Oregon Vortex
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oregon Vortex clear sky chart from 2005-11-06 to 2025-09-19.

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	10% 6% 2 6% 4 4 4 3 5% 6% 51%	135410
	Night	13% 8% 2 6% 4 4 4 3 5% 6% 46%	131042
Transparency	Day	34% 6% 11% 23% 18% 9%	130226
	Night	39% 7% 12% 22% 14% 6%	127220
Seeing	Day	14% 9% 21% 26% 20% 9%	16036
	Night	17% 4 12% 35% 28% 4	25032

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	25% 13% 3 11% 7% 6% 6% 4 6% 6% 14%	3639
	11th to 20th	Day	22% 11% 3 7% 4 4 4 3 5% 5% 33%	3888
	21st to end	Day	21% 8% 2 6% 3 3 3 3 5% 7% 39%	4510
February	1st to 10th	Day	17% 12% 4 9% 6% 6% 5% 5% 6% 6% 25%	1800
	11th to 20th	Day	16% 11% 3 9% 6% 5% 6% 4 5% 6% 29%	1755
	21st to end	Day	17% 14% 4 10% 6% 4 5% 4 5% 7% 25%	1594
March	1st to 10th	Day	16% 12% 4 9% 5% 5% 5% 4 6% 8% 27%	4863
	11th to 20th	Day	15% 12% 3 9% 6% 5% 4 4 5% 6% 31%	5000
	21st to end	Day	16% 12% 4 10% 6% 5% 6% 4 6% 8% 22%	5527
April	1st to 10th	Day	15% 10% 5% 10% 7% 5% 6% 4 5% 8% 26%	2400
	11th to 20th	Day	15% 10% 3 7% 5% 5% 4 3 6% 6% 35%	2369
	21st to end	Day	10% 9% 3 8% 5% 4 4 4 6% 8% 39%	2530
May	1st to 10th	Day	9% 5% 2 5% 3 4 4 3 5% 6% 52%	5941
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 4 6% 6% 53%	6277
	21st to end	Day	6% 4 1 4 3 3 3 3 5% 7% 61%	6902
June	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 6% 8% 48%	2800
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 5% 7% 57%	2800
	21st to end	Day	4 3 1 2 3 2 3 2 4 6% 70%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 86%	6319
	11th to 20th	Day	1 2 1 1 1 1 2 2 3 85%	6320
	21st to end	Day	1 1 1 1 1 2 1 1 2 3 85%	6535
August	1st to 10th	Day	2 2 1 2 1 2 2 2 4 6% 77%	5777
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 77%	5796
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 74%	6072
September	1st to 10th	Day	2 2 1 3 2 2 3 2 4 5% 76%	2400
	11th to 20th	Day	4 4 2 4 4 3 4 3 5% 6% 61%	2352
	21st to end	Day	7% 3 2 5% 3 3 3 2 4 7% 61%	2012
October	1st to 10th	Day	5% 3 1 3 3 3 3 2 4 5% 67%	3180
	11th to 20th	Day	7% 5% 3 5% 5% 5% 6% 4 6% 9% 46%	3161
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 5% 8% 36%	3448
November	1st to 10th	Day	19% 9% 4 9% 6% 5% 6% 3 5% 7% 26%	1791
	11th to 20th	Day	20% 12% 5% 9% 5% 6% 5% 4 6% 8% 22%	1600
	21st to end	Day	18% 10% 3 10% 8% 5% 5% 3 4 7% 28%	1600
December	1st to 10th	Day	11% 6% 3 7% 4 5% 5% 4 6% 8% 40%	2620
	11th to 20th	Day	27% 13% 4 9% 6% 6% 6% 4 6% 6% 13%	3028
	21st to end	Day	26% 14% 5% 14% 8% 7% 5% 3 4 4 9%	4004
January	1st to 10th	Night	27% 14% 4 9% 6% 5% 5% 4 6% 5% 15%	6306
	11th to 20th	Night	20% 10% 3 7% 4 5% 5% 3 6% 7% 30%	6080
	21st to end	Night	24% 9% 3 6% 4 3 3 3 4 5% 37%	6468
February	1st to 10th	Night	17% 13% 4 8% 5% 3 5% 4 6% 6% 30%	2600
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 5% 6% 32%	2532
