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 2026-01-02.

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	11% 6% 2 5% 4 4 4 4 5% 7% 49%	127265
	Night	12% 6% 2 5% 3 3 3 3 5% 7% 52%	124377
Transparency	Day	33% 6% 10% 21% 20% 10%	131032
	Night	32% 5% 10% 20% 20% 13%	128673
Seeing	Day	13% 8% 23% 30% 21% 6%	16409
	Night	14% 7% 27% 36% 15% 1	26378

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% 9% 2 5% 3 3 3 3 4 5% 50%	4086
	11th to 20th	Day	13% 5% 2 4 2 2 3 3 5% 6% 55%	4080
	21st to end	Day	23% 8% 2 6% 4 2 3 1 3 5% 44%	4745
February	1st to 10th	Day	17% 7% 3 5% 3 3 2 2 4 6% 48%	2000
	11th to 20th	Day	16% 7% 1 5% 3 3 4 3 5% 6% 46%	1949
	21st to end	Day	19% 6% 2 4 3 3 3 3 4 5% 51%	1650
March	1st to 10th	Day	11% 7% 2 6% 3 3 3 2 5% 6% 51%	4867
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 4 6% 52%	4980
	21st to end	Day	12% 7% 3 6% 4 4 4 3 5% 7% 47%	5462
April	1st to 10th	Day	12% 9% 2 6% 4 4 4 3 5% 6% 44%	2200
	11th to 20th	Day	15% 7% 2 6% 4 3 4 3 4 5% 45%	2189
	21st to end	Day	13% 6% 2 5% 4 4 4 4 4 6% 47%	2570
May	1st to 10th	Day	18% 7% 2 6% 4 4 5% 4 7% 8% 35%	5862
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 6% 7% 45%	5862
	21st to end	Day	12% 8% 3 6% 5% 5% 6% 5% 7% 8% 35%	6434
June	1st to 10th	Day	5% 4 2 5% 4 5% 5% 5% 6% 8% 52%	2600
	11th to 20th	Day	6% 4 2 5% 4 4 5% 3 6% 9% 53%	2600
	21st to end	Day	6% 5% 2 5% 5% 5% 6% 5% 7% 9% 46%	2600
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	5900
	11th to 20th	Day	4 3 2 5% 4 4 5% 5% 7% 11% 51%	5900
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 8% 11% 46%	6422
August	1st to 10th	Day	1 1 1 2 3 3 4 3 5% 8% 70%	4234
	11th to 20th	Day	3 2 1 4 3 3 3 3 6% 10% 61%	3886
	21st to end	Day	3 3 1 5% 4 4 5% 5% 8% 11% 50%	4147
September	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 6% 7% 49%	2200
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 45%	2200
	21st to end	Day	6% 4 2 5% 4 3 5% 4 6% 7% 54%	2200
October	1st to 10th	Day	5% 4 2 4 3 3 4 3 5% 6% 61%	2200
	11th to 20th	Day	11% 5% 1 4 3 3 3 3 4 5% 58%	2055
	21st to end	Day	11% 4 2 5% 3 3 3 2 3 4 61%	2200
November	1st to 10th	Day	13% 8% 2 5% 3 3 3 2 3 5% 52%	2058
	11th to 20th	Day	8% 5% 2 6% 4 4 3 3 4 6% 55%	1939
	21st to end	Day	15% 6% 2 6% 3 3 4 3 4 5% 49%	1890
December	1st to 10th	Day	19% 8% 3 7% 3 2 3 2 3 4 44%	3030
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 4 6% 45%	3481
	21st to end	Day	14% 6% 2 3 2 2 2 2 3 4 59%	4587
January	1st to 10th	Night	16% 6% 2 4 2 2 2 2 3 4 55%	5902
	11th to 20th	Night	11% 3 1 3 2 1 2 2 4 5% 65%	5900
	21st to end	Night	25% 6% 2 6% 3 3 3 2 3 5% 42%	6263
February	1st to 10th	Night	15% 8% 2 5% 3 2 3 2 3 5% 51%	2400
	11th to 20th	Night	17% 7% 2 4 3 3 3 2 4 5% 50%	2338
	21st to end	Night	18% 7% 2 5% 4 3 2 2 3 5% 50%	1980
