Great Falls
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Falls 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% 8% 3 6% 4 4 4 4 6% 8% 46%	135675
	Night	8% 9% 3 7% 5% 4 4 4 6% 7% 43%	131369
Transparency	Day	37% 5% 12% 24% 16% 6%	130356
	Night	40% 5% 13% 24% 14% 4	127693
Seeing	Day	12% 9% 25% 27% 20% 7%	16764
	Night	13% 8% 29% 33% 15% 1	26034

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	8% 16% 5% 11% 7% 5% 4 3 5% 7% 29%	3179
	11th to 20th	Day	4 15% 5% 12% 7% 7% 7% 3 4 6% 30%	3340
	21st to end	Day	3 9% 5% 8% 5% 4 4 4 5% 7% 46%	4161
February	1st to 10th	Day	8% 17% 5% 11% 7% 5% 5% 3 5% 6% 27%	1800
	11th to 20th	Day	7% 18% 6% 10% 6% 4 5% 3 5% 6% 31%	1755
	21st to end	Day	4 12% 6% 11% 6% 5% 6% 5% 6% 6% 34%	1545
March	1st to 10th	Day	5% 8% 3 7% 4 4 4 4 6% 9% 44%	4738
	11th to 20th	Day	3 7% 4 10% 6% 6% 6% 5% 7% 8% 39%	5000
	21st to end	Day	9% 18% 5% 11% 6% 6% 6% 4 6% 8% 21%	5486
April	1st to 10th	Day	11% 12% 3 7% 5% 5% 5% 5% 7% 7% 33%	2370
	11th to 20th	Day	12% 13% 4 8% 6% 5% 4 4 6% 6% 32%	2567
	21st to end	Day	14% 10% 4 6% 5% 5% 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	16% 11% 3 6% 3 4 3 3 5% 8% 40%	5966
	11th to 20th	Day	13% 9% 3 7% 5% 6% 5% 4 7% 8% 33%	6298
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	7309
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 9% 38%	3000
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 5% 6% 8% 38%	3000
	21st to end	Day	5% 3 1 4 3 4 4 4 7% 10% 54%	3000
July	1st to 10th	Day	4 3 1 4 4 4 4 4 7% 8% 57%	6779
	11th to 20th	Day	7% 3 1 3 2 2 3 3 4 7% 65%	6780
	21st to end	Day	2 2 1 3 3 3 3 3 6% 9% 65%	7020
August	1st to 10th	Day	5% 4 2 5% 4 5% 6% 5% 8% 9% 47%	5779
	11th to 20th	Day	3 2 1 3 3 3 3 4 6% 10% 63%	5798
	21st to end	Day	6% 3 2 5% 3 3 3 3 5% 7% 60%	6468
September	1st to 10th	Day	9% 3 1 4 3 3 3 3 5% 7% 60%	2441
	11th to 20th	Day	10% 5% 2 6% 5% 3 5% 4 5% 6% 50%	2200
	21st to end	Day	12% 5% 2 4 2 3 3 3 4 6% 57%	2200
October	1st to 10th	Day	9% 7% 2 5% 3 3 3 3 4 6% 54%	3319
	11th to 20th	Day	6% 6% 2 5% 3 4 4 4 5% 7% 54%	3230
	21st to end	Day	6% 8% 3 8% 5% 4 5% 4 6% 7% 44%	3180
November	1st to 10th	Day	12% 12% 4 7% 5% 4 4 2 5% 6% 40%	1625
	11th to 20th	Day	6% 10% 4 8% 6% 5% 5% 4 7% 9% 36%	1600
	21st to end	Day	6% 11% 4 7% 6% 6% 4 3 6% 7% 40%	1600
December	1st to 10th	Day	4 8% 4 9% 5% 4 5% 4 7% 8% 44%	2399
	11th to 20th	Day	4 10% 4 10% 5% 5% 5% 4 6% 8% 39%	2645
	21st to end	Day	5% 13% 3 9% 5% 4 5% 4 5% 6% 40%	3498
January	1st to 10th	Night	7% 15% 4 10% 5% 5% 5% 4 6% 8% 33%	6766
	11th to 20th	Night	6% 15% 4 11% 6% 6% 5% 4 5% 6% 33%	6665
	21st to end	Night	4 10% 4 9% 5% 4 4 4 5% 6% 44%	6800
February	1st to 10th	Night	10% 17% 5% 11% 5% 4 4 4 6% 6% 28%	2600
	11th to 20th	Night	7% 17% 5% 10% 6% 4 5% 4 5% 6% 31%	2532
	21st to end	Night	5% 13% 6% 11% 6% 5% 5% 4 5% 7% 33%	2084
