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 2024-11-05.

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	8% 8% 3 6% 4 4 4 4 6% 8% 45%	105441
	Night	8% 9% 3 7% 5% 4 4 4 6% 7% 43%	102269
Transparency	Day	36% 6% 13% 23% 16% 6%	105170
	Night	39% 6% 13% 23% 14% 4	102130
Seeing	Day	12% 9% 25% 26% 20% 7%	15478
	Night	13% 8% 29% 33% 15% 1	24800

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% 13% 4 11% 6% 4 5% 3 6% 9% 34%	2219
	11th to 20th	Day	5% 13% 5% 11% 7% 8% 8% 3 5% 6% 29%	2324
	21st to end	Day	4 9% 3 7% 5% 4 5% 4 5% 8% 44%	2895
February	1st to 10th	Day	8% 17% 5% 11% 6% 5% 5% 3 5% 7% 27%	1710
	11th to 20th	Day	7% 18% 5% 9% 6% 4 5% 3 5% 5% 31%	1665
	21st to end	Day	4 13% 6% 11% 6% 5% 6% 5% 6% 5% 33%	1470
March	1st to 10th	Day	4 7% 3 8% 4 5% 5% 4 7% 10% 42%	3293
	11th to 20th	Day	3 7% 3 8% 5% 5% 4 4 6% 8% 47%	3490
	21st to end	Day	8% 19% 6% 11% 5% 5% 5% 4 5% 8% 23%	3825
April	1st to 10th	Day	11% 12% 4 8% 6% 5% 5% 5% 7% 7% 32%	2251
	11th to 20th	Day	12% 13% 4 8% 6% 5% 4 4 6% 6% 31%	2437
	21st to end	Day	14% 10% 4 6% 5% 5% 5% 3 5% 7% 35%	2470
May	1st to 10th	Day	20% 14% 3 7% 4 4 3 4 6% 7% 28%	4168
	11th to 20th	Day	9% 7% 2 7% 5% 6% 5% 5% 7% 9% 39%	4388
	21st to end	Day	14% 7% 3 7% 5% 6% 5% 4 7% 8% 33%	5092
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 8% 9% 38%	2850
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 5% 6% 7% 38%	2850
	21st to end	Day	5% 3 1 5% 4 4 4 4 7% 10% 54%	2850
July	1st to 10th	Day	4 3 1 4 4 4 4 4 6% 8% 58%	4740
	11th to 20th	Day	3 2 1 2 2 2 3 3 4 7% 71%	4740
	21st to end	Day	2 1 1 3 2 3 3 3 5% 7% 70%	4886
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 7% 8% 52%	4092
	11th to 20th	Day	4 3 1 3 3 3 4 5% 7% 9% 58%	4028
	21st to end	Day	5% 2 1 4 3 3 4 3 6% 8% 61%	4521
September	1st to 10th	Day	9% 3 1 4 3 3 3 3 5% 7% 60%	2319
	11th to 20th	Day	10% 5% 2 6% 5% 3 5% 4 5% 6% 49%	2090
	21st to end	Day	12% 5% 2 4 2 3 3 3 4 6% 56%	2090
October	1st to 10th	Day	9% 7% 2 5% 3 3 4 3 4 6% 54%	3217
	11th to 20th	Day	6% 6% 2 5% 3 4 4 4 5% 7% 55%	3150
	21st to end	Day	6% 8% 3 8% 5% 4 5% 4 6% 7% 44%	3180
November	1st to 10th	Day	12% 11% 4 8% 5% 4 4 2 5% 6% 39%	1577
	11th to 20th	Day	6% 11% 4 8% 6% 5% 5% 4 7% 9% 37%	1520
	21st to end	Day	6% 11% 4 7% 6% 6% 4 3 6% 7% 41%	1520
December	1st to 10th	Day	6% 10% 5% 11% 6% 5% 6% 4 6% 6% 36%	1397
	11th to 20th	Day	5% 12% 3 10% 6% 5% 5% 3 6% 8% 38%	1685
	21st to end	Day	5% 10% 2 8% 5% 5% 5% 4 6% 7% 44%	2442
January	1st to 10th	Night	4 11% 3 8% 5% 5% 5% 4 7% 9% 39%	4726
	11th to 20th	Night	7% 13% 4 10% 7% 6% 5% 4 5% 6% 32%	4667
	21st to end	Night	5% 9% 4 8% 5% 5% 5% 4 6% 7% 41%	4762
