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 2024-01-16.

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% 3 5% 5% 3 5% 5% 6% 8% 44%	74895
	Night	12% 5% 2 4 4 2 4 4 5% 8% 50%	74939
Transparency	Day	38% 6% 11% 20% 16% 8%	74620
	Night	34% 6% 14% 21% 16% 10%	74734
Seeing	Day	11% 6% 22% 30% 23% 8%	13527
	Night	14% 6% 26% 37% 16% 1	23601

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	18% 6% 2 5% 5% 2 4 3 4 5% 46%	1707
	11th to 20th	Day	20% 7% 2 4 5% 3 4 3 4 5% 44%	1670
	21st to end	Day	17% 6% 3 5% 5% 3 3 4 5% 5% 44%	1948
February	1st to 10th	Day	20% 7% 3 6% 5% 2 3 3 4 5% 42%	1800
	11th to 20th	Day	19% 8% 3 5% 3 2 4 5% 4 5% 42%	1749
	21st to end	Day	18% 6% 2 5% 4 3 4 3 5% 5% 45%	1488
March	1st to 10th	Day	19% 7% 2 4 5% 3 4 4 6% 7% 38%	1980
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 6% 40%	1980
	21st to end	Day	12% 6% 3 5% 4 3 4 4 5% 6% 48%	2178
April	1st to 10th	Day	12% 6% 3 6% 5% 3 6% 5% 6% 7% 42%	1980
	11th to 20th	Day	16% 7% 4 5% 5% 3 4 4 5% 6% 41%	1956
	21st to end	Day	13% 7% 3 5% 5% 2 5% 4 5% 5% 46%	2191
May	1st to 10th	Day	9% 6% 3 7% 6% 4 5% 4 4 8% 44%	2340
	11th to 20th	Day	9% 6% 3 5% 7% 5% 6% 5% 7% 8% 40%	2302
	21st to end	Day	9% 5% 3 6% 7% 4 6% 6% 7% 10% 36%	2531
June	1st to 10th	Day	5% 3 2 4 6% 4 6% 6% 7% 10% 47%	2340
	11th to 20th	Day	4 4 3 5% 6% 4 5% 7% 7% 10% 46%	2340
	21st to end	Day	8% 5% 2 6% 7% 6% 6% 8% 8% 10% 35%	2340
July	1st to 10th	Day	10% 5% 3 7% 7% 5% 7% 6% 7% 9% 34%	2340
	11th to 20th	Day	4 4 2 5% 5% 4 6% 6% 7% 10% 48%	2340
	21st to end	Day	4 3 2 4 5% 4 6% 7% 7% 12% 45%	2546
August	1st to 10th	Day	2 2 1 4 5% 4 5% 4 6% 10% 57%	2285
	11th to 20th	Day	5% 3 2 4 5% 3 6% 7% 7% 12% 46%	2123
	21st to end	Day	6% 4 2 6% 7% 5% 8% 7% 7% 11% 36%	2236
September	1st to 10th	Day	9% 5% 3 6% 7% 5% 7% 7% 7% 9% 36%	1980
	11th to 20th	Day	10% 5% 3 7% 6% 4 6% 6% 6% 8% 39%	1980
	21st to end	Day	7% 5% 2 5% 6% 4 6% 6% 5% 8% 45%	1980
October	1st to 10th	Day	5% 4 2 5% 5% 4 5% 6% 6% 8% 49%	1980
	11th to 20th	Day	11% 6% 2 4 4 3 4 5% 5% 7% 50%	1864
	21st to end	Day	9% 4 2 3 4 3 3 4 4 5% 58%	1980
November	1st to 10th	Day	10% 5% 2 6% 6% 3 4 4 5% 8% 45%	1858
	11th to 20th	Day	14% 5% 3 4 5% 2 3 3 5% 6% 48%	1867
	21st to end	Day	17% 8% 2 6% 4 3 4 4 5% 6% 41%	1710
December	1st to 10th	Day	17% 7% 3 6% 5% 3 3 3 4 6% 43%	1710
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 4 5% 6% 41%	2161
	21st to end	Day	13% 6% 2 6% 5% 2 4 4 4 6% 47%	3135
January	1st to 10th	Night	18% 6% 2 4 4 2 3 3 4 6% 48%	2458
	11th to 20th	Night	19% 5% 2 5% 3 2 3 3 4 5% 49%	2418
	21st to end	Night	18% 5% 2 4 4 3 4 4 5% 6% 45%	2421
February	1st to 10th	Night	17% 8% 3 4 5% 3 4 4 4 7% 42%	2160
	11th to 20th	Night	19% 6% 2 5% 3 1 3 3 4 6% 47%	2098
	21st to end	Night	19% 7% 3 4 3 2 4 4 4 5% 46%	1766
