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 2025-02-06.

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	11% 11% 3 7% 5% 4 5% 4 5% 7% 38%	111911
	Night	13% 13% 4 7% 4 4 4 3 5% 6% 35%	114988
Transparency	Day	25% 45% 9% 12% 7% 2	111676
	Night	29% 44% 9% 11% 5% 2	114869
Seeing	Day	17% 14% 28% 25% 13% 3	15129
	Night	20% 6% 22% 33% 18% 2	24367

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% 9% 15% 6% 4 2 2 2 3 16%	3575
	11th to 20th	Day	13% 12% 2 6% 5% 5% 5% 3 5% 6% 38%	3616
	21st to end	Day	20% 21% 3 5% 3 3 4 3 4 5% 28%	4004
February	1st to 10th	Day	15% 17% 5% 11% 6% 5% 4 4 5% 6% 23%	1560
	11th to 20th	Day	13% 17% 5% 10% 5% 4 5% 3 6% 7% 26%	1713
	21st to end	Day	12% 15% 5% 11% 7% 6% 5% 4 5% 6% 23%	1570
March	1st to 10th	Day	12% 19% 5% 9% 6% 5% 6% 4 4 6% 23%	3138
	11th to 20th	Day	11% 12% 3 6% 4 3 3 3 4 6% 43%	3242
	21st to end	Day	15% 17% 5% 11% 7% 6% 7% 5% 6% 6% 16%	4160
April	1st to 10th	Day	14% 16% 4 9% 7% 5% 5% 3 6% 6% 23%	2280
	11th to 20th	Day	13% 12% 4 10% 7% 7% 6% 4 6% 6% 26%	2326
	21st to end	Day	12% 12% 4 10% 7% 5% 6% 4 6% 8% 27%	2484
May	1st to 10th	Day	9% 9% 4 10% 7% 6% 5% 5% 6% 7% 31%	4420
	11th to 20th	Day	8% 9% 3 8% 6% 7% 6% 5% 8% 10% 30%	4377
	21st to end	Day	12% 11% 5% 12% 7% 7% 6% 5% 6% 7% 22%	4999
June	1st to 10th	Day	19% 9% 3 8% 7% 4 5% 4 5% 6% 29%	2850
	11th to 20th	Day	14% 9% 4 9% 8% 7% 6% 6% 6% 7% 24%	2850
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 4 7% 6% 44%	2850
July	1st to 10th	Day	3 2 1 3 3 4 4 4 5% 6% 66%	4650
	11th to 20th	Day	3 2 1 3 3 3 3 2 4 5% 72%	4420
	21st to end	Day	2 2 1 3 2 3 3 4 5% 7% 68%	4790
August	1st to 10th	Day	3 3 1 3 3 3 4 3 6% 8% 64%	4401
	11th to 20th	Day	3 3 1 3 2 3 4 4 6% 8% 62%	4221
	21st to end	Day	6% 3 1 5% 4 4 5% 4 6% 8% 54%	4285
September	1st to 10th	Day	5% 4 1 5% 3 4 5% 4 6% 7% 56%	2280
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 6% 7% 47%	2193
	21st to end	Day	11% 8% 3 6% 5% 4 5% 3 6% 7% 42%	2090
October	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 7% 47%	3310
	11th to 20th	Day	13% 7% 2 7% 4 4 5% 5% 6% 8% 39%	2954
	21st to end	Day	17% 13% 4 7% 4 4 5% 5% 5% 7% 29%	3101
November	1st to 10th	Day	22% 17% 3 7% 6% 4 4 3 5% 6% 22%	1764
	11th to 20th	Day	22% 18% 5% 9% 5% 3 4 3 4 5% 23%	1608
	21st to end	Day	20% 18% 6% 12% 5% 5% 4 3 4 5% 18%	1400
December	1st to 10th	Day	12% 12% 3 10% 5% 3 4 3 5% 7% 37%	2290
	11th to 20th	Day	17% 27% 5% 9% 5% 4 3 3 4 5% 17%	2642
	21st to end	Day	18% 31% 9% 9% 4 3 3 2 4 4 12%	3498
January	1st to 10th	Night	17% 28% 8% 12% 5% 3 3 2 3 3 17%	6398
	11th to 20th	Night	11% 13% 4 8% 5% 4 4 3 5% 7% 35%	6282
	21st to end	Night	19% 22% 4 6% 4 2 3 3 5% 4 28%	6952
February	1st to 10th	Night	15% 18% 5% 9% 5% 5% 4 4 5% 5% 24%	2730
	11th to 20th	Night	14% 17% 5% 9% 5% 4 4 3 4 6% 30%	2350
