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 2024-04-10.

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 6% 6% 4 5% 5% 5% 7% 35%	83199
	Night	16% 8% 3 6% 5% 3 5% 4 5% 7% 37%	85587
Transparency	Day	47% 7% 12% 20% 12% 2	82909
	Night	46% 7% 12% 21% 13% 2	85396
Seeing	Day	25% 14% 26% 21% 11% 4	13976
	Night	22% 13% 23% 29% 12% 1	23575

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	15% 12% 5% 7% 7% 4 4 4 4 5% 32%	2517
	11th to 20th	Day	12% 16% 8% 12% 8% 4 4 4 4 5% 22%	2812
	21st to end	Day	18% 18% 6% 9% 7% 4 5% 4 4 4 21%	3124
February	1st to 10th	Day	15% 14% 5% 8% 7% 3 5% 5% 5% 5% 28%	1710
	11th to 20th	Day	14% 9% 4 10% 8% 4 5% 5% 5% 5% 31%	1663
	21st to end	Day	19% 10% 4 8% 5% 3 4 4 5% 5% 32%	1570
March	1st to 10th	Day	23% 10% 3 6% 6% 3 4 4 4 6% 31%	3390
	11th to 20th	Day	12% 9% 4 7% 7% 3 6% 4 5% 8% 35%	3480
	21st to end	Day	16% 9% 3 6% 5% 3 4 4 4 5% 40%	3867
April	1st to 10th	Day	17% 9% 4 7% 5% 3 4 4 5% 7% 35%	2270
	11th to 20th	Day	17% 8% 2 6% 6% 3 5% 5% 5% 6% 37%	2134
	21st to end	Day	21% 6% 3 5% 7% 3 5% 4 5% 6% 35%	2200
May	1st to 10th	Day	20% 6% 3 5% 7% 4 5% 4 4 8% 34%	2340
	11th to 20th	Day	21% 8% 3 5% 6% 4 5% 5% 5% 7% 34%	2455
	21st to end	Day	16% 6% 2 5% 5% 3 5% 5% 7% 8% 38%	2725
June	1st to 10th	Day	13% 6% 3 5% 6% 5% 6% 6% 6% 7% 37%	2520
	11th to 20th	Day	15% 4 3 6% 6% 4 5% 5% 6% 9% 37%	2520
	21st to end	Day	11% 4 2 5% 6% 4 6% 6% 6% 10% 40%	2520
July	1st to 10th	Day	11% 6% 2 6% 7% 3 5% 6% 7% 8% 38%	2520
	11th to 20th	Day	8% 5% 3 6% 6% 4 6% 6% 6% 10% 41%	2515
	21st to end	Day	11% 5% 3 7% 7% 4 6% 5% 7% 9% 37%	2547
August	1st to 10th	Day	11% 5% 2 6% 7% 4 7% 6% 8% 9% 34%	2327
	11th to 20th	Day	14% 5% 3 7% 6% 4 5% 6% 6% 9% 37%	2263
	21st to end	Day	14% 5% 2 6% 5% 3 5% 6% 6% 8% 39%	2376
September	1st to 10th	Day	18% 6% 2 4 5% 3 4 4 4 6% 43%	2160
	11th to 20th	Day	15% 5% 2 3 4 3 4 4 5% 7% 49%	2160
	21st to end	Day	19% 6% 3 6% 4 2 4 5% 5% 7% 39%	1882
October	1st to 10th	Day	19% 6% 2 5% 5% 3 4 5% 5% 7% 39%	1800
	11th to 20th	Day	20% 5% 2 5% 5% 3 5% 4 5% 6% 38%	1800
	21st to end	Day	19% 6% 3 5% 6% 2 4 5% 6% 7% 36%	1980
November	1st to 10th	Day	11% 6% 3 5% 5% 3 4 4 5% 7% 46%	1731
	11th to 20th	Day	16% 9% 3 6% 6% 3 5% 5% 5% 6% 36%	1599
	21st to end	Day	17% 10% 3 5% 5% 5% 5% 4 6% 6% 33%	1520
December	1st to 10th	Day	19% 11% 5% 8% 7% 2 5% 4 6% 7% 26%	1520
	11th to 20th	Day	19% 12% 5% 7% 5% 3 4 4 5% 6% 30%	1910
	21st to end	Day	22% 12% 4 10% 8% 5% 5% 5% 4 5% 20%	2772
January	1st to 10th	Night	14% 12% 4 9% 7% 3 4 4 5% 7% 30%	4429
	11th to 20th	Night	16% 12% 7% 9% 6% 3 5% 5% 5% 7% 27%	4144
	21st to end	Night	23% 12% 4 10% 6% 3 4 4 4 5% 25%	4532
February	1st to 10th	Night	16% 9% 4 7% 6% 3 6% 4 6% 7% 33%	2470
	11th to 20th	Night	16% 9% 4 7% 6% 4 5% 5% 6% 7% 32%	2402
	21st to end	Night	17% 10% 4 7% 6% 3 5% 5% 5% 5% 34%	1884
