Montana Learning Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montana Learning Center clear sky chart from 2005-11-06 to 2025-11-18.

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	6% 8% 3 8% 5% 5% 5% 4 6% 8% 43%	135850
	Night	7% 9% 4 8% 5% 5% 5% 4 6% 7% 41%	131843
Transparency	Day	36% 6% 15% 23% 15% 5%	130537
	Night	39% 7% 17% 23% 12% 3	128160
Seeing	Day	12% 12% 29% 29% 15% 3	16792
	Night	12% 8% 26% 37% 15% 1	26143

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	6% 14% 6% 16% 8% 6% 6% 5% 7% 8% 18%	3179
	11th to 20th	Day	4 10% 5% 16% 8% 6% 5% 4 5% 6% 30%	3501
	21st to end	Day	3 8% 4 10% 6% 5% 5% 5% 7% 8% 39%	4202
February	1st to 10th	Day	9% 18% 6% 14% 7% 4 5% 4 7% 7% 20%	1800
	11th to 20th	Day	4 17% 6% 13% 7% 5% 4 5% 6% 8% 25%	1755
	21st to end	Day	6% 14% 6% 10% 7% 7% 5% 5% 6% 7% 26%	1582
March	1st to 10th	Day	4 9% 3 9% 4 4 4 4 6% 9% 44%	4728
	11th to 20th	Day	4 11% 4 9% 6% 6% 6% 4 6% 7% 37%	5000
	21st to end	Day	8% 15% 5% 12% 7% 7% 6% 4 7% 7% 22%	5486
April	1st to 10th	Day	7% 13% 4 9% 7% 6% 5% 5% 6% 8% 29%	2353
	11th to 20th	Day	8% 12% 4 11% 6% 5% 5% 5% 6% 6% 30%	2567
	21st to end	Day	10% 12% 4 10% 6% 6% 5% 4 6% 6% 31%	2600
May	1st to 10th	Day	11% 13% 3 6% 4 4 4 4 6% 7% 38%	5880
	11th to 20th	Day	10% 8% 3 7% 6% 6% 6% 5% 8% 9% 31%	6242
	21st to end	Day	8% 7% 2 7% 4 4 4 3 6% 8% 46%	7262
June	1st to 10th	Day	13% 8% 2 7% 5% 6% 5% 5% 7% 9% 34%	3000
	11th to 20th	Day	13% 7% 3 7% 6% 6% 6% 5% 7% 7% 35%	3000
	21st to end	Day	5% 4 2 6% 4 4 5% 4 7% 8% 51%	3000
July	1st to 10th	Day	3 2 1 4 4 4 5% 4 7% 8% 58%	6779
	11th to 20th	Day	5% 3 1 3 3 2 3 2 4 5% 68%	6780
	21st to end	Day	2 2 1 4 4 4 4 4 7% 9% 59%	6796
August	1st to 10th	Day	3 3 2 5% 5% 6% 6% 5% 8% 9% 47%	5779
	11th to 20th	Day	2 2 1 4 3 3 4 4 7% 8% 61%	5798
	21st to end	Day	5% 4 2 4 3 3 4 3 5% 7% 59%	6468
September	1st to 10th	Day	6% 4 1 5% 3 3 4 3 4 6% 61%	2412
	11th to 20th	Day	8% 6% 3 7% 4 5% 4 3 5% 7% 49%	2200
	21st to end	Day	10% 6% 2 5% 3 3 3 4 5% 7% 54%	2200
October	1st to 10th	Day	7% 7% 2 5% 3 4 3 3 4 6% 57%	3355
	11th to 20th	Day	6% 8% 2 6% 4 4 3 3 4 7% 53%	3250
	21st to end	Day	6% 7% 3 10% 5% 5% 6% 4 6% 7% 41%	3278
November	1st to 10th	Day	10% 12% 3 9% 5% 5% 5% 5% 6% 8% 31%	1764
	11th to 20th	Day	8% 13% 4 9% 7% 4 5% 5% 7% 9% 28%	1656
	21st to end	Day	6% 14% 4 9% 5% 4 5% 4 6% 10% 32%	1600
December	1st to 10th	Day	4 9% 4 10% 5% 5% 5% 4 5% 7% 43%	2455
	11th to 20th	Day	5% 13% 5% 11% 7% 6% 5% 4 7% 9% 29%	2645
	21st to end	Day	6% 13% 5% 11% 6% 5% 5% 4 7% 7% 32%	3498
January	1st to 10th	Night	6% 13% 6% 12% 7% 5% 5% 4 6% 8% 29%	6766
	11th to 20th	Night	5% 11% 5% 12% 7% 6% 5% 4 5% 7% 32%	6504
	21st to end	Night	3 10% 4 9% 5% 5% 5% 4 6% 8% 41%	6761
February	1st to 10th	Night	9% 17% 6% 13% 6% 6% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	6% 15% 5% 12% 7% 6% 5% 4 7% 7% 27%	2532
