Eugene
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eugene clear sky chart from 2005-11-06 to 2025-07-06.

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	17% 8% 3 7% 4 4 4 3 5% 6% 38%	124229
	Night	20% 9% 3 7% 4 4 4 3 4 5% 35%	123397
Transparency	Day	46% 5% 12% 19% 13% 5%	123977
	Night	50% 6% 12% 17% 11% 4	123236
Seeing	Day	21% 9% 22% 27% 16% 5%	15577
	Night	24% 6% 13% 31% 23% 3	24576

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	37% 14% 4 7% 6% 6% 6% 4 4 4 8%	3639
	11th to 20th	Day	28% 10% 3 8% 6% 5% 6% 4 5% 7% 19%	3707
	21st to end	Day	23% 9% 3 7% 5% 4 5% 4 6% 7% 28%	4510
February	1st to 10th	Day	27% 12% 4 9% 6% 5% 6% 5% 5% 6% 16%	1800
	11th to 20th	Day	28% 13% 3 8% 6% 5% 4 3 5% 4 19%	1755
	21st to end	Day	28% 15% 4 8% 6% 4 4 3 4 5% 19%	1570
March	1st to 10th	Day	18% 12% 5% 9% 6% 6% 6% 4 6% 7% 21%	4817
	11th to 20th	Day	26% 13% 4 8% 5% 4 4 2 4 4 27%	5000
	21st to end	Day	24% 11% 4 10% 7% 6% 5% 5% 6% 7% 16%	5561
April	1st to 10th	Day	25% 14% 4 10% 6% 5% 5% 3 4 5% 19%	2400
	11th to 20th	Day	23% 11% 4 7% 6% 4 5% 3 4 5% 28%	2369
	21st to end	Day	22% 9% 3 8% 5% 4 4 4 6% 7% 29%	2575
May	1st to 10th	Day	14% 7% 2 6% 4 5% 4 4 5% 6% 42%	6073
	11th to 20th	Day	14% 8% 2 7% 4 4 5% 4 6% 7% 39%	6277
	21st to end	Day	12% 6% 2 7% 6% 6% 5% 4 6% 7% 39%	6902
June	1st to 10th	Day	20% 9% 3 7% 5% 4 3 3 6% 6% 37%	2800
	11th to 20th	Day	16% 7% 3 7% 5% 5% 4 4 5% 8% 37%	2800
	21st to end	Day	9% 4 2 4 3 3 3 3 5% 6% 58%	2800
July	1st to 10th	Day	3 2 1 2 2 1 2 2 2 3 80%	4491
	11th to 20th	Day	4 3 1 2 2 2 2 2 2 4 77%	4420
	21st to end	Day	3 2 1 2 2 2 2 2 3 4 78%	4680
August	1st to 10th	Day	2 3 1 3 2 3 3 2 4 5% 72%	4091
	11th to 20th	Day	4 4 1 4 3 3 3 3 4 6% 64%	4026
	21st to end	Day	7% 5% 1 4 3 3 3 2 4 6% 63%	4283
September	1st to 10th	Day	6% 3 1 3 2 3 2 2 4 5% 70%	2280
	11th to 20th	Day	10% 7% 2 6% 4 3 4 3 5% 5% 49%	2259
	21st to end	Day	13% 6% 2 6% 4 4 4 3 4 6% 49%	2009
October	1st to 10th	Day	12% 6% 2 6% 3 3 3 2 4 6% 52%	3180
	11th to 20th	Day	17% 7% 2 7% 4 5% 6% 4 6% 8% 33%	3161
	21st to end	Day	22% 9% 3 9% 5% 5% 6% 3 6% 8% 23%	3448
November	1st to 10th	Day	31% 9% 3 9% 5% 5% 4 3 5% 6% 19%	1694
	11th to 20th	Day	28% 11% 4 9% 5% 5% 4 4 6% 7% 17%	1600
	21st to end	Day	26% 10% 4 9% 5% 4 5% 4 7% 7% 20%	1600
December	1st to 10th	Day	21% 7% 2 6% 5% 6% 6% 5% 7% 8% 27%	2620
	11th to 20th	Day	30% 14% 4 11% 7% 6% 5% 3 6% 6% 9%	3028
	21st to end	Day	35% 16% 5% 11% 6% 6% 5% 3 4 4 6%	4004
January	1st to 10th	Night	36% 15% 5% 11% 6% 5% 4 3 3 3 9%	6306
	11th to 20th	Night	28% 13% 4 8% 7% 6% 6% 4 4 5% 15%	6254
	21st to end	Night	27% 10% 3 7% 4 3 3 4 4 5% 29%	6468
February	1st to 10th	Night	28% 14% 4 9% 5% 5% 4 3 4 6% 18%	2600
	11th to 20th	Night	28% 12% 3 6% 5% 4 4 3 5% 6% 23%	2532
	21st to end	Night	26% 12% 4 7% 5% 4 5% 3 5% 6% 23%	2060
