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 2024-11-05.

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%	101337
	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 37%	97657
Transparency	Day	46% 7% 12% 22% 13% 2	104976
	Night	45% 6% 12% 22% 14% 2	102157
Seeing	Day	24% 14% 26% 21% 11% 4	14811
	Night	22% 12% 23% 29% 12% 1	24230

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	14% 13% 4 9% 6% 5% 4 3 4 5% 32%	2517
	11th to 20th	Day	12% 17% 8% 13% 6% 5% 4 3 4 5% 21%	2812
	21st to end	Day	18% 18% 5% 10% 6% 5% 5% 3 5% 4 21%	3124
February	1st to 10th	Day	15% 15% 5% 8% 6% 4 6% 3 5% 5% 28%	1710
	11th to 20th	Day	14% 9% 4 11% 7% 5% 5% 4 5% 5% 31%	1663
	21st to end	Day	19% 11% 3 9% 5% 4 4 3 5% 5% 31%	1570
March	1st to 10th	Day	23% 10% 3 7% 5% 4 4 3 4 6% 31%	3390
	11th to 20th	Day	12% 10% 3 8% 6% 5% 5% 3 5% 8% 35%	3480
	21st to end	Day	16% 10% 3 7% 4 4 4 3 4 5% 40%	3867
April	1st to 10th	Day	17% 10% 3 8% 5% 4 4 3 5% 7% 35%	2280
	11th to 20th	Day	16% 9% 2 7% 5% 4 5% 4 5% 6% 37%	2254
	21st to end	Day	20% 7% 3 6% 6% 4 4 4 6% 7% 35%	2323
May	1st to 10th	Day	29% 6% 2 6% 5% 4 4 3 4 6% 30%	4082
	11th to 20th	Day	24% 8% 3 7% 5% 5% 5% 3 5% 7% 29%	4361
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	4823
June	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 5% 6% 7% 37%	2660
	11th to 20th	Day	14% 5% 2 6% 6% 6% 4 4 6% 9% 37%	2660
	21st to end	Day	11% 5% 2 6% 6% 5% 5% 5% 7% 10% 40%	2660
July	1st to 10th	Day	8% 6% 2 7% 6% 5% 5% 5% 8% 9% 39%	4440
	11th to 20th	Day	7% 5% 2 7% 5% 6% 5% 6% 7% 11% 40%	4421
	21st to end	Day	11% 7% 3 7% 6% 5% 5% 4 7% 8% 38%	4466
August	1st to 10th	Day	20% 5% 2 7% 6% 5% 5% 4 7% 8% 31%	4104
	11th to 20th	Day	14% 5% 2 7% 6% 6% 6% 4 6% 8% 37%	3962
	21st to end	Day	16% 4 2 5% 4 4 4 4 5% 7% 45%	4180
September	1st to 10th	Day	17% 6% 1 5% 5% 4 4 3 4 6% 45%	2280
	11th to 20th	Day	14% 5% 2 4 3 4 4 3 5% 6% 50%	2280
	21st to end	Day	22% 6% 2 7% 3 3 4 4 5% 7% 37%	1986
October	1st to 10th	Day	19% 6% 2 5% 5% 4 5% 4 5% 6% 40%	1900
	11th to 20th	Day	19% 6% 2 6% 5% 5% 5% 3 6% 6% 39%	1900
	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 3 4 5% 7% 46%	1771
	11th to 20th	Day	15% 10% 3 7% 5% 4 5% 4 5% 6% 36%	1599
	21st to end	Day	17% 11% 2 6% 5% 5% 5% 3 5% 6% 33%	1520
December	1st to 10th	Day	19% 12% 4 9% 6% 4 5% 3 6% 7% 26%	1520
	11th to 20th	Day	19% 12% 5% 7% 5% 5% 4 3 5% 6% 30%	1910
	21st to end	Day	21% 13% 4 11% 7% 6% 4 4 5% 5% 20%	2772
January	1st to 10th	Night	14% 13% 4 10% 6% 5% 4 3 5% 7% 30%	4429
	11th to 20th	Night	16% 12% 6% 10% 4 5% 4 4 5% 6% 27%	4144
	21st to end	Night	23% 13% 4 11% 5% 3 4 3 4 5% 25%	4532
February	1st to 10th	Night	15% 10% 3 8% 6% 5% 5% 3 6% 6% 32%	2470
	11th to 20th	Night	15% 10% 3 7% 6% 5% 4 4 6% 7% 32%	2402
	21st to end	Night	17% 11% 3 8% 5% 4 5% 4 5% 5% 34%	1884
March	1st to 10th	Night	23% 11% 3 7% 4 4 3 3 4 4 34%	3461
	11th to 20th	Night	11% 10% 3 8% 5% 4 4 4 5% 7% 38%	3480
