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 2024-11-08.

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% 42%	105417
	Night	7% 10% 4 8% 5% 5% 5% 4 6% 7% 41%	102338
Transparency	Day	36% 8% 15% 22% 14% 5%	105151
	Night	38% 8% 17% 22% 12% 3	102194
Seeing	Day	12% 13% 29% 29% 15% 2	15426
	Night	12% 8% 26% 38% 14% 1	24789

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	5% 12% 6% 15% 7% 6% 6% 4 7% 9% 23%	2219
	11th to 20th	Day	6% 14% 6% 13% 8% 6% 5% 4 6% 7% 25%	2443
	21st to end	Day	4 9% 4 10% 6% 5% 5% 5% 7% 8% 36%	2922
February	1st to 10th	Day	9% 18% 6% 13% 7% 4 5% 4 7% 7% 21%	1710
	11th to 20th	Day	4 17% 6% 14% 7% 5% 4 5% 6% 8% 24%	1665
	21st to end	Day	6% 15% 7% 10% 7% 7% 6% 5% 6% 7% 25%	1505
March	1st to 10th	Day	3 8% 3 9% 5% 5% 5% 5% 7% 10% 40%	3283
	11th to 20th	Day	4 10% 3 7% 5% 4 5% 4 6% 7% 45%	3490
	21st to end	Day	6% 15% 6% 13% 7% 6% 6% 5% 6% 7% 23%	3825
April	1st to 10th	Day	7% 14% 4 9% 7% 6% 5% 5% 6% 8% 28%	2235
	11th to 20th	Day	8% 13% 5% 11% 6% 5% 5% 5% 6% 6% 29%	2437
	21st to end	Day	10% 13% 4 10% 6% 6% 5% 4 6% 6% 30%	2470
May	1st to 10th	Day	13% 16% 4 8% 5% 5% 5% 5% 6% 7% 27%	4110
	11th to 20th	Day	7% 6% 2 6% 6% 6% 7% 6% 9% 10% 36%	4346
	21st to end	Day	11% 10% 3 9% 5% 5% 5% 4 7% 8% 34%	5058
June	1st to 10th	Day	13% 9% 2 7% 5% 5% 5% 5% 7% 9% 33%	2850
	11th to 20th	Day	13% 7% 3 6% 6% 5% 6% 5% 7% 7% 35%	2850
	21st to end	Day	5% 4 2 6% 4 4 5% 4 7% 8% 51%	2850
July	1st to 10th	Day	2 2 1 3 4 4 5% 4 7% 9% 60%	4740
	11th to 20th	Day	3 2 1 3 3 3 3 3 4 5% 70%	4740
	21st to end	Day	2 2 1 4 4 3 4 4 5% 7% 65%	4731
August	1st to 10th	Day	4 4 2 5% 5% 5% 5% 5% 8% 8% 50%	4092
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 8% 56%	4028
	21st to end	Day	4 2 1 4 3 3 4 4 6% 8% 61%	4521
September	1st to 10th	Day	6% 4 1 5% 3 3 3 3 4 6% 60%	2292
	11th to 20th	Day	9% 6% 3 7% 4 4 4 3 5% 7% 49%	2090
	21st to end	Day	10% 6% 2 5% 3 3 3 4 5% 7% 53%	2090
October	1st to 10th	Day	7% 7% 2 5% 3 4 3 3 4 6% 56%	3252
	11th to 20th	Day	6% 8% 2 6% 4 4 4 3 4 7% 54%	3150
	21st to end	Day	6% 7% 4 10% 5% 5% 6% 4 6% 7% 40%	3176
November	1st to 10th	Day	11% 13% 3 10% 5% 5% 5% 5% 6% 8% 31%	1655
	11th to 20th	Day	9% 14% 4 8% 7% 4 5% 5% 7% 9% 28%	1520
	21st to end	Day	6% 13% 4 9% 5% 4 5% 4 6% 10% 33%	1520
December	1st to 10th	Day	7% 13% 4 11% 6% 5% 6% 5% 7% 8% 27%	1425
	11th to 20th	Day	5% 15% 6% 11% 8% 6% 5% 4 6% 8% 26%	1685
	21st to end	Day	6% 10% 4 9% 6% 5% 5% 4 8% 8% 35%	2442
January	1st to 10th	Night	6% 12% 5% 11% 6% 5% 5% 4 7% 9% 30%	4726
	11th to 20th	Night	6% 13% 5% 11% 7% 6% 5% 4 5% 8% 27%	4548