March	1st to 10th	Night	23% 10% 4 6% 5% 3 3 4 4 5% 34%	3461
	11th to 20th	Night	11% 9% 3 7% 6% 3 4 6% 5% 7% 38%	3480
	21st to end	Night	16% 6% 3 6% 5% 3 4 4 4 6% 43%	3764
April	1st to 10th	Night	18% 8% 4 8% 6% 3 4 4 5% 6% 33%	1900
	11th to 20th	Night	16% 6% 3 5% 5% 3 5% 5% 5% 8% 39%	1775
	21st to end	Night	18% 5% 2 5% 5% 3 5% 4 5% 6% 40%	1774
May	1st to 10th	Night	19% 8% 2 6% 6% 2 5% 5% 5% 5% 36%	1620
	11th to 20th	Night	18% 6% 3 5% 4 3 5% 5% 5% 7% 41%	1436
	21st to end	Night	14% 7% 2 5% 4 3 5% 4 6% 9% 42%	1550
June	1st to 10th	Night	12% 6% 3 5% 4 3 5% 5% 5% 8% 44%	1440
	11th to 20th	Night	13% 5% 3 6% 6% 3 4 5% 5% 9% 40%	1440
	21st to end	Night	11% 4 2 4 4 3 5% 5% 6% 9% 45%	1440
July	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 5% 5% 10% 43%	1440
	11th to 20th	Night	8% 3 2 5% 5% 3 5% 6% 8% 10% 46%	1442
	21st to end	Night	9% 5% 2 5% 6% 4 5% 5% 8% 10% 42%	1771
August	1st to 10th	Night	10% 6% 2 6% 5% 3 5% 5% 6% 9% 43%	1611
	11th to 20th	Night	13% 7% 2 5% 5% 3 5% 5% 5% 8% 42%	1619
	21st to end	Night	14% 6% 2 4 4 3 5% 5% 6% 8% 43%	1980
September	1st to 10th	Night	17% 4 2 5% 6% 3 4 3 4 7% 46%	1800
	11th to 20th	Night	13% 5% 2 4 4 2 4 4 4 7% 52%	1800
	21st to end	Night	19% 4 2 5% 5% 3 4 4 6% 7% 43%	2074
October	1st to 10th	Night	19% 6% 2 4 3 2 3 4 5% 7% 44%	2160
	11th to 20th	Night	17% 6% 3 4 5% 4 4 4 5% 6% 43%	2160
	21st to end	Night	20% 7% 2 5% 4 2 4 4 5% 7% 40%	2376
November	1st to 10th	Night	10% 6% 3 5% 5% 2 5% 3 5% 5% 52%	2345
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 5% 7% 39%	2579
	21st to end	Night	17% 7% 4 6% 6% 3 4 5% 5% 7% 37%	2660
December	1st to 10th	Night	17% 10% 5% 6% 5% 4 5% 5% 6% 8% 30%	2660
	11th to 20th	Night	18% 10% 3 6% 6% 3 5% 5% 5% 6% 32%	3285
	21st to end	Night	19% 10% 3 9% 8% 4 5% 5% 6% 7% 24%	4884

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	51% 7% 15% 19% 6% 2	2518
	11th to 20th	Day	63% 5% 10% 11% 8% 1	2812
	21st to end	Day	63% 8% 8% 15% 4 1	3124
February	1st to 10th	Day	56% 5% 11% 17% 8% 1	1710
	11th to 20th	Day	53% 6% 10% 22% 8% 1	1672
	21st to end	Day	54% 5% 10% 19% 9% 3	1570
March	1st to 10th	Day	53% 5% 11% 23% 7% 1	3390
	11th to 20th	Day	44% 8% 14% 21% 11% 2	3480
	21st to end	Day	46% 6% 11% 22% 14% 2	3867
April	1st to 10th	Day	49% 4 11% 19% 14% 3	2270
	11th to 20th	Day	46% 6% 9% 19% 17% 3	2134
	21st to end	Day	48% 6% 9% 20% 14% 3	2200
May	1st to 10th	Day	49% 7% 8% 20% 13% 2	2340
	11th to 20th	Day	50% 7% 11% 16% 13% 3	2459
	21st to end	Day	42% 9% 14% 21% 11% 2	2725
June	1st to 10th	Day	43% 9% 12% 21% 14% 2	2520
	11th to 20th	Day	44% 8% 12% 21% 14% 2	2520
	21st to end	Day	37% 9% 13% 27% 14% 1	2520
July	1st to 10th	Day	41% 10% 12% 20% 14% 2	2450
	11th to 20th	Day	36% 10% 14% 22% 17% 2	2440
	21st to end	Day	41% 8% 12% 21% 16% 1	2405
August	1st to 10th	Day	41% 10% 15% 22% 12%	2314
	11th to 20th	Day	43% 9% 12% 22% 13% 1	2263
