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 2024-01-15.

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	6% 8% 4 8% 7% 4 6% 6% 6% 8% 38%	74657
	Night	5% 7% 4 7% 7% 4 6% 6% 7% 9% 40%	75110
Transparency	Day	43% 7% 13% 21% 13% 4	74352
	Night	38% 7% 16% 23% 12% 3	74937
Seeing	Day	12% 16% 31% 26% 13% 3	14147
	Night	9% 10% 29% 37% 15% 1	23717

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	2 8% 5% 10% 9% 4 8% 8% 8% 10% 29%	1355
	11th to 20th	Day	1 6% 4 10% 9% 6% 6% 5% 6% 9% 38%	1472
	21st to end	Day	2 9% 5% 11% 9% 5% 6% 6% 7% 10% 30%	1634
February	1st to 10th	Day	2 9% 6% 11% 9% 6% 8% 7% 7% 8% 27%	1620
	11th to 20th	Day	1 8% 6% 12% 10% 6% 6% 7% 7% 8% 28%	1575
	21st to end	Day	2 9% 5% 15% 11% 6% 6% 7% 6% 7% 26%	1471
March	1st to 10th	Day	3 9% 6% 10% 9% 6% 8% 5% 8% 8% 29%	1854
	11th to 20th	Day	5% 10% 4 10% 9% 5% 6% 6% 6% 9% 30%	1980
	21st to end	Day	3 13% 6% 12% 9% 5% 7% 6% 6% 7% 25%	2178
April	1st to 10th	Day	6% 12% 6% 11% 8% 5% 6% 6% 8% 8% 24%	2114
	11th to 20th	Day	7% 12% 6% 10% 8% 4 5% 5% 5% 6% 29%	2307
	21st to end	Day	10% 13% 5% 9% 7% 4 5% 6% 5% 7% 28%	2340
May	1st to 10th	Day	9% 10% 5% 8% 8% 5% 6% 6% 7% 8% 29%	2340
	11th to 20th	Day	11% 10% 4 9% 8% 4 6% 6% 7% 7% 27%	2371
	21st to end	Day	16% 11% 5% 8% 9% 5% 6% 6% 5% 8% 20%	2869
June	1st to 10th	Day	14% 8% 3 8% 8% 4 6% 6% 7% 7% 28%	2700
	11th to 20th	Day	12% 8% 4 7% 7% 4 6% 6% 7% 7% 32%	2700
	21st to end	Day	5% 4 3 6% 5% 3 6% 6% 6% 8% 47%	2700
July	1st to 10th	Day	5% 3 2 4 5% 4 6% 7% 7% 8% 49%	2700
	11th to 20th	Day	5% 3 1 3 4 3 5% 5% 5% 7% 59%	2634
	21st to end	Day	4 3 2 5% 5% 4 5% 6% 6% 7% 54%	2705
August	1st to 10th	Day	5% 4 2 4 5% 4 6% 6% 8% 6% 49%	2328
	11th to 20th	Day	4 3 2 3 4 3 5% 6% 7% 7% 57%	2299
	21st to end	Day	5% 4 2 4 5% 3 6% 5% 6% 7% 55%	2574
September	1st to 10th	Day	6% 4 2 4 5% 3 4 4 4 6% 60%	2142
	11th to 20th	Day	7% 6% 3 5% 5% 3 5% 4 5% 7% 49%	1980
	21st to end	Day	9% 7% 2 5% 6% 3 3 3 4 7% 50%	1980
October	1st to 10th	Day	11% 10% 3 7% 7% 3 5% 5% 4 6% 39%	1887
	11th to 20th	Day	7% 11% 4 8% 6% 3 4 4 4 6% 44%	1800
	21st to end	Day	5% 10% 4 9% 7% 4 6% 7% 5% 8% 34%	1805
November	1st to 10th	Day	6% 11% 4 9% 7% 4 5% 6% 6% 7% 34%	1665
	11th to 20th	Day	3 9% 4 9% 8% 5% 6% 7% 8% 8% 34%	1520
	21st to end	Day	3 8% 5% 8% 7% 5% 6% 6% 5% 8% 39%	1520
December	1st to 10th	Day	2 6% 4 10% 8% 7% 7% 7% 6% 9% 34%	1411
	11th to 20th	Day	2 7% 4 8% 9% 5% 7% 7% 9% 11% 31%	1685
	21st to end	Day	2 6% 4 9% 6% 4 7% 6% 7% 8% 42%	2442
January	1st to 10th	Night	1 6% 4 10% 9% 6% 8% 8% 8% 10% 31%	2827
	11th to 20th	Night	2 6% 4 8% 8% 6% 7% 7% 8% 12% 34%	2576
	21st to end	Night	1 6% 5% 10% 8% 5% 8% 6% 8% 10% 33%	2720
February	1st to 10th	Night	2 9% 6% 12% 8% 6% 8% 7% 7% 8% 27%	2340
