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 2024-12-02.

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	15% 6% 2 6% 5% 4 5% 4 6% 8% 41%	101221
	Night	16% 6% 2 5% 4 4 4 4 6% 7% 43%	98343
Transparency	Day	40% 7% 12% 18% 17% 5%	105041
	Night	38% 7% 12% 19% 19% 6%	102662
Seeing	Day	19% 12% 23% 22% 16% 8%	14980
	Night	18% 11% 21% 27% 19% 3	25020

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% 5% 2 8% 7% 6% 7% 6% 9% 12% 29%	4120
	11th to 20th	Day	10% 5% 2 8% 7% 7% 8% 5% 8% 10% 30%	4120
	21st to end	Day	13% 8% 4 9% 8% 9% 8% 7% 8% 9% 18%	4466
August	1st to 10th	Day	8% 6% 2 6% 4 5% 6% 5% 9% 11% 38%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 5% 7% 10% 41%	3868
	21st to end	Day	9% 4 2 6% 4 4 5% 4 6% 7% 48%	4165
September	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	2090
	11th to 20th	Day	18% 6% 2 6% 5% 5% 5% 4 6% 6% 37%	2090
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 5% 6% 44%	2090
October	1st to 10th	Day	14% 5% 2 3 3 3 3 3 4 5% 54%	2090
	11th to 20th	Day	12% 5% 1 4 3 2 3 3 4 5% 57%	1961
	21st to end	Day	15% 4 1 4 3 3 3 3 4 5% 55%	2090
November	1st to 10th	Day	15% 4 1 4 3 2 3 3 4 6% 55%	1783
	11th to 20th	Day	16% 5% 1 4 3 3 3 2 4 6% 53%	1800
	21st to end	Day	16% 6% 2 5% 3 3 4 3 4 7% 49%	1800
December	1st to 10th	Day	24% 7% 2 4 3 3 4 3 5% 5% 39%	1719
	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% 5% 1 5% 5% 5% 6% 5% 7% 10% 44%	1710
July	1st to 10th	Night	14% 7% 3 8% 6% 5% 6% 4 7% 9% 31%	2840
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 6% 8% 11% 36%	2840
	21st to end	Night	14% 8% 3 8% 5% 6% 6% 7% 9% 9% 24%	3087
August	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 11% 39%	2825
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 8% 46%	2943
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 8% 50%	3476
September	1st to 10th	Night	10% 7% 2 7% 5% 4 4 4 7% 8% 42%	2071
	11th to 20th	Night	16% 6% 2 5% 5% 4 4 5% 5% 7% 41%	2090
	21st to end	Night	15% 5% 2 5% 3 3 4 3 4 6% 49%	2090
October	1st to 10th	Night	15% 4 1 3 3 3 3 2 3 5% 57%	2090
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 4 6% 55%	2218
	21st to end	Night	14% 4 1 4 3 2 3 3 4 6% 56%	2508
November	1st to 10th	Night	15% 5% 2 4 3 2 3 3 4 6% 55%	2512
	11th to 20th	Night	16% 5% 2 3 3 3 3 3 5% 5% 53%	2600
	21st to end	Night	17% 5% 2 4 4 3 4 4 5% 6% 46%	2600
December	1st to 10th	Night	22% 6% 2 6% 3 4 3 3 5% 6% 40%	2495
	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	44% 13% 21% 17% 5% 1	4055
	11th to 20th	Day	46% 12% 21% 14% 4 1	4051
	21st to end	Day	60% 12% 15% 10% 4	4324
August	1st to 10th	Day	37% 10% 15% 29% 8%	4091
	11th to 20th	Day	35% 10% 22% 16% 14% 3	3907
	21st to end	Day	34% 9% 13% 26% 17% 1	4165
September	1st to 10th	Day	41% 9% 14% 16% 16% 3	2090
	11th to 20th	Day	45% 8% 11% 16% 19% 1	2090
	21st to end	Day	39% 7% 11% 16% 21% 6%	2090
October	1st to 10th	Day	25% 5% 6% 15% 38% 11%	3480