	21st to end	Night	5% 14% 6% 11% 6% 4 4 5% 6% 7% 30%	2045
March	1st to 10th	Night	3 7% 3 8% 5% 5% 6% 5% 7% 8% 41%	5166
	11th to 20th	Night	5% 9% 4 11% 7% 7% 6% 5% 6% 6% 35%	5000
	21st to end	Night	12% 18% 6% 11% 6% 5% 5% 4 6% 7% 20%	5489
April	1st to 10th	Night	7% 13% 4 8% 6% 5% 4 5% 6% 6% 35%	2042
	11th to 20th	Night	10% 13% 4 9% 5% 5% 6% 5% 6% 7% 31%	1781
	21st to end	Night	12% 10% 3 8% 6% 4 5% 4 6% 7% 36%	1800
May	1st to 10th	Night	11% 11% 4 6% 4 4 3 3 4 5% 43%	4100
	11th to 20th	Night	12% 9% 4 7% 4 4 4 4 6% 8% 37%	3673
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 5% 7% 47%	3616
June	1st to 10th	Night	14% 8% 2 7% 5% 5% 4 5% 5% 7% 39%	1400
	11th to 20th	Night	14% 7% 2 6% 4 5% 5% 4 5% 8% 40%	1400
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 9% 56%	1400
July	1st to 10th	Night	3 2 1 3 2 3 4 4 5% 9% 65%	3180
	11th to 20th	Night	5% 2 1 3 2 2 3 3 5% 6% 69%	3180
	21st to end	Night	3 3 2 5% 4 5% 5% 4 7% 8% 54%	4030
August	1st to 10th	Night	4 3 2 5% 5% 5% 6% 5% 8% 10% 47%	4033
	11th to 20th	Night	3 2 1 4 3 4 4 4 5% 7% 61%	4051
	21st to end	Night	7% 4 1 4 3 3 5% 3 5% 7% 56%	4509
September	1st to 10th	Night	8% 5% 2 4 3 3 4 3 5% 7% 56%	1983
	11th to 20th	Night	10% 6% 1 5% 5% 4 5% 3 5% 7% 48%	2200
	21st to end	Night	12% 5% 2 5% 4 3 3 3 4 6% 53%	2200
October	1st to 10th	Night	8% 6% 2 4 3 3 3 3 4 7% 57%	3824
	11th to 20th	Night	8% 8% 3 6% 3 3 3 2 5% 6% 52%	3898
	21st to end	Night	6% 9% 3 8% 5% 5% 5% 3 7% 7% 42%	4518
November	1st to 10th	Night	11% 12% 4 7% 4 4 5% 3 6% 7% 35%	2733
	11th to 20th	Night	8% 12% 5% 10% 6% 5% 5% 4 6% 8% 31%	2899
	21st to end	Night	6% 11% 4 8% 5% 4 4 4 7% 9% 37%	2800
December	1st to 10th	Night	4 12% 4 8% 4 4 4 4 6% 7% 42%	4726
	11th to 20th	Night	5% 14% 5% 12% 6% 5% 5% 4 6% 8% 30%	5546
	21st to end	Night	5% 13% 4 11% 6% 5% 5% 4 6% 8% 34%	7458

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	57% 8% 18% 15% 2	3179
	11th to 20th	Day	51% 7% 19% 14% 7% 1	3501
	21st to end	Day	38% 8% 21% 25% 6%	4202
February	1st to 10th	Day	58% 10% 14% 14% 4 1	1800
	11th to 20th	Day	53% 7% 20% 15% 5%	1764
	21st to end	Day	51% 8% 14% 17% 8% 2	1582
March	1st to 10th	Day	32% 6% 16% 26% 19% 1	4728
	11th to 20th	Day	41% 6% 17% 26% 9%	5000
	21st to end	Day	53% 11% 14% 20% 3	5486
April	1st to 10th	Day	47% 9% 17% 19% 8%	2353
	11th to 20th	Day	50% 7% 15% 20% 7% 1	2570
	21st to end	Day	49% 8% 14% 19% 9% 1	2600
May	1st to 10th	Day	41% 5% 14% 26% 12% 2	5880
	11th to 20th	Day	42% 8% 16% 22% 10% 3	6246
	21st to end	Day	33% 7% 15% 27% 17% 1	7263
June	1st to 10th	Day	43% 9% 19% 20% 9% 1	3000
	11th to 20th	Day	42% 9% 16% 16% 13% 5%	3000
	21st to end	Day	26% 7% 13% 29% 22% 4	3000
July	1st to 10th	Day	22% 3 9% 29% 27% 11%	6706
	11th to 20th	Day	19% 3 4 21% 27% 27%	6699
	21st to end	Day	20% 3 14% 20% 26% 16%	6598
August	1st to 10th	Day	28% 4 12% 23% 20% 13%	4208