	11th to 20th	Night	1 8% 6% 11% 10% 5% 7% 6% 6% 9% 31%	2272
	21st to end	Night	2 8% 6% 11% 8% 6% 8% 7% 7% 8% 30%	1782
March	1st to 10th	Night	3 7% 5% 9% 8% 5% 7% 7% 8% 10% 32%	2106
	11th to 20th	Night	3 9% 4 6% 5% 4 7% 6% 7% 10% 36%	1980
	21st to end	Night	4 10% 5% 10% 8% 6% 7% 6% 7% 9% 29%	2178
April	1st to 10th	Night	5% 9% 5% 7% 8% 4 6% 6% 7% 8% 34%	1843
	11th to 20th	Night	7% 11% 6% 8% 7% 4 6% 5% 6% 7% 32%	1601
	21st to end	Night	9% 12% 4 8% 6% 3 5% 5% 5% 8% 35%	1620
May	1st to 10th	Night	7% 7% 3 7% 7% 3 6% 7% 7% 9% 36%	1620
	11th to 20th	Night	11% 8% 3 7% 6% 4 6% 6% 5% 8% 35%	1520
	21st to end	Night	15% 9% 5% 7% 7% 4 4 6% 5% 7% 31%	1402
June	1st to 10th	Night	11% 8% 3 7% 6% 5% 5% 6% 7% 8% 34%	1260
	11th to 20th	Night	12% 4 3 6% 6% 3 5% 6% 7% 7% 41%	1260
	21st to end	Night	6% 2 1 5% 4 3 5% 5% 5% 7% 56%	1260
July	1st to 10th	Night	4 3 2 3 4 3 3 4 6% 8% 62%	1260
	11th to 20th	Night	3 2 2 2 2 2 4 5% 6% 8% 64%	1322
	21st to end	Night	3 2 1 3 3 2 4 5% 6% 6% 66%	1612
August	1st to 10th	Night	4 3 2 4 5% 2 5% 5% 7% 9% 55%	1611
	11th to 20th	Night	4 2 1 3 3 2 4 4 6% 7% 63%	1602
	21st to end	Night	8% 3 2 4 4 3 4 5% 6% 7% 54%	1782
September	1st to 10th	Night	6% 4 2 4 5% 2 5% 5% 5% 7% 57%	1816
	11th to 20th	Night	8% 5% 2 5% 4 2 4 5% 5% 8% 52%	1980
	21st to end	Night	9% 6% 2 5% 4 2 4 5% 5% 8% 51%	1980
October	1st to 10th	Night	8% 9% 3 7% 4 3 5% 5% 5% 7% 43%	2072
	11th to 20th	Night	7% 8% 4 6% 5% 3 5% 5% 5% 7% 46%	2160
	21st to end	Night	5% 8% 3 8% 7% 4 5% 5% 6% 8% 40%	2547
November	1st to 10th	Night	6% 9% 4 9% 7% 5% 6% 6% 7% 7% 35%	2405
	11th to 20th	Night	3 8% 5% 8% 7% 4 6% 8% 8% 9% 35%	2649
	21st to end	Night	2 6% 4 6% 7% 4 6% 7% 7% 8% 41%	2660
December	1st to 10th	Night	1 5% 4 9% 9% 5% 7% 8% 8% 9% 37%	2765
	11th to 20th	Night	2 8% 4 8% 8% 4 7% 6% 7% 10% 36%	3506
	21st to end	Night	2 7% 4 8% 7% 4 6% 6% 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% 10% 18% 21% 7% 1	1355
	11th to 20th	Day	40% 11% 21% 18% 8% 2	1472
	21st to end	Day	46% 8% 14% 21% 10% 2	1634
February	1st to 10th	Day	52% 8% 16% 17% 6% 1	1620
	11th to 20th	Day	51% 8% 18% 16% 7% 1	1584
	21st to end	Day	55% 7% 10% 15% 10% 2	1471
March	1st to 10th	Day	50% 8% 16% 19% 7%	1854
	11th to 20th	Day	50% 7% 14% 21% 7%	1980
	21st to end	Day	58% 7% 14% 16% 4 1	2178
April	1st to 10th	Day	54% 8% 13% 18% 6%	2114
	11th to 20th	Day	55% 6% 12% 18% 9%	2310
	21st to end	Day	55% 6% 12% 20% 7%	2340
May	1st to 10th	Day	51% 6% 11% 24% 7% 1	2340
	11th to 20th	Day	53% 7% 13% 18% 8% 1	2375
	21st to end	Day	61% 7% 14% 12% 4 1	2869
June	1st to 10th	Day	52% 9% 15% 18% 6% 1	2700
	11th to 20th	Day	49% 7% 11% 16% 12% 5%	2700
	21st to end	Day	33% 6% 11% 24% 22% 3	2700
