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 2024-03-01.

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	8% 9% 4 7% 6% 3 5% 4 5% 6% 43%	78388
	Night	9% 11% 4 7% 6% 3 4 5% 5% 7% 39%	81041
Transparency	Day	42% 6% 11% 19% 16% 7%	78091
	Night	44% 7% 12% 19% 14% 5%	80859
Seeing	Day	12% 10% 25% 28% 19% 7%	13552
	Night	14% 5% 15% 41% 23% 3	22872

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	10% 16% 9% 12% 10% 4 6% 5% 6% 5% 18%	2539
	11th to 20th	Day	11% 21% 8% 10% 6% 4 5% 5% 4 5% 22%	2540
	21st to end	Day	15% 16% 5% 8% 8% 3 5% 5% 5% 7% 23%	2794
February	1st to 10th	Day	8% 15% 6% 9% 7% 4 6% 5% 5% 7% 29%	1710
	11th to 20th	Day	7% 14% 7% 10% 8% 5% 5% 5% 4 7% 29%	1772
	21st to end	Day	6% 16% 8% 12% 9% 4 5% 4 6% 7% 24%	1570
March	1st to 10th	Day	9% 11% 8% 12% 10% 4 5% 6% 5% 6% 23%	1801
	11th to 20th	Day	8% 13% 6% 9% 8% 4 6% 5% 5% 8% 28%	1872
	21st to end	Day	9% 16% 8% 11% 9% 5% 6% 6% 5% 5% 21%	2360
April	1st to 10th	Day	8% 16% 6% 9% 8% 4 6% 4 6% 6% 28%	2160
	11th to 20th	Day	9% 12% 6% 9% 8% 4 5% 5% 4 6% 33%	2129
	21st to end	Day	10% 12% 5% 7% 6% 3 4 5% 4 6% 38%	2267
May	1st to 10th	Day	7% 11% 4 6% 5% 3 5% 6% 5% 7% 42%	2451
	11th to 20th	Day	11% 8% 4 7% 7% 4 6% 5% 5% 7% 36%	2477
	21st to end	Day	14% 9% 4 7% 7% 3 5% 5% 6% 6% 33%	2723
June	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 4 4 7% 39%	2520
	11th to 20th	Day	9% 6% 2 6% 5% 3 5% 4 5% 7% 48%	2520
	21st to end	Day	4 3 1 3 3 3 3 4 4 6% 65%	2520
July	1st to 10th	Day	2 2 1 3 3 3 3 4 5% 5% 70%	2520
	11th to 20th	Day	2 1 1 3 2 1 3 3 4 5% 75%	2520
	21st to end	Day	1 1 1 2 2 1 3 3 4 5% 77%	2741
August	1st to 10th	Day	3 2 1 2 3 3 4 4 6% 6% 67%	2507
	11th to 20th	Day	3 2 1 2 3 2 2 2 3 5% 74%	2221
	21st to end	Day	3 2 1 3 3 2 3 3 4 6% 72%	2376
September	1st to 10th	Day	2 2 1 3 3 2 2 3 4 6% 72%	2160
	11th to 20th	Day	5% 4 1 4 5% 3 4 4 5% 6% 59%	2111
	21st to end	Day	6% 7% 2 5% 5% 3 4 3 5% 6% 53%	1980
October	1st to 10th	Day	10% 9% 4 6% 5% 2 4 4 4 4 47%	1843
	11th to 20th	Day	9% 7% 2 6% 5% 2 4 4 6% 6% 48%	1800
	21st to end	Day	11% 9% 3 7% 6% 3 5% 5% 5% 6% 40%	1792
November	1st to 10th	Day	13% 12% 4 6% 6% 4 5% 5% 6% 8% 31%	1674
	11th to 20th	Day	12% 11% 5% 8% 8% 5% 5% 5% 6% 7% 27%	1656
	21st to end	Day	9% 13% 6% 7% 8% 3 4 5% 5% 7% 34%	1520
December	1st to 10th	Day	9% 13% 5% 7% 7% 4 6% 5% 5% 7% 32%	1520
	11th to 20th	Day	10% 13% 5% 10% 8% 4 5% 6% 7% 8% 24%	1928
	21st to end	Day	9% 13% 7% 11% 8% 5% 6% 6% 5% 8% 22%	2794
January	1st to 10th	Night	12% 17% 7% 12% 7% 5% 6% 5% 6% 6% 17%	4406
	11th to 20th	Night	16% 21% 6% 10% 7% 3 4 3 3 5% 21%	4420
	21st to end	Night	13% 14% 5% 9% 7% 4 5% 5% 7% 7% 23%	4861
February	1st to 10th	Night	10% 14% 7% 10% 8% 4 5% 5% 4 6% 27%	2470
	11th to 20th	Night	9% 13% 7% 9% 8% 4 5% 5% 5% 6% 29%	2296
	21st to end	Night	7% 14% 6% 10% 8% 5% 6% 6% 6% 7% 25%	1884