February	1st to 10th	Night	10% 17% 5% 11% 5% 4 4 4 6% 6% 28%	2470
	11th to 20th	Night	7% 17% 4 10% 6% 4 5% 4 5% 6% 31%	2402
	21st to end	Night	5% 13% 6% 11% 6% 5% 5% 4 5% 7% 33%	1983
March	1st to 10th	Night	4 9% 4 9% 6% 5% 5% 4 7% 8% 37%	3581
	11th to 20th	Night	3 8% 3 7% 5% 5% 4 4 6% 8% 47%	3490
	21st to end	Night	13% 20% 6% 10% 6% 5% 5% 3 5% 6% 20%	3828
April	1st to 10th	Night	11% 12% 4 8% 6% 4 4 4 7% 7% 32%	1927
	11th to 20th	Night	12% 12% 4 9% 5% 4 6% 3 5% 6% 33%	1691
	21st to end	Night	16% 8% 2 6% 4 5% 4 3 6% 8% 38%	1710
May	1st to 10th	Night	20% 15% 3 7% 5% 4 4 3 4 5% 30%	2801
	11th to 20th	Night	13% 9% 3 7% 4 4 4 4 6% 8% 39%	2517
	21st to end	Night	14% 6% 2 7% 4 6% 5% 5% 9% 10% 33%	2481
June	1st to 10th	Night	12% 5% 2 7% 4 5% 5% 4 8% 9% 39%	1330
	11th to 20th	Night	15% 6% 2 5% 5% 4 4 4 6% 7% 42%	1330
	21st to end	Night	6% 4 2 5% 4 4 4 3 6% 9% 55%	1330
July	1st to 10th	Night	5% 3 2 4 3 4 4 3 6% 7% 59%	2220
	11th to 20th	Night	3 2 1 3 2 3 2 3 4 5% 73%	2220
	21st to end	Night	2 2 1 4 3 4 4 3 5% 6% 65%	2665
August	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 6% 7% 52%	2847
	11th to 20th	Night	5% 3 1 6% 5% 5% 6% 5% 8% 8% 47%	2811
	21st to end	Night	9% 5% 2 5% 3 3 4 3 4 6% 57%	3146
September	1st to 10th	Night	8% 4 2 5% 3 4 3 3 5% 8% 55%	1857
	11th to 20th	Night	12% 5% 2 6% 4 3 4 2 4 7% 50%	2090
	21st to end	Night	13% 6% 2 4 3 3 4 3 4 6% 52%	2090
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 3 6% 57%	3753
	11th to 20th	Night	10% 6% 2 6% 3 3 3 3 4 6% 53%	3778
	21st to end	Night	7% 7% 3 6% 4 4 4 4 5% 7% 49%	4365
November	1st to 10th	Night	12% 11% 2 6% 4 4 4 4 6% 8% 39%	2583
	11th to 20th	Night	8% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	2660
	21st to end	Night	6% 11% 3 6% 5% 5% 5% 3 5% 8% 45%	2660
December	1st to 10th	Night	6% 14% 4 9% 7% 5% 5% 4 6% 7% 34%	2778
	11th to 20th	Night	6% 11% 4 8% 5% 5% 4 3 5% 7% 40%	3506
	21st to end	Night	4 10% 3 7% 5% 5% 4 4 6% 6% 46%	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% 8% 15% 25% 6%	2219
	11th to 20th	Day	56% 7% 11% 11% 13% 2	2324
	21st to end	Day	35% 9% 24% 23% 9% 1	2895
February	1st to 10th	Day	55% 7% 14% 16% 7% 1	1710
	11th to 20th	Day	54% 6% 14% 19% 6% 1	1674
	21st to end	Day	49% 8% 12% 19% 9% 2	1470
March	1st to 10th	Day	35% 5% 14% 27% 19%	3293
	11th to 20th	Day	33% 7% 24% 29% 7%	3490
	21st to end	Day	56% 9% 14% 16% 4	3825
April	1st to 10th	Day	49% 7% 15% 20% 9% 1	2251
	11th to 20th	Day	52% 6% 12% 19% 10% 1	2440
	21st to end	Day	48% 6% 13% 23% 10% 1	2470
May	1st to 10th	Day	53% 7% 11% 22% 6% 1	4168
	11th to 20th	Day	37% 8% 15% 24% 13% 3	4392
	21st to end	Day	43% 9% 18% 21% 9% 1	5092
