Billings
Clear Sky Chart
History

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

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	8% 7% 2 6% 4 4 5% 4 6% 8% 47%	136945
	Night	8% 8% 3 7% 4 4 4 4 6% 7% 44%	130576
Transparency	Day	35% 5% 11% 26% 18% 5%	131697
	Night	39% 5% 12% 25% 15% 4	126828
Seeing	Day	12% 8% 27% 31% 18% 4	17317
	Night	13% 7% 30% 37% 12% 1	26778

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	8% 12% 4 10% 6% 5% 6% 4 6% 6% 32%	3638
	11th to 20th	Day	4 9% 4 13% 9% 6% 6% 5% 5% 7% 33%	3640
	21st to end	Day	3 7% 4 7% 4 4 4 4 5% 7% 52%	4488
February	1st to 10th	Day	9% 15% 4 9% 6% 5% 6% 4 5% 7% 30%	1800
	11th to 20th	Day	5% 12% 4 11% 6% 6% 5% 4 7% 8% 31%	1755
	21st to end	Day	5% 12% 4 9% 6% 4 4 5% 7% 7% 36%	1525
March	1st to 10th	Day	5% 8% 4 7% 6% 5% 4 4 5% 7% 46%	4814
	11th to 20th	Day	8% 8% 3 8% 5% 4 6% 5% 7% 8% 39%	5000
	21st to end	Day	12% 16% 4 11% 5% 4 5% 4 6% 8% 26%	5529
April	1st to 10th	Day	12% 12% 3 6% 5% 3 4 4 6% 8% 37%	2400
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 5% 6% 8% 34%	2418
	21st to end	Day	15% 11% 3 6% 4 5% 4 4 6% 7% 35%	2600
May	1st to 10th	Day	17% 8% 3 6% 3 4 4 3 4 7% 40%	6174
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 5% 8% 10% 35%	6298
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	6925
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 5% 7% 8% 44%	2800
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	2955
	21st to end	Day	4 3 1 4 3 4 5% 4 7% 9% 56%	3000
July	1st to 10th	Day	3 2 1 4 4 5% 5% 4 7% 8% 56%	6779
	11th to 20th	Day	6% 2 1 2 2 2 3 3 6% 8% 65%	6341
	21st to end	Day	5% 3 1 4 3 4 4 4 7% 10% 56%	6870
August	1st to 10th	Day	3 3 1 4 3 4 5% 5% 7% 10% 56%	5779
	11th to 20th	Day	3 2 1 3 3 4 4 4 6% 7% 62%	5798
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 6% 58%	6249
September	1st to 10th	Day	7% 3 1 4 2 3 4 3 4 7% 62%	2400
	11th to 20th	Day	10% 4 2 5% 3 3 4 3 5% 7% 54%	2358
	21st to end	Day	11% 5% 1 3 3 2 3 2 5% 6% 59%	2130
October	1st to 10th	Day	7% 5% 2 4 3 3 3 2 4 5% 61%	3270
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 3 5% 58%	3250
	21st to end	Day	7% 7% 2 6% 4 4 5% 4 5% 9% 47%	3488
November	1st to 10th	Day	12% 11% 2 5% 3 4 3 4 5% 7% 44%	1624
	11th to 20th	Day	8% 10% 3 6% 4 5% 4 4 5% 7% 43%	1600
	21st to end	Day	6% 10% 3 8% 5% 4 5% 4 7% 7% 43%	1600
December	1st to 10th	Day	3 7% 3 6% 4 4 4 3 5% 8% 52%	2620
	11th to 20th	Day	5% 8% 3 7% 5% 5% 6% 5% 7% 9% 41%	3026
	21st to end	Day	6% 13% 4 8% 4 4 4 3 5% 6% 41%	4004
January	1st to 10th	Night	7% 13% 5% 10% 5% 5% 4 4 5% 6% 36%	6327
	11th to 20th	Night	6% 12% 5% 13% 7% 5% 4 3 4 6% 35%	6340
	21st to end	Night	4 7% 3 7% 4 4 5% 4 6% 8% 48%	6532
February	1st to 10th	Night	8% 15% 6% 11% 5% 5% 5% 4 5% 6% 30%	2600
	11th to 20th	Night	7% 16% 5% 10% 5% 5% 5% 4 6% 6% 32%	2532
	21st to end	Night	6% 14% 5% 9% 6% 4 4 5% 6% 6% 36%	2105
