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 2024-01-17.

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	2 2 1 3 3 2 3 4 5% 7% 69%	74948
	Night	3 3 1 3 3 2 3 4 5% 7% 67%	74917
Transparency	Day	17% 4 6% 17% 28% 28%	74675
	Night	18% 4 6% 17% 28% 27%	74712
Seeing	Day	5% 10% 27% 35% 19% 3	13946
	Night	4 4 18% 42% 28% 3	24224

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 3 2 5% 5% 3 5% 5% 5% 6% 59%	1708
	11th to 20th	Day	2 3 1 3 3 3 3 4 5% 7% 64%	1674
	21st to end	Day	5% 4 2 4 3 2 5% 4 4 6% 62%	1782
February	1st to 10th	Day	2 3 1 3 3 2 4 4 4 8% 66%	1714
	11th to 20th	Day	4 3 2 4 3 2 3 4 4 7% 63%	1750
	21st to end	Day	4 3 1 4 4 3 4 3 6% 6% 62%	1480
March	1st to 10th	Day	4 4 1 3 4 3 3 4 6% 6% 63%	1865
	11th to 20th	Day	3 4 2 3 4 2 4 5% 6% 6% 60%	1980
	21st to end	Day	3 3 2 3 3 2 5% 5% 6% 8% 61%	2178
April	1st to 10th	Day	2 2 1 3 3 3 5% 5% 5% 8% 64%	2011
	11th to 20th	Day	3 2 1 2 3 2 4 3 4 6% 70%	2183
	21st to end	Day	2 2 1 2 3 2 3 4 4 6% 71%	2340
May	1st to 10th	Day	1 2 1 2 3 2 3 4 5% 9% 68%	2340
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 6% 74%	2301
	21st to end	Day	2 2 1 2 3 2 3 4 4 7% 71%	2530
June	1st to 10th	Day	1 1 1 1 2 1 2 3 4 6% 80%	2340
	11th to 20th	Day	1 1 1 1 2 1 2 2 2 5% 81%	2340
	21st to end	Day	1 1 1 1 1 1 2 3 3 4 82%	2340
July	1st to 10th	Day	2 1 1 1 2 2 4 4 4 6% 73%	2340
	11th to 20th	Day	2 2 1 2 3 3 3 5% 6% 7% 66%	2340
	21st to end	Day	4 3 2 4 4 3 5% 6% 8% 10% 52%	2545
August	1st to 10th	Day	2 1 1 2 2 3 4 4 6% 7% 69%	2328
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 7% 72%	2299
	21st to end	Day	3 1 1 2 2 2 3 4 4 6% 73%	2394
September	1st to 10th	Day	2 1 1 3 3 2 4 4 4 5% 71%	2004
	11th to 20th	Day	2 1 1 1 2 2 2 2 2 4 82%	1980
	21st to end	Day	2 1 1 2 2 1 2 2 3 5% 79%	1980
October	1st to 10th	Day	3 2 1 2 2 1 3 3 4 6% 72%	1980
	11th to 20th	Day	2 1 1 2 2 1 2 2 3 5% 79%	1829
	21st to end	Day	1 1 1 2 2 1 3 2 3 5% 79%	1954
November	1st to 10th	Day	1 2 1 2 3 1 3 3 4 6% 75%	1674
	11th to 20th	Day	1 2 1 2 3 1 3 3 4 6% 74%	1710
	21st to end	Day	3 3 2 2 3 2 3 3 4 7% 68%	1710
December	1st to 10th	Day	3 3 2 4 4 2 4 3 5% 5% 65%	1710
	11th to 20th	Day	5% 5% 2 4 4 4 5% 5% 6% 10% 51%	2169
	21st to end	Day	4 4 1 4 5% 3 5% 5% 7% 9% 53%	3146
January	1st to 10th	Night	5% 4 1 4 4 3 4 4 4 7% 60%	2457
	11th to 20th	Night	3 3 1 3 3 3 3 4 5% 7% 65%	2430
	21st to end	Night	4 4 2 4 5% 3 4 5% 5% 7% 57%	2574
February	1st to 10th	Night	3 3 1 4 4 2 4 4 5% 7% 63%	2244
	11th to 20th	Night	5% 5% 2 3 3 3 3 4 5% 6% 62%	2098
	21st to end	Night	4 5% 2 4 3 3 4 4 5% 7% 59%	1776
March	1st to 10th	Night	3 3 2 3 3 2 4 5% 6% 9% 58%	2093
	11th to 20th	Night	5% 4 2 3 4 3 4 4 6% 6% 58%	1980
	21st to end	Night	3 2 1 3 4 2 4 4 6% 7% 63%	2178
