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 2026-09-14.

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 9% 5% 5% 5% 4 6% 7% 33%	153759
	Night	15% 10% 4 9% 5% 5% 5% 4 5% 6% 31%	146069
Transparency	Day	47% 7% 16% 20% 8% 1	153473
	Night	52% 8% 14% 18% 7% 1	145937
Seeing	Day	19% 10% 21% 26% 19% 5%	17580
	Night	22% 9% 18% 27% 20% 4	25800

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% 17% 6% 14% 8% 6% 5% 4 5% 4 10%	4125
	11th to 20th	Day	12% 10% 5% 12% 8% 6% 6% 5% 7% 8% 23%	4571
	21st to end	Day	17% 14% 5% 12% 6% 6% 5% 4 5% 5% 21%	5362
February	1st to 10th	Day	18% 14% 5% 9% 7% 6% 6% 5% 7% 7% 16%	1890
	11th to 20th	Day	15% 13% 5% 11% 7% 6% 5% 5% 7% 7% 19%	1845
	21st to end	Day	15% 14% 4 11% 8% 5% 6% 4 6% 5% 23%	1610
March	1st to 10th	Day	12% 12% 5% 11% 7% 7% 8% 5% 7% 7% 18%	4753
	11th to 20th	Day	13% 13% 4 10% 6% 5% 6% 4 6% 5% 26%	5110
	21st to end	Day	14% 11% 4 13% 9% 8% 8% 5% 7% 7% 15%	5775
April	1st to 10th	Day	16% 13% 3 10% 7% 6% 5% 5% 6% 7% 21%	2520
	11th to 20th	Day	16% 10% 4 9% 7% 5% 5% 4 7% 7% 27%	2539
	21st to end	Day	14% 11% 3 8% 6% 6% 6% 5% 6% 7% 27%	2770
May	1st to 10th	Day	11% 8% 3 7% 5% 5% 5% 4 6% 6% 41%	8195
	11th to 20th	Day	14% 11% 4 10% 6% 5% 5% 5% 7% 8% 27%	8160
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 7% 10% 30%	8993
June	1st to 10th	Day	16% 8% 2 9% 6% 6% 6% 4 6% 7% 31%	3045
	11th to 20th	Day	15% 10% 3 9% 6% 7% 6% 5% 6% 7% 25%	3150
	21st to end	Day	10% 8% 3 8% 5% 4 5% 5% 7% 7% 37%	3150
July	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 6% 53%	8820
	11th to 20th	Day	5% 4 2 4 3 3 3 3 4 5% 63%	8580
	21st to end	Day	8% 6% 2 5% 3 4 4 3 5% 8% 51%	8916
August	1st to 10th	Day	5% 4 1 5% 3 3 3 3 4 6% 62%	6312
	11th to 20th	Day	6% 7% 3 7% 4 4 4 4 6% 8% 47%	5923
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 7% 40%	6329
September	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 5% 8% 50%	2520
	11th to 20th	Day	11% 8% 3 8% 5% 5% 4 4 6% 7% 40%	2437
	21st to end	Day	12% 8% 3 7% 6% 5% 4 4 6% 7% 39%	2109
October	1st to 10th	Day	11% 8% 3 8% 5% 5% 5% 5% 7% 10% 34%	3280
	11th to 20th	Day	20% 11% 4 10% 5% 5% 5% 3 5% 7% 25%	3261
	21st to end	Day	16% 12% 5% 13% 7% 6% 6% 4 7% 7% 19%	3370
November	1st to 10th	Day	19% 13% 5% 11% 6% 7% 6% 5% 6% 6% 17%	1670
	11th to 20th	Day	20% 13% 5% 10% 6% 5% 4 4 6% 7% 20%	1680
	21st to end	Day	22% 12% 4 10% 6% 5% 5% 4 6% 8% 18%	1680
December	1st to 10th	Day	14% 10% 4 14% 8% 8% 7% 5% 7% 7% 17%	2700
	11th to 20th	Day	20% 15% 6% 14% 7% 6% 6% 4 5% 5% 12%	3034
	21st to end	Day	20% 15% 5% 13% 8% 7% 5% 4 6% 5% 12%	3575
January	1st to 10th	Night	20% 17% 6% 13% 8% 7% 6% 4 5% 5% 10%	8770
	11th to 20th	Night	14% 11% 4 9% 5% 5% 6% 4 5% 7% 30%	8323
	21st to end	Night	18% 10% 4 9% 6% 6% 6% 4 6% 7% 24%	8823
February	1st to 10th	Night	17% 16% 4 10% 6% 5% 5% 4 5% 8% 20%	2730
	11th to 20th	Night	15% 13% 5% 10% 6% 6% 5% 4 6% 6% 25%	2662