March	1st to 10th	Night	5% 10% 4 8% 5% 5% 4 4 5% 7% 44%	5072
	11th to 20th	Night	8% 9% 3 6% 4 4 4 4 6% 9% 42%	5000
	21st to end	Night	13% 15% 5% 10% 6% 5% 5% 4 5% 7% 25%	5444
April	1st to 10th	Night	13% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	2000
	11th to 20th	Night	12% 12% 4 8% 5% 5% 5% 5% 5% 7% 32%	1929
	21st to end	Night	17% 8% 2 8% 6% 5% 5% 3 6% 6% 35%	1800
May	1st to 10th	Night	15% 8% 3 6% 5% 4 4 3 5% 6% 41%	3806
	11th to 20th	Night	14% 7% 3 7% 5% 5% 6% 5% 6% 9% 33%	3617
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	3972
June	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 5% 9% 9% 39%	1600
	11th to 20th	Night	10% 5% 2 7% 4 5% 6% 4 7% 8% 42%	1439
	21st to end	Night	6% 5% 2 4 4 4 5% 3 5% 8% 54%	1400
July	1st to 10th	Night	4 3 1 6% 4 4 5% 5% 8% 9% 51%	3180
	11th to 20th	Night	7% 3 1 5% 5% 4 5% 4 6% 7% 54%	3634
	21st to end	Night	5% 3 2 5% 4 4 5% 4 8% 10% 50%	3987
August	1st to 10th	Night	3 3 1 6% 4 5% 6% 5% 8% 10% 48%	4033
	11th to 20th	Night	3 3 1 4 3 4 5% 6% 8% 9% 53%	4051
	21st to end	Night	8% 5% 2 5% 3 4 4 3 5% 6% 56%	4727
September	1st to 10th	Night	6% 5% 2 4 2 3 3 3 5% 6% 60%	2000
	11th to 20th	Night	11% 4 1 5% 3 4 4 3 5% 9% 51%	2040
	21st to end	Night	12% 5% 1 4 2 3 4 3 4 6% 56%	2270
October	1st to 10th	Night	9% 4 1 4 3 2 3 2 4 6% 61%	3920
	11th to 20th	Night	12% 7% 2 5% 2 3 3 3 4 5% 55%	3898
	21st to end	Night	10% 6% 2 5% 3 3 3 3 4 7% 54%	4279
November	1st to 10th	Night	10% 10% 3 7% 4 4 4 4 6% 7% 41%	2717
	11th to 20th	Night	9% 10% 3 6% 4 4 4 4 5% 7% 44%	2800
	21st to end	Night	6% 10% 3 7% 5% 4 4 4 6% 8% 44%	2800
December	1st to 10th	Night	4 10% 4 8% 5% 5% 5% 4 5% 6% 46%	4570
	11th to 20th	Night	4 9% 3 9% 5% 5% 5% 4 6% 6% 43%	5181
	21st to end	Night	5% 10% 3 7% 4 4 4 4 5% 7% 46%	6974

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	50% 6% 10% 27% 7%	3639
	11th to 20th	Day	51% 5% 13% 17% 14% 1	3640
	21st to end	Day	32% 5% 14% 30% 18% 1	4488
February	1st to 10th	Day	52% 5% 15% 18% 10%	1800
	11th to 20th	Day	50% 6% 12% 21% 9% 2	1764
	21st to end	Day	43% 5% 14% 23% 13% 2	1525
March	1st to 10th	Day	36% 5% 8% 25% 22% 3	4814
	11th to 20th	Day	41% 5% 13% 29% 11% 1	5000
	21st to end	Day	55% 6% 12% 22% 4	5529
April	1st to 10th	Day	44% 7% 12% 23% 12% 2	2400
	11th to 20th	Day	47% 7% 13% 23% 10% 1	2421
	21st to end	Day	48% 6% 12% 22% 11% 1	2600
May	1st to 10th	Day	44% 6% 9% 25% 15% 2	6174
	11th to 20th	Day	39% 8% 16% 26% 10% 1	6302
	21st to end	Day	35% 6% 13% 33% 13%	6926
June	1st to 10th	Day	35% 8% 18% 26% 13% 1	2800
	11th to 20th	Day	35% 7% 17% 22% 15% 3	2955
	21st to end	Day	22% 7% 15% 27% 23% 7%	3000
July	1st to 10th	Day	24% 4 9% 30% 22% 11%	6706
	11th to 20th	Day	19% 3 5% 23% 36% 14%	6265
	21st to end	Day	25% 3 10% 21% 28% 14%	6672
August	1st to 10th	Day	27% 3 10% 23% 22% 14%	4208
