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 2024-01-12.

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	9% 8% 3 6% 5% 3 5% 5% 6% 8% 43%	74668
	Night	9% 9% 3 6% 5% 3 5% 5% 5% 7% 42%	75051
Transparency	Day	38% 7% 13% 23% 14% 5%	74357
	Night	40% 7% 14% 22% 13% 4	74884
Seeing	Day	13% 9% 25% 26% 20% 7%	14305
	Night	13% 8% 29% 33% 15% 2	23825

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% 12% 4 7% 7% 3 4 4 6% 9% 36%	1329
	11th to 20th	Day	7% 14% 5% 7% 5% 2 5% 5% 5% 9% 35%	1329
	21st to end	Day	6% 14% 5% 9% 7% 4 5% 6% 6% 7% 31%	1643
February	1st to 10th	Day	8% 15% 6% 10% 8% 4 5% 5% 5% 7% 28%	1620
	11th to 20th	Day	8% 18% 5% 9% 7% 3 5% 4 5% 5% 31%	1575
	21st to end	Day	4 12% 6% 11% 8% 4 6% 6% 6% 6% 32%	1386
March	1st to 10th	Day	7% 9% 5% 9% 7% 3 4 5% 7% 9% 36%	1890
	11th to 20th	Day	6% 10% 4 8% 6% 4 5% 6% 6% 8% 36%	1980
	21st to end	Day	8% 10% 4 9% 7% 4 6% 6% 6% 10% 30%	2178
April	1st to 10th	Day	11% 10% 4 7% 7% 4 6% 6% 6% 8% 33%	2132
	11th to 20th	Day	13% 12% 4 7% 7% 4 5% 5% 5% 6% 30%	2307
	21st to end	Day	15% 10% 4 6% 6% 4 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	11% 9% 3 6% 4 3 5% 5% 7% 8% 38%	2370
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 5% 7% 8% 39%	2478
	21st to end	Day	20% 7% 3 6% 5% 4 5% 5% 6% 9% 30%	2874
June	1st to 10th	Day	13% 6% 2 5% 6% 4 6% 6% 7% 9% 37%	2700
	11th to 20th	Day	16% 5% 3 5% 5% 3 6% 6% 6% 8% 38%	2700
	21st to end	Day	5% 3 1 4 4 3 4 4 6% 10% 55%	2700
July	1st to 10th	Day	5% 2 1 3 3 2 4 5% 6% 8% 59%	2700
	11th to 20th	Day	5% 3 1 3 3 2 3 4 5% 8% 63%	2700
	21st to end	Day	4 2 1 3 3 2 3 4 5% 7% 66%	2803
August	1st to 10th	Day	4 3 1 3 4 3 4 4 6% 8% 60%	2328
	11th to 20th	Day	6% 3 1 2 3 2 4 4 6% 8% 61%	2299
	21st to end	Day	7% 3 2 4 3 3 4 4 5% 8% 57%	2574
September	1st to 10th	Day	9% 4 1 3 3 2 3 4 5% 7% 59%	2198
	11th to 20th	Day	10% 4 2 4 5% 2 4 5% 5% 6% 51%	1980
	21st to end	Day	13% 5% 2 3 3 2 4 3 4 6% 54%	1980
October	1st to 10th	Day	13% 8% 3 5% 5% 3 4 5% 5% 8% 42%	1833
	11th to 20th	Day	10% 8% 3 6% 6% 3 4 6% 5% 6% 43%	1800
	21st to end	Day	9% 8% 2 6% 6% 3 5% 6% 7% 10% 39%	1836
November	1st to 10th	Day	12% 11% 4 7% 5% 3 4 3 5% 6% 39%	1542
	11th to 20th	Day	6% 10% 4 7% 7% 3 5% 5% 6% 9% 37%	1520
	21st to end	Day	7% 9% 4 7% 6% 4 5% 4 6% 7% 41%	1520
December	1st to 10th	Day	6% 9% 5% 9% 8% 3 6% 5% 6% 6% 36%	1397
	11th to 20th	Day	5% 11% 4 9% 7% 4 5% 4 6% 8% 38%	1685
	21st to end	Day	5% 10% 3 6% 6% 3 5% 5% 5% 7% 44%	2442
January	1st to 10th	Night	7% 11% 4 7% 6% 3 5% 5% 7% 9% 36%	2836
	11th to 20th	Night	9% 12% 4 7% 7% 3 5% 5% 5% 7% 37%	2677
	21st to end	Night	7% 11% 4 7% 6% 4 5% 5% 7% 8% 36%	2711
February	1st to 10th	Night	10% 16% 6% 9% 6% 3 4 5% 6% 7% 29%	2340
	11th to 20th	Night	8% 16% 5% 10% 7% 3 6% 4 5% 6% 31%	2272
	21st to end	Night	5% 13% 7% 10% 7% 4 6% 5% 5% 6% 32%	1870
