Ketchikan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ketchikan clear sky chart from 2005-11-06 to 2026-09-14.

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	33% 13% 4 8% 5% 4 4 3 4 5% 19%	154644
	Night	35% 15% 4 8% 4 3 3 2 3 4 18%	145153
Transparency	Day	68% 5% 10% 12% 4 1	154323
	Night	70% 4 9% 11% 5% 1	145057
Seeing	Day	43% 8% 16% 19% 11% 3	19015
	Night	46% 8% 11% 19% 13% 3	26046

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% 19% 5% 8% 4 4 4 3 3 3 15%	3527
	11th to 20th	Day	37% 11% 2 6% 4 3 4 3 4 4 21%	3522
	21st to end	Day	50% 13% 3 8% 3 3 2 1 3 3 12%	4859
February	1st to 10th	Day	37% 15% 4 6% 4 3 3 2 2 3 21%	1680
	11th to 20th	Day	32% 17% 4 9% 4 3 3 2 3 4 20%	1640
	21st to end	Day	25% 18% 3 9% 5% 4 4 2 3 3 25%	1692
March	1st to 10th	Day	25% 24% 4 8% 4 4 3 3 3 5% 18%	4596
	11th to 20th	Day	28% 23% 4 10% 6% 4 5% 2 3 4 12%	4817
	21st to end	Day	20% 20% 4 9% 4 4 4 3 5% 4 24%	6006
April	1st to 10th	Day	31% 20% 4 9% 5% 4 3 3 3 4 14%	2541
	11th to 20th	Day	29% 16% 4 7% 4 4 3 3 3 4 21%	2698
	21st to end	Day	32% 13% 3 8% 5% 4 4 3 4 5% 19%	2904
May	1st to 10th	Day	32% 12% 4 9% 5% 4 4 3 5% 6% 17%	8522
	11th to 20th	Day	30% 14% 4 9% 5% 5% 4 3 5% 5% 14%	8757
	21st to end	Day	36% 14% 4 10% 5% 4 4 3 4 4 13%	10058
June	1st to 10th	Day	34% 13% 4 8% 5% 4 4 3 4 5% 16%	3360
	11th to 20th	Day	31% 11% 3 8% 5% 5% 4 3 5% 5% 19%	3360
	21st to end	Day	38% 11% 4 9% 5% 4 4 3 5% 5% 13%	3360
July	1st to 10th	Day	32% 10% 3 8% 5% 4 4 3 5% 6% 21%	9420
	11th to 20th	Day	31% 8% 3 7% 4 4 4 3 4 5% 27%	9420
	21st to end	Day	31% 12% 4 9% 5% 4 4 2 3 4 20%	9808
August	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 37%	6492
	11th to 20th	Day	23% 8% 3 6% 4 3 3 2 5% 5% 38%	6387
	21st to end	Day	43% 9% 3 6% 4 4 4 3 4 5% 15%	6666
September	1st to 10th	Day	37% 10% 4 8% 4 5% 4 3 5% 5% 16%	2564
	11th to 20th	Day	35% 9% 3 6% 3 3 3 2 5% 5% 26%	2337
	21st to end	Day	53% 10% 3 7% 5% 3 2 2 3 3 10%	2200
October	1st to 10th	Day	42% 12% 3 7% 4 4 4 3 4 4 14%	3323
	11th to 20th	Day	50% 11% 4 8% 4 3 3 2 3 4 8%	2933
	21st to end	Day	36% 12% 4 8% 5% 4 4 3 4 4 15%	3200
November	1st to 10th	Day	43% 14% 4 6% 5% 4 3 2 4 3 12%	1541
	11th to 20th	Day	35% 19% 5% 9% 4 3 3 2 4 4 12%	1470
	21st to end	Day	39% 15% 4 8% 6% 4 3 2 3 2 13%	1470
December	1st to 10th	Day	49% 14% 4 8% 4 3 2 2 2 3 10%	2131
	11th to 20th	Day	28% 19% 5% 10% 5% 4 4 3 3 4 14%	2325
	21st to end	Day	45% 21% 4 8% 3 3 2 1 2 2 8%	3058
January	1st to 10th	Night	35% 20% 5% 9% 4 3 3 2 3 3 15%	9368
	11th to 20th	Night	34% 12% 3 7% 4 4 4 3 5% 4 21%	9376
	21st to end	Night	47% 16% 4 7% 3 2 2 2 2 3 13%	9339
February	1st to 10th	Night	37% 15% 3 7% 3 3 3 2 2 4 21%	2940
	11th to 20th	Night	31% 17% 4 9% 4 3 3 2 4 4 20%	2867
