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 2026-08-30.

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	5% 8% 3 7% 4 4 4 3 5% 6% 50%	152933
	Night	7% 9% 3 7% 5% 4 4 3 5% 7% 46%	146578
Transparency	Day	34% 5% 10% 22% 19% 10%	147698
	Night	37% 6% 11% 23% 17% 6%	142814
Seeing	Day	12% 10% 25% 27% 19% 7%	16816
	Night	13% 5% 16% 42% 22% 2	25881

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	8% 15% 7% 12% 7% 6% 6% 4 7% 7% 20%	4722
	11th to 20th	Day	6% 13% 4 7% 3 3 4 3 4 7% 46%	4740
	21st to end	Day	9% 10% 3 7% 5% 4 4 3 5% 7% 43%	5187
February	1st to 10th	Day	8% 15% 6% 10% 6% 6% 5% 4 5% 7% 29%	1890
	11th to 20th	Day	6% 15% 6% 11% 7% 6% 5% 4 5% 6% 28%	1964
	21st to end	Day	6% 17% 6% 12% 7% 6% 4 4 6% 7% 26%	1730
March	1st to 10th	Day	6% 12% 5% 12% 7% 6% 6% 4 6% 8% 29%	4618
	11th to 20th	Day	5% 11% 4 10% 7% 6% 5% 4 6% 7% 35%	4840
	21st to end	Day	5% 14% 6% 12% 7% 7% 7% 5% 6% 7% 23%	6071
April	1st to 10th	Day	7% 16% 5% 9% 6% 5% 5% 3 6% 7% 30%	2520
	11th to 20th	Day	8% 12% 4 10% 6% 5% 5% 3 5% 6% 35%	2489
	21st to end	Day	9% 12% 4 7% 6% 4 5% 4 5% 6% 39%	2645
May	1st to 10th	Day	6% 8% 3 5% 3 3 4 3 4 6% 54%	7983
	11th to 20th	Day	6% 6% 3 6% 4 5% 5% 4 6% 7% 49%	8160
	21st to end	Day	6% 7% 2 5% 4 4 5% 3 5% 8% 52%	8992
June	1st to 10th	Day	13% 8% 2 7% 4 5% 4 3 5% 7% 42%	2940
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 6% 7% 50%	2940
	21st to end	Day	4 3 1 4 3 4 3 4 4 6% 64%	2940
July	1st to 10th	Day	1 2 1 3 2 2 3 3 4 5% 76%	8219
	11th to 20th	Day	2 2 1 3 2 2 3 2 4 5% 74%	8220
	21st to end	Day	1 1 1 2 2 2 3 3 4 6% 75%	8922
August	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 6% 63%	6350
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 77%	5727
	21st to end	Day	3 3 1 4 3 3 3 3 5% 7% 65%	6093
September	1st to 10th	Day	2 2 1 4 2 3 3 3 4 6% 70%	2400
	11th to 20th	Day	5% 5% 2 5% 4 4 4 3 5% 6% 58%	2345
	21st to end	Day	5% 7% 2 5% 4 3 4 3 5% 6% 56%	2200
October	1st to 10th	Day	6% 6% 2 5% 3 3 3 2 3 4 63%	3351
	11th to 20th	Day	6% 6% 2 7% 4 4 4 4 6% 7% 51%	3261
	21st to end	Day	8% 10% 3 9% 6% 6% 6% 4 6% 7% 34%	3210
November	1st to 10th	Day	12% 13% 4 8% 5% 6% 5% 4 6% 8% 31%	1854
	11th to 20th	Day	12% 13% 5% 9% 7% 5% 6% 4 7% 7% 26%	1830
	21st to end	Day	8% 14% 5% 8% 7% 4 4 4 5% 7% 34%	1680
December	1st to 10th	Day	6% 10% 3 7% 5% 4 4 3 4 6% 48%	2700
	11th to 20th	Day	10% 14% 4 11% 6% 5% 5% 5% 7% 9% 23%	3108
	21st to end	Day	10% 13% 6% 13% 7% 7% 6% 5% 6% 8% 20%	4092
January	1st to 10th	Night	10% 16% 6% 12% 7% 7% 6% 4 6% 7% 18%	8172
	11th to 20th	Night	8% 13% 4 7% 4 3 4 2 3 7% 45%	8220
	21st to end	Night	7% 10% 4 8% 5% 5% 5% 4 6% 8% 39%	8980
February	1st to 10th	Night	9% 15% 6% 11% 7% 6% 4 4 5% 6% 27%	2730
	11th to 20th	Night	8% 13% 6% 11% 7% 6% 5% 4 6% 6% 29%	2544
	21st to end	Night	7% 15% 5% 10% 7% 7% 5% 5% 6% 7% 26%	2076
