Central Washington University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Central Washington University Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 6% 2 6% 4 4 4 4 6% 7% 52%	147998
	Night	6% 7% 2 6% 4 4 4 4 6% 8% 48%	142613
Transparency	Day	29% 7% 12% 26% 19% 7%	147724
	Night	33% 8% 15% 25% 15% 4	142468
Seeing	Day	12% 16% 29% 26% 14% 4	16591
	Night	10% 12% 31% 31% 14% 2	25503

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	9% 14% 5% 10% 7% 6% 6% 6% 8% 8% 22%	4723
	11th to 20th	Day	8% 9% 2 5% 3 3 5% 4 8% 10% 42%	4716
	21st to end	Day	12% 12% 4 8% 5% 5% 5% 4 6% 8% 32%	5186
February	1st to 10th	Day	7% 13% 4 10% 6% 5% 4 4 6% 8% 32%	1874
	11th to 20th	Day	7% 11% 3 8% 5% 5% 5% 4 7% 8% 37%	1845
	21st to end	Day	6% 10% 4 7% 6% 5% 5% 5% 8% 8% 36%	1720
March	1st to 10th	Day	4 6% 3 8% 6% 6% 7% 5% 8% 10% 38%	4618
	11th to 20th	Day	5% 8% 3 8% 6% 6% 5% 3 6% 7% 44%	5014
	21st to end	Day	6% 8% 4 11% 7% 7% 7% 5% 7% 8% 30%	5954
April	1st to 10th	Day	6% 9% 3 8% 6% 7% 6% 6% 6% 8% 36%	2520
	11th to 20th	Day	5% 9% 3 7% 5% 6% 4 4 7% 8% 42%	2492
	21st to end	Day	5% 6% 2 6% 4 4 5% 4 8% 9% 48%	2748
May	1st to 10th	Day	4 4 2 4 3 3 4 4 5% 8% 59%	8195
	11th to 20th	Day	4 4 2 5% 4 4 4 4 6% 9% 54%	8160
	21st to end	Day	5% 5% 2 5% 4 4 4 4 6% 9% 50%	8993
June	1st to 10th	Day	7% 4 2 7% 5% 5% 5% 4 6% 7% 48%	2940
	11th to 20th	Day	4 4 1 4 4 5% 5% 4 6% 8% 57%	2940
	21st to end	Day	3 4 1 4 4 3 3 3 6% 7% 63%	2940
July	1st to 10th	Day	1 1 1 1 2 2 2 3 5% 82%	8220
	11th to 20th	Day	1 2 1 2 2 2 2 2 2 3 81%	8220
	21st to end	Day	1 2 1 2 2 1 2 2 4 5% 78%	8916
August	1st to 10th	Day	2 2 1 2 2 2 3 2 4 6% 75%	4681
	11th to 20th	Day	1 1 1 3 2 3 3 3 5% 7% 71%	4259
	21st to end	Day	4 4 1 5% 3 3 3 3 3 6% 64%	4323
September	1st to 10th	Day	2 2 1 3 3 2 3 3 4 7% 70%	2400
	11th to 20th	Day	3 4 2 5% 4 5% 4 4 6% 6% 58%	2400
	21st to end	Day	4 5% 1 6% 4 4 4 3 5% 7% 58%	2150
October	1st to 10th	Day	4 4 1 4 4 3 3 3 5% 7% 62%	3280
	11th to 20th	Day	5% 5% 2 6% 4 5% 5% 4 8% 9% 48%	3261
	21st to end	Day	7% 6% 2 7% 4 6% 6% 5% 7% 7% 42%	3242
November	1st to 10th	Day	8% 11% 3 8% 5% 5% 5% 4 7% 8% 36%	1811
	11th to 20th	Day	7% 12% 4 10% 6% 6% 5% 4 6% 7% 32%	1680
	21st to end	Day	9% 12% 4 8% 6% 5% 6% 5% 6% 7% 32%	1680
December	1st to 10th	Day	8% 9% 3 7% 4 4 5% 5% 7% 9% 38%	2700
	11th to 20th	Day	10% 16% 6% 13% 8% 6% 6% 4 5% 7% 19%	3105
	21st to end	Day	10% 15% 7% 12% 6% 6% 4 5% 6% 6% 23%	4092
January	1st to 10th	Night	9% 16% 5% 13% 6% 5% 6% 5% 8% 6% 20%	8172
	11th to 20th	Night	7% 8% 2 6% 4 4 4 3 10% 11% 40%	8182
	21st to end	Night	11% 12% 4 10% 6% 5% 5% 4 7% 9% 28%	8994
February	1st to 10th	Night	7% 13% 3 9% 5% 5% 5% 5% 6% 8% 33%	2745
	11th to 20th	Night	8% 11% 3 8% 4 4 4 4 6% 8% 42%	2662
	21st to end	Night	6% 12% 3 8% 5% 5% 5% 4 6% 8% 39%	2086
March	1st to 10th	Night	4 7% 2 6% 5% 6% 7% 5% 9% 11% 40%	5496
