Corvallis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Corvallis 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	15% 8% 3 8% 5% 5% 4 4 5% 6% 37%	124230
	Night	17% 9% 3 8% 5% 4 4 3 5% 6% 36%	123390
Transparency	Day	48% 4 12% 20% 13% 4	123977
	Night	49% 4 13% 20% 11% 3	123230
Seeing	Day	17% 8% 18% 26% 22% 9%	15537
	Night	20% 5% 17% 35% 22% 2	24510

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	30% 16% 6% 14% 6% 5% 4 3 4 3 8%	3639
	11th to 20th	Day	26% 9% 4 12% 8% 5% 5% 4 4 5% 19%	3668
	21st to end	Day	19% 11% 4 10% 5% 5% 5% 4 5% 6% 26%	4510
February	1st to 10th	Day	24% 13% 5% 12% 8% 6% 5% 4 5% 5% 13%	1800
	11th to 20th	Day	23% 12% 5% 10% 6% 7% 5% 4 5% 5% 18%	1755
	21st to end	Day	22% 17% 5% 11% 7% 4 5% 4 4 4 18%	1554
March	1st to 10th	Day	15% 11% 4 11% 7% 7% 7% 5% 7% 6% 19%	4820
	11th to 20th	Day	24% 12% 3 8% 6% 5% 4 4 4 5% 25%	5000
	21st to end	Day	20% 12% 5% 12% 7% 7% 7% 5% 5% 6% 14%	5545
April	1st to 10th	Day	21% 13% 4 11% 7% 6% 4 5% 5% 6% 18%	2400
	11th to 20th	Day	19% 11% 3 9% 6% 5% 5% 4 5% 6% 27%	2371
	21st to end	Day	17% 9% 3 8% 6% 5% 6% 5% 5% 7% 27%	2600
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 5% 6% 41%	6095
	11th to 20th	Day	10% 8% 3 8% 5% 5% 5% 4 6% 7% 41%	6277
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 8% 38%	6902
June	1st to 10th	Day	16% 9% 4 7% 4 3 4 3 6% 6% 37%	2800
	11th to 20th	Day	12% 9% 2 7% 6% 5% 5% 5% 7% 8% 34%	2800
	21st to end	Day	7% 4 2 5% 4 4 4 4 5% 7% 55%	2800
July	1st to 10th	Day	3 2 1 2 2 2 2 2 3 4 76%	4491
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 3 77%	4420
	21st to end	Day	3 2 1 2 2 2 2 1 3 4 78%	4706
August	1st to 10th	Day	1 2 1 3 3 3 3 3 4 6% 71%	4091
	11th to 20th	Day	4 3 2 5% 3 3 4 3 6% 8% 58%	4026
	21st to end	Day	6% 5% 2 4 3 4 4 3 5% 6% 58%	4313
September	1st to 10th	Day	5% 3 1 3 3 2 2 2 4 7% 66%	2280
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 5% 8% 45%	2250
	21st to end	Day	13% 5% 3 6% 5% 4 4 3 5% 6% 47%	2016
October	1st to 10th	Day	11% 6% 2 7% 3 4 5% 3 5% 5% 48%	3180
	11th to 20th	Day	13% 7% 4 9% 7% 7% 6% 4 6% 7% 30%	3161
	21st to end	Day	19% 12% 4 10% 5% 7% 6% 4 5% 7% 22%	3448
November	1st to 10th	Day	23% 14% 5% 11% 6% 6% 7% 4 5% 4 15%	1660
	11th to 20th	Day	26% 12% 4 11% 7% 4 4 4 5% 6% 17%	1600
	21st to end	Day	22% 13% 5% 11% 7% 5% 4 4 5% 5% 18%	1600
December	1st to 10th	Day	18% 8% 4 12% 8% 5% 4 3 5% 6% 26%	2620
	11th to 20th	Day	28% 14% 5% 13% 6% 7% 5% 4 5% 5% 8%	3028
	21st to end	Day	30% 16% 8% 18% 7% 4 4 3 3 3 5%	4004
January	1st to 10th	Night	31% 16% 6% 11% 6% 5% 5% 3 4 4 12%	6306
	11th to 20th	Night	23% 12% 4 9% 6% 4 5% 3 5% 6% 22%	6278
	21st to end	Night	24% 11% 3 9% 4 4 3 3 3 5% 31%	6468
February	1st to 10th	Night	21% 14% 4 10% 6% 5% 5% 5% 6% 5% 18%	2600
	11th to 20th	Night	23% 12% 4 8% 4 5% 5% 4 5% 6% 24%	2532