March	1st to 10th	Night	18% 14% 4 10% 6% 5% 5% 4 6% 7% 20%	5074
	11th to 20th	Night	21% 14% 4 9% 5% 4 3 3 4 4 28%	5000
	21st to end	Night	25% 14% 4 10% 5% 5% 5% 3 5% 6% 17%	5412
April	1st to 10th	Night	26% 14% 3 8% 5% 4 5% 4 4 4 23%	2000
	11th to 20th	Night	20% 11% 3 8% 4 4 4 3 5% 6% 32%	1979
	21st to end	Night	22% 9% 2 6% 5% 5% 5% 4 4 7% 31%	1820
May	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 4 6% 45%	3882
	11th to 20th	Night	13% 9% 3 8% 4 4 4 3 5% 5% 41%	3616
	21st to end	Night	13% 5% 3 6% 4 3 4 3 5% 7% 46%	3972
June	1st to 10th	Night	18% 7% 3 4 4 3 4 4 5% 6% 42%	1600
	11th to 20th	Night	12% 8% 2 5% 3 4 3 4 5% 8% 47%	1600
	21st to end	Night	9% 4 2 3 2 2 3 2 5% 5% 64%	1600
July	1st to 10th	Night	3 2 1 2 1 1 2 1 2 2 83%	2580
	11th to 20th	Night	2 2 1 2 2 1 2 2 2 3 80%	2540
	21st to end	Night	3 2 1 2 1 1 1 1 2 3 82%	2872
August	1st to 10th	Night	2 2 1 2 2 2 3 2 3 5% 76%	2848
	11th to 20th	Night	3 3 1 4 2 3 3 3 4 6% 67%	2812
	21st to end	Night	6% 4 2 3 3 2 3 2 3 4 69%	3392
September	1st to 10th	Night	6% 2 1 2 2 2 3 2 4 4 72%	1900
	11th to 20th	Night	10% 4 2 4 4 4 4 2 5% 6% 56%	1918
	21st to end	Night	11% 5% 2 5% 3 3 4 3 5% 7% 52%	2171
October	1st to 10th	Night	11% 5% 2 6% 3 3 3 3 4 6% 56%	3800
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	3777
	21st to end	Night	22% 12% 4 9% 5% 5% 4 3 5% 6% 26%	4121
November	1st to 10th	Night	27% 9% 3 8% 5% 5% 5% 4 5% 5% 23%	2603
	11th to 20th	Night	31% 10% 4 9% 4 4 4 4 4 5% 20%	2800
	21st to end	Night	23% 11% 3 8% 5% 5% 5% 4 5% 5% 27%	2800
December	1st to 10th	Night	18% 7% 2 6% 5% 6% 9% 8% 7% 6% 26%	4560
	11th to 20th	Night	33% 12% 5% 10% 6% 5% 5% 3 4 4 11%	5176
	21st to end	Night	33% 12% 4 12% 6% 5% 5% 3 4 5% 11%	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	76% 5% 11% 6% 2 1	3639
	11th to 20th	Day	60% 7% 10% 14% 8% 2	3707
	21st to end	Day	53% 5% 10% 14% 13% 5%	4510
February	1st to 10th	Day	67% 6% 11% 10% 6%	1800
	11th to 20th	Day	66% 6% 12% 10% 4 2	1764
	21st to end	Day	68% 4 9% 10% 7% 2	1570
March	1st to 10th	Day	58% 6% 13% 18% 5%	4817
	11th to 20th	Day	61% 5% 9% 17% 8% 1	5000
	21st to end	Day	66% 6% 15% 10% 2	5561
April	1st to 10th	Day	68% 5% 11% 12% 4	2400
	11th to 20th	Day	59% 6% 11% 15% 7% 3	2372
	21st to end	Day	55% 7% 14% 16% 7% 1	2575
May	1st to 10th	Day	42% 6% 12% 25% 15%	6073
	11th to 20th	Day	43% 7% 15% 24% 11%	6280
	21st to end	Day	41% 7% 25% 22% 4	6904
June	1st to 10th	Day	49% 6% 15% 23% 7% 1	2800
	11th to 20th	Day	45% 5% 19% 22% 8%	2800
	21st to end	Day	28% 4 17% 33% 15% 3	2800
July	1st to 10th	Day	12% 2 7% 17% 25% 35%	4422
	11th to 20th	Day	14% 2 5% 17% 33% 29%	4345
	21st to end	Day	12% 3 6% 27% 40% 12%	4532
August	1st to 10th	Day	16% 3 9% 27% 33% 12%	4076
	11th to 20th	Day	22% 4 12% 39% 20% 4	4065
	21st to end	Day	23% 4 13% 25% 26% 8%	4282
September	1st to 10th	Day	19% 3 9% 32% 24% 13%	2280
