Tualatin River National Wildlife Refuge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tualatin River National Wildlife Refuge clear sky chart from 2005-11-06 to 2026-01-02.

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	16% 8% 3 8% 5% 4 4 3 5% 6% 38%	136513
	Night	18% 9% 3 7% 5% 4 4 3 5% 5% 36%	132160
Transparency	Day	46% 8% 12% 18% 12% 4	131282
	Night	48% 8% 12% 17% 11% 3	128388
Seeing	Day	21% 10% 20% 27% 18% 5%	16030
	Night	23% 7% 18% 32% 18% 1	25041

By Month

Percentage Cloud Cover SW (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	35% 15% 4 12% 6% 4 5% 3 3 3 9%	3647
	11th to 20th	Day	27% 11% 3 8% 7% 6% 5% 3 4 5% 21%	3640
	21st to end	Day	24% 10% 3 8% 5% 4 4 3 4 4 29%	4399
February	1st to 10th	Day	28% 12% 4 11% 6% 5% 4 5% 6% 6% 15%	1800
	11th to 20th	Day	27% 15% 4 11% 7% 5% 4 3 4 3 17%	1755
	21st to end	Day	28% 14% 4 9% 6% 5% 4 2 4 5% 19%	1560
March	1st to 10th	Day	16% 13% 4 11% 7% 6% 6% 5% 6% 6% 19%	4738
	11th to 20th	Day	23% 13% 5% 9% 5% 4 4 3 4 5% 25%	5000
	21st to end	Day	21% 13% 5% 11% 7% 6% 7% 4 6% 7% 13%	5590
April	1st to 10th	Day	25% 12% 4 11% 7% 4 5% 3 5% 6% 18%	2400
	11th to 20th	Day	21% 11% 4 9% 6% 5% 4 4 4 6% 27%	2369
	21st to end	Day	18% 10% 3 9% 6% 5% 5% 5% 6% 7% 25%	2598
May	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 4 6% 41%	6219
	11th to 20th	Day	13% 8% 3 9% 6% 6% 5% 4 5% 7% 34%	6277
	21st to end	Day	15% 7% 3 7% 6% 6% 5% 4 6% 7% 33%	6902
June	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 34%	2800
	11th to 20th	Day	16% 10% 3 7% 6% 6% 6% 5% 6% 7% 28%	2800
	21st to end	Day	8% 6% 2 6% 5% 4 4 4 6% 7% 49%	2800
July	1st to 10th	Day	3 3 1 3 2 2 2 2 3 4 72%	6319
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 79%	6320
	21st to end	Day	4 3 1 3 2 2 2 2 3 5% 72%	6880
August	1st to 10th	Day	3 3 1 4 3 3 4 3 4 6% 64%	5806
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 8% 53%	5796
	21st to end	Day	7% 6% 2 5% 4 4 4 3 6% 7% 52%	6165
September	1st to 10th	Day	6% 3 1 4 4 4 4 3 5% 7% 58%	2400
	11th to 20th	Day	11% 8% 3 7% 4 4 5% 4 4 6% 43%	2398
	21st to end	Day	14% 5% 3 7% 4 4 4 3 4 6% 45%	2104
October	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 45%	3280
	11th to 20th	Day	16% 8% 4 9% 6% 6% 5% 4 6% 8% 28%	3261
	21st to end	Day	23% 10% 4 10% 5% 5% 5% 4 6% 8% 21%	3494
November	1st to 10th	Day	28% 14% 4 10% 5% 5% 5% 3 5% 5% 14%	1736