March	1st to 10th	Night	18% 7% 3 5% 5% 3 3 4 5% 7% 41%	1980
	11th to 20th	Night	16% 9% 3 5% 4 3 4 3 4 7% 42%	1980
	21st to end	Night	13% 6% 2 4 4 2 4 4 4 7% 51%	2178
April	1st to 10th	Night	12% 6% 2 5% 5% 3 6% 6% 6% 8% 41%	1980
	11th to 20th	Night	15% 7% 3 5% 4 2 4 4 5% 7% 45%	1953
	21st to end	Night	14% 7% 3 6% 5% 3 4 4 5% 7% 42%	1762
May	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 5% 7% 9% 43%	1620
	11th to 20th	Night	10% 5% 2 4 4 3 4 5% 5% 11% 46%	1593
	21st to end	Night	8% 5% 2 4 5% 4 6% 6% 7% 12% 40%	1745
June	1st to 10th	Night	5% 4 2 3 3 2 3 4 6% 10% 58%	1620
	11th to 20th	Night	3 2 1 4 4 3 4 4 6% 9% 59%	1620
	21st to end	Night	5% 3 2 4 5% 3 5% 5% 8% 11% 48%	1620
July	1st to 10th	Night	6% 4 3 5% 5% 3 5% 4 7% 9% 49%	1620
	11th to 20th	Night	3 2 1 2 2 2 4 4 6% 11% 62%	1620
	21st to end	Night	3 1 1 3 3 2 4 4 5% 12% 63%	1772
August	1st to 10th	Night	2 1 1 2 2 1 2 4 5% 8% 72%	1648
	11th to 20th	Night	4 3 1 3 3 2 4 4 6% 11% 59%	1778
	21st to end	Night	5% 4 2 4 4 3 5% 5% 6% 11% 53%	2119
September	1st to 10th	Night	6% 5% 2 5% 5% 3 4 6% 7% 8% 49%	1980
	11th to 20th	Night	9% 6% 2 4 5% 3 4 4 5% 9% 49%	1980
	21st to end	Night	6% 5% 2 5% 5% 2 4 4 5% 8% 55%	1980
October	1st to 10th	Night	4 3 1 3 3 2 3 4 5% 6% 64%	1980
	11th to 20th	Night	10% 4 2 4 4 2 3 4 4 6% 58%	2078
	21st to end	Night	10% 4 1 3 3 2 2 2 3 6% 64%	2376
November	1st to 10th	Night	9% 5% 2 4 5% 3 4 3 5% 8% 52%	2220
	11th to 20th	Night	13% 5% 2 4 4 2 3 4 3 5% 53%	2308
	21st to end	Night	15% 6% 2 4 5% 3 5% 4 4 6% 46%	2470
December	1st to 10th	Night	17% 6% 2 4 4 2 3 3 4 6% 47%	2470
	11th to 20th	Night	18% 7% 3 5% 6% 3 3 4 5% 6% 41%	3036
	21st to end	Night	14% 7% 2 5% 4 2 3 4 5% 7% 47%	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	42% 4 6% 14% 20% 15%	1708
	11th to 20th	Day	44% 4 6% 14% 16% 16%	1670
	21st to end	Day	42% 4 6% 15% 20% 13%	1948
February	1st to 10th	Day	47% 4 6% 10% 20% 14%	1800
	11th to 20th	Day	44% 4 8% 16% 16% 11%	1759
	21st to end	Day	42% 5% 8% 16% 17% 13%	1488
March	1st to 10th	Day	45% 5% 11% 16% 15% 8%	1980
	11th to 20th	Day	46% 5% 9% 11% 16% 13%	1980
	21st to end	Day	37% 4 12% 17% 17% 13%	2178
April	1st to 10th	Day	40% 6% 10% 16% 18% 10%	1980
	11th to 20th	Day	44% 5% 8% 17% 15% 12%	1956
	21st to end	Day	39% 5% 8% 19% 17% 11%	2191
May	1st to 10th	Day	39% 6% 11% 20% 15% 9%	2340
	11th to 20th	Day	39% 7% 10% 19% 19% 5%	2306
	21st to end	Day	40% 8% 11% 23% 16% 1	2531
June	1st to 10th	Day	29% 9% 15% 27% 18% 2	2340
	11th to 20th	Day	31% 8% 11% 27% 19% 4	2340
	21st to end	Day	40% 11% 14% 24% 10% 2	2340
July	1st to 10th	Day	43% 9% 16% 24% 9%	2276
	11th to 20th	Day	30% 8% 19% 26% 16% 1	2271
	21st to end	Day	27% 10% 22% 33% 8% 1	2405
August	1st to 10th	Day	22% 8% 18% 37% 14% 1	2271
	11th to 20th	Day	28% 10% 21% 30% 11% 2	2123
	21st to end	Day	39% 10% 19% 22% 9% 2	2236
