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 2026-03-03.

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	15% 6% 2 6% 4 4 5% 4 6% 7% 41%	131091
	Night	16% 6% 2 5% 4 4 4 3 5% 7% 44%	130067
Transparency	Day	40% 7% 11% 18% 17% 6%	134867
	Night	38% 6% 11% 19% 19% 7%	134361
Seeing	Day	20% 13% 23% 22% 15% 7%	16588
	Night	18% 11% 22% 27% 18% 3	26696

By Month

Percentage Cloud Cover SE (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	22% 8% 3 5% 3 3 3 2 3 4 45%	5258
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 3 4 61%	5280
	21st to end	Day	24% 8% 3 7% 4 3 3 3 4 5% 38%	5882
February	1st to 10th	Day	18% 7% 3 5% 3 4 4 3 4 7% 41%	2100
	11th to 20th	Day	25% 6% 2 3 3 3 4 3 5% 5% 41%	2048
	21st to end	Day	23% 6% 2 5% 4 5% 2 3 5% 6% 40%	1730
March	1st to 10th	Day	24% 7% 2 5% 4 4 4 3 4 6% 37%	4674
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 7% 53%	4980
	21st to end	Day	18% 5% 2 6% 4 4 4 3 4 5% 45%	5465
April	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 4 6% 7% 43%	2240
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 6% 47%	2374
	21st to end	Day	13% 5% 1 5% 4 4 4 4 6% 8% 47%	2590
May	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 36%	5862
	11th to 20th	Day	21% 7% 2 7% 6% 5% 5% 5% 7% 8% 27%	5861
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 6% 8% 38%	6433
June	1st to 10th	Day	12% 6% 2 7% 6% 6% 5% 6% 7% 10% 32%	2600
	11th to 20th	Day	7% 5% 2 6% 6% 6% 6% 6% 9% 11% 36%	2600
	21st to end	Day	7% 5% 2 6% 6% 7% 7% 5% 8% 11% 37%	2600
July	1st to 10th	Day	8% 4 2 7% 6% 6% 6% 5% 8% 11% 36%	5900
	11th to 20th	Day	8% 4 1 7% 6% 7% 8% 6% 9% 13% 30%	5900
	21st to end	Day	11% 7% 3 8% 7% 8% 8% 7% 8% 9% 23%	6424
August	1st to 10th	Day	9% 6% 2 6% 4 5% 6% 5% 8% 11% 37%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	3993
	21st to end	Day	9% 4 2 6% 4 4 5% 4 6% 7% 48%	4297
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 8% 40%	2200
	11th to 20th	Day	17% 5% 2 6% 5% 5% 5% 4 6% 7% 38%	2200
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 6% 44%	2200
October	1st to 10th	Day	14% 5% 2 3 3 4 3 3 5% 5% 54%	2200
	11th to 20th	Day	12% 5% 1 4 2 2 3 3 4 5% 59%	2064
	21st to end	Day	16% 4 1 4 4 3 3 3 4 5% 55%	2200
November	1st to 10th	Day	14% 4 2 4 3 2 2 3 4 6% 55%	1874
	11th to 20th	Day	15% 4 1 4 3 3 3 2 4 6% 54%	1890
