Big Sky Observatory Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Sky Observatory Site clear sky chart from 2005-11-06 to 2025-08-05.

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 7% 5% 5% 5% 4 6% 8% 41%	129939
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 39%	126832
Transparency	Day	40% 5% 14% 26% 13% 2	129641
	Night	41% 5% 14% 24% 13% 2	126708
Seeing	Day	12% 9% 26% 29% 17% 6%	16361
	Night	11% 7% 30% 35% 15% 2	25425

By Month

Percentage Cloud Cover NW (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% 7% 14% 6% 5% 4 4 6% 7% 31%	3179
	11th to 20th	Day	3 8% 4 11% 7% 5% 5% 5% 6% 6% 39%	3269
	21st to end	Day	2 6% 3 9% 6% 5% 4 4 7% 11% 44%	4004
February	1st to 10th	Day	5% 12% 4 12% 7% 7% 6% 5% 6% 7% 28%	1641
	11th to 20th	Day	4 12% 5% 9% 8% 6% 5% 5% 7% 8% 30%	1755
	21st to end	Day	4 14% 5% 10% 7% 6% 5% 5% 5% 8% 32%	1595
March	1st to 10th	Day	4 11% 3 10% 6% 5% 4 3 5% 11% 38%	4518
	11th to 20th	Day	5% 8% 4 9% 6% 6% 5% 4 6% 8% 41%	4998
	21st to end	Day	9% 17% 6% 12% 5% 5% 5% 4 6% 8% 23%	5486
April	1st to 10th	Day	8% 14% 4 8% 6% 5% 5% 4 5% 8% 32%	2464
	11th to 20th	Day	10% 13% 4 8% 5% 4 5% 5% 7% 8% 32%	2570
	21st to end	Day	10% 10% 3 7% 6% 5% 4 4 7% 8% 35%	2600
May	1st to 10th	Day	15% 10% 3 6% 4 3 4 3 5% 7% 41%	6042
	11th to 20th	Day	12% 7% 3 9% 6% 6% 7% 5% 7% 9% 29%	6734
	21st to end	Day	11% 7% 2 7% 5% 6% 6% 4 6% 8% 38%	7405
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 6% 5% 7% 8% 35%	3000
	11th to 20th	Day	17% 7% 2 6% 6% 5% 6% 4 7% 8% 31%	3000
	21st to end	Day	8% 5% 2 4 4 4 5% 5% 7% 8% 47%	3000
July	1st to 10th	Day	7% 4 2 5% 3 4 4 4 5% 8% 54%	6780
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 6% 9% 50%	6780
	21st to end	Day	7% 5% 2 4 3 3 3 3 5% 8% 57%	7163
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 5% 8% 10% 45%	4415
	11th to 20th	Day	6% 4 2 5% 3 4 4 4 7% 10% 50%	4028
	21st to end	Day	8% 3 2 4 3 3 4 4 6% 8% 57%	4521
September	1st to 10th	Day	9% 5% 2 5% 3 4 4 3 5% 6% 54%	2345
	11th to 20th	Day	13% 6% 2 6% 4 4 3 2 5% 7% 47%	2090
	21st to end	Day	9% 7% 2 6% 3 4 4 3 6% 8% 48%	2090
October	1st to 10th	Day	8% 7% 2 7% 4 5% 5% 5% 7% 9% 42%	3330
	11th to 20th	Day	6% 7% 2 5% 3 4 4 4 7% 10% 49%	2976
	21st to end	Day	7% 6% 2 6% 5% 4 5% 5% 6% 8% 45%	3101
November	1st to 10th	Day	9% 14% 4 7% 5% 4 6% 4 6% 7% 35%	1639
	11th to 20th	Day	5% 11% 3 8% 6% 4 4 4 6% 8% 40%	1591
	21st to end	Day	5% 11% 4 9% 5% 4 5% 5% 8% 10% 36%	1400
December	1st to 10th	Day	3 11% 5% 11% 6% 6% 4 3 5% 9% 36%	2290
	11th to 20th	Day	3 8% 5% 11% 7% 7% 6% 5% 7% 9% 33%	2642
	21st to end	Day	3 9% 4 9% 5% 5% 5% 5% 6% 8% 41%	3498
January	1st to 10th	Night	3 15% 7% 12% 7% 6% 6% 4 6% 8% 25%	6766
	11th to 20th	Night	3 9% 5% 12% 6% 6% 6% 5% 6% 8% 35%	6675
	21st to end	Night	3 8% 4 9% 5% 5% 5% 4 6% 8% 42%	6952
February	1st to 10th	Night	5% 11% 4 13% 7% 6% 5% 4 7% 8% 31%	2758