	21st to end	Night	15% 13% 3 7% 5% 5% 4 3 6% 8% 31%	2035
March	1st to 10th	Night	17% 14% 4 10% 5% 4 4 3 7% 8% 25%	5029
	11th to 20th	Night	15% 10% 3 9% 5% 5% 4 3 5% 7% 35%	5000
	21st to end	Night	15% 13% 5% 11% 6% 6% 6% 4 5% 7% 23%	5446
April	1st to 10th	Night	17% 10% 3 8% 5% 6% 5% 4 6% 7% 30%	2000
	11th to 20th	Night	13% 9% 3 6% 5% 4 5% 4 5% 6% 40%	1979
	21st to end	Night	12% 8% 3 6% 4 4 4 4 7% 8% 40%	1867
May	1st to 10th	Night	10% 5% 2 4 3 4 3 3 4 5% 58%	4036
	11th to 20th	Night	6% 6% 2 6% 4 4 3 3 4 6% 56%	3616
	21st to end	Night	6% 3 1 4 3 3 3 3 4 6% 64%	3972
June	1st to 10th	Night	10% 5% 2 6% 3 4 4 3 5% 7% 52%	1600
	11th to 20th	Night	6% 3 2 4 3 3 3 3 4 6% 63%	1600
	21st to end	Night	3 2 1 2 2 2 3 2 3 4 76%	1600
July	1st to 10th	Night	1 1 1 2 1 2 2 2 3 4 80%	3640
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 2 88%	3640
	21st to end	Night	1 2 1 2 1 1 2 1 2 4 83%	4328
August	1st to 10th	Night	1 1 1 2 1 2 2 2 3 5% 81%	4035
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 5% 75%	4052
	21st to end	Night	3 2 1 3 2 2 3 3 4 6% 72%	4923
September	1st to 10th	Night	3 2 2 1 2 2 2 3 5% 79%	2000
	11th to 20th	Night	4 2 1 3 2 3 3 3 4 7% 66%	2003
	21st to end	Night	7% 3 1 4 3 3 3 2 3 6% 64%	2167
October	1st to 10th	Night	4 3 1 4 3 3 2 2 3 4 73%	3800
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 6% 6% 53%	3777
	21st to end	Night	13% 8% 3 8% 5% 5% 6% 4 6% 6% 36%	4121
November	1st to 10th	Night	19% 8% 3 9% 6% 4 5% 3 6% 5% 32%	2502
	11th to 20th	Night	20% 9% 4 9% 7% 6% 5% 4 6% 7% 24%	2800
	21st to end	Night	18% 10% 3 8% 6% 5% 4 4 5% 7% 33%	2800
December	1st to 10th	Night	12% 7% 3 6% 5% 5% 5% 4 6% 6% 42%	4560
	11th to 20th	Night	25% 14% 5% 11% 7% 5% 5% 4 5% 6% 14%	5176
	21st to end	Night	26% 13% 3 11% 7% 7% 5% 4 5% 5% 14%	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	63% 10% 11% 12% 3	3639
	11th to 20th	Day	50% 8% 7% 23% 9% 2	3888
	21st to end	Day	42% 8% 9% 14% 16% 11%	4510
February	1st to 10th	Day	55% 7% 15% 16% 7% 1	1800
	11th to 20th	Day	52% 8% 13% 19% 7% 2	1764
	21st to end	Day	55% 8% 12% 16% 8% 2	1594
March	1st to 10th	Day	51% 8% 12% 23% 6%	4863
	11th to 20th	Day	49% 9% 10% 16% 15% 1	5000
	21st to end	Day	54% 10% 16% 17% 3	5527
April	1st to 10th	Day	55% 7% 14% 16% 7% 1	2400
	11th to 20th	Day	48% 7% 16% 15% 11% 4	2372
	21st to end	Day	41% 8% 17% 21% 11% 1	2530
May	1st to 10th	Day	31% 6% 11% 25% 23% 4	5941
	11th to 20th	Day	28% 7% 14% 29% 18% 3	6280
	21st to end	Day	24% 5% 21% 38% 13%	6904
June	1st to 10th	Day	33% 6% 17% 30% 13% 1	2800
	11th to 20th	Day	27% 6% 14% 34% 18% 2	2800
	21st to end	Day	18% 3 12% 34% 25% 8%	2800
July	1st to 10th	Day	7% 1 6% 23% 30% 32%	6251
	11th to 20th	Day	8% 2 4 15% 32% 39%	6245
	21st to end	Day	8% 2 5% 31% 39% 15%	6346
August	1st to 10th	Day	12% 2 6% 22% 37% 20%	4206
	11th to 20th	Day	12% 3 7% 34% 31% 13%	4195
