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 2024-01-14.

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	7% 11% 5% 9% 7% 4 5% 5% 5% 6% 35%	74693
	Night	5% 10% 5% 9% 7% 4 5% 5% 6% 7% 37%	75062
Transparency	Day	47% 8% 12% 17% 12% 4	74394
	Night	44% 10% 14% 18% 11% 3	74883
Seeing	Day	18% 23% 31% 19% 8% 1	14265
	Night	13% 18% 31% 25% 11% 1	23936

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	3 13% 9% 17% 10% 3 5% 4 6% 6% 24%	1329
	11th to 20th	Day	1 13% 7% 15% 9% 5% 6% 5% 4 5% 30%	1455
	21st to end	Day	2 14% 8% 18% 7% 4 6% 4 4 6% 26%	1760
February	1st to 10th	Day	3 13% 12% 14% 9% 5% 5% 6% 6% 6% 20%	1620
	11th to 20th	Day	2 14% 10% 18% 8% 5% 5% 5% 4 6% 22%	1575
	21st to end	Day	2 15% 10% 18% 12% 4 6% 5% 4 5% 20%	1426
March	1st to 10th	Day	2 15% 8% 15% 9% 3 5% 5% 6% 6% 25%	1906
	11th to 20th	Day	3 15% 7% 13% 8% 5% 6% 6% 5% 6% 26%	1980
	21st to end	Day	5% 20% 10% 14% 9% 4 6% 5% 4 6% 18%	2178
April	1st to 10th	Day	7% 22% 7% 11% 7% 4 5% 4 5% 5% 22%	2078
	11th to 20th	Day	6% 17% 8% 13% 8% 4 4 4 5% 6% 24%	2307
	21st to end	Day	10% 22% 6% 9% 6% 3 4 4 6% 6% 24%	2340
May	1st to 10th	Day	10% 18% 6% 8% 7% 4 6% 5% 5% 6% 26%	2340
	11th to 20th	Day	11% 12% 4 8% 8% 4 7% 6% 7% 8% 25%	2410
	21st to end	Day	17% 18% 5% 9% 7% 4 5% 5% 5% 7% 18%	2739
June	1st to 10th	Day	14% 10% 4 7% 7% 4 7% 5% 7% 7% 27%	2700
	11th to 20th	Day	15% 10% 4 7% 7% 4 6% 4 5% 6% 32%	2700
	21st to end	Day	6% 5% 2 5% 6% 4 6% 5% 6% 7% 49%	2700
July	1st to 10th	Day	4 3 1 4 5% 4 6% 8% 7% 8% 50%	2700
	11th to 20th	Day	4 3 1 4 5% 3 5% 5% 5% 7% 58%	2676
	21st to end	Day	5% 3 2 4 6% 3 4 7% 5% 6% 54%	2590
August	1st to 10th	Day	7% 4 2 5% 4 4 5% 6% 6% 8% 48%	2328
	11th to 20th	Day	5% 4 1 5% 4 3 5% 5% 5% 7% 56%	2299
	21st to end	Day	6% 4 2 6% 5% 3 5% 5% 6% 7% 51%	2574
September	1st to 10th	Day	8% 6% 2 4 3 3 4 4 4 7% 56%	2148
	11th to 20th	Day	10% 7% 3 5% 5% 3 4 4 6% 6% 48%	1980
	21st to end	Day	9% 8% 2 4 5% 3 3 4 5% 8% 48%	1980
October	1st to 10th	Day	11% 14% 4 7% 6% 3 5% 4 4 5% 37%	1846
	11th to 20th	Day	10% 14% 5% 7% 4 3 3 4 4 5% 40%	1800
	21st to end	Day	7% 14% 4 7% 7% 4 5% 5% 6% 7% 35%	1883
November	1st to 10th	Day	7% 13% 6% 10% 7% 4 5% 6% 4 6% 33%	1621
	11th to 20th	Day	5% 13% 8% 12% 6% 3 6% 6% 4 7% 31%	1520
	21st to end	Day	3 10% 7% 11% 8% 4 6% 5% 4 6% 37%	1520
December	1st to 10th	Day	3 10% 6% 13% 8% 5% 6% 5% 6% 8% 30%	1520
	11th to 20th	Day	2 12% 7% 14% 11% 5% 6% 6% 7% 8% 23%	1723
	21st to end	Day	3 10% 6% 13% 10% 3 5% 5% 5% 6% 36%	2442
January	1st to 10th	Night	3 10% 7% 15% 9% 5% 5% 6% 6% 7% 27%	2836
	11th to 20th	Night	2 12% 7% 13% 9% 4 5% 5% 4 7% 31%	2598
	21st to end	Night	2 11% 8% 13% 9% 5% 5% 5% 5% 7% 29%	2593
