OAS Port Gamble Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the OAS Port Gamble Site 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	14% 10% 4 9% 6% 5% 5% 4 6% 7% 31%	117737
	Night	16% 11% 4 9% 6% 5% 5% 4 5% 6% 29%	119918
Transparency	Day	50% 8% 14% 18% 8% 1	117492
	Night	54% 8% 13% 17% 7% 1	119809
Seeing	Day	18% 10% 21% 26% 19% 5%	15745
	Night	21% 9% 19% 26% 20% 4	24241

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	21% 16% 6% 14% 8% 6% 5% 4 5% 4 11%	3179
	11th to 20th	Day	13% 9% 5% 13% 8% 6% 6% 4 7% 9% 21%	3499
	21st to end	Day	17% 14% 5% 11% 7% 6% 5% 4 5% 5% 21%	4138
February	1st to 10th	Day	18% 14% 5% 9% 7% 6% 6% 5% 7% 7% 17%	1800
	11th to 20th	Day	15% 13% 5% 11% 7% 6% 4 5% 7% 6% 19%	1755
	21st to end	Day	15% 14% 4 11% 8% 5% 6% 4 6% 5% 23%	1535
March	1st to 10th	Day	12% 12% 5% 11% 7% 7% 8% 5% 7% 7% 18%	4650
	11th to 20th	Day	13% 13% 4 10% 7% 5% 6% 4 6% 5% 27%	5000
	21st to end	Day	14% 11% 4 13% 9% 8% 8% 5% 7% 7% 15%	5651
April	1st to 10th	Day	16% 14% 3 10% 7% 6% 5% 5% 6% 7% 20%	2400
	11th to 20th	Day	16% 11% 4 9% 7% 5% 5% 4 6% 7% 28%	2417
	21st to end	Day	14% 11% 3 9% 6% 6% 6% 5% 6% 8% 26%	2638
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 3 6% 6% 38%	4560
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 6% 8% 37%	4380
	21st to end	Day	23% 12% 3 8% 6% 6% 5% 4 5% 7% 21%	4813
June	1st to 10th	Day	17% 8% 2 9% 6% 6% 6% 5% 6% 7% 29%	2755
	11th to 20th	Day	16% 11% 3 9% 6% 7% 6% 5% 6% 7% 23%	2850
	21st to end	Day	10% 7% 2 7% 5% 4 5% 5% 7% 7% 39%	2850
July	1st to 10th	Day	6% 5% 2 4 3 3 3 2 4 5% 63%	4740
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 5% 64%	4612
	21st to end	Day	11% 6% 2 5% 3 3 4 3 4 7% 53%	4790
August	1st to 10th	Day	5% 4 1 5% 3 3 4 4 5% 7% 60%	4314
	11th to 20th	Day	8% 9% 3 7% 4 4 5% 4 6% 8% 41%	4026
	21st to end	Day	14% 8% 3 8% 4 4 4 3 5% 5% 43%	4351
September	1st to 10th	Day	9% 6% 2 5% 3 4 4 3 5% 7% 52%	2280
	11th to 20th	Day	11% 8% 3 8% 5% 5% 4 4 6% 7% 39%	2280
	21st to end	Day	12% 8% 2 7% 6% 5% 4 4 6% 7% 39%	2004
October	1st to 10th	Day	11% 8% 3 8% 5% 5% 5% 5% 7% 10% 34%	3180
	11th to 20th	Day	20% 11% 4 10% 6% 5% 5% 3 5% 7% 25%	3161
	21st to end	Day	15% 12% 5% 13% 7% 6% 6% 4 7% 6% 20%	3266
November	1st to 10th	Day	19% 13% 5% 11% 6% 7% 5% 5% 6% 6% 18%	1589
	11th to 20th	Day	18% 13% 5% 10% 7% 5% 4 5% 6% 7% 21%	1600
	21st to end	Day	21% 12% 4 10% 6% 5% 5% 4 6% 8% 19%	1600
December	1st to 10th	Day	14% 9% 4 14% 8% 8% 7% 5% 7% 7% 18%	2620
	11th to 20th	Day	20% 15% 6% 14% 7% 6% 6% 4 5% 5% 12%	2956
	21st to end	Day	20% 15% 5% 13% 8% 7% 5% 4 6% 5% 12%	3498
January	1st to 10th	Night	19% 17% 5% 13% 8% 7% 6% 4 5% 5% 11%	6766
	11th to 20th	Night	13% 9% 4 10% 6% 6% 7% 4 6% 9% 26%	6395
	21st to end	Night	19% 11% 4 9% 6% 5% 6% 4 6% 6% 24%	6820
February	1st to 10th	Night	18% 16% 4 10% 6% 5% 5% 4 5% 7% 20%	2600