	21st to end	Day	13% 3 8% 29% 34% 13%	4375
September	1st to 10th	Day	12% 3 5% 26% 32% 21%	2400
	11th to 20th	Day	23% 5% 10% 28% 24% 9%	2352
	21st to end	Day	23% 5% 9% 27% 23% 12%	2012
October	1st to 10th	Day	20% 4 10% 19% 25% 22%	3180
	11th to 20th	Day	32% 7% 18% 26% 14% 4	3161
	21st to end	Day	43% 6% 10% 21% 16% 4	3448
November	1st to 10th	Day	54% 8% 12% 15% 10% 2	1791
	11th to 20th	Day	56% 9% 12% 13% 8% 3	1600
	21st to end	Day	54% 7% 11% 12% 13% 4	1600
December	1st to 10th	Day	38% 7% 9% 24% 17% 4	2620
	11th to 20th	Day	65% 10% 10% 10% 4	3028
	21st to end	Day	69% 15% 6% 8% 2	4004
January	1st to 10th	Night	64% 10% 11% 11% 3 1	6306
	11th to 20th	Night	48% 9% 10% 24% 7% 2	6080
	21st to end	Night	48% 5% 8% 14% 15% 11%	6468
February	1st to 10th	Night	52% 8% 13% 17% 8% 1	2600
	11th to 20th	Night	52% 7% 13% 20% 6% 2	2534
	21st to end	Night	48% 10% 15% 19% 6% 2	2035
March	1st to 10th	Night	52% 8% 18% 17% 5%	5029
	11th to 20th	Night	45% 9% 12% 22% 10% 2	5000
	21st to end	Night	56% 11% 15% 15% 3	5446
April	1st to 10th	Night	51% 7% 18% 17% 6% 1	2000
	11th to 20th	Night	43% 8% 19% 19% 8% 3	1979
	21st to end	Night	40% 9% 22% 21% 7% 1	1867
May	1st to 10th	Night	28% 6% 14% 30% 20% 1	4033
	11th to 20th	Night	29% 6% 20% 32% 11% 1	3616
	21st to end	Night	23% 5% 31% 35% 6%	3972
June	1st to 10th	Night	33% 7% 18% 34% 7%	1600
	11th to 20th	Night	24% 6% 18% 36% 15% 1	1600
	21st to end	Night	15% 3 15% 41% 22% 4	1600
July	1st to 10th	Night	11% 2 6% 29% 23% 30%	3600
	11th to 20th	Night	6% 2 3 18% 36% 36%	3592
	21st to end	Night	9% 2 5% 40% 34% 10%	4205
August	1st to 10th	Night	10% 2 7% 25% 47% 9%	2929
	11th to 20th	Night	10% 3 11% 40% 26% 10%	2929
	21st to end	Night	15% 3 8% 32% 35% 8%	3542
September	1st to 10th	Night	10% 2 7% 31% 33% 16%	2000
	11th to 20th	Night	19% 5% 14% 32% 24% 7%	2003
	21st to end	Night	22% 4 13% 28% 21% 12%	2167
October	1st to 10th	Night	18% 4 10% 21% 28% 19%	3800
	11th to 20th	Night	31% 6% 16% 32% 12% 4	3777
	21st to end	Night	45% 8% 12% 20% 11% 4	4121
November	1st to 10th	Night	51% 7% 14% 14% 12% 2	2502
	11th to 20th	Night	56% 7% 13% 12% 9% 2	2800
	21st to end	Night	49% 7% 12% 15% 12% 4	2800
December	1st to 10th	Night	38% 8% 11% 24% 17% 2	4560
	11th to 20th	Night	64% 10% 11% 11% 3	5176
	21st to end	Night	62% 14% 9% 12% 3 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	35% 8% 19% 23% 14% 4	200
	11th to 20th	Day	30% 5% 11% 25% 16% 13%	305
	21st to end	Day	25% 7% 14% 23% 20% 11%	440
February	1st to 10th	Day	23% 7% 16% 28% 20% 6%	400
	11th to 20th	Day	27% 9% 23% 22% 14% 4	395
	21st to end	Day	24% 12% 20% 24% 17% 3	387
March	1st to 10th	Day	28% 11% 20% 24% 14% 3	500
	11th to 20th	Day	23% 9% 20% 26% 17% 5%	500
	21st to end	Day	24% 16% 21% 22% 15% 3	550
April	1st to 10th	Day	21% 12% 23% 27% 12% 4	500
	11th to 20th	Day	20% 14% 25% 23% 13% 5%	492
	21st to end	Day	16% 17% 24% 23% 14% 6%	500