February	1st to 10th	Night	3 13% 9% 15% 9% 4 6% 5% 5% 6% 23%	2340
	11th to 20th	Night	2 12% 8% 14% 10% 5% 6% 5% 5% 6% 25%	2272
	21st to end	Night	2 11% 8% 16% 10% 6% 6% 5% 5% 6% 26%	1830
March	1st to 10th	Night	2 12% 7% 13% 9% 4 5% 5% 5% 7% 32%	2053
	11th to 20th	Night	3 9% 5% 9% 8% 4 5% 6% 8% 8% 35%	1980
	21st to end	Night	4 12% 9% 10% 9% 4 6% 6% 8% 6% 25%	2178
April	1st to 10th	Night	5% 16% 7% 10% 6% 4 6% 5% 6% 7% 28%	1879
	11th to 20th	Night	6% 12% 5% 10% 7% 4 5% 6% 6% 7% 31%	1601
	21st to end	Night	9% 15% 5% 8% 6% 3 5% 5% 5% 6% 32%	1620
May	1st to 10th	Night	8% 14% 7% 8% 5% 4 5% 5% 6% 6% 31%	1620
	11th to 20th	Night	10% 10% 4 7% 5% 4 6% 6% 8% 6% 35%	1481
	21st to end	Night	14% 13% 4 6% 7% 3 6% 6% 7% 9% 26%	1534
June	1st to 10th	Night	11% 10% 3 7% 6% 3 5% 5% 8% 10% 33%	1260
	11th to 20th	Night	10% 11% 3 6% 7% 4 5% 6% 5% 7% 36%	1260
	21st to end	Night	6% 4 2 3 4 3 5% 5% 5% 6% 57%	1260
July	1st to 10th	Night	5% 3 1 4 3 3 4 4 7% 9% 58%	1260
	11th to 20th	Night	2 4 2 3 4 2 4 5% 6% 7% 62%	1282
	21st to end	Night	4 3 2 4 3 2 3 5% 7% 7% 60%	1724
August	1st to 10th	Night	5% 5% 2 4 4 3 6% 5% 7% 8% 51%	1611
	11th to 20th	Night	4 3 2 3 4 2 5% 5% 6% 9% 58%	1602
	21st to end	Night	6% 4 2 3 4 3 5% 5% 6% 9% 53%	1782
September	1st to 10th	Night	7% 5% 2 4 5% 2 4 4 6% 6% 55%	1808
	11th to 20th	Night	7% 5% 2 4 5% 3 5% 5% 6% 6% 52%	1980
	21st to end	Night	9% 9% 2 4 4 2 3 3 4 6% 54%	1980
October	1st to 10th	Night	10% 12% 4 5% 4 3 5% 4 5% 6% 42%	2113
	11th to 20th	Night	9% 10% 5% 6% 4 2 4 4 5% 6% 45%	2160
	21st to end	Night	6% 11% 5% 8% 7% 4 5% 5% 5% 8% 38%	2470
November	1st to 10th	Night	6% 13% 4 8% 7% 4 6% 5% 5% 7% 35%	2438
	11th to 20th	Night	5% 11% 6% 11% 8% 4 6% 6% 7% 6% 30%	2660
	21st to end	Night	2 9% 6% 9% 6% 4 5% 4 7% 8% 40%	2660
December	1st to 10th	Night	2 11% 6% 13% 8% 5% 6% 7% 5% 6% 30%	2660
	11th to 20th	Night	2 10% 5% 13% 8% 5% 6% 6% 7% 9% 30%	3463
	21st to end	Night	2 10% 6% 12% 11% 4 5% 6% 5% 6% 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	60% 8% 13% 13% 5% 1	1329
	11th to 20th	Day	56% 9% 13% 11% 9% 3	1455
	21st to end	Day	59% 7% 10% 14% 8% 2	1760
February	1st to 10th	Day	63% 9% 11% 11% 4	1620
	11th to 20th	Day	63% 9% 11% 12% 4 1	1584
	21st to end	Day	66% 7% 7% 10% 8% 3	1426
March	1st to 10th	Day	58% 9% 13% 14% 6% 1	1906
	11th to 20th	Day	57% 9% 12% 13% 8% 1	1980
	21st to end	Day	65% 10% 12% 11% 2	2178
April	1st to 10th	Day	62% 8% 11% 15% 5%	2078
	11th to 20th	Day	59% 8% 13% 16% 5%	2310
	21st to end	Day	59% 7% 13% 16% 4	2340
May	1st to 10th	Day	54% 9% 12% 19% 5%	2340
	11th to 20th	Day	52% 11% 15% 16% 5%	2414
	21st to end	Day	64% 10% 12% 10% 4	2739