	11th to 20th	Day	35% 5% 12% 25% 16% 7%	2259
	21st to end	Day	36% 6% 12% 23% 17% 6%	2009
October	1st to 10th	Day	34% 5% 8% 19% 23% 11%	3180
	11th to 20th	Day	46% 6% 16% 20% 10% 2	3161
	21st to end	Day	55% 7% 11% 15% 10% 1	3448
November	1st to 10th	Day	65% 4 12% 13% 5%	1694
	11th to 20th	Day	64% 5% 12% 11% 6% 2	1600
	21st to end	Day	62% 5% 12% 7% 12% 2	1600
December	1st to 10th	Day	50% 7% 11% 15% 17% 1	2620
	11th to 20th	Day	74% 5% 11% 9% 2	3028
	21st to end	Day	79% 4 10% 5% 1	4004
January	1st to 10th	Night	79% 5% 7% 5% 4 1	6306
	11th to 20th	Night	66% 9% 7% 11% 5% 1	6254
	21st to end	Night	56% 5% 7% 14% 14% 4	6468
February	1st to 10th	Night	69% 5% 9% 12% 5% 1	2600
	11th to 20th	Night	62% 6% 12% 12% 6% 2	2534
	21st to end	Night	61% 5% 13% 13% 6% 2	2060
March	1st to 10th	Night	60% 6% 16% 12% 5% 1	5074
	11th to 20th	Night	58% 5% 12% 17% 6% 1	5000
	21st to end	Night	67% 7% 15% 10% 1	5412
April	1st to 10th	Night	65% 6% 12% 13% 5%	2000
	11th to 20th	Night	54% 6% 15% 16% 6% 3	1979
	21st to end	Night	54% 8% 15% 16% 5% 2	1820
May	1st to 10th	Night	41% 6% 12% 28% 12% 1	3879
	11th to 20th	Night	43% 8% 12% 28% 8% 1	3616
	21st to end	Night	37% 8% 29% 22% 3	3972
June	1st to 10th	Night	43% 8% 15% 30% 5%	1600
	11th to 20th	Night	36% 6% 21% 28% 8% 1	1600
	21st to end	Night	24% 5% 18% 35% 15% 3	1600
July	1st to 10th	Night	11% 2 7% 20% 26% 34%	2540
	11th to 20th	Night	13% 2 5% 19% 33% 28%	2492
	21st to end	Night	11% 2 9% 28% 42% 8%	2782
August	1st to 10th	Night	12% 3 9% 35% 31% 10%	2839
	11th to 20th	Night	19% 4 18% 36% 19% 4	2839
	21st to end	Night	21% 3 11% 29% 29% 6%	3392
September	1st to 10th	Night	17% 4 9% 32% 26% 13%	1900
	11th to 20th	Night	30% 4 14% 29% 15% 8%	1918
	21st to end	Night	32% 5% 13% 24% 17% 8%	2171
October	1st to 10th	Night	31% 5% 9% 23% 28% 5%	3800
	11th to 20th	Night	39% 7% 23% 20% 9% 2	3777
	21st to end	Night	57% 7% 11% 13% 9% 2	4121
November	1st to 10th	Night	60% 6% 11% 14% 8%	2603
	11th to 20th	Night	66% 6% 9% 11% 6% 2	2800
	21st to end	Night	58% 5% 10% 12% 10% 4	2800
December	1st to 10th	Night	45% 14% 12% 17% 11% 1	4560
	11th to 20th	Night	76% 5% 9% 8% 3	5176
	21st to end	Night	73% 6% 13% 7% 2	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	50% 10% 14% 14% 12% 2	200
	11th to 20th	Day	33% 10% 14% 23% 15% 4	228
	21st to end	Day	28% 10% 20% 21% 17% 4	440
February	1st to 10th	Day	33% 9% 20% 24% 11% 3	400
	11th to 20th	Day	36% 9% 26% 18% 9% 2	395
	21st to end	Day	34% 15% 17% 22% 10% 2	374
March	1st to 10th	Day	32% 13% 25% 19% 9% 1	500
	11th to 20th	Day	30% 11% 23% 23% 10% 3	500
	21st to end	Day	31% 15% 25% 19% 10% 1	550
April	1st to 10th	Day	30% 16% 19% 21% 11% 3	500
	11th to 20th	Day	29% 14% 24% 18% 10% 5%	492
	21st to end	Day	28% 13% 22% 24% 11% 3	500
May	1st to 10th	Day	19% 14% 25% 25% 13% 4	503