	11th to 20th	Day	23% 4 8% 35% 25% 5%	4197
	21st to end	Day	20% 4 10% 29% 28% 9%	4663
September	1st to 10th	Day	23% 4 11% 26% 25% 10%	2412
	11th to 20th	Day	32% 6% 12% 27% 19% 4	2200
	21st to end	Day	28% 5% 16% 27% 20% 4	2200
October	1st to 10th	Day	28% 5% 9% 33% 24% 1	3355
	11th to 20th	Day	30% 4 19% 30% 16% 1	3250
	21st to end	Day	38% 6% 14% 27% 12% 2	3278
November	1st to 10th	Day	46% 8% 17% 19% 9% 2	1764
	11th to 20th	Day	46% 11% 19% 18% 5%	1656
	21st to end	Day	43% 9% 18% 22% 7%	1600
December	1st to 10th	Day	37% 6% 27% 24% 6%	2455
	11th to 20th	Day	47% 7% 26% 16% 3	2644
	21st to end	Day	47% 7% 17% 19% 10%	3498
January	1st to 10th	Night	48% 8% 22% 20% 2	6766
	11th to 20th	Night	50% 6% 19% 16% 8% 1	6504
	21st to end	Night	36% 10% 23% 23% 8%	6761
February	1st to 10th	Night	58% 8% 14% 15% 6%	2600
	11th to 20th	Night	52% 8% 18% 16% 5%	2533
	21st to end	Night	48% 10% 15% 17% 9% 1	2045
March	1st to 10th	Night	35% 6% 15% 26% 17% 1	5166
	11th to 20th	Night	42% 9% 16% 27% 6%	5000
	21st to end	Night	57% 9% 14% 16% 4	5489
April	1st to 10th	Night	45% 7% 18% 21% 7% 1	2042
	11th to 20th	Night	49% 10% 13% 19% 7% 1	1781
	21st to end	Night	44% 9% 16% 22% 8%	1800
May	1st to 10th	Night	42% 6% 13% 28% 11% 1	4097
	11th to 20th	Night	41% 6% 17% 23% 13%	3674
	21st to end	Night	33% 7% 19% 27% 14% 1	3616
June	1st to 10th	Night	41% 9% 17% 22% 10%	1400
	11th to 20th	Night	40% 8% 18% 18% 13% 4	1400
	21st to end	Night	24% 7% 16% 30% 21% 1	1400
July	1st to 10th	Night	17% 3 13% 30% 28% 9%	3145
	11th to 20th	Night	16% 2 7% 26% 25% 25%	3138
	21st to end	Night	27% 3 13% 20% 21% 16%	3916
August	1st to 10th	Night	29% 3 12% 22% 21% 12%	2928
	11th to 20th	Night	27% 3 10% 33% 20% 7%	2928
	21st to end	Night	22% 4 11% 35% 22% 6%	3245
September	1st to 10th	Night	25% 5% 12% 24% 20% 14%	1983
	11th to 20th	Night	33% 5% 13% 26% 19% 4	2200
	21st to end	Night	31% 5% 14% 24% 22% 4	2200
October	1st to 10th	Night	28% 4 10% 32% 24% 2	3824
	11th to 20th	Night	33% 4 20% 27% 15% 1	3898
	21st to end	Night	38% 5% 15% 23% 17% 2	4518
November	1st to 10th	Night	43% 8% 16% 24% 7% 2	2733
	11th to 20th	Night	45% 10% 20% 19% 6% 1	2899
	21st to end	Night	40% 8% 20% 22% 8% 1	2800
December	1st to 10th	Night	38% 7% 25% 23% 6%	4726
	11th to 20th	Night	49% 8% 25% 16% 3	5547
	21st to end	Night	44% 7% 21% 18% 10%	7458

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% 12% 26% 35% 12% 4	200
	11th to 20th	Day	11% 11% 39% 25% 12% 4	200
	21st to end	Day	12% 11% 35% 33% 7% 2	261
February	1st to 10th	Day	18% 17% 39% 21% 5%	300
	11th to 20th	Day	10% 18% 37% 28% 6%	300
	21st to end	Day	12% 18% 39% 24% 6% 1	346
March	1st to 10th	Day	11% 18% 38% 27% 7%	500
	11th to 20th	Day	13% 15% 32% 29% 11% 1	500
	21st to end	Day	16% 17% 32% 24% 9% 1	550
April	1st to 10th	Day	16% 17% 37% 22% 8% 1	500
	11th to 20th	Day	16% 23% 33% 22% 5% 1	492