	21st to end	Night	11% 16% 4 9% 5% 5% 5% 4 6% 8% 27%	1884
March	1st to 10th	Night	11% 17% 6% 9% 6% 4 5% 4 5% 6% 27%	3736
	11th to 20th	Night	11% 13% 3 6% 4 3 3 2 3 5% 47%	3737
	21st to end	Night	14% 16% 5% 10% 7% 6% 6% 4 5% 6% 20%	3491
April	1st to 10th	Night	12% 13% 3 7% 6% 5% 5% 5% 6% 7% 31%	1900
	11th to 20th	Night	13% 10% 3 8% 7% 6% 5% 4 5% 7% 33%	1804
	21st to end	Night	10% 10% 3 9% 5% 5% 6% 4 5% 7% 34%	1694
May	1st to 10th	Night	10% 6% 2 6% 6% 6% 7% 6% 7% 9% 34%	2540
	11th to 20th	Night	8% 6% 3 6% 5% 4 5% 4 5% 8% 46%	2516
	21st to end	Night	16% 11% 4 9% 6% 5% 5% 4 7% 7% 25%	2576
June	1st to 10th	Night	14% 9% 3 7% 5% 5% 6% 4 6% 8% 33%	1330
	11th to 20th	Night	12% 7% 3 9% 6% 7% 5% 6% 8% 7% 31%	1330
	21st to end	Night	8% 4 2 5% 5% 3 5% 5% 6% 8% 51%	1330
July	1st to 10th	Night	2 1 1 3 3 3 3 3 4 5% 73%	2306
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 75%	2540
	21st to end	Night	2 1 1 2 2 2 3 3 5% 8% 72%	2762
August	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 8% 66%	2529
	11th to 20th	Night	2 2 1 3 3 2 4 4 6% 11% 62%	2609
	21st to end	Night	5% 3 1 3 4 4 5% 5% 6% 9% 55%	3388
September	1st to 10th	Night	4 3 1 4 3 4 3 2 6% 8% 61%	1900
	11th to 20th	Night	7% 6% 2 5% 5% 4 4 4 7% 7% 48%	1986
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 5% 8% 49%	2090
October	1st to 10th	Night	7% 4 2 4 3 4 4 4 6% 8% 54%	3666
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 8% 43%	3976
	21st to end	Night	15% 9% 3 8% 4 5% 5% 4 5% 6% 35%	4457
November	1st to 10th	Night	23% 17% 4 8% 3 4 5% 4 4 6% 22%	2535
	11th to 20th	Night	22% 19% 3 9% 5% 4 4 3 5% 5% 22%	2771
	21st to end	Night	21% 20% 5% 9% 5% 4 3 3 4 6% 20%	2995
December	1st to 10th	Night	11% 12% 5% 10% 5% 4 5% 3 5% 6% 34%	4890
	11th to 20th	Night	17% 26% 7% 10% 5% 4 5% 4 5% 4 14%	5550
	21st to end	Night	20% 28% 8% 11% 5% 3 3 3 3 3 12%	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	55% 29% 6% 8% 1	3575
	11th to 20th	Day	31% 29% 19% 15% 6% 1	3640
	21st to end	Day	43% 31% 9% 13% 3	4004
February	1st to 10th	Day	24% 65% 3 3 3 1	1560
	11th to 20th	Day	24% 66% 5% 3 2	1722
	21st to end	Day	19% 66% 5% 6% 4	1570
March	1st to 10th	Day	35% 45% 4 8% 8%	3138
	11th to 20th	Day	19% 41% 17% 20% 3	3242
	21st to end	Day	40% 46% 6% 6% 3	4160
April	1st to 10th	Day	20% 66% 7% 6% 2	2280
	11th to 20th	Day	19% 67% 4 9% 2	2326
	21st to end	Day	19% 67% 5% 6% 3	2484
May	1st to 10th	Day	25% 47% 10% 13% 5%	4420
	11th to 20th	Day	21% 48% 11% 15% 4	4380
	21st to end	Day	32% 47% 9% 10% 2	5000
June	1st to 10th	Day	15% 67% 5% 8% 4 1	2850
	11th to 20th	Day	18% 66% 5% 7% 4	2850
	21st to end	Day	15% 65% 5% 8% 6%	2850
July	1st to 10th	Day	9% 36% 5% 14% 23% 13%	4578
	11th to 20th	Day	8% 36% 3 7% 24% 23%	4345
	21st to end	Day	12% 34% 6% 17% 18% 12%	4639
August	1st to 10th	Day	10% 37% 4 23% 21% 5%	4384
	11th to 20th	Day	14% 37% 13% 28% 7% 1	4263
