Black Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Mountain Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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 6% 4 4 5% 4 6% 8% 53%	117903
	Night	2 4 2 4 3 3 3 3 5% 7% 64%	119967
Transparency	Day	25% 8% 11% 23% 24% 10%	117653
	Night	19% 6% 13% 27% 27% 9%	119807
Seeing	Day	13% 25% 26% 22% 11% 3	16129
	Night	6% 21% 34% 26% 12% 2	26307

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 2 7% 3 2 3 3 5% 8% 63%	3638
	11th to 20th	Day	1 1 3 3 4 7% 5% 8% 8% 61%	3985
	21st to end	Day	4 3 7% 4 4 4 4 6% 7% 58%	4510
February	1st to 10th	Day	3 3 6% 4 4 5% 3 6% 8% 59%	1800
	11th to 20th	Day	3 1 5% 4 4 4 4 7% 8% 61%	1879
	21st to end	Day	2 3 7% 6% 5% 5% 4 5% 6% 58%	1650
March	1st to 10th	Day	4 3 7% 4 4 5% 4 5% 7% 57%	4628
	11th to 20th	Day	1 8% 4 8% 4 4 5% 3 7% 9% 46%	5000
	21st to end	Day	1 5% 3 6% 4 5% 6% 5% 7% 8% 51%	5703
April	1st to 10th	Day	1 5% 2 5% 4 4 4 4 7% 8% 55%	2400
	11th to 20th	Day	2 9% 3 8% 5% 4 5% 4 6% 8% 45%	2371
	21st to end	Day	3 8% 2 7% 4 5% 5% 5% 7% 9% 46%	2400
May	1st to 10th	Day	4 11% 3 7% 5% 5% 5% 5% 7% 8% 41%	4056
	11th to 20th	Day	5% 10% 3 6% 5% 5% 6% 5% 7% 7% 42%	4390
	21st to end	Day	3 5% 2 4 3 4 5% 5% 7% 10% 52%	4825
June	1st to 10th	Day	3 5% 2 6% 5% 4 5% 5% 7% 9% 48%	2660
	11th to 20th	Day	4 5% 2 4 4 4 4 4 6% 8% 53%	2660
	21st to end	Day	4 6% 2 5% 4 5% 6% 4 7% 8% 48%	2660
July	1st to 10th	Day	4 5% 2 6% 5% 5% 5% 4 5% 5% 55%	4430
	11th to 20th	Day	6% 5% 2 6% 6% 6% 7% 6% 8% 9% 39%	4430
	21st to end	Day	7% 6% 3 8% 5% 6% 5% 5% 6% 7% 42%	4518
August	1st to 10th	Day	9% 7% 2 8% 7% 7% 6% 5% 7% 10% 31%	4092
	11th to 20th	Day	5% 5% 2 7% 5% 6% 7% 5% 7% 9% 42%	3789
	21st to end	Day	5% 5% 2 7% 5% 7% 7% 6% 7% 9% 41%	4180
September	1st to 10th	Day	6% 6% 2 7% 5% 5% 5% 4 5% 6% 50%	2280
	11th to 20th	Day	5% 5% 2 5% 5% 4 4 3 5% 6% 55%	2280
	21st to end	Day	4 5% 2 5% 3 3 4 3 4 6% 61%	2056
October	1st to 10th	Day	2 3 1 4 3 2 3 3 4 5% 69%	3170
	11th to 20th	Day	4 4 1 4 3 4 5% 4 6% 9% 55%	3150
	21st to end	Day	2 4 1 4 2 3 3 2 5% 6% 69%	3438
November	1st to 10th	Day	1 3 1 4 2 2 2 3 5% 6% 71%	1770
	11th to 20th	Day	2 1 2 2 2 2 2 4 8% 74%	1800
	21st to end	Day	3 2 5% 3 2 3 3 5% 7% 67%	1655
December	1st to 10th	Day	1 1 4 3 3 2 3 4 7% 73%	2620
	11th to 20th	Day	2 1 3 2 2 3 3 4 9% 70%	3026
	21st to end	Day	3 2 5% 3 3 4 3 6% 9% 62%	4004
January	1st to 10th	Night	2 2 9% 4 3 3 3 5% 7% 62%	6327
	11th to 20th	Night	1 3 3 3 4 3 5% 6% 71%	6036
	21st to end	Night	5% 3 5% 3 3 3 2 4 6% 66%	6468
February	1st to 10th	Night	2 1 5% 3 4 3 3 5% 7% 68%	2600
	11th to 20th	Night	2 2 5% 3 3 2 4 5% 7% 66%	2406
	21st to end	Night	3 2 4 4 2 3 3 4 8% 66%	1980
March	1st to 10th	Night	4 3 6% 3 4 3 3 5% 7% 62%	5247
	11th to 20th	Night	1 8% 4 5% 3 3 3 3 5% 7% 59%	5000
