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 2025-04-11.

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%	117371
	Night	7% 8% 3 7% 4 4 4 4 6% 8% 46%	119643
Transparency	Day	32% 7% 13% 24% 17% 7%	117138
	Night	35% 8% 15% 24% 14% 4	119521
Seeing	Day	12% 17% 30% 26% 13% 3	14834
	Night	10% 12% 31% 30% 14% 2	23990

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	11% 16% 5% 10% 6% 5% 6% 5% 7% 7% 22%	3639
	11th to 20th	Day	10% 11% 3 6% 4 3 4 4 7% 9% 40%	3616
	21st to end	Day	14% 13% 4 8% 5% 4 5% 3 6% 8% 32%	4004
February	1st to 10th	Day	7% 13% 4 10% 6% 5% 4 4 6% 7% 33%	1785
	11th to 20th	Day	8% 11% 3 8% 5% 5% 4 4 7% 8% 38%	1755
	21st to end	Day	5% 10% 4 8% 7% 5% 5% 5% 8% 8% 36%	1640
March	1st to 10th	Day	4 6% 3 8% 6% 6% 7% 5% 8% 10% 37%	4518
	11th to 20th	Day	5% 8% 3 8% 6% 6% 5% 3 5% 7% 44%	4906
	21st to end	Day	6% 8% 4 11% 7% 7% 7% 5% 7% 8% 29%	5826
April	1st to 10th	Day	6% 9% 3 8% 7% 7% 6% 6% 6% 8% 35%	2400
	11th to 20th	Day	6% 9% 3 7% 5% 6% 5% 5% 7% 7% 41%	2254
	21st to end	Day	5% 6% 2 7% 5% 4 5% 4 8% 9% 45%	2486
May	1st to 10th	Day	6% 5% 2 4 4 4 4 3 5% 7% 56%	4420
	11th to 20th	Day	4 4 1 4 3 3 3 3 5% 7% 63%	4377
	21st to end	Day	8% 6% 3 6% 5% 5% 5% 5% 6% 9% 42%	4813
June	1st to 10th	Day	7% 5% 2 7% 5% 5% 5% 5% 6% 7% 47%	2660
	11th to 20th	Day	4 4 2 4 4 5% 5% 4 7% 8% 55%	2660
	21st to end	Day	3 4 1 3 3 3 3 3 6% 7% 64%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 3 82%	4420
	11th to 20th	Day	2 2 2 2 2 2 2 3 4 81%	4420
	21st to end	Day	1 1 1 2 2 1 2 2 3 4 80%	4790
August	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 76%	4401
	11th to 20th	Day	1 1 1 3 2 3 3 3 5% 7% 71%	4072
	21st to end	Day	4 4 1 4 3 3 3 3 3 5% 65%	4191
September	1st to 10th	Day	2 2 1 3 3 2 3 3 4 7% 71%	2280
	11th to 20th	Day	3 4 2 5% 4 5% 4 4 6% 6% 56%	2280
	21st to end	Day	4 5% 1 5% 4 4 4 3 6% 6% 58%	2043
October	1st to 10th	Day	4 4 1 4 4 3 3 3 5% 7% 62%	3180
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 5% 8% 9% 48%	3161
	21st to end	Day	7% 6% 2 7% 4 6% 6% 5% 7% 7% 43%	3142
November	1st to 10th	Day	8% 10% 3 8% 5% 5% 6% 4 7% 9% 35%	1723
	11th to 20th	Day	8% 12% 4 10% 6% 6% 5% 4 6% 7% 33%	1600
	21st to end	Day	9% 12% 3 8% 6% 5% 6% 5% 6% 7% 32%	1600
December	1st to 10th	Day	8% 9% 4 7% 4 4 5% 5% 7% 9% 38%	2620
	11th to 20th	Day	10% 16% 6% 13% 8% 6% 6% 4 5% 7% 19%	3025
	21st to end	Day	10% 16% 7% 12% 6% 6% 4 5% 6% 6% 22%	4004
January	1st to 10th	Night	10% 18% 5% 13% 6% 5% 6% 6% 8% 5% 18%	6306
	11th to 20th	Night	9% 10% 3 6% 4 4 4 3 7% 9% 41%	6282
	21st to end	Night	12% 13% 4 11% 6% 4 4 3 6% 8% 28%	6952
February	1st to 10th	Night	7% 14% 4 9% 5% 5% 5% 4 6% 8% 33%	2615
	11th to 20th	Night	8% 10% 3 8% 4 4 4 4 6% 7% 42%	2532
	21st to end	Night	6% 12% 3 8% 5% 4 5% 5% 6% 8% 38%	1990
March	1st to 10th	Night	4 6% 2 6% 5% 6% 7% 5% 9% 11% 40%	5376
