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 2024-11-04.

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% 2 6% 4 4 4 4 6% 7% 50%	105376
	Night	8% 8% 3 7% 5% 4 4 4 6% 7% 45%	102316
Transparency	Day	32% 10% 14% 21% 16% 8%	105119
	Night	35% 11% 15% 21% 13% 5%	102162
Seeing	Day	11% 16% 29% 27% 14% 3	14538
	Night	10% 10% 25% 35% 17% 2	23516

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% 6% 11% 7% 6% 6% 6% 6% 7% 13%	2539
	11th to 20th	Day	19% 18% 5% 9% 5% 5% 5% 5% 6% 6% 17%	2540
	21st to end	Day	21% 19% 7% 13% 7% 6% 4 3 4 5% 10%	2899
February	1st to 10th	Day	10% 13% 4 10% 7% 6% 6% 5% 7% 8% 24%	1710
	11th to 20th	Day	9% 11% 4 8% 5% 5% 7% 6% 8% 8% 30%	1665
	21st to end	Day	8% 10% 3 7% 5% 7% 6% 6% 9% 9% 30%	1555
March	1st to 10th	Day	5% 10% 4 10% 7% 6% 7% 6% 8% 10% 28%	3151
	11th to 20th	Day	5% 8% 3 5% 5% 5% 5% 4 5% 8% 46%	3490
	21st to end	Day	7% 9% 3 9% 7% 6% 7% 6% 8% 9% 29%	4006
April	1st to 10th	Day	8% 8% 3 7% 6% 6% 7% 5% 7% 8% 35%	2280
	11th to 20th	Day	6% 8% 3 7% 6% 5% 6% 5% 6% 8% 40%	2249
	21st to end	Day	4 5% 2 5% 5% 4 5% 6% 8% 10% 46%	2416
May	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 6% 7% 51%	4420
	11th to 20th	Day	4 3 1 3 3 3 3 2 4 7% 66%	4377
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 5% 7% 9% 47%	4813
June	1st to 10th	Day	7% 5% 2 5% 5% 4 5% 4 6% 8% 48%	2660
	11th to 20th	Day	4 3 1 5% 4 4 4 4 6% 8% 56%	2660
	21st to end	Day	3 2 1 3 3 2 3 3 6% 7% 68%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 3 3 86%	4420
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 85%	4420
	21st to end	Day	1 1 1 1 1 2 2 2 3 4 83%	4790
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% 27%	1678
	11th to 20th	Day	10% 13% 5% 12% 7% 7% 7% 5% 8% 7% 20%	1520
	21st to end	Day	10% 11% 4 9% 6% 7% 7% 4 7% 7% 27%	1520
December	1st to 10th	Day	9% 17% 6% 10% 7% 6% 5% 4 5% 6% 26%	1520
	11th to 20th	Day	14% 17% 7% 17% 7% 5% 6% 5% 6% 5% 12%	1928
	21st to end	Day	15% 20% 7% 14% 7% 6% 5% 3 4 5% 14%	2794
January	1st to 10th	Night	16% 19% 7% 11% 6% 5% 5% 4 7% 6% 14%	4406
	11th to 20th	Night	21% 17% 5% 9% 6% 4 5% 3 5% 8% 17%	4420
	21st to end	Night	21% 19% 5% 13% 8% 7% 4 3 4 4 13%	4755
February	1st to 10th	Night	9% 13% 4 10% 6% 4 6% 5% 7% 8% 29%	2470
	11th to 20th	Night	10% 10% 3 8% 6% 5% 5% 4 6% 6% 36%	2402
	21st to end	Night	9% 11% 3 8% 5% 5% 5% 5% 8% 8% 33%	1899
March	1st to 10th	Night	6% 7% 3 8% 6% 6% 6% 6% 8% 12% 31%	3719
	11th to 20th	Night	5% 6% 2 6% 4 4 4 4 6% 6% 52%	3490
	21st to end	Night	8% 7% 3 8% 7% 6% 7% 6% 9% 10% 31%	3646
April	1st to 10th	Night	7% 9% 2 6% 5% 5% 5% 6% 8% 8% 38%	1900
	11th to 20th	Night	6% 5% 3 6% 5% 4 5% 5% 6% 8% 45%	1879
	21st to end	Night	6% 4 2 6% 4 4 4 4 8% 10% 47%	1758
May	1st to 10th	Night	9% 6% 2 4 3 3 3 3 5% 6% 56%	2540
	11th to 20th	Night	3 3 1 2 3 3 3 3 5% 7% 67%	2516
	21st to end	Night	3 3 1 3 4 4 4 5% 8% 11% 53%	2762
June	1st to 10th	Night	7% 6% 1 5% 3 4 5% 5% 6% 7% 48%	1520
	11th to 20th	Night	4 4 1 4 3 3 4 3 5% 7% 61%	1520
	21st to end	Night	2 3 1 3 3 3 3 3 4 5% 70%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 4 87%	2540
	11th to 20th	Night	1 2 1 1 1 1 2 2 3 3 84%	2540
	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% 9% 3 9% 5% 6% 6% 4 7% 7% 33%	2472
