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 2024-01-17.

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% 6% 2 4 4 3 4 4 5% 7% 52%	74760
	Night	10% 7% 2 5% 4 3 4 4 5% 7% 50%	75076
Transparency	Day	32% 7% 12% 20% 19% 10%	74471
	Night	34% 7% 14% 22% 16% 6%	74887
Seeing	Day	11% 10% 20% 27% 22% 10%	13826
	Night	12% 4 16% 39% 26% 3	23048

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	14% 11% 4 8% 6% 3 6% 5% 6% 11% 26%	1519
	11th to 20th	Day	17% 9% 3 5% 5% 4 4 5% 7% 9% 31%	1575
	21st to end	Day	13% 9% 2 6% 5% 3 4 5% 7% 8% 37%	1782
February	1st to 10th	Day	8% 9% 3 6% 6% 4 5% 6% 8% 11% 34%	1620
	11th to 20th	Day	10% 10% 3 6% 6% 3 5% 5% 6% 8% 37%	1575
	21st to end	Day	11% 13% 5% 7% 6% 3 6% 5% 6% 5% 32%	1466
March	1st to 10th	Day	12% 12% 4 8% 7% 3 5% 5% 6% 7% 29%	1908
	11th to 20th	Day	12% 10% 3 7% 6% 4 5% 6% 6% 8% 33%	1980
	21st to end	Day	13% 9% 4 7% 6% 4 6% 6% 6% 8% 31%	2216
April	1st to 10th	Day	12% 10% 4 8% 8% 4 5% 5% 4 6% 33%	2160
	11th to 20th	Day	10% 8% 4 6% 6% 3 5% 4 6% 8% 40%	2129
	21st to end	Day	6% 7% 2 5% 6% 3 5% 4 5% 8% 49%	2219
May	1st to 10th	Day	7% 5% 2 3 5% 3 4 4 6% 7% 55%	2376
	11th to 20th	Day	9% 7% 2 4 4 2 4 5% 6% 8% 50%	2477
	21st to end	Day	10% 7% 2 4 4 3 4 4 5% 7% 50%	2723
June	1st to 10th	Day	9% 5% 3 3 4 2 4 4 6% 8% 51%	2520
	11th to 20th	Day	4 3 2 3 3 2 3 4 3 6% 67%	2520
	21st to end	Day	4 2 1 2 2 2 3 3 3 5% 74%	2520
July	1st to 10th	Day	2 1 1 1 2 1 2 3 3 4 79%	2520
	11th to 20th	Day	2 1 1 1 2 1 2 2 3 3 82%	2520
	21st to end	Day	2 1 2 2 1 2 3 3 4 81%	2616
August	1st to 10th	Day	2 1 1 2 2 1 3 4 4 5% 75%	2328
	11th to 20th	Day	1 1 1 1 2 1 2 3 3 4 81%	2294
	21st to end	Day	2 1 1 1 1 2 2 2 4 84%	2376
September	1st to 10th	Day	1 1 1 2 1 2 2 3 5% 82%	2160
	11th to 20th	Day	3 3 1 3 3 2 3 3 5% 6% 68%	2160
	21st to end	Day	4 2 1 3 4 2 3 3 5% 7% 67%	1898
October	1st to 10th	Day	6% 4 2 3 4 2 4 4 4 6% 62%	1800
	11th to 20th	Day	7% 5% 2 4 4 2 4 4 6% 7% 55%	1800
	21st to end	Day	9% 6% 2 6% 5% 3 4 5% 5% 7% 49%	1980
November	1st to 10th	Day	13% 9% 4 6% 6% 3 5% 4 5% 9% 38%	1707
	11th to 20th	Day	13% 7% 3 7% 6% 4 6% 5% 7% 11% 32%	1554
	21st to end	Day	11% 11% 3 6% 5% 4 5% 6% 7% 8% 35%	1520
December	1st to 10th	Day	12% 9% 3 5% 5% 4 5% 6% 7% 7% 38%	1520
	11th to 20th	Day	18% 11% 3 7% 7% 3 6% 5% 6% 9% 24%	1928
	21st to end	Day	17% 12% 4 9% 8% 4 6% 6% 5% 6% 21%	2794
January	1st to 10th	Night	14% 9% 3 8% 6% 5% 7% 6% 7% 8% 28%	2646
	11th to 20th	Night	12% 11% 4 6% 5% 3 4 5% 5% 7% 36%	2522
	21st to end	Night	13% 10% 3 6% 5% 3 4 4 5% 8% 39%	2574
February	1st to 10th	Night	12% 9% 3 5% 5% 4 5% 4 5% 8% 40%	2340
	11th to 20th	Night	11% 11% 3 5% 6% 3 4 5% 5% 6% 40%	2272
	21st to end	Night	10% 11% 4 6% 5% 3 6% 5% 6% 8% 38%	1790
March	1st to 10th	Night	14% 11% 5% 7% 6% 4 5% 5% 6% 7% 32%	2048
