Vernonia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vernonia clear sky chart from 2005-11-06 to 2025-12-18.

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	15% 11% 4 8% 5% 5% 4 3 5% 6% 34%	132134
	Night	15% 11% 4 9% 5% 5% 4 3 5% 5% 34%	127637
Transparency	Day	49% 7% 12% 17% 12% 3	131854
	Night	50% 7% 12% 17% 11% 3	127494
Seeing	Day	22% 12% 26% 24% 12% 3	16453
	Night	23% 8% 22% 30% 16% 1	24928

By Month

Percentage Cloud Cover NW (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	27% 19% 6% 11% 7% 5% 5% 3 4 3 9%	3639
	11th to 20th	Day	19% 15% 3 8% 5% 4 4 4 6% 5% 26%	3616
	21st to end	Day	19% 14% 4 9% 6% 4 4 3 4 4 29%	4396
February	1st to 10th	Day	22% 18% 5% 11% 6% 5% 6% 4 5% 5% 15%	1800
	11th to 20th	Day	22% 20% 6% 11% 6% 5% 3 3 3 4 17%	1755
	21st to end	Day	22% 19% 5% 12% 6% 6% 4 3 3 5% 14%	1537
March	1st to 10th	Day	14% 18% 5% 12% 7% 7% 6% 4 6% 5% 17%	4777
	11th to 20th	Day	20% 19% 5% 9% 5% 4 4 2 3 4 25%	5000
	21st to end	Day	20% 18% 5% 12% 8% 7% 6% 4 5% 5% 10%	5568
April	1st to 10th	Day	23% 18% 5% 9% 6% 6% 4 3 5% 5% 15%	2400
	11th to 20th	Day	22% 15% 5% 9% 5% 5% 5% 3 3 4 23%	2402
	21st to end	Day	21% 14% 4 9% 6% 5% 4 4 6% 5% 21%	2600
May	1st to 10th	Day	15% 10% 3 8% 5% 4 4 3 5% 6% 37%	6205
	11th to 20th	Day	12% 11% 4 9% 6% 6% 6% 5% 7% 7% 27%	6277
	21st to end	Day	15% 9% 3 9% 6% 5% 5% 5% 6% 7% 29%	6903
June	1st to 10th	Day	21% 10% 3 7% 6% 5% 4 3 5% 6% 30%	2800
	11th to 20th	Day	16% 12% 4 9% 6% 6% 6% 4 7% 7% 22%	2918
	21st to end	Day	11% 8% 3 7% 5% 5% 4 4 6% 8% 42%	3000
July	1st to 10th	Day	4 4 2 5% 3 3 3 2 4 5% 66%	6693
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 6% 74%	6320
	21st to end	Day	5% 4 2 3 3 3 3 3 5% 7% 63%	6874
August	1st to 10th	Day	4 3 2 4 3 3 3 3 4 8% 63%	4238
	11th to 20th	Day	6% 5% 2 8% 5% 5% 5% 4 7% 8% 43%	4156
	21st to end	Day	9% 7% 3 7% 5% 4 4 4 5% 6% 47%	4481
September	1st to 10th	Day	6% 6% 2 6% 3 4 5% 4 6% 7% 52%	2400
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 4 5% 7% 41%	2376
	21st to end	Day	14% 9% 3 7% 4 4 4 4 4 7% 41%	2116
October	1st to 10th	Day	14% 7% 2 5% 4 4 4 3 6% 8% 44%	3280
	11th to 20th	Day	17% 9% 3 9% 5% 5% 5% 4 6% 7% 28%	3261
	21st to end	Day	19% 12% 4 10% 6% 5% 6% 4 5% 6% 23%	3499
November	1st to 10th	Day	23% 16% 5% 10% 6% 6% 5% 4 5% 5% 15%	1702
	11th to 20th	Day	25% 15% 5% 10% 6% 6% 5% 2 4 5% 18%	1680
	21st to end	Day	21% 14% 5% 10% 6% 5% 5% 4 4 4 21%	1680
December	1st to 10th	Day	17% 11% 3 8% 5% 6% 6% 3 5% 5% 30%	2700
	11th to 20th	Day	23% 17% 6% 12% 6% 7% 5% 3 4 4 12%	3081
	21st to end	Day	26% 20% 6% 12% 6% 5% 5% 3 4 4 8%	4004
January	1st to 10th	Night	28% 16% 5% 11% 7% 6% 5% 4 4 4 11%	6306
	11th to 20th	Night	17% 15% 5% 10% 5% 4 4 4 4 4 29%	6282
	21st to end	Night	23% 14% 4 7% 4 4 3 2 3 4 31%	6573
February	1st to 10th	Night	18% 18% 5% 10% 8% 6% 5% 4 4 5% 18%	2600
