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 2024-12-24.

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%	105766
	Night	8% 9% 3 7% 5% 4 4 4 5% 6% 46%	103022
Transparency	Day	33% 5% 11% 22% 19% 11%	105509
	Night	37% 6% 13% 22% 15% 6%	102868
Seeing	Day	12% 11% 23% 27% 20% 8%	14751
	Night	12% 6% 20% 38% 21% 2	23880

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% 12% 4 10% 6% 7% 6% 4 7% 8% 22%	2539
	11th to 20th	Day	19% 17% 5% 10% 5% 5% 5% 4 5% 5% 20%	2540
	21st to end	Day	11% 14% 5% 11% 6% 5% 5% 4 6% 7% 26%	2968
February	1st to 10th	Day	10% 13% 4 8% 6% 5% 5% 4 5% 9% 31%	1710
	11th to 20th	Day	9% 13% 3 8% 5% 5% 5% 4 6% 9% 33%	1683
	21st to end	Day	8% 14% 4 11% 6% 5% 5% 6% 7% 7% 28%	1570
March	1st to 10th	Day	7% 11% 5% 10% 7% 6% 6% 4 6% 8% 30%	3151
	11th to 20th	Day	6% 10% 3 7% 4 4 4 3 4 6% 49%	3490
	21st to end	Day	8% 13% 4 11% 7% 6% 6% 5% 6% 7% 27%	4010
April	1st to 10th	Day	10% 13% 5% 10% 6% 4 5% 5% 5% 6% 30%	2280
	11th to 20th	Day	10% 11% 4 8% 4 5% 4 3 5% 6% 37%	2249
	21st to end	Day	7% 9% 2 7% 5% 5% 4 3 7% 8% 41%	2335
May	1st to 10th	Day	12% 8% 3 6% 5% 4 3 4 5% 5% 45%	4420
	11th to 20th	Day	6% 5% 1 4 3 3 3 3 4 6% 62%	4377
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 7% 47%	4813
June	1st to 10th	Day	12% 8% 2 7% 5% 4 4 3 6% 6% 43%	2660
	11th to 20th	Day	7% 5% 2 6% 4 4 4 4 5% 7% 53%	2660
	21st to end	Day	4 4 1 4 3 3 2 3 5% 5% 66%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 2 3 85%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 81%	4420
	21st to end	Day	1 1 1 2 1 2 2 2 4 4 80%	4790
August	1st to 10th	Day	3 3 1 3 2 2 3 3 4 5% 70%	4229
	11th to 20th	Day	2 1 1 3 2 2 3 3 4 6% 73%	3978
	21st to end	Day	3 3 1 3 2 2 3 2 3 5% 74%	4180
September	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 76%	2280
	11th to 20th	Day	4 3 1 5% 3 4 3 4 5% 7% 60%	2280
	21st to end	Day	7% 5% 2 4 2 3 4 3 4 6% 59%	2039
October	1st to 10th	Day	5% 4 2 4 3 2 3 3 3 6% 64%	3180
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 8% 48%	3161
	21st to end	Day	10% 9% 3 8% 6% 6% 5% 4 5% 7% 38%	3269
November	1st to 10th	Day	12% 10% 3 9% 5% 5% 5% 4 5% 8% 33%	1764
	11th to 20th	Day	14% 8% 3 10% 7% 5% 6% 5% 7% 7% 28%	1635
	21st to end	Day	9% 12% 4 7% 5% 5% 6% 4 6% 7% 35%	1600
December	1st to 10th	Day	10% 9% 3 7% 6% 4 5% 4 7% 7% 38%	1600
	11th to 20th	Day	12% 12% 3 8% 7% 6% 5% 5% 8% 8% 25%	2008
	21st to end	Day	10% 14% 5% 10% 7% 6% 6% 4 6% 7% 27%	2818
January	1st to 10th	Night	11% 14% 5% 11% 7% 6% 6% 5% 6% 7% 21%	4406
	11th to 20th	Night	16% 17% 5% 10% 6% 6% 5% 4 5% 6% 20%	4420
	21st to end	Night	12% 15% 5% 10% 7% 5% 5% 4 6% 6% 24%	4677
February	1st to 10th	Night	10% 13% 4 9% 6% 5% 5% 4 6% 7% 29%	2470
	11th to 20th	Night	9% 13% 3 8% 5% 4 5% 4 6% 7% 35%	2383
	21st to end	Night	9% 14% 4 9% 6% 7% 6% 5% 5% 7% 28%	1884