	21st to end	Day	42% 8% 10% 22% 16% 3	2376
September	1st to 10th	Day	42% 7% 11% 21% 16% 3	2160
	11th to 20th	Day	35% 6% 11% 25% 21% 3	2160
	21st to end	Day	44% 6% 11% 19% 19% 1	1882
October	1st to 10th	Day	44% 8% 11% 22% 14% 1	1800
	11th to 20th	Day	44% 6% 12% 23% 14%	1800
	21st to end	Day	46% 6% 12% 22% 13% 1	1980
November	1st to 10th	Day	37% 6% 14% 24% 17% 3	1731
	11th to 20th	Day	47% 6% 11% 21% 13% 2	1599
	21st to end	Day	50% 7% 12% 23% 9% 1	1520
December	1st to 10th	Day	57% 7% 12% 16% 7%	1520
	11th to 20th	Day	55% 5% 13% 21% 5%	1906
	21st to end	Day	64% 8% 12% 12% 4	2772
January	1st to 10th	Night	51% 7% 12% 21% 8% 1	4429
	11th to 20th	Night	56% 7% 13% 15% 8% 1	4144
	21st to end	Night	60% 7% 8% 17% 7% 1	4532
February	1st to 10th	Night	50% 6% 13% 22% 9% 1	2470
	11th to 20th	Night	49% 6% 12% 18% 13% 1	2404
	21st to end	Night	51% 6% 9% 20% 12% 2	1884
March	1st to 10th	Night	53% 5% 10% 21% 10% 1	3461
	11th to 20th	Night	42% 8% 12% 26% 10% 1	3480
	21st to end	Night	42% 4 11% 21% 20% 3	3764
April	1st to 10th	Night	51% 5% 10% 18% 14% 2	1900
	11th to 20th	Night	42% 7% 9% 23% 16% 3	1775
	21st to end	Night	43% 6% 11% 21% 15% 4	1774
May	1st to 10th	Night	48% 7% 9% 20% 12% 4	1620
	11th to 20th	Night	43% 8% 11% 17% 18% 2	1437
	21st to end	Night	40% 7% 15% 22% 14% 3	1551
June	1st to 10th	Night	39% 8% 11% 23% 17% 2	1440
	11th to 20th	Night	41% 8% 12% 22% 16% 2	1440
	21st to end	Night	34% 7% 14% 28% 15% 1	1440
July	1st to 10th	Night	39% 9% 12% 22% 14% 4	1400
	11th to 20th	Night	31% 11% 14% 23% 19% 2	1394
	21st to end	Night	34% 10% 15% 22% 16% 3	1672
August	1st to 10th	Night	35% 8% 18% 23% 15% 1	1600
	11th to 20th	Night	41% 7% 14% 20% 17% 1	1619
	21st to end	Night	37% 8% 13% 22% 16% 3	1980
September	1st to 10th	Night	40% 6% 11% 22% 18% 3	1800
	11th to 20th	Night	32% 6% 9% 29% 21% 2	1800
	21st to end	Night	41% 6% 10% 22% 20% 2	2074
October	1st to 10th	Night	40% 6% 13% 25% 15% 1	2160
	11th to 20th	Night	41% 6% 11% 26% 14% 2	2160
	21st to end	Night	44% 6% 10% 23% 16% 1	2376
November	1st to 10th	Night	36% 5% 11% 26% 19% 3	2345
	11th to 20th	Night	46% 5% 9% 22% 15% 3	2579
	21st to end	Night	46% 6% 13% 24% 10% 1	2660
December	1st to 10th	Night	50% 8% 14% 19% 8%	2660
	11th to 20th	Night	51% 6% 12% 22% 8% 1	3288
	21st to end	Night	56% 7% 14% 17% 5%	4884

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	37% 23% 24% 13% 3 1	180
	11th to 20th	Day	43% 19% 26% 11% 1	263
	21st to end	Day	32% 25% 29% 11% 2 1	297
February	1st to 10th	Day	31% 24% 29% 14% 2	270
	11th to 20th	Day	40% 22% 34% 4	270
	21st to end	Day	35% 19% 26% 18% 2	382
March	1st to 10th	Day	33% 17% 30% 16% 4	460
	11th to 20th	Day	32% 23% 25% 14% 4 1	460
	21st to end	Day	32% 21% 25% 15% 5% 2	506
April	1st to 10th	Day	33% 20% 25% 16% 5% 1	457
	11th to 20th	Day	26% 22% 21% 17% 9% 4	412
	21st to end	Day	29% 18% 31% 13% 7% 2	420
