Observatory of The Dalles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory of The Dalles clear sky chart from 2005-11-06 to 2026-11-13.

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% 53%	162965
	Night	7% 7% 3 7% 4 4 5% 4 6% 7% 47%	155475
Transparency	Day	29% 8% 13% 24% 18% 7%	157714
	Night	32% 9% 14% 24% 15% 5%	151728
Seeing	Day	10% 16% 28% 27% 15% 3	17085
	Night	10% 10% 25% 36% 17% 2	26033

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	14% 15% 7% 14% 8% 7% 7% 5% 6% 6% 11%	4722
	11th to 20th	Day	10% 10% 3 6% 4 4 5% 6% 8% 7% 35%	4740
	21st to end	Day	12% 13% 6% 12% 7% 6% 5% 4 6% 7% 24%	5403
February	1st to 10th	Day	9% 13% 4 10% 7% 7% 6% 5% 7% 8% 23%	1890
	11th to 20th	Day	9% 11% 4 8% 5% 5% 7% 6% 8% 7% 30%	1845
	21st to end	Day	8% 10% 3 7% 5% 6% 6% 6% 8% 9% 31%	1714
March	1st to 10th	Day	6% 9% 3 9% 6% 5% 7% 6% 8% 11% 31%	4631
	11th to 20th	Day	7% 10% 3 9% 6% 6% 6% 4 5% 7% 37%	5110
	21st to end	Day	7% 8% 4 10% 7% 7% 8% 6% 8% 9% 27%	5872
April	1st to 10th	Day	7% 7% 3 7% 5% 6% 7% 5% 7% 9% 36%	2520
	11th to 20th	Day	6% 7% 3 6% 6% 5% 6% 5% 6% 8% 42%	2489
	21st to end	Day	4 5% 2 5% 4 4 5% 6% 7% 10% 48%	2670
May	1st to 10th	Day	6% 3 1 4 3 3 3 3 6% 8% 60%	8195
	11th to 20th	Day	3 3 1 4 3 3 4 3 6% 9% 60%	8160
	21st to end	Day	4 4 1 4 3 3 4 4 6% 9% 57%	8992
June	1st to 10th	Day	6% 4 2 5% 5% 4 5% 4 6% 8% 51%	2940
	11th to 20th	Day	3 3 1 4 4 4 4 4 6% 8% 59%	2940
	21st to end	Day	3 2 1 3 3 2 3 3 6% 8% 67%	2940
July	1st to 10th	Day	1 1 1 1 1 2 2 3 4 84%	8219
	11th to 20th	Day	1 1 1 2 1 1 1 2 2 3 84%	8220
	21st to end	Day	1 1 1 2 1 2 2 2 3 4 83%	8922
August	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 79%	7942
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 7% 75%	7563
	21st to end	Day	3 3 1 3 3 3 3 2 4 6% 68%	7773
September	1st to 10th	Day	2 1 1 2 3 2 3 2 4 7% 73%	2520
	11th to 20th	Day	3 3 1 4 3 4 5% 3 6% 8% 61%	2520
	21st to end	Day	6% 3 2 5% 3 4 4 3 5% 6% 59%	2247
October	1st to 10th	Day	3 3 1 3 2 3 3 4 6% 8% 65%	4660
	11th to 20th	Day	3 4 2 6% 4 5% 5% 5% 8% 10% 49%	4639
	21st to end	Day	6% 5% 3 9% 7% 7% 7% 5% 8% 8% 35%	4774
November	1st to 10th	Day	10% 10% 4 8% 7% 6% 6% 5% 9% 8% 27%	1917
	11th to 20th	Day	10% 13% 5% 12% 7% 8% 7% 5% 7% 7% 20%	1696
	21st to end	Day	10% 12% 4 9% 6% 7% 7% 4 7% 7% 27%	1680
December	1st to 10th	Day	8% 11% 5% 9% 6% 6% 6% 6% 5% 6% 32%	2700
	11th to 20th	Day	16% 16% 6% 16% 8% 6% 6% 4 6% 5% 11%	3108
	21st to end	Day	13% 18% 6% 13% 7% 5% 6% 4 6% 7% 14%	4092
January	1st to 10th	Night	16% 19% 7% 14% 6% 6% 6% 4 5% 5% 12%	8172
	11th to 20th	Night	11% 9% 3 6% 5% 4 5% 5% 6% 8% 37%	8220
	21st to end	Night	13% 13% 5% 11% 6% 6% 4 3 7% 6% 26%	8784
February	1st to 10th	Night	9% 13% 4 10% 6% 5% 6% 5% 6% 8% 28%	2730
	11th to 20th	Night	10% 11% 3 8% 6% 5% 5% 4 6% 7% 35%	2662
	21st to end	Night	9% 11% 3 8% 5% 5% 5% 5% 8% 8% 34%	2091
March	1st to 10th	Night	6% 6% 3 8% 6% 6% 6% 6% 8% 11% 35%	5479
	11th to 20th	Night	6% 7% 3 9% 6% 5% 5% 5% 6% 7% 41%	5110
	21st to end	Night	8% 7% 3 8% 7% 7% 7% 7% 8% 10% 28%	5344