	21st to end	Night	22% 12% 3 9% 6% 5% 5% 3 5% 6% 23%	2075
March	1st to 10th	Night	15% 10% 4 10% 7% 7% 7% 5% 8% 6% 20%	5069
	11th to 20th	Night	19% 13% 4 9% 5% 4 4 3 4 5% 30%	5000
	21st to end	Night	21% 12% 5% 11% 7% 6% 5% 4 5% 6% 17%	5429
April	1st to 10th	Night	22% 11% 3 9% 5% 5% 5% 4 6% 6% 24%	2000
	11th to 20th	Night	15% 9% 4 7% 5% 4 4 4 6% 6% 36%	1974
	21st to end	Night	15% 8% 3 8% 6% 5% 5% 4 5% 7% 35%	1800
May	1st to 10th	Night	15% 6% 2 6% 4 3 3 3 4 5% 48%	3875
	11th to 20th	Night	9% 7% 3 7% 4 4 4 2 5% 6% 48%	3616
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 10% 46%	3972
June	1st to 10th	Night	13% 8% 2 6% 4 3 4 5% 6% 6% 45%	1600
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 8% 8% 47%	1600
	21st to end	Night	5% 5% 1 4 3 3 3 3 4 6% 63%	1600
July	1st to 10th	Night	2 2 1 2 1 1 1 2 2 3 83%	2580
	11th to 20th	Night	3 3 1 2 2 2 2 2 2 3 80%	2540
	21st to end	Night	2 2 1 2 1 1 2 2 2 3 82%	2844
August	1st to 10th	Night	1 2 1 1 2 2 2 2 3 5% 78%	2848
	11th to 20th	Night	3 3 1 4 2 3 4 3 4 7% 65%	2812
	21st to end	Night	6% 4 1 4 2 2 2 2 4 5% 66%	3360
September	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 5% 70%	1900
	11th to 20th	Night	7% 5% 2 4 4 4 5% 3 6% 6% 55%	1928
	21st to end	Night	9% 4 2 5% 3 4 4 3 5% 7% 54%	2163
October	1st to 10th	Night	8% 5% 1 6% 4 4 4 4 6% 7% 52%	3800
	11th to 20th	Night	12% 6% 3 7% 5% 6% 6% 4 7% 7% 37%	3777
	21st to end	Night	17% 10% 4 9% 5% 5% 5% 4 6% 6% 28%	4121
November	1st to 10th	Night	22% 9% 3 8% 6% 5% 5% 4 5% 6% 26%	2635
	11th to 20th	Night	26% 12% 3 9% 7% 5% 4 3 4 5% 21%	2800
	21st to end	Night	18% 10% 3 11% 6% 5% 5% 4 6% 6% 27%	2800
December	1st to 10th	Night	16% 8% 3 10% 5% 5% 4 4 7% 10% 29%	4560
	11th to 20th	Night	29% 14% 6% 14% 6% 5% 4 3 4 4 10%	5176
	21st to end	Night	27% 13% 7% 14% 7% 6% 5% 4 4 4 10%	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	80% 3 7% 7% 2	3639
	11th to 20th	Day	68% 4 8% 13% 6% 1	3668
	21st to end	Day	59% 3 8% 12% 15% 3	4510
February	1st to 10th	Day	72% 4 9% 10% 5%	1800
	11th to 20th	Day	70% 3 10% 11% 4 2	1764
	21st to end	Day	70% 3 9% 11% 6% 1	1554
March	1st to 10th	Day	62% 4 11% 19% 4	4820
	11th to 20th	Day	61% 4 8% 18% 8%	5000
	21st to end	Day	69% 4 14% 10% 3	5545
April	1st to 10th	Day	66% 3 13% 14% 4	2400
	11th to 20th	Day	56% 4 12% 17% 9% 2	2374
	21st to end	Day	54% 5% 16% 17% 7% 1	2600
May	1st to 10th	Day	43% 2 11% 27% 16%	6095
	11th to 20th	Day	41% 6% 15% 27% 10% 1	6280
	21st to end	Day	41% 4 28% 23% 4	6904
June	1st to 10th	Day	48% 2 17% 24% 8% 1	2800
	11th to 20th	Day	45% 3 19% 24% 7%	2800
	21st to end	Day	29% 3 18% 32% 13% 4	2800
July	1st to 10th	Day	13% 2 9% 19% 25% 32%	4422
	11th to 20th	Day	14% 2 5% 19% 36% 24%	4345
	21st to end	Day	12% 2 9% 29% 38% 9%	4557
August	1st to 10th	Day	15% 3 9% 32% 31% 11%	4076
	11th to 20th	Day	22% 5% 18% 34% 17% 4	4065
