Skywalker Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skywalker Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	8% 8% 3 7% 4 4 4 4 5% 7% 45%	130064
	Night	10% 10% 3 7% 4 4 4 4 5% 7% 41%	128034
Transparency	Day	39% 4 11% 23% 17% 6%	129790
	Night	42% 4 11% 22% 15% 5%	127885
Seeing	Day	14% 13% 26% 28% 15% 3	15883
	Night	16% 7% 17% 36% 22% 3	24934

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	16% 19% 8% 14% 7% 5% 5% 3 4 5% 14%	3639
	11th to 20th	Day	14% 9% 2 8% 5% 5% 5% 3 5% 7% 37%	3616
	21st to end	Day	20% 16% 5% 9% 5% 4 3 2 3 5% 28%	4004
February	1st to 10th	Day	14% 14% 4 11% 6% 6% 5% 4 6% 7% 25%	1625
	11th to 20th	Day	12% 14% 3 8% 6% 4 5% 4 6% 7% 30%	1833
	21st to end	Day	14% 13% 4 8% 5% 5% 5% 5% 6% 6% 29%	1650
March	1st to 10th	Day	8% 10% 3 6% 5% 6% 6% 5% 7% 9% 36%	4518
	11th to 20th	Day	9% 9% 3 7% 5% 4 4 3 5% 7% 45%	4661
	21st to end	Day	11% 11% 4 10% 6% 6% 5% 5% 7% 8% 28%	5969
April	1st to 10th	Day	12% 9% 3 9% 7% 6% 5% 5% 7% 9% 29%	2400
	11th to 20th	Day	8% 8% 3 8% 6% 5% 5% 4 6% 7% 40%	2416
	21st to end	Day	8% 8% 3 8% 6% 5% 6% 4 7% 8% 38%	2603
May	1st to 10th	Day	5% 6% 3 6% 4 5% 5% 4 6% 8% 47%	6320
	11th to 20th	Day	6% 7% 3 8% 6% 5% 5% 5% 7% 10% 38%	6277
	21st to end	Day	6% 6% 3 7% 4 5% 5% 4 6% 8% 44%	6903
June	1st to 10th	Day	13% 7% 3 6% 5% 4 4 3 6% 7% 41%	2960
	11th to 20th	Day	10% 5% 2 8% 6% 5% 6% 5% 6% 7% 39%	3000
	21st to end	Day	5% 4 2 5% 4 4 5% 3 6% 7% 55%	2974
July	1st to 10th	Day	2 2 1 3 2 3 3 2 5% 5% 72%	6320
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 74%	6874
August	1st to 10th	Day	3 2 1 4 3 3 3 3 4 6% 68%	4541
	11th to 20th	Day	2 2 1 4 3 3 4 3 5% 8% 66%	4352
	21st to end	Day	3 4 1 5% 4 4 5% 3 4 6% 61%	4345
September	1st to 10th	Day	3 3 1 3 3 3 4 3 4 6% 66%	2400
	11th to 20th	Day	5% 5% 2 5% 4 4 5% 4 6% 7% 53%	2320
	21st to end	Day	7% 5% 2 5% 4 3 5% 4 6% 8% 51%	2200
October	1st to 10th	Day	6% 5% 2 5% 4 3 3 3 6% 7% 56%	3320
	11th to 20th	Day	7% 4 2 6% 5% 5% 7% 5% 6% 8% 44%	3010
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	3101
November	1st to 10th	Day	19% 11% 4 9% 5% 4 5% 4 4 7% 28%	1764
	11th to 20th	Day	18% 14% 4 9% 5% 5% 4 4 5% 5% 28%	1643
	21st to end	Day	16% 18% 6% 10% 7% 5% 4 4 5% 6% 19%	1503
December	1st to 10th	Day	12% 11% 4 11% 5% 4 4 4 7% 8% 29%	2290
	11th to 20th	Day	22% 22% 5% 11% 4 4 4 3 4 5% 14%	2642
	21st to end	Day	16% 23% 7% 11% 6% 6% 6% 4 5% 5% 11%	3751
January	1st to 10th	Night	20% 19% 7% 11% 5% 5% 4 3 5% 5% 17%	6306
	11th to 20th	Night	13% 11% 2 8% 5% 5% 5% 4 6% 7% 35%	6282
	21st to end	Night	17% 18% 6% 7% 4 4 4 3 3 4 29%	6952
February	1st to 10th	Night	13% 16% 4 10% 5% 4 5% 4 6% 7% 26%	2772
	11th to 20th	Night	11% 12% 5% 9% 5% 5% 5% 4 5% 6% 32%	2453
	21st to end	Night	11% 11% 4 7% 5% 5% 4 4 7% 8% 34%	1980
March	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 4 6% 8% 41%	5376
