Lewistown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lewistown clear sky chart from 2005-11-06 to 2025-10-19.

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	16% 8% 3 7% 5% 5% 5% 4 6% 7% 35%	126732
	Night	16% 8% 3 8% 5% 4 4 4 5% 7% 36%	122959
Transparency	Day	46% 7% 11% 21% 13% 2	130327
	Night	46% 6% 12% 21% 14% 2	127433
Seeing	Day	25% 14% 26% 21% 11% 4	16037
	Night	22% 12% 23% 29% 12% 1	25436

By Month

Percentage Cloud Cover SE (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	12% 13% 4 15% 5% 5% 4 3 4 4 30%	3597
	11th to 20th	Day	12% 15% 8% 15% 8% 6% 5% 3 5% 7% 18%	4032
	21st to end	Day	15% 18% 5% 8% 5% 4 4 3 5% 5% 27%	4466
February	1st to 10th	Day	16% 15% 5% 8% 6% 4 6% 4 5% 5% 28%	1800
	11th to 20th	Day	14% 10% 4 11% 7% 5% 5% 4 5% 5% 30%	1753
	21st to end	Day	18% 11% 3 9% 5% 4 4 3 6% 5% 31%	1650
March	1st to 10th	Day	19% 10% 2 6% 4 5% 4 3 4 5% 37%	4862
	11th to 20th	Day	13% 9% 3 7% 5% 5% 5% 3 5% 8% 37%	4980
	21st to end	Day	13% 9% 3 7% 5% 5% 4 4 5% 5% 39%	5545
April	1st to 10th	Day	18% 10% 3 7% 5% 4 4 3 5% 7% 34%	2400
	11th to 20th	Day	16% 10% 2 7% 5% 4 5% 4 5% 6% 37%	2374
	21st to end	Day	19% 6% 3 6% 6% 3 4 4 5% 7% 36%	2446
May	1st to 10th	Day	26% 6% 2 6% 5% 5% 5% 3 5% 7% 29%	5862
	11th to 20th	Day	25% 7% 2 6% 5% 5% 4 3 5% 7% 29%	6267
	21st to end	Day	19% 6% 2 6% 4 5% 5% 5% 7% 8% 33%	6934
June	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 5% 6% 7% 38%	2800
	11th to 20th	Day	16% 5% 2 6% 6% 6% 4 4 6% 9% 37%	2800
	21st to end	Day	11% 4 2 6% 6% 5% 5% 5% 7% 10% 40%	2800
July	1st to 10th	Day	7% 5% 2 8% 6% 6% 6% 5% 7% 9% 40%	6360
	11th to 20th	Day	9% 6% 2 8% 6% 6% 6% 6% 8% 11% 32%	6327
	21st to end	Day	11% 6% 2 7% 5% 4 5% 3 7% 8% 41%	6424
August	1st to 10th	Day	20% 5% 2 7% 6% 5% 5% 4 7% 8% 32%	4234
	11th to 20th	Day	14% 5% 2 7% 6% 6% 6% 4 6% 8% 37%	4090
	21st to end	Day	15% 4 2 5% 4 4 4 4 6% 8% 45%	4312
September	1st to 10th	Day	16% 6% 1 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Day	14% 5% 2 4 3 4 4 3 6% 7% 50%	2400
	21st to end	Day	22% 6% 2 7% 3 3 4 4 5% 7% 36%	2090
October	1st to 10th	Day	18% 6% 2 5% 4 4 4 4 5% 6% 42%	2000
	11th to 20th	Day	19% 6% 2 6% 5% 4 5% 3 6% 6% 39%	1980
	21st to end	Day	18% 7% 2 6% 5% 4 4 4 6% 7% 38%	2090
November	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 47%	1824
	11th to 20th	Day	15% 10% 3 7% 5% 4 5% 4 5% 6% 36%	1683