	11th to 20th	Day	25% 3 7% 29% 30% 6%	4197
	21st to end	Day	21% 3 6% 29% 29% 11%	4504
September	1st to 10th	Day	22% 3 7% 25% 30% 13%	2400
	11th to 20th	Day	29% 6% 10% 25% 23% 7%	2358
	21st to end	Day	27% 4 9% 29% 24% 8%	2130
October	1st to 10th	Day	27% 3 8% 19% 33% 11%	3270
	11th to 20th	Day	31% 3 9% 29% 24% 4	3250
	21st to end	Day	36% 3 13% 29% 17% 2	3488
November	1st to 10th	Day	40% 5% 12% 23% 16% 3	1624
	11th to 20th	Day	40% 4 15% 24% 14% 2	1600
	21st to end	Day	39% 5% 15% 23% 15% 2	1600
December	1st to 10th	Day	33% 4 17% 27% 15% 3	2620
	11th to 20th	Day	38% 5% 22% 25% 10% 1	3024
	21st to end	Day	44% 4 7% 26% 15% 4	4004
January	1st to 10th	Night	49% 4 13% 26% 7% 1	6327
	11th to 20th	Night	52% 4 12% 19% 11% 2	6340
	21st to end	Night	35% 5% 13% 30% 15% 3	6532
February	1st to 10th	Night	53% 5% 12% 19% 10% 1	2600
	11th to 20th	Night	53% 5% 11% 21% 9% 1	2533
	21st to end	Night	46% 6% 13% 20% 12% 3	2105
March	1st to 10th	Night	40% 5% 7% 30% 16% 2	5072
	11th to 20th	Night	36% 6% 17% 28% 13% 1	5000
	21st to end	Night	57% 6% 14% 15% 7%	5444
April	1st to 10th	Night	46% 8% 14% 19% 12% 2	2000
	11th to 20th	Night	49% 6% 12% 25% 8%	1929
	21st to end	Night	49% 6% 12% 20% 11% 1	1800
May	1st to 10th	Night	42% 6% 11% 23% 17% 1	3803
	11th to 20th	Night	45% 6% 14% 25% 8% 2	3618
	21st to end	Night	34% 7% 14% 34% 11%	3972
June	1st to 10th	Night	38% 9% 18% 23% 10% 2	1600
	11th to 20th	Night	38% 8% 16% 21% 14% 3	1439
	21st to end	Night	27% 6% 15% 25% 22% 5%	1400
July	1st to 10th	Night	27% 3 13% 33% 18% 6%	3145
	11th to 20th	Night	29% 3 6% 20% 30% 11%	3587
	21st to end	Night	27% 4 13% 24% 23% 9%	3873
August	1st to 10th	Night	31% 4 14% 22% 17% 12%	2928
	11th to 20th	Night	27% 4 11% 31% 22% 5%	2928
	21st to end	Night	24% 4 8% 25% 30% 10%	3403
September	1st to 10th	Night	24% 4 9% 22% 27% 14%	2000
	11th to 20th	Night	31% 5% 11% 25% 21% 6%	2040
	21st to end	Night	29% 4 9% 26% 24% 7%	2270
October	1st to 10th	Night	25% 4 7% 25% 27% 12%	3920
	11th to 20th	Night	34% 3 8% 24% 26% 4	3898
	21st to end	Night	32% 3 12% 31% 15% 7%	4279
November	1st to 10th	Night	41% 6% 11% 24% 15% 3	2717
	11th to 20th	Night	39% 5% 12% 28% 14% 2	2800
	21st to end	Night	37% 5% 15% 26% 14% 3	2800
December	1st to 10th	Night	39% 4 15% 26% 15% 2	4570
	11th to 20th	Night	41% 4 15% 29% 9% 2	5182
	21st to end	Night	38% 5% 10% 30% 14% 4	6974

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	13% 10% 37% 30% 8% 2	300
	11th to 20th	Day	12% 12% 35% 28% 9% 3	300
	21st to end	Day	12% 13% 40% 26% 7% 1	330
February	1st to 10th	Day	19% 13% 42% 23% 3 1	300
	11th to 20th	Day	11% 13% 41% 26% 7% 1	300
	21st to end	Day	10% 14% 42% 25% 8% 1	249
March	1st to 10th	Day	11% 11% 35% 31% 11% 1	433
	11th to 20th	Day	13% 11% 34% 27% 12% 3	500
	21st to end	Day	19% 14% 28% 27% 10% 2	550
April	1st to 10th	Day	17% 15% 32% 24% 9% 2	500
	11th to 20th	Day	17% 14% 32% 26% 9% 1	492