	21st to end	Night	28% 17% 4 7% 4 3 2 2 3 4 24%	2110
March	1st to 10th	Night	21% 22% 5% 10% 5% 4 4 3 4 3 19%	5498
	11th to 20th	Night	24% 21% 5% 11% 6% 5% 5% 3 4 4 11%	5389
	21st to end	Night	23% 16% 4 9% 5% 3 4 3 4 5% 25%	5190
April	1st to 10th	Night	31% 17% 4 9% 5% 4 3 3 4 3 17%	2078
	11th to 20th	Night	28% 16% 3 10% 5% 3 4 2 4 4 22%	1871
	21st to end	Night	29% 14% 3 7% 5% 5% 3 3 3 5% 22%	1712
May	1st to 10th	Night	34% 10% 3 7% 4 3 4 3 6% 6% 20%	4410
	11th to 20th	Night	34% 14% 3 8% 4 3 4 3 4 4 18%	4105
	21st to end	Night	40% 14% 4 8% 4 3 4 2 3 3 15%	4110
June	1st to 10th	Night	36% 9% 4 7% 5% 4 5% 3 5% 4 16%	1260
	11th to 20th	Night	29% 10% 4 9% 5% 5% 4 3 4 6% 20%	1260
	21st to end	Night	37% 11% 5% 9% 4 4 5% 3 4 4 15%	1260
July	1st to 10th	Night	33% 10% 3 8% 4 3 3 2 4 5% 25%	3540
	11th to 20th	Night	28% 9% 2 8% 4 4 4 3 4 5% 32%	3540
	21st to end	Night	31% 12% 4 9% 4 4 4 2 3 4 22%	4260
August	1st to 10th	Night	17% 6% 2 7% 4 3 4 3 5% 7% 42%	3662
	11th to 20th	Night	19% 7% 3 9% 5% 4 3 3 4 8% 36%	3678
	21st to end	Night	41% 10% 3 7% 4 4 4 2 3 5% 15%	4482
September	1st to 10th	Night	34% 10% 4 8% 5% 4 4 3 4 5% 18%	2053
	11th to 20th	Night	34% 9% 3 6% 3 3 3 2 3 4 28%	2130
	21st to end	Night	49% 11% 3 7% 4 4 3 2 3 3 12%	2200
October	1st to 10th	Night	44% 15% 4 7% 4 3 3 2 3 3 12%	3868
	11th to 20th	Night	48% 14% 4 9% 4 3 2 2 3 3 8%	4215
	21st to end	Night	36% 13% 3 8% 4 3 3 2 2 3 22%	4601
November	1st to 10th	Night	44% 15% 4 7% 4 3 3 2 3 3 13%	2973
	11th to 20th	Night	37% 19% 4 8% 4 4 3 3 2 4 11%	3150
	21st to end	Night	42% 17% 3 8% 3 3 3 2 3 3 13%	3150
December	1st to 10th	Night	51% 12% 4 6% 4 3 3 2 2 3 11%	5268
	11th to 20th	Night	34% 18% 4 9% 4 3 3 2 3 3 16%	6100
	21st to end	Night	43% 20% 4 8% 3 3 2 2 2 3 9%	8140

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% 3 5% 9% 9% 1	3527
	11th to 20th	Day	64% 4 8% 9% 11% 3	3540
	21st to end	Day	79% 4 5% 6% 5% 1	4860
February	1st to 10th	Day	72% 2 4 7% 10% 5%	1680
	11th to 20th	Day	69% 3 9% 10% 7% 2	1648
	21st to end	Day	65% 3 7% 13% 10% 2	1692
March	1st to 10th	Day	70% 4 4 14% 7% 1	4596
	11th to 20th	Day	77% 3 4 9% 6% 1	4817
	21st to end	Day	63% 3 8% 21% 5%	6006
April	1st to 10th	Day	75% 4 7% 9% 5% 1	2541
	11th to 20th	Day	67% 4 7% 14% 7% 1	2698
	21st to end	Day	68% 5% 9% 12% 6%	2904
May	1st to 10th	Day	67% 6% 12% 14% 1	8522
	11th to 20th	Day	70% 7% 10% 11% 2	8760
	21st to end	Day	73% 6% 9% 10% 2	10059
June	1st to 10th	Day	73% 4 10% 11% 3	3360
	11th to 20th	Day	66% 5% 14% 13% 1	3360
	21st to end	Day	75% 5% 11% 8% 2	3360
July	1st to 10th	Day	60% 7% 15% 14% 4	9341
	11th to 20th	Day	58% 6% 18% 16% 2	9333
	21st to end	Day	67% 7% 13% 11% 2	9596
August	1st to 10th	Day	45% 8% 15% 19% 11% 2	6474
	11th to 20th	Day	47% 7% 18% 21% 5% 1	6429
	21st to end	Day	71% 7% 12% 9% 2	6665