	11th to 20th	Night	5% 7% 2 6% 4 4 5% 5% 6% 7% 48%	5092
	21st to end	Night	7% 7% 3 7% 5% 7% 7% 5% 8% 10% 35%	5148
April	1st to 10th	Night	6% 6% 3 6% 4 5% 5% 5% 8% 10% 42%	2000
	11th to 20th	Night	5% 6% 2 7% 4 4 5% 4 5% 8% 51%	1883
	21st to end	Night	5% 5% 2 6% 4 5% 5% 4 5% 9% 50%	1690
May	1st to 10th	Night	5% 4 2 5% 4 4 4 3 4 6% 59%	2540
	11th to 20th	Night	4 3 1 4 3 3 3 3 4 7% 66%	2516
	21st to end	Night	3 3 1 5% 3 4 4 4 7% 11% 55%	2762
June	1st to 10th	Night	5% 4 2 4 3 4 5% 5% 7% 8% 52%	1520
	11th to 20th	Night	4 3 1 4 3 5% 4 4 5% 7% 59%	1520
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 67%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 4 85%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 4 84%	2540
	21st to end	Night	1 1 1 1 2 1 1 3 4 86%	2762
August	1st to 10th	Night	1 2 1 2 1 2 2 2 4 5% 78%	2529
	11th to 20th	Night	1 1 1 3 2 2 2 3 4 7% 74%	2751
	21st to end	Night	3 2 1 3 2 3 3 3 4 6% 70%	3485
September	1st to 10th	Night	3 2 1 3 2 2 2 2 3 6% 75%	1900
	11th to 20th	Night	3 3 1 4 3 4 5% 4 5% 6% 63%	1900
	21st to end	Night	4 3 1 3 3 4 3 3 6% 7% 62%	2132
October	1st to 10th	Night	3 3 1 3 3 2 4 3 6% 10% 63%	3800
	11th to 20th	Night	4 4 1 4 4 4 6% 4 7% 10% 53%	3777
	21st to end	Night	8% 5% 2 7% 4 5% 4 4 7% 8% 47%	4416
November	1st to 10th	Night	11% 9% 2 7% 5% 5% 5% 5% 7% 8% 37%	2575
	11th to 20th	Night	10% 9% 3 8% 5% 5% 5% 5% 6% 7% 38%	2800
	21st to end	Night	9% 13% 3 8% 6% 5% 4 4 6% 7% 35%	2800
December	1st to 10th	Night	6% 8% 2 6% 4 4 5% 7% 13% 9% 34%	4560
	11th to 20th	Night	12% 17% 6% 15% 7% 6% 6% 4 5% 6% 17%	5180
	21st to end	Night	13% 18% 6% 10% 5% 5% 4 4 5% 6% 23%	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	58% 11% 13% 14% 4	3639
	11th to 20th	Day	40% 8% 19% 27% 6%	3640
	21st to end	Day	52% 9% 11% 19% 9%	4004
February	1st to 10th	Day	49% 11% 17% 16% 5% 2	1785
	11th to 20th	Day	45% 9% 18% 20% 8% 1	1764
	21st to end	Day	41% 13% 19% 19% 8% 1	1640
March	1st to 10th	Day	37% 9% 14% 22% 18% 1	4518
	11th to 20th	Day	36% 6% 11% 34% 12%	4906
	21st to end	Day	46% 10% 18% 21% 5%	5826
April	1st to 10th	Day	44% 10% 15% 21% 9%	2400
	11th to 20th	Day	37% 8% 15% 24% 13% 2	2254
	21st to end	Day	33% 9% 15% 26% 15% 2	2486
May	1st to 10th	Day	25% 6% 11% 25% 25% 8%	4420
	11th to 20th	Day	20% 6% 13% 35% 23% 2	4380
	21st to end	Day	36% 9% 19% 28% 8% 1	4814
June	1st to 10th	Day	33% 8% 13% 27% 15% 5%	2660
	11th to 20th	Day	25% 6% 15% 32% 19% 3	2660
	21st to end	Day	19% 5% 13% 33% 22% 7%	2660
July	1st to 10th	Day	9% 2 6% 16% 28% 38%	4351
	11th to 20th	Day	10% 3 4 11% 24% 48%	4345
	21st to end	Day	9% 3 4 22% 39% 24%	4639
August	1st to 10th	Day	12% 3 6% 17% 40% 22%	4384
	11th to 20th	Day	13% 3 10% 40% 29% 6%	4112
	21st to end	Day	23% 4 10% 25% 32% 6%	4190
September	1st to 10th	Day	15% 4 9% 29% 31% 10%	2280
