Black Rock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Rock clear sky chart from 2005-11-06 to 2025-01-07.

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	2 2 1 3 2 3 3 3 5% 6% 70%	109289
	Night	3 3 1 3 3 3 3 3 5% 6% 68%	108561
Transparency	Day	15% 4 6% 17% 27% 31%	109056
	Night	17% 4 6% 19% 27% 28%	108384
Seeing	Day	5% 10% 27% 35% 19% 4	15263
	Night	4 5% 18% 42% 28% 3	25517

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	2 4 2 6% 4 5% 6% 4 5% 7% 56%	2912
	11th to 20th	Day	2 3 1 4 3 4 4 4 6% 8% 59%	2860
	21st to end	Day	8% 6% 2 5% 4 4 3 3 4 7% 54%	3146
February	1st to 10th	Day	3 4 1 4 3 3 4 3 5% 7% 63%	1809
	11th to 20th	Day	4 4 2 4 2 3 3 3 4 7% 62%	1850
	21st to end	Day	4 3 1 6% 3 3 4 3 6% 6% 61%	1570
March	1st to 10th	Day	2 3 1 3 4 4 4 4 6% 6% 62%	3255
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 6% 7% 55%	3490
	21st to end	Day	4 4 1 3 4 4 5% 5% 7% 9% 53%	3825
April	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 8% 64%	2123
	11th to 20th	Day	2 3 1 3 2 3 3 3 5% 6% 70%	2307
	21st to end	Day	2 3 1 2 3 2 3 3 4 6% 71%	2470
May	1st to 10th	Day	1 2 1 2 3 2 3 2 4 6% 74%	4110
	11th to 20th	Day	1 1 2 2 2 3 2 4 6% 77%	4071
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 80%	4477
June	1st to 10th	Day	1 1 2 1 3 2 4 6% 79%	2470
	11th to 20th	Day	1 1 2 2 1 2 2 2 5% 82%	2470
	21st to end	Day	1 1 1 1 2 2 2 4 4 83%	2470
July	1st to 10th	Day	1 1 1 1 1 2 2 3 4 84%	4110
	11th to 20th	Day	1 1 1 2 2 3 3 3 5% 6% 73%	4110
	21st to end	Day	2 2 1 2 2 3 3 3 6% 7% 68%	4454
August	1st to 10th	Day	2 1 1 2 2 3 4 3 6% 7% 70%	4092
	11th to 20th	Day	1 1 2 2 3 2 2 4 6% 77%	4028
	21st to end	Day	2 1 1 1 2 3 3 4 5% 79%	4211
September	1st to 10th	Day	2 1 1 3 3 3 4 3 4 5% 71%	2115
	11th to 20th	Day	2 1 1 1 1 3 2 1 2 4 82%	2090
	21st to end	Day	2 1 1 2 1 2 2 2 3 4 80%	2090
October	1st to 10th	Day	2 1 1 1 1 2 1 3 4 83%	3490
	11th to 20th	Day	1 2 2 2 2 2 2 3 5% 78%	3193
	21st to end	Day	1 1 2 2 3 3 2 2 4 79%	3388
November	1st to 10th	Day	1 2 1 2 2 1 3 3 4 6% 74%	1764
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 6% 75%	1800
	21st to end	Day	3 4 1 3 2 3 3 3 4 7% 67%	1800
December	1st to 10th	Day	2 2 1 3 3 3 5% 3 7% 8% 63%	2950
	11th to 20th	Day	3 3 1 3 2 4 4 5% 6% 10% 58%	3409
	21st to end	Day	3 4 1 4 4 5% 5% 4 7% 8% 55%	4510
January	1st to 10th	Night	4 4 3 6% 4 3 4 3 4 6% 60%	4175
	11th to 20th	Night	3 4 2 4 3 4 4 4 6% 8% 60%	4110
	21st to end	Night	8% 6% 2 5% 4 4 3 3 5% 7% 52%	4521
February	1st to 10th	Night	4 3 1 5% 4 4 4 4 5% 7% 60%	2369
	11th to 20th	Night	5% 5% 1 3 3 3 4 4 5% 6% 61%	2218
	21st to end	Night	4 6% 2 4 3 4 4 3 5% 7% 58%	1884
March	1st to 10th	Night	3 3 1 3 3 4 5% 4 6% 8% 60%	3617
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 4 6% 6% 51%	3490
