Lolo Pass Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lolo Pass Visitor 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% 12% 4 9% 5% 4 4 3 5% 6% 40%	135554
	Night	5% 12% 5% 11% 5% 5% 5% 4 5% 6% 38%	132105
Transparency	Day	24% 40% 8% 14% 9% 4	130198
	Night	26% 40% 9% 14% 8% 3	128468
Seeing	Day	18% 20% 29% 21% 11% 3	16449
	Night	13% 15% 25% 30% 15% 2	25689

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	3 21% 12% 22% 6% 5% 4 3 4 5% 15%	3479
	11th to 20th	Day	3 13% 8% 20% 7% 6% 4 3 4 4 27%	3640
	21st to end	Day	3 17% 6% 11% 5% 5% 4 4 5% 6% 34%	4004
February	1st to 10th	Day	6% 24% 9% 18% 8% 5% 4 3 4 3 17%	1700
	11th to 20th	Day	5% 21% 9% 16% 8% 5% 4 3 5% 5% 18%	1839
	21st to end	Day	4 21% 8% 18% 9% 6% 5% 3 3 4 19%	1650
March	1st to 10th	Day	3 14% 5% 13% 7% 5% 5% 4 7% 8% 29%	4518
	11th to 20th	Day	3 16% 8% 15% 6% 5% 4 3 4 4 31%	4946
	21st to end	Day	6% 24% 9% 15% 7% 6% 5% 4 5% 6% 13%	5563
April	1st to 10th	Day	8% 22% 7% 13% 6% 4 5% 3 6% 7% 19%	2400
	11th to 20th	Day	8% 21% 6% 10% 6% 5% 4 4 5% 5% 26%	2538
	21st to end	Day	13% 20% 5% 11% 5% 5% 4 3 5% 5% 25%	2600
May	1st to 10th	Day	9% 17% 4 7% 4 4 4 3 5% 5% 38%	5880
	11th to 20th	Day	9% 15% 5% 11% 6% 5% 5% 4 6% 7% 28%	6118
	21st to end	Day	10% 14% 4 8% 4 4 4 3 5% 6% 38%	7332
June	1st to 10th	Day	17% 12% 3 8% 6% 5% 4 4 6% 7% 27%	3000
	11th to 20th	Day	18% 12% 4 8% 5% 5% 4 4 5% 6% 30%	3000
	21st to end	Day	6% 6% 2 6% 5% 4 5% 4 6% 6% 51%	3000
July	1st to 10th	Day	3 2 1 3 3 3 4 3 5% 7% 66%	6765
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 4 77%	6320
	21st to end	Day	3 2 1 3 2 3 4 4 5% 7% 66%	6880
August	1st to 10th	Day	4 4 2 5% 4 5% 6% 5% 7% 9% 51%	6006
	11th to 20th	Day	3 2 1 3 3 4 4 4 6% 8% 62%	5798
	21st to end	Day	5% 4 2 4 3 3 3 3 5% 6% 62%	6424
September	1st to 10th	Day	6% 4 2 5% 4 3 4 3 5% 7% 58%	2370
	11th to 20th	Day	8% 7% 2 7% 4 5% 5% 4 6% 6% 45%	2200
	21st to end	Day	9% 8% 3 6% 3 3 4 4 6% 7% 47%	2200
October	1st to 10th	Day	6% 10% 3 6% 4 3 3 2 4 6% 53%	3551
	11th to 20th	Day	7% 13% 3 7% 4 4 4 3 5% 7% 44%	3075
	21st to end	Day	6% 18% 6% 12% 6% 5% 4 3 4 5% 32%	3200
November	1st to 10th	Day	8% 21% 7% 13% 7% 5% 5% 3 4 6% 22%	1854
	11th to 20th	Day	6% 20% 5% 14% 6% 5% 5% 4 5% 6% 24%	1761
	21st to end	Day	5% 16% 7% 12% 6% 5% 5% 5% 5% 7% 27%	1510
December	1st to 10th	Day	1 13% 8% 11% 5% 4 5% 4 4 7% 38%	2290
	11th to 20th	Day	2 17% 10% 16% 9% 8% 6% 4 6% 7% 16%	2645
	21st to end	Day	3 21% 8% 17% 6% 6% 5% 3 5% 5% 22%	3498
January	1st to 10th	Night	3 18% 11% 21% 7% 5% 5% 4 6% 5% 16%	6508
	11th to 20th	Night	3 12% 8% 20% 8% 5% 6% 4 4 5% 25%	6320
	21st to end	Night	3 14% 7% 12% 5% 5% 5% 4 5% 7% 33%	6952
February	1st to 10th	Night	6% 20% 8% 19% 7% 5% 4 3 4 5% 17%	2699
	11th to 20th	Night	3 18% 9% 16% 8% 5% 6% 4 4 5% 22%	2445
	21st to end	Night	3 17% 8% 16% 8% 7% 5% 4 6% 5% 22%	1980
