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 2026-04-17.

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	7% 6% 2 5% 3 3 4 3 5% 6% 56%	140668
	Night	9% 8% 2 5% 4 3 4 3 4 6% 52%	138868
Transparency	Day	29% 6% 11% 22% 20% 12%	135509
	Night	33% 7% 12% 23% 17% 7%	135021
Seeing	Day	11% 10% 19% 26% 23% 10%	16723
	Night	12% 5% 16% 39% 25% 3	25944

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	15% 14% 4 10% 7% 5% 6% 4 5% 7% 23%	4722
	11th to 20th	Day	14% 8% 2 4 3 3 2 2 4 6% 53%	5022
	21st to end	Day	12% 7% 2 7% 4 4 4 4 5% 7% 44%	5843
February	1st to 10th	Day	9% 11% 3 6% 5% 4 5% 5% 8% 10% 34%	1890
	11th to 20th	Day	10% 12% 3 8% 6% 5% 4 4 6% 7% 34%	1845
	21st to end	Day	11% 13% 4 8% 5% 5% 6% 3 6% 6% 34%	1714
March	1st to 10th	Day	11% 15% 4 9% 6% 4 4 3 5% 7% 32%	4900
	11th to 20th	Day	9% 12% 3 7% 5% 5% 4 3 5% 6% 40%	5110
	21st to end	Day	11% 14% 3 8% 6% 6% 6% 4 6% 8% 29%	5711
April	1st to 10th	Day	11% 10% 3 8% 6% 5% 5% 5% 5% 7% 35%	2520
	11th to 20th	Day	10% 9% 3 6% 5% 4 4 3 6% 8% 42%	2442
	21st to end	Day	7% 7% 2 7% 5% 4 5% 3 5% 8% 48%	2467
May	1st to 10th	Day	6% 5% 1 4 3 3 3 3 5% 5% 61%	5968
	11th to 20th	Day	4 4 1 3 3 3 4 3 5% 7% 63%	6277
	21st to end	Day	4 3 1 3 2 2 3 3 4 7% 68%	6902
June	1st to 10th	Day	8% 6% 2 4 3 3 4 4 6% 8% 53%	2800
	11th to 20th	Day	4 4 1 3 3 3 3 3 4 6% 67%	2800
	21st to end	Day	3 2 1 2 1 2 2 2 3 5% 75%	2800
July	1st to 10th	Day	1 1 1 1 2 2 2 2 3 3 83%	6319
	11th to 20th	Day	1 1 1 1 2 2 1 3 3 84%	6320
	21st to end	Day	1 1 2 1 2 2 2 3 5% 80%	6563
August	1st to 10th	Day	2 1 1 2 2 2 3 2 4 5% 76%	5777
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 4 80%	5779
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	5984
September	1st to 10th	Day	1 1 1 1 2 1 2 2 3 5% 80%	2400
	11th to 20th	Day	3 3 2 3 3 3 3 3 5% 6% 67%	2400
	21st to end	Day	4 2 1 4 3 3 3 2 5% 6% 68%	2107
October	1st to 10th	Day	3 3 1 2 2 2 3 2 4 5% 73%	3280
	11th to 20th	Day	6% 5% 2 4 3 3 3 4 6% 9% 56%	3261
	21st to end	Day	9% 8% 3 6% 4 4 4 3 5% 8% 44%	3558
November	1st to 10th	Day	13% 9% 3 6% 5% 4 4 3 5% 9% 38%	1890
	11th to 20th	Day	12% 8% 2 8% 6% 5% 6% 4 7% 10% 30%	1717
	21st to end	Day	9% 11% 3 6% 5% 5% 5% 4 7% 8% 37%	1680
December	1st to 10th	Day	8% 6% 2 6% 4 4 4 4 6% 8% 49%	2700
	11th to 20th	Day	17% 11% 3 9% 7% 6% 6% 4 6% 8% 22%	3108
	21st to end	Day	19% 13% 4 11% 7% 6% 6% 5% 5% 6% 17%	4092
January	1st to 10th	Night	16% 16% 4 9% 6% 5% 5% 4 5% 7% 24%	8172
	11th to 20th	Night	11% 9% 3 5% 2 2 2 2 3 5% 55%	7939
	21st to end	Night	13% 9% 3 8% 5% 5% 4 3 5% 6% 41%	8371
February	1st to 10th	Night	12% 10% 3 7% 4 5% 5% 3 5% 7% 38%	2730
	11th to 20th	Night	11% 12% 3 7% 6% 5% 4 4 5% 6% 36%	2662
	21st to end	Night	10% 12% 3 6% 4 4 6% 4 5% 9% 38%	2091
March	1st to 10th	Night	12% 14% 4 8% 6% 4 4 3 6% 8% 31%	5210