	21st to end	Day	16% 5% 2 5% 3 3 4 3 4 7% 49%	1890
December	1st to 10th	Day	22% 6% 1 3 3 3 3 3 4 5% 47%	3020
	11th to 20th	Day	21% 7% 2 5% 4 4 4 3 4 4 43%	3463
	21st to end	Day	25% 4 1 4 3 3 3 3 5% 7% 42%	4565
January	1st to 10th	Night	20% 6% 2 4 3 3 3 2 4 5% 47%	7635
	11th to 20th	Night	15% 6% 2 5% 3 2 3 2 4 5% 52%	7680
	21st to end	Night	22% 7% 2 6% 3 3 3 2 4 4 44%	8282
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 6% 45%	2520
	11th to 20th	Night	23% 6% 2 5% 4 4 3 2 5% 4 41%	2458
	21st to end	Night	21% 5% 2 7% 3 4 5% 3 5% 6% 39%	2076
March	1st to 10th	Night	24% 8% 2 5% 3 3 4 4 4 6% 36%	5224
	11th to 20th	Night	11% 4 2 4 3 3 3 4 6% 7% 54%	4980
	21st to end	Night	18% 6% 2 5% 4 3 3 3 4 6% 45%	5467
April	1st to 10th	Night	15% 4 1 4 3 4 4 4 5% 6% 50%	2175
	11th to 20th	Night	13% 6% 1 5% 4 3 3 3 5% 7% 50%	1975
	21st to end	Night	12% 5% 3 5% 3 3 3 3 5% 6% 50%	1807
May	1st to 10th	Night	15% 6% 3 6% 5% 4 5% 4 6% 9% 37%	4033
	11th to 20th	Night	24% 6% 2 6% 5% 4 5% 4 6% 8% 30%	4032
	21st to end	Night	16% 4 2 5% 4 4 5% 4 6% 9% 42%	4421
June	1st to 10th	Night	10% 6% 3 7% 6% 6% 6% 5% 8% 9% 36%	1800
	11th to 20th	Night	7% 4 2 6% 4 4 5% 5% 9% 12% 41%	1800
	21st to end	Night	7% 4 1 5% 4 5% 6% 5% 7% 11% 44%	1800
July	1st to 10th	Night	11% 6% 3 7% 6% 5% 6% 5% 7% 9% 35%	4058
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 6% 8% 11% 35%	4057
	21st to end	Night	14% 7% 3 8% 5% 6% 5% 6% 8% 8% 31%	4427
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 11% 38%	2915
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 8% 45%	3038
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 8% 50%	3586
September	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 7% 9% 41%	2180
	11th to 20th	Night	16% 6% 2 5% 5% 4 4 5% 5% 7% 42%	2200
	21st to end	Night	15% 5% 2 5% 4 3 4 4 4 7% 47%	2200
October	1st to 10th	Night	15% 4 1 4 3 3 3 3 3 5% 56%	2200
	11th to 20th	Night	12% 4 2 5% 3 3 3 3 4 5% 57%	2335
	21st to end	Night	15% 4 1 4 3 2 3 3 4 5% 55%	2640
November	1st to 10th	Night	15% 5% 2 4 3 2 3 3 4 6% 55%	2641
	11th to 20th	Night	16% 5% 2 3 3 2 3 3 5% 5% 54%	2730
	21st to end	Night	18% 5% 2 4 4 3 4 4 4 6% 46%	2730
December	1st to 10th	Night	19% 6% 1 5% 3 3 3 3 4 5% 49%	4380
	11th to 20th	Night	22% 5% 2 5% 3 3 3 3 4 5% 45%	4952
	21st to end	Night	25% 5% 1 4 3 3 3 3 4 5% 41%	6633

