Mt Vernon (OR)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt Vernon (OR) 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	6% 9% 3 7% 5% 4 4 3 5% 6% 46%	117473
	Night	7% 10% 4 8% 5% 4 4 3 5% 6% 43%	119921
Transparency	Day	37% 5% 10% 21% 17% 10%	117217
	Night	40% 6% 12% 21% 15% 6%	119766
Seeing	Day	12% 10% 25% 28% 19% 7%	15044
	Night	13% 5% 15% 41% 22% 3	24343

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	9% 15% 8% 14% 8% 5% 5% 4 6% 6% 20%	3639
	11th to 20th	Day	7% 17% 6% 9% 4 4 4 4 5% 6% 34%	3640
	21st to end	Day	11% 12% 3 7% 4 3 3 3 4 6% 41%	4004
February	1st to 10th	Day	8% 16% 6% 10% 6% 6% 5% 4 5% 7% 28%	1800
	11th to 20th	Day	7% 15% 6% 11% 7% 6% 5% 4 5% 6% 28%	1868
	21st to end	Day	6% 17% 6% 13% 7% 6% 4 4 6% 7% 25%	1650
March	1st to 10th	Day	6% 12% 5% 12% 7% 6% 6% 4 6% 8% 30%	4518
	11th to 20th	Day	5% 11% 4 10% 7% 6% 5% 4 6% 7% 35%	4736
	21st to end	Day	5% 14% 6% 12% 7% 7% 7% 5% 6% 7% 23%	5940
April	1st to 10th	Day	7% 17% 5% 10% 6% 5% 5% 3 6% 6% 29%	2400
	11th to 20th	Day	8% 13% 5% 10% 7% 5% 5% 3 5% 6% 35%	2369
	21st to end	Day	9% 13% 4 8% 6% 4 4 4 5% 6% 37%	2456
May	1st to 10th	Day	9% 14% 5% 8% 4 4 4 4 5% 6% 38%	4312
	11th to 20th	Day	6% 6% 2 5% 4 3 4 3 4 6% 58%	4377
	21st to end	Day	8% 10% 3 8% 6% 4 5% 3 6% 7% 40%	4813
June	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 4 5% 7% 39%	2660
	11th to 20th	Day	9% 7% 2 7% 5% 4 4 4 6% 6% 48%	2660
	21st to end	Day	4 3 1 4 3 4 3 4 4 6% 65%	2660
July	1st to 10th	Day	1 1 2 2 2 2 2 3 4 81%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 78%	4420
	21st to end	Day	1 1 1 2 2 2 3 2 4 5% 77%	4790
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 6% 66%	4400
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 6% 74%	3883
	21st to end	Day	2 2 1 3 2 3 3 2 4 5% 72%	4180
September	1st to 10th	Day	2 2 1 3 2 3 2 2 4 6% 72%	2280
	11th to 20th	Day	5% 5% 2 5% 4 4 4 3 5% 6% 58%	2228
	21st to end	Day	6% 7% 2 5% 4 3 4 3 5% 6% 55%	2090
October	1st to 10th	Day	6% 6% 2 5% 3 3 3 2 3 4 63%	3249
	11th to 20th	Day	6% 6% 2 7% 4 4 4 4 6% 7% 51%	3161
	21st to end	Day	8% 11% 3 9% 6% 6% 6% 4 6% 7% 34%	3111
November	1st to 10th	Day	13% 13% 4 7% 5% 5% 4 4 6% 8% 31%	1764
	11th to 20th	Day	12% 13% 5% 9% 7% 5% 6% 4 7% 7% 27%	1743
	21st to end	Day	8% 14% 5% 8% 7% 4 4 4 5% 7% 34%	1600
December	1st to 10th	Day	5% 10% 3 7% 5% 4 4 3 4 6% 49%	2620
	11th to 20th	Day	10% 15% 4 12% 6% 5% 5% 5% 7% 9% 22%	3028
	21st to end	Day	10% 13% 6% 13% 7% 7% 6% 5% 6% 8% 20%	4004
January	1st to 10th	Night	12% 18% 7% 12% 6% 6% 5% 4 6% 7% 17%	6306
	11th to 20th	Night	11% 17% 5% 10% 5% 4 4 2 3 6% 34%	6320
	21st to end	Night	9% 11% 4 8% 5% 4 4 3 5% 6% 40%	6938
February	1st to 10th	Night	10% 15% 6% 12% 7% 6% 4 4 5% 6% 26%	2600
	11th to 20th	Night	8% 14% 6% 11% 7% 5% 5% 4 6% 6% 29%	2420
	21st to end	Night	7% 15% 5% 11% 7% 7% 5% 4 6% 7% 26%	1980