March	1st to 10th	Night	12% 7% 2 5% 4 3 4 4 5% 8% 47%	4975
	11th to 20th	Night	12% 7% 2 5% 3 3 4 3 5% 6% 51%	4980
	21st to end	Night	13% 9% 3 6% 4 3 3 3 4 6% 47%	5467
April	1st to 10th	Night	11% 6% 3 5% 4 4 3 3 5% 7% 50%	2200
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 6% 6% 47%	2154
	21st to end	Night	14% 7% 2 5% 3 3 4 3 5% 6% 49%	1830
May	1st to 10th	Night	20% 8% 3 6% 4 3 4 3 4 7% 40%	4053
	11th to 20th	Night	7% 5% 3 7% 5% 4 5% 4 6% 7% 49%	4053
	21st to end	Night	12% 7% 2 6% 4 5% 4 4 6% 9% 40%	4443
June	1st to 10th	Night	3 5% 1 4 3 3 3 4 6% 9% 59%	1800
	11th to 20th	Night	5% 4 1 3 3 3 3 4 6% 8% 59%	1800
	21st to end	Night	5% 5% 1 3 3 4 4 4 6% 7% 57%	1800
July	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 7% 10% 44%	4078
	11th to 20th	Night	3 3 1 5% 3 3 4 3 7% 11% 58%	4077
	21st to end	Night	4 6% 2 5% 4 3 5% 4 8% 10% 50%	4449
August	1st to 10th	Night	1 1 2 1 2 2 3 5% 7% 77%	2926
	11th to 20th	Night	3 2 1 2 2 3 4 3 6% 10% 63%	3167
	21st to end	Night	3 3 1 4 3 4 4 4 7% 10% 57%	3754
September	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 3 5% 7% 51%	2200
	11th to 20th	Night	9% 6% 3 6% 4 5% 4 3 5% 8% 48%	2200
	21st to end	Night	6% 4 2 4 3 3 3 3 6% 9% 57%	2200
October	1st to 10th	Night	6% 3 1 3 2 3 2 2 4 7% 66%	2200
	11th to 20th	Night	10% 4 2 4 3 2 3 3 4 6% 58%	2322
	21st to end	Night	13% 5% 2 3 2 2 3 2 4 4 60%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 2 2 2 4 5% 56%	2460
	11th to 20th	Night	8% 5% 2 4 3 3 3 2 4 6% 59%	2677
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 6% 49%	2730
December	1st to 10th	Night	19% 6% 3 5% 3 3 4 3 5% 6% 44%	4380
	11th to 20th	Night	18% 10% 3 5% 3 2 2 2 3 4 48%	4946
	21st to end	Night	15% 7% 2 4 2 2 2 2 2 4 58%	6633

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	37% 4 4 23% 21% 10%	4087
	11th to 20th	Day	30% 3 5% 17% 27% 18%	4080
	21st to end	Day	47% 3 3 9% 24% 14%	4745
February	1st to 10th	Day	39% 5% 6% 12% 21% 17%	2000
	11th to 20th	Day	39% 5% 7% 18% 19% 13%	1959
	21st to end	Day	38% 4 5% 16% 21% 17%	1650
March	1st to 10th	Day	34% 4 4 17% 18% 22%	4867
	11th to 20th	Day	35% 6% 6% 8% 14% 30%	4980
	21st to end	Day	37% 6% 9% 18% 22% 9%	5464
April	1st to 10th	Day	40% 7% 8% 17% 15% 14%	2200
	11th to 20th	Day	41% 5% 8% 16% 19% 11%	2189
	21st to end	Day	37% 6% 7% 20% 21% 9%	2570
May	1st to 10th	Day	44% 8% 10% 18% 16% 4	5900
	11th to 20th	Day	37% 6% 11% 15% 24% 7%	5866
	21st to end	Day	40% 10% 12% 21% 13% 3	6447
June	1st to 10th	Day	27% 9% 13% 28% 20% 3	2600
	11th to 20th	Day	27% 7% 14% 28% 19% 6%	2600
	21st to end	Day	30% 11% 13% 23% 19% 3	2600
July	1st to 10th	Day	40% 7% 19% 27% 6% 1	5836
	11th to 20th	Day	26% 7% 17% 33% 15% 2	5831
	21st to end	Day	30% 7% 18% 28% 16% 2	6240
August	1st to 10th	Day	13% 4 12% 39% 31% 1	4220
	11th to 20th	Day	18% 5% 22% 38% 13% 3	3923
