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 2024-07-03.

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	16% 6% 2 6% 4 4 5% 4 6% 7% 40%	89611
	Night	16% 6% 2 5% 4 4 4 4 5% 7% 43%	89536
Transparency	Day	42% 7% 11% 18% 17% 5%	89385
	Night	39% 7% 12% 19% 18% 6%	89366
Seeing	Day	19% 12% 23% 22% 16% 8%	14402
	Night	18% 11% 21% 27% 19% 4	24469

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	20% 7% 3 5% 4 4 4 3 5% 6% 39%	2836
	11th to 20th	Day	17% 5% 2 5% 4 3 4 3 4 5% 50%	2840
	21st to end	Day	27% 7% 2 4 3 3 2 3 3 4 42%	3170
February	1st to 10th	Day	20% 8% 3 5% 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Day	24% 6% 2 3 3 3 4 3 5% 5% 41%	1848
	21st to end	Day	25% 6% 1 6% 4 5% 3 3 5% 5% 38%	1570
March	1st to 10th	Day	27% 7% 2 5% 3 3 3 3 4 6% 37%	3238
	11th to 20th	Day	14% 5% 2 5% 4 3 3 3 4 6% 51%	3480
	21st to end	Day	18% 5% 2 7% 4 5% 4 3 4 6% 41%	3815
April	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 7% 7% 44%	2128
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 6% 6% 45%	2254
	21st to end	Day	12% 4 1 5% 4 4 4 4 6% 8% 48%	2460
May	1st to 10th	Day	14% 7% 3 6% 4 4 5% 4 5% 8% 39%	4082
	11th to 20th	Day	19% 7% 2 7% 5% 5% 4 4 6% 7% 33%	4081
	21st to end	Day	10% 5% 2 6% 5% 4 6% 5% 7% 10% 40%	4476
June	1st to 10th	Day	12% 6% 2 7% 6% 6% 5% 6% 7% 10% 32%	2470
	11th to 20th	Day	6% 5% 2 6% 6% 6% 6% 6% 9% 11% 37%	2470
	21st to end	Day	8% 5% 2 6% 6% 7% 7% 5% 8% 11% 36%	2470
July	1st to 10th	Day	9% 6% 3 8% 8% 7% 7% 5% 9% 10% 27%	2366
	11th to 20th	Day	10% 7% 2 8% 8% 7% 9% 6% 9% 11% 24%	2340
	21st to end	Day	7% 6% 2 7% 7% 8% 7% 7% 9% 10% 29%	2547
August	1st to 10th	Day	11% 6% 2 7% 6% 6% 8% 6% 9% 10% 29%	2327
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 10% 32%	2211
	21st to end	Day	15% 6% 3 7% 6% 6% 6% 6% 8% 8% 29%	2375
September	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	1980
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 6% 38%	1980
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 5% 6% 46%	1980
October	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 5% 53%	1980
	11th to 20th	Day	13% 5% 1 5% 3 2 3 3 4 5% 56%	1858
	21st to end	Day	16% 4 1 4 3 3 3 3 4 5% 55%	1980
November	1st to 10th	Day	14% 4 1 4 3 2 2 3 4 5% 56%	1692
	11th to 20th	Day	16% 5% 1 4 3 3 3 3 4 6% 53%	1710
	21st to end	Day	17% 6% 2 5% 2 3 4 3 4 7% 47%	1710
December	1st to 10th	Day	25% 7% 2 4 4 3 4 3 5% 5% 39%	1710
	11th to 20th	Day	18% 6% 2 4 4 4 3 3 5% 5% 46%	2153
	21st to end	Day	24% 6% 2 5% 3 3 3 4 5% 7% 39%	3124
January	1st to 10th	Night	20% 7% 2 5% 3 3 4 3 5% 6% 42%	4109
	11th to 20th	Night	15% 8% 2 6% 3 3 4 3 5% 6% 45%	4120
	21st to end	Night	28% 8% 2 5% 3 3 3 2 4 4 40%	4469
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 6% 44%	2280
	11th to 20th	Night	22% 6% 2 5% 4 4 3 2 5% 4 42%	2218
	21st to end	Night	22% 5% 2 7% 3 4 5% 3 5% 5% 38%	1884