	11th to 20th	Day	20% 3 7% 16% 27% 28%	3240
	21st to end	Day	30% 4 5% 21% 26% 14%	3428
November	1st to 10th	Day	33% 3 8% 15% 30% 11%	1783
	11th to 20th	Day	34% 4 6% 19% 27% 11%	1800
	21st to end	Day	37% 5% 7% 16% 27% 7%	1800
December	1st to 10th	Day	47% 5% 7% 17% 17% 6%	1719
	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	48% 9% 19% 18% 6%	2795
	11th to 20th	Night	36% 14% 25% 20% 5% 1	2786
	21st to end	Night	52% 15% 16% 12% 4 1	2988
August	1st to 10th	Night	37% 9% 21% 26% 6%	2814
	11th to 20th	Night	35% 8% 20% 21% 15% 1	2970
	21st to end	Night	30% 8% 15% 25% 20% 1	3476
September	1st to 10th	Night	37% 10% 15% 18% 16% 4	2071
	11th to 20th	Night	41% 9% 13% 18% 17% 2	2090
	21st to end	Night	36% 7% 12% 21% 18% 7%	2090
October	1st to 10th	Night	24% 4 11% 16% 35% 10%	3480
	11th to 20th	Night	19% 3 6% 18% 30% 24%	3678
	21st to end	Night	27% 4 5% 20% 27% 15%	4120
November	1st to 10th	Night	33% 4 7% 19% 25% 11%	2512
	11th to 20th	Night	34% 4 7% 17% 26% 12%	2600
	21st to end	Night	38% 5% 7% 19% 23% 7%	2600
December	1st to 10th	Night	46% 4 9% 18% 18% 6%	2495
	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% 20% 12%	490
July	1st to 10th	Day	11% 2 12% 27% 29% 19%	485
	11th to 20th	Day	8% 3 12% 25% 33% 19%	484
	21st to end	Day	7% 3 13% 24% 32% 21%	532
August	1st to 10th	Day	14% 6% 16% 23% 22% 18%	486
	11th to 20th	Day	13% 4 13% 28% 29% 14%	479
	21st to end	Day	17% 3 11% 22% 31% 16%	539
September	1st to 10th	Day	12% 4 18% 27% 27% 13%	490
	11th to 20th	Day	19% 4 14% 25% 31% 7%	490
	21st to end	Day	17% 6% 20% 26% 24% 7%	490
October	1st to 10th	Day	17% 5% 25% 29% 16% 7%	490
	11th to 20th	Day	15% 14% 23% 29% 16% 3	490
	21st to end	Day	19% 11% 27% 26% 14% 2	539
November	1st to 10th	Day	22% 13% 27% 24% 11% 2	321
	11th to 20th	Day	20% 24% 33% 19% 3	300
	21st to end	Day	17% 21% 37% 18% 6% 1	300
December	1st to 10th	Day	33% 23% 26% 14% 3 1	273
	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	12% 1 7% 26% 45% 9%	555
	11th to 20th	Night	10% 2 8% 22% 47% 11%	552
	21st to end	Night	8% 1 9% 25% 46% 11%	624
August	1st to 10th	Night	13% 1 9% 25% 37% 14%	567
	11th to 20th	Night	15% 3 9% 25% 39% 9%	564
	21st to end	Night	17% 2 8% 26% 39% 8%	627
September	1st to 10th	Night	12% 3 14% 26% 36% 9%	741
	11th to 20th	Night	20% 2 15% 27% 31% 4	760
	21st to end	Night	17% 5% 13% 34% 29% 3	760
October	1st to 10th	Night	16% 6% 19% 31% 21% 7%	760
	11th to 20th	Night	14% 11% 21% 32% 19% 2	760
	21st to end	Night	17% 11% 19% 33% 17% 2	836
November	1st to 10th	Night	18% 10% 23% 29% 18% 2	780
	11th to 20th	Night	18% 13% 27% 29% 12% 1	800
	21st to end	Night	21% 16% 25% 25% 12% 1	800
December	1st to 10th	Night	25% 17% 25% 26% 6% 1	768
	11th to 20th	Night	21% 15% 28% 27% 7% 1	760
	21st to end	Night	31% 14% 24% 25% 6%	836

Page generated at 2024-11-29 10:43:10. Version ac07

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