	21st to end	Day	17% 12% 3 6% 5% 5% 5% 4 6% 6% 32%	1600
December	1st to 10th	Day	15% 9% 6% 11% 6% 4 5% 3 6% 7% 27%	2600
	11th to 20th	Day	22% 13% 4 6% 4 4 4 3 5% 5% 28%	2990
	21st to end	Day	19% 14% 4 11% 7% 7% 5% 4 5% 5% 20%	3960
January	1st to 10th	Night	12% 11% 4 17% 5% 5% 4 3 5% 7% 26%	6349
	11th to 20th	Night	13% 11% 6% 13% 5% 5% 5% 4 6% 7% 25%	5924
	21st to end	Night	18% 11% 3 9% 5% 5% 5% 4 5% 6% 29%	6490
February	1st to 10th	Night	15% 11% 3 8% 5% 4 5% 3 6% 6% 32%	2600
	11th to 20th	Night	16% 10% 3 7% 6% 5% 4 4 6% 7% 31%	2531
	21st to end	Night	16% 11% 3 8% 5% 5% 5% 4 5% 5% 33%	1980
March	1st to 10th	Night	19% 10% 4 7% 4 4 4 3 4 5% 36%	4975
	11th to 20th	Night	11% 8% 2 7% 5% 4 4 4 5% 7% 44%	4980
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	5386
April	1st to 10th	Night	18% 10% 3 9% 6% 5% 4 3 5% 6% 33%	2000
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	1975
	21st to end	Night	17% 6% 2 6% 4 4 5% 3 5% 7% 40%	1970
May	1st to 10th	Night	30% 7% 2 6% 5% 4 4 4 5% 6% 28%	4033
	11th to 20th	Night	26% 7% 2 6% 4 4 4 3 4 6% 34%	3624
	21st to end	Night	21% 6% 2 6% 4 4 4 3 6% 8% 36%	3920
June	1st to 10th	Night	13% 7% 3 5% 4 4 5% 4 5% 7% 43%	1600
	11th to 20th	Night	14% 5% 2 6% 6% 5% 4 4 5% 9% 40%	1600
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 7% 10% 45%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 10% 40%	3598
	11th to 20th	Night	11% 5% 2 9% 6% 5% 6% 5% 7% 10% 33%	3613
	21st to end	Night	11% 5% 2 6% 4 5% 4 3 6% 9% 44%	4427
August	1st to 10th	Night	19% 6% 2 7% 4 4 5% 4 5% 8% 36%	2915
	11th to 20th	Night	17% 6% 2 6% 3 3 4 4 5% 7% 42%	2925
	21st to end	Night	16% 4 2 4 3 4 4 3 5% 7% 47%	3586
September	1st to 10th	Night	16% 5% 2 6% 5% 4 3 3 4 7% 47%	2000
	11th to 20th	Night	12% 6% 1 5% 3 4 4 3 4 7% 52%	2000
	21st to end	Night	22% 5% 2 6% 4 4 4 3 5% 7% 39%	2305
October	1st to 10th	Night	18% 6% 2 4 3 3 3 3 4 7% 46%	2400
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 5% 6% 44%	2377
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 7% 41%	2508
November	1st to 10th	Night	10% 6% 3 5% 4 3 4 3 5% 5% 52%	2472
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 3 5% 6% 38%	2715
	21st to end	Night	17% 8% 4 7% 5% 4 4 4 5% 7% 35%	2800
December	1st to 10th	Night	12% 9% 6% 7% 5% 5% 5% 4 6% 8% 33%	4580
	11th to 20th	Night	21% 10% 3 7% 5% 5% 5% 4 5% 6% 30%	5205
	21st to end	Night	19% 13% 3 10% 6% 5% 5% 3 5% 6% 24%	6996

