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 2024-01-15.

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	7% 8% 3 6% 6% 3 5% 5% 5% 7% 45%	74802
	Night	7% 8% 3 6% 6% 3 5% 5% 6% 7% 44%	75000
Transparency	Day	38% 9% 11% 18% 16% 8%	74505
	Night	38% 9% 12% 19% 15% 5%	74818
Seeing	Day	12% 15% 27% 26% 15% 5%	13476
	Night	11% 10% 21% 35% 21% 3	22706

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	12% 14% 4 10% 9% 5% 7% 8% 7% 7% 18%	1519
	11th to 20th	Day	12% 11% 4 7% 8% 5% 7% 5% 7% 8% 27%	1475
	21st to end	Day	8% 12% 5% 9% 8% 3 6% 6% 7% 8% 27%	1639
February	1st to 10th	Day	6% 11% 6% 9% 7% 5% 6% 6% 8% 9% 27%	1620
	11th to 20th	Day	7% 12% 5% 8% 7% 4 6% 6% 7% 8% 31%	1631
	21st to end	Day	5% 12% 5% 9% 9% 6% 8% 8% 7% 8% 24%	1480
March	1st to 10th	Day	8% 13% 6% 10% 9% 5% 6% 6% 6% 6% 25%	1800
	11th to 20th	Day	8% 14% 5% 8% 8% 5% 6% 5% 5% 7% 30%	1930
	21st to end	Day	9% 15% 6% 9% 9% 5% 7% 6% 4 6% 24%	2320
April	1st to 10th	Day	9% 12% 6% 9% 7% 5% 7% 5% 6% 6% 29%	2160
	11th to 20th	Day	9% 10% 5% 9% 7% 3 4 6% 5% 7% 36%	2129
	21st to end	Day	7% 10% 3 7% 7% 3 5% 5% 6% 7% 40%	2215
May	1st to 10th	Day	8% 7% 3 6% 5% 3 5% 5% 5% 7% 45%	2507
	11th to 20th	Day	8% 8% 4 7% 7% 4 5% 6% 5% 7% 38%	2477
	21st to end	Day	9% 10% 4 7% 8% 4 6% 5% 5% 7% 35%	2723
June	1st to 10th	Day	13% 7% 4 5% 5% 4 5% 5% 4 8% 39%	2520
	11th to 20th	Day	7% 5% 3 5% 5% 3 4 5% 6% 8% 50%	2520
	21st to end	Day	4 3 1 3 3 3 4 4 5% 6% 65%	2520
July	1st to 10th	Day	2 2 1 3 3 2 3 3 5% 6% 71%	2520
	11th to 20th	Day	2 2 1 2 3 1 3 3 3 4 75%	2520
	21st to end	Day	1 1 1 2 2 1 2 3 2 5% 79%	2741
August	1st to 10th	Day	2 3 1 3 4 2 3 4 5% 7% 67%	2472
	11th to 20th	Day	3 2 1 2 2 2 3 3 4 5% 74%	2236
	21st to end	Day	3 2 1 2 2 1 2 3 4 4 74%	2376
September	1st to 10th	Day	2 2 1 2 3 2 2 3 3 5% 75%	2160
	11th to 20th	Day	5% 4 2 4 5% 2 4 4 5% 7% 58%	2152
	21st to end	Day	6% 6% 2 5% 4 3 3 4 5% 7% 55%	1962
October	1st to 10th	Day	8% 8% 3 5% 4 3 4 5% 5% 6% 50%	1800
	11th to 20th	Day	8% 6% 3 4 5% 4 5% 5% 5% 8% 47%	1800
	21st to end	Day	11% 7% 3 6% 6% 3 4 5% 6% 6% 42%	1841
November	1st to 10th	Day	11% 9% 4 6% 7% 4 6% 4 6% 9% 33%	1674
	11th to 20th	Day	10% 11% 4 8% 8% 4 7% 6% 6% 8% 29%	1601
	21st to end	Day	7% 10% 4 8% 8% 5% 5% 6% 5% 9% 33%	1520
December	1st to 10th	Day	9% 11% 5% 10% 8% 5% 5% 6% 6% 7% 29%	1520
	11th to 20th	Day	12% 13% 5% 8% 8% 5% 6% 6% 8% 9% 21%	1928
	21st to end	Day	9% 15% 6% 13% 11% 6% 6% 6% 5% 6% 17%	2794
January	1st to 10th	Night	8% 14% 5% 9% 8% 5% 7% 8% 5% 8% 23%	2646
	11th to 20th	Night	10% 10% 4 6% 7% 5% 6% 6% 6% 9% 29%	2590
	21st to end	Night	8% 11% 4 6% 7% 4 7% 6% 6% 9% 31%	2714
February	1st to 10th	Night	7% 10% 4 9% 8% 4 5% 6% 6% 8% 32%	2340
	11th to 20th	Night	8% 10% 4 7% 6% 4 5% 6% 6% 9% 34%	2214
	21st to end	Night	6% 10% 5% 9% 6% 4 8% 6% 8% 9% 29%	1776
