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 2024-11-05.

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	14% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	105562
	Night	16% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	102073
Transparency	Day	26% 48% 9% 11% 5% 1	105315
	Night	29% 49% 8% 10% 4 1	101966
Seeing	Day	17% 9% 20% 26% 21% 6%	15262
	Night	21% 9% 19% 26% 21% 5%	23531

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% 18% 5% 12% 8% 5% 5% 4 5% 5% 13%	2219
	11th to 20th	Day	18% 13% 4 10% 8% 5% 5% 5% 5% 7% 18%	2354
	21st to end	Day	24% 15% 5% 12% 8% 7% 5% 4 5% 5% 11%	2872
February	1st to 10th	Day	20% 13% 4 9% 7% 6% 6% 5% 6% 6% 18%	1710
	11th to 20th	Day	18% 14% 4 9% 7% 5% 5% 5% 6% 7% 20%	1665
	21st to end	Day	17% 12% 4 9% 8% 6% 6% 5% 5% 6% 23%	1451
March	1st to 10th	Day	13% 11% 3 9% 7% 7% 8% 5% 7% 9% 22%	3248
	11th to 20th	Day	13% 8% 3 7% 5% 6% 5% 5% 5% 7% 38%	3490
	21st to end	Day	16% 11% 4 10% 7% 7% 6% 4 6% 7% 22%	3934
April	1st to 10th	Day	15% 11% 4 10% 7% 5% 6% 6% 7% 7% 23%	2280
	11th to 20th	Day	15% 10% 3 8% 7% 5% 6% 5% 6% 6% 29%	2310
	21st to end	Day	17% 10% 3 8% 6% 6% 5% 4 7% 7% 28%	2505
May	1st to 10th	Day	16% 7% 3 6% 4 4 4 3 5% 6% 43%	4420
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 9% 43%	4377
	21st to end	Day	25% 9% 3 8% 4 4 4 4 5% 6% 26%	4813
June	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 3 6% 7% 33%	2796
	11th to 20th	Day	15% 8% 3 8% 8% 6% 6% 4 6% 6% 30%	2850
	21st to end	Day	11% 7% 2 7% 4 5% 5% 4 7% 9% 39%	2850
July	1st to 10th	Day	5% 3 1 3 3 3 3 2 3 4 69%	4740
	11th to 20th	Day	6% 4 1 4 3 3 2 2 4 5% 66%	4676
	21st to end	Day	10% 5% 2 4 3 3 3 3 4 5% 57%	4790
August	1st to 10th	Day	4 4 2 4 3 3 4 3 5% 8% 60%	4315
	11th to 20th	Day	9% 7% 3 7% 4 4 4 4 5% 8% 45%	4026
	21st to end	Day	13% 7% 3 7% 4 3 4 3 4 6% 47%	4376
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 8% 51%	2280
	11th to 20th	Day	10% 9% 3 8% 5% 4 4 4 6% 6% 41%	2280
	21st to end	Day	12% 9% 3 8% 5% 5% 4 4 6% 7% 37%	2001
October	1st to 10th	Day	13% 6% 3 8% 6% 6% 6% 5% 8% 8% 31%	3180
	11th to 20th	Day	20% 11% 4 10% 5% 5% 4 3 5% 6% 26%	3161
	21st to end	Day	16% 10% 4 12% 7% 7% 6% 5% 6% 6% 22%	3268
November	1st to 10th	Day	21% 14% 4 8% 7% 5% 5% 5% 6% 6% 18%	1525
	11th to 20th	Day	19% 12% 5% 9% 5% 7% 5% 4 5% 7% 23%	1520
	21st to end	Day	20% 12% 3 10% 6% 6% 5% 5% 6% 7% 19%	1520
December	1st to 10th	Day	15% 15% 4 8% 7% 7% 6% 3 6% 7% 22%	1520
	11th to 20th	Day	17% 14% 5% 15% 8% 7% 6% 5% 6% 5% 13%	1798
	21st to end	Day	18% 17% 5% 11% 7% 6% 6% 5% 5% 6% 14%	2442
January	1st to 10th	Night	19% 16% 6% 11% 7% 7% 6% 4 5% 5% 15%	4726
	11th to 20th	Night	20% 13% 4 9% 5% 5% 4 4 5% 8% 22%	4541
	21st to end	Night	23% 14% 4 10% 7% 5% 6% 5% 5% 6% 14%	4771
February	1st to 10th	Night	19% 16% 4 9% 6% 6% 5% 4 5% 5% 21%	2470
