Butte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Butte clear sky chart from 2005-11-06 to 2026-02-16.

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	5% 7% 3 8% 5% 5% 5% 4 6% 8% 43%	139423
	Night	4 7% 3 8% 5% 5% 5% 4 7% 8% 43%	138784
Transparency	Day	37% 8% 12% 22% 16% 5%	134112
	Night	35% 8% 15% 25% 14% 4	135101
Seeing	Day	11% 15% 31% 27% 13% 3	16888
	Night	8% 10% 29% 37% 14% 1	26451

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	1 9% 5% 15% 7% 7% 9% 6% 8% 9% 22%	4219
	11th to 20th	Day	4 2 10% 9% 6% 5% 5% 6% 9% 44%	4740
	21st to end	Day	1 5% 2 9% 6% 6% 6% 5% 7% 9% 44%	5363
February	1st to 10th	Day	2 11% 5% 13% 8% 8% 7% 6% 7% 7% 26%	1890
	11th to 20th	Day	1 8% 6% 14% 8% 7% 6% 6% 7% 8% 29%	1800
	21st to end	Day	2 10% 4 16% 9% 7% 7% 6% 6% 6% 27%	1640
March	1st to 10th	Day	1 9% 4 8% 5% 6% 5% 4 7% 10% 42%	4650
	11th to 20th	Day	2 10% 4 10% 7% 6% 5% 5% 6% 8% 36%	5000
	21st to end	Day	5% 14% 6% 13% 8% 7% 7% 5% 7% 7% 20%	5486
April	1st to 10th	Day	5% 13% 5% 12% 7% 6% 5% 4 8% 8% 26%	2350
	11th to 20th	Day	7% 13% 5% 11% 7% 5% 5% 4 5% 7% 30%	2567
	21st to end	Day	10% 14% 5% 10% 6% 5% 5% 4 5% 7% 29%	2600
May	1st to 10th	Day	9% 10% 3 8% 5% 5% 5% 5% 6% 8% 35%	5880
	11th to 20th	Day	10% 9% 3 8% 5% 6% 7% 5% 7% 9% 30%	6037
	21st to end	Day	8% 8% 3 7% 5% 5% 5% 4 5% 7% 42%	7304
June	1st to 10th	Day	13% 8% 3 8% 7% 6% 6% 5% 7% 7% 30%	3000
	11th to 20th	Day	11% 9% 3 8% 6% 5% 6% 4 7% 7% 34%	3000
	21st to end	Day	5% 5% 2 6% 5% 5% 5% 5% 7% 8% 48%	3000
July	1st to 10th	Day	3 2 1 3 4 4 4 4 6% 7% 61%	6779
	11th to 20th	Day	6% 3 1 3 2 4 4 3 4 5% 65%	6602
	21st to end	Day	4 3 1 4 4 5% 5% 5% 7% 9% 54%	6794
August	1st to 10th	Day	3 3 1 5% 5% 6% 6% 5% 8% 8% 49%	5779
	11th to 20th	Day	3 2 1 4 3 4 5% 5% 7% 7% 61%	5798
	21st to end	Day	5% 4 1 4 4 4 4 3 6% 6% 58%	6468
September	1st to 10th	Day	6% 3 2 5% 4 4 3 3 4 7% 60%	2379
	11th to 20th	Day	7% 7% 2 6% 5% 4 5% 3 6% 7% 48%	2200
	21st to end	Day	8% 7% 2 6% 4 4 3 2 4 7% 54%	2200
October	1st to 10th	Day	7% 8% 2 6% 4 3 3 2 3 5% 57%	3418
	11th to 20th	Day	5% 8% 3 7% 4 3 3 3 4 6% 55%	3250
	21st to end	Day	3 11% 4 9% 5% 5% 5% 4 6% 7% 41%	3236
November	1st to 10th	Day	5% 12% 4 9% 6% 5% 5% 5% 7% 7% 35%	1843
	11th to 20th	Day	3 9% 4 10% 6% 6% 6% 6% 8% 8% 33%	1680
	21st to end	Day	2 9% 4 9% 6% 6% 6% 4 5% 8% 39%	1680
December	1st to 10th	Day	1 5% 3 9% 6% 6% 6% 4 5% 7% 48%	2501
	11th to 20th	Day	2 8% 4 10% 8% 8% 6% 5% 8% 9% 31%	2715
	21st to end	Day	2 10% 5% 10% 6% 6% 6% 5% 6% 7% 37%	3575
January	1st to 10th	Night	1 9% 4 13% 8% 7% 7% 6% 8% 9% 27%	8731
	11th to 20th	Night	1 5% 3 9% 7% 7% 6% 5% 7% 10% 42%	8220
	21st to end	Night	1 4 3 9% 6% 6% 6% 5% 7% 8% 46%	8825
February	1st to 10th	Night	2 11% 5% 14% 7% 8% 7% 5% 7% 8% 26%	2730
	11th to 20th	Night	1 9% 6% 12% 8% 7% 7% 5% 7% 9% 30%	2597
	21st to end	Night	1 9% 5% 12% 7% 7% 7% 6% 7% 8% 31%	1987
