Missoula
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Missoula clear sky chart from 2005-11-06 to 2025-10-19.

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	7% 10% 3 8% 5% 4 5% 4 5% 7% 42%	135365
	Night	7% 12% 4 9% 5% 5% 5% 4 5% 7% 38%	131640
Transparency	Day	40% 5% 14% 23% 14% 5%	130004
	Night	43% 6% 16% 22% 10% 3	128007
Seeing	Day	15% 14% 26% 28% 14% 3	16447
	Night	14% 7% 22% 39% 17% 1	25662

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% 23% 10% 16% 8% 7% 5% 4 4 5% 13%	3342
	11th to 20th	Day	6% 15% 6% 17% 7% 5% 5% 3 5% 4 28%	3640
	21st to end	Day	7% 13% 5% 12% 7% 5% 5% 3 4 6% 34%	4004
February	1st to 10th	Day	8% 22% 7% 14% 7% 5% 5% 3 4 5% 17%	1725
	11th to 20th	Day	6% 20% 8% 12% 7% 6% 6% 4 5% 6% 22%	1804
	21st to end	Day	8% 18% 5% 12% 8% 6% 5% 4 5% 7% 21%	1650
March	1st to 10th	Day	5% 12% 4 9% 5% 5% 5% 4 8% 10% 33%	4518
	11th to 20th	Day	6% 12% 6% 11% 6% 6% 5% 4 5% 7% 32%	4994
	21st to end	Day	8% 16% 5% 15% 8% 6% 6% 5% 6% 7% 17%	5486
April	1st to 10th	Day	9% 15% 6% 12% 6% 6% 5% 4 6% 7% 24%	2397
	11th to 20th	Day	10% 15% 5% 10% 6% 5% 5% 4 6% 6% 29%	2567
	21st to end	Day	12% 16% 4 10% 6% 4 4 5% 6% 6% 28%	2600
May	1st to 10th	Day	11% 11% 3 7% 4 5% 5% 4 5% 6% 39%	5880
	11th to 20th	Day	9% 9% 3 8% 6% 6% 7% 5% 7% 10% 29%	6133
	21st to end	Day	10% 8% 2 6% 4 4 5% 3 6% 8% 43%	7404
June	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 8% 33%	3000
	11th to 20th	Day	15% 10% 3 7% 5% 5% 5% 4 6% 7% 33%	3000
	21st to end	Day	6% 5% 2 5% 5% 4 5% 4 6% 7% 52%	3000
July	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 7% 67%	6779
	11th to 20th	Day	3 2 1 3 1 2 2 2 3 4 77%	6469
	21st to end	Day	2 3 1 3 2 3 3 3 5% 8% 67%	6880
August	1st to 10th	Day	4 3 2 5% 4 5% 6% 4 7% 9% 52%	5958
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 8% 64%	5798
	21st to end	Day	5% 4 1 4 3 3 4 3 5% 7% 62%	6468
September	1st to 10th	Day	6% 3 1 4 4 3 3 3 5% 7% 61%	2370
	11th to 20th	Day	7% 7% 2 6% 5% 5% 5% 4 5% 8% 47%	2200
	21st to end	Day	9% 6% 3 5% 4 3 3 3 6% 7% 50%	2200
October	1st to 10th	Day	8% 6% 2 6% 3 3 3 3 4 7% 54%	3511
	11th to 20th	Day	8% 8% 3 7% 4 4 4 3 5% 7% 48%	3110
	21st to end	Day	10% 11% 5% 10% 5% 4 5% 4 4 6% 35%	3101
November	1st to 10th	Day	14% 18% 5% 10% 4 5% 5% 3 5% 7% 24%	1764
	11th to 20th	Day	11% 18% 6% 11% 6% 5% 4 4 5% 6% 24%	1646
	21st to end	Day	7% 17% 5% 10% 6% 5% 5% 4 5% 7% 28%	1534
December	1st to 10th	Day	6% 14% 5% 10% 4 4 4 3 4 7% 38%	2290
	11th to 20th	Day	9% 20% 5% 14% 7% 5% 6% 3 6% 6% 18%	2645
	21st to end	Day	6% 17% 6% 14% 7% 7% 6% 4 6% 6% 20%	3498
January	1st to 10th	Night	7% 21% 7% 15% 8% 7% 5% 4 5% 5% 15%	6646
	11th to 20th	Night	6% 15% 6% 15% 7% 6% 5% 3 5% 6% 26%	6320
	21st to end	Night	8% 15% 5% 10% 7% 4 4 4 5% 6% 32%	6952
February	1st to 10th	Night	10% 23% 8% 12% 6% 5% 4 4 4 6% 19%	2675