	11th to 20th	Night	20% 15% 5% 9% 6% 5% 5% 4 5% 5% 23%	2532
	21st to end	Night	18% 17% 6% 10% 7% 5% 5% 3 4 5% 20%	2091
March	1st to 10th	Night	12% 14% 5% 12% 7% 7% 7% 4 5% 5% 22%	5113
	11th to 20th	Night	15% 16% 5% 11% 5% 4 4 3 4 4 28%	4999
	21st to end	Night	20% 19% 5% 12% 7% 6% 5% 4 4 5% 12%	5407
April	1st to 10th	Night	19% 15% 5% 11% 7% 5% 5% 3 4 6% 21%	2000
	11th to 20th	Night	15% 14% 3 9% 6% 5% 4 3 5% 6% 28%	1945
	21st to end	Night	17% 12% 4 7% 4 4 4 4 5% 7% 30%	1800
May	1st to 10th	Night	15% 9% 3 6% 3 3 4 3 5% 6% 44%	3760
	11th to 20th	Night	11% 12% 4 8% 5% 5% 5% 4 6% 7% 33%	3616
	21st to end	Night	9% 10% 2 8% 5% 5% 5% 5% 7% 9% 35%	3972
June	1st to 10th	Night	15% 8% 4 8% 6% 4 4 4 5% 6% 38%	1600
	11th to 20th	Night	10% 11% 4 7% 4 4 5% 3 6% 9% 38%	1475
	21st to end	Night	6% 6% 3 5% 4 4 3 2 5% 7% 55%	1400
July	1st to 10th	Night	2 3 2 6% 3 3 2 2 4 6% 68%	3262
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 76%	3640
	21st to end	Night	2 2 1 4 3 3 3 3 4 6% 69%	3969
August	1st to 10th	Night	2 2 1 3 2 2 3 2 4 6% 72%	2905
	11th to 20th	Night	6% 5% 2 8% 3 4 3 3 5% 7% 55%	2902
	21st to end	Night	10% 5% 2 5% 3 3 2 3 5% 6% 55%	3433
September	1st to 10th	Night	6% 3 2 5% 3 3 3 3 5% 7% 59%	2000
	11th to 20th	Night	9% 5% 3 7% 5% 5% 4 3 5% 6% 49%	2020
	21st to end	Night	9% 6% 2 6% 5% 4 4 4 5% 8% 48%	2283
October	1st to 10th	Night	10% 7% 2 5% 4 4 4 3 5% 7% 49%	3920
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	3897
	21st to end	Night	18% 9% 4 10% 5% 6% 5% 4 5% 5% 29%	4310
November	1st to 10th	Night	23% 12% 4 10% 6% 5% 5% 4 5% 5% 20%	2815
	11th to 20th	Night	25% 13% 5% 10% 6% 6% 5% 3 4 5% 18%	2940
	21st to end	Night	20% 14% 4 10% 6% 5% 5% 4 4 5% 25%	2940
December	1st to 10th	Night	13% 11% 4 8% 5% 4 5% 4 5% 6% 35%	4700
	11th to 20th	Night	23% 17% 6% 14% 7% 6% 5% 3 4 4 11%	5278
	21st to end	Night	25% 17% 6% 12% 6% 6% 5% 3 4 3 13%	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	79% 6% 6% 6% 4	3639
	11th to 20th	Day	57% 6% 8% 17% 10% 2	3640
	21st to end	Day	59% 5% 6% 12% 15% 4	4396
February	1st to 10th	Day	70% 6% 10% 10% 4	1800
	11th to 20th	Day	71% 7% 7% 9% 5% 1	1764
	21st to end	Day	73% 4 9% 7% 6% 1	1537
March	1st to 10th	Day	66% 7% 12% 10% 5%	4777
	11th to 20th	Day	64% 5% 7% 19% 5%	5000
	21st to end	Day	74% 8% 10% 6% 2	5568
April	1st to 10th	Day	69% 7% 12% 11% 2	2400
	11th to 20th	Day	62% 6% 9% 13% 8% 1	2402
	21st to end	Day	61% 7% 11% 13% 6% 1	2600
May	1st to 10th	Day	45% 6% 11% 24% 14%	6205
	11th to 20th	Day	48% 10% 19% 17% 6%	6280
	21st to end	Day	50% 10% 21% 16% 3	6904
June	1st to 10th	Day	53% 6% 16% 16% 9%	2800
	11th to 20th	Day	55% 8% 16% 15% 5%	2918
	21st to end	Day	37% 8% 17% 25% 10% 3	3000
July	1st to 10th	Day	20% 5% 12% 24% 19% 20%	6620
	11th to 20th	Day	13% 3 9% 23% 40% 12%	6245
	21st to end	Day	18% 6% 17% 35% 22% 3	6681
August	1st to 10th	Day	19% 5% 10% 25% 28% 12%	4221
