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 2025-04-26.

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	7% 7% 3 7% 4 4 4 4 6% 7% 47%	117655
	Night	10% 8% 3 7% 4 4 4 4 6% 7% 42%	119752
Transparency	Day	35% 8% 13% 22% 16% 7%	117398
	Night	38% 8% 15% 22% 12% 5%	119598
Seeing	Day	11% 16% 29% 27% 14% 3	15042
	Night	10% 10% 25% 35% 17% 2	24208

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	19% 18% 7% 13% 7% 7% 5% 4 5% 6% 10%	3639
	11th to 20th	Day	15% 13% 4 7% 4 5% 5% 3 5% 7% 32%	3640
	21st to end	Day	16% 14% 5% 11% 6% 4 4 4 4 7% 25%	4165
February	1st to 10th	Day	12% 13% 4 11% 7% 5% 6% 5% 7% 7% 23%	1800
	11th to 20th	Day	11% 10% 3 9% 5% 6% 6% 5% 8% 7% 30%	1755
	21st to end	Day	9% 10% 3 8% 6% 5% 6% 6% 8% 8% 30%	1635
March	1st to 10th	Day	7% 8% 4 8% 5% 6% 6% 6% 9% 11% 29%	4518
	11th to 20th	Day	9% 10% 3 9% 6% 5% 5% 4 5% 7% 37%	5000
	21st to end	Day	9% 9% 3 9% 7% 7% 7% 6% 7% 9% 26%	5746
April	1st to 10th	Day	8% 8% 2 8% 6% 5% 7% 5% 7% 9% 35%	2400
	11th to 20th	Day	6% 8% 2 6% 5% 5% 6% 4 6% 7% 42%	2369
	21st to end	Day	5% 5% 2 5% 4 5% 5% 5% 8% 10% 46%	2480
May	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 6% 7% 51%	4420
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 8% 66%	4377
	21st to end	Day	6% 5% 2 6% 4 5% 5% 5% 7% 9% 46%	4813
June	1st to 10th	Day	7% 5% 3 5% 4 4 5% 4 7% 7% 49%	2660
	11th to 20th	Day	4 3 2 5% 4 4 4 4 6% 7% 57%	2660
	21st to end	Day	3 2 1 3 3 2 3 3 5% 7% 68%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 3 4 86%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 2 2 3 84%	4420
	21st to end	Day	1 1 1 2 1 1 2 2 3 4 83%	4790
August	1st to 10th	Day	2 2 1 3 2 2 3 2 3 5% 76%	4309
	11th to 20th	Day	1 1 1 3 2 2 3 3 5% 7% 73%	4026
	21st to end	Day	3 3 1 4 3 3 3 3 4 5% 69%	4190
September	1st to 10th	Day	2 2 1 2 1 2 2 2 4 6% 76%	2280
	11th to 20th	Day	3 3 1 4 3 4 4 4 6% 7% 61%	2280
	21st to end	Day	6% 4 1 5% 3 4 3 4 5% 6% 58%	2034
October	1st to 10th	Day	5% 4 1 4 3 3 3 4 6% 7% 61%	3180
	11th to 20th	Day	5% 5% 3 6% 5% 5% 5% 5% 8% 9% 43%	3161
	21st to end	Day	9% 6% 3 9% 6% 6% 5% 5% 7% 8% 35%	3237
November	1st to 10th	Day	13% 10% 3 10% 6% 5% 6% 5% 9% 8% 25%	1739
	11th to 20th	Day	13% 15% 4 11% 7% 6% 8% 6% 7% 6% 19%	1600
	21st to end	Day	13% 11% 5% 10% 6% 6% 6% 5% 7% 7% 25%	1600
December	1st to 10th	Day	10% 10% 5% 11% 5% 4 5% 3 7% 9% 31%	2620
	11th to 20th	Day	18% 16% 6% 17% 8% 6% 5% 4 5% 5% 10%	3028
	21st to end	Day	15% 19% 6% 14% 6% 6% 5% 5% 6% 5% 12%	4004
January	1st to 10th	Night	20% 18% 6% 15% 6% 5% 5% 4 5% 5% 10%	6306
	11th to 20th	Night	16% 11% 3 8% 4 5% 3 3 5% 8% 33%	6320
	21st to end	Night	18% 14% 5% 9% 5% 4 4 3 4 5% 30%	6794
February	1st to 10th	Night	13% 13% 4 9% 5% 5% 5% 5% 6% 7% 27%	2600
	11th to 20th	Night	12% 11% 4 9% 5% 5% 4 3 6% 8% 34%	2532
	21st to end	Night	9% 12% 3 9% 5% 5% 4 5% 8% 9% 31%	1995
