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 2024-11-12.

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	5% 12% 4 10% 5% 5% 5% 4 5% 6% 39%	105406
	Night	4 10% 4 10% 6% 5% 5% 4 6% 7% 39%	102455
Transparency	Day	42% 9% 12% 18% 15% 5%	105147
	Night	40% 10% 13% 19% 14% 4	102304
Seeing	Day	17% 23% 32% 19% 8% 1	15454
	Night	13% 18% 31% 26% 11% 1	24938

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	2 12% 9% 21% 9% 5% 5% 4 6% 6% 22%	2219
	11th to 20th	Day	1 12% 6% 17% 10% 8% 5% 4 4 5% 29%	2518
	21st to end	Day	1 17% 7% 15% 5% 5% 4 4 5% 6% 32%	3110
February	1st to 10th	Day	3 16% 10% 17% 8% 5% 5% 5% 6% 5% 19%	1710
	11th to 20th	Day	2 15% 8% 20% 7% 6% 5% 4 4 6% 22%	1665
	21st to end	Day	2 18% 8% 19% 11% 5% 5% 4 4 5% 19%	1513
March	1st to 10th	Day	1 12% 7% 16% 6% 4 4 3 5% 6% 36%	3322
	11th to 20th	Day	2 15% 6% 12% 5% 4 4 4 5% 4 40%	3490
	21st to end	Day	4 23% 11% 20% 8% 5% 5% 3 4 5% 13%	3825
April	1st to 10th	Day	6% 24% 6% 12% 6% 5% 5% 3 5% 5% 23%	2194
	11th to 20th	Day	6% 19% 6% 15% 6% 5% 4 3 5% 6% 25%	2437
	21st to end	Day	9% 23% 6% 11% 5% 4 4 3 6% 6% 24%	2470
May	1st to 10th	Day	10% 25% 6% 10% 5% 4 5% 3 5% 5% 23%	4110
	11th to 20th	Day	7% 14% 3 8% 6% 6% 6% 5% 6% 8% 30%	4278
	21st to end	Day	12% 19% 5% 8% 5% 5% 4 4 5% 6% 27%	4853
June	1st to 10th	Day	13% 11% 3 8% 7% 5% 6% 4 7% 7% 28%	2850
	11th to 20th	Day	14% 11% 3 7% 6% 6% 5% 3 5% 6% 33%	2850
	21st to end	Day	5% 5% 2 6% 5% 5% 5% 4 6% 7% 51%	2850
July	1st to 10th	Day	3 3 1 3 3 4 6% 5% 7% 7% 58%	4740
	11th to 20th	Day	3 3 1 4 3 4 5% 4 6% 7% 61%	4688
	21st to end	Day	3 3 1 5% 5% 5% 6% 6% 7% 7% 54%	4525
August	1st to 10th	Day	5% 3 2 5% 3 5% 5% 6% 9% 10% 47%	4092
	11th to 20th	Day	4 3 1 5% 3 5% 6% 4 7% 8% 55%	4028
	21st to end	Day	5% 4 1 5% 4 4 5% 4 6% 6% 56%	4521
September	1st to 10th	Day	8% 6% 2 4 3 4 4 3 5% 7% 56%	2267
	11th to 20th	Day	10% 8% 3 6% 4 4 5% 4 6% 5% 47%	2090
	21st to end	Day	9% 8% 2 5% 4 3 4 3 5% 7% 51%	2090
October	1st to 10th	Day	6% 9% 2 5% 3 3 4 2 3 4 58%	3244
	11th to 20th	Day	6% 11% 3 6% 3 3 2 2 3 6% 54%	3150
	21st to end	Day	4 13% 4 7% 5% 4 5% 4 7% 7% 39%	3266
November	1st to 10th	Day	6% 14% 5% 11% 6% 5% 4 4 4 6% 34%	1708
	11th to 20th	Day	4 15% 6% 14% 5% 4 6% 5% 4 7% 30%	1528
	21st to end	Day	3 12% 6% 13% 7% 5% 5% 4 4 6% 37%	1520
December	1st to 10th	Day	3 11% 5% 14% 8% 6% 6% 4 6% 8% 30%	1520
	11th to 20th	Day	1 13% 7% 16% 9% 6% 5% 4 7% 8% 23%	1723
	21st to end	Day	2 11% 5% 16% 7% 5% 5% 4 5% 6% 36%	2442
January	1st to 10th	Night	2 9% 6% 22% 8% 6% 5% 4 5% 6% 26%	4726
	11th to 20th	Night	2 12% 6% 16% 12% 6% 6% 4 5% 7% 25%	4470
	21st to end	Night	1 18% 6% 15% 7% 6% 5% 3 5% 6% 29%	4553
February	1st to 10th	Night	2 15% 9% 18% 7% 6% 6% 4 5% 6% 23%	2470
	11th to 20th	Night	2 13% 8% 16% 8% 7% 5% 4 5% 6% 25%	2402
	21st to end	Night	2 14% 5% 18% 8% 7% 6% 3 5% 6% 26%	1941