	11th to 20th	Night	5% 7% 2 6% 4 4 5% 5% 6% 7% 48%	5204
	21st to end	Night	7% 6% 3 7% 5% 7% 7% 5% 8% 9% 36%	5262
April	1st to 10th	Night	6% 6% 3 5% 4 5% 5% 5% 8% 10% 43%	2100
	11th to 20th	Night	5% 6% 2 6% 4 4 5% 4 5% 8% 52%	2079
	21st to end	Night	5% 5% 2 6% 4 5% 5% 4 5% 9% 51%	1867
May	1st to 10th	Night	3 3 1 4 3 3 3 3 6% 7% 63%	4724
	11th to 20th	Night	2 3 1 4 3 4 4 4 6% 8% 61%	4700
	21st to end	Night	4 3 1 4 3 3 4 3 6% 11% 57%	5178
June	1st to 10th	Night	5% 4 2 4 3 4 5% 4 7% 8% 53%	1680
	11th to 20th	Night	3 3 1 4 2 5% 4 4 5% 7% 61%	1680
	21st to end	Night	2 2 1 4 3 3 3 3 5% 7% 66%	1680
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 5% 85%	4740
	11th to 20th	Night	1 1 1 2 1 2 2 2 4 4 80%	4740
	21st to end	Night	1 1 1 2 2 2 2 2 3 5% 80%	5152
August	1st to 10th	Night	1 2 1 2 1 1 2 2 4 5% 78%	2689
	11th to 20th	Night	1 1 1 3 2 2 2 3 5% 7% 73%	2873
	21st to end	Night	3 2 1 3 2 3 3 3 4 6% 69%	3595
September	1st to 10th	Night	3 2 1 3 2 2 3 2 3 6% 74%	2000
	11th to 20th	Night	3 3 1 4 3 4 5% 4 5% 6% 64%	2000
	21st to end	Night	4 3 1 3 3 4 4 3 5% 7% 63%	2245
October	1st to 10th	Night	3 3 1 3 3 2 4 3 6% 9% 63%	3920
	11th to 20th	Night	4 4 1 4 3 4 5% 4 7% 10% 53%	3897
	21st to end	Night	7% 5% 2 7% 4 5% 4 4 7% 8% 47%	4558
November	1st to 10th	Night	11% 9% 2 7% 5% 5% 5% 5% 7% 8% 37%	2707
	11th to 20th	Night	10% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	2940
	21st to end	Night	9% 12% 3 8% 7% 4 4 4 6% 7% 35%	2940
December	1st to 10th	Night	6% 8% 2 6% 4 4 5% 7% 13% 9% 34%	4700
	11th to 20th	Night	12% 17% 6% 15% 7% 6% 6% 4 5% 6% 17%	5320
	21st to end	Night	13% 17% 6% 10% 5% 5% 5% 4 5% 7% 23%	7106

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	56% 11% 15% 14% 4	4723
	11th to 20th	Day	36% 7% 19% 30% 6% 1	4740
	21st to end	Day	50% 8% 13% 20% 9%	5187
February	1st to 10th	Day	50% 10% 17% 17% 4 2	1874
	11th to 20th	Day	44% 9% 18% 20% 8% 1	1854
	21st to end	Day	41% 12% 19% 20% 8% 1	1720
March	1st to 10th	Day	36% 9% 14% 23% 18% 1	4618
	11th to 20th	Day	37% 6% 11% 33% 12%	5014
	21st to end	Day	46% 10% 17% 21% 5%	5954
April	1st to 10th	Day	43% 10% 16% 22% 10%	2520
	11th to 20th	Day	36% 8% 15% 25% 14% 2	2492
	21st to end	Day	31% 9% 15% 28% 16% 2	2748
May	1st to 10th	Day	22% 7% 7% 32% 27% 6%	8195
	11th to 20th	Day	22% 6% 15% 34% 22% 1	8163
	21st to end	Day	28% 7% 17% 36% 12% 1	8994
June	1st to 10th	Day	31% 7% 13% 27% 17% 5%	2940
	11th to 20th	Day	23% 6% 13% 32% 22% 3	2940
	21st to end	Day	20% 4 14% 34% 21% 7%	2940
July	1st to 10th	Day	8% 2 5% 28% 31% 26%	8151
	11th to 20th	Day	11% 2 2 16% 29% 39%	8145
	21st to end	Day	10% 2 4 27% 40% 17%	8723
August	1st to 10th	Day	12% 3 6% 18% 40% 22%	4664
	11th to 20th	Day	12% 3 10% 40% 29% 6%	4299
	21st to end	Day	23% 4 10% 25% 32% 6%	4322
September	1st to 10th	Day	16% 4 9% 29% 31% 10%	2400
