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 2024-01-12.

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	6% 7% 3 6% 5% 3 5% 5% 6% 8% 47%	74769
	Night	8% 8% 3 6% 6% 3 5% 5% 6% 7% 43%	74972
Transparency	Day	36% 8% 12% 22% 17% 5%	74472
	Night	40% 8% 13% 21% 14% 4	74787
Seeing	Day	9% 14% 25% 29% 18% 5%	13198
	Night	11% 7% 21% 37% 21% 3	22311

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	14% 17% 6% 9% 8% 5% 5% 5% 6% 5% 19%	1519
	11th to 20th	Day	11% 15% 7% 10% 7% 4 5% 5% 4 8% 24%	1451
	21st to end	Day	9% 14% 6% 11% 7% 4 6% 5% 5% 6% 27%	1584
February	1st to 10th	Day	8% 12% 3 7% 7% 4 7% 7% 6% 9% 29%	1554
	11th to 20th	Day	8% 10% 3 6% 6% 3 5% 5% 5% 8% 39%	1575
	21st to end	Day	5% 9% 4 9% 8% 4 5% 6% 7% 10% 33%	1479
March	1st to 10th	Day	7% 10% 4 6% 6% 4 5% 5% 7% 8% 37%	1800
	11th to 20th	Day	7% 8% 3 7% 6% 5% 6% 7% 6% 9% 36%	1908
	21st to end	Day	7% 8% 3 7% 8% 4 6% 7% 8% 8% 34%	2330
April	1st to 10th	Day	7% 7% 3 6% 6% 4 7% 7% 8% 8% 36%	2160
	11th to 20th	Day	5% 7% 3 6% 6% 4 7% 6% 7% 9% 41%	2132
	21st to end	Day	4 5% 3 6% 7% 4 6% 6% 6% 10% 44%	2390
May	1st to 10th	Day	4 5% 2 5% 5% 3 5% 6% 7% 11% 50%	2520
	11th to 20th	Day	7% 5% 3 6% 6% 4 6% 7% 7% 10% 41%	2477
	21st to end	Day	8% 5% 3 6% 6% 4 6% 6% 6% 7% 43%	2723
June	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 6% 7% 9% 45%	2520
	11th to 20th	Day	6% 4 2 5% 5% 3 5% 5% 7% 8% 51%	2520
	21st to end	Day	3 3 2 4 4 2 3 5% 5% 8% 61%	2520
July	1st to 10th	Day	2 1 1 3 3 2 4 4 5% 7% 66%	2520
	11th to 20th	Day	2 2 2 2 3 2 4 4 4 5% 69%	2520
	21st to end	Day	1 1 1 1 1 2 3 4 4 81%	2741
August	1st to 10th	Day	2 2 1 2 2 2 3 4 4 7% 71%	2507
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 6% 72%	2386
	21st to end	Day	2 2 1 2 3 3 4 5% 6% 7% 65%	2383
September	1st to 10th	Day	3 2 1 2 3 2 2 4 5% 8% 68%	2160
	11th to 20th	Day	4 3 2 4 5% 3 5% 5% 5% 8% 55%	2152
	21st to end	Day	6% 4 2 5% 4 3 4 4 5% 6% 57%	1952
October	1st to 10th	Day	6% 5% 2 5% 5% 3 4 5% 6% 8% 53%	1800
	11th to 20th	Day	6% 6% 3 5% 6% 3 5% 5% 6% 8% 48%	1793
	21st to end	Day	11% 6% 3 6% 6% 3 4 5% 6% 7% 43%	1782
November	1st to 10th	Day	10% 9% 5% 6% 7% 4 4 6% 7% 7% 35%	1632
	11th to 20th	Day	8% 11% 5% 9% 7% 3 4 6% 7% 5% 34%	1520
	21st to end	Day	9% 12% 7% 10% 7% 3 5% 6% 6% 7% 29%	1520
December	1st to 10th	Day	11% 12% 6% 8% 6% 3 6% 6% 5% 7% 30%	1520
	11th to 20th	Day	13% 15% 8% 13% 7% 4 5% 4 5% 6% 20%	1925
	21st to end	Day	11% 20% 6% 12% 8% 5% 5% 4 5% 6% 19%	2794
January	1st to 10th	Night	13% 17% 6% 11% 8% 5% 5% 4 5% 6% 19%	2646
	11th to 20th	Night	13% 14% 6% 10% 8% 4 6% 5% 6% 7% 22%	2548
	21st to end	Night	10% 13% 7% 12% 8% 4 7% 6% 5% 6% 23%	2772
February	1st to 10th	Night	9% 11% 4 8% 8% 3 6% 7% 6% 7% 31%	2403
	11th to 20th	Night	10% 6% 4 6% 6% 4 5% 7% 6% 8% 38%	2272
	21st to end	Night	7% 10% 5% 7% 6% 4 5% 5% 7% 8% 35%	1776
