Trail
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trail clear sky chart from 2005-11-06 to 2026-06-01.

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	10% 11% 3 7% 5% 5% 5% 4 5% 7% 39%	135762
	Night	12% 13% 4 7% 5% 4 4 3 5% 6% 37%	136107
Transparency	Day	28% 38% 10% 15% 7% 2	135486
	Night	31% 38% 10% 14% 6% 2	135962
Seeing	Day	17% 14% 28% 25% 13% 2	16865
	Night	20% 6% 22% 33% 17% 2	26013

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	13% 27% 8% 14% 6% 4 3 3 3 4 14%	4646
	11th to 20th	Day	11% 11% 2 6% 5% 6% 5% 3 4 6% 43%	4716
	21st to end	Day	18% 19% 3 5% 3 3 4 4 4 5% 31%	5186
February	1st to 10th	Day	14% 18% 5% 10% 5% 5% 4 4 5% 6% 23%	1680
	11th to 20th	Day	12% 17% 5% 10% 5% 4 4 3 6% 7% 26%	1899
	21st to end	Day	13% 15% 5% 11% 7% 5% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	9% 15% 5% 8% 6% 5% 6% 5% 6% 7% 29%	4618
	11th to 20th	Day	11% 15% 4 8% 5% 4 4 3 4 6% 36%	4750
	21st to end	Day	14% 16% 5% 10% 6% 6% 6% 4 6% 7% 19%	6096
April	1st to 10th	Day	14% 15% 4 9% 7% 5% 5% 3 6% 7% 25%	2520
	11th to 20th	Day	12% 12% 4 10% 6% 6% 6% 4 6% 7% 28%	2574
	21st to end	Day	11% 11% 4 9% 7% 5% 6% 4 6% 8% 29%	2746
May	1st to 10th	Day	7% 7% 3 8% 6% 6% 5% 5% 6% 7% 40%	6460
	11th to 20th	Day	9% 11% 4 10% 7% 7% 7% 5% 8% 9% 24%	6417
	21st to end	Day	9% 8% 4 10% 6% 6% 6% 4 6% 7% 35%	7333
June	1st to 10th	Day	18% 9% 3 8% 6% 4 5% 4 5% 6% 31%	3001
	11th to 20th	Day	14% 8% 4 9% 7% 7% 6% 5% 6% 7% 26%	3000
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 5% 7% 6% 43%	3000
July	1st to 10th	Day	3 3 1 3 3 3 4 4 5% 6% 66%	6648
	11th to 20th	Day	3 2 1 3 2 3 3 3 4 6% 72%	6320
	21st to end	Day	4 3 1 3 3 3 4 4 5% 7% 64%	6874
August	1st to 10th	Day	3 3 1 3 3 3 4 4 6% 8% 63%	4541
	11th to 20th	Day	4 3 1 3 2 3 4 4 6% 8% 62%	4358
	21st to end	Day	6% 3 1 4 4 4 5% 4 6% 8% 54%	4420
September	1st to 10th	Day	5% 4 1 4 3 4 5% 4 6% 7% 57%	2400
	11th to 20th	Day	9% 6% 2 6% 4 5% 4 4 6% 7% 48%	2308
	21st to end	Day	11% 7% 3 6% 4 4 5% 3 6% 7% 43%	2200
October	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 7% 47%	3414
	11th to 20th	Day	12% 8% 2 7% 4 4 5% 4 6% 8% 39%	3048
	21st to end	Day	17% 12% 4 7% 4 4 5% 5% 5% 7% 30%	3200
November	1st to 10th	Day	22% 16% 3 7% 6% 4 4 3 5% 6% 23%	1854
	11th to 20th	Day	22% 18% 5% 9% 5% 4 4 3 4 5% 22%	1688
	21st to end	Day	20% 18% 6% 12% 5% 5% 3 3 4 5% 19%	1470
December	1st to 10th	Day	13% 13% 4 9% 5% 3 4 3 5% 7% 36%	2360
	11th to 20th	Day	17% 27% 5% 9% 5% 4 4 3 4 5% 16%	2712
	21st to end	Day	18% 31% 9% 9% 4 3 3 2 4 4 13%	3575
January	1st to 10th	Night	16% 27% 7% 12% 5% 4 4 2 3 3 16%	8289
	11th to 20th	Night	9% 11% 3 8% 5% 5% 5% 3 5% 7% 39%	8182
	21st to end	Night	16% 21% 4 6% 4 2 3 3 5% 4 32%	8994
February	1st to 10th	Night	15% 18% 5% 10% 5% 5% 4 4 5% 5% 25%	2940
	11th to 20th	Night	13% 17% 5% 9% 5% 4 4 3 4 6% 30%	2604