April	1st to 10th	Night	7% 8% 2 6% 5% 5% 5% 6% 8% 8% 40%	2100
	11th to 20th	Night	6% 5% 3 6% 5% 4 5% 4 6% 8% 46%	2079
	21st to end	Night	6% 4 2 6% 4 4 4 3 8% 9% 49%	1944
May	1st to 10th	Night	6% 4 1 3 3 3 3 3 4 7% 63%	4723
	11th to 20th	Night	3 3 2 4 3 4 4 4 6% 8% 59%	4700
	21st to end	Night	5% 3 1 3 3 3 3 4 7% 10% 59%	5181
June	1st to 10th	Night	7% 6% 1 5% 3 4 5% 5% 6% 7% 50%	1680
	11th to 20th	Night	4 4 1 3 3 3 4 3 5% 8% 62%	1680
	21st to end	Night	2 3 1 3 3 3 3 3 4 6% 70%	1680
July	1st to 10th	Night	1 1 1 1 2 2 1 3 5% 84%	4740
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 80%	4740
	21st to end	Night	1 1 1 1 1 1 1 2 3 5% 83%	5153
August	1st to 10th	Night	1 1 1 2 2 2 3 2 4 5% 77%	4855
	11th to 20th	Night	2 1 1 2 2 3 4 3 4 6% 72%	5292
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 72%	6474
September	1st to 10th	Night	1 2 1 3 2 2 3 3 4 6% 74%	2100
	11th to 20th	Night	4 2 1 3 3 3 4 4 6% 8% 63%	2100
	21st to end	Night	5% 3 1 4 4 4 3 3 5% 7% 62%	2369
October	1st to 10th	Night	3 2 1 2 2 2 3 3 4 9% 70%	5560
	11th to 20th	Night	4 4 1 4 4 4 4 4 7% 11% 52%	5537
	21st to end	Night	7% 5% 3 7% 5% 5% 5% 5% 7% 9% 41%	6345
November	1st to 10th	Night	11% 9% 3 9% 5% 6% 6% 4 7% 8% 31%	2821
	11th to 20th	Night	11% 12% 4 9% 6% 6% 7% 5% 7% 7% 26%	2968
	21st to end	Night	9% 11% 4 10% 5% 6% 7% 5% 6% 8% 30%	2940
December	1st to 10th	Night	7% 9% 4 10% 6% 5% 9% 6% 5% 7% 32%	4700
	11th to 20th	Night	18% 17% 7% 14% 8% 7% 6% 4 5% 5% 10%	5316
	21st to end	Night	15% 16% 7% 12% 6% 5% 6% 4 5% 6% 18%	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	61% 16% 13% 8% 2	4723
	11th to 20th	Day	36% 10% 15% 27% 10% 2	4740
	21st to end	Day	48% 12% 11% 19% 9% 1	5403
February	1st to 10th	Day	52% 14% 18% 12% 4	1890
	11th to 20th	Day	45% 15% 18% 15% 6%	1854
	21st to end	Day	46% 12% 18% 13% 10% 1	1714
March	1st to 10th	Day	38% 12% 18% 21% 12%	4631
	11th to 20th	Day	44% 9% 14% 25% 8%	5110
	21st to end	Day	48% 16% 16% 15% 5%	5872
April	1st to 10th	Day	42% 12% 19% 20% 7%	2520
	11th to 20th	Day	40% 10% 18% 16% 13% 2	2492
	21st to end	Day	31% 12% 19% 24% 12% 2	2670
May	1st to 10th	Day	22% 7% 11% 35% 24% 2	8195
	11th to 20th	Day	21% 8% 19% 32% 18% 2	8163
	21st to end	Day	23% 11% 22% 31% 13% 1	8994
June	1st to 10th	Day	32% 8% 14% 28% 14% 3	2940
	11th to 20th	Day	25% 7% 15% 33% 15% 3	2940
	21st to end	Day	18% 5% 15% 35% 20% 8%	2940
July	1st to 10th	Day	7% 2 6% 26% 33% 25%	8151
	11th to 20th	Day	9% 2 4 14% 31% 41%	8145
	21st to end	Day	8% 2 8% 31% 36% 15%	8723
August	1st to 10th	Day	9% 3 5% 16% 38% 30%	6290
	11th to 20th	Day	11% 5% 12% 39% 27% 7%	5962
	21st to end	Day	16% 4 11% 35% 29% 5%	6100
September	1st to 10th	Day	14% 4 11% 31% 27% 13%	2520
	11th to 20th	Day	23% 7% 14% 28% 22% 6%	2520
	21st to end	Day	26% 7% 15% 28% 18% 6%	2247
October	1st to 10th	Day	17% 5% 11% 31% 30% 6%	4659
	11th to 20th	Day	27% 10% 17% 26% 19% 2	4641
	21st to end	Day	40% 11% 16% 25% 8% 1	4772
November	1st to 10th	Day	47% 13% 18% 16% 6%	1917
	11th to 20th	Day	56% 17% 13% 11% 3	1696
