Oregon Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oregon Star Party clear sky chart from 2005-11-06 to 2025-11-03.

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	6% 8% 3 6% 5% 4 4 3 5% 6% 49%	135835
	Night	7% 8% 3 6% 4 4 4 4 5% 7% 48%	131506
Transparency	Day	34% 7% 10% 20% 18% 11%	130621
	Night	36% 8% 12% 21% 16% 7%	127716
Seeing	Day	12% 15% 26% 26% 16% 5%	15859
	Night	11% 10% 21% 35% 21% 3	24911

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% 15% 5% 11% 8% 7% 7% 5% 7% 7% 18%	3639
	11th to 20th	Day	9% 13% 5% 7% 5% 5% 5% 4 5% 5% 37%	3640
	21st to end	Day	12% 13% 3 8% 5% 3 4 3 5% 6% 39%	4157
February	1st to 10th	Day	7% 12% 5% 11% 7% 6% 6% 5% 8% 8% 26%	1800
	11th to 20th	Day	7% 14% 4 10% 7% 6% 5% 4 7% 7% 30%	1818
	21st to end	Day	6% 12% 4 10% 8% 7% 7% 6% 6% 8% 26%	1650
March	1st to 10th	Day	7% 13% 5% 10% 6% 5% 5% 4 6% 9% 30%	4518
	11th to 20th	Day	7% 14% 4 7% 5% 6% 5% 4 5% 5% 38%	4885
	21st to end	Day	8% 15% 5% 11% 7% 6% 7% 4 5% 7% 25%	5835
April	1st to 10th	Day	9% 13% 5% 10% 6% 6% 6% 4 6% 6% 30%	2400
	11th to 20th	Day	8% 11% 4 8% 6% 4 4 4 5% 6% 38%	2369
	21st to end	Day	7% 10% 3 8% 6% 5% 5% 4 6% 7% 41%	2462
May	1st to 10th	Day	9% 8% 2 5% 3 4 4 4 4 6% 51%	6307
	11th to 20th	Day	6% 8% 2 6% 4 4 4 4 5% 6% 50%	6277
	21st to end	Day	5% 7% 2 5% 4 4 4 3 5% 8% 51%	6902
June	1st to 10th	Day	11% 8% 3 6% 4 5% 5% 4 5% 8% 42%	2800
	11th to 20th	Day	6% 6% 2 6% 4 4 4 4 6% 8% 51%	2800
	21st to end	Day	3 3 1 4 3 4 4 4 5% 6% 64%	2800
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 5% 78%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 3 83%	6320
	21st to end	Day	2 1 1 2 2 2 3 3 3 5% 76%	6880
August	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 7% 64%	6111
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 77%	5630
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 5% 64%	5984
September	1st to 10th	Day	2 2 1 3 3 3 3 3 3 6% 72%	2400
	11th to 20th	Day	5% 4 1 5% 4 3 4 3 5% 7% 58%	2390
	21st to end	Day	5% 5% 2 6% 3 4 3 3 5% 6% 58%	2180
October	1st to 10th	Day	4 5% 2 4 3 3 3 2 4 6% 64%	3280
	11th to 20th	Day	5% 5% 2 5% 5% 5% 6% 4 6% 8% 48%	3261
	21st to end	Day	9% 10% 4 8% 5% 5% 6% 4 6% 7% 37%	3302
November	1st to 10th	Day	11% 10% 4 8% 6% 5% 5% 4 6% 9% 33%	1782
	11th to 20th	Day	10% 12% 3 9% 7% 6% 6% 5% 7% 7% 27%	1685
	21st to end	Day	7% 11% 3 8% 7% 7% 5% 5% 5% 9% 32%	1600
December	1st to 10th	Day	6% 8% 3 9% 6% 4 4 3 5% 5% 45%	2620
	11th to 20th	Day	11% 15% 5% 11% 7% 7% 6% 5% 7% 8% 18%	3028
	21st to end	Day	10% 14% 5% 15% 10% 8% 6% 5% 5% 6% 16%	4004
January	1st to 10th	Night	12% 15% 4 10% 7% 6% 6% 5% 6% 8% 22%	6306
	11th to 20th	Night	9% 13% 3 7% 4 5% 5% 3 4 5% 42%	6320
	21st to end	Night	12% 10% 3 7% 5% 5% 5% 3 4 6% 40%	6803
February	1st to 10th	Night	7% 13% 4 10% 7% 6% 5% 4 6% 7% 30%	2600
	11th to 20th	Night	8% 11% 4 9% 6% 5% 5% 5% 6% 9% 33%	2467