	11th to 20th	Day	25% 7% 13% 26% 24% 7%	2400
	21st to end	Day	26% 6% 16% 29% 19% 5%	2150
October	1st to 10th	Day	22% 5% 12% 39% 21% 1	3280
	11th to 20th	Day	31% 9% 18% 25% 17% 1	3261
	21st to end	Day	36% 7% 13% 26% 12% 5%	3242
November	1st to 10th	Day	45% 9% 18% 20% 8% 1	1811
	11th to 20th	Day	50% 10% 18% 16% 5% 1	1680
	21st to end	Day	49% 11% 17% 15% 7% 1	1680
December	1st to 10th	Day	39% 11% 22% 23% 6%	2700
	11th to 20th	Day	63% 11% 12% 11% 4	3108
	21st to end	Day	59% 9% 14% 14% 4	4092
January	1st to 10th	Night	58% 11% 15% 11% 4	8172
	11th to 20th	Night	34% 9% 21% 29% 7%	8220
	21st to end	Night	50% 9% 14% 20% 6%	8994
February	1st to 10th	Night	48% 10% 19% 17% 5% 1	2745
	11th to 20th	Night	42% 9% 17% 22% 9% 1	2664
	21st to end	Night	43% 9% 17% 21% 10%	2086
March	1st to 10th	Night	32% 9% 15% 27% 15% 2	5496
	11th to 20th	Night	32% 7% 16% 33% 11%	5205
	21st to end	Night	41% 11% 19% 24% 5%	5262
April	1st to 10th	Night	34% 9% 22% 24% 10% 1	2100
	11th to 20th	Night	30% 7% 18% 26% 16% 2	2079
	21st to end	Night	30% 7% 17% 32% 13% 1	1867
May	1st to 10th	Night	20% 6% 7% 34% 30% 3	4721
	11th to 20th	Night	19% 5% 16% 37% 22% 1	4700
	21st to end	Night	21% 6% 25% 36% 11% 1	5182
June	1st to 10th	Night	27% 8% 15% 30% 17% 3	1680
	11th to 20th	Night	20% 7% 18% 32% 21% 2	1680
	21st to end	Night	18% 4 15% 36% 22% 5%	1680
July	1st to 10th	Night	6% 2 6% 32% 29% 24%	4700
	11th to 20th	Night	9% 3 3 20% 30% 35%	4692
	21st to end	Night	9% 3 6% 29% 38% 14%	5040
August	1st to 10th	Night	11% 2 6% 16% 46% 19%	2681
	11th to 20th	Night	13% 4 15% 40% 22% 7%	2898
	21st to end	Night	17% 4 14% 28% 29% 8%	3595
September	1st to 10th	Night	14% 3 12% 30% 29% 12%	2000
	11th to 20th	Night	20% 6% 15% 27% 24% 8%	2000
	21st to end	Night	21% 6% 16% 31% 20% 6%	2245
October	1st to 10th	Night	19% 6% 12% 44% 18% 1	3920
	11th to 20th	Night	26% 9% 17% 30% 16% 1	3897
	21st to end	Night	34% 7% 14% 30% 15% 1	4558
November	1st to 10th	Night	43% 10% 16% 20% 10% 1	2707
	11th to 20th	Night	46% 11% 16% 20% 6% 1	2940
	21st to end	Night	48% 10% 17% 17% 8%	2940
December	1st to 10th	Night	38% 13% 20% 23% 6%	4700
	11th to 20th	Night	63% 13% 12% 9% 3	5316
	21st to end	Night	59% 9% 13% 14% 5%	7106

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	22% 10% 21% 29% 14% 4	220
	11th to 20th	Day	20% 12% 20% 29% 15% 5%	220
	21st to end	Day	16% 7% 24% 30% 19% 4	242
February	1st to 10th	Day	19% 16% 32% 23% 8% 1	414
	11th to 20th	Day	18% 17% 34% 22% 8% 1	425
	21st to end	Day	15% 17% 36% 23% 8% 1	441
March	1st to 10th	Day	15% 21% 30% 24% 9% 1	540
	11th to 20th	Day	15% 19% 30% 24% 10% 2	540
	21st to end	Day	17% 21% 31% 23% 8%	594
April	1st to 10th	Day	16% 25% 26% 22% 9% 3	540
	11th to 20th	Day	13% 27% 27% 21% 8% 3	532
	21st to end	Day	13% 25% 30% 21% 9% 3	540
May	1st to 10th	Day	12% 19% 31% 25% 10% 2	543
