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 2024-12-24.

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% 7% 2 5% 4 3 3 3 5% 6% 55%	105732
	Night	10% 8% 2 5% 4 3 4 3 4 6% 51%	103062
Transparency	Day	30% 6% 11% 20% 20% 13%	105483
	Night	34% 7% 12% 22% 17% 8%	102901
Seeing	Day	11% 10% 20% 26% 22% 10%	15063
	Night	12% 4 16% 39% 25% 3	24223

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% 18% 4 10% 6% 5% 5% 4 5% 8% 21%	2539
	11th to 20th	Day	26% 16% 3 7% 4 5% 3 3 5% 6% 22%	2682
	21st to end	Day	20% 12% 3 9% 4 4 4 3 6% 7% 29%	3146
February	1st to 10th	Day	8% 10% 3 6% 5% 5% 5% 5% 8% 10% 33%	1710
	11th to 20th	Day	10% 12% 3 8% 5% 5% 4 4 6% 7% 36%	1665
	21st to end	Day	11% 14% 4 8% 5% 5% 6% 3 6% 5% 33%	1555
March	1st to 10th	Day	13% 17% 4 10% 6% 5% 5% 3 4 5% 27%	3336
	11th to 20th	Day	8% 8% 2 6% 5% 4 4 3 5% 6% 51%	3490
	21st to end	Day	12% 15% 3 9% 6% 5% 6% 5% 6% 8% 25%	3901
April	1st to 10th	Day	12% 11% 3 8% 7% 4 5% 4 4 6% 34%	2280
	11th to 20th	Day	11% 9% 3 6% 6% 4 4 3 6% 8% 40%	2249
	21st to end	Day	6% 7% 2 7% 5% 4 5% 3 5% 8% 48%	2343
May	1st to 10th	Day	8% 7% 2 5% 4 3 3 3 4 5% 56%	4172
	11th to 20th	Day	5% 4 1 3 2 2 2 2 4 6% 69%	4377
	21st to end	Day	6% 5% 1 3 3 3 3 3 5% 7% 61%	4813
June	1st to 10th	Day	8% 6% 2 4 3 3 4 4 6% 8% 51%	2660
	11th to 20th	Day	4 4 1 3 3 3 3 3 4 6% 67%	2660
	21st to end	Day	3 2 1 2 1 2 2 2 3 5% 75%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 88%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 3 81%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	4564
August	1st to 10th	Day	3 2 1 3 2 2 3 2 4 4 75%	4091
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 79%	4009
	21st to end	Day	3 2 1 2 2 2 2 2 2 4 81%	4180
September	1st to 10th	Day	1 1 1 1 1 2 2 3 5% 81%	2280
	11th to 20th	Day	3 3 2 3 3 3 3 3 5% 6% 67%	2280
	21st to end	Day	4 2 1 4 3 2 3 2 5% 6% 68%	2002
October	1st to 10th	Day	3 3 1 2 2 2 3 2 4 5% 73%	3180
	11th to 20th	Day	5% 4 2 4 3 3 3 4 6% 9% 56%	3161
	21st to end	Day	9% 8% 3 7% 4 4 5% 3 5% 8% 44%	3448
November	1st to 10th	Day	13% 9% 3 7% 5% 5% 4 3 5% 9% 38%	1798
	11th to 20th	Day	13% 9% 2 8% 6% 5% 6% 4 7% 10% 30%	1635
	21st to end	Day	10% 12% 3 6% 5% 5% 5% 5% 6% 8% 36%	1600
December	1st to 10th	Day	11% 9% 2 6% 4 5% 5% 4 7% 7% 38%	1600
	11th to 20th	Day	18% 12% 3 9% 6% 5% 6% 4 6% 9% 24%	2008
	21st to end	Day	17% 12% 4 10% 7% 6% 6% 5% 5% 6% 21%	2818
January	1st to 10th	Night	14% 16% 4 9% 6% 6% 5% 4 6% 7% 22%	4406
	11th to 20th	Night	21% 17% 5% 8% 4 3 3 3 4 5% 27%	4269
	21st to end	Night	22% 14% 4 9% 4 5% 3 3 5% 5% 26%	4521
February	1st to 10th	Night	12% 10% 3 7% 4 5% 5% 3 6% 8% 38%	2470
	11th to 20th	Night	11% 12% 3 6% 6% 5% 4 4 5% 6% 38%	2402
	21st to end	Night	9% 12% 3 6% 4 4 6% 4 6% 8% 38%	1899
March	1st to 10th	Night	14% 15% 4 8% 6% 4 4 4 5% 7% 28%	3534