	11th to 20th	Night	15% 13% 5% 10% 6% 6% 5% 5% 6% 5% 25%	2532
	21st to end	Night	15% 16% 4 8% 6% 5% 4 5% 6% 7% 24%	2093
March	1st to 10th	Night	12% 12% 5% 11% 7% 7% 6% 6% 7% 8% 20%	5230
	11th to 20th	Night	12% 12% 4 10% 6% 5% 5% 4 6% 6% 31%	4999
	21st to end	Night	18% 15% 5% 11% 6% 6% 6% 4 6% 6% 17%	5324
April	1st to 10th	Night	16% 12% 4 10% 7% 5% 6% 4 6% 7% 21%	2000
	11th to 20th	Night	16% 9% 4 7% 6% 5% 5% 4 6% 7% 31%	1930
	21st to end	Night	17% 10% 3 9% 6% 5% 5% 4 6% 7% 27%	1760
May	1st to 10th	Night	13% 9% 2 5% 4 4 4 4 5% 5% 44%	2620
	11th to 20th	Night	14% 7% 2 6% 5% 5% 4 3 5% 6% 44%	2519
	21st to end	Night	26% 10% 4 8% 5% 5% 4 3 4 6% 25%	2762
June	1st to 10th	Night	17% 9% 2 9% 5% 5% 5% 4 5% 7% 32%	1420
	11th to 20th	Night	19% 11% 4 8% 5% 5% 5% 4 6% 6% 27%	1330
	21st to end	Night	10% 6% 2 7% 4 5% 4 4 6% 9% 42%	1330
July	1st to 10th	Night	9% 5% 2 4 3 3 3 2 3 4 63%	2220
	11th to 20th	Night	4 2 1 3 2 2 3 2 4 5% 72%	2343
	21st to end	Night	15% 5% 2 5% 3 3 3 3 4 5% 53%	2762
August	1st to 10th	Night	4 3 1 3 2 3 4 3 5% 6% 66%	2615
	11th to 20th	Night	11% 7% 3 7% 4 3 3 3 5% 7% 48%	2812
	21st to end	Night	15% 7% 3 6% 3 3 3 3 4 5% 48%	3321
September	1st to 10th	Night	7% 5% 2 6% 3 4 3 3 5% 7% 55%	1900
	11th to 20th	Night	12% 7% 3 8% 5% 5% 4 3 5% 6% 41%	1900
	21st to end	Night	12% 7% 3 8% 5% 4 4 3 6% 6% 41%	2174
October	1st to 10th	Night	11% 7% 3 7% 5% 5% 6% 4 7% 7% 37%	3800
	11th to 20th	Night	20% 9% 4 8% 5% 5% 4 4 5% 6% 31%	3777
	21st to end	Night	19% 10% 4 11% 5% 5% 5% 4 5% 6% 25%	4296
November	1st to 10th	Night	22% 10% 3 8% 5% 5% 6% 4 5% 6% 25%	2706
	11th to 20th	Night	20% 11% 4 9% 7% 5% 5% 5% 6% 6% 23%	2800
	21st to end	Night	16% 13% 4 10% 7% 6% 6% 4 7% 7% 21%	2800
December	1st to 10th	Night	13% 12% 3 11% 7% 7% 7% 5% 6% 6% 23%	4560
	11th to 20th	Night	23% 13% 5% 13% 7% 6% 5% 4 6% 6% 13%	5244
	21st to end	Night	19% 16% 6% 12% 7% 6% 6% 4 5% 6% 13%	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	75% 9% 6% 8% 2	3179
	11th to 20th	Day	59% 7% 14% 16% 4 1	3523
	21st to end	Day	64% 6% 10% 13% 6%	4138
February	1st to 10th	Day	67% 9% 11% 8% 4 1	1800
	11th to 20th	Day	67% 8% 9% 9% 7% 1	1764
	21st to end	Day	63% 8% 13% 11% 5%	1535
March	1st to 10th	Day	63% 8% 13% 12% 4	4650
	11th to 20th	Day	55% 9% 12% 20% 3	5000
	21st to end	Day	64% 10% 14% 11% 1	5651
April	1st to 10th	Day	60% 10% 17% 12% 1	2400
	11th to 20th	Day	54% 8% 14% 16% 7% 2	2417
	21st to end	Day	54% 8% 16% 16% 6% 1	2638
May	1st to 10th	Day	42% 9% 15% 24% 9% 1	4560
	11th to 20th	Day	39% 10% 24% 20% 6%	4383
	21st to end	Day	59% 10% 18% 10% 2	4814
June	1st to 10th	Day	48% 9% 20% 15% 6% 1	2755
	11th to 20th	Day	54% 8% 20% 14% 4	2850
	21st to end	Day	39% 9% 22% 21% 7% 1	2850
July	1st to 10th	Day	22% 5% 10% 24% 25% 14%	4666
	11th to 20th	Day	18% 5% 10% 31% 32% 4	4531
	21st to end	Day	27% 5% 18% 32% 15% 2	4639
August	1st to 10th	Day	20% 6% 15% 39% 18% 2	4297