	21st to end	Day	48% 12% 18% 15% 5% 1	1680
December	1st to 10th	Day	41% 9% 16% 25% 9%	2700
	11th to 20th	Day	62% 15% 14% 8% 2	3108
	21st to end	Day	58% 15% 15% 10% 2	4092
January	1st to 10th	Night	64% 13% 12% 9% 2	8172
	11th to 20th	Night	35% 8% 16% 29% 10% 1	8220
	21st to end	Night	47% 11% 11% 20% 11%	8784
February	1st to 10th	Night	48% 13% 20% 16% 3	2730
	11th to 20th	Night	43% 12% 18% 19% 7%	2664
	21st to end	Night	43% 13% 17% 18% 8% 1	2091
March	1st to 10th	Night	36% 10% 16% 26% 11% 1	5479
	11th to 20th	Night	36% 12% 17% 27% 9%	5110
	21st to end	Night	43% 18% 19% 16% 3	5344
April	1st to 10th	Night	37% 13% 18% 23% 8%	2100
	11th to 20th	Night	34% 10% 20% 21% 12% 3	2079
	21st to end	Night	31% 11% 18% 26% 12% 2	1944
May	1st to 10th	Night	22% 6% 10% 35% 26% 2	4721
	11th to 20th	Night	22% 8% 17% 33% 18% 2	4700
	21st to end	Night	20% 10% 26% 28% 15%	5182
June	1st to 10th	Night	33% 10% 13% 30% 12% 2	1680
	11th to 20th	Night	22% 6% 16% 36% 17% 3	1680
	21st to end	Night	18% 5% 12% 34% 23% 8%	1680
July	1st to 10th	Night	7% 3 7% 23% 38% 24%	4700
	11th to 20th	Night	11% 3 6% 13% 30% 38%	4692
	21st to end	Night	8% 2 7% 31% 36% 16%	5040
August	1st to 10th	Night	9% 2 5% 13% 39% 31%	3837
	11th to 20th	Night	13% 5% 11% 33% 30% 8%	4169
	21st to end	Night	11% 4 13% 35% 30% 8%	5067
September	1st to 10th	Night	12% 4 13% 29% 27% 14%	2100
	11th to 20th	Night	20% 8% 13% 28% 20% 11%	2100
	21st to end	Night	21% 6% 14% 31% 21% 7%	2369
October	1st to 10th	Night	14% 4 12% 32% 33% 6%	5560
	11th to 20th	Night	24% 8% 19% 24% 22% 3	5537
	21st to end	Night	33% 11% 15% 31% 9% 1	6346
November	1st to 10th	Night	45% 13% 15% 20% 7%	2821
	11th to 20th	Night	50% 14% 16% 16% 4	2968
	21st to end	Night	43% 13% 19% 19% 5%	2940
December	1st to 10th	Night	37% 10% 17% 30% 6%	4700
	11th to 20th	Night	63% 13% 14% 8% 2	5316
	21st to end	Night	55% 15% 16% 11% 4	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%	627
September	1st to 10th	Day	4 9% 23% 32% 26% 6%	570
	11th to 20th	Day	5% 11% 25% 34% 23% 3	570
	21st to end	Day	8% 13% 25% 30% 20% 4	570
October	1st to 10th	Day	10% 12% 26% 27% 23% 2	570
	11th to 20th	Day	10% 16% 24% 30% 16% 4	570
	21st to end	Day	13% 18% 30% 24% 12% 3	491
November	1st to 10th	Day	17% 16% 25% 30% 12% 1	345
	11th to 20th	Day	14% 19% 31% 23% 11% 2	224
	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	684
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% 50% 23% 3	476
	11th to 20th	Night	3 1 13% 42% 34% 7%	624
	21st to end	Night	2 4 25% 45% 21% 3	911
September	1st to 10th	Night	2 5% 20% 43% 28% 3	840
	11th to 20th	Night	4 3 20% 40% 29% 4	840
	21st to end	Night	5% 7% 20% 38% 26% 3	840
October	1st to 10th	Night	10% 4 23% 39% 22% 2	840
	11th to 20th	Night	8% 7% 23% 37% 22% 3	840
	21st to end	Night	12% 10% 26% 34% 16% 3	924
November	1st to 10th	Night	13% 10% 25% 37% 14% 1	860
	11th to 20th	Night	15% 16% 25% 31% 12% 1	848
	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 2026-11-10 00:55:17. Version ac07

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