	21st to end	Night	15% 16% 4 9% 6% 5% 4 5% 6% 7% 23%	2194
March	1st to 10th	Night	12% 12% 5% 11% 7% 7% 6% 6% 7% 8% 20%	5347
	11th to 20th	Night	13% 12% 5% 10% 6% 5% 5% 4 6% 5% 30%	5109
	21st to end	Night	18% 14% 5% 11% 6% 6% 6% 4 6% 6% 17%	5442
April	1st to 10th	Night	16% 12% 4 10% 7% 5% 6% 4 6% 7% 22%	2100
	11th to 20th	Night	16% 9% 4 8% 6% 5% 5% 4 6% 7% 30%	2028
	21st to end	Night	17% 10% 3 9% 6% 5% 4 4 6% 7% 29%	1848
May	1st to 10th	Night	10% 8% 2 6% 4 4 5% 4 5% 5% 47%	4724
	11th to 20th	Night	17% 11% 4 10% 5% 5% 5% 4 5% 6% 29%	4700
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	5178
June	1st to 10th	Night	17% 9% 2 9% 5% 5% 5% 4 5% 7% 33%	1570
	11th to 20th	Night	18% 10% 4 9% 5% 5% 5% 4 6% 7% 29%	1470
	21st to end	Night	11% 6% 2 7% 4 5% 4 4 6% 9% 41%	1470
July	1st to 10th	Night	13% 6% 2 6% 3 3 3 2 4 4 53%	4140
	11th to 20th	Night	6% 3 1 3 2 2 3 2 4 5% 69%	4375
	21st to end	Night	9% 4 2 5% 3 4 4 4 6% 7% 52%	5152
August	1st to 10th	Night	5% 3 1 3 2 2 3 2 4 5% 70%	3840
	11th to 20th	Night	8% 5% 2 6% 4 3 3 3 5% 8% 53%	4142
	21st to end	Night	15% 8% 3 6% 3 3 3 3 4 5% 46%	4829
September	1st to 10th	Night	7% 5% 2 6% 3 4 3 3 5% 7% 54%	2100
	11th to 20th	Night	12% 8% 3 8% 5% 5% 4 3 5% 6% 41%	2030
	21st to end	Night	12% 7% 3 8% 5% 4 4 3 6% 6% 41%	2289
October	1st to 10th	Night	11% 7% 3 7% 5% 5% 6% 4 7% 7% 37%	3920
	11th to 20th	Night	20% 9% 4 8% 5% 5% 4 4 5% 6% 32%	3897
	21st to end	Night	19% 10% 4 11% 5% 5% 5% 4 5% 6% 25%	4434
November	1st to 10th	Night	22% 11% 3 8% 5% 5% 6% 5% 5% 6% 24%	2845
	11th to 20th	Night	21% 11% 4 9% 7% 5% 5% 5% 5% 6% 22%	2940
	21st to end	Night	17% 13% 4 10% 7% 6% 6% 4 6% 6% 20%	2940
December	1st to 10th	Night	14% 12% 4 11% 7% 7% 7% 4 6% 6% 23%	4700
	11th to 20th	Night	23% 13% 5% 12% 7% 6% 5% 4 6% 6% 13%	5385
	21st to end	Night	19% 16% 6% 12% 7% 6% 6% 4 5% 6% 13%	7623

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	77% 8% 6% 7% 1	4125
	11th to 20th	Day	57% 7% 12% 16% 8%	4595
	21st to end	Day	64% 6% 10% 15% 6%	5363
February	1st to 10th	Day	67% 9% 11% 8% 4 1	1890
	11th to 20th	Day	67% 7% 9% 10% 7%	1854
	21st to end	Day	63% 8% 13% 11% 5%	1610
March	1st to 10th	Day	63% 8% 13% 13% 4	4753
	11th to 20th	Day	56% 9% 12% 19% 3	5110
	21st to end	Day	64% 10% 14% 11% 1	5775
April	1st to 10th	Day	59% 10% 18% 13% 1	2520
	11th to 20th	Day	54% 8% 14% 16% 7% 2	2539
	21st to end	Day	54% 8% 15% 16% 6% 1	2770
May	1st to 10th	Day	39% 7% 18% 26% 9% 1	8195
	11th to 20th	Day	49% 10% 19% 18% 3	8163
	21st to end	Day	45% 9% 26% 17% 3	8994
June	1st to 10th	Day	46% 9% 20% 17% 6% 1	3045
	11th to 20th	Day	51% 8% 19% 17% 5%	3150
	21st to end	Day	41% 8% 22% 21% 7% 1	3150
July	1st to 10th	Day	28% 7% 18% 21% 19% 7%	8746
	11th to 20th	Day	19% 4 13% 32% 27% 4	8499
	21st to end	Day	26% 5% 24% 32% 12% 1	8723
August	1st to 10th	Day	20% 5% 15% 33% 22% 4	6295
