Port Townsend
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Townsend clear sky chart from 2005-11-06 to 2025-10-19.

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	13% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	130860
	Night	15% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	127551
Transparency	Day	30% 40% 11% 12% 6% 1	130571
	Night	34% 41% 10% 11% 4 1	127417
Seeing	Day	17% 9% 19% 26% 22% 7%	16501
	Night	21% 8% 19% 26% 21% 5%	24717

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	20% 17% 6% 13% 8% 6% 5% 4 5% 4 12%	3179
	11th to 20th	Day	13% 12% 5% 12% 8% 6% 5% 5% 6% 8% 21%	3384
	21st to end	Day	18% 13% 5% 11% 7% 6% 5% 4 5% 5% 21%	4124
February	1st to 10th	Day	19% 13% 4 10% 8% 6% 6% 5% 6% 6% 18%	1800
	11th to 20th	Day	17% 14% 4 9% 6% 5% 5% 5% 6% 7% 20%	1755
	21st to end	Day	17% 12% 4 9% 8% 6% 6% 4 5% 6% 22%	1525
March	1st to 10th	Day	13% 11% 3 10% 7% 7% 8% 5% 7% 8% 20%	4667
	11th to 20th	Day	13% 12% 4 9% 6% 6% 5% 5% 6% 6% 29%	5000
	21st to end	Day	14% 13% 5% 12% 7% 7% 7% 5% 5% 6% 17%	5634
April	1st to 10th	Day	15% 11% 3 10% 7% 5% 6% 6% 7% 7% 22%	2400
	11th to 20th	Day	15% 10% 3 8% 6% 5% 6% 5% 6% 6% 30%	2433
	21st to end	Day	16% 10% 3 8% 6% 6% 5% 4 7% 7% 29%	2637
May	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 5% 6% 45%	6320
	11th to 20th	Day	13% 8% 3 9% 5% 5% 5% 4 6% 8% 34%	6277
	21st to end	Day	21% 8% 3 7% 4 4 4 5% 6% 8% 30%	6903
June	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 3 6% 7% 35%	2943
	11th to 20th	Day	14% 8% 3 9% 7% 6% 6% 4 6% 7% 30%	3000
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 7% 9% 39%	3000
July	1st to 10th	Day	6% 3 1 3 3 3 3 2 4 5% 65%	6780
	11th to 20th	Day	5% 4 1 3 2 3 2 2 4 5% 69%	6688
	21st to end	Day	9% 5% 2 5% 3 4 3 3 4 6% 56%	6874
August	1st to 10th	Day	5% 4 2 4 3 3 4 3 5% 8% 59%	4452
	11th to 20th	Day	10% 7% 3 7% 4 4 4 3 5% 8% 44%	4156
	21st to end	Day	13% 7% 3 7% 4 3 4 3 5% 6% 46%	4514
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 8% 50%	2400
	11th to 20th	Day	10% 9% 3 8% 5% 4 4 4 6% 5% 42%	2400
	21st to end	Day	12% 9% 3 7% 5% 5% 4 4 6% 8% 38%	2106
October	1st to 10th	Day	13% 6% 3 8% 6% 6% 6% 5% 8% 8% 31%	3280
	11th to 20th	Day	19% 11% 4 10% 5% 5% 4 3 5% 6% 27%	3242
	21st to end	Day	16% 10% 4 12% 7% 7% 6% 5% 6% 6% 22%	3268
November	1st to 10th	Day	21% 13% 4 8% 6% 6% 5% 6% 6% 6% 18%	1573
	11th to 20th	Day	19% 12% 5% 9% 5% 7% 6% 4 5% 7% 22%	1600
	21st to end	Day	20% 12% 3 10% 6% 6% 5% 5% 6% 8% 19%	1600
December	1st to 10th	Day	12% 11% 3 14% 9% 8% 7% 4 6% 7% 18%	2620
	11th to 20th	Day	20% 14% 5% 14% 8% 6% 6% 4 6% 5% 13%	2828
	21st to end	Day	20% 17% 5% 11% 7% 5% 5% 5% 5% 6% 13%	3498
January	1st to 10th	Night	20% 16% 6% 12% 7% 7% 5% 4 4 5% 13%	6766
	11th to 20th	Night	14% 11% 4 9% 5% 5% 5% 4 5% 9% 29%	6511
	21st to end	Night	18% 12% 3 9% 6% 5% 6% 4 5% 6% 26%	6822