	11th to 20th	Day	36% 9% 21% 24% 8% 2	4065
	21st to end	Day	42% 5% 13% 28% 11%	4350
September	1st to 10th	Day	32% 6% 18% 26% 13% 5%	2280
	11th to 20th	Day	44% 8% 15% 17% 15% 1	2280
	21st to end	Day	46% 7% 18% 18% 9% 2	2004
October	1st to 10th	Day	42% 9% 19% 25% 5%	3180
	11th to 20th	Day	61% 6% 13% 14% 6%	3161
	21st to end	Day	63% 8% 12% 11% 6% 1	3266
November	1st to 10th	Day	65% 9% 12% 10% 4	1589
	11th to 20th	Day	65% 7% 10% 12% 5% 2	1600
	21st to end	Day	66% 9% 9% 11% 6%	1600
December	1st to 10th	Day	62% 8% 10% 14% 6%	2620
	11th to 20th	Day	73% 8% 9% 8% 1	2959
	21st to end	Day	73% 8% 11% 7% 2	3498
January	1st to 10th	Night	76% 8% 7% 7% 2	6766
	11th to 20th	Night	50% 10% 14% 22% 4	6433
	21st to end	Night	59% 8% 11% 17% 5%	6820
February	1st to 10th	Night	64% 8% 11% 11% 5% 1	2600
	11th to 20th	Night	61% 7% 12% 11% 7% 1	2534
	21st to end	Night	60% 7% 13% 14% 4	2093
March	1st to 10th	Night	58% 11% 12% 14% 5%	5230
	11th to 20th	Night	51% 10% 15% 21% 2	5000
	21st to end	Night	66% 10% 13% 10% 1	5324
April	1st to 10th	Night	58% 12% 14% 14% 2	2000
	11th to 20th	Night	52% 7% 15% 15% 8% 2	1930
	21st to end	Night	55% 9% 15% 14% 6% 1	1760
May	1st to 10th	Night	42% 7% 14% 26% 11% 1	2620
	11th to 20th	Night	39% 10% 24% 19% 7% 1	2519
	21st to end	Night	59% 15% 15% 10% 2	2762
June	1st to 10th	Night	50% 10% 19% 15% 5% 1	1420
	11th to 20th	Night	57% 9% 16% 15% 3	1330
	21st to end	Night	40% 6% 24% 20% 7% 3	1330
July	1st to 10th	Night	29% 4 9% 22% 21% 15%	2185
	11th to 20th	Night	16% 4 10% 30% 38% 2	2301
	21st to end	Night	31% 6% 13% 28% 19% 3	2674
August	1st to 10th	Night	19% 4 14% 39% 21% 2	2607
	11th to 20th	Night	34% 7% 24% 23% 10% 2	2839
	21st to end	Night	39% 6% 15% 28% 11% 1	3321
September	1st to 10th	Night	29% 7% 17% 27% 15% 5%	1900
	11th to 20th	Night	44% 7% 12% 18% 15% 4	1900
	21st to end	Night	45% 7% 13% 25% 10% 1	2174
October	1st to 10th	Night	42% 8% 19% 27% 5%	3800
	11th to 20th	Night	55% 6% 14% 20% 5%	3777
	21st to end	Night	61% 8% 12% 13% 6%	4296
November	1st to 10th	Night	62% 7% 15% 12% 4 1	2706
	11th to 20th	Night	64% 7% 12% 11% 5% 2	2800
	21st to end	Night	64% 7% 11% 11% 6%	2800
December	1st to 10th	Night	60% 8% 11% 16% 5%	4560
	11th to 20th	Night	73% 8% 11% 6% 2	5240
	21st to end	Night	72% 9% 9% 8% 2	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	29% 15% 25% 19% 10% 3	200
	11th to 20th	Day	25% 18% 28% 11% 18% 2	200
	21st to end	Day	30% 15% 25% 16% 11% 3	276
February	1st to 10th	Day	27% 20% 24% 18% 10% 1	400
	11th to 20th	Day	30% 15% 25% 20% 8% 2	395
	21st to end	Day	23% 20% 25% 18% 12% 1	357
March	1st to 10th	Day	24% 16% 25% 23% 11% 2	500
	11th to 20th	Day	24% 17% 19% 21% 13% 5%	500
	21st to end	Day	22% 16% 29% 21% 10% 2	550
April	1st to 10th	Day	24% 15% 21% 24% 12% 3	500
	11th to 20th	Day	18% 16% 29% 21% 11% 4	492
	21st to end	Day	19% 11% 25% 28% 14% 3	500