	11th to 20th	Night	18% 13% 4 9% 6% 6% 5% 3 5% 6% 25%	2402
	21st to end	Night	18% 13% 4 9% 6% 5% 5% 4 6% 7% 23%	2002
March	1st to 10th	Night	12% 13% 4 10% 8% 7% 8% 5% 7% 7% 19%	3623
	11th to 20th	Night	10% 9% 3 6% 5% 4 5% 4 6% 8% 40%	3489
	21st to end	Night	21% 14% 5% 9% 6% 5% 5% 4 6% 6% 19%	3719
April	1st to 10th	Night	18% 12% 4 8% 5% 5% 6% 4 7% 8% 22%	1900
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	1819
	21st to end	Night	19% 9% 4 8% 6% 6% 5% 4 6% 7% 26%	1673
May	1st to 10th	Night	16% 7% 2 6% 4 4 3 3 4 6% 46%	2540
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 7% 7% 47%	2516
	21st to end	Night	22% 9% 3 7% 5% 4 5% 4 7% 7% 27%	2762
June	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 6% 32%	1380
	11th to 20th	Night	17% 11% 3 8% 6% 6% 4 4 5% 7% 30%	1330
	21st to end	Night	12% 6% 1 5% 5% 4 5% 4 6% 8% 43%	1330
July	1st to 10th	Night	7% 3 1 3 2 3 2 2 3 4 70%	2220
	11th to 20th	Night	5% 3 1 2 1 2 2 2 3 5% 74%	2282
	21st to end	Night	12% 3 2 3 2 3 3 3 4 5% 59%	2762
August	1st to 10th	Night	5% 2 1 3 3 4 3 3 5% 5% 67%	2614
	11th to 20th	Night	13% 6% 2 7% 4 4 3 3 4 6% 49%	2812
	21st to end	Night	14% 6% 2 5% 3 4 3 3 4 6% 50%	3296
September	1st to 10th	Night	9% 5% 1 5% 4 3 3 3 5% 7% 55%	1900
	11th to 20th	Night	13% 8% 3 7% 5% 3 4 4 5% 6% 42%	1900
	21st to end	Night	13% 6% 2 7% 4 3 3 3 5% 6% 46%	2177
October	1st to 10th	Night	13% 5% 3 7% 5% 4 5% 4 7% 7% 40%	3800
	11th to 20th	Night	20% 10% 3 7% 4 4 4 3 5% 7% 33%	3777
	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% 4 4 5% 6% 23%	2649
	11th to 20th	Night	17% 12% 5% 9% 7% 6% 5% 4 5% 5% 24%	2660
	21st to end	Night	20% 12% 4 9% 7% 5% 5% 4 6% 6% 23%	2660
December	1st to 10th	Night	17% 15% 4 10% 6% 4 4 4 5% 5% 25%	2660
	11th to 20th	Night	17% 16% 5% 13% 7% 5% 5% 5% 6% 7% 14%	3403
	21st to end	Night	21% 17% 4 10% 6% 4 5% 4 5% 6% 17%	5214

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	46% 42% 5% 5% 1	2219
	11th to 20th	Day	35% 41% 12% 11% 2	2377
	21st to end	Day	48% 45% 4 3	2872
February	1st to 10th	Day	25% 67% 3 3 2 1	1710
	11th to 20th	Day	27% 65% 3 3 2	1674
	21st to end	Day	22% 66% 5% 4 3	1451
March	1st to 10th	Day	30% 47% 12% 8% 3	3248
	11th to 20th	Day	23% 43% 12% 21% 1	3490
	21st to end	Day	35% 47% 11% 7% 1	3934
April	1st to 10th	Day	22% 67% 6% 5%	2280
	11th to 20th	Day	19% 66% 6% 6% 2 1	2310
	21st to end	Day	20% 66% 5% 6% 2	2505
May	1st to 10th	Day	23% 43% 10% 16% 8% 1	4420
	11th to 20th	Day	21% 45% 18% 13% 3	4380
	21st to end	Day	36% 45% 13% 5% 1	4814
June	1st to 10th	Day	15% 67% 9% 7% 3	2796
	11th to 20th	Day	17% 66% 7% 7% 3	2850
	21st to end	Day	15% 64% 9% 9% 2	2850
July	1st to 10th	Day	10% 36% 4 17% 21% 12%	4666
	11th to 20th	Day	8% 37% 5% 21% 26% 2	4595
	21st to end	Day	21% 37% 16% 19% 7%	4639
August	1st to 10th	Day	11% 38% 8% 29% 14% 1	4298
