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 2024-11-30.

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% 8% 3 8% 6% 5% 5% 4 6% 7% 42%	105518
	Night	4 7% 3 8% 6% 5% 5% 5% 7% 8% 42%	102750
Transparency	Day	38% 7% 12% 22% 16% 5%	105253
	Night	35% 8% 15% 24% 14% 4	102605
Seeing	Day	12% 16% 31% 26% 13% 3	15383
	Night	9% 10% 29% 37% 14% 1	24802

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 8% 5% 13% 8% 7% 9% 7% 9% 10% 25%	2258
	11th to 20th	Day	1 6% 3 15% 10% 9% 7% 4 6% 9% 31%	2540
	21st to end	Day	1 7% 3 10% 6% 6% 6% 5% 8% 10% 38%	2886
February	1st to 10th	Day	3 11% 5% 13% 8% 8% 7% 5% 7% 7% 26%	1710
	11th to 20th	Day	1 8% 6% 14% 9% 7% 6% 6% 7% 8% 28%	1665
	21st to end	Day	2 11% 4 17% 9% 7% 7% 6% 6% 6% 25%	1560
March	1st to 10th	Day	1 7% 4 10% 6% 7% 6% 4 7% 8% 39%	3231
	11th to 20th	Day	3 8% 3 9% 6% 5% 5% 4 6% 9% 42%	3490
	21st to end	Day	3 15% 7% 14% 8% 8% 7% 6% 8% 7% 19%	3825
April	1st to 10th	Day	5% 14% 5% 12% 7% 6% 5% 4 8% 8% 25%	2232
	11th to 20th	Day	7% 13% 5% 11% 7% 5% 5% 4 5% 7% 30%	2437
	21st to end	Day	10% 14% 5% 10% 6% 5% 6% 4 5% 7% 28%	2470
May	1st to 10th	Day	10% 14% 5% 10% 6% 7% 6% 5% 6% 8% 25%	4110
	11th to 20th	Day	7% 8% 3 8% 6% 6% 6% 5% 7% 9% 36%	4211
	21st to end	Day	11% 11% 4 8% 7% 6% 6% 5% 7% 8% 28%	5087
June	1st to 10th	Day	13% 9% 3 8% 7% 6% 6% 5% 7% 8% 29%	2850
	11th to 20th	Day	12% 9% 3 8% 6% 5% 6% 4 7% 7% 33%	2850
	21st to end	Day	5% 5% 2 6% 4 5% 5% 5% 7% 8% 48%	2850
July	1st to 10th	Day	3 2 1 3 3 4 4 4 6% 7% 64%	4740
	11th to 20th	Day	3 2 1 3 3 4 5% 3 5% 6% 64%	4618
	21st to end	Day	4 3 1 4 3 4 4 4 6% 8% 59%	4730
August	1st to 10th	Day	4 4 2 6% 5% 6% 6% 5% 7% 7% 50%	4092
	11th to 20th	Day	3 2 1 4 4 4 5% 6% 7% 7% 56%	4028
	21st to end	Day	3 3 1 4 4 4 5% 4 6% 6% 60%	4521
September	1st to 10th	Day	6% 4 2 5% 4 4 3 3 4 7% 60%	2260
	11th to 20th	Day	8% 7% 2 6% 4 4 5% 3 6% 7% 48%	2090
	21st to end	Day	8% 7% 2 6% 4 3 3 2 4 7% 52%	2090
October	1st to 10th	Day	7% 8% 2 6% 4 3 3 2 3 5% 57%	3313
	11th to 20th	Day	5% 8% 2 6% 3 3 3 3 4 6% 56%	3150
	21st to end	Day	3 11% 4 10% 5% 5% 5% 5% 6% 7% 40%	3136
November	1st to 10th	Day	5% 12% 4 10% 6% 5% 5% 5% 6% 7% 34%	1754
	11th to 20th	Day	3 10% 4 11% 6% 7% 6% 6% 7% 8% 33%	1600
	21st to end	Day	2 9% 4 9% 6% 6% 6% 4 5% 8% 39%	1596
December	1st to 10th	Day	2 7% 3 11% 8% 8% 7% 5% 7% 9% 34%	1411
	11th to 20th	Day	2 7% 4 10% 8% 7% 6% 6% 9% 10% 31%	1685
	21st to end	Day	1 7% 4 10% 6% 6% 6% 5% 7% 7% 42%	2442
January	1st to 10th	Night	1 5% 3 13% 8% 8% 7% 6% 9% 10% 30%	4717
	11th to 20th	Night	1 8% 3 13% 9% 9% 7% 5% 8% 10% 26%	4420
	21st to end	Night	1 6% 4 10% 6% 7% 7% 5% 8% 9% 37%	4771
February	1st to 10th	Night	2 11% 5% 14% 7% 8% 7% 6% 7% 8% 26%	2470
	11th to 20th	Night	1 8% 6% 12% 8% 7% 7% 5% 7% 9% 30%	2402
	21st to end	Night	1 9% 5% 12% 7% 8% 7% 6% 7% 8% 30%	1891