	11th to 20th	Night	10% 10% 2 4 3 3 3 2 4 6% 51%	3490
	21st to end	Night	10% 14% 3 8% 7% 6% 6% 5% 6% 8% 27%	3751
April	1st to 10th	Night	11% 12% 3 7% 5% 4 4 4 6% 8% 37%	1900
	11th to 20th	Night	10% 9% 2 5% 4 4 4 3 5% 7% 47%	1879
	21st to end	Night	12% 7% 2 5% 3 2 3 3 6% 10% 48%	1834
May	1st to 10th	Night	10% 8% 1 4 2 3 3 2 4 5% 58%	2780
	11th to 20th	Night	4 3 1 3 1 2 3 3 4 4 72%	2516
	21st to end	Night	6% 4 1 2 3 2 3 2 4 6% 67%	2762
June	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 6% 6% 52%	1520
	11th to 20th	Night	5% 3 1 4 2 2 3 2 4 5% 69%	1520
	21st to end	Night	3 3 1 2 2 2 2 2 3 4 78%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2540
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 2 80%	2540
	21st to end	Night	1 1 1 1 1 1 2 2 88%	2979
August	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 78%	2848
	11th to 20th	Night	1 1 2 1 1 2 1 2 4 84%	2828
	21st to end	Night	6% 2 1 1 1 1 1 2 2 3 79%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 5% 85%	1900
	11th to 20th	Night	4 2 1 3 2 2 2 2 4 6% 71%	1900
	21st to end	Night	5% 2 1 3 2 3 3 2 3 5% 70%	2173
October	1st to 10th	Night	2 2 1 2 3 2 3 2 3 4 77%	3800
	11th to 20th	Night	8% 4 1 3 3 3 3 3 4 6% 61%	3777
	21st to end	Night	6% 5% 1 6% 4 4 5% 4 7% 9% 48%	4121
November	1st to 10th	Night	13% 10% 2 7% 4 4 5% 3 6% 7% 39%	2499
	11th to 20th	Night	16% 7% 2 8% 6% 5% 5% 4 6% 7% 34%	2760
	21st to end	Night	11% 9% 3 6% 4 4 4 4 5% 7% 44%	2800
December	1st to 10th	Night	12% 8% 2 6% 4 4 4 4 6% 7% 42%	2800
	11th to 20th	Night	18% 13% 3 8% 5% 5% 5% 4 7% 7% 26%	3416
	21st to end	Night	16% 13% 4 9% 6% 5% 5% 3 4 6% 30%	4910

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% 11% 17% 10% 3 1	2539
	11th to 20th	Day	62% 10% 10% 12% 4 2	2682
	21st to end	Day	51% 9% 16% 17% 5% 2	3146
February	1st to 10th	Day	43% 11% 18% 18% 9% 1	1710
	11th to 20th	Day	44% 9% 15% 22% 8% 2	1674
	21st to end	Day	49% 9% 13% 18% 9% 2	1555
March	1st to 10th	Day	56% 13% 11% 15% 5% 1	3336
	11th to 20th	Day	33% 8% 14% 21% 22% 2	3490
	21st to end	Day	52% 11% 15% 17% 4 1	3901
April	1st to 10th	Day	50% 8% 12% 18% 11% 2	2280
	11th to 20th	Day	42% 7% 16% 20% 11% 5%	2252
	21st to end	Day	35% 7% 17% 24% 15% 3	2343
May	1st to 10th	Day	28% 7% 13% 25% 20% 6%	4172
	11th to 20th	Day	18% 4 9% 25% 30% 13%	4380
	21st to end	Day	22% 8% 17% 36% 16% 2	4814
June	1st to 10th	Day	30% 7% 15% 28% 17% 3	2660
	11th to 20th	Day	19% 5% 11% 31% 26% 7%	2660
	21st to end	Day	14% 3 8% 27% 30% 18%	2660
July	1st to 10th	Day	6% 2 3 12% 21% 55%	4351
	11th to 20th	Day	10% 2 5% 11% 23% 48%	4345
	21st to end	Day	6% 1 4 14% 39% 35%	4420
August	1st to 10th	Day	13% 3 5% 12% 32% 35%	4076
	11th to 20th	Day	9% 2 7% 25% 35% 21%	4048
	21st to end	Day	11% 2 5% 25% 37% 20%	4179
September	1st to 10th	Day	8% 3 4 17% 32% 36%	2280