	21st to end	Night	4 5% 2 5% 4 4 5% 4 6% 7% 55%	3828
April	1st to 10th	Night	2 2 2 3 2 2 3 4 5% 6% 67%	2052
	11th to 20th	Night	2 2 1 2 3 3 3 3 5% 8% 69%	1819
	21st to end	Night	2 3 1 2 3 2 2 3 5% 8% 68%	1710
May	1st to 10th	Night	1 2 1 3 2 3 4 3 4 5% 73%	2860
	11th to 20th	Night	2 2 2 2 2 2 2 4 5% 76%	2834
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 76%	3110
June	1st to 10th	Night	1 1 1 1 1 2 3 4 6% 80%	1710
	11th to 20th	Night	1 1 1 2 2 2 3 2 3 5% 81%	1710
	21st to end	Night	1 1 2 2 1 2 2 3 5% 82%	1710
July	1st to 10th	Night	1 1 1 1 2 3 3 4 83%	2860
	11th to 20th	Night	2 2 1 4 3 3 3 3 6% 7% 66%	2860
	21st to end	Night	3 2 1 3 4 4 5% 5% 6% 8% 61%	3109
August	1st to 10th	Night	1 1 1 2 2 3 4 3 5% 7% 72%	2847
	11th to 20th	Night	1 1 2 2 1 2 2 5% 6% 77%	2811
	21st to end	Night	2 1 1 2 2 2 2 4 5% 78%	3461
September	1st to 10th	Night	2 2 1 3 2 3 2 2 5% 6% 73%	2062
	11th to 20th	Night	2 1 2 1 1 2 2 2 4 81%	2090
	21st to end	Night	1 1 1 2 2 2 2 2 4 3 81%	2090
October	1st to 10th	Night	2 1 1 2 1 1 2 1 3 3 82%	3490
	11th to 20th	Night	2 1 2 2 2 3 3 5% 5% 75%	3734
	21st to end	Night	2 1 2 2 2 2 3 5% 5% 76%	4166
November	1st to 10th	Night	2 2 1 2 2 3 3 2 4 6% 73%	2535
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 7% 74%	2600
	21st to end	Night	3 3 1 4 4 3 3 3 4 7% 67%	2600
December	1st to 10th	Night	3 3 1 3 2 3 6% 3 5% 7% 63%	4240
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 10% 61%	4811
	21st to end	Night	4 3 1 5% 4 4 4 4 5% 8% 59%	6468

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	26% 7% 11% 32% 23% 3	2913
	11th to 20th	Day	22% 6% 14% 30% 22% 6%	2860
	21st to end	Day	30% 5% 8% 30% 22% 4	3146
February	1st to 10th	Day	22% 5% 9% 22% 30% 12%	1809
	11th to 20th	Day	22% 4 6% 24% 29% 15%	1860
	21st to end	Day	23% 5% 9% 20% 31% 12%	1570
March	1st to 10th	Day	20% 7% 6% 25% 30% 12%	3255
	11th to 20th	Day	23% 8% 8% 24% 28% 8%	3490
	21st to end	Day	23% 8% 8% 25% 27% 8%	3825
April	1st to 10th	Day	19% 6% 6% 20% 28% 22%	2123
	11th to 20th	Day	16% 3 5% 16% 31% 29%	2310
	21st to end	Day	15% 3 5% 16% 36% 24%	2470
May	1st to 10th	Day	12% 4 5% 12% 35% 33%	4110
	11th to 20th	Day	8% 4 4 11% 35% 37%	4074
	21st to end	Day	8% 3 4 9% 36% 40%	4477
June	1st to 10th	Day	8% 3 3 9% 27% 50%	2470
	11th to 20th	Day	9% 2 3 6% 17% 64%	2470
	21st to end	Day	7% 2 2 5% 18% 67%	2470
July	1st to 10th	Day	7% 2 3 5% 8% 76%	4046
	11th to 20th	Day	13% 4 3 10% 24% 46%	4041
	21st to end	Day	15% 4 8% 15% 19% 40%	4314
August	1st to 10th	Day	14% 3 5% 15% 32% 31%	4078
	11th to 20th	Day	11% 2 7% 15% 22% 44%	4067
	21st to end	Day	10% 2 4 10% 20% 54%	4210
September	1st to 10th	Day	15% 3 5% 9% 21% 46%	2115
	11th to 20th	Day	10% 2 4 13% 25% 47%	2090