	21st to end	Night	12% 16% 4 9% 5% 5% 5% 4 6% 8% 27%	2076
March	1st to 10th	Night	9% 14% 5% 8% 6% 5% 5% 3 5% 6% 34%	5496
	11th to 20th	Night	11% 16% 4 9% 5% 3 3 3 4 5% 38%	5469
	21st to end	Night	14% 15% 4 9% 7% 7% 7% 5% 6% 6% 20%	5119
April	1st to 10th	Night	11% 12% 3 6% 5% 5% 5% 5% 6% 7% 32%	2100
	11th to 20th	Night	12% 10% 3 8% 6% 5% 5% 4 5% 7% 35%	1996
	21st to end	Night	10% 9% 3 8% 5% 5% 6% 4 5% 7% 38%	1872
May	1st to 10th	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 9% 44%	3720
	11th to 20th	Night	9% 8% 3 8% 6% 6% 6% 5% 6% 7% 35%	3696
	21st to end	Night	11% 8% 3 7% 5% 4 5% 4 6% 7% 39%	3784
June	1st to 10th	Night	13% 9% 3 6% 5% 5% 6% 4 6% 8% 35%	1400
	11th to 20th	Night	11% 7% 3 9% 6% 6% 5% 6% 8% 8% 32%	1400
	21st to end	Night	8% 5% 2 5% 5% 3 5% 4 6% 8% 50%	1400
July	1st to 10th	Night	3 2 1 2 2 2 3 2 4 6% 72%	3308
	11th to 20th	Night	3 2 1 3 2 2 3 2 4 5% 73%	3640
	21st to end	Night	3 2 1 2 2 3 3 3 5% 8% 68%	3969
August	1st to 10th	Night	3 2 1 3 3 3 3 3 6% 8% 65%	2609
	11th to 20th	Night	2 2 1 3 3 2 4 4 6% 11% 62%	2692
	21st to end	Night	5% 3 1 4 4 4 5% 5% 6% 9% 55%	3494
September	1st to 10th	Night	4 3 1 4 3 4 3 2 6% 8% 62%	2000
	11th to 20th	Night	7% 6% 2 5% 4 4 4 4 7% 7% 49%	2091
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 7% 50%	2200
October	1st to 10th	Night	7% 4 2 4 3 4 4 4 6% 8% 54%	3782
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 7% 44%	4102
	21st to end	Night	15% 9% 3 8% 4 5% 5% 4 5% 6% 35%	4600
November	1st to 10th	Night	23% 16% 4 8% 4 4 5% 3 4 6% 23%	2665
	11th to 20th	Night	22% 18% 3 8% 5% 4 4 3 5% 5% 21%	2910
	21st to end	Night	21% 20% 5% 9% 5% 4 3 3 4 6% 20%	3145
December	1st to 10th	Night	11% 13% 5% 10% 5% 4 4 3 5% 6% 33%	5040
	11th to 20th	Night	17% 25% 7% 10% 5% 4 5% 4 5% 4 14%	5700
	21st to end	Night	20% 28% 8% 11% 5% 3 3 3 3 3 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	60% 24% 7% 8% 1	4646
	11th to 20th	Day	32% 23% 22% 18% 5% 1	4740
	21st to end	Day	44% 25% 12% 15% 4	5187
February	1st to 10th	Day	27% 61% 3 4 5% 1	1680
	11th to 20th	Day	27% 59% 6% 5% 2	1908
	21st to end	Day	23% 60% 7% 6% 4	1730
March	1st to 10th	Day	37% 32% 8% 11% 11%	4618
	11th to 20th	Day	34% 29% 14% 19% 4	4750
	21st to end	Day	46% 33% 8% 10% 3	6096
April	1st to 10th	Day	21% 59% 9% 8% 2	2520
	11th to 20th	Day	21% 61% 6% 10% 2	2574
	21st to end	Day	20% 61% 5% 10% 4 1	2746
May	1st to 10th	Day	25% 32% 9% 19% 13% 1	6460
	11th to 20th	Day	39% 33% 11% 14% 3	6420
	21st to end	Day	30% 32% 14% 20% 3	7334
June	1st to 10th	Day	15% 63% 5% 9% 6% 1	3001
	11th to 20th	Day	18% 63% 5% 9% 6%	3000
	21st to end	Day	17% 62% 6% 9% 6%	3000
July	1st to 10th	Day	13% 25% 8% 20% 22% 11%	6576
	11th to 20th	Day	11% 25% 8% 20% 20% 16%	6245
	21st to end	Day	19% 24% 10% 24% 15% 9%	6681
August	1st to 10th	Day	11% 35% 5% 24% 21% 5%	4524
