Walla Walla
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walla Walla clear sky chart from 2005-11-06 to 2025-12-18.

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	10% 7% 2 6% 4 4 4 3 5% 7% 47%	136175
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	132148
Transparency	Day	22% 40% 7% 14% 12% 6%	130881
	Night	26% 40% 7% 13% 9% 4	128445
Seeing	Day	13% 11% 24% 29% 18% 5%	15722
	Night	17% 9% 21% 31% 18% 4	24867

By Month

Percentage Cloud Cover SW (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	24% 18% 6% 15% 7% 5% 5% 4 4 4 8%	3639
	11th to 20th	Day	15% 13% 5% 10% 5% 5% 6% 5% 6% 7% 23%	3640
	21st to end	Day	17% 11% 4 9% 5% 5% 5% 4 5% 6% 29%	4004
February	1st to 10th	Day	21% 14% 4 8% 6% 4 5% 4 5% 6% 22%	1700
	11th to 20th	Day	21% 13% 4 9% 4 4 3 4 4 6% 27%	1868
	21st to end	Day	16% 13% 4 8% 5% 6% 6% 5% 5% 6% 26%	1650
March	1st to 10th	Day	9% 10% 3 8% 5% 5% 5% 5% 8% 10% 32%	4518
	11th to 20th	Day	13% 10% 3 7% 5% 4 5% 4 5% 7% 38%	4677
	21st to end	Day	17% 11% 4 10% 6% 6% 5% 5% 6% 8% 22%	5969
April	1st to 10th	Day	13% 9% 3 8% 5% 6% 5% 5% 7% 10% 29%	2400
	11th to 20th	Day	10% 8% 2 7% 5% 4 5% 4 6% 7% 41%	2369
	21st to end	Day	12% 6% 3 7% 5% 4 5% 4 6% 6% 41%	2594
May	1st to 10th	Day	11% 4 2 4 3 3 3 4 6% 8% 53%	6187
	11th to 20th	Day	8% 6% 3 7% 4 5% 4 4 5% 6% 47%	6277
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 7% 8% 51%	6902
June	1st to 10th	Day	12% 7% 2 6% 5% 4 4 4 5% 7% 45%	2800
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 49%	2800
	21st to end	Day	4 3 1 4 3 2 3 3 5% 6% 66%	2800
July	1st to 10th	Day	2 2 1 3 2 2 2 2 3 4 79%	6319
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 82%	6320
	21st to end	Day	2 2 2 1 2 2 2 4 7% 78%	6880
August	1st to 10th	Day	4 4 1 4 3 3 3 3 4 6% 66%	6210
	11th to 20th	Day	3 2 1 2 2 2 3 2 4 6% 74%	5846
	21st to end	Day	3 3 1 4 3 4 3 3 4 6% 65%	5984
September	1st to 10th	Day	3 2 1 2 2 2 2 3 4 7% 72%	2400
	11th to 20th	Day	7% 5% 1 4 3 3 4 3 5% 6% 57%	2320
	21st to end	Day	8% 5% 2 5% 3 4 3 3 5% 7% 55%	2200
October	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 6% 61%	3408
	11th to 20th	Day	9% 6% 2 5% 3 4 5% 4 9% 8% 44%	3171
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 4 6% 7% 35%	3200
November	1st to 10th	Day	22% 9% 2 7% 5% 4 6% 4 5% 8% 28%	1854
	11th to 20th	Day	19% 11% 4 10% 7% 6% 6% 5% 6% 7% 19%	1801
	21st to end	Day	18% 12% 4 10% 7% 4 6% 5% 6% 6% 21%	1680
December	1st to 10th	Day	15% 9% 3 11% 7% 9% 7% 6% 7% 10% 18%	2700
	11th to 20th	Day	22% 13% 6% 15% 9% 6% 5% 4 4 4 12%	3084
	21st to end	Day	23% 11% 5% 12% 9% 7% 6% 4 4 5% 15%	4004
January	1st to 10th	Night	28% 19% 6% 15% 6% 5% 3 2 3 3 8%	6306
	11th to 20th	Night	17% 14% 5% 9% 5% 4 5% 3 5% 6% 27%	6320
	21st to end	Night	15% 9% 4 8% 5% 5% 5% 4 5% 6% 32%	6952
February	1st to 10th	Night	22% 15% 4 9% 5% 4 4 3 5% 6% 24%	2696