March	1st to 10th	Night	9% 9% 4 6% 6% 3 5% 6% 7% 10% 35%	2160
	11th to 20th	Night	7% 6% 3 5% 6% 4 6% 6% 7% 10% 41%	2049
	21st to end	Night	7% 8% 3 6% 7% 5% 6% 6% 7% 9% 37%	2026
April	1st to 10th	Night	5% 7% 3 6% 7% 4 6% 6% 6% 9% 42%	1800
	11th to 20th	Night	4 8% 3 6% 5% 4 5% 5% 5% 7% 48%	1779
	21st to end	Night	7% 6% 1 4 4 4 5% 5% 5% 9% 50%	1566
May	1st to 10th	Night	4 3 2 5% 5% 3 4 4 6% 9% 56%	1440
	11th to 20th	Night	8% 4 2 5% 5% 3 5% 5% 6% 9% 48%	1416
	21st to end	Night	7% 4 2 4 4 3 6% 6% 9% 10% 47%	1552
June	1st to 10th	Night	6% 4 2 5% 5% 4 5% 6% 6% 8% 50%	1440
	11th to 20th	Night	5% 3 2 5% 4 3 6% 5% 6% 7% 54%	1440
	21st to end	Night	2 2 1 3 4 3 3 4 4 8% 65%	1440
July	1st to 10th	Night	2 2 1 3 3 3 3 4 5% 7% 69%	1440
	11th to 20th	Night	1 1 1 1 3 2 3 3 3 6% 76%	1440
	21st to end	Night	1 2 1 2 2 4 4 84%	1576
August	1st to 10th	Night	2 2 2 2 2 2 3 3 6% 76%	1432
	11th to 20th	Night	1 2 1 2 2 2 2 3 4 7% 75%	1510
	21st to end	Night	2 1 1 2 3 2 4 4 5% 7% 69%	1971
September	1st to 10th	Night	3 1 1 2 3 2 4 4 5% 8% 67%	1800
	11th to 20th	Night	4 3 2 3 4 2 4 4 5% 8% 61%	1808
	21st to end	Night	5% 3 1 4 3 2 4 5% 6% 8% 58%	2006
October	1st to 10th	Night	6% 6% 2 5% 5% 3 4 4 6% 7% 52%	2160
	11th to 20th	Night	5% 5% 2 5% 5% 3 5% 5% 7% 7% 51%	2165
	21st to end	Night	10% 7% 3 5% 7% 3 4 5% 6% 8% 43%	2574
November	1st to 10th	Night	13% 8% 3 6% 6% 3 6% 6% 6% 8% 34%	2443
	11th to 20th	Night	9% 10% 4 6% 7% 4 5% 5% 7% 7% 35%	2660
	21st to end	Night	10% 12% 7% 9% 6% 4 5% 4 5% 6% 31%	2660
December	1st to 10th	Night	11% 13% 6% 9% 6% 4 6% 4 6% 7% 28%	2660
	11th to 20th	Night	12% 15% 6% 16% 9% 5% 6% 5% 5% 6% 15%	3280
	21st to end	Night	15% 21% 7% 9% 7% 3 5% 4 4 5% 20%	4862

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% 9% 10% 8% 6% 1	1519
	11th to 20th	Day	62% 9% 12% 14% 3	1451
	21st to end	Day	58% 9% 13% 15% 5%	1584
February	1st to 10th	Day	52% 12% 15% 16% 4 1	1554
	11th to 20th	Day	44% 10% 17% 19% 9% 1	1584
	21st to end	Day	45% 12% 16% 18% 9% 1	1479
March	1st to 10th	Day	42% 10% 16% 20% 12% 1	1800
	11th to 20th	Day	43% 9% 15% 22% 11%	1908
	21st to end	Day	45% 11% 17% 19% 7% 1	2330
April	1st to 10th	Day	42% 9% 16% 22% 9% 2	2160
	11th to 20th	Day	37% 8% 16% 21% 14% 3	2132
	21st to end	Day	34% 9% 13% 26% 14% 4	2390
May	1st to 10th	Day	27% 8% 17% 30% 15% 4	2520
	11th to 20th	Day	36% 8% 16% 26% 14% 1	2480
	21st to end	Day	38% 8% 14% 25% 13% 1	2724
June	1st to 10th	Day	35% 8% 13% 24% 16% 6%	2520
	11th to 20th	Day	30% 8% 13% 29% 17% 4	2520
	21st to end	Day	23% 6% 11% 28% 24% 8%	2520
July	1st to 10th	Day	17% 5% 10% 25% 29% 14%	2451
	11th to 20th	Day	17% 4 6% 20% 32% 21%	2445
	21st to end	Day	9% 2 5% 21% 40% 23%	2589
August	1st to 10th	Day	14% 5% 9% 24% 33% 15%	2490
	11th to 20th	Day	13% 5% 9% 26% 36% 10%	2386
	21st to end	Day	18% 5% 10% 30% 30% 6%	2383
