Chelan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chelan clear sky chart from 2005-11-06 to 2025-09-19.

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	5% 7% 3 6% 4 4 5% 4 6% 7% 49%	130434
	Night	7% 9% 3 7% 5% 4 5% 4 6% 7% 43%	127329
Transparency	Day	33% 7% 12% 23% 18% 7%	130161
	Night	39% 8% 12% 22% 14% 5%	127179
Seeing	Day	9% 13% 25% 29% 18% 6%	15386
	Night	11% 7% 20% 37% 21% 3	24306

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% 18% 6% 13% 8% 6% 5% 3 5% 5% 18%	3639
	11th to 20th	Day	8% 12% 4 8% 4 4 5% 5% 7% 8% 34%	3616
	21st to end	Day	14% 15% 6% 10% 5% 4 6% 4 6% 6% 24%	4004
February	1st to 10th	Day	8% 13% 3 9% 7% 6% 6% 5% 7% 8% 28%	1727
	11th to 20th	Day	8% 11% 3 9% 5% 4 5% 4 6% 8% 37%	1755
	21st to end	Day	5% 11% 3 10% 7% 5% 6% 5% 7% 10% 32%	1649
March	1st to 10th	Day	4 8% 3 7% 6% 6% 6% 6% 8% 8% 38%	4518
	11th to 20th	Day	4 7% 2 7% 6% 5% 5% 4 6% 8% 46%	4824
	21st to end	Day	8% 8% 3 9% 7% 6% 6% 5% 8% 9% 31%	5871
April	1st to 10th	Day	7% 8% 3 6% 6% 6% 7% 6% 8% 8% 36%	2400
	11th to 20th	Day	4 7% 2 6% 6% 5% 5% 5% 7% 9% 44%	2372
	21st to end	Day	4 6% 2 7% 6% 5% 5% 5% 6% 9% 44%	2656
May	1st to 10th	Day	4 4 2 5% 4 4 4 4 6% 7% 56%	6320
	11th to 20th	Day	4 4 2 6% 5% 5% 5% 5% 6% 9% 49%	6277
	21st to end	Day	6% 6% 2 6% 4 5% 5% 4 6% 8% 49%	6903
June	1st to 10th	Day	7% 6% 2 5% 5% 4 5% 5% 7% 8% 47%	2800
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 7% 7% 51%	2800
	21st to end	Day	3 3 1 5% 4 3 3 4 5% 7% 60%	2800
July	1st to 10th	Day	1 1 1 2 2 2 3 2 3 5% 78%	6320
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 79%	6320
	21st to end	Day	2 2 1 2 1 2 3 3 4 6% 73%	6874
August	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 71%	4541
	11th to 20th	Day	1 2 1 2 3 3 4 3 6% 8% 66%	4311
	21st to end	Day	4 4 1 4 3 4 4 3 5% 6% 62%	4319
September	1st to 10th	Day	3 2 1 3 2 3 3 3 5% 8% 67%	2400
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 5% 8% 56%	2368
	21st to end	Day	6% 4 2 5% 3 4 4 4 5% 7% 58%	2060
October	1st to 10th	Day	4 4 2 5% 3 3 3 3 5% 9% 59%	3180
	11th to 20th	Day	4 6% 3 6% 5% 5% 5% 4 6% 9% 45%	3141
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 4 6% 7% 45%	3101
November	1st to 10th	Day	9% 10% 4 7% 6% 5% 5% 4 6% 7% 35%	1719
	11th to 20th	Day	8% 13% 4 11% 5% 5% 4 5% 7% 5% 32%	1600
	21st to end	Day	9% 13% 7% 11% 6% 5% 5% 5% 6% 7% 28%	1600
December	1st to 10th	Day	6% 10% 4 10% 6% 6% 6% 6% 7% 8% 29%	2620
	11th to 20th	Day	12% 20% 8% 14% 6% 5% 4 3 5% 5% 18%	3025
	21st to end	Day	10% 21% 6% 14% 7% 7% 5% 3 4 5% 17%	4004
January	1st to 10th	Night	12% 21% 7% 13% 7% 7% 5% 3 5% 4 15%	6306
	11th to 20th	Night	9% 11% 3 7% 5% 5% 6% 5% 9% 10% 30%	6282
	21st to end	Night	13% 17% 7% 12% 5% 4 5% 4 6% 9% 19%	6952
February	1st to 10th	Night	9% 13% 4 9% 6% 5% 6% 5% 7% 7% 28%	2670
	11th to 20th	Night	10% 8% 4 8% 5% 5% 4 6% 6% 8% 36%	2532
