Big Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Sky Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	5% 10% 4 9% 5% 5% 5% 4 6% 7% 41%	136026
	Night	3 9% 4 10% 5% 5% 5% 4 6% 8% 41%	132375
Transparency	Day	40% 9% 12% 19% 15% 5%	130724
	Night	38% 10% 14% 21% 13% 4	128681
Seeing	Day	18% 23% 32% 19% 7% 1	16899
	Night	13% 19% 31% 25% 10% 1	26433

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	1 9% 8% 21% 9% 7% 6% 5% 7% 6% 20%	3179
	11th to 20th	Day	1 9% 4 19% 10% 9% 5% 3 4 5% 32%	3618
	21st to end	Day	1 12% 5% 13% 5% 5% 4 4 6% 7% 38%	4460
February	1st to 10th	Day	3 16% 10% 16% 9% 6% 5% 5% 6% 6% 19%	1800
	11th to 20th	Day	2 15% 8% 21% 7% 6% 5% 4 4 6% 22%	1755
	21st to end	Day	2 18% 7% 19% 10% 5% 5% 4 4 6% 21%	1590
March	1st to 10th	Day	2 12% 6% 14% 5% 3 3 3 5% 7% 41%	4780
	11th to 20th	Day	1 16% 6% 13% 5% 5% 5% 4 5% 5% 35%	5000
	21st to end	Day	3 19% 9% 17% 7% 6% 6% 4 5% 6% 17%	5486
April	1st to 10th	Day	5% 23% 6% 12% 6% 5% 5% 3 5% 5% 24%	2310
	11th to 20th	Day	6% 19% 6% 15% 6% 5% 4 4 5% 5% 26%	2567
	21st to end	Day	9% 23% 6% 11% 5% 4 5% 3 6% 6% 23%	2600
May	1st to 10th	Day	9% 18% 5% 8% 4 4 4 3 5% 7% 34%	5880
	11th to 20th	Day	9% 16% 4 9% 6% 5% 5% 5% 6% 8% 28%	6146
	21st to end	Day	9% 14% 4 6% 4 5% 5% 4 5% 7% 38%	6966
June	1st to 10th	Day	13% 11% 3 8% 6% 5% 6% 4 7% 7% 30%	3000
	11th to 20th	Day	14% 11% 3 7% 6% 6% 5% 3 5% 6% 34%	3000
	21st to end	Day	5% 5% 2 6% 5% 5% 6% 4 6% 7% 51%	3000
July	1st to 10th	Day	2 2 1 4 4 5% 6% 5% 7% 7% 57%	6779
	11th to 20th	Day	2 3 1 3 3 4 4 4 5% 6% 65%	6700
	21st to end	Day	2 2 1 4 4 5% 6% 6% 7% 8% 54%	6498
August	1st to 10th	Day	3 3 1 5% 3 5% 5% 6% 9% 11% 50%	5779
	11th to 20th	Day	3 2 1 4 3 5% 6% 5% 7% 8% 57%	5798
	21st to end	Day	8% 5% 1 5% 3 4 4 4 5% 6% 55%	6468
September	1st to 10th	Day	7% 6% 2 4 3 4 4 3 5% 7% 56%	2386
	11th to 20th	Day	9% 8% 2 6% 4 4 5% 4 6% 5% 47%	2200
	21st to end	Day	8% 8% 2 5% 4 3 3 3 5% 7% 51%	2200
October	1st to 10th	Day	7% 8% 2 5% 3 3 4 3 3 4 58%	3346
	11th to 20th	Day	6% 12% 3 6% 3 3 2 2 3 6% 53%	3250
	21st to end	Day	4 13% 4 7% 4 4 5% 4 7% 7% 40%	3370
November	1st to 10th	Day	6% 14% 5% 10% 6% 5% 5% 4 4 6% 35%	1795
	11th to 20th	Day	4 14% 6% 13% 5% 4 6% 5% 4 7% 30%	1680
	21st to end	Day	3 11% 5% 12% 6% 5% 5% 4 4 6% 38%	1680
December	1st to 10th	Day	2 7% 3 14% 5% 4 5% 4 5% 7% 44%	2700
	11th to 20th	Day	1 9% 5% 13% 7% 7% 6% 5% 10% 10% 26%	2762
	21st to end	Day	2 10% 4 15% 8% 6% 6% 5% 7% 7% 30%	3498
January	1st to 10th	Night	1 7% 5% 21% 8% 6% 6% 4 6% 7% 28%	6766
	11th to 20th	Night	1 8% 5% 15% 11% 6% 6% 4 5% 7% 29%	6384
	21st to end	Night	1 12% 5% 13% 7% 5% 5% 4 5% 6% 36%	6513
February	1st to 10th	Night	2 15% 8% 17% 7% 6% 6% 4 6% 6% 22%	2600
	11th to 20th	Night	2 13% 7% 16% 8% 7% 5% 4 6% 6% 25%	2532
	21st to end	Night	2 13% 5% 17% 8% 7% 6% 3 5% 7% 27%	2039
