Orion Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Orion Ranch Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	11% 5% 2 4 4 3 4 4 5% 7% 50%	74890
	Night	11% 5% 2 4 4 2 3 4 5% 7% 54%	74879
Transparency	Day	34% 6% 10% 20% 20% 10%	74615
	Night	31% 6% 11% 20% 20% 12%	74674
Seeing	Day	13% 7% 22% 31% 22% 6%	13769
	Night	14% 7% 27% 36% 15% 1	23816

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	12% 6% 2 4 4 2 3 3 4 6% 55%	1707
	11th to 20th	Day	17% 5% 2 4 3 1 3 4 5% 5% 50%	1643
	21st to end	Day	14% 5% 2 4 3 2 4 3 4 6% 53%	1885
February	1st to 10th	Day	16% 6% 3 5% 3 2 3 2 4 7% 49%	1800
	11th to 20th	Day	17% 6% 2 4 4 2 4 3 4 6% 47%	1749
	21st to end	Day	19% 5% 2 4 3 2 3 3 3 5% 50%	1480
March	1st to 10th	Day	18% 7% 2 6% 4 3 4 3 4 6% 42%	1948
	11th to 20th	Day	14% 7% 3 5% 5% 3 3 4 4 5% 47%	1980
	21st to end	Day	14% 5% 3 4 4 2 3 3 5% 6% 50%	2178
April	1st to 10th	Day	12% 8% 3 5% 5% 3 4 3 5% 7% 44%	1980
	11th to 20th	Day	16% 7% 2 6% 4 3 4 3 4 6% 45%	1967
	21st to end	Day	13% 6% 2 5% 4 2 4 4 4 6% 49%	2312
May	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 5% 6% 7% 42%	2340
	11th to 20th	Day	12% 8% 2 6% 5% 4 5% 6% 5% 6% 41%	2302
	21st to end	Day	14% 7% 3 5% 6% 3 6% 6% 7% 8% 35%	2531
June	1st to 10th	Day	5% 3 2 4 4 4 5% 6% 6% 8% 53%	2340
	11th to 20th	Day	6% 4 2 5% 4 3 4 4 6% 9% 54%	2340
	21st to end	Day	7% 4 3 4 6% 3 6% 6% 7% 9% 44%	2340
July	1st to 10th	Day	9% 4 3 5% 7% 4 6% 6% 7% 9% 41%	2340
	11th to 20th	Day	5% 3 1 4 4 2 4 6% 6% 9% 56%	2340
	21st to end	Day	4 2 2 4 4 3 4 5% 7% 12% 55%	2546
August	1st to 10th	Day	2 1 1 3 4 2 4 4 4 10% 65%	2328
	11th to 20th	Day	6% 3 2 5% 6% 3 4 5% 6% 9% 51%	2149
	21st to end	Day	5% 3 2 5% 5% 4 6% 7% 8% 9% 47%	2286
September	1st to 10th	Day	9% 5% 2 5% 6% 3 5% 5% 6% 7% 49%	1980
	11th to 20th	Day	10% 6% 2 5% 6% 4 5% 5% 6% 8% 43%	1980
	21st to end	Day	7% 4 2 4 4 2 4 6% 6% 7% 53%	1980
October	1st to 10th	Day	6% 4 2 4 5% 3 4 4 5% 6% 57%	1980
	11th to 20th	Day	12% 5% 1 3 4 2 3 4 4 6% 55%	1850
	21st to end	Day	13% 4 2 4 3 2 3 3 3 4 60%	1980
November	1st to 10th	Day	13% 7% 3 4 4 3 4 3 3 6% 51%	1858
	11th to 20th	Day	9% 5% 2 5% 5% 3 3 4 4 6% 53%	1755
	21st to end	Day	16% 6% 3 5% 4 2 3 4 4 5% 49%	1710
December	1st to 10th	Day	14% 7% 2 4 4 2 3 3 4 6% 51%	1710
	11th to 20th	Day	14% 7% 3 4 5% 2 3 5% 4 6% 46%	2161
	21st to end	Day	16% 6% 2 3 3 2 2 3 3 4 56%	3135
January	1st to 10th	Night	15% 5% 2 3 2 2 3 3 3 5% 57%	2458
	11th to 20th	Night	15% 5% 3 4 4 2 4 3 4 6% 52%	2379
	21st to end	Night	15% 6% 2 4 4 2 3 3 3 5% 52%	2487
February	1st to 10th	Night	15% 7% 2 4 3 2 3 3 3 5% 52%	2160
	11th to 20th	Night	17% 6% 3 4 4 2 3 3 4 5% 51%	2098
	21st to end	Night	19% 6% 2 4 4 2 3 3 3 5% 49%	1776
March	1st to 10th	Night	16% 6% 3 4 4 2 4 4 4 8% 45%	2010
