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 2024-11-06.

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%	106145
	Night	8% 8% 3 7% 4 4 4 4 6% 7% 44%	101655
Transparency	Day	34% 6% 11% 25% 18% 6%	105883
	Night	38% 6% 12% 25% 15% 4	101507
Seeing	Day	12% 8% 27% 31% 18% 4	15937
	Night	13% 7% 30% 37% 12% 1	25454

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	5% 10% 4 9% 6% 5% 6% 4 7% 7% 36%	2538
	11th to 20th	Day	5% 8% 3 11% 8% 7% 7% 5% 6% 8% 32%	2540
	21st to end	Day	4 8% 2 6% 4 4 5% 4 6% 8% 50%	3124
February	1st to 10th	Day	9% 14% 4 9% 5% 5% 6% 4 5% 7% 31%	1710
	11th to 20th	Day	6% 13% 4 11% 6% 6% 6% 4 7% 8% 31%	1665
	21st to end	Day	5% 13% 4 9% 6% 4 4 5% 7% 7% 35%	1451
March	1st to 10th	Day	4 7% 4 6% 6% 5% 5% 4 6% 8% 43%	3346
	11th to 20th	Day	8% 7% 3 7% 4 3 5% 3 6% 8% 47%	3490
	21st to end	Day	13% 17% 4 11% 4 4 5% 4 6% 7% 25%	3855
April	1st to 10th	Day	12% 12% 3 6% 5% 3 4 4 6% 8% 36%	2280
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 5% 6% 8% 33%	2295
	21st to end	Day	15% 11% 3 6% 4 4 4 4 6% 6% 36%	2470
May	1st to 10th	Day	20% 10% 4 7% 4 5% 5% 4 5% 6% 31%	4320
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 10% 38%	4388
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 7% 9% 38%	4825
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 43%	2660
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	2807
	21st to end	Day	4 3 1 4 3 4 5% 4 7% 9% 56%	2850
July	1st to 10th	Day	4 2 1 4 3 5% 5% 4 7% 9% 57%	4740
	11th to 20th	Day	2 1 1 2 2 3 4 3 6% 9% 68%	4431
	21st to end	Day	2 2 1 3 3 3 4 4 6% 10% 63%	4783
August	1st to 10th	Day	4 3 2 5% 4 4 5% 5% 7% 10% 51%	4092
	11th to 20th	Day	4 3 2 4 4 4 5% 4 7% 8% 57%	4028
	21st to end	Day	6% 2 1 3 3 3 4 3 5% 7% 63%	4367
September	1st to 10th	Day	8% 3 1 4 2 3 4 3 4 7% 62%	2280
	11th to 20th	Day	10% 4 2 5% 3 3 4 3 5% 6% 55%	2240
	21st to end	Day	12% 5% 1 3 2 2 3 2 5% 6% 58%	2024
October	1st to 10th	Day	7% 5% 2 4 3 3 3 2 4 5% 62%	3170
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 3 5% 59%	3150
	21st to end	Day	7% 7% 3 6% 4 4 5% 4 6% 9% 46%	3380
November	1st to 10th	Day	12% 11% 3 6% 3 4 4 4 5% 7% 42%	1566
	11th to 20th	Day	8% 10% 3 6% 4 5% 5% 4 6% 8% 43%	1520
	21st to end	Day	6% 10% 3 7% 5% 4 4 4 7% 6% 44%	1520
December	1st to 10th	Day	6% 11% 4 9% 6% 5% 5% 4 5% 7% 39%	1520
	11th to 20th	Day	4 8% 2 8% 5% 4 5% 5% 8% 10% 41%	1926
	21st to end	Day	5% 10% 3 7% 4 5% 5% 3 6% 6% 46%	2794
January	1st to 10th	Night	5% 10% 4 8% 5% 5% 5% 4 6% 6% 40%	4417
	11th to 20th	Night	6% 11% 5% 12% 7% 5% 5% 3 5% 7% 34%	4430
	21st to end	Night	6% 9% 3 7% 5% 5% 5% 4 6% 9% 42%	4571
February	1st to 10th	Night	8% 15% 6% 10% 5% 5% 5% 4 5% 6% 30%	2470
	11th to 20th	Night	8% 15% 5% 10% 5% 4 5% 4 5% 7% 32%	2402
	21st to end	Night	7% 14% 5% 9% 6% 4 4 5% 6% 6% 35%	2003
March	1st to 10th	Night	4 10% 5% 10% 6% 5% 4 4 6% 7% 39%	3523
