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 2025-07-21.

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% 3 6% 4 4 5% 4 6% 7% 49%	124319
	Night	8% 8% 3 7% 5% 4 5% 4 6% 7% 44%	123603
Transparency	Day	32% 9% 14% 22% 16% 7%	124063
	Night	35% 10% 15% 22% 13% 5%	123446
Seeing	Day	11% 16% 29% 27% 14% 3	15386
	Night	10% 10% 25% 35% 17% 2	24381

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	15% 17% 8% 13% 8% 6% 6% 5% 5% 5% 11%	3639
	11th to 20th	Day	14% 13% 3 7% 4 4 5% 5% 7% 7% 31%	3640
	21st to end	Day	15% 14% 5% 11% 6% 5% 4 3 6% 6% 25%	4165
February	1st to 10th	Day	10% 13% 4 10% 7% 6% 6% 5% 7% 8% 23%	1800
	11th to 20th	Day	9% 11% 4 8% 5% 5% 7% 6% 8% 7% 30%	1755
	21st to end	Day	8% 10% 3 7% 5% 7% 6% 6% 9% 9% 30%	1635
March	1st to 10th	Day	6% 9% 3 9% 6% 5% 7% 6% 8% 11% 31%	4531
	11th to 20th	Day	7% 10% 3 8% 6% 6% 6% 4 5% 7% 38%	5000
	21st to end	Day	7% 8% 4 10% 7% 7% 8% 6% 8% 9% 27%	5745
April	1st to 10th	Day	8% 7% 3 7% 6% 6% 7% 5% 7% 8% 35%	2400
	11th to 20th	Day	6% 7% 3 6% 6% 5% 6% 5% 6% 8% 42%	2369
	21st to end	Day	4 5% 2 5% 5% 4 5% 6% 7% 10% 47%	2543
May	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 7% 58%	6320
	11th to 20th	Day	3 4 1 4 3 3 4 3 6% 9% 59%	6277
	21st to end	Day	4 4 1 4 3 4 4 4 6% 8% 57%	6902
June	1st to 10th	Day	6% 5% 2 5% 5% 4 5% 4 6% 8% 50%	2800
	11th to 20th	Day	4 3 1 5% 4 4 4 4 6% 8% 57%	2800
	21st to end	Day	3 2 1 3 3 2 3 3 6% 7% 68%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 3 85%	4560
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 85%	4560
	21st to end	Day	1 1 1 1 1 2 2 2 3 4 83%	4791
August	1st to 10th	Day	1 2 1 3 2 2 3 2 4 5% 76%	4308
	11th to 20th	Day	1 1 1 2 2 3 3 3 4 6% 73%	4026
	21st to end	Day	3 3 1 4 3 3 3 2 4 6% 69%	4191
September	1st to 10th	Day	2 1 1 2 2 2 2 2 3 6% 77%	2280
	11th to 20th	Day	2 3 1 4 3 4 5% 3 6% 8% 61%	2280
	21st to end	Day	6% 3 2 5% 4 4 4 3 5% 6% 59%	2033
October	1st to 10th	Day	5% 3 1 4 2 4 4 4 5% 7% 61%	3180
	11th to 20th	Day	4 4 2 6% 4 5% 5% 5% 8% 10% 44%	3161
	21st to end	Day	7% 6% 2 9% 7% 6% 7% 5% 8% 8% 37%	3237
November	1st to 10th	Day	11% 10% 4 8% 7% 6% 5% 5% 9% 8% 26%	1739
	11th to 20th	Day	10% 13% 5% 12% 7% 8% 7% 5% 7% 7% 20%	1600
	21st to end	Day	10% 11% 5% 9% 6% 7% 7% 4 7% 7% 27%	1600
December	1st to 10th	Day	8% 11% 5% 9% 6% 6% 7% 6% 5% 6% 32%	2620
	11th to 20th	Day	16% 17% 6% 16% 8% 6% 6% 4 6% 5% 11%	3028
	21st to end	Day	13% 18% 6% 13% 7% 5% 6% 5% 6% 7% 14%	4004
January	1st to 10th	Night	18% 20% 8% 15% 6% 6% 5% 4 5% 5% 10%	6306
	11th to 20th	Night	15% 12% 3 7% 5% 4 5% 4 6% 8% 32%	6320
	21st to end	Night	15% 15% 4 11% 6% 5% 3 2 6% 5% 28%	6793
February	1st to 10th	Night	9% 13% 4 10% 6% 4 6% 5% 6% 8% 29%	2600
	11th to 20th	Night	10% 11% 3 9% 6% 5% 5% 4 6% 7% 36%	2532
	21st to end	Night	9% 11% 3 8% 5% 5% 5% 5% 8% 8% 33%	1995
March	1st to 10th	Night	6% 6% 3 8% 6% 6% 6% 6% 8% 11% 35%	5359
	11th to 20th	Night	7% 7% 3 9% 6% 5% 5% 4 6% 6% 41%	5000
	21st to end	Night	8% 7% 3 8% 7% 7% 7% 7% 9% 10% 28%	5229