	21st to end	Night	4 11% 4 10% 6% 5% 6% 5% 7% 8% 34%	4736
February	1st to 10th	Night	9% 17% 6% 12% 6% 5% 5% 4 5% 5% 25%	2470
	11th to 20th	Night	6% 14% 5% 12% 7% 6% 5% 4 7% 7% 27%	2402
	21st to end	Night	5% 14% 6% 12% 6% 4 4 5% 6% 7% 29%	1946
March	1st to 10th	Night	4 9% 4 8% 6% 5% 6% 5% 7% 8% 40%	3591
	11th to 20th	Night	5% 8% 4 8% 5% 5% 5% 4 6% 7% 43%	3490
	21st to end	Night	9% 18% 6% 12% 6% 6% 5% 4 6% 7% 21%	3828
April	1st to 10th	Night	8% 13% 4 8% 6% 5% 5% 5% 6% 6% 34%	1940
	11th to 20th	Night	10% 13% 4 9% 6% 5% 6% 5% 6% 7% 29%	1691
	21st to end	Night	13% 11% 3 8% 6% 4 5% 4 6% 7% 35%	1710
May	1st to 10th	Night	13% 15% 6% 8% 5% 5% 4 4 4 5% 31%	2860
	11th to 20th	Night	7% 7% 3 6% 4 4 4 3 6% 9% 46%	2559
	21st to end	Night	11% 8% 3 7% 6% 5% 6% 5% 6% 8% 34%	2518
June	1st to 10th	Night	14% 8% 2 7% 5% 5% 4 5% 5% 7% 37%	1330
	11th to 20th	Night	15% 7% 2 6% 4 5% 5% 4 5% 8% 40%	1330
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 9% 56%	1330
July	1st to 10th	Night	3 2 1 2 2 3 4 3 5% 9% 66%	2220
	11th to 20th	Night	2 2 1 3 2 2 3 3 5% 7% 70%	2220
	21st to end	Night	3 2 1 4 4 4 4 3 5% 6% 64%	2807
August	1st to 10th	Night	4 4 2 6% 5% 4 5% 5% 7% 8% 51%	2847
	11th to 20th	Night	4 3 1 5% 5% 5% 5% 5% 6% 8% 53%	2811
	21st to end	Night	5% 3 1 4 3 3 5% 3 5% 7% 60%	3146
September	1st to 10th	Night	8% 5% 2 4 3 4 4 3 5% 7% 55%	1883
	11th to 20th	Night	11% 6% 1 5% 5% 4 5% 3 5% 7% 48%	2090
	21st to end	Night	12% 5% 2 5% 4 3 3 3 4 7% 52%	2090
October	1st to 10th	Night	8% 6% 2 4 3 3 3 3 4 7% 57%	3707
	11th to 20th	Night	8% 7% 3 6% 3 3 3 2 5% 6% 53%	3778
	21st to end	Night	7% 9% 3 8% 5% 5% 5% 3 7% 7% 41%	4378
November	1st to 10th	Night	12% 13% 4 8% 4 4 5% 3 6% 7% 35%	2565
	11th to 20th	Night	8% 12% 5% 10% 6% 4 4 4 7% 9% 31%	2660
	21st to end	Night	6% 11% 4 7% 5% 4 4 4 7% 9% 38%	2660
December	1st to 10th	Night	6% 15% 4 10% 5% 5% 5% 5% 7% 7% 30%	2751
	11th to 20th	Night	6% 13% 5% 12% 5% 4 5% 3 5% 8% 32%	3506
	21st to end	Night	5% 10% 4 10% 5% 4 4 4 6% 7% 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	48% 11% 21% 18% 2	2219
	11th to 20th	Day	53% 10% 16% 11% 8% 1	2443
	21st to end	Day	40% 11% 26% 19% 4	2922
February	1st to 10th	Day	57% 10% 14% 15% 3	1710
	11th to 20th	Day	53% 8% 20% 15% 4	1674
	21st to end	Day	51% 8% 14% 17% 8% 2	1505
March	1st to 10th	Day	33% 8% 18% 26% 14%	3283
	11th to 20th	Day	33% 8% 24% 29% 5%	3490
	21st to end	Day	52% 14% 15% 16% 3	3825
April	1st to 10th	Day	48% 10% 17% 18% 7%	2235
	11th to 20th	Day	51% 7% 15% 19% 7% 1	2440
	21st to end	Day	50% 8% 14% 19% 8% 1	2470
May	1st to 10th	Day	50% 8% 15% 20% 7% 1	4110