	21st to end	Night	6% 11% 4 9% 6% 6% 7% 5% 8% 9% 29%	1980
March	1st to 10th	Night	5% 10% 4 8% 5% 5% 6% 5% 7% 9% 35%	5376
	11th to 20th	Night	6% 11% 3 6% 5% 4 5% 4 6% 7% 44%	5099
	21st to end	Night	7% 11% 3 9% 6% 6% 7% 6% 8% 8% 29%	5138
April	1st to 10th	Night	9% 11% 3 7% 5% 4 5% 4 5% 7% 38%	2000
	11th to 20th	Night	6% 10% 3 6% 6% 5% 4 3 6% 6% 44%	1979
	21st to end	Night	6% 8% 3 6% 5% 4 5% 5% 6% 9% 43%	1936
May	1st to 10th	Night	6% 9% 2 5% 3 3 4 3 5% 6% 54%	3650
	11th to 20th	Night	6% 6% 2 5% 4 4 4 3 5% 7% 54%	3616
	21st to end	Night	5% 5% 2 4 3 3 3 4 5% 6% 60%	3972
June	1st to 10th	Night	10% 5% 2 5% 4 3 4 5% 5% 6% 51%	1600
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 5% 7% 58%	1600
	21st to end	Night	3 4 1 3 3 3 2 3 4 7% 67%	1600
July	1st to 10th	Night	1 1 1 3 2 2 2 2 3 4 78%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 3 84%	3640
	21st to end	Night	3 2 1 3 2 3 3 3 3 4 74%	3970
August	1st to 10th	Night	3 2 2 5% 3 4 4 3 5% 6% 63%	3670
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 4 74%	4226
	21st to end	Night	4 2 1 3 2 3 2 3 4 6% 70%	5015
September	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 6% 72%	2000
	11th to 20th	Night	3 3 1 4 3 3 4 4 5% 7% 63%	2010
	21st to end	Night	5% 4 1 4 3 4 3 3 5% 5% 63%	2216
October	1st to 10th	Night	4 3 2 4 3 2 2 2 4 5% 69%	3920
	11th to 20th	Night	6% 6% 2 5% 4 3 4 3 5% 7% 55%	3897
	21st to end	Night	6% 7% 3 7% 4 5% 5% 5% 6% 8% 46%	4501
November	1st to 10th	Night	11% 10% 3 8% 5% 6% 5% 4 5% 7% 35%	2561
	11th to 20th	Night	11% 9% 3 8% 7% 6% 5% 5% 7% 7% 33%	2710
	21st to end	Night	7% 10% 3 7% 6% 5% 6% 5% 6% 8% 37%	2800
December	1st to 10th	Night	6% 6% 2 6% 4 4 4 3 4 7% 51%	4560
	11th to 20th	Night	13% 15% 5% 13% 7% 6% 6% 5% 6% 7% 19%	5176
	21st to end	Night	12% 14% 4 12% 8% 6% 5% 4 5% 7% 24%	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	61% 11% 13% 12% 3	3639
	11th to 20th	Day	48% 7% 14% 20% 10% 1	3640
	21st to end	Day	43% 9% 9% 17% 16% 6%	4157
February	1st to 10th	Day	53% 12% 14% 15% 6%	1800
	11th to 20th	Day	51% 11% 13% 15% 8% 3	1827
	21st to end	Day	51% 15% 13% 12% 7% 1	1650
March	1st to 10th	Day	45% 11% 15% 18% 8% 3	4518
	11th to 20th	Day	44% 9% 10% 26% 10%	4885
	21st to end	Day	53% 13% 15% 14% 4	5835
April	1st to 10th	Day	51% 10% 14% 15% 8% 1	2400
	11th to 20th	Day	44% 10% 12% 19% 13% 2	2372
	21st to end	Day	40% 9% 13% 23% 12% 2	2462
May	1st to 10th	Day	32% 7% 9% 25% 24% 2	6307
	11th to 20th	Day	33% 6% 12% 23% 22% 4	6280
	21st to end	Day	29% 9% 15% 30% 15% 2	6904
June	1st to 10th	Day	40% 7% 11% 28% 13% 1	2800
	11th to 20th	Day	29% 7% 12% 27% 20% 4	2800
	21st to end	Day	20% 6% 10% 24% 29% 11%	2800
July	1st to 10th	Day	11% 3 5% 14% 26% 42%	6251
	11th to 20th	Day	9% 2 4 8% 23% 55%	6245
	21st to end	Day	12% 3 4 23% 35% 23%	6681
August	1st to 10th	Day	18% 4 5% 14% 30% 28%	4448
	11th to 20th	Day	12% 3 5% 23% 37% 19%	4077