	11th to 20th	Night	10% 11% 3 7% 4 3 3 3 4 6% 45%	5334
	21st to end	Night	11% 10% 3 9% 7% 7% 7% 6% 8% 10% 24%	5006
April	1st to 10th	Night	9% 8% 3 8% 6% 6% 5% 4 7% 8% 36%	2000
	11th to 20th	Night	8% 9% 2 8% 6% 5% 5% 3 5% 6% 43%	1935
	21st to end	Night	5% 8% 3 8% 6% 5% 5% 4 7% 7% 43%	1794
May	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 6% 8% 48%	3640
	11th to 20th	Night	6% 6% 3 8% 5% 5% 5% 4 7% 8% 43%	3616
	21st to end	Night	6% 5% 2 6% 4 4 4 4 6% 9% 50%	3972
June	1st to 10th	Night	9% 5% 3 5% 5% 4 4 4 7% 9% 45%	1435
	11th to 20th	Night	7% 6% 3 5% 4 5% 5% 5% 7% 8% 45%	1400
	21st to end	Night	4 4 1 4 4 4 4 3 5% 7% 59%	1419
July	1st to 10th	Night	2 2 1 2 2 2 2 2 4 6% 75%	3640
	11th to 20th	Night	2 1 2 2 1 2 2 3 4 81%	3640
	21st to end	Night	2 2 1 2 2 2 2 2 3 6% 77%	3969
August	1st to 10th	Night	3 3 1 3 2 2 3 3 4 7% 70%	2609
	11th to 20th	Night	2 3 1 4 3 3 3 3 6% 7% 64%	2699
	21st to end	Night	3 2 1 5% 3 4 4 4 5% 6% 63%	3570
September	1st to 10th	Night	2 2 1 3 3 3 3 4 5% 8% 66%	2000
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 54%	2080
	21st to end	Night	8% 4 2 5% 4 3 3 4 6% 8% 55%	2200
October	1st to 10th	Night	6% 3 1 4 2 3 3 4 6% 9% 59%	3726
	11th to 20th	Night	9% 6% 2 6% 3 4 4 4 6% 9% 47%	3922
	21st to end	Night	13% 7% 3 5% 4 4 5% 3 6% 7% 42%	4457
November	1st to 10th	Night	18% 12% 3 7% 5% 5% 5% 5% 7% 8% 28%	2535
	11th to 20th	Night	17% 12% 3 7% 6% 5% 6% 5% 6% 6% 27%	2755
	21st to end	Night	18% 14% 4 11% 6% 6% 4 4 4 5% 23%	2874
December	1st to 10th	Night	8% 10% 3 10% 5% 5% 4 3 6% 7% 38%	4890
	11th to 20th	Night	20% 19% 7% 12% 6% 5% 4 4 4 5% 15%	5550
	21st to end	Night	20% 21% 6% 11% 6% 5% 6% 4 5% 4 13%	7246

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	73% 5% 9% 11% 3	3639
	11th to 20th	Day	48% 5% 19% 21% 6% 2	3640
	21st to end	Day	61% 3 11% 19% 5%	4004
February	1st to 10th	Day	59% 5% 15% 11% 8% 2	1625
	11th to 20th	Day	53% 5% 15% 17% 9% 1	1842
	21st to end	Day	53% 6% 13% 17% 10% 1	1650
March	1st to 10th	Day	45% 5% 11% 21% 17% 1	4518
	11th to 20th	Day	40% 4 15% 30% 10%	4661
	21st to end	Day	54% 5% 17% 17% 6% 1	5969
April	1st to 10th	Day	53% 6% 13% 19% 8% 1	2400
	11th to 20th	Day	44% 5% 14% 21% 13% 3	2416
	21st to end	Day	44% 4 11% 25% 12% 3	2603
May	1st to 10th	Day	35% 4 6% 27% 28% 1	6320
	11th to 20th	Day	39% 5% 9% 29% 15% 1	6280
	21st to end	Day	38% 3 15% 31% 12% 1	6904
June	1st to 10th	Day	43% 3 9% 22% 19% 4	2960
	11th to 20th	Day	42% 5% 10% 23% 16% 5%	3000
	21st to end	Day	28% 4 11% 29% 23% 5%	2974
July	1st to 10th	Day	15% 3 6% 21% 26% 28%	6251
	11th to 20th	Day	11% 2 4 21% 29% 32%	6245
	21st to end	Day	15% 2 4 28% 32% 19%	6681
August	1st to 10th	Day	17% 3 6% 23% 36% 15%	4524
	11th to 20th	Day	17% 4 9% 39% 26% 5%	4394
	21st to end	Day	25% 4 9% 31% 26% 5%	4344