	11th to 20th	Night	19% 14% 3 7% 6% 3 3 4 5% 5% 31%	2420
	21st to end	Night	17% 17% 4 8% 6% 4 4 3 4 6% 27%	1980
March	1st to 10th	Night	11% 9% 2 8% 4 4 4 5% 7% 8% 37%	5376
	11th to 20th	Night	15% 13% 3 7% 4 4 3 3 4 5% 39%	5319
	21st to end	Night	16% 12% 4 9% 5% 6% 5% 4 7% 8% 24%	5006
April	1st to 10th	Night	17% 12% 2 7% 4 5% 5% 3 5% 7% 32%	2000
	11th to 20th	Night	14% 8% 2 6% 5% 4 4 3 5% 7% 43%	1979
	21st to end	Night	12% 8% 3 7% 5% 4 5% 4 6% 7% 39%	1805
May	1st to 10th	Night	11% 4 1 4 4 4 4 3 5% 6% 53%	3763
	11th to 20th	Night	15% 7% 3 7% 4 3 3 3 5% 6% 43%	3616
	21st to end	Night	14% 7% 2 5% 4 3 3 3 4 6% 48%	3972
June	1st to 10th	Night	14% 6% 2 5% 4 4 3 4 6% 8% 44%	1600
	11th to 20th	Night	13% 6% 3 5% 5% 3 4 4 4 6% 47%	1600
	21st to end	Night	6% 3 1 4 3 3 4 3 5% 6% 60%	1600
July	1st to 10th	Night	2 2 1 3 2 2 3 3 4 5% 73%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 84%	3640
	21st to end	Night	3 2 1 2 1 1 2 2 4 7% 76%	3970
August	1st to 10th	Night	3 3 1 4 3 3 4 3 5% 7% 65%	3582
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 5% 71%	3995
	21st to end	Night	4 4 2 4 3 3 3 2 4 6% 65%	5015
September	1st to 10th	Night	3 2 2 3 2 3 3 2 3 6% 72%	2000
	11th to 20th	Night	6% 4 1 5% 3 3 3 3 6% 7% 58%	2080
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 7% 57%	2200
October	1st to 10th	Night	7% 4 2 5% 3 3 3 2 3 5% 63%	3788
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 7% 7% 47%	3983
	21st to end	Night	15% 7% 3 6% 3 4 5% 3 7% 7% 38%	4600
November	1st to 10th	Night	22% 10% 3 7% 4 4 4 4 4 6% 33%	2665
	11th to 20th	Night	23% 10% 4 9% 6% 4 4 4 5% 5% 25%	2814
	21st to end	Night	19% 11% 4 7% 7% 5% 4 3 6% 6% 27%	2940
December	1st to 10th	Night	13% 8% 3 8% 6% 6% 5% 6% 7% 7% 30%	4700
	11th to 20th	Night	22% 17% 5% 14% 8% 7% 5% 4 4 4 11%	5274
	21st to end	Night	21% 13% 4 12% 9% 6% 5% 4 5% 6% 16%	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	57% 34% 6% 2 1	3639
	11th to 20th	Day	38% 32% 15% 13% 2	3640
	21st to end	Day	36% 32% 7% 18% 7%	4004
February	1st to 10th	Day	28% 64% 4 2 1	1700
	11th to 20th	Day	27% 64% 4 4 1	1877
	21st to end	Day	23% 64% 4 5% 3 1	1650
March	1st to 10th	Day	29% 32% 9% 20% 10% 1	4518
	11th to 20th	Day	29% 31% 8% 24% 8%	4677
	21st to end	Day	39% 35% 10% 12% 3	5969
April	1st to 10th	Day	18% 64% 8% 7% 2	2400
	11th to 20th	Day	15% 65% 5% 8% 6% 1	2372
	21st to end	Day	15% 64% 5% 10% 6%	2594
May	1st to 10th	Day	17% 33% 7% 16% 23% 4	6187
	11th to 20th	Day	24% 31% 9% 19% 14% 3	6280
	21st to end	Day	17% 33% 14% 20% 14% 1	6904
June	1st to 10th	Day	10% 63% 5% 12% 8% 2	2800
	11th to 20th	Day	12% 63% 4 10% 9% 2	2800
	21st to end	Day	8% 61% 4 12% 12% 3	2800
July	1st to 10th	Day	9% 25% 3 11% 22% 30%	6251
	11th to 20th	Day	6% 25% 2 9% 25% 33%	6245
	21st to end	Day	8% 25% 5% 21% 25% 16%	6681
August	1st to 10th	Day	12% 35% 3 6% 22% 22%	4524