	11th to 20th	Day	28% 12% 5% 10% 6% 5% 4 3 4 6% 18%	1680
	21st to end	Day	27% 12% 4 11% 6% 4 4 4 5% 4 20%	1680
December	1st to 10th	Day	22% 9% 3 9% 5% 4 3 3 5% 6% 30%	2700
	11th to 20th	Day	29% 14% 4 12% 6% 5% 4 4 5% 5% 11%	3108
	21st to end	Day	31% 16% 5% 13% 8% 5% 4 3 4 4 7%	4092
January	1st to 10th	Night	33% 15% 4 11% 6% 5% 4 3 3 4 13%	6320
	11th to 20th	Night	26% 12% 3 8% 5% 5% 5% 3 4 4 24%	6320
	21st to end	Night	26% 11% 3 7% 4 3 3 2 4 4 32%	6573
February	1st to 10th	Night	23% 14% 4 10% 6% 5% 4 4 5% 5% 19%	2600
	11th to 20th	Night	28% 11% 4 8% 5% 4 4 3 4 4 24%	2532
	21st to end	Night	27% 14% 3 9% 4 4 4 4 4 5% 22%	2070
March	1st to 10th	Night	15% 14% 4 10% 6% 6% 6% 4 6% 6% 22%	5139
	11th to 20th	Night	19% 14% 3 8% 5% 5% 4 3 5% 5% 30%	5000
	21st to end	Night	21% 13% 5% 11% 8% 6% 6% 4 5% 6% 14%	5372
April	1st to 10th	Night	23% 13% 4 9% 6% 5% 4 3 5% 5% 23%	2000
	11th to 20th	Night	17% 11% 3 7% 4 4 4 3 5% 7% 34%	1978
	21st to end	Night	21% 10% 3 7% 5% 3 4 3 5% 7% 31%	1800
May	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	3742
	11th to 20th	Night	12% 7% 3 7% 4 3 4 3 4 7% 45%	3616
	21st to end	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	3972
June	1st to 10th	Night	15% 8% 3 8% 4 5% 4 3 4 7% 39%	1600
	11th to 20th	Night	12% 9% 3 8% 6% 4 5% 3 5% 6% 39%	1600
	21st to end	Night	7% 5% 2 4 3 4 3 3 5% 6% 59%	1600
July	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 77%	3640
	11th to 20th	Night	2 2 1 2 2 2 2 1 2 3 82%	3640
	21st to end	Night	3 2 1 2 2 2 2 2 4 6% 74%	3970
August	1st to 10th	Night	4 3 2 4 3 3 3 3 4 6% 67%	3989
	11th to 20th	Night	11% 5% 2 5% 3 3 3 2 4 5% 55%	4052
	21st to end	Night	8% 5% 2 5% 4 4 4 2 4 5% 58%	4826
September	1st to 10th	Night	6% 4 2 4 2 2 3 2 4 6% 64%	2000
	11th to 20th	Night	10% 6% 3 6% 5% 4 4 4 5% 5% 49%	2000
	21st to end	Night	11% 4 2 5% 4 3 4 3 5% 7% 52%	2295
October	1st to 10th	Night	11% 5% 2 6% 3 4 4 4 5% 7% 50%	3920
	11th to 20th	Night	16% 6% 3 7% 4 4 5% 4 5% 9% 37%	3897
	21st to end	Night	21% 10% 4 9% 5% 5% 5% 3 5% 6% 28%	4315
November	1st to 10th	Night	28% 11% 3 9% 6% 5% 4 3 5% 5% 22%	2780
	11th to 20th	Night	33% 11% 4 9% 5% 4 3 3 4 4 20%	2940
	21st to end	Night	24% 10% 3 10% 6% 4 5% 3 5% 5% 25%	2940
December	1st to 10th	Night	20% 9% 3 6% 4 4 3 4 6% 7% 33%	4700
	11th to 20th	Night	33% 15% 5% 10% 5% 4 4 3 4 5% 12%	5316
	21st to end	Night	31% 13% 5% 10% 7% 5% 5% 3 4 4 13%	7106