	21st to end	Day	25% 5% 12% 25% 26% 6%	4312
September	1st to 10th	Day	19% 2 10% 35% 22% 12%	2280
	11th to 20th	Day	36% 4 14% 24% 15% 6%	2250
	21st to end	Day	39% 3 13% 25% 16% 6%	2016
October	1st to 10th	Day	37% 5% 8% 23% 24% 4	3180
	11th to 20th	Day	51% 5% 16% 17% 10% 1	3161
	21st to end	Day	57% 7% 9% 17% 9% 1	3448
November	1st to 10th	Day	70% 4 10% 12% 4	1660
	11th to 20th	Day	69% 4 10% 10% 6% 2	1600
	21st to end	Day	68% 3 8% 9% 11% 1	1600
December	1st to 10th	Day	58% 5% 10% 14% 12%	2620
	11th to 20th	Day	79% 4 10% 7% 1	3028
	21st to end	Day	85% 3 5% 4 2	4004
January	1st to 10th	Night	77% 4 8% 7% 3 1	6306
	11th to 20th	Night	63% 6% 8% 16% 7% 1	6278
	21st to end	Night	57% 2 7% 15% 15% 3	6468
February	1st to 10th	Night	67% 5% 11% 13% 4 1	2600
	11th to 20th	Night	61% 3 14% 13% 7% 2	2534
	21st to end	Night	62% 4 12% 15% 6% 2	2075
March	1st to 10th	Night	60% 4 13% 17% 5%	5069
	11th to 20th	Night	56% 4 12% 20% 7% 1	5000
	21st to end	Night	67% 5% 15% 12% 2	5429
April	1st to 10th	Night	61% 3 15% 14% 6%	2000
	11th to 20th	Night	50% 4 16% 20% 7% 3	1974
	21st to end	Night	49% 4 16% 22% 6% 2	1800
May	1st to 10th	Night	39% 2 10% 34% 14% 1	3872
	11th to 20th	Night	37% 4 17% 30% 10% 2	3616
	21st to end	Night	35% 5% 33% 24% 3	3972
June	1st to 10th	Night	41% 4 17% 32% 5% 1	1600
	11th to 20th	Night	35% 3 21% 32% 8%	1600
	21st to end	Night	23% 2 20% 36% 14% 4	1600
July	1st to 10th	Night	10% 1 8% 23% 25% 33%	2540
	11th to 20th	Night	13% 1 7% 20% 31% 29%	2492
	21st to end	Night	9% 3 10% 31% 38% 9%	2755
August	1st to 10th	Night	10% 2 10% 38% 30% 10%	2839
	11th to 20th	Night	19% 4 21% 38% 14% 4	2839
	21st to end	Night	21% 4 12% 31% 26% 7%	3360
September	1st to 10th	Night	18% 2 11% 33% 25% 11%	1900
	11th to 20th	Night	30% 3 16% 28% 15% 8%	1928
	21st to end	Night	31% 2 17% 25% 18% 7%	2163
October	1st to 10th	Night	30% 6% 9% 29% 24% 3	3800
	11th to 20th	Night	42% 7% 21% 19% 9% 2	3777
	21st to end	Night	53% 7% 13% 15% 10% 1	4121
November	1st to 10th	Night	57% 5% 12% 19% 7%	2635
	11th to 20th	Night	66% 4 11% 11% 7% 2	2800
	21st to end	Night	58% 3 12% 13% 12% 2	2800
December	1st to 10th	Night	48% 10% 11% 19% 11%	4560
	11th to 20th	Night	78% 3 8% 8% 2	5176
	21st to end	Night	78% 4 8% 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	38% 14% 19% 17% 10% 3	200
	11th to 20th	Day	31% 9% 13% 29% 12% 5%	215
	21st to end	Day	29% 11% 17% 19% 16% 8%	440
February	1st to 10th	Day	30% 9% 22% 19% 16% 5%	400
	11th to 20th	Day	28% 9% 27% 22% 11% 4	395
	21st to end	Day	25% 16% 22% 21% 14% 4	367
March	1st to 10th	Day	27% 11% 24% 25% 10% 3	500
	11th to 20th	Day	27% 10% 21% 20% 17% 4	500
	21st to end	Day	24% 14% 21% 25% 13% 3	550
April	1st to 10th	Day	27% 10% 19% 24% 14% 6%	500
	11th to 20th	Day	20% 13% 23% 24% 13% 6%	492
	21st to end	Day	20% 10% 22% 24% 19% 5%	500
May	1st to 10th	Day	13% 11% 24% 25% 19% 7%	503
