Starr Butte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starr Butte clear sky chart from 2005-11-06 to 2025-12-03.

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	11% 9% 3 6% 4 4 4 3 5% 6% 44%	130479
	Night	13% 10% 3 7% 4 4 4 3 5% 6% 40%	128898
Transparency	Day	40% 6% 12% 22% 15% 5%	130205
	Night	44% 6% 12% 20% 14% 5%	128749
Seeing	Day	17% 11% 23% 25% 18% 6%	16150
	Night	19% 8% 18% 32% 20% 3	25338

By Month

Percentage Cloud Cover NW (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	24% 23% 8% 11% 4 4 4 2 4 4 12%	3639
	11th to 20th	Day	16% 10% 4 9% 5% 5% 4 3 5% 7% 32%	3616
	21st to end	Day	18% 15% 3 8% 4 4 4 3 4 4 33%	4004
February	1st to 10th	Day	20% 17% 5% 9% 4 4 4 4 5% 6% 21%	1600
	11th to 20th	Day	18% 19% 6% 8% 4 3 4 4 6% 5% 24%	1889
	21st to end	Day	18% 20% 6% 9% 4 4 4 3 4 5% 22%	1650
March	1st to 10th	Day	9% 13% 3 8% 6% 5% 5% 5% 8% 8% 30%	4518
	11th to 20th	Day	11% 12% 3 7% 5% 5% 5% 3 4 6% 37%	4724
	21st to end	Day	17% 16% 5% 10% 7% 5% 5% 4 6% 6% 20%	5876
April	1st to 10th	Day	14% 14% 3 8% 5% 6% 5% 5% 6% 8% 25%	2400
	11th to 20th	Day	12% 11% 3 8% 6% 5% 4 3 5% 7% 35%	2447
	21st to end	Day	13% 9% 3 8% 6% 5% 5% 4 6% 7% 36%	2600
May	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 47%	6136
	11th to 20th	Day	10% 6% 3 7% 5% 4 6% 4 7% 9% 37%	6277
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	6921
June	1st to 10th	Day	15% 8% 2 6% 5% 5% 4 3 5% 7% 40%	3000
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 4 6% 8% 40%	3000
	21st to end	Day	6% 4 2 4 4 4 4 4 6% 7% 56%	3000
July	1st to 10th	Day	4 2 1 3 2 2 3 2 4 5% 73%	6332
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 80%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 73%	6874
August	1st to 10th	Day	3 3 1 3 2 3 3 2 4 6% 70%	4541
	11th to 20th	Day	4 3 1 3 3 3 4 3 5% 7% 65%	4248
	21st to end	Day	3 3 1 4 3 3 3 3 5% 6% 65%	4413
September	1st to 10th	Day	5% 3 1 3 2 3 3 3 4 7% 66%	2400
	11th to 20th	Day	9% 5% 1 5% 4 4 4 3 5% 8% 51%	2274
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 7% 49%	2200
October	1st to 10th	Day	9% 4 2 5% 3 3 3 3 5% 7% 57%	3505
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 3 5% 7% 43%	3043
	21st to end	Day	17% 11% 5% 7% 4 4 4 3 5% 6% 35%	3200
November	1st to 10th	Day	23% 13% 4 7% 4 5% 4 3 4 7% 26%	1854
	11th to 20th	Day	22% 16% 4 8% 6% 4 5% 4 5% 5% 23%	1816
