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 2024-01-13.

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% 9% 4 7% 6% 4 5% 5% 6% 8% 39%	74665
	Night	8% 10% 4 7% 6% 3 5% 5% 6% 8% 39%	75076
Transparency	Day	39% 9% 15% 21% 12% 4	74359
	Night	40% 9% 17% 20% 11% 3	74904
Seeing	Day	12% 13% 29% 28% 15% 2	14249
	Night	13% 8% 26% 37% 14% 1	23810

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	7% 12% 6% 9% 8% 3 7% 5% 7% 9% 28%	1329
	11th to 20th	Day	8% 13% 7% 9% 7% 3 5% 5% 5% 9% 29%	1399
	21st to end	Day	6% 13% 6% 12% 7% 4 5% 6% 5% 7% 28%	1656
February	1st to 10th	Day	9% 16% 7% 12% 7% 3 5% 5% 6% 7% 21%	1620
	11th to 20th	Day	5% 16% 7% 12% 8% 4 4 5% 6% 8% 25%	1575
	21st to end	Day	6% 12% 8% 9% 8% 4 7% 6% 7% 8% 25%	1419
March	1st to 10th	Day	6% 10% 4 9% 7% 5% 6% 6% 8% 11% 28%	1880
	11th to 20th	Day	8% 12% 5% 8% 7% 4 6% 6% 7% 9% 30%	1980
	21st to end	Day	7% 13% 5% 10% 8% 4 6% 6% 7% 8% 26%	2178
April	1st to 10th	Day	8% 12% 5% 8% 8% 5% 5% 6% 6% 8% 28%	2117
	11th to 20th	Day	9% 12% 5% 10% 7% 4 6% 6% 6% 6% 28%	2307
	21st to end	Day	10% 12% 5% 9% 7% 4 6% 5% 6% 6% 30%	2340
May	1st to 10th	Day	7% 8% 4 8% 7% 4 6% 6% 6% 8% 35%	2340
	11th to 20th	Day	10% 7% 3 6% 8% 4 6% 7% 7% 9% 33%	2450
	21st to end	Day	15% 9% 4 9% 6% 4 6% 6% 7% 8% 25%	2853
June	1st to 10th	Day	14% 8% 3 6% 5% 4 6% 6% 7% 9% 32%	2700
	11th to 20th	Day	14% 7% 3 6% 6% 4 6% 7% 7% 7% 34%	2700
	21st to end	Day	6% 4 2 5% 5% 3 5% 5% 7% 8% 51%	2700
July	1st to 10th	Day	4 3 1 3 5% 3 5% 5% 7% 9% 55%	2700
	11th to 20th	Day	5% 3 1 3 4 2 4 5% 6% 6% 61%	2700
	21st to end	Day	3 2 1 3 5% 3 4 6% 6% 7% 61%	2717
August	1st to 10th	Day	4 4 1 4 6% 3 5% 5% 6% 7% 55%	2328
	11th to 20th	Day	4 3 1 4 4 3 4 5% 6% 8% 59%	2299
	21st to end	Day	6% 3 1 4 4 2 5% 5% 6% 9% 56%	2574
September	1st to 10th	Day	7% 4 1 5% 4 3 3 4 4 6% 60%	2172
	11th to 20th	Day	8% 5% 3 5% 5% 3 4 4 5% 7% 51%	1980
	21st to end	Day	11% 6% 2 4 4 3 3 5% 5% 7% 51%	1980
October	1st to 10th	Day	11% 10% 2 6% 5% 5% 5% 5% 5% 7% 39%	1854
	11th to 20th	Day	10% 10% 3 6% 6% 4 5% 5% 5% 7% 40%	1800
	21st to end	Day	10% 8% 4 8% 6% 4 6% 5% 6% 8% 34%	1832
November	1st to 10th	Day	11% 12% 4 9% 6% 3 5% 5% 6% 8% 31%	1594
	11th to 20th	Day	9% 13% 4 7% 8% 3 5% 6% 7% 9% 28%	1520
	21st to end	Day	6% 13% 4 8% 6% 3 5% 5% 6% 10% 33%	1520
December	1st to 10th	Day	7% 13% 4 10% 7% 4 7% 5% 7% 8% 28%	1425
	11th to 20th	Day	5% 14% 6% 9% 9% 5% 6% 5% 6% 9% 26%	1685
	21st to end	Day	6% 10% 5% 8% 6% 4 5% 5% 7% 8% 35%	2442
January	1st to 10th	Night	8% 12% 6% 9% 7% 3 5% 5% 7% 9% 29%	2836
	11th to 20th	Night	9% 13% 5% 8% 6% 3 5% 5% 6% 8% 32%	2635
	21st to end	Night	5% 13% 5% 9% 8% 3 6% 6% 6% 8% 31%	2698
February	1st to 10th	Night	9% 16% 7% 11% 7% 4 5% 5% 5% 5% 26%	2340
	11th to 20th	Night	7% 13% 5% 11% 9% 5% 5% 4 6% 7% 27%	2272
	21st to end	Night	5% 12% 7% 11% 8% 3 5% 6% 6% 8% 29%	1834