March	1st to 10th	Night	8% 14% 6% 9% 6% 3 5% 6% 6% 9% 28%	2165
	11th to 20th	Night	9% 11% 4 8% 8% 5% 6% 5% 6% 7% 30%	2085
	21st to end	Night	8% 13% 6% 10% 7% 5% 5% 6% 6% 7% 26%	1995
April	1st to 10th	Night	9% 13% 6% 8% 6% 3 4 5% 5% 6% 34%	1800
	11th to 20th	Night	8% 11% 4 8% 7% 3 4 5% 5% 5% 38%	1779
	21st to end	Night	9% 11% 4 7% 6% 3 5% 4 5% 7% 39%	1692
May	1st to 10th	Night	6% 9% 3 6% 5% 3 5% 4 5% 7% 47%	1507
	11th to 20th	Night	10% 7% 3 6% 5% 3 4 5% 6% 8% 44%	1416
	21st to end	Night	12% 10% 4 7% 6% 4 4 5% 5% 5% 39%	1552
June	1st to 10th	Night	10% 8% 3 5% 4 3 3 6% 7% 8% 44%	1440
	11th to 20th	Night	11% 8% 2 5% 4 2 3 3 4 7% 53%	1440
	21st to end	Night	3 3 1 3 3 3 4 3 6% 6% 65%	1440
July	1st to 10th	Night	1 1 1 2 3 2 3 4 5% 7% 71%	1440
	11th to 20th	Night	1 1 1 1 2 2 3 5% 81%	1440
	21st to end	Night	1 1 1 2 2 1 3 3 5% 4 77%	1576
August	1st to 10th	Night	2 3 1 2 3 2 3 5% 7% 6% 66%	1432
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 6% 76%	1675
	21st to end	Night	2 2 1 2 2 2 2 3 4 5% 74%	1980
September	1st to 10th	Night	3 2 1 2 2 2 2 3 4 5% 74%	1800
	11th to 20th	Night	3 4 1 4 4 2 4 5% 5% 7% 62%	1849
	21st to end	Night	7% 6% 2 5% 5% 2 3 3 4 6% 57%	1980
October	1st to 10th	Night	8% 7% 2 6% 5% 2 4 4 4 5% 53%	2114
	11th to 20th	Night	9% 6% 2 5% 5% 2 4 4 5% 6% 51%	2160
	21st to end	Night	9% 9% 3 7% 6% 4 4 4 5% 6% 43%	2562
November	1st to 10th	Night	14% 11% 3 6% 7% 4 5% 4 5% 6% 34%	2405
	11th to 20th	Night	15% 10% 4 6% 7% 4 5% 6% 6% 7% 30%	2522
	21st to end	Night	10% 12% 5% 8% 6% 3 4 5% 6% 8% 33%	2660
December	1st to 10th	Night	11% 11% 5% 7% 6% 4 6% 5% 5% 8% 31%	2660
	11th to 20th	Night	10% 14% 5% 9% 7% 5% 5% 6% 7% 9% 23%	3276
	21st to end	Night	10% 14% 6% 9% 8% 4 5% 5% 7% 8% 24%	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% 8% 12% 12% 2	2539
	11th to 20th	Day	63% 7% 11% 12% 5% 1	2540
	21st to end	Day	60% 7% 14% 13% 4 1	2794
February	1st to 10th	Day	55% 7% 15% 18% 5%	1710
	11th to 20th	Day	55% 7% 12% 16% 8% 1	1781
	21st to end	Day	59% 7% 13% 13% 7% 1	1570
March	1st to 10th	Day	59% 7% 13% 14% 6% 1	1801
	11th to 20th	Day	54% 6% 11% 22% 7%	1872
	21st to end	Day	64% 7% 12% 12% 4	2360
April	1st to 10th	Day	56% 6% 14% 15% 8% 1	2160
	11th to 20th	Day	52% 5% 10% 19% 10% 3	2132
	21st to end	Day	48% 6% 10% 22% 13% 1	2267
May	1st to 10th	Day	41% 5% 12% 26% 15% 1	2451
	11th to 20th	Day	46% 6% 12% 23% 12% 1	2480
	21st to end	Day	51% 6% 13% 20% 9% 1	2724
June	1st to 10th	Day	44% 6% 10% 29% 10%	2520
	11th to 20th	Day	35% 5% 10% 28% 17% 5%	2520
	21st to end	Day	22% 4 10% 26% 28% 10%	2520
July	1st to 10th	Day	15% 5% 7% 20% 28% 25%	2451
	11th to 20th	Day	12% 4 7% 15% 30% 32%	2445
	21st to end	Day	11% 3 4 12% 38% 32%	2589
August	1st to 10th	Day	17% 4 7% 22% 31% 18%	2490
	11th to 20th	Day	14% 2 6% 22% 35% 20%	2221
	21st to end	Day	15% 3 6% 23% 36% 17%	2376
