The Dalles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Dalles clear sky chart from 2005-11-06 to 2026-08-30.

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% 6% 2 6% 4 4 4 4 5% 7% 51%	153282
	Night	9% 8% 3 7% 4 4 4 4 6% 7% 45%	146250
Transparency	Day	31% 7% 13% 24% 18% 7%	148032
	Night	36% 8% 15% 23% 14% 5%	142502
Seeing	Day	10% 16% 29% 27% 15% 3	16807
	Night	10% 10% 25% 36% 17% 2	25734

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	18% 17% 7% 13% 8% 7% 6% 4 5% 6% 10%	4722
	11th to 20th	Day	12% 10% 3 7% 4 4 5% 3 6% 9% 36%	4740
	21st to end	Day	14% 13% 5% 12% 6% 4 5% 4 5% 8% 24%	5403
February	1st to 10th	Day	12% 12% 4 11% 8% 5% 6% 5% 7% 7% 23%	1890
	11th to 20th	Day	11% 10% 3 9% 6% 6% 6% 5% 8% 7% 29%	1845
	21st to end	Day	10% 11% 3 8% 6% 5% 6% 6% 8% 8% 30%	1714
March	1st to 10th	Day	7% 8% 4 8% 5% 6% 7% 6% 9% 10% 29%	4618
	11th to 20th	Day	9% 10% 3 9% 6% 5% 5% 4 5% 7% 37%	5110
	21st to end	Day	9% 9% 3 9% 7% 7% 7% 6% 7% 9% 27%	5873
April	1st to 10th	Day	8% 8% 2 8% 6% 5% 7% 4 7% 9% 36%	2520
	11th to 20th	Day	6% 8% 2 6% 5% 5% 6% 4 6% 7% 42%	2489
	21st to end	Day	5% 5% 2 4 4 5% 5% 5% 8% 10% 47%	2671
May	1st to 10th	Day	6% 4 1 4 3 3 4 3 6% 7% 59%	8195
	11th to 20th	Day	3 4 1 4 4 3 4 4 6% 9% 58%	8160
	21st to end	Day	4 3 1 4 3 4 4 4 6% 9% 56%	8992
June	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 7% 51%	2940
	11th to 20th	Day	3 3 1 4 4 4 4 4 6% 7% 59%	2940
	21st to end	Day	3 2 1 3 3 3 3 3 5% 8% 67%	2940
July	1st to 10th	Day	1 1 1 1 1 2 2 3 4 84%	8219
	11th to 20th	Day	2 1 1 2 1 1 2 1 2 3 83%	8220
	21st to end	Day	2 1 1 2 1 1 2 2 3 4 82%	8922
August	1st to 10th	Day	2 2 1 3 2 2 3 2 4 6% 73%	6219
	11th to 20th	Day	1 1 1 3 2 2 3 2 4 6% 73%	5926
	21st to end	Day	3 3 1 4 3 3 3 3 4 6% 65%	6104
September	1st to 10th	Day	2 2 1 2 2 2 3 2 4 6% 74%	2400
	11th to 20th	Day	3 3 1 4 3 4 4 4 5% 7% 62%	2400
	21st to end	Day	6% 4 1 5% 3 4 3 4 5% 6% 59%	2141
October	1st to 10th	Day	5% 4 1 4 3 3 3 4 6% 7% 60%	3280
	11th to 20th	Day	5% 5% 3 7% 5% 5% 5% 5% 8% 9% 43%	3261
	21st to end	Day	9% 6% 3 9% 6% 6% 5% 5% 7% 8% 35%	3340
November	1st to 10th	Day	13% 10% 3 10% 7% 5% 6% 5% 8% 8% 25%	1828
	11th to 20th	Day	12% 15% 4 11% 7% 6% 8% 5% 7% 6% 19%	1680
	21st to end	Day	13% 11% 5% 10% 6% 6% 6% 5% 7% 7% 25%	1680
December	1st to 10th	Day	11% 10% 5% 11% 5% 4 5% 3 7% 9% 31%	2700
	11th to 20th	Day	18% 16% 6% 17% 8% 6% 5% 4 5% 5% 10%	3108
	21st to end	Day	15% 19% 6% 14% 6% 6% 5% 5% 6% 5% 13%	4092
January	1st to 10th	Night	20% 17% 6% 15% 6% 5% 5% 4 5% 5% 12%	8172
	11th to 20th	Night	13% 9% 3 7% 4 4 3 3 6% 8% 40%	8220
	21st to end	Night	16% 13% 5% 10% 5% 4 4 3 5% 6% 30%	8785
February	1st to 10th	Night	12% 13% 4 9% 5% 5% 5% 5% 7% 7% 27%	2730
	11th to 20th	Night	12% 11% 4 9% 5% 5% 4 3 6% 8% 34%	2662
	21st to end	Night	10% 12% 3 9% 5% 5% 4 4 8% 9% 31%	2091