	21st to end	Day	17% 18% 32% 22% 10% 1	500
May	1st to 10th	Day	15% 12% 29% 28% 14% 2	503
	11th to 20th	Day	17% 8% 26% 27% 19% 3	695
	21st to end	Day	16% 11% 22% 24% 22% 4	789
June	1st to 10th	Day	12% 12% 25% 29% 19% 4	730
	11th to 20th	Day	16% 16% 26% 28% 12% 2	730
	21st to end	Day	8% 10% 25% 32% 23% 2	730
July	1st to 10th	Day	5% 7% 23% 38% 24% 4	721
	11th to 20th	Day	6% 7% 27% 35% 23% 2	718
	21st to end	Day	3 5% 25% 40% 23% 4	607
August	1st to 10th	Day	6% 9% 29% 34% 19% 4	527
	11th to 20th	Day	6% 6% 25% 38% 21% 4	517
	21st to end	Day	7% 9% 24% 33% 22% 5%	583
September	1st to 10th	Day	8% 8% 29% 26% 22% 8%	530
	11th to 20th	Day	11% 10% 26% 27% 21% 5%	530
	21st to end	Day	14% 13% 24% 28% 17% 5%	530
October	1st to 10th	Day	15% 9% 29% 30% 14% 4	389
	11th to 20th	Day	16% 11% 27% 27% 12% 6%	330
	21st to end	Day	13% 17% 36% 24% 9% 2	363
November	1st to 10th	Day	18% 13% 34% 25% 9%	330
	11th to 20th	Day	13% 16% 40% 22% 8% 1	321
	21st to end	Day	12% 12% 34% 32% 8%	300
December	1st to 10th	Day	11% 16% 36% 26% 11%	250
	11th to 20th	Day	17% 16% 35% 23% 9% 1	200
	21st to end	Day	16% 10% 35% 26% 10% 3	220
January	1st to 10th	Night	14% 12% 32% 34% 8% 1	1000
	11th to 20th	Night	15% 10% 34% 30% 9% 2	880
	21st to end	Night	10% 10% 32% 39% 8% 1	880
February	1st to 10th	Night	16% 16% 31% 30% 7% 1	800
	11th to 20th	Night	13% 11% 32% 34% 9% 1	784
	21st to end	Night	12% 15% 31% 30% 10% 2	660
March	1st to 10th	Night	8% 6% 37% 39% 9% 1	800
	11th to 20th	Night	11% 10% 28% 38% 13% 1	800
	21st to end	Night	14% 10% 29% 35% 12% 1	880
April	1st to 10th	Night	15% 8% 26% 39% 10% 1	706
	11th to 20th	Night	13% 11% 29% 36% 10% 1	593
	21st to end	Night	17% 9% 24% 35% 14% 1	600
May	1st to 10th	Night	13% 9% 23% 35% 19% 1	600
	11th to 20th	Night	16% 8% 19% 32% 23% 2	591
	21st to end	Night	18% 7% 17% 29% 26% 3	502
June	1st to 10th	Night	17% 4 17% 35% 25% 3	400
	11th to 20th	Night	15% 6% 28% 37% 13%	400
	21st to end	Night	6% 3 17% 49% 25% 1	400
July	1st to 10th	Night	5% 1 12% 51% 29% 1	390
	11th to 20th	Night	4 3 16% 57% 19%	388
	21st to end	Night	5% 1 13% 51% 28% 1	498
August	1st to 10th	Night	6% 2 17% 50% 24% 1	597
	11th to 20th	Night	5% 2 17% 47% 27% 2	594
	21st to end	Night	8% 4 17% 44% 26% 1	660
September	1st to 10th	Night	9% 2 18% 42% 26% 3	636
	11th to 20th	Night	11% 4 15% 39% 31% 2	800
	21st to end	Night	15% 5% 16% 40% 22% 3	800
October	1st to 10th	Night	16% 5% 19% 41% 18% 2	800
	11th to 20th	Night	13% 8% 25% 36% 16% 2	800
	21st to end	Night	14% 11% 28% 35% 12% 1	880
November	1st to 10th	Night	16% 9% 29% 38% 7% 1	888
	11th to 20th	Night	14% 15% 33% 31% 7%	1036
	21st to end	Night	10% 10% 32% 40% 7%	1000
December	1st to 10th	Night	13% 12% 30% 35% 11% 1	1000
	11th to 20th	Night	15% 8% 35% 34% 8% 1	1000
	21st to end	Night	13% 10% 35% 33% 9% 1	1100

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