	11th to 20th	Night	5% 12% 5% 11% 7% 6% 6% 5% 6% 8% 28%	2532
	21st to end	Night	4 11% 5% 12% 8% 6% 4 4 5% 7% 34%	2030
March	1st to 10th	Night	2 8% 4 12% 6% 6% 5% 4 7% 10% 36%	5376
	11th to 20th	Night	3 9% 3 9% 6% 5% 5% 4 6% 10% 39%	5001
	21st to end	Night	10% 18% 6% 11% 6% 5% 5% 4 6% 8% 22%	5489
April	1st to 10th	Night	9% 11% 4 10% 5% 6% 5% 4 6% 7% 34%	1933
	11th to 20th	Night	9% 11% 4 9% 6% 6% 5% 5% 5% 6% 34%	1781
	21st to end	Night	11% 9% 3 8% 5% 6% 5% 4 6% 8% 34%	1800
May	1st to 10th	Night	14% 11% 3 7% 4 4 4 4 5% 6% 36%	3931
	11th to 20th	Night	11% 8% 3 8% 6% 7% 6% 4 7% 8% 33%	3159
	21st to end	Night	11% 5% 2 7% 5% 5% 5% 5% 5% 8% 41%	3470
June	1st to 10th	Night	14% 7% 3 6% 5% 5% 4 4 7% 7% 39%	1400
	11th to 20th	Night	18% 7% 3 7% 6% 4 5% 4 5% 8% 34%	1400
	21st to end	Night	8% 3 1 5% 4 5% 4 4 8% 9% 48%	1400
July	1st to 10th	Night	5% 4 2 6% 4 4 4 4 7% 9% 51%	3180
	11th to 20th	Night	7% 4 2 5% 3 4 5% 4 8% 10% 48%	3180
	21st to end	Night	7% 4 2 6% 4 3 4 4 6% 7% 54%	3664
August	1st to 10th	Night	7% 5% 2 7% 4 5% 5% 4 6% 8% 47%	2592
	11th to 20th	Night	5% 4 2 6% 5% 4 6% 4 8% 10% 45%	2811
	21st to end	Night	6% 3 1 4 3 3 3 4 6% 8% 60%	3146
September	1st to 10th	Night	10% 4 2 6% 4 4 3 3 5% 7% 52%	1833
	11th to 20th	Night	12% 6% 2 5% 3 2 4 2 5% 7% 53%	2090
	21st to end	Night	8% 7% 2 5% 4 4 4 4 6% 7% 49%	2090
October	1st to 10th	Night	6% 5% 2 5% 4 3 4 4 5% 8% 55%	3645
	11th to 20th	Night	6% 6% 2 5% 4 4 4 4 6% 8% 50%	3948
	21st to end	Night	5% 6% 2 6% 5% 4 5% 5% 7% 8% 47%	4457
November	1st to 10th	Night	8% 12% 4 7% 6% 5% 5% 5% 7% 7% 35%	2640
	11th to 20th	Night	4 9% 3 8% 5% 4 5% 5% 7% 8% 41%	2805
	21st to end	Night	5% 11% 3 7% 5% 5% 5% 4 7% 10% 39%	3000
December	1st to 10th	Night	3 9% 4 9% 6% 6% 7% 6% 7% 8% 34%	4890
	11th to 20th	Night	3 10% 5% 10% 7% 6% 6% 5% 7% 9% 31%	5550
	21st to end	Night	4 10% 4 9% 5% 4 5% 5% 6% 8% 41%	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	52% 5% 11% 22% 8% 2	3179
	11th to 20th	Day	44% 4 12% 19% 18% 2	3290
	21st to end	Day	34% 5% 13% 34% 13% 1	4004
February	1st to 10th	Day	52% 8% 12% 17% 10% 2	1641
	11th to 20th	Day	48% 8% 15% 19% 8% 2	1764
	21st to end	Day	49% 6% 12% 18% 13% 2	1595
March	1st to 10th	Day	41% 4 14% 23% 17% 1	4518
	11th to 20th	Day	41% 5% 13% 24% 16% 1	4999
	21st to end	Day	58% 5% 15% 16% 6%	5486
April	1st to 10th	Day	49% 5% 14% 21% 9% 1	2464
	11th to 20th	Day	47% 7% 11% 21% 11% 2	2570
	21st to end	Day	45% 6% 14% 22% 11% 1	2600
May	1st to 10th	Day	43% 5% 8% 23% 17% 4	6042
	11th to 20th	Day	48% 7% 13% 23% 8%	6738
	21st to end	Day	42% 6% 15% 24% 12% 1	7406
June	1st to 10th	Day	43% 8% 16% 21% 11% 2	3000
	11th to 20th	Day	48% 8% 13% 20% 9% 2	3000
	21st to end	Day	32% 7% 18% 29% 11% 2	3000
July	1st to 10th	Day	28% 4 15% 28% 21% 5%	6706
	11th to 20th	Day	31% 4 19% 30% 14% 2	6699
	21st to end	Day	28% 3 13% 27% 18% 11%	6961
August	1st to 10th	Day	33% 5% 13% 35% 11% 2	4398