March	1st to 10th	Night	9% 10% 5% 8% 6% 4 6% 7% 6% 8% 30%	2160
	11th to 20th	Night	7% 10% 4 7% 7% 4 7% 7% 7% 9% 32%	2027
	21st to end	Night	7% 10% 4 8% 7% 4 6% 7% 8% 9% 30%	2034
April	1st to 10th	Night	10% 10% 4 7% 6% 4 5% 6% 6% 7% 36%	1800
	11th to 20th	Night	7% 10% 4 6% 7% 4 5% 4 5% 6% 41%	1779
	21st to end	Night	6% 8% 3 6% 5% 3 5% 6% 6% 8% 43%	1743
May	1st to 10th	Night	5% 7% 2 5% 4 3 5% 5% 6% 8% 50%	1450
	11th to 20th	Night	7% 4 2 5% 7% 3 5% 5% 6% 10% 46%	1416
	21st to end	Night	7% 6% 3 6% 5% 3 5% 6% 6% 9% 44%	1552
June	1st to 10th	Night	10% 5% 3 5% 4 3 4 6% 6% 7% 48%	1440
	11th to 20th	Night	5% 4 2 4 4 3 3 4 5% 7% 58%	1440
	21st to end	Night	4 3 1 3 4 3 3 4 4 7% 66%	1440
July	1st to 10th	Night	2 1 1 2 2 2 3 4 4 6% 72%	1440
	11th to 20th	Night	1 1 1 1 2 1 2 3 4 5% 78%	1440
	21st to end	Night	2 2 2 2 1 3 3 3 4 79%	1576
August	1st to 10th	Night	2 2 1 2 2 2 4 4 5% 7% 70%	1465
	11th to 20th	Night	2 2 1 2 2 2 3 3 4 5% 75%	1664
	21st to end	Night	2 1 1 2 2 2 2 3 3 5% 77%	1980
September	1st to 10th	Night	2 1 1 3 2 1 3 3 4 6% 75%	1800
	11th to 20th	Night	3 3 2 3 4 2 4 4 5% 6% 64%	1808
	21st to end	Night	5% 4 2 3 4 3 4 4 5% 5% 61%	1994
October	1st to 10th	Night	6% 5% 3 4 4 3 3 4 6% 7% 56%	2160
	11th to 20th	Night	8% 6% 2 4 4 3 4 4 5% 7% 54%	2160
	21st to end	Night	8% 7% 3 6% 5% 4 5% 5% 5% 6% 45%	2515
November	1st to 10th	Night	11% 9% 3 7% 6% 4 6% 6% 5% 7% 35%	2405
	11th to 20th	Night	11% 8% 4 7% 8% 4 5% 6% 7% 8% 33%	2574
	21st to end	Night	6% 10% 3 6% 6% 4 6% 6% 7% 8% 37%	2660
December	1st to 10th	Night	10% 10% 4 9% 7% 3 6% 5% 5% 9% 32%	2660
	11th to 20th	Night	13% 13% 5% 9% 7% 4 6% 5% 6% 8% 24%	3276
	21st to end	Night	8% 13% 5% 11% 10% 4 6% 6% 5% 7% 25%	4862

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	62% 11% 12% 11% 4 1	1519
	11th to 20th	Day	53% 10% 13% 13% 7% 3	1475
	21st to end	Day	52% 9% 15% 14% 8% 3	1639
February	1st to 10th	Day	52% 11% 15% 16% 6%	1620
	11th to 20th	Day	50% 11% 13% 16% 8% 3	1640
	21st to end	Day	53% 15% 13% 11% 7% 2	1480
March	1st to 10th	Day	57% 11% 13% 13% 5% 1	1800
	11th to 20th	Day	51% 10% 11% 19% 9%	1930
	21st to end	Day	58% 13% 12% 13% 4	2320
April	1st to 10th	Day	53% 11% 13% 14% 9% 2	2160
	11th to 20th	Day	46% 10% 12% 18% 11% 3	2132
	21st to end	Day	42% 9% 12% 22% 13% 2	2215
May	1st to 10th	Day	36% 9% 13% 26% 14% 1	2507
	11th to 20th	Day	44% 8% 12% 22% 13% 2	2480
	21st to end	Day	45% 11% 13% 19% 10% 1	2724
June	1st to 10th	Day	42% 8% 11% 28% 10% 1	2520
	11th to 20th	Day	31% 8% 12% 27% 18% 4	2520
	21st to end	Day	21% 6% 10% 24% 29% 10%	2520
July	1st to 10th	Day	14% 5% 9% 19% 27% 26%	2451
	11th to 20th	Day	13% 4 7% 12% 30% 33%	2445
	21st to end	Day	10% 3 4 12% 38% 32%	2589
August	1st to 10th	Day	16% 5% 9% 21% 30% 19%	2455
	11th to 20th	Day	13% 3 6% 22% 34% 22%	2236
	21st to end	Day	13% 4 6% 23% 36% 18%	2376
September	1st to 10th	Day	13% 4 6% 24% 29% 24%	2160