	11th to 20th	Day	32% 12% 16% 23% 12% 4	4195
	21st to end	Day	36% 7% 11% 22% 18% 6%	4480
September	1st to 10th	Day	28% 7% 13% 25% 18% 9%	2400
	11th to 20th	Day	40% 8% 14% 18% 16% 5%	2376
	21st to end	Day	42% 6% 14% 19% 16% 4	2116
October	1st to 10th	Day	37% 6% 9% 27% 19% 2	3280
	11th to 20th	Day	54% 8% 12% 14% 11% 1	3261
	21st to end	Day	59% 7% 9% 15% 9%	3499
November	1st to 10th	Day	70% 7% 10% 10% 3	1702
	11th to 20th	Day	71% 4 10% 8% 5% 2	1680
	21st to end	Day	65% 7% 8% 8% 11%	1680
December	1st to 10th	Day	54% 6% 7% 12% 19% 1	2700
	11th to 20th	Day	76% 6% 9% 8% 2	3084
	21st to end	Day	79% 6% 9% 4 2	4004
January	1st to 10th	Night	76% 5% 8% 6% 4 1	6306
	11th to 20th	Night	56% 7% 8% 18% 11% 1	6320
	21st to end	Night	58% 3 6% 14% 14% 6%	6573
February	1st to 10th	Night	67% 8% 8% 12% 4	2600
	11th to 20th	Night	63% 7% 11% 11% 6% 2	2534
	21st to end	Night	65% 8% 10% 10% 6%	2091
March	1st to 10th	Night	61% 8% 12% 13% 5% 1	5113
	11th to 20th	Night	59% 7% 10% 17% 6%	5000
	21st to end	Night	71% 8% 12% 8% 1	5407
April	1st to 10th	Night	65% 5% 14% 12% 4	2000
	11th to 20th	Night	54% 7% 12% 16% 8% 2	1945
	21st to end	Night	52% 8% 15% 18% 6% 1	1800
May	1st to 10th	Night	38% 5% 13% 31% 13%	3757
	11th to 20th	Night	41% 9% 20% 19% 9% 2	3616
	21st to end	Night	41% 11% 27% 17% 4	3972
June	1st to 10th	Night	44% 7% 18% 23% 7% 1	1600
	11th to 20th	Night	40% 8% 21% 25% 7%	1475
	21st to end	Night	27% 7% 21% 31% 11% 4	1400
July	1st to 10th	Night	13% 7% 12% 27% 20% 21%	3226
	11th to 20th	Night	12% 2 9% 26% 35% 17%	3592
	21st to end	Night	13% 6% 18% 35% 23% 5%	3857
August	1st to 10th	Night	12% 5% 12% 28% 30% 13%	2897
	11th to 20th	Night	23% 11% 23% 28% 10% 6%	2929
	21st to end	Night	28% 6% 17% 23% 21% 5%	3433
September	1st to 10th	Night	23% 6% 14% 27% 20% 10%	2000
	11th to 20th	Night	34% 8% 15% 20% 16% 7%	2020
	21st to end	Night	33% 6% 16% 21% 18% 6%	2283
October	1st to 10th	Night	31% 6% 11% 31% 20% 1	3920
	11th to 20th	Night	46% 7% 16% 20% 10% 1	3897
	21st to end	Night	55% 7% 9% 18% 9% 1	4310
November	1st to 10th	Night	64% 7% 10% 12% 6%	2815
	11th to 20th	Night	67% 6% 10% 9% 6% 1	2940
	21st to end	Night	61% 7% 10% 10% 11% 1	2940
December	1st to 10th	Night	48% 7% 9% 16% 20% 1	4700
	11th to 20th	Night	77% 6% 7% 7% 4	5274
	21st to end	Night	75% 7% 10% 7% 1	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	38% 16% 28% 11% 7% 2	200
	11th to 20th	Day	31% 16% 30% 12% 9% 4	200
	21st to end	Day	31% 12% 22% 22% 11% 2	391
February	1st to 10th	Day	38% 15% 20% 16% 10% 1	400
	11th to 20th	Day	39% 15% 27% 14% 4 1	395
	21st to end	Day	40% 17% 21% 17% 4 1	359
March	1st to 10th	Day	34% 14% 30% 15% 8%	500
	11th to 20th	Day	32% 16% 26% 16% 7% 3	500
	21st to end	Day	36% 19% 19% 19% 7% 1	550
April	1st to 10th	Day	35% 14% 24% 16% 10% 3	500
	11th to 20th	Day	29% 16% 27% 17% 8% 3	492
	21st to end	Day	27% 14% 29% 19% 8% 3	500