	11th to 20th	Day	18% 5% 9% 22% 31% 14%	2280
	21st to end	Day	18% 4 9% 25% 26% 17%	2002
October	1st to 10th	Day	15% 4 11% 15% 29% 26%	3180
	11th to 20th	Day	24% 5% 16% 32% 19% 4	3161
	21st to end	Day	36% 8% 10% 23% 18% 5%	3448
November	1st to 10th	Day	44% 7% 14% 21% 13% 2	1798
	11th to 20th	Day	48% 10% 18% 13% 10% 1	1635
	21st to end	Day	44% 11% 13% 16% 14% 2	1600
December	1st to 10th	Day	43% 9% 14% 19% 13% 2	1600
	11th to 20th	Day	55% 12% 16% 12% 4	2008
	21st to end	Day	61% 12% 13% 11% 4	2818
January	1st to 10th	Night	58% 13% 14% 11% 4 1	4406
	11th to 20th	Night	60% 11% 10% 11% 6% 2	4269
	21st to end	Night	60% 7% 9% 15% 8% 1	4521
February	1st to 10th	Night	43% 9% 15% 21% 11% 2	2470
	11th to 20th	Night	45% 7% 15% 22% 9% 1	2404
	21st to end	Night	42% 7% 20% 18% 9% 3	1899
March	1st to 10th	Night	53% 9% 16% 15% 7%	3534
	11th to 20th	Night	34% 9% 13% 20% 23% 1	3490
	21st to end	Night	51% 12% 16% 15% 6% 1	3751
April	1st to 10th	Night	44% 9% 19% 19% 8% 2	1900
	11th to 20th	Night	36% 7% 22% 19% 11% 5%	1879
	21st to end	Night	33% 8% 21% 27% 9% 2	1834
May	1st to 10th	Night	29% 6% 14% 30% 16% 4	2780
	11th to 20th	Night	16% 5% 11% 40% 22% 5%	2516
	21st to end	Night	19% 12% 25% 38% 6%	2762
June	1st to 10th	Night	31% 8% 18% 33% 9% 1	1520
	11th to 20th	Night	17% 6% 13% 39% 22% 2	1520
	21st to end	Night	13% 3 9% 40% 28% 8%	1520
July	1st to 10th	Night	5% 3 5% 17% 20% 51%	2500
	11th to 20th	Night	12% 2 3 13% 32% 37%	2492
	21st to end	Night	6% 2 4 18% 44% 27%	2886
August	1st to 10th	Night	12% 2 4 21% 42% 19%	2839
	11th to 20th	Night	8% 2 6% 39% 30% 15%	2855
	21st to end	Night	14% 2 6% 33% 31% 14%	3498
September	1st to 10th	Night	6% 1 5% 21% 35% 32%	1900
	11th to 20th	Night	15% 4 11% 26% 30% 13%	1900
	21st to end	Night	18% 4 12% 25% 22% 19%	2173
October	1st to 10th	Night	14% 3 9% 17% 32% 24%	3800
	11th to 20th	Night	24% 5% 13% 38% 15% 6%	3777
	21st to end	Night	29% 7% 15% 26% 16% 6%	4121
November	1st to 10th	Night	43% 8% 14% 17% 14% 4	2499
	11th to 20th	Night	48% 8% 16% 16% 11% 1	2760
	21st to end	Night	41% 9% 14% 19% 14% 4	2800
December	1st to 10th	Night	40% 9% 13% 23% 15% 1	2800
	11th to 20th	Night	55% 9% 12% 19% 5%	3416
	21st to end	Night	54% 12% 12% 16% 5% 1	4910

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	25% 10% 13% 28% 14% 9%	270
	21st to end	Day	19% 8% 18% 25% 20% 10%	407
February	1st to 10th	Day	15% 12% 16% 30% 22% 5%	370
	11th to 20th	Day	18% 15% 19% 24% 18% 5%	365
	21st to end	Day	17% 11% 23% 29% 15% 5%	375
March	1st to 10th	Day	22% 15% 23% 25% 11% 4	460
	11th to 20th	Day	17% 13% 21% 25% 18% 6%	460
	21st to end	Day	19% 18% 23% 24% 14% 2	506
April	1st to 10th	Day	17% 17% 20% 25% 15% 4	460
	11th to 20th	Day	18% 15% 21% 23% 17% 6%	452
	21st to end	Day	11% 16% 27% 23% 18% 7%	460
May	1st to 10th	Day	11% 19% 24% 25% 16% 6%	463