March	1st to 10th	Night	5% 12% 4 9% 5% 5% 5% 4 7% 9% 34%	5496
	11th to 20th	Night	7% 11% 4 9% 6% 5% 5% 3 5% 5% 40%	5364
	21st to end	Night	6% 12% 5% 13% 7% 7% 6% 5% 6% 7% 25%	5145
April	1st to 10th	Night	8% 13% 5% 9% 5% 4 4 4 5% 6% 36%	2100
	11th to 20th	Night	7% 12% 3 9% 5% 4 5% 4 5% 5% 40%	2079
	21st to end	Night	8% 11% 4 7% 5% 5% 5% 4 5% 7% 40%	1974
May	1st to 10th	Night	4 8% 2 5% 3 3 4 3 5% 6% 56%	4930
	11th to 20th	Night	5% 7% 3 7% 4 4 3 3 4 6% 55%	4700
	21st to end	Night	8% 7% 2 5% 3 3 3 3 4 5% 57%	5181
June	1st to 10th	Night	9% 8% 2 5% 4 3 4 4 7% 7% 46%	1680
	11th to 20th	Night	10% 7% 2 5% 3 2 4 3 4 7% 54%	1680
	21st to end	Night	2 3 1 4 3 3 4 3 6% 6% 64%	1680
July	1st to 10th	Night	1 1 1 2 2 2 3 2 4 6% 77%	4740
	11th to 20th	Night	1 1 1 3 3 3 3 3 4 5% 75%	4740
	21st to end	Night	2 2 1 2 2 3 3 3 5% 4 73%	5153
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 6% 7% 62%	3662
	11th to 20th	Night	2 2 1 3 2 2 3 2 5% 5% 74%	4346
	21st to end	Night	3 3 1 4 3 3 3 3 5% 6% 66%	5105
September	1st to 10th	Night	3 3 1 3 2 2 2 3 4 6% 71%	2000
	11th to 20th	Night	3 4 1 5% 3 3 4 4 4 7% 60%	2055
	21st to end	Night	6% 6% 2 5% 3 3 3 2 4 6% 59%	2200
October	1st to 10th	Night	5% 5% 2 6% 4 2 2 2 2 4 67%	3846
	11th to 20th	Night	6% 6% 2 6% 4 4 4 3 4 7% 54%	3897
	21st to end	Night	6% 7% 3 8% 5% 5% 4 4 6% 7% 44%	4588
November	1st to 10th	Night	13% 11% 3 8% 6% 5% 5% 3 5% 7% 34%	2665
	11th to 20th	Night	14% 10% 4 8% 7% 5% 5% 5% 6% 7% 30%	2788
	21st to end	Night	8% 13% 4 9% 6% 5% 4 4 6% 8% 33%	2940
December	1st to 10th	Night	7% 8% 3 6% 4 4 4 3 4 7% 49%	4700
	11th to 20th	Night	10% 15% 5% 11% 7% 7% 6% 4 6% 8% 19%	5316
	21st to end	Night	13% 15% 6% 11% 6% 6% 5% 4 6% 7% 21%	7106

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	60% 8% 15% 15% 3	4723
	11th to 20th	Day	38% 6% 11% 24% 18% 4	4740
	21st to end	Day	41% 6% 11% 20% 19% 3	5187
February	1st to 10th	Day	54% 6% 15% 19% 6%	1890
	11th to 20th	Day	55% 7% 12% 16% 8% 1	1973
	21st to end	Day	56% 7% 13% 14% 8% 1	1730
March	1st to 10th	Day	49% 6% 11% 22% 9% 4	4618
	11th to 20th	Day	44% 7% 11% 26% 12%	4840
	21st to end	Day	55% 9% 15% 16% 5%	6071
April	1st to 10th	Day	53% 6% 14% 19% 8% 1	2520
	11th to 20th	Day	49% 6% 11% 20% 12% 2	2492
	21st to end	Day	46% 6% 11% 23% 14% 1	2645
May	1st to 10th	Day	30% 4 8% 29% 26% 2	7983
	11th to 20th	Day	31% 6% 11% 29% 21% 3	8163
	21st to end	Day	29% 6% 14% 29% 19% 3	8994
June	1st to 10th	Day	41% 6% 9% 29% 14% 1	2940
	11th to 20th	Day	33% 5% 9% 28% 20% 5%	2940
	21st to end	Day	22% 4 10% 26% 28% 11%	2940
July	1st to 10th	Day	11% 3 4 19% 30% 34%	8151
	11th to 20th	Day	14% 2 4 10% 25% 45%	8145
	21st to end	Day	11% 3 4 21% 35% 26%	8723
August	1st to 10th	Day	19% 3 5% 15% 31% 28%	4664
	11th to 20th	Day	13% 3 4 23% 37% 20%	4175
	21st to end	Day	15% 3 7% 26% 28% 21%	4420
September	1st to 10th	Day	16% 3 7% 24% 30% 21%	2400
