Mineral Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mineral Ranch Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	131963
	Night	13% 6% 2 5% 4 3 3 3 5% 7% 48%	130529
Transparency	Day	37% 6% 11% 21% 16% 9%	135742
	Night	35% 5% 12% 21% 16% 11%	134814
Seeing	Day	11% 6% 23% 30% 22% 8%	16443
	Night	14% 6% 26% 37% 16% 1	26516

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	16% 8% 3 7% 4 3 3 2 3 5% 47%	5289
	11th to 20th	Day	14% 7% 2 6% 3 3 3 2 4 5% 51%	5310
	21st to end	Day	25% 8% 3 6% 3 3 3 3 5% 6% 37%	6366
February	1st to 10th	Day	18% 7% 2 7% 4 4 2 3 4 5% 44%	2100
	11th to 20th	Day	18% 8% 2 6% 3 3 4 3 4 5% 42%	2049
	21st to end	Day	17% 7% 2 5% 3 4 4 3 5% 4 47%	1740
March	1st to 10th	Day	13% 7% 2 7% 4 4 4 3 6% 7% 44%	5046
	11th to 20th	Day	14% 7% 2 7% 4 3 3 2 4 5% 48%	5090
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 46%	5583
April	1st to 10th	Day	11% 6% 3 6% 5% 4 5% 5% 6% 6% 43%	2310
	11th to 20th	Day	15% 7% 3 7% 5% 4 4 4 5% 6% 40%	2242
	21st to end	Day	12% 8% 2 7% 5% 4 4 4 5% 5% 44%	2435
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	5862
	11th to 20th	Day	7% 5% 2 6% 5% 5% 4 4 6% 7% 50%	5862
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 4 7% 10% 40%	6434
June	1st to 10th	Day	5% 3 2 5% 5% 5% 6% 5% 7% 10% 47%	2600
	11th to 20th	Day	5% 4 2 6% 5% 6% 5% 6% 7% 9% 45%	2600
	21st to end	Day	7% 5% 2 6% 6% 7% 6% 7% 9% 10% 35%	2600
July	1st to 10th	Day	9% 7% 3 9% 6% 6% 6% 5% 7% 9% 33%	5900
	11th to 20th	Day	4 6% 2 7% 5% 5% 6% 6% 8% 11% 39%	5900
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 6% 7% 11% 38%	6422
August	1st to 10th	Day	1 1 1 3 3 3 4 3 6% 9% 64%	4146
	11th to 20th	Day	3 2 1 4 3 4 6% 5% 9% 13% 50%	3846
	21st to end	Day	5% 3 2 7% 6% 6% 8% 6% 9% 11% 37%	4052
September	1st to 10th	Day	9% 6% 2 7% 6% 7% 6% 5% 7% 8% 36%	2200
	11th to 20th	Day	10% 5% 3 7% 5% 6% 6% 5% 6% 8% 39%	2200
	21st to end	Day	7% 4 2 6% 5% 5% 6% 4 6% 8% 47%	2200
October	1st to 10th	Day	5% 5% 1 5% 5% 5% 5% 5% 6% 7% 52%	2200
	11th to 20th	Day	10% 6% 2 4 4 4 4 4 4 7% 52%	2070
	21st to end	Day	8% 4 2 4 4 3 3 3 4 5% 59%	2200
November	1st to 10th	Day	10% 6% 2 7% 5% 5% 4 3 5% 7% 48%	2058
	11th to 20th	Day	13% 5% 2 5% 4 3 3 3 5% 6% 51%	2063
	21st to end	Day	17% 8% 2 7% 3 4 4 4 5% 6% 41%	1890
December	1st to 10th	Day	29% 11% 3 7% 4 3 3 2 3 4 32%	3030
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 4 5% 6% 41%	3481
	21st to end	Day	11% 6% 2 6% 4 3 4 3 5% 6% 52%	4587
January	1st to 10th	Night	17% 8% 2 5% 3 3 3 2 4 5% 49%	7634
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 6% 55%	7680
	21st to end	Night	28% 7% 2 6% 3 3 3 2 5% 5% 36%	7889
February	1st to 10th	Night	16% 9% 2 5% 4 4 3 3 4 6% 44%	2520
	11th to 20th	Night	19% 7% 2 5% 3 2 3 2 4 6% 46%	2458
	21st to end	Night	17% 8% 3 5% 3 3 3 3 4 5% 47%	2063