March	1st to 10th	Night	7% 7% 3 8% 6% 5% 6% 6% 9% 10% 33%	5360
	11th to 20th	Night	8% 8% 3 9% 5% 5% 5% 4 6% 7% 40%	5000
	21st to end	Night	10% 7% 3 8% 6% 7% 7% 6% 9% 10% 27%	5228
April	1st to 10th	Night	8% 8% 2 6% 5% 4 6% 6% 8% 8% 39%	2000
	11th to 20th	Night	6% 7% 2 6% 5% 5% 5% 5% 6% 7% 47%	1979
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 9% 49%	1812
May	1st to 10th	Night	9% 6% 2 4 3 3 3 3 5% 6% 54%	2540
	11th to 20th	Night	4 3 1 3 3 3 3 3 4 8% 67%	2516
	21st to end	Night	3 4 1 5% 4 3 5% 4 6% 10% 54%	2762
June	1st to 10th	Night	9% 5% 1 5% 4 6% 5% 4 6% 7% 48%	1520
	11th to 20th	Night	4 4 1 4 3 4 3 3 6% 8% 60%	1520
	21st to end	Night	2 4 1 3 3 3 4 2 4 5% 70%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	2540
	11th to 20th	Night	1 1 1 2 1 2 2 3 4 83%	2540
	21st to end	Night	1 1 1 1 1 1 1 3 3 87%	2761
August	1st to 10th	Night	2 2 1 2 1 3 3 2 4 5% 74%	2619
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 5% 75%	2812
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 74%	3487
September	1st to 10th	Night	1 1 1 2 2 1 3 2 4 5% 77%	1900
	11th to 20th	Night	4 2 1 3 3 4 4 4 5% 7% 62%	1900
	21st to end	Night	5% 4 1 4 3 3 3 3 5% 7% 62%	2142
October	1st to 10th	Night	5% 3 1 3 2 3 3 3 5% 8% 65%	3800
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 8% 9% 48%	3777
	21st to end	Night	9% 5% 2 8% 4 5% 6% 6% 7% 8% 39%	4325
November	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 5% 8% 7% 31%	2557
	11th to 20th	Night	15% 12% 4 8% 7% 5% 6% 4 7% 6% 25%	2800
	21st to end	Night	10% 11% 3 9% 5% 6% 6% 6% 7% 7% 29%	2800
December	1st to 10th	Night	9% 9% 4 11% 5% 4 5% 4 7% 8% 33%	4560
	11th to 20th	Night	21% 17% 6% 16% 7% 5% 5% 3 5% 4 10%	5176
	21st to end	Night	18% 16% 6% 13% 6% 5% 5% 3 5% 6% 17%	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	67% 14% 11% 6% 2	3639
	11th to 20th	Day	49% 6% 17% 22% 5%	3640
	21st to end	Day	52% 9% 11% 18% 9% 2	4165
February	1st to 10th	Day	54% 11% 19% 13% 3	1800
	11th to 20th	Day	49% 12% 16% 17% 6%	1764
	21st to end	Day	49% 11% 17% 14% 9%	1635
March	1st to 10th	Day	41% 9% 16% 21% 13%	4518
	11th to 20th	Day	45% 8% 13% 25% 8%	5000
	21st to end	Day	51% 14% 14% 16% 4	5746
April	1st to 10th	Day	44% 10% 17% 20% 9%	2400
	11th to 20th	Day	41% 8% 17% 18% 14% 2	2372
	21st to end	Day	34% 10% 16% 25% 13% 2	2480
May	1st to 10th	Day	33% 7% 11% 26% 22% 1	4420
	11th to 20th	Day	20% 5% 11% 33% 27% 4	4380
	21st to end	Day	35% 11% 21% 26% 7%	4814
June	1st to 10th	Day	35% 8% 14% 28% 13% 3	2660
	11th to 20th	Day	28% 8% 13% 34% 14% 3	2660
	21st to end	Day	18% 4 13% 35% 22% 8%	2660
July	1st to 10th	Day	6% 2 6% 17% 26% 42%	4351
	11th to 20th	Day	8% 2 5% 12% 25% 48%	4345
	21st to end	Day	8% 3 8% 20% 36% 25%	4639
August	1st to 10th	Day	13% 3 7% 19% 34% 25%	4293
	11th to 20th	Day	12% 5% 12% 37% 25% 8%	4065
	21st to end	Day	18% 4 12% 26% 32% 8%	4189