	21st to end	Day	21% 10% 29% 23% 14% 3	500
May	1st to 10th	Day	18% 7% 22% 27% 21% 5%	628
	11th to 20th	Day	20% 5% 22% 30% 18% 6%	725
	21st to end	Day	17% 8% 18% 28% 22% 8%	789
June	1st to 10th	Day	9% 6% 22% 35% 22% 6%	730
	11th to 20th	Day	10% 12% 28% 32% 16% 2	730
	21st to end	Day	6% 6% 22% 38% 25% 4	730
July	1st to 10th	Day	5% 3 19% 37% 30% 6%	721
	11th to 20th	Day	4 5% 24% 39% 25% 4	718
	21st to end	Day	4 2 19% 42% 26% 6%	786
August	1st to 10th	Day	6% 5% 24% 38% 22% 6%	527
	11th to 20th	Day	7% 2 23% 34% 25% 8%	517
	21st to end	Day	6% 5% 22% 32% 27% 9%	583
September	1st to 10th	Day	6% 6% 22% 37% 23% 6%	530
	11th to 20th	Day	12% 5% 19% 34% 23% 8%	530
	21st to end	Day	13% 6% 21% 28% 25% 6%	460
October	1st to 10th	Day	15% 6% 18% 29% 25% 6%	330
	11th to 20th	Day	16% 8% 25% 32% 14% 5%	330
	21st to end	Day	13% 10% 34% 26% 13% 3	363
November	1st to 10th	Day	20% 8% 29% 28% 12% 3	306
	11th to 20th	Day	12% 11% 36% 33% 6% 1	300
	21st to end	Day	12% 7% 39% 31% 10%	300
December	1st to 10th	Day	8% 10% 46% 29% 7%	300
	11th to 20th	Day	13% 12% 36% 33% 6%	300
	21st to end	Day	15% 9% 39% 24% 12% 2	330
January	1st to 10th	Night	13% 9% 40% 31% 7% 1	1000
	11th to 20th	Night	14% 10% 43% 26% 7% 1	1000
	21st to end	Night	13% 9% 44% 26% 7% 1	905
February	1st to 10th	Night	15% 10% 39% 28% 7% 1	800
	11th to 20th	Night	15% 8% 38% 31% 7% 1	784
	21st to end	Night	13% 10% 37% 30% 9% 1	660
March	1st to 10th	Night	12% 7% 37% 33% 12%	800
	11th to 20th	Night	12% 7% 32% 37% 12% 2	800
	21st to end	Night	18% 7% 28% 33% 13% 1	860
April	1st to 10th	Night	17% 10% 27% 37% 9% 1	600
	11th to 20th	Night	17% 7% 30% 40% 7%	593
	21st to end	Night	20% 6% 22% 38% 13% 1	600
May	1st to 10th	Night	18% 7% 20% 38% 16% 1	600
	11th to 20th	Night	17% 7% 19% 34% 21% 1	591
	21st to end	Night	19% 5% 20% 36% 19% 1	650
June	1st to 10th	Night	11% 5% 26% 38% 17% 2	600
	11th to 20th	Night	13% 6% 27% 41% 14%	439
	21st to end	Night	7% 2 22% 50% 19%	400
July	1st to 10th	Night	5% 2 12% 53% 28%	390
	11th to 20th	Night	5% 4 26% 50% 15%	577
	21st to end	Night	4 3 22% 55% 16%	657
August	1st to 10th	Night	6% 3 21% 51% 19% 1	597
	11th to 20th	Night	6% 3 18% 54% 19% 1	594
	21st to end	Night	8% 4 18% 52% 18% 1	660
September	1st to 10th	Night	7% 3 18% 51% 20% 1	600
	11th to 20th	Night	13% 2 18% 48% 19% 1	640
	21st to end	Night	13% 4 19% 41% 22% 2	800
October	1st to 10th	Night	18% 4 21% 39% 16% 2	800
	11th to 20th	Night	18% 7% 26% 33% 16% 1	800
	21st to end	Night	13% 5% 34% 37% 11% 1	895
November	1st to 10th	Night	14% 9% 32% 36% 8% 1	986
	11th to 20th	Night	13% 10% 39% 32% 6%	1000
	21st to end	Night	11% 8% 39% 37% 6%	1000
December	1st to 10th	Night	11% 8% 42% 32% 7%	1000
	11th to 20th	Night	11% 8% 42% 32% 7% 1	1000
	21st to end	Night	13% 10% 38% 30% 9% 1	1100

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