	11th to 20th	Night	10% 11% 2 5% 4 4 4 3 5% 8% 43%	5110
	21st to end	Night	11% 14% 3 9% 6% 5% 6% 4 6% 8% 28%	5505
April	1st to 10th	Night	11% 11% 3 6% 4 4 4 4 6% 8% 37%	2100
	11th to 20th	Night	10% 9% 2 5% 4 3 4 3 5% 7% 48%	2039
	21st to end	Night	12% 7% 2 5% 2 2 3 3 6% 10% 48%	1930
May	1st to 10th	Night	8% 7% 1 3 2 3 2 2 4 5% 64%	3992
	11th to 20th	Night	5% 6% 2 4 2 3 3 3 5% 5% 62%	3616
	21st to end	Night	4 3 1 2 2 2 2 2 3 5% 73%	3972
June	1st to 10th	Night	9% 5% 2 5% 3 4 4 4 6% 6% 54%	1600
	11th to 20th	Night	5% 3 1 4 2 2 3 2 4 5% 69%	1600
	21st to end	Night	3 2 1 2 2 2 2 2 3 4 78%	1600
July	1st to 10th	Night	1 1 2 2 2 3 2 3 3 82%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 2 2 85%	3640
	21st to end	Night	1 1 2 1 2 2 2 3 5% 80%	4286
August	1st to 10th	Night	1 2 1 2 1 2 2 2 3 5% 79%	4035
	11th to 20th	Night	1 1 1 3 2 2 2 1 2 3 83%	4068
	21st to end	Night	5% 2 1 2 2 2 2 2 3 4 75%	5015
September	1st to 10th	Night	1 1 1 1 2 1 1 2 3 5% 82%	2000
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 6% 70%	2000
	21st to end	Night	5% 2 1 3 2 3 3 2 3 6% 70%	2288
October	1st to 10th	Night	3 2 1 2 3 2 3 2 3 4 76%	3920
	11th to 20th	Night	8% 4 1 3 3 3 3 3 4 6% 62%	3897
	21st to end	Night	6% 5% 1 6% 4 4 5% 4 7% 9% 48%	4253
November	1st to 10th	Night	14% 9% 2 7% 4 4 5% 3 5% 8% 39%	2627
	11th to 20th	Night	16% 7% 2 8% 5% 5% 5% 4 6% 7% 33%	2898
	21st to end	Night	10% 9% 3 6% 4 4 4 4 5% 7% 44%	2940
December	1st to 10th	Night	9% 5% 1 5% 3 3 4 3 5% 7% 54%	4700
	11th to 20th	Night	17% 13% 3 9% 6% 6% 6% 4 7% 7% 23%	5316
	21st to end	Night	19% 13% 4 9% 6% 5% 4 3 4 6% 26%	7106

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	58% 9% 17% 14% 2	4723
	11th to 20th	Day	34% 6% 8% 26% 21% 5%	5022
	21st to end	Day	38% 6% 12% 19% 18% 7%	5843
February	1st to 10th	Day	43% 10% 17% 18% 10% 1	1890
	11th to 20th	Day	46% 9% 15% 21% 8% 1	1854
	21st to end	Day	48% 8% 14% 18% 9% 2	1714
March	1st to 10th	Day	48% 11% 11% 20% 8% 1	4900
	11th to 20th	Day	42% 9% 15% 16% 17% 2	5110
	21st to end	Day	49% 9% 16% 19% 6% 1	5711
April	1st to 10th	Day	48% 8% 13% 19% 11% 2	2520
	11th to 20th	Day	40% 7% 15% 20% 13% 5%	2445
	21st to end	Day	35% 7% 16% 24% 15% 3	2467
May	1st to 10th	Day	24% 5% 10% 26% 27% 8%	5968
	11th to 20th	Day	20% 5% 14% 27% 23% 10%	6280
	21st to end	Day	17% 6% 14% 42% 21% 1	6904
June	1st to 10th	Day	28% 7% 15% 28% 19% 3	2800
	11th to 20th	Day	18% 5% 11% 31% 27% 7%	2800
	21st to end	Day	14% 3 8% 27% 31% 17%	2800
July	1st to 10th	Day	8% 2 4 18% 27% 40%	6251
	11th to 20th	Day	8% 2 4 12% 26% 48%	6245
	21st to end	Day	9% 2 5% 24% 35% 25%	6374
August	1st to 10th	Day	13% 3 5% 14% 31% 34%	4206
	11th to 20th	Day	9% 2 7% 25% 35% 21%	4178
	21st to end	Day	11% 2 5% 25% 37% 20%	4311
September	1st to 10th	Day	8% 3 5% 18% 31% 35%	2400
	11th to 20th	Day	18% 5% 9% 22% 31% 14%	2400