	11th to 20th	Day	27% 40% 15% 14% 4 1	4065
	21st to end	Day	29% 37% 8% 18% 7%	4375
September	1st to 10th	Day	17% 61% 9% 7% 5% 1	2280
	11th to 20th	Day	20% 61% 7% 5% 6%	2280
	21st to end	Day	19% 61% 8% 8% 4	2001
October	1st to 10th	Day	29% 41% 14% 14% 2	3180
	11th to 20th	Day	42% 37% 8% 9% 4	3161
	21st to end	Day	42% 39% 8% 7% 4	3268
November	1st to 10th	Day	29% 59% 6% 4 1	1525
	11th to 20th	Day	23% 61% 6% 5% 3 2	1520
	21st to end	Day	24% 60% 4 6% 4	1520
December	1st to 10th	Day	26% 60% 2 8% 3	1520
	11th to 20th	Day	39% 50% 5% 4 1	1800
	21st to end	Day	47% 41% 8% 3 1	2442
January	1st to 10th	Night	45% 42% 6% 4 2	4726
	11th to 20th	Night	33% 39% 11% 14% 2	4580
	21st to end	Night	47% 45% 5% 3 1	4771
February	1st to 10th	Night	24% 66% 4 3 3 1	2470
	11th to 20th	Night	26% 64% 4 4 1 1	2404
	21st to end	Night	22% 65% 5% 6% 2	2002
March	1st to 10th	Night	37% 47% 7% 6% 3	3623
	11th to 20th	Night	18% 45% 14% 22% 1 1	3490
	21st to end	Night	40% 45% 8% 6% 1	3719
April	1st to 10th	Night	21% 66% 7% 5% 1	1900
	11th to 20th	Night	20% 66% 5% 6% 2 1	1819
	21st to end	Night	22% 66% 5% 5% 2	1673
May	1st to 10th	Night	20% 43% 8% 19% 9% 1	2540
	11th to 20th	Night	19% 44% 19% 13% 4	2516
	21st to end	Night	34% 49% 11% 5% 1	2762
June	1st to 10th	Night	17% 66% 9% 7% 2	1380
	11th to 20th	Night	19% 65% 6% 7% 2	1330
	21st to end	Night	16% 64% 10% 8% 2	1330
July	1st to 10th	Night	12% 36% 5% 18% 18% 12%	2185
	11th to 20th	Night	7% 35% 5% 19% 33% 1	2240
	21st to end	Night	22% 38% 11% 18% 10%	2674
August	1st to 10th	Night	9% 38% 8% 29% 14% 1	2606
	11th to 20th	Night	28% 39% 16% 13% 4 1	2839
	21st to end	Night	27% 38% 9% 18% 7%	3296
September	1st to 10th	Night	16% 62% 8% 8% 6% 1	1900
	11th to 20th	Night	21% 61% 5% 6% 6% 1	1900
	21st to end	Night	18% 61% 6% 11% 3	2177
October	1st to 10th	Night	27% 39% 15% 17% 2	3800
	11th to 20th	Night	36% 39% 9% 13% 3	3777
	21st to end	Night	41% 39% 9% 7% 4	4294
November	1st to 10th	Night	28% 58% 6% 6% 1	2649
	11th to 20th	Night	25% 61% 4 5% 3 2	2660
	21st to end	Night	25% 61% 4 5% 5% 1	2660
December	1st to 10th	Night	26% 61% 3 6% 3	2660
	11th to 20th	Night	36% 53% 5% 3 2	3400
	21st to end	Night	43% 44% 6% 6% 2	5214

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	26% 18% 23% 18% 11% 3	180
	11th to 20th	Day	20% 17% 27% 21% 13% 2	180
	21st to end	Day	27% 16% 26% 18% 12% 2	250
February	1st to 10th	Day	29% 14% 26% 19% 11% 1	370
	11th to 20th	Day	26% 14% 26% 24% 9% 1	365
	21st to end	Day	21% 18% 24% 22% 13% 1	325
March	1st to 10th	Day	23% 13% 25% 25% 12% 2	460
	11th to 20th	Day	19% 15% 21% 22% 18% 5%	460
	21st to end	Day	25% 11% 27% 23% 11% 2	506
April	1st to 10th	Day	18% 18% 23% 24% 13% 3	460
	11th to 20th	Day	17% 15% 26% 25% 12% 5%	452
	21st to end	Day	20% 12% 25% 23% 18% 2	460
May	1st to 10th	Day	13% 11% 21% 31% 22% 3	463