	21st to end	Night	7% 11% 4 9% 5% 5% 5% 4 7% 8% 34%	1980
March	1st to 10th	Night	4 7% 3 6% 5% 5% 7% 6% 9% 10% 38%	5376
	11th to 20th	Night	6% 8% 3 7% 4 4 4 3 5% 7% 49%	5160
	21st to end	Night	8% 9% 3 8% 6% 6% 7% 5% 7% 9% 32%	5104
April	1st to 10th	Night	5% 7% 3 6% 6% 5% 6% 5% 6% 9% 43%	2000
	11th to 20th	Night	4 8% 3 6% 5% 4 5% 4 6% 7% 49%	1979
	21st to end	Night	6% 6% 1 5% 4 5% 4 4 5% 9% 50%	1739
May	1st to 10th	Night	4 3 1 4 3 3 4 4 5% 7% 62%	3640
	11th to 20th	Night	4 5% 3 6% 4 5% 5% 4 5% 7% 53%	3616
	21st to end	Night	5% 3 1 3 3 3 5% 4 8% 10% 55%	3972
June	1st to 10th	Night	5% 4 2 5% 4 4 5% 5% 6% 8% 51%	1600
	11th to 20th	Night	4 4 2 5% 4 4 5% 5% 6% 7% 55%	1600
	21st to end	Night	2 3 1 3 4 4 4 3 5% 8% 64%	1600
July	1st to 10th	Night	1 2 1 2 1 2 1 1 3 4 82%	3640
	11th to 20th	Night	1 1 1 1 2 2 3 4 85%	3640
	21st to end	Night	1 1 1 2 2 2 3 3 5% 5% 76%	3969
August	1st to 10th	Night	1 2 1 4 2 3 4 3 5% 6% 69%	2609
	11th to 20th	Night	2 2 1 3 3 3 3 4 5% 8% 68%	2738
	21st to end	Night	4 3 1 4 3 3 3 3 4 6% 66%	3599
September	1st to 10th	Night	2 2 1 3 3 3 4 3 5% 8% 67%	2000
	11th to 20th	Night	4 3 2 5% 3 3 4 3 5% 8% 61%	1989
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 8% 59%	2118
October	1st to 10th	Night	4 4 1 4 3 3 4 5% 7% 8% 57%	3800
	11th to 20th	Night	4 5% 2 6% 4 5% 5% 4 7% 8% 50%	3794
	21st to end	Night	8% 7% 3 9% 5% 4 4 4 6% 6% 44%	4457
November	1st to 10th	Night	13% 9% 3 7% 5% 5% 5% 5% 6% 8% 34%	2576
	11th to 20th	Night	10% 11% 3 7% 6% 5% 5% 4 7% 7% 34%	2800
	21st to end	Night	9% 15% 5% 10% 6% 5% 5% 4 5% 6% 30%	2800
December	1st to 10th	Night	6% 9% 4 10% 6% 7% 8% 6% 9% 7% 28%	4560
	11th to 20th	Night	13% 17% 7% 17% 7% 6% 5% 4 5% 5% 12%	5180
	21st to end	Night	16% 20% 6% 11% 7% 6% 5% 3 4 4 18%	6952

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	66% 8% 9% 10% 7%	3639
	11th to 20th	Day	45% 9% 18% 22% 5% 1	3640
	21st to end	Day	58% 9% 10% 19% 5%	4004
February	1st to 10th	Day	53% 12% 14% 15% 5% 2	1727
	11th to 20th	Day	46% 10% 17% 18% 9% 1	1764
	21st to end	Day	45% 13% 15% 17% 8% 1	1649
March	1st to 10th	Day	38% 9% 13% 21% 19% 1	4518
	11th to 20th	Day	35% 6% 12% 32% 13% 2	4824
	21st to end	Day	45% 10% 18% 17% 9% 1	5871
April	1st to 10th	Day	41% 9% 16% 23% 9% 2	2400
	11th to 20th	Day	34% 8% 16% 22% 15% 4	2372
	21st to end	Day	34% 9% 13% 26% 15% 3	2656
May	1st to 10th	Day	26% 4 10% 25% 29% 6%	6320
	11th to 20th	Day	30% 5% 15% 28% 17% 4	6280
	21st to end	Day	31% 8% 13% 30% 15% 2	6904
June	1st to 10th	Day	33% 7% 12% 23% 18% 6%	2800
	11th to 20th	Day	29% 7% 12% 27% 19% 5%	2800
	21st to end	Day	24% 5% 11% 30% 23% 7%	2800
July	1st to 10th	Day	11% 3 6% 21% 29% 30%	6251
	11th to 20th	Day	12% 2 3 20% 31% 32%	6245
	21st to end	Day	14% 2 5% 27% 34% 19%	6681
August	1st to 10th	Day	13% 3 6% 23% 37% 18%	4524
	11th to 20th	Day	16% 5% 12% 34% 27% 6%	4353
	21st to end	Day	24% 4 10% 27% 29% 7%	4318