	11th to 20th	Day	26% 9% 17% 29% 15% 4	528
	21st to end	Day	18% 12% 21% 27% 17% 5%	573
June	1st to 10th	Day	20% 10% 20% 29% 17% 4	530
	11th to 20th	Day	16% 15% 28% 27% 12% 2	530
	21st to end	Day	8% 4 25% 39% 19% 5%	530
July	1st to 10th	Day	7% 5% 24% 40% 20% 4	507
	11th to 20th	Day	4 5% 29% 40% 17% 6%	484
	21st to end	Day	2 3 25% 40% 24% 5%	531
August	1st to 10th	Day	4 2 21% 42% 25% 7%	487
	11th to 20th	Day	3 3 18% 40% 28% 9%	477
	21st to end	Day	6% 6% 26% 33% 23% 6%	539
September	1st to 10th	Day	7% 4 25% 34% 23% 8%	490
	11th to 20th	Day	15% 7% 22% 31% 22% 5%	490
	21st to end	Day	16% 8% 24% 28% 17% 6%	490
October	1st to 10th	Day	17% 8% 24% 24% 20% 8%	490
	11th to 20th	Day	20% 9% 16% 28% 19% 8%	490
	21st to end	Day	26% 13% 21% 21% 15% 4	539
November	1st to 10th	Day	29% 12% 19% 22% 14% 5%	270
	11th to 20th	Day	30% 12% 21% 17% 13% 8%	200
	21st to end	Day	31% 10% 14% 20% 20% 6%	200
December	1st to 10th	Day	30% 10% 16% 20% 14% 11%	200
	11th to 20th	Day	39% 10% 19% 19% 12% 3	200
	21st to end	Day	39% 12% 15% 18% 15% 1	220
January	1st to 10th	Night	39% 8% 16% 22% 13% 3	800
	11th to 20th	Night	34% 8% 13% 24% 18% 4	800
	21st to end	Night	30% 4 13% 29% 19% 4	880
February	1st to 10th	Night	34% 7% 12% 29% 17% 1	800
	11th to 20th	Night	33% 6% 13% 30% 17% 1	784
	21st to end	Night	30% 12% 13% 29% 15%	660
March	1st to 10th	Night	31% 9% 14% 32% 15% 1	800
	11th to 20th	Night	26% 7% 16% 32% 18% 1	800
	21st to end	Night	32% 9% 15% 28% 16% 1	880
April	1st to 10th	Night	31% 8% 15% 26% 19% 1	800
	11th to 20th	Night	25% 7% 17% 29% 19% 2	791
	21st to end	Night	26% 5% 15% 30% 22% 2	620
May	1st to 10th	Night	19% 6% 18% 32% 23% 3	512
	11th to 20th	Night	20% 3 12% 34% 28% 3	394
	21st to end	Night	19% 3 14% 34% 29% 2	433
June	1st to 10th	Night	20% 4 13% 38% 24% 2	400
	11th to 20th	Night	16% 3 23% 34% 22% 2	400
	21st to end	Night	11% 2 13% 45% 30% 1	400
July	1st to 10th	Night	6% 2 13% 44% 33% 1	380
	11th to 20th	Night	5% 3 21% 46% 25% 1	368
	21st to end	Night	3 2 17% 38% 38% 2	478
August	1st to 10th	Night	4 1 10% 36% 43% 7%	567
	11th to 20th	Night	4 1 7% 39% 44% 6%	564
	21st to end	Night	6% 4 14% 40% 33% 4	769
September	1st to 10th	Night	7% 2 14% 37% 37% 4	760
	11th to 20th	Night	11% 3 10% 31% 38% 7%	760
	21st to end	Night	13% 4 12% 34% 33% 4	760
October	1st to 10th	Night	15% 3 13% 34% 30% 5%	760
	11th to 20th	Night	19% 4 8% 33% 30% 6%	760
	21st to end	Night	29% 7% 11% 28% 20% 5%	836
November	1st to 10th	Night	30% 7% 13% 30% 18% 2	780
	11th to 20th	Night	36% 8% 13% 25% 17% 2	800
	21st to end	Night	28% 6% 12% 31% 20% 4	800
December	1st to 10th	Night	27% 7% 13% 27% 22% 4	800
	11th to 20th	Night	40% 7% 10% 26% 17% 1	800
	21st to end	Night	43% 8% 13% 26% 9% 1	880

Page generated at 2025-07-02 22:30:17. Version ac07

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