March	1st to 10th	Night	5% 12% 4 9% 5% 5% 5% 4 7% 9% 35%	5376
	11th to 20th	Night	7% 11% 4 9% 6% 5% 4 3 5% 5% 40%	5248
	21st to end	Night	6% 13% 5% 13% 7% 7% 6% 5% 7% 7% 25%	5034
April	1st to 10th	Night	8% 14% 5% 9% 5% 4 4 4 5% 6% 35%	2000
	11th to 20th	Night	7% 12% 3 9% 5% 4 5% 4 5% 6% 41%	1979
	21st to end	Night	8% 11% 4 7% 5% 5% 5% 4 5% 7% 39%	1839
May	1st to 10th	Night	8% 15% 4 9% 5% 4 4 3 4 6% 38%	2649
	11th to 20th	Night	5% 5% 2 4 3 3 3 2 4 6% 62%	2516
	21st to end	Night	10% 11% 3 7% 4 4 3 3 4 5% 47%	2762
June	1st to 10th	Night	10% 8% 2 5% 4 3 4 4 8% 7% 44%	1520
	11th to 20th	Night	10% 8% 2 5% 3 2 3 3 4 7% 53%	1520
	21st to end	Night	2 3 1 3 3 3 4 3 6% 6% 66%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 3 4 83%	2540
	11th to 20th	Night	1 2 2 3 3 2 4 5% 78%	2540
	21st to end	Night	1 1 1 2 2 2 3 2 4 4 77%	2761
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 6% 6% 63%	2529
	11th to 20th	Night	1 1 1 2 2 2 3 2 5% 6% 74%	2956
	21st to end	Night	2 2 1 3 2 2 3 2 5% 5% 74%	3498
September	1st to 10th	Night	3 2 1 2 2 2 2 3 4 5% 73%	1900
	11th to 20th	Night	3 5% 1 5% 3 3 4 4 5% 7% 60%	1952
	21st to end	Night	6% 6% 2 5% 3 3 3 2 4 6% 58%	2090
October	1st to 10th	Night	5% 5% 2 6% 4 2 2 2 2 4 67%	3728
	11th to 20th	Night	6% 5% 2 6% 4 3 4 3 4 7% 55%	3777
	21st to end	Night	6% 7% 3 8% 6% 5% 4 4 5% 8% 43%	4445
November	1st to 10th	Night	13% 11% 3 7% 6% 5% 5% 3 5% 6% 34%	2535
	11th to 20th	Night	15% 10% 4 8% 7% 5% 5% 4 6% 6% 30%	2655
	21st to end	Night	9% 13% 4 9% 6% 5% 4 4 6% 8% 33%	2800
December	1st to 10th	Night	6% 8% 3 6% 4 4 4 3 4 7% 49%	4560
	11th to 20th	Night	10% 15% 5% 12% 7% 7% 6% 5% 6% 8% 19%	5176
	21st to end	Night	13% 15% 6% 11% 6% 6% 5% 4 6% 7% 21%	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	63% 7% 11% 15% 4	3639
	11th to 20th	Day	49% 7% 13% 21% 9% 1	3640
	21st to end	Day	44% 5% 10% 18% 18% 4	4004
February	1st to 10th	Day	55% 7% 14% 18% 6%	1800
	11th to 20th	Day	56% 7% 12% 16% 9% 1	1877
	21st to end	Day	57% 7% 14% 13% 8% 1	1650
March	1st to 10th	Day	49% 6% 11% 22% 9% 4	4518
	11th to 20th	Day	44% 7% 11% 27% 12%	4736
	21st to end	Day	55% 9% 15% 16% 5%	5940
April	1st to 10th	Day	54% 6% 14% 17% 8% 1	2400
	11th to 20th	Day	49% 6% 10% 20% 12% 2	2372
	21st to end	Day	47% 6% 11% 22% 13% 2	2456
May	1st to 10th	Day	45% 5% 11% 25% 12% 1	4312
	11th to 20th	Day	27% 5% 8% 29% 26% 5%	4380
	21st to end	Day	41% 8% 17% 25% 10% 1	4814
June	1st to 10th	Day	44% 6% 10% 29% 11% 1	2660
	11th to 20th	Day	34% 5% 10% 28% 18% 5%	2660
	21st to end	Day	21% 4 9% 26% 29% 11%	2660
July	1st to 10th	Day	9% 3 4 12% 22% 50%	4351
	11th to 20th	Day	10% 3 4 10% 21% 52%	4345
	21st to end	Day	11% 3 3 15% 37% 31%	4639
August	1st to 10th	Day	19% 3 5% 15% 31% 27%	4384
	11th to 20th	Day	13% 3 4 22% 37% 20%	3923
	21st to end	Day	14% 3 8% 26% 28% 21%	4179
