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 2024-10-25.

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% 2 6% 4 4 4 4 5% 7% 49%	101469
	Night	9% 8% 3 7% 4 4 4 4 6% 7% 43%	97481
Transparency	Day	34% 8% 13% 22% 16% 8%	101212
	Night	38% 8% 15% 21% 13% 6%	97327
Seeing	Day	11% 16% 29% 27% 14% 3	14510
	Night	10% 10% 25% 35% 17% 2	23479

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	16% 18% 6% 12% 7% 6% 5% 4 6% 7% 13%	2539
	11th to 20th	Day	22% 17% 6% 9% 5% 5% 6% 3 4 7% 16%	2540
	21st to end	Day	22% 18% 6% 14% 7% 5% 5% 4 4 5% 9%	2899
February	1st to 10th	Day	12% 13% 4 11% 8% 5% 6% 5% 7% 7% 23%	1710
	11th to 20th	Day	10% 10% 3 8% 6% 6% 6% 5% 8% 7% 29%	1665
	21st to end	Day	9% 10% 3 8% 6% 5% 7% 6% 8% 8% 29%	1555
March	1st to 10th	Day	7% 10% 4 9% 6% 7% 7% 6% 9% 10% 26%	3138
	11th to 20th	Day	7% 7% 2 7% 5% 4 5% 4 6% 8% 45%	3490
	21st to end	Day	8% 9% 3 9% 7% 6% 6% 6% 8% 9% 29%	4007
April	1st to 10th	Day	8% 9% 2 8% 6% 5% 7% 4 7% 8% 35%	2280
	11th to 20th	Day	7% 9% 3 7% 6% 6% 6% 4 6% 7% 40%	2249
	21st to end	Day	5% 5% 2 5% 4 6% 5% 5% 8% 10% 45%	2417
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	7% 5% 2 5% 3 4 4 4 6% 7% 52%	1900
	11th to 20th	Day	7% 5% 3 7% 6% 5% 4 4 7% 8% 44%	1900
	21st to end	Day	10% 7% 3 9% 5% 6% 5% 4 7% 7% 37%	1908
November	1st to 10th	Day	13% 10% 3 10% 6% 5% 6% 5% 9% 8% 25%	1651
	11th to 20th	Day	12% 15% 4 11% 7% 6% 8% 5% 7% 6% 20%	1520
	21st to end	Day	14% 11% 4 10% 6% 6% 6% 4 6% 7% 26%	1520
December	1st to 10th	Day	12% 15% 5% 12% 6% 5% 5% 4 5% 7% 24%	1520
	11th to 20th	Day	15% 17% 7% 16% 8% 6% 5% 5% 5% 5% 10%	1928
	21st to end	Day	15% 21% 7% 16% 6% 6% 4 3 4 4 13%	2794
January	1st to 10th	Night	18% 18% 6% 13% 6% 5% 6% 5% 5% 5% 13%	4406
	11th to 20th	Night	24% 16% 5% 10% 5% 5% 3 3 5% 7% 17%	4420
	21st to end	Night	23% 17% 6% 13% 7% 5% 5% 3 4 4 13%	4756
February	1st to 10th	Night	13% 13% 4 9% 5% 5% 5% 5% 6% 7% 28%	2470
	11th to 20th	Night	11% 11% 4 9% 5% 5% 4 3 6% 8% 34%	2402
	21st to end	Night	9% 13% 3 9% 5% 5% 4 5% 8% 9% 30%	1899
March	1st to 10th	Night	7% 8% 3 8% 7% 6% 7% 6% 10% 10% 28%	3720
	11th to 20th	Night	6% 6% 2 6% 4 4 4 4 6% 7% 51%	3490
	21st to end	Night	8% 7% 3 8% 6% 7% 7% 6% 9% 9% 30%	3645
April	1st to 10th	Night	8% 8% 2 7% 5% 4 6% 6% 8% 8% 38%	1900
	11th to 20th	Night	6% 7% 3 6% 5% 5% 5% 5% 6% 7% 46%	1879
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 9% 48%	1759
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	8% 5% 2 5% 3 3 3 3 5% 6% 56%	2280
	11th to 20th	Night	7% 5% 2 5% 5% 5% 5% 4 6% 7% 49%	2280
	21st to end	Night	10% 6% 2 7% 4 4 5% 5% 5% 8% 43%	2553
November	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 8% 8% 32%	2425
	11th to 20th	Night	14% 12% 4 8% 7% 5% 6% 4 8% 6% 25%	2660
	21st to end	Night	10% 11% 3 9% 5% 6% 6% 6% 7% 7% 30%	2660
December	1st to 10th	Night	15% 15% 4 11% 6% 5% 6% 4 5% 6% 23%	2660
	11th to 20th	Night	17% 16% 6% 15% 8% 6% 6% 4 5% 5% 12%	3276
	21st to end	Night	15% 18% 7% 14% 6% 5% 5% 3 5% 5% 16%	4862