March	1st to 10th	Night	1 10% 5% 13% 6% 5% 5% 4 5% 6% 42%	5113
	11th to 20th	Night	2 11% 4 10% 5% 4 4 4 7% 9% 40%	5000
	21st to end	Night	2 12% 7% 11% 6% 6% 6% 6% 9% 10% 27%	5489
April	1st to 10th	Night	4 16% 6% 11% 5% 5% 5% 4 6% 7% 30%	2087
	11th to 20th	Night	6% 13% 4 11% 5% 5% 5% 4 7% 7% 32%	1781
	21st to end	Night	9% 17% 5% 8% 5% 4 5% 4 6% 6% 31%	1800
May	1st to 10th	Night	7% 16% 5% 9% 4 4 4 3 4 5% 39%	4100
	11th to 20th	Night	10% 13% 4 8% 4 4 4 4 6% 7% 35%	3765
	21st to end	Night	7% 12% 3 6% 5% 5% 5% 4 7% 9% 38%	3912
June	1st to 10th	Night	10% 10% 2 7% 5% 5% 4 5% 8% 8% 36%	1400
	11th to 20th	Night	9% 11% 3 7% 6% 5% 5% 5% 5% 7% 38%	1400
	21st to end	Night	6% 4 2 3 4 4 4 4 5% 6% 58%	1400
July	1st to 10th	Night	2 2 1 4 3 4 4 4 7% 9% 60%	3180
	11th to 20th	Night	2 3 1 3 3 3 4 3 5% 6% 65%	3244
	21st to end	Night	2 2 1 4 3 4 4 4 7% 10% 59%	4367
August	1st to 10th	Night	3 3 1 3 3 4 5% 5% 8% 10% 54%	4033
	11th to 20th	Night	2 2 1 3 3 4 4 4 6% 7% 64%	4051
	21st to end	Night	7% 6% 1 4 3 3 4 3 4 7% 58%	4509
September	1st to 10th	Night	6% 5% 2 5% 4 4 4 2 5% 6% 57%	2010
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 4 6% 6% 50%	2200
	21st to end	Night	8% 9% 1 4 3 2 3 2 4 6% 57%	2200
October	1st to 10th	Night	6% 7% 2 3 3 3 3 2 5% 5% 61%	3845
	11th to 20th	Night	7% 10% 3 5% 3 3 3 2 4 6% 53%	3898
	21st to end	Night	4 11% 5% 8% 4 5% 5% 4 6% 8% 41%	4426
November	1st to 10th	Night	5% 13% 3 9% 5% 5% 6% 4 6% 8% 36%	2702
	11th to 20th	Night	4 11% 5% 12% 6% 5% 6% 5% 7% 7% 31%	2940
	21st to end	Night	2 9% 5% 10% 5% 5% 5% 4 8% 8% 39%	2940
December	1st to 10th	Night	1 9% 4 12% 5% 5% 5% 4 5% 6% 45%	4710
	11th to 20th	Night	1 8% 3 11% 5% 6% 7% 6% 9% 10% 33%	5581
	21st to end	Night	1 10% 5% 12% 8% 6% 6% 5% 7% 9% 30%	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	58% 11% 12% 13% 5%	3179
	11th to 20th	Day	51% 9% 15% 10% 10% 5%	3618
	21st to end	Day	41% 11% 13% 16% 15% 3	4460
February	1st to 10th	Day	64% 10% 11% 11% 4	1800
	11th to 20th	Day	62% 9% 11% 11% 4 2	1764
	21st to end	Day	62% 8% 7% 11% 9% 2	1590
March	1st to 10th	Day	42% 6% 12% 17% 16% 6%	4780
	11th to 20th	Day	48% 8% 17% 20% 7%	5000
	21st to end	Day	62% 13% 13% 11% 1	5486
April	1st to 10th	Day	59% 8% 11% 15% 6%	2310
	11th to 20th	Day	56% 8% 13% 17% 5%	2570
	21st to end	Day	59% 8% 13% 16% 5%	2600
May	1st to 10th	Day	44% 9% 10% 26% 11% 1	5880
	11th to 20th	Day	47% 12% 13% 18% 9% 1	6150
	21st to end	Day	40% 11% 15% 24% 10%	6967
June	1st to 10th	Day	49% 10% 17% 18% 6%	3000
	11th to 20th	Day	49% 7% 11% 18% 12% 4	3000
	21st to end	Day	30% 8% 12% 25% 22% 4	3000
July	1st to 10th	Day	20% 7% 11% 20% 27% 15%	6706
	11th to 20th	Day	18% 5% 8% 18% 30% 22%	6619
	21st to end	Day	21% 8% 9% 19% 30% 12%	6311
August	1st to 10th	Day	26% 8% 11% 21% 24% 10%	4208
	11th to 20th	Day	22% 6% 11% 26% 27% 8%	4197
	21st to end	Day	26% 5% 8% 22% 28% 11%	4663
