Southwest Montana Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southwest Montana Astronomical Society clear sky chart from 2005-11-06 to 2025-09-04.

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% 9% 3 7% 4 4 5% 4 6% 7% 44%	135153
	Night	6% 11% 4 8% 5% 4 4 4 6% 7% 41%	130942
Transparency	Day	37% 6% 12% 23% 16% 5%	129852
	Night	41% 7% 13% 24% 12% 3	127250
Seeing	Day	15% 11% 27% 28% 15% 3	16610
	Night	16% 9% 25% 37% 13% 1	26010

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	5% 21% 8% 13% 6% 4 5% 3 6% 7% 22%	3179
	11th to 20th	Day	3 14% 6% 16% 7% 5% 4 4 6% 5% 29%	3525
	21st to end	Day	3 11% 5% 10% 6% 4 4 3 5% 6% 43%	4460
February	1st to 10th	Day	8% 20% 5% 12% 7% 5% 4 5% 5% 7% 22%	1800
	11th to 20th	Day	4 18% 7% 13% 8% 5% 5% 3 6% 5% 26%	1755
	21st to end	Day	4 15% 6% 14% 8% 5% 6% 5% 5% 6% 27%	1564
March	1st to 10th	Day	4 12% 4 9% 4 4 4 4 5% 7% 43%	4814
	11th to 20th	Day	3 14% 5% 10% 6% 5% 5% 5% 6% 6% 35%	5000
	21st to end	Day	7% 16% 6% 12% 7% 6% 6% 4 7% 7% 22%	5486
April	1st to 10th	Day	7% 16% 5% 9% 5% 5% 5% 4 5% 7% 30%	2314
	11th to 20th	Day	8% 13% 5% 10% 6% 5% 6% 4 6% 7% 29%	2567
	21st to end	Day	11% 15% 4 8% 5% 5% 4 4 7% 7% 30%	2600
May	1st to 10th	Day	12% 13% 3 6% 4 4 4 3 5% 7% 39%	5880
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 5% 8% 10% 31%	6240
	21st to end	Day	10% 8% 2 5% 4 4 4 3 6% 8% 46%	6983
June	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 6% 7% 9% 35%	3000
	11th to 20th	Day	12% 7% 3 7% 6% 5% 5% 4 6% 8% 36%	3000
	21st to end	Day	4 4 2 5% 4 4 5% 4 6% 8% 55%	3000
July	1st to 10th	Day	2 2 1 3 3 4 5% 5% 6% 7% 61%	6779
	11th to 20th	Day	3 2 1 3 3 4 4 4 6% 7% 64%	6780
	21st to end	Day	2 2 1 3 3 4 5% 5% 7% 8% 60%	6549
August	1st to 10th	Day	3 3 1 4 4 5% 6% 5% 8% 10% 53%	5779
	11th to 20th	Day	3 2 1 3 3 4 5% 5% 7% 7% 62%	5798
	21st to end	Day	7% 4 1 4 3 3 4 3 6% 7% 58%	6465
September	1st to 10th	Day	6% 4 1 3 3 3 3 3 5% 8% 61%	2319
	11th to 20th	Day	9% 5% 2 5% 3 4 5% 3 6% 7% 50%	2090
	21st to end	Day	9% 5% 2 6% 3 3 4 3 5% 7% 53%	2090
October	1st to 10th	Day	8% 7% 2 5% 3 3 2 2 3 5% 60%	3216
	11th to 20th	Day	8% 8% 2 5% 3 3 3 2 4 7% 55%	3150
	21st to end	Day	7% 12% 4 8% 4 5% 6% 4 6% 6% 39%	3286
November	1st to 10th	Day	10% 14% 4 8% 5% 4 4 3 5% 7% 36%	1674
	11th to 20th	Day	7% 15% 5% 8% 6% 6% 4 4 5% 8% 33%	1600
	21st to end	Day	6% 15% 5% 9% 5% 4 4 3 4 6% 38%	1600
December	1st to 10th	Day	3 10% 5% 10% 6% 5% 4 3 5% 6% 44%	2620
	11th to 20th	Day	4 14% 6% 12% 6% 5% 6% 5% 7% 8% 28%	2693
	21st to end	Day	5% 15% 5% 11% 5% 5% 5% 4 5% 6% 33%	3498
January	1st to 10th	Night	4 16% 8% 12% 5% 5% 5% 4 5% 6% 30%	6766
	11th to 20th	Night	4 16% 7% 14% 8% 5% 4 3 4 5% 28%	6479
	21st to end	Night	4 12% 4 10% 5% 5% 5% 3 5% 6% 40%	6513
February	1st to 10th	Night	7% 22% 7% 14% 6% 4 5% 3 4 5% 23%	2600
	11th to 20th	Night	5% 20% 7% 12% 5% 4 4 4 5% 6% 28%	2532