September	1st to 10th	Day	15% 3 6% 24% 30% 21%	2160
	11th to 20th	Day	26% 5% 8% 23% 26% 13%	2111
	21st to end	Day	31% 5% 9% 23% 22% 10%	1980
October	1st to 10th	Day	41% 5% 8% 19% 20% 8%	1843
	11th to 20th	Day	35% 6% 11% 25% 19% 4	1800
	21st to end	Day	44% 6% 11% 21% 17% 1	1792
November	1st to 10th	Day	50% 5% 13% 21% 10% 1	1674
	11th to 20th	Day	53% 9% 14% 20% 5%	1656
	21st to end	Day	49% 7% 13% 17% 14% 1	1520
December	1st to 10th	Day	51% 8% 16% 15% 8% 2	1520
	11th to 20th	Day	57% 7% 16% 14% 5%	1928
	21st to end	Day	58% 9% 16% 15% 3	2794
January	1st to 10th	Night	65% 10% 12% 11% 2	4406
	11th to 20th	Night	65% 7% 11% 11% 6% 2	4420
	21st to end	Night	57% 9% 15% 12% 5%	4861
February	1st to 10th	Night	58% 7% 14% 14% 7%	2470
	11th to 20th	Night	54% 7% 14% 16% 8% 2	2298
	21st to end	Night	56% 9% 11% 16% 7% 1	1884
March	1st to 10th	Night	52% 7% 13% 18% 8% 2	2165
	11th to 20th	Night	50% 9% 14% 23% 4 1	2085
	21st to end	Night	56% 7% 15% 18% 4	1995
April	1st to 10th	Night	51% 6% 14% 21% 7% 1	1800
	11th to 20th	Night	47% 6% 13% 23% 10% 2	1779
	21st to end	Night	44% 9% 13% 19% 14%	1692
May	1st to 10th	Night	36% 6% 14% 30% 14%	1507
	11th to 20th	Night	38% 7% 18% 27% 10% 1	1416
	21st to end	Night	46% 7% 16% 22% 9%	1552
June	1st to 10th	Night	36% 9% 15% 29% 12%	1440
	11th to 20th	Night	33% 6% 13% 30% 15% 4	1440
	21st to end	Night	20% 4 13% 31% 25% 7%	1440
July	1st to 10th	Night	15% 4 9% 25% 28% 20%	1400
	11th to 20th	Night	7% 3 10% 18% 34% 27%	1392
	21st to end	Night	9% 3 4 17% 40% 26%	1488
August	1st to 10th	Night	16% 5% 8% 26% 30% 15%	1424
	11th to 20th	Night	12% 3 7% 23% 35% 20%	1675
	21st to end	Night	13% 3 7% 25% 37% 16%	1980
September	1st to 10th	Night	14% 3 5% 26% 33% 19%	1800
	11th to 20th	Night	20% 6% 9% 24% 28% 13%	1849
	21st to end	Night	29% 4 9% 24% 23% 11%	1980
October	1st to 10th	Night	33% 6% 9% 23% 22% 7%	2114
	11th to 20th	Night	34% 5% 12% 21% 23% 5%	2160
	21st to end	Night	42% 5% 14% 18% 18% 2	2562
November	1st to 10th	Night	49% 8% 10% 19% 13% 2	2405
	11th to 20th	Night	52% 7% 15% 18% 8%	2522
	21st to end	Night	48% 6% 12% 17% 16% 1	2660
December	1st to 10th	Night	50% 9% 15% 15% 9% 2	2660
	11th to 20th	Night	56% 9% 14% 16% 6%	3276
	21st to end	Night	56% 9% 15% 13% 7%	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	20% 11% 23% 26% 16% 4	180
	11th to 20th	Day	17% 9% 25% 25% 18% 6%	180
	21st to end	Day	15% 6% 26% 30% 21% 3	198
February	1st to 10th	Day	17% 14% 27% 27% 11% 4	370
	11th to 20th	Day	17% 14% 31% 24% 13% 2	416
	21st to end	Day	16% 15% 26% 29% 12% 2	382
March	1st to 10th	Day	19% 14% 29% 27% 10% 1	421
	11th to 20th	Day	15% 16% 27% 27% 14% 1	420
	21st to end	Day	20% 14% 27% 26% 11% 1	462
April	1st to 10th	Day	18% 20% 24% 23% 12% 3	420
	11th to 20th	Day	17% 17% 30% 19% 12% 5%	412
	21st to end	Day	18% 15% 25% 24% 16% 3	420