June	1st to 10th	Day	39% 8% 19% 23% 10% 1	2850
	11th to 20th	Day	42% 8% 12% 19% 15% 5%	2850
	21st to end	Day	24% 6% 13% 32% 22% 3	2850
July	1st to 10th	Day	24% 4 8% 30% 21% 13%	4666
	11th to 20th	Day	15% 3 5% 18% 29% 31%	4659
	21st to end	Day	14% 2 5% 18% 30% 30%	4731
August	1st to 10th	Day	30% 4 10% 19% 24% 13%	4078
	11th to 20th	Day	22% 3 10% 37% 22% 7%	4067
	21st to end	Day	20% 3 8% 28% 31% 10%	4520
September	1st to 10th	Day	24% 4 9% 26% 24% 11%	2319
	11th to 20th	Day	34% 6% 11% 26% 17% 6%	2090
	21st to end	Day	30% 4 10% 27% 21% 7%	2090
October	1st to 10th	Day	32% 3 8% 28% 25% 5%	3217
	11th to 20th	Day	29% 4 14% 30% 19% 3	3150
	21st to end	Day	39% 5% 10% 24% 16% 5%	3180
November	1st to 10th	Day	46% 5% 12% 21% 12% 3	1577
	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	40% 8% 16% 26% 9% 1	4726
	11th to 20th	Night	52% 6% 13% 15% 12% 2	4667
	21st to end	Night	39% 9% 18% 24% 9% 1	4762
February	1st to 10th	Night	56% 6% 14% 16% 7% 1	2470
	11th to 20th	Night	53% 7% 14% 18% 7% 1	2403
	21st to end	Night	51% 6% 13% 18% 9% 2	1983
March	1st to 10th	Night	40% 8% 12% 22% 17%	3581
	11th to 20th	Night	33% 7% 23% 29% 7% 1	3490
	21st to end	Night	61% 9% 12% 13% 3 1	3828
April	1st to 10th	Night	49% 6% 14% 22% 8% 1	1927
	11th to 20th	Night	51% 7% 12% 20% 9% 1	1691
	21st to end	Night	45% 6% 13% 26% 10% 1	1710
May	1st to 10th	Night	55% 5% 9% 20% 9% 2	2801
	11th to 20th	Night	39% 9% 13% 24% 14% 1	2518
	21st to end	Night	40% 10% 16% 25% 7% 1	2481
June	1st to 10th	Night	39% 9% 19% 22% 9% 1	1330
	11th to 20th	Night	41% 7% 10% 23% 15% 4	1330
	21st to end	Night	26% 6% 13% 30% 22% 2	1330
July	1st to 10th	Night	23% 4 7% 32% 27% 7%	2185
	11th to 20th	Night	14% 3 4 19% 33% 26%	2178
	21st to end	Night	18% 4 5% 19% 29% 25%	2582
August	1st to 10th	Night	32% 3 10% 22% 20% 12%	2838
	11th to 20th	Night	32% 3 9% 29% 20% 6%	2838
	21st to end	Night	29% 4 7% 29% 21% 10%	3146
September	1st to 10th	Night	27% 4 10% 22% 23% 14%	1857
	11th to 20th	Night	35% 4 10% 27% 17% 7%	2090
	21st to end	Night	33% 6% 11% 24% 20% 6%	2090
October	1st to 10th	Night	30% 3 8% 30% 23% 7%	3753
	11th to 20th	Night	33% 4 15% 26% 21% 2	3778
	21st to end	Night	35% 5% 11% 23% 23% 4	4365
November	1st to 10th	Night	43% 7% 15% 20% 12% 3	2583
	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% 16% 34% 16% 19% 8%	180
	11th to 20th	Day	13% 9% 33% 27% 10% 8%	180
	21st to end	Day	11% 14% 41% 24% 7% 4	228
February	1st to 10th	Day	18% 19% 33% 25% 4 3	270
	11th to 20th	Day	12% 20% 26% 27% 13% 2	270
	21st to end	Day	10% 18% 34% 27% 10% 2	282
March	1st to 10th	Day	12% 14% 31% 29% 12% 2	460
	11th to 20th	Day	10% 12% 26% 32% 16% 5%	460
