Battle Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Battle Mountain Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	3 5% 2 5% 3 3 4 3 5% 7% 59%	134629
	Night	4 6% 2 6% 4 4 4 4 6% 7% 53%	131769
Transparency	Day	24% 6% 9% 17% 22% 22%	129542
	Night	29% 7% 11% 17% 21% 16%	127842
Seeing	Day	5% 5% 17% 30% 28% 15%	15895
	Night	7% 3 15% 44% 27% 4	25282

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	4 11% 5% 11% 6% 6% 6% 5% 6% 9% 31%	3639
	11th to 20th	Day	3 8% 4 8% 7% 6% 4 4 5% 6% 45%	3640
	21st to end	Day	3 10% 4 9% 7% 6% 6% 5% 7% 7% 35%	4004
February	1st to 10th	Day	5% 8% 4 9% 5% 5% 5% 4 7% 11% 37%	1982
	11th to 20th	Day	3 9% 4 8% 7% 5% 5% 5% 8% 12% 35%	1950
	21st to end	Day	2 7% 3 8% 5% 3 4 4 8% 11% 43%	1650
March	1st to 10th	Day	4 15% 4 9% 5% 5% 6% 4 7% 11% 30%	4518
	11th to 20th	Day	2 8% 3 7% 4 4 5% 4 8% 13% 42%	4770
	21st to end	Day	3 9% 4 9% 6% 6% 6% 5% 6% 8% 38%	5793
April	1st to 10th	Day	4 11% 3 7% 5% 4 5% 4 7% 10% 41%	2400
	11th to 20th	Day	4 7% 2 6% 5% 5% 4 4 5% 9% 50%	2369
	21st to end	Day	7% 7% 2 5% 4 4 4 3 6% 8% 51%	2550
May	1st to 10th	Day	4 5% 2 4 3 3 3 3 5% 8% 62%	5880
	11th to 20th	Day	3 5% 2 4 3 3 4 3 4 6% 62%	5841
	21st to end	Day	3 3 1 4 3 3 3 3 5% 6% 67%	6813
June	1st to 10th	Day	4 3 2 4 4 4 4 4 5% 9% 59%	2800
	11th to 20th	Day	4 3 1 3 2 2 4 3 4 5% 70%	2800
	21st to end	Day	2 2 1 2 3 2 3 2 4 5% 75%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 3 3 85%	6319
	11th to 20th	Day	1 2 1 2 2 2 3 2 3 5% 78%	6320
	21st to end	Day	1 1 1 2 2 3 3 3 4 6% 74%	6880
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 77%	5784
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 83%	5496
	21st to end	Day	2 2 1 3 2 3 3 3 4 6% 73%	5984
September	1st to 10th	Day	2 2 1 2 1 2 2 3 3 5% 78%	2400
	11th to 20th	Day	1 3 1 4 3 3 3 2 4 5% 72%	2203
	21st to end	Day	3 3 1 3 2 2 3 2 3 4 73%	2090
October	1st to 10th	Day	2 3 1 3 3 2 3 2 4 6% 71%	3490
	11th to 20th	Day	4 4 1 3 2 2 3 2 3 4 71%	3177
	21st to end	Day	2 4 1 4 3 4 3 3 6% 8% 60%	3101
November	1st to 10th	Day	5% 7% 2 5% 5% 4 4 4 6% 7% 50%	1764
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 10% 49%	1800
	21st to end	Day	5% 8% 2 6% 5% 4 4 4 6% 9% 48%	1800
December	1st to 10th	Day	4 5% 2 5% 3 2 3 3 4 7% 63%	2790
	11th to 20th	Day	5% 11% 3 8% 6% 6% 6% 5% 7% 11% 34%	3028
	21st to end	Day	5% 9% 4 10% 6% 6% 5% 5% 7% 9% 35%	4004
January	1st to 10th	Night	3 11% 4 10% 5% 5% 6% 5% 8% 10% 33%	6306
	11th to 20th	Night	4 7% 3 11% 7% 6% 5% 4 6% 7% 41%	6320
	21st to end	Night	4 10% 3 7% 5% 6% 6% 5% 9% 7% 38%	6952
February	1st to 10th	Night	4 10% 4 7% 5% 5% 4 3 6% 10% 40%	2415
	11th to 20th	Night	4 8% 4 8% 5% 4 5% 5% 7% 10% 40%	2338
	21st to end	Night	2 8% 4 8% 5% 5% 5% 5% 8% 9% 41%	1980
March	1st to 10th	Night	4 14% 4 9% 4 4 5% 4 7% 10% 34%	5376
