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 2025-05-11.

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	34% 14% 4 8% 5% 4 4 3 4 4 18%	116820
	Night	36% 15% 4 8% 4 3 3 2 3 4 17%	120789
Transparency	Day	70% 4 9% 11% 5% 1	116543
	Night	72% 4 8% 10% 5% 1	120713
Seeing	Day	44% 8% 16% 19% 11% 3	17073
	Night	47% 7% 11% 19% 13% 3	24447

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% 21% 5% 8% 4 4 4 3 3 3 13%	2719
	11th to 20th	Day	39% 13% 2 7% 4 4 3 2 3 3 18%	2702
	21st to end	Day	51% 13% 4 9% 4 3 2 2 2 2 9%	3748
February	1st to 10th	Day	35% 15% 4 6% 4 3 3 2 3 3 22%	1600
	11th to 20th	Day	33% 16% 4 8% 4 3 3 3 3 4 20%	1560
	21st to end	Day	24% 18% 3 9% 5% 4 3 2 3 3 25%	1613
March	1st to 10th	Day	25% 24% 4 8% 4 4 3 3 3 5% 18%	4496
	11th to 20th	Day	28% 23% 4 10% 6% 4 4 2 3 3 12%	4714
	21st to end	Day	21% 19% 4 8% 4 4 4 3 4 4 25%	5876
April	1st to 10th	Day	32% 19% 4 9% 5% 4 3 3 3 4 14%	2420
	11th to 20th	Day	30% 16% 4 7% 4 4 3 3 4 4 21%	2568
	21st to end	Day	33% 14% 4 8% 5% 4 4 3 4 5% 17%	2766
May	1st to 10th	Day	28% 13% 4 9% 6% 5% 5% 3 5% 5% 17%	4744
	11th to 20th	Day	31% 13% 4 9% 5% 4 4 3 5% 6% 17%	4700
	21st to end	Day	30% 14% 4 9% 5% 4 4 3 5% 5% 17%	5374
June	1st to 10th	Day	36% 14% 3 7% 5% 4 3 3 4 4 15%	3040
	11th to 20th	Day	32% 12% 3 8% 6% 5% 4 4 5% 5% 18%	3040
	21st to end	Day	38% 11% 4 8% 5% 4 4 3 5% 5% 13%	3040
July	1st to 10th	Day	24% 8% 2 6% 5% 4 3 3 4 6% 34%	5060
	11th to 20th	Day	47% 12% 3 7% 5% 4 3 2 3 3 11%	5060
	21st to end	Day	38% 12% 4 9% 5% 4 4 3 4 4 14%	5270
August	1st to 10th	Day	19% 6% 2 6% 4 3 3 3 5% 7% 42%	4428
	11th to 20th	Day	29% 9% 3 6% 4 3 3 2 4 5% 32%	4340
	21st to end	Day	44% 9% 2 6% 4 4 4 3 4 6% 14%	4590
September	1st to 10th	Day	39% 10% 4 8% 4 5% 4 3 4 4 14%	2320
	11th to 20th	Day	35% 8% 3 6% 3 3 4 2 5% 5% 26%	2185
	21st to end	Day	53% 10% 3 7% 4 3 2 2 3 3 10%	2090
October	1st to 10th	Day	42% 12% 3 7% 4 4 4 3 4 4 13%	3222
	11th to 20th	Day	51% 11% 4 7% 4 3 3 2 3 4 8%	2843
	21st to end	Day	35% 12% 4 8% 5% 4 4 3 4 4 15%	3101
November	1st to 10th	Day	43% 14% 4 6% 5% 3 3 3 4 3 12%	1466
	11th to 20th	Day	35% 19% 5% 8% 5% 3 3 2 4 4 13%	1400
	21st to end	Day	40% 15% 4 8% 6% 4 3 3 3 2 13%	1400
December	1st to 10th	Day	49% 14% 3 8% 4 3 2 2 2 3 10%	2068
	11th to 20th	Day	29% 18% 5% 10% 5% 4 4 3 3 4 14%	2265
	21st to end	Day	45% 22% 4 8% 3 3 2 1 2 2 8%	2992
January	1st to 10th	Night	38% 19% 5% 8% 4 3 3 2 2 3 12%	7226
	11th to 20th	Night	33% 14% 4 8% 4 4 3 2 3 3 21%	7196
	21st to end	Night	49% 16% 4 8% 3 2 2 2 2 2 10%	7219
February	1st to 10th	Night	36% 15% 3 7% 3 3 3 2 2 4 22%	2800
	11th to 20th	Night	31% 17% 4 9% 4 3 3 2 4 4 19%	2727
	21st to end	Night	28% 17% 4 7% 4 3 2 2 3 4 24%	2013