	21st to end	Night	4 15% 7% 15% 7% 4 5% 3 5% 6% 29%	2064
March	1st to 10th	Night	3 9% 4 9% 6% 5% 5% 4 7% 7% 42%	5079
	11th to 20th	Night	5% 12% 4 8% 5% 5% 5% 4 6% 7% 37%	5000
	21st to end	Night	7% 18% 5% 12% 7% 6% 6% 5% 6% 7% 21%	5489
April	1st to 10th	Night	8% 16% 5% 9% 5% 5% 4 4 5% 7% 31%	2083
	11th to 20th	Night	8% 13% 4 8% 6% 5% 5% 4 7% 8% 32%	1781
	21st to end	Night	15% 14% 4 7% 5% 4 4 4 5% 6% 33%	1800
May	1st to 10th	Night	13% 14% 4 7% 4 4 4 3 4 6% 37%	4100
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	3673
	21st to end	Night	9% 10% 4 8% 5% 5% 5% 4 7% 8% 36%	3894
June	1st to 10th	Night	14% 7% 3 7% 6% 5% 5% 5% 5% 7% 36%	1400
	11th to 20th	Night	11% 10% 3 8% 5% 5% 5% 4 6% 6% 37%	1400
	21st to end	Night	6% 3 1 4 3 5% 4 4 5% 8% 57%	1400
July	1st to 10th	Night	4 3 1 4 4 4 4 4 6% 9% 56%	3180
	11th to 20th	Night	3 2 1 4 3 4 4 4 6% 8% 61%	3180
	21st to end	Night	2 2 1 4 3 3 5% 5% 9% 11% 55%	4301
August	1st to 10th	Night	3 3 1 4 3 5% 6% 5% 8% 10% 53%	4033
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 7% 9% 58%	4051
	21st to end	Night	7% 4 2 5% 3 4 4 3 5% 6% 58%	4509
September	1st to 10th	Night	6% 4 1 4 5% 4 5% 3 5% 8% 56%	1924
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 6% 8% 50%	2090
	21st to end	Night	9% 7% 2 6% 3 3 3 3 4 5% 55%	2090
October	1st to 10th	Night	8% 5% 2 4 3 3 3 3 4 6% 61%	3754
	11th to 20th	Night	9% 10% 3 5% 3 3 3 3 4 6% 53%	3778
	21st to end	Night	9% 12% 3 6% 3 4 4 3 6% 7% 44%	4271
November	1st to 10th	Night	10% 15% 3 7% 5% 4 3 3 6% 7% 36%	2605
	11th to 20th	Night	8% 16% 5% 10% 4 5% 4 3 5% 6% 34%	2800
	21st to end	Night	6% 12% 4 8% 5% 4 5% 4 5% 8% 40%	2800
December	1st to 10th	Night	5% 13% 8% 7% 4 3 3 3 4 6% 45%	4570
	11th to 20th	Night	4 13% 5% 11% 5% 5% 5% 4 6% 8% 33%	5495
	21st to end	Night	5% 14% 6% 11% 5% 5% 4 4 5% 7% 34%	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	59% 8% 14% 17% 3	3179
	11th to 20th	Day	54% 6% 14% 15% 11% 1	3525
	21st to end	Day	39% 6% 17% 25% 12%	4460
February	1st to 10th	Day	60% 6% 12% 16% 5%	1800
	11th to 20th	Day	58% 6% 12% 16% 7% 1	1764
	21st to end	Day	55% 7% 9% 18% 10% 2	1564
March	1st to 10th	Day	37% 7% 9% 27% 16% 4	4814
	11th to 20th	Day	44% 8% 15% 25% 9%	5000
	21st to end	Day	56% 8% 16% 17% 2	5486
April	1st to 10th	Day	50% 8% 14% 20% 9%	2314
	11th to 20th	Day	51% 6% 13% 22% 7%	2570
	21st to end	Day	51% 5% 15% 22% 7% 1	2600
May	1st to 10th	Day	42% 7% 10% 28% 12% 1	5880
	11th to 20th	Day	43% 9% 16% 24% 8% 1	6244
	21st to end	Day	34% 7% 17% 26% 15% 1	6984
June	1st to 10th	Day	42% 7% 19% 23% 9% 1	3000
	11th to 20th	Day	44% 6% 14% 20% 14% 4	3000
	21st to end	Day	25% 5% 13% 28% 23% 6%	3000
July	1st to 10th	Day	17% 7% 9% 25% 29% 13%	6706
	11th to 20th	Day	16% 5% 7% 21% 30% 21%	6699
	21st to end	Day	18% 7% 12% 20% 27% 15%	6360
August	1st to 10th	Day	25% 6% 10% 24% 23% 13%	4208
