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 2024-11-05.

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% 2 6% 4 4 4 4 6% 7% 50%	105339
	Night	7% 9% 3 7% 4 4 4 4 6% 7% 45%	102296
Transparency	Day	33% 7% 12% 22% 18% 8%	105108
	Night	38% 8% 13% 22% 14% 5%	102173
Seeing	Day	10% 13% 25% 29% 18% 5%	14309
	Night	11% 7% 21% 37% 21% 3	23270

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% 20% 6% 9% 6% 5% 4 3 6% 5% 22%	2539
	11th to 20th	Day	12% 17% 6% 10% 5% 4 5% 4 4 6% 26%	2516
	21st to end	Day	20% 21% 7% 12% 5% 4 4 3 4 4 16%	2794
February	1st to 10th	Day	8% 13% 3 9% 7% 5% 7% 5% 7% 8% 28%	1640
	11th to 20th	Day	8% 11% 3 8% 5% 4 5% 4 6% 8% 38%	1665
	21st to end	Day	5% 10% 3 10% 7% 5% 5% 5% 7% 10% 33%	1569
March	1st to 10th	Day	5% 10% 3 7% 5% 5% 5% 5% 7% 7% 40%	3138
	11th to 20th	Day	4 6% 2 5% 4 4 4 4 5% 7% 55%	3366
	21st to end	Day	9% 8% 3 8% 6% 5% 6% 5% 7% 8% 35%	4093
April	1st to 10th	Day	7% 8% 3 6% 5% 5% 7% 6% 8% 8% 36%	2280
	11th to 20th	Day	5% 7% 2 7% 6% 6% 5% 5% 7% 9% 42%	2252
	21st to end	Day	4 7% 3 7% 6% 5% 5% 5% 7% 9% 43%	2523
May	1st to 10th	Day	5% 5% 2 5% 3 3 4 4 6% 8% 55%	4420
	11th to 20th	Day	4 3 1 4 4 4 4 4 6% 9% 55%	4377
	21st to end	Day	8% 7% 3 7% 5% 6% 6% 4 7% 7% 40%	4813
June	1st to 10th	Day	7% 6% 2 5% 5% 4 5% 5% 7% 8% 45%	2660
	11th to 20th	Day	5% 5% 2 5% 4 5% 5% 4 7% 7% 51%	2660
	21st to end	Day	3 3 1 5% 4 3 3 4 5% 8% 60%	2660
July	1st to 10th	Day	1 1 1 2 2 2 3 2 4 4 78%	4420
	11th to 20th	Day	1 2 1 2 2 2 3 2 3 4 78%	4420
	21st to end	Day	2 1 1 1 2 2 2 3 4 80%	4790
August	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 72%	4401
	11th to 20th	Day	1 2 1 2 3 3 4 3 6% 8% 66%	4176
	21st to end	Day	4 4 1 4 3 4 4 3 5% 6% 63%	4187
September	1st to 10th	Day	3 2 1 2 2 3 3 3 5% 8% 68%	2280
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 6% 8% 55%	2271
	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	10% 10% 4 7% 7% 5% 5% 4 6% 7% 35%	1668
	11th to 20th	Day	8% 13% 4 10% 5% 5% 4 5% 7% 5% 34%	1520
	21st to end	Day	8% 13% 6% 11% 6% 5% 4 5% 6% 7% 29%	1520
December	1st to 10th	Day	10% 14% 5% 9% 5% 5% 5% 5% 5% 7% 29%	1520
	11th to 20th	Day	12% 16% 8% 15% 6% 5% 4 3 5% 6% 20%	1925
	21st to end	Day	11% 21% 5% 14% 7% 6% 5% 3 5% 5% 19%	2794
January	1st to 10th	Night	13% 19% 6% 12% 6% 5% 4 4 5% 6% 20%	4406
	11th to 20th	Night	13% 15% 4 8% 5% 5% 5% 4 5% 7% 28%	4382
	21st to end	Night	17% 22% 10% 15% 5% 4 4 3 3 3 13%	4862
February	1st to 10th	Night	9% 13% 4 9% 7% 5% 6% 5% 6% 7% 29%	2537
	11th to 20th	Night	10% 8% 3 8% 5% 5% 4 6% 6% 8% 37%	2402
	21st to end	Night	7% 12% 4 9% 5% 5% 5% 4 7% 8% 34%	1884
March	1st to 10th	Night	6% 9% 3 5% 5% 5% 6% 5% 8% 10% 38%	3736
	11th to 20th	Night	5% 6% 2 4 3 4 3 3 5% 7% 58%	3598
	21st to end	Night	8% 9% 3 9% 7% 6% 6% 5% 7% 8% 33%	3560
April	1st to 10th	Night	5% 7% 3 6% 6% 5% 6% 5% 6% 9% 42%	1900
	11th to 20th	Night	4 8% 3 7% 5% 5% 5% 4 6% 7% 47%	1879
	21st to end	Night	6% 7% 1 5% 4 5% 4 4 6% 9% 50%	1652
May	1st to 10th	Night	5% 4 2 5% 3 3 3 4 5% 7% 58%	2540
	11th to 20th	Night	4 3 1 4 3 4 4 4 6% 8% 58%	2516
	21st to end	Night	4 3 1 4 3 4 5% 5% 8% 10% 51%	2762
June	1st to 10th	Night	6% 4 2 6% 5% 4 5% 5% 6% 8% 50%	1520