March	1st to 10th	Night	27% 7% 2 5% 3 3 4 3 4 5% 36%	3615
	11th to 20th	Night	11% 4 2 4 3 3 3 4 7% 7% 51%	3480
	21st to end	Night	19% 6% 2 6% 5% 3 3 3 4 6% 42%	3817
April	1st to 10th	Night	14% 4 1 4 3 4 4 4 5% 6% 51%	2066
	11th to 20th	Night	14% 6% 1 6% 4 3 3 3 5% 7% 49%	1875
	21st to end	Night	11% 5% 3 4 3 3 3 3 5% 7% 52%	1717
May	1st to 10th	Night	15% 6% 3 6% 4 4 5% 4 6% 8% 40%	2813
	11th to 20th	Night	19% 5% 2 6% 4 4 4 4 6% 8% 39%	2812
	21st to end	Night	9% 4 2 4 5% 4 5% 4 7% 10% 46%	3079
June	1st to 10th	Night	10% 6% 2 7% 6% 6% 6% 5% 8% 9% 35%	1710
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 9% 12% 42%	1710
	21st to end	Night	7% 4 1 5% 5% 5% 6% 5% 7% 10% 44%	1710
July	1st to 10th	Night	10% 6% 2 6% 7% 5% 6% 5% 8% 11% 35%	1638
	11th to 20th	Night	9% 7% 2 6% 5% 5% 7% 7% 7% 12% 35%	1620
	21st to end	Night	6% 5% 2 5% 5% 6% 6% 7% 10% 11% 36%	1771
August	1st to 10th	Night	11% 6% 2 7% 6% 5% 6% 5% 7% 10% 37%	1611
	11th to 20th	Night	12% 7% 2 6% 5% 4 5% 4 6% 10% 39%	1686
	21st to end	Night	14% 8% 3 7% 4 5% 5% 5% 7% 9% 35%	1980
September	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 8% 8% 42%	1962
	11th to 20th	Night	15% 6% 2 5% 5% 4 4 5% 5% 8% 42%	1980
	21st to end	Night	14% 5% 1 5% 3 3 4 3 4 6% 50%	1980
October	1st to 10th	Night	15% 4 1 3 3 3 3 3 3 5% 57%	1980
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 6% 53%	2101
	21st to end	Night	14% 4 1 4 3 2 3 3 4 6% 55%	2376
November	1st to 10th	Night	14% 5% 1 3 3 2 3 3 4 6% 57%	2383
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 5% 5% 53%	2470
	21st to end	Night	18% 5% 2 4 4 3 4 4 5% 6% 45%	2470
December	1st to 10th	Night	22% 6% 2 6% 3 4 3 3 5% 6% 39%	2470
	11th to 20th	Night	18% 5% 2 5% 2 3 3 3 5% 5% 49%	3042
	21st to end	Night	24% 6% 2 5% 4 4 4 4 6% 7% 35%	4532

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% 6% 6% 16% 21% 7%	2837
	11th to 20th	Day	38% 4 8% 18% 24% 8%	2840
	21st to end	Day	46% 4 5% 18% 20% 7%	3170
February	1st to 10th	Day	45% 5% 8% 18% 19% 5%	1900
	11th to 20th	Day	45% 4 7% 18% 19% 7%	1858
	21st to end	Day	49% 3 8% 14% 19% 8%	1570
March	1st to 10th	Day	48% 6% 8% 14% 14% 10%	3238
	11th to 20th	Day	35% 6% 7% 18% 23% 11%	3480
	21st to end	Day	44% 5% 9% 18% 20% 4	3815
April	1st to 10th	Day	37% 7% 11% 21% 19% 5%	2128
	11th to 20th	Day	39% 5% 9% 17% 23% 7%	2254
	21st to end	Day	34% 7% 12% 26% 18% 3	2460
May	1st to 10th	Day	42% 6% 15% 19% 16% 2	4120
	11th to 20th	Day	47% 8% 13% 20% 11% 1	4085
	21st to end	Day	37% 9% 15% 20% 17% 1	4489
June	1st to 10th	Day	45% 9% 18% 18% 7% 2	2470
	11th to 20th	Day	37% 10% 19% 21% 11% 2	2470
	21st to end	Day	39% 11% 16% 19% 12% 3	2470
July	1st to 10th	Day	47% 11% 19% 15% 6% 1	2301
	11th to 20th	Day	49% 14% 17% 11% 7% 3	2271
	21st to end	Day	44% 13% 20% 16% 7%	2405
August	1st to 10th	Day	45% 13% 18% 19% 5% 1	2314
	11th to 20th	Day	46% 11% 17% 17% 8% 2	2211