March	1st to 10th	Night	7% 7% 3 8% 6% 5% 6% 6% 9% 10% 33%	5480
	11th to 20th	Night	8% 8% 3 9% 5% 5% 5% 4 6% 7% 40%	5110
	21st to end	Night	10% 7% 3 8% 6% 7% 7% 6% 9% 10% 27%	5343
April	1st to 10th	Night	8% 8% 2 6% 5% 4 6% 6% 8% 8% 39%	2100
	11th to 20th	Night	6% 7% 2 6% 5% 5% 5% 5% 6% 7% 46%	2079
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 9% 50%	1945
May	1st to 10th	Night	6% 4 2 4 3 3 3 3 5% 7% 61%	4723
	11th to 20th	Night	4 4 2 4 3 4 4 4 5% 9% 57%	4700
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 10% 59%	5181
June	1st to 10th	Night	9% 5% 1 4 4 5% 5% 4 6% 7% 50%	1680
	11th to 20th	Night	4 4 1 4 4 3 3 3 6% 9% 62%	1680
	21st to end	Night	2 4 1 3 3 3 4 2 4 5% 69%	1680
July	1st to 10th	Night	1 1 1 1 1 2 2 3 4 84%	4740
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 4 80%	4740
	21st to end	Night	1 1 1 2 1 1 1 2 3 5% 82%	5153
August	1st to 10th	Night	2 2 1 3 2 3 3 3 4 7% 70%	3795
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 5% 73%	4142
	21st to end	Night	2 3 1 3 3 3 4 3 5% 6% 68%	5093
September	1st to 10th	Night	1 2 1 3 2 2 3 3 4 6% 75%	2000
	11th to 20th	Night	4 2 1 3 3 3 4 4 5% 7% 63%	2000
	21st to end	Night	5% 4 2 4 3 3 3 3 5% 7% 62%	2255
October	1st to 10th	Night	5% 3 1 4 2 3 3 3 5% 8% 64%	3920
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 5% 8% 9% 48%	3897
	21st to end	Night	9% 5% 2 8% 4 5% 6% 6% 7% 8% 39%	4464
November	1st to 10th	Night	15% 8% 3 7% 5% 5% 6% 5% 8% 8% 30%	2688
	11th to 20th	Night	14% 13% 4 9% 7% 6% 6% 4 7% 6% 24%	2940
	21st to end	Night	11% 11% 3 9% 5% 6% 6% 5% 8% 7% 28%	2940
December	1st to 10th	Night	9% 9% 4 11% 5% 4 5% 4 7% 8% 33%	4700
	11th to 20th	Night	21% 17% 6% 16% 7% 5% 5% 3 5% 5% 10%	5316
	21st to end	Night	18% 16% 6% 13% 6% 5% 5% 4 5% 6% 17%	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	66% 13% 12% 7% 2	4723
	11th to 20th	Day	40% 6% 15% 27% 9% 2	4740
	21st to end	Day	51% 9% 12% 19% 8% 1	5403
February	1st to 10th	Day	53% 11% 19% 13% 3	1890
	11th to 20th	Day	49% 12% 16% 17% 7%	1854
	21st to end	Day	49% 11% 16% 14% 10% 1	1714
March	1st to 10th	Day	40% 9% 16% 21% 13%	4618
	11th to 20th	Day	45% 8% 13% 25% 8%	5110
	21st to end	Day	51% 14% 14% 16% 5%	5873
April	1st to 10th	Day	43% 10% 17% 21% 8%	2520
	11th to 20th	Day	40% 8% 17% 18% 14% 2	2492
	21st to end	Day	33% 10% 16% 26% 13% 2	2671
May	1st to 10th	Day	24% 6% 9% 34% 25% 2	8195
	11th to 20th	Day	22% 8% 18% 31% 19% 2	8163
	21st to end	Day	25% 9% 20% 32% 13% 1	8994
June	1st to 10th	Day	33% 8% 14% 28% 15% 3	2940
	11th to 20th	Day	27% 7% 12% 33% 17% 4	2940
	21st to end	Day	18% 5% 13% 35% 21% 8%	2940
July	1st to 10th	Day	7% 2 7% 26% 33% 24%	8151
	11th to 20th	Day	9% 2 3 14% 31% 41%	8145
	21st to end	Day	9% 3 7% 31% 36% 15%	8723
August	1st to 10th	Day	12% 3 7% 20% 34% 24%	4567
	11th to 20th	Day	12% 5% 12% 37% 25% 9%	4325
	21st to end	Day	18% 4 11% 27% 32% 8%	4431