	11th to 20th	Night	12% 10% 4 5% 5% 4 5% 4 5% 9% 36%	1980
	21st to end	Night	11% 10% 4 6% 6% 4 6% 6% 6% 8% 33%	2139
April	1st to 10th	Night	12% 11% 4 6% 6% 3 5% 5% 5% 8% 36%	1800
	11th to 20th	Night	10% 8% 3 5% 4 3 4 4 5% 7% 47%	1779
	21st to end	Night	12% 7% 2 4 3 1 3 4 5% 10% 49%	1738
May	1st to 10th	Night	8% 5% 2 4 3 3 4 3 5% 6% 57%	1582
	11th to 20th	Night	7% 5% 1 5% 2 3 4 6% 7% 7% 52%	1416
	21st to end	Night	10% 7% 2 4 4 2 4 4 5% 7% 52%	1552
June	1st to 10th	Night	10% 4 2 4 4 3 5% 5% 6% 6% 52%	1440
	11th to 20th	Night	5% 3 1 3 3 2 3 3 4 5% 68%	1440
	21st to end	Night	3 3 1 2 2 2 2 2 3 4 77%	1440
July	1st to 10th	Night	1 1 1 1 1 3 3 4 5% 79%	1440
	11th to 20th	Night	3 1 1 2 2 1 2 3 3 3 81%	1440
	21st to end	Night	1 1 1 1 1 2 2 2 3 86%	1699
August	1st to 10th	Night	2 2 1 2 2 1 2 2 4 5% 77%	1611
	11th to 20th	Night	1 1 2 2 1 2 2 2 4 83%	1604
	21st to end	Night	2 2 1 1 1 1 2 3 3 83%	1980
September	1st to 10th	Night	1 1 1 1 1 2 3 5% 85%	1800
	11th to 20th	Night	4 2 1 2 3 1 2 3 4 7% 72%	1800
	21st to end	Night	5% 2 1 2 3 2 3 3 4 6% 69%	2058
October	1st to 10th	Night	4 3 1 3 4 2 3 3 4 6% 67%	2160
	11th to 20th	Night	7% 4 1 3 4 2 3 4 4 7% 60%	2160
	21st to end	Night	8% 5% 1 4 4 2 3 4 5% 7% 55%	2376
November	1st to 10th	Night	13% 9% 3 6% 5% 3 5% 5% 5% 7% 39%	2370
	11th to 20th	Night	17% 6% 2 6% 7% 4 5% 5% 6% 7% 35%	2622
	21st to end	Night	12% 8% 3 5% 5% 3 4 5% 5% 7% 43%	2660
December	1st to 10th	Night	13% 8% 3 6% 5% 3 5% 5% 5% 8% 40%	2660
	11th to 20th	Night	18% 12% 3 7% 6% 4 5% 5% 6% 7% 26%	3276
	21st to end	Night	16% 12% 4 7% 7% 4 5% 4 5% 6% 31%	4862

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% 11% 21% 11% 4 1	1519
	11th to 20th	Day	47% 9% 14% 19% 7% 3	1575
	21st to end	Day	43% 9% 13% 23% 9% 3	1782
February	1st to 10th	Day	42% 11% 18% 19% 9% 1	1620
	11th to 20th	Day	43% 9% 16% 22% 8% 2	1584
	21st to end	Day	50% 9% 13% 17% 9% 2	1466
March	1st to 10th	Day	51% 10% 15% 16% 7% 1	1908
	11th to 20th	Day	46% 8% 15% 18% 11% 1	1980
	21st to end	Day	48% 10% 18% 15% 7% 2	2216
April	1st to 10th	Day	52% 8% 11% 17% 10% 2	2160
	11th to 20th	Day	42% 7% 16% 19% 11% 5%	2132
	21st to end	Day	35% 7% 17% 24% 15% 3	2219
May	1st to 10th	Day	29% 6% 15% 27% 20% 3	2376
	11th to 20th	Day	32% 7% 14% 26% 17% 4	2480
	21st to end	Day	34% 7% 17% 28% 13% 2	2724
June	1st to 10th	Day	30% 7% 15% 27% 17% 3	2520
	11th to 20th	Day	19% 6% 11% 32% 25% 7%	2520
	21st to end	Day	14% 3 8% 28% 30% 17%	2520
July	1st to 10th	Day	11% 3 6% 22% 30% 27%	2451
	11th to 20th	Day	9% 3 6% 18% 31% 31%	2445
	21st to end	Day	10% 2 5% 15% 35% 33%	2471
August	1st to 10th	Day	12% 5% 7% 20% 34% 22%	2312
	11th to 20th	Day	9% 2 8% 19% 38% 26%	2294
	21st to end	Day	8% 2 5% 22% 39% 25%	2376
September	1st to 10th	Day	8% 3 5% 17% 32% 36%	2160
	11th to 20th	Day	18% 5% 9% 21% 32% 15%	2160