March	1st to 10th	Night	6% 11% 4 9% 7% 4 5% 5% 6% 8% 34%	2070
	11th to 20th	Night	5% 10% 4 7% 7% 4 5% 5% 6% 10% 37%	1980
	21st to end	Night	11% 14% 5% 7% 7% 4 5% 6% 6% 7% 29%	2178
April	1st to 10th	Night	10% 11% 4 8% 7% 3 5% 5% 7% 7% 32%	1826
	11th to 20th	Night	13% 11% 4 8% 6% 4 6% 4 4 7% 33%	1601
	21st to end	Night	16% 8% 2 5% 5% 3 5% 4 6% 8% 38%	1620
May	1st to 10th	Night	13% 8% 3 6% 7% 3 5% 5% 5% 7% 39%	1589
	11th to 20th	Night	14% 7% 2 5% 4 3 5% 4 6% 8% 42%	1417
	21st to end	Night	20% 6% 3 5% 5% 4 4 5% 7% 9% 32%	1397
June	1st to 10th	Night	12% 4 3 6% 5% 3 6% 5% 8% 10% 38%	1260
	11th to 20th	Night	16% 5% 2 5% 5% 3 5% 5% 6% 7% 41%	1260
	21st to end	Night	7% 3 2 4 4 3 4 4 5% 9% 55%	1260
July	1st to 10th	Night	5% 2 2 3 3 2 4 4 6% 8% 62%	1260
	11th to 20th	Night	4 2 1 3 3 3 3 4 5% 7% 65%	1260
	21st to end	Night	4 2 1 3 3 3 3 4 5% 7% 65%	1511
August	1st to 10th	Night	5% 4 2 4 4 2 4 5% 5% 7% 59%	1611
	11th to 20th	Night	7% 3 1 3 5% 2 4 4 6% 7% 57%	1602
	21st to end	Night	9% 5% 2 4 4 3 4 4 4 6% 55%	1782
September	1st to 10th	Night	9% 4 2 4 4 3 3 4 5% 8% 54%	1759
	11th to 20th	Night	11% 5% 2 5% 5% 2 4 3 4 7% 52%	1980
	21st to end	Night	13% 6% 2 4 4 2 4 4 4 6% 50%	1980
October	1st to 10th	Night	14% 7% 2 6% 4 3 3 5% 4 7% 45%	2126
	11th to 20th	Night	11% 8% 2 5% 4 3 3 4 5% 7% 47%	2160
	21st to end	Night	10% 9% 3 6% 5% 3 4 5% 5% 7% 42%	2520
November	1st to 10th	Night	12% 11% 2 6% 4 3 4 5% 5% 8% 39%	2518
	11th to 20th	Night	8% 8% 3 7% 5% 4 4 5% 6% 8% 42%	2660
	21st to end	Night	6% 10% 3 5% 6% 4 5% 4 5% 7% 45%	2660
December	1st to 10th	Night	6% 12% 5% 8% 8% 4 5% 5% 6% 7% 34%	2778
	11th to 20th	Night	6% 10% 5% 7% 6% 3 4 4 5% 7% 40%	3506
	21st to end	Night	4 9% 4 6% 6% 3 5% 5% 5% 6% 47%	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% 7% 15% 24% 7% 1	1329
	11th to 20th	Day	45% 7% 16% 19% 10% 2	1329
	21st to end	Day	50% 9% 15% 18% 7% 2	1643
February	1st to 10th	Day	55% 7% 14% 16% 7% 1	1620
	11th to 20th	Day	54% 6% 14% 19% 7% 1	1584
	21st to end	Day	51% 7% 12% 18% 10% 2	1386
March	1st to 10th	Day	43% 7% 17% 23% 9% 1	1890
	11th to 20th	Day	44% 7% 15% 24% 9% 1	1980
	21st to end	Day	47% 9% 16% 20% 8%	2178
April	1st to 10th	Day	49% 7% 15% 20% 9% 1	2132
	11th to 20th	Day	53% 6% 11% 18% 10% 1	2310
	21st to end	Day	49% 6% 12% 23% 9% 1	2340
May	1st to 10th	Day	42% 7% 13% 26% 10% 2	2370
	11th to 20th	Day	41% 9% 15% 23% 11% 1	2482
	21st to end	Day	49% 9% 17% 19% 5% 1	2874
June	1st to 10th	Day	41% 8% 19% 22% 9%	2700
	11th to 20th	Day	43% 8% 13% 19% 14% 4	2700
	21st to end	Day	24% 7% 12% 33% 22% 2	2700
July	1st to 10th	Day	21% 6% 10% 31% 22% 9%	2626
	11th to 20th	Day	20% 4 7% 25% 23% 19%	2619
	21st to end	Day	18% 3 7% 23% 31% 17%	2647
August	1st to 10th	Day	22% 6% 11% 24% 26% 12%	2313
	11th to 20th	Day	21% 5% 9% 29% 25% 10%	2299
	21st to end	Day	24% 5% 9% 25% 24% 13%	2574