September	1st to 10th	Day	13% 4 10% 32% 27% 14%	2400
	11th to 20th	Day	23% 7% 13% 28% 23% 7%	2400
	21st to end	Day	26% 6% 15% 28% 19% 6%	2141
October	1st to 10th	Day	23% 6% 10% 32% 25% 3	3280
	11th to 20th	Day	32% 11% 19% 23% 15% 1	3261
	21st to end	Day	41% 11% 15% 25% 8% 1	3340
November	1st to 10th	Day	51% 10% 16% 16% 6%	1828
	11th to 20th	Day	60% 12% 13% 12% 3	1680
	21st to end	Day	51% 11% 18% 14% 6% 1	1680
December	1st to 10th	Day	42% 10% 16% 24% 8%	2700
	11th to 20th	Day	65% 12% 15% 7% 1	3108
	21st to end	Day	64% 11% 14% 9% 2	4092
January	1st to 10th	Night	68% 11% 11% 8% 2	8172
	11th to 20th	Night	38% 6% 17% 28% 10% 1	8220
	21st to end	Night	51% 7% 12% 19% 10% 1	8785
February	1st to 10th	Night	50% 10% 21% 15% 3	2730
	11th to 20th	Night	46% 9% 19% 18% 7% 1	2664
	21st to end	Night	46% 10% 17% 17% 9% 1	2091
March	1st to 10th	Night	39% 9% 15% 26% 12%	5480
	11th to 20th	Night	37% 9% 17% 26% 10%	5110
	21st to end	Night	48% 14% 18% 17% 3	5343
April	1st to 10th	Night	39% 11% 18% 24% 8% 1	2100
	11th to 20th	Night	35% 8% 19% 23% 11% 3	2079
	21st to end	Night	31% 10% 17% 27% 12% 2	1945
May	1st to 10th	Night	23% 6% 9% 35% 25% 2	4721
	11th to 20th	Night	23% 8% 18% 33% 17% 2	4700
	21st to end	Night	21% 9% 25% 29% 15%	5182
June	1st to 10th	Night	34% 8% 14% 30% 12% 3	1680
	11th to 20th	Night	24% 6% 14% 37% 17% 2	1680
	21st to end	Night	19% 4 13% 35% 23% 7%	1680
July	1st to 10th	Night	7% 3 7% 24% 37% 22%	4700
	11th to 20th	Night	11% 3 6% 13% 32% 35%	4692
	21st to end	Night	8% 3 7% 32% 36% 16%	5040
August	1st to 10th	Night	13% 3 6% 19% 35% 24%	2777
	11th to 20th	Night	13% 5% 13% 36% 26% 8%	3019
	21st to end	Night	13% 5% 14% 29% 28% 11%	3686
September	1st to 10th	Night	12% 3 14% 29% 26% 16%	2000
	11th to 20th	Night	21% 7% 13% 28% 20% 11%	2000
	21st to end	Night	23% 5% 12% 31% 21% 7%	2255
October	1st to 10th	Night	19% 5% 12% 34% 27% 2	3920
	11th to 20th	Night	28% 11% 21% 24% 14% 2	3897
	21st to end	Night	37% 11% 14% 27% 11%	4464
November	1st to 10th	Night	48% 9% 15% 20% 7%	2688
	11th to 20th	Night	54% 11% 15% 16% 4	2940
	21st to end	Night	47% 9% 20% 18% 5%	2940
December	1st to 10th	Night	39% 9% 19% 28% 5%	4700
	11th to 20th	Night	68% 10% 14% 6% 2	5316
	21st to end	Night	60% 10% 16% 10% 3	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	21% 17% 29% 20% 10% 2	220
	11th to 20th	Day	16% 14% 23% 32% 13% 2	220
	21st to end	Day	16% 11% 28% 31% 13% 2	316
February	1st to 10th	Day	18% 19% 27% 28% 9%	430
	11th to 20th	Day	15% 22% 30% 24% 9% 1	425
	21st to end	Day	16% 21% 30% 26% 7% 1	438
March	1st to 10th	Day	15% 21% 34% 23% 7% 1	540
	11th to 20th	Day	15% 20% 30% 25% 10% 1	540
	21st to end	Day	19% 20% 31% 23% 7%	594
April	1st to 10th	Day	17% 21% 29% 18% 12% 2	540
	11th to 20th	Day	14% 23% 31% 20% 8% 3	532
	21st to end	Day	10% 21% 34% 24% 10% 2	540