	21st to end	Day	18% 4 10% 25% 26% 17%	1898
October	1st to 10th	Day	24% 6% 13% 19% 25% 13%	1800
	11th to 20th	Day	27% 7% 14% 23% 21% 8%	1800
	21st to end	Day	34% 9% 12% 18% 19% 7%	1980
November	1st to 10th	Day	45% 6% 14% 20% 13% 2	1707
	11th to 20th	Day	46% 10% 18% 14% 11% 1	1554
	21st to end	Day	45% 11% 13% 15% 13% 3	1520
December	1st to 10th	Day	44% 9% 14% 18% 12% 2	1520
	11th to 20th	Day	55% 13% 16% 12% 4	1928
	21st to end	Day	61% 12% 12% 11% 4	2794
January	1st to 10th	Night	51% 11% 19% 12% 6% 1	2646
	11th to 20th	Night	46% 10% 14% 16% 10% 3	2522
	21st to end	Night	44% 6% 12% 21% 14% 2	2574
February	1st to 10th	Night	42% 8% 16% 21% 11% 2	2340
	11th to 20th	Night	44% 7% 15% 23% 10% 2	2274
	21st to end	Night	42% 7% 20% 18% 10% 3	1790
March	1st to 10th	Night	51% 8% 15% 15% 10% 1	2048
	11th to 20th	Night	45% 8% 15% 18% 13% 1	1980
	21st to end	Night	47% 11% 18% 17% 6% 1	2139
April	1st to 10th	Night	45% 8% 18% 19% 8% 2	1800
	11th to 20th	Night	37% 7% 21% 19% 11% 5%	1779
	21st to end	Night	33% 7% 21% 27% 9% 2	1738
May	1st to 10th	Night	29% 5% 18% 31% 15% 2	1582
	11th to 20th	Night	28% 8% 20% 31% 10% 3	1416
	21st to end	Night	31% 9% 24% 26% 9%	1552
June	1st to 10th	Night	31% 8% 18% 33% 9% 1	1440
	11th to 20th	Night	18% 7% 14% 39% 20% 2	1440
	21st to end	Night	13% 3 9% 40% 27% 7%	1440
July	1st to 10th	Night	9% 5% 9% 30% 28% 20%	1400
	11th to 20th	Night	11% 3 4 23% 37% 22%	1392
	21st to end	Night	7% 2 4 20% 45% 22%	1605
August	1st to 10th	Night	11% 4 6% 33% 32% 14%	1602
	11th to 20th	Night	9% 2 7% 26% 39% 17%	1604
	21st to end	Night	8% 3 7% 27% 37% 19%	1980
September	1st to 10th	Night	6% 2 6% 22% 34% 32%	1800
	11th to 20th	Night	14% 5% 10% 26% 31% 14%	1800
	21st to end	Night	19% 4 12% 24% 21% 20%	2058
October	1st to 10th	Night	20% 4 14% 20% 27% 14%	2160
	11th to 20th	Night	24% 7% 13% 25% 23% 10%	2160
	21st to end	Night	28% 8% 16% 20% 18% 11%	2376
November	1st to 10th	Night	43% 8% 13% 17% 14% 4	2370
	11th to 20th	Night	47% 8% 16% 16% 12% 1	2622
	21st to end	Night	41% 9% 14% 18% 14% 4	2660
December	1st to 10th	Night	41% 9% 13% 22% 14% 1	2660
	11th to 20th	Night	54% 9% 12% 19% 5%	3276
	21st to end	Night	54% 12% 12% 16% 5% 1	4862

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	23% 11% 15% 34% 14% 3	180
	11th to 20th	Day	24% 11% 14% 28% 15% 10%	259
	21st to end	Day	18% 7% 18% 25% 20% 10%	374
February	1st to 10th	Day	15% 12% 15% 29% 23% 6%	340
	11th to 20th	Day	18% 14% 20% 24% 19% 5%	335
	21st to end	Day	16% 12% 24% 30% 14% 4	340
March	1st to 10th	Day	21% 14% 23% 26% 11% 5%	420
	11th to 20th	Day	17% 13% 20% 27% 18% 5%	420
	21st to end	Day	19% 16% 23% 24% 15% 2	462
April	1st to 10th	Day	18% 18% 20% 25% 14% 4	420
	11th to 20th	Day	18% 16% 21% 23% 16% 6%	412
	21st to end	Day	11% 15% 26% 23% 17% 6%	420
May	1st to 10th	Day	11% 18% 24% 25% 17% 6%	423