March	1st to 10th	Night	20% 22% 4 10% 5% 4 4 3 4 3 20%	5378
	11th to 20th	Night	24% 21% 5% 11% 6% 6% 5% 3 4 4 11%	5272
	21st to end	Night	23% 16% 4 8% 4 3 4 3 4 5% 25%	5078
April	1st to 10th	Night	31% 17% 4 9% 5% 4 3 3 4 3 17%	1979
	11th to 20th	Night	29% 16% 3 10% 5% 3 4 2 4 4 22%	1781
	21st to end	Night	30% 14% 4 7% 5% 5% 4 3 3 5% 20%	1630
May	1st to 10th	Night	31% 9% 3 7% 4 4 4 3 6% 5% 23%	2439
	11th to 20th	Night	33% 12% 4 8% 4 3 4 3 4 4 21%	2201
	21st to end	Night	30% 15% 4 8% 5% 4 5% 3 4 4 21%	2198
June	1st to 10th	Night	38% 10% 4 6% 5% 4 5% 3 5% 4 16%	1140
	11th to 20th	Night	30% 10% 4 9% 5% 5% 4 3 5% 6% 18%	1140
	21st to end	Night	38% 12% 5% 9% 4 4 5% 3 3 4 14%	1140
July	1st to 10th	Night	24% 8% 3 6% 4 3 3 2 4 6% 38%	1900
	11th to 20th	Night	42% 13% 3 7% 4 4 4 3 4 4 13%	1900
	21st to end	Night	36% 12% 4 9% 4 4 4 2 3 4 18%	2282
August	1st to 10th	Night	16% 5% 2 5% 3 3 4 3 5% 7% 48%	2497
	11th to 20th	Night	22% 7% 3 9% 5% 4 3 3 4 7% 32%	2498
	21st to end	Night	44% 10% 4 6% 4 3 3 2 4 5% 15%	3077
September	1st to 10th	Night	36% 11% 4 8% 5% 4 4 3 4 5% 17%	1857
	11th to 20th	Night	33% 8% 3 6% 3 4 4 2 3 5% 29%	1995
	21st to end	Night	49% 11% 3 7% 4 4 3 2 3 3 12%	2090
October	1st to 10th	Night	45% 15% 4 7% 3 3 3 2 3 3 12%	3750
	11th to 20th	Night	49% 14% 4 9% 4 3 2 2 3 3 7%	4085
	21st to end	Night	36% 13% 3 8% 4 3 3 2 2 3 22%	4458
November	1st to 10th	Night	44% 14% 4 7% 4 3 3 2 3 3 13%	2828
	11th to 20th	Night	37% 19% 4 8% 4 4 3 3 2 4 11%	3000
	21st to end	Night	43% 17% 3 8% 3 3 2 2 3 3 13%	3000
December	1st to 10th	Night	51% 11% 4 6% 4 3 3 2 2 3 11%	5111
	11th to 20th	Night	34% 18% 4 9% 4 3 3 2 3 3 16%	5940
	21st to end	Night	43% 21% 4 8% 3 3 2 2 2 3 9%	7964

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	76% 3 5% 10% 6% 1	2719
	11th to 20th	Day	71% 4 8% 6% 8% 4	2720
	21st to end	Day	81% 4 6% 4 4 1	3748
February	1st to 10th	Day	70% 3 4 7% 11% 5%	1600
	11th to 20th	Day	69% 4 9% 10% 7% 2	1568
	21st to end	Day	65% 3 7% 13% 10% 3	1613
March	1st to 10th	Day	69% 4 4 14% 8% 1	4496
	11th to 20th	Day	78% 3 4 9% 6% 1	4714
	21st to end	Day	63% 3 8% 21% 5%	5876
April	1st to 10th	Day	75% 4 7% 9% 5% 1	2420
	11th to 20th	Day	68% 4 7% 13% 7% 1	2568
	21st to end	Day	70% 5% 8% 12% 6%	2766
May	1st to 10th	Day	68% 5% 8% 17% 1	4744
	11th to 20th	Day	68% 5% 11% 12% 3	4703
	21st to end	Day	68% 6% 10% 14% 3	5375
June	1st to 10th	Day	74% 3 10% 10% 3	3040
	11th to 20th	Day	68% 5% 14% 11% 1	3040
	21st to end	Day	75% 5% 11% 7% 2	3040
July	1st to 10th	Day	48% 6% 20% 20% 7%	4981
	11th to 20th	Day	81% 4 10% 4 1	4973
	21st to end	Day	72% 6% 13% 9% 1	5103
August	1st to 10th	Day	43% 5% 15% 19% 16% 3	4410
	11th to 20th	Day	55% 4 15% 17% 7% 2	4382
	21st to end	Day	72% 6% 12% 9% 2	4589