	21st to end	Day	18% 37% 16% 18% 9% 1	4284
September	1st to 10th	Day	12% 61% 7% 12% 6% 1	2280
	11th to 20th	Day	17% 61% 5% 11% 5%	2193
	21st to end	Day	17% 62% 8% 8% 5%	2090
October	1st to 10th	Day	19% 37% 16% 23% 5%	3310
	11th to 20th	Day	29% 40% 7% 17% 7%	2954
	21st to end	Day	37% 37% 13% 10% 3	3101
November	1st to 10th	Day	27% 60% 7% 6%	1764
	11th to 20th	Day	29% 58% 4 5% 2 1	1608
	21st to end	Day	29% 57% 7% 5% 3	1400
December	1st to 10th	Day	30% 40% 20% 8% 1	2290
	11th to 20th	Day	50% 34% 9% 5% 1	2644
	21st to end	Day	59% 29% 6% 4 2	3498
January	1st to 10th	Night	56% 29% 7% 7% 1	6398
	11th to 20th	Night	30% 33% 13% 16% 7% 1	6320
	21st to end	Night	43% 31% 12% 11% 3	6952
February	1st to 10th	Night	24% 64% 4 4 3 1	2730
	11th to 20th	Night	23% 64% 4 6% 2 1	2352
	21st to end	Night	16% 66% 7% 6% 4	1884
March	1st to 10th	Night	29% 47% 9% 9% 5%	3736
	11th to 20th	Night	18% 42% 16% 20% 3	3738
	21st to end	Night	37% 47% 9% 5% 2	3491
April	1st to 10th	Night	18% 66% 7% 6% 3	1900
	11th to 20th	Night	16% 67% 4 10% 2	1804
	21st to end	Night	16% 68% 4 8% 5%	1694
May	1st to 10th	Night	22% 46% 10% 14% 7%	2540
	11th to 20th	Night	16% 44% 9% 22% 8% 1	2516
	21st to end	Night	30% 45% 12% 10% 3	2576
June	1st to 10th	Night	13% 67% 5% 10% 5% 1	1330
	11th to 20th	Night	16% 66% 6% 7% 4 1	1330
	21st to end	Night	11% 66% 5% 10% 8% 1	1330
July	1st to 10th	Night	7% 35% 3 17% 21% 18%	2269
	11th to 20th	Night	6% 36% 4 6% 21% 27%	2492
	21st to end	Night	9% 34% 4 18% 21% 14%	2674
August	1st to 10th	Night	12% 36% 3 21% 23% 6%	2521
	11th to 20th	Night	13% 37% 13% 28% 9% 1	2633
	21st to end	Night	20% 36% 14% 17% 12% 1	3388
September	1st to 10th	Night	11% 61% 6% 11% 9% 2	1900
	11th to 20th	Night	15% 61% 7% 10% 7%	1986
	21st to end	Night	16% 61% 8% 9% 5%	2090
October	1st to 10th	Night	15% 37% 17% 25% 6%	3666
	11th to 20th	Night	26% 39% 10% 20% 5%	3976
	21st to end	Night	33% 38% 12% 15% 2	4457
November	1st to 10th	Night	29% 58% 7% 5% 1	2535
	11th to 20th	Night	27% 60% 6% 4 2 1	2771
	21st to end	Night	27% 57% 6% 6% 3	2995
December	1st to 10th	Night	31% 42% 16% 11% 1	4890
	11th to 20th	Night	53% 34% 9% 3 1	5547
	21st to end	Night	57% 31% 5% 5% 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	28% 16% 25% 20% 10% 3	200
	11th to 20th	Day	28% 8% 31% 20% 10% 4	200
	21st to end	Day	29% 7% 33% 21% 10%	220
February	1st to 10th	Day	18% 17% 39% 21% 5% 1	190
	11th to 20th	Day	21% 17% 33% 21% 8% 1	375
	21st to end	Day	16% 17% 34% 25% 7% 1	382
March	1st to 10th	Day	27% 18% 29% 16% 9% 1	460
	11th to 20th	Day	22% 16% 24% 22% 15% 1	460
	21st to end	Day	24% 20% 25% 18% 11% 2	506
April	1st to 10th	Day	26% 22% 23% 20% 8% 2	460
	11th to 20th	Day	23% 19% 26% 19% 9% 3	452
	21st to end	Day	16% 21% 28% 21% 11% 2	460
May	1st to 10th	Day	16% 14% 29% 26% 12% 2	463
