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 2024-11-01.

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% 51%	101286
	Night	2 4 2 4 3 3 3 4 6% 8% 61%	97907
Transparency	Day	26% 9% 12% 22% 22% 9%	101036
	Night	21% 7% 15% 27% 24% 8%	97747
Seeing	Day	12% 25% 26% 23% 12% 3	15489
	Night	6% 20% 33% 26% 13% 2	25515

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	3 2 5% 3 2 4 4 6% 9% 63%	2538
	11th to 20th	Day	2 1 4 3 3 7% 5% 8% 10% 57%	2773
	21st to end	Day	5% 4 8% 4 4 4 4 6% 8% 54%	3146
February	1st to 10th	Day	3 3 6% 4 4 5% 4 6% 8% 57%	1710
	11th to 20th	Day	3 1 5% 4 4 4 3 7% 8% 61%	1782
	21st to end	Day	2 3 7% 6% 5% 5% 4 5% 6% 57%	1570
March	1st to 10th	Day	5% 4 8% 4 4 5% 4 5% 6% 55%	3216
	11th to 20th	Day	1 11% 6% 10% 4 3 4 3 6% 7% 46%	3490
	21st to end	Day	1 6% 3 6% 4 5% 6% 6% 8% 9% 46%	3977
April	1st to 10th	Day	1 6% 2 5% 4 4 4 4 7% 8% 54%	2280
	11th to 20th	Day	2 9% 3 8% 5% 4 5% 4 6% 8% 44%	2251
	21st to end	Day	3 8% 2 7% 4 5% 5% 5% 7% 9% 45%	2280
May	1st to 10th	Day	4 11% 3 7% 5% 5% 5% 4 7% 8% 41%	3932
	11th to 20th	Day	5% 10% 3 6% 5% 5% 6% 5% 7% 7% 42%	4388
	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	4 5% 2 5% 5% 3 4 3 5% 6% 57%	1900
	11th to 20th	Day	2 4 1 3 2 3 3 3 5% 7% 66%	1900
	21st to end	Day	2 5% 2 4 3 3 4 2 4 6% 64%	2090
November	1st to 10th	Day	1 3 1 3 2 2 2 3 5% 6% 72%	1680
	11th to 20th	Day	2 1 2 2 2 2 2 4 8% 74%	1710
	21st to end	Day	1 3 2 4 3 2 3 3 6% 7% 66%	1573
December	1st to 10th	Day	2 2 5% 4 3 2 4 6% 9% 64%	1520
	11th to 20th	Day	1 3 1 4 4 3 4 4 6% 12% 58%	1926
	21st to end	Day	3 2 7% 3 3 4 3 7% 10% 58%	2794
January	1st to 10th	Night	2 2 7% 4 4 3 3 5% 8% 61%	4417
	11th to 20th	Night	2 3 3 3 5% 4 6% 7% 68%	4224
	21st to end	Night	6% 3 6% 3 3 3 3 4 6% 63%	4521
February	1st to 10th	Night	2 2 5% 3 4 3 3 5% 7% 67%	2470
	11th to 20th	Night	3 2 5% 3 3 2 4 5% 7% 67%	2283
	21st to end	Night	3 3 4 4 2 3 3 5% 8% 66%	1884
March	1st to 10th	Night	5% 3 5% 3 4 3 4 5% 7% 63%	3646
	11th to 20th	Night	1 11% 6% 6% 3 3 2 3 4 7% 55%	3490
	21st to end	Night	1 5% 2 5% 4 4 4 5% 7% 9% 55%	3669
April	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 8% 67%	1900
	11th to 20th	Night	3 8% 3 5% 3 3 4 4 6% 7% 56%	1877
	21st to end	Night	4 8% 2 4 3 3 3 3 4 7% 59%	1900
May	1st to 10th	Night	4 10% 2 4 3 3 3 4 6% 8% 54%	3035
	11th to 20th	Night	5% 12% 3 5% 3 4 4 4 6% 7% 47%	2517
	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	4 5% 1 3 2 3 3 3 4 6% 64%	2280
	11th to 20th	Night	1 3 1 2 2 2 2 3 4 6% 75%	2280
	21st to end	Night	2 5% 2 4 3 2 2 2 5% 6% 68%	2508
November	1st to 10th	Night	3 1 2 2 2 2 3 4 6% 75%	2383
	11th to 20th	Night	1 1 1 2 2 2 2 5% 7% 76%	2470
	21st to end	Night	1 4 2 4 2 2 2 2 6% 8% 67%	2603
December	1st to 10th	Night	2 1 3 3 2 2 3 4 8% 71%	2660
	11th to 20th	Night	1 4 1 3 3 3 3 3 6% 11% 61%	3271
	21st to end	Night	3 2 6% 4 3 4 4 5% 8% 62%	4873