	21st to end	Day	27% 9% 18% 25% 18% 2	4146
September	1st to 10th	Day	35% 6% 11% 21% 17% 10%	2200
	11th to 20th	Day	36% 8% 12% 21% 19% 6%	2200
	21st to end	Day	27% 7% 10% 21% 21% 15%	2200
October	1st to 10th	Day	15% 4 6% 13% 25% 37%	3590
	11th to 20th	Day	19% 3 7% 20% 30% 20%	3321
	21st to end	Day	24% 4 3 20% 35% 14%	3538
November	1st to 10th	Day	36% 4 7% 18% 24% 11%	2058
	11th to 20th	Day	32% 4 8% 19% 21% 16%	1939
	21st to end	Day	40% 5% 5% 17% 20% 13%	1890
December	1st to 10th	Day	45% 4 6% 15% 21% 9%	3030
	11th to 20th	Day	40% 6% 6% 12% 23% 14%	3479
	21st to end	Day	31% 3 4 17% 28% 17%	4587
January	1st to 10th	Night	34% 4 4 21% 25% 12%	5902
	11th to 20th	Night	23% 2 4 18% 30% 22%	5900
	21st to end	Night	47% 4 3 10% 20% 16%	6263
February	1st to 10th	Night	38% 4 6% 12% 21% 19%	2400
	11th to 20th	Night	38% 4 7% 17% 19% 15%	2338
	21st to end	Night	40% 3 6% 13% 20% 18%	1980
March	1st to 10th	Night	34% 6% 6% 17% 21% 15%	4975
	11th to 20th	Night	34% 6% 7% 10% 15% 26%	4980
	21st to end	Night	38% 7% 7% 17% 21% 10%	5467
April	1st to 10th	Night	35% 6% 8% 17% 17% 16%	2200
	11th to 20th	Night	37% 6% 10% 14% 18% 15%	2154
	21st to end	Night	37% 6% 9% 19% 20% 10%	1830
May	1st to 10th	Night	45% 6% 8% 19% 15% 7%	4080
	11th to 20th	Night	33% 6% 12% 20% 18% 11%	4054
	21st to end	Night	39% 8% 14% 25% 10% 4	4452
June	1st to 10th	Night	20% 8% 18% 30% 22% 3	1800
	11th to 20th	Night	21% 7% 15% 30% 21% 6%	1800
	21st to end	Night	25% 8% 16% 27% 20% 4	1800
July	1st to 10th	Night	35% 7% 24% 28% 5% 1	4035
	11th to 20th	Night	21% 6% 22% 36% 14% 1	4026
	21st to end	Night	28% 7% 22% 26% 14% 3	4320
August	1st to 10th	Night	8% 4 13% 42% 32% 1	2916
	11th to 20th	Night	17% 5% 30% 33% 12% 2	3196
	21st to end	Night	21% 10% 21% 27% 18% 2	3754
September	1st to 10th	Night	31% 7% 12% 23% 18% 9%	2200
	11th to 20th	Night	33% 8% 15% 24% 15% 7%	2200
	21st to end	Night	25% 5% 13% 25% 18% 14%	2200
October	1st to 10th	Night	13% 3 7% 15% 22% 40%	3590
	11th to 20th	Night	18% 4 9% 17% 32% 20%	3782
	21st to end	Night	23% 5% 4 19% 33% 16%	4252
November	1st to 10th	Night	32% 5% 7% 19% 24% 14%	2460
	11th to 20th	Night	27% 5% 8% 20% 22% 17%	2677
	21st to end	Night	38% 4 7% 16% 20% 15%	2730
December	1st to 10th	Night	43% 4 6% 17% 20% 11%	4380
	11th to 20th	Night	43% 3 5% 9% 23% 17%	4947
	21st to end	Night	33% 3 4 16% 28% 17%	6633

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	21% 13% 37% 24% 5%	303
	11th to 20th	Day	24% 14% 31% 23% 7% 1	300
	21st to end	Day	19% 22% 33% 21% 5%	330
February	1st to 10th	Day	26% 17% 32% 21% 4	300
	11th to 20th	Day	24% 18% 37% 18% 3	300
	21st to end	Day	27% 18% 37% 18% 1	249
March	1st to 10th	Day	23% 16% 38% 21% 3	300
	11th to 20th	Day	21% 19% 37% 20% 4	300
	21st to end	Day	17% 18% 43% 18% 4	330
April	1st to 10th	Day	18% 20% 40% 17% 4	300
