Powell Butte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Powell Butte clear sky chart from 2005-11-06 to 2024-01-16.

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	8% 7% 3 6% 5% 3 4 4 5% 7% 48%	74799
	Night	9% 8% 3 6% 5% 3 5% 4 5% 7% 44%	75026
Transparency	Day	36% 6% 12% 22% 18% 8%	74502
	Night	38% 7% 14% 22% 15% 5%	74844
Seeing	Day	12% 11% 23% 27% 19% 8%	13527
	Night	12% 6% 20% 38% 21% 3	22720

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	14% 11% 5% 9% 8% 5% 6% 5% 6% 8% 24%	1519
	11th to 20th	Day	13% 10% 4 8% 6% 3 5% 6% 5% 7% 32%	1483
	21st to end	Day	9% 13% 4 8% 6% 4 4 5% 5% 7% 35%	1688
February	1st to 10th	Day	10% 12% 4 8% 6% 4 5% 5% 5% 9% 32%	1620
	11th to 20th	Day	9% 12% 5% 8% 6% 4 4 6% 5% 9% 34%	1592
	21st to end	Day	9% 12% 5% 10% 6% 4 5% 7% 8% 7% 28%	1480
March	1st to 10th	Day	10% 13% 5% 9% 8% 5% 6% 5% 5% 8% 26%	1800
	11th to 20th	Day	10% 13% 5% 8% 6% 4 6% 5% 5% 8% 31%	1980
	21st to end	Day	10% 13% 5% 9% 8% 5% 6% 6% 5% 6% 27%	2286
April	1st to 10th	Day	11% 12% 5% 9% 7% 3 5% 6% 5% 7% 29%	2160
	11th to 20th	Day	11% 10% 6% 7% 6% 4 5% 4 5% 6% 37%	2129
	21st to end	Day	8% 9% 3 6% 6% 3 4 4 7% 8% 43%	2211
May	1st to 10th	Day	9% 7% 3 5% 5% 3 4 5% 6% 6% 47%	2520
	11th to 20th	Day	11% 8% 3 6% 5% 3 5% 5% 6% 7% 41%	2477
	21st to end	Day	12% 7% 3 7% 5% 3 5% 5% 5% 7% 41%	2723
June	1st to 10th	Day	12% 7% 3 6% 5% 3 5% 5% 5% 6% 42%	2520
	11th to 20th	Day	7% 4 3 5% 5% 3 4 5% 5% 7% 53%	2520
	21st to end	Day	4 3 1 3 3 2 3 3 5% 6% 66%	2520
July	1st to 10th	Day	1 2 1 2 2 2 3 3 3 5% 75%	2520
	11th to 20th	Day	3 2 1 2 2 2 2 3 3 5% 76%	2520
	21st to end	Day	2 1 1 2 2 1 2 3 3 4 80%	2741
August	1st to 10th	Day	2 2 1 2 3 2 3 3 5% 6% 72%	2414
	11th to 20th	Day	2 1 1 2 3 1 2 3 3 5% 76%	2276
	21st to end	Day	2 2 1 2 2 1 3 2 2 5% 77%	2376
September	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 77%	2160
	11th to 20th	Day	5% 3 1 4 4 3 4 4 5% 7% 61%	2160
	21st to end	Day	7% 6% 2 4 3 2 4 4 5% 6% 58%	1932
October	1st to 10th	Day	9% 6% 2 5% 5% 2 4 5% 4 5% 51%	1800
	11th to 20th	Day	10% 6% 2 5% 5% 3 4 4 6% 7% 48%	1800
	21st to end	Day	11% 7% 3 7% 6% 4 5% 4 4 5% 44%	1882
November	1st to 10th	Day	13% 9% 3 7% 7% 4 5% 5% 5% 8% 34%	1674
	11th to 20th	Day	14% 7% 3 8% 7% 5% 6% 6% 7% 7% 29%	1554
	21st to end	Day	9% 11% 5% 6% 6% 3 6% 6% 6% 8% 35%	1520
December	1st to 10th	Day	11% 8% 4 6% 7% 4 5% 5% 6% 8% 36%	1520
	11th to 20th	Day	12% 12% 4 7% 8% 4 6% 6% 8% 8% 26%	1928
	21st to end	Day	10% 13% 5% 9% 7% 4 6% 6% 6% 7% 27%	2794
January	1st to 10th	Night	12% 12% 6% 9% 7% 4 6% 6% 6% 7% 26%	2646
	11th to 20th	Night	11% 11% 3 7% 7% 4 6% 6% 6% 8% 31%	2604
	21st to end	Night	10% 11% 4 8% 7% 4 5% 5% 6% 6% 34%	2665
February	1st to 10th	Night	11% 11% 5% 7% 7% 4 6% 6% 5% 8% 31%	2340
	11th to 20th	Night	9% 11% 4 7% 5% 3 5% 5% 5% 8% 37%	2254