	21st to end	Day	17% 17% 5% 10% 5% 4 4 4 5% 5% 22%	1534
December	1st to 10th	Day	16% 12% 5% 9% 4 4 3 4 6% 9% 28%	2304
	11th to 20th	Day	21% 23% 6% 12% 6% 4 4 3 5% 3 13%	2642
	21st to end	Day	22% 21% 6% 10% 5% 5% 4 3 4 5% 14%	3682
January	1st to 10th	Night	24% 19% 6% 10% 5% 5% 5% 4 5% 4 12%	6306
	11th to 20th	Night	14% 12% 4 9% 4 4 4 3 5% 7% 34%	6282
	21st to end	Night	17% 14% 4 7% 4 4 4 3 4 6% 34%	6952
February	1st to 10th	Night	19% 19% 4 8% 5% 4 3 3 4 5% 25%	2800
	11th to 20th	Night	17% 15% 4 9% 5% 4 4 3 5% 5% 29%	2395
	21st to end	Night	14% 15% 3 8% 5% 5% 5% 4 6% 6% 29%	1980
March	1st to 10th	Night	10% 10% 3 8% 5% 5% 4 4 6% 7% 39%	5376
	11th to 20th	Night	12% 14% 4 7% 4 4 4 3 4 5% 39%	5274
	21st to end	Night	14% 10% 4 10% 7% 6% 6% 5% 8% 8% 22%	5094
April	1st to 10th	Night	11% 11% 3 7% 6% 5% 6% 4 8% 8% 32%	2000
	11th to 20th	Night	12% 10% 3 7% 6% 4 4 4 5% 5% 41%	1903
	21st to end	Night	10% 10% 2 7% 5% 5% 5% 4 6% 6% 39%	1800
May	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 5% 8% 47%	3829
	11th to 20th	Night	12% 9% 3 7% 5% 4 5% 4 5% 8% 38%	3616
	21st to end	Night	10% 5% 2 6% 3 4 4 4 6% 8% 47%	3939
June	1st to 10th	Night	12% 6% 1 6% 4 4 5% 4 5% 9% 44%	1400
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 5% 7% 8% 42%	1400
	21st to end	Night	8% 4 2 5% 3 5% 5% 3 6% 7% 53%	1400
July	1st to 10th	Night	3 1 1 2 2 2 2 2 4 6% 76%	3609
	11th to 20th	Night	2 2 1 2 2 1 2 1 3 3 81%	3640
	21st to end	Night	3 3 1 2 2 2 2 2 4 6% 73%	3969
August	1st to 10th	Night	4 3 1 3 2 2 3 3 6% 6% 66%	2609
	11th to 20th	Night	3 3 1 4 4 3 4 4 5% 6% 62%	2806
	21st to end	Night	3 3 1 4 3 3 3 3 5% 6% 65%	3500
September	1st to 10th	Night	4 3 1 4 3 2 3 3 5% 6% 66%	2000
	11th to 20th	Night	6% 6% 2 4 4 4 4 4 5% 6% 55%	2125
	21st to end	Night	9% 5% 1 5% 4 3 3 3 5% 8% 54%	2200
October	1st to 10th	Night	7% 4 2 5% 3 3 3 4 5% 7% 58%	3691
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 5% 7% 47%	4106
	21st to end	Night	14% 7% 2 7% 4 4 4 4 5% 7% 42%	4600
November	1st to 10th	Night	21% 12% 3 7% 4 4 4 3 5% 7% 29%	2665
	11th to 20th	Night	22% 13% 4 8% 6% 4 4 4 4 5% 27%	2798
	21st to end	Night	20% 13% 4 8% 5% 4 4 3 5% 6% 26%	3061
December	1st to 10th	Night	13% 10% 3 7% 4 4 5% 4 7% 11% 33%	4921
	11th to 20th	Night	22% 20% 6% 10% 5% 5% 4 3 5% 5% 14%	5550
	21st to end	Night	22% 17% 6% 11% 5% 5% 4 4 5% 5% 16%	7302