September	1st to 10th	Day	74% 5% 13% 6% 1	2320
	11th to 20th	Day	61% 3 14% 15% 7%	2185
	21st to end	Day	82% 2 7% 5% 2 1	2090
October	1st to 10th	Day	74% 5% 10% 8% 3	3222
	11th to 20th	Day	81% 3 6% 7% 2	2843
	21st to end	Day	71% 6% 9% 10% 4	3101
November	1st to 10th	Day	80% 2 6% 8% 3	1466
	11th to 20th	Day	78% 3 5% 10% 4 1	1400
	21st to end	Day	79% 3 4 7% 5% 2	1400
December	1st to 10th	Day	83% 2 4 6% 5%	2068
	11th to 20th	Day	74% 5% 4 10% 5% 1	2268
	21st to end	Day	85% 3 3 5% 2 2	2992
January	1st to 10th	Night	79% 2 6% 9% 4	7226
	11th to 20th	Night	68% 3 7% 8% 8% 6%	7240
	21st to end	Night	80% 4 6% 6% 3 1	7219
February	1st to 10th	Night	69% 3 6% 8% 9% 6%	2800
	11th to 20th	Night	70% 4 6% 11% 7% 2	2730
	21st to end	Night	65% 3 7% 12% 9% 3	2013
March	1st to 10th	Night	68% 4 7% 12% 7% 1	5378
	11th to 20th	Night	76% 3 7% 8% 5%	5274
	21st to end	Night	61% 3 9% 22% 4	5078
April	1st to 10th	Night	72% 4 8% 10% 5%	1979
	11th to 20th	Night	67% 3 8% 13% 8% 1	1781
	21st to end	Night	68% 4 9% 14% 5%	1630
May	1st to 10th	Night	62% 5% 10% 19% 3	2439
	11th to 20th	Night	66% 5% 12% 11% 5% 1	2201
	21st to end	Night	66% 5% 11% 15% 2	2198
June	1st to 10th	Night	72% 4 12% 9% 4	1140
	11th to 20th	Night	67% 4 16% 12% 1	1140
	21st to end	Night	75% 5% 13% 7% 1	1140
July	1st to 10th	Night	47% 4 18% 25% 6%	1870
	11th to 20th	Night	75% 7% 12% 6%	1864
	21st to end	Night	68% 7% 14% 10% 1	2210
August	1st to 10th	Night	37% 7% 18% 16% 18% 4	2490
	11th to 20th	Night	50% 6% 20% 16% 7% 1	2522
	21st to end	Night	71% 4 13% 11% 1	3077
September	1st to 10th	Night	70% 5% 17% 8% 1	1857
	11th to 20th	Night	60% 5% 12% 16% 6%	1995
	21st to end	Night	79% 3 7% 9% 2 1	2090
October	1st to 10th	Night	80% 3 6% 7% 3 1	3750
	11th to 20th	Night	82% 5% 6% 5% 2	4085
	21st to end	Night	69% 4 5% 16% 5%	4458
November	1st to 10th	Night	79% 3 5% 8% 5% 1	2828
	11th to 20th	Night	81% 3 4 8% 4 1	3000
	21st to end	Night	79% 2 6% 7% 6% 2	3000
December	1st to 10th	Night	80% 3 6% 7% 4	5111
	11th to 20th	Night	76% 2 5% 10% 6% 1	5936
	21st to end	Night	84% 2 3 6% 3 1	7964

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	51% 10% 16% 14% 5% 5%	200
	11th to 20th	Day	57% 12% 13% 11% 6% 2	200
	21st to end	Day	59% 9% 14% 11% 7%	358
February	1st to 10th	Day	45% 11% 21% 13% 8% 3	400
	11th to 20th	Day	47% 8% 16% 18% 8% 3	395
	21st to end	Day	44% 13% 21% 12% 7% 3	331
March	1st to 10th	Day	42% 11% 16% 18% 9% 5%	400
	11th to 20th	Day	49% 9% 17% 17% 6% 2	436
	21st to end	Day	42% 10% 15% 19% 11% 3	550
April	1st to 10th	Day	45% 12% 18% 17% 8% 1	500
	11th to 20th	Day	37% 8% 17% 20% 15% 3	492
	21st to end	Day	43% 9% 18% 22% 7% 2	666
May	1st to 10th	Day	41% 10% 16% 22% 11% 1	703
	11th to 20th	Day	33% 6% 19% 24% 14% 4	677
	21st to end	Day	34% 9% 20% 23% 11% 3	735