	11th to 20th	Night	14% 6% 3 4 5% 2 4 4 5% 6% 46%	1980
	21st to end	Night	12% 4 2 4 4 3 4 4 4 7% 52%	2178
April	1st to 10th	Night	10% 5% 3 5% 5% 2 3 4 5% 7% 50%	1980
	11th to 20th	Night	15% 6% 2 5% 4 2 4 4 6% 6% 47%	1936
	21st to end	Night	13% 6% 3 4 3 2 3 4 4 6% 51%	1648
May	1st to 10th	Night	9% 6% 3 4 4 3 4 4 5% 9% 49%	1620
	11th to 20th	Night	9% 5% 3 5% 5% 3 3 4 7% 7% 49%	1593
	21st to end	Night	12% 5% 2 4 4 3 4 4 7% 9% 47%	1745
June	1st to 10th	Night	3 4 1 3 3 2 4 4 6% 9% 60%	1620
	11th to 20th	Night	4 3 2 2 4 1 4 4 6% 9% 61%	1620
	21st to end	Night	5% 4 2 3 3 2 5% 4 6% 8% 56%	1620
July	1st to 10th	Night	8% 4 1 4 4 2 4 5% 6% 12% 49%	1620
	11th to 20th	Night	3 2 1 2 3 1 3 3 6% 10% 66%	1620
	21st to end	Night	2 2 1 2 3 2 4 4 7% 13% 60%	1772
August	1st to 10th	Night	1 1 1 1 1 2 3 5% 9% 74%	1611
	11th to 20th	Night	6% 3 2 3 4 2 5% 5% 5% 9% 56%	1747
	21st to end	Night	4 4 1 4 4 3 5% 5% 7% 10% 52%	2067
September	1st to 10th	Night	9% 4 2 4 5% 3 4 5% 4 8% 52%	1980
	11th to 20th	Night	10% 6% 3 4 5% 4 5% 4 5% 8% 48%	1980
	21st to end	Night	7% 4 2 3 4 2 3 4 6% 8% 55%	1980
October	1st to 10th	Night	7% 3 1 3 3 2 2 3 5% 7% 63%	1980
	11th to 20th	Night	11% 4 2 3 3 2 3 4 4 7% 55%	2089
	21st to end	Night	13% 4 2 3 3 2 3 3 3 4 59%	2376
November	1st to 10th	Night	12% 6% 2 4 4 2 3 3 4 6% 56%	2220
	11th to 20th	Night	8% 5% 2 4 4 2 3 4 4 6% 58%	2421
	21st to end	Night	17% 6% 2 4 4 2 4 4 4 6% 48%	2470
December	1st to 10th	Night	13% 6% 2 4 3 2 4 3 4 7% 53%	2470
	11th to 20th	Night	15% 8% 3 5% 4 2 3 4 4 5% 48%	3036
	21st to end	Night	18% 6% 1 3 3 2 2 3 3 4 56%	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	32% 4 6% 20% 24% 13%	1708
	11th to 20th	Day	37% 3 7% 15% 25% 14%	1643
	21st to end	Day	35% 3 6% 13% 27% 16%	1885
February	1st to 10th	Day	39% 5% 6% 12% 21% 16%	1800
	11th to 20th	Day	39% 4 7% 18% 19% 13%	1759
	21st to end	Day	38% 4 6% 16% 20% 17%	1480
March	1st to 10th	Day	44% 6% 7% 13% 20% 11%	1948
	11th to 20th	Day	40% 5% 7% 12% 19% 16%	1980
	21st to end	Day	35% 6% 9% 17% 21% 12%	2178
April	1st to 10th	Day	40% 6% 8% 18% 16% 12%	1980
	11th to 20th	Day	41% 5% 8% 16% 19% 11%	1967
	21st to end	Day	37% 6% 6% 20% 22% 9%	2312
May	1st to 10th	Day	40% 7% 9% 16% 19% 8%	2340
	11th to 20th	Day	43% 8% 9% 18% 18% 4	2306
	21st to end	Day	43% 10% 13% 19% 15% 1	2531
June	1st to 10th	Day	27% 9% 13% 28% 20% 4	2340
	11th to 20th	Day	27% 7% 14% 28% 18% 6%	2340
	21st to end	Day	32% 11% 13% 22% 18% 3	2340
July	1st to 10th	Day	36% 10% 17% 24% 12% 1	2276
	11th to 20th	Day	22% 8% 17% 27% 22% 3	2271
	21st to end	Day	20% 8% 22% 34% 15% 1	2405
August	1st to 10th	Day	15% 6% 16% 38% 22% 2	2314
	11th to 20th	Day	28% 8% 17% 29% 14% 4	2149
	21st to end	Day	29% 10% 16% 26% 15% 4	2286
September	1st to 10th	Day	34% 7% 11% 22% 17% 9%	1980