September	1st to 10th	Day	16% 4 10% 30% 30% 9%	2160
	11th to 20th	Day	26% 8% 11% 24% 24% 8%	2152
	21st to end	Day	28% 6% 16% 27% 18% 5%	1952
October	1st to 10th	Day	29% 8% 13% 31% 16% 2	1800
	11th to 20th	Day	34% 9% 17% 26% 14% 1	1793
	21st to end	Day	41% 8% 13% 25% 12% 1	1782
November	1st to 10th	Day	48% 10% 14% 17% 10% 1	1632
	11th to 20th	Day	50% 11% 14% 16% 7% 2	1520
	21st to end	Day	54% 11% 13% 13% 8% 1	1520
December	1st to 10th	Day	54% 10% 14% 15% 7%	1520
	11th to 20th	Day	66% 11% 10% 8% 4	1928
	21st to end	Day	67% 9% 7% 10% 5%	2794
January	1st to 10th	Night	66% 9% 10% 9% 5% 1	2646
	11th to 20th	Night	61% 10% 12% 13% 3	2548
	21st to end	Night	59% 10% 12% 14% 4 1	2772
February	1st to 10th	Night	50% 12% 17% 14% 5% 1	2403
	11th to 20th	Night	41% 12% 19% 18% 8% 1	2274
	21st to end	Night	44% 12% 14% 19% 10% 1	1776
March	1st to 10th	Night	43% 10% 16% 20% 10% 1	2160
	11th to 20th	Night	38% 11% 13% 26% 11% 1	2050
	21st to end	Night	43% 10% 17% 22% 6% 1	2026
April	1st to 10th	Night	38% 9% 16% 24% 10% 2	1800
	11th to 20th	Night	34% 7% 16% 24% 17% 2	1779
	21st to end	Night	32% 8% 13% 29% 17% 2	1566
May	1st to 10th	Night	25% 7% 15% 31% 18% 4	1440
	11th to 20th	Night	32% 9% 15% 28% 15% 2	1416
	21st to end	Night	30% 9% 16% 31% 13% 1	1552
June	1st to 10th	Night	31% 9% 11% 27% 18% 3	1440
	11th to 20th	Night	29% 6% 16% 30% 17% 3	1440
	21st to end	Night	19% 6% 13% 32% 24% 5%	1440
July	1st to 10th	Night	17% 4 12% 26% 29% 12%	1400
	11th to 20th	Night	12% 4 8% 26% 31% 20%	1392
	21st to end	Night	6% 3 4 24% 40% 23%	1488
August	1st to 10th	Night	12% 3 10% 27% 30% 18%	1424
	11th to 20th	Night	12% 3 11% 28% 35% 11%	1510
	21st to end	Night	15% 5% 9% 27% 34% 8%	1971
September	1st to 10th	Night	16% 5% 11% 27% 32% 10%	1800
	11th to 20th	Night	22% 6% 10% 25% 28% 9%	1808
	21st to end	Night	23% 7% 13% 33% 17% 7%	2006
October	1st to 10th	Night	32% 8% 10% 30% 19% 1	2160
	11th to 20th	Night	32% 9% 14% 28% 16% 1	2165
	21st to end	Night	39% 8% 11% 30% 11% 1	2574
November	1st to 10th	Night	47% 9% 17% 17% 9% 2	2443
	11th to 20th	Night	47% 11% 15% 18% 7% 2	2660
	21st to end	Night	54% 9% 12% 15% 8% 1	2660
December	1st to 10th	Night	54% 8% 15% 18% 4	2660
	11th to 20th	Night	67% 14% 9% 7% 3	3276
	21st to end	Night	65% 9% 8% 9% 7% 1	4862

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	25% 11% 18% 26% 14% 5%	180
	11th to 20th	Day	22% 7% 20% 29% 16% 6%	163
	21st to end	Day	20% 7% 18% 32% 18% 4	176
February	1st to 10th	Day	11% 12% 34% 31% 11% 1	274
	11th to 20th	Day	13% 13% 33% 30% 8% 4	335
	21st to end	Day	8% 16% 32% 28% 14% 2	346
March	1st to 10th	Day	10% 16% 33% 27% 12% 2	420
	11th to 20th	Day	11% 18% 21% 28% 19% 2	420
	21st to end	Day	10% 23% 27% 26% 12% 1	462
April	1st to 10th	Day	13% 21% 28% 24% 10% 4	420
	11th to 20th	Day	9% 27% 28% 22% 9% 5%	412
	21st to end	Day	11% 19% 29% 23% 15% 4	420