	21st to end	Night	16% 7% 3 6% 4 3 4 3 4 6% 43%	3764
April	1st to 10th	Night	17% 9% 3 9% 6% 5% 4 3 5% 6% 34%	1900
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 5% 7% 39%	1875
	21st to end	Night	17% 6% 2 7% 4 4 5% 3 5% 7% 40%	1872
May	1st to 10th	Night	28% 8% 2 7% 5% 4 4 4 4 5% 30%	2813
	11th to 20th	Night	26% 8% 2 6% 4 4 4 4 5% 6% 31%	2530
	21st to end	Night	14% 6% 2 6% 4 4 4 4 7% 10% 40%	2732
June	1st to 10th	Night	12% 6% 3 5% 4 4 5% 4 5% 7% 43%	1520
	11th to 20th	Night	13% 5% 2 7% 5% 4 4 4 5% 9% 41%	1520
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 7% 9% 45%	1520
July	1st to 10th	Night	8% 6% 2 8% 5% 5% 5% 4 6% 9% 41%	2520
	11th to 20th	Night	8% 3 2 7% 5% 5% 6% 5% 8% 11% 42%	2536
	21st to end	Night	13% 6% 3 6% 5% 5% 5% 3 6% 8% 40%	3087
August	1st to 10th	Night	19% 6% 2 7% 4 4 5% 4 5% 8% 36%	2825
	11th to 20th	Night	17% 6% 2 6% 3 3 4 4 5% 7% 42%	2834
	21st to end	Night	16% 5% 2 4 3 4 4 3 5% 7% 46%	3476
September	1st to 10th	Night	16% 5% 2 6% 5% 4 3 3 4 7% 47%	1900
	11th to 20th	Night	12% 6% 1 4 3 4 4 3 4 7% 53%	1900
	21st to end	Night	22% 4 2 6% 4 4 4 3 5% 7% 40%	2190
October	1st to 10th	Night	19% 6% 2 4 3 3 3 3 4 7% 45%	2280
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 5% 6% 44%	2280
	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% 6% 51%	2405
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 3 5% 6% 39%	2579
	21st to end	Night	17% 8% 3 6% 5% 4 4 4 5% 7% 36%	2660
December	1st to 10th	Night	17% 11% 4 7% 4 5% 5% 4 6% 8% 29%	2660
	11th to 20th	Night	18% 10% 3 7% 5% 5% 5% 4 6% 6% 32%	3285
	21st to end	Night	19% 11% 3 10% 7% 5% 5% 4 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	2280
	11th to 20th	Day	46% 6% 9% 19% 16% 3	2254
	21st to end	Day	47% 6% 9% 20% 14% 3	2323
May	1st to 10th	Day	54% 6% 8% 20% 10% 1	4120
	11th to 20th	Day	54% 8% 12% 16% 8% 2	4365
	21st to end	Day	37% 9% 13% 24% 15% 2	4837
June	1st to 10th	Day	42% 9% 12% 22% 14% 2	2660
	11th to 20th	Day	43% 8% 13% 21% 14% 2	2660
	21st to end	Day	37% 9% 13% 27% 14% 1	2660
July	1st to 10th	Day	39% 9% 13% 25% 11% 2	4370
	11th to 20th	Day	36% 9% 16% 23% 16% 1	4346
	21st to end	Day	42% 7% 12% 22% 16% 1	4324
August	1st to 10th	Day	48% 9% 14% 21% 9%	4091
	11th to 20th	Day	43% 7% 10% 28% 11% 1	4001
	21st to end	Day	37% 6% 10% 29% 17% 2	4180
September	1st to 10th	Day	40% 6% 10% 22% 18% 3	2280
	11th to 20th	Day	34% 6% 11% 25% 21% 3	2280
	21st to end	Day	46% 6% 11% 18% 18% 1	1986
October	1st to 10th	Day	40% 6% 8% 26% 17% 4	3160
	11th to 20th	Day	37% 7% 10% 25% 20% 2	3140
	21st to end	Day	29% 4 16% 31% 19% 1	3428
November	1st to 10th	Day	36% 6% 14% 23% 17% 3	1771
	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% 9% 19% 14% 2	1900
	11th to 20th	Night	43% 7% 9% 22% 16% 3	1875
	21st to end	Night	42% 7% 10% 22% 15% 4	1872
May	1st to 10th	Night	55% 6% 7% 18% 11% 2	2840
	11th to 20th	Night	53% 9% 12% 15% 10% 1	2531
	21st to end	Night	38% 8% 17% 21% 12% 4	2739
June	1st to 10th	Night	39% 8% 11% 23% 17% 2	1520