	11th to 20th	Day	33% 11% 17% 23% 12% 4	4350
	21st to end	Day	42% 11% 16% 21% 9% 1	5058
June	1st to 10th	Day	43% 10% 19% 19% 8% 1	2850
	11th to 20th	Day	42% 9% 15% 16% 13% 5%	2850
	21st to end	Day	26% 8% 13% 29% 22% 3	2850
July	1st to 10th	Day	19% 5% 7% 29% 30% 11%	4666
	11th to 20th	Day	15% 4 4 16% 25% 35%	4659
	21st to end	Day	17% 3 6% 19% 31% 23%	4581
August	1st to 10th	Day	29% 4 11% 22% 21% 13%	4078
	11th to 20th	Day	24% 4 8% 35% 24% 5%	4067
	21st to end	Day	19% 4 10% 29% 28% 9%	4520
September	1st to 10th	Day	24% 4 11% 25% 25% 11%	2292
	11th to 20th	Day	32% 6% 12% 27% 19% 4	2090
	21st to end	Day	29% 5% 17% 27% 19% 4	2090
October	1st to 10th	Day	28% 5% 9% 33% 25% 1	3252
	11th to 20th	Day	29% 4 20% 30% 16% 1	3150
	21st to end	Day	39% 6% 14% 26% 12% 2	3176
November	1st to 10th	Day	47% 8% 15% 20% 8% 2	1655
	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	42% 11% 23% 22% 3	4726
	11th to 20th	Night	52% 9% 17% 15% 7% 1	4548
	21st to end	Night	40% 14% 25% 16% 4	4736
February	1st to 10th	Night	57% 9% 14% 15% 5%	2470
	11th to 20th	Night	52% 9% 19% 16% 4	2403
	21st to end	Night	48% 10% 15% 17% 9% 1	1946
March	1st to 10th	Night	34% 8% 18% 21% 18%	3591
	11th to 20th	Night	32% 12% 22% 29% 5%	3490
	21st to end	Night	55% 12% 16% 14% 2	3828
April	1st to 10th	Night	46% 8% 19% 20% 7% 1	1940
	11th to 20th	Night	50% 10% 14% 18% 7% 1	1691
	21st to end	Night	44% 10% 16% 22% 8%	1710
May	1st to 10th	Night	52% 8% 13% 19% 7% 1	2860
	11th to 20th	Night	30% 9% 17% 25% 19% 1	2560
	21st to end	Night	41% 10% 20% 21% 7% 1	2518
June	1st to 10th	Night	43% 10% 18% 21% 8%	1330
	11th to 20th	Night	39% 8% 17% 19% 12% 4	1330
	21st to end	Night	24% 7% 17% 29% 21% 2	1330
July	1st to 10th	Night	16% 3 11% 30% 29% 11%	2185
	11th to 20th	Night	13% 3 7% 16% 26% 35%	2178
	21st to end	Night	19% 3 8% 23% 24% 23%	2719
August	1st to 10th	Night	29% 3 12% 22% 22% 12%	2838
	11th to 20th	Night	27% 3 10% 33% 20% 6%	2838
	21st to end	Night	22% 4 11% 36% 22% 6%	3146
September	1st to 10th	Night	26% 6% 12% 22% 20% 14%	1883
	11th to 20th	Night	33% 5% 13% 25% 19% 5%	2090
	21st to end	Night	31% 5% 14% 25% 21% 4	2090
October	1st to 10th	Night	28% 4 10% 32% 24% 2	3707
	11th to 20th	Night	32% 4 20% 27% 16% 1	3778
	21st to end	Night	38% 5% 15% 22% 17% 2	4378
November	1st to 10th	Night	44% 8% 16% 24% 7% 2	2565
	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	12% 9% 35% 27% 13% 4	180
	21st to end	Day	12% 11% 33% 34% 8% 2	234
February	1st to 10th	Day	19% 16% 39% 22% 5%	270
	11th to 20th	Day	9% 18% 39% 29% 6%	270
	21st to end	Day	11% 20% 37% 25% 6% 1	317
March	1st to 10th	Day	11% 20% 38% 26% 6%	460