	11th to 20th	Night	6% 7% 2 6% 5% 4 4 5% 6% 9% 46%	3490
	21st to end	Night	14% 17% 5% 10% 6% 5% 5% 4 6% 6% 22%	3797
April	1st to 10th	Night	13% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	1900
	11th to 20th	Night	12% 12% 4 8% 6% 5% 4 5% 5% 7% 32%	1832
	21st to end	Night	16% 8% 2 8% 6% 5% 4 3 6% 6% 36%	1710
May	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 31%	2650
	11th to 20th	Night	10% 6% 3 7% 5% 5% 6% 4 7% 9% 39%	2517
	21st to end	Night	13% 7% 2 7% 5% 5% 6% 5% 7% 10% 34%	2762
June	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 5% 8% 9% 39%	1520
	11th to 20th	Night	11% 5% 2 7% 4 4 6% 4 7% 8% 42%	1367
	21st to end	Night	6% 5% 2 4 4 4 5% 3 5% 8% 54%	1330
July	1st to 10th	Night	4 3 1 5% 4 4 4 4 7% 9% 55%	2220
	11th to 20th	Night	3 2 1 4 4 4 5% 4 6% 8% 58%	2534
	21st to end	Night	3 3 2 4 4 4 5% 4 8% 10% 53%	2778
August	1st to 10th	Night	4 4 2 6% 4 6% 5% 5% 6% 9% 49%	2847
	11th to 20th	Night	4 3 2 5% 3 5% 6% 6% 9% 9% 48%	2811
	21st to end	Night	7% 3 1 4 3 3 4 3 5% 6% 60%	3299
September	1st to 10th	Night	6% 5% 2 5% 2 3 3 3 5% 6% 59%	1900
	11th to 20th	Night	11% 4 1 5% 3 4 3 3 5% 9% 52%	1938
	21st to end	Night	13% 5% 1 4 2 3 4 3 4 6% 55%	2156
October	1st to 10th	Night	9% 4 1 3 3 2 3 2 4 6% 62%	3800
	11th to 20th	Night	12% 7% 2 4 2 3 3 3 4 5% 56%	3778
	21st to end	Night	10% 6% 2 5% 3 3 3 3 4 7% 53%	4146
November	1st to 10th	Night	10% 11% 3 7% 4 4 5% 4 6% 7% 40%	2633
	11th to 20th	Night	9% 10% 3 5% 4 4 4 4 6% 7% 44%	2660
	21st to end	Night	6% 10% 3 6% 5% 4 4 4 6% 8% 45%	2660
December	1st to 10th	Night	5% 12% 5% 9% 6% 6% 5% 4 5% 6% 38%	2660
	11th to 20th	Night	5% 7% 3 9% 5% 4 4 4 6% 7% 47%	3271
	21st to end	Night	5% 9% 3 6% 4 4 4 4 5% 7% 50%	4873

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	43% 7% 12% 31% 6%	2539
	11th to 20th	Day	48% 6% 13% 19% 12% 1	2540
	21st to end	Day	31% 7% 19% 28% 14% 2	3124
February	1st to 10th	Day	51% 5% 15% 19% 9%	1710
	11th to 20th	Day	50% 6% 13% 21% 8% 2	1674
	21st to end	Day	44% 6% 14% 22% 12% 2	1451
March	1st to 10th	Day	35% 7% 11% 25% 21% 1	3346
	11th to 20th	Day	35% 6% 17% 30% 10% 1	3490
	21st to end	Day	56% 8% 13% 18% 4	3855
April	1st to 10th	Day	44% 7% 13% 22% 11% 2	2280
	11th to 20th	Day	47% 7% 13% 22% 9% 1	2298
	21st to end	Day	48% 6% 11% 22% 12% 1	2470
May	1st to 10th	Day	51% 8% 9% 22% 8% 2	4320
	11th to 20th	Day	34% 10% 13% 30% 11% 1	4392
	21st to end	Day	40% 8% 14% 30% 8%	4825
June	1st to 10th	Day	36% 8% 18% 25% 12% 1	2660
	11th to 20th	Day	35% 8% 16% 22% 15% 4	2807
	21st to end	Day	22% 7% 15% 26% 22% 7%	2850
July	1st to 10th	Day	23% 4 8% 31% 24% 10%	4666
	11th to 20th	Day	15% 3 5% 20% 39% 19%	4355
	21st to end	Day	16% 3 5% 22% 33% 20%	4632
August	1st to 10th	Day	28% 3 10% 23% 22% 15%	4078
	11th to 20th	Day	25% 3 7% 28% 30% 6%	4067
	21st to end	Day	21% 3 6% 29% 29% 11%	4366