April	1st to 10th	Night	3 2 1 3 3 2 3 5% 5% 6% 66%	1944
	11th to 20th	Night	2 2 1 2 3 2 3 4 5% 8% 69%	1723
	21st to end	Night	2 3 1 2 3 2 3 3 5% 8% 68%	1620
May	1st to 10th	Night	2 2 1 3 3 2 3 3 5% 7% 70%	1620
	11th to 20th	Night	2 2 1 2 3 2 3 3 4 5% 75%	1594
	21st to end	Night	2 2 1 3 2 2 3 3 4 6% 72%	1746
June	1st to 10th	Night	1 1 1 1 1 2 3 4 6% 80%	1620
	11th to 20th	Night	1 1 1 1 2 2 3 2 3 5% 81%	1620
	21st to end	Night	1 1 1 1 2 1 2 2 2 6% 82%	1620
July	1st to 10th	Night	1 1 1 1 3 1 3 5% 5% 6% 73%	1620
	11th to 20th	Night	2 3 1 3 4 3 4 4 5% 7% 64%	1620
	21st to end	Night	5% 3 1 3 5% 2 5% 6% 7% 9% 54%	1773
August	1st to 10th	Night	2 2 1 3 2 2 4 4 4 6% 70%	1611
	11th to 20th	Night	2 1 1 3 3 1 3 3 6% 5% 71%	1602
	21st to end	Night	3 2 2 2 2 3 3 4 7% 72%	1957
September	1st to 10th	Night	2 2 1 2 2 2 2 3 5% 6% 73%	1953
	11th to 20th	Night	2 1 2 1 1 2 3 2 5% 80%	1980
	21st to end	Night	1 1 1 2 2 1 2 2 4 3 81%	1980
October	1st to 10th	Night	3 2 1 3 3 2 2 2 5% 5% 72%	1980
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 3 81%	2129
	21st to end	Night	2 1 2 2 1 2 3 4 6% 77%	2398
November	1st to 10th	Night	2 2 1 2 2 2 3 3 4 6% 73%	2405
	11th to 20th	Night	2 1 1 2 2 2 3 3 3 7% 74%	2470
	21st to end	Night	3 3 1 4 4 3 3 3 4 6% 67%	2470
December	1st to 10th	Night	6% 4 2 3 4 2 3 4 5% 6% 60%	2470
	11th to 20th	Night	5% 4 2 4 5% 3 4 4 6% 10% 53%	3041
	21st to end	Night	5% 4 2 5% 5% 3 5% 5% 5% 8% 53%	4521

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	25% 7% 9% 30% 26% 4	1709
	11th to 20th	Day	21% 4 10% 30% 28% 7%	1674
	21st to end	Day	24% 5% 8% 31% 26% 5%	1782
February	1st to 10th	Day	19% 5% 10% 23% 31% 12%	1714
	11th to 20th	Day	22% 4 6% 23% 30% 16%	1760
	21st to end	Day	23% 5% 9% 20% 31% 12%	1480
March	1st to 10th	Day	22% 4 6% 22% 33% 13%	1865
	11th to 20th	Day	23% 6% 5% 19% 35% 12%	1980
	21st to end	Day	21% 6% 7% 26% 29% 11%	2178
April	1st to 10th	Day	19% 5% 6% 20% 27% 22%	2011
	11th to 20th	Day	17% 3 5% 16% 29% 30%	2186
	21st to end	Day	15% 3 5% 16% 35% 24%	2340
May	1st to 10th	Day	14% 4 6% 14% 34% 28%	2340
	11th to 20th	Day	11% 4 5% 14% 35% 32%	2304
	21st to end	Day	14% 3 6% 12% 32% 33%	2530
June	1st to 10th	Day	8% 3 3 9% 27% 51%	2340
	11th to 20th	Day	9% 2 3 6% 18% 62%	2340
	21st to end	Day	7% 2 2 4 18% 67%	2340
July	1st to 10th	Day	12% 4 5% 9% 13% 57%	2276
	11th to 20th	Day	17% 5% 6% 14% 23% 35%	2271
	21st to end	Day	24% 6% 12% 18% 17% 23%	2404
August	1st to 10th	Day	14% 5% 7% 12% 21% 40%	2313
	11th to 20th	Day	14% 3 6% 13% 21% 42%	2299
	21st to end	Day	14% 4 5% 12% 27% 38%	2394
September	1st to 10th	Day	16% 3 5% 9% 21% 46%	2004
	11th to 20th	Day	10% 2 4 13% 25% 46%	1980
	21st to end	Day	11% 3 3 11% 30% 42%	1980