September	1st to 10th	Day	17% 4 10% 30% 30% 9%	2400
	11th to 20th	Day	26% 7% 11% 26% 23% 7%	2368
	21st to end	Day	27% 6% 16% 28% 17% 5%	2060
October	1st to 10th	Day	23% 5% 14% 36% 20% 2	3180
	11th to 20th	Day	34% 7% 17% 23% 18%	3141
	21st to end	Day	39% 7% 12% 25% 14% 4	3101
November	1st to 10th	Day	48% 9% 15% 17% 10% 1	1719
	11th to 20th	Day	51% 11% 14% 16% 7% 2	1600
	21st to end	Day	55% 11% 13% 12% 8% 1	1600
December	1st to 10th	Day	47% 12% 17% 19% 5%	2620
	11th to 20th	Day	69% 9% 8% 10% 4	3028
	21st to end	Day	69% 9% 8% 10% 4	4004
January	1st to 10th	Night	72% 8% 9% 8% 4	6306
	11th to 20th	Night	46% 12% 16% 20% 6%	6320
	21st to end	Night	60% 11% 12% 14% 3	6952
February	1st to 10th	Night	51% 11% 17% 14% 6% 1	2670
	11th to 20th	Night	43% 11% 18% 18% 8% 1	2534
	21st to end	Night	45% 12% 14% 18% 9% 1	1980
March	1st to 10th	Night	36% 11% 13% 24% 14% 2	5376
	11th to 20th	Night	34% 6% 14% 29% 16% 1	5161
	21st to end	Night	44% 13% 17% 20% 6% 1	5104
April	1st to 10th	Night	37% 8% 17% 25% 10% 2	2000
	11th to 20th	Night	33% 7% 16% 25% 18% 3	1979
	21st to end	Night	31% 8% 12% 30% 18% 2	1739
May	1st to 10th	Night	21% 5% 9% 25% 36% 4	3637
	11th to 20th	Night	28% 5% 13% 31% 21% 1	3616
	21st to end	Night	22% 9% 17% 37% 13% 1	3972
June	1st to 10th	Night	29% 9% 11% 27% 19% 5%	1600
	11th to 20th	Night	28% 6% 15% 29% 20% 3	1600
	21st to end	Night	20% 6% 13% 32% 24% 5%	1600
July	1st to 10th	Night	10% 2 6% 24% 28% 31%	3600
	11th to 20th	Night	6% 2 4 23% 33% 32%	3592
	21st to end	Night	12% 3 6% 29% 31% 20%	3857
August	1st to 10th	Night	17% 2 8% 24% 29% 20%	2601
	11th to 20th	Night	16% 4 16% 35% 23% 6%	2762
	21st to end	Night	21% 4 10% 29% 28% 8%	3599
September	1st to 10th	Night	16% 5% 11% 27% 31% 10%	2000
	11th to 20th	Night	22% 6% 10% 28% 26% 8%	1989
	21st to end	Night	23% 7% 14% 33% 17% 6%	2118
October	1st to 10th	Night	24% 6% 12% 40% 18% 1	3800
	11th to 20th	Night	32% 7% 15% 29% 16% 1	3794
	21st to end	Night	40% 6% 12% 27% 13% 2	4457
November	1st to 10th	Night	47% 9% 17% 17% 9% 2	2576
	11th to 20th	Night	47% 11% 14% 18% 7% 2	2800
	21st to end	Night	56% 9% 12% 15% 8% 1	2800
December	1st to 10th	Night	44% 17% 16% 19% 3	4560
	11th to 20th	Night	71% 11% 8% 6% 3	5176
	21st to end	Night	68% 8% 7% 10% 6% 1	6952

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	24% 11% 20% 28% 15% 5%	200
	11th to 20th	Day	20% 7% 21% 29% 18% 6%	200
	21st to end	Day	20% 7% 18% 33% 18% 5%	220
February	1st to 10th	Day	11% 11% 32% 30% 13% 2	327
	11th to 20th	Day	12% 12% 31% 30% 11% 4	395
	21st to end	Day	10% 16% 31% 26% 14% 2	414
March	1st to 10th	Day	10% 14% 30% 28% 16% 2	500
	11th to 20th	Day	11% 17% 22% 27% 19% 3	500
	21st to end	Day	11% 21% 27% 27% 12% 1	550
April	1st to 10th	Day	14% 20% 26% 26% 10% 4	500
	11th to 20th	Day	9% 26% 27% 23% 11% 4	492
	21st to end	Day	10% 18% 28% 23% 16% 5%	500