	21st to end	Night	9% 13% 5% 7% 7% 5% 7% 6% 5% 7% 28%	1776
March	1st to 10th	Night	10% 14% 4 8% 6% 4 5% 5% 6% 8% 30%	2156
	11th to 20th	Night	8% 11% 4 8% 7% 4 7% 6% 6% 8% 31%	1980
	21st to end	Night	8% 13% 5% 9% 7% 4 6% 6% 7% 7% 28%	2070
April	1st to 10th	Night	12% 12% 4 7% 6% 3 5% 4 5% 7% 35%	1800
	11th to 20th	Night	12% 11% 4 7% 5% 4 4 4 6% 6% 37%	1779
	21st to end	Night	10% 9% 2 6% 6% 4 5% 6% 7% 8% 38%	1745
May	1st to 10th	Night	8% 6% 3 5% 4 2 5% 6% 5% 8% 48%	1440
	11th to 20th	Night	8% 6% 2 5% 5% 4 6% 5% 6% 8% 45%	1416
	21st to end	Night	10% 8% 3 6% 6% 3 5% 4 7% 7% 41%	1552
June	1st to 10th	Night	13% 6% 2 5% 4 3 4 5% 6% 6% 47%	1440
	11th to 20th	Night	7% 6% 2 4 3 2 3 4 5% 8% 57%	1440
	21st to end	Night	4 3 1 3 3 2 3 3 5% 6% 68%	1440
July	1st to 10th	Night	2 1 1 2 2 2 3 3 4 6% 75%	1440
	11th to 20th	Night	1 1 1 2 1 1 2 3 3 4 81%	1440
	21st to end	Night	1 1 1 2 2 1 2 2 3 4 82%	1576
August	1st to 10th	Night	1 2 1 3 3 2 2 3 4 6% 74%	1523
	11th to 20th	Night	1 1 1 2 3 2 2 3 3 5% 76%	1624
	21st to end	Night	1 2 1 2 2 1 2 2 3 5% 78%	1980
September	1st to 10th	Night	3 1 2 2 1 2 2 3 5% 78%	1800
	11th to 20th	Night	4 3 2 5% 4 2 3 4 4 6% 63%	1800
	21st to end	Night	7% 4 2 4 4 2 2 4 5% 6% 60%	2022
October	1st to 10th	Night	9% 5% 2 5% 4 3 3 4 4 6% 56%	2160
	11th to 20th	Night	10% 5% 2 5% 4 2 4 3 5% 7% 54%	2160
	21st to end	Night	10% 7% 3 6% 6% 3 4 4 5% 6% 45%	2473
November	1st to 10th	Night	15% 8% 4 7% 7% 4 6% 5% 6% 5% 35%	2405
	11th to 20th	Night	14% 8% 3 8% 8% 4 6% 5% 6% 7% 30%	2622
	21st to end	Night	10% 10% 3 6% 5% 3 5% 5% 5% 8% 39%	2660
December	1st to 10th	Night	10% 11% 4 7% 6% 3 5% 5% 6% 8% 36%	2660
	11th to 20th	Night	11% 12% 4 9% 8% 4 7% 6% 6% 8% 25%	3276
	21st to end	Night	11% 13% 5% 8% 7% 4 6% 5% 5% 6% 30%	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	54% 8% 17% 16% 4 1	1519
	11th to 20th	Day	50% 6% 16% 17% 7% 5%	1483
	21st to end	Day	47% 6% 15% 21% 9% 2	1688
February	1st to 10th	Day	48% 7% 17% 21% 7%	1620
	11th to 20th	Day	45% 8% 15% 20% 9% 2	1601
	21st to end	Day	51% 8% 16% 16% 8% 1	1480
March	1st to 10th	Day	53% 7% 14% 16% 9% 1	1800
	11th to 20th	Day	50% 6% 12% 23% 8%	1980
	21st to end	Day	54% 8% 15% 17% 6%	2286
April	1st to 10th	Day	52% 6% 14% 17% 10% 1	2160
	11th to 20th	Day	47% 6% 13% 18% 12% 4	2132
	21st to end	Day	37% 7% 15% 26% 13% 3	2211
May	1st to 10th	Day	36% 5% 12% 27% 18% 1	2520
	11th to 20th	Day	41% 6% 12% 26% 13% 3	2480
	21st to end	Day	41% 7% 14% 24% 13% 1	2724
June	1st to 10th	Day	41% 5% 11% 29% 13% 1	2520
	11th to 20th	Day	30% 6% 11% 31% 19% 3	2520
	21st to end	Day	20% 3 9% 28% 29% 10%	2520
July	1st to 10th	Day	13% 4 8% 22% 28% 26%	2451
	11th to 20th	Day	14% 3 6% 13% 32% 32%	2445
	21st to end	Day	11% 2 3 13% 39% 33%	2589
August	1st to 10th	Day	14% 3 6% 25% 33% 19%	2397
	11th to 20th	Day	13% 3 5% 22% 34% 23%	2276
	21st to end	Day	13% 2 5% 24% 38% 18%	2376