March	1st to 10th	Night	4 9% 3 8% 5% 5% 5% 4 8% 10% 40%	5156
	11th to 20th	Night	3 10% 3 9% 6% 6% 5% 4 6% 8% 39%	5000
	21st to end	Night	13% 17% 5% 9% 5% 5% 6% 4 6% 7% 22%	5489
April	1st to 10th	Night	10% 12% 4 8% 6% 4 4 4 7% 7% 33%	2028
	11th to 20th	Night	12% 12% 4 9% 5% 4 6% 3 5% 6% 34%	1781
	21st to end	Night	16% 8% 2 6% 4 5% 4 3 6% 8% 39%	1800
May	1st to 10th	Night	17% 11% 2 5% 4 3 3 3 4 6% 41%	4013
	11th to 20th	Night	15% 10% 3 8% 5% 5% 4 4 6% 8% 30%	3617
	21st to end	Night	10% 5% 2 6% 4 5% 5% 4 7% 8% 44%	3565
June	1st to 10th	Night	12% 5% 2 7% 5% 5% 5% 4 8% 9% 40%	1400
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 4 6% 8% 41%	1400
	21st to end	Night	7% 4 2 5% 3 4 4 3 6% 9% 55%	1400
July	1st to 10th	Night	4 3 2 4 3 4 4 3 6% 7% 60%	3180
	11th to 20th	Night	7% 2 1 3 2 3 3 3 4 6% 65%	3180
	21st to end	Night	4 3 1 5% 4 4 4 3 6% 9% 56%	3818
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	4033
	11th to 20th	Night	4 3 1 5% 4 4 5% 4 7% 8% 56%	4051
	21st to end	Night	9% 5% 2 5% 3 3 4 3 4 6% 57%	4509
September	1st to 10th	Night	8% 4 2 5% 3 4 3 3 5% 8% 55%	1955
	11th to 20th	Night	12% 5% 2 6% 4 3 4 2 4 7% 50%	2200
	21st to end	Night	12% 6% 2 4 3 3 4 3 4 6% 53%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 3 6% 58%	3871
	11th to 20th	Night	10% 6% 2 6% 3 3 3 3 5% 6% 53%	3874
	21st to end	Night	7% 7% 3 6% 4 4 4 4 5% 7% 49%	4365
November	1st to 10th	Night	11% 12% 2 6% 4 4 4 4 5% 8% 39%	2654
	11th to 20th	Night	7% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	2800
	21st to end	Night	5% 11% 3 7% 6% 5% 5% 3 5% 8% 43%	2800
December	1st to 10th	Night	4 10% 3 8% 6% 4 4 5% 6% 6% 43%	4779
	11th to 20th	Night	6% 11% 4 9% 6% 5% 5% 4 6% 7% 36%	5546
	21st to end	Night	4 12% 4 8% 5% 5% 5% 4 6% 7% 41%	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	55% 6% 12% 24% 4	3179
	11th to 20th	Day	56% 5% 12% 14% 12% 2	3340
	21st to end	Day	36% 7% 18% 27% 12% 1	4161
February	1st to 10th	Day	56% 7% 13% 15% 7% 1	1800
	11th to 20th	Day	53% 6% 14% 19% 7% 1	1764
	21st to end	Day	48% 7% 13% 20% 10% 2	1545
March	1st to 10th	Day	35% 4 10% 26% 19% 5%	4738
	11th to 20th	Day	40% 6% 17% 25% 11% 1	5000
	21st to end	Day	58% 6% 12% 20% 4	5486
April	1st to 10th	Day	48% 7% 15% 20% 9% 1	2370
	11th to 20th	Day	52% 6% 12% 20% 10% 1	2570
	21st to end	Day	48% 6% 12% 23% 10% 1	2600
May	1st to 10th	Day	43% 5% 10% 25% 14% 3	5966
	11th to 20th	Day	45% 6% 15% 23% 9% 2	6302
	21st to end	Day	35% 6% 14% 26% 17% 1	7310
June	1st to 10th	Day	39% 8% 19% 23% 10% 1	3000
	11th to 20th	Day	42% 7% 13% 19% 15% 5%	3000
	21st to end	Day	24% 6% 12% 32% 22% 3	3000
July	1st to 10th	Day	24% 3 9% 29% 22% 13%	6706
	11th to 20th	Day	21% 2 5% 22% 28% 22%	6699
	21st to end	Day	17% 2 12% 20% 25% 23%	6816
August	1st to 10th	Day	30% 4 11% 20% 24% 13%	4208
	11th to 20th	Day	21% 3 10% 36% 22% 7%	4197
	21st to end	Day	21% 3 8% 27% 31% 9%	4663