October	1st to 10th	Day	15% 4 5% 16% 25% 35%	1980
	11th to 20th	Day	11% 3 3 12% 34% 37%	1829
	21st to end	Day	11% 3 5% 17% 38% 27%	1954
November	1st to 10th	Day	12% 2 5% 23% 36% 22%	1674
	11th to 20th	Day	13% 4 6% 20% 40% 17%	1710
	21st to end	Day	19% 3 7% 21% 35% 16%	1710
December	1st to 10th	Day	23% 5% 9% 24% 28% 11%	1710
	11th to 20th	Day	28% 8% 12% 26% 22% 3	2168
	21st to end	Day	26% 7% 12% 25% 25% 5%	3146
January	1st to 10th	Night	25% 5% 8% 31% 27% 5%	2457
	11th to 20th	Night	20% 5% 7% 31% 28% 9%	2430
	21st to end	Night	25% 6% 7% 28% 27% 7%	2574
February	1st to 10th	Night	20% 5% 9% 24% 30% 12%	2244
	11th to 20th	Night	24% 4 5% 19% 28% 20%	2099
	21st to end	Night	26% 4 8% 17% 32% 14%	1776
March	1st to 10th	Night	23% 6% 7% 20% 34% 11%	2093
	11th to 20th	Night	24% 6% 6% 17% 32% 15%	1980
	21st to end	Night	21% 6% 7% 24% 29% 14%	2178
April	1st to 10th	Night	18% 5% 6% 18% 31% 22%	1944
	11th to 20th	Night	15% 4 6% 17% 29% 28%	1723
	21st to end	Night	15% 4 5% 16% 36% 24%	1620
May	1st to 10th	Night	16% 4 5% 12% 33% 30%	1620
	11th to 20th	Night	13% 4 5% 13% 32% 33%	1595
	21st to end	Night	15% 3 4 11% 31% 35%	1746
June	1st to 10th	Night	7% 3 3 8% 24% 55%	1620
	11th to 20th	Night	8% 2 3 8% 14% 64%	1620
	21st to end	Night	7% 2 3 3 15% 71%	1620
July	1st to 10th	Night	12% 4 5% 7% 12% 60%	1575
	11th to 20th	Night	20% 4 7% 12% 19% 38%	1566
	21st to end	Night	25% 7% 9% 17% 20% 22%	1674
August	1st to 10th	Night	16% 4 5% 13% 19% 43%	1602
	11th to 20th	Night	14% 4 6% 12% 21% 43%	1602
	21st to end	Night	14% 3 5% 12% 24% 43%	1957
September	1st to 10th	Night	13% 3 5% 12% 21% 45%	1953
	11th to 20th	Night	10% 3 5% 10% 24% 49%	1980
	21st to end	Night	9% 2 4 11% 28% 46%	1980
October	1st to 10th	Night	15% 4 4 13% 26% 38%	1980
	11th to 20th	Night	11% 3 3 10% 31% 42%	2129
	21st to end	Night	10% 3 4 16% 34% 33%	2398
November	1st to 10th	Night	13% 3 5% 20% 37% 22%	2405
	11th to 20th	Night	13% 3 6% 16% 41% 20%	2470
	21st to end	Night	20% 3 5% 17% 35% 20%	2470
December	1st to 10th	Night	24% 5% 8% 24% 25% 14%	2470
	11th to 20th	Night	27% 6% 10% 26% 26% 5%	3041
	21st to end	Night	31% 5% 11% 20% 28% 6%	4521

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	9% 11% 33% 41% 6%	270
	11th to 20th	Day	7% 8% 32% 38% 13% 2	258
	21st to end	Day	6% 6% 25% 39% 22% 2	264
February	1st to 10th	Day	5% 12% 28% 37% 15% 2	334
	11th to 20th	Day	10% 15% 29% 30% 13% 2	415
	21st to end	Day	10% 15% 36% 30% 8% 1	346
March	1st to 10th	Day	12% 19% 29% 25% 14% 1	420
	11th to 20th	Day	9% 17% 31% 29% 11% 3	420
	21st to end	Day	11% 19% 33% 29% 8%	462
April	1st to 10th	Day	8% 18% 30% 30% 11% 3	420
	11th to 20th	Day	11% 23% 31% 25% 9% 1	412
	21st to end	Day	8% 20% 27% 29% 14% 2	420
