
January	1st to 10th	Night	41% 12% 14% 17% 13% 3	831
	11th to 20th	Night	35% 10% 12% 23% 18% 3	800
	21st to end	Night	32% 8% 15% 24% 17% 3	880
February	1st to 10th	Night	36% 8% 16% 24% 14% 2	800
	11th to 20th	Night	34% 10% 15% 23% 16% 2	784
	21st to end	Night	35% 11% 18% 24% 11% 1	660
March	1st to 10th	Night	31% 12% 17% 24% 15% 2	800
	11th to 20th	Night	29% 13% 18% 24% 15% 2	800
	21st to end	Night	31% 13% 21% 25% 10% 1	862
April	1st to 10th	Night	28% 9% 21% 24% 16% 2	760
	11th to 20th	Night	22% 12% 21% 26% 17% 2	727
	21st to end	Night	22% 11% 17% 33% 15% 2	546
May	1st to 10th	Night	18% 9% 19% 32% 19% 2	380
	11th to 20th	Night	19% 5% 18% 30% 26% 2	374
	21st to end	Night	17% 8% 18% 27% 26% 3	411
June	1st to 10th	Night	16% 8% 20% 29% 25% 1	380
	11th to 20th	Night	19% 8% 20% 31% 21% 1	380
	21st to end	Night	11% 4 19% 35% 28% 2	380
July	1st to 10th	Night	9% 2 14% 34% 36% 4	370
	11th to 20th	Night	6% 4 11% 43% 34% 3	368
	21st to end	Night	5% 2 11% 35% 46% 1	416
August	1st to 10th	Night	7% 3 10% 30% 44% 6%	445
	11th to 20th	Night	7% 2 8% 27% 47% 9%	564
	21st to end	Night	11% 4 12% 34% 34% 4	741
September	1st to 10th	Night	11% 3 13% 35% 34% 5%	760
	11th to 20th	Night	17% 4 13% 25% 33% 8%	760
	21st to end	Night	18% 6% 16% 31% 26% 4	760
October	1st to 10th	Night	21% 5% 14% 28% 27% 5%	760
	11th to 20th	Night	27% 7% 14% 26% 21% 6%	760
	21st to end	Night	31% 6% 18% 24% 17% 3	836
November	1st to 10th	Night	36% 8% 19% 25% 12% 1	780
	11th to 20th	Night	36% 11% 16% 21% 13% 3	800
	21st to end	Night	32% 10% 19% 21% 15% 4	800
December	1st to 10th	Night	30% 8% 15% 26% 18% 3	800
	11th to 20th	Night	38% 10% 17% 22% 12% 2	800
	21st to end	Night	43% 10% 17% 19% 9% 1	905

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