	11th to 20th	Day	21% 9% 16% 26% 17% 12%	528
	21st to end	Day	11% 8% 18% 25% 25% 13%	573
June	1st to 10th	Day	17% 8% 16% 25% 27% 8%	530
	11th to 20th	Day	13% 7% 24% 33% 18% 4	530
	21st to end	Day	5% 4 15% 34% 33% 10%	530
July	1st to 10th	Day	6% 2 15% 33% 31% 12%	507
	11th to 20th	Day	2 2 18% 35% 30% 13%	484
	21st to end	Day	3 1 11% 33% 37% 15%	531
August	1st to 10th	Day	1 1 11% 30% 39% 19%	487
	11th to 20th	Day	3 2 10% 28% 38% 20%	477
	21st to end	Day	5% 3 14% 35% 31% 12%	539
September	1st to 10th	Day	6% 3 16% 32% 31% 13%	490
	11th to 20th	Day	11% 4 14% 28% 29% 14%	490
	21st to end	Day	15% 5% 19% 28% 26% 7%	490
October	1st to 10th	Day	18% 5% 20% 24% 22% 12%	490
	11th to 20th	Day	17% 7% 17% 23% 26% 10%	490
	21st to end	Day	24% 12% 21% 22% 15% 6%	539
November	1st to 10th	Day	28% 8% 23% 23% 14% 5%	250
	11th to 20th	Day	25% 11% 22% 23% 15% 6%	200
	21st to end	Day	27% 7% 24% 18% 19% 6%	200
December	1st to 10th	Day	27% 10% 17% 24% 16% 8%	200
	11th to 20th	Day	36% 8% 18% 26% 12% 3	200
	21st to end	Day	33% 12% 19% 22% 12% 1	220
January	1st to 10th	Night	34% 9% 19% 23% 12% 2	800
	11th to 20th	Night	31% 8% 18% 24% 16% 3	800
	21st to end	Night	27% 5% 16% 32% 18% 3	880
February	1st to 10th	Night	26% 7% 20% 27% 18% 2	800
	11th to 20th	Night	27% 7% 21% 28% 17% 1	784
	21st to end	Night	25% 10% 19% 32% 14%	660
March	1st to 10th	Night	26% 6% 19% 35% 15% 1	800
	11th to 20th	Night	25% 5% 18% 34% 16% 2	800
	21st to end	Night	26% 6% 22% 30% 14% 1	880
April	1st to 10th	Night	26% 7% 19% 29% 19% 1	800
	11th to 20th	Night	19% 6% 22% 37% 16% 1	786
	21st to end	Night	18% 5% 19% 37% 20% 1	600
May	1st to 10th	Night	12% 5% 21% 42% 19% 2	505
	11th to 20th	Night	14% 2 17% 40% 26% 1	394
	21st to end	Night	14% 4 18% 39% 23% 2	433
June	1st to 10th	Night	15% 4 15% 43% 23% 1	400
	11th to 20th	Night	11% 5% 19% 48% 17% 1	400
	21st to end	Night	7% 2 15% 53% 23% 1	400
July	1st to 10th	Night	4 1 11% 53% 30% 1	380
	11th to 20th	Night	4 1 18% 51% 26% 1	368
	21st to end	Night	2 1 11% 49% 36% 1	463
August	1st to 10th	Night	2 1 7% 43% 42% 6%	567
	11th to 20th	Night	4 5% 38% 49% 3	564
	21st to end	Night	6% 1 13% 45% 33% 2	750
September	1st to 10th	Night	6% 1 13% 47% 32% 1	760
	11th to 20th	Night	9% 3 11% 37% 34% 7%	760
	21st to end	Night	11% 2 15% 42% 28% 3	760
October	1st to 10th	Night	13% 3 17% 36% 27% 3	760
	11th to 20th	Night	17% 4 12% 35% 28% 4	760
	21st to end	Night	23% 6% 19% 32% 18% 4	836
November	1st to 10th	Night	24% 5% 18% 34% 18% 1	780
	11th to 20th	Night	31% 7% 20% 26% 15% 1	800
	21st to end	Night	21% 5% 22% 31% 20% 2	800
December	1st to 10th	Night	24% 6% 18% 31% 19% 3	800
	11th to 20th	Night	30% 8% 21% 24% 16% 1	800
	21st to end	Night	36% 6% 21% 26% 11% 1	880

Page generated at 2025-07-02 22:30:12. Version ac07

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