May	1st to 10th	Day	7% 17% 39% 22% 14% 1	423
	11th to 20th	Day	6% 15% 31% 32% 13% 2	448
	21st to end	Day	6% 16% 29% 29% 18% 2	485
June	1st to 10th	Day	3 10% 31% 36% 17% 3	450
	11th to 20th	Day	3 15% 28% 32% 20% 3	450
	21st to end	Day	2 8% 32% 33% 23% 3	450
July	1st to 10th	Day	1 2 22% 47% 21% 6%	446
	11th to 20th	Day	2 1 20% 44% 26% 6%	444
	21st to end	Day	3 1 16% 44% 30% 6%	487
August	1st to 10th	Day	1 3 22% 43% 26% 4	447
	11th to 20th	Day	2 4 22% 36% 32% 5%	437
	21st to end	Day	2 2 20% 38% 30% 8%	495
September	1st to 10th	Day	2 3 23% 39% 27% 6%	450
	11th to 20th	Day	2 7% 28% 36% 23% 4	450
	21st to end	Day	3 9% 25% 34% 25% 4	450
October	1st to 10th	Day	4 10% 23% 36% 24% 3	450
	11th to 20th	Day	4 7% 23% 41% 19% 6%	299
	21st to end	Day	6% 5% 23% 38% 23% 5%	297
November	1st to 10th	Day	7% 7% 21% 39% 23% 3	270
	11th to 20th	Day	4 6% 24% 40% 22% 4	270
	21st to end	Day	5% 7% 30% 36% 20% 1	270
December	1st to 10th	Day	5% 8% 29% 41% 17%	270
	11th to 20th	Day	4 5% 30% 40% 20%	270
	21st to end	Day	14% 10% 30% 34% 10% 1	297
January	1st to 10th	Night	7% 4 21% 45% 21% 2	760
	11th to 20th	Night	6% 4 21% 49% 18% 2	748
	21st to end	Night	7% 5% 21% 45% 21% 2	792
February	1st to 10th	Night	4 3 24% 49% 19% 1	720
	11th to 20th	Night	8% 5% 23% 44% 19% 1	704
	21st to end	Night	8% 7% 24% 42% 17% 2	592
March	1st to 10th	Night	7% 7% 20% 48% 17% 1	720
	11th to 20th	Night	7% 6% 21% 38% 25% 3	720
	21st to end	Night	4 8% 25% 46% 17% 1	792
April	1st to 10th	Night	3 11% 23% 43% 19% 1	720
	11th to 20th	Night	4 10% 26% 38% 19% 2	655
	21st to end	Night	4 12% 23% 37% 20% 4	540
May	1st to 10th	Night	4 9% 25% 43% 17% 2	540
	11th to 20th	Night	3 6% 21% 38% 30% 2	531
	21st to end	Night	4 9% 21% 38% 27% 3	584
June	1st to 10th	Night	2 3 19% 46% 29% 2	540
	11th to 20th	Night	1 5% 24% 37% 29% 3	540
	21st to end	Night	3 15% 41% 37% 4	540
July	1st to 10th	Night	1 1 13% 35% 47% 3	525
	11th to 20th	Night	4 1 7% 40% 43% 4	522
	21st to end	Night	6% 1 5% 32% 53% 4	591
August	1st to 10th	Night	3 2 9% 40% 44% 2	537
	11th to 20th	Night	2 1 8% 38% 49% 1	534
	21st to end	Night	4 1 6% 33% 49% 7%	769
September	1st to 10th	Night	3 3 8% 33% 46% 6%	720
	11th to 20th	Night	2 2 12% 41% 38% 5%	720
	21st to end	Night	2 2 14% 36% 41% 5%	720
October	1st to 10th	Night	3 5% 16% 48% 27% 1	720
	11th to 20th	Night	2 3 13% 47% 31% 4	720
	21st to end	Night	2 3 19% 40% 31% 4	792
November	1st to 10th	Night	3 4 14% 44% 32% 2	740
	11th to 20th	Night	3 3 16% 49% 28% 2	760
	21st to end	Night	4 4 19% 46% 25% 2	760
December	1st to 10th	Night	9% 3 17% 49% 21% 1	760
	11th to 20th	Night	8% 3 19% 49% 20% 2	760
	21st to end	Night	10% 5% 25% 44% 17%	836

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