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 6% 7% 54%	1520
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 65%	1520
July	1st to 10th	Night	1 1 2 2 2 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 1 2 1 2 2 2 4 84%	2540
	21st to end	Night	1 1 1 1 2 2 2 4 4 82%	2762
August	1st to 10th	Night	1 2 1 4 2 3 4 3 5% 6% 69%	2529
	11th to 20th	Night	1 2 1 3 3 3 3 4 5% 8% 68%	2653
	21st to end	Night	3 3 1 4 3 3 3 2 4 6% 67%	3489
September	1st to 10th	Night	3 1 1 3 3 3 4 3 5% 8% 67%	1900
	11th to 20th	Night	4 3 2 5% 3 3 4 3 5% 8% 60%	1909
	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% 7% 34%	2507
	11th to 20th	Night	9% 11% 3 7% 6% 5% 5% 4 7% 7% 35%	2660
	21st to end	Night	9% 15% 5% 10% 5% 5% 5% 4 5% 6% 31%	2660
December	1st to 10th	Night	10% 14% 5% 10% 6% 5% 5% 4 6% 7% 28%	2660
	11th to 20th	Night	12% 15% 6% 17% 8% 6% 6% 4 5% 6% 15%	3280
	21st to end	Night	14% 22% 6% 11% 6% 5% 4 3 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	62% 9% 11% 9% 8% 1	2539
	11th to 20th	Day	59% 7% 15% 16% 2 1	2540
	21st to end	Day	73% 8% 7% 9% 3	2794
February	1st to 10th	Day	53% 13% 14% 16% 3 1	1640
	11th to 20th	Day	45% 11% 17% 18% 9% 1	1674
	21st to end	Day	44% 13% 16% 17% 9% 1	1569
March	1st to 10th	Day	39% 9% 13% 22% 17% 1	3138
	11th to 20th	Day	28% 7% 14% 40% 10% 1	3366
	21st to end	Day	43% 10% 18% 17% 10% 1	4093
April	1st to 10th	Day	41% 9% 16% 22% 9% 2	2280
	11th to 20th	Day	36% 8% 16% 21% 14% 4	2252
	21st to end	Day	35% 9% 14% 25% 14% 3	2523
May	1st to 10th	Day	25% 6% 12% 22% 29% 6%	4420
	11th to 20th	Day	22% 6% 16% 32% 19% 5%	4380
	21st to end	Day	39% 10% 14% 25% 10% 1	4814
June	1st to 10th	Day	35% 7% 12% 23% 17% 6%	2660
	11th to 20th	Day	30% 7% 12% 28% 17% 5%	2660
	21st to end	Day	24% 5% 11% 29% 24% 7%	2660
July	1st to 10th	Day	11% 3 6% 16% 27% 37%	4351
	11th to 20th	Day	12% 3 4 12% 25% 45%	4345
	21st to end	Day	11% 2 4 21% 36% 26%	4639
August	1st to 10th	Day	12% 3 6% 22% 38% 18%	4384
	11th to 20th	Day	16% 5% 12% 34% 27% 6%	4218
	21st to end	Day	24% 4 10% 27% 28% 6%	4186
September	1st to 10th	Day	16% 4 10% 30% 31% 9%	2280
	11th to 20th	Day	26% 7% 11% 25% 24% 7%	2271
	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% 14% 17% 10% 1	1668
	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	63% 10% 11% 10% 5% 1	4406
	11th to 20th	Night	54% 7% 16% 17% 5%	4420
	21st to end	Night	75% 7% 7% 8% 2 1	4862
February	1st to 10th	Night	52% 12% 17% 14% 5% 1	2537
	11th to 20th	Night	42% 12% 19% 18% 9% 1	2404
	21st to end	Night	45% 12% 14% 19% 10% 1	1884
March	1st to 10th	Night	36% 10% 15% 22% 14% 2	3736
	11th to 20th	Night	25% 7% 17% 37% 13% 1	3599
	21st to end	Night	44% 14% 15% 22% 4 1	3560
April	1st to 10th	Night	38% 9% 17% 24% 10% 2	1900
	11th to 20th	Night	34% 7% 16% 24% 17% 3	1879
	21st to end	Night	31% 8% 13% 28% 17% 2	1652
May	1st to 10th	Night	24% 6% 12% 21% 34% 4	2540
	11th to 20th	Night	21% 6% 14% 32% 24% 2	2516
	21st to end	Night	23% 13% 17% 36% 11% 1	2762
June	1st to 10th	Night	31% 9% 11% 27% 18% 4	1520
	11th to 20th	Night	29% 6% 15% 29% 18% 3	1520
	21st to end	Night	19% 6% 12% 32% 25% 5%	1520