	11th to 20th	Day	15% 35% 13% 28% 7% 1	4400
	21st to end	Day	19% 36% 16% 18% 10% 2	4419
September	1st to 10th	Day	12% 58% 7% 13% 8% 1	2400
	11th to 20th	Day	17% 58% 6% 12% 6%	2308
	21st to end	Day	18% 58% 9% 10% 5%	2200
October	1st to 10th	Day	20% 36% 17% 23% 5%	3414
	11th to 20th	Day	29% 39% 7% 16% 8%	3048
	21st to end	Day	37% 37% 13% 10% 3	3200
November	1st to 10th	Day	28% 57% 8% 7%	1854
	11th to 20th	Day	32% 55% 5% 4 2 1	1688
	21st to end	Day	31% 54% 7% 5% 3	1470
December	1st to 10th	Day	32% 39% 20% 8% 1	2360
	11th to 20th	Day	51% 34% 9% 5% 1	2714
	21st to end	Day	59% 28% 6% 4 3	3575
January	1st to 10th	Night	60% 23% 9% 6% 1	8290
	11th to 20th	Night	33% 27% 17% 17% 5% 1	8220
	21st to end	Night	44% 25% 13% 16% 3	8994
February	1st to 10th	Night	27% 59% 4 5% 4 1	2940
	11th to 20th	Night	26% 58% 5% 7% 3 1	2606
	21st to end	Night	21% 60% 8% 7% 4	2076
March	1st to 10th	Night	33% 34% 10% 14% 10%	5496
	11th to 20th	Night	33% 31% 12% 19% 4	5470
	21st to end	Night	45% 34% 10% 8% 2	5119
April	1st to 10th	Night	20% 60% 9% 8% 3	2100
	11th to 20th	Night	18% 61% 6% 12% 3	1996
	21st to end	Night	16% 62% 4 11% 7% 1	1872
May	1st to 10th	Night	20% 32% 8% 25% 14% 1	3717
	11th to 20th	Night	35% 30% 9% 20% 5%	3696
	21st to end	Night	27% 31% 18% 20% 4	3784
June	1st to 10th	Night	13% 64% 5% 11% 7% 1	1400
	11th to 20th	Night	17% 63% 5% 8% 6% 1	1400
	21st to end	Night	13% 62% 6% 11% 8% 1	1400
July	1st to 10th	Night	11% 25% 7% 19% 23% 15%	3271
	11th to 20th	Night	12% 25% 8% 19% 17% 19%	3592
	21st to end	Night	14% 24% 7% 26% 19% 10%	3857
August	1st to 10th	Night	13% 35% 4 21% 22% 5%	2601
	11th to 20th	Night	13% 35% 14% 28% 9% 1	2716
	21st to end	Night	20% 35% 14% 17% 13% 1	3494
September	1st to 10th	Night	12% 58% 7% 13% 9% 2	2000
	11th to 20th	Night	15% 58% 8% 11% 7%	2091
	21st to end	Night	17% 58% 9% 11% 6%	2200
October	1st to 10th	Night	15% 36% 18% 26% 6%	3782
	11th to 20th	Night	27% 38% 10% 20% 5%	4102
	21st to end	Night	34% 37% 12% 15% 2	4600
November	1st to 10th	Night	30% 56% 8% 5% 1	2665
	11th to 20th	Night	30% 57% 6% 4 2 1	2910
	21st to end	Night	29% 55% 7% 6% 3	3145
December	1st to 10th	Night	32% 40% 15% 11% 1	5040
	11th to 20th	Night	53% 33% 9% 4 1	5697
	21st to end	Night	57% 30% 5% 5% 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	28% 16% 25% 19% 10% 2	220
	11th to 20th	Day	26% 7% 34% 20% 10% 4	220
	21st to end	Day	28% 7% 32% 23% 9% 1	242
February	1st to 10th	Day	19% 15% 35% 23% 5% 2	220
	11th to 20th	Day	21% 16% 32% 22% 8% 1	441
	21st to end	Day	19% 17% 33% 24% 7% 1	446
March	1st to 10th	Day	25% 18% 29% 18% 9% 1	540
	11th to 20th	Day	25% 17% 23% 20% 14% 1	540
	21st to end	Day	26% 20% 25% 18% 10% 2	594
April	1st to 10th	Day	25% 21% 23% 21% 9% 2	540
	11th to 20th	Day	23% 19% 27% 19% 10% 3	532
	21st to end	Day	15% 19% 28% 24% 12% 2	540
May	1st to 10th	Day	16% 14% 30% 27% 11% 2	543