	11th to 20th	Day	31% 8% 26% 28% 6% 1	5962
	21st to end	Day	40% 7% 17% 28% 8%	6328
September	1st to 10th	Day	33% 7% 19% 25% 12% 4	2520
	11th to 20th	Day	43% 8% 15% 18% 14% 1	2437
	21st to end	Day	46% 7% 18% 19% 9% 2	2109
October	1st to 10th	Day	42% 9% 19% 25% 5%	3280
	11th to 20th	Day	60% 6% 13% 14% 6%	3261
	21st to end	Day	63% 8% 12% 11% 6% 1	3370
November	1st to 10th	Day	65% 9% 13% 10% 4	1670
	11th to 20th	Day	65% 7% 11% 11% 5% 1	1680
	21st to end	Day	66% 9% 9% 10% 5%	1680
December	1st to 10th	Day	62% 8% 10% 13% 6%	2700
	11th to 20th	Day	73% 8% 9% 8% 1	3037
	21st to end	Day	72% 9% 11% 7% 1	3575
January	1st to 10th	Night	77% 8% 7% 7% 1	8770
	11th to 20th	Night	50% 8% 12% 22% 8%	8361
	21st to end	Night	58% 7% 11% 18% 6%	8823
February	1st to 10th	Night	64% 8% 11% 11% 6% 1	2730
	11th to 20th	Night	61% 7% 12% 12% 7% 1	2664
	21st to end	Night	61% 7% 13% 14% 4	2194
March	1st to 10th	Night	58% 11% 12% 14% 5%	5347
	11th to 20th	Night	52% 10% 14% 21% 2	5110
	21st to end	Night	66% 10% 13% 11% 1	5442
April	1st to 10th	Night	57% 12% 15% 14% 2	2100
	11th to 20th	Night	53% 8% 15% 15% 8% 2	2028
	21st to end	Night	53% 9% 16% 14% 7% 1	1848
May	1st to 10th	Night	39% 6% 13% 32% 10% 1	4721
	11th to 20th	Night	52% 10% 16% 18% 4	4700
	21st to end	Night	51% 12% 21% 15% 2	5182
June	1st to 10th	Night	49% 9% 19% 16% 6% 1	1570
	11th to 20th	Night	54% 8% 15% 18% 3	1470
	21st to end	Night	40% 6% 23% 20% 7% 3	1470
July	1st to 10th	Night	34% 6% 16% 20% 16% 8%	4105
	11th to 20th	Night	17% 4 14% 29% 34% 2	4333
	21st to end	Night	27% 6% 21% 31% 12% 3	5040
August	1st to 10th	Night	17% 3 15% 39% 21% 5%	3832
	11th to 20th	Night	28% 6% 30% 26% 8% 2	4169
	21st to end	Night	40% 6% 17% 27% 9% 1	4829
September	1st to 10th	Night	29% 7% 19% 27% 13% 4	2100
	11th to 20th	Night	44% 7% 12% 19% 15% 4	2030
	21st to end	Night	45% 7% 13% 25% 10% 1	2289
October	1st to 10th	Night	42% 8% 19% 27% 5%	3920
	11th to 20th	Night	54% 6% 14% 20% 6%	3897
	21st to end	Night	60% 8% 12% 13% 6%	4434
November	1st to 10th	Night	62% 7% 14% 12% 4 1	2845
	11th to 20th	Night	65% 7% 11% 10% 4 2	2940
	21st to end	Night	65% 7% 11% 11% 6%	2940
December	1st to 10th	Night	61% 8% 11% 16% 5%	4700
	11th to 20th	Night	73% 8% 10% 7% 2	5381
	21st to end	Night	72% 9% 9% 9% 2	7623

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	30% 14% 27% 18% 9% 2	220
	11th to 20th	Day	25% 17% 28% 11% 18% 1	220
	21st to end	Day	30% 15% 24% 16% 12% 3	301
February	1st to 10th	Day	27% 19% 23% 20% 10% 1	430
	11th to 20th	Day	29% 16% 25% 21% 8% 1	425
	21st to end	Day	22% 20% 27% 18% 11% 1	384
March	1st to 10th	Day	25% 16% 25% 21% 10% 2	540
	11th to 20th	Day	27% 17% 18% 20% 12% 5%	540
	21st to end	Day	23% 15% 29% 21% 10% 2	594
April	1st to 10th	Day	23% 14% 21% 24% 14% 3	540
	11th to 20th	Day	19% 15% 29% 21% 11% 5%	532
	21st to end	Day	19% 11% 25% 28% 14% 3	540
May	1st to 10th	Day	13% 7% 25% 33% 18% 4	543