March	1st to 10th	Night	1 12% 5% 15% 7% 5% 4 3 5% 6% 37%	3551
	11th to 20th	Night	2 8% 5% 11% 5% 4 4 4 6% 7% 46%	3490
	21st to end	Night	2 14% 9% 13% 7% 7% 6% 5% 8% 7% 21%	3828
April	1st to 10th	Night	4 17% 7% 11% 5% 5% 5% 4 6% 7% 29%	1983
	11th to 20th	Night	6% 13% 4 11% 6% 5% 5% 5% 7% 7% 31%	1691
	21st to end	Night	9% 17% 5% 8% 5% 4 4 4 5% 6% 32%	1710
May	1st to 10th	Night	7% 20% 6% 11% 5% 5% 5% 3 5% 5% 28%	2860
	11th to 20th	Night	8% 12% 4 8% 4 4 4 4 6% 5% 41%	2623
	21st to end	Night	9% 16% 4 7% 5% 5% 5% 4 7% 9% 28%	2716
June	1st to 10th	Night	11% 11% 3 8% 5% 5% 4 5% 8% 8% 33%	1330
	11th to 20th	Night	9% 11% 2 7% 6% 5% 5% 5% 5% 7% 38%	1330
	21st to end	Night	6% 4 2 3 4 4 4 4 5% 6% 58%	1330
July	1st to 10th	Night	3 2 1 5% 3 5% 5% 4 6% 6% 61%	2220
	11th to 20th	Night	2 3 1 2 4 3 5% 4 6% 7% 64%	2270
	21st to end	Night	3 2 1 3 2 3 4 4 7% 10% 61%	3032
August	1st to 10th	Night	3 3 1 4 4 5% 6% 5% 8% 9% 53%	2847
	11th to 20th	Night	3 2 1 3 3 4 5% 5% 7% 8% 58%	2811
	21st to end	Night	5% 4 1 3 3 3 4 3 4 7% 63%	3146
September	1st to 10th	Night	6% 5% 2 5% 4 4 4 2 5% 6% 56%	1909
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 4 6% 6% 50%	2090
	21st to end	Night	9% 9% 2 5% 3 2 3 3 4 6% 56%	2090
October	1st to 10th	Night	6% 7% 2 3 3 3 3 2 5% 5% 61%	3727
	11th to 20th	Night	7% 10% 3 5% 3 3 3 2 4 6% 54%	3778
	21st to end	Night	4 11% 5% 8% 4 5% 6% 4 6% 8% 40%	4289
November	1st to 10th	Night	6% 14% 4 9% 6% 5% 5% 3 6% 7% 35%	2570
	11th to 20th	Night	4 12% 6% 12% 7% 5% 6% 5% 7% 6% 30%	2675
	21st to end	Night	2 10% 5% 10% 5% 5% 5% 3 7% 7% 40%	2660
December	1st to 10th	Night	2 13% 6% 15% 6% 6% 6% 5% 5% 6% 30%	2660
	11th to 20th	Night	2 11% 5% 14% 6% 6% 5% 5% 7% 9% 29%	3463
	21st to end	Night	2 11% 6% 14% 9% 6% 5% 5% 5% 5% 33%	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	56% 11% 13% 15% 5%	2219
	11th to 20th	Day	52% 12% 14% 7% 10% 5%	2518
	21st to end	Day	49% 12% 14% 16% 8% 1	3110
February	1st to 10th	Day	64% 9% 11% 11% 4	1710
	11th to 20th	Day	62% 9% 11% 12% 4 1	1674
	21st to end	Day	65% 8% 7% 10% 7% 2	1513
March	1st to 10th	Day	47% 7% 12% 15% 17% 1	3322
	11th to 20th	Day	46% 9% 18% 22% 6% 1	3490
	21st to end	Day	66% 13% 11% 8% 1	3825
April	1st to 10th	Day	61% 9% 11% 15% 5%	2194
	11th to 20th	Day	57% 8% 13% 17% 5%	2440
	21st to end	Day	59% 8% 13% 16% 5%	2470
May	1st to 10th	Day	56% 10% 10% 18% 6% 1	4110
	11th to 20th	Day	42% 14% 15% 20% 9% 1	4282
	21st to end	Day	51% 12% 14% 18% 5%	4853
June	1st to 10th	Day	50% 10% 17% 17% 5%	2850
	11th to 20th	Day	50% 7% 10% 17% 12% 4	2850
	21st to end	Day	30% 8% 12% 25% 21% 4	2850
July	1st to 10th	Day	19% 7% 12% 16% 33% 13%	4666
	11th to 20th	Day	19% 5% 9% 15% 25% 27%	4607
	21st to end	Day	24% 7% 8% 16% 29% 16%	4382
August	1st to 10th	Day	26% 8% 11% 21% 24% 10%	4078
	11th to 20th	Day	22% 6% 11% 26% 27% 8%	4067