	21st to end	Day	48% 12% 14% 16% 8% 2	2375
September	1st to 10th	Day	42% 9% 15% 16% 15% 4	1980
	11th to 20th	Day	44% 8% 11% 16% 20% 1	1980
	21st to end	Day	38% 7% 11% 17% 22% 6%	1980
October	1st to 10th	Day	34% 6% 10% 16% 27% 8%	1980
	11th to 20th	Day	31% 5% 10% 22% 22% 10%	1858
	21st to end	Day	33% 5% 5% 20% 27% 11%	1980
November	1st to 10th	Day	31% 3 8% 15% 32% 11%	1692
	11th to 20th	Day	34% 4 6% 19% 26% 11%	1710
	21st to end	Day	38% 5% 7% 17% 26% 7%	1710
December	1st to 10th	Day	47% 5% 7% 17% 18% 6%	1710
	11th to 20th	Day	41% 3 7% 16% 24% 8%	2150
	21st to end	Day	43% 6% 8% 20% 19% 4	3124
January	1st to 10th	Night	41% 6% 7% 14% 25% 6%	4109
	11th to 20th	Night	39% 6% 8% 17% 19% 10%	4120
	21st to end	Night	49% 4 5% 14% 21% 7%	4469
February	1st to 10th	Night	40% 5% 9% 18% 21% 7%	2280
	11th to 20th	Night	46% 4 6% 17% 20% 6%	2219
	21st to end	Night	48% 5% 7% 16% 17% 7%	1884
March	1st to 10th	Night	50% 5% 9% 14% 15% 6%	3615
	11th to 20th	Night	30% 6% 10% 19% 24% 10%	3480
	21st to end	Night	44% 4 8% 20% 17% 7%	3817
April	1st to 10th	Night	33% 6% 12% 20% 23% 6%	2066
	11th to 20th	Night	35% 4 8% 22% 21% 9%	1875
	21st to end	Night	32% 7% 11% 27% 20% 3	1717
May	1st to 10th	Night	40% 9% 16% 20% 14% 1	2840
	11th to 20th	Night	42% 9% 17% 21% 11%	2813
	21st to end	Night	30% 9% 18% 27% 14% 2	3087
June	1st to 10th	Night	41% 11% 18% 19% 10% 2	1710
	11th to 20th	Night	30% 11% 23% 23% 12% 1	1710
	21st to end	Night	33% 11% 18% 21% 14% 4	1710
July	1st to 10th	Night	41% 11% 19% 18% 10% 1	1593
	11th to 20th	Night	36% 15% 24% 15% 8% 2	1566
	21st to end	Night	35% 16% 22% 19% 8% 1	1672
August	1st to 10th	Night	39% 13% 22% 19% 6% 1	1600
	11th to 20th	Night	40% 11% 18% 16% 14% 1	1686
	21st to end	Night	42% 11% 17% 18% 9% 2	1980
September	1st to 10th	Night	37% 10% 15% 18% 15% 4	1962
	11th to 20th	Night	39% 9% 13% 18% 18% 2	1980
	21st to end	Night	34% 7% 12% 21% 19% 7%	1980
October	1st to 10th	Night	31% 5% 11% 19% 28% 7%	1980
	11th to 20th	Night	33% 6% 10% 21% 21% 9%	2101
	21st to end	Night	31% 4 8% 19% 27% 11%	2376
November	1st to 10th	Night	31% 4 7% 20% 26% 12%	2383
	11th to 20th	Night	34% 4 8% 18% 24% 12%	2470
	21st to end	Night	39% 6% 7% 19% 22% 7%	2470
December	1st to 10th	Night	46% 4 9% 17% 18% 6%	2470
	11th to 20th	Night	36% 5% 10% 17% 25% 7%	3044
	21st to end	Night	46% 6% 10% 20% 15% 3	4532

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	30% 28% 33% 7% 2	270
	11th to 20th	Day	33% 23% 27% 13% 4	270
	21st to end	Day	29% 32% 28% 9% 2	297
February	1st to 10th	Day	30% 25% 31% 12% 1	270
	11th to 20th	Day	45% 22% 25% 8%	270
	21st to end	Day	43% 20% 31% 6% 1	225
March	1st to 10th	Day	27% 19% 31% 17% 6%	336
	11th to 20th	Day	28% 22% 27% 19% 5% 1	460
	21st to end	Day	24% 22% 32% 16% 6% 1	506
April	1st to 10th	Day	23% 18% 28% 20% 10% 1	460
	11th to 20th	Day	16% 21% 32% 20% 9% 2	452