September	1st to 10th	Day	12% 3 5% 25% 32% 23%	2160
	11th to 20th	Day	23% 5% 8% 24% 26% 14%	2160
	21st to end	Day	27% 4 10% 25% 24% 10%	1932
October	1st to 10th	Day	36% 4 11% 19% 21% 9%	1800
	11th to 20th	Day	34% 5% 14% 22% 19% 5%	1800
	21st to end	Day	40% 6% 11% 22% 18% 2	1882
November	1st to 10th	Day	47% 6% 12% 24% 11% 1	1674
	11th to 20th	Day	50% 8% 18% 17% 8%	1554
	21st to end	Day	43% 9% 16% 17% 14% 1	1520
December	1st to 10th	Day	45% 8% 16% 22% 8% 1	1520
	11th to 20th	Day	51% 8% 18% 18% 4	1928
	21st to end	Day	53% 8% 14% 20% 3 1	2794
January	1st to 10th	Night	54% 9% 18% 14% 5%	2646
	11th to 20th	Night	49% 8% 14% 17% 8% 5%	2604
	21st to end	Night	49% 6% 14% 20% 10% 1	2665
February	1st to 10th	Night	50% 7% 18% 15% 9%	2340
	11th to 20th	Night	44% 8% 17% 21% 9% 2	2256
	21st to end	Night	52% 7% 14% 19% 7% 1	1776
March	1st to 10th	Night	50% 8% 13% 18% 9% 1	2156
	11th to 20th	Night	46% 8% 18% 22% 6%	1980
	21st to end	Night	52% 10% 15% 17% 5%	2070
April	1st to 10th	Night	49% 6% 17% 18% 9% 1	1800
	11th to 20th	Night	47% 5% 15% 19% 12% 2	1779
	21st to end	Night	41% 9% 15% 22% 10% 2	1745
May	1st to 10th	Night	32% 6% 17% 30% 15% 1	1440
	11th to 20th	Night	36% 8% 17% 27% 10% 3	1416
	21st to end	Night	40% 7% 18% 25% 10%	1552
June	1st to 10th	Night	37% 7% 13% 31% 11% 1	1440
	11th to 20th	Night	26% 6% 17% 35% 14% 3	1440
	21st to end	Night	19% 4 12% 34% 25% 7%	1440
July	1st to 10th	Night	12% 4 11% 27% 28% 20%	1400
	11th to 20th	Night	9% 2 8% 20% 32% 29%	1392
	21st to end	Night	9% 2 4 18% 41% 27%	1488
August	1st to 10th	Night	12% 4 7% 28% 36% 13%	1515
	11th to 20th	Night	11% 4 7% 27% 35% 17%	1624
	21st to end	Night	12% 3 6% 27% 39% 14%	1980
September	1st to 10th	Night	11% 2 7% 28% 35% 18%	1800
	11th to 20th	Night	22% 6% 9% 24% 27% 13%	1800
	21st to end	Night	25% 4 10% 25% 24% 12%	2022
October	1st to 10th	Night	30% 5% 10% 24% 24% 7%	2160
	11th to 20th	Night	30% 5% 16% 23% 21% 5%	2160
	21st to end	Night	39% 7% 13% 20% 18% 3	2473
November	1st to 10th	Night	48% 7% 11% 19% 13% 2	2405
	11th to 20th	Night	49% 9% 15% 21% 6%	2622
	21st to end	Night	41% 7% 16% 19% 16% 1	2660
December	1st to 10th	Night	45% 8% 17% 21% 8% 1	2660
	11th to 20th	Night	53% 9% 14% 17% 6%	3276
	21st to end	Night	52% 8% 14% 21% 4 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	24% 13% 31% 20% 11% 1	180
	11th to 20th	Day	18% 11% 30% 23% 15% 4	171
	21st to end	Day	16% 13% 24% 29% 16% 2	280
February	1st to 10th	Day	19% 16% 23% 29% 11% 1	340
	11th to 20th	Day	19% 17% 28% 23% 11% 2	343
	21st to end	Day	21% 15% 26% 27% 10% 1	346
March	1st to 10th	Day	20% 16% 27% 28% 9% 1	420
	11th to 20th	Day	18% 15% 25% 28% 13% 2	420
	21st to end	Day	18% 19% 23% 26% 12% 2	462
April	1st to 10th	Day	25% 14% 23% 22% 10% 6%	420
	11th to 20th	Day	15% 20% 25% 23% 11% 6%	412
	21st to end	Day	12% 18% 25% 22% 17% 5%	420