	11th to 20th	Day	17% 13% 22% 22% 17% 10%	488
	21st to end	Day	12% 12% 21% 23% 23% 9%	529
June	1st to 10th	Day	10% 13% 22% 24% 22% 9%	490
	11th to 20th	Day	6% 17% 26% 25% 19% 6%	490
	21st to end	Day	2 9% 23% 27% 24% 14%	490
July	1st to 10th	Day	4 6% 22% 30% 24% 13%	486
	11th to 20th	Day	2 6% 23% 29% 27% 13%	484
	21st to end	Day	1 5% 20% 28% 31% 16%	531
August	1st to 10th	Day	2 4 22% 25% 28% 19%	487
	11th to 20th	Day	1 8% 19% 24% 27% 20%	477
	21st to end	Day	3 5% 18% 26% 28% 20%	539
September	1st to 10th	Day	2 4 17% 28% 34% 14%	490
	11th to 20th	Day	5% 7% 19% 27% 30% 12%	490
	21st to end	Day	5% 7% 19% 28% 30% 11%	490
October	1st to 10th	Day	8% 8% 17% 24% 28% 16%	490
	11th to 20th	Day	11% 8% 13% 24% 30% 13%	490
	21st to end	Day	12% 9% 15% 30% 21% 12%	539
November	1st to 10th	Day	14% 11% 13% 31% 22% 9%	334
	11th to 20th	Day	14% 6% 13% 35% 24% 8%	217
	21st to end	Day	13% 7% 17% 29% 26% 10%	200
December	1st to 10th	Day	16% 8% 11% 35% 19% 12%	200
	11th to 20th	Day	20% 7% 15% 33% 20% 7%	200
	21st to end	Day	19% 8% 19% 25% 27% 2	204
January	1st to 10th	Night	17% 6% 19% 39% 17% 2	760
	11th to 20th	Night	17% 7% 17% 35% 20% 3	760
	21st to end	Night	19% 5% 16% 36% 20% 3	836
February	1st to 10th	Night	18% 5% 17% 41% 17% 2	760
	11th to 20th	Night	15% 6% 17% 42% 16% 2	744
	21st to end	Night	14% 7% 20% 44% 15%	628
March	1st to 10th	Night	18% 10% 21% 35% 15% 2	760
	11th to 20th	Night	15% 8% 19% 35% 21% 3	760
	21st to end	Night	15% 9% 25% 35% 16% 1	836
April	1st to 10th	Night	16% 8% 20% 39% 16% 1	760
	11th to 20th	Night	13% 7% 24% 35% 18% 3	751
	21st to end	Night	15% 5% 19% 37% 22% 2	694
May	1st to 10th	Night	10% 4 18% 42% 24% 2	529
	11th to 20th	Night	9% 3 17% 40% 29% 2	374
	21st to end	Night	11% 4 19% 36% 29% 1	411
June	1st to 10th	Night	11% 3 15% 42% 28% 2	380
	11th to 20th	Night	5% 3 18% 49% 23% 1	380
	21st to end	Night	3 1 12% 48% 35% 1	380
July	1st to 10th	Night	1 9% 41% 47% 1	370
	11th to 20th	Night	2 1 10% 48% 38% 1	368
	21st to end	Night	6% 37% 52% 5%	546
August	1st to 10th	Night	3 4 39% 48% 5%	567
	11th to 20th	Night	2 1 3 41% 46% 7%	567
	21st to end	Night	3 1 14% 39% 39% 4	836
September	1st to 10th	Night	1 1 14% 41% 38% 4	760
	11th to 20th	Night	5% 2 11% 40% 37% 6%	760
	21st to end	Night	5% 3 13% 39% 34% 6%	760
October	1st to 10th	Night	5% 1 15% 44% 32% 3	760
	11th to 20th	Night	9% 3 13% 39% 33% 4	760
	21st to end	Night	10% 4 13% 43% 23% 7%	836
November	1st to 10th	Night	14% 5% 16% 41% 20% 4	780
	11th to 20th	Night	18% 5% 18% 41% 16% 3	800
	21st to end	Night	14% 4 18% 39% 23% 2	800
December	1st to 10th	Night	15% 5% 16% 38% 22% 5%	800
	11th to 20th	Night	21% 6% 17% 36% 19% 3	800
	21st to end	Night	23% 7% 21% 37% 11% 1	850

Page generated at 2024-12-21 01:13:47. Version ac07

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