March	1st to 10th	Night	1 6% 3 8% 5% 6% 6% 5% 8% 10% 43%	5244
	11th to 20th	Night	2 9% 4 10% 6% 6% 6% 4 7% 7% 39%	5000
	21st to end	Night	5% 13% 6% 12% 8% 7% 6% 5% 7% 8% 24%	5489
April	1st to 10th	Night	4 11% 4 8% 7% 6% 6% 4 7% 8% 35%	2046
	11th to 20th	Night	6% 11% 5% 9% 7% 5% 6% 5% 6% 7% 33%	1781
	21st to end	Night	9% 12% 3 9% 5% 5% 4 4 6% 8% 35%	1800
May	1st to 10th	Night	9% 8% 3 7% 6% 4 5% 4 5% 7% 43%	4100
	11th to 20th	Night	11% 8% 2 6% 4 5% 5% 4 5% 7% 41%	3874
	21st to end	Night	9% 7% 3 6% 4 5% 4 4 6% 7% 45%	3570
June	1st to 10th	Night	10% 8% 3 8% 6% 5% 5% 5% 6% 7% 38%	1400
	11th to 20th	Night	10% 5% 3 7% 5% 5% 4 4 7% 6% 42%	1400
	21st to end	Night	6% 3 1 5% 3 4 4 4 5% 7% 57%	1400
July	1st to 10th	Night	3 2 1 3 3 3 4 3 5% 6% 67%	3180
	11th to 20th	Night	5% 2 1 2 2 3 3 3 5% 6% 68%	3354
	21st to end	Night	2 2 1 4 3 5% 6% 5% 7% 8% 57%	4060
August	1st to 10th	Night	3 3 1 5% 5% 4 5% 4 8% 10% 53%	4033
	11th to 20th	Night	2 2 1 3 2 4 4 3 6% 7% 67%	4051
	21st to end	Night	6% 3 2 5% 3 4 4 4 6% 6% 57%	4509
September	1st to 10th	Night	6% 4 2 5% 4 3 4 4 5% 7% 58%	2018
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 8% 50%	2200
	21st to end	Night	8% 6% 2 5% 3 3 4 3 5% 7% 54%	2200
October	1st to 10th	Night	5% 6% 2 6% 3 4 3 3 4 6% 60%	3775
	11th to 20th	Night	6% 8% 3 7% 4 3 4 3 4 7% 52%	3898
	21st to end	Night	4 8% 3 8% 5% 5% 4 4 6% 8% 45%	4560
November	1st to 10th	Night	5% 10% 4 9% 6% 6% 5% 5% 8% 7% 35%	2665
	11th to 20th	Night	3 9% 4 9% 6% 5% 6% 6% 8% 9% 35%	2929
	21st to end	Night	2 8% 3 7% 6% 5% 6% 5% 7% 9% 41%	2940
December	1st to 10th	Night	2 6% 3 8% 6% 5% 5% 5% 6% 9% 46%	4899
	11th to 20th	Night	2 9% 3 9% 7% 6% 7% 6% 8% 10% 34%	5696
	21st to end	Night	2 9% 4 10% 6% 6% 6% 5% 8% 9% 34%	7623

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	48% 12% 17% 19% 5%	4220
	11th to 20th	Day	32% 13% 21% 19% 12% 4	4740
	21st to end	Day	34% 9% 18% 25% 13% 2	5363
February	1st to 10th	Day	53% 8% 15% 17% 6% 1	1890
	11th to 20th	Day	50% 8% 17% 16% 9% 1	1809
	21st to end	Day	53% 9% 10% 16% 10% 2	1640
March	1st to 10th	Day	36% 8% 12% 21% 17% 6%	4650
	11th to 20th	Day	43% 9% 13% 24% 9%	5000
	21st to end	Day	57% 12% 12% 16% 2	5486
April	1st to 10th	Day	52% 9% 13% 19% 7%	2350
	11th to 20th	Day	52% 7% 12% 19% 10%	2570
	21st to end	Day	53% 7% 12% 20% 8%	2600
May	1st to 10th	Day	44% 9% 10% 25% 11% 1	5880
	11th to 20th	Day	46% 9% 12% 21% 10% 2	6041
	21st to end	Day	38% 8% 14% 26% 13% 1	7305
June	1st to 10th	Day	50% 9% 14% 18% 8% 1	3000
	11th to 20th	Day	47% 7% 11% 17% 14% 5%	3000
	21st to end	Day	32% 6% 10% 24% 23% 3	3000
July	1st to 10th	Day	21% 6% 6% 23% 30% 14%	6706
	11th to 20th	Day	21% 4 4 20% 26% 25%	6521
	21st to end	Day	24% 7% 9% 19% 29% 13%	6597
August	1st to 10th	Day	31% 6% 7% 22% 22% 12%	4208
	11th to 20th	Day	23% 7% 8% 30% 26% 6%	4197
	21st to end	Day	24% 5% 7% 27% 28% 9%	4663