May	1st to 10th	Day	13% 12% 26% 26% 17% 7%	423
	11th to 20th	Day	20% 11% 22% 22% 18% 8%	448
	21st to end	Day	14% 8% 19% 27% 22% 11%	485
June	1st to 10th	Day	12% 10% 22% 22% 20% 13%	450
	11th to 20th	Day	9% 14% 28% 24% 19% 6%	450
	21st to end	Day	4 8% 18% 30% 28% 13%	450
July	1st to 10th	Day	1 6% 26% 32% 24% 11%	446
	11th to 20th	Day	2 7% 24% 25% 28% 13%	444
	21st to end	Day	2 5% 25% 26% 27% 15%	487
August	1st to 10th	Day	1 5% 21% 29% 26% 17%	447
	11th to 20th	Day	2 3 18% 31% 27% 20%	437
	21st to end	Day	5% 7% 21% 26% 27% 16%	495
September	1st to 10th	Day	2 5% 19% 29% 29% 15%	450
	11th to 20th	Day	7% 6% 17% 32% 28% 10%	450
	21st to end	Day	11% 6% 20% 28% 27% 8%	450
October	1st to 10th	Day	13% 10% 19% 28% 22% 9%	450
	11th to 20th	Day	14% 9% 20% 24% 26% 7%	450
	21st to end	Day	17% 10% 22% 23% 21% 7%	397
November	1st to 10th	Day	17% 10% 22% 34% 14% 2	270
	11th to 20th	Day	16% 13% 21% 29% 17% 5%	196
	21st to end	Day	16% 11% 24% 33% 14% 2	180
December	1st to 10th	Day	13% 12% 29% 28% 14% 4	180
	11th to 20th	Day	19% 14% 28% 34% 4	180
	21st to end	Day	20% 12% 25% 31% 10% 2	198
January	1st to 10th	Night	16% 9% 31% 28% 15% 2	760
	11th to 20th	Night	16% 8% 26% 29% 18% 2	744
	21st to end	Night	16% 6% 22% 38% 16% 2	792
February	1st to 10th	Night	16% 9% 25% 34% 14% 1	720
	11th to 20th	Night	15% 6% 24% 39% 14% 2	704
	21st to end	Night	16% 9% 27% 34% 13% 1	592
March	1st to 10th	Night	16% 12% 25% 34% 12% 1	720
	11th to 20th	Night	14% 8% 21% 40% 15% 2	720
	21st to end	Night	15% 8% 27% 33% 15% 1	792
April	1st to 10th	Night	18% 9% 20% 32% 19% 1	720
	11th to 20th	Night	15% 9% 25% 30% 17% 3	711
	21st to end	Night	15% 8% 22% 35% 18% 2	665
May	1st to 10th	Night	11% 6% 16% 44% 21% 2	360
	11th to 20th	Night	10% 5% 17% 41% 27% 1	354
	21st to end	Night	13% 6% 10% 43% 25% 3	389
June	1st to 10th	Night	15% 4 16% 43% 21% 2	360
	11th to 20th	Night	9% 4 23% 43% 21% 1	360
	21st to end	Night	6% 2 13% 52% 27% 1	360
July	1st to 10th	Night	2 2 10% 51% 34% 1	350
	11th to 20th	Night	2 1 14% 57% 25% 1	348
	21st to end	Night	1 1 10% 55% 31% 2	394
August	1st to 10th	Night	2 1 7% 46% 41% 4	449
	11th to 20th	Night	2 6% 40% 44% 8%	556
	21st to end	Night	2 3 15% 44% 33% 4	792
September	1st to 10th	Night	4 2 14% 44% 32% 5%	720
	11th to 20th	Night	5% 3 11% 40% 35% 7%	720
	21st to end	Night	9% 4 13% 41% 28% 5%	720
October	1st to 10th	Night	11% 4 13% 41% 28% 3	720
	11th to 20th	Night	12% 4 18% 37% 24% 5%	720
	21st to end	Night	14% 6% 20% 37% 18% 5%	792
November	1st to 10th	Night	18% 6% 20% 41% 14% 1	740
	11th to 20th	Night	16% 9% 24% 35% 15% 2	760
	21st to end	Night	15% 8% 22% 37% 16% 3	760
December	1st to 10th	Night	16% 7% 22% 34% 19% 2	760
	11th to 20th	Night	18% 8% 26% 33% 14% 1	760
	21st to end	Night	22% 9% 29% 28% 10% 1	836

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