September	1st to 10th	Day	20% 2 10% 30% 28% 10%	2400
	11th to 20th	Day	30% 4 12% 26% 23% 5%	2320
	21st to end	Day	32% 5% 17% 29% 13% 4	2200
October	1st to 10th	Day	28% 4 15% 36% 17%	3320
	11th to 20th	Day	37% 6% 18% 23% 16%	3010
	21st to end	Day	42% 6% 16% 22% 13% 1	3101
November	1st to 10th	Day	57% 6% 14% 17% 5% 1	1764
	11th to 20th	Day	58% 6% 14% 15% 5% 2	1643
	21st to end	Day	64% 5% 13% 13% 4 1	1503
December	1st to 10th	Day	51% 8% 23% 14% 3	2290
	11th to 20th	Day	73% 5% 11% 8% 3	2644
	21st to end	Day	72% 5% 11% 8% 4	3751
January	1st to 10th	Night	69% 6% 9% 14% 2	6306
	11th to 20th	Night	47% 4 19% 19% 10% 1	6320
	21st to end	Night	59% 4 13% 19% 5%	6952
February	1st to 10th	Night	56% 7% 15% 13% 8% 2	2772
	11th to 20th	Night	52% 5% 15% 17% 10% 1	2455
	21st to end	Night	47% 5% 15% 20% 12% 1	1980
March	1st to 10th	Night	41% 3 11% 25% 19% 1	5376
	11th to 20th	Night	41% 3 13% 27% 15%	5335
	21st to end	Night	54% 6% 13% 20% 6% 1	5006
April	1st to 10th	Night	46% 4 13% 25% 12% 1	2000
	11th to 20th	Night	43% 3 9% 25% 19% 2	1935
	21st to end	Night	40% 3 10% 25% 19% 3	1794
May	1st to 10th	Night	33% 4 5% 25% 29% 3	3637
	11th to 20th	Night	36% 5% 10% 30% 18% 2	3616
	21st to end	Night	32% 3 15% 36% 14% 1	3972
June	1st to 10th	Night	35% 4 8% 26% 20% 6%	1435
	11th to 20th	Night	36% 5% 9% 23% 21% 6%	1400
	21st to end	Night	25% 4 7% 29% 26% 8%	1419
July	1st to 10th	Night	12% 3 4 21% 32% 29%	3600
	11th to 20th	Night	9% 2 4 22% 26% 37%	3592
	21st to end	Night	12% 2 2 25% 34% 24%	3857
August	1st to 10th	Night	15% 3 5% 22% 33% 22%	2601
	11th to 20th	Night	19% 3 6% 34% 31% 7%	2723
	21st to end	Night	22% 4 9% 26% 30% 9%	3570
September	1st to 10th	Night	18% 3 6% 28% 30% 15%	2000
	11th to 20th	Night	28% 4 9% 25% 25% 8%	2080
	21st to end	Night	29% 3 13% 33% 17% 6%	2200
October	1st to 10th	Night	23% 4 11% 41% 20% 1	3726
	11th to 20th	Night	34% 5% 14% 25% 20% 1	3922
	21st to end	Night	41% 5% 12% 29% 10% 2	4457
November	1st to 10th	Night	53% 6% 17% 16% 7% 1	2535
	11th to 20th	Night	55% 5% 16% 16% 7% 2	2755
	21st to end	Night	62% 4 14% 14% 7%	2874
December	1st to 10th	Night	44% 9% 19% 24% 3	4890
	11th to 20th	Night	72% 6% 12% 8% 3	5547
	21st to end	Night	71% 5% 10% 10% 4	7246

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	31% 8% 22% 27% 11% 2	200
	11th to 20th	Day	27% 11% 20% 25% 13% 5%	200
	21st to end	Day	31% 5% 26% 25% 12% 1	220
February	1st to 10th	Day	20% 16% 30% 24% 8%	225
	11th to 20th	Day	22% 12% 30% 26% 9% 1	435
	21st to end	Day	19% 18% 29% 23% 10% 1	414
March	1st to 10th	Day	19% 19% 29% 21% 10% 2	500
	11th to 20th	Day	20% 16% 22% 25% 15% 1	500
	21st to end	Day	21% 21% 27% 20% 10% 1	550
April	1st to 10th	Day	20% 17% 30% 22% 9% 2	500
	11th to 20th	Day	15% 27% 25% 22% 9% 2	492
	21st to end	Day	14% 17% 27% 28% 11% 3	500
