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 2026-04-17.

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% 7% 2 6% 4 4 4 3 5% 6% 51%	140610
	Night	8% 8% 3 7% 4 4 4 3 5% 6% 47%	138856
Transparency	Day	33% 5% 10% 22% 20% 10%	135418
	Night	37% 6% 12% 23% 16% 6%	135068
Seeing	Day	12% 11% 23% 27% 20% 8%	16386
	Night	12% 6% 20% 38% 20% 2	25582

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% 15% 4 11% 7% 7% 6% 4 6% 7% 20%	4722
	11th to 20th	Day	10% 9% 3 6% 3 4 3 3 4 5% 51%	4740
	21st to end	Day	7% 9% 3 8% 6% 5% 5% 4 7% 8% 37%	5526
February	1st to 10th	Day	9% 13% 4 9% 5% 5% 5% 4 5% 9% 31%	1890
	11th to 20th	Day	9% 13% 4 9% 5% 5% 5% 4 5% 8% 32%	1865
	21st to end	Day	8% 13% 4 10% 6% 5% 5% 6% 7% 7% 29%	1730
March	1st to 10th	Day	8% 11% 4 9% 6% 6% 5% 4 6% 9% 33%	4631
	11th to 20th	Day	8% 14% 4 9% 6% 5% 4 3 5% 6% 36%	5110
	21st to end	Day	8% 12% 5% 11% 7% 6% 6% 5% 6% 8% 27%	5876
April	1st to 10th	Day	9% 12% 4 10% 6% 4 6% 5% 5% 7% 30%	2520
	11th to 20th	Day	10% 11% 4 8% 4 5% 4 3 5% 6% 39%	2442
	21st to end	Day	7% 9% 2 7% 5% 5% 4 3 7% 8% 43%	2458
May	1st to 10th	Day	10% 6% 2 5% 4 4 3 4 5% 6% 52%	6320
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 5% 7% 50%	6277
	21st to end	Day	6% 5% 2 5% 3 4 5% 4 6% 8% 53%	6902
June	1st to 10th	Day	11% 7% 2 7% 4 4 4 3 5% 6% 45%	2800
	11th to 20th	Day	7% 5% 2 6% 4 3 4 4 5% 7% 54%	2800
	21st to end	Day	4 4 1 4 3 3 2 3 5% 5% 66%	2800
July	1st to 10th	Day	1 2 1 2 2 2 2 2 3 4 81%	6319
	11th to 20th	Day	1 1 1 2 1 1 2 2 3 3 84%	6320
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 79%	6880
August	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 7% 68%	5965
	11th to 20th	Day	3 1 1 3 2 2 2 2 3 5% 75%	5722
	21st to end	Day	4 4 1 4 2 3 3 3 4 6% 65%	5984
September	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 74%	2400
	11th to 20th	Day	4 3 1 5% 3 4 3 3 5% 7% 61%	2400
	21st to end	Day	7% 5% 2 4 2 3 3 3 4 6% 60%	2146
October	1st to 10th	Day	6% 4 2 5% 3 2 3 3 3 5% 64%	3280
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 8% 48%	3261
	21st to end	Day	10% 9% 3 8% 6% 6% 5% 4 5% 7% 38%	3373
November	1st to 10th	Day	12% 11% 3 9% 5% 5% 5% 4 6% 8% 33%	1854
	11th to 20th	Day	13% 8% 3 10% 7% 5% 6% 5% 7% 7% 28%	1717
	21st to end	Day	9% 11% 4 7% 5% 5% 5% 4 7% 7% 35%	1680
December	1st to 10th	Day	8% 7% 2 5% 4 3 4 3 5% 7% 50%	2700
	11th to 20th	Day	12% 14% 4 10% 7% 6% 5% 5% 7% 7% 24%	3108
	21st to end	Day	10% 14% 5% 11% 7% 6% 7% 5% 6% 7% 23%	4092
January	1st to 10th	Night	12% 15% 5% 12% 6% 6% 6% 5% 6% 7% 19%	8172
	11th to 20th	Night	9% 9% 3 6% 4 4 4 3 4 6% 49%	8220
	21st to end	Night	8% 10% 4 9% 6% 6% 5% 3 6% 6% 36%	8656
February	1st to 10th	Night	10% 13% 4 9% 6% 5% 5% 4 6% 7% 30%	2730
	11th to 20th	Night	8% 13% 4 9% 5% 4 5% 4 6% 7% 34%	2641
	21st to end	Night	9% 14% 4 8% 6% 7% 6% 5% 5% 8% 29%	2076