	21st to end	Night	1 4 2 4 3 3 4 4 6% 9% 61%	5260
April	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 8% 67%	2000
	11th to 20th	Night	3 8% 3 5% 3 3 3 3 5% 7% 56%	1977
	21st to end	Night	3 8% 2 4 3 3 3 3 4 7% 60%	2000
May	1st to 10th	Night	4 10% 2 4 3 3 3 4 6% 8% 53%	3130
	11th to 20th	Night	5% 12% 3 5% 3 4 4 4 6% 7% 47%	2521
	21st to end	Night	3 5% 1 4 2 3 3 3 5% 7% 63%	2762
June	1st to 10th	Night	2 5% 1 3 2 2 3 4 6% 10% 62%	1520
	11th to 20th	Night	4 2 1 3 2 3 3 3 6% 8% 65%	1520
	21st to end	Night	2 3 1 4 3 3 4 4 7% 8% 61%	1520
July	1st to 10th	Night	4 5% 1 4 3 3 4 4 5% 7% 59%	2540
	11th to 20th	Night	2 3 1 4 3 4 5% 4 9% 11% 55%	2540
	21st to end	Night	4 4 1 4 3 4 5% 5% 6% 8% 55%	3040
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 5% 9% 11% 44%	2847
	11th to 20th	Night	4 4 1 3 3 4 4 5% 9% 11% 53%	3050
	21st to end	Night	3 4 1 6% 5% 5% 6% 5% 8% 10% 46%	3487
September	1st to 10th	Night	5% 4 2 5% 3 3 4 3 7% 7% 56%	1900
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 8% 62%	1900
	21st to end	Night	5% 5% 1 3 3 3 3 3 5% 5% 64%	2120
October	1st to 10th	Night	3 3 1 2 1 2 2 3 3 4 76%	3800
	11th to 20th	Night	3 4 1 3 2 2 3 3 5% 7% 67%	3778
	21st to end	Night	1 3 1 3 2 2 2 2 4 5% 75%	4120
November	1st to 10th	Night	3 1 3 2 2 2 2 4 6% 73%	2506
	11th to 20th	Night	1 1 2 2 2 2 2 4 7% 77%	2600
	21st to end	Night	1 3 2 4 2 2 3 2 6% 8% 67%	2740
December	1st to 10th	Night	1 1 2 2 1 2 2 3 6% 80%	4570
	11th to 20th	Night	2 1 2 2 2 3 2 5% 8% 73%	5181
	21st to end	Night	3 2 5% 4 3 4 4 5% 8% 63%	6974

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	18% 5% 14% 28% 25% 10%	3639
	11th to 20th	Day	14% 7% 17% 20% 32% 11%	3985
	21st to end	Day	20% 8% 9% 26% 23% 14%	4510
February	1st to 10th	Day	20% 8% 15% 27% 20% 10%	1800
	11th to 20th	Day	18% 8% 11% 26% 28% 9%	1888
	21st to end	Day	23% 8% 10% 20% 24% 15%	1650
March	1st to 10th	Day	21% 6% 7% 17% 33% 15%	4628
	11th to 20th	Day	29% 9% 8% 27% 22% 4	5000
	21st to end	Day	22% 12% 10% 37% 17% 2	5703
April	1st to 10th	Day	23% 8% 13% 25% 22% 9%	2400
	11th to 20th	Day	32% 8% 12% 21% 22% 4	2374
	21st to end	Day	30% 9% 12% 28% 19% 2	2400
May	1st to 10th	Day	33% 8% 12% 19% 21% 7%	4056
	11th to 20th	Day	35% 12% 13% 22% 16% 2	4394
	21st to end	Day	22% 7% 12% 19% 32% 8%	4825
June	1st to 10th	Day	28% 10% 13% 20% 21% 8%	2660
	11th to 20th	Day	26% 8% 8% 18% 24% 17%	2660
	21st to end	Day	28% 10% 9% 20% 21% 13%	2660
July	1st to 10th	Day	25% 10% 8% 13% 16% 27%	4361
	11th to 20th	Day	33% 12% 11% 23% 14% 8%	4355
	21st to end	Day	35% 11% 11% 15% 13% 15%	4375
August	1st to 10th	Day	42% 12% 17% 14% 11% 4	4078
	11th to 20th	Day	33% 10% 14% 14% 20% 9%	3826
	21st to end	Day	33% 11% 12% 16% 23% 6%	4179
September	1st to 10th	Day	31% 8% 9% 15% 24% 13%	2280
	11th to 20th	Day	26% 7% 11% 18% 27% 12%	2280