February	1st to 10th	Night	19% 16% 4 9% 6% 6% 5% 4 5% 5% 21%	2600
	11th to 20th	Night	18% 13% 4 10% 6% 6% 5% 3 5% 6% 25%	2532
	21st to end	Night	18% 13% 4 9% 6% 5% 5% 4 6% 7% 23%	2104
March	1st to 10th	Night	11% 12% 4 11% 7% 7% 7% 5% 7% 7% 21%	5211
	11th to 20th	Night	10% 12% 4 8% 6% 5% 6% 4 6% 7% 32%	4999
	21st to end	Night	21% 14% 5% 9% 6% 5% 5% 4 6% 6% 18%	5341
April	1st to 10th	Night	18% 12% 4 8% 6% 5% 6% 4 7% 8% 22%	2000
	11th to 20th	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	1915
	21st to end	Night	18% 9% 3 8% 6% 6% 5% 4 6% 7% 27%	1761
May	1st to 10th	Night	16% 7% 2 7% 4 4 3 3 4 5% 46%	3640
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 6% 37%	3616
	21st to end	Night	16% 8% 3 8% 5% 5% 6% 4 8% 8% 29%	3972
June	1st to 10th	Night	19% 7% 3 8% 5% 5% 5% 4 6% 6% 34%	1452
	11th to 20th	Night	17% 10% 3 8% 6% 6% 4 4 5% 6% 30%	1400
	21st to end	Night	13% 6% 2 5% 5% 4 5% 4 6% 8% 43%	1400
July	1st to 10th	Night	9% 4 2 5% 3 3 2 2 3 4 65%	3180
	11th to 20th	Night	5% 3 1 2 2 2 2 2 3 4 75%	3270
	21st to end	Night	9% 3 2 3 3 3 4 3 5% 7% 59%	3969
August	1st to 10th	Night	5% 2 1 3 3 4 3 3 4 6% 66%	2697
	11th to 20th	Night	13% 6% 2 7% 4 4 3 3 4 5% 49%	2902
	21st to end	Night	14% 6% 2 5% 3 4 4 3 4 6% 50%	3400
September	1st to 10th	Night	9% 5% 1 5% 4 4 4 3 5% 7% 54%	2000
	11th to 20th	Night	12% 8% 3 7% 5% 3 4 4 6% 6% 43%	2000
	21st to end	Night	13% 7% 2 7% 4 3 3 3 5% 6% 45%	2292
October	1st to 10th	Night	13% 5% 3 7% 5% 4 5% 4 7% 7% 40%	3920
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 5% 7% 33%	3873
	21st to end	Night	17% 10% 4 9% 6% 4 5% 4 6% 7% 28%	4294
November	1st to 10th	Night	22% 11% 4 9% 6% 5% 5% 4 5% 6% 23%	2721
	11th to 20th	Night	18% 12% 5% 9% 7% 6% 5% 4 5% 5% 23%	2800
	21st to end	Night	19% 12% 4 10% 7% 5% 5% 4 6% 6% 22%	2800
December	1st to 10th	Night	14% 12% 4 11% 6% 6% 5% 4 6% 7% 25%	4560
	11th to 20th	Night	18% 15% 6% 13% 7% 6% 5% 4 5% 6% 13%	5373
	21st to end	Night	19% 16% 5% 11% 7% 5% 5% 4 5% 6% 17%	7458

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	56% 31% 6% 6% 1	3179
	11th to 20th	Day	39% 32% 11% 15% 4	3407
	21st to end	Day	44% 33% 6% 13% 3	4124
February	1st to 10th	Day	26% 64% 4 3 3 1	1800
	11th to 20th	Day	29% 62% 3 3 3	1764
	21st to end	Day	25% 63% 5% 5% 2	1525
March	1st to 10th	Day	43% 35% 10% 9% 3	4667
	11th to 20th	Day	39% 32% 11% 16% 1	5000
	21st to end	Day	46% 35% 11% 7% 1	5634
April	1st to 10th	Day	23% 64% 7% 6%	2400
	11th to 20th	Day	19% 63% 7% 8% 3 1	2433
	21st to end	Day	20% 63% 6% 8% 2	2637
May	1st to 10th	Day	28% 31% 13% 19% 9% 1	6320
	11th to 20th	Day	37% 34% 16% 11% 2	6280
	21st to end	Day	34% 35% 18% 12% 2	6904
June	1st to 10th	Day	15% 64% 9% 9% 3	2943
	11th to 20th	Day	19% 63% 8% 8% 3	3000
	21st to end	Day	17% 61% 10% 9% 2	3000
July	1st to 10th	Day	15% 27% 16% 16% 18% 8%	6706
	11th to 20th	Day	10% 27% 12% 23% 25% 2	6607