	11th to 20th	Day	38% 8% 12% 19% 17% 6%	1980
	21st to end	Day	28% 8% 10% 20% 21% 13%	1980
October	1st to 10th	Day	26% 6% 10% 21% 23% 13%	1980
	11th to 20th	Day	32% 5% 7% 17% 22% 18%	1850
	21st to end	Day	29% 4 5% 15% 26% 21%	1980
November	1st to 10th	Day	36% 4 8% 20% 22% 10%	1858
	11th to 20th	Day	33% 5% 8% 18% 21% 15%	1755
	21st to end	Day	40% 5% 5% 15% 21% 14%	1710
December	1st to 10th	Day	35% 4 9% 19% 23% 9%	1710
	11th to 20th	Day	39% 7% 7% 12% 23% 13%	2159
	21st to end	Day	33% 4 6% 15% 28% 15%	3135
January	1st to 10th	Night	31% 3 5% 17% 25% 18%	2458
	11th to 20th	Night	36% 3 7% 16% 22% 17%	2379
	21st to end	Night	36% 3 5% 14% 22% 20%	2487
February	1st to 10th	Night	38% 3 7% 13% 21% 18%	2160
	11th to 20th	Night	37% 4 7% 16% 20% 16%	2098
	21st to end	Night	40% 3 6% 14% 18% 19%	1776
March	1st to 10th	Night	38% 6% 9% 14% 23% 10%	2010
	11th to 20th	Night	39% 6% 8% 12% 20% 14%	1980
	21st to end	Night	33% 6% 10% 17% 20% 13%	2178
April	1st to 10th	Night	35% 6% 8% 18% 18% 14%	1980
	11th to 20th	Night	37% 6% 10% 14% 18% 15%	1936
	21st to end	Night	36% 5% 9% 20% 20% 10%	1648
May	1st to 10th	Night	33% 8% 10% 21% 19% 9%	1620
	11th to 20th	Night	34% 7% 12% 26% 16% 4	1594
	21st to end	Night	34% 7% 16% 28% 14% 2	1746
June	1st to 10th	Night	19% 7% 18% 29% 22% 4	1620
	11th to 20th	Night	19% 7% 16% 31% 21% 6%	1620
	21st to end	Night	26% 8% 15% 27% 19% 4	1620
July	1st to 10th	Night	27% 10% 21% 28% 13% 1	1575
	11th to 20th	Night	14% 8% 22% 32% 23% 2	1566
	21st to end	Night	14% 8% 26% 39% 13%	1673
August	1st to 10th	Night	9% 6% 20% 40% 22% 2	1601
	11th to 20th	Night	24% 8% 21% 29% 13% 4	1747
	21st to end	Night	24% 9% 21% 28% 14% 4	2067
September	1st to 10th	Night	29% 8% 13% 25% 18% 8%	1980
	11th to 20th	Night	33% 8% 15% 23% 14% 6%	1980
	21st to end	Night	27% 6% 14% 23% 18% 13%	1980
October	1st to 10th	Night	21% 5% 12% 25% 22% 14%	1980
	11th to 20th	Night	29% 6% 9% 14% 22% 20%	2089
	21st to end	Night	30% 4 6% 16% 22% 22%	2376
November	1st to 10th	Night	32% 5% 8% 19% 23% 14%	2220
	11th to 20th	Night	29% 5% 8% 20% 22% 16%	2421
	21st to end	Night	39% 4 7% 15% 20% 15%	2470
December	1st to 10th	Night	33% 5% 9% 20% 22% 12%	2470
	11th to 20th	Night	40% 4 6% 11% 22% 17%	3037
	21st to end	Night	34% 3 5% 15% 29% 14%	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	20% 12% 37% 26% 5%	270
	11th to 20th	Day	25% 12% 29% 24% 8% 1	248
	21st to end	Day	17% 20% 36% 22% 4	264
February	1st to 10th	Day	25% 18% 33% 20% 4	240
	11th to 20th	Day	26% 16% 36% 19% 3	240
	21st to end	Day	29% 18% 35% 18%	198
March	1st to 10th	Day	23% 16% 37% 22% 3	240
	11th to 20th	Day	20% 17% 35% 23% 5%	240
	21st to end	Day	17% 18% 41% 19% 5%	264
April	1st to 10th	Day	19% 18% 42% 18% 3	240
	11th to 20th	Day	18% 17% 37% 22% 7%	245