	11th to 20th	Day	29% 5% 21% 34% 9% 2	4067
	21st to end	Day	27% 3 13% 40% 15% 2	4520
September	1st to 10th	Day	31% 6% 13% 26% 18% 8%	2345
	11th to 20th	Day	37% 5% 13% 28% 13% 3	2090
	21st to end	Day	34% 6% 13% 31% 14% 3	2090
October	1st to 10th	Day	37% 4 11% 36% 11% 1	3330
	11th to 20th	Day	31% 4 21% 27% 16% 3	2976
	21st to end	Day	37% 5% 11% 32% 12% 3	3101
November	1st to 10th	Day	48% 7% 12% 22% 10% 1	1639
	11th to 20th	Day	41% 7% 17% 24% 11% 1	1591
	21st to end	Day	42% 6% 18% 21% 10% 2	1400
December	1st to 10th	Day	48% 5% 12% 26% 7% 2	2290
	11th to 20th	Day	48% 5% 20% 21% 6% 1	2644
	21st to end	Day	40% 5% 12% 22% 19% 2	3498
January	1st to 10th	Night	55% 6% 11% 21% 6% 1	6766
	11th to 20th	Night	47% 4 13% 19% 14% 3	6716
	21st to end	Night	39% 5% 14% 28% 14% 1	6952
February	1st to 10th	Night	49% 6% 13% 17% 13% 2	2758
	11th to 20th	Night	50% 8% 12% 19% 10% 2	2533
	21st to end	Night	49% 6% 13% 21% 10% 2	2030
March	1st to 10th	Night	42% 6% 16% 22% 12% 2	5376
	11th to 20th	Night	40% 6% 16% 25% 14%	5001
	21st to end	Night	59% 5% 14% 14% 7% 1	5489
April	1st to 10th	Night	49% 7% 10% 22% 12% 1	1933
	11th to 20th	Night	48% 6% 13% 21% 10% 1	1781
	21st to end	Night	46% 7% 12% 23% 9% 2	1800
May	1st to 10th	Night	49% 4 9% 22% 13% 3	3928
	11th to 20th	Night	46% 6% 13% 28% 6% 1	3159
	21st to end	Night	40% 8% 15% 25% 11% 2	3470
June	1st to 10th	Night	42% 8% 13% 24% 12% 1	1400
	11th to 20th	Night	47% 6% 16% 20% 8% 2	1400
	21st to end	Night	29% 9% 19% 27% 14% 2	1400
July	1st to 10th	Night	29% 5% 18% 22% 22% 4	3145
	11th to 20th	Night	32% 4 19% 30% 12% 3	3138
	21st to end	Night	30% 4 14% 26% 16% 9%	3562
August	1st to 10th	Night	35% 5% 15% 27% 15% 3	2584
	11th to 20th	Night	33% 4 20% 29% 12% 2	2838
	21st to end	Night	23% 3 13% 40% 17% 4	3146
September	1st to 10th	Night	30% 6% 13% 27% 18% 6%	1833
	11th to 20th	Night	32% 5% 14% 27% 17% 5%	2090
	21st to end	Night	33% 6% 15% 28% 15% 4	2090
October	1st to 10th	Night	29% 4 11% 32% 21% 3	3645
	11th to 20th	Night	32% 5% 17% 25% 18% 3	3948
	21st to end	Night	34% 5% 12% 33% 16%	4457
November	1st to 10th	Night	46% 7% 12% 23% 11% 1	2640
	11th to 20th	Night	38% 8% 16% 21% 16% 1	2805
	21st to end	Night	40% 7% 18% 20% 11% 3	3000
December	1st to 10th	Night	46% 6% 19% 19% 9% 2	4890
	11th to 20th	Night	48% 6% 22% 18% 6%	5547
	21st to end	Night	40% 5% 12% 25% 15% 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	6% 9% 30% 31% 14% 11%	200
	11th to 20th	Day	13% 6% 33% 28% 15% 7%	200
	21st to end	Day	4 10% 34% 34% 16% 2	220
February	1st to 10th	Day	10% 10% 37% 31% 10% 3	221
	11th to 20th	Day	6% 12% 29% 34% 15% 4	300
	21st to end	Day	13% 13% 31% 26% 15% 3	359
March	1st to 10th	Day	11% 15% 32% 26% 11% 4	500
	11th to 20th	Day	9% 11% 33% 30% 14% 3	500
	21st to end	Day	13% 16% 32% 24% 11% 4	550
April	1st to 10th	Day	15% 14% 31% 26% 11% 2	500
	11th to 20th	Day	14% 15% 27% 26% 14% 3	492