March	1st to 10th	Night	19% 8% 2 7% 4 4 3 3 4 6% 39%	5029
	11th to 20th	Night	17% 8% 2 6% 3 3 3 2 3 4 49%	5090
	21st to end	Night	18% 8% 2 4 3 3 3 3 4 6% 47%	5588
April	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	2310
	11th to 20th	Night	15% 7% 3 6% 3 3 4 3 5% 7% 44%	2239
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 6% 39%	1957
May	1st to 10th	Night	19% 10% 3 8% 5% 4 4 4 6% 7% 31%	4053
	11th to 20th	Night	9% 6% 2 5% 3 4 3 3 5% 9% 50%	4053
	21st to end	Night	10% 7% 3 8% 6% 5% 5% 4 6% 9% 36%	4443
June	1st to 10th	Night	5% 5% 1 4 3 3 3 3 7% 10% 57%	1800
	11th to 20th	Night	4 3 1 4 4 4 4 4 6% 9% 58%	1800
	21st to end	Night	5% 3 2 4 4 4 5% 5% 8% 11% 50%	1800
July	1st to 10th	Night	6% 6% 3 8% 5% 5% 5% 4 7% 9% 43%	4078
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4077
	21st to end	Night	6% 4 2 6% 4 5% 5% 4 6% 10% 48%	4449
August	1st to 10th	Night	1 2 1 1 2 3 4 7% 79%	3007
	11th to 20th	Night	3 2 1 3 3 4 4 3 6% 12% 59%	3213
	21st to end	Night	3 3 1 4 2 4 4 3 6% 13% 58%	3848
September	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 7% 8% 48%	2200
	11th to 20th	Night	8% 6% 2 5% 4 4 4 4 6% 9% 50%	2200
	21st to end	Night	6% 5% 2 6% 4 3 4 3 5% 7% 56%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 5% 6% 66%	2200
	11th to 20th	Night	9% 4 1 4 3 2 3 3 4 5% 61%	2310
	21st to end	Night	10% 4 1 4 2 2 2 2 4 6% 65%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 4 2 5% 7% 53%	2460
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 3 5% 55%	2552
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 6% 45%	2730
December	1st to 10th	Night	32% 11% 3 7% 3 2 2 1 3 4 32%	4380
	11th to 20th	Night	14% 7% 3 7% 4 4 3 2 4 6% 46%	4946
	21st to end	Night	12% 7% 2 5% 3 2 3 2 4 5% 55%	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	42% 3 8% 17% 19% 10%	5290
	11th to 20th	Day	37% 3 4 17% 25% 15%	5310
	21st to end	Day	50% 3 6% 13% 20% 9%	6366
February	1st to 10th	Day	45% 3 6% 11% 20% 15%	2100
	11th to 20th	Day	44% 4 8% 16% 16% 11%	2059
	21st to end	Day	40% 4 7% 18% 19% 12%	1740
March	1st to 10th	Day	38% 5% 8% 20% 12% 18%	5046
	11th to 20th	Day	40% 4 5% 9% 17% 25%	5090
	21st to end	Day	39% 4 9% 20% 17% 10%	5585
April	1st to 10th	Day	40% 6% 10% 16% 17% 12%	2310
	11th to 20th	Day	44% 5% 8% 17% 14% 12%	2242
	21st to end	Day	40% 6% 8% 19% 16% 10%	2435
May	1st to 10th	Day	46% 6% 13% 20% 11% 4	5900
	11th to 20th	Day	32% 6% 11% 24% 18% 9%	5866
	21st to end	Day	36% 7% 10% 34% 11% 1	6447
June	1st to 10th	Day	28% 9% 15% 28% 18% 2	2600
	11th to 20th	Day	32% 8% 11% 26% 20% 4	2600
	21st to end	Day	38% 11% 15% 25% 10% 1	2600
July	1st to 10th	Day	44% 8% 19% 25% 4	5836
	11th to 20th	Day	34% 9% 23% 26% 8%	5831
	21st to end	Day	36% 9% 18% 25% 11% 1	6240
August	1st to 10th	Day	16% 6% 14% 46% 18% 1	4132
	11th to 20th	Day	22% 9% 28% 32% 7% 1	3882