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	76% 6% 11% 6% 1	3639
	11th to 20th	Day	51% 6% 18% 18% 5% 1	3640
	21st to end	Day	55% 5% 13% 19% 7%	4004
February	1st to 10th	Day	63% 9% 13% 10% 5% 1	1600
	11th to 20th	Day	63% 8% 12% 11% 6%	1899
	21st to end	Day	66% 5% 8% 15% 5%	1650
March	1st to 10th	Day	48% 6% 13% 20% 13% 1	4518
	11th to 20th	Day	48% 6% 13% 27% 5%	4724
	21st to end	Day	64% 7% 12% 13% 4	5876
April	1st to 10th	Day	56% 8% 17% 15% 4	2400
	11th to 20th	Day	48% 7% 15% 18% 11% 2	2447
	21st to end	Day	48% 7% 15% 20% 9% 1	2600
May	1st to 10th	Day	35% 5% 12% 30% 19%	6136
	11th to 20th	Day	39% 7% 16% 28% 9% 1	6280
	21st to end	Day	35% 8% 20% 28% 9%	6922
June	1st to 10th	Day	44% 6% 13% 23% 13% 1	3000
	11th to 20th	Day	40% 7% 16% 23% 12% 2	3000
	21st to end	Day	26% 5% 15% 29% 20% 3	3000
July	1st to 10th	Day	15% 3 8% 22% 29% 23%	6263
	11th to 20th	Day	11% 2 5% 21% 31% 30%	6245
	21st to end	Day	14% 3 5% 30% 30% 17%	6681
August	1st to 10th	Day	16% 4 5% 21% 38% 16%	4524
	11th to 20th	Day	20% 4 8% 37% 26% 5%	4289
	21st to end	Day	19% 4 11% 32% 27% 7%	4412
September	1st to 10th	Day	19% 4 11% 29% 25% 11%	2400
	11th to 20th	Day	32% 6% 12% 23% 22% 5%	2274
	21st to end	Day	33% 6% 17% 26% 13% 4	2200
October	1st to 10th	Day	28% 4 13% 34% 20% 1	3505
	11th to 20th	Day	42% 5% 17% 22% 13% 1	3043
	21st to end	Day	51% 6% 14% 19% 9% 1	3200
November	1st to 10th	Day	60% 6% 15% 16% 3 1	1854
	11th to 20th	Day	64% 7% 16% 10% 3 1	1816
	21st to end	Day	62% 8% 13% 13% 3 1	1534
December	1st to 10th	Day	52% 9% 20% 16% 2	2304
	11th to 20th	Day	75% 6% 11% 7% 1	2644
	21st to end	Day	71% 6% 13% 7% 3	3682
January	1st to 10th	Night	72% 7% 12% 8% 1	6306
	11th to 20th	Night	50% 6% 18% 16% 9% 1	6320
	21st to end	Night	52% 6% 13% 21% 8%	6952
February	1st to 10th	Night	63% 6% 12% 12% 5% 1	2800
	11th to 20th	Night	57% 6% 15% 13% 8% 1	2396
	21st to end	Night	54% 7% 14% 18% 7% 1	1980
March	1st to 10th	Night	42% 6% 11% 21% 18% 1	5376
	11th to 20th	Night	47% 5% 10% 28% 9%	5275
	21st to end	Night	56% 9% 13% 19% 4	5094
April	1st to 10th	Night	47% 8% 15% 22% 8%	2000
	11th to 20th	Night	44% 5% 11% 24% 14% 2	1903
	21st to end	Night	42% 7% 13% 22% 14% 2	1800
May	1st to 10th	Night	33% 6% 10% 25% 25% 1	3826
	11th to 20th	Night	41% 6% 12% 26% 14% 1	3616
	21st to end	Night	33% 7% 17% 29% 14%	3939
June	1st to 10th	Night	37% 7% 10% 23% 20% 3	1400
	11th to 20th	Night	37% 6% 12% 25% 15% 5%	1400
	21st to end	Night	29% 6% 9% 27% 22% 7%	1400
July	1st to 10th	Night	11% 2 5% 18% 35% 29%	3569
	11th to 20th	Night	11% 2 4 15% 29% 39%	3592
	21st to end	Night	14% 3 4 22% 33% 25%	3857
August	1st to 10th	Night	18% 3 5% 16% 35% 23%	2601
	11th to 20th	Night	21% 5% 7% 29% 30% 9%	2831
	21st to end	Night	20% 4 9% 24% 30% 13%	3500
September	1st to 10th	Night	19% 3 10% 26% 27% 15%	2000
	11th to 20th	Night	30% 4 11% 23% 24% 8%	2125
	21st to end	Night	29% 5% 14% 29% 16% 7%	2200
October	1st to 10th	Night	27% 4 9% 34% 24% 2	3691
	11th to 20th	Night	37% 5% 14% 23% 20% 1	4106
	21st to end	Night	42% 6% 11% 28% 11% 2	4600
November	1st to 10th	Night	55% 6% 17% 16% 5% 1	2665
	11th to 20th	Night	60% 6% 15% 13% 5% 1	2798
	21st to end	Night	58% 7% 15% 15% 6%	3061
December	1st to 10th	Night	45% 9% 23% 19% 5%	4921
	11th to 20th	Night	71% 6% 13% 9% 1	5547
	21st to end	Night	67% 6% 14% 9% 3	7302