	11th to 20th	Night	3 8% 4 8% 5% 5% 4 4 6% 11% 44%	5230
	21st to end	Night	4 8% 3 7% 5% 5% 5% 4 6% 10% 42%	5165
April	1st to 10th	Night	4 9% 2 6% 5% 5% 5% 4 6% 10% 43%	2000
	11th to 20th	Night	5% 7% 2 4 4 4 4 4 6% 7% 52%	1979
	21st to end	Night	9% 8% 2 4 4 3 4 4 5% 7% 50%	1847
May	1st to 10th	Night	5% 6% 2 5% 3 3 3 3 4 6% 61%	4100
	11th to 20th	Night	4 5% 2 6% 3 4 4 3 4 6% 58%	4074
	21st to end	Night	4 3 2 5% 3 3 4 3 5% 7% 61%	4048
June	1st to 10th	Night	4 3 1 4 4 4 5% 5% 5% 7% 57%	1600
	11th to 20th	Night	4 2 1 3 3 3 3 3 3 6% 68%	1600
	21st to end	Night	1 2 1 3 2 3 4 3 4 5% 73%	1600
July	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 78%	3640
	11th to 20th	Night	2 2 1 4 3 3 4 3 6% 6% 66%	3640
	21st to end	Night	3 3 1 4 4 4 5% 4 6% 8% 58%	3970
August	1st to 10th	Night	2 2 1 3 2 3 3 3 4 5% 71%	4025
	11th to 20th	Night	2 2 1 3 2 3 3 2 3 5% 74%	4364
	21st to end	Night	3 3 1 4 3 4 4 3 4 4 66%	5015
September	1st to 10th	Night	3 3 1 3 2 2 2 2 4 5% 73%	2000
	11th to 20th	Night	4 3 1 3 2 4 3 2 4 5% 69%	2153
	21st to end	Night	4 2 1 3 2 2 2 2 3 5% 74%	2090
October	1st to 10th	Night	3 3 1 2 2 3 2 2 4 4 73%	3490
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 5% 68%	3742
	21st to end	Night	3 3 1 5% 3 4 4 3 4 6% 64%	4457
November	1st to 10th	Night	6% 6% 2 5% 4 4 4 4 6% 7% 52%	2535
	11th to 20th	Night	5% 6% 2 5% 4 4 4 4 6% 7% 53%	2600
	21st to end	Night	3 8% 3 6% 5% 5% 4 4 5% 9% 48%	2600
December	1st to 10th	Night	3 5% 2 5% 3 3 4 3 5% 6% 60%	4390
	11th to 20th	Night	5% 10% 3 8% 5% 5% 6% 6% 8% 11% 34%	5176
	21st to end	Night	7% 11% 4 10% 7% 7% 6% 5% 8% 10% 26%	6952

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	44% 11% 18% 18% 8%	3639
	11th to 20th	Day	35% 11% 14% 20% 20% 1	3640
	21st to end	Day	39% 16% 12% 7% 21% 5%	4004
February	1st to 10th	Day	38% 11% 16% 23% 9% 3	1982
	11th to 20th	Day	38% 9% 18% 22% 12% 2	1960
	21st to end	Day	33% 9% 19% 24% 11% 5%	1650
March	1st to 10th	Day	42% 10% 13% 20% 11% 4	4518
	11th to 20th	Day	30% 10% 21% 25% 13% 1	4770
	21st to end	Day	38% 12% 14% 20% 13% 3	5793
April	1st to 10th	Day	37% 6% 15% 22% 15% 4	2400
	11th to 20th	Day	31% 7% 13% 25% 19% 5%	2372
	21st to end	Day	30% 6% 14% 27% 19% 4	2550
May	1st to 10th	Day	20% 7% 11% 23% 33% 6%	5880
	11th to 20th	Day	22% 4 8% 22% 35% 8%	5844
	21st to end	Day	18% 5% 7% 19% 38% 13%	6814
June	1st to 10th	Day	22% 6% 10% 19% 28% 14%	2800
	11th to 20th	Day	17% 4 6% 13% 33% 28%	2800
	21st to end	Day	12% 3 4 11% 29% 42%	2800
July	1st to 10th	Day	8% 2 2 5% 15% 67%	6251
	11th to 20th	Day	11% 3 2 4 12% 68%	6245
	21st to end	Day	11% 3 3 7% 27% 49%	6681
August	1st to 10th	Day	15% 2 3 8% 20% 52%	4212
	11th to 20th	Day	9% 2 3 6% 17% 63%	3979
	21st to end	Day	11% 1 2 9% 28% 49%	4311
September	1st to 10th	Day	12% 3 3 9% 28% 45%	2400