	11th to 20th	Day	19% 7% 20% 30% 17% 8%	568
	21st to end	Day	20% 8% 17% 28% 20% 8%	617
June	1st to 10th	Day	17% 8% 23% 29% 18% 4	675
	11th to 20th	Day	16% 7% 23% 32% 19% 3	780
	21st to end	Day	13% 5% 18% 36% 23% 6%	780
July	1st to 10th	Day	11% 4 18% 33% 28% 7%	771
	11th to 20th	Day	7% 5% 17% 33% 31% 8%	684
	21st to end	Day	8% 3 11% 31% 35% 12%	619
August	1st to 10th	Day	8% 3 10% 30% 35% 13%	567
	11th to 20th	Day	6% 3 10% 33% 34% 14%	557
	21st to end	Day	13% 4 16% 33% 27% 7%	627
September	1st to 10th	Day	13% 6% 17% 32% 27% 5%	570
	11th to 20th	Day	15% 6% 17% 29% 28% 3	543
	21st to end	Day	16% 8% 24% 30% 20% 2	530
October	1st to 10th	Day	19% 9% 22% 23% 21% 6%	530
	11th to 20th	Day	23% 12% 22% 21% 18% 4	530
	21st to end	Day	25% 19% 20% 23% 12% 2	470
November	1st to 10th	Day	33% 12% 24% 19% 10%	231
	11th to 20th	Day	29% 18% 23% 13% 10% 6%	220
	21st to end	Day	30% 14% 25% 15% 9% 7%	220
December	1st to 10th	Day	30% 15% 18% 14% 17% 7%	220
	11th to 20th	Day	31% 20% 30% 13% 5% 1	220
	21st to end	Day	31% 17% 24% 18% 7% 2	242
January	1st to 10th	Night	31% 14% 23% 19% 11% 2	950
	11th to 20th	Night	26% 13% 17% 22% 19% 2	865
	21st to end	Night	25% 12% 19% 24% 17% 4	924
February	1st to 10th	Night	27% 14% 22% 23% 11% 3	840
	11th to 20th	Night	23% 13% 21% 25% 17% 1	824
	21st to end	Night	25% 13% 22% 25% 12% 2	692
March	1st to 10th	Night	22% 13% 23% 26% 14% 2	840
	11th to 20th	Night	26% 15% 19% 24% 14% 2	840
	21st to end	Night	24% 12% 24% 27% 11% 1	924
April	1st to 10th	Night	24% 8% 20% 28% 17% 2	840
	11th to 20th	Night	22% 8% 22% 28% 18% 3	780
	21st to end	Night	22% 7% 19% 34% 15% 3	588
May	1st to 10th	Night	16% 6% 18% 35% 21% 4	420
	11th to 20th	Night	20% 3 16% 29% 30% 3	414
	21st to end	Night	22% 5% 14% 30% 24% 4	455
June	1st to 10th	Night	21% 5% 18% 32% 22% 2	420
	11th to 20th	Night	19% 10% 15% 34% 20% 2	420
	21st to end	Night	15% 3 13% 36% 28% 4	420
July	1st to 10th	Night	13% 3 10% 30% 39% 5%	410
	11th to 20th	Night	8% 3 11% 36% 38% 4	408
	21st to end	Night	9% 3 7% 32% 44% 6%	460
August	1st to 10th	Night	10% 2 5% 27% 46% 10%	471
	11th to 20th	Night	10% 2 6% 27% 44% 12%	624
	21st to end	Night	11% 4 12% 33% 36% 5%	800
September	1st to 10th	Night	11% 3 10% 37% 33% 7%	840
	11th to 20th	Night	17% 4 12% 23% 33% 11%	812
	21st to end	Night	15% 6% 19% 29% 25% 5%	800
October	1st to 10th	Night	15% 8% 18% 29% 26% 6%	800
	11th to 20th	Night	21% 10% 18% 26% 21% 5%	800
	21st to end	Night	23% 10% 21% 28% 16% 3	880
November	1st to 10th	Night	28% 12% 21% 25% 12%	820
	11th to 20th	Night	28% 12% 23% 21% 13% 3	840
	21st to end	Night	26% 13% 22% 21% 14% 4	840
December	1st to 10th	Night	26% 10% 18% 25% 17% 3	840
	11th to 20th	Night	28% 17% 22% 21% 11% 2	854
	21st to end	Night	31% 16% 24% 19% 9% 1	1045

Page generated at 2026-09-11 01:11:53. Version ac07

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