	11th to 20th	Night	11% 12% 5% 9% 6% 6% 7% 5% 6% 7% 27%	2660
	21st to end	Night	8% 11% 3 9% 6% 6% 6% 5% 6% 8% 31%	2660
December	1st to 10th	Night	12% 15% 5% 11% 7% 4 5% 4 5% 7% 25%	2660
	11th to 20th	Night	16% 16% 7% 14% 7% 6% 5% 4 5% 6% 13%	3276
	21st to end	Night	13% 19% 8% 13% 7% 5% 5% 4 4 5% 17%	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	57% 19% 14% 8% 2	2539
	11th to 20th	Day	61% 14% 15% 8% 1	2540
	21st to end	Day	61% 18% 12% 7% 2	2899
February	1st to 10th	Day	52% 14% 18% 12% 3	1710
	11th to 20th	Day	44% 16% 18% 16% 5%	1674
	21st to end	Day	47% 13% 19% 13% 8%	1555
March	1st to 10th	Day	40% 12% 19% 23% 7%	3151
	11th to 20th	Day	34% 9% 17% 31% 9%	3490
	21st to end	Day	47% 17% 15% 15% 5%	4006
April	1st to 10th	Day	44% 12% 18% 18% 8%	2280
	11th to 20th	Day	42% 10% 19% 14% 13% 2	2252
	21st to end	Day	33% 13% 19% 22% 11% 2	2416
May	1st to 10th	Day	31% 9% 13% 27% 20%	4420
	11th to 20th	Day	20% 6% 11% 33% 26% 4	4380
	21st to end	Day	32% 15% 22% 24% 7%	4814
June	1st to 10th	Day	34% 9% 15% 27% 12% 3	2660
	11th to 20th	Day	27% 8% 17% 34% 12% 3	2660
	21st to end	Day	18% 5% 14% 35% 21% 8%	2660
July	1st to 10th	Day	7% 3 7% 16% 25% 42%	4351
	11th to 20th	Day	8% 3 5% 11% 25% 48%	4345
	21st to end	Day	8% 3 8% 20% 36% 25%	4639
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%	1678
	11th to 20th	Day	54% 18% 13% 12% 3	1520
	21st to end	Day	48% 12% 17% 15% 6% 1	1520
December	1st to 10th	Day	54% 12% 14% 15% 5%	1520
	11th to 20th	Day	59% 18% 14% 7% 2	1928
	21st to end	Day	62% 16% 12% 8% 2	2794
January	1st to 10th	Night	61% 16% 13% 7% 3	4406
	11th to 20th	Night	59% 11% 17% 10% 3	4420
	21st to end	Night	61% 17% 12% 9% 2	4755
February	1st to 10th	Night	48% 13% 20% 16% 3	2470
	11th to 20th	Night	43% 13% 19% 19% 7%	2404
	21st to end	Night	43% 14% 18% 18% 7% 1	1899
March	1st to 10th	Night	37% 11% 17% 26% 7% 1	3719
	11th to 20th	Night	27% 11% 17% 34% 10%	3490
	21st to end	Night	42% 20% 17% 17% 3	3646
April	1st to 10th	Night	40% 13% 18% 20% 8% 1	1900
	11th to 20th	Night	35% 10% 21% 20% 11% 3	1879
	21st to end	Night	31% 12% 19% 24% 11% 2	1758
May	1st to 10th	Night	30% 7% 13% 31% 17% 1	2540
	11th to 20th	Night	18% 6% 12% 37% 23% 3	2516
	21st to end	Night	22% 15% 34% 22% 6% 1	2762
June	1st to 10th	Night	35% 10% 12% 30% 11% 2	1520
	11th to 20th	Night	24% 7% 18% 36% 14% 2	1520
	21st to end	Night	18% 6% 11% 35% 23% 8%	1520
July	1st to 10th	Night	6% 2 7% 15% 29% 39%	2500
	11th to 20th	Night	9% 2 6% 14% 22% 46%	2492
	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% 13% 21% 7%	2472
	11th to 20th	Night	50% 14% 15% 16% 4	2660
	21st to end	Night	43% 14% 18% 20% 5%	2660
December	1st to 10th	Night	54% 13% 15% 14% 4	2660
	11th to 20th	Night	59% 15% 15% 9% 2	3276
	21st to end	Night	56% 16% 13% 12% 3	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	13% 19% 30% 22% 12% 4	416
November	1st to 10th	Day	19% 16% 26% 26% 10% 1	268
	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	618
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 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% 26% 35% 15% 1	754
	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 2024-11-01 19:47:35. Version ac07

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