September	1st to 10th	Day	15% 3 6% 24% 30% 22%	2280
	11th to 20th	Day	27% 5% 9% 23% 25% 12%	2228
	21st to end	Day	29% 5% 9% 24% 23% 10%	2090
October	1st to 10th	Day	27% 3 7% 19% 31% 13%	3249
	11th to 20th	Day	32% 5% 14% 28% 19% 3	3161
	21st to end	Day	46% 7% 9% 23% 14% 1	3111
November	1st to 10th	Day	51% 6% 13% 20% 10% 1	1764
	11th to 20th	Day	53% 9% 14% 19% 5%	1743
	21st to end	Day	48% 7% 13% 18% 13% 1	1600
December	1st to 10th	Day	36% 6% 12% 33% 12% 1	2620
	11th to 20th	Day	57% 7% 20% 12% 3	3028
	21st to end	Day	57% 8% 17% 15% 3	4004
January	1st to 10th	Night	64% 8% 12% 13% 2	6306
	11th to 20th	Night	51% 6% 13% 20% 9% 1	6320
	21st to end	Night	44% 7% 11% 19% 15% 4	6938
February	1st to 10th	Night	59% 7% 13% 14% 7%	2600
	11th to 20th	Night	54% 7% 14% 16% 8% 2	2422
	21st to end	Night	55% 8% 11% 17% 7% 1	1980
March	1st to 10th	Night	42% 7% 12% 24% 11% 4	5376
	11th to 20th	Night	44% 6% 11% 31% 8%	5248
	21st to end	Night	52% 12% 14% 18% 5%	5034
April	1st to 10th	Night	48% 6% 15% 23% 6% 1	2000
	11th to 20th	Night	43% 6% 13% 25% 10% 2	1979
	21st to end	Night	42% 10% 13% 20% 15%	1839
May	1st to 10th	Night	45% 6% 13% 24% 12%	2649
	11th to 20th	Night	23% 5% 11% 38% 21% 1	2516
	21st to end	Night	39% 7% 21% 27% 7%	2762
June	1st to 10th	Night	36% 9% 14% 29% 12% 1	1520
	11th to 20th	Night	33% 5% 12% 30% 16% 4	1520
	21st to end	Night	19% 4 13% 31% 26% 7%	1520
July	1st to 10th	Night	8% 2 5% 14% 25% 46%	2500
	11th to 20th	Night	9% 3 6% 12% 26% 44%	2492
	21st to end	Night	11% 2 3 16% 39% 28%	2674
August	1st to 10th	Night	19% 4 6% 17% 35% 20%	2521
	11th to 20th	Night	11% 3 6% 27% 38% 15%	2982
	21st to end	Night	13% 3 8% 28% 30% 18%	3498
September	1st to 10th	Night	14% 3 5% 26% 33% 19%	1900
	11th to 20th	Night	21% 7% 9% 25% 26% 12%	1952
	21st to end	Night	28% 4 9% 25% 24% 11%	2090
October	1st to 10th	Night	23% 3 6% 23% 34% 11%	3728
	11th to 20th	Night	30% 4 19% 21% 23% 3	3777
	21st to end	Night	38% 5% 11% 28% 16% 1	4445
November	1st to 10th	Night	48% 8% 10% 19% 13% 2	2535
	11th to 20th	Night	52% 7% 15% 18% 8%	2655
	21st to end	Night	48% 6% 12% 17% 15% 1	2800
December	1st to 10th	Night	34% 7% 12% 33% 12% 1	4560
	11th to 20th	Night	60% 8% 13% 15% 4	5176
	21st to end	Night	58% 9% 15% 12% 5%	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	20% 12% 22% 29% 15% 4	200
	11th to 20th	Day	15% 9% 26% 27% 19% 6%	200
	21st to end	Day	14% 6% 24% 31% 21% 4	220
February	1st to 10th	Day	16% 16% 28% 26% 11% 3	400
	11th to 20th	Day	17% 13% 31% 25% 13% 2	452
	21st to end	Day	16% 15% 27% 29% 11% 2	414
March	1st to 10th	Day	18% 15% 27% 28% 11% 1	500
	11th to 20th	Day	16% 16% 26% 26% 13% 3	500
	21st to end	Day	19% 15% 28% 25% 11% 1	550
April	1st to 10th	Day	18% 18% 24% 24% 12% 5%	500
	11th to 20th	Day	15% 15% 30% 21% 13% 5%	492
	21st to end	Day	16% 14% 25% 24% 17% 4	481