	11th to 20th	Day	20% 14% 24% 25% 15% 2	488
	21st to end	Day	17% 14% 22% 27% 17% 3	637
June	1st to 10th	Day	20% 14% 28% 24% 12% 2	680
	11th to 20th	Day	16% 16% 27% 25% 13% 3	680
	21st to end	Day	12% 9% 26% 29% 20% 4	680
July	1st to 10th	Day	6% 10% 28% 36% 18% 2	622
	11th to 20th	Day	6% 15% 33% 30% 14% 2	484
	21st to end	Day	6% 10% 32% 31% 17% 4	531
August	1st to 10th	Day	7% 9% 31% 32% 16% 5%	487
	11th to 20th	Day	4 8% 30% 31% 21% 5%	477
	21st to end	Day	8% 17% 27% 27% 17% 4	539
September	1st to 10th	Day	11% 12% 27% 28% 19% 3	490
	11th to 20th	Day	13% 12% 29% 26% 17% 2	490
	21st to end	Day	18% 19% 27% 22% 11% 3	490
October	1st to 10th	Day	15% 12% 31% 27% 14% 1	490
	11th to 20th	Day	21% 13% 26% 24% 13% 3	373
	21st to end	Day	27% 15% 26% 23% 6% 2	330
November	1st to 10th	Day	26% 13% 24% 28% 8% 1	300
	11th to 20th	Day	32% 12% 23% 20% 9% 3	213
	21st to end	Day	29% 9% 26% 21% 12% 5%	200
December	1st to 10th	Day	27% 10% 20% 22% 18% 5%	200
	11th to 20th	Day	23% 11% 29% 28% 9% 1	200
	21st to end	Day	24% 13% 22% 32% 7% 1	220
January	1st to 10th	Night	34% 8% 24% 26% 8% 1	840
	11th to 20th	Night	26% 8% 27% 29% 10% 1	800
	21st to end	Night	25% 4 28% 33% 9% 1	880
February	1st to 10th	Night	22% 9% 31% 29% 9%	780
	11th to 20th	Night	22% 6% 27% 33% 11%	744
	21st to end	Night	19% 8% 24% 32% 16% 2	628
March	1st to 10th	Night	23% 7% 22% 31% 15% 1	760
	11th to 20th	Night	22% 9% 20% 31% 17% 1	760
	21st to end	Night	21% 6% 25% 31% 15% 1	836
April	1st to 10th	Night	18% 7% 24% 31% 17% 2	760
	11th to 20th	Night	18% 7% 24% 35% 14% 2	676
	21st to end	Night	16% 10% 22% 34% 16% 1	554
May	1st to 10th	Night	11% 7% 18% 37% 26% 1	380
	11th to 20th	Night	14% 5% 19% 29% 32% 2	374
	21st to end	Night	16% 8% 19% 30% 24% 2	411
June	1st to 10th	Night	15% 7% 19% 38% 19% 2	380
	11th to 20th	Night	14% 7% 18% 39% 19% 3	380
	21st to end	Night	10% 3 17% 41% 24% 4	380
July	1st to 10th	Night	4 2 12% 43% 38% 2	370
	11th to 20th	Night	3 3 19% 40% 32% 3	368
	21st to end	Night	3 2 13% 42% 37% 3	416
August	1st to 10th	Night	5% 3 10% 35% 43% 5%	378
	11th to 20th	Night	5% 2 12% 37% 38% 6%	436
	21st to end	Night	7% 3 14% 39% 34% 3	765
September	1st to 10th	Night	6% 3 18% 39% 29% 4	760
	11th to 20th	Night	12% 4 14% 29% 36% 5%	760
	21st to end	Night	12% 6% 18% 40% 21% 4	760
October	1st to 10th	Night	16% 4 20% 36% 24% 1	760
	11th to 20th	Night	20% 5% 19% 34% 20% 1	760
	21st to end	Night	24% 5% 26% 30% 12% 2	836
November	1st to 10th	Night	32% 7% 21% 33% 7%	780
	11th to 20th	Night	31% 9% 22% 26% 12% 1	800
	21st to end	Night	31% 7% 19% 29% 12% 1	995
December	1st to 10th	Night	24% 7% 25% 29% 14% 1	1000
	11th to 20th	Night	31% 6% 26% 30% 7%	1000
	21st to end	Night	31% 8% 27% 29% 5%	1100

Page generated at 2025-02-03 00:05:51. Version ac07

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