	11th to 20th	Day	25% 7% 13% 24% 24% 7%	2280
	21st to end	Day	26% 6% 17% 28% 18% 5%	2043
October	1st to 10th	Day	22% 5% 12% 38% 21% 1	3180
	11th to 20th	Day	31% 9% 18% 25% 17% 1	3161
	21st to end	Day	36% 7% 13% 26% 12% 5%	3142
November	1st to 10th	Day	45% 9% 17% 20% 8% 1	1723
	11th to 20th	Day	50% 11% 17% 16% 5% 1	1600
	21st to end	Day	48% 11% 17% 15% 8% 1	1600
December	1st to 10th	Day	38% 10% 22% 24% 6%	2620
	11th to 20th	Day	64% 11% 11% 11% 3	3028
	21st to end	Day	59% 9% 14% 14% 4	4004
January	1st to 10th	Night	61% 12% 14% 8% 4	6306
	11th to 20th	Night	37% 10% 19% 26% 7%	6320
	21st to end	Night	53% 9% 13% 20% 6%	6952
February	1st to 10th	Night	48% 10% 20% 16% 5% 1	2615
	11th to 20th	Night	42% 10% 18% 21% 9% 1	2534
	21st to end	Night	43% 9% 17% 20% 10%	1990
March	1st to 10th	Night	32% 9% 15% 27% 15% 2	5376
	11th to 20th	Night	32% 7% 16% 34% 12%	5093
	21st to end	Night	41% 11% 19% 24% 5%	5148
April	1st to 10th	Night	35% 9% 21% 24% 10% 1	2000
	11th to 20th	Night	31% 8% 18% 25% 16% 2	1883
	21st to end	Night	31% 8% 18% 29% 13% 2	1690
May	1st to 10th	Night	26% 6% 11% 30% 24% 4	2540
	11th to 20th	Night	18% 5% 14% 38% 23% 2	2516
	21st to end	Night	22% 8% 31% 31% 8% 1	2762
June	1st to 10th	Night	29% 8% 16% 29% 15% 3	1520
	11th to 20th	Night	22% 7% 20% 34% 17% 1	1520
	21st to end	Night	17% 4 14% 37% 22% 6%	1520
July	1st to 10th	Night	7% 2 8% 19% 28% 36%	2500
	11th to 20th	Night	7% 2 5% 15% 27% 44%	2492
	21st to end	Night	5% 3 6% 24% 37% 25%	2674
August	1st to 10th	Night	11% 2 6% 16% 46% 19%	2521
	11th to 20th	Night	12% 4 16% 39% 22% 6%	2776
	21st to end	Night	16% 4 14% 29% 30% 7%	3485
September	1st to 10th	Night	14% 3 12% 29% 30% 12%	1900
	11th to 20th	Night	21% 6% 15% 26% 24% 8%	1900
	21st to end	Night	21% 6% 16% 31% 19% 6%	2132
October	1st to 10th	Night	19% 6% 12% 44% 19% 1	3800
	11th to 20th	Night	26% 9% 18% 30% 16% 1	3777
	21st to end	Night	34% 7% 13% 30% 15% 1	4416
November	1st to 10th	Night	43% 10% 16% 20% 10% 1	2575
	11th to 20th	Night	45% 11% 17% 20% 6% 1	2800
	21st to end	Night	47% 9% 17% 17% 8%	2800
December	1st to 10th	Night	38% 13% 20% 23% 6%	4560
	11th to 20th	Night	64% 13% 11% 9% 3	5176
	21st to end	Night	59% 9% 13% 14% 5%	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% 22% 25% 14% 4	200
	11th to 20th	Day	22% 13% 21% 28% 13% 4	200
	21st to end	Day	17% 7% 26% 29% 17% 4	220
February	1st to 10th	Day	19% 17% 34% 22% 7% 1	385
	11th to 20th	Day	19% 18% 34% 21% 7% 1	395
	21st to end	Day	14% 18% 37% 22% 9% 1	409
March	1st to 10th	Day	14% 21% 32% 24% 8% 2	500
	11th to 20th	Day	14% 19% 29% 24% 11% 2	500
	21st to end	Day	17% 21% 32% 23% 7%	550
April	1st to 10th	Day	16% 27% 27% 21% 6% 3	500
	11th to 20th	Day	12% 29% 28% 19% 7% 4	453
	21st to end	Day	13% 27% 30% 21% 7% 2	460
May	1st to 10th	Day	12% 20% 34% 24% 8% 1	463