	11th to 20th	Day	16% 4 6% 14% 28% 32%	2203
	21st to end	Day	16% 2 5% 16% 31% 31%	2090
October	1st to 10th	Day	17% 3 5% 11% 32% 33%	3490
	11th to 20th	Day	18% 3 5% 20% 36% 18%	3177
	21st to end	Day	22% 4 7% 31% 25% 11%	3101
November	1st to 10th	Day	30% 8% 11% 23% 22% 6%	1764
	11th to 20th	Day	29% 7% 14% 30% 18% 2	1800
	21st to end	Day	33% 7% 14% 26% 17% 3	1800
December	1st to 10th	Day	23% 5% 10% 26% 31% 5%	2790
	11th to 20th	Day	42% 10% 15% 20% 12% 1	3028
	21st to end	Day	41% 10% 19% 22% 8% 1	4004
January	1st to 10th	Night	41% 12% 18% 21% 8%	6306
	11th to 20th	Night	39% 10% 14% 15% 19% 2	6320
	21st to end	Night	34% 15% 13% 9% 24% 4	6952
February	1st to 10th	Night	38% 7% 16% 26% 10% 2	2415
	11th to 20th	Night	36% 9% 17% 24% 13% 1	2339
	21st to end	Night	34% 9% 18% 24% 12% 4	1980
March	1st to 10th	Night	39% 10% 14% 19% 12% 7%	5376
	11th to 20th	Night	30% 8% 18% 24% 18% 1	5230
	21st to end	Night	33% 12% 21% 23% 10% 2	5165
April	1st to 10th	Night	34% 7% 18% 22% 15% 3	2000
	11th to 20th	Night	29% 6% 14% 28% 16% 6%	1979
	21st to end	Night	32% 7% 13% 25% 19% 4	1847
May	1st to 10th	Night	24% 7% 11% 17% 34% 8%	4097
	11th to 20th	Night	25% 4 7% 22% 34% 8%	4075
	21st to end	Night	21% 6% 8% 16% 36% 13%	4048
June	1st to 10th	Night	22% 8% 11% 18% 29% 12%	1600
	11th to 20th	Night	19% 3 5% 15% 33% 25%	1600
	21st to end	Night	14% 3 6% 10% 28% 40%	1600
July	1st to 10th	Night	12% 2 3 4 13% 65%	3600
	11th to 20th	Night	19% 4 3 4 12% 58%	3592
	21st to end	Night	23% 4 4 10% 24% 35%	3857
August	1st to 10th	Night	22% 3 4 8% 15% 48%	2921
	11th to 20th	Night	16% 2 3 5% 15% 58%	3150
	21st to end	Night	15% 2 3 7% 20% 52%	3608
September	1st to 10th	Night	16% 3 4 8% 25% 44%	2000
	11th to 20th	Night	19% 3 5% 12% 26% 35%	2153
	21st to end	Night	16% 2 4 13% 27% 38%	2090
October	1st to 10th	Night	17% 2 3 11% 32% 35%	3490
	11th to 20th	Night	20% 3 4 14% 38% 21%	3742
	21st to end	Night	21% 3 6% 27% 33% 10%	4457
November	1st to 10th	Night	29% 6% 13% 20% 23% 8%	2535
	11th to 20th	Night	27% 6% 14% 27% 23% 3	2600
	21st to end	Night	31% 6% 14% 26% 21% 3	2600
December	1st to 10th	Night	25% 6% 9% 21% 33% 6%	4390
	11th to 20th	Night	41% 8% 20% 18% 10% 2	5176
	21st to end	Night	47% 11% 16% 18% 7%	6952

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	9% 4 18% 38% 25% 8%	200
	11th to 20th	Day	6% 5% 10% 36% 30% 15%	200
	21st to end	Day	4 10% 35% 33% 18%	220
February	1st to 10th	Day	8% 5% 18% 37% 23% 9%	491
	11th to 20th	Day	8% 5% 21% 37% 25% 4	495
	21st to end	Day	4 6% 22% 39% 24% 6%	414
March	1st to 10th	Day	9% 8% 17% 37% 25% 4	500
	11th to 20th	Day	8% 7% 22% 33% 24% 7%	500
	21st to end	Day	8% 9% 21% 35% 21% 6%	550
April	1st to 10th	Day	8% 9% 17% 33% 24% 11%	500
	11th to 20th	Day	7% 9% 18% 35% 24% 7%	492
	21st to end	Day	8% 8% 17% 29% 26% 12%	500