September	1st to 10th	Day	43% 10% 14% 15% 14% 4	1980
	11th to 20th	Day	41% 8% 15% 19% 13% 4	1980
	21st to end	Day	35% 7% 14% 17% 18% 9%	1980
October	1st to 10th	Day	31% 7% 10% 21% 23% 8%	1980
	11th to 20th	Day	34% 6% 10% 18% 18% 14%	1864
	21st to end	Day	29% 4 7% 17% 23% 19%	1980
November	1st to 10th	Day	39% 5% 9% 18% 19% 10%	1858
	11th to 20th	Day	37% 4 10% 17% 19% 13%	1867
	21st to end	Day	45% 4 7% 16% 17% 11%	1710
December	1st to 10th	Day	44% 3 8% 17% 19% 9%	1710
	11th to 20th	Day	45% 5% 7% 11% 18% 15%	2159
	21st to end	Day	39% 4 6% 18% 20% 12%	3135
January	1st to 10th	Night	39% 3 7% 14% 20% 17%	2458
	11th to 20th	Night	40% 3 8% 17% 16% 17%	2418
	21st to end	Night	41% 3 8% 15% 19% 15%	2421
February	1st to 10th	Night	44% 3 9% 11% 16% 17%	2160
	11th to 20th	Night	40% 3 9% 18% 17% 13%	2098
	21st to end	Night	42% 4 8% 18% 16% 12%	1766
March	1st to 10th	Night	43% 5% 12% 15% 15% 10%	1980
	11th to 20th	Night	45% 4 10% 14% 14% 13%	1980
	21st to end	Night	35% 6% 13% 17% 17% 13%	2178
April	1st to 10th	Night	40% 7% 12% 18% 14% 9%	1980
	11th to 20th	Night	40% 5% 10% 16% 14% 15%	1953
	21st to end	Night	43% 5% 10% 17% 15% 9%	1762
May	1st to 10th	Night	38% 7% 12% 20% 14% 9%	1620
	11th to 20th	Night	34% 6% 14% 25% 18% 4	1594
	21st to end	Night	35% 8% 17% 27% 12% 1	1746
June	1st to 10th	Night	22% 6% 19% 32% 19% 2	1620
	11th to 20th	Night	23% 6% 13% 34% 20% 3	1620
	21st to end	Night	27% 10% 21% 29% 11% 1	1620
July	1st to 10th	Night	30% 9% 22% 31% 7% 1	1575
	11th to 20th	Night	15% 9% 29% 32% 14%	1566
	21st to end	Night	15% 8% 31% 37% 8%	1673
August	1st to 10th	Night	11% 7% 29% 42% 11%	1638
	11th to 20th	Night	19% 8% 29% 37% 6%	1778
	21st to end	Night	24% 10% 27% 29% 9% 1	2119
September	1st to 10th	Night	31% 9% 22% 21% 12% 4	1980
	11th to 20th	Night	32% 8% 18% 24% 15% 3	1980
	21st to end	Night	28% 7% 15% 25% 18% 7%	1980
October	1st to 10th	Night	21% 5% 14% 30% 20% 10%	1980
	11th to 20th	Night	28% 5% 11% 21% 20% 15%	2078
	21st to end	Night	25% 3 9% 19% 24% 20%	2376
November	1st to 10th	Night	31% 6% 11% 19% 22% 10%	2220
	11th to 20th	Night	34% 5% 13% 17% 18% 13%	2308
	21st to end	Night	39% 4 10% 19% 17% 11%	2470
December	1st to 10th	Night	40% 4 9% 19% 18% 10%	2470
	11th to 20th	Night	45% 4 6% 14% 15% 16%	3037
	21st to end	Night	38% 5% 7% 15% 22% 12%	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	19% 10% 38% 24% 7% 2	270
	11th to 20th	Day	22% 9% 32% 26% 11% 1	257
	21st to end	Day	20% 10% 37% 23% 8% 2	264
February	1st to 10th	Day	25% 13% 26% 26% 10% 1	240
	11th to 20th	Day	22% 12% 38% 20% 8%	240
	21st to end	Day	25% 13% 36% 22% 4	198
March	1st to 10th	Day	20% 9% 41% 23% 7%	240
	11th to 20th	Day	17% 14% 40% 24% 6%	240
	21st to end	Day	16% 12% 42% 25% 5%	264
April	1st to 10th	Day	14% 10% 42% 25% 9%	240