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	29% 18% 18% 17% 16% 4	200
	11th to 20th	Day	29% 13% 19% 21% 12% 7%	200
	21st to end	Day	28% 12% 27% 19% 13% 1	220
February	1st to 10th	Day	29% 17% 29% 18% 7% 1	200
	11th to 20th	Day	33% 15% 23% 21% 8% 1	461
	21st to end	Day	28% 15% 25% 22% 10% 1	414
March	1st to 10th	Day	24% 19% 25% 20% 9% 3	500
	11th to 20th	Day	22% 13% 22% 24% 15% 3	500
	21st to end	Day	28% 19% 23% 21% 9% 1	550
April	1st to 10th	Day	22% 15% 29% 22% 9% 4	500
	11th to 20th	Day	18% 18% 28% 19% 12% 4	492
	21st to end	Day	16% 14% 27% 24% 15% 4	500
May	1st to 10th	Day	13% 10% 25% 32% 17% 3	503
	11th to 20th	Day	15% 10% 26% 22% 18% 9%	528
	21st to end	Day	11% 11% 21% 25% 21% 11%	578
June	1st to 10th	Day	16% 9% 19% 28% 21% 8%	730
	11th to 20th	Day	14% 13% 29% 25% 14% 4	730
	21st to end	Day	7% 6% 24% 28% 28% 8%	730
July	1st to 10th	Day	4 3 22% 35% 28% 6%	538
	11th to 20th	Day	3 6% 23% 37% 25% 6%	524
	21st to end	Day	3 5% 19% 35% 29% 9%	575
August	1st to 10th	Day	7% 4 22% 31% 27% 9%	527
	11th to 20th	Day	4 3 20% 32% 27% 14%	517
	21st to end	Day	8% 7% 25% 25% 24% 11%	583
September	1st to 10th	Day	8% 6% 22% 26% 24% 14%	530
	11th to 20th	Day	11% 9% 23% 27% 20% 10%	530
	21st to end	Day	12% 11% 24% 28% 20% 6%	530
October	1st to 10th	Day	13% 9% 26% 27% 19% 5%	530
	11th to 20th	Day	20% 14% 20% 22% 17% 7%	402
	21st to end	Day	25% 13% 23% 23% 12% 3	363
November	1st to 10th	Day	30% 19% 15% 25% 10% 1	330
	11th to 20th	Day	34% 14% 16% 20% 13% 3	291
	21st to end	Day	27% 10% 29% 19% 14% 1	220
December	1st to 10th	Day	28% 13% 19% 18% 14% 8%	204
	11th to 20th	Day	40% 13% 22% 17% 8% 2	200
	21st to end	Day	35% 10% 23% 20% 10% 3	220
January	1st to 10th	Night	36% 11% 17% 23% 11% 2	800
	11th to 20th	Night	28% 9% 18% 29% 14% 3	800
	21st to end	Night	24% 7% 19% 31% 18% 1	880
February	1st to 10th	Night	29% 11% 22% 27% 10% 2	800
	11th to 20th	Night	26% 10% 18% 30% 14% 2	784
	21st to end	Night	23% 11% 20% 28% 16% 3	660
March	1st to 10th	Night	21% 10% 19% 32% 18% 1	800
	11th to 20th	Night	20% 12% 19% 30% 19% 1	800
	21st to end	Night	21% 8% 24% 33% 12% 2	880
April	1st to 10th	Night	18% 10% 20% 33% 18% 2	800
	11th to 20th	Night	15% 10% 25% 33% 14% 2	715
	21st to end	Night	14% 7% 24% 34% 19% 2	600
May	1st to 10th	Night	10% 7% 23% 33% 25% 3	485
	11th to 20th	Night	13% 6% 15% 31% 28% 8%	394
	21st to end	Night	14% 6% 15% 31% 27% 6%	433
June	1st to 10th	Night	15% 5% 16% 34% 26% 6%	400
	11th to 20th	Night	12% 8% 23% 34% 21% 3	400
	21st to end	Night	10% 4 15% 38% 31% 5%	400
July	1st to 10th	Night	2 2 13% 41% 38% 4	390
	11th to 20th	Night	4 3 13% 47% 31% 1	388
	21st to end	Night	1 1 13% 40% 38% 6%	438
August	1st to 10th	Night	4 3 10% 40% 38% 6%	398
	11th to 20th	Night	3 2 9% 37% 40% 9%	535
	21st to end	Night	5% 5% 13% 37% 36% 4	811
September	1st to 10th	Night	5% 3 13% 39% 32% 7%	800
	11th to 20th	Night	9% 4 15% 31% 33% 9%	800
	21st to end	Night	11% 5% 19% 33% 27% 5%	800
October	1st to 10th	Night	14% 6% 14% 38% 25% 3	800
	11th to 20th	Night	17% 8% 18% 30% 24% 4	800
	21st to end	Night	21% 8% 21% 31% 15% 4	880
November	1st to 10th	Night	29% 9% 16% 33% 13%	820
	11th to 20th	Night	30% 10% 19% 28% 13% 1	840
	21st to end	Night	28% 9% 17% 28% 16% 3	961
December	1st to 10th	Night	25% 8% 18% 32% 17% 1	1011
	11th to 20th	Night	34% 9% 17% 29% 11%	1000
	21st to end	Night	36% 11% 17% 26% 9% 1	1035

Page generated at 2025-11-29 22:00:54. Version ac07

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