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	54% 6% 12% 18% 9% 1	3598
	11th to 20th	Day	63% 6% 9% 12% 8% 2	4032
	21st to end	Day	57% 7% 9% 17% 10% 1	4466
February	1st to 10th	Day	56% 5% 11% 17% 9% 1	1800
	11th to 20th	Day	54% 6% 10% 22% 8% 1	1762
	21st to end	Day	54% 5% 10% 19% 9% 3	1650
March	1st to 10th	Day	48% 5% 11% 22% 10% 4	4862
	11th to 20th	Day	44% 7% 14% 23% 9% 3	4980
	21st to end	Day	45% 6% 10% 19% 16% 3	5545
April	1st to 10th	Day	50% 4 11% 19% 14% 3	2400
	11th to 20th	Day	46% 6% 9% 19% 16% 3	2374
	21st to end	Day	46% 6% 9% 21% 15% 3	2446
May	1st to 10th	Day	54% 7% 10% 17% 10% 2	5897
	11th to 20th	Day	53% 7% 10% 19% 10% 1	6271
	21st to end	Day	44% 9% 12% 22% 12% 2	6949
June	1st to 10th	Day	42% 9% 11% 22% 14% 2	2800
	11th to 20th	Day	44% 8% 12% 21% 14% 2	2800
	21st to end	Day	37% 9% 13% 26% 15% 1	2800
July	1st to 10th	Day	39% 9% 11% 25% 15% 1	6290
	11th to 20th	Day	42% 10% 15% 20% 12% 1	6252
	21st to end	Day	39% 6% 10% 23% 20% 1	6240
August	1st to 10th	Day	47% 8% 14% 21% 9%	4221
	11th to 20th	Day	43% 7% 10% 28% 11% 1	4129
	21st to end	Day	36% 6% 10% 29% 17% 2	4312
September	1st to 10th	Day	39% 6% 10% 23% 19% 3	2400
	11th to 20th	Day	34% 6% 11% 26% 21% 3	2400
	21st to end	Day	48% 6% 10% 17% 18% 1	2090
October	1st to 10th	Day	39% 5% 8% 26% 18% 4	3260
	11th to 20th	Day	37% 7% 9% 25% 20% 2	3220
	21st to end	Day	29% 4 16% 31% 19% 1	3428
November	1st to 10th	Day	36% 6% 14% 23% 18% 3	1824
	11th to 20th	Day	48% 6% 11% 21% 13% 2	1683
	21st to end	Day	50% 7% 12% 22% 9% 1	1600
December	1st to 10th	Day	55% 7% 11% 19% 6% 1	2600
	11th to 20th	Day	56% 6% 12% 21% 5%	2986
	21st to end	Day	62% 7% 13% 11% 5% 1	3960
January	1st to 10th	Night	54% 7% 10% 18% 10% 1	6349
	11th to 20th	Night	56% 7% 13% 13% 10% 2	5924
	21st to end	Night	53% 6% 10% 18% 12% 1	6490
February	1st to 10th	Night	51% 6% 13% 22% 8% 1	2600
	11th to 20th	Night	50% 6% 12% 19% 13% 1	2533
	21st to end	Night	51% 6% 9% 20% 12% 2	1980
March	1st to 10th	Night	50% 5% 11% 20% 11% 3	4975
	11th to 20th	Night	38% 6% 16% 26% 9% 4	4980
	21st to end	Night	44% 5% 10% 18% 21% 3	5386
April	1st to 10th	Night	52% 5% 9% 18% 14% 2	2000
	11th to 20th	Night	44% 6% 9% 22% 16% 3	1975
	21st to end	Night	42% 7% 10% 22% 16% 4	1970
May	1st to 10th	Night	56% 6% 9% 15% 11% 3	4060
	11th to 20th	Night	50% 8% 11% 18% 12% 1	3625
	21st to end	Night	45% 6% 13% 19% 13% 3	3927
June	1st to 10th	Night	39% 8% 11% 23% 17% 2	1600
	11th to 20th	Night	40% 8% 13% 22% 15% 2	1600
	21st to end	Night	34% 7% 15% 27% 16% 1	1600
July	1st to 10th	Night	39% 8% 13% 22% 15% 3	3560
	11th to 20th	Night	43% 10% 14% 19% 12% 1	3568
	21st to end	Night	36% 8% 11% 22% 23% 1	4297
August	1st to 10th	Night	45% 8% 15% 22% 10% 1	2904
	11th to 20th	Night	41% 6% 11% 27% 15%	2952
	21st to end	Night	36% 6% 12% 30% 14% 2	3586
September	1st to 10th	Night	39% 6% 10% 23% 19% 3	2000
	11th to 20th	Night	32% 5% 10% 30% 21% 2	2000
	21st to end	Night	45% 6% 10% 20% 18% 2	2305
October	1st to 10th	Night	37% 5% 11% 24% 21% 1	3920
	11th to 20th	Night	31% 5% 10% 28% 22% 4	3876
	21st to end	Night	30% 4 13% 29% 22% 2	4120
November	1st to 10th	Night	35% 5% 11% 26% 20% 3	2472
	11th to 20th	Night	46% 5% 9% 22% 14% 3	2715
	21st to end	Night	47% 6% 13% 23% 10% 1	2800
December	1st to 10th	Night	47% 7% 15% 25% 6%	4580
	11th to 20th	Night	54% 5% 14% 19% 7% 1	5208
	21st to end	Night	58% 6% 11% 16% 8%	6996