	11th to 20th	Night	7% 21% 6% 12% 7% 5% 4 3 5% 5% 25%	2482
	21st to end	Night	7% 18% 6% 12% 7% 6% 6% 4 4 6% 23%	1980
March	1st to 10th	Night	3 11% 4 10% 5% 5% 5% 5% 6% 8% 36%	5376
	11th to 20th	Night	7% 12% 4 11% 6% 5% 5% 4 5% 6% 35%	5005
	21st to end	Night	12% 18% 5% 11% 6% 5% 6% 4 7% 7% 16%	5489
April	1st to 10th	Night	9% 15% 4 7% 7% 5% 5% 4 6% 8% 30%	2000
	11th to 20th	Night	11% 11% 4 10% 6% 5% 5% 4 6% 6% 33%	1781
	21st to end	Night	10% 11% 4 8% 5% 5% 4 4 7% 8% 35%	1800
May	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 5% 6% 42%	4100
	11th to 20th	Night	10% 9% 3 7% 5% 5% 5% 5% 7% 9% 36%	3751
	21st to end	Night	10% 7% 3 8% 4 4 5% 4 6% 6% 43%	3470
June	1st to 10th	Night	15% 8% 3 5% 6% 4 4 4 6% 7% 39%	1400
	11th to 20th	Night	15% 7% 3 5% 5% 4 4 4 6% 7% 38%	1400
	21st to end	Night	5% 4 1 4 4 4 4 3 6% 9% 56%	1400
July	1st to 10th	Night	3 2 1 2 1 1 3 3 4 8% 73%	3180
	11th to 20th	Night	2 2 1 3 3 2 2 2 4 5% 73%	3474
	21st to end	Night	1 2 1 3 3 3 3 2 5% 8% 68%	3970
August	1st to 10th	Night	4 4 2 4 4 4 4 4 6% 10% 55%	3844
	11th to 20th	Night	2 2 1 4 3 4 4 3 5% 6% 66%	4051
	21st to end	Night	4 3 1 3 2 3 4 4 6% 7% 63%	4509
September	1st to 10th	Night	6% 5% 2 4 3 2 3 3 5% 7% 60%	2025
	11th to 20th	Night	9% 5% 1 6% 4 4 5% 3 6% 7% 51%	2200
	21st to end	Night	10% 5% 2 6% 3 3 3 2 5% 6% 54%	2200
October	1st to 10th	Night	8% 7% 3 7% 4 3 3 3 4 5% 54%	3686
	11th to 20th	Night	8% 7% 2 6% 4 3 5% 4 5% 7% 48%	3992
	21st to end	Night	9% 11% 3 8% 5% 4 4 4 5% 8% 39%	4457
November	1st to 10th	Night	15% 15% 4 8% 5% 4 4 4 4 7% 29%	2535
	11th to 20th	Night	11% 18% 5% 10% 5% 5% 4 4 5% 6% 27%	2731
	21st to end	Night	8% 13% 4 11% 6% 5% 5% 4 7% 7% 30%	2865
December	1st to 10th	Night	6% 16% 5% 8% 5% 4 4 3 5% 7% 39%	4890
	11th to 20th	Night	7% 18% 7% 14% 7% 5% 6% 4 6% 8% 19%	5546
	21st to end	Night	7% 18% 6% 14% 6% 7% 7% 6% 6% 7% 16%	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	68% 6% 15% 10% 1	3342
	11th to 20th	Day	56% 5% 20% 16% 3 1	3640
	21st to end	Day	48% 5% 17% 23% 6%	4004
February	1st to 10th	Day	66% 6% 14% 11% 2 1	1725
	11th to 20th	Day	59% 5% 17% 13% 6%	1813
	21st to end	Day	58% 5% 11% 18% 6% 1	1650
March	1st to 10th	Day	41% 6% 15% 21% 16% 1	4518
	11th to 20th	Day	48% 6% 16% 25% 5%	4994
	21st to end	Day	61% 7% 15% 14% 3	5486
April	1st to 10th	Day	55% 6% 17% 18% 4	2397
	11th to 20th	Day	52% 5% 13% 22% 7% 1	2570
	21st to end	Day	54% 6% 13% 20% 7%	2600
May	1st to 10th	Day	44% 5% 11% 23% 16% 1	5880
	11th to 20th	Day	45% 6% 16% 23% 9%	6136
	21st to end	Day	37% 5% 17% 28% 12% 1	7406
June	1st to 10th	Day	47% 7% 17% 20% 8% 1	3000
	11th to 20th	Day	47% 4 12% 21% 12% 3	3000
	21st to end	Day	30% 5% 13% 32% 19% 3	3000
July	1st to 10th	Day	17% 4 8% 21% 31% 19%	6706
	11th to 20th	Day	14% 2 5% 18% 29% 33%	6390
	21st to end	Day	15% 4 9% 25% 31% 17%	6681
August	1st to 10th	Day	21% 5% 8% 28% 27% 11%	4338