	11th to 20th	Day	15% 17% 28% 26% 11% 2	488
	21st to end	Day	13% 17% 31% 25% 12% 2	529
June	1st to 10th	Day	13% 20% 30% 22% 13% 1	490
	11th to 20th	Day	12% 25% 37% 17% 7% 2	490
	21st to end	Day	6% 19% 28% 27% 17% 3	490
July	1st to 10th	Day	5% 14% 33% 34% 12% 2	486
	11th to 20th	Day	4 16% 34% 29% 14% 2	484
	21st to end	Day	3 11% 31% 32% 20% 3	531
August	1st to 10th	Day	5% 11% 28% 33% 19% 5%	487
	11th to 20th	Day	3 8% 27% 34% 22% 6%	477
	21st to end	Day	6% 15% 26% 31% 14% 7%	539
September	1st to 10th	Day	5% 14% 29% 28% 16% 8%	490
	11th to 20th	Day	7% 15% 28% 30% 19% 2	490
	21st to end	Day	10% 17% 27% 25% 18% 3	490
October	1st to 10th	Day	13% 13% 29% 22% 18% 5%	490
	11th to 20th	Day	12% 15% 24% 27% 17% 6%	490
	21st to end	Day	15% 15% 26% 27% 13% 4	359
November	1st to 10th	Day	16% 16% 29% 26% 11% 2	279
	11th to 20th	Day	18% 15% 35% 14% 14% 5%	200
	21st to end	Day	17% 10% 28% 24% 19% 4	200
December	1st to 10th	Day	20% 10% 20% 21% 17% 13%	200
	11th to 20th	Day	17% 10% 30% 31% 12% 2	200
	21st to end	Day	15% 9% 31% 28% 16% 1	220
January	1st to 10th	Night	19% 8% 27% 30% 15% 2	800
	11th to 20th	Night	15% 8% 27% 32% 16% 2	800
	21st to end	Night	17% 6% 23% 34% 17% 3	880
February	1st to 10th	Night	14% 12% 33% 28% 12% 1	800
	11th to 20th	Night	13% 11% 30% 31% 13% 2	784
	21st to end	Night	13% 13% 29% 30% 13% 2	660
March	1st to 10th	Night	12% 9% 31% 36% 12% 1	800
	11th to 20th	Night	8% 12% 30% 33% 16% 1	800
	21st to end	Night	10% 16% 33% 31% 9% 1	880
April	1st to 10th	Night	9% 15% 35% 29% 12% 1	800
	11th to 20th	Night	9% 18% 35% 29% 8% 2	753
	21st to end	Night	8% 18% 39% 25% 10% 1	550
May	1st to 10th	Night	7% 20% 37% 28% 7% 1	380
	11th to 20th	Night	7% 13% 36% 30% 13% 1	374
	21st to end	Night	8% 12% 40% 29% 11% 1	411
June	1st to 10th	Night	9% 13% 44% 24% 10%	380
	11th to 20th	Night	7% 24% 43% 22% 4	380
	21st to end	Night	4 16% 38% 28% 12% 1	380
July	1st to 10th	Night	4 15% 41% 31% 8%	370
	11th to 20th	Night	1 18% 51% 22% 7% 1	368
	21st to end	Night	2 17% 37% 33% 9% 1	416
August	1st to 10th	Night	2 12% 41% 30% 14% 1	378
	11th to 20th	Night	2 8% 34% 34% 17% 5%	527
	21st to end	Night	3 15% 33% 27% 18% 3	823
September	1st to 10th	Night	4 13% 31% 29% 20% 4	760
	11th to 20th	Night	5% 9% 29% 27% 23% 6%	760
	21st to end	Night	5% 12% 29% 29% 21% 4	760
October	1st to 10th	Night	8% 11% 27% 32% 21% 2	760
	11th to 20th	Night	9% 10% 25% 32% 21% 4	760
	21st to end	Night	13% 12% 28% 30% 13% 4	836
November	1st to 10th	Night	14% 7% 32% 34% 12% 1	780
	11th to 20th	Night	15% 11% 31% 28% 14% 2	800
	21st to end	Night	14% 8% 27% 32% 16% 4	800
December	1st to 10th	Night	14% 9% 26% 32% 15% 4	800
	11th to 20th	Night	20% 8% 27% 33% 12% 1	800
	21st to end	Night	19% 10% 28% 34% 9% 1	880

Page generated at 2025-04-08 06:17:26. Version ac07

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