September	1st to 10th	Day	26% 5% 8% 23% 27% 10%	2386
	11th to 20th	Day	36% 7% 10% 23% 19% 5%	2200
	21st to end	Day	31% 6% 12% 27% 19% 5%	2200
October	1st to 10th	Day	31% 6% 7% 24% 30% 3	3346
	11th to 20th	Day	33% 5% 15% 27% 16% 3	3250
	21st to end	Day	40% 11% 11% 25% 13% 1	3370
November	1st to 10th	Day	48% 8% 13% 21% 8% 2	1795
	11th to 20th	Day	52% 11% 15% 15% 6% 2	1680
	21st to end	Day	47% 9% 14% 16% 11% 2	1680
December	1st to 10th	Day	39% 9% 18% 22% 10% 2	2700
	11th to 20th	Day	46% 20% 16% 15% 3 1	2761
	21st to end	Day	49% 11% 11% 16% 5% 8%	3498
January	1st to 10th	Night	49% 13% 12% 17% 8%	6766
	11th to 20th	Night	49% 10% 15% 15% 7% 5%	6384
	21st to end	Night	43% 11% 13% 15% 12% 6%	6513
February	1st to 10th	Night	60% 11% 11% 12% 5% 1	2600
	11th to 20th	Night	58% 9% 13% 12% 7% 2	2533
	21st to end	Night	53% 10% 12% 13% 10% 2	2039
March	1st to 10th	Night	38% 9% 15% 18% 15% 4	5113
	11th to 20th	Night	36% 11% 20% 26% 7%	5000
	21st to end	Night	44% 19% 14% 18% 4	5489
April	1st to 10th	Night	50% 11% 14% 18% 6%	2087
	11th to 20th	Night	45% 11% 18% 19% 7%	1781
	21st to end	Night	50% 9% 16% 20% 5%	1800
May	1st to 10th	Night	43% 9% 13% 25% 10%	4097
	11th to 20th	Night	43% 12% 14% 23% 6% 1	3766
	21st to end	Night	37% 12% 18% 25% 8%	3912
June	1st to 10th	Night	41% 13% 16% 23% 6% 1	1400
	11th to 20th	Night	41% 10% 14% 19% 12% 3	1400
	21st to end	Night	24% 8% 13% 27% 23% 6%	1400
July	1st to 10th	Night	18% 6% 10% 28% 31% 7%	3145
	11th to 20th	Night	18% 4 10% 22% 29% 18%	3202
	21st to end	Night	17% 6% 10% 28% 29% 10%	4242
August	1st to 10th	Night	23% 7% 12% 26% 25% 8%	2928
	11th to 20th	Night	20% 5% 11% 29% 28% 7%	2928
	21st to end	Night	21% 5% 10% 24% 32% 8%	3245
September	1st to 10th	Night	27% 6% 9% 23% 25% 10%	2010
	11th to 20th	Night	31% 8% 12% 24% 19% 6%	2200
	21st to end	Night	29% 6% 12% 27% 23% 3	2200
October	1st to 10th	Night	25% 6% 9% 28% 29% 3	3845
	11th to 20th	Night	33% 6% 16% 24% 18% 2	3898
	21st to end	Night	39% 8% 12% 25% 15% 1	4426
November	1st to 10th	Night	45% 10% 13% 21% 8% 2	2702
	11th to 20th	Night	48% 12% 16% 16% 6% 1	2940
	21st to end	Night	42% 9% 17% 17% 12% 3	2940
December	1st to 10th	Night	38% 9% 22% 21% 9% 2	4710
	11th to 20th	Night	42% 17% 22% 15% 4	5582
	21st to end	Night	44% 14% 15% 16% 5% 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	22% 24% 36% 16% 3 1	200
	11th to 20th	Day	15% 39% 29% 10% 7% 2	200
	21st to end	Day	16% 26% 37% 17% 4	318
February	1st to 10th	Day	27% 40% 26% 6% 2	300
	11th to 20th	Day	23% 30% 31% 12% 3 1	300
	21st to end	Day	24% 31% 32% 10% 3	354
March	1st to 10th	Day	22% 36% 32% 8% 2	500
	11th to 20th	Day	21% 30% 29% 16% 4	500
	21st to end	Day	24% 31% 28% 14% 3	550
April	1st to 10th	Day	27% 29% 28% 13% 1	500
	11th to 20th	Day	24% 32% 30% 11% 3	492
	21st to end	Day	26% 25% 28% 17% 3 2	500