March	1st to 10th	Night	6% 10% 4 8% 7% 4 6% 7% 6% 9% 33%	2080
	11th to 20th	Night	8% 10% 5% 7% 6% 5% 5% 6% 6% 9% 33%	1980
	21st to end	Night	8% 14% 6% 9% 7% 4 6% 6% 6% 7% 27%	2178
April	1st to 10th	Night	8% 12% 5% 7% 7% 4 5% 6% 6% 6% 34%	1839
	11th to 20th	Night	10% 12% 4 8% 6% 4 6% 6% 6% 7% 30%	1601
	21st to end	Night	13% 10% 3 6% 6% 3 5% 5% 6% 7% 36%	1620
May	1st to 10th	Night	9% 8% 4 5% 7% 3 6% 6% 7% 7% 39%	1620
	11th to 20th	Night	11% 8% 3 6% 5% 3 5% 5% 7% 10% 37%	1445
	21st to end	Night	16% 9% 4 5% 6% 3 6% 5% 6% 8% 32%	1420
June	1st to 10th	Night	15% 7% 3 6% 6% 4 5% 5% 5% 7% 38%	1260
	11th to 20th	Night	15% 7% 3 5% 4 4 5% 5% 5% 8% 39%	1260
	21st to end	Night	6% 3 1 3 5% 3 4 5% 5% 9% 55%	1260
July	1st to 10th	Night	5% 3 1 2 3 2 3 4 5% 11% 61%	1260
	11th to 20th	Night	4 2 1 3 3 2 3 5% 5% 9% 63%	1260
	21st to end	Night	4 2 1 3 4 2 4 4 5% 7% 63%	1597
August	1st to 10th	Night	5% 4 2 4 4 3 4 5% 6% 8% 55%	1611
	11th to 20th	Night	5% 3 1 3 3 3 4 5% 6% 8% 60%	1602
	21st to end	Night	8% 4 1 4 5% 3 4 5% 7% 7% 52%	1782
September	1st to 10th	Night	8% 5% 2 4 4 2 3 5% 5% 7% 54%	1783
	11th to 20th	Night	10% 6% 2 5% 5% 3 4 4 5% 7% 50%	1980
	21st to end	Night	13% 5% 2 4 5% 3 3 4 4 7% 51%	1980
October	1st to 10th	Night	12% 7% 3 6% 4 3 4 4 5% 8% 44%	2106
	11th to 20th	Night	10% 9% 4 5% 4 2 4 4 5% 8% 45%	2160
	21st to end	Night	10% 10% 4 7% 6% 3 5% 4 7% 7% 37%	2520
November	1st to 10th	Night	12% 12% 4 7% 5% 3 5% 5% 6% 7% 35%	2466
	11th to 20th	Night	8% 12% 5% 9% 7% 3 4 5% 6% 9% 31%	2660
	21st to end	Night	6% 10% 4 6% 6% 3 4 5% 7% 9% 38%	2660
December	1st to 10th	Night	6% 14% 5% 9% 6% 4 5% 6% 7% 8% 30%	2751
	11th to 20th	Night	7% 13% 5% 11% 7% 3 5% 4 5% 8% 33%	3506
	21st to end	Night	6% 10% 4 8% 6% 3 4 4 6% 8% 41%	5214

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	46% 11% 20% 20% 3	1329
	11th to 20th	Day	50% 9% 21% 14% 5% 1	1399
	21st to end	Day	51% 8% 16% 19% 5%	1656
February	1st to 10th	Day	56% 11% 15% 15% 3	1620
	11th to 20th	Day	53% 7% 20% 16% 4	1584
	21st to end	Day	52% 9% 14% 16% 8% 2	1419
March	1st to 10th	Day	44% 12% 20% 17% 7%	1880
	11th to 20th	Day	44% 11% 17% 22% 7%	1980
	21st to end	Day	52% 11% 16% 17% 4	2178
April	1st to 10th	Day	48% 10% 17% 17% 7% 1	2117
	11th to 20th	Day	52% 7% 14% 19% 7%	2310
	21st to end	Day	50% 8% 14% 19% 7% 1	2340
May	1st to 10th	Day	41% 10% 15% 26% 8% 1	2340
	11th to 20th	Day	43% 13% 16% 19% 7% 2	2454
	21st to end	Day	52% 12% 17% 15% 4 1	2853
June	1st to 10th	Day	44% 10% 19% 19% 7%	2700
	11th to 20th	Day	42% 10% 16% 16% 12% 5%	2700
	21st to end	Day	26% 8% 12% 29% 22% 3	2700
July	1st to 10th	Day	22% 8% 11% 29% 24% 6%	2626
	11th to 20th	Day	20% 6% 8% 22% 24% 20%	2619
	21st to end	Day	18% 5% 10% 20% 28% 18%	2566
August	1st to 10th	Day	25% 7% 11% 26% 22% 10%	2313
	11th to 20th	Day	21% 6% 10% 27% 28% 8%	2299