	21st to end	Day	22% 28% 23% 21% 6%	6681
August	1st to 10th	Day	12% 37% 8% 29% 13% 1	4435
	11th to 20th	Day	28% 39% 14% 14% 4 1	4195
	21st to end	Day	29% 37% 8% 18% 8% 1	4513
September	1st to 10th	Day	18% 59% 10% 7% 5% 1	2400
	11th to 20th	Day	20% 59% 7% 7% 7%	2400
	21st to end	Day	20% 58% 9% 9% 4	2106
October	1st to 10th	Day	30% 40% 15% 14% 2	3280
	11th to 20th	Day	42% 36% 9% 9% 4	3242
	21st to end	Day	42% 39% 8% 7% 4	3268
November	1st to 10th	Day	30% 58% 7% 4 1	1573
	11th to 20th	Day	25% 58% 6% 6% 3 2	1600
	21st to end	Day	26% 58% 5% 6% 4	1600
December	1st to 10th	Day	41% 39% 6% 9% 4	2620
	11th to 20th	Day	52% 35% 7% 5% 1	2830
	21st to end	Day	57% 31% 9% 2 1	3498
January	1st to 10th	Night	55% 31% 7% 5% 2	6766
	11th to 20th	Night	34% 31% 13% 19% 3	6550
	21st to end	Night	42% 33% 8% 13% 4	6822
February	1st to 10th	Night	25% 63% 4 3 4 1	2600
	11th to 20th	Night	28% 61% 4 5% 2 1	2534
	21st to end	Night	23% 63% 5% 6% 2	2104
March	1st to 10th	Night	43% 35% 10% 9% 4	5211
	11th to 20th	Night	35% 33% 12% 18% 1	5000
	21st to end	Night	49% 33% 10% 8% 1	5341
April	1st to 10th	Night	24% 63% 7% 6% 1	2000
	11th to 20th	Night	21% 63% 6% 7% 3 1	1915
	21st to end	Night	22% 63% 5% 7% 2	1761
May	1st to 10th	Night	28% 31% 8% 25% 8%	3637
	11th to 20th	Night	34% 33% 17% 12% 3	3616
	21st to end	Night	36% 38% 18% 8% 1	3972
June	1st to 10th	Night	17% 63% 10% 8% 3	1452
	11th to 20th	Night	20% 63% 6% 8% 2	1400
	21st to end	Night	18% 62% 10% 9% 2	1400
July	1st to 10th	Night	21% 26% 13% 18% 15% 8%	3145
	11th to 20th	Night	9% 26% 12% 23% 29% 1	3228
	21st to end	Night	21% 28% 20% 22% 8%	3857
August	1st to 10th	Night	10% 38% 9% 29% 14% 1	2689
	11th to 20th	Night	29% 38% 15% 13% 4 1	2929
	21st to end	Night	27% 37% 9% 19% 8% 1	3400
September	1st to 10th	Night	17% 59% 9% 8% 6% 1	2000
	11th to 20th	Night	21% 59% 6% 8% 6% 1	2000
	21st to end	Night	20% 59% 6% 13% 3	2292
October	1st to 10th	Night	27% 39% 16% 17% 2	3920
	11th to 20th	Night	36% 38% 9% 13% 3	3873
	21st to end	Night	41% 39% 9% 7% 4	4294
November	1st to 10th	Night	29% 57% 7% 7% 1	2721
	11th to 20th	Night	28% 58% 4 5% 3 2	2800
	21st to end	Night	27% 58% 4 5% 5% 1	2800
December	1st to 10th	Night	39% 39% 7% 12% 3	4560
	11th to 20th	Night	50% 36% 7% 5% 1	5370
	21st to end	Night	52% 34% 7% 6% 1	7458

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	27% 18% 23% 20% 10% 3	200
	11th to 20th	Day	19% 16% 27% 22% 15% 2	200
	21st to end	Day	25% 15% 26% 19% 12% 4	275
February	1st to 10th	Day	28% 14% 26% 20% 11% 2	400
	11th to 20th	Day	26% 13% 25% 24% 10% 1	395
	21st to end	Day	22% 18% 23% 22% 13% 1	351
March	1st to 10th	Day	23% 12% 24% 25% 13% 2	500
	11th to 20th	Day	19% 15% 21% 23% 18% 4	500
	21st to end	Day	26% 11% 27% 23% 11% 2	550
April	1st to 10th	Day	18% 18% 22% 24% 14% 3	500
	11th to 20th	Day	16% 14% 25% 25% 14% 5%	492
	21st to end	Day	19% 12% 23% 22% 20% 4	500