	11th to 20th	Day	9% 36% 4 28% 19% 3	4236
	21st to end	Day	13% 36% 5% 19% 20% 7%	4311
September	1st to 10th	Day	7% 58% 6% 13% 12% 4	2400
	11th to 20th	Day	14% 58% 6% 9% 11% 2	2320
	21st to end	Day	14% 59% 5% 13% 7% 1	2200
October	1st to 10th	Day	15% 37% 5% 23% 18% 3	3408
	11th to 20th	Day	23% 38% 11% 19% 9%	3171
	21st to end	Day	30% 39% 5% 20% 6%	3200
November	1st to 10th	Day	26% 56% 8% 9% 1	1854
	11th to 20th	Day	30% 58% 6% 5% 1	1801
	21st to end	Day	28% 56% 8% 6% 2	1680
December	1st to 10th	Day	38% 42% 13% 7%	2700
	11th to 20th	Day	47% 40% 7% 5%	3084
	21st to end	Day	45% 34% 11% 9% 1	4004
January	1st to 10th	Night	60% 32% 5% 3	6306
	11th to 20th	Night	39% 31% 15% 11% 4	6320
	21st to end	Night	33% 33% 10% 16% 8%	6952
February	1st to 10th	Night	30% 63% 4 2 1	2696
	11th to 20th	Night	26% 62% 3 7% 2	2422
	21st to end	Night	24% 63% 3 7% 2 1	1980
March	1st to 10th	Night	27% 32% 5% 19% 16% 1	5376
	11th to 20th	Night	32% 31% 4 25% 8%	5319
	21st to end	Night	39% 35% 9% 14% 3	5006
April	1st to 10th	Night	21% 63% 6% 7% 3	2000
	11th to 20th	Night	15% 63% 5% 9% 6% 1	1979
	21st to end	Night	15% 64% 4 9% 8% 1	1805
May	1st to 10th	Night	21% 31% 8% 15% 22% 4	3760
	11th to 20th	Night	29% 31% 7% 23% 10% 1	3616
	21st to end	Night	22% 33% 10% 22% 12%	3972
June	1st to 10th	Night	9% 63% 3 13% 10% 2	1600
	11th to 20th	Night	14% 62% 3 10% 8% 3	1600
	21st to end	Night	11% 61% 3 11% 9% 4	1600
July	1st to 10th	Night	10% 25% 3 11% 18% 33%	3600
	11th to 20th	Night	4 24% 3 7% 24% 38%	3592
	21st to end	Night	9% 24% 3 18% 27% 20%	3857
August	1st to 10th	Night	12% 35% 3 7% 17% 26%	2601
	11th to 20th	Night	10% 36% 4 26% 21% 4	2882
	21st to end	Night	11% 35% 5% 22% 15% 12%	3608
September	1st to 10th	Night	9% 57% 5% 12% 11% 6%	2000
	11th to 20th	Night	13% 58% 5% 11% 10% 4	2080
	21st to end	Night	13% 58% 5% 13% 10% 1	2200
October	1st to 10th	Night	15% 36% 5% 19% 24% 2	3788
	11th to 20th	Night	20% 38% 10% 15% 16% 1	3983
	21st to end	Night	29% 38% 5% 21% 7% 1	4600
November	1st to 10th	Night	25% 56% 6% 10% 3	2665
	11th to 20th	Night	26% 58% 5% 8% 2	2814
	21st to end	Night	25% 56% 7% 8% 3	2940
December	1st to 10th	Night	29% 41% 16% 13% 2	4700
	11th to 20th	Night	50% 37% 8% 4 1	5274
	21st to end	Night	41% 36% 11% 10% 3	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	31% 16% 19% 19% 13% 3	200
	11th to 20th	Day	23% 10% 23% 29% 12% 5%	200
	21st to end	Day	26% 10% 22% 25% 15% 2	220
February	1st to 10th	Day	29% 17% 28% 16% 9% 1	300
	11th to 20th	Day	30% 18% 21% 21% 10% 1	452
	21st to end	Day	21% 14% 25% 27% 11% 1	414
March	1st to 10th	Day	19% 17% 29% 23% 11% 1	500
	11th to 20th	Day	16% 17% 22% 25% 17% 3	500
	21st to end	Day	17% 18% 32% 23% 8% 2	550
April	1st to 10th	Day	19% 15% 27% 22% 12% 5%	500
	11th to 20th	Day	14% 18% 31% 23% 11% 3	492
	21st to end	Day	11% 18% 26% 26% 14% 5%	500
May	1st to 10th	Day	10% 14% 26% 29% 16% 4	503