	11th to 20th	Day	19% 15% 21% 20% 17% 9%	448
	21st to end	Day	13% 12% 21% 23% 22% 8%	485
June	1st to 10th	Day	10% 13% 21% 25% 22% 9%	450
	11th to 20th	Day	6% 18% 26% 25% 19% 6%	450
	21st to end	Day	2 9% 23% 28% 24% 14%	450
July	1st to 10th	Day	4 7% 24% 31% 24% 11%	446
	11th to 20th	Day	2 6% 24% 29% 26% 14%	444
	21st to end	Day	1 5% 21% 29% 30% 15%	487
August	1st to 10th	Day	2 4 22% 26% 27% 19%	447
	11th to 20th	Day	1 7% 19% 24% 28% 21%	437
	21st to end	Day	3 5% 18% 27% 28% 19%	495
September	1st to 10th	Day	2 4 17% 28% 34% 14%	450
	11th to 20th	Day	5% 7% 18% 26% 31% 13%	450
	21st to end	Day	6% 7% 20% 28% 29% 10%	450
October	1st to 10th	Day	9% 8% 18% 24% 26% 15%	450
	11th to 20th	Day	11% 8% 14% 24% 30% 13%	450
	21st to end	Day	13% 8% 16% 29% 22% 13%	495
November	1st to 10th	Day	15% 11% 13% 30% 23% 8%	303
	11th to 20th	Day	10% 6% 11% 37% 27% 9%	196
	21st to end	Day	13% 6% 16% 31% 27% 7%	180
December	1st to 10th	Day	17% 8% 12% 37% 16% 11%	180
	11th to 20th	Day	19% 5% 16% 33% 20% 7%	180
	21st to end	Day	19% 7% 18% 26% 28% 2	198
January	1st to 10th	Night	17% 6% 19% 39% 17% 2	760
	11th to 20th	Night	17% 6% 18% 35% 21% 3	746
	21st to end	Night	18% 4 16% 37% 21% 4	792
February	1st to 10th	Night	17% 5% 17% 42% 17% 2	720
	11th to 20th	Night	15% 6% 17% 42% 17% 2	704
	21st to end	Night	14% 7% 20% 44% 15% 1	592
March	1st to 10th	Night	18% 10% 20% 35% 15% 2	720
	11th to 20th	Night	15% 8% 18% 36% 20% 3	720
	21st to end	Night	15% 8% 23% 35% 17% 1	792
April	1st to 10th	Night	16% 8% 19% 39% 16% 2	720
	11th to 20th	Night	13% 7% 24% 35% 18% 3	711
	21st to end	Night	16% 5% 18% 37% 22% 2	658
May	1st to 10th	Night	10% 4 16% 42% 25% 2	502
	11th to 20th	Night	10% 3 17% 39% 30% 2	354
	21st to end	Night	12% 4 17% 36% 29% 2	389
June	1st to 10th	Night	11% 3 15% 42% 28% 2	360
	11th to 20th	Night	6% 3 17% 49% 24% 1	360
	21st to end	Night	3 1 12% 46% 37% 1	360
July	1st to 10th	Night	1 9% 41% 47% 1	350
	11th to 20th	Night	2 1 10% 48% 38% 1	348
	21st to end	Night	6% 36% 53% 4	517
August	1st to 10th	Night	3 4 39% 49% 6%	537
	11th to 20th	Night	2 3 40% 48% 8%	536
	21st to end	Night	3 1 14% 40% 39% 4	792
September	1st to 10th	Night	2 1 14% 40% 39% 4	720
	11th to 20th	Night	5% 1 10% 40% 38% 6%	720
	21st to end	Night	5% 3 12% 39% 34% 7%	720
October	1st to 10th	Night	5% 2 16% 43% 32% 3	720
	11th to 20th	Night	9% 3 12% 40% 33% 4	720
	21st to end	Night	10% 4 13% 42% 24% 7%	792
November	1st to 10th	Night	14% 4 15% 41% 20% 4	740
	11th to 20th	Night	17% 5% 16% 42% 17% 3	760
	21st to end	Night	14% 4 17% 40% 23% 2	760
December	1st to 10th	Night	16% 5% 16% 38% 21% 4	760
	11th to 20th	Night	21% 6% 16% 36% 19% 3	760
	21st to end	Night	23% 7% 21% 37% 11% 1	836

Page generated at 2024-01-13 20:56:39. Version ac07

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