	21st to end	Day	20% 13% 26% 23% 13% 4	460
May	1st to 10th	Day	18% 14% 31% 20% 11% 6%	463
	11th to 20th	Day	19% 9% 20% 27% 15% 10%	487
	21st to end	Day	16% 7% 23% 27% 15% 12%	529
June	1st to 10th	Day	15% 6% 21% 28% 19% 11%	490
	11th to 20th	Day	8% 4 19% 28% 23% 18%	490
	21st to end	Day	9% 5% 24% 29% 21% 12%	490
July	1st to 10th	Day	11% 2 12% 26% 29% 19%	453
	11th to 20th	Day	8% 3 12% 23% 33% 20%	444
	21st to end	Day	5% 2 12% 25% 33% 23%	488
August	1st to 10th	Day	15% 5% 16% 22% 22% 20%	446
	11th to 20th	Day	13% 4 11% 26% 31% 15%	439
	21st to end	Day	18% 4 11% 22% 29% 16%	495
September	1st to 10th	Day	12% 4 16% 26% 28% 14%	450
	11th to 20th	Day	17% 4 13% 26% 32% 7%	450
	21st to end	Day	14% 5% 20% 27% 26% 8%	450
October	1st to 10th	Day	18% 5% 24% 27% 18% 8%	450
	11th to 20th	Day	16% 13% 24% 28% 16% 3	450
	21st to end	Day	20% 11% 27% 26% 13% 3	495
November	1st to 10th	Day	21% 13% 27% 25% 12% 2	290
	11th to 20th	Day	19% 26% 32% 20% 3	270
	21st to end	Day	15% 20% 39% 18% 7% 1	270
December	1st to 10th	Day	33% 23% 26% 14% 3 1	270
	11th to 20th	Day	26% 21% 33% 16% 3	270
	21st to end	Day	35% 19% 28% 14% 3 1	297
January	1st to 10th	Night	27% 22% 25% 21% 5%	760
	11th to 20th	Night	23% 21% 28% 22% 7%	760
	21st to end	Night	23% 21% 33% 21% 3	836
February	1st to 10th	Night	20% 18% 30% 26% 6%	760
	11th to 20th	Night	27% 21% 29% 21% 2	743
	21st to end	Night	25% 22% 32% 18% 3	628
March	1st to 10th	Night	23% 17% 31% 25% 4	760
	11th to 20th	Night	19% 17% 32% 23% 7% 1	760
	21st to end	Night	22% 16% 27% 27% 8%	836
April	1st to 10th	Night	17% 18% 29% 27% 9% 1	736
	11th to 20th	Night	17% 15% 25% 27% 14% 1	563
	21st to end	Night	13% 9% 25% 33% 18% 2	570
May	1st to 10th	Night	14% 13% 20% 31% 21% 1	570
	11th to 20th	Night	16% 7% 17% 32% 23% 4	561
	21st to end	Night	16% 2 13% 31% 32% 6%	617
June	1st to 10th	Night	11% 4 14% 34% 32% 5%	570
	11th to 20th	Night	8% 2 15% 29% 33% 13%	570
	21st to end	Night	8% 2 11% 36% 34% 9%	570
July	1st to 10th	Night	11% 2 8% 26% 45% 9%	531
	11th to 20th	Night	10% 2 8% 22% 46% 12%	522
	21st to end	Night	6% 1 10% 24% 47% 12%	591
August	1st to 10th	Night	14% 1 8% 24% 38% 15%	537
	11th to 20th	Night	14% 3 8% 25% 40% 9%	534
	21st to end	Night	18% 2 8% 25% 39% 8%	594
September	1st to 10th	Night	11% 3 13% 25% 38% 9%	702
	11th to 20th	Night	19% 2 14% 28% 33% 4	720
	21st to end	Night	16% 4 13% 35% 29% 3	720
October	1st to 10th	Night	17% 7% 19% 29% 21% 7%	720
	11th to 20th	Night	15% 11% 22% 31% 20% 2	720
	21st to end	Night	17% 12% 20% 33% 16% 3	792
November	1st to 10th	Night	17% 10% 24% 28% 18% 2	740
	11th to 20th	Night	18% 14% 26% 29% 12% 1	760
	21st to end	Night	20% 16% 26% 25% 12% 1	760
December	1st to 10th	Night	25% 18% 24% 26% 6% 1	760
	11th to 20th	Night	21% 15% 28% 27% 7% 1	760
	21st to end	Night	31% 14% 24% 25% 6%	836

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