	11th to 20th	Day	24% 7% 9% 20% 25% 14%	2152
	21st to end	Day	29% 7% 11% 20% 23% 10%	1962
October	1st to 10th	Day	35% 9% 10% 18% 20% 8%	1800
	11th to 20th	Day	36% 8% 10% 23% 19% 5%	1800
	21st to end	Day	41% 8% 12% 20% 17% 2	1841
November	1st to 10th	Day	46% 11% 14% 19% 10% 1	1674
	11th to 20th	Day	51% 12% 13% 16% 7% 1	1601
	21st to end	Day	47% 12% 11% 14% 13% 3	1520
December	1st to 10th	Day	54% 10% 12% 15% 8% 1	1520
	11th to 20th	Day	56% 11% 13% 14% 5% 1	1928
	21st to end	Day	60% 13% 11% 12% 4	2794
January	1st to 10th	Night	58% 12% 13% 11% 5% 1	2646
	11th to 20th	Night	50% 10% 11% 16% 9% 4	2590
	21st to end	Night	49% 9% 12% 17% 11% 2	2714
February	1st to 10th	Night	50% 10% 16% 15% 8%	2340
	11th to 20th	Night	48% 10% 13% 17% 9% 3	2216
	21st to end	Night	49% 14% 12% 15% 9% 2	1776
March	1st to 10th	Night	50% 10% 13% 18% 7% 2	2160
	11th to 20th	Night	46% 12% 15% 19% 6% 1	2027
	21st to end	Night	48% 15% 15% 18% 5%	2034
April	1st to 10th	Night	46% 10% 16% 19% 9% 1	1800
	11th to 20th	Night	43% 9% 14% 23% 10% 1	1779
	21st to end	Night	37% 12% 16% 22% 12% 1	1743
May	1st to 10th	Night	29% 10% 14% 31% 15%	1450
	11th to 20th	Night	33% 10% 17% 27% 12% 2	1416
	21st to end	Night	36% 11% 19% 25% 9% 1	1552
June	1st to 10th	Night	34% 10% 13% 30% 12% 1	1440
	11th to 20th	Night	26% 8% 15% 32% 16% 3	1440
	21st to end	Night	19% 5% 12% 29% 26% 9%	1440
July	1st to 10th	Night	12% 6% 11% 24% 26% 20%	1400
	11th to 20th	Night	8% 5% 9% 18% 31% 29%	1392
	21st to end	Night	10% 3 4 15% 42% 26%	1488
August	1st to 10th	Night	14% 5% 9% 27% 32% 13%	1457
	11th to 20th	Night	12% 5% 6% 25% 33% 20%	1664
	21st to end	Night	12% 4 7% 24% 36% 18%	1980
September	1st to 10th	Night	11% 4 6% 27% 33% 19%	1800
	11th to 20th	Night	19% 8% 9% 23% 28% 14%	1808
	21st to end	Night	24% 6% 9% 24% 22% 14%	1994
October	1st to 10th	Night	28% 8% 11% 21% 24% 7%	2160
	11th to 20th	Night	31% 7% 13% 21% 22% 6%	2160
	21st to end	Night	38% 11% 12% 16% 20% 4	2515
November	1st to 10th	Night	46% 11% 11% 15% 15% 2	2405
	11th to 20th	Night	47% 11% 15% 17% 9% 1	2574
	21st to end	Night	43% 11% 12% 15% 17% 3	2660
December	1st to 10th	Night	49% 11% 14% 15% 9% 2	2660
	11th to 20th	Night	55% 10% 12% 16% 7%	3276
	21st to end	Night	52% 13% 13% 13% 7% 1	4862

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	32% 13% 29% 13% 12% 1	180
	11th to 20th	Day	21% 16% 27% 23% 9% 3	169
	21st to end	Day	12% 21% 28% 26% 11% 2	231
February	1st to 10th	Day	15% 24% 31% 21% 8% 1	340
	11th to 20th	Day	16% 26% 29% 23% 7%	361
	21st to end	Day	14% 24% 32% 23% 7%	346
March	1st to 10th	Day	15% 23% 33% 22% 5%	420
	11th to 20th	Day	18% 19% 30% 22% 10% 1	420
	21st to end	Day	20% 23% 29% 21% 6% 1	462
April	1st to 10th	Day	23% 20% 27% 19% 8% 3	420
	11th to 20th	Day	17% 25% 29% 16% 9% 4	412
	21st to end	Day	12% 24% 30% 22% 10% 2	420