March	1st to 10th	Night	2 6% 3 8% 6% 6% 6% 5% 7% 9% 43%	3643
	11th to 20th	Night	2 7% 3 7% 5% 5% 5% 4 7% 8% 47%	3490
	21st to end	Night	3 13% 5% 12% 8% 8% 7% 5% 7% 8% 23%	3828
April	1st to 10th	Night	4 11% 4 8% 7% 6% 6% 5% 7% 8% 34%	1944
	11th to 20th	Night	6% 11% 5% 9% 7% 5% 6% 5% 6% 7% 32%	1691
	21st to end	Night	9% 13% 3 9% 5% 5% 4 4 6% 8% 34%	1710
May	1st to 10th	Night	10% 11% 4 10% 7% 5% 6% 5% 5% 7% 30%	2860
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 47%	2690
	21st to end	Night	12% 9% 4 7% 5% 5% 5% 5% 6% 8% 33%	2486
June	1st to 10th	Night	10% 8% 3 9% 6% 6% 5% 4 7% 7% 35%	1330
	11th to 20th	Night	11% 5% 3 7% 5% 5% 4 4 7% 6% 42%	1330
	21st to end	Night	6% 2 1 5% 3 4 4 4 5% 7% 57%	1330
July	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 6% 68%	2220
	11th to 20th	Night	3 2 1 3 2 3 3 3 5% 7% 67%	2338
	21st to end	Night	2 2 1 3 2 3 4 4 6% 6% 66%	2823
August	1st to 10th	Night	3 3 1 5% 4 4 5% 4 8% 9% 54%	2847
	11th to 20th	Night	3 2 1 4 3 4 4 4 7% 8% 60%	2811
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 6% 62%	3146
September	1st to 10th	Night	6% 4 2 5% 4 4 4 4 4 7% 58%	1917
	11th to 20th	Night	8% 6% 2 5% 3 4 4 4 6% 8% 50%	2090
	21st to end	Night	8% 6% 2 5% 3 3 4 3 5% 8% 53%	2090
October	1st to 10th	Night	5% 6% 2 6% 3 3 3 3 4 6% 60%	3660
	11th to 20th	Night	6% 7% 3 7% 4 3 4 3 4 6% 52%	3778
	21st to end	Night	4 9% 3 8% 5% 5% 4 4 6% 8% 45%	4418
November	1st to 10th	Night	5% 10% 4 9% 6% 6% 6% 5% 7% 7% 35%	2535
	11th to 20th	Night	3 9% 4 9% 7% 5% 6% 6% 8% 8% 35%	2789
	21st to end	Night	2 8% 3 7% 6% 6% 6% 5% 7% 9% 41%	2800
December	1st to 10th	Night	1 6% 3 10% 8% 7% 6% 6% 8% 9% 37%	2765
	11th to 20th	Night	2 8% 3 9% 7% 6% 6% 5% 7% 10% 36%	3506
	21st to end	Night	1 8% 4 9% 6% 6% 5% 5% 8% 9% 39%	5214

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	44% 11% 17% 22% 6%	2258
	11th to 20th	Day	42% 16% 15% 13% 11% 2	2540
	21st to end	Day	37% 10% 23% 22% 7% 1	2886
February	1st to 10th	Day	54% 8% 15% 17% 6% 1	1710
	11th to 20th	Day	51% 8% 17% 16% 7% 1	1674
	21st to end	Day	54% 9% 10% 15% 9% 2	1560
March	1st to 10th	Day	38% 9% 13% 23% 17%	3231
	11th to 20th	Day	36% 9% 18% 31% 6%	3490
	21st to end	Day	57% 12% 14% 14% 3	3825
April	1st to 10th	Day	53% 8% 13% 19% 6%	2232
	11th to 20th	Day	53% 6% 12% 19% 9%	2440
	21st to end	Day	54% 6% 12% 20% 8%	2470
May	1st to 10th	Day	55% 8% 8% 22% 6%	4110
	11th to 20th	Day	40% 8% 13% 24% 13% 3	4215
	21st to end	Day	48% 9% 14% 19% 9% 1	5087
June	1st to 10th	Day	51% 9% 14% 18% 7% 1	2850
	11th to 20th	Day	48% 7% 10% 17% 13% 5%	2850
	21st to end	Day	32% 6% 10% 24% 23% 3	2850
July	1st to 10th	Day	19% 6% 6% 18% 35% 15%	4666
	11th to 20th	Day	19% 5% 6% 14% 24% 33%	4537
	21st to end	Day	22% 5% 4 17% 33% 18%	4581
August	1st to 10th	Day	31% 6% 6% 22% 23% 12%	4078
	11th to 20th	Day	23% 7% 8% 30% 25% 6%	4067