	11th to 20th	Day	21% 14% 29% 26% 10% 1	234
	21st to end	Day	12% 14% 40% 27% 6%	451
May	1st to 10th	Day	10% 14% 43% 26% 7%	603
	11th to 20th	Day	11% 7% 36% 31% 12% 2	625
	21st to end	Day	10% 5% 30% 36% 17% 1	679
June	1st to 10th	Day	7% 1 16% 38% 32% 6%	630
	11th to 20th	Day	5% 1 18% 47% 23% 5%	630
	21st to end	Day	9% 3 12% 33% 33% 11%	630
July	1st to 10th	Day	7% 3 10% 33% 35% 12%	621
	11th to 20th	Day	4 1 6% 33% 44% 12%	618
	21st to end	Day	3 2 5% 36% 40% 15%	684
August	1st to 10th	Day	3 1 7% 35% 40% 16%	583
	11th to 20th	Day	4 1 6% 36% 35% 19%	437
	21st to end	Day	6% 2 8% 24% 32% 28%	355
September	1st to 10th	Day	10% 1 4 23% 39% 24%	270
	11th to 20th	Day	8% 5% 14% 42% 31%	270
	21st to end	Day	8% 9% 24% 39% 20%	270
October	1st to 10th	Day	6% 1 11% 31% 37% 14%	270
	11th to 20th	Day	13% 2 11% 36% 28% 10%	270
	21st to end	Day	15% 8% 23% 30% 18% 6%	297
November	1st to 10th	Day	15% 3 16% 33% 29% 4	270
	11th to 20th	Day	20% 5% 20% 29% 21% 4	270
	21st to end	Day	24% 10% 30% 21% 11% 3	270
December	1st to 10th	Day	16% 6% 30% 28% 15% 5%	270
	11th to 20th	Day	19% 11% 30% 27% 12% 1	270
	21st to end	Day	20% 8% 30% 31% 10% 2	297
January	1st to 10th	Night	21% 7% 30% 33% 9% 1	950
	11th to 20th	Night	23% 8% 28% 34% 7%	930
	21st to end	Night	19% 9% 31% 33% 7%	837
February	1st to 10th	Night	23% 12% 31% 29% 5%	720
	11th to 20th	Night	25% 8% 35% 27% 5%	704
	21st to end	Night	24% 11% 41% 21% 3	582
March	1st to 10th	Night	23% 9% 37% 28% 4	540
	11th to 20th	Night	21% 9% 36% 29% 4	540
	21st to end	Night	13% 10% 45% 28% 4	594
April	1st to 10th	Night	13% 7% 50% 26% 4	540
	11th to 20th	Night	20% 8% 39% 28% 6%	533
	21st to end	Night	17% 10% 42% 26% 5%	540
May	1st to 10th	Night	12% 6% 50% 28% 3	540
	11th to 20th	Night	12% 4 38% 36% 9%	531
	21st to end	Night	9% 4 35% 42% 10%	584
June	1st to 10th	Night	4 2 19% 56% 18% 1	540
	11th to 20th	Night	5% 1 24% 56% 14%	540
	21st to end	Night	8% 1 15% 49% 24% 2	540
July	1st to 10th	Night	5% 2 13% 48% 30% 2	525
	11th to 20th	Night	2 1 9% 51% 34% 2	522
	21st to end	Night	2 1 7% 55% 33% 2	591
August	1st to 10th	Night	1 7% 54% 34% 4	537
	11th to 20th	Night	5% 1 7% 50% 35% 3	534
	21st to end	Night	5% 1 6% 39% 46% 3	594
September	1st to 10th	Night	8% 1 4 40% 42% 5%	540
	11th to 20th	Night	11% 1 7% 34% 43% 4	540
	21st to end	Night	8% 1 11% 37% 38% 4	540
October	1st to 10th	Night	4 1 11% 46% 34% 4	540
	11th to 20th	Night	11% 3 21% 45% 17% 3	638
	21st to end	Night	11% 6% 25% 37% 19% 2	792
November	1st to 10th	Night	11% 3 23% 43% 18% 2	740
	11th to 20th	Night	15% 5% 24% 37% 17% 1	788
	21st to end	Night	18% 7% 28% 32% 14% 1	950
December	1st to 10th	Night	21% 6% 24% 34% 13% 1	950
	11th to 20th	Night	23% 9% 27% 33% 7%	950
	21st to end	Night	21% 8% 27% 35% 9% 1	1045

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