	11th to 20th	Day	16% 19% 28% 24% 11% 2	568
	21st to end	Day	12% 16% 30% 26% 13% 3	617
June	1st to 10th	Day	14% 20% 29% 21% 14% 2	570
	11th to 20th	Day	12% 25% 35% 18% 9% 2	570
	21st to end	Day	6% 18% 29% 26% 18% 4	570
July	1st to 10th	Day	5% 14% 32% 33% 13% 3	566
	11th to 20th	Day	4 15% 35% 29% 13% 3	564
	21st to end	Day	3 11% 30% 30% 21% 4	619
August	1st to 10th	Day	5% 10% 27% 33% 19% 5%	567
	11th to 20th	Day	3 8% 28% 33% 22% 6%	534
	21st to end	Day	7% 14% 24% 30% 17% 8%	583
September	1st to 10th	Day	5% 13% 28% 27% 17% 11%	530
	11th to 20th	Day	6% 14% 27% 30% 20% 3	530
	21st to end	Day	10% 17% 27% 24% 19% 4	530
October	1st to 10th	Day	12% 13% 28% 23% 19% 5%	530
	11th to 20th	Day	12% 15% 25% 27% 16% 5%	530
	21st to end	Day	15% 14% 25% 28% 14% 4	393
November	1st to 10th	Day	17% 15% 28% 26% 11% 2	307
	11th to 20th	Day	16% 14% 34% 15% 15% 5%	220
	21st to end	Day	16% 10% 28% 26% 17% 3	220
December	1st to 10th	Day	19% 10% 20% 23% 15% 12%	220
	11th to 20th	Day	16% 10% 30% 31% 11% 1	220
	21st to end	Day	14% 8% 30% 30% 16% 3	242
January	1st to 10th	Night	19% 8% 27% 30% 15% 2	840
	11th to 20th	Night	14% 8% 26% 32% 17% 2	840
	21st to end	Night	16% 6% 23% 35% 17% 3	924
February	1st to 10th	Night	13% 11% 32% 30% 13% 2	840
	11th to 20th	Night	13% 11% 30% 32% 13% 2	824
	21st to end	Night	13% 13% 29% 30% 13% 1	692
March	1st to 10th	Night	12% 9% 30% 35% 12% 1	840
	11th to 20th	Night	8% 12% 31% 33% 15% 1	840
	21st to end	Night	10% 16% 32% 31% 9% 1	924
April	1st to 10th	Night	9% 14% 35% 29% 12% 1	840
	11th to 20th	Night	9% 17% 35% 28% 10% 1	831
	21st to end	Night	9% 18% 37% 24% 11% 1	607
May	1st to 10th	Night	7% 19% 35% 29% 9% 1	420
	11th to 20th	Night	7% 14% 38% 29% 12% 1	414
	21st to end	Night	8% 12% 38% 30% 11% 1	455
June	1st to 10th	Night	9% 14% 43% 24% 10%	420
	11th to 20th	Night	7% 25% 41% 21% 5%	420
	21st to end	Night	4 16% 40% 26% 13% 1	420
July	1st to 10th	Night	4 15% 43% 30% 9%	410
	11th to 20th	Night	2 19% 51% 21% 6% 1	408
	21st to end	Night	2 17% 38% 33% 9% 1	460
August	1st to 10th	Night	2 12% 42% 30% 13% 1	418
	11th to 20th	Night	2 8% 35% 33% 17% 5%	565
	21st to end	Night	3 14% 32% 28% 19% 3	867
September	1st to 10th	Night	4 12% 30% 29% 21% 4	800
	11th to 20th	Night	5% 9% 30% 27% 23% 6%	800
	21st to end	Night	5% 12% 28% 29% 22% 4	800
October	1st to 10th	Night	8% 11% 26% 33% 21% 2	800
	11th to 20th	Night	9% 10% 26% 32% 20% 4	800
	21st to end	Night	12% 11% 29% 31% 13% 4	880
November	1st to 10th	Night	14% 7% 31% 34% 12% 1	820
	11th to 20th	Night	15% 11% 30% 29% 14% 2	840
	21st to end	Night	14% 8% 26% 33% 16% 3	840
December	1st to 10th	Night	14% 8% 27% 32% 15% 4	840
	11th to 20th	Night	20% 8% 27% 34% 11% 1	840
	21st to end	Night	18% 9% 28% 35% 9% 1	924

Page generated at 2026-08-12 00:37:19. Version ac07

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