September	1st to 10th	Day	25% 4 7% 24% 28% 12%	2379
	11th to 20th	Day	35% 5% 9% 24% 21% 5%	2200
	21st to end	Day	32% 3 11% 29% 19% 5%	2200
October	1st to 10th	Day	32% 3 7% 31% 25% 1	3418
	11th to 20th	Day	32% 4 16% 26% 21% 1	3250
	21st to end	Day	40% 8% 9% 28% 15% 1	3236
November	1st to 10th	Day	44% 7% 14% 22% 11% 2	1843
	11th to 20th	Day	43% 7% 20% 21% 8% 1	1680
	21st to end	Day	40% 7% 17% 24% 11% 1	1680
December	1st to 10th	Day	33% 6% 23% 25% 10% 2	2501
	11th to 20th	Day	43% 10% 22% 21% 4	2714
	21st to end	Day	44% 7% 11% 25% 9% 5%	3575
January	1st to 10th	Night	44% 11% 15% 24% 6%	8732
	11th to 20th	Night	31% 9% 23% 26% 7% 4	8220
	21st to end	Night	29% 9% 19% 25% 15% 3	8825
February	1st to 10th	Night	51% 8% 15% 17% 8%	2730
	11th to 20th	Night	46% 7% 19% 18% 10% 1	2598
	21st to end	Night	44% 10% 14% 20% 11% 2	1987
March	1st to 10th	Night	32% 8% 12% 24% 20% 5%	5244
	11th to 20th	Night	40% 9% 17% 25% 9%	5000
	21st to end	Night	51% 13% 12% 19% 5%	5489
April	1st to 10th	Night	44% 7% 15% 23% 10% 1	2046
	11th to 20th	Night	48% 8% 12% 22% 9% 1	1781
	21st to end	Night	46% 7% 17% 23% 6%	1800
May	1st to 10th	Night	40% 7% 11% 31% 11%	4097
	11th to 20th	Night	39% 7% 12% 28% 12% 2	3875
	21st to end	Night	35% 8% 15% 29% 12% 1	3570
June	1st to 10th	Night	44% 9% 15% 22% 9%	1400
	11th to 20th	Night	39% 8% 15% 20% 14% 4	1400
	21st to end	Night	26% 5% 14% 30% 23% 2	1400
July	1st to 10th	Night	18% 5% 6% 32% 25% 13%	3145
	11th to 20th	Night	17% 4 6% 23% 25% 25%	3312
	21st to end	Night	23% 7% 9% 22% 27% 12%	3945
August	1st to 10th	Night	25% 7% 8% 25% 24% 11%	2928
	11th to 20th	Night	20% 6% 9% 32% 27% 6%	2928
	21st to end	Night	23% 6% 7% 34% 26% 5%	3245
September	1st to 10th	Night	26% 5% 8% 25% 24% 11%	2018
	11th to 20th	Night	30% 6% 11% 28% 20% 5%	2200
	21st to end	Night	30% 4 11% 26% 24% 4	2200
October	1st to 10th	Night	27% 4 8% 32% 26% 3	3775
	11th to 20th	Night	34% 5% 15% 25% 19% 2	3898
	21st to end	Night	35% 7% 13% 23% 20% 1	4560
November	1st to 10th	Night	41% 9% 13% 25% 10% 2	2665
	11th to 20th	Night	38% 9% 20% 23% 7% 1	2929
	21st to end	Night	36% 6% 19% 25% 11% 2	2940
December	1st to 10th	Night	32% 8% 24% 25% 9% 2	4899
	11th to 20th	Night	40% 10% 24% 22% 4 1	5697
	21st to end	Night	40% 8% 15% 23% 9% 5%	7623

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	10% 21% 25% 28% 14% 2	220
	11th to 20th	Day	5% 15% 39% 29% 8% 4	220
	21st to end	Day	7% 13% 37% 32% 10% 1	273
February	1st to 10th	Day	11% 24% 42% 19% 4 1	330
	11th to 20th	Day	6% 20% 37% 28% 8% 1	315
	21st to end	Day	10% 22% 42% 20% 6%	404
March	1st to 10th	Day	9% 23% 41% 21% 6%	500
	11th to 20th	Day	14% 19% 29% 26% 10% 1	500
	21st to end	Day	13% 24% 37% 19% 7% 1	550
April	1st to 10th	Day	16% 22% 35% 21% 6%	500
	11th to 20th	Day	15% 24% 33% 20% 6% 2	492
	21st to end	Day	20% 19% 32% 21% 8% 1	500