September	1st to 10th	Day	71% 6% 13% 7% 3	2564
	11th to 20th	Day	61% 3 13% 15% 6%	2337
	21st to end	Day	82% 2 8% 5% 2 1	2200
October	1st to 10th	Day	73% 5% 11% 8% 3	3323
	11th to 20th	Day	81% 4 6% 7% 2	2933
	21st to end	Day	71% 6% 9% 10% 4	3200
November	1st to 10th	Day	80% 2 6% 8% 3	1541
	11th to 20th	Day	78% 3 5% 9% 4 1	1470
	21st to end	Day	79% 3 4 8% 5% 2	1470
December	1st to 10th	Day	83% 2 4 6% 4	2131
	11th to 20th	Day	74% 5% 4 10% 6% 1	2328
	21st to end	Day	84% 3 3 6% 3 2	3058
January	1st to 10th	Night	76% 2 5% 8% 7%	9368
	11th to 20th	Night	65% 4 7% 12% 8% 5%	9420
	21st to end	Night	77% 3 5% 9% 5% 1	9339
February	1st to 10th	Night	70% 3 6% 8% 8% 5%	2940
	11th to 20th	Night	70% 4 6% 11% 7% 2	2870
	21st to end	Night	65% 3 7% 12% 9% 3	2110
March	1st to 10th	Night	69% 4 7% 12% 7% 1	5498
	11th to 20th	Night	75% 3 7% 8% 6%	5391
	21st to end	Night	62% 3 8% 22% 4	5190
April	1st to 10th	Night	72% 5% 9% 10% 5%	2078
	11th to 20th	Night	67% 3 8% 13% 8% 1	1871
	21st to end	Night	65% 4 11% 15% 5%	1712
May	1st to 10th	Night	62% 6% 15% 15% 2	4407
	11th to 20th	Night	67% 7% 12% 10% 4	4105
	21st to end	Night	75% 4 8% 12% 1	4114
June	1st to 10th	Night	71% 4 11% 10% 3	1260
	11th to 20th	Night	65% 4 15% 14% 2	1260
	21st to end	Night	73% 5% 14% 7% 1	1260
July	1st to 10th	Night	58% 5% 15% 18% 3	3510
	11th to 20th	Night	52% 8% 21% 19%	3504
	21st to end	Night	64% 8% 17% 8% 3	4167
August	1st to 10th	Night	40% 8% 22% 15% 13% 3	3655
	11th to 20th	Night	45% 9% 21% 19% 5% 1	3702
	21st to end	Night	70% 6% 14% 9% 1	4482
September	1st to 10th	Night	67% 6% 17% 9% 2	2053
	11th to 20th	Night	62% 5% 11% 16% 6%	2130
	21st to end	Night	80% 3 7% 8% 2 1	2200
October	1st to 10th	Night	79% 3 7% 8% 3 1	3868
	11th to 20th	Night	81% 5% 6% 5% 2	4215
	21st to end	Night	69% 4 5% 16% 5%	4601
November	1st to 10th	Night	79% 3 5% 8% 4 1	2973
	11th to 20th	Night	81% 3 4 8% 4 1	3150
	21st to end	Night	79% 2 6% 6% 5% 1	3150
December	1st to 10th	Night	80% 3 6% 7% 3	5268
	11th to 20th	Night	76% 2 5% 10% 7% 1	6096
	21st to end	Night	84% 2 4 6% 3 1	8140

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	50% 10% 16% 13% 5% 5%	220
	11th to 20th	Day	55% 11% 12% 13% 8% 1	220
	21st to end	Day	59% 8% 13% 12% 7% 1	387
February	1st to 10th	Day	49% 10% 20% 12% 7% 3	430
	11th to 20th	Day	47% 9% 16% 17% 8% 3	425
	21st to end	Day	44% 14% 21% 11% 6% 3	355
March	1st to 10th	Day	44% 11% 15% 17% 8% 4	430
	11th to 20th	Day	48% 10% 17% 17% 6% 1	469
	21st to end	Day	42% 11% 15% 19% 10% 3	594
April	1st to 10th	Day	44% 12% 19% 17% 7% 1	540
	11th to 20th	Day	36% 8% 17% 20% 15% 3	532
	21st to end	Day	40% 9% 19% 23% 7% 2	714
May	1st to 10th	Day	40% 10% 16% 22% 10% 1	753
	11th to 20th	Day	32% 7% 20% 23% 13% 4	775