	21st to end	Day	16% 4 7% 25% 26% 22%	4311
September	1st to 10th	Day	14% 4 6% 24% 29% 23%	2400
	11th to 20th	Day	25% 7% 10% 21% 25% 13%	2390
	21st to end	Day	27% 6% 11% 21% 24% 11%	2180
October	1st to 10th	Day	23% 6% 9% 22% 27% 13%	3280
	11th to 20th	Day	33% 6% 16% 24% 18% 3	3261
	21st to end	Day	45% 6% 11% 22% 14% 2	3302
November	1st to 10th	Day	46% 10% 14% 19% 10% 1	1782
	11th to 20th	Day	52% 12% 13% 16% 7% 1	1685
	21st to end	Day	47% 11% 11% 15% 13% 3	1600
December	1st to 10th	Day	39% 9% 12% 24% 15% 1	2620
	11th to 20th	Day	60% 9% 15% 12% 4	3028
	21st to end	Day	61% 11% 12% 13% 4	4004
January	1st to 10th	Night	58% 12% 15% 10% 4	6306
	11th to 20th	Night	45% 8% 17% 18% 12% 2	6320
	21st to end	Night	43% 9% 12% 16% 14% 6%	6803
February	1st to 10th	Night	52% 10% 15% 15% 8% 1	2600
	11th to 20th	Night	48% 10% 13% 17% 10% 2	2469
	21st to end	Night	48% 14% 13% 15% 9% 1	1980
March	1st to 10th	Night	39% 10% 16% 22% 12% 2	5376
	11th to 20th	Night	37% 8% 14% 30% 11%	5099
	21st to end	Night	48% 16% 14% 17% 4 1	5138
April	1st to 10th	Night	44% 10% 16% 21% 8% 1	2000
	11th to 20th	Night	40% 9% 14% 25% 11% 1	1979
	21st to end	Night	36% 12% 15% 24% 12% 1	1936
May	1st to 10th	Night	29% 7% 12% 26% 25% 1	3647
	11th to 20th	Night	29% 7% 14% 30% 20% 1	3616
	21st to end	Night	24% 7% 23% 33% 12%	3972
June	1st to 10th	Night	33% 9% 13% 30% 15% 1	1600
	11th to 20th	Night	26% 7% 13% 31% 19% 3	1600
	21st to end	Night	19% 5% 12% 29% 28% 9%	1600
July	1st to 10th	Night	12% 3 5% 16% 27% 36%	3600
	11th to 20th	Night	7% 2 5% 10% 26% 49%	3592
	21st to end	Night	16% 2 4 24% 33% 22%	3857
August	1st to 10th	Night	19% 4 6% 18% 29% 23%	2664
	11th to 20th	Night	14% 4 6% 30% 34% 13%	3051
	21st to end	Night	12% 2 10% 28% 27% 21%	3608
September	1st to 10th	Night	13% 4 7% 26% 32% 18%	2000
	11th to 20th	Night	20% 7% 9% 24% 27% 13%	2010
	21st to end	Night	23% 5% 9% 25% 24% 14%	2216
October	1st to 10th	Night	20% 4 8% 22% 34% 12%	3920
	11th to 20th	Night	30% 5% 19% 21% 23% 3	3897
	21st to end	Night	35% 7% 11% 28% 16% 2	4501
November	1st to 10th	Night	45% 11% 12% 15% 15% 2	2561
	11th to 20th	Night	48% 11% 14% 17% 10% 1	2710
	21st to end	Night	43% 11% 12% 15% 16% 3	2800
December	1st to 10th	Night	32% 8% 14% 27% 17% 1	4560
	11th to 20th	Night	63% 8% 11% 13% 5%	5176
	21st to end	Night	56% 11% 14% 13% 6% 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	31% 13% 30% 15% 11% 1	200
	11th to 20th	Day	20% 15% 25% 27% 10% 4	200
	21st to end	Day	13% 18% 28% 26% 13% 2	282
February	1st to 10th	Day	16% 25% 30% 21% 8% 1	400
	11th to 20th	Day	16% 25% 31% 21% 7%	428
	21st to end	Day	15% 23% 30% 24% 8%	414
March	1st to 10th	Day	15% 23% 33% 22% 6%	500
	11th to 20th	Day	17% 20% 30% 22% 10% 1	500
	21st to end	Day	20% 23% 29% 20% 7% 1	550
April	1st to 10th	Day	21% 19% 28% 19% 10% 3	500
	11th to 20th	Day	15% 23% 30% 17% 11% 4	492