March	1st to 10th	Night	2 11% 5% 12% 7% 6% 5% 4 5% 6% 37%	5376
	11th to 20th	Night	3 13% 6% 13% 7% 6% 6% 4 5% 5% 32%	5054
	21st to end	Night	6% 22% 8% 15% 7% 6% 5% 5% 5% 6% 15%	5410
April	1st to 10th	Night	7% 16% 6% 12% 7% 6% 5% 4 6% 7% 26%	2000
	11th to 20th	Night	7% 14% 4 10% 6% 6% 5% 4 6% 6% 33%	1809
	21st to end	Night	9% 14% 4 8% 5% 5% 5% 5% 6% 6% 33%	1800
May	1st to 10th	Night	10% 14% 3 6% 3 3 3 3 5% 7% 42%	4100
	11th to 20th	Night	10% 13% 3 8% 5% 4 4 4 6% 8% 34%	3766
	21st to end	Night	10% 10% 4 7% 4 4 3 2 4 6% 44%	3542
June	1st to 10th	Night	16% 9% 3 5% 6% 4 5% 4 7% 7% 35%	1400
	11th to 20th	Night	14% 11% 3 6% 4 4 5% 5% 6% 6% 36%	1400
	21st to end	Night	5% 5% 2 5% 3 5% 5% 3 6% 8% 53%	1400
July	1st to 10th	Night	2 1 2 1 1 2 2 5% 7% 75%	3180
	11th to 20th	Night	3 1 2 3 2 2 3 2 3 5% 75%	3636
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 68%	3970
August	1st to 10th	Night	3 3 2 5% 3 4 5% 3 7% 8% 57%	3796
	11th to 20th	Night	2 2 1 4 3 3 3 3 5% 7% 67%	4051
	21st to end	Night	5% 4 1 4 3 3 3 3 4 6% 63%	4551
September	1st to 10th	Night	4 4 1 5% 3 3 3 3 5% 7% 61%	2030
	11th to 20th	Night	8% 6% 2 6% 3 4 5% 4 6% 7% 50%	2200
	21st to end	Night	8% 7% 2 6% 4 4 3 3 5% 6% 53%	2200
October	1st to 10th	Night	6% 9% 3 6% 4 4 3 3 4 6% 53%	3646
	11th to 20th	Night	6% 10% 2 7% 4 5% 4 3 5% 7% 46%	4070
	21st to end	Night	5% 15% 6% 8% 5% 5% 5% 3 6% 7% 36%	4600
November	1st to 10th	Night	9% 15% 5% 11% 6% 6% 5% 4 5% 7% 28%	2665
	11th to 20th	Night	6% 18% 6% 11% 7% 6% 5% 4 5% 6% 25%	2770
	21st to end	Night	3 13% 7% 13% 6% 6% 6% 4 6% 8% 29%	2885
December	1st to 10th	Night	3 12% 7% 11% 5% 4 5% 3 5% 7% 39%	4890
	11th to 20th	Night	2 14% 8% 17% 7% 7% 6% 5% 6% 7% 19%	5546
	21st to end	Night	3 16% 9% 16% 8% 6% 5% 4 5% 6% 22%	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	52% 32% 6% 9% 1	3479
	11th to 20th	Day	43% 30% 11% 7% 8%	3640
	21st to end	Day	30% 32% 12% 18% 8%	4004
February	1st to 10th	Day	29% 63% 4 2 1	1700
	11th to 20th	Day	26% 62% 5% 4 2	1848
	21st to end	Day	25% 63% 4 4 3 1	1650
March	1st to 10th	Day	29% 34% 8% 11% 14% 4	4518
	11th to 20th	Day	33% 33% 12% 20% 2	4946
	21st to end	Day	45% 36% 9% 9% 1	5563
April	1st to 10th	Day	23% 63% 8% 5% 1	2400
	11th to 20th	Day	20% 63% 5% 9% 2 1	2541
	21st to end	Day	23% 63% 4 8% 3	2600
May	1st to 10th	Day	30% 31% 8% 20% 11%	5880
	11th to 20th	Day	33% 36% 13% 15% 3	6121
	21st to end	Day	25% 32% 12% 21% 8%	7334
June	1st to 10th	Day	14% 62% 9% 12% 3	3000
	11th to 20th	Day	18% 63% 5% 7% 6% 1	3000
	21st to end	Day	14% 62% 4 10% 9% 1	3000
July	1st to 10th	Day	10% 27% 7% 14% 26% 15%	6692
	11th to 20th	Day	8% 25% 3 16% 17% 30%	6245
	21st to end	Day	11% 27% 9% 17% 24% 12%	6681
August	1st to 10th	Day	12% 37% 6% 17% 19% 9%	4375
	11th to 20th	Day	11% 40% 10% 23% 15% 1	4197