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% 9% 4 6% 3	3647
	11th to 20th	Day	64% 5% 7% 16% 7% 1	3640
	21st to end	Day	57% 8% 6% 12% 15% 3	4399
February	1st to 10th	Day	67% 11% 10% 8% 4	1800
	11th to 20th	Day	68% 10% 7% 8% 4 2	1764
	21st to end	Day	67% 7% 10% 8% 6% 1	1560
March	1st to 10th	Day	57% 13% 14% 12% 4	4738
	11th to 20th	Day	60% 8% 7% 18% 7%	5000
	21st to end	Day	66% 13% 12% 7% 3	5590
April	1st to 10th	Day	64% 10% 12% 11% 3	2400
	11th to 20th	Day	57% 9% 10% 13% 9% 2	2372
	21st to end	Day	56% 11% 12% 12% 8% 1	2598
May	1st to 10th	Day	40% 9% 11% 22% 17%	6219
	11th to 20th	Day	43% 12% 18% 19% 7% 1	6280
	21st to end	Day	44% 15% 21% 17% 4	6904
June	1st to 10th	Day	49% 9% 16% 18% 7% 1	2800
	11th to 20th	Day	50% 12% 17% 15% 6%	2800
	21st to end	Day	33% 7% 18% 27% 10% 4	2800
July	1st to 10th	Day	16% 5% 10% 28% 19% 22%	6251
	11th to 20th	Day	12% 3 7% 21% 39% 18%	6245
	21st to end	Day	14% 5% 16% 36% 25% 4	6681
August	1st to 10th	Day	19% 5% 11% 27% 25% 13%	4222
	11th to 20th	Day	26% 9% 18% 29% 13% 5%	4195
	21st to end	Day	29% 7% 12% 25% 20% 7%	4444
September	1st to 10th	Day	24% 7% 14% 26% 18% 12%	2400
	11th to 20th	Day	38% 9% 15% 18% 14% 6%	2398
	21st to end	Day	38% 9% 14% 19% 15% 5%	2104
October	1st to 10th	Day	39% 6% 7% 25% 21% 2	3280
	11th to 20th	Day	52% 9% 14% 13% 11% 1	3261
	21st to end	Day	60% 6% 8% 16% 9% 1	3494
November	1st to 10th	Day	69% 8% 10% 10% 3	1736
	11th to 20th	Day	68% 7% 10% 8% 5% 1	1680
	21st to end	Day	66% 7% 7% 9% 10% 1	1680
December	1st to 10th	Day	55% 5% 10% 16% 14% 1	2700
	11th to 20th	Day	73% 9% 9% 8% 1	3108
	21st to end	Day	78% 9% 7% 5% 2	4092
January	1st to 10th	Night	73% 9% 6% 6% 4 1	6320
	11th to 20th	Night	60% 6% 8% 17% 7% 1	6320
	21st to end	Night	55% 6% 7% 12% 15% 5%	6573
February	1st to 10th	Night	64% 11% 10% 10% 4 1	2600
	11th to 20th	Night	61% 8% 12% 10% 6% 2	2534
	21st to end	Night	62% 9% 12% 10% 5% 2	2070
March	1st to 10th	Night	56% 11% 14% 14% 5%	5139
	11th to 20th	Night	54% 10% 11% 17% 8% 1	5000
	21st to end	Night	65% 12% 12% 9% 1	5372
April	1st to 10th	Night	62% 9% 13% 11% 5%	2000
	11th to 20th	Night	48% 11% 16% 15% 8% 3	1978
	21st to end	Night	51% 10% 15% 15% 7% 2	1800
May	1st to 10th	Night	34% 9% 13% 30% 13% 1	3739
	11th to 20th	Night	37% 7% 22% 27% 6% 1	3616
	21st to end	Night	38% 15% 27% 16% 4	3972
June	1st to 10th	Night	44% 8% 18% 23% 6% 1	1600
	11th to 20th	Night	44% 9% 19% 23% 6%	1600
	21st to end	Night	26% 8% 19% 31% 12% 4	1600
July	1st to 10th	Night	12% 5% 11% 28% 24% 21%	3600
	11th to 20th	Night	11% 3 8% 18% 41% 19%	3592
	21st to end	Night	13% 4 19% 33% 27% 5%	3857
August	1st to 10th	Night	12% 3 11% 34% 30% 9%	2897
	11th to 20th	Night	27% 7% 21% 29% 11% 5%	2929
	21st to end	Night	27% 5% 14% 27% 20% 7%	3471
September	1st to 10th	Night	20% 7% 15% 27% 19% 11%	2000
	11th to 20th	Night	34% 8% 14% 22% 13% 8%	2000
	21st to end	Night	30% 7% 18% 21% 17% 6%	2295
October	1st to 10th	Night	32% 6% 10% 31% 19% 2	3920
	11th to 20th	Night	43% 8% 18% 19% 11% 2	3897
	21st to end	Night	55% 7% 11% 15% 10% 1	4315
November	1st to 10th	Night	61% 10% 9% 14% 6%	2780
	11th to 20th	Night	66% 8% 9% 8% 7% 1	2940
	21st to end	Night	59% 10% 9% 10% 11% 2	2940
December	1st to 10th	Night	48% 6% 10% 18% 17% 1	4700
	11th to 20th	Night	74% 8% 7% 8% 2	5316
	21st to end	Night	71% 12% 10% 6% 1	7106