	21st to end	Day	16% 12% 29% 24% 15% 4	500
May	1st to 10th	Day	15% 11% 23% 29% 15% 7%	538
	11th to 20th	Day	18% 7% 20% 27% 21% 6%	725
	21st to end	Day	17% 8% 17% 30% 20% 8%	789
June	1st to 10th	Day	13% 8% 28% 29% 17% 3	730
	11th to 20th	Day	18% 10% 25% 28% 17% 2	730
	21st to end	Day	11% 5% 22% 32% 22% 7%	730
July	1st to 10th	Day	9% 4 21% 35% 26% 6%	721
	11th to 20th	Day	10% 5% 22% 34% 22% 6%	718
	21st to end	Day	6% 4 24% 33% 25% 7%	708
August	1st to 10th	Day	9% 4 23% 33% 22% 9%	504
	11th to 20th	Day	8% 6% 20% 33% 25% 9%	477
	21st to end	Day	9% 8% 25% 30% 18% 10%	539
September	1st to 10th	Day	10% 8% 28% 26% 20% 8%	490
	11th to 20th	Day	16% 7% 25% 29% 17% 6%	490
	21st to end	Day	16% 15% 29% 22% 14% 4	490
October	1st to 10th	Day	17% 8% 22% 24% 22% 7%	395
	11th to 20th	Day	13% 13% 28% 23% 16% 7%	300
	21st to end	Day	14% 15% 28% 28% 12% 3	330
November	1st to 10th	Day	20% 8% 29% 26% 13% 3	300
	11th to 20th	Day	12% 14% 35% 26% 11% 2	295
	21st to end	Day	5% 17% 29% 30% 12% 9%	200
December	1st to 10th	Day	10% 12% 29% 31% 14% 6%	200
	11th to 20th	Day	8% 11% 41% 24% 13% 4	200
	21st to end	Day	10% 7% 23% 34% 18% 7%	220
January	1st to 10th	Night	9% 12% 34% 31% 13% 2	1000
	11th to 20th	Night	8% 8% 36% 35% 11% 2	975
	21st to end	Night	9% 8% 37% 32% 13% 1	880
February	1st to 10th	Night	10% 11% 29% 33% 15% 3	800
	11th to 20th	Night	10% 6% 32% 38% 13% 2	784
	21st to end	Night	9% 8% 27% 33% 21% 2	660
March	1st to 10th	Night	8% 7% 35% 34% 14% 2	800
	11th to 20th	Night	10% 8% 38% 31% 12% 2	800
	21st to end	Night	12% 9% 30% 33% 12% 3	880
April	1st to 10th	Night	14% 8% 34% 31% 12% 2	733
	11th to 20th	Night	15% 8% 34% 33% 10% 1	593
	21st to end	Night	15% 9% 29% 34% 13% 1	600
May	1st to 10th	Night	15% 8% 25% 34% 18% 1	570
	11th to 20th	Night	13% 7% 21% 37% 21% 1	394
	21st to end	Night	18% 5% 21% 33% 21% 1	433
June	1st to 10th	Night	16% 6% 26% 36% 16% 1	400
	11th to 20th	Night	22% 6% 27% 31% 14%	400
	21st to end	Night	10% 3 23% 43% 21% 2	400
July	1st to 10th	Night	7% 3 20% 43% 27% 1	390
	11th to 20th	Night	8% 2 22% 45% 22% 1	388
	21st to end	Night	6% 2 23% 43% 24% 2	438
August	1st to 10th	Night	10% 2 15% 42% 29% 2	404
	11th to 20th	Night	7% 2 21% 46% 23% 2	564
	21st to end	Night	9% 5% 23% 41% 20% 1	627
September	1st to 10th	Night	11% 3 23% 40% 21% 2	651
	11th to 20th	Night	16% 3 20% 37% 22% 2	760
	21st to end	Night	11% 7% 29% 35% 14% 4	760
October	1st to 10th	Night	14% 6% 24% 34% 19% 2	760
	11th to 20th	Night	12% 6% 34% 30% 15% 3	760
	21st to end	Night	11% 7% 34% 33% 14% 1	836
November	1st to 10th	Night	15% 9% 37% 30% 8% 1	885
	11th to 20th	Night	10% 12% 40% 27% 9% 2	1000
	21st to end	Night	10% 9% 35% 33% 11% 2	1000
December	1st to 10th	Night	9% 9% 30% 36% 14% 3	1000
	11th to 20th	Night	8% 8% 36% 37% 10% 2	1000
	21st to end	Night	12% 8% 32% 34% 12% 2	1100

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