June	1st to 10th	Day	49% 7% 14% 18% 10% 2	680
	11th to 20th	Day	36% 7% 14% 27% 15% 2	680
	21st to end	Day	41% 5% 14% 21% 14% 5%	680
July	1st to 10th	Day	34% 4 17% 26% 16% 4	671
	11th to 20th	Day	43% 6% 10% 24% 14% 4	668
	21st to end	Day	43% 8% 13% 21% 12% 3	739
August	1st to 10th	Day	30% 6% 15% 28% 18% 4	676
	11th to 20th	Day	42% 5% 15% 26% 12% 1	665
	21st to end	Day	48% 6% 16% 16% 10% 4	582
September	1st to 10th	Day	45% 6% 14% 21% 13% 1	490
	11th to 20th	Day	40% 7% 14% 18% 15% 6%	490
	21st to end	Day	63% 7% 11% 10% 7% 1	490
October	1st to 10th	Day	45% 8% 17% 17% 10% 3	426
	11th to 20th	Day	60% 6% 15% 13% 5% 1	390
	21st to end	Day	51% 8% 16% 16% 8% 2	429
November	1st to 10th	Day	57% 9% 14% 11% 7% 1	254
	11th to 20th	Day	50% 9% 19% 13% 7% 3	200
	21st to end	Day	54% 9% 16% 15% 6% 1	200
December	1st to 10th	Day	49% 9% 13% 16% 14% 1	200
	11th to 20th	Day	49% 10% 18% 16% 7% 2	200
	21st to end	Day	45% 10% 15% 15% 11% 4	220
January	1st to 10th	Night	53% 11% 13% 15% 7% 2	900
	11th to 20th	Night	58% 9% 12% 15% 5%	900
	21st to end	Night	58% 6% 12% 14% 8% 1	990
February	1st to 10th	Night	47% 12% 12% 17% 10% 2	900
	11th to 20th	Night	45% 8% 12% 23% 11% 2	884
	21st to end	Night	41% 11% 14% 21% 10% 3	665
March	1st to 10th	Night	38% 9% 16% 22% 12% 3	800
	11th to 20th	Night	47% 8% 15% 19% 9% 1	800
	21st to end	Night	43% 8% 10% 18% 17% 3	702
April	1st to 10th	Night	44% 7% 13% 19% 15% 3	600
	11th to 20th	Night	37% 7% 10% 23% 20% 3	593
	21st to end	Night	40% 9% 9% 21% 19% 3	600
May	1st to 10th	Night	40% 9% 7% 23% 16% 5%	497
	11th to 20th	Night	35% 6% 12% 22% 22% 4	375
	21st to end	Night	35% 6% 9% 26% 22% 3	411
June	1st to 10th	Night	43% 6% 7% 21% 18% 6%	380
	11th to 20th	Night	34% 6% 9% 22% 24% 4	380
	21st to end	Night	40% 8% 6% 20% 21% 6%	380
July	1st to 10th	Night	35% 3 11% 21% 24% 5%	370
	11th to 20th	Night	37% 5% 6% 19% 25% 8%	368
	21st to end	Night	42% 4 6% 22% 21% 4	416
August	1st to 10th	Night	28% 5% 8% 21% 31% 7%	547
	11th to 20th	Night	37% 3 8% 21% 26% 5%	564
	21st to end	Night	47% 5% 6% 20% 19% 4	627
September	1st to 10th	Night	43% 4 6% 23% 21% 4	570
	11th to 20th	Night	40% 4 9% 19% 22% 7%	665
	21st to end	Night	58% 5% 11% 14% 9% 3	760
October	1st to 10th	Night	46% 6% 8% 21% 16% 2	760
	11th to 20th	Night	56% 7% 9% 16% 10% 2	760
	21st to end	Night	53% 7% 11% 18% 10% 2	836
November	1st to 10th	Night	57% 8% 9% 18% 6% 1	857
	11th to 20th	Night	52% 10% 14% 14% 8% 2	900
	21st to end	Night	59% 7% 13% 13% 6% 2	900
December	1st to 10th	Night	48% 8% 12% 18% 11% 2	900
	11th to 20th	Night	47% 9% 16% 20% 7% 1	900
	21st to end	Night	52% 9% 13% 16% 8% 2	990

Page generated at 2025-05-08 05:34:01. Version ac07

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