May	1st to 10th	Day	31% 17% 28% 13% 7% 3	423
	11th to 20th	Day	25% 17% 26% 19% 9% 2	447
	21st to end	Day	21% 10% 26% 22% 16% 5%	485
June	1st to 10th	Day	17% 16% 27% 25% 11% 3	450
	11th to 20th	Day	21% 13% 27% 23% 13% 3	450
	21st to end	Day	15% 10% 33% 25% 14% 4	450
July	1st to 10th	Day	12% 8% 24% 30% 18% 8%	445
	11th to 20th	Day	8% 6% 28% 31% 20% 7%	444
	21st to end	Day	13% 8% 29% 29% 16% 5%	488
August	1st to 10th	Day	11% 6% 25% 27% 24% 7%	446
	11th to 20th	Day	16% 5% 24% 28% 19% 9%	439
	21st to end	Day	16% 7% 24% 26% 17% 11%	495
September	1st to 10th	Day	21% 9% 19% 24% 19% 8%	450
	11th to 20th	Day	18% 6% 21% 26% 20% 8%	450
	21st to end	Day	25% 12% 22% 20% 16% 5%	450
October	1st to 10th	Day	26% 6% 21% 24% 18% 5%	450
	11th to 20th	Day	23% 15% 28% 24% 8% 2	450
	21st to end	Day	26% 14% 28% 23% 8%	495
November	1st to 10th	Day	21% 14% 23% 27% 14% 2	395
	11th to 20th	Day	28% 22% 24% 19% 7% 1	259
	21st to end	Day	30% 21% 30% 16% 3 1	180
December	1st to 10th	Day	37% 21% 28% 9% 4 1	180
	11th to 20th	Day	34% 15% 35% 11% 4 1	180
	21st to end	Day	36% 17% 28% 15% 4	198
January	1st to 10th	Night	25% 20% 33% 18% 4	760
	11th to 20th	Night	29% 16% 28% 23% 4	760
	21st to end	Night	23% 22% 27% 25% 2	836
February	1st to 10th	Night	27% 17% 27% 28% 2	760
	11th to 20th	Night	31% 23% 29% 17% 1	743
	21st to end	Night	31% 16% 28% 24% 2	628
March	1st to 10th	Night	28% 17% 28% 23% 4	760
	11th to 20th	Night	27% 19% 28% 21% 5% 1	760
	21st to end	Night	23% 17% 31% 22% 6% 1	836
April	1st to 10th	Night	28% 17% 29% 21% 6%	760
	11th to 20th	Night	21% 15% 29% 26% 8% 1	711
	21st to end	Night	24% 13% 29% 26% 8%	694
May	1st to 10th	Night	27% 9% 20% 30% 14% 1	540
	11th to 20th	Night	21% 8% 28% 33% 10%	375
	21st to end	Night	17% 7% 18% 34% 23% 1	389
June	1st to 10th	Night	14% 6% 21% 39% 20%	360
	11th to 20th	Night	15% 8% 21% 39% 18%	360
	21st to end	Night	12% 3 23% 43% 19% 1	360
July	1st to 10th	Night	12% 5% 18% 40% 22% 3	350
	11th to 20th	Night	9% 1 15% 44% 28% 2	350
	21st to end	Night	12% 3 16% 44% 24% 1	591
August	1st to 10th	Night	12% 1 13% 42% 30% 2	537
	11th to 20th	Night	17% 5% 10% 37% 27% 5%	555
	21st to end	Night	15% 4 13% 35% 28% 3	792
September	1st to 10th	Night	19% 4 15% 34% 24% 2	720
	11th to 20th	Night	16% 6% 14% 30% 29% 5%	720
	21st to end	Night	20% 9% 16% 30% 21% 4	720
October	1st to 10th	Night	21% 8% 19% 30% 20% 3	720
	11th to 20th	Night	20% 13% 22% 33% 12% 1	720
	21st to end	Night	26% 13% 23% 29% 8% 1	792
November	1st to 10th	Night	16% 12% 24% 36% 12% 1	740
	11th to 20th	Night	25% 18% 20% 27% 8% 2	760
	21st to end	Night	25% 16% 26% 28% 5% 1	760
December	1st to 10th	Night	28% 16% 28% 21% 6%	760
	11th to 20th	Night	26% 16% 28% 27% 4	760
	21st to end	Night	25% 16% 26% 30% 3	836

Page generated at 2024-04-07 09:28:13. Version ac07

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