	21st to end	Day	36% 11% 18% 23% 12% 1	4051
September	1st to 10th	Day	43% 10% 13% 15% 14% 5%	2200
	11th to 20th	Day	39% 9% 15% 20% 14% 4	2200
	21st to end	Day	33% 7% 13% 19% 17% 11%	2200
October	1st to 10th	Day	22% 4 7% 13% 21% 34%	3590
	11th to 20th	Day	25% 5% 10% 19% 22% 20%	3349
	21st to end	Day	25% 4 7% 18% 28% 17%	3538
November	1st to 10th	Day	37% 4 9% 18% 20% 12%	2058
	11th to 20th	Day	36% 3 10% 17% 19% 15%	2063
	21st to end	Day	44% 4 7% 17% 16% 11%	1890
December	1st to 10th	Day	58% 3 5% 11% 17% 6%	3030
	11th to 20th	Day	43% 5% 6% 17% 16% 13%	3479
	21st to end	Day	34% 4 5% 21% 20% 15%	4587
January	1st to 10th	Night	40% 3 9% 16% 18% 15%	7635
	11th to 20th	Night	33% 2 5% 19% 22% 19%	7680
	21st to end	Night	51% 3 5% 14% 17% 11%	7889
February	1st to 10th	Night	43% 3 9% 11% 17% 17%	2520
	11th to 20th	Night	40% 3 9% 18% 17% 13%	2458
	21st to end	Night	41% 3 8% 19% 18% 11%	2063
March	1st to 10th	Night	46% 5% 7% 16% 13% 13%	5029
	11th to 20th	Night	41% 3 6% 11% 16% 23%	5090
	21st to end	Night	40% 4 11% 18% 18% 10%	5588
April	1st to 10th	Night	41% 6% 11% 18% 13% 10%	2310
	11th to 20th	Night	41% 5% 10% 16% 14% 14%	2239
	21st to end	Night	45% 5% 10% 17% 14% 8%	1957
May	1st to 10th	Night	53% 5% 10% 16% 11% 4	4080
	11th to 20th	Night	32% 5% 14% 23% 19% 7%	4054
	21st to end	Night	44% 6% 16% 27% 6%	4452
June	1st to 10th	Night	23% 6% 20% 32% 18% 2	1800
	11th to 20th	Night	24% 6% 13% 33% 22% 3	1800
	21st to end	Night	26% 10% 23% 30% 10% 1	1800
July	1st to 10th	Night	37% 8% 25% 27% 3	4035
	11th to 20th	Night	28% 9% 29% 27% 7%	4026
	21st to end	Night	32% 8% 23% 25% 11% 1	4320
August	1st to 10th	Night	8% 4 17% 53% 17%	2997
	11th to 20th	Night	17% 7% 37% 34% 4	3243
	21st to end	Night	19% 11% 29% 31% 9% 1	3848
September	1st to 10th	Night	32% 9% 22% 21% 12% 4	2200
	11th to 20th	Night	31% 7% 18% 26% 16% 3	2200
	21st to end	Night	27% 6% 14% 27% 17% 9%	2200
October	1st to 10th	Night	14% 3 9% 19% 21% 33%	3590
	11th to 20th	Night	20% 4 10% 21% 27% 18%	3757
	21st to end	Night	20% 3 8% 24% 30% 16%	4252
November	1st to 10th	Night	31% 6% 11% 18% 23% 12%	2460
	11th to 20th	Night	32% 5% 12% 18% 19% 15%	2552
	21st to end	Night	40% 4 10% 18% 17% 11%	2730
December	1st to 10th	Night	60% 2 5% 11% 13% 8%	4380
	11th to 20th	Night	41% 4 9% 15% 16% 15%	4947
	21st to end	Night	33% 4 6% 18% 23% 16%	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	20% 10% 37% 22% 9% 2	330
	11th to 20th	Day	20% 9% 35% 25% 9% 1	330
	21st to end	Day	23% 9% 34% 23% 9% 1	363
February	1st to 10th	Day	21% 12% 26% 28% 12% 1	330
	11th to 20th	Day	22% 12% 36% 22% 7%	330
	21st to end	Day	22% 12% 38% 23% 5%	273
March	1st to 10th	Day	18% 11% 41% 24% 6%	330
	11th to 20th	Day	15% 12% 43% 23% 7%	330
	21st to end	Day	14% 12% 39% 27% 7% 1	363
April	1st to 10th	Day	14% 15% 38% 25% 8%	330
	11th to 20th	Day	18% 13% 34% 24% 10% 1	312