May	1st to 10th	Day	11% 20% 29% 25% 13% 2	543
	11th to 20th	Day	15% 19% 28% 23% 13% 3	568
	21st to end	Day	10% 16% 29% 27% 15% 3	617
June	1st to 10th	Day	10% 20% 26% 26% 13% 5%	570
	11th to 20th	Day	11% 22% 28% 26% 10% 2	570
	21st to end	Day	5% 14% 32% 30% 17% 3	570
July	1st to 10th	Day	2 9% 37% 32% 19% 2	566
	11th to 20th	Day	2 13% 31% 32% 18% 4	564
	21st to end	Day	1 9% 31% 31% 21% 8%	619
August	1st to 10th	Day	2 8% 27% 30% 24% 9%	567
	11th to 20th	Day	1 7% 25% 33% 25% 8%	557
	21st to end	Day	4 11% 25% 31% 21% 7%	620
September	1st to 10th	Day	3 10% 23% 32% 27% 5%	530
	11th to 20th	Day	4 11% 25% 34% 23% 3	530
	21st to end	Day	8% 14% 25% 29% 20% 3	530
October	1st to 10th	Day	11% 12% 27% 27% 21% 2	530
	11th to 20th	Day	11% 16% 24% 29% 16% 4	530
	21st to end	Day	13% 18% 31% 23% 12% 3	453
November	1st to 10th	Day	18% 17% 26% 27% 12% 1	316
	11th to 20th	Day	15% 19% 31% 23% 10% 2	220
	21st to end	Day	12% 17% 23% 32% 15% 1	220
December	1st to 10th	Day	13% 17% 21% 29% 16% 3	220
	11th to 20th	Day	16% 19% 34% 25% 4 1	220
	21st to end	Day	14% 20% 26% 31% 8% 1	242
January	1st to 10th	Night	19% 15% 24% 29% 12% 1	840
	11th to 20th	Night	16% 13% 24% 33% 14% 1	840
	21st to end	Night	15% 11% 22% 34% 17% 2	924
February	1st to 10th	Night	13% 15% 25% 34% 13% 1	840
	11th to 20th	Night	15% 14% 26% 30% 15% 1	824
	21st to end	Night	16% 15% 27% 32% 9% 1	692
March	1st to 10th	Night	13% 13% 27% 32% 14%	840
	11th to 20th	Night	11% 12% 26% 35% 14%	840
	21st to end	Night	11% 15% 29% 33% 10% 1	924
April	1st to 10th	Night	12% 13% 27% 33% 14% 1	840
	11th to 20th	Night	10% 15% 31% 27% 15% 2	831
	21st to end	Night	8% 12% 33% 32% 14% 1	685
May	1st to 10th	Night	7% 12% 28% 35% 17% 1	420
	11th to 20th	Night	8% 10% 25% 35% 20% 2	414
	21st to end	Night	5% 6% 25% 44% 16% 2	455
June	1st to 10th	Night	10% 10% 26% 39% 15% 1	420
	11th to 20th	Night	6% 11% 34% 38% 11%	420
	21st to end	Night	5% 4 30% 44% 17% 1	420
July	1st to 10th	Night	1 3 25% 52% 19% 1	410
	11th to 20th	Night	1 2 31% 50% 14% 1	408
	21st to end	Night	2 2 25% 49% 21% 2	460
August	1st to 10th	Night	1 2 21% 49% 23% 3	475
	11th to 20th	Night	3 1 13% 42% 34% 7%	624
	21st to end	Night	2 4 25% 45% 21% 3	904
September	1st to 10th	Night	2 5% 20% 43% 29% 3	800
	11th to 20th	Night	4 3 20% 39% 30% 5%	800
	21st to end	Night	5% 7% 20% 37% 27% 3	800
October	1st to 10th	Night	10% 4 23% 39% 22% 2	800
	11th to 20th	Night	8% 8% 23% 35% 22% 3	800
	21st to end	Night	12% 9% 27% 33% 16% 3	880
November	1st to 10th	Night	13% 10% 26% 36% 15% 1	820
	11th to 20th	Night	15% 16% 25% 31% 12% 1	840
	21st to end	Night	11% 10% 24% 37% 15% 2	840
December	1st to 10th	Night	15% 11% 25% 31% 16% 2	840
	11th to 20th	Night	17% 13% 25% 32% 12%	840
	21st to end	Night	17% 15% 29% 29% 9% 1	924

Page generated at 2026-08-27 01:07:16. Version ac07

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