	21st to end	Day	18% 4 9% 25% 27% 18%	2107
October	1st to 10th	Day	16% 4 11% 15% 29% 25%	3280
	11th to 20th	Day	24% 5% 16% 32% 19% 4	3261
	21st to end	Day	36% 8% 11% 23% 18% 4	3558
November	1st to 10th	Day	44% 7% 15% 21% 12% 2	1890
	11th to 20th	Day	47% 10% 19% 13% 10% 1	1717
	21st to end	Day	42% 11% 14% 17% 13% 2	1680
December	1st to 10th	Day	32% 8% 13% 28% 18% 1	2700
	11th to 20th	Day	57% 12% 16% 11% 4	3108
	21st to end	Day	64% 12% 12% 9% 3	4092
January	1st to 10th	Night	58% 10% 16% 12% 4	8172
	11th to 20th	Night	34% 7% 8% 23% 25% 3	7939
	21st to end	Night	45% 5% 9% 17% 16% 8%	8371
February	1st to 10th	Night	43% 8% 15% 21% 12% 2	2730
	11th to 20th	Night	46% 8% 16% 20% 9% 1	2664
	21st to end	Night	42% 7% 20% 18% 11% 2	2091
March	1st to 10th	Night	49% 8% 16% 18% 8% 1	5210
	11th to 20th	Night	38% 8% 16% 19% 18% 1	5110
	21st to end	Night	50% 11% 16% 17% 6% 1	5505
April	1st to 10th	Night	43% 9% 19% 20% 8% 2	2100
	11th to 20th	Night	35% 7% 21% 19% 13% 5%	2039
	21st to end	Night	33% 8% 21% 27% 9% 2	1930
May	1st to 10th	Night	25% 5% 10% 29% 28% 3	3989
	11th to 20th	Night	22% 6% 14% 37% 17% 4	3616
	21st to end	Night	16% 9% 20% 46% 10%	3972
June	1st to 10th	Night	29% 8% 18% 34% 10% 2	1600
	11th to 20th	Night	17% 6% 13% 39% 23% 2	1600
	21st to end	Night	13% 3 9% 39% 27% 8%	1600
July	1st to 10th	Night	9% 4 5% 26% 21% 35%	3600
	11th to 20th	Night	9% 1 3 14% 38% 36%	3592
	21st to end	Night	9% 2 6% 33% 31% 19%	4164
August	1st to 10th	Night	12% 2 5% 22% 41% 19%	2929
	11th to 20th	Night	8% 2 6% 38% 31% 15%	2945
	21st to end	Night	14% 2 6% 33% 30% 14%	3608
September	1st to 10th	Night	7% 2 6% 21% 34% 31%	2000
	11th to 20th	Night	16% 4 11% 26% 30% 13%	2000
	21st to end	Night	18% 4 12% 25% 22% 20%	2288
October	1st to 10th	Night	15% 3 9% 17% 32% 24%	3920
	11th to 20th	Night	24% 5% 13% 38% 15% 5%	3897
	21st to end	Night	29% 7% 15% 27% 16% 6%	4253
November	1st to 10th	Night	43% 8% 15% 17% 13% 4	2627
	11th to 20th	Night	48% 8% 16% 16% 11% 1	2898
	21st to end	Night	39% 9% 15% 20% 14% 3	2940
December	1st to 10th	Night	30% 6% 13% 31% 20% 1	4700
	11th to 20th	Night	57% 9% 14% 16% 4	5316
	21st to end	Night	56% 13% 13% 15% 4	7106

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	24% 12% 15% 33% 14% 3	220
	11th to 20th	Day	21% 9% 12% 28% 18% 12%	320
	21st to end	Day	17% 8% 17% 27% 21% 10%	473
February	1st to 10th	Day	17% 12% 14% 30% 21% 6%	430
	11th to 20th	Day	18% 15% 20% 24% 18% 4	425
	21st to end	Day	18% 11% 22% 29% 16% 5%	438
March	1st to 10th	Day	21% 15% 22% 25% 14% 4	540
	11th to 20th	Day	17% 13% 20% 27% 19% 5%	540
	21st to end	Day	19% 17% 23% 25% 15% 2	594
April	1st to 10th	Day	16% 16% 19% 26% 16% 6%	540
	11th to 20th	Day	16% 15% 22% 24% 16% 6%	517
	21st to end	Day	11% 15% 26% 23% 19% 7%	500
May	1st to 10th	Day	11% 18% 24% 25% 17% 6%	503