September	1st to 10th	Day	24% 4 10% 28% 24% 11%	2441
	11th to 20th	Day	33% 6% 11% 27% 17% 6%	2200
	21st to end	Day	29% 4 10% 27% 22% 8%	2200
October	1st to 10th	Day	32% 3 8% 28% 25% 5%	3319
	11th to 20th	Day	30% 4 14% 30% 19% 3	3230
	21st to end	Day	39% 5% 10% 24% 16% 5%	3180
November	1st to 10th	Day	46% 5% 12% 22% 12% 3	1625
	11th to 20th	Day	42% 8% 17% 22% 9% 2	1600
	21st to end	Day	44% 6% 16% 23% 10% 2	1600
December	1st to 10th	Day	38% 4 22% 23% 11% 2	2399
	11th to 20th	Day	42% 5% 26% 19% 7% 1	2644
	21st to end	Day	44% 5% 11% 22% 14% 5%	3498
January	1st to 10th	Night	48% 6% 13% 25% 7% 1	6766
	11th to 20th	Night	53% 4 14% 17% 10% 2	6665
	21st to end	Night	40% 6% 13% 27% 11% 2	6800
February	1st to 10th	Night	57% 6% 13% 16% 8% 1	2600
	11th to 20th	Night	53% 6% 14% 18% 8% 1	2533
	21st to end	Night	50% 5% 13% 19% 9% 2	2084
March	1st to 10th	Night	38% 5% 10% 28% 16% 3	5156
	11th to 20th	Night	41% 6% 17% 28% 9% 1	5000
	21st to end	Night	59% 6% 13% 17% 5% 1	5489
April	1st to 10th	Night	48% 6% 14% 23% 8% 1	2028
	11th to 20th	Night	51% 6% 11% 22% 9% 1	1781
	21st to end	Night	44% 6% 13% 26% 10% 1	1800
May	1st to 10th	Night	44% 4 8% 26% 14% 4	4010
	11th to 20th	Night	48% 6% 14% 21% 10% 1	3618
	21st to end	Night	33% 7% 15% 28% 15% 1	3565
June	1st to 10th	Night	38% 9% 18% 22% 11% 1	1400
	11th to 20th	Night	41% 7% 11% 23% 15% 4	1400
	21st to end	Night	26% 6% 13% 31% 22% 2	1400
July	1st to 10th	Night	23% 3 9% 28% 27% 9%	3145
	11th to 20th	Night	22% 2 5% 23% 29% 18%	3138
	21st to end	Night	26% 3 12% 17% 24% 18%	3711
August	1st to 10th	Night	32% 3 11% 22% 20% 12%	2928
	11th to 20th	Night	32% 3 10% 30% 20% 6%	2928
	21st to end	Night	29% 4 7% 29% 22% 10%	3245
September	1st to 10th	Night	27% 4 10% 23% 23% 13%	1955
	11th to 20th	Night	35% 4 10% 28% 17% 7%	2200
	21st to end	Night	32% 6% 11% 23% 22% 6%	2200
October	1st to 10th	Night	30% 3 8% 30% 22% 7%	3871
	11th to 20th	Night	33% 4 15% 26% 21% 2	3874
	21st to end	Night	35% 5% 11% 23% 23% 4	4365
November	1st to 10th	Night	43% 7% 15% 21% 12% 3	2654
	11th to 20th	Night	39% 8% 17% 24% 11% 1	2800
	21st to end	Night	40% 6% 17% 25% 10% 3	2800
December	1st to 10th	Night	40% 4 18% 22% 12% 3	4779
	11th to 20th	Night	47% 5% 21% 19% 7% 1	5547
	21st to end	Night	41% 5% 12% 25% 12% 6%	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	11% 15% 34% 16% 19% 8%	200
	11th to 20th	Day	14% 11% 36% 25% 9% 7%	200
	21st to end	Day	10% 18% 40% 23% 6% 3	254
February	1st to 10th	Day	18% 17% 35% 24% 4 2	300
	11th to 20th	Day	11% 19% 27% 29% 12% 2	300
	21st to end	Day	12% 20% 33% 25% 9% 2	309
March	1st to 10th	Day	13% 14% 30% 29% 11% 3	500
	11th to 20th	Day	11% 12% 28% 30% 15% 4	500
	21st to end	Day	15% 12% 26% 29% 16% 2	550
April	1st to 10th	Day	17% 13% 34% 26% 10% 1	500
	11th to 20th	Day	19% 13% 27% 23% 15% 3	492
	21st to end	Day	20% 8% 25% 25% 16% 6%	500