September	1st to 10th	Day	12% 4 9% 32% 28% 15%	2280
	11th to 20th	Day	24% 7% 13% 26% 22% 7%	2280
	21st to end	Day	27% 6% 15% 28% 18% 6%	2034
October	1st to 10th	Day	23% 6% 10% 32% 25% 4	3180
	11th to 20th	Day	32% 11% 19% 23% 15% 1	3161
	21st to end	Day	41% 11% 15% 25% 8% 1	3237
November	1st to 10th	Day	51% 10% 16% 17% 6%	1739
	11th to 20th	Day	60% 12% 13% 12% 3	1600
	21st to end	Day	52% 10% 17% 14% 6% 1	1600
December	1st to 10th	Day	41% 11% 16% 24% 8%	2620
	11th to 20th	Day	66% 12% 15% 7% 1	3028
	21st to end	Day	64% 11% 14% 9% 2	4004
January	1st to 10th	Night	70% 11% 10% 6% 2	6306
	11th to 20th	Night	45% 6% 20% 23% 5%	6320
	21st to end	Night	52% 7% 11% 18% 10% 1	6794
February	1st to 10th	Night	51% 10% 21% 15% 4	2600
	11th to 20th	Night	46% 9% 19% 18% 7% 1	2534
	21st to end	Night	46% 11% 17% 18% 7% 1	1995
March	1st to 10th	Night	38% 9% 15% 26% 12%	5360
	11th to 20th	Night	37% 9% 17% 27% 10%	5000
	21st to end	Night	48% 14% 18% 17% 3	5228
April	1st to 10th	Night	40% 11% 18% 22% 8% 1	2000
	11th to 20th	Night	35% 8% 19% 23% 12% 3	1979
	21st to end	Night	32% 11% 18% 25% 12% 2	1812
May	1st to 10th	Night	31% 7% 12% 32% 18% 1	2540
	11th to 20th	Night	18% 6% 12% 38% 23% 3	2516
	21st to end	Night	25% 14% 31% 23% 7% 1	2762
June	1st to 10th	Night	36% 8% 14% 30% 10% 3	1520
	11th to 20th	Night	25% 6% 15% 38% 14% 1	1520
	21st to end	Night	19% 4 12% 36% 23% 7%	1520
July	1st to 10th	Night	7% 2 7% 16% 31% 37%	2500
	11th to 20th	Night	9% 2 6% 15% 24% 44%	2492
	21st to end	Night	6% 2 7% 20% 38% 27%	2674
August	1st to 10th	Night	14% 3 6% 19% 35% 24%	2611
	11th to 20th	Night	12% 5% 13% 36% 26% 8%	2839
	21st to end	Night	13% 5% 15% 29% 28% 11%	3487
September	1st to 10th	Night	10% 3 13% 30% 27% 16%	1900
	11th to 20th	Night	22% 7% 13% 28% 19% 12%	1900
	21st to end	Night	23% 6% 13% 30% 21% 7%	2142
October	1st to 10th	Night	18% 5% 13% 34% 28% 2	3800
	11th to 20th	Night	28% 11% 21% 24% 14% 2	3777
	21st to end	Night	37% 10% 14% 27% 11% 1	4325
November	1st to 10th	Night	48% 9% 15% 21% 8%	2557
	11th to 20th	Night	53% 11% 16% 16% 4	2800
	21st to end	Night	47% 10% 20% 18% 5%	2800
December	1st to 10th	Night	39% 9% 19% 29% 5%	4560
	11th to 20th	Night	69% 10% 14% 6% 1	5176
	21st to end	Night	61% 10% 15% 10% 4	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	22% 17% 31% 20% 10% 2	200
	11th to 20th	Day	18% 16% 26% 29% 11% 2	200
	21st to end	Day	16% 12% 27% 31% 12% 2	290
February	1st to 10th	Day	18% 19% 27% 27% 9% 1	400
	11th to 20th	Day	15% 22% 30% 24% 8%	395
	21st to end	Day	16% 21% 29% 26% 7% 1	407
March	1st to 10th	Day	15% 20% 34% 23% 7% 1	500
	11th to 20th	Day	15% 20% 29% 25% 10% 1	500
	21st to end	Day	19% 20% 30% 23% 7%	550
April	1st to 10th	Day	18% 23% 30% 17% 11% 2	500
	11th to 20th	Day	14% 24% 32% 18% 8% 3	492
	21st to end	Day	10% 22% 33% 23% 10% 2	481
May	1st to 10th	Day	12% 22% 30% 23% 12% 2	463