May	1st to 10th	Day	14% 7% 25% 34% 17% 3	503
	11th to 20th	Day	18% 6% 19% 30% 18% 9%	489
	21st to end	Day	20% 8% 17% 28% 19% 8%	529
June	1st to 10th	Day	18% 8% 24% 29% 17% 4	585
	11th to 20th	Day	18% 7% 23% 31% 18% 3	680
	21st to end	Day	12% 5% 19% 35% 22% 6%	680
July	1st to 10th	Day	10% 4 19% 32% 28% 7%	671
	11th to 20th	Day	6% 5% 16% 32% 33% 9%	592
	21st to end	Day	8% 3 11% 32% 35% 11%	531
August	1st to 10th	Day	7% 3 10% 31% 35% 14%	487
	11th to 20th	Day	5% 2 11% 35% 34% 13%	477
	21st to end	Day	12% 4 17% 33% 25% 8%	539
September	1st to 10th	Day	13% 6% 18% 33% 25% 5%	490
	11th to 20th	Day	15% 7% 18% 29% 28% 4	490
	21st to end	Day	16% 9% 23% 29% 21% 2	490
October	1st to 10th	Day	20% 9% 22% 23% 21% 6%	490
	11th to 20th	Day	23% 12% 22% 21% 18% 4	490
	21st to end	Day	24% 19% 19% 23% 13% 2	432
November	1st to 10th	Day	34% 11% 24% 19% 10%	210
	11th to 20th	Day	27% 20% 24% 13% 11% 7%	200
	21st to end	Day	27% 15% 26% 15% 10% 8%	200
December	1st to 10th	Day	25% 17% 19% 14% 19% 8%	200
	11th to 20th	Day	29% 21% 30% 14% 6% 2	200
	21st to end	Day	30% 18% 25% 19% 6% 2	220
January	1st to 10th	Night	29% 14% 24% 19% 11% 3	900
	11th to 20th	Night	26% 13% 17% 22% 19% 2	823
	21st to end	Night	25% 12% 19% 23% 16% 4	880
February	1st to 10th	Night	27% 14% 22% 23% 11% 3	800
	11th to 20th	Night	23% 12% 22% 25% 17% 1	784
	21st to end	Night	25% 13% 23% 24% 13% 2	660
March	1st to 10th	Night	21% 14% 23% 26% 14% 2	800
	11th to 20th	Night	24% 15% 20% 25% 15% 2	800
	21st to end	Night	24% 12% 25% 26% 11% 1	880
April	1st to 10th	Night	25% 9% 20% 28% 17% 2	800
	11th to 20th	Night	21% 8% 23% 27% 17% 3	742
	21st to end	Night	22% 7% 20% 32% 16% 3	560
May	1st to 10th	Night	16% 6% 19% 36% 20% 4	400
	11th to 20th	Night	18% 3 17% 28% 31% 3	375
	21st to end	Night	23% 6% 14% 29% 23% 5%	411
June	1st to 10th	Night	22% 5% 19% 32% 21% 2	380
	11th to 20th	Night	20% 11% 16% 32% 19% 2	380
	21st to end	Night	15% 3 14% 36% 28% 4	380
July	1st to 10th	Night	13% 3 10% 31% 38% 5%	370
	11th to 20th	Night	8% 3 12% 34% 40% 4	368
	21st to end	Night	10% 3 6% 31% 45% 6%	416
August	1st to 10th	Night	10% 2 6% 26% 47% 10%	425
	11th to 20th	Night	8% 2 6% 27% 45% 12%	564
	21st to end	Night	11% 3 13% 32% 36% 5%	724
September	1st to 10th	Night	11% 2 10% 37% 33% 7%	760
	11th to 20th	Night	17% 4 13% 23% 33% 11%	760
	21st to end	Night	15% 6% 19% 29% 26% 5%	760
October	1st to 10th	Night	15% 7% 18% 28% 26% 6%	760
	11th to 20th	Night	21% 8% 18% 26% 22% 5%	760
	21st to end	Night	23% 10% 22% 27% 16% 3	836
November	1st to 10th	Night	28% 12% 22% 25% 12% 1	780
	11th to 20th	Night	27% 12% 23% 22% 14% 3	800
	21st to end	Night	24% 14% 22% 21% 15% 4	800
December	1st to 10th	Night	24% 11% 19% 25% 18% 3	800
	11th to 20th	Night	26% 16% 23% 22% 11% 2	813
	21st to end	Night	32% 16% 24% 19% 9% 1	990

Page generated at 2025-05-08 06:33:56. Version ac07

Copyright © 2025 Attilla Danko.