	21st to end	Day	11% 2 3 10% 29% 44%	2090
October	1st to 10th	Day	9% 2 3 9% 21% 56%	3490
	11th to 20th	Day	12% 2 3 17% 34% 33%	3193
	21st to end	Day	13% 2 4 15% 41% 26%	3388
November	1st to 10th	Day	13% 2 5% 22% 36% 22%	1764
	11th to 20th	Day	13% 3 6% 20% 40% 17%	1800
	21st to end	Day	19% 3 7% 20% 35% 16%	1800
December	1st to 10th	Day	18% 4 7% 27% 31% 12%	2950
	11th to 20th	Day	21% 5% 9% 34% 27% 2	3408
	21st to end	Day	26% 5% 13% 29% 23% 4	4510
January	1st to 10th	Night	26% 5% 7% 34% 25% 3	4175
	11th to 20th	Night	22% 6% 11% 33% 22% 6%	4110
	21st to end	Night	30% 5% 8% 30% 21% 6%	4521
February	1st to 10th	Night	23% 5% 9% 23% 29% 11%	2369
	11th to 20th	Night	24% 4 6% 20% 28% 19%	2219
	21st to end	Night	26% 4 7% 16% 33% 13%	1884
March	1st to 10th	Night	20% 7% 8% 26% 28% 11%	3617
	11th to 20th	Night	27% 9% 9% 22% 23% 9%	3490
	21st to end	Night	24% 8% 8% 20% 29% 11%	3828
April	1st to 10th	Night	17% 5% 6% 18% 33% 21%	2052
	11th to 20th	Night	15% 4 6% 17% 30% 28%	1819
	21st to end	Night	15% 4 4 15% 36% 25%	1710
May	1st to 10th	Night	14% 4 4 8% 38% 32%	2860
	11th to 20th	Night	10% 4 5% 9% 33% 39%	2835
	21st to end	Night	11% 4 4 8% 30% 44%	3110
June	1st to 10th	Night	6% 3 3 8% 25% 54%	1710
	11th to 20th	Night	8% 2 3 8% 14% 66%	1710
	21st to end	Night	7% 2 2 3 15% 71%	1710
July	1st to 10th	Night	7% 2 3 4 7% 77%	2815
	11th to 20th	Night	17% 3 4 10% 22% 44%	2806
	21st to end	Night	22% 4 7% 15% 15% 37%	3011
August	1st to 10th	Night	14% 3 3 17% 27% 36%	2838
	11th to 20th	Night	9% 2 6% 18% 21% 43%	2838
	21st to end	Night	10% 2 3 12% 16% 56%	3461
September	1st to 10th	Night	13% 3 5% 12% 22% 45%	2062
	11th to 20th	Night	10% 3 4 10% 24% 50%	2090
	21st to end	Night	10% 2 4 10% 27% 47%	2090
October	1st to 10th	Night	9% 3 2 7% 19% 60%	3490
	11th to 20th	Night	13% 2 2 11% 34% 37%	3734
	21st to end	Night	12% 2 3 14% 40% 29%	4166
November	1st to 10th	Night	13% 3 4 20% 37% 23%	2535
	11th to 20th	Night	13% 3 6% 17% 41% 20%	2600
	21st to end	Night	20% 3 5% 17% 35% 19%	2600
December	1st to 10th	Night	20% 5% 5% 27% 28% 15%	4240
	11th to 20th	Night	19% 4 8% 35% 30% 4	4811
	21st to end	Night	26% 4 13% 29% 25% 4	6468

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	8% 11% 32% 42% 7%	288
	11th to 20th	Day	7% 9% 32% 38% 13% 1	270
	21st to end	Day	7% 7% 24% 40% 21% 2	297
February	1st to 10th	Day	6% 15% 28% 34% 14% 2	369
	11th to 20th	Day	10% 15% 30% 30% 13% 2	455
	21st to end	Day	10% 15% 37% 29% 8% 1	382
March	1st to 10th	Day	12% 20% 29% 26% 13% 1	460
	11th to 20th	Day	9% 16% 33% 28% 11% 3	460
	21st to end	Day	12% 19% 32% 29% 8%	506
April	1st to 10th	Day	9% 18% 30% 30% 11% 3	460
	11th to 20th	Day	11% 22% 32% 25% 10% 1	452
	21st to end	Day	7% 19% 28% 29% 13% 2	460
May	1st to 10th	Day	8% 17% 38% 22% 13% 1	463