	11th to 20th	Day	18% 5% 12% 36% 24% 5%	4197
	21st to end	Day	20% 3 9% 33% 28% 6%	4663
September	1st to 10th	Day	23% 4 10% 31% 23% 9%	2370
	11th to 20th	Day	34% 5% 14% 29% 15% 3	2200
	21st to end	Day	31% 4 19% 29% 16% 2	2200
October	1st to 10th	Day	31% 4 13% 39% 14%	3511
	11th to 20th	Day	35% 4 21% 32% 9%	3110
	21st to end	Day	48% 4 12% 27% 8% 1	3101
November	1st to 10th	Day	57% 6% 17% 16% 4	1764
	11th to 20th	Day	56% 7% 19% 15% 3	1646
	21st to end	Day	49% 6% 23% 18% 4	1534
December	1st to 10th	Day	45% 5% 26% 20% 4	2290
	11th to 20th	Day	63% 5% 20% 12% 1	2644
	21st to end	Day	59% 7% 16% 14% 5%	3498
January	1st to 10th	Night	66% 8% 15% 10% 1	6646
	11th to 20th	Night	56% 6% 19% 14% 5% 1	6320
	21st to end	Night	50% 7% 17% 20% 5%	6952
February	1st to 10th	Night	64% 5% 15% 13% 3	2675
	11th to 20th	Night	58% 5% 19% 15% 3	2483
	21st to end	Night	57% 5% 14% 18% 6% 1	1980
March	1st to 10th	Night	39% 6% 16% 24% 14% 1	5376
	11th to 20th	Night	46% 6% 19% 24% 5%	5005
	21st to end	Night	60% 8% 15% 15% 3	5489
April	1st to 10th	Night	47% 7% 20% 22% 4	2000
	11th to 20th	Night	49% 5% 15% 23% 7% 1	1781
	21st to end	Night	44% 6% 17% 23% 9%	1800
May	1st to 10th	Night	40% 5% 13% 28% 14% 1	4097
	11th to 20th	Night	41% 7% 17% 25% 10%	3751
	21st to end	Night	37% 6% 19% 23% 15%	3470
June	1st to 10th	Night	43% 6% 18% 24% 9% 1	1400
	11th to 20th	Night	43% 5% 15% 22% 12% 3	1400
	21st to end	Night	23% 5% 14% 34% 21% 3	1400
July	1st to 10th	Night	11% 4 9% 26% 31% 19%	3145
	11th to 20th	Night	14% 2 6% 23% 21% 33%	3430
	21st to end	Night	16% 2 12% 24% 23% 22%	3857
August	1st to 10th	Night	19% 3 9% 30% 26% 12%	2791
	11th to 20th	Night	21% 4 12% 36% 21% 5%	2928
	21st to end	Night	17% 3 11% 35% 28% 5%	3245
September	1st to 10th	Night	24% 3 12% 27% 24% 10%	2025
	11th to 20th	Night	31% 5% 14% 28% 18% 4	2200
	21st to end	Night	30% 4 16% 29% 18% 2	2200
October	1st to 10th	Night	33% 3 13% 33% 17%	3686
	11th to 20th	Night	34% 5% 22% 30% 9%	3992
	21st to end	Night	41% 5% 18% 27% 8% 1	4457
November	1st to 10th	Night	52% 5% 18% 20% 4 1	2535
	11th to 20th	Night	54% 6% 20% 16% 4	2731
	21st to end	Night	47% 6% 23% 20% 5%	2865
December	1st to 10th	Night	43% 5% 26% 21% 4	4890
	11th to 20th	Night	60% 7% 22% 11% 1	5547
	21st to end	Night	59% 8% 15% 12% 5%	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	16% 14% 24% 28% 17% 2	200
	11th to 20th	Day	18% 9% 24% 31% 14% 6%	200
	21st to end	Day	15% 14% 26% 31% 10% 3	220
February	1st to 10th	Day	23% 17% 30% 24% 6%	263
	11th to 20th	Day	15% 20% 26% 27% 11% 1	349
	21st to end	Day	22% 16% 26% 28% 8%	414
March	1st to 10th	Day	20% 18% 27% 25% 8% 1	500
	11th to 20th	Day	18% 16% 25% 23% 16% 1	500
	21st to end	Day	21% 18% 30% 24% 7% 1	550
April	1st to 10th	Day	20% 19% 31% 22% 9%	500
	11th to 20th	Day	22% 20% 32% 19% 7% 1	492