May	1st to 10th	Day	8% 14% 28% 31% 16% 3	503
	11th to 20th	Day	13% 14% 23% 26% 19% 5%	528
	21st to end	Day	9% 14% 21% 29% 19% 7%	573
June	1st to 10th	Day	10% 16% 28% 25% 17% 5%	530
	11th to 20th	Day	8% 17% 33% 26% 12% 3	530
	21st to end	Day	6% 11% 24% 29% 22% 8%	530
July	1st to 10th	Day	5% 9% 24% 37% 22% 3	526
	11th to 20th	Day	3 10% 27% 29% 24% 6%	524
	21st to end	Day	3 9% 19% 36% 23% 10%	575
August	1st to 10th	Day	5% 5% 22% 34% 24% 9%	527
	11th to 20th	Day	3 7% 22% 32% 25% 11%	517
	21st to end	Day	4 11% 22% 31% 21% 10%	583
September	1st to 10th	Day	4 9% 22% 28% 24% 12%	530
	11th to 20th	Day	7% 10% 22% 30% 23% 8%	523
	21st to end	Day	9% 13% 24% 30% 19% 4	490
October	1st to 10th	Day	8% 16% 22% 27% 18% 8%	490
	11th to 20th	Day	10% 12% 21% 27% 21% 8%	482
	21st to end	Day	15% 12% 26% 29% 14% 3	330
November	1st to 10th	Day	16% 12% 22% 31% 18% 1	277
	11th to 20th	Day	15% 18% 23% 20% 16% 9%	200
	21st to end	Day	13% 8% 21% 31% 22% 7%	200
December	1st to 10th	Day	20% 9% 17% 21% 20% 15%	200
	11th to 20th	Day	20% 9% 21% 34% 13% 5%	200
	21st to end	Day	16% 5% 21% 36% 17% 4	220
January	1st to 10th	Night	22% 9% 23% 28% 16% 3	800
	11th to 20th	Night	20% 8% 17% 37% 17% 3	800
	21st to end	Night	18% 4 18% 36% 20% 3	880
February	1st to 10th	Night	15% 6% 23% 39% 14% 2	800
	11th to 20th	Night	14% 5% 24% 36% 18% 2	784
	21st to end	Night	11% 10% 26% 33% 16% 3	660
March	1st to 10th	Night	13% 6% 21% 40% 18% 2	800
	11th to 20th	Night	9% 9% 25% 35% 21% 2	800
	21st to end	Night	12% 9% 27% 38% 13% 1	880
April	1st to 10th	Night	9% 10% 22% 36% 21% 2	800
	11th to 20th	Night	8% 10% 29% 34% 17% 2	791
	21st to end	Night	9% 9% 29% 32% 18% 2	539
May	1st to 10th	Night	5% 6% 26% 41% 20% 2	400
	11th to 20th	Night	10% 5% 18% 39% 26% 2	394
	21st to end	Night	9% 5% 14% 42% 28% 3	433
June	1st to 10th	Night	7% 7% 21% 43% 21% 1	400
	11th to 20th	Night	5% 8% 26% 42% 19% 1	400
	21st to end	Night	2 6% 20% 43% 27% 2	400
July	1st to 10th	Night	2 3 15% 47% 30% 2	390
	11th to 20th	Night	2 4 20% 47% 27% 1	388
	21st to end	Night	1 3 15% 45% 35% 2	438
August	1st to 10th	Night	2 3 12% 39% 42% 3	398
	11th to 20th	Night	3 2 11% 38% 40% 6%	492
	21st to end	Night	4 4 16% 41% 31% 4	871
September	1st to 10th	Night	3 4 18% 37% 34% 5%	800
	11th to 20th	Night	6% 5% 15% 30% 35% 9%	792
	21st to end	Night	6% 6% 18% 39% 26% 6%	760
October	1st to 10th	Night	11% 6% 16% 38% 26% 3	760
	11th to 20th	Night	7% 8% 21% 37% 23% 4	760
	21st to end	Night	14% 8% 22% 36% 16% 3	836
November	1st to 10th	Night	19% 6% 22% 40% 12%	780
	11th to 20th	Night	17% 8% 23% 34% 15% 3	800
	21st to end	Night	16% 8% 18% 34% 19% 5%	800
December	1st to 10th	Night	17% 6% 17% 37% 20% 3	800
	11th to 20th	Night	19% 6% 19% 40% 14% 2	800
	21st to end	Night	21% 8% 19% 40% 10% 1	880

Page generated at 2025-09-15 22:49:14. Version ac07

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