March	1st to 10th	Night	6% 11% 3 7% 5% 5% 5% 5% 8% 9% 34%	5479
	11th to 20th	Night	7% 10% 3 8% 5% 5% 5% 4 6% 8% 40%	5110
	21st to end	Night	8% 12% 4 10% 7% 7% 6% 5% 7% 7% 27%	5341
April	1st to 10th	Night	10% 13% 4 7% 6% 4 4 4 5% 7% 36%	2100
	11th to 20th	Night	11% 12% 4 8% 5% 5% 4 3 6% 6% 38%	2039
	21st to end	Night	9% 9% 2 6% 6% 5% 4 5% 6% 8% 40%	1937
May	1st to 10th	Night	8% 7% 2 6% 4 3 4 4 5% 6% 51%	3640
	11th to 20th	Night	6% 6% 3 7% 4 4 4 3 4 7% 52%	3616
	21st to end	Night	8% 6% 2 4 3 3 3 3 5% 6% 57%	3972
June	1st to 10th	Night	12% 6% 2 5% 3 4 4 4 6% 5% 49%	1600
	11th to 20th	Night	6% 6% 2 4 4 3 4 3 6% 7% 56%	1600
	21st to end	Night	4 4 1 3 2 3 2 3 4 5% 68%	1600
July	1st to 10th	Night	2 1 1 2 2 2 3 2 3 4 78%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 86%	3640
	21st to end	Night	2 2 1 3 2 2 2 2 3 4 77%	3970
August	1st to 10th	Night	2 3 1 6% 3 3 3 3 5% 6% 64%	3824
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 75%	4116
	21st to end	Night	3 3 2 4 2 2 3 3 4 6% 68%	5015
September	1st to 10th	Night	2 2 3 2 2 2 2 4 5% 75%	2000
	11th to 20th	Night	4 3 2 6% 3 3 4 3 5% 6% 61%	2000
	21st to end	Night	6% 4 1 5% 3 3 3 3 5% 6% 62%	2248
October	1st to 10th	Night	6% 4 1 4 3 2 3 2 3 4 68%	3920
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 6% 8% 51%	3897
	21st to end	Night	8% 7% 3 8% 6% 5% 5% 4 6% 7% 41%	4431
November	1st to 10th	Night	14% 9% 3 8% 6% 5% 5% 4 6% 6% 34%	2665
	11th to 20th	Night	14% 10% 4 10% 7% 5% 5% 4 6% 7% 29%	2899
	21st to end	Night	10% 11% 3 7% 5% 5% 4 4 5% 8% 39%	2940
December	1st to 10th	Night	6% 7% 2 5% 3 3 3 3 5% 7% 54%	4700
	11th to 20th	Night	12% 14% 4 12% 7% 6% 6% 4 6% 7% 21%	5316
	21st to end	Night	13% 14% 4 10% 6% 5% 5% 4 5% 6% 28%	7106

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	59% 8% 14% 15% 4	4723
	11th to 20th	Day	36% 5% 9% 21% 23% 6%	4740
	21st to end	Day	41% 6% 11% 15% 21% 5%	5526
February	1st to 10th	Day	48% 6% 16% 21% 9% 1	1890
	11th to 20th	Day	48% 7% 14% 19% 10% 1	1874
	21st to end	Day	50% 7% 16% 16% 9% 2	1730
March	1st to 10th	Day	44% 6% 10% 25% 12% 3	4631
	11th to 20th	Day	47% 6% 10% 24% 13%	5110
	21st to end	Day	51% 8% 15% 20% 6% 1	5876
April	1st to 10th	Day	49% 6% 14% 19% 10% 1	2520
	11th to 20th	Day	45% 6% 13% 18% 14% 3	2445
	21st to end	Day	36% 7% 14% 26% 14% 3	2458
May	1st to 10th	Day	32% 5% 8% 27% 25% 3	6320
	11th to 20th	Day	30% 6% 12% 26% 23% 3	6280
	21st to end	Day	27% 6% 17% 34% 16% 1	6904
June	1st to 10th	Day	38% 5% 10% 30% 15% 2	2800
	11th to 20th	Day	29% 5% 11% 30% 21% 4	2800
	21st to end	Day	20% 3 10% 28% 29% 10%	2800
July	1st to 10th	Day	9% 2 4 16% 30% 39%	6251
	11th to 20th	Day	8% 1 4 9% 23% 55%	6245
	21st to end	Day	10% 2 4 26% 36% 22%	6681
August	1st to 10th	Day	16% 2 6% 16% 36% 24%	4347
	11th to 20th	Day	14% 3 6% 28% 33% 16%	4145
	21st to end	Day	16% 2 6% 27% 28% 20%	4311