May	1st to 10th	Day	15% 15% 24% 25% 17% 6%	503
	11th to 20th	Day	16% 12% 24% 26% 16% 7%	528
	21st to end	Day	13% 11% 23% 25% 19% 9%	573
June	1st to 10th	Day	11% 11% 26% 23% 21% 7%	530
	11th to 20th	Day	11% 14% 29% 25% 15% 6%	530
	21st to end	Day	4 9% 26% 29% 22% 10%	530
July	1st to 10th	Day	3 7% 25% 32% 21% 12%	526
	11th to 20th	Day	2 5% 27% 30% 23% 12%	524
	21st to end	Day	1 4 24% 32% 24% 15%	575
August	1st to 10th	Day	2 4 22% 33% 21% 18%	527
	11th to 20th	Day	3 5% 20% 31% 25% 17%	517
	21st to end	Day	5% 6% 21% 27% 26% 16%	583
September	1st to 10th	Day	3 7% 18% 30% 26% 15%	530
	11th to 20th	Day	6% 8% 20% 31% 24% 11%	523
	21st to end	Day	7% 8% 19% 29% 25% 12%	490
October	1st to 10th	Day	11% 8% 19% 26% 24% 13%	490
	11th to 20th	Day	13% 8% 15% 24% 26% 14%	490
	21st to end	Day	19% 10% 17% 24% 19% 11%	539
November	1st to 10th	Day	17% 8% 15% 31% 21% 8%	339
	11th to 20th	Day	22% 9% 13% 26% 22% 10%	200
	21st to end	Day	20% 4 17% 24% 26% 10%	200
December	1st to 10th	Day	20% 5% 14% 25% 21% 15%	200
	11th to 20th	Day	33% 5% 17% 22% 19% 6%	200
	21st to end	Day	28% 7% 18% 26% 19% 3	220
January	1st to 10th	Night	30% 3 15% 34% 16% 3	800
	11th to 20th	Night	27% 3 14% 31% 21% 4	800
	21st to end	Night	24% 3 12% 34% 23% 4	880
February	1st to 10th	Night	23% 4 14% 36% 22% 2	800
	11th to 20th	Night	24% 5% 14% 35% 20% 2	784
	21st to end	Night	21% 6% 16% 39% 16% 1	660
March	1st to 10th	Night	27% 7% 16% 33% 17% 1	800
	11th to 20th	Night	25% 5% 12% 36% 20% 2	800
	21st to end	Night	23% 8% 18% 33% 17% 1	880
April	1st to 10th	Night	22% 6% 17% 34% 20% 2	800
	11th to 20th	Night	17% 7% 20% 32% 20% 3	791
	21st to end	Night	16% 4 16% 36% 25% 3	667
May	1st to 10th	Night	13% 4 13% 39% 27% 3	575
	11th to 20th	Night	11% 5% 15% 34% 32% 3	394
	21st to end	Night	12% 3 15% 36% 31% 3	433
June	1st to 10th	Night	12% 3 11% 35% 38% 2	400
	11th to 20th	Night	7% 4 16% 40% 32% 2	400
	21st to end	Night	4 2 11% 39% 44% 1	400
July	1st to 10th	Night	3 1 8% 36% 51% 2	390
	11th to 20th	Night	2 1 3 47% 46% 1	388
	21st to end	Night	1 3 29% 56% 10%	591
August	1st to 10th	Night	2 1 2 32% 54% 9%	597
	11th to 20th	Night	2 1 3 31% 54% 9%	594
	21st to end	Night	2 1 12% 36% 43% 6%	840
September	1st to 10th	Night	4 2 11% 33% 44% 6%	800
	11th to 20th	Night	5% 3 11% 31% 41% 9%	792
	21st to end	Night	9% 3 10% 37% 34% 8%	760
October	1st to 10th	Night	9% 2 11% 39% 32% 7%	760
	11th to 20th	Night	13% 3 10% 36% 34% 5%	760
	21st to end	Night	17% 3 12% 35% 25% 7%	836
November	1st to 10th	Night	22% 4 10% 41% 20% 4	780
	11th to 20th	Night	23% 5% 14% 36% 20% 3	800
	21st to end	Night	23% 4 13% 35% 23% 3	800
December	1st to 10th	Night	21% 4 11% 35% 24% 5%	800
	11th to 20th	Night	29% 5% 13% 32% 18% 3	800
	21st to end	Night	33% 5% 16% 33% 12% 2	880

Page generated at 2025-09-15 22:49:12. Version ac07

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