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 2024-11-02.

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% 13% 3 8% 5% 4 3 3 4 4 19%	102262
	Night	36% 14% 4 8% 4 3 3 2 3 4 19%	96762
Transparency	Day	69% 4 10% 12% 5% 1	101985
	Night	71% 3 8% 11% 5% 1	96686
Seeing	Day	44% 7% 16% 20% 11% 3	16511
	Night	47% 7% 11% 19% 14% 3	23630

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	31% 17% 4 8% 4 4 5% 3 3 4 16%	1899
	11th to 20th	Day	38% 12% 3 7% 5% 4 4 2 3 3 21%	1882
	21st to end	Day	62% 14% 3 5% 3 2 2 1 2 1 6%	2608
February	1st to 10th	Day	37% 15% 4 6% 4 3 3 2 3 3 19%	1520
	11th to 20th	Day	33% 16% 4 9% 4 3 3 3 3 4 20%	1480
	21st to end	Day	22% 18% 3 9% 5% 4 4 2 3 3 26%	1534
March	1st to 10th	Day	22% 20% 3 7% 5% 4 3 3 4 6% 24%	3126
	11th to 20th	Day	31% 21% 4 10% 6% 5% 4 2 2 3 11%	3288
	21st to end	Day	21% 17% 4 8% 4 3 4 3 4 4 28%	4098
April	1st to 10th	Day	32% 19% 4 9% 5% 4 3 3 3 4 14%	2299
	11th to 20th	Day	31% 16% 4 7% 4 3 3 3 3 4 22%	2438
	21st to end	Day	33% 13% 3 8% 5% 4 3 3 4 5% 18%	2627
May	1st to 10th	Day	27% 13% 4 9% 6% 5% 5% 3 5% 6% 18%	4599
	11th to 20th	Day	31% 13% 4 9% 5% 4 4 3 5% 6% 17%	4696
	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	37% 10% 3 8% 4 5% 4 3 5% 4 18%	1937
	11th to 20th	Day	53% 10% 3 6% 4 3 2 2 3 4 9%	1710
	21st to end	Day	42% 11% 4 7% 4 3 4 3 3 5% 14%	1881
November	1st to 10th	Day	43% 13% 4 6% 5% 3 3 3 4 3 13%	1402
	11th to 20th	Day	35% 19% 5% 8% 4 3 3 2 4 4 13%	1330
	21st to end	Day	41% 15% 4 8% 6% 4 3 2 3 2 12%	1330
December	1st to 10th	Day	35% 14% 4 8% 4 4 3 3 3 5% 17%	1206
	11th to 20th	Day	34% 20% 5% 9% 5% 3 3 1 2 4 14%	1445
	21st to end	Day	44% 19% 4 8% 4 3 2 2 2 2 10%	2090
January	1st to 10th	Night	36% 19% 5% 9% 4 3 3 2 3 3 14%	5046
	11th to 20th	Night	34% 13% 4 7% 4 4 3 2 3 3 24%	5016
	21st to end	Night	60% 16% 3 5% 2 2 2 1 1 1 7%	5052
February	1st to 10th	Night	38% 16% 3 6% 3 3 3 2 2 4 20%	2660
	11th to 20th	Night	32% 17% 4 9% 4 3 3 2 4 4 18%	2587
	21st to end	Night	26% 17% 5% 8% 5% 3 2 2 3 4 26%	1916
March	1st to 10th	Night	17% 17% 4 10% 5% 5% 4 3 4 4 26%	3738
	11th to 20th	Night	27% 21% 5% 10% 6% 5% 4 3 4 4 10%	3684
	21st to end	Night	25% 15% 4 8% 4 3 3 2 4 4 29%	3543
April	1st to 10th	Night	31% 16% 4 9% 5% 4 3 3 4 3 17%	1880
	11th to 20th	Night	28% 16% 3 9% 5% 3 3 2 4 4 22%	1691
	21st to end	Night	31% 14% 4 7% 5% 5% 4 3 3 5% 21%	1549
May	1st to 10th	Night	30% 9% 3 7% 4 4 5% 3 6% 6% 23%	2364
	11th to 20th	Night	33% 12% 4 8% 5% 3 4 4 4 4 21%	2199
	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	37% 11% 3 7% 4 3 3 2 4 5% 19%	2242
	11th to 20th	Night	46% 12% 4 7% 4 4 2 2 3 4 11%	2470
	21st to end	Night	41% 11% 3 7% 4 3 3 2 3 3 19%	2717
November	1st to 10th	Night	44% 14% 4 6% 4 3 3 2 3 3 14%	2695
	11th to 20th	Night	37% 19% 4 8% 4 4 3 3 2 4 11%	2850
	21st to end	Night	45% 16% 3 8% 3 3 3 2 3 3 12%	2850
December	1st to 10th	Night	35% 14% 5% 7% 5% 4 3 2 4 4 17%	2973
	11th to 20th	Night	38% 17% 4 7% 4 3 3 3 3 3 15%	3760
	21st to end	Night	41% 19% 4 8% 4 3 3 2 2 4 10%	5566