March	1st to 10th	Night	6% 13% 4 9% 6% 5% 5% 5% 7% 8% 33%	3719
	11th to 20th	Night	6% 8% 2 6% 4 4 5% 3 5% 6% 51%	3490
	21st to end	Night	8% 12% 4 10% 7% 7% 7% 5% 7% 7% 25%	3643
April	1st to 10th	Night	11% 14% 4 7% 6% 4 4 3 5% 7% 35%	1900
	11th to 20th	Night	11% 12% 4 8% 5% 5% 4 3 6% 6% 37%	1879
	21st to end	Night	10% 9% 2 7% 6% 5% 4 5% 6% 8% 38%	1841
May	1st to 10th	Night	11% 9% 3 7% 4 4 5% 5% 5% 6% 41%	2540
	11th to 20th	Night	5% 3 1 4 3 4 3 3 5% 6% 63%	2516
	21st to end	Night	9% 6% 2 5% 4 3 4 3 6% 6% 52%	2762
June	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 6% 5% 47%	1520
	11th to 20th	Night	7% 6% 2 4 4 2 3 3 5% 7% 57%	1520
	21st to end	Night	4 3 1 3 2 3 2 3 4 5% 68%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 2 3 85%	2540
	11th to 20th	Night	2 2 1 2 2 2 2 2 2 3 81%	2540
	21st to end	Night	1 1 1 2 1 1 2 1 3 3 83%	2761
August	1st to 10th	Night	1 2 1 5% 2 2 2 3 4 5% 71%	2704
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 6% 74%	2862
	21st to end	Night	2 3 1 3 2 2 2 2 4 5% 76%	3498
September	1st to 10th	Night	2 1 2 2 2 2 2 4 5% 78%	1900
	11th to 20th	Night	4 3 2 6% 3 3 4 4 5% 6% 61%	1900
	21st to end	Night	7% 4 2 5% 3 2 3 3 5% 6% 61%	2135
October	1st to 10th	Night	6% 4 1 4 3 2 3 2 2 4 69%	3800
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 6% 8% 51%	3777
	21st to end	Night	8% 7% 3 8% 6% 5% 5% 4 6% 7% 41%	4293
November	1st to 10th	Night	15% 9% 3 8% 6% 5% 5% 4 6% 5% 35%	2535
	11th to 20th	Night	14% 9% 3 10% 6% 5% 6% 4 6% 7% 29%	2761
	21st to end	Night	10% 11% 3 7% 5% 5% 4 4 5% 8% 39%	2800
December	1st to 10th	Night	9% 11% 4 8% 5% 5% 4 4 6% 7% 38%	2800
	11th to 20th	Night	11% 13% 4 11% 6% 6% 6% 5% 7% 8% 24%	3416
	21st to end	Night	10% 14% 4 10% 6% 6% 5% 3 5% 6% 30%	4910

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	55% 10% 15% 15% 3	2539
	11th to 20th	Day	62% 8% 13% 11% 4 3	2540
	21st to end	Day	54% 9% 14% 16% 5% 1	2968
February	1st to 10th	Day	49% 7% 16% 21% 7%	1710
	11th to 20th	Day	46% 8% 15% 20% 10% 2	1692
	21st to end	Day	51% 8% 17% 16% 8% 1	1570
March	1st to 10th	Day	47% 7% 10% 22% 13% 1	3151
	11th to 20th	Day	35% 6% 11% 31% 16%	3490
	21st to end	Day	51% 8% 14% 20% 6%	4010
April	1st to 10th	Day	51% 6% 14% 17% 10% 1	2280
	11th to 20th	Day	46% 6% 13% 19% 13% 4	2252
	21st to end	Day	37% 7% 14% 25% 13% 3	2335
May	1st to 10th	Day	38% 6% 10% 26% 19% 1	4420
	11th to 20th	Day	23% 4 8% 28% 32% 5%	4380
	21st to end	Day	32% 7% 18% 29% 13% 1	4814
June	1st to 10th	Day	40% 5% 11% 29% 14% 2	2660
	11th to 20th	Day	30% 5% 11% 30% 20% 4	2660
	21st to end	Day	20% 3 9% 28% 29% 10%	2660
July	1st to 10th	Day	7% 2 4 13% 22% 52%	4351
	11th to 20th	Day	10% 2 4 9% 22% 54%	4345
	21st to end	Day	9% 2 4 15% 38% 32%	4639
August	1st to 10th	Day	16% 2 5% 15% 36% 25%	4213
	11th to 20th	Day	14% 3 6% 28% 33% 16%	4017