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% 4 5% 21% 20% 6%	5259
	11th to 20th	Day	28% 3 5% 15% 33% 17%	5280
	21st to end	Day	50% 3 4 16% 17% 10%	5882
February	1st to 10th	Day	43% 5% 7% 20% 20% 5%	2100
	11th to 20th	Day	46% 4 7% 19% 19% 6%	2058
	21st to end	Day	46% 3 8% 14% 21% 8%	1730
March	1st to 10th	Day	47% 5% 6% 13% 20% 9%	4674
	11th to 20th	Day	33% 5% 6% 14% 22% 19%	4980
	21st to end	Day	42% 4 7% 23% 19% 5%	5465
April	1st to 10th	Day	38% 6% 11% 21% 18% 5%	2240
	11th to 20th	Day	38% 5% 9% 18% 23% 7%	2374
	21st to end	Day	35% 6% 13% 25% 17% 3	2590
May	1st to 10th	Day	46% 6% 15% 19% 13% 2	5897
	11th to 20th	Day	53% 7% 15% 16% 8% 1	5865
	21st to end	Day	42% 7% 12% 20% 16% 2	6447
June	1st to 10th	Day	45% 9% 18% 19% 7% 2	2600
	11th to 20th	Day	38% 10% 19% 20% 11% 2	2600
	21st to end	Day	38% 11% 17% 19% 12% 3	2600
July	1st to 10th	Day	39% 12% 20% 22% 7% 1	5835
	11th to 20th	Day	43% 13% 27% 13% 3 1	5831
	21st to end	Day	53% 12% 17% 14% 4	6240
August	1st to 10th	Day	38% 10% 15% 28% 8%	4221
	11th to 20th	Day	36% 10% 22% 16% 13% 3	4032
	21st to end	Day	35% 9% 13% 26% 17% 1	4297
September	1st to 10th	Day	42% 9% 14% 16% 16% 3	2200
	11th to 20th	Day	44% 8% 12% 17% 18% 1	2200
	21st to end	Day	39% 7% 12% 16% 20% 6%	2200
October	1st to 10th	Day	25% 5% 7% 15% 37% 11%	3590
	11th to 20th	Day	20% 3 7% 16% 27% 27%	3343
	21st to end	Day	31% 4 5% 21% 26% 13%	3538
November	1st to 10th	Day	32% 3 8% 16% 30% 11%	1874
	11th to 20th	Day	32% 4 7% 20% 27% 10%	1890
	21st to end	Day	37% 5% 7% 17% 27% 7%	1890
December	1st to 10th	Day	40% 3 6% 16% 25% 10%	3020
	11th to 20th	Day	47% 4 6% 16% 20% 8%	3460
	21st to end	Day	40% 5% 9% 27% 17% 3	4565
January	1st to 10th	Night	38% 4 10% 19% 22% 7%	7636
	11th to 20th	Night	34% 4 5% 16% 26% 13%	7680
	21st to end	Night	45% 3 4 16% 22% 11%	8282
February	1st to 10th	Night	39% 5% 9% 20% 21% 6%	2520
	11th to 20th	Night	46% 4 6% 18% 20% 6%	2459
	21st to end	Night	46% 5% 7% 17% 18% 8%	2076
March	1st to 10th	Night	48% 5% 7% 14% 20% 6%	5224
	11th to 20th	Night	29% 5% 8% 17% 25% 16%	4980
	21st to end	Night	41% 3 8% 23% 17% 7%	5467
April	1st to 10th	Night	34% 6% 12% 20% 23% 6%	2175
	11th to 20th	Night	34% 4 8% 23% 21% 9%	1975
	21st to end	Night	34% 7% 11% 26% 19% 3	1807
May	1st to 10th	Night	43% 7% 17% 21% 11% 1	4060
	11th to 20th	Night	51% 7% 18% 16% 8%	4033
	21st to end	Night	37% 7% 17% 25% 13% 2	4429
June	1st to 10th	Night	41% 11% 17% 19% 10% 2	1800
	11th to 20th	Night	31% 11% 22% 22% 12% 1	1800
	21st to end	Night	32% 11% 19% 22% 13% 3	1800
July	1st to 10th	Night	43% 10% 18% 23% 6%	4015
	11th to 20th	Night	37% 14% 28% 17% 3 1	4006
	21st to end	Night	48% 12% 18% 17% 5% 1	4297
August	1st to 10th	Night	39% 9% 21% 25% 6%	2904
	11th to 20th	Night	36% 8% 20% 21% 15% 1	3065
	21st to end	Night	30% 8% 15% 25% 20% 2	3586
September	1st to 10th	Night	38% 10% 15% 18% 15% 4	2180
	11th to 20th	Night	40% 9% 13% 19% 17% 2	2200
	21st to end	Night	37% 7% 12% 20% 18% 7%	2200
October	1st to 10th	Night	25% 4 11% 16% 35% 10%	3590
	11th to 20th	Night	19% 3 6% 18% 31% 24%	3795
	21st to end	Night	28% 4 5% 20% 27% 15%	4252
November	1st to 10th	Night	33% 4 7% 19% 25% 11%	2641
	11th to 20th	Night	32% 4 8% 18% 27% 12%	2730
	21st to end	Night	38% 5% 7% 20% 23% 8%	2730
December	1st to 10th	Night	39% 3 6% 16% 26% 9%	4380
	11th to 20th	Night	42% 5% 9% 16% 21% 7%	4954
	21st to end	Night	43% 5% 9% 26% 15% 2	6633