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% 2 6% 12% 7% 1	1899
	11th to 20th	Day	71% 2 4 6% 12% 6%	1900
	21st to end	Day	89% 1 3 3 3	2608
February	1st to 10th	Day	73% 3 4 7% 10% 4	1520
	11th to 20th	Day	70% 3 8% 10% 7% 2	1488
	21st to end	Day	63% 3 7% 14% 10% 3	1534
March	1st to 10th	Day	63% 3 4 17% 11% 2	3126
	11th to 20th	Day	79% 3 4 11% 4	3288
	21st to end	Day	61% 2 8% 24% 5%	4098
April	1st to 10th	Day	76% 4 6% 9% 5% 1	2299
	11th to 20th	Day	68% 3 7% 13% 8% 1	2438
	21st to end	Day	70% 4 8% 12% 6%	2627
May	1st to 10th	Day	67% 5% 9% 18% 1	4599
	11th to 20th	Day	68% 5% 11% 12% 3	4699
	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	68% 4 12% 10% 5% 1	1937
	11th to 20th	Day	82% 3 6% 7% 1	1710
	21st to end	Day	73% 5% 6% 12% 4	1881
November	1st to 10th	Day	79% 2 6% 9% 4	1402
	11th to 20th	Day	78% 3 5% 10% 4 1	1330
	21st to end	Day	81% 2 4 6% 5% 2	1330
December	1st to 10th	Day	72% 3 6% 11% 8%	1206
	11th to 20th	Day	78% 3 4 6% 8% 1	1448
	21st to end	Day	82% 3 3 6% 3 2	2090
January	1st to 10th	Night	78% 2 7% 9% 4	5046
	11th to 20th	Night	69% 2 4 6% 11% 9%	5060
	21st to end	Night	89% 1 2 5% 2	5052
February	1st to 10th	Night	71% 2 6% 8% 8% 4	2660
	11th to 20th	Night	71% 3 6% 11% 6% 2	2590
	21st to end	Night	64% 3 8% 13% 10% 3	1916
March	1st to 10th	Night	62% 4 5% 17% 10% 2	3738
	11th to 20th	Night	78% 3 8% 8% 3	3686
	21st to end	Night	60% 3 8% 23% 6%	3543
April	1st to 10th	Night	73% 4 8% 10% 5%	1880
	11th to 20th	Night	67% 3 8% 13% 8% 1	1691
	21st to end	Night	68% 4 9% 15% 5%	1549
May	1st to 10th	Night	62% 5% 11% 19% 4	2364
	11th to 20th	Night	66% 5% 12% 11% 6% 1	2199
	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	69% 4 9% 12% 4 1	2242
	11th to 20th	Night	80% 3 8% 7% 2	2470
	21st to end	Night	71% 4 6% 14% 4 1	2717
November	1st to 10th	Night	78% 2 5% 8% 5% 1	2695
	11th to 20th	Night	80% 3 4 8% 4 1	2850
	21st to end	Night	80% 2 6% 5% 5% 2	2850
December	1st to 10th	Night	71% 4 7% 11% 6%	2973
	11th to 20th	Night	77% 2 5% 8% 6% 1	3756
	21st to end	Night	81% 2 4 6% 4 2	5566

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% 8% 15% 15% 6% 6%	180
	11th to 20th	Day	57% 12% 14% 9% 6% 2	180
	21st to end	Day	61% 9% 12% 11% 7%	329
February	1st to 10th	Day	46% 11% 22% 12% 6% 3	370
	11th to 20th	Day	47% 8% 17% 19% 6% 2	365
	21st to end	Day	41% 13% 23% 13% 7% 4	307
March	1st to 10th	Day	41% 11% 16% 18% 9% 5%	370
	11th to 20th	Day	50% 9% 16% 16% 7% 2	402
	21st to end	Day	43% 9% 15% 20% 11% 3	506
April	1st to 10th	Day	46% 10% 17% 18% 7% 1	460
	11th to 20th	Day	38% 6% 17% 20% 15% 3	452
	21st to end	Day	43% 7% 17% 23% 8% 2	617
May	1st to 10th	Day	40% 9% 16% 23% 11% 1	653
	11th to 20th	Day	32% 6% 19% 24% 14% 4	675
	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% 12% 7% 1	236
	11th to 20th	Day	49% 8% 19% 13% 7% 3	180
	21st to end	Day	56% 8% 14% 16% 6%	180
December	1st to 10th	Day	46% 9% 13% 16% 15% 1	180
	11th to 20th	Day	51% 9% 17% 14% 7% 2	180
	21st to end	Day	44% 10% 15% 15% 12% 4	198
January	1st to 10th	Night	53% 10% 13% 15% 7% 2	850
	11th to 20th	Night	59% 9% 12% 14% 6%	850
	21st to end	Night	59% 6% 11% 14% 8% 1	935
February	1st to 10th	Night	49% 12% 12% 16% 9% 2	850
	11th to 20th	Night	46% 7% 12% 24% 9% 2	834
	21st to end	Night	39% 11% 15% 22% 11% 3	633
March	1st to 10th	Night	38% 10% 16% 22% 12% 3	760
	11th to 20th	Night	47% 8% 15% 19% 9% 1	760
	21st to end	Night	44% 8% 9% 19% 18% 4	667
April	1st to 10th	Night	43% 6% 12% 19% 16% 4	570
	11th to 20th	Night	37% 7% 9% 24% 21% 3	563
	21st to end	Night	40% 8% 9% 20% 19% 3	570
May	1st to 10th	Night	39% 9% 7% 23% 17% 5%	472
	11th to 20th	Night	35% 6% 12% 22% 22% 4	374
	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% 7% 9% 18% 7% 1	813
	11th to 20th	Night	52% 9% 14% 14% 8% 2	850
	21st to end	Night	60% 7% 12% 14% 6% 2	850
December	1st to 10th	Night	46% 9% 12% 18% 12% 2	850
	11th to 20th	Night	48% 8% 16% 20% 7% 1	850
	21st to end	Night	51% 9% 14% 17% 8% 2	935

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