	11th to 20th	Day	20% 8% 9% 27% 29% 8%	4197
	21st to end	Day	21% 5% 7% 26% 31% 10%	4663
September	1st to 10th	Day	22% 3 9% 26% 28% 12%	2319
	11th to 20th	Day	32% 5% 10% 23% 23% 6%	2090
	21st to end	Day	30% 4 10% 31% 20% 6%	2090
October	1st to 10th	Day	30% 3 7% 26% 31% 4	3216
	11th to 20th	Day	30% 4 17% 28% 19% 2	3150
	21st to end	Day	42% 7% 12% 27% 10% 2	3286
November	1st to 10th	Day	47% 5% 12% 24% 10% 2	1674
	11th to 20th	Day	49% 5% 17% 20% 9% 1	1600
	21st to end	Day	46% 5% 13% 23% 13% 1	1600
December	1st to 10th	Day	39% 7% 19% 25% 9% 1	2620
	11th to 20th	Day	49% 8% 18% 21% 3	2692
	21st to end	Day	50% 6% 10% 22% 9% 4	3498
January	1st to 10th	Night	51% 8% 11% 26% 3	6766
	11th to 20th	Night	57% 5% 13% 17% 7% 2	6479
	21st to end	Night	42% 7% 15% 25% 11% 1	6513
February	1st to 10th	Night	62% 6% 11% 15% 6%	2600
	11th to 20th	Night	56% 5% 15% 16% 7% 1	2533
	21st to end	Night	54% 6% 10% 18% 11% 1	2064
March	1st to 10th	Night	36% 8% 9% 27% 16% 4	5079
	11th to 20th	Night	41% 9% 18% 27% 5%	5000
	21st to end	Night	56% 8% 14% 17% 4	5489
April	1st to 10th	Night	49% 8% 14% 21% 7%	2083
	11th to 20th	Night	46% 7% 12% 26% 8%	1781
	21st to end	Night	50% 6% 15% 22% 7%	1800
May	1st to 10th	Night	45% 7% 13% 23% 10% 2	4097
	11th to 20th	Night	47% 10% 13% 22% 7% 2	3674
	21st to end	Night	39% 11% 15% 28% 6% 1	3894
June	1st to 10th	Night	46% 8% 17% 20% 9% 1	1400
	11th to 20th	Night	45% 7% 13% 21% 11% 4	1400
	21st to end	Night	24% 6% 14% 28% 21% 5%	1400
July	1st to 10th	Night	22% 6% 13% 31% 24% 5%	3145
	11th to 20th	Night	19% 7% 7% 22% 28% 17%	3138
	21st to end	Night	18% 9% 16% 25% 23% 10%	4178
August	1st to 10th	Night	22% 8% 13% 28% 19% 10%	2928
	11th to 20th	Night	22% 8% 11% 30% 23% 7%	2928
	21st to end	Night	21% 5% 9% 33% 26% 7%	3245
September	1st to 10th	Night	27% 4 8% 23% 25% 13%	1924
	11th to 20th	Night	30% 6% 12% 27% 20% 6%	2090
	21st to end	Night	32% 3 9% 27% 22% 5%	2090
October	1st to 10th	Night	26% 4 9% 26% 30% 6%	3754
	11th to 20th	Night	34% 4 14% 26% 18% 4	3778
	21st to end	Night	39% 6% 12% 25% 16% 2	4271
November	1st to 10th	Night	45% 7% 11% 24% 11% 2	2605
	11th to 20th	Night	49% 6% 14% 23% 7% 1	2800
	21st to end	Night	41% 5% 16% 24% 12% 2	2800
December	1st to 10th	Night	41% 5% 20% 24% 9% 1	4570
	11th to 20th	Night	45% 9% 21% 22% 4	5496
	21st to end	Night	47% 7% 11% 23% 8% 4	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	15% 10% 21% 34% 17% 3	200
	11th to 20th	Day	20% 9% 26% 30% 10% 6%	200
	21st to end	Day	13% 8% 31% 33% 13% 2	318
February	1st to 10th	Day	24% 16% 35% 20% 6%	300
	11th to 20th	Day	20% 14% 34% 23% 7% 2	300
	21st to end	Day	19% 16% 34% 20% 10% 1	328
March	1st to 10th	Day	20% 16% 31% 23% 8% 1	500
	11th to 20th	Day	20% 16% 28% 24% 10% 2	500
	21st to end	Day	23% 16% 28% 25% 8% 1	550
April	1st to 10th	Day	21% 19% 31% 21% 8% 1	500
	11th to 20th	Day	19% 16% 34% 23% 7% 1	492