	11th to 20th	Day	15% 17% 28% 24% 13% 3	488
	21st to end	Day	11% 16% 30% 26% 14% 3	529
June	1st to 10th	Day	10% 20% 27% 26% 12% 5%	490
	11th to 20th	Day	12% 24% 28% 26% 9% 1	490
	21st to end	Day	5% 14% 32% 30% 16% 3	490
July	1st to 10th	Day	2 9% 37% 32% 17% 1	486
	11th to 20th	Day	2 14% 31% 32% 17% 4	484
	21st to end	Day	1 10% 31% 31% 20% 7%	531
August	1st to 10th	Day	2 8% 29% 29% 24% 8%	487
	11th to 20th	Day	1 8% 25% 33% 25% 8%	477
	21st to end	Day	5% 11% 27% 30% 21% 7%	539
September	1st to 10th	Day	3 10% 24% 32% 25% 5%	490
	11th to 20th	Day	4 11% 27% 33% 22% 2	490
	21st to end	Day	8% 14% 26% 29% 19% 3	490
October	1st to 10th	Day	11% 12% 27% 27% 21% 2	490
	11th to 20th	Day	10% 16% 23% 29% 17% 5%	490
	21st to end	Day	13% 19% 30% 22% 12% 4	416
November	1st to 10th	Day	19% 17% 25% 26% 12% 1	287
	11th to 20th	Day	16% 19% 31% 23% 10% 2	200
	21st to end	Day	14% 16% 24% 32% 14% 1	200
December	1st to 10th	Day	13% 16% 21% 30% 18% 4	200
	11th to 20th	Day	17% 19% 35% 26% 4 1	200
	21st to end	Day	15% 20% 25% 30% 8%	220
January	1st to 10th	Night	19% 14% 25% 29% 12% 1	800
	11th to 20th	Night	16% 13% 24% 32% 13% 1	800
	21st to end	Night	16% 11% 22% 33% 16% 2	880
February	1st to 10th	Night	13% 15% 25% 33% 13% 1	800
	11th to 20th	Night	15% 14% 26% 29% 15% 1	784
	21st to end	Night	16% 16% 26% 33% 10% 1	660
March	1st to 10th	Night	13% 13% 27% 33% 14% 1	800
	11th to 20th	Night	11% 12% 25% 36% 15% 1	800
	21st to end	Night	11% 15% 29% 33% 11% 1	880
April	1st to 10th	Night	13% 13% 28% 32% 13% 1	800
	11th to 20th	Night	10% 15% 31% 26% 15% 2	791
	21st to end	Night	8% 12% 32% 32% 15% 1	640
May	1st to 10th	Night	7% 12% 31% 33% 16% 1	380
	11th to 20th	Night	8% 10% 24% 35% 21% 2	374
	21st to end	Night	5% 7% 26% 43% 16% 2	411
June	1st to 10th	Night	11% 10% 25% 40% 14% 1	380
	11th to 20th	Night	6% 12% 33% 37% 11%	380
	21st to end	Night	5% 4 31% 43% 17% 1	380
July	1st to 10th	Night	1 3 25% 52% 18% 1	370
	11th to 20th	Night	1 2 30% 51% 14% 2	368
	21st to end	Night	2 2 26% 47% 21% 2	416
August	1st to 10th	Night	2 2 21% 48% 24% 3	429
	11th to 20th	Night	2 1 14% 41% 35% 7%	564
	21st to end	Night	1 4 26% 43% 22% 4	825
September	1st to 10th	Night	2 5% 20% 43% 28% 3	760
	11th to 20th	Night	4 3 20% 39% 30% 5%	760
	21st to end	Night	5% 7% 21% 37% 27% 4	760
October	1st to 10th	Night	10% 4 23% 38% 22% 3	760
	11th to 20th	Night	8% 8% 22% 36% 23% 3	760
	21st to end	Night	13% 9% 26% 32% 17% 3	836
November	1st to 10th	Night	13% 10% 25% 36% 15% 1	780
	11th to 20th	Night	16% 16% 25% 30% 12% 1	800
	21st to end	Night	11% 10% 24% 38% 16% 2	800
December	1st to 10th	Night	15% 10% 24% 32% 17% 2	800
	11th to 20th	Night	17% 13% 25% 32% 13% 1	800
	21st to end	Night	17% 16% 29% 29% 9% 1	880

Page generated at 2025-04-23 06:25:06. Version ac07

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