July	1st to 10th	Day	30% 8% 8% 24% 23% 8%	2626
	11th to 20th	Day	24% 5% 8% 19% 23% 22%	2553
	21st to end	Day	26% 6% 5% 17% 26% 20%	2555
August	1st to 10th	Day	31% 6% 9% 23% 23% 9%	2313
	11th to 20th	Day	24% 6% 9% 24% 26% 11%	2299
	21st to end	Day	28% 5% 9% 24% 25% 10%	2574
September	1st to 10th	Day	26% 4 7% 22% 28% 13%	2142
	11th to 20th	Day	34% 5% 8% 24% 23% 6%	1980
	21st to end	Day	35% 4 12% 29% 18% 4	1980
October	1st to 10th	Day	48% 4 12% 23% 12% 2	1887
	11th to 20th	Day	42% 4 15% 25% 12% 2	1800
	21st to end	Day	44% 9% 13% 25% 8% 1	1805
November	1st to 10th	Day	45% 7% 13% 22% 11% 2	1665
	11th to 20th	Day	43% 6% 20% 22% 8% 1	1520
	21st to end	Day	40% 7% 16% 24% 12% 1	1520
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	41% 10% 21% 20% 7% 1	2827
	11th to 20th	Night	38% 11% 18% 23% 8% 2	2576
	21st to end	Night	40% 8% 19% 22% 9% 2	2720
February	1st to 10th	Night	50% 8% 16% 19% 7%	2340
	11th to 20th	Night	47% 6% 21% 18% 8%	2273
	21st to end	Night	46% 9% 14% 19% 11% 2	1782
March	1st to 10th	Night	42% 10% 17% 22% 8% 1	2106
	11th to 20th	Night	38% 8% 20% 23% 10%	1980
	21st to end	Night	49% 8% 18% 20% 5%	2178
April	1st to 10th	Night	45% 7% 16% 22% 9% 1	1843
	11th to 20th	Night	49% 8% 13% 21% 9% 1	1601
	21st to end	Night	47% 7% 18% 23% 5%	1620
May	1st to 10th	Night	40% 9% 17% 26% 8%	1620
	11th to 20th	Night	45% 7% 18% 22% 7%	1521
	21st to end	Night	51% 8% 20% 15% 5%	1402
June	1st to 10th	Night	47% 9% 16% 21% 7%	1260
	11th to 20th	Night	40% 8% 15% 20% 13% 4	1260
	21st to end	Night	26% 5% 14% 31% 22% 2	1260
July	1st to 10th	Night	21% 6% 12% 30% 23% 8%	1225
	11th to 20th	Night	17% 6% 10% 23% 26% 18%	1280
	21st to end	Night	17% 5% 8% 23% 31% 16%	1522
August	1st to 10th	Night	25% 6% 10% 26% 24% 9%	1602
	11th to 20th	Night	20% 5% 12% 27% 26% 10%	1602
	21st to end	Night	28% 6% 9% 25% 25% 7%	1782
September	1st to 10th	Night	27% 5% 8% 24% 24% 12%	1816
	11th to 20th	Night	29% 6% 11% 28% 21% 5%	1980
	21st to end	Night	33% 4 12% 27% 21% 3	1980
October	1st to 10th	Night	40% 6% 13% 26% 14% 2	2072
	11th to 20th	Night	37% 5% 16% 26% 15% 2	2160
	21st to end	Night	40% 7% 15% 25% 12% 1	2547
November	1st to 10th	Night	42% 8% 13% 24% 10% 2	2405
	11th to 20th	Night	38% 9% 20% 24% 8% 1	2649
	21st to end	Night	36% 6% 18% 24% 13% 3	2660
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	7% 15% 38% 27% 9% 5%	168
	21st to end	Day	9% 16% 36% 29% 10% 1	198
February	1st to 10th	Day	13% 23% 45% 18% 1	240
	11th to 20th	Day	7% 25% 38% 24% 5% 1	240
	21st to end	Day	10% 20% 43% 20% 7%	337
March	1st to 10th	Day	9% 24% 42% 20% 5%	420
	11th to 20th	Day	14% 20% 30% 27% 9%	420
	21st to end	Day	11% 25% 36% 19% 7% 1	462