September	1st to 10th	Day	25% 4 10% 26% 24% 11%	2198
	11th to 20th	Day	32% 6% 11% 27% 18% 7%	1980
	21st to end	Day	31% 5% 10% 27% 20% 7%	1980
October	1st to 10th	Day	41% 5% 13% 25% 14% 2	1833
	11th to 20th	Day	40% 7% 16% 22% 12% 3	1800
	21st to end	Day	39% 8% 16% 26% 9% 2	1836
November	1st to 10th	Day	46% 6% 11% 21% 12% 3	1542
	11th to 20th	Day	42% 9% 17% 22% 9% 2	1520
	21st to end	Day	42% 6% 16% 24% 11% 2	1520
December	1st to 10th	Day	46% 6% 17% 20% 8% 3	1397
	11th to 20th	Day	43% 8% 24% 18% 6% 1	1684
	21st to end	Day	38% 6% 14% 23% 12% 7%	2442
January	1st to 10th	Night	44% 9% 16% 22% 8% 1	2836
	11th to 20th	Night	46% 7% 16% 19% 10% 2	2677
	21st to end	Night	45% 8% 15% 20% 11% 1	2711
February	1st to 10th	Night	55% 6% 14% 16% 7% 1	2340
	11th to 20th	Night	52% 7% 14% 19% 7%	2273
	21st to end	Night	52% 5% 13% 17% 10% 3	1870
March	1st to 10th	Night	47% 8% 16% 19% 10%	2070
	11th to 20th	Night	43% 7% 17% 23% 9% 1	1980
	21st to end	Night	52% 9% 16% 17% 6% 1	2178
April	1st to 10th	Night	49% 6% 15% 22% 8% 1	1826
	11th to 20th	Night	53% 6% 11% 20% 9% 1	1601
	21st to end	Night	45% 6% 13% 26% 9%	1620
May	1st to 10th	Night	43% 7% 13% 26% 9% 2	1589
	11th to 20th	Night	39% 5% 15% 24% 16% 2	1418
	21st to end	Night	48% 9% 17% 19% 8%	1397
June	1st to 10th	Night	39% 10% 20% 22% 8% 1	1260
	11th to 20th	Night	41% 8% 10% 24% 14% 3	1260
	21st to end	Night	27% 7% 13% 31% 21% 2	1260
July	1st to 10th	Night	19% 6% 9% 33% 24% 9%	1225
	11th to 20th	Night	20% 4 7% 24% 30% 15%	1218
	21st to end	Night	18% 5% 8% 22% 31% 16%	1427
August	1st to 10th	Night	25% 5% 9% 27% 23% 11%	1602
	11th to 20th	Night	25% 5% 10% 24% 25% 11%	1602
	21st to end	Night	29% 6% 8% 23% 22% 12%	1782
September	1st to 10th	Night	28% 4 10% 22% 22% 13%	1759
	11th to 20th	Night	34% 5% 9% 27% 18% 7%	1980
	21st to end	Night	34% 6% 11% 24% 19% 5%	1980
October	1st to 10th	Night	39% 5% 13% 25% 15% 3	2126
	11th to 20th	Night	37% 6% 14% 25% 14% 3	2160
	21st to end	Night	41% 7% 16% 21% 13% 2	2520
November	1st to 10th	Night	43% 7% 15% 20% 12% 3	2518
	11th to 20th	Night	39% 9% 17% 23% 11% 1	2660
	21st to end	Night	38% 6% 17% 26% 11% 3	2660
December	1st to 10th	Night	49% 7% 15% 18% 8% 3	2778
	11th to 20th	Night	42% 8% 21% 21% 7% 1	3507
	21st to end	Night	36% 6% 13% 25% 12% 8%	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	7% 17% 34% 16% 19% 8%	180
	11th to 20th	Day	14% 11% 33% 24% 10% 9%	162
	21st to end	Day	12% 15% 44% 19% 6% 4	203
February	1st to 10th	Day	17% 20% 33% 25% 3 2	240
	11th to 20th	Day	12% 23% 27% 24% 12% 3	240
	21st to end	Day	10% 16% 35% 27% 11% 2	252
March	1st to 10th	Day	13% 14% 32% 28% 10% 3	420
	11th to 20th	Day	11% 13% 25% 31% 15% 5%	420
	21st to end	Day	15% 11% 26% 29% 16% 3	462
April	1st to 10th	Day	15% 12% 35% 25% 11% 1	420
	11th to 20th	Day	20% 13% 27% 24% 14% 3	412
	21st to end	Day	22% 8% 25% 22% 16% 6%	420