	21st to end	Day	14% 11% 27% 29% 16% 3	506
April	1st to 10th	Day	15% 13% 36% 25% 11% 1	460
	11th to 20th	Day	19% 12% 26% 25% 15% 4	452
	21st to end	Day	21% 8% 26% 23% 16% 6%	460
May	1st to 10th	Day	17% 8% 18% 27% 20% 10%	495
	11th to 20th	Day	17% 6% 17% 25% 25% 10%	675
	21st to end	Day	18% 7% 19% 22% 23% 11%	734
June	1st to 10th	Day	13% 7% 21% 28% 21% 11%	680
	11th to 20th	Day	19% 11% 25% 24% 16% 4	680
	21st to end	Day	7% 6% 19% 30% 29% 9%	680
July	1st to 10th	Day	7% 2 20% 29% 31% 11%	671
	11th to 20th	Day	6% 4 21% 32% 30% 6%	668
	21st to end	Day	3 2 20% 34% 31% 11%	638
August	1st to 10th	Day	7% 4 22% 28% 28% 12%	487
	11th to 20th	Day	7% 2 19% 28% 31% 14%	477
	21st to end	Day	5% 5% 21% 30% 27% 12%	539
September	1st to 10th	Day	9% 5% 23% 27% 26% 10%	490
	11th to 20th	Day	13% 6% 23% 24% 22% 11%	490
	21st to end	Day	16% 10% 22% 26% 19% 7%	490
October	1st to 10th	Day	19% 6% 21% 24% 19% 11%	334
	11th to 20th	Day	11% 11% 30% 22% 18% 9%	300
	21st to end	Day	14% 18% 31% 20% 13% 5%	330
November	1st to 10th	Day	22% 16% 29% 21% 11% 1	282
	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% 12% 39% 25% 9% 1	903
	21st to end	Night	12% 13% 41% 25% 7% 2	836
February	1st to 10th	Night	18% 14% 33% 26% 8% 3	760
	11th to 20th	Night	16% 10% 35% 28% 9% 2	744
	21st to end	Night	11% 11% 33% 26% 16% 2	628
March	1st to 10th	Night	10% 7% 35% 31% 16% 1	760
	11th to 20th	Night	10% 7% 32% 35% 14% 2	760
	21st to end	Night	17% 6% 31% 31% 14% 2	836
April	1st to 10th	Night	17% 8% 27% 36% 12%	646
	11th to 20th	Night	19% 7% 27% 33% 14%	563
	21st to end	Night	19% 6% 24% 34% 16% 1	570
May	1st to 10th	Night	18% 6% 19% 37% 20% 1	570
	11th to 20th	Night	17% 6% 16% 35% 24% 2	561
	21st to end	Night	22% 4 17% 32% 23% 3	453
June	1st to 10th	Night	14% 4 20% 36% 23% 2	380
	11th to 20th	Night	17% 7% 27% 36% 13% 1	380
	21st to end	Night	8% 3 19% 46% 24%	380
July	1st to 10th	Night	6% 1 15% 52% 26% 1	370
	11th to 20th	Night	5% 2 15% 60% 18%	368
	21st to end	Night	4 2 15% 54% 25% 1	459
August	1st to 10th	Night	7% 2 15% 50% 24% 2	567
	11th to 20th	Night	8% 2 15% 47% 26% 2	564
	21st to end	Night	10% 2 18% 46% 23% 2	627
September	1st to 10th	Night	10% 2 18% 46% 22% 2	570
	11th to 20th	Night	14% 2 17% 39% 24% 4	760
	21st to end	Night	15% 4 23% 35% 20% 3	760
October	1st to 10th	Night	16% 5% 20% 37% 19% 2	760
	11th to 20th	Night	16% 9% 26% 30% 18% 2	760
	21st to end	Night	14% 9% 34% 30% 10% 2	836
November	1st to 10th	Night	18% 9% 36% 30% 7%	874
	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

Page generated at 2024-11-03 00:11:15. Version ac07

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