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	33% 29% 29% 7% 2	330
	11th to 20th	Day	34% 26% 25% 12% 4	330
	21st to end	Day	31% 29% 28% 9% 2	363
February	1st to 10th	Day	27% 25% 35% 12% 1	330
	11th to 20th	Day	45% 22% 25% 7%	330
	21st to end	Day	39% 21% 31% 8% 1 1	273
March	1st to 10th	Day	29% 19% 30% 16% 6%	376
	11th to 20th	Day	28% 21% 27% 18% 4 1	500
	21st to end	Day	25% 22% 31% 16% 5% 1	550
April	1st to 10th	Day	24% 18% 29% 19% 9% 1	500
	11th to 20th	Day	16% 21% 34% 19% 9% 2	492
	21st to end	Day	21% 12% 26% 24% 12% 4	500
May	1st to 10th	Day	19% 14% 31% 20% 11% 6%	503
	11th to 20th	Day	20% 11% 20% 26% 14% 9%	527
	21st to end	Day	18% 8% 23% 25% 14% 11%	573
June	1st to 10th	Day	14% 7% 22% 28% 19% 10%	530
	11th to 20th	Day	9% 4 20% 28% 23% 17%	530
	21st to end	Day	8% 5% 23% 30% 21% 13%	530
July	1st to 10th	Day	11% 2 13% 26% 30% 18%	525
	11th to 20th	Day	8% 3 11% 25% 35% 19%	524
	21st to end	Day	6% 3 12% 25% 32% 20%	576
August	1st to 10th	Day	16% 6% 16% 24% 21% 17%	526
	11th to 20th	Day	13% 4 12% 27% 29% 15%	519
	21st to end	Day	16% 4 11% 24% 29% 15%	583
September	1st to 10th	Day	11% 4 18% 29% 26% 12%	530
	11th to 20th	Day	17% 4 13% 27% 31% 7%	530
	21st to end	Day	16% 6% 20% 27% 24% 7%	530
October	1st to 10th	Day	17% 5% 26% 29% 16% 7%	530
	11th to 20th	Day	14% 13% 23% 30% 16% 3	530
	21st to end	Day	21% 11% 28% 25% 13% 2	583
November	1st to 10th	Day	22% 14% 28% 23% 10% 2	352
	11th to 20th	Day	19% 24% 35% 18% 3	330
	21st to end	Day	18% 22% 36% 16% 6% 1	330
December	1st to 10th	Day	34% 25% 27% 12% 2 1	330
	11th to 20th	Day	28% 21% 32% 16% 3	330
	21st to end	Day	34% 18% 29% 15% 3 1	363
January	1st to 10th	Night	29% 21% 25% 21% 5%	840
	11th to 20th	Night	23% 21% 29% 20% 6%	840
	21st to end	Night	23% 21% 34% 20% 2	924
February	1st to 10th	Night	20% 18% 31% 26% 5%	840
	11th to 20th	Night	29% 21% 29% 20% 2	823
	21st to end	Night	25% 21% 33% 19% 2	692
March	1st to 10th	Night	22% 18% 31% 25% 4	809
	11th to 20th	Night	20% 18% 32% 23% 7% 1	800
	21st to end	Night	22% 16% 28% 27% 8%	880
April	1st to 10th	Night	18% 18% 28% 26% 9% 1	775
	11th to 20th	Night	17% 15% 25% 28% 14% 1	593
	21st to end	Night	14% 9% 25% 33% 18% 2	600
May	1st to 10th	Night	14% 12% 22% 31% 20% 1	600
	11th to 20th	Night	18% 7% 17% 32% 22% 4	591
	21st to end	Night	17% 4 14% 30% 30% 6%	650
June	1st to 10th	Night	11% 4 14% 36% 31% 5%	600
	11th to 20th	Night	9% 3 14% 29% 33% 13%	600
	21st to end	Night	8% 2 11% 36% 35% 9%	600
July	1st to 10th	Night	11% 1 7% 26% 46% 8%	585
	11th to 20th	Night	10% 2 8% 21% 48% 11%	582
	21st to end	Night	8% 2 9% 25% 46% 11%	657
August	1st to 10th	Night	15% 2 9% 25% 36% 13%	597
	11th to 20th	Night	16% 4 9% 24% 39% 9%	594
	21st to end	Night	17% 2 9% 26% 38% 8%	660
September	1st to 10th	Night	12% 4 14% 27% 35% 8%	780
	11th to 20th	Night	20% 3 15% 28% 32% 4	800
	21st to end	Night	17% 5% 13% 35% 29% 2	800
October	1st to 10th	Night	16% 6% 19% 32% 20% 6%	800
	11th to 20th	Night	14% 11% 22% 33% 19% 2	800
	21st to end	Night	18% 11% 21% 32% 16% 2	880
November	1st to 10th	Night	18% 10% 23% 29% 17% 2	820
	11th to 20th	Night	18% 13% 27% 30% 11% 1	840
	21st to end	Night	21% 17% 25% 25% 11% 1	840
December	1st to 10th	Night	25% 18% 26% 25% 6% 1	840
	11th to 20th	Night	22% 15% 28% 27% 7% 1	840
	21st to end	Night	31% 14% 24% 26% 5%	924

Page generated at 2026-02-28 01:40:22. Version ac07

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