May	1st to 10th	Day	20% 23% 29% 21% 6% 1	503
	11th to 20th	Day	23% 18% 30% 19% 8% 2	651
	21st to end	Day	23% 15% 26% 24% 11% 2	789
June	1st to 10th	Day	17% 17% 28% 22% 14% 2	730
	11th to 20th	Day	24% 24% 27% 18% 6%	730
	21st to end	Day	10% 16% 33% 28% 12% 2	730
July	1st to 10th	Day	6% 15% 32% 32% 14% 2	721
	11th to 20th	Day	5% 15% 41% 26% 13% 1	690
	21st to end	Day	7% 14% 39% 28% 11% 1	575
August	1st to 10th	Day	9% 15% 39% 27% 10% 1	527
	11th to 20th	Day	9% 14% 33% 29% 13% 2	517
	21st to end	Day	9% 17% 39% 22% 11% 3	583
September	1st to 10th	Day	9% 20% 36% 23% 9% 2	530
	11th to 20th	Day	17% 19% 33% 21% 8% 2	530
	21st to end	Day	19% 20% 31% 19% 9% 2	530
October	1st to 10th	Day	16% 26% 31% 16% 9% 1	380
	11th to 20th	Day	24% 24% 29% 12% 9% 3	330
	21st to end	Day	22% 30% 31% 12% 4	363
November	1st to 10th	Day	24% 30% 30% 10% 5% 2	330
	11th to 20th	Day	21% 33% 32% 10% 4	330
	21st to end	Day	16% 29% 35% 18% 2	330
December	1st to 10th	Day	24% 28% 27% 16% 5%	330
	11th to 20th	Day	26% 38% 24% 9% 3	236
	21st to end	Day	25% 30% 30% 12% 4	220
January	1st to 10th	Night	14% 26% 37% 19% 4	1000
	11th to 20th	Night	13% 26% 38% 14% 7% 1	825
	21st to end	Night	10% 25% 41% 17% 7%	880
February	1st to 10th	Night	15% 31% 34% 15% 5%	800
	11th to 20th	Night	13% 27% 36% 18% 6% 1	784
	21st to end	Night	16% 25% 35% 19% 6%	660
March	1st to 10th	Night	12% 25% 38% 20% 5%	800
	11th to 20th	Night	12% 24% 31% 25% 8% 1	800
	21st to end	Night	13% 20% 34% 24% 8% 1	880
April	1st to 10th	Night	16% 18% 36% 22% 7% 1	705
	11th to 20th	Night	16% 18% 38% 21% 6% 1	593
	21st to end	Night	17% 19% 25% 26% 12% 1	600
May	1st to 10th	Night	17% 16% 23% 31% 12% 1	600
	11th to 20th	Night	16% 11% 26% 27% 19% 1	591
	21st to end	Night	17% 10% 25% 31% 15%	629
June	1st to 10th	Night	16% 9% 22% 35% 18% 1	400
	11th to 20th	Night	13% 14% 32% 34% 8%	400
	21st to end	Night	8% 4 24% 45% 19% 1	400
July	1st to 10th	Night	5% 2 17% 46% 26% 3	390
	11th to 20th	Night	3 3 24% 51% 18%	388
	21st to end	Night	4 5% 24% 43% 23% 2	604
August	1st to 10th	Night	7% 5% 24% 40% 23% 1	597
	11th to 20th	Night	6% 4 19% 46% 23% 2	594
	21st to end	Night	7% 10% 22% 39% 20% 2	660
September	1st to 10th	Night	8% 6% 27% 34% 22% 3	630
	11th to 20th	Night	10% 7% 22% 35% 23% 4	800
	21st to end	Night	15% 11% 26% 28% 17% 3	800
October	1st to 10th	Night	15% 11% 30% 30% 13% 2	800
	11th to 20th	Night	17% 16% 29% 24% 13% 2	800
	21st to end	Night	15% 20% 34% 23% 6% 2	880
November	1st to 10th	Night	18% 20% 36% 21% 5%	857
	11th to 20th	Night	18% 29% 35% 16% 2	1050
	21st to end	Night	12% 27% 35% 21% 5%	1050
December	1st to 10th	Night	15% 26% 35% 20% 4	1050
	11th to 20th	Night	17% 27% 37% 14% 4 1	1036
	21st to end	Night	17% 27% 35% 16% 4 1	1100

Page generated at 2025-12-14 23:13:56. Version ac07

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