	11th to 20th	Day	18% 18% 39% 20% 6%	307
	21st to end	Day	17% 14% 42% 20% 7%	670
May	1st to 10th	Day	16% 12% 37% 29% 6%	703
	11th to 20th	Day	16% 8% 33% 31% 10% 2	725
	21st to end	Day	16% 7% 29% 32% 14% 2	789
June	1st to 10th	Day	8% 3 18% 42% 23% 6%	730
	11th to 20th	Day	8% 2 18% 47% 21% 4	730
	21st to end	Day	6% 2 13% 40% 32% 8%	730
July	1st to 10th	Day	10% 3 9% 36% 32% 10%	721
	11th to 20th	Day	3 2 6% 36% 44% 8%	718
	21st to end	Day	4 1 6% 34% 44% 12%	794
August	1st to 10th	Day	2 2 34% 47% 14%	726
	11th to 20th	Day	4 1 6% 34% 42% 13%	545
	21st to end	Day	5% 1 4 28% 42% 20%	483
September	1st to 10th	Day	10% 2 6% 25% 39% 18%	330
	11th to 20th	Day	7% 1 6% 28% 45% 12%	330
	21st to end	Day	10% 1 8% 35% 30% 16%	330
October	1st to 10th	Day	5% 1 13% 37% 35% 9%	330
	11th to 20th	Day	9% 6% 24% 33% 24% 5%	330
	21st to end	Day	20% 10% 22% 27% 19% 1	363
November	1st to 10th	Day	19% 6% 28% 32% 10% 4	330
	11th to 20th	Day	14% 9% 31% 32% 12% 3	330
	21st to end	Day	20% 17% 34% 17% 10% 2	330
December	1st to 10th	Day	17% 9% 35% 26% 8% 3	330
	11th to 20th	Day	22% 12% 33% 25% 6% 2	330
	21st to end	Day	20% 11% 37% 23% 8% 1	363
January	1st to 10th	Night	19% 9% 35% 30% 7%	1006
	11th to 20th	Night	19% 12% 32% 31% 6%	1000
	21st to end	Night	20% 10% 34% 30% 6%	1003
February	1st to 10th	Night	23% 11% 34% 29% 4	800
	11th to 20th	Night	24% 12% 36% 25% 2	784
	21st to end	Night	23% 12% 43% 20% 2	660
March	1st to 10th	Night	20% 13% 36% 28% 3	632
	11th to 20th	Night	17% 11% 37% 31% 4	600
	21st to end	Night	15% 11% 44% 27% 3	660
April	1st to 10th	Night	14% 12% 43% 29% 2	600
	11th to 20th	Night	17% 13% 43% 24% 3	593
	21st to end	Night	15% 11% 44% 23% 7% 1	600
May	1st to 10th	Night	14% 11% 43% 27% 5%	600
	11th to 20th	Night	9% 8% 37% 33% 12% 1	591
	21st to end	Night	11% 7% 36% 35% 11% 1	650
June	1st to 10th	Night	4 2 25% 45% 22% 2	600
	11th to 20th	Night	6% 2 28% 51% 12% 1	600
	21st to end	Night	5% 3 17% 50% 24% 3	600
July	1st to 10th	Night	9% 3 10% 47% 29% 3	585
	11th to 20th	Night	4 9% 55% 30% 2	582
	21st to end	Night	4 1 10% 50% 32% 3	657
August	1st to 10th	Night	2 5% 53% 36% 4	597
	11th to 20th	Night	6% 6% 51% 34% 3	594
	21st to end	Night	4 1 6% 43% 40% 5%	660
September	1st to 10th	Night	9% 2 10% 42% 34% 4	600
	11th to 20th	Night	11% 2 9% 41% 35% 3	600
	21st to end	Night	9% 2 11% 40% 36% 3	600
October	1st to 10th	Night	6% 1 13% 48% 28% 4	600
	11th to 20th	Night	12% 4 23% 40% 19% 2	722
	21st to end	Night	14% 8% 26% 31% 19% 1	880
November	1st to 10th	Night	15% 4 23% 41% 15% 1	820
	11th to 20th	Night	10% 8% 28% 38% 14% 1	997
	21st to end	Night	20% 9% 29% 30% 12%	1050
December	1st to 10th	Night	16% 9% 31% 35% 8% 1	1050
	11th to 20th	Night	22% 9% 30% 33% 6%	1050
	21st to end	Night	19% 8% 30% 35% 7%	1155

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