May	1st to 10th	Day	15% 14% 26% 22% 17% 5%	463
	11th to 20th	Day	21% 11% 24% 21% 17% 6%	488
	21st to end	Day	15% 9% 20% 24% 21% 11%	529
June	1st to 10th	Day	13% 7% 27% 24% 18% 11%	490
	11th to 20th	Day	13% 12% 28% 26% 16% 5%	490
	21st to end	Day	4 8% 27% 29% 22% 10%	490
July	1st to 10th	Day	2 6% 28% 32% 25% 7%	486
	11th to 20th	Day	2 6% 28% 31% 23% 9%	484
	21st to end	Day	2 5% 26% 34% 25% 8%	531
August	1st to 10th	Day	5% 4 26% 30% 24% 11%	487
	11th to 20th	Day	3 4 21% 31% 26% 15%	477
	21st to end	Day	5% 6% 20% 31% 25% 13%	539
September	1st to 10th	Day	4 4 22% 27% 30% 13%	490
	11th to 20th	Day	7% 7% 21% 30% 24% 11%	490
	21st to end	Day	10% 6% 21% 31% 25% 7%	490
October	1st to 10th	Day	12% 9% 19% 29% 21% 11%	490
	11th to 20th	Day	13% 7% 25% 25% 21% 9%	490
	21st to end	Day	17% 10% 20% 29% 16% 8%	340
November	1st to 10th	Day	18% 9% 22% 30% 18% 2	300
	11th to 20th	Day	16% 8% 24% 32% 15% 4	271
	21st to end	Day	13% 10% 20% 37% 17% 4	200
December	1st to 10th	Day	17% 9% 20% 24% 19% 13%	200
	11th to 20th	Day	23% 9% 25% 31% 13%	200
	21st to end	Day	15% 8% 28% 30% 15% 3	220
January	1st to 10th	Night	19% 7% 21% 35% 17% 1	800
	11th to 20th	Night	22% 6% 18% 34% 18% 2	800
	21st to end	Night	17% 5% 15% 42% 19% 2	880
February	1st to 10th	Night	19% 7% 21% 39% 14% 1	800
	11th to 20th	Night	15% 6% 22% 36% 19% 2	784
	21st to end	Night	15% 10% 22% 39% 13% 2	660
March	1st to 10th	Night	15% 7% 21% 40% 16% 1	800
	11th to 20th	Night	16% 6% 19% 39% 19% 1	800
	21st to end	Night	15% 9% 21% 39% 15% 1	880
April	1st to 10th	Night	16% 8% 17% 40% 19% 1	800
	11th to 20th	Night	12% 7% 20% 39% 20% 2	791
	21st to end	Night	14% 6% 18% 41% 20% 1	667
May	1st to 10th	Night	11% 5% 19% 42% 22% 1	450
	11th to 20th	Night	13% 4 9% 37% 34% 3	374
	21st to end	Night	14% 6% 10% 39% 27% 4	411
June	1st to 10th	Night	12% 4 12% 46% 24% 1	380
	11th to 20th	Night	15% 5% 13% 44% 22% 1	380
	21st to end	Night	4 2 11% 49% 34% 1	380
July	1st to 10th	Night	2 1 6% 56% 32% 2	370
	11th to 20th	Night	1 11% 60% 27% 1	368
	21st to end	Night	1 6% 55% 36% 1	416
August	1st to 10th	Night	4 1 5% 52% 37% 2	378
	11th to 20th	Night	2 2 7% 39% 43% 7%	642
	21st to end	Night	3 2 9% 49% 34% 3	836
September	1st to 10th	Night	5% 1 7% 44% 39% 4	760
	11th to 20th	Night	6% 3 8% 41% 36% 7%	760
	21st to end	Night	10% 3 8% 42% 29% 8%	760
October	1st to 10th	Night	11% 3 11% 45% 28% 3	760
	11th to 20th	Night	12% 4 13% 42% 24% 5%	760
	21st to end	Night	15% 4 15% 42% 20% 4	836
November	1st to 10th	Night	18% 6% 14% 44% 18% 1	780
	11th to 20th	Night	21% 5% 16% 39% 16% 2	800
	21st to end	Night	15% 6% 18% 40% 17% 5%	800
December	1st to 10th	Night	16% 3 18% 42% 17% 3	800
	11th to 20th	Night	20% 6% 21% 37% 15% 1	800
	21st to end	Night	24% 9% 20% 36% 11%	880

Page generated at 2025-04-23 06:25:08. Version ac07

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