September	1st to 10th	Day	22% 3 7% 24% 30% 14%	2280
	11th to 20th	Day	29% 6% 10% 25% 24% 7%	2240
	21st to end	Day	27% 4 9% 29% 23% 8%	2024
October	1st to 10th	Day	27% 3 8% 18% 33% 11%	3170
	11th to 20th	Day	30% 3 9% 30% 24% 4	3150
	21st to end	Day	36% 3 13% 28% 17% 2	3380
November	1st to 10th	Day	41% 6% 12% 23% 16% 4	1566
	11th to 20th	Day	40% 4 15% 24% 14% 2	1520
	21st to end	Day	38% 5% 14% 24% 16% 3	1520
December	1st to 10th	Day	44% 6% 12% 23% 12% 2	1520
	11th to 20th	Day	36% 7% 24% 23% 9% 1	1924
	21st to end	Day	38% 5% 9% 27% 15% 6%	2794
January	1st to 10th	Night	43% 5% 15% 27% 9% 1	4417
	11th to 20th	Night	51% 5% 13% 18% 10% 3	4430
	21st to end	Night	39% 6% 17% 27% 10% 1	4571
February	1st to 10th	Night	53% 5% 13% 19% 9% 1	2470
	11th to 20th	Night	52% 6% 11% 21% 9% 1	2403
	21st to end	Night	47% 6% 12% 20% 11% 3	2003
March	1st to 10th	Night	41% 6% 10% 24% 17% 2	3523
	11th to 20th	Night	31% 8% 22% 27% 11% 1	3490
	21st to end	Night	60% 8% 14% 14% 4	3797
April	1st to 10th	Night	46% 8% 14% 19% 11% 2	1900
	11th to 20th	Night	49% 7% 12% 25% 8%	1832
	21st to end	Night	48% 7% 11% 21% 12% 1	1710
May	1st to 10th	Night	50% 8% 7% 22% 11% 2	2650
	11th to 20th	Night	38% 9% 13% 30% 8% 3	2518
	21st to end	Night	41% 10% 15% 28% 6%	2762
June	1st to 10th	Night	38% 9% 19% 23% 9% 2	1520
	11th to 20th	Night	37% 8% 16% 21% 14% 3	1367
	21st to end	Night	27% 6% 15% 25% 21% 5%	1330
July	1st to 10th	Night	24% 4 12% 35% 19% 7%	2185
	11th to 20th	Night	24% 4 7% 19% 31% 16%	2487
	21st to end	Night	23% 4 8% 22% 31% 13%	2691
August	1st to 10th	Night	31% 4 14% 22% 18% 12%	2838
	11th to 20th	Night	28% 4 11% 31% 22% 5%	2838
	21st to end	Night	23% 4 8% 25% 30% 10%	3299
September	1st to 10th	Night	25% 4 9% 21% 26% 15%	1900
	11th to 20th	Night	30% 5% 11% 25% 22% 7%	1938
	21st to end	Night	30% 5% 9% 27% 23% 7%	2156
October	1st to 10th	Night	25% 4 7% 25% 27% 13%	3800
	11th to 20th	Night	33% 3 8% 25% 26% 4	3778
	21st to end	Night	33% 3 12% 30% 15% 7%	4146
November	1st to 10th	Night	42% 6% 12% 23% 14% 3	2633
	11th to 20th	Night	38% 5% 13% 28% 13% 2	2660
	21st to end	Night	37% 5% 14% 27% 15% 3	2660
December	1st to 10th	Night	46% 6% 12% 21% 12% 3	2660
	11th to 20th	Night	35% 6% 17% 32% 10% 1	3272
	21st to end	Night	34% 6% 11% 29% 16% 5%	4873

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	11% 11% 38% 30% 8% 3	270
	11th to 20th	Day	12% 11% 34% 29% 10% 4	270
	21st to end	Day	12% 12% 40% 27% 7% 1	297
February	1st to 10th	Day	19% 14% 42% 22% 3 1	270
	11th to 20th	Day	11% 15% 42% 26% 5% 1	270
	21st to end	Day	11% 13% 42% 25% 9% 1	225
March	1st to 10th	Day	11% 11% 35% 32% 10% 1	396
	11th to 20th	Day	13% 11% 33% 28% 12% 3	460
	21st to end	Day	19% 14% 26% 28% 11% 2	506
April	1st to 10th	Day	17% 15% 33% 24% 8% 2	460
	11th to 20th	Day	17% 14% 32% 27% 9% 1	452
	21st to end	Day	21% 10% 27% 23% 15% 4	460