	21st to end	Day	24% 5% 7% 27% 28% 9%	4520
September	1st to 10th	Day	25% 4 6% 23% 29% 13%	2260
	11th to 20th	Day	35% 5% 9% 24% 21% 6%	2090
	21st to end	Day	33% 4 11% 29% 18% 5%	2090
October	1st to 10th	Day	32% 3 7% 31% 25% 1	3313
	11th to 20th	Day	31% 4 17% 26% 22% 1	3150
	21st to end	Day	41% 8% 9% 27% 15% 1	3136
November	1st to 10th	Day	45% 7% 13% 23% 11% 2	1754
	11th to 20th	Day	44% 6% 20% 21% 9% 1	1600
	21st to end	Day	40% 7% 16% 24% 12% 1	1596
December	1st to 10th	Day	43% 7% 20% 20% 8% 2	1411
	11th to 20th	Day	41% 9% 25% 21% 4 1	1684
	21st to end	Day	37% 6% 13% 26% 10% 7%	2442
January	1st to 10th	Night	37% 11% 17% 27% 8%	4717
	11th to 20th	Night	44% 10% 16% 19% 9% 1	4420
	21st to end	Night	35% 10% 23% 23% 8% 1	4771
February	1st to 10th	Night	51% 7% 16% 18% 7%	2470
	11th to 20th	Night	46% 6% 21% 18% 8%	2403
	21st to end	Night	44% 10% 14% 20% 10% 2	1891
March	1st to 10th	Night	32% 10% 12% 23% 22% 2	3643
	11th to 20th	Night	30% 10% 22% 30% 8%	3490
	21st to end	Night	50% 13% 16% 18% 3	3828
April	1st to 10th	Night	45% 7% 16% 22% 9% 1	1944
	11th to 20th	Night	49% 8% 12% 21% 9% 1	1691
	21st to end	Night	47% 7% 17% 23% 6%	1710
May	1st to 10th	Night	50% 9% 13% 21% 8%	2860
	11th to 20th	Night	33% 8% 13% 29% 15% 2	2691
	21st to end	Night	43% 10% 17% 21% 8%	2486
June	1st to 10th	Night	46% 9% 15% 21% 8%	1330
	11th to 20th	Night	39% 8% 15% 21% 13% 4	1330
	21st to end	Night	25% 5% 14% 30% 23% 2	1330
July	1st to 10th	Night	17% 5% 8% 25% 30% 15%	2185
	11th to 20th	Night	16% 5% 7% 17% 22% 34%	2296
	21st to end	Night	16% 5% 6% 23% 33% 17%	2734
August	1st to 10th	Night	25% 7% 7% 25% 25% 11%	2838
	11th to 20th	Night	20% 6% 9% 32% 27% 6%	2838
	21st to end	Night	23% 6% 7% 34% 26% 5%	3146
September	1st to 10th	Night	26% 5% 8% 24% 25% 11%	1917
	11th to 20th	Night	31% 6% 11% 27% 20% 5%	2090
	21st to end	Night	31% 4 12% 27% 22% 4	2090
October	1st to 10th	Night	27% 4 8% 32% 26% 4	3660
	11th to 20th	Night	34% 5% 16% 25% 19% 2	3778
	21st to end	Night	36% 7% 12% 23% 20% 1	4418
November	1st to 10th	Night	42% 8% 13% 25% 10% 2	2535
	11th to 20th	Night	38% 9% 20% 24% 8% 1	2789
	21st to end	Night	36% 7% 18% 24% 12% 3	2800
December	1st to 10th	Night	38% 9% 20% 22% 8% 3	2765
	11th to 20th	Night	37% 8% 24% 24% 5% 1	3507
	21st to end	Night	35% 7% 15% 25% 10% 8%	5214

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	9% 21% 25% 27% 16% 2	180
	11th to 20th	Day	6% 16% 37% 28% 8% 5%	180
	21st to end	Day	8% 15% 36% 30% 10% 1	223
February	1st to 10th	Day	12% 24% 44% 17% 3	270
	11th to 20th	Day	6% 23% 39% 26% 5% 1	270
	21st to end	Day	10% 21% 42% 19% 7%	372
March	1st to 10th	Day	9% 23% 41% 21% 5%	460
	11th to 20th	Day	13% 19% 30% 27% 11% 1	460
	21st to end	Day	11% 24% 38% 20% 7% 1	506
April	1st to 10th	Day	17% 23% 36% 20% 5%	460
	11th to 20th	Day	15% 25% 33% 20% 6% 1	452