April	1st to 10th	Night	7% 8% 2 6% 5% 5% 5% 6% 8% 8% 39%	2000
	11th to 20th	Night	6% 5% 3 6% 5% 4 5% 5% 6% 8% 47%	1979
	21st to end	Night	6% 4 2 6% 4 4 4 4 7% 10% 49%	1851
May	1st to 10th	Night	8% 5% 1 3 3 2 3 3 4 6% 63%	3640
	11th to 20th	Night	3 3 2 4 4 4 4 3 6% 8% 60%	3616
	21st to end	Night	5% 3 1 3 3 3 4 4 7% 10% 56%	3972
June	1st to 10th	Night	7% 6% 1 5% 3 4 5% 5% 6% 7% 51%	1600
	11th to 20th	Night	4 4 1 3 3 3 4 3 5% 8% 62%	1600
	21st to end	Night	2 3 1 3 2 3 3 4 4 5% 70%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 4 86%	2620
	11th to 20th	Night	1 2 1 1 1 1 2 2 3 3 84%	2620
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	2761
August	1st to 10th	Night	2 2 1 2 2 2 3 2 4 5% 75%	2620
	11th to 20th	Night	1 1 1 2 2 3 3 3 4 6% 74%	2812
	21st to end	Night	2 1 2 2 2 3 3 4 6% 75%	3486
September	1st to 10th	Night	1 1 1 2 2 2 2 3 4 5% 78%	1900
	11th to 20th	Night	4 2 1 3 3 3 4 4 6% 8% 62%	1900
	21st to end	Night	5% 3 1 4 4 3 3 3 5% 6% 63%	2143
October	1st to 10th	Night	4 3 1 3 2 2 3 3 4 8% 67%	3800
	11th to 20th	Night	4 4 2 4 4 5% 5% 5% 8% 10% 50%	3777
	21st to end	Night	8% 5% 3 7% 5% 5% 5% 6% 7% 9% 41%	4325
November	1st to 10th	Night	12% 8% 3 9% 5% 5% 6% 4 7% 7% 33%	2559
	11th to 20th	Night	12% 12% 5% 9% 6% 6% 7% 5% 6% 7% 26%	2800
	21st to end	Night	8% 11% 4 10% 6% 6% 6% 5% 6% 8% 31%	2800
December	1st to 10th	Night	7% 9% 4 9% 6% 5% 9% 6% 4 7% 32%	4560
	11th to 20th	Night	19% 17% 7% 14% 8% 7% 6% 4 5% 5% 10%	5176
	21st to end	Night	15% 17% 7% 12% 6% 5% 6% 4 4 5% 18%	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	62% 17% 13% 7% 2	3639
	11th to 20th	Day	44% 11% 16% 21% 7%	3640
	21st to end	Day	47% 13% 10% 19% 9% 2	4165
February	1st to 10th	Day	52% 14% 18% 13% 4	1800
	11th to 20th	Day	45% 16% 18% 15% 6%	1764
	21st to end	Day	46% 13% 19% 13% 9%	1635
March	1st to 10th	Day	38% 11% 18% 21% 12%	4531
	11th to 20th	Day	44% 9% 14% 25% 8%	5000
	21st to end	Day	49% 16% 16% 15% 4	5745
April	1st to 10th	Day	44% 12% 19% 18% 8%	2400
	11th to 20th	Day	40% 9% 19% 16% 13% 2	2372
	21st to end	Day	32% 13% 19% 23% 11% 2	2543
May	1st to 10th	Day	25% 7% 11% 30% 24% 2	6320
	11th to 20th	Day	23% 8% 17% 30% 19% 3	6280
	21st to end	Day	24% 12% 23% 31% 10%	6904
June	1st to 10th	Day	33% 9% 15% 27% 14% 3	2800
	11th to 20th	Day	26% 7% 16% 33% 14% 3	2800
	21st to end	Day	18% 5% 15% 35% 20% 8%	2800
July	1st to 10th	Day	7% 3 7% 17% 26% 41%	4491
	11th to 20th	Day	8% 3 5% 11% 26% 47%	4485
	21st to end	Day	8% 3 8% 20% 36% 25%	4640
August	1st to 10th	Day	12% 4 6% 20% 34% 24%	4292
	11th to 20th	Day	13% 5% 12% 37% 26% 7%	4065
	21st to end	Day	18% 4 12% 26% 33% 7%	4190
September	1st to 10th	Day	12% 3 10% 32% 29% 14%	2280
	11th to 20th	Day	24% 7% 14% 26% 22% 7%	2280
	21st to end	Day	27% 7% 15% 28% 18% 6%	2033
October	1st to 10th	Day	22% 7% 11% 31% 26% 3	3180
	11th to 20th	Day	30% 12% 20% 22% 15% 1	3161
	21st to end	Day	41% 11% 14% 25% 8% 1	3237
November	1st to 10th	Day	48% 13% 16% 16% 6%	1739
	11th to 20th	Day	56% 17% 13% 12% 3	1600
	21st to end	Day	48% 12% 17% 16% 6% 1	1600