June	1st to 10th	Day	51% 10% 18% 16% 4	2700
	11th to 20th	Day	51% 8% 11% 17% 11% 3	2700
	21st to end	Day	31% 8% 13% 24% 20% 4	2700
July	1st to 10th	Day	26% 10% 14% 23% 21% 7%	2626
	11th to 20th	Day	24% 6% 9% 20% 25% 17%	2595
	21st to end	Day	26% 8% 8% 16% 27% 15%	2446
August	1st to 10th	Day	31% 8% 10% 21% 22% 8%	2313
	11th to 20th	Day	25% 6% 11% 24% 26% 7%	2299
	21st to end	Day	28% 7% 10% 24% 22% 9%	2574
September	1st to 10th	Day	28% 5% 9% 21% 27% 10%	2148
	11th to 20th	Day	35% 7% 10% 23% 20% 6%	1980
	21st to end	Day	33% 7% 12% 28% 17% 3	1980
October	1st to 10th	Day	49% 8% 11% 19% 13% 1	1846
	11th to 20th	Day	46% 7% 14% 21% 10% 2	1800
	21st to end	Day	47% 11% 14% 19% 8% 2	1883
November	1st to 10th	Day	50% 8% 12% 20% 8% 2	1621
	11th to 20th	Day	53% 10% 13% 15% 6% 2	1520
	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	54% 11% 14% 13% 7% 1	2836
	11th to 20th	Night	53% 10% 14% 13% 8% 3	2598
	21st to end	Night	54% 9% 13% 13% 9% 2	2593
February	1st to 10th	Night	60% 11% 11% 13% 5% 1	2340
	11th to 20th	Night	60% 8% 13% 12% 7% 1	2273
	21st to end	Night	57% 9% 11% 11% 9% 3	1830
March	1st to 10th	Night	51% 11% 14% 18% 6% 1	2053
	11th to 20th	Night	44% 13% 16% 17% 9% 1	1980
	21st to end	Night	54% 13% 17% 13% 4	2178
April	1st to 10th	Night	53% 11% 14% 16% 6%	1879
	11th to 20th	Night	46% 11% 18% 17% 7%	1601
	21st to end	Night	50% 9% 17% 19% 5%	1620
May	1st to 10th	Night	49% 9% 17% 18% 6%	1620
	11th to 20th	Night	43% 13% 17% 20% 7%	1482
	21st to end	Night	51% 13% 18% 14% 4	1534
June	1st to 10th	Night	44% 13% 16% 22% 4 1	1260
	11th to 20th	Night	43% 10% 14% 20% 10% 3	1260
	21st to end	Night	25% 9% 13% 27% 20% 6%	1260
July	1st to 10th	Night	21% 8% 14% 29% 20% 7%	1225
	11th to 20th	Night	18% 5% 14% 21% 24% 18%	1240
	21st to end	Night	18% 8% 11% 22% 29% 12%	1628
August	1st to 10th	Night	27% 9% 14% 23% 20% 7%	1602
	11th to 20th	Night	21% 7% 14% 27% 24% 8%	1602
	21st to end	Night	24% 8% 13% 23% 25% 7%	1782
September	1st to 10th	Night	28% 6% 9% 21% 25% 11%	1808
	11th to 20th	Night	29% 8% 13% 23% 20% 6%	1980
	21st to end	Night	32% 6% 12% 28% 20% 2	1980
October	1st to 10th	Night	40% 9% 12% 23% 15% 1	2113
	11th to 20th	Night	39% 8% 15% 26% 11% 2	2160
	21st to end	Night	44% 9% 14% 21% 10% 1	2470
November	1st to 10th	Night	48% 10% 13% 19% 8% 3	2438
	11th to 20th	Night	51% 12% 15% 15% 7% 1	2660
	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	17% 36% 26% 11% 8% 2	166
	21st to end	Day	17% 28% 37% 15% 2	254
February	1st to 10th	Day	24% 42% 29% 5%	240
	11th to 20th	Day	25% 28% 31% 12% 3	240
	21st to end	Day	22% 31% 34% 10% 3	292
March	1st to 10th	Day	23% 37% 31% 7% 2	420