	11th to 20th	Day	17% 9% 25% 26% 18% 5%	528
	21st to end	Day	10% 9% 24% 28% 21% 8%	573
June	1st to 10th	Day	15% 10% 18% 28% 22% 7%	530
	11th to 20th	Day	11% 15% 31% 28% 13% 2	530
	21st to end	Day	4 7% 26% 32% 24% 6%	530
July	1st to 10th	Day	3 4 21% 41% 27% 3	526
	11th to 20th	Day	2 4 26% 41% 22% 4	524
	21st to end	Day	2 5% 20% 42% 25% 6%	575
August	1st to 10th	Day	3 4 20% 39% 26% 7%	527
	11th to 20th	Day	2 4 18% 36% 32% 7%	517
	21st to end	Day	6% 9% 21% 31% 23% 9%	583
September	1st to 10th	Day	5% 5% 19% 33% 27% 11%	530
	11th to 20th	Day	9% 8% 23% 29% 25% 7%	530
	21st to end	Day	9% 8% 23% 29% 26% 5%	530
October	1st to 10th	Day	10% 9% 23% 29% 23% 5%	530
	11th to 20th	Day	14% 13% 20% 27% 20% 6%	478
	21st to end	Day	21% 10% 24% 27% 12% 4	363
November	1st to 10th	Day	25% 15% 26% 23% 9% 2	330
	11th to 20th	Day	23% 13% 22% 27% 13% 2	283
	21st to end	Day	19% 10% 22% 28% 19% 2	220
December	1st to 10th	Day	23% 16% 16% 21% 14% 10%	220
	11th to 20th	Day	30% 15% 24% 20% 10% 1	214
	21st to end	Day	26% 13% 25% 22% 11% 2	220
January	1st to 10th	Night	33% 12% 17% 24% 12% 2	800
	11th to 20th	Night	24% 11% 20% 28% 14% 3	800
	21st to end	Night	23% 6% 21% 32% 16% 2	880
February	1st to 10th	Night	28% 12% 21% 27% 10% 2	800
	11th to 20th	Night	26% 11% 19% 29% 14% 2	784
	21st to end	Night	22% 16% 21% 28% 10% 2	660
March	1st to 10th	Night	18% 11% 27% 29% 13% 3	800
	11th to 20th	Night	17% 13% 22% 27% 19% 2	800
	21st to end	Night	21% 13% 27% 27% 10% 1	880
April	1st to 10th	Night	21% 10% 23% 31% 12% 2	800
	11th to 20th	Night	15% 12% 27% 27% 16% 3	791
	21st to end	Night	14% 12% 25% 30% 16% 3	605
May	1st to 10th	Night	11% 9% 28% 31% 19% 2	458
	11th to 20th	Night	14% 7% 18% 33% 22% 6%	394
	21st to end	Night	12% 9% 17% 35% 21% 6%	433
June	1st to 10th	Night	12% 9% 23% 34% 19% 4	400
	11th to 20th	Night	13% 12% 30% 31% 14% 2	400
	21st to end	Night	6% 6% 23% 38% 26% 3	400
July	1st to 10th	Night	4 3 22% 40% 28% 4	390
	11th to 20th	Night	3 4 28% 40% 22% 4	388
	21st to end	Night	1 6% 19% 40% 30% 4	438
August	1st to 10th	Night	5% 2 19% 42% 26% 6%	398
	11th to 20th	Night	3 3 16% 37% 31% 11%	560
	21st to end	Night	5% 4 19% 38% 28% 7%	880
September	1st to 10th	Night	3 3 18% 38% 29% 8%	800
	11th to 20th	Night	8% 3 18% 31% 30% 10%	800
	21st to end	Night	10% 6% 18% 30% 28% 8%	800
October	1st to 10th	Night	13% 2 19% 39% 22% 4	800
	11th to 20th	Night	15% 9% 19% 28% 22% 6%	800
	21st to end	Night	18% 10% 21% 31% 14% 6%	880
November	1st to 10th	Night	24% 10% 22% 29% 13% 1	820
	11th to 20th	Night	26% 11% 21% 28% 10% 3	840
	21st to end	Night	22% 9% 18% 31% 16% 4	840
December	1st to 10th	Night	23% 8% 20% 28% 18% 4	840
	11th to 20th	Night	28% 12% 20% 29% 11% 1	828
	21st to end	Night	32% 12% 22% 24% 9% 1	880

Page generated at 2025-12-14 22:13:54. Version ac07

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