	21st to end	Day	11% 22% 31% 24% 10% 3	500
May	1st to 10th	Day	14% 20% 29% 20% 12% 4	503
	11th to 20th	Day	16% 21% 24% 22% 14% 4	528
	21st to end	Day	13% 12% 24% 25% 18% 7%	573
June	1st to 10th	Day	14% 13% 25% 24% 17% 8%	530
	11th to 20th	Day	10% 20% 28% 25% 14% 3	530
	21st to end	Day	3 10% 27% 30% 22% 8%	530
July	1st to 10th	Day	3 6% 28% 34% 23% 5%	526
	11th to 20th	Day	2 8% 27% 33% 23% 7%	524
	21st to end	Day	2 6% 26% 36% 21% 9%	575
August	1st to 10th	Day	2 6% 24% 34% 23% 10%	527
	11th to 20th	Day	3 5% 19% 33% 27% 13%	517
	21st to end	Day	5% 8% 22% 30% 26% 9%	583
September	1st to 10th	Day	4 6% 23% 30% 28% 9%	530
	11th to 20th	Day	7% 9% 21% 32% 23% 8%	530
	21st to end	Day	10% 11% 23% 29% 22% 5%	530
October	1st to 10th	Day	11% 14% 23% 28% 17% 7%	530
	11th to 20th	Day	12% 15% 20% 28% 19% 5%	530
	21st to end	Day	14% 17% 23% 23% 18% 5%	428
November	1st to 10th	Day	16% 16% 26% 27% 14%	306
	11th to 20th	Day	21% 14% 28% 22% 14% 2	243
	21st to end	Day	15% 15% 24% 35% 10% 2	200
December	1st to 10th	Day	17% 17% 27% 20% 12% 8%	200
	11th to 20th	Day	19% 19% 37% 20% 6%	200
	21st to end	Day	21% 15% 29% 25% 7% 2	220
January	1st to 10th	Night	17% 13% 26% 26% 16% 2	800
	11th to 20th	Night	16% 14% 21% 27% 19% 3	800
	21st to end	Night	15% 10% 20% 34% 18% 3	880
February	1st to 10th	Night	12% 17% 24% 31% 15% 1	800
	11th to 20th	Night	14% 12% 24% 33% 16% 2	784
	21st to end	Night	14% 14% 23% 37% 12% 1	660
March	1st to 10th	Night	12% 15% 26% 31% 14% 2	800
	11th to 20th	Night	12% 12% 25% 33% 17% 1	800
	21st to end	Night	12% 16% 25% 33% 13% 1	880
April	1st to 10th	Night	15% 13% 24% 30% 17% 2	800
	11th to 20th	Night	10% 16% 27% 28% 16% 3	791
	21st to end	Night	9% 12% 27% 33% 17% 2	736
May	1st to 10th	Night	7% 10% 29% 32% 20% 2	410
	11th to 20th	Night	9% 10% 19% 32% 28% 2	394
	21st to end	Night	9% 8% 22% 34% 24% 3	433
June	1st to 10th	Night	11% 6% 21% 39% 22% 1	400
	11th to 20th	Night	6% 8% 25% 44% 16% 1	400
	21st to end	Night	5% 4 19% 44% 25% 2	400
July	1st to 10th	Night	2 3 16% 48% 29% 2	390
	11th to 20th	Night	1 1 21% 50% 25% 2	388
	21st to end	Night	2 2 19% 46% 29% 3	438
August	1st to 10th	Night	3 13% 48% 33% 3	435
	11th to 20th	Night	3 2 12% 41% 36% 6%	664
	21st to end	Night	4 3 18% 42% 31% 3	880
September	1st to 10th	Night	2 3 17% 42% 33% 3	800
	11th to 20th	Night	4 5% 16% 37% 32% 7%	800
	21st to end	Night	6% 6% 15% 36% 33% 4	800
October	1st to 10th	Night	8% 5% 19% 38% 26% 4	800
	11th to 20th	Night	10% 9% 19% 35% 22% 5%	800
	21st to end	Night	11% 10% 20% 36% 18% 5%	880
November	1st to 10th	Night	14% 12% 21% 35% 17% 1	788
	11th to 20th	Night	15% 15% 22% 33% 14% 2	800
	21st to end	Night	13% 13% 19% 33% 17% 4	800
December	1st to 10th	Night	15% 11% 20% 32% 20% 3	800
	11th to 20th	Night	17% 14% 23% 30% 16% 1	800
	21st to end	Night	19% 16% 27% 28% 10% 1	880

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