	11th to 20th	Day	26% 5% 9% 23% 25% 12%	2345
	21st to end	Day	29% 5% 9% 24% 23% 11%	2200
October	1st to 10th	Day	27% 3 7% 20% 30% 13%	3351
	11th to 20th	Day	32% 5% 13% 28% 19% 3	3261
	21st to end	Day	45% 7% 10% 23% 14% 1	3210
November	1st to 10th	Day	50% 6% 14% 20% 9% 1	1854
	11th to 20th	Day	53% 9% 15% 19% 5%	1830
	21st to end	Day	48% 7% 14% 18% 13% 1	1680
December	1st to 10th	Day	37% 6% 12% 32% 12% 1	2700
	11th to 20th	Day	57% 7% 20% 13% 4	3108
	21st to end	Day	57% 8% 17% 15% 3	4092
January	1st to 10th	Night	60% 9% 15% 13% 2	8172
	11th to 20th	Night	40% 5% 11% 25% 16% 3	8220
	21st to end	Night	43% 7% 12% 20% 15% 3	8980
February	1st to 10th	Night	57% 7% 14% 15% 7%	2730
	11th to 20th	Night	54% 7% 14% 16% 8% 2	2546
	21st to end	Night	54% 8% 11% 17% 8% 1	2076
March	1st to 10th	Night	42% 7% 12% 24% 11% 4	5496
	11th to 20th	Night	44% 6% 11% 31% 8% 1	5364
	21st to end	Night	51% 12% 14% 18% 5% 1	5145
April	1st to 10th	Night	47% 6% 15% 25% 6% 1	2100
	11th to 20th	Night	44% 7% 13% 25% 10% 2	2079
	21st to end	Night	41% 9% 13% 21% 15%	1974
May	1st to 10th	Night	28% 5% 10% 29% 27% 1	4928
	11th to 20th	Night	30% 5% 13% 35% 16% 1	4700
	21st to end	Night	30% 6% 16% 34% 14%	5182
June	1st to 10th	Night	34% 8% 13% 29% 15% 1	1680
	11th to 20th	Night	32% 5% 11% 29% 19% 4	1680
	21st to end	Night	20% 5% 12% 30% 25% 7%	1680
July	1st to 10th	Night	12% 2 4 23% 31% 28%	4700
	11th to 20th	Night	11% 3 6% 12% 30% 38%	4692
	21st to end	Night	14% 3 5% 22% 35% 21%	5040
August	1st to 10th	Night	18% 4 6% 18% 34% 21%	2681
	11th to 20th	Night	12% 3 6% 27% 38% 15%	3170
	21st to end	Night	14% 3 8% 28% 29% 18%	3698
September	1st to 10th	Night	15% 3 6% 25% 32% 18%	2000
	11th to 20th	Night	21% 6% 9% 24% 27% 12%	2055
	21st to end	Night	27% 4 9% 25% 25% 12%	2200
October	1st to 10th	Night	24% 3 6% 23% 33% 11%	3846
	11th to 20th	Night	31% 4 18% 21% 23% 3	3897
	21st to end	Night	37% 5% 11% 29% 16% 1	4588
November	1st to 10th	Night	48% 8% 11% 20% 12% 2	2665
	11th to 20th	Night	52% 7% 15% 18% 8%	2788
	21st to end	Night	49% 6% 13% 17% 15% 1	2940
December	1st to 10th	Night	35% 7% 13% 33% 12% 1	4700
	11th to 20th	Night	60% 8% 13% 15% 4	5316
	21st to end	Night	58% 9% 15% 12% 5%	7106

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% 13% 22% 27% 14% 4	220
	11th to 20th	Day	14% 8% 24% 28% 21% 6%	220
	21st to end	Day	13% 5% 25% 33% 20% 3	242
February	1st to 10th	Day	15% 16% 27% 26% 12% 3	430
	11th to 20th	Day	16% 13% 32% 24% 13% 2	488
	21st to end	Day	17% 17% 26% 28% 11% 2	446
March	1st to 10th	Day	19% 16% 27% 28% 10% 1	540
	11th to 20th	Day	16% 16% 26% 26% 12% 3	540
	21st to end	Day	19% 16% 27% 25% 11% 1	594
April	1st to 10th	Day	17% 17% 23% 24% 13% 5%	540
	11th to 20th	Day	16% 15% 30% 22% 12% 5%	532
	21st to end	Day	15% 14% 26% 25% 16% 4	540
May	1st to 10th	Day	14% 13% 26% 24% 17% 5%	543