July	1st to 10th	Night	9% 2 7% 17% 27% 38%	2500
	11th to 20th	Night	7% 2 5% 15% 28% 44%	2492
	21st to end	Night	7% 2 5% 26% 31% 29%	2674
August	1st to 10th	Night	17% 2 8% 23% 29% 21%	2521
	11th to 20th	Night	16% 4 16% 35% 23% 6%	2677
	21st to end	Night	20% 4 11% 29% 28% 8%	3489
September	1st to 10th	Night	16% 5% 10% 27% 32% 10%	1900
	11th to 20th	Night	23% 6% 10% 27% 26% 8%	1909
	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% 8% 17% 17% 9% 2	2507
	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% 25% 16% 5%	180
	11th to 20th	Day	22% 8% 19% 28% 17% 6%	180
	21st to end	Day	22% 8% 17% 31% 18% 4	198
February	1st to 10th	Day	12% 11% 32% 31% 12% 2	300
	11th to 20th	Day	12% 13% 33% 29% 10% 4	365
	21st to end	Day	9% 17% 32% 25% 14% 2	382
March	1st to 10th	Day	10% 15% 32% 27% 14% 2	460
	11th to 20th	Day	10% 17% 21% 28% 21% 3	460
	21st to end	Day	10% 22% 27% 27% 13% 1	506
April	1st to 10th	Day	14% 21% 27% 25% 10% 3	460
	11th to 20th	Day	9% 27% 28% 22% 10% 5%	452
	21st to end	Day	10% 19% 29% 23% 15% 4	460
May	1st to 10th	Day	8% 14% 29% 31% 15% 3	463
	11th to 20th	Day	13% 14% 22% 27% 19% 5%	488
	21st to end	Day	9% 14% 22% 29% 19% 7%	529
June	1st to 10th	Day	11% 17% 29% 24% 17% 3	490
	11th to 20th	Day	9% 18% 33% 27% 11% 3	490
	21st to end	Day	6% 12% 24% 30% 22% 7%	490
July	1st to 10th	Day	5% 9% 24% 37% 22% 3	486
	11th to 20th	Day	3 10% 27% 30% 24% 6%	484
	21st to end	Day	3 10% 19% 37% 23% 8%	531
August	1st to 10th	Day	5% 5% 22% 34% 24% 9%	487
	11th to 20th	Day	3 6% 22% 33% 26% 11%	477
	21st to end	Day	4 12% 23% 31% 20% 10%	539
September	1st to 10th	Day	4 9% 23% 29% 23% 10%	490
	11th to 20th	Day	7% 11% 22% 30% 22% 7%	490
	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% 13% 23% 30% 17% 1	262
	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	21% 8% 16% 36% 17% 3	760
	21st to end	Night	19% 5% 18% 36% 20% 3	836
February	1st to 10th	Night	16% 6% 22% 39% 14% 2	760
	11th to 20th	Night	14% 6% 24% 36% 19% 2	744
	21st to end	Night	11% 10% 27% 31% 16% 4	628
March	1st to 10th	Night	14% 6% 22% 40% 17% 2	760
	11th to 20th	Night	9% 9% 24% 34% 21% 2	760
	21st to end	Night	12% 9% 28% 38% 13% 1	836
April	1st to 10th	Night	9% 10% 22% 36% 21% 2	760
	11th to 20th	Night	8% 9% 29% 33% 18% 2	751
	21st to end	Night	9% 9% 30% 33% 18% 2	512
May	1st to 10th	Night	5% 6% 26% 41% 19% 2	380
	11th to 20th	Night	10% 5% 17% 39% 26% 2	374
	21st to end	Night	8% 5% 14% 42% 28% 2	411
June	1st to 10th	Night	8% 7% 21% 43% 21% 1	380
	11th to 20th	Night	5% 8% 25% 42% 19% 1	380
	21st to end	Night	2 6% 20% 43% 27% 2	380
July	1st to 10th	Night	2 3 15% 49% 30% 1	370
	11th to 20th	Night	2 4 20% 47% 27% 1	368
	21st to end	Night	1 4 15% 46% 34% 1	416
August	1st to 10th	Night	2 3 12% 39% 42% 3	378
	11th to 20th	Night	2 1 12% 37% 41% 6%	467
	21st to end	Night	3 4 17% 41% 30% 4	827
September	1st to 10th	Night	3 4 18% 38% 32% 5%	760
	11th to 20th	Night	6% 5% 16% 30% 34% 9%	760
	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	20% 6% 22% 40% 12%	760
	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

Page generated at 2024-11-02 23:11:20. Version ac07

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