	21st to end	Day	24% 21% 26% 19% 9% 2	500
May	1st to 10th	Day	13% 18% 28% 29% 9% 2	503
	11th to 20th	Day	22% 14% 25% 23% 13% 2	553
	21st to end	Day	19% 14% 19% 28% 17% 4	789
June	1st to 10th	Day	16% 14% 22% 26% 18% 4	730
	11th to 20th	Day	22% 18% 26% 22% 11% 2	730
	21st to end	Day	10% 14% 23% 31% 20% 2	730
July	1st to 10th	Day	5% 10% 28% 31% 24% 3	721
	11th to 20th	Day	7% 14% 30% 31% 16% 2	591
	21st to end	Day	4 10% 32% 34% 18% 3	575
August	1st to 10th	Day	6% 13% 28% 31% 17% 5%	527
	11th to 20th	Day	6% 9% 26% 32% 21% 5%	517
	21st to end	Day	9% 10% 27% 32% 18% 4	583
September	1st to 10th	Day	8% 12% 24% 27% 24% 5%	530
	11th to 20th	Day	11% 12% 24% 28% 20% 5%	530
	21st to end	Day	15% 15% 20% 25% 21% 4	530
October	1st to 10th	Day	18% 9% 22% 33% 16% 3	480
	11th to 20th	Day	19% 11% 23% 27% 14% 6%	324
	21st to end	Day	20% 11% 26% 33% 7% 2	330
November	1st to 10th	Day	22% 13% 22% 34% 9%	300
	11th to 20th	Day	21% 11% 26% 31% 11% 1	300
	21st to end	Day	13% 12% 21% 40% 12% 1	266
December	1st to 10th	Day	22% 9% 26% 26% 14% 4	200
	11th to 20th	Day	19% 12% 28% 32% 9% 1	200
	21st to end	Day	16% 12% 30% 30% 10% 2	220
January	1st to 10th	Night	19% 9% 23% 37% 11% 1	945
	11th to 20th	Night	17% 9% 26% 35% 11% 2	800
	21st to end	Night	15% 6% 25% 41% 12% 1	880
February	1st to 10th	Night	21% 11% 29% 31% 8% 1	800
	11th to 20th	Night	17% 9% 28% 35% 9% 1	784
	21st to end	Night	14% 11% 28% 33% 12% 1	660
March	1st to 10th	Night	13% 8% 28% 39% 11% 1	800
	11th to 20th	Night	14% 9% 26% 37% 14%	800
	21st to end	Night	17% 8% 24% 38% 11% 1	880
April	1st to 10th	Night	15% 10% 24% 38% 14% 1	800
	11th to 20th	Night	14% 11% 26% 38% 11%	593
	21st to end	Night	14% 9% 23% 38% 15% 1	600
May	1st to 10th	Night	14% 10% 23% 35% 19%	600
	11th to 20th	Night	11% 6% 18% 38% 24% 2	493
	21st to end	Night	17% 6% 17% 28% 30% 1	433
June	1st to 10th	Night	16% 7% 16% 36% 22% 4	400
	11th to 20th	Night	18% 9% 19% 37% 17% 1	400
	21st to end	Night	7% 3 16% 43% 31% 1	400
July	1st to 10th	Night	4 1 9% 51% 33% 1	390
	11th to 20th	Night	4 2 11% 57% 25%	388
	21st to end	Night	3 1 10% 55% 31% 1	438
August	1st to 10th	Night	4 3 13% 50% 31% 1	510
	11th to 20th	Night	4 3 11% 48% 32% 2	594
	21st to end	Night	7% 4 14% 47% 27% 1	660
September	1st to 10th	Night	8% 2 15% 44% 29% 3	775
	11th to 20th	Night	12% 2 12% 38% 34% 2	800
	21st to end	Night	13% 6% 16% 39% 24% 3	800
October	1st to 10th	Night	13% 6% 18% 42% 19% 1	800
	11th to 20th	Night	17% 5% 17% 41% 18% 3	792
	21st to end	Night	19% 7% 21% 37% 14% 1	836
November	1st to 10th	Night	22% 6% 21% 40% 10%	780
	11th to 20th	Night	22% 10% 24% 35% 9% 1	931
	21st to end	Night	13% 9% 25% 42% 12%	1000
December	1st to 10th	Night	17% 9% 24% 37% 12% 2	1000
	11th to 20th	Night	16% 8% 29% 38% 9%	1000
	21st to end	Night	17% 11% 28% 34% 9% 1	1100

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