May	1st to 10th	Day	16% 8% 18% 28% 20% 9%	537
	11th to 20th	Day	18% 6% 18% 25% 24% 10%	725
	21st to end	Day	17% 6% 19% 23% 24% 11%	789
June	1st to 10th	Day	12% 7% 21% 28% 21% 11%	730
	11th to 20th	Day	18% 11% 25% 25% 17% 4	730
	21st to end	Day	8% 7% 19% 30% 28% 8%	730
July	1st to 10th	Day	6% 2 22% 29% 30% 11%	721
	11th to 20th	Day	7% 4 21% 33% 30% 6%	718
	21st to end	Day	3 2 20% 33% 31% 10%	687
August	1st to 10th	Day	6% 4 22% 28% 28% 11%	527
	11th to 20th	Day	7% 2 19% 28% 30% 13%	517
	21st to end	Day	5% 4 21% 31% 27% 12%	583
September	1st to 10th	Day	9% 5% 22% 29% 25% 11%	530
	11th to 20th	Day	12% 6% 22% 24% 24% 12%	530
	21st to end	Day	15% 9% 22% 27% 20% 8%	530
October	1st to 10th	Day	19% 6% 22% 25% 18% 10%	366
	11th to 20th	Day	13% 11% 29% 21% 17% 8%	324
	21st to end	Day	14% 18% 31% 20% 13% 5%	330
November	1st to 10th	Day	21% 16% 30% 21% 11% 1	300
	11th to 20th	Day	11% 18% 40% 23% 8%	300
	21st to end	Day	12% 18% 30% 27% 13% 1	300
December	1st to 10th	Day	9% 17% 39% 22% 10% 3	235
	11th to 20th	Day	16% 22% 28% 25% 10% 1	200
	21st to end	Day	13% 16% 33% 21% 11% 5%	220
January	1st to 10th	Night	13% 16% 37% 23% 10% 1	1000
	11th to 20th	Night	15% 13% 38% 25% 8% 1	950
	21st to end	Night	13% 13% 42% 24% 7% 1	880
February	1st to 10th	Night	17% 14% 33% 26% 8% 3	800
	11th to 20th	Night	16% 9% 33% 29% 10% 2	784
	21st to end	Night	12% 12% 33% 25% 15% 2	660
March	1st to 10th	Night	10% 7% 35% 31% 16% 1	800
	11th to 20th	Night	10% 8% 34% 34% 14% 2	800
	21st to end	Night	18% 7% 30% 30% 13% 2	880
April	1st to 10th	Night	16% 8% 27% 36% 12%	680
	11th to 20th	Night	19% 7% 27% 33% 14%	593
	21st to end	Night	19% 6% 24% 35% 16% 1	600
May	1st to 10th	Night	17% 6% 19% 38% 20% 1	600
	11th to 20th	Night	17% 6% 16% 35% 24% 2	591
	21st to end	Night	21% 4 17% 33% 23% 3	477
June	1st to 10th	Night	14% 5% 20% 37% 23% 2	400
	11th to 20th	Night	16% 6% 27% 37% 14% 1	400
	21st to end	Night	8% 3 19% 46% 24%	400
July	1st to 10th	Night	6% 1 15% 52% 25% 1	390
	11th to 20th	Night	6% 2 15% 59% 18%	388
	21st to end	Night	4 2 14% 54% 25% 1	483
August	1st to 10th	Night	8% 2 16% 50% 23% 2	597
	11th to 20th	Night	8% 2 16% 47% 25% 2	594
	21st to end	Night	10% 2 18% 45% 23% 2	660
September	1st to 10th	Night	10% 2 17% 46% 23% 2	600
	11th to 20th	Night	14% 2 16% 39% 25% 3	800
	21st to end	Night	15% 4 23% 35% 21% 3	800
October	1st to 10th	Night	16% 5% 20% 38% 19% 2	800
	11th to 20th	Night	16% 9% 26% 30% 17% 2	792
	21st to end	Night	14% 9% 34% 30% 10% 2	836
November	1st to 10th	Night	18% 9% 36% 30% 7%	899
	11th to 20th	Night	11% 15% 40% 26% 7% 1	1000
	21st to end	Night	9% 12% 38% 33% 7% 1	1000
December	1st to 10th	Night	13% 12% 35% 28% 11% 2	1000
	11th to 20th	Night	12% 14% 38% 25% 10% 1	1000
	21st to end	Night	13% 14% 38% 25% 10% 1	1100

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