	11th to 20th	Day	6% 14% 31% 32% 15% 2	488
	21st to end	Day	5% 16% 30% 29% 18% 2	529
June	1st to 10th	Day	2 10% 30% 36% 19% 3	490
	11th to 20th	Day	2 14% 29% 31% 20% 3	490
	21st to end	Day	1 7% 32% 33% 24% 3	490
July	1st to 10th	Day	1 2 22% 47% 21% 8%	486
	11th to 20th	Day	2 1 19% 43% 25% 9%	484
	21st to end	Day	3 1 16% 43% 30% 7%	531
August	1st to 10th	Day	1 3 21% 42% 26% 7%	487
	11th to 20th	Day	2 3 22% 36% 32% 5%	477
	21st to end	Day	2 2 21% 37% 29% 9%	539
September	1st to 10th	Day	2 3 21% 38% 27% 8%	490
	11th to 20th	Day	2 8% 29% 35% 22% 4	490
	21st to end	Day	3 9% 23% 33% 26% 6%	490
October	1st to 10th	Day	4 9% 22% 34% 26% 6%	490
	11th to 20th	Day	3 7% 22% 42% 20% 6%	330
	21st to end	Day	5% 5% 22% 37% 24% 7%	330
November	1st to 10th	Day	6% 7% 21% 38% 23% 4	300
	11th to 20th	Day	4 6% 24% 42% 21% 4	300
	21st to end	Day	5% 7% 30% 38% 19% 1	300
December	1st to 10th	Day	4 7% 26% 41% 19% 2	300
	11th to 20th	Day	4 4 28% 41% 23%	300
	21st to end	Day	12% 9% 29% 36% 12% 1	330
January	1st to 10th	Night	7% 4 22% 46% 20% 2	784
	11th to 20th	Night	6% 4 21% 49% 18% 2	760
	21st to end	Night	7% 5% 21% 45% 20% 2	836
February	1st to 10th	Night	5% 4 25% 47% 18% 1	760
	11th to 20th	Night	8% 5% 24% 44% 18% 1	744
	21st to end	Night	8% 8% 25% 41% 16% 1	628
March	1st to 10th	Night	7% 7% 21% 48% 17% 1	760
	11th to 20th	Night	7% 7% 22% 38% 24% 3	760
	21st to end	Night	4 8% 25% 45% 16% 1	836
April	1st to 10th	Night	4 10% 24% 42% 19% 1	760
	11th to 20th	Night	4 11% 26% 38% 19% 2	691
	21st to end	Night	4 11% 24% 38% 19% 4	570
May	1st to 10th	Night	4 9% 25% 43% 16% 2	570
	11th to 20th	Night	3 6% 22% 39% 28% 2	561
	21st to end	Night	4 8% 22% 38% 26% 2	617
June	1st to 10th	Night	2 3 19% 46% 28% 2	570
	11th to 20th	Night	1 5% 24% 39% 28% 3	570
	21st to end	Night	3 14% 42% 36% 3	570
July	1st to 10th	Night	1 1 13% 37% 45% 3	555
	11th to 20th	Night	4 1 7% 39% 44% 5%	552
	21st to end	Night	6% 1 5% 33% 52% 3	624
August	1st to 10th	Night	3 2 9% 40% 44% 3	567
	11th to 20th	Night	2 1 8% 39% 49% 1	564
	21st to end	Night	4 1 7% 33% 49% 7%	812
September	1st to 10th	Night	3 3 8% 33% 47% 6%	760
	11th to 20th	Night	2 2 13% 42% 37% 5%	760
	21st to end	Night	2 2 14% 37% 40% 5%	760
October	1st to 10th	Night	3 4 16% 47% 29% 1	760
	11th to 20th	Night	2 3 13% 48% 30% 4	760
	21st to end	Night	2 3 19% 41% 32% 4	836
November	1st to 10th	Night	3 4 15% 44% 32% 2	780
	11th to 20th	Night	3 3 17% 49% 27% 2	800
	21st to end	Night	5% 4 19% 47% 24% 2	800
December	1st to 10th	Night	9% 3 17% 50% 21% 1	800
	11th to 20th	Night	7% 3 19% 49% 20% 2	800
	21st to end	Night	9% 5% 25% 44% 17%	880

Page generated at 2025-01-04 01:24:03. Version ac07

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