April	1st to 10th	Day	17% 22% 35% 20% 5%	420
	11th to 20th	Day	17% 26% 32% 18% 6% 1	412
	21st to end	Day	21% 18% 32% 19% 8% 1	420
May	1st to 10th	Day	12% 18% 31% 26% 12% 2	423
	11th to 20th	Day	20% 15% 27% 20% 14% 3	518
	21st to end	Day	22% 12% 20% 23% 18% 5%	679
June	1st to 10th	Day	13% 13% 25% 27% 18% 4	630
	11th to 20th	Day	20% 17% 28% 23% 11% 2	630
	21st to end	Day	9% 9% 25% 34% 20% 3	630
July	1st to 10th	Day	5% 9% 25% 39% 19% 4	621
	11th to 20th	Day	7% 12% 27% 34% 18% 1	552
	21st to end	Day	5% 9% 30% 33% 20% 3	487
August	1st to 10th	Day	10% 8% 30% 31% 17% 4	447
	11th to 20th	Day	6% 8% 27% 34% 19% 6%	437
	21st to end	Day	6% 12% 31% 31% 16% 5%	495
September	1st to 10th	Day	9% 11% 34% 27% 16% 4	450
	11th to 20th	Day	12% 11% 32% 26% 14% 4	450
	21st to end	Day	16% 16% 27% 24% 14% 3	450
October	1st to 10th	Day	17% 12% 30% 22% 14% 4	357
	11th to 20th	Day	12% 18% 28% 24% 13% 6%	270
	21st to end	Day	9% 21% 35% 25% 7% 2	297
November	1st to 10th	Day	14% 20% 32% 25% 9% 1	270
	11th to 20th	Day	11% 19% 32% 28% 9% 1	270
	21st to end	Day	6% 18% 32% 35% 9%	270
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% 14% 36% 32% 11% 2	736
	21st to end	Night	5% 12% 35% 38% 10% 1	792
February	1st to 10th	Night	6% 15% 39% 29% 9% 2	720
	11th to 20th	Night	6% 14% 37% 33% 8% 2	704
	21st to end	Night	8% 12% 35% 34% 10% 1	592
March	1st to 10th	Night	6% 10% 37% 37% 9% 1	720
	11th to 20th	Night	8% 14% 28% 37% 13%	720
	21st to end	Night	9% 11% 34% 35% 10% 1	792
April	1st to 10th	Night	11% 10% 30% 34% 14% 1	675
	11th to 20th	Night	12% 13% 34% 30% 10%	533
	21st to end	Night	17% 12% 27% 31% 12% 1	540
May	1st to 10th	Night	10% 13% 26% 32% 18% 1	540
	11th to 20th	Night	16% 10% 19% 33% 20% 2	531
	21st to end	Night	19% 7% 17% 31% 24% 2	434
June	1st to 10th	Night	14% 5% 18% 38% 22% 2	360
	11th to 20th	Night	12% 9% 28% 38% 13% 1	360
	21st to end	Night	6% 3 18% 50% 23% 1	360
July	1st to 10th	Night	5% 2 13% 51% 27% 2	350
	11th to 20th	Night	4 3 14% 56% 23%	348
	21st to end	Night	4 2 12% 50% 30% 1	430
August	1st to 10th	Night	6% 2 17% 50% 24% 1	537
	11th to 20th	Night	5% 3 15% 50% 25% 1	534
	21st to end	Night	9% 5% 16% 43% 26% 1	594
September	1st to 10th	Night	7% 3 20% 44% 22% 4	628
	11th to 20th	Night	10% 3 13% 42% 27% 4	720
	21st to end	Night	13% 6% 22% 34% 21% 5%	720
October	1st to 10th	Night	13% 6% 23% 38% 18% 2	720
	11th to 20th	Night	11% 8% 25% 35% 17% 3	720
	21st to end	Night	11% 11% 32% 32% 12% 3	792
November	1st to 10th	Night	13% 9% 31% 36% 10% 1	740
	11th to 20th	Night	11% 15% 34% 33% 8%	939
	21st to end	Night	6% 13% 33% 37% 10% 1	950
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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