	21st to end	Day	23% 7% 12% 24% 24% 10%	2574
September	1st to 10th	Day	25% 5% 11% 24% 25% 11%	2172
	11th to 20th	Day	30% 6% 12% 27% 20% 4	1980
	21st to end	Day	30% 5% 17% 26% 18% 3	1980
October	1st to 10th	Day	41% 8% 15% 25% 11% 1	1854
	11th to 20th	Day	40% 7% 19% 23% 9% 2	1800
	21st to end	Day	42% 10% 18% 24% 6%	1832
November	1st to 10th	Day	47% 8% 15% 20% 8% 2	1594
	11th to 20th	Day	47% 11% 18% 18% 5%	1520
	21st to end	Day	42% 10% 18% 22% 8%	1520
December	1st to 10th	Day	47% 9% 20% 17% 6%	1425
	11th to 20th	Day	49% 10% 26% 12% 2	1684
	21st to end	Day	41% 10% 21% 16% 12%	2442
January	1st to 10th	Night	46% 11% 20% 19% 3	2836
	11th to 20th	Night	46% 11% 18% 18% 6% 1	2635
	21st to end	Night	46% 11% 18% 17% 7% 1	2698
February	1st to 10th	Night	57% 9% 15% 15% 4	2340
	11th to 20th	Night	51% 9% 20% 16% 4	2273
	21st to end	Night	50% 10% 14% 16% 9% 1	1834
March	1st to 10th	Night	43% 12% 18% 18% 9%	2080
	11th to 20th	Night	41% 12% 17% 22% 8%	1980
	21st to end	Night	51% 11% 18% 17% 4	2178
April	1st to 10th	Night	46% 7% 19% 20% 7% 1	1839
	11th to 20th	Night	51% 10% 14% 17% 7% 1	1601
	21st to end	Night	44% 10% 17% 23% 7%	1620
May	1st to 10th	Night	39% 12% 16% 25% 7% 1	1620
	11th to 20th	Night	39% 11% 17% 21% 11% 1	1446
	21st to end	Night	47% 11% 20% 17% 6%	1420
June	1st to 10th	Night	44% 10% 18% 21% 8%	1260
	11th to 20th	Night	40% 8% 17% 19% 11% 4	1260
	21st to end	Night	24% 8% 16% 30% 21% 1	1260
July	1st to 10th	Night	18% 6% 16% 29% 22% 9%	1225
	11th to 20th	Night	17% 4 11% 23% 27% 17%	1218
	21st to end	Night	19% 4 10% 24% 26% 17%	1508
August	1st to 10th	Night	24% 6% 12% 24% 23% 11%	1602
	11th to 20th	Night	21% 6% 12% 25% 27% 10%	1602
	21st to end	Night	27% 7% 11% 24% 23% 9%	1782
September	1st to 10th	Night	26% 6% 12% 22% 20% 14%	1783
	11th to 20th	Night	32% 6% 12% 25% 20% 5%	1980
	21st to end	Night	33% 6% 14% 24% 20% 3	1980
October	1st to 10th	Night	39% 6% 16% 25% 12% 2	2106
	11th to 20th	Night	36% 7% 18% 26% 11% 2	2160
	21st to end	Night	42% 8% 19% 19% 11% 1	2520
November	1st to 10th	Night	44% 9% 16% 23% 7% 2	2466
	11th to 20th	Night	44% 10% 20% 19% 6% 1	2660
	21st to end	Night	38% 9% 20% 23% 9% 1	2660
December	1st to 10th	Night	47% 11% 18% 17% 6% 1	2751
	11th to 20th	Night	46% 11% 23% 17% 3	3507
	21st to end	Night	38% 9% 23% 17% 12%	5214

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	12% 12% 23% 36% 12% 4	180
	11th to 20th	Day	13% 10% 31% 28% 13% 4	164
	21st to end	Day	14% 12% 35% 32% 5% 2	208
February	1st to 10th	Day	18% 18% 40% 21% 3	240
	11th to 20th	Day	10% 20% 39% 26% 5%	240
	21st to end	Day	10% 17% 39% 26% 7% 1	285
March	1st to 10th	Day	11% 20% 39% 25% 5%	420
	11th to 20th	Day	13% 17% 29% 28% 11% 1	420
	21st to end	Day	17% 18% 31% 24% 9% 1	462
April	1st to 10th	Day	16% 17% 38% 21% 7%	420
	11th to 20th	Day	17% 25% 33% 19% 5% 1	412