May	1st to 10th	Day	12% 10% 20% 32% 23% 3	503
	11th to 20th	Day	17% 7% 18% 28% 22% 9%	528
	21st to end	Day	16% 8% 18% 29% 19% 8%	573
June	1st to 10th	Day	16% 7% 21% 29% 22% 5%	673
	11th to 20th	Day	14% 7% 22% 34% 18% 4	730
	21st to end	Day	13% 3 18% 31% 28% 8%	730
July	1st to 10th	Day	10% 2 15% 32% 30% 10%	721
	11th to 20th	Day	8% 4 15% 29% 34% 11%	678
	21st to end	Day	6% 3 7% 28% 40% 15%	575
August	1st to 10th	Day	8% 3 7% 24% 41% 17%	527
	11th to 20th	Day	8% 2 8% 30% 37% 16%	517
	21st to end	Day	11% 5% 12% 31% 31% 10%	583
September	1st to 10th	Day	12% 3 11% 34% 30% 10%	530
	11th to 20th	Day	15% 5% 12% 28% 33% 7%	530
	21st to end	Day	15% 7% 20% 29% 24% 5%	530
October	1st to 10th	Day	19% 5% 20% 24% 25% 7%	530
	11th to 20th	Day	22% 12% 21% 22% 17% 7%	523
	21st to end	Day	27% 14% 21% 22% 12% 5%	434
November	1st to 10th	Day	31% 15% 21% 18% 14%	203
	11th to 20th	Day	27% 10% 28% 13% 16% 7%	200
	21st to end	Day	24% 16% 25% 18% 8% 10%	200
December	1st to 10th	Day	25% 13% 25% 10% 19% 9%	200
	11th to 20th	Day	33% 18% 27% 15% 7% 1	200
	21st to end	Day	27% 15% 30% 20% 6% 1	220
January	1st to 10th	Night	30% 13% 22% 20% 12% 2	900
	11th to 20th	Night	26% 12% 19% 22% 19% 2	850
	21st to end	Night	27% 8% 20% 22% 18% 5%	880
February	1st to 10th	Night	26% 10% 24% 26% 12% 3	800
	11th to 20th	Night	23% 9% 22% 27% 16% 2	784
	21st to end	Night	26% 14% 21% 23% 12% 4	660
March	1st to 10th	Night	22% 13% 21% 24% 17% 3	800
	11th to 20th	Night	21% 13% 24% 24% 15% 3	800
	21st to end	Night	25% 11% 23% 28% 12% 3	880
April	1st to 10th	Night	24% 10% 18% 28% 17% 3	800
	11th to 20th	Night	18% 8% 25% 31% 15% 4	727
	21st to end	Night	22% 8% 23% 29% 15% 5%	561
May	1st to 10th	Night	14% 7% 18% 34% 23% 4	400
	11th to 20th	Night	16% 7% 14% 27% 30% 6%	394
	21st to end	Night	19% 6% 15% 30% 26% 4	433
June	1st to 10th	Night	20% 6% 19% 31% 23% 2	400
	11th to 20th	Night	18% 6% 21% 35% 17% 4	400
	21st to end	Night	17% 2 15% 32% 30% 4	400
July	1st to 10th	Night	11% 3 14% 32% 35% 4	390
	11th to 20th	Night	9% 3 16% 29% 39% 5%	388
	21st to end	Night	7% 3 10% 32% 42% 7%	438
August	1st to 10th	Night	10% 2 8% 24% 46% 10%	447
	11th to 20th	Night	11% 1 5% 24% 46% 14%	594
	21st to end	Night	11% 3 12% 28% 39% 6%	750
September	1st to 10th	Night	10% 2 9% 31% 36% 10%	800
	11th to 20th	Night	16% 3 12% 23% 31% 15%	800
	21st to end	Night	16% 4 16% 29% 28% 7%	800
October	1st to 10th	Night	18% 5% 14% 25% 29% 9%	800
	11th to 20th	Night	22% 7% 18% 25% 21% 7%	792
	21st to end	Night	22% 10% 21% 26% 17% 4	836
November	1st to 10th	Night	28% 10% 22% 26% 14% 1	780
	11th to 20th	Night	25% 16% 20% 23% 14% 3	800
	21st to end	Night	25% 13% 22% 22% 13% 6%	800
December	1st to 10th	Night	25% 9% 20% 24% 18% 5%	800
	11th to 20th	Night	30% 11% 27% 21% 10% 1	843
	21st to end	Night	32% 15% 25% 19% 8% 1	990

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