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	46% 10% 18% 15% 10% 1	202
	11th to 20th	Day	37% 10% 21% 21% 10% 3	200
	21st to end	Day	32% 10% 20% 20% 14% 3	394
February	1st to 10th	Day	33% 14% 19% 19% 13% 3	400
	11th to 20th	Day	33% 14% 26% 16% 9% 2	395
	21st to end	Day	35% 15% 21% 20% 9% 1	369
March	1st to 10th	Day	30% 15% 24% 21% 8% 2	500
	11th to 20th	Day	27% 13% 23% 21% 11% 4	500
	21st to end	Day	31% 17% 24% 18% 10% 1	550
April	1st to 10th	Day	29% 14% 23% 21% 8% 4	500
	11th to 20th	Day	27% 14% 26% 21% 9% 3	492
	21st to end	Day	22% 13% 26% 23% 15% 2	500
May	1st to 10th	Day	19% 13% 22% 27% 15% 4	503
	11th to 20th	Day	26% 10% 18% 27% 15% 4	528
	21st to end	Day	18% 9% 18% 30% 19% 6%	573
June	1st to 10th	Day	20% 8% 21% 27% 20% 5%	530
	11th to 20th	Day	16% 12% 26% 28% 15% 3	530
	21st to end	Day	7% 6% 19% 37% 25% 5%	530
July	1st to 10th	Day	6% 4 18% 36% 28% 7%	526
	11th to 20th	Day	4 4 21% 40% 26% 6%	524
	21st to end	Day	4 3 15% 36% 32% 10%	575
August	1st to 10th	Day	5% 3 11% 37% 32% 11%	527
	11th to 20th	Day	4 3 13% 34% 34% 12%	517
	21st to end	Day	8% 4 20% 33% 27% 9%	583
September	1st to 10th	Day	7% 4 22% 30% 29% 8%	530
	11th to 20th	Day	13% 6% 18% 31% 26% 7%	530
	21st to end	Day	15% 8% 21% 33% 20% 3	530
October	1st to 10th	Day	20% 8% 21% 26% 18% 7%	530
	11th to 20th	Day	22% 9% 17% 28% 17% 6%	530
	21st to end	Day	30% 11% 24% 20% 11% 4	543
November	1st to 10th	Day	31% 10% 24% 19% 12% 3	267
	11th to 20th	Day	29% 19% 19% 19% 11% 3	220
	21st to end	Day	28% 12% 20% 20% 19% 1	220
December	1st to 10th	Day	36% 11% 15% 20% 14% 4	220
	11th to 20th	Day	40% 15% 22% 15% 6% 1	220
	21st to end	Day	40% 13% 19% 15% 12% 1	242
January	1st to 10th	Night	41% 9% 18% 20% 11% 1	804
	11th to 20th	Night	34% 10% 16% 24% 15% 1	800
	21st to end	Night	32% 7% 16% 30% 13% 2	880
February	1st to 10th	Night	31% 9% 17% 28% 14% 1	800
	11th to 20th	Night	32% 10% 20% 22% 15%	784
	21st to end	Night	32% 11% 19% 30% 8%	660
March	1st to 10th	Night	27% 9% 21% 29% 13% 1	800
	11th to 20th	Night	26% 10% 19% 31% 13% 1	800
	21st to end	Night	28% 9% 23% 27% 12% 1	880
April	1st to 10th	Night	30% 8% 17% 28% 16% 1	800
	11th to 20th	Night	22% 9% 23% 31% 14% 1	790
	21st to end	Night	26% 6% 21% 31% 16% 1	600
May	1st to 10th	Night	16% 6% 25% 34% 17% 1	450
	11th to 20th	Night	18% 5% 18% 35% 22% 2	394
	21st to end	Night	17% 6% 22% 34% 21%	433
June	1st to 10th	Night	17% 5% 24% 34% 20% 1	400
	11th to 20th	Night	14% 6% 26% 41% 12% 1	400
	21st to end	Night	10% 2 23% 46% 20%	400
July	1st to 10th	Night	5% 3 15% 50% 25% 1	390
	11th to 20th	Night	6% 1 23% 46% 23% 1	388
	21st to end	Night	2 2 16% 50% 29% 1	438
August	1st to 10th	Night	5% 1 10% 41% 38% 5%	577
	11th to 20th	Night	6% 2 8% 37% 43% 4	594
	21st to end	Night	8% 3 15% 43% 29% 2	795
September	1st to 10th	Night	7% 3 17% 41% 30% 2	800
	11th to 20th	Night	12% 3 14% 34% 34% 4	800
	21st to end	Night	12% 4 16% 41% 25% 3	800
October	1st to 10th	Night	18% 4 20% 32% 24% 3	800
	11th to 20th	Night	18% 7% 15% 32% 25% 3	800
	21st to end	Night	25% 9% 19% 29% 15% 3	880
November	1st to 10th	Night	30% 8% 17% 31% 13%	820
	11th to 20th	Night	36% 10% 18% 24% 12% 1	840
	21st to end	Night	28% 6% 19% 29% 17% 1	840
December	1st to 10th	Night	31% 8% 18% 28% 14% 1	840
	11th to 20th	Night	39% 7% 19% 22% 12% 1	840
	21st to end	Night	37% 10% 22% 22% 9% 1	924

Page generated at 2025-12-29 22:14:06. Version ac07

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