	21st to end	Day	19% 18% 32% 20% 10% 1	420
May	1st to 10th	Day	14% 13% 29% 28% 13% 2	423
	11th to 20th	Day	17% 8% 25% 27% 20% 3	597
	21st to end	Day	18% 10% 23% 23% 22% 4	679
June	1st to 10th	Day	12% 12% 24% 29% 20% 3	630
	11th to 20th	Day	18% 17% 27% 26% 11% 1	630
	21st to end	Day	8% 10% 24% 32% 23% 2	630
July	1st to 10th	Day	6% 7% 23% 38% 24% 3	621
	11th to 20th	Day	6% 8% 28% 36% 21% 1	618
	21st to end	Day	3 6% 25% 42% 21% 3	517
August	1st to 10th	Day	6% 10% 29% 33% 19% 3	447
	11th to 20th	Day	7% 6% 26% 37% 19% 5%	437
	21st to end	Day	6% 10% 26% 34% 19% 4	495
September	1st to 10th	Day	9% 10% 32% 26% 19% 3	450
	11th to 20th	Day	11% 11% 29% 25% 19% 4	450
	21st to end	Day	16% 14% 24% 26% 16% 4	450
October	1st to 10th	Day	16% 9% 28% 29% 13% 5%	324
	11th to 20th	Day	17% 12% 28% 27% 11% 5%	270
	21st to end	Day	15% 18% 33% 24% 9% 2	297
November	1st to 10th	Day	19% 13% 34% 24% 10%	270
	11th to 20th	Day	12% 15% 41% 23% 8%	270
	21st to end	Day	11% 13% 34% 33% 9%	270
December	1st to 10th	Day	12% 15% 33% 27% 12%	225
	11th to 20th	Day	16% 14% 35% 24% 10% 1	180
	21st to end	Day	16% 11% 33% 28% 9% 3	198
January	1st to 10th	Night	13% 12% 32% 34% 9% 1	950
	11th to 20th	Night	15% 10% 33% 31% 9% 2	807
	21st to end	Night	11% 11% 32% 38% 8% 1	792
February	1st to 10th	Night	15% 15% 30% 31% 7% 1	720
	11th to 20th	Night	12% 12% 32% 34% 8% 1	704
	21st to end	Night	11% 14% 31% 31% 11% 2	592
March	1st to 10th	Night	9% 6% 37% 40% 8% 1	720
	11th to 20th	Night	11% 10% 28% 38% 13% 1	720
	21st to end	Night	13% 10% 29% 34% 13% 1	792
April	1st to 10th	Night	16% 7% 26% 39% 11% 1	636
	11th to 20th	Night	14% 11% 28% 36% 10% 1	533
	21st to end	Night	18% 9% 24% 34% 14% 1	540
May	1st to 10th	Night	12% 10% 24% 34% 19% 1	540
	11th to 20th	Night	15% 8% 18% 33% 24% 2	531
	21st to end	Night	19% 6% 17% 28% 26% 3	452
June	1st to 10th	Night	17% 3 18% 34% 26% 3	360
	11th to 20th	Night	16% 6% 28% 36% 13%	360
	21st to end	Night	6% 3 17% 48% 26%	360
July	1st to 10th	Night	6% 1 13% 52% 27% 1	350
	11th to 20th	Night	3 3 17% 59% 18%	348
	21st to end	Night	5% 1 13% 53% 27% 1	448
August	1st to 10th	Night	6% 2 17% 50% 24% 1	537
	11th to 20th	Night	5% 2 18% 46% 27% 2	534
	21st to end	Night	8% 5% 17% 43% 27% 1	594
September	1st to 10th	Night	10% 2 19% 44% 22% 3	572
	11th to 20th	Night	10% 4 16% 38% 30% 2	720
	21st to end	Night	16% 5% 16% 40% 20% 3	720
October	1st to 10th	Night	16% 5% 20% 39% 18% 2	720
	11th to 20th	Night	13% 8% 24% 37% 16% 2	720
	21st to end	Night	15% 11% 27% 34% 12% 1	792
November	1st to 10th	Night	17% 9% 29% 38% 7% 1	801
	11th to 20th	Night	13% 16% 33% 31% 7%	950
	21st to end	Night	10% 10% 31% 41% 7%	950
December	1st to 10th	Night	13% 12% 28% 35% 11% 1	950
	11th to 20th	Night	15% 8% 34% 34% 8% 1	950
	21st to end	Night	13% 10% 34% 34% 9% 1	1045

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