	21st to end	Day	16% 2 6% 27% 29% 20%	4179
September	1st to 10th	Day	12% 3 5% 24% 33% 23%	2280
	11th to 20th	Day	23% 5% 9% 25% 25% 13%	2280
	21st to end	Day	26% 4 10% 26% 24% 10%	2039
October	1st to 10th	Day	24% 3 10% 20% 25% 17%	3180
	11th to 20th	Day	31% 5% 18% 24% 19% 3	3161
	21st to end	Day	42% 5% 11% 23% 17% 2	3269
November	1st to 10th	Day	47% 6% 12% 24% 11% 1	1764
	11th to 20th	Day	51% 8% 17% 16% 8%	1635
	21st to end	Day	43% 9% 15% 18% 14% 1	1600
December	1st to 10th	Day	43% 8% 16% 23% 9% 1	1600
	11th to 20th	Day	52% 8% 18% 18% 3	2008
	21st to end	Day	53% 8% 14% 20% 3 1	2818
January	1st to 10th	Night	58% 10% 16% 12% 4	4406
	11th to 20th	Night	62% 9% 11% 11% 5% 3	4420
	21st to end	Night	56% 9% 15% 14% 6% 1	4677
February	1st to 10th	Night	51% 8% 18% 15% 8%	2470
	11th to 20th	Night	45% 8% 16% 20% 9% 2	2385
	21st to end	Night	51% 8% 14% 19% 7% 1	1884
March	1st to 10th	Night	44% 9% 10% 26% 11% 1	3719
	11th to 20th	Night	32% 6% 16% 35% 11%	3490
	21st to end	Night	51% 13% 16% 16% 4	3643
April	1st to 10th	Night	48% 6% 19% 18% 8% 1	1900
	11th to 20th	Night	46% 6% 15% 19% 12% 2	1879
	21st to end	Night	40% 9% 15% 23% 10% 2	1841
May	1st to 10th	Night	38% 8% 13% 29% 12%	2540
	11th to 20th	Night	22% 6% 10% 39% 21% 2	2516
	21st to end	Night	31% 6% 31% 26% 6%	2762
June	1st to 10th	Night	37% 7% 13% 30% 12% 1	1520
	11th to 20th	Night	26% 6% 17% 35% 14% 3	1520
	21st to end	Night	18% 4 12% 33% 26% 7%	1520
July	1st to 10th	Night	7% 2 6% 15% 27% 44%	2500
	11th to 20th	Night	11% 1 5% 14% 21% 48%	2492
	21st to end	Night	9% 2 4 18% 38% 29%	2674
August	1st to 10th	Night	16% 2 5% 18% 38% 21%	2696
	11th to 20th	Night	12% 3 7% 39% 29% 10%	2889
	21st to end	Night	14% 3 9% 29% 29% 16%	3498
September	1st to 10th	Night	11% 2 7% 27% 35% 17%	1900
	11th to 20th	Night	22% 6% 10% 24% 26% 12%	1900
	21st to end	Night	24% 4 10% 26% 25% 12%	2135
October	1st to 10th	Night	21% 3 8% 24% 32% 11%	3800
	11th to 20th	Night	31% 5% 21% 22% 19% 3	3777
	21st to end	Night	39% 6% 11% 28% 14% 2	4293
November	1st to 10th	Night	48% 6% 12% 19% 13% 2	2535
	11th to 20th	Night	50% 9% 15% 21% 6%	2761
	21st to end	Night	41% 7% 16% 20% 16% 1	2800
December	1st to 10th	Night	43% 8% 16% 23% 9% 1	2800
	11th to 20th	Night	54% 8% 14% 17% 6%	3416
	21st to end	Night	52% 8% 14% 20% 4 1	4910

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	19% 12% 29% 22% 14% 3	180
	21st to end	Day	16% 14% 25% 28% 15% 2	307
February	1st to 10th	Day	19% 15% 23% 29% 12% 1	370
	11th to 20th	Day	18% 17% 28% 24% 11% 2	374
	21st to end	Day	21% 14% 25% 27% 11% 1	382
March	1st to 10th	Day	19% 16% 27% 28% 8% 1	460
	11th to 20th	Day	17% 14% 24% 28% 14% 3	460
	21st to end	Day	17% 19% 24% 26% 12% 2	506
April	1st to 10th	Day	24% 14% 22% 23% 11% 6%	460
	11th to 20th	Day	14% 19% 25% 23% 12% 6%	452
	21st to end	Day	12% 18% 25% 23% 17% 5%	460