	11th to 20th	Day	12% 16% 30% 29% 12% 1	460
	21st to end	Day	17% 18% 31% 24% 9% 1	506
April	1st to 10th	Day	16% 17% 38% 21% 7%	460
	11th to 20th	Day	16% 23% 32% 22% 5% 1	452
	21st to end	Day	18% 19% 32% 21% 10% 1	460
May	1st to 10th	Day	16% 13% 28% 28% 13% 2	463
	11th to 20th	Day	16% 8% 26% 27% 20% 3	646
	21st to end	Day	17% 12% 23% 23% 21% 4	734
June	1st to 10th	Day	12% 12% 24% 29% 19% 3	680
	11th to 20th	Day	17% 17% 26% 26% 12% 1	680
	21st to end	Day	8% 10% 25% 32% 23% 2	680
July	1st to 10th	Day	5% 7% 23% 37% 24% 3	671
	11th to 20th	Day	6% 7% 28% 36% 21% 2	668
	21st to end	Day	3 5% 25% 41% 22% 3	562
August	1st to 10th	Day	6% 9% 29% 33% 19% 3	487
	11th to 20th	Day	6% 6% 26% 38% 19% 4	477
	21st to end	Day	6% 10% 25% 34% 21% 4	539
September	1st to 10th	Day	9% 9% 30% 26% 21% 6%	490
	11th to 20th	Day	12% 10% 28% 27% 18% 4	490
	21st to end	Day	15% 13% 25% 27% 16% 4	490
October	1st to 10th	Day	15% 10% 28% 29% 14% 4	356
	11th to 20th	Day	16% 11% 27% 27% 13% 6%	300
	21st to end	Day	13% 18% 35% 24% 9% 2	330
November	1st to 10th	Day	18% 13% 34% 25% 10%	291
	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	836
	21st to end	Night	11% 10% 31% 39% 8% 1	836
February	1st to 10th	Night	16% 15% 31% 30% 7% 1	760
	11th to 20th	Night	12% 12% 32% 35% 8% 1	744
	21st to end	Night	11% 15% 30% 31% 11% 2	628
March	1st to 10th	Night	9% 6% 37% 39% 8% 1	760
	11th to 20th	Night	11% 10% 28% 39% 13% 1	760
	21st to end	Night	13% 10% 29% 35% 12% 1	836
April	1st to 10th	Night	15% 8% 27% 39% 10% 1	671
	11th to 20th	Night	13% 11% 28% 36% 10% 1	563
	21st to end	Night	17% 9% 25% 34% 14% 1	570
May	1st to 10th	Night	13% 10% 24% 34% 18% 1	570
	11th to 20th	Night	14% 8% 19% 33% 24% 2	561
	21st to end	Night	19% 7% 17% 29% 25% 3	477
June	1st to 10th	Night	17% 4 17% 35% 24% 2	380
	11th to 20th	Night	16% 6% 29% 36% 13%	380
	21st to end	Night	6% 3 17% 50% 25%	380
July	1st to 10th	Night	6% 1 12% 51% 29% 1	370
	11th to 20th	Night	3 3 16% 58% 19%	368
	21st to end	Night	5% 1 13% 52% 28% 1	473
August	1st to 10th	Night	6% 2 16% 50% 25% 1	567
	11th to 20th	Night	5% 2 17% 47% 26% 2	564
	21st to end	Night	8% 5% 17% 43% 26% 1	627
September	1st to 10th	Night	10% 2 19% 43% 24% 3	604
	11th to 20th	Night	11% 4 15% 39% 29% 2	760
	21st to end	Night	15% 5% 16% 40% 21% 2	760
October	1st to 10th	Night	16% 5% 19% 40% 18% 2	760
	11th to 20th	Night	13% 8% 24% 37% 17% 2	760
	21st to end	Night	14% 11% 28% 34% 12% 1	836
November	1st to 10th	Night	17% 9% 29% 37% 7% 1	833
	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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