	11th to 20th	Night	39% 8% 13% 23% 15% 2	1520
	21st to end	Night	34% 7% 14% 28% 15% 1	1520
July	1st to 10th	Night	39% 8% 13% 19% 15% 4	2480
	11th to 20th	Night	33% 10% 15% 24% 16% 1	2488
	21st to end	Night	41% 8% 11% 16% 21% 2	2988
August	1st to 10th	Night	45% 8% 15% 21% 10%	2814
	11th to 20th	Night	41% 5% 11% 27% 15%	2861
	21st to end	Night	36% 6% 12% 30% 14% 2	3476
September	1st to 10th	Night	39% 6% 10% 23% 19% 3	1900
	11th to 20th	Night	32% 5% 9% 29% 22% 2	1900
	21st to end	Night	44% 6% 9% 21% 19% 2	2190
October	1st to 10th	Night	38% 5% 12% 24% 21% 1	3800
	11th to 20th	Night	31% 5% 10% 28% 22% 4	3779
	21st to end	Night	30% 4 13% 29% 22% 2	4120
November	1st to 10th	Night	35% 5% 12% 26% 19% 3	2405
	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	460
	11th to 20th	Day	27% 23% 21% 17% 9% 3	452
	21st to end	Day	28% 17% 32% 13% 7% 2	460
May	1st to 10th	Day	33% 16% 28% 13% 7% 3	463
	11th to 20th	Day	25% 17% 26% 19% 10% 3	487
	21st to end	Day	21% 10% 26% 23% 15% 5%	529
June	1st to 10th	Day	19% 16% 26% 24% 12% 3	490
	11th to 20th	Day	20% 12% 27% 24% 13% 4	490
	21st to end	Day	15% 11% 33% 24% 13% 3	490
July	1st to 10th	Day	12% 8% 25% 31% 17% 7%	485
	11th to 20th	Day	8% 7% 28% 31% 19% 7%	484
	21st to end	Day	13% 8% 29% 30% 16% 5%	532
August	1st to 10th	Day	13% 6% 25% 27% 23% 7%	486
	11th to 20th	Day	16% 5% 24% 27% 19% 8%	479
	21st to end	Day	16% 6% 23% 26% 17% 11%	539
September	1st to 10th	Day	20% 10% 19% 24% 19% 8%	490
	11th to 20th	Day	17% 6% 21% 27% 21% 9%	490
	21st to end	Day	30% 11% 22% 19% 15% 4	490
October	1st to 10th	Day	27% 7% 21% 23% 18% 4	490
	11th to 20th	Day	22% 14% 28% 24% 10% 2	490
	21st to end	Day	24% 14% 29% 24% 9%	539
November	1st to 10th	Day	20% 14% 23% 27% 13% 2	411
	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	27% 17% 29% 21% 6%	760
	11th to 20th	Night	22% 15% 29% 26% 8% 1	751
	21st to end	Night	23% 13% 29% 27% 8%	732
May	1st to 10th	Night	28% 9% 21% 29% 13% 1	570
	11th to 20th	Night	21% 8% 28% 33% 11%	396
	21st to end	Night	17% 7% 18% 34% 23% 1	411
June	1st to 10th	Night	14% 7% 20% 39% 20%	380
	11th to 20th	Night	14% 7% 20% 40% 19%	380
	21st to end	Night	12% 3 23% 42% 19% 1	380
July	1st to 10th	Night	12% 5% 18% 40% 23% 3	370
	11th to 20th	Night	8% 1 16% 45% 28% 2	371
	21st to end	Night	13% 3 16% 44% 23% 1	624
August	1st to 10th	Night	13% 1 13% 42% 29% 2	567
	11th to 20th	Night	16% 5% 11% 38% 26% 4	587
	21st to end	Night	16% 4 13% 36% 29% 3	836
September	1st to 10th	Night	19% 4 16% 34% 24% 2	760
	11th to 20th	Night	15% 6% 13% 31% 29% 5%	760
	21st to end	Night	23% 8% 15% 29% 20% 3	760
October	1st to 10th	Night	22% 7% 19% 29% 20% 3	760
	11th to 20th	Night	19% 13% 23% 33% 12% 1	760
	21st to end	Night	24% 13% 23% 31% 9% 1	836
November	1st to 10th	Night	16% 12% 25% 36% 12% 1	760
	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 2024-11-03 02:11:07. Version ac07

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