	11th to 20th	Day	22% 12% 26% 19% 16% 5%	568
	21st to end	Day	13% 11% 20% 23% 22% 11%	617
June	1st to 10th	Day	13% 9% 26% 24% 18% 11%	570
	11th to 20th	Day	11% 12% 30% 26% 17% 5%	570
	21st to end	Day	4 9% 27% 28% 22% 10%	570
July	1st to 10th	Day	3 5% 27% 34% 24% 7%	566
	11th to 20th	Day	2 6% 30% 31% 23% 8%	564
	21st to end	Day	2 5% 26% 33% 25% 8%	619
August	1st to 10th	Day	5% 4 25% 31% 25% 10%	567
	11th to 20th	Day	2 4 22% 31% 27% 13%	557
	21st to end	Day	5% 6% 20% 30% 26% 12%	620
September	1st to 10th	Day	4 4 22% 26% 30% 14%	530
	11th to 20th	Day	7% 7% 20% 30% 25% 12%	530
	21st to end	Day	9% 6% 21% 30% 26% 8%	530
October	1st to 10th	Day	12% 9% 21% 28% 20% 10%	530
	11th to 20th	Day	13% 8% 25% 25% 21% 8%	530
	21st to end	Day	16% 10% 20% 29% 17% 7%	373
November	1st to 10th	Day	17% 10% 25% 29% 17% 2	330
	11th to 20th	Day	16% 9% 24% 32% 15% 3	298
	21st to end	Day	12% 10% 24% 34% 17% 4	220
December	1st to 10th	Day	16% 11% 20% 23% 18% 12%	220
	11th to 20th	Day	24% 9% 24% 30% 14%	220
	21st to end	Day	16% 10% 27% 29% 14% 3	242
January	1st to 10th	Night	18% 8% 22% 35% 16% 1	840
	11th to 20th	Night	21% 6% 18% 35% 18% 2	840
	21st to end	Night	16% 5% 16% 43% 18% 2	924
February	1st to 10th	Night	18% 6% 21% 39% 14% 1	840
	11th to 20th	Night	14% 6% 23% 37% 19% 2	824
	21st to end	Night	15% 10% 23% 38% 12% 2	692
March	1st to 10th	Night	15% 8% 22% 39% 15% 1	840
	11th to 20th	Night	15% 7% 21% 38% 18% 1	840
	21st to end	Night	15% 10% 21% 39% 14% 1	924
April	1st to 10th	Night	15% 7% 17% 41% 18% 1	840
	11th to 20th	Night	12% 7% 20% 39% 19% 2	831
	21st to end	Night	14% 6% 18% 41% 20% 1	714
May	1st to 10th	Night	10% 5% 19% 43% 22% 1	497
	11th to 20th	Night	13% 5% 12% 37% 31% 3	414
	21st to end	Night	13% 6% 11% 39% 26% 4	455
June	1st to 10th	Night	11% 5% 13% 45% 24% 1	420
	11th to 20th	Night	14% 5% 14% 45% 21% 1	420
	21st to end	Night	4 3 12% 48% 33% 1	420
July	1st to 10th	Night	2 1 6% 56% 32% 2	410
	11th to 20th	Night	1 1 12% 60% 26% 1	408
	21st to end	Night	1 7% 55% 35% 1	460
August	1st to 10th	Night	3 1 5% 53% 36% 2	418
	11th to 20th	Night	2 2 8% 42% 40% 6%	710
	21st to end	Night	3 2 10% 49% 33% 3	916
September	1st to 10th	Night	5% 2 7% 44% 39% 4	800
	11th to 20th	Night	6% 2 8% 42% 36% 7%	800
	21st to end	Night	10% 3 9% 42% 30% 7%	800
October	1st to 10th	Night	11% 3 12% 45% 27% 3	800
	11th to 20th	Night	12% 5% 14% 42% 23% 5%	800
	21st to end	Night	15% 5% 15% 42% 19% 4	880
November	1st to 10th	Night	18% 6% 15% 44% 17% 1	820
	11th to 20th	Night	21% 6% 16% 40% 16% 2	840
	21st to end	Night	14% 7% 18% 40% 16% 4	840
December	1st to 10th	Night	16% 4 19% 41% 17% 3	840
	11th to 20th	Night	19% 5% 22% 38% 15% 1	840
	21st to end	Night	23% 9% 20% 36% 11%	924

Page generated at 2026-08-27 01:07:15. Version ac07

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