May	1st to 10th	Day	11% 16% 31% 26% 14% 2	503
	11th to 20th	Day	16% 15% 26% 27% 13% 3	528
	21st to end	Day	12% 14% 26% 26% 16% 6%	573
June	1st to 10th	Day	18% 11% 22% 29% 17% 3	690
	11th to 20th	Day	12% 16% 27% 28% 14% 3	730
	21st to end	Day	7% 11% 23% 33% 22% 5%	704
July	1st to 10th	Day	4 11% 33% 37% 14% 2	526
	11th to 20th	Day	5% 11% 29% 33% 17% 5%	524
	21st to end	Day	4 8% 26% 38% 19% 5%	575
August	1st to 10th	Day	6% 8% 21% 38% 21% 6%	527
	11th to 20th	Day	4 5% 24% 35% 24% 7%	517
	21st to end	Day	6% 11% 27% 29% 21% 7%	583
September	1st to 10th	Day	5% 8% 27% 29% 24% 7%	530
	11th to 20th	Day	7% 12% 27% 30% 21% 5%	530
	21st to end	Day	12% 15% 29% 24% 16% 5%	530
October	1st to 10th	Day	12% 12% 29% 27% 17% 3	503
	11th to 20th	Day	17% 10% 26% 25% 18% 3	403
	21st to end	Day	22% 10% 28% 27% 10% 3	330
November	1st to 10th	Day	23% 11% 23% 29% 13%	300
	11th to 20th	Day	29% 16% 18% 19% 15% 3	221
	21st to end	Day	26% 9% 22% 28% 14% 3	200
December	1st to 10th	Day	22% 11% 23% 22% 19% 5%	200
	11th to 20th	Day	29% 13% 24% 26% 7% 2	200
	21st to end	Day	29% 13% 22% 27% 10%	220
January	1st to 10th	Night	31% 9% 20% 28% 12% 1	800
	11th to 20th	Night	27% 7% 20% 31% 14% 2	800
	21st to end	Night	23% 5% 19% 37% 15% 1	880
February	1st to 10th	Night	22% 8% 24% 32% 13% 2	800
	11th to 20th	Night	18% 8% 21% 34% 18% 1	784
	21st to end	Night	18% 8% 18% 35% 17% 3	660
March	1st to 10th	Night	17% 9% 19% 34% 19% 2	800
	11th to 20th	Night	17% 9% 18% 33% 21% 2	800
	21st to end	Night	17% 9% 20% 37% 15% 3	880
April	1st to 10th	Night	14% 7% 19% 35% 22% 3	800
	11th to 20th	Night	12% 8% 25% 36% 16% 3	747
	21st to end	Night	9% 10% 20% 37% 22% 2	594
May	1st to 10th	Night	8% 8% 21% 35% 26% 3	400
	11th to 20th	Night	9% 9% 15% 31% 30% 6%	394
	21st to end	Night	10% 5% 17% 35% 27% 6%	433
June	1st to 10th	Night	10% 7% 12% 43% 25% 3	400
	11th to 20th	Night	10% 7% 18% 42% 20% 4	400
	21st to end	Night	6% 5% 12% 41% 33% 3	400
July	1st to 10th	Night	3 3 10% 41% 39% 3	390
	11th to 20th	Night	2 4 13% 45% 35% 1	388
	21st to end	Night	1 3 8% 40% 43% 5%	438
August	1st to 10th	Night	3 4 11% 33% 43% 8%	398
	11th to 20th	Night	2 5% 11% 35% 38% 9%	466
	21st to end	Night	4 5% 12% 39% 35% 5%	855
September	1st to 10th	Night	4 4 12% 42% 33% 7%	800
	11th to 20th	Night	7% 5% 12% 32% 38% 7%	800
	21st to end	Night	11% 4 16% 38% 27% 4	800
October	1st to 10th	Night	13% 5% 12% 39% 29% 2	772
	11th to 20th	Night	14% 7% 16% 33% 27% 3	760
	21st to end	Night	19% 7% 15% 40% 16% 3	836
November	1st to 10th	Night	25% 9% 12% 42% 12%	780
	11th to 20th	Night	24% 9% 19% 32% 14% 2	800
	21st to end	Night	26% 7% 15% 32% 18% 2	874
December	1st to 10th	Night	21% 6% 21% 36% 15% 2	1000
	11th to 20th	Night	29% 8% 19% 33% 11% 1	1000
	21st to end	Night	29% 9% 17% 36% 8% 1	1005

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