	11th to 20th	Day	16% 7% 18% 28% 23% 8%	488
	21st to end	Day	16% 9% 19% 28% 19% 8%	529
June	1st to 10th	Day	17% 8% 23% 29% 20% 4	626
	11th to 20th	Day	15% 7% 23% 34% 17% 3	680
	21st to end	Day	12% 3 19% 32% 27% 7%	680
July	1st to 10th	Day	10% 2 16% 32% 29% 10%	671
	11th to 20th	Day	8% 4 16% 29% 33% 11%	630
	21st to end	Day	6% 3 8% 30% 38% 15%	531
August	1st to 10th	Day	7% 3 8% 25% 41% 16%	487
	11th to 20th	Day	7% 1 8% 30% 36% 17%	477
	21st to end	Day	11% 5% 13% 31% 29% 10%	539
September	1st to 10th	Day	12% 3 12% 37% 28% 9%	490
	11th to 20th	Day	15% 5% 13% 29% 31% 7%	490
	21st to end	Day	15% 8% 20% 28% 24% 5%	490
October	1st to 10th	Day	19% 5% 21% 23% 25% 7%	490
	11th to 20th	Day	23% 12% 21% 21% 17% 7%	490
	21st to end	Day	27% 14% 21% 22% 12% 5%	434
November	1st to 10th	Day	32% 16% 22% 18% 13%	191
	11th to 20th	Day	26% 10% 27% 13% 16% 7%	180
	21st to end	Day	25% 15% 25% 18% 7% 10%	180
December	1st to 10th	Day	26% 14% 27% 10% 16% 8%	180
	11th to 20th	Day	31% 19% 27% 14% 7% 1	180
	21st to end	Day	27% 16% 30% 20% 6% 2	198
January	1st to 10th	Night	30% 14% 22% 19% 12% 2	850
	11th to 20th	Night	28% 12% 19% 21% 19% 2	805
	21st to end	Night	27% 9% 20% 22% 18% 4	836
February	1st to 10th	Night	26% 10% 23% 27% 12% 3	760
	11th to 20th	Night	23% 9% 23% 27% 17% 2	744
	21st to end	Night	25% 14% 21% 22% 13% 4	628
March	1st to 10th	Night	23% 13% 21% 23% 16% 3	760
	11th to 20th	Night	21% 13% 23% 24% 16% 3	760
	21st to end	Night	24% 11% 23% 28% 12% 3	836
April	1st to 10th	Night	24% 11% 18% 28% 17% 3	760
	11th to 20th	Night	18% 8% 24% 31% 15% 4	691
	21st to end	Night	22% 8% 24% 29% 14% 4	533
May	1st to 10th	Night	13% 8% 18% 34% 23% 4	380
	11th to 20th	Night	16% 7% 14% 27% 31% 5%	374
	21st to end	Night	19% 6% 15% 28% 27% 5%	411
June	1st to 10th	Night	21% 6% 19% 31% 21% 2	380
	11th to 20th	Night	18% 7% 21% 34% 17% 4	380
	21st to end	Night	16% 2 16% 33% 30% 4	380
July	1st to 10th	Night	11% 3 13% 31% 37% 5%	370
	11th to 20th	Night	9% 3 16% 28% 39% 5%	368
	21st to end	Night	7% 3 10% 31% 41% 7%	416
August	1st to 10th	Night	9% 2 7% 24% 46% 11%	424
	11th to 20th	Night	10% 1 5% 24% 47% 15%	564
	21st to end	Night	11% 3 13% 28% 39% 6%	712
September	1st to 10th	Night	10% 2 10% 33% 36% 10%	760
	11th to 20th	Night	16% 3 12% 23% 31% 14%	760
	21st to end	Night	16% 4 16% 28% 28% 7%	760
October	1st to 10th	Night	18% 6% 14% 24% 29% 10%	760
	11th to 20th	Night	22% 7% 17% 24% 22% 7%	760
	21st to end	Night	22% 10% 21% 26% 17% 4	836
November	1st to 10th	Night	27% 10% 23% 26% 14% 1	760
	11th to 20th	Night	25% 16% 19% 23% 15% 3	760
	21st to end	Night	25% 13% 22% 22% 13% 6%	760
December	1st to 10th	Night	26% 9% 21% 24% 16% 4	760
	11th to 20th	Night	30% 11% 27% 21% 10% 1	798
	21st to end	Night	32% 15% 24% 19% 9% 1	935

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