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	16% 6% 19% 27% 24% 8%	2539
	11th to 20th	Day	16% 7% 23% 26% 25% 4	2773
	21st to end	Day	24% 10% 11% 30% 20% 6%	3146
February	1st to 10th	Day	21% 9% 16% 27% 19% 8%	1710
	11th to 20th	Day	18% 8% 12% 26% 27% 9%	1791
	21st to end	Day	23% 8% 11% 20% 24% 13%	1570
March	1st to 10th	Day	25% 7% 9% 22% 31% 7%	3216
	11th to 20th	Day	31% 10% 10% 32% 14% 3	3490
	21st to end	Day	24% 13% 12% 31% 19% 3	3977
April	1st to 10th	Day	24% 8% 13% 24% 22% 9%	2280
	11th to 20th	Day	33% 8% 12% 22% 21% 4	2254
	21st to end	Day	30% 9% 11% 28% 19% 2	2280
May	1st to 10th	Day	33% 8% 12% 19% 21% 7%	3932
	11th to 20th	Day	34% 12% 13% 22% 16% 2	4392
	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	24% 8% 8% 21% 29% 11%	1900
	11th to 20th	Day	18% 4 6% 25% 36% 11%	1900
	21st to end	Day	20% 5% 8% 30% 28% 10%	2090
November	1st to 10th	Day	12% 5% 12% 23% 36% 11%	1680
	11th to 20th	Day	11% 4 10% 37% 28% 10%	1710
	21st to end	Day	17% 4 11% 28% 27% 13%	1573
December	1st to 10th	Day	17% 7% 17% 31% 22% 7%	1520
	11th to 20th	Day	19% 7% 14% 33% 24% 4	1924
	21st to end	Day	18% 8% 12% 25% 26% 11%	2794
January	1st to 10th	Night	22% 5% 14% 32% 23% 4	4417
	11th to 20th	Night	11% 7% 18% 33% 23% 7%	4224
	21st to end	Night	22% 6% 14% 29% 24% 5%	4521
February	1st to 10th	Night	18% 4 15% 33% 23% 7%	2470
	11th to 20th	Night	17% 5% 13% 30% 28% 6%	2284
	21st to end	Night	18% 4 10% 29% 29% 10%	1884
March	1st to 10th	Night	20% 5% 10% 26% 35% 5%	3646
	11th to 20th	Night	30% 7% 12% 29% 19% 3	3490
	21st to end	Night	21% 8% 18% 33% 18% 2	3669
April	1st to 10th	Night	16% 6% 16% 30% 27% 6%	1900
	11th to 20th	Night	28% 5% 11% 29% 23% 4	1877
	21st to end	Night	27% 5% 14% 32% 19% 2	1900
May	1st to 10th	Night	25% 8% 12% 24% 23% 8%	3035
	11th to 20th	Night	34% 7% 16% 26% 16% 1	2518
	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	21% 7% 11% 23% 26% 12%	2280
	11th to 20th	Night	11% 4 10% 28% 36% 12%	2280
	21st to end	Night	17% 4 10% 29% 31% 8%	2508
November	1st to 10th	Night	11% 5% 12% 27% 34% 11%	2383
	11th to 20th	Night	9% 4 11% 37% 30% 9%	2470
	21st to end	Night	16% 4 11% 30% 27% 11%	2603
December	1st to 10th	Night	14% 4 19% 30% 25% 7%	2660
	11th to 20th	Night	17% 6% 16% 27% 28% 6%	3272
	21st to end	Night	20% 6% 14% 24% 22% 13%	4873

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% 33% 26% 20% 6% 1	270
	11th to 20th	Day	16% 39% 26% 13% 5%	270
	21st to end	Day	6% 40% 32% 18% 3 1	297
February	1st to 10th	Day	10% 57% 24% 7% 1 1	270
	11th to 20th	Day	18% 46% 23% 10% 3	270
	21st to end	Day	27% 40% 23% 8% 2	225
March	1st to 10th	Day	13% 41% 27% 15% 3	317
	11th to 20th	Day	13% 34% 32% 16% 5%	460
	21st to end	Day	17% 47% 26% 8% 2	506
April	1st to 10th	Day	17% 41% 28% 11% 4	460
	11th to 20th	Day	22% 42% 23% 8% 4 1	452
	21st to end	Day	20% 36% 28% 13% 3 1	460
May	1st to 10th	Day	19% 35% 26% 13% 5% 1	491
	11th to 20th	Day	16% 21% 28% 23% 10% 3	675
	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	270
	11th to 20th	Day	10% 34% 30% 18% 5% 2	270
	21st to end	Day	6% 33% 39% 19% 3 1	270
December	1st to 10th	Day	16% 35% 30% 13% 4 1	270
	11th to 20th	Day	12% 43% 27% 13% 5%	270
	21st to end	Day	16% 38% 28% 14% 3 1	297
January	1st to 10th	Night	6% 33% 34% 20% 6% 1	950
	11th to 20th	Night	7% 37% 33% 17% 5% 1	822
	21st to end	Night	3 31% 42% 18% 5% 1	836
February	1st to 10th	Night	3 35% 42% 15% 4	760
	11th to 20th	Night	5% 33% 40% 18% 4	744
	21st to end	Night	5% 30% 46% 17% 2	628
March	1st to 10th	Night	4 27% 41% 23% 5%	760
	11th to 20th	Night	4 22% 40% 26% 8%	760
	21st to end	Night	4 28% 44% 20% 4	741
April	1st to 10th	Night	8% 24% 45% 20% 3	570
	11th to 20th	Night	9% 24% 38% 23% 6% 1	563
	21st to end	Night	9% 21% 37% 24% 8% 1	570
May	1st to 10th	Night	10% 15% 40% 24% 9% 1	570
	11th to 20th	Night	12% 9% 28% 33% 15% 2	561
	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% 42% 25% 8% 1	899
	11th to 20th	Night	3 29% 41% 19% 7% 1	950
	21st to end	Night	4 28% 37% 26% 5% 1	950
December	1st to 10th	Night	6% 30% 40% 20% 4	950
	11th to 20th	Night	6% 34% 35% 20% 6%	950
	21st to end	Night	8% 33% 37% 18% 4	1045

Page generated at 2024-10-29 09:20:44. Version ac07

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