May	1st to 10th	Day	12% 22% 30% 22% 13% 3	423
	11th to 20th	Day	17% 20% 23% 22% 14% 4	448
	21st to end	Day	14% 13% 24% 26% 17% 7%	485
June	1st to 10th	Day	14% 13% 26% 22% 17% 8%	450
	11th to 20th	Day	11% 21% 28% 24% 13% 3	450
	21st to end	Day	4 10% 27% 31% 21% 7%	450
July	1st to 10th	Day	4 7% 29% 35% 22% 4	446
	11th to 20th	Day	2 9% 28% 33% 22% 6%	444
	21st to end	Day	2 6% 27% 36% 21% 9%	487
August	1st to 10th	Day	2 6% 25% 36% 22% 10%	447
	11th to 20th	Day	3 4 18% 33% 27% 14%	437
	21st to end	Day	5% 8% 24% 30% 26% 7%	495
September	1st to 10th	Day	4 6% 25% 31% 26% 8%	450
	11th to 20th	Day	7% 10% 22% 33% 21% 7%	450
	21st to end	Day	11% 12% 24% 30% 21% 3	450
October	1st to 10th	Day	12% 15% 23% 28% 16% 7%	450
	11th to 20th	Day	13% 16% 20% 26% 19% 6%	450
	21st to end	Day	13% 19% 24% 22% 18% 5%	356
November	1st to 10th	Day	17% 15% 27% 28% 13%	270
	11th to 20th	Day	18% 14% 27% 24% 15% 2	219
	21st to end	Day	14% 15% 24% 34% 10% 2	180
December	1st to 10th	Day	18% 18% 29% 19% 9% 6%	180
	11th to 20th	Day	18% 20% 36% 20% 6%	180
	21st to end	Day	19% 16% 30% 26% 8% 2	198
January	1st to 10th	Night	16% 14% 26% 25% 16% 2	760
	11th to 20th	Night	16% 14% 20% 27% 19% 4	740
	21st to end	Night	16% 10% 18% 34% 19% 3	792
February	1st to 10th	Night	11% 16% 24% 31% 15% 2	720
	11th to 20th	Night	13% 12% 23% 33% 17% 2	704
	21st to end	Night	13% 15% 24% 36% 11% 1	592
March	1st to 10th	Night	13% 15% 25% 32% 14% 2	720
	11th to 20th	Night	12% 12% 25% 34% 17% 1	720
	21st to end	Night	12% 16% 24% 33% 14% 1	792
April	1st to 10th	Night	15% 13% 23% 30% 16% 2	720
	11th to 20th	Night	11% 16% 26% 27% 17% 3	711
	21st to end	Night	10% 13% 26% 32% 17% 2	663
May	1st to 10th	Night	7% 9% 27% 34% 21% 2	370
	11th to 20th	Night	9% 9% 18% 31% 31% 2	354
	21st to end	Night	10% 8% 21% 33% 25% 3	389
June	1st to 10th	Night	12% 6% 20% 39% 22% 1	360
	11th to 20th	Night	6% 8% 24% 43% 17% 1	360
	21st to end	Night	5% 4 18% 44% 26% 3	360
July	1st to 10th	Night	1 3 16% 47% 30% 2	350
	11th to 20th	Night	1 1 22% 48% 25% 2	348
	21st to end	Night	2 2 18% 46% 29% 3	394
August	1st to 10th	Night	3 1 12% 48% 34% 3	391
	11th to 20th	Night	3 1 10% 39% 40% 7%	596
	21st to end	Night	3 3 17% 42% 32% 3	792
September	1st to 10th	Night	2 3 17% 42% 33% 3	720
	11th to 20th	Night	4 5% 15% 36% 33% 8%	720
	21st to end	Night	7% 6% 15% 36% 33% 5%	720
October	1st to 10th	Night	9% 5% 19% 37% 26% 4	720
	11th to 20th	Night	9% 10% 18% 34% 24% 5%	720
	21st to end	Night	12% 10% 19% 35% 19% 5%	792
November	1st to 10th	Night	15% 12% 20% 35% 17% 1	740
	11th to 20th	Night	14% 14% 21% 34% 15% 2	760
	21st to end	Night	12% 14% 20% 33% 17% 4	760
December	1st to 10th	Night	15% 11% 20% 32% 18% 3	760
	11th to 20th	Night	16% 13% 23% 30% 16% 1	760
	21st to end	Night	18% 16% 27% 28% 10% 1	836

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