May	1st to 10th	Day	18% 17% 27% 24% 11% 3	503
	11th to 20th	Day	25% 13% 27% 22% 10% 3	528
	21st to end	Day	21% 11% 28% 23% 13% 4	573
June	1st to 10th	Day	22% 11% 27% 26% 12% 2	530
	11th to 20th	Day	19% 18% 34% 22% 6% 1	648
	21st to end	Day	11% 11% 33% 32% 12% 2	730
July	1st to 10th	Day	9% 8% 34% 36% 11% 3	687
	11th to 20th	Day	7% 9% 30% 37% 13% 4	524
	21st to end	Day	6% 5% 25% 41% 18% 4	575
August	1st to 10th	Day	8% 7% 22% 39% 18% 5%	527
	11th to 20th	Day	5% 5% 23% 38% 22% 7%	517
	21st to end	Day	11% 8% 28% 32% 16% 5%	583
September	1st to 10th	Day	9% 8% 26% 36% 17% 4	530
	11th to 20th	Day	14% 9% 26% 30% 17% 4	530
	21st to end	Day	18% 13% 24% 28% 14% 3	530
October	1st to 10th	Day	20% 12% 23% 24% 18% 3	530
	11th to 20th	Day	25% 12% 23% 20% 17% 3	530
	21st to end	Day	32% 12% 25% 18% 9% 4	548
November	1st to 10th	Day	32% 18% 26% 16% 8% 1	249
	11th to 20th	Day	29% 16% 30% 16% 8% 2	220
	21st to end	Day	25% 17% 26% 22% 10% 1	220
December	1st to 10th	Day	32% 16% 24% 16% 10% 2	220
	11th to 20th	Day	38% 13% 28% 18% 4	214
	21st to end	Day	40% 16% 23% 14% 7% 1	220
January	1st to 10th	Night	37% 13% 21% 17% 11% 2	800
	11th to 20th	Night	34% 11% 20% 19% 15% 1	800
	21st to end	Night	30% 7% 18% 29% 15% 2	880
February	1st to 10th	Night	27% 11% 24% 25% 12% 1	800
	11th to 20th	Night	29% 9% 21% 27% 14%	784
	21st to end	Night	28% 12% 23% 27% 10%	660
March	1st to 10th	Night	27% 10% 24% 26% 13% 1	800
	11th to 20th	Night	28% 11% 19% 29% 13% 1	800
	21st to end	Night	31% 12% 20% 26% 10% 1	880
April	1st to 10th	Night	28% 13% 19% 26% 12% 2	800
	11th to 20th	Night	24% 9% 25% 29% 11% 2	757
	21st to end	Night	23% 9% 22% 31% 15% 1	600
May	1st to 10th	Night	17% 8% 27% 33% 15% 1	455
	11th to 20th	Night	17% 7% 22% 34% 19% 1	394
	21st to end	Night	16% 7% 24% 34% 18%	433
June	1st to 10th	Night	19% 4 27% 39% 12% 1	400
	11th to 20th	Night	14% 8% 29% 37% 11% 1	400
	21st to end	Night	9% 6% 24% 45% 15% 1	400
July	1st to 10th	Night	4 3 28% 48% 16%	390
	11th to 20th	Night	5% 5% 34% 40% 15% 1	388
	21st to end	Night	3 2 28% 48% 18%	438
August	1st to 10th	Night	5% 3 16% 46% 27% 3	577
	11th to 20th	Night	7% 3 15% 42% 31% 3	594
	21st to end	Night	9% 5% 22% 40% 22% 2	770
September	1st to 10th	Night	7% 5% 20% 44% 22% 1	800
	11th to 20th	Night	11% 3 18% 35% 28% 5%	800
	21st to end	Night	13% 6% 20% 38% 22% 2	800
October	1st to 10th	Night	18% 4 19% 35% 21% 3	800
	11th to 20th	Night	21% 7% 18% 30% 22% 4	800
	21st to end	Night	26% 9% 20% 26% 17% 2	880
November	1st to 10th	Night	29% 10% 21% 27% 12% 1	820
	11th to 20th	Night	34% 12% 20% 24% 10% 1	840
	21st to end	Night	26% 11% 22% 26% 13% 1	840
December	1st to 10th	Night	28% 9% 22% 24% 15% 2	840
	11th to 20th	Night	34% 12% 22% 21% 10% 1	828
	21st to end	Night	37% 14% 23% 17% 8% 1	880

Page generated at 2025-12-14 22:13:51. Version ac07

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