May	1st to 10th	Day	8% 14% 30% 31% 14% 3	423
	11th to 20th	Day	14% 14% 21% 27% 20% 5%	448
	21st to end	Day	9% 14% 21% 30% 19% 7%	485
June	1st to 10th	Day	10% 17% 29% 23% 17% 4	450
	11th to 20th	Day	8% 17% 35% 26% 11% 2	450
	21st to end	Day	6% 12% 24% 29% 22% 7%	450
July	1st to 10th	Day	5% 9% 24% 37% 22% 3	446
	11th to 20th	Day	3 10% 28% 30% 22% 6%	444
	21st to end	Day	2 10% 21% 38% 22% 8%	487
August	1st to 10th	Day	6% 6% 23% 33% 24% 8%	447
	11th to 20th	Day	3 6% 22% 33% 26% 11%	437
	21st to end	Day	3 12% 23% 33% 21% 8%	495
September	1st to 10th	Day	5% 10% 24% 30% 23% 8%	450
	11th to 20th	Day	7% 11% 22% 31% 22% 7%	450
	21st to end	Day	9% 14% 24% 30% 19% 4	450
October	1st to 10th	Day	8% 16% 23% 27% 18% 8%	450
	11th to 20th	Day	10% 13% 22% 27% 21% 7%	443
	21st to end	Day	17% 12% 26% 27% 15% 3	297
November	1st to 10th	Day	16% 13% 22% 30% 18% 1	250
	11th to 20th	Day	14% 19% 22% 19% 16% 9%	180
	21st to end	Day	12% 8% 23% 31% 21% 6%	180
December	1st to 10th	Day	21% 9% 18% 22% 19% 11%	180
	11th to 20th	Day	16% 9% 22% 36% 13% 5%	180
	21st to end	Day	18% 6% 19% 35% 18% 5%	198
January	1st to 10th	Night	21% 9% 23% 28% 15% 3	760
	11th to 20th	Night	20% 8% 16% 35% 17% 3	728
	21st to end	Night	18% 4 19% 36% 20% 3	792
February	1st to 10th	Night	15% 6% 23% 39% 14% 2	720
	11th to 20th	Night	13% 6% 25% 36% 18% 2	704
	21st to end	Night	11% 10% 27% 32% 17% 4	592
March	1st to 10th	Night	14% 6% 22% 40% 16% 2	720
	11th to 20th	Night	9% 9% 25% 34% 21% 2	720
	21st to end	Night	12% 8% 28% 38% 12% 2	792
April	1st to 10th	Night	9% 10% 22% 36% 22% 2	720
	11th to 20th	Night	9% 9% 29% 32% 19% 2	711
	21st to end	Night	9% 9% 30% 32% 18% 2	486
May	1st to 10th	Night	4 6% 27% 41% 19% 3	360
	11th to 20th	Night	10% 5% 16% 38% 28% 3	354
	21st to end	Night	9% 4 14% 42% 29% 3	389
June	1st to 10th	Night	8% 6% 21% 42% 22% 1	360
	11th to 20th	Night	5% 8% 25% 43% 19% 1	360
	21st to end	Night	3 5% 21% 43% 27% 2	360
July	1st to 10th	Night	2 3 15% 48% 31% 1	350
	11th to 20th	Night	2 4 21% 48% 25% 1	348
	21st to end	Night	1 3 15% 47% 34% 1	394
August	1st to 10th	Night	2 3 13% 38% 42% 3	358
	11th to 20th	Night	2 2 12% 38% 40% 7%	442
	21st to end	Night	3 4 17% 41% 30% 4	783
September	1st to 10th	Night	4 4 19% 39% 30% 5%	720
	11th to 20th	Night	6% 5% 15% 30% 35% 10%	720
	21st to end	Night	7% 6% 17% 39% 26% 6%	720
October	1st to 10th	Night	11% 5% 16% 37% 26% 4	720
	11th to 20th	Night	7% 7% 21% 38% 23% 4	720
	21st to end	Night	15% 8% 22% 35% 16% 3	792
November	1st to 10th	Night	19% 6% 23% 40% 12%	740
	11th to 20th	Night	16% 8% 24% 33% 16% 4	760
	21st to end	Night	16% 8% 19% 35% 19% 4	760
December	1st to 10th	Night	18% 7% 18% 37% 18% 2	760
	11th to 20th	Night	18% 6% 19% 41% 14% 1	760
	21st to end	Night	21% 8% 19% 40% 10% 1	836

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