September	1st to 10th	Day	13% 3 5% 24% 33% 22%	2400
	11th to 20th	Day	23% 5% 9% 26% 25% 13%	2400
	21st to end	Day	26% 4 10% 26% 24% 11%	2146
October	1st to 10th	Day	24% 3 10% 21% 25% 16%	3280
	11th to 20th	Day	31% 5% 18% 24% 19% 3	3261
	21st to end	Day	42% 5% 10% 24% 17% 2	3373
November	1st to 10th	Day	47% 6% 13% 23% 10% 1	1854
	11th to 20th	Day	51% 8% 17% 17% 8%	1717
	21st to end	Day	43% 8% 16% 18% 13% 1	1680
December	1st to 10th	Day	34% 5% 13% 26% 21% 2	2700
	11th to 20th	Day	56% 7% 16% 17% 4	3108
	21st to end	Day	56% 7% 15% 18% 4	4092
January	1st to 10th	Night	60% 8% 15% 13% 3	8172
	11th to 20th	Night	36% 6% 8% 23% 22% 6%	8220
	21st to end	Night	47% 6% 10% 14% 19% 5%	8656
February	1st to 10th	Night	51% 7% 17% 16% 9%	2730
	11th to 20th	Night	46% 8% 16% 20% 9% 1	2643
	21st to end	Night	50% 7% 13% 20% 8% 1	2076
March	1st to 10th	Night	41% 8% 10% 27% 12% 2	5479
	11th to 20th	Night	39% 6% 16% 28% 10% 1	5110
	21st to end	Night	50% 12% 15% 18% 5%	5341
April	1st to 10th	Night	46% 6% 18% 21% 8% 1	2100
	11th to 20th	Night	45% 6% 15% 20% 13% 1	2039
	21st to end	Night	39% 9% 16% 23% 11% 2	1937
May	1st to 10th	Night	32% 6% 10% 27% 24% 1	3637
	11th to 20th	Night	31% 6% 15% 31% 15% 1	3616
	21st to end	Night	27% 5% 28% 31% 9%	3972
June	1st to 10th	Night	35% 6% 13% 31% 13% 1	1600
	11th to 20th	Night	26% 6% 16% 34% 15% 3	1600
	21st to end	Night	19% 3 12% 34% 25% 7%	1600
July	1st to 10th	Night	13% 2 6% 21% 27% 32%	3600
	11th to 20th	Night	8% 1 5% 13% 24% 49%	3592
	21st to end	Night	14% 2 4 30% 30% 20%	3857
August	1st to 10th	Night	17% 2 5% 19% 37% 20%	2781
	11th to 20th	Night	12% 3 7% 39% 29% 10%	2980
	21st to end	Night	14% 3 9% 29% 29% 16%	3608
September	1st to 10th	Night	13% 3 7% 26% 34% 17%	2000
	11th to 20th	Night	22% 6% 10% 25% 26% 12%	2000
	21st to end	Night	23% 3 10% 26% 25% 12%	2248
October	1st to 10th	Night	22% 3 8% 24% 32% 11%	3920
	11th to 20th	Night	31% 5% 21% 23% 18% 3	3897
	21st to end	Night	39% 6% 11% 28% 14% 2	4431
November	1st to 10th	Night	48% 7% 13% 19% 12% 2	2665
	11th to 20th	Night	50% 8% 15% 20% 6%	2899
	21st to end	Night	41% 7% 16% 20% 15% 1	2940
December	1st to 10th	Night	30% 5% 12% 30% 23% 1	4700
	11th to 20th	Night	59% 7% 14% 15% 5%	5316
	21st to end	Night	54% 8% 16% 18% 5%	7106

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	25% 13% 33% 20% 9% 1	220
	11th to 20th	Day	16% 10% 29% 25% 16% 4	220
	21st to end	Day	15% 12% 26% 27% 17% 3	363
February	1st to 10th	Day	19% 15% 23% 29% 13% 1	430
	11th to 20th	Day	19% 16% 28% 24% 11% 2	436
	21st to end	Day	23% 14% 26% 26% 9% 1	446
March	1st to 10th	Day	19% 17% 26% 29% 8% 1	540
	11th to 20th	Day	18% 16% 25% 26% 12% 2	540
	21st to end	Day	18% 19% 24% 26% 12% 1	594
April	1st to 10th	Day	22% 14% 21% 24% 12% 7%	540
	11th to 20th	Day	14% 19% 25% 24% 13% 5%	517
	21st to end	Day	12% 17% 25% 24% 18% 5%	500
May	1st to 10th	Day	13% 13% 24% 26% 17% 7%	503