	21st to end	Day	12% 37% 5% 25% 18% 4	4631
September	1st to 10th	Day	11% 57% 6% 15% 9% 2	2370
	11th to 20th	Day	19% 58% 6% 9% 7% 1	2200
	21st to end	Day	18% 58% 6% 11% 6% 1	2200
October	1st to 10th	Day	16% 36% 8% 25% 14%	3551
	11th to 20th	Day	20% 38% 12% 19% 11%	3075
	21st to end	Day	32% 37% 6% 17% 6% 2	3200
November	1st to 10th	Day	30% 55% 7% 7% 1	1854
	11th to 20th	Day	23% 59% 8% 7% 2 1	1761
	21st to end	Day	22% 59% 7% 7% 5% 1	1510
December	1st to 10th	Day	24% 39% 15% 16% 5% 1	2290
	11th to 20th	Day	38% 41% 10% 10% 1	2644
	21st to end	Day	38% 31% 13% 11% 4 4	3498
January	1st to 10th	Night	47% 31% 10% 9% 3	6508
	11th to 20th	Night	40% 31% 12% 10% 7% 1	6320
	21st to end	Night	31% 32% 9% 18% 9%	6952
February	1st to 10th	Night	28% 63% 5% 2 2	2699
	11th to 20th	Night	24% 62% 6% 5% 2 1	2446
	21st to end	Night	23% 63% 5% 5% 2 1	1980
March	1st to 10th	Night	25% 34% 9% 13% 16% 4	5376
	11th to 20th	Night	32% 33% 13% 20% 2	5054
	21st to end	Night	46% 35% 8% 10% 2	5410
April	1st to 10th	Night	18% 64% 10% 6% 2	2000
	11th to 20th	Night	17% 65% 7% 8% 3	1809
	21st to end	Night	18% 64% 5% 9% 4	1800
May	1st to 10th	Night	26% 31% 10% 22% 10%	4097
	11th to 20th	Night	31% 34% 12% 19% 4	3766
	21st to end	Night	23% 31% 14% 21% 10%	3542
June	1st to 10th	Night	11% 63% 8% 14% 3	1400
	11th to 20th	Night	15% 63% 6% 9% 6% 2	1400
	21st to end	Night	12% 62% 6% 10% 10% 1	1400
July	1st to 10th	Night	5% 25% 6% 20% 27% 17%	3145
	11th to 20th	Night	9% 26% 5% 20% 11% 30%	3589
	21st to end	Night	12% 26% 9% 19% 17% 16%	3857
August	1st to 10th	Night	11% 37% 6% 16% 19% 11%	2755
	11th to 20th	Night	11% 38% 7% 30% 13% 1	2928
	21st to end	Night	11% 37% 6% 25% 16% 5%	3274
September	1st to 10th	Night	11% 57% 7% 12% 9% 3	2030
	11th to 20th	Night	15% 58% 7% 11% 8% 1	2200
	21st to end	Night	15% 58% 8% 11% 7% 1	2200
October	1st to 10th	Night	18% 38% 6% 19% 19% 1	3646
	11th to 20th	Night	18% 40% 12% 18% 11%	4070
	21st to end	Night	24% 41% 10% 17% 7% 1	4600
November	1st to 10th	Night	27% 55% 8% 7% 2	2665
	11th to 20th	Night	24% 58% 9% 6% 2 1	2770
	21st to end	Night	19% 59% 7% 8% 7%	2885
December	1st to 10th	Night	20% 41% 16% 15% 7%	4890
	11th to 20th	Night	39% 38% 13% 8% 1 1	5547
	21st to end	Night	37% 33% 12% 10% 5% 3	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	20% 22% 23% 23% 11% 3	200
	11th to 20th	Day	18% 27% 25% 16% 9% 6%	200
	21st to end	Day	20% 24% 28% 17% 9% 2	220
February	1st to 10th	Day	28% 30% 28% 11% 2	250
	11th to 20th	Day	24% 26% 30% 14% 5%	384
	21st to end	Day	27% 25% 29% 14% 4	414
March	1st to 10th	Day	19% 31% 30% 14% 5% 1	500
	11th to 20th	Day	20% 21% 28% 22% 8%	500
	21st to end	Day	23% 26% 33% 13% 5% 1	550
April	1st to 10th	Day	24% 25% 30% 16% 5%	500
	11th to 20th	Day	27% 23% 30% 13% 5% 2	492
	21st to end	Day	27% 24% 27% 15% 5% 2	500
May	1st to 10th	Day	17% 19% 29% 23% 10% 1	503