	11th to 20th	Day	21% 14% 26% 23% 13% 2	568
	21st to end	Day	18% 13% 23% 28% 16% 3	737
June	1st to 10th	Day	19% 14% 28% 25% 12% 2	731
	11th to 20th	Day	16% 16% 28% 26% 13% 2	730
	21st to end	Day	12% 8% 27% 29% 20% 4	730
July	1st to 10th	Day	6% 10% 28% 35% 17% 2	669
	11th to 20th	Day	6% 15% 33% 30% 14% 2	524
	21st to end	Day	6% 10% 32% 30% 18% 4	575
August	1st to 10th	Day	7% 8% 31% 32% 16% 5%	527
	11th to 20th	Day	5% 9% 31% 31% 20% 5%	517
	21st to end	Day	8% 16% 26% 28% 18% 4	583
September	1st to 10th	Day	10% 12% 25% 29% 20% 3	530
	11th to 20th	Day	13% 12% 28% 29% 17% 2	530
	21st to end	Day	18% 18% 27% 24% 11% 3	530
October	1st to 10th	Day	15% 12% 32% 27% 13% 2	530
	11th to 20th	Day	22% 14% 26% 23% 13% 2	407
	21st to end	Day	26% 15% 27% 23% 6% 2	363
November	1st to 10th	Day	27% 13% 23% 29% 7% 1	330
	11th to 20th	Day	34% 12% 23% 20% 9% 3	234
	21st to end	Day	30% 9% 26% 20% 10% 4	220
December	1st to 10th	Day	27% 10% 22% 20% 16% 5%	220
	11th to 20th	Day	26% 10% 28% 27% 8% 1	220
	21st to end	Day	24% 12% 24% 32% 7% 1	242
January	1st to 10th	Night	34% 8% 23% 26% 8% 1	882
	11th to 20th	Night	25% 8% 27% 30% 9% 1	840
	21st to end	Night	24% 4 29% 33% 9% 1	924
February	1st to 10th	Night	21% 10% 29% 30% 10%	840
	11th to 20th	Night	21% 6% 27% 34% 12%	824
	21st to end	Night	20% 8% 24% 31% 15% 2	692
March	1st to 10th	Night	23% 7% 22% 33% 15% 1	840
	11th to 20th	Night	24% 9% 20% 31% 16% 1	840
	21st to end	Night	22% 6% 27% 30% 14% 1	924
April	1st to 10th	Night	18% 6% 24% 33% 17% 2	840
	11th to 20th	Night	18% 7% 25% 35% 14% 2	748
	21st to end	Night	14% 10% 22% 35% 17% 1	612
May	1st to 10th	Night	11% 6% 19% 38% 25% 1	420
	11th to 20th	Night	15% 5% 20% 29% 30% 2	414
	21st to end	Night	16% 7% 19% 31% 24% 2	455
June	1st to 10th	Night	15% 7% 18% 39% 20% 2	400
	11th to 20th	Night	14% 7% 18% 40% 18% 3	400
	21st to end	Night	11% 3 18% 41% 24% 4	400
July	1st to 10th	Night	4 2 13% 43% 37% 2	390
	11th to 20th	Night	3 3 20% 40% 31% 3	388
	21st to end	Night	3 2 13% 42% 36% 3	438
August	1st to 10th	Night	6% 3 9% 36% 42% 5%	398
	11th to 20th	Night	5% 2 12% 39% 36% 6%	459
	21st to end	Night	8% 3 14% 40% 33% 3	805
September	1st to 10th	Night	6% 3 18% 40% 30% 4	800
	11th to 20th	Night	12% 4 14% 30% 36% 5%	800
	21st to end	Night	12% 6% 18% 40% 21% 4	800
October	1st to 10th	Night	16% 5% 20% 36% 23% 1	800
	11th to 20th	Night	20% 6% 20% 33% 19% 1	800
	21st to end	Night	25% 5% 26% 30% 12% 2	880
November	1st to 10th	Night	32% 6% 22% 33% 7%	820
	11th to 20th	Night	32% 9% 21% 26% 11% 1	840
	21st to end	Night	32% 7% 20% 29% 12% 1	1045
December	1st to 10th	Night	24% 6% 25% 30% 14% 1	1050
	11th to 20th	Night	32% 6% 26% 29% 7%	1050
	21st to end	Night	31% 8% 27% 29% 5%	1155

Page generated at 2026-05-28 22:53:31. Version ac07

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