	11th to 20th	Day	19% 11% 23% 22% 17% 8%	528
	21st to end	Day	12% 9% 19% 27% 22% 11%	573
June	1st to 10th	Day	11% 10% 22% 23% 21% 14%	530
	11th to 20th	Day	8% 15% 28% 24% 20% 6%	530
	21st to end	Day	4 9% 18% 28% 29% 13%	530
July	1st to 10th	Day	1 6% 25% 31% 25% 12%	526
	11th to 20th	Day	2 6% 23% 26% 29% 13%	524
	21st to end	Day	2 4 25% 26% 27% 15%	575
August	1st to 10th	Day	2 6% 20% 28% 28% 16%	527
	11th to 20th	Day	3 4 18% 30% 26% 19%	517
	21st to end	Day	5% 7% 20% 25% 28% 16%	583
September	1st to 10th	Day	2 6% 18% 28% 31% 15%	530
	11th to 20th	Day	6% 6% 17% 31% 28% 11%	530
	21st to end	Day	11% 7% 18% 27% 28% 9%	530
October	1st to 10th	Day	12% 10% 19% 28% 23% 9%	530
	11th to 20th	Day	13% 9% 21% 26% 25% 6%	530
	21st to end	Day	17% 11% 22% 24% 20% 6%	473
November	1st to 10th	Day	17% 12% 21% 34% 13% 2	330
	11th to 20th	Day	17% 14% 21% 28% 16% 4	239
	21st to end	Day	15% 10% 26% 33% 14% 2	220
December	1st to 10th	Day	13% 12% 28% 27% 15% 5%	220
	11th to 20th	Day	19% 15% 30% 32% 5%	220
	21st to end	Day	21% 14% 26% 29% 9% 2	242
January	1st to 10th	Night	17% 10% 31% 27% 14% 1	840
	11th to 20th	Night	15% 8% 25% 31% 18% 2	840
	21st to end	Night	15% 6% 23% 38% 16% 2	924
February	1st to 10th	Night	16% 10% 25% 35% 13% 1	840
	11th to 20th	Night	14% 7% 26% 39% 13% 1	824
	21st to end	Night	17% 10% 27% 35% 11%	692
March	1st to 10th	Night	15% 12% 26% 35% 11% 1	840
	11th to 20th	Night	14% 8% 22% 39% 15% 1	840
	21st to end	Night	15% 10% 27% 34% 14% 1	924
April	1st to 10th	Night	16% 9% 21% 34% 19% 1	840
	11th to 20th	Night	14% 9% 26% 32% 16% 3	815
	21st to end	Night	14% 8% 22% 37% 18% 1	737
May	1st to 10th	Night	12% 6% 17% 45% 20% 2	400
	11th to 20th	Night	10% 5% 18% 40% 26% 1	394
	21st to end	Night	12% 5% 11% 44% 25% 3	433
June	1st to 10th	Night	14% 4 17% 44% 21% 2	400
	11th to 20th	Night	8% 4 23% 44% 20% 1	400
	21st to end	Night	6% 2 13% 53% 26% 1	400
July	1st to 10th	Night	2 2 10% 52% 33% 1	390
	11th to 20th	Night	2 2 13% 57% 26% 1	388
	21st to end	Night	1 1 9% 56% 31% 2	438
August	1st to 10th	Night	2 1 8% 47% 39% 4	500
	11th to 20th	Night	2 1 6% 43% 41% 7%	619
	21st to end	Night	2 3 15% 44% 32% 3	880
September	1st to 10th	Night	4 2 13% 44% 32% 5%	800
	11th to 20th	Night	5% 3 12% 41% 34% 7%	800
	21st to end	Night	8% 4 13% 41% 29% 5%	800
October	1st to 10th	Night	11% 4 14% 41% 27% 3	800
	11th to 20th	Night	12% 5% 17% 39% 23% 4	800
	21st to end	Night	13% 6% 22% 38% 17% 4	880
November	1st to 10th	Night	17% 7% 20% 40% 14% 1	820
	11th to 20th	Night	17% 9% 24% 35% 14% 2	840
	21st to end	Night	15% 9% 21% 36% 16% 3	840
December	1st to 10th	Night	15% 7% 22% 33% 20% 3	840
	11th to 20th	Night	18% 9% 27% 32% 13% 1	840
	21st to end	Night	22% 10% 29% 28% 10% 1	924

Page generated at 2026-04-13 23:44:27. Version ac07

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