	21st to end	Day	23% 14% 27% 23% 10% 3	500
May	1st to 10th	Day	20% 13% 22% 26% 16% 3	503
	11th to 20th	Day	20% 10% 26% 24% 17% 4	693
	21st to end	Day	20% 10% 20% 24% 20% 6%	789
June	1st to 10th	Day	16% 8% 26% 26% 21% 5%	730
	11th to 20th	Day	18% 15% 28% 25% 12% 2	730
	21st to end	Day	8% 8% 26% 34% 20% 3	730
July	1st to 10th	Day	5% 7% 26% 35% 22% 5%	721
	11th to 20th	Day	5% 8% 27% 35% 24% 2	718
	21st to end	Day	5% 8% 27% 36% 19% 5%	598
August	1st to 10th	Day	6% 7% 31% 30% 21% 4	527
	11th to 20th	Day	6% 5% 29% 33% 19% 7%	517
	21st to end	Day	7% 9% 29% 28% 22% 7%	583
September	1st to 10th	Day	7% 7% 29% 30% 21% 6%	503
	11th to 20th	Day	16% 9% 27% 30% 14% 3	490
	21st to end	Day	17% 11% 23% 25% 20% 4	490
October	1st to 10th	Day	18% 12% 22% 23% 20% 5%	333
	11th to 20th	Day	21% 10% 26% 23% 12% 8%	300
	21st to end	Day	20% 12% 30% 25% 11% 2	330
November	1st to 10th	Day	21% 11% 27% 27% 12% 3	300
	11th to 20th	Day	19% 13% 31% 27% 8% 1	300
	21st to end	Day	17% 13% 25% 36% 8% 1	300
December	1st to 10th	Day	13% 15% 26% 30% 14% 2	300
	11th to 20th	Day	22% 16% 26% 28% 8%	217
	21st to end	Day	20% 16% 23% 25% 14% 2	220
January	1st to 10th	Night	18% 12% 28% 34% 8%	1000
	11th to 20th	Night	19% 13% 27% 29% 10% 2	868
	21st to end	Night	16% 13% 27% 34% 9% 1	880
February	1st to 10th	Night	20% 15% 31% 27% 8% 1	800
	11th to 20th	Night	18% 12% 28% 34% 7% 1	784
	21st to end	Night	14% 14% 33% 27% 10% 2	660
March	1st to 10th	Night	13% 12% 29% 38% 8%	800
	11th to 20th	Night	14% 12% 27% 37% 11% 1	800
	21st to end	Night	21% 11% 24% 34% 9% 2	880
April	1st to 10th	Night	20% 9% 27% 33% 10% 1	688
	11th to 20th	Night	16% 10% 28% 37% 8% 1	593
	21st to end	Night	22% 10% 22% 32% 14% 1	600
May	1st to 10th	Night	20% 8% 22% 34% 16% 1	600
	11th to 20th	Night	18% 7% 20% 34% 19% 1	591
	21st to end	Night	21% 6% 18% 32% 21% 1	624
June	1st to 10th	Night	18% 4 19% 35% 23% 1	400
	11th to 20th	Night	18% 8% 25% 39% 10%	400
	21st to end	Night	8% 4 20% 49% 20%	400
July	1st to 10th	Night	6% 1 16% 49% 28% 1	390
	11th to 20th	Night	5% 3 17% 53% 22%	388
	21st to end	Night	4 3 20% 48% 24% 1	603
August	1st to 10th	Night	6% 4 18% 53% 19% 2	597
	11th to 20th	Night	6% 2 18% 49% 23% 2	594
	21st to end	Night	9% 4 22% 41% 23% 1	660
September	1st to 10th	Night	7% 3 20% 46% 22% 2	584
	11th to 20th	Night	12% 4 15% 44% 24% 2	760
	21st to end	Night	14% 7% 17% 40% 20% 3	760
October	1st to 10th	Night	18% 4 19% 43% 16% 1	760
	11th to 20th	Night	19% 7% 21% 36% 15% 2	760
	21st to end	Night	20% 8% 23% 36% 10% 1	836
November	1st to 10th	Night	22% 8% 25% 37% 8% 1	850
	11th to 20th	Night	20% 13% 29% 32% 6%	1000
	21st to end	Night	12% 12% 32% 37% 8%	1000
December	1st to 10th	Night	17% 12% 28% 33% 9% 1	1000
	11th to 20th	Night	18% 11% 31% 32% 7% 1	1000
	21st to end	Night	17% 15% 29% 30% 8% 1	1100

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