	11th to 20th	Day	18% 32% 31% 16% 4	420
	21st to end	Day	23% 30% 29% 15% 3	462
April	1st to 10th	Day	30% 29% 28% 11% 1	420
	11th to 20th	Day	25% 34% 28% 10% 3	412
	21st to end	Day	27% 24% 26% 17% 4 2	420
May	1st to 10th	Day	19% 24% 29% 21% 6% 1	423
	11th to 20th	Day	22% 17% 29% 19% 9% 3	557
	21st to end	Day	24% 14% 25% 23% 11% 2	679
June	1st to 10th	Day	19% 15% 27% 22% 15% 3	630
	11th to 20th	Day	25% 23% 27% 18% 7%	630
	21st to end	Day	9% 13% 34% 30% 13% 2	630
July	1st to 10th	Day	5% 14% 31% 33% 15% 2	621
	11th to 20th	Day	6% 14% 40% 26% 13% 1	594
	21st to end	Day	7% 13% 37% 29% 12% 1	487
August	1st to 10th	Day	10% 13% 37% 28% 11% 1	447
	11th to 20th	Day	9% 12% 31% 31% 14% 3	437
	21st to end	Day	7% 17% 40% 23% 11% 3	495
September	1st to 10th	Day	10% 21% 37% 22% 8% 1	450
	11th to 20th	Day	18% 20% 32% 21% 8% 2	450
	21st to end	Day	20% 19% 33% 19% 8% 1	450
October	1st to 10th	Day	17% 24% 31% 17% 9% 1	316
	11th to 20th	Day	21% 25% 31% 12% 9% 2	270
	21st to end	Day	21% 32% 33% 11% 4	297
November	1st to 10th	Day	24% 30% 29% 11% 5% 1	270
	11th to 20th	Day	20% 33% 33% 10% 4	270
	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% 38% 13% 8% 1	760
	21st to end	Night	10% 24% 42% 17% 7%	792
February	1st to 10th	Night	14% 32% 34% 15% 4	720
	11th to 20th	Night	14% 27% 34% 19% 6% 1	704
	21st to end	Night	15% 24% 35% 19% 7%	592
March	1st to 10th	Night	11% 25% 38% 20% 5%	720
	11th to 20th	Night	11% 24% 30% 26% 8% 1	720
	21st to end	Night	12% 20% 36% 23% 8% 1	792
April	1st to 10th	Night	16% 17% 37% 21% 7% 1	635
	11th to 20th	Night	16% 19% 36% 20% 7% 1	533
	21st to end	Night	17% 19% 24% 27% 13% 1	540
May	1st to 10th	Night	17% 15% 24% 30% 13% 1	540
	11th to 20th	Night	15% 10% 25% 28% 20% 1	531
	21st to end	Night	19% 11% 24% 31% 16%	566
June	1st to 10th	Night	16% 9% 20% 35% 19% 1	360
	11th to 20th	Night	14% 13% 30% 34% 9%	360
	21st to end	Night	8% 3 23% 45% 21% 1	360
July	1st to 10th	Night	5% 2 18% 44% 27% 3	350
	11th to 20th	Night	3 4 24% 51% 18%	348
	21st to end	Night	5% 4 23% 42% 24% 2	542
August	1st to 10th	Night	7% 5% 23% 39% 24% 1	537
	11th to 20th	Night	7% 4 17% 46% 25% 2	534
	21st to end	Night	7% 8% 23% 39% 21% 2	594
September	1st to 10th	Night	8% 6% 28% 35% 20% 3	566
	11th to 20th	Night	11% 7% 21% 34% 23% 4	720
	21st to end	Night	15% 11% 26% 28% 17% 3	720
October	1st to 10th	Night	16% 11% 29% 30% 13% 2	720
	11th to 20th	Night	17% 15% 30% 24% 13% 2	720
	21st to end	Night	16% 19% 33% 23% 6% 3	792
November	1st to 10th	Night	19% 20% 36% 21% 5% 1	773
	11th to 20th	Night	17% 29% 35% 16% 2	950
	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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