	21st to end	Day	22% 7% 7% 24% 32% 8%	2056
October	1st to 10th	Day	16% 6% 6% 19% 34% 19%	3170
	11th to 20th	Day	21% 7% 8% 32% 25% 6%	3150
	21st to end	Day	14% 5% 6% 29% 38% 7%	3438
November	1st to 10th	Day	14% 5% 12% 23% 36% 10%	1770
	11th to 20th	Day	11% 5% 10% 36% 29% 11%	1800
	21st to end	Day	17% 4 11% 28% 28% 12%	1655
December	1st to 10th	Day	13% 4 10% 33% 31% 9%	2620
	11th to 20th	Day	12% 5% 12% 37% 28% 6%	3024
	21st to end	Day	16% 7% 10% 28% 29% 10%	4004
January	1st to 10th	Night	21% 5% 11% 28% 28% 7%	6327
	11th to 20th	Night	10% 6% 14% 27% 27% 17%	6036
	21st to end	Night	19% 6% 11% 25% 27% 12%	6468
February	1st to 10th	Night	17% 4 15% 32% 23% 9%	2600
	11th to 20th	Night	17% 6% 13% 31% 28% 6%	2407
	21st to end	Night	17% 4 10% 29% 29% 11%	1980
March	1st to 10th	Night	20% 5% 8% 19% 37% 11%	5247
	11th to 20th	Night	24% 6% 9% 21% 32% 8%	5000
	21st to end	Night	19% 6% 17% 41% 16% 1	5260
April	1st to 10th	Night	16% 6% 15% 31% 26% 5%	2000
	11th to 20th	Night	27% 5% 11% 29% 23% 4	1977
	21st to end	Night	27% 5% 14% 32% 20% 2	2000
May	1st to 10th	Night	25% 8% 12% 24% 23% 8%	3130
	11th to 20th	Night	33% 7% 16% 26% 16% 1	2522
	21st to end	Night	20% 6% 13% 23% 31% 6%	2762
June	1st to 10th	Night	18% 7% 18% 28% 20% 9%	1520
	11th to 20th	Night	17% 7% 12% 22% 25% 18%	1520
	21st to end	Night	18% 7% 13% 23% 27% 12%	1520
July	1st to 10th	Night	24% 8% 10% 19% 16% 22%	2500
	11th to 20th	Night	20% 9% 20% 31% 14% 7%	2492
	21st to end	Night	22% 11% 20% 25% 7% 15%	2945
August	1st to 10th	Night	31% 12% 26% 16% 11% 4	2838
	11th to 20th	Night	22% 9% 26% 15% 20% 8%	3079
	21st to end	Night	29% 11% 16% 19% 21% 3	3487
September	1st to 10th	Night	25% 8% 13% 22% 22% 10%	1900
	11th to 20th	Night	21% 6% 13% 23% 23% 13%	1900
	21st to end	Night	22% 5% 9% 27% 31% 6%	2120
October	1st to 10th	Night	13% 5% 8% 23% 35% 15%	3800
	11th to 20th	Night	16% 5% 10% 36% 26% 7%	3778
	21st to end	Night	12% 4 7% 27% 42% 9%	4120
November	1st to 10th	Night	13% 5% 12% 26% 34% 11%	2506
	11th to 20th	Night	9% 4 11% 36% 31% 10%	2600
	21st to end	Night	16% 4 11% 30% 28% 11%	2740
December	1st to 10th	Night	9% 3 11% 35% 34% 7%	4570
	11th to 20th	Night	11% 4 11% 30% 37% 6%	5182
	21st to end	Night	19% 6% 14% 25% 27% 10%	6974

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	14% 35% 26% 19% 6% 1	300
	11th to 20th	Day	16% 38% 28% 13% 4	300
	21st to end	Day	8% 40% 32% 17% 3 1	330
February	1st to 10th	Day	16% 53% 23% 6% 1 1	300
	11th to 20th	Day	20% 46% 22% 9% 3	300
	21st to end	Day	26% 40% 24% 8% 2	249
March	1st to 10th	Day	13% 40% 29% 15% 3	350
	11th to 20th	Day	15% 35% 31% 16% 4	500
	21st to end	Day	17% 46% 26% 8% 2	550
April	1st to 10th	Day	16% 41% 28% 11% 4	500
	11th to 20th	Day	22% 41% 23% 9% 4 1	492
	21st to end	Day	19% 36% 28% 13% 3 1	500
May	1st to 10th	Day	19% 34% 27% 14% 5% 1	533