	21st to end	Day	17% 14% 42% 19% 8%	572
May	1st to 10th	Day	15% 11% 37% 31% 6%	603
	11th to 20th	Day	17% 6% 31% 32% 11% 2	625
	21st to end	Day	17% 7% 27% 32% 15% 2	679
June	1st to 10th	Day	8% 2 16% 42% 26% 7%	630
	11th to 20th	Day	7% 1 17% 50% 21% 4	630
	21st to end	Day	7% 1 13% 41% 30% 8%	630
July	1st to 10th	Day	9% 2 10% 36% 33% 10%	621
	11th to 20th	Day	3 1 6% 35% 46% 10%	618
	21st to end	Day	3 1 4 34% 46% 12%	684
August	1st to 10th	Day	2 3 36% 46% 13%	626
	11th to 20th	Day	5% 1 6% 34% 41% 13%	463
	21st to end	Day	5% 1 4 28% 40% 21%	405
September	1st to 10th	Day	10% 1 3 22% 43% 21%	270
	11th to 20th	Day	9% 1 6% 27% 44% 13%	270
	21st to end	Day	11% 1 8% 31% 31% 17%	270
October	1st to 10th	Day	6% 1 14% 38% 33% 8%	270
	11th to 20th	Day	10% 7% 24% 31% 24% 4	270
	21st to end	Day	23% 10% 23% 25% 18% 1	297
November	1st to 10th	Day	19% 6% 29% 33% 10% 4	270
	11th to 20th	Day	15% 10% 30% 31% 11% 3	270
	21st to end	Day	21% 18% 33% 19% 8% 1	270
December	1st to 10th	Day	14% 9% 36% 28% 9% 4	270
	11th to 20th	Day	23% 11% 37% 23% 6%	270
	21st to end	Day	22% 9% 40% 21% 7% 1	297
January	1st to 10th	Night	18% 9% 35% 31% 7%	950
	11th to 20th	Night	20% 10% 31% 33% 7%	915
	21st to end	Night	18% 10% 35% 31% 6%	903
February	1st to 10th	Night	22% 11% 34% 28% 4	720
	11th to 20th	Night	23% 11% 37% 26% 3	704
	21st to end	Night	24% 11% 44% 19% 2	592
March	1st to 10th	Night	20% 12% 38% 28% 3	570
	11th to 20th	Night	17% 10% 36% 32% 5%	540
	21st to end	Night	14% 10% 45% 27% 3	594
April	1st to 10th	Night	12% 12% 45% 29% 2	540
	11th to 20th	Night	17% 11% 44% 24% 3	533
	21st to end	Night	14% 10% 46% 22% 7% 1	540
May	1st to 10th	Night	12% 10% 45% 28% 5%	540
	11th to 20th	Night	10% 7% 35% 35% 13% 1	531
	21st to end	Night	11% 6% 34% 36% 12% 1	584
June	1st to 10th	Night	4 2 24% 46% 23% 1	540
	11th to 20th	Night	5% 1 27% 53% 13% 1	540
	21st to end	Night	5% 3 17% 48% 25% 3	540
July	1st to 10th	Night	8% 2 11% 47% 29% 3	525
	11th to 20th	Night	3 8% 56% 31% 2	522
	21st to end	Night	3 1 9% 52% 33% 3	591
August	1st to 10th	Night	2 6% 54% 34% 4	537
	11th to 20th	Night	7% 6% 49% 35% 3	534
	21st to end	Night	5% 1 6% 43% 39% 6%	594
September	1st to 10th	Night	9% 1 10% 40% 36% 5%	540
	11th to 20th	Night	12% 1 8% 40% 35% 3	540
	21st to end	Night	10% 1 11% 38% 37% 3	540
October	1st to 10th	Night	7% 1 14% 48% 26% 4	540
	11th to 20th	Night	14% 4 24% 39% 18% 2	649
	21st to end	Night	15% 8% 26% 30% 19% 1	792
November	1st to 10th	Night	14% 4 23% 42% 16% 1	740
	11th to 20th	Night	11% 8% 28% 38% 14% 1	901
	21st to end	Night	20% 9% 29% 30% 11% 1	950
December	1st to 10th	Night	15% 8% 31% 36% 9% 1	950
	11th to 20th	Night	22% 9% 31% 32% 6%	950
	21st to end	Night	19% 8% 31% 35% 7%	1045

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