	21st to end	Day	26% 5% 9% 22% 28% 11%	4520
September	1st to 10th	Day	27% 5% 9% 21% 28% 10%	2267
	11th to 20th	Day	35% 7% 10% 23% 19% 6%	2090
	21st to end	Day	32% 6% 12% 28% 19% 4	2090
October	1st to 10th	Day	31% 6% 7% 23% 30% 2	3244
	11th to 20th	Day	32% 5% 16% 27% 17% 3	3150
	21st to end	Day	41% 11% 11% 24% 12% 1	3266
November	1st to 10th	Day	49% 8% 11% 21% 8% 2	1708
	11th to 20th	Day	53% 10% 14% 15% 6% 2	1528
	21st to end	Day	49% 7% 13% 17% 11% 2	1520
December	1st to 10th	Day	51% 10% 14% 13% 10% 3	1520
	11th to 20th	Day	51% 16% 15% 14% 4 1	1722
	21st to end	Day	46% 9% 9% 18% 6% 11%	2442
January	1st to 10th	Night	50% 14% 12% 15% 9% 1	4726
	11th to 20th	Night	54% 12% 12% 10% 9% 4	4470
	21st to end	Night	50% 13% 14% 14% 7% 1	4553
February	1st to 10th	Night	60% 11% 10% 13% 5% 1	2470
	11th to 20th	Night	59% 9% 13% 12% 7% 1	2403
	21st to end	Night	55% 10% 11% 13% 9% 2	1941
March	1st to 10th	Night	41% 11% 14% 17% 16% 1	3551
	11th to 20th	Night	34% 12% 21% 26% 6%	3490
	21st to end	Night	49% 19% 16% 13% 3	3828
April	1st to 10th	Night	52% 11% 14% 17% 6%	1983
	11th to 20th	Night	45% 11% 18% 18% 7%	1691
	21st to end	Night	51% 9% 16% 19% 5%	1710
May	1st to 10th	Night	52% 11% 15% 15% 7%	2860
	11th to 20th	Night	37% 13% 14% 25% 8% 2	2624
	21st to end	Night	44% 13% 18% 21% 4	2716
June	1st to 10th	Night	43% 14% 16% 22% 5% 1	1330
	11th to 20th	Night	42% 10% 14% 20% 11% 3	1330
	21st to end	Night	24% 8% 13% 27% 22% 6%	1330
July	1st to 10th	Night	21% 7% 11% 22% 34% 6%	2185
	11th to 20th	Night	18% 4 11% 16% 29% 23%	2228
	21st to end	Night	16% 6% 9% 27% 29% 14%	2937
August	1st to 10th	Night	23% 7% 12% 25% 25% 8%	2838
	11th to 20th	Night	20% 5% 11% 30% 28% 6%	2838
	21st to end	Night	20% 5% 10% 24% 33% 8%	3146
September	1st to 10th	Night	27% 6% 9% 22% 26% 10%	1909
	11th to 20th	Night	30% 8% 13% 23% 19% 6%	2090
	21st to end	Night	30% 6% 12% 28% 21% 3	2090
October	1st to 10th	Night	25% 6% 9% 28% 29% 3	3727
	11th to 20th	Night	32% 6% 16% 24% 19% 2	3778
	21st to end	Night	40% 8% 12% 24% 15% 1	4289
November	1st to 10th	Night	47% 10% 13% 20% 9% 2	2570
	11th to 20th	Night	51% 12% 15% 15% 7% 1	2675
	21st to end	Night	43% 8% 17% 17% 12% 3	2660
December	1st to 10th	Night	51% 11% 13% 14% 7% 3	2660
	11th to 20th	Night	48% 13% 19% 15% 6%	3464
	21st to end	Night	45% 11% 10% 18% 6% 9%	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	21% 22% 37% 17% 3 1	180
	11th to 20th	Day	15% 38% 28% 11% 7% 2	180
	21st to end	Day	16% 27% 39% 16% 3	286
February	1st to 10th	Day	23% 41% 28% 6% 2	270
	11th to 20th	Day	24% 30% 31% 11% 3 1	270
	21st to end	Day	23% 31% 33% 10% 3	325
March	1st to 10th	Day	23% 36% 32% 8% 2	460
	11th to 20th	Day	17% 31% 30% 17% 4	460
	21st to end	Day	22% 31% 29% 15% 3	506
April	1st to 10th	Day	28% 31% 28% 11% 1	460
	11th to 20th	Day	24% 33% 30% 11% 3	452