	21st to end	Day	21% 19% 31% 20% 8% 1	460
May	1st to 10th	Day	13% 18% 31% 25% 12% 2	463
	11th to 20th	Day	20% 15% 28% 20% 14% 3	563
	21st to end	Day	21% 12% 21% 24% 18% 4	734
June	1st to 10th	Day	13% 14% 25% 27% 17% 4	680
	11th to 20th	Day	19% 18% 27% 24% 11% 2	680
	21st to end	Day	9% 10% 26% 33% 19% 3	680
July	1st to 10th	Day	4 9% 25% 39% 19% 4	671
	11th to 20th	Day	7% 12% 27% 34% 19% 2	598
	21st to end	Day	5% 9% 31% 32% 20% 3	531
August	1st to 10th	Day	9% 8% 30% 31% 17% 4	487
	11th to 20th	Day	6% 8% 28% 34% 19% 6%	477
	21st to end	Day	6% 12% 31% 30% 16% 5%	539
September	1st to 10th	Day	8% 10% 32% 27% 18% 6%	490
	11th to 20th	Day	12% 11% 32% 27% 13% 4	490
	21st to end	Day	16% 16% 27% 24% 14% 4	490
October	1st to 10th	Day	17% 12% 30% 24% 14% 4	391
	11th to 20th	Day	11% 16% 27% 23% 14% 8%	300
	21st to end	Day	9% 22% 35% 25% 7% 2	330
November	1st to 10th	Day	12% 20% 31% 26% 9% 2	300
	11th to 20th	Day	10% 20% 34% 27% 8% 1	300
	21st to end	Day	7% 17% 33% 34% 9%	299
December	1st to 10th	Day	8% 20% 37% 22% 11% 2	219
	11th to 20th	Day	13% 21% 33% 24% 8%	180
	21st to end	Day	8% 18% 40% 24% 8% 3	198
January	1st to 10th	Night	6% 14% 34% 38% 8% 1	941
	11th to 20th	Night	6% 15% 36% 31% 11% 2	760
	21st to end	Night	4 12% 35% 39% 9% 1	836
February	1st to 10th	Night	7% 15% 38% 29% 9% 1	760
	11th to 20th	Night	6% 13% 37% 33% 8% 2	744
	21st to end	Night	8% 12% 36% 33% 10% 1	628
March	1st to 10th	Night	6% 10% 38% 37% 9% 1	760
	11th to 20th	Night	8% 13% 28% 38% 13%	760
	21st to end	Night	8% 11% 34% 35% 10% 1	836
April	1st to 10th	Night	11% 11% 31% 34% 13% 1	712
	11th to 20th	Night	12% 13% 34% 32% 10%	563
	21st to end	Night	16% 12% 28% 31% 12% 1	570
May	1st to 10th	Night	11% 14% 26% 32% 17% 1	570
	11th to 20th	Night	16% 10% 20% 32% 20% 2	561
	21st to end	Night	19% 8% 17% 32% 23% 2	458
June	1st to 10th	Night	14% 6% 19% 38% 21% 2	380
	11th to 20th	Night	11% 9% 28% 38% 12% 1	380
	21st to end	Night	6% 3 18% 51% 22% 1	380
July	1st to 10th	Night	5% 2 13% 52% 26% 2	370
	11th to 20th	Night	4 3 14% 56% 23%	368
	21st to end	Night	4 2 13% 51% 29% 1	454
August	1st to 10th	Night	6% 2 17% 50% 24% 1	567
	11th to 20th	Night	4 3 16% 51% 24% 1	564
	21st to end	Night	8% 5% 17% 43% 26% 1	627
September	1st to 10th	Night	7% 3 19% 44% 24% 3	663
	11th to 20th	Night	11% 4 13% 42% 26% 4	760
	21st to end	Night	12% 6% 22% 35% 21% 4	760
October	1st to 10th	Night	13% 6% 23% 38% 18% 2	760
	11th to 20th	Night	11% 8% 25% 36% 17% 3	760
	21st to end	Night	11% 11% 32% 33% 11% 3	836
November	1st to 10th	Night	12% 10% 31% 36% 10% 1	780
	11th to 20th	Night	10% 15% 35% 32% 7%	989
	21st to end	Night	6% 13% 34% 37% 10% 1	1000
December	1st to 10th	Night	5% 15% 36% 32% 10% 1	950
	11th to 20th	Night	7% 11% 40% 32% 9% 1	950
	21st to end	Night	7% 14% 40% 29% 8% 1	1045

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