May	1st to 10th	Day	15% 8% 19% 28% 20% 10%	453
	11th to 20th	Day	18% 5% 16% 25% 25% 10%	625
	21st to end	Day	19% 6% 19% 22% 23% 11%	679
June	1st to 10th	Day	13% 7% 21% 27% 21% 12%	630
	11th to 20th	Day	19% 11% 25% 24% 17% 4	630
	21st to end	Day	7% 5% 18% 30% 30% 9%	630
July	1st to 10th	Day	7% 2 20% 29% 31% 11%	621
	11th to 20th	Day	7% 4 21% 32% 30% 6%	618
	21st to end	Day	3 1 18% 35% 32% 11%	589
August	1st to 10th	Day	5% 4 23% 27% 28% 13%	447
	11th to 20th	Day	7% 2 19% 27% 30% 14%	437
	21st to end	Day	5% 5% 20% 29% 28% 12%	495
September	1st to 10th	Day	10% 5% 25% 27% 24% 9%	450
	11th to 20th	Day	13% 6% 22% 23% 24% 12%	450
	21st to end	Day	16% 9% 22% 26% 20% 8%	450
October	1st to 10th	Day	20% 5% 21% 23% 20% 12%	303
	11th to 20th	Day	13% 10% 30% 22% 17% 9%	270
	21st to end	Day	15% 15% 30% 21% 14% 5%	297
November	1st to 10th	Day	22% 16% 29% 21% 11% 1	270
	11th to 20th	Day	11% 16% 41% 23% 9%	270
	21st to end	Day	11% 19% 30% 26% 12% 1	270
December	1st to 10th	Day	8% 17% 41% 22% 9% 3	212
	11th to 20th	Day	15% 21% 29% 24% 10% 1	180
	21st to end	Day	12% 13% 34% 24% 11% 6%	198
January	1st to 10th	Night	12% 16% 37% 23% 10% 1	950
	11th to 20th	Night	15% 13% 39% 24% 9% 1	863
	21st to end	Night	13% 13% 42% 24% 7% 2	792
February	1st to 10th	Night	17% 14% 33% 25% 8% 3	720
	11th to 20th	Night	16% 10% 36% 27% 9% 2	704
	21st to end	Night	12% 10% 34% 26% 16% 2	592
March	1st to 10th	Night	10% 7% 36% 30% 15% 1	720
	11th to 20th	Night	10% 7% 33% 34% 14% 2	720
	21st to end	Night	17% 6% 31% 30% 14% 2	792
April	1st to 10th	Night	16% 8% 27% 36% 13%	612
	11th to 20th	Night	20% 7% 28% 31% 14%	533
	21st to end	Night	20% 6% 24% 33% 16% 1	540
May	1st to 10th	Night	16% 6% 19% 37% 21% 1	540
	11th to 20th	Night	17% 6% 15% 35% 25% 2	531
	21st to end	Night	23% 4 17% 31% 24% 3	429
June	1st to 10th	Night	14% 3 20% 36% 24% 2	360
	11th to 20th	Night	17% 6% 27% 36% 14% 1	360
	21st to end	Night	8% 3 19% 45% 25%	360
July	1st to 10th	Night	5% 1 15% 51% 26% 1	350
	11th to 20th	Night	6% 2 15% 58% 19%	348
	21st to end	Night	4 1 13% 55% 25% 1	434
August	1st to 10th	Night	7% 2 14% 50% 25% 2	537
	11th to 20th	Night	8% 1 15% 47% 26% 2	534
	21st to end	Night	9% 2 16% 47% 24% 2	594
September	1st to 10th	Night	11% 2 17% 46% 22% 2	540
	11th to 20th	Night	13% 2 17% 39% 25% 4	720
	21st to end	Night	16% 3 22% 36% 20% 3	720
October	1st to 10th	Night	17% 5% 20% 37% 19% 2	720
	11th to 20th	Night	16% 9% 27% 30% 17% 2	720
	21st to end	Night	15% 9% 34% 29% 11% 2	792
November	1st to 10th	Night	18% 9% 35% 30% 7%	853
	11th to 20th	Night	11% 14% 41% 26% 7% 1	950
	21st to end	Night	9% 12% 38% 33% 7% 1	950
December	1st to 10th	Night	13% 12% 34% 28% 12% 2	950
	11th to 20th	Night	12% 15% 38% 25% 9% 1	950
	21st to end	Night	13% 13% 38% 26% 10% 1	1045

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