	11th to 20th	Day	25% 20% 24% 19% 11% 2	528
	21st to end	Day	22% 15% 21% 23% 15% 4	756
June	1st to 10th	Day	18% 18% 25% 22% 13% 5%	730
	11th to 20th	Day	23% 21% 26% 18% 10% 2	730
	21st to end	Day	10% 16% 28% 27% 17% 3	730
July	1st to 10th	Day	4 16% 29% 29% 18% 3	720
	11th to 20th	Day	8% 17% 39% 22% 12% 2	524
	21st to end	Day	4 13% 35% 32% 13% 3	575
August	1st to 10th	Day	6% 16% 30% 27% 17% 4	527
	11th to 20th	Day	5% 12% 33% 31% 14% 5%	517
	21st to end	Day	10% 15% 34% 22% 14% 5%	583
September	1st to 10th	Day	9% 15% 28% 26% 17% 6%	530
	11th to 20th	Day	14% 16% 29% 24% 12% 5%	530
	21st to end	Day	18% 19% 28% 20% 14% 3	530
October	1st to 10th	Day	19% 15% 31% 21% 12% 2	507
	11th to 20th	Day	23% 17% 25% 18% 11% 5%	330
	21st to end	Day	23% 22% 29% 18% 5% 2	363
November	1st to 10th	Day	26% 22% 25% 19% 8%	330
	11th to 20th	Day	25% 21% 31% 13% 8% 2	321
	21st to end	Day	18% 28% 22% 24% 7% 1	255
December	1st to 10th	Day	20% 25% 26% 17% 10% 4	200
	11th to 20th	Day	24% 26% 27% 22% 3	200
	21st to end	Day	24% 24% 28% 16% 7% 1	220
January	1st to 10th	Night	17% 20% 26% 27% 10% 1	885
	11th to 20th	Night	15% 19% 31% 23% 10% 3	800
	21st to end	Night	14% 18% 31% 26% 10% 2	880
February	1st to 10th	Night	18% 25% 27% 23% 7% 1	800
	11th to 20th	Night	15% 21% 31% 22% 9% 2	784
	21st to end	Night	17% 20% 30% 22% 9% 2	660
March	1st to 10th	Night	11% 20% 29% 30% 10% 1	800
	11th to 20th	Night	13% 20% 27% 28% 12% 1	800
	21st to end	Night	16% 16% 28% 30% 9% 1	880
April	1st to 10th	Night	14% 17% 27% 29% 12% 2	800
	11th to 20th	Night	14% 17% 32% 24% 11% 1	621
	21st to end	Night	16% 15% 26% 30% 11% 2	600
May	1st to 10th	Night	15% 12% 24% 31% 17% 2	600
	11th to 20th	Night	14% 9% 21% 31% 21% 3	466
	21st to end	Night	18% 10% 18% 27% 26% 1	433
June	1st to 10th	Night	17% 8% 19% 32% 22% 3	400
	11th to 20th	Night	17% 12% 24% 33% 14% 1	400
	21st to end	Night	6% 6% 17% 43% 29% 1	400
July	1st to 10th	Night	2 3 18% 43% 33% 2	390
	11th to 20th	Night	5% 3 16% 52% 23% 1	388
	21st to end	Night	2 2 14% 48% 32% 1	438
August	1st to 10th	Night	4 4 16% 44% 31% 1	487
	11th to 20th	Night	3 2 19% 43% 28% 5%	594
	21st to end	Night	7% 6% 17% 41% 28% 1	676
September	1st to 10th	Night	6% 4 21% 36% 28% 5%	800
	11th to 20th	Night	9% 7% 17% 33% 27% 6%	800
	21st to end	Night	12% 8% 19% 33% 22% 5%	800
October	1st to 10th	Night	13% 10% 21% 37% 17% 2	800
	11th to 20th	Night	14% 11% 24% 31% 17% 4	800
	21st to end	Night	17% 16% 27% 25% 11% 3	880
November	1st to 10th	Night	17% 18% 28% 26% 10% 1	820
	11th to 20th	Night	21% 20% 27% 24% 7%	907
	21st to end	Night	14% 20% 28% 26% 11% 2	1000
December	1st to 10th	Night	16% 21% 24% 28% 10% 2	1000
	11th to 20th	Night	14% 21% 34% 24% 7% 1	1000
	21st to end	Night	18% 25% 29% 20% 7% 1	1100

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