May	1st to 10th	Day	13% 13% 25% 26% 16% 7%	463
	11th to 20th	Day	18% 11% 22% 23% 18% 9%	488
	21st to end	Day	13% 9% 19% 26% 22% 11%	529
June	1st to 10th	Day	11% 11% 22% 23% 20% 13%	490
	11th to 20th	Day	9% 14% 28% 23% 20% 6%	490
	21st to end	Day	4 9% 19% 29% 28% 12%	490
July	1st to 10th	Day	1 6% 26% 32% 24% 12%	486
	11th to 20th	Day	2 7% 24% 26% 29% 13%	484
	21st to end	Day	2 5% 26% 26% 27% 15%	531
August	1st to 10th	Day	2 5% 21% 29% 28% 16%	487
	11th to 20th	Day	2 4 18% 30% 27% 19%	477
	21st to end	Day	5% 7% 20% 25% 27% 16%	539
September	1st to 10th	Day	2 5% 18% 28% 30% 15%	490
	11th to 20th	Day	7% 6% 18% 31% 29% 9%	490
	21st to end	Day	11% 7% 19% 28% 27% 8%	490
October	1st to 10th	Day	12% 9% 19% 28% 23% 9%	490
	11th to 20th	Day	13% 9% 20% 24% 26% 7%	490
	21st to end	Day	17% 11% 22% 23% 20% 6%	435
November	1st to 10th	Day	17% 12% 21% 33% 14% 2	300
	11th to 20th	Day	18% 14% 21% 27% 15% 4	217
	21st to end	Day	16% 11% 25% 33% 14% 2	200
December	1st to 10th	Day	13% 12% 28% 27% 16% 6%	200
	11th to 20th	Day	19% 15% 31% 32% 4	200
	21st to end	Day	21% 12% 25% 30% 10% 2	204
January	1st to 10th	Night	16% 9% 31% 28% 15% 2	760
	11th to 20th	Night	17% 9% 25% 29% 18% 2	760
	21st to end	Night	16% 6% 22% 38% 16% 2	836
February	1st to 10th	Night	16% 10% 26% 34% 13% 1	760
	11th to 20th	Night	15% 6% 25% 39% 14% 1	744
	21st to end	Night	16% 9% 27% 36% 12%	628
March	1st to 10th	Night	15% 12% 25% 34% 12% 1	760
	11th to 20th	Night	14% 8% 20% 40% 16% 2	760
	21st to end	Night	15% 8% 28% 33% 14% 1	836
April	1st to 10th	Night	17% 9% 21% 33% 19% 1	760
	11th to 20th	Night	15% 9% 26% 31% 17% 3	751
	21st to end	Night	14% 8% 22% 36% 18% 2	701
May	1st to 10th	Night	12% 6% 17% 44% 20% 2	380
	11th to 20th	Night	9% 5% 17% 41% 27% 1	374
	21st to end	Night	13% 6% 10% 43% 26% 3	411
June	1st to 10th	Night	14% 4 16% 43% 20% 2	380
	11th to 20th	Night	9% 4 22% 43% 21% 1	380
	21st to end	Night	6% 2 13% 53% 26% 1	380
July	1st to 10th	Night	2 2 10% 53% 33% 1	370
	11th to 20th	Night	2 2 13% 57% 25% 1	368
	21st to end	Night	1 1 10% 56% 31% 2	416
August	1st to 10th	Night	2 1 7% 46% 39% 4	475
	11th to 20th	Night	2 1 6% 42% 42% 7%	588
	21st to end	Night	2 3 15% 43% 33% 3	836
September	1st to 10th	Night	4 2 13% 44% 32% 5%	760
	11th to 20th	Night	5% 3 11% 41% 33% 7%	760
	21st to end	Night	8% 4 13% 41% 29% 5%	760
October	1st to 10th	Night	10% 4 13% 42% 28% 3	760
	11th to 20th	Night	12% 4 18% 38% 24% 4	760
	21st to end	Night	14% 6% 21% 37% 18% 4	836
November	1st to 10th	Night	17% 6% 21% 41% 14% 1	780
	11th to 20th	Night	17% 9% 24% 34% 14% 2	800
	21st to end	Night	15% 8% 21% 36% 16% 3	800
December	1st to 10th	Night	15% 7% 22% 34% 20% 3	800
	11th to 20th	Night	18% 9% 26% 32% 13% 1	800
	21st to end	Night	22% 10% 30% 28% 10% 1	850

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