	11th to 20th	Day	16% 21% 28% 23% 10% 3	676
	21st to end	Day	15% 20% 25% 26% 11% 3	734
June	1st to 10th	Day	6% 16% 26% 28% 20% 4	680
	11th to 20th	Day	8% 18% 30% 29% 12% 3	680
	21st to end	Day	5% 14% 31% 25% 19% 6%	680
July	1st to 10th	Day	7% 9% 20% 36% 22% 6%	671
	11th to 20th	Day	8% 7% 17% 37% 23% 8%	668
	21st to end	Day	11% 8% 17% 35% 23% 5%	569
August	1st to 10th	Day	16% 10% 23% 29% 18% 4	487
	11th to 20th	Day	8% 7% 26% 33% 21% 5%	477
	21st to end	Day	12% 10% 23% 33% 18% 4	539
September	1st to 10th	Day	10% 11% 25% 30% 19% 5%	490
	11th to 20th	Day	10% 16% 32% 28% 13% 3	490
	21st to end	Day	9% 15% 27% 27% 19% 3	324
October	1st to 10th	Day	8% 24% 21% 23% 22% 2	300
	11th to 20th	Day	13% 29% 24% 22% 10% 2	300
	21st to end	Day	15% 28% 25% 23% 7% 1	330
November	1st to 10th	Day	13% 36% 27% 17% 7% 1	300
	11th to 20th	Day	11% 35% 30% 18% 4 2	300
	21st to end	Day	8% 33% 38% 18% 3 1	300
December	1st to 10th	Day	16% 33% 29% 16% 5% 1	300
	11th to 20th	Day	15% 42% 27% 12% 5%	300
	21st to end	Day	17% 37% 28% 14% 3 1	330
January	1st to 10th	Night	6% 34% 34% 19% 6% 1	1000
	11th to 20th	Night	6% 37% 34% 17% 5% 1	865
	21st to end	Night	3 31% 43% 18% 5% 1	880
February	1st to 10th	Night	4 37% 41% 14% 4	800
	11th to 20th	Night	5% 35% 39% 17% 4	784
	21st to end	Night	6% 30% 47% 16% 2	660
March	1st to 10th	Night	4 27% 41% 23% 5%	800
	11th to 20th	Night	4 23% 40% 25% 8%	800
	21st to end	Night	4 28% 44% 20% 4	780
April	1st to 10th	Night	8% 24% 45% 20% 3	600
	11th to 20th	Night	8% 24% 38% 22% 6% 1	593
	21st to end	Night	8% 20% 38% 25% 8% 1	600
May	1st to 10th	Night	10% 15% 40% 25% 9% 1	600
	11th to 20th	Night	12% 9% 28% 33% 15% 2	563
	21st to end	Night	9% 12% 30% 31% 15% 2	617
June	1st to 10th	Night	6% 8% 27% 36% 21% 2	570
	11th to 20th	Night	6% 7% 35% 38% 13% 1	570
	21st to end	Night	4 4 24% 38% 27% 4	570
July	1st to 10th	Night	8% 4 12% 36% 35% 5%	555
	11th to 20th	Night	4 2 10% 42% 36% 6%	552
	21st to end	Night	7% 4 8% 32% 44% 5%	624
August	1st to 10th	Night	10% 4 11% 35% 38% 3	567
	11th to 20th	Night	7% 4 15% 42% 27% 5%	564
	21st to end	Night	6% 3 16% 36% 34% 4	627
September	1st to 10th	Night	6% 5% 17% 41% 25% 6%	570
	11th to 20th	Night	8% 3 27% 39% 21% 3	570
	21st to end	Night	7% 9% 28% 34% 20% 3	624
October	1st to 10th	Night	6% 11% 37% 31% 12% 3	760
	11th to 20th	Night	3 19% 37% 29% 11% 2	760
	21st to end	Night	5% 20% 40% 24% 9% 2	836
November	1st to 10th	Night	4 21% 43% 25% 7% 1	946
	11th to 20th	Night	3 29% 42% 19% 6% 1	1000
	21st to end	Night	5% 28% 38% 25% 4 1	1000
December	1st to 10th	Night	6% 30% 40% 21% 4	1000
	11th to 20th	Night	6% 35% 35% 19% 5%	1000
	21st to end	Night	8% 33% 38% 17% 4	1100

Page generated at 2025-05-08 07:33:59. Version ac07

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