December	1st to 10th	Day	40% 9% 16% 25% 10%	2620
	11th to 20th	Day	62% 15% 14% 8% 1	3028
	21st to end	Day	58% 15% 14% 10% 2	4004
January	1st to 10th	Night	67% 14% 11% 6% 2	6306
	11th to 20th	Night	42% 9% 18% 25% 6%	6320
	21st to end	Night	48% 12% 10% 19% 11%	6793
February	1st to 10th	Night	48% 13% 20% 16% 3	2600
	11th to 20th	Night	43% 12% 19% 18% 7%	2534
	21st to end	Night	43% 13% 17% 19% 7% 1	1995
March	1st to 10th	Night	35% 10% 17% 26% 11% 1	5359
	11th to 20th	Night	36% 11% 16% 27% 9%	5000
	21st to end	Night	43% 19% 19% 16% 3	5229
April	1st to 10th	Night	39% 13% 18% 22% 8% 1	2000
	11th to 20th	Night	34% 10% 20% 22% 12% 3	1979
	21st to end	Night	31% 12% 19% 25% 11% 2	1851
May	1st to 10th	Night	24% 6% 11% 30% 28% 1	3637
	11th to 20th	Night	22% 8% 18% 33% 17% 2	3616
	21st to end	Night	22% 11% 29% 28% 10%	3972
June	1st to 10th	Night	33% 10% 12% 31% 12% 3	1600
	11th to 20th	Night	23% 6% 17% 36% 16% 2	1600
	21st to end	Night	18% 6% 12% 35% 22% 7%	1600
July	1st to 10th	Night	7% 2 7% 16% 29% 38%	2580
	11th to 20th	Night	9% 2 6% 14% 24% 46%	2572
	21st to end	Night	5% 2 7% 19% 38% 28%	2674
August	1st to 10th	Night	13% 3 6% 18% 37% 23%	2612
	11th to 20th	Night	12% 5% 12% 37% 26% 8%	2839
	21st to end	Night	12% 5% 16% 28% 29% 10%	3486
September	1st to 10th	Night	10% 4 13% 29% 28% 15%	1900
	11th to 20th	Night	21% 9% 12% 27% 18% 12%	1900
	21st to end	Night	21% 6% 14% 30% 21% 7%	2143
October	1st to 10th	Night	18% 5% 13% 35% 27% 2	3800
	11th to 20th	Night	26% 10% 23% 24% 14% 2	3777
	21st to end	Night	36% 11% 15% 27% 11% 1	4325
November	1st to 10th	Night	45% 12% 14% 21% 8%	2559
	11th to 20th	Night	51% 14% 16% 16% 4	2800
	21st to end	Night	43% 14% 19% 19% 5%	2800
December	1st to 10th	Night	36% 10% 17% 31% 6%	4560
	11th to 20th	Night	64% 13% 14% 7% 1	5176
	21st to end	Night	55% 15% 15% 11% 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	500
May	1st to 10th	Day	12% 21% 29% 24% 13% 2	503
	11th to 20th	Day	15% 18% 28% 23% 13% 3	528
	21st to end	Day	10% 16% 29% 26% 15% 3	573
June	1st to 10th	Day	9% 20% 25% 27% 13% 5%	530
	11th to 20th	Day	12% 23% 29% 26% 9% 1	530
	21st to end	Day	5% 14% 31% 30% 17% 3	530
July	1st to 10th	Day	3 9% 37% 32% 18% 2	526
	11th to 20th	Day	2 13% 30% 32% 18% 5%	524
	21st to end	Day	1 10% 31% 31% 20% 7%	532
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% 33% 32% 14% 1	651
May	1st to 10th	Night	7% 12% 30% 34% 17% 1	400
	11th to 20th	Night	8% 10% 25% 35% 20% 2	394
	21st to end	Night	6% 6% 26% 43% 17% 2	433
June	1st to 10th	Night	10% 10% 25% 40% 15% 1	400
	11th to 20th	Night	6% 12% 35% 37% 11%	400
	21st to end	Night	5% 4 30% 43% 17% 1	400
July	1st to 10th	Night	1 3 25% 51% 18% 1	390
	11th to 20th	Night	1 2 30% 51% 15% 2	388
	21st to end	Night	2 2 26% 47% 21% 2	416
August	1st to 10th	Night	2 2 20% 48% 24% 3	430
	11th to 20th	Night	2 1 14% 41% 35% 7%	564
	21st to end	Night	1 4 26% 43% 22% 4	824
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 2025-07-17 22:10:03. Version ac07

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