	21st to end	Day	36% 10% 20% 21% 9% 3	845
June	1st to 10th	Day	46% 9% 15% 18% 10% 2	780
	11th to 20th	Day	35% 10% 16% 24% 14% 1	780
	21st to end	Day	41% 5% 15% 21% 13% 4	780
July	1st to 10th	Day	36% 5% 18% 24% 14% 3	771
	11th to 20th	Day	39% 7% 12% 24% 14% 4	768
	21st to end	Day	41% 9% 16% 20% 11% 3	849
August	1st to 10th	Day	30% 6% 17% 27% 16% 3	776
	11th to 20th	Day	41% 5% 15% 25% 13% 1	765
	21st to end	Day	45% 6% 16% 18% 12% 4	674
September	1st to 10th	Day	41% 6% 14% 22% 14% 2	570
	11th to 20th	Day	40% 8% 15% 17% 14% 6%	543
	21st to end	Day	63% 8% 12% 10% 6% 1	530
October	1st to 10th	Day	44% 7% 19% 17% 11% 3	459
	11th to 20th	Day	59% 8% 15% 12% 5% 1	420
	21st to end	Day	52% 8% 15% 16% 7% 2	462
November	1st to 10th	Day	58% 11% 13% 10% 6% 1	277
	11th to 20th	Day	52% 10% 19% 12% 6% 2	220
	21st to end	Day	52% 9% 18% 15% 5%	220
December	1st to 10th	Day	51% 9% 13% 14% 12%	220
	11th to 20th	Day	48% 9% 20% 15% 7% 2	220
	21st to end	Day	45% 10% 16% 16% 10% 4	242
January	1st to 10th	Night	52% 11% 13% 15% 7% 2	950
	11th to 20th	Night	57% 9% 12% 15% 7%	950
	21st to end	Night	57% 6% 12% 15% 8% 1	1045
February	1st to 10th	Night	49% 11% 12% 17% 9% 2	950
	11th to 20th	Night	45% 8% 12% 22% 11% 2	934
	21st to end	Night	41% 11% 15% 20% 10% 3	697
March	1st to 10th	Night	40% 10% 15% 21% 12% 2	840
	11th to 20th	Night	46% 9% 15% 20% 10% 1	840
	21st to end	Night	41% 10% 10% 18% 17% 3	737
April	1st to 10th	Night	43% 7% 14% 19% 15% 3	630
	11th to 20th	Night	37% 8% 10% 23% 19% 3	623
	21st to end	Night	39% 8% 11% 21% 18% 3	630
May	1st to 10th	Night	39% 8% 8% 23% 16% 5%	522
	11th to 20th	Night	36% 6% 13% 22% 20% 4	414
	21st to end	Night	37% 6% 9% 25% 20% 3	455
June	1st to 10th	Night	41% 7% 8% 21% 19% 5%	420
	11th to 20th	Night	33% 7% 10% 23% 23% 4	420
	21st to end	Night	40% 8% 7% 20% 19% 5%	420
July	1st to 10th	Night	37% 4 11% 20% 23% 5%	410
	11th to 20th	Night	34% 5% 8% 20% 25% 8%	408
	21st to end	Night	40% 5% 8% 22% 20% 4	460
August	1st to 10th	Night	28% 5% 9% 21% 29% 7%	605
	11th to 20th	Night	36% 3 8% 21% 26% 5%	624
	21st to end	Night	45% 4 6% 20% 20% 4	693
September	1st to 10th	Night	40% 4 7% 23% 23% 4	630
	11th to 20th	Night	41% 4 10% 19% 20% 6%	709
	21st to end	Night	58% 6% 11% 14% 9% 3	800
October	1st to 10th	Night	45% 7% 9% 21% 17% 2	800
	11th to 20th	Night	56% 8% 9% 16% 10% 2	800
	21st to end	Night	53% 7% 10% 18% 10% 2	880
November	1st to 10th	Night	58% 8% 9% 18% 6% 1	905
	11th to 20th	Night	53% 10% 14% 14% 7% 2	950
	21st to end	Night	57% 8% 13% 14% 6% 2	950
December	1st to 10th	Night	49% 9% 12% 17% 11% 2	950
	11th to 20th	Night	46% 9% 17% 20% 7% 1	950
	21st to end	Night	51% 9% 14% 17% 8% 2	1045

Page generated at 2026-09-11 00:11:54. Version ac07

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