May	1st to 10th	Day	14% 14% 26% 24% 18% 5%	423
	11th to 20th	Day	23% 10% 24% 21% 16% 6%	448
	21st to end	Day	15% 9% 19% 24% 21% 11%	485
June	1st to 10th	Day	13% 7% 27% 24% 17% 11%	450
	11th to 20th	Day	13% 12% 28% 27% 15% 5%	450
	21st to end	Day	4 7% 27% 30% 22% 10%	450
July	1st to 10th	Day	2 5% 28% 33% 24% 7%	446
	11th to 20th	Day	2 7% 27% 32% 23% 9%	444
	21st to end	Day	2 5% 25% 34% 25% 9%	487
August	1st to 10th	Day	5% 4 25% 31% 24% 12%	447
	11th to 20th	Day	3 4 20% 31% 27% 17%	437
	21st to end	Day	5% 5% 20% 32% 25% 12%	495
September	1st to 10th	Day	4 5% 22% 27% 30% 12%	450
	11th to 20th	Day	7% 7% 20% 30% 25% 11%	450
	21st to end	Day	10% 6% 22% 30% 25% 7%	450
October	1st to 10th	Day	12% 9% 20% 28% 20% 11%	450
	11th to 20th	Day	14% 7% 25% 24% 20% 9%	450
	21st to end	Day	16% 10% 20% 28% 18% 9%	307
November	1st to 10th	Day	17% 9% 23% 31% 17% 3	270
	11th to 20th	Day	14% 6% 25% 34% 16% 4	244
	21st to end	Day	12% 9% 21% 36% 18% 4	180
December	1st to 10th	Day	18% 10% 19% 23% 17% 12%	180
	11th to 20th	Day	23% 7% 23% 32% 14%	180
	21st to end	Day	15% 7% 27% 32% 16% 4	198
January	1st to 10th	Night	19% 7% 21% 35% 18% 1	760
	11th to 20th	Night	23% 6% 18% 32% 18% 2	760
	21st to end	Night	18% 6% 14% 43% 18% 2	836
February	1st to 10th	Night	19% 6% 20% 39% 15% 1	760
	11th to 20th	Night	15% 6% 22% 36% 20% 2	744
	21st to end	Night	15% 10% 22% 39% 13% 2	628
March	1st to 10th	Night	16% 7% 20% 39% 16% 2	722
	11th to 20th	Night	16% 5% 19% 40% 19% 1	720
	21st to end	Night	16% 8% 20% 40% 15% 1	792
April	1st to 10th	Night	17% 8% 16% 40% 18% 2	720
	11th to 20th	Night	13% 7% 20% 37% 20% 2	711
	21st to end	Night	15% 6% 18% 40% 20% 1	612
May	1st to 10th	Night	10% 5% 18% 43% 23% 1	427
	11th to 20th	Night	14% 4 9% 37% 34% 3	354
	21st to end	Night	14% 7% 10% 38% 27% 4	389
June	1st to 10th	Night	13% 5% 11% 45% 25% 1	360
	11th to 20th	Night	15% 5% 12% 44% 23% 1	360
	21st to end	Night	4 2 10% 48% 35% 1	360
July	1st to 10th	Night	2 1 6% 56% 33% 2	350
	11th to 20th	Night	1 10% 59% 28% 1	348
	21st to end	Night	1 6% 55% 37% 2	394
August	1st to 10th	Night	4 1 5% 51% 38% 2	358
	11th to 20th	Night	2 1 6% 38% 45% 7%	607
	21st to end	Night	3 2 8% 49% 35% 4	792
September	1st to 10th	Night	5% 1 7% 44% 39% 4	720
	11th to 20th	Night	6% 2 7% 40% 38% 7%	720
	21st to end	Night	11% 3 8% 42% 29% 8%	720
October	1st to 10th	Night	11% 2 11% 44% 28% 3	720
	11th to 20th	Night	12% 4 13% 40% 25% 5%	720
	21st to end	Night	16% 4 14% 41% 20% 5%	792
November	1st to 10th	Night	18% 5% 13% 45% 18% 1	740
	11th to 20th	Night	21% 5% 15% 40% 17% 2	760
	21st to end	Night	15% 6% 18% 40% 17% 5%	760
December	1st to 10th	Night	17% 3 18% 42% 16% 3	760
	11th to 20th	Night	19% 5% 21% 38% 16% 1	760
	21st to end	Night	24% 8% 19% 37% 11%	836

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