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	38% 24% 24% 12% 3 1	200
	11th to 20th	Day	44% 18% 26% 10% 1	293
	21st to end	Day	31% 25% 30% 11% 2 1	330
February	1st to 10th	Day	33% 24% 28% 13% 2	300
	11th to 20th	Day	42% 22% 32% 4	300
	21st to end	Day	34% 20% 27% 17% 2	414
March	1st to 10th	Day	34% 17% 30% 15% 4	500
	11th to 20th	Day	32% 23% 26% 15% 4 1	500
	21st to end	Day	31% 21% 26% 15% 5% 1	550
April	1st to 10th	Day	36% 19% 25% 15% 5% 1	500
	11th to 20th	Day	28% 23% 21% 17% 8% 3	492
	21st to end	Day	28% 19% 31% 13% 8% 2	500
May	1st to 10th	Day	33% 16% 29% 12% 7% 3	503
	11th to 20th	Day	27% 17% 26% 19% 10% 2	527
	21st to end	Day	23% 10% 26% 22% 14% 5%	573
June	1st to 10th	Day	19% 16% 26% 23% 12% 4	530
	11th to 20th	Day	22% 12% 27% 23% 13% 3	530
	21st to end	Day	15% 10% 32% 25% 14% 4	530
July	1st to 10th	Day	12% 8% 24% 32% 17% 7%	525
	11th to 20th	Day	9% 7% 28% 30% 19% 7%	524
	21st to end	Day	13% 7% 28% 30% 16% 5%	576
August	1st to 10th	Day	13% 6% 25% 27% 22% 7%	526
	11th to 20th	Day	17% 5% 23% 28% 19% 8%	519
	21st to end	Day	16% 7% 23% 26% 16% 11%	583
September	1st to 10th	Day	20% 10% 18% 25% 20% 8%	530
	11th to 20th	Day	16% 5% 20% 27% 22% 9%	530
	21st to end	Day	30% 10% 22% 20% 15% 4	530
October	1st to 10th	Day	25% 6% 21% 25% 17% 5%	530
	11th to 20th	Day	22% 15% 28% 24% 9% 2	522
	21st to end	Day	24% 14% 29% 24% 9%	539
November	1st to 10th	Day	20% 14% 24% 26% 13% 2	428
	11th to 20th	Day	27% 22% 27% 18% 6% 1	283
	21st to end	Day	34% 20% 29% 15% 3 1	200
December	1st to 10th	Day	36% 21% 29% 9% 4 1	200
	11th to 20th	Day	36% 15% 36% 11% 4 1	200
	21st to end	Day	35% 17% 28% 16% 3	220
January	1st to 10th	Night	26% 21% 32% 18% 4	800
	11th to 20th	Night	30% 16% 28% 22% 4	800
	21st to end	Night	23% 22% 28% 25% 2	880
February	1st to 10th	Night	27% 17% 27% 27% 2	800
	11th to 20th	Night	32% 22% 28% 17% 1	783
	21st to end	Night	30% 15% 30% 23% 2	660
March	1st to 10th	Night	28% 17% 28% 23% 4	800
	11th to 20th	Night	27% 19% 28% 21% 5% 1	800
	21st to end	Night	23% 17% 31% 22% 6% 1	880
April	1st to 10th	Night	29% 17% 29% 21% 5%	800
	11th to 20th	Night	23% 14% 29% 25% 7% 1	791
	21st to end	Night	24% 12% 28% 28% 8%	770
May	1st to 10th	Night	28% 9% 21% 30% 13% 1	600
	11th to 20th	Night	22% 7% 28% 33% 10%	417
	21st to end	Night	19% 7% 18% 33% 22% 1	433
June	1st to 10th	Night	14% 6% 21% 39% 21%	400
	11th to 20th	Night	16% 7% 19% 40% 18%	400
	21st to end	Night	13% 3 22% 42% 20% 1	400
July	1st to 10th	Night	12% 5% 17% 41% 23% 3	390
	11th to 20th	Night	9% 1 15% 44% 28% 2	391
	21st to end	Night	12% 3 15% 44% 25% 1	657
August	1st to 10th	Night	13% 1 13% 42% 30% 2	597
	11th to 20th	Night	16% 4 11% 37% 28% 4	618
	21st to end	Night	15% 4 13% 36% 28% 3	880
September	1st to 10th	Night	18% 4 16% 35% 25% 2	800
	11th to 20th	Night	15% 5% 13% 32% 30% 5%	800
	21st to end	Night	24% 8% 15% 30% 20% 3	800
October	1st to 10th	Night	21% 7% 19% 30% 21% 3	800
	11th to 20th	Night	20% 12% 23% 33% 11% 1	793
	21st to end	Night	24% 13% 23% 31% 9% 1	836
November	1st to 10th	Night	16% 11% 25% 36% 11% 1	780
	11th to 20th	Night	26% 17% 21% 28% 8% 2	800
	21st to end	Night	25% 17% 26% 27% 5% 1	800
December	1st to 10th	Night	27% 17% 28% 21% 6%	800
	11th to 20th	Night	26% 16% 27% 27% 4	800
	21st to end	Night	25% 16% 26% 31% 3	880

Page generated at 2025-10-16 01:50:28. Version ac07

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