May	1st to 10th	Day	5% 7% 20% 34% 24% 11%	503
	11th to 20th	Day	10% 7% 18% 24% 26% 15%	528
	21st to end	Day	8% 7% 15% 23% 28% 20%	573
June	1st to 10th	Day	4 6% 17% 27% 27% 19%	530
	11th to 20th	Day	5% 9% 22% 29% 22% 14%	530
	21st to end	Day	2 4 16% 32% 29% 18%	530
July	1st to 10th	Day	1 3 17% 32% 29% 18%	526
	11th to 20th	Day	2 2 12% 32% 30% 22%	524
	21st to end	Day	2 1 13% 25% 35% 23%	575
August	1st to 10th	Day	4 5% 13% 25% 31% 22%	527
	11th to 20th	Day	2 3 16% 27% 29% 23%	517
	21st to end	Day	3 3 13% 26% 35% 21%	583
September	1st to 10th	Day	2 2 16% 22% 33% 25%	530
	11th to 20th	Day	3 5% 15% 26% 30% 21%	523
	21st to end	Day	3 4 14% 27% 34% 18%	490
October	1st to 10th	Day	4 6% 16% 22% 30% 22%	490
	11th to 20th	Day	4 6% 15% 21% 35% 19%	451
	21st to end	Day	5% 4 16% 33% 29% 14%	330
November	1st to 10th	Day	9% 5% 16% 30% 28% 12%	300
	11th to 20th	Day	4 4 13% 33% 32% 14%	300
	21st to end	Day	8% 3 17% 29% 31% 12%	300
December	1st to 10th	Day	6% 2 14% 31% 30% 18%	253
	11th to 20th	Day	9% 3 13% 37% 30% 10%	200
	21st to end	Day	11% 3 16% 38% 22% 10%	220
January	1st to 10th	Night	8% 5% 18% 42% 24% 4	800
	11th to 20th	Night	8% 2 16% 43% 27% 4	800
	21st to end	Night	10% 3 13% 45% 24% 6%	880
February	1st to 10th	Night	9% 5% 17% 44% 23% 3	800
	11th to 20th	Night	8% 2 17% 50% 21% 2	784
	21st to end	Night	4 3 23% 48% 20% 2	660
March	1st to 10th	Night	9% 4 19% 46% 20% 2	800
	11th to 20th	Night	6% 4 17% 46% 24% 3	800
	21st to end	Night	9% 6% 22% 44% 18% 1	880
April	1st to 10th	Night	9% 5% 20% 45% 21% 2	800
	11th to 20th	Night	7% 5% 20% 43% 20% 3	791
	21st to end	Night	11% 5% 14% 40% 26% 4	647
May	1st to 10th	Night	6% 5% 20% 43% 26% 2	600
	11th to 20th	Night	9% 4 14% 38% 30% 4	591
	21st to end	Night	8% 3 14% 38% 31% 5%	471
June	1st to 10th	Night	6% 2 14% 43% 31% 4	400
	11th to 20th	Night	5% 3 14% 47% 31% 1	400
	21st to end	Night	3 1 12% 56% 26% 3	400
July	1st to 10th	Night	3 2 8% 51% 35% 1	390
	11th to 20th	Night	2 8% 53% 36% 1	388
	21st to end	Night	4 2 6% 47% 40% 2	438
August	1st to 10th	Night	4 3 11% 49% 33% 1	589
	11th to 20th	Night	3 2 8% 48% 35% 4	725
	21st to end	Night	3 1 7% 44% 40% 4	880
September	1st to 10th	Night	4 1 8% 43% 39% 4	800
	11th to 20th	Night	4 2 9% 39% 37% 9%	792
	21st to end	Night	5% 2 10% 38% 36% 9%	760
October	1st to 10th	Night	8% 1 11% 43% 33% 5%	760
	11th to 20th	Night	5% 2 12% 42% 33% 5%	760
	21st to end	Night	5% 2 14% 45% 28% 6%	836
November	1st to 10th	Night	8% 2 15% 43% 29% 4	780
	11th to 20th	Night	8% 2 14% 49% 22% 4	800
	21st to end	Night	6% 3 18% 41% 27% 5%	800
December	1st to 10th	Night	8% 3 16% 46% 21% 5%	800
	11th to 20th	Night	12% 4 15% 45% 22% 2	800
	21st to end	Night	13% 6% 20% 44% 15% 3	880

Page generated at 2025-09-15 22:49:10. Version ac07

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