	11th to 20th	Day	17% 14% 22% 22% 16% 9%	528
	21st to end	Day	12% 11% 21% 24% 23% 9%	573
June	1st to 10th	Day	9% 13% 21% 24% 23% 10%	530
	11th to 20th	Day	6% 17% 25% 25% 20% 6%	530
	21st to end	Day	2 9% 22% 27% 25% 15%	530
July	1st to 10th	Day	4 6% 22% 30% 25% 13%	526
	11th to 20th	Day	2 6% 22% 27% 28% 15%	524
	21st to end	Day	1 4 19% 27% 31% 17%	575
August	1st to 10th	Day	2 4 20% 25% 29% 20%	527
	11th to 20th	Day	2 7% 19% 24% 28% 21%	517
	21st to end	Day	3 5% 17% 25% 29% 21%	583
September	1st to 10th	Day	2 4 17% 28% 34% 15%	530
	11th to 20th	Day	5% 7% 18% 26% 31% 14%	530
	21st to end	Day	5% 7% 18% 27% 31% 12%	530
October	1st to 10th	Day	8% 9% 17% 25% 26% 15%	530
	11th to 20th	Day	11% 8% 14% 24% 30% 13%	530
	21st to end	Day	13% 8% 15% 30% 22% 12%	583
November	1st to 10th	Day	15% 11% 13% 30% 22% 9%	366
	11th to 20th	Day	13% 6% 13% 36% 25% 7%	239
	21st to end	Day	11% 7% 16% 29% 27% 10%	220
December	1st to 10th	Day	14% 7% 10% 38% 20% 11%	220
	11th to 20th	Day	20% 7% 14% 32% 20% 8%	220
	21st to end	Day	20% 8% 20% 24% 24% 3	242
January	1st to 10th	Night	18% 6% 20% 39% 16% 2	840
	11th to 20th	Night	15% 6% 17% 36% 23% 3	840
	21st to end	Night	18% 4 17% 37% 21% 3	924
February	1st to 10th	Night	18% 5% 18% 41% 16% 2	840
	11th to 20th	Night	15% 7% 19% 42% 15% 2	824
	21st to end	Night	14% 8% 21% 43% 14% 1	692
March	1st to 10th	Night	17% 10% 21% 36% 15% 1	840
	11th to 20th	Night	14% 7% 21% 36% 19% 2	840
	21st to end	Night	15% 9% 24% 36% 15% 1	924
April	1st to 10th	Night	15% 8% 19% 40% 17% 2	840
	11th to 20th	Night	13% 7% 25% 35% 18% 2	815
	21st to end	Night	15% 5% 18% 37% 22% 2	730
May	1st to 10th	Night	10% 4 18% 41% 25% 2	557
	11th to 20th	Night	9% 4 18% 39% 28% 2	394
	21st to end	Night	11% 4 19% 36% 28% 1	433
June	1st to 10th	Night	10% 3 15% 43% 28% 2	400
	11th to 20th	Night	6% 3 18% 50% 23% 1	400
	21st to end	Night	3 1 12% 48% 35% 1	400
July	1st to 10th	Night	2 9% 42% 46% 1	390
	11th to 20th	Night	2 1 10% 48% 38% 2	388
	21st to end	Night	1 6% 37% 52% 5%	575
August	1st to 10th	Night	3 4 40% 48% 6%	597
	11th to 20th	Night	2 1 3 41% 46% 8%	597
	21st to end	Night	3 1 13% 39% 39% 4	880
September	1st to 10th	Night	2 1 14% 41% 38% 4	800
	11th to 20th	Night	5% 2 11% 39% 37% 6%	800
	21st to end	Night	5% 3 13% 39% 35% 6%	800
October	1st to 10th	Night	6% 2 15% 43% 32% 3	800
	11th to 20th	Night	10% 3 12% 40% 32% 4	800
	21st to end	Night	9% 4 13% 43% 24% 6%	880
November	1st to 10th	Night	15% 5% 15% 40% 20% 4	820
	11th to 20th	Night	17% 5% 18% 40% 17% 3	840
	21st to end	Night	13% 4 18% 40% 23% 2	840
December	1st to 10th	Night	15% 5% 16% 38% 22% 4	840
	11th to 20th	Night	21% 6% 16% 35% 19% 3	840
	21st to end	Night	23% 8% 21% 36% 11% 1	924

Page generated at 2026-04-13 23:44:27. Version ac07

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