May	1st to 10th	Day	13% 17% 30% 25% 13% 2	503
	11th to 20th	Day	20% 15% 28% 20% 14% 2	607
	21st to end	Day	20% 12% 21% 25% 18% 5%	789
June	1st to 10th	Day	12% 14% 25% 27% 17% 4	730
	11th to 20th	Day	18% 17% 28% 25% 11% 2	730
	21st to end	Day	9% 10% 27% 33% 19% 3	730
July	1st to 10th	Day	4 9% 26% 38% 19% 3	721
	11th to 20th	Day	7% 11% 27% 34% 19% 2	644
	21st to end	Day	5% 9% 30% 32% 21% 4	575
August	1st to 10th	Day	8% 8% 30% 32% 17% 4	527
	11th to 20th	Day	5% 8% 28% 34% 19% 6%	517
	21st to end	Day	7% 11% 30% 30% 16% 6%	583
September	1st to 10th	Day	8% 9% 30% 28% 18% 7%	530
	11th to 20th	Day	12% 11% 30% 28% 15% 5%	530
	21st to end	Day	15% 16% 25% 25% 15% 5%	530
October	1st to 10th	Day	16% 13% 30% 23% 13% 5%	426
	11th to 20th	Day	13% 16% 28% 23% 14% 7%	330
	21st to end	Day	9% 20% 34% 26% 8% 2	363
November	1st to 10th	Day	14% 20% 31% 25% 9% 2	330
	11th to 20th	Day	10% 20% 33% 26% 10% 1	330
	21st to end	Day	7% 18% 32% 35% 8%	330
December	1st to 10th	Day	8% 19% 35% 23% 13% 2	267
	11th to 20th	Day	18% 20% 30% 24% 8%	220
	21st to end	Day	9% 18% 39% 24% 8% 2	242
January	1st to 10th	Night	6% 14% 34% 38% 7% 1	1039
	11th to 20th	Night	5% 14% 37% 32% 10% 2	840
	21st to end	Night	4 11% 35% 40% 9% 1	924
February	1st to 10th	Night	7% 15% 38% 30% 9% 1	840
	11th to 20th	Night	6% 13% 37% 35% 8% 2	804
	21st to end	Night	8% 12% 36% 33% 10% 1	660
March	1st to 10th	Night	6% 10% 37% 37% 10% 1	800
	11th to 20th	Night	8% 13% 29% 37% 13%	800
	21st to end	Night	8% 11% 34% 35% 10% 1	880
April	1st to 10th	Night	10% 10% 31% 34% 13% 1	749
	11th to 20th	Night	11% 13% 34% 32% 9%	593
	21st to end	Night	16% 12% 28% 32% 12% 1	600
May	1st to 10th	Night	11% 14% 25% 32% 18% 1	600
	11th to 20th	Night	16% 10% 21% 32% 19% 2	591
	21st to end	Night	18% 8% 17% 32% 24% 1	482
June	1st to 10th	Night	14% 6% 19% 38% 22% 2	400
	11th to 20th	Night	11% 9% 28% 40% 12% 1	400
	21st to end	Night	6% 4 18% 51% 22% 1	400
July	1st to 10th	Night	5% 2 13% 52% 26% 2	390
	11th to 20th	Night	4 3 14% 56% 23%	388
	21st to end	Night	4 2 12% 52% 29% 1	478
August	1st to 10th	Night	6% 2 17% 50% 24% 1	597
	11th to 20th	Night	4 3 16% 52% 24% 1	594
	21st to end	Night	8% 5% 17% 43% 25% 1	660
September	1st to 10th	Night	6% 3 18% 44% 24% 3	698
	11th to 20th	Night	10% 4 13% 42% 28% 4	800
	21st to end	Night	12% 6% 22% 35% 21% 4	800
October	1st to 10th	Night	13% 6% 23% 39% 18% 2	800
	11th to 20th	Night	11% 9% 25% 36% 16% 3	800
	21st to end	Night	10% 11% 32% 33% 11% 2	880
November	1st to 10th	Night	12% 10% 32% 36% 10% 1	820
	11th to 20th	Night	10% 15% 34% 32% 8%	1039
	21st to end	Night	6% 13% 33% 38% 10% 1	1050
December	1st to 10th	Night	5% 14% 36% 33% 10% 1	1050
	11th to 20th	Night	8% 12% 40% 32% 8% 1	1050
	21st to end	Night	7% 14% 41% 29% 8% 1	1155

Page generated at 2026-02-12 23:42:14. Version ac07

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