	21st to end	Day	13% 14% 40% 26% 6%	535
May	1st to 10th	Day	12% 14% 43% 25% 6%	703
	11th to 20th	Day	10% 8% 37% 31% 12% 2	725
	21st to end	Day	9% 6% 32% 36% 15% 1	789
June	1st to 10th	Day	6% 2 18% 38% 30% 6%	730
	11th to 20th	Day	6% 1 19% 46% 23% 5%	730
	21st to end	Day	8% 3 12% 33% 32% 12%	730
July	1st to 10th	Day	7% 3 11% 33% 34% 11%	721
	11th to 20th	Day	4 1 6% 35% 42% 12%	718
	21st to end	Day	4 2 6% 36% 38% 14%	794
August	1st to 10th	Day	3 6% 33% 38% 19%	677
	11th to 20th	Day	3 1 6% 36% 35% 19%	517
	21st to end	Day	5% 2 7% 23% 35% 29%	427
September	1st to 10th	Day	10% 2 6% 24% 36% 22%	330
	11th to 20th	Day	7% 5% 18% 42% 28%	330
	21st to end	Day	6% 8% 26% 38% 20%	330
October	1st to 10th	Day	5% 1 10% 30% 38% 16%	330
	11th to 20th	Day	10% 2 11% 34% 32% 11%	330
	21st to end	Day	13% 7% 21% 29% 22% 7%	363
November	1st to 10th	Day	13% 5% 20% 32% 25% 6%	330
	11th to 20th	Day	17% 5% 22% 29% 23% 4	330
	21st to end	Day	22% 11% 30% 22% 13% 2	330
December	1st to 10th	Day	19% 7% 30% 27% 13% 4	330
	11th to 20th	Day	19% 9% 27% 29% 14% 1	330
	21st to end	Day	18% 7% 29% 32% 12% 1	363
January	1st to 10th	Night	20% 8% 30% 33% 9%	1050
	11th to 20th	Night	21% 8% 30% 34% 7%	1050
	21st to end	Night	22% 9% 30% 32% 6%	976
February	1st to 10th	Night	23% 11% 31% 30% 5%	840
	11th to 20th	Night	25% 9% 34% 27% 5%	824
	21st to end	Night	23% 10% 41% 23% 3	679
March	1st to 10th	Night	25% 9% 37% 26% 3	630
	11th to 20th	Night	21% 11% 36% 27% 5%	630
	21st to end	Night	15% 10% 44% 27% 3	693
April	1st to 10th	Night	15% 9% 48% 25% 3	630
	11th to 20th	Night	20% 9% 39% 27% 5%	611
	21st to end	Night	19% 11% 41% 25% 5%	600
May	1st to 10th	Night	15% 7% 48% 27% 3	600
	11th to 20th	Night	12% 5% 39% 35% 8%	591
	21st to end	Night	9% 5% 36% 40% 9%	650
June	1st to 10th	Night	6% 3 21% 53% 17% 1	600
	11th to 20th	Night	5% 1 24% 55% 14%	600
	21st to end	Night	7% 1 16% 48% 26% 2	600
July	1st to 10th	Night	5% 2 14% 48% 29% 2	585
	11th to 20th	Night	3 1 10% 51% 33% 2	582
	21st to end	Night	3 2 7% 54% 33% 2	657
August	1st to 10th	Night	1 6% 53% 35% 4	597
	11th to 20th	Night	4 1 7% 49% 36% 3	594
	21st to end	Night	4 1 6% 39% 47% 4	660
September	1st to 10th	Night	9% 1 5% 39% 41% 6%	600
	11th to 20th	Night	11% 1 7% 34% 44% 4	600
	21st to end	Night	8% 1 11% 37% 39% 4	600
October	1st to 10th	Night	4 1 11% 46% 35% 4	600
	11th to 20th	Night	10% 3 22% 45% 18% 2	710
	21st to end	Night	10% 6% 26% 38% 19% 1	880
November	1st to 10th	Night	12% 4 22% 42% 18% 1	820
	11th to 20th	Night	14% 5% 25% 37% 17% 1	872
	21st to end	Night	18% 7% 28% 32% 13% 1	1050
December	1st to 10th	Night	22% 6% 24% 33% 13% 1	1050
	11th to 20th	Night	22% 9% 27% 33% 8%	1050
	21st to end	Night	21% 8% 26% 36% 9% 1	1155

Page generated at 2026-04-14 01:44:25. Version ac07

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