	21st to end	Day	26% 24% 27% 17% 3 2	460
May	1st to 10th	Day	21% 24% 29% 20% 6% 1	463
	11th to 20th	Day	23% 18% 29% 20% 9% 2	604
	21st to end	Day	24% 15% 26% 23% 10% 2	734
June	1st to 10th	Day	18% 17% 27% 22% 14% 3	680
	11th to 20th	Day	25% 23% 27% 18% 7%	680
	21st to end	Day	9% 15% 34% 28% 12% 2	680
July	1st to 10th	Day	6% 15% 31% 32% 14% 2	671
	11th to 20th	Day	5% 14% 40% 26% 13% 1	642
	21st to end	Day	7% 14% 38% 28% 12% 1	531
August	1st to 10th	Day	9% 14% 38% 28% 10% 1	487
	11th to 20th	Day	9% 14% 32% 30% 13% 3	477
	21st to end	Day	8% 18% 39% 22% 10% 3	539
September	1st to 10th	Day	9% 20% 37% 22% 9% 2	490
	11th to 20th	Day	18% 20% 33% 20% 7% 1	490
	21st to end	Day	20% 20% 32% 19% 9% 1	490
October	1st to 10th	Day	17% 24% 32% 17% 9% 1	348
	11th to 20th	Day	19% 24% 30% 13% 10% 3	300
	21st to end	Day	22% 31% 32% 11% 4	330
November	1st to 10th	Day	22% 30% 30% 11% 5% 2	300
	11th to 20th	Day	20% 32% 33% 10% 4	273
	21st to end	Day	14% 28% 35% 21% 1	270
December	1st to 10th	Day	23% 29% 26% 18% 5%	270
	11th to 20th	Day	22% 40% 25% 10% 4	198
	21st to end	Day	23% 30% 30% 13% 4	198
January	1st to 10th	Night	15% 26% 35% 20% 4	950
	11th to 20th	Night	13% 27% 37% 14% 8% 1	784
	21st to end	Night	10% 24% 42% 17% 7%	836
February	1st to 10th	Night	14% 31% 34% 16% 5%	760
	11th to 20th	Night	13% 27% 35% 19% 6% 1	744
	21st to end	Night	15% 25% 35% 19% 6%	628
March	1st to 10th	Night	12% 25% 38% 21% 5%	760
	11th to 20th	Night	11% 24% 31% 26% 8% 1	760
	21st to end	Night	12% 19% 35% 25% 8% 1	836
April	1st to 10th	Night	16% 18% 37% 21% 7% 1	670
	11th to 20th	Night	16% 18% 37% 21% 7% 1	563
	21st to end	Night	17% 19% 25% 26% 12% 1	570
May	1st to 10th	Night	17% 16% 24% 29% 12% 1	570
	11th to 20th	Night	15% 10% 25% 28% 20% 1	561
	21st to end	Night	18% 11% 25% 31% 15%	597
June	1st to 10th	Night	16% 9% 21% 35% 18% 1	380
	11th to 20th	Night	13% 14% 30% 34% 8%	380
	21st to end	Night	8% 3 24% 45% 19% 1	380
July	1st to 10th	Night	5% 2 18% 44% 27% 3	370
	11th to 20th	Night	3 4 24% 51% 18%	368
	21st to end	Night	5% 5% 23% 42% 24% 2	573
August	1st to 10th	Night	7% 5% 23% 40% 24% 1	567
	11th to 20th	Night	7% 4 19% 45% 24% 2	564
	21st to end	Night	7% 10% 22% 39% 20% 2	627
September	1st to 10th	Night	8% 6% 27% 34% 22% 3	598
	11th to 20th	Night	11% 7% 23% 34% 22% 4	760
	21st to end	Night	15% 11% 26% 28% 17% 3	760
October	1st to 10th	Night	16% 11% 29% 30% 13% 2	760
	11th to 20th	Night	17% 14% 29% 24% 13% 2	760
	21st to end	Night	16% 19% 33% 23% 6% 3	836
November	1st to 10th	Night	18% 20% 35% 21% 5%	815
	11th to 20th	Night	17% 29% 35% 16% 2	956
	21st to end	Night	11% 25% 35% 22% 5% 1	950
December	1st to 10th	Night	14% 26% 35% 21% 4 1	950
	11th to 20th	Night	16% 26% 38% 15% 5% 1	950
	21st to end	Night	16% 27% 36% 17% 4 1	1045

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