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	65% 14% 12% 8% 1	2539
	11th to 20th	Day	67% 7% 16% 9% 1	2540
	21st to end	Day	67% 10% 13% 7% 2	2899
February	1st to 10th	Day	54% 11% 19% 12% 3	1710
	11th to 20th	Day	48% 12% 17% 17% 6%	1674
	21st to end	Day	49% 11% 17% 14% 8%	1555
March	1st to 10th	Day	43% 9% 16% 23% 8%	3138
	11th to 20th	Day	36% 8% 16% 31% 10%	3490
	21st to end	Day	48% 16% 13% 17% 6%	4007
April	1st to 10th	Day	45% 10% 17% 19% 9%	2280
	11th to 20th	Day	42% 8% 17% 17% 13% 2	2252
	21st to end	Day	34% 10% 16% 25% 12% 2	2417
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	31% 6% 14% 27% 19% 3	1900
	11th to 20th	Day	34% 10% 18% 21% 15% 1	1900
	21st to end	Day	42% 9% 16% 23% 9% 1	1908
November	1st to 10th	Day	51% 10% 15% 17% 6%	1651
	11th to 20th	Day	59% 12% 13% 12% 3	1520
	21st to end	Day	52% 11% 17% 14% 6% 1	1520
December	1st to 10th	Day	55% 12% 13% 15% 5%	1520
	11th to 20th	Day	64% 13% 15% 7% 1	1928
	21st to end	Day	69% 9% 12% 8% 2	2794
January	1st to 10th	Night	67% 12% 12% 7% 3	4406
	11th to 20th	Night	64% 7% 16% 10% 3	4420
	21st to end	Night	67% 10% 13% 8% 2	4756
February	1st to 10th	Night	50% 10% 22% 15% 3	2470
	11th to 20th	Night	46% 9% 20% 18% 7%	2404
	21st to end	Night	47% 11% 17% 17% 7% 1	1899
March	1st to 10th	Night	41% 9% 15% 26% 8% 1	3720
	11th to 20th	Night	29% 8% 18% 33% 11%	3490
	21st to end	Night	47% 15% 16% 18% 3	3645
April	1st to 10th	Night	42% 11% 18% 21% 8% 1	1900
	11th to 20th	Night	36% 8% 19% 22% 11% 3	1879
	21st to end	Night	33% 11% 18% 25% 11% 2	1759
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	28% 6% 12% 28% 23% 3	2280
	11th to 20th	Night	33% 9% 15% 25% 15% 3	2280
	21st to end	Night	38% 9% 14% 23% 16% 1	2553
November	1st to 10th	Night	48% 9% 15% 21% 8%	2425
	11th to 20th	Night	54% 11% 16% 16% 4	2660
	21st to end	Night	47% 10% 19% 19% 5%	2660
December	1st to 10th	Night	57% 11% 14% 14% 4	2660
	11th to 20th	Night	65% 10% 17% 7% 1	3276
	21st to end	Night	64% 9% 13% 11% 4	4862

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	24% 16% 29% 18% 11% 2	180
	11th to 20th	Day	19% 16% 27% 27% 11% 1	180
	21st to end	Day	17% 13% 28% 30% 11% 2	264
February	1st to 10th	Day	19% 19% 26% 27% 9% 1	370
	11th to 20th	Day	16% 22% 29% 24% 8%	365
	21st to end	Day	15% 22% 29% 26% 7% 1	375
March	1st to 10th	Day	16% 20% 34% 21% 7% 1	460
	11th to 20th	Day	14% 20% 28% 25% 11% 1	460
	21st to end	Day	20% 20% 29% 24% 7%	506
April	1st to 10th	Day	19% 23% 30% 17% 10% 1	460
	11th to 20th	Day	15% 24% 33% 17% 8% 3	452
	21st to end	Day	11% 23% 33% 22% 9% 2	460
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	14% 20% 30% 21% 12% 4	397
November	1st to 10th	Day	19% 17% 26% 27% 10% 1	259
	11th to 20th	Day	17% 17% 29% 24% 11% 2	180
	21st to end	Day	14% 17% 26% 28% 13% 1	180
December	1st to 10th	Day	13% 17% 23% 29% 15% 3	180
	11th to 20th	Day	16% 18% 35% 26% 3 1	180
	21st to end	Day	15% 20% 25% 31% 9% 1	198
January	1st to 10th	Night	19% 14% 25% 28% 12% 1	760
	11th to 20th	Night	17% 13% 23% 31% 13% 1	760
	21st to end	Night	16% 11% 22% 33% 16% 2	836
February	1st to 10th	Night	13% 16% 24% 33% 13% 1	760
	11th to 20th	Night	15% 14% 26% 29% 15% 1	744
	21st to end	Night	15% 16% 26% 32% 10% 1	628
March	1st to 10th	Night	13% 14% 27% 31% 14% 1	760
	11th to 20th	Night	11% 11% 24% 36% 16% 1	760
	21st to end	Night	11% 15% 29% 32% 11% 1	836
April	1st to 10th	Night	13% 14% 28% 32% 13% 1	760
	11th to 20th	Night	11% 15% 31% 25% 15% 3	751
	21st to end	Night	8% 12% 33% 32% 14% 1	619
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% 31% 17% 3	812
November	1st to 10th	Night	13% 9% 25% 36% 15% 1	740
	11th to 20th	Night	15% 16% 24% 30% 13% 1	760
	21st to end	Night	10% 10% 25% 37% 15% 3	760
December	1st to 10th	Night	16% 10% 24% 32% 15% 2	760
	11th to 20th	Night	17% 13% 25% 32% 13% 1	760
	21st to end	Night	17% 16% 29% 29% 9% 1	836

Page generated at 2024-10-22 17:21:03. Version ac07

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