May	1st to 10th	Day	18% 7% 23% 27% 20% 4	582
	11th to 20th	Day	19% 5% 21% 29% 19% 6%	675
	21st to end	Day	18% 9% 18% 27% 21% 8%	734
June	1st to 10th	Day	10% 5% 22% 35% 23% 6%	680
	11th to 20th	Day	10% 12% 28% 31% 17% 2	680
	21st to end	Day	6% 6% 22% 37% 25% 4	680
July	1st to 10th	Day	5% 3 19% 36% 31% 6%	671
	11th to 20th	Day	3 5% 24% 40% 25% 4	668
	21st to end	Day	3 2 19% 42% 27% 6%	732
August	1st to 10th	Day	7% 5% 22% 38% 22% 6%	487
	11th to 20th	Day	8% 2 23% 34% 25% 9%	477
	21st to end	Day	6% 5% 22% 32% 27% 9%	539
September	1st to 10th	Day	6% 6% 23% 37% 22% 6%	490
	11th to 20th	Day	12% 6% 20% 33% 22% 8%	490
	21st to end	Day	14% 7% 21% 29% 25% 4	424
October	1st to 10th	Day	15% 7% 18% 28% 27% 6%	300
	11th to 20th	Day	16% 8% 24% 33% 15% 5%	300
	21st to end	Day	14% 11% 35% 23% 14% 3	330
November	1st to 10th	Day	20% 8% 29% 28% 12% 2	285
	11th to 20th	Day	12% 11% 38% 30% 7% 1	270
	21st to end	Day	13% 8% 41% 29% 10%	270
December	1st to 10th	Day	7% 10% 45% 30% 7%	270
	11th to 20th	Day	13% 12% 36% 31% 7%	270
	21st to end	Day	14% 9% 39% 25% 11% 2	297
January	1st to 10th	Night	12% 9% 41% 31% 7% 1	950
	11th to 20th	Night	13% 11% 43% 26% 7% 1	950
	21st to end	Night	14% 9% 43% 26% 7% 1	860
February	1st to 10th	Night	15% 10% 39% 28% 8% 1	760
	11th to 20th	Night	15% 9% 39% 30% 6% 1	744
	21st to end	Night	13% 10% 36% 31% 9% 1	628
March	1st to 10th	Night	12% 7% 36% 33% 12%	760
	11th to 20th	Night	12% 7% 32% 36% 12% 2	760
	21st to end	Night	18% 8% 27% 34% 13% 1	817
April	1st to 10th	Night	17% 10% 27% 38% 9% 1	570
	11th to 20th	Night	17% 7% 29% 39% 7%	563
	21st to end	Night	20% 6% 21% 38% 14% 1	570
May	1st to 10th	Night	18% 7% 20% 37% 16% 1	570
	11th to 20th	Night	16% 7% 19% 34% 22% 1	561
	21st to end	Night	20% 5% 20% 36% 19% 1	617
June	1st to 10th	Night	11% 5% 26% 38% 17% 2	570
	11th to 20th	Night	12% 6% 27% 40% 15%	417
	21st to end	Night	7% 2 22% 50% 19%	380
July	1st to 10th	Night	6% 2 12% 52% 28%	370
	11th to 20th	Night	4 4 26% 51% 15%	547
	21st to end	Night	4 3 21% 55% 16%	624
August	1st to 10th	Night	6% 3 21% 51% 19% 1	567
	11th to 20th	Night	7% 2 18% 53% 19% 1	564
	21st to end	Night	7% 4 18% 52% 19% 1	627
September	1st to 10th	Night	7% 4 18% 51% 19% 1	570
	11th to 20th	Night	13% 2 18% 48% 18% 1	608
	21st to end	Night	14% 4 19% 40% 21% 2	760
October	1st to 10th	Night	18% 4 21% 38% 16% 2	760
	11th to 20th	Night	16% 7% 25% 34% 17% 1	760
	21st to end	Night	13% 5% 33% 36% 11% 1	850
November	1st to 10th	Night	15% 9% 31% 36% 8% 1	955
	11th to 20th	Night	13% 10% 38% 32% 7%	950
	21st to end	Night	11% 8% 39% 36% 6%	950
December	1st to 10th	Night	12% 9% 41% 31% 7%	950
	11th to 20th	Night	10% 